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BioResources

BioResources is proud to host the archives of the Pulp and Paper Fundamental Research Symposia. More information on this organization can be found here.

FRC Symposium Proceedings

  • 2017
    Oxford
    pp 3-26Experimental and Numerical Verification of 3D FormingAbstractPDF

    Motivated by sustainability arguments there is a recent interest in forming of advanced structures in paper and paperboard. Therefore, in this paper, hydro-forming of papers and the effect of different fibre raw materials, beating, strength additives (PVAm), grammage and wet and dry papers have been investigated experimentally and numerically.
    The experiments were carried out in laboratory hydro-forming device. Softwood sheets performed better than hardwood sheets, since they had higher strain at break. The ability of paper to withstand hydro-forming successfully was primarily dependent of the strain at break of the paper in relation to the straining required to fill the mould. Forming of wet sheets were also investigated; overall the wet sheets formed better than the dry sheets, which was due to higher strain at break and lower elastic energy. Since the forming was displacement controlled, there was no significant difference in the effects of beating, amount of PVAm or grammage.
    Finite element modelling was performed to identify local strains and predict problematic regions. Simulations were also performed to determine how anisotropic sheets would behave, as well as to compare the process of hydro-forming with press-forming. The papers could be strained to higher strain levels than the measured strain at break because the paper is supported by the membrane and mould during the forming operation. The maximum strain a paper can withstand can be increased if the paper can slide into the mould, i.e. by having a lower coefficient of friction between the steel mould and the paperboard.
    During hydro-forming the paper is supported by a rubber membrane, which gives lower strain levels than the corresponding press-forming operation due to the difference in how the paper is deformed. Press-forming therefore required paper with higher strain at break. Higher friction results in more paper being pulled into the mould, which contributes to wrinkling of the paper. Simulation of tray forming of a creased sample was performed, which showed that high friction or compliant creases decreased the circumferential compression.

  • 2017
    Oxford
    pp 27-41Enhanced Test Devices for the Development of Novel Paper-like Materials for Sandwich-StructuresAbstractPDF

    High performance sandwich components have a great significance in aerospace applications. Particularly, lightweight sandwich structures made of honeycomb or foldcores show excellent load carrying capabilities. Both types of cores are usually made of aramid paper coated with phenolic resin. Therefore, the development of improved paper-like materials seems to be a promising approach to improve the mechanical performance of this kind of cores. An essential part of this development process is the evaluation of the new materials by the complete characterisation of the mechanical properties. This is still a challenging task, since the resulting papers are orthotropic and most of the existing testing procedures and devices are not suitable for very thin sheet materials. This is particularly true for investigating stiffness and strength properties under compressive and shear loading.
    The paper presents a novel single-curved compression test device as well as an adapted shear-frame for the in-plane characterisation of very thin specimens. These devices have been applied in the development process of a new paper-like material that consists of three layers in order to increase the stiffness and strength of honeycomb- and foldcores. The performance of this material was evaluated by comparing relevant mechanical properties to that of state of the art paper materials. Based on the experimental results the benefits of the new paper-like material could be shown.

  • 2017
    Oxford
    pp 43-68Prediction of Curl BehaviourAbstractPDF

    Curl is unwanted dynamic behaviour that appears already in the manufacturing process and evolves through the end-use of paper or board due to moisture content changes and mechanical treatments. In this paper, the analytical and numerical approaches are used to reveal the sensitivity of the curl tendency to fundamental variables affecting deformation behaviour of paper. Good agreement between the measured curvatures induced into the sheet by the photocopying process and the simulated curvatures is achieved. Paper is treated as a multi-layered material, and finite element simulations are performed by using hygro-elasto-plastic and hygro-viscoelastic material models. Also analytical calculations are carried out to support the conclusions. The results show that the prediction and control of curl is not straightforward; curl depends not only on the fibre orientation structure but interacts in a complex way with the through-thickness dry solids content profiles during moisture content changes and external mechanical forces acting on paper. Despite the complexity of the phenomenon, the simplified computational approaches presented in this paper can be used for analysing and optimising the paper structure and process parameters to prevent detrimental curl.

  • 2017
    Oxford
    pp 69-136Creasing and FoldingAbstractPDF

    NA

  • 2017
    Oxford
    pp 139-158Development of Cellulose Nanofibre Quality with Mechanical Energy: Effect of Starting MaterialAbstractPDF

    Energy efficient production of nanocellulose fibres is key to establishing this highly-promoted materials in an industrial scale. In this work, we attempt to explain how the mechanical energy input and the chemical composition of the raw materials affect the quality of nanofibres. Bleached eucalyptus Kraft (BEK) pulp, a commercially availble microfibrillated nanocellulose from cotton, and whitewater fines collected from a radiata pine thermomechanical pulping (TMP) mill were used to produce cellulose nanofibres. BEK was the most responsive to mechanical fibrillation due to low crystallinity and it produced high aspect ratio nanofibres, while TMP whitewater fines were the most difficult to process and resulted in low aspect ratio nanofibres. Nanofibres were then added to TMP newsprint to evaluate the effect on tnesile strength. Nanofibres produced from BEK were able to increase the tensile strength the most, while nanofibres from TMP whitewater fines had the least effect. The results showed that a high aspect ratio and a surface chemical composition favouring more hydrogen bonds i.e. pure cellulose, are the key criteria when selecting nanofibre for strength improvement in paper.

  • 2017
    Oxford
    pp 159-173The Effect of Fibrous Materials on the Rheology of Aqueous FoamsAbstractPDF

    We studied fully developed pipe flow of fibre-laden aqueous foams and decoupled their bulk rheological properties boundary effects like slippage at the pipe wall. The air volume fraction of the foams varied between 70% and 75%. The addition of hardwood fibres at the consistency 20 g/kg to plain aqueous foam increased viscosity more than 100%, while with microfibrillated cellulose at a consistency of 25 g/kg the increase was about 30%. The effect of synthetic (cellulosic)rayon fibres was negligible at the consistency of 20 g/kg. All the studied foams could be described as shear-thinning power-law fluids with significant slippage at the pipe wall by particles size and interactions between particles and bubbles.

  • 2017
    Oxford
    pp 175-194Refining Impulse Controls the Morphological Modifications of FibersAbstractPDF

    Considering the analogy between the pressing of a paper sheet and the refining of a pulp suspension, the refining impulse is introduced. For beaters, disc or conical refiners, whatever the running mode (continuous or batch), the refining impulse is found to be a controlling variable for the pulp properties, and consequently for the paper properties. In a Valley beater, different normal forces were applied. The SR evolution versus the refining impulse exhibits a unique curve whatever the experimental conditions. For disc and conical refiners, the refining impulse depends on the net power, the rotation speed, the bar width, or the average bar angle. A unique parameter is used to fit each set of trials to obtain a single curve of the SR evolution. This parameter corresponds to the global friction coefficient f. he fiber length and the swelling (WRV) depend also on the refining impulse. However, as in pressing theories, the applied pressure has also to be introduced as a complementary parameter. Consequently, the paper properties are shown to depend also on both the refining impulse and the applied pressure.

  • 2017
    Oxford
    pp 195-207A New Representation for Low Consistency Refining DataAbstractPDF

    The standard method of representing refining data is to plot fibre or sheet properties as a function of refiner Specific Energy Consumption (SEC), for separate refining trials done at different Specifics EDG Loads (SEL). This approach does not allow for refining outcomes to be predicted when refining at other values of SEL and does not allow for refining conditions to be optimized to satisfy multiple constraints. In addition, the change in fibre properties is determined by the number of impacts on a fibre and the energy used in each impact, while SEC is the product of number and energy used in each impact. The paper describes a new representation of refining data where the two axes of the plot are SEC/SEL, which is proportional to the number of impacts, and 1/SEL, which is proportional to the inverse of the energy used in each impact. Data from refining trials are then plotted as lines of equal value. The paper shows how flow and power limited for a low consistency refiner are represented on such a plot. The utility of the approach is demonstrated with refining data of a CTMP pulp with three different refining plates and three different speeds.

  • 2017
    Oxford
    pp 209-228Fractionation of Bifer Pulp in a Hydrodynamic Fractionation Device: Influence of Reynolds Number and Accept Flow RateAbstractPDF

    Fibre fractionation in the Hydrodynamic Fractionation Device (HDF) was studied for changing suspension flow parameters, i.e. different channel Reynolds numbers Re and accept flow rates up to 20% of the feed flow rate. The suspension flow behaviour was described using images recored with a high-speed camera system. Fractionation performance was determined based on mass balances for a variety of length fractions of the pulp. Low Reynolds number flow characterised by Re = 1300 led to the formation of a fluid gap between the wall and the fibres located at the chaneel centre. Best fractionation performance was achieved for flow at this Reynolds number: no fiber removal was observed at 10% accept flow rate, and only 1% of the fibres were removed at 20% accept flow rate. A design space was established that highlights the optimum settings for fractionation in an HDF, which at low Re and high accept flow rate. Surprisingly, we found a significant increase of fines mass flow rate in the accept upon an increase of the Reynolds number. We speculate that a flow regime-dependent interaction of fines with the fibres exists in the HDF that critically affects the amount of fines in the fluid gap near the wall.

  • 2017
    Oxford
    pp 231-254The Effect of Microfibrillated Cellulose on the Strength and Light Scattering of Highly Filled PapersAbstractPDF

    There has been much recent interest in the use of microfibrillated and nanofibrillated cellulose as additives to improve the mechanical properties of paper. Most of the original methods used to make these materials are to costly for this purpose, but now purely mechanical processed are becoming available which have been made it a more practical possibility. The tensile strength of unfilled paper and its relation to light scattering have been the subject of extensive theoretical and experimental research a, and the effect of addition of fibrillated cellulose have been considered by several authors in the light of this work. However, much less theoretical work have been dedicated to the properties of papers with high filler contents.
    In the FiberLean process, fibres are mixed with filler and found together until fibres are converted into microfibrillated cellulose, a few percent of which can be added to paper to increase its strength and allow a substantially higher filler content. We build on the work of Bown to develop a model for the effect of filler on paper tensile strength and light scattering, and use this to investigate mechanism by which mfc improves these in highly filled paper. We further demonstrate some of the advantages of its use over the conventional refining approach. There include process flexibility and some specific paper properties such as increased elasticity and higher resistance to tearing.

  • 2017
    Oxford
    pp 255-280Experimental Study of Filtration of Fiber Suspensions: Part I: Time-resolved Measurements of the Formation of a Fiber NetworkAbstractPDF

    We present a new laboratory apparatus designed to study the filtration of a fiber suspension in a parameter range close to that of the real papermaking process. An experimental procedure that combines index-of-refraction matching, high-speed imaging, and article Image Velocimetry (PIV) is used to measure glow and particles dynamics. In particular we presents results for: flow above a mesh similar to that used for paper forming; flow above and in the forming fiber network; evolution of the structure of the fiber network itself. Experiments are performed varying filtration velocity and fiber length. The results show that flow perturbations in the proximity of the mesh and the forming network are confined in a thin region and the height of this region are likely to scale with the pore size. Snapshot reveal qualitative difference between the networks formed by fibers of different lengths.

  • 2017
    Oxford
    pp 281-298Experimental Study of Filtration of Fiber Suspensions: Part II: Combined PIV and Pressure Drop Measurements.AbstractPDF

    The filtration of a fiber suspension has been studied experimentally. Typical application where pressure filtration occurs are: papermaking, air cleaners, production of composite materials, etc. In particular, in papermaking, the quality of the final product depends on the fiber orientation and mass distribution in the filtered materials Micro-variations of there properties can strongly affect the quality of the final product and they can occur during filtration, thus it is important to predict how this can happen. However, this is not an easy task, first because the filtered cake is a non-homogeneous compressible porous media, second because the filtration flow is non-stationary, since the cake is continuously evolving in time. Therefor in this work we focus on the filtration flow through formed steady fiber networks. For each grammage (i.e. mass of fibers per unit area), we simultaneously measure the pressure drop across the network and velocity field on top and below the fiber network using Particle Image Velocimetry (PIV). Compression of the fiber network can also be extracted form the PIV images normalized filtration resistance was found to be decreasing with increasing network thickness, as well as network compressibility. From the PIV data the influence of the formed fiber network on the flow field was analyzed and characteristic scales of the flow structure are quantified.

  • 2017
    Oxford
    pp 299-322Characterization of Fines Quality and Their Independent Effect on Sheet PropertiesAbstractPDF

    It is widely accepted that pulp fines (particles passing a 200 mesh screen) largely affect pulp properties, sheet consolidation and the final paper properties. Especially fines produced during refining – so called secondary fines- showing a more fibrillar character compared to primary fines already present after the pulping process, have a positive effect on strength properties. Although this is common knowledge within the paper physics community, it is still largely unclear which detailed properties of fines influence pulp and paper properties to what extent. As fines show some similarity to MFC, this question is also of interest regarding the sue of MFC as an additive in papermaking. We apply established and new methods for fines characterization, such as the secondary fines content, the swelling ability and data on fibrillation and fibrillary material together with a suitable experimental setup t isolate the technological impact of fines in the final product Thus we are able to evaluate the technological effect of fines with different characteristics in therms of the above mentioned properties. Our results clearly show that categorizing primary and secondary fines is not sufficient when it comes to their technological impact and only in depth analysis of the fines present in a given pulp allows to understand their effect on paper properties.

  • 2017
    Oxford
    pp 323-343The Effect of Press Nip Geometry on Dryness, Density and Paper PropertiesAbstractPDF

    NA

  • 2017
    Oxford
    pp 345-365Chemical Modification of Pulp Fibres for the Production of Thermoformable PaperAbstractPDF

    Enhancing the usability of paper based materials in forming processes is one major objective to pave the way for a sustainable and bio-based economy. Therefore, modifications of either the forming process or the paper material will provide future opportunities. The present study focuses on the concept of chemical pulp fibre modification as a factor influencing the formability of paper materials made thereof.Specifically, covalent attachment of ester moieties with different chain lengths onto cellulose onto fibres, a surface polymer grafting and a two-step glycol cleavage-reduction approach were characterised regarding their chemical composition and thermomechanical properties. To evaluate their formability, a temperature controlled tensile test was used.

  • 2017
    Oxford
    pp 367-381Control of Porous Structure of Paper in a Continuous ProcessAbstractPDF

    Microfibrillated cellulose (MFC) has unique properties which can lead to interesting new paper applications. Some applications strongly rely on defined pore structure, like filter paper for water purification or separator paper in batteries. The pore structure modification of a paper sheet through MFC is investigated, the combination of MFC and polyelectrolytes (PEs) enables a pore dimension reduction of an existing softwood softwood base layer. In both handsheet and pilot paper machine trials this effect was accomplished and reproducible. It was shown that MFC grammage, MFC consistency and PE type are significant factors that affect the pore dimension.

  • 2017
    Oxford
    pp 383-407Simulation of Sloshing of Condensate Water Inside a Drying Cylinder Using Particle Finite Element MethodAbstractPDF

    Energy consumption is a key issue in paper making due to its high costs and for ecological reasons. In this paper, we focus on the simulation of the drying section, with a special emphasis on the heat transfer inside a steam-heated drying cylinder, accounting for the sloshing of condensate water inside the cylinder. It is known that the condensate creates a barrier reducing the efficiency of heat transfer through the cylinder surface, a shortcoming that can be overcome by the addition of turbulator bars that increase the convective heat transfer through the water layer. In this study, we simulate the fluid flow and heat transfer in the drying roll for both the water and steam phase, but for simplicity, neglect the phase change.The Particle Finite Element Method with a fixed mesh (PFEM-2) is used to compute the numerical solution. Our aim is to show the capability of this method for solving complex sloshing phenomena with adequate qualitative accuracy and computational efficiency.

  • 2017
    Oxford
    pp 409-449Quantitative Studies of Ambient Gases in Pulp and Paper Mills and Their Degradation with Photo-catalytic Oxidation Technology.AbstractPDF

    In order to reduce gaseous pollution emissions and achieve the goal of cleaner production in paper industry, in this study, (I) First, the concentrations of four potential compositions of gaseous pollutants, TVOC, HCHO, H2S and CxHy, in the ambient air on 30 sampling sites in 5 pulp and paper mills were analysed. The analysed results were discussed in the following aspects: (a) the levels of four gaseous pollutants on all the sampling sites in five mills; (b) gaseous pollution differences due to different production processes; (c) gaseous pollution comparisons on the common sites. (II) Secondly, the compositions of VOCs in a secondary fiber paper mill were determined with GC-MS method. The main identified substances in the four sites were as follows: (a) waste paper sorting room: alkanes, phenols and esters; (b) paper machine hall: benzene homologues, alkanes, ethers and phenols; (c) vacuum pulp outlet: benzene homologues and phenols; (d) office area: benzene homologues and phenols. (III) Third and last, aiming a the detected formaldehyde and benzene pollutants, a photo-catalytic reactor was developed and its performance with respect to degradation was studied. The performance tests of reactor showed that both formaldehyde and benzene could be completely degraded, but the degradation time for benzene was much longer than that for formaldehyde.

  • 2017
    Oxford
    pp 451-485Prediction of Mineral Deposits in Kraft Pulp Bleaching Lines Through Chemical Process SimulationAbstractPDF

    The general tendency in the pulp industry towards reduced fresh water consumption and minimum effluent causes major deposit problems in mills. Chemical pulp bleach plants are affected by several types of mineral deposits, the most frequent being calcite, calcium oxalate and barite. We present a coupled chemical process simulation of kraft pulp bleaching line, which handles chemical equilibria, together with dissolution and precipitation effects. The simulation could adequately predict formation of mineral deposits throughout a D0(EP)D1D2 bleaching line. Strategies to help reduce formation of calcium oxalate and barite scales could be evaluated. Partial substitution of sodium hydroxide by a magnesium source at extraction stage is anticipated to inhibit formation of calcium oxalate throughout the line. Also, using sulfuric acid instead of spent acid for pH regulation at D0 would reduce but not suppress barite deposits.

  • 2017
    Oxford
    pp 489-594Three-dimensional Deformation and Damage Mechanisms in Forming of Advanced Structures in PaperAbstractPDF

    There is a large potential for wood-fiber based materials such as paper and board to contribute to lightweight structures in several applications, particularly packaging. Fiber-based packaging materials have important advantages in comparison to fossil-based plastics regarding biodegradability, recyclability and renewability. Individualisation has become a crucial criterion for the use of packaging solutions and forming of advanced paperboard structure is a key technology for manufacturing of such packaging shapes. New sustainable packaging concepts are creating a need for paper materials with considerably enhanced properties.

    Paper and board are in manufacturing of geometrically advances structures in general subjected to complex and often little known multi-axial states of loading and deformation that are necessarily quantified by conventional measures for paper performance. Today, commercial paperboard is optimised for folding and printing, and not for applications involving forming of advanced structures. It is like-wise important to design the manufacturing process to meet the particular properties of paperboard. Manufacturing methods that are suitable for metals and plastics are inevitably not suitable for paper and board since the deformation and damage mechanisms of fibre network materials are different from metals and plastics.

    In this paper recent findings in the literature on 3D forming of paper and paperboard structures are reviewed. In particular, deformation and damage mechanisms involved in pertinent forming operations and how they are related to paper and board properties in order to enhance the development of new advanced paper materials and structure are analysed.

    In the last decade, there have been major advancements in the development of geometrically advanced 3D paperboard structures including technological advances of various forming process, enriched understanding of the importance and influence of process parameters, and new paperboard materials with significant improved forming properties. However, there is still a lack of knowledge regarding the deformation mechanisms of these complex systems and particularly regarding the influence of friction. One remedy would be the enhancement of numerical simulation tools. Optimisation of existing forming processes and development of new ones as well as tailored paper and board materials with properties customised to the demands of exiting and new 3D forming processes will also play important roles. This development is only in its beginning and major progress is expected in the near future.

  • 2017
    Oxford
    pp 595-609Focused Ion Beam Tomography as A Means for Characterization of CNF in A Paper Matrix.AbstractPDF

    A method to study Cellulose Nanofibril (CNF) distribution in three dimensions within a paper matrix-in-situ-was developed. Focused Ion Beam (FIB)/Scanning Electron Microscopy (SEM) tomography was used to investigate the distribution of cellulose nanofibres in thee dimensions within a paper structure. Sufficient resolution and material contrast was contained using both secondary and back-scattered electrons in volumes as large as 103 um3. Challenges and approaches to achieve this are discussed, both with respect to the microscopy technique and with respect to image processing and volume reconstruction. A range of recorded images and reconstructed 3D volumes show the technique capable of resolving CNF in a paper matrix. Results presented show CNF within the paper matrix forming capsules enclosing filler particles. These capsules are seen to only infrequently be in physical contact with the enclosed particles. Similar separation between CNF and enclosed filler particles was tested and confirmed in CNF films with 10 wt% added ground calcium carbonate.

  • 2017
    Oxford
    pp 611-625Synchrotron Tomographic Imaging of Softwood Paper: A 4D Investigation of Deformation and Failure MechanismAbstractPDF

    Fibre pull-out and fibre fracture are the two dominant failure mechanisms of softwood paper. For the first time, 4D synchrotron X-ray tomographic imaging (three spatial directions plus time) was performed to observe these mechanism in-situ during tensile deformation of softwood paper handsheets. The experiments were conducted on three handsheets, produced from pulp that was low consistency refined at 0 k Wh/t and 100 kWh/t and wither air-dried in restraint or freeze-dried. The fibre deformation was found to be highly complex; initially being accommodated via straightening of the fibres resulting in fibre separation and then complete fibre pull-out or fibre fracture. The 3D strain fields, computed by Digital Volume Correlation, revealed increasing out-of =plane deformation in samples with decreasing inter-fibre bonding. Further, the relation between the L2 norms of the out-of-lane strain fields with displacement was computed and found to follow a second order polynomial, with an increasing slope in samples with reduced inter-fibre bonding. It was then shown that the accumulated out-of plane deformations could be used as a metric to quantify the relative contribution of inter-fibre bond breakage, and subsequently, fibre pull-out during tensile deformation of handsheets. The results demonstrate that 4D imaging provides new insights into paper deformation mechanisms.

  • 2017
    Oxford
    pp 627-649A Micro-mechanical Modelling Study of Drying Restraint Effects on the Hygro-mechanics of Paper SheetsAbstractPDF

    In this contribution we show how fibre activation and micro-buckling of fibre walls may explain, quantitatively, differences in the hydro-mechanical response of paper sheets due to the presence or absence of mechanical restraint during their fabrication. To this end, both effects are incorporated in an idealised micro-mechanical model of the fibre network. The model is used to predict the response of the network to wetting-drying cycles, as a function of the degree of restraint during production. Restrained-dried networks are predicted to exhibit an irreversible hydroscopic strain upon first wetting and a different reversible hygro-expansivity coefficient, compared with free-dried networks, which match well with experimental values reposted in the literature.

  • 2017
    Oxford
    pp 651-668Computational Design of Fibre Network by Discrete Element MethodAbstractPDF

    Soft fibre networks, typically seen in bathroom tissues, kitchen towels, and personal-care products, have properties that are intricately affected by the details of fibre geometry, 3D-network structures, and processing conditions. Designing such materials and products for better performance, while controlling cost, is especially a challenge in today’s fast paced product development. This paper concerns the development of a new, robust computational design platform for the design of soft fibre networks.
    We have used particle-based methods, particularly, Discrete Element Method (DEM), to model fibres, fibre networks, their properties and performance, and also unit processes for manufacturing. Unlike other computational methods, this method has advantages to model discrete and non-homogeneous materials, complex geometries, and highly non-linear dynamic problems, such as large deformation (flow), contact/non/contact, fracture, and fragmentation.
    With this approach, fibres are represented by a series of connected spherical particles in different lengths and geometries (curl, kinks, twists). Fiber networks are created by the deposition of those fibres under gravity, followed by the subsequent consolidation under pressure. here processes have shown an interesting transition phenomenon from a highly fluidic granular system to a fragile soft solid. The network is then subjected to a creping process, a critical process of tissue-making. The model was able, not only to reproduce unique crepe frequencies, but also unprecedented details of the destruction of fibre network structures and fibre failure (dusting) during creping. Typical tensile tests, thickness-direction compression tests, and softness tests have been also performed to demonstrate unique deformation characteristics of low-density, low-basis weight fibre networks.
    This computational design system based on DEM provides a promising platform for exploring large parameter space of new material/product design.

  • 2017
    Oxford
    pp 669-682Linking Paper Structure to Local Distribution of Deformation and DamageAbstractPDF

    A method for quantitatively investigating the relationship between local structural properties (local basis weight, local thickness, local density and local load carrying factor (local fiber orientation)) and local tensile deformation (local strain and local temperature increase (thermal energy dissipation)) was introduced. It was found by utilizing the method for 70 g/m2 sack paper strips that relative basis weight, relative thickness, relative density and relative load carrying factor combined explain 31% and 26% of total variation in relative strain and relative temperature increase, respectively. Best single predictors for relative strain were relative basis weight (R2=0.14) and relative load carrying factor (R2=0.11). On the other hand, relative basis weight alone was the best predictor for relative temperature increase deformation distribution parameters from the perspective that high relative temperature increase is preceded by the high relative strain, it could be said that the relative strain explains 45% of the total variation in the relative temperature increase (R2=0.45). Thus, these two parameters describe the deformation in partially different ways. Based on this study, it can be concluded that the introduced method offers a promising tool to quantitatively investigate the separate/combined influence of local structure properties on the local deformation accumulation initializing the failure of paper.

  • 2017
    Oxford
    pp 683-719The Effect of Geometry Changes on the Mechanical Stiffness of Fiber-Fiber BondsAbstractPDF

    In this work, we discuss the effect of geometry on the compliance of the fibre bond regions against normal and tangent loads. Since the fibre bonds play a key role in defining the paper strength, the compliance of the bond regions can affect the amount of elastic energy stored in the bonds and thus change not only the strength but also the stiffness of paper products under certain conditions. Using finite element simulation tools, we overcome the major difficulty of performing controlled mechanical testing of the isolated bond region and reveal the key geometrical factors affecting the compliance of the bond region. Specifically, we show that the compliance of the fiber-fiber bond is strongly governed by its geometric configuration after pressing. Among the strongest factors is the collapse of the lumen and the crossing angle.
    Using the range of obtained stiffness values, we demonstrated the effect the bond stiffness has on the stiffness of the network using fiber-level simulation tools. We show how the dependence of tangent bond stiffness on fiber-to-fiber angle further softens the more compliance cross-machine direction.

  • 2017
    Oxford
    pp 723-737A Versatile Tool for Conductive PaperAbstractPDF

    NA

  • 2017
    Oxford
    pp 739-749Paper-based Electronics and Sensors Fabricated by Using Printing TechnologyAbstractPDF

    Paper is seen as a potential substrate for devices such as electronics and sensor because of environmental friendliness being a dualistic material with flexibility and rigidity, and a possibility for mass production at low cost. In this study, two such applications as devices will be introduced. First, a power generator has been developed to convert sonic vibration into electric energy. Secondly, a corona discharge-treated polytetrafluoroethylene sheet, as an electret, was attached to a paperboard with a back electrode. Another paperboard with a counter electrode was mechanically vibrated to simulate a sound. During vibration, electric power was successfully generated by electrostatic induction. Insertion of nano0cellulose paper further more enhanced the output voltage. A simple, quick, sensitive and ion species-selective paper-based sensor with a quinone derivative dye ink-jet printed has been developed to detect Cu2+ ions at 2 ppm, a maximum allowed for drinking water, by colour change observation. A fluorescence spectrum of the dye provided higher resolution to permit quantitative detection of Cu2+ concentrations.

  • 2017
    Oxford
    pp 751-773Paper-based Chemical Detecting Sensors for Surface-enhanced Raman ScatteringAbstractPDF

    In this study, we fabricated a paper-based molecule-detecting sensor for the surface-enhanced Raman scattering (SERS) technique. SERS phenomenon is based on the face that the low intensity of Raman scattering is dramatically increased when the molecules are adsorbed on novel metal surface. To improve the applicability of paper substrate as a base for SERS several trials were made. The smoothness of the filter paper was improved through a calendaring process. To prevent the spreading of the chemical solution on the aper the hydrophobicity of paper was increased by treating with an alkyl ketene dimer (AKD). Onto the smooth and hydrophobic filter paper a silver nanoparticle (AgNP) solution was applied with a simple drop and dry method, and analyte was treated in the same manner on the AgNP decorated area for SERS measurement. To improve the reproducibility of the SERS intensity, an area scanning method that used a dual axis galvanometric mirror was introduced. A 4-aminothiophenol molecule could be detected at the femtomolar level using the hydrophobic-treated filter paper. Coating of cellulose nanofibrils (CNF) made from pulp fibers reduced the surface pore sizes and increased the uniformity of the surface of the filter paper, which improved the reproducibility and sensitivity of the molecule-detecting sensor. The use of a high magnification objective lens for increased SERS intensity allowed for the detection of a strong SERS signal, and the application of a CNF coating to the filter paper improved the reproducibility. Pesticides were detected using the paper-based substrate as SERS substrate.

  • 2017
    Oxford
    pp 777-784Developments of Manufacturing Techniques and Applications of Cellulose NanofibreAbstractPDF

    Nippon Paper industries has been developing Cellulose Nanofibre (CNF) products prepared by the fibrillation of chemically modified pulp since 2007.

    In November 2013 we started to operate a pre-commercial plant in order to provide CNF for the collaborators, potential users and internal use, after 5 years of fundamental research together with out collaborators with the aim of commercialization of products using CNF. In our pre-commercial plant, mainly TEMPO oxidized CNF is produced, a process developed by the research group of Prof. A. Isogai at The University of Tokyo. In addition t TEMPO oxidation, other chemical modifications such as carboxymethylation are carried out in out plant as well. Thus collaborators can choose the type of CNF samples, depending on their target applications.

    In 2016, NPI announced the plan to install a CNF full-scale production facilities at the Ishinomaki Mill and the Gotsu Mill in Japan. At the Ishinomaki Mill, CNF prepared by TEMPO oxidation will be produced, and at the Gotsu Mill, CNF prepared by carboxymethylation will be produced.

    In this brief paper we will present our developments of manufacturing techniques and applications of chemically modified CNF

  • 2017
    Oxford
    pp 785-800Preparation and Characterization of Spherical Nanosized Cellulose by Enzymatic Hydrolysis of Pulp FibersAbstractPDF

    In this work, the pulp fibers were enzymolyzed to prepare the nano-sized cellulose (NC). The as-prepared samples were characterized by optical microscopy, electron microscopy, and Raman spectra. The experimental results indicated that enzymatic hydrolysis of pulp fibers could produce the spherical NC with a mean particle size of about 30 nm, which has the excellent monodispersity and uniformity. When the concentration of complex enzymes was 20 u/mL (cellulase: xylanase = 9:1), the yield of NC was 13.6%. The single cellulase was used, even if the concentration and time reached up to 200 u/mL, only a mixture of trip and granular flocculation were obtained. The positive synergistic effect between xylanase and cellulase could be due to the enzymolysis of hemicellulose located on the cellulose microfibers to favorable of cutting and splitting of the microfibers by the endoglycannase in cellulase. Otherwise, the additive copper sulfate could decrease formation of reducing sugar effectively.

  • 2017
    Oxford
    pp 801-811Determination of Length and Width of Nanocelluloses from Their Dilute DispersionsAbstractPDF
    Length/width and their distribution of nanocelluloses, prepared from wood pulps with or without chemical pretreatment, are key factors in application to high-strength and light-weight composites, transparent optical films, gas-barrier films, electronic devices, etc. Although microscopy images provide some length/width information, the number of measurable nanocellulose elements is limited. In this paper, three methods to determine nanocellulose lengths and widths are presented. The field-flow-fractionation (FFF) method combined with static light scattering was applied to dilute aqueous TEMPO-oxidised cellulose nanofibril (TOCN) dispersions to obtain avarage lengths, length distributions, and widths of different TOCNs. Although TOCN elements with lengths > 300 nm coiuld not be separated properly according to their lengths by the FFF system, TOCNs with lengths  < 300 nm were adequately separated, depending on the lengths by the FFF system, and provided length/length distributions and widths, well corresponding to those obtained from microscopy images. Intrinsic viscosities of TOCNs with different lengths and widths were obtained viscosities of TOCNs with different lengths and widths were obtained uusing shear viscosity measurement of dilute aqueous TOCN dispersions . When the obtained intrinsic viscosities of TOCN dispersions were compared with aspect ratios p of TOCNs measured from their microscopy images, these two factors had good relationship, when the constant value of TOCN density (),  =0.15*p1.9 was used. Compared to length measurement from microscopy images, the viscosity methods provide average lengths og numerous amounts of TOCN elements present in the dispersions. Seven types of nanocelluloses with different average widths and width distributions as well as those with different network strictures were prepared via different mechanical and chemical/mechanical processes, and the turbidities of their dilute dispersions were measured. The widths of nanocelluloses were calculated from the respective turbidity plots based on the theory of light scattering for thin and long particles. The turbidity-derived widths of the seven nanocelluloses ranges from 2 to 10 nm, and showed good correlations with the thickness of nanocelluloses measured from their microscopy images.
  • 2017
    Oxford
    pp 813-821Preparation and Utilization of Highly Transparent and Viscous Dispersion of Phosphorylated Cellulose NanofibersAbstractPDF

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  • 2017
    Oxford
    pp 823-836Investigating Silica Nanoparticle-polyelectrolyte Structures in Microfi Brillated Cellulose Flms by Scattering TechniquesAbstractPDF

    We report the cationic polyelectrolyte (CPAM)-SiO2 nanoparticle (NP) interactions in suspension and in a sheet form, when mixed with microfibrillated (MFC), using dynamic light scattering (DLS) and small angle X-ray scattering (SAXS) techniques. The CPAM-SiO2 NP suspensions were prepared by adding NPs into CPAM drop wise and composites were prepared by adding CPAM-SiO2 suspension into MFC and through standard paper making procedure. DLS revealed that increase in CPAM dosage creates larger sized CPAM-NP aggragates because more NPs can be picked up by stretched CPAM chains. SAXS study revealed that CPAM-SiO2 NP assembly in the formed nanopaper fits well with a sperical core shell model (with SiO2 partially covered with CPAM) and sphere model (SiO2 alone) combined together. Understanding the interaction between polyelectrolyte-NP system through such scattering techniques enables us to engineer novel cellulose based composites for specific applications.

  • 2017
    Oxford
    pp 839-863Fiber-fiber Bond Formation and Failure: Mechanisms and Analytical TechniquesAbstractPDF

    In this paper we give a literature overview on three different aspects of pulp fiber-fiber bonding. First we are reviewing how the adhesion between the pulp fibers is created by the capillary pressure during drying of a sheet. Second we are discussing the individual mechanisms relevant for fiber-fiber bonding. They can be grouped in three different groups: (a) The area in molecular contact, which also includes interdiffusion; (b) the intermolecular bonding mechanisms hydrogen bonding, Van der waals forces and coulomb interaction; (c) the mechanical bonding mechanisms which are capillary bridges and mechanical interlocking. The third and last part of the review discusses the failure process of fiber-fiber bonds and related single fiber-fiber bond testing methods. The general emphasis of the paper is set on providing a general understanding of the processes responsible for how bonds between fibers are created, how they work and how they are failing.

  • 2017
    Oxford
    pp 865-894Towards Wet Resilient Paper – Fiber Modifications and Test Method DevelopmentAbstractPDF

    Wet paper that has been crumpled into a ball shows little tendency to recover to a planar shape when the applied pressure is released – a characteristic called poor wet resiliency. We report the results of an investigation into approaches to improve paper wet resiliency through the choice of fiber types and fiber treatments. Following the lead of the textile industry and the patent literature, wet recovery angle (WRA)  was used as a measure of wet resiliency. In this technique, wet strips of paper are folded, without creasing, pressed, and then released. The WRA was measured after the paper relaxed – the greater the WRA, the more resilient the paper.

    All of our sheets with a WRA > 0° contained PAE, a standard wet strength resin. Except for sheets based on Abaca fibers, standard PAE or PAE + CMC (carboxymethyl cellulose) applications gave very low WRA. Instead, conventional fibers had to undergo wither TEMPO oxidation or CMC grafting before PAE application. Both fiber treatments substantially increase fiber surface charge density, promoting PAE adsorption and, more importantly, giving covalent bonding sites on the fiber surfaces for grafting PAE.

    When comparing the WRA values from treated fibers (TEMPO oxidation or CMC grafting), the ranking was Abaca >>Lyocell > bleached southern softwood kraft > bleached eucalyptus kraft. For pulp mixtures, treated fiber contents of 20-40% had a much bigger influence on WRA compared to wet tensile strength.

    Wet-resiliency is due to the swelling of the “hinge region” in folded wet paper, in combination with a sufficiently strong fiber network that can translate the swelling forces into shape recovery. Limited data suggests that the extent of recovery from z-directional wet compression is directly correlated with WRA values. By contrast, wet tensile strength is weakly correlated with WRA.

     

  • 2017
    Oxford
    pp 895-907Biomacromolecule Adsorption at the Cellulose–liquid InterfaceAbstractPDF

    A novel methodology is developed to visualize and quantify biomolecules adsorption at the cellulose film0liquid interface. Hydrogenated cellulose (HC) films were made from cellulose acetate and deuterated cellulose (DC) films produced using deuterated bacterial cellulose. Deuterated bacterial cellulose was obtained by growing the Gluconacetobacter xylinus strain ATCC 53524 in D2O media. Horse Radish Peroxidase (HRP), a robust and well knw enzyme, was selected as model functional biomacromolecule to adsorb at the cellulose interface. The film thickness and quantification of adsorbed HRP molecules were characterized by X-ray and neutron reflectivity (NR) measurements. Reflectivity data analysis reveals the cellulose films to be smooth (low roughness) and uniform. The HC and DC films are 206 A and 92 A thick, respectively, and both films swell in the aqueous buffer solution. In NR measurements, it is difficult to trace the adsorbed HRP layer on HC film due to the small scaterring length density (SLD) difference between HC and HRP providing no contrast. However, using deuterated cellulose (DC) film provides sufficient SLD difference (contrast) with respect to the SLD of HRP. The adsorbed HRP layer is 110A thick and occupies a volume fraction of 20%. Using deuterated cellulose films enabled the quantification of thin and partial layers of proteins at the liquid interface. Quantifying and controlling the morphology and functionality of biomolecules at the cellulose interface enables to efficiently develop and optimize low cost cellulose based diagnostics devices with superior functionalization.

  • 2017
    Oxford
    pp 909-927Novel CED-based Rheological Test to Evaluate Pulp ReactivityAbstractPDF

    Reactivity is an important quality parameter form some grades of pulp, but its estimation is usually complex and time demanding. The study verified the possibility to utilize an alternative rheological approach to investigate pulp reactivity rapidly and with limited effort. 12.5 mL of pulp at 1.5% consistency was dissolved in 12.5 mL bis(ethylenediamine) pulp copper (II) hydroxide solution (CED) 1M The reaction was monitored by a torque rheogram and its two indices: the optimal dissolution time (ODT) and the initial dissolution rate (IDR). The method used to assess pulps either hornified to different extents or subjected to enzymatic hydrolysis. ODT was shown to decrease with increasing pulp swelling, while IDR reported the opposite trend. Both the indices were shown to be sensitive to pulp treatments. However, the ODT showed larger differences between hardwood and softwood pulps. The method had reasonable statistical reliability and required only 1.5 h per measurement.

  • 2017
    Oxford
    pp 931-935Supramolecular Engineering Strategies of BiomaterialsAbstractPDF

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  • 1965
    Cambridge
    pp 1-10The Humanity of PaperAbstractPDF

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  • 1965
    Cambridge
    pp 13-22The Meaning of ConsolidationAbstractPDF

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  • 1965
    Cambridge
    pp 23-37Structure and Properties of Heterophase SolidsAbstractPDF

    A classification of the wide variety of solids according to physical, structural and chemical characteristics shows the category of heterophase solids to be the largest group of natural and artificial substances. Most manufacturing and manufactured materials fall in this category.

    The structure of such materials can be described by the shape, size, concentration, orientation and topology of the constituent phases. Statistical methods in particular, geometric probability sometimes supplemented by certain physical ortopological restrictions, provide adequate tools for the description of naturally grown or artificially mixed systems.

    Mechanical modulus properties as well as thermal and electrical properties can be treated in a fairly general manner,based either on field distortion of one phase by the presence of another phase or by means of phase models that give upper and lower limits for the property.

    The problem of strength properties can be seen in terms of fracture mechanisms or as a statistical one, in which case extreme value theory provides a suitable method.

  • 1965
    Cambridge
    pp 38-56Web Formation with Synthetic Organic PolymersAbstractPDF

    Organic polymers have always been useful for the making of fibres, films, rubbers, plastics, adhesives and coatings and cover a wide range of properties. For several years, systematic efforts have been made to explore the use of fully synthetic fibres on a paper machine either alone or together with cellulose materials. Several sheet properties can be substantially improved by this approach. More recently, the preparation of polymer systems has been studied that occupy an intermediate position between a uniform fibre and a homogeneous film. They have been called `fibrids’ and permit one to prepare two-dimensional sheet-like entities without the use of a liquid vehicle (such as water on a papermachine) that range in their properties from paper to textiles.

  • 1965
    Cambridge
    pp 59-84The Cellulose/water RelationshipAbstractPDF

    This paper is a condensed survey of the interactions between water and cellulose materials. It is introduced by some general remarks about the interaction of water with solids, with special attention to hydrogen bonds, solutions and gels, then about the chemical and physical properties of cellulose fibres. The main discussion is devoted to the interaction of water below and above the saturation point, dimensional changes and restricted swelling, drying and hysteresis and, finally, the quantitative measurements of the absorbed water.

  • 1965
    Cambridge
    pp 90-115Measurement and Significance of the Water Retention Properties of Papermaking FibresAbstractPDF

    Equilibrium water retention isotherms have been determined for papermaking fibres through the range of moisture contents that is of greatest papermaking interest.

    The experimental method provides not only a measure of the swelling of the fibres and the equilibrium water retention at any force of water removal, but, conversely, the method measures the force with which the water is held and the force with which fibres or fibre elements are held together by residual water.

    Factors affecting the mechanism of water retention and the amount of water held by fibres are demonstrated by the isotherms of model and papermaking fibres of widely varying properties and treatments. The part played by the mechanism and extent of water retention on the development of wet web properties during drying is discussed and the need is demonstrated for more information than is provided by the isotherm.

    Finally, consideration is given to the role of swelling and water retention in pulp evaluation and some general principles are suggested by which the running and papermaking properties of a pulp might be predicted.

     

  • 1965
    Cambridge
    pp 119-144Transverse Shrinkage of Individual Fibres by Micro-radiographyAbstractPDF

    Micro-radiography has been used to determine the width and moisture content of individual pulp fibres during their drying from the swollen state. The method, which employs soft X-rays to produce contact radiographs of the fibres, is described and results are given for a spruce sulphite wood pulp. An assessment of changes in the thickness of the fibres while drying is also given.

  • 1965
    Cambridge
    pp 145-166Influence of Drying on the Pore Structures of the Cell WallAbstractPDF

    Using a recently proposed concept of cell wall structure based on a multiplicity of lamellae coaxial with the cell, the effect of drying on the structure of the cell wall of bleached spruce sulphite tracheids has been examined by means of nitrogen adsorption. The data suggest that in the fully water-swollen wall there are up to several hundred lamellae, each of the order of 100 Å thick, with a median separation of about 35 Å. During drying, the lamellae draw together progressively into thicker and thicker aggregates, decreasing the total pore volume, but leaving approximately the same median separation in the spaces that remain. At dryness, the pore volume remaining in the wall is negligible.

    During drying,the first pores to close  do not reopen when the fibres are treated with water,whereas the pores that close during the later stages of drying do so. The lamellae separation after drying at 25°C and reswelling remains at a median value of about 35 Å, but it drops after drying at 105°C and reswelling to 25 Å, owing to a greater permanency of pore closure in the pores of larger size. It is tentatively suggested that a fibre dries radially inwards towards the lumen and that the pores that tend to remain closed after drying are located towards the outside of the fibre, whereas the pores that reopen easily upon rewetting after drying are located towards the lumen.

    Fibres of very different composition, chemical treatment and morphology, as well as native and regenerated cellulose fibres, possess a wide range of pore volumes, yet are shown to have very similar pore size distributions and it is suggested that this distribution is not therefore of biological origin, but is based on a property of the cellulose molecule. No obvious correlation was found between the fractional extent of irreversible pore closure upon drying and the composition of the fibres.

    In the swollen cell wall of spruce sulphite pulp fibres, which contain almost 1 cm3 water per gram of dry material, about 20 percent of the water is present in macro reticular pores (spaces between lamellae) and 80 per cent in micro reticular pores(spaces within lamellae). This ratio persists throughout the drying cycle.

    The relationship between cell wall porosity and the papermaking properties of fibres is discussed briefly in terms of the loss of porosity during drying and its regain during beating.

  • 1965
    Cambridge
    pp 177-186Formation and Properties of Fibre NetworksAbstractPDF

    When elongated particles such as fibres are dispersed in water, they form a continuous network, provided the fibre concentration is above a certain level. A measuring technique utilising a concentric cylinder elasto-viscometer has been developed and used for studies of the mechanical properties of such networks.

    Networks generally exhibited the same characteristic properties as solid visco-elastic bodies, hence should be characterised by methods used for such materials rather than by hydrodynamic methods. Nevertheless, a close connection was found between the mechanical properties of fibre networks as measured by quasi-static methods and the hydrodynamic behaviour of the same material, then considered as a fibre suspension.

    A mechanism for the formation of fibre networks is proposed, in which the network is considered to derive its strength from the energy stored in the fibres when, after being bent in a turbulent shear field-such as is produced during agitation of the fibre suspension-they are prevented from straightening out by their interaction.

    A mathematical model of random three-dimensional fibre networks of low concentration has been evolved. The fibre concentration, the length-to-radius ratio and the modulus of elasticity on bending are shown to be the most significant of the parameters determining the rigidity of the network. Experimental study of the shear modulus of model fibre networks substantiated the qualitative validity of the fibre network model.

  • 1965
    Cambridge
    pp 189-214Detection and Cause of the Layered Structure of PaperAbstractPDF

    A method is described of determining the distribution of fibres in the thickness of a sheet of paper. It relies on observing the disposition of a small proportion of dyed fibres in a transparentised sheet. All the samples examined show a highly layered structure.

    Density profiles of the boundary of a sheet being formed in a drainage apparatus have been measured and the movement of single fibres was observed in approaching the forming zone in a model of the wire part of a paper machine. Both experiments show that a diffuse zone exists at the boundary of the forming mat, in which thickening of the stock takes place. The length of this zone is only a few millimetres and most of the formation takes place by filtration rather than by thickening. This finding is confirmed by computations based on a theory of formation, which includes both thickening and filtration. An argument is advanced that the layered structure of paper is the inevitable result of this mechanism of formation, which takes place at the usual papermaking consistencies. Much higher consistencies would be needed in order to produce a more felted structure of the sheet.

     

  • 1965
    Cambridge
    pp 216-238High-speed Filtration of Compressible Fibrous MediaAbstractPDF

    Experiments with synthetic fibres and a special flow apparatus, yielding data for the water permeation of fibre mats in the viscous-turbulent flow regime are briefly described. It is found that, within the range of the variables concerned,the results conform well to a recently established empirical equation relating the flow resistance of a pad to the flow speed, pad porosity and fibre specific surface. This empirical expression is then used, along with an equation representing wet mat compression characteristics, to construct a theoretical model of high-speed filtration. The result is a system of non-linear partial differential equations for the suspension kinematics and the flow rate/density distributions within the forming mat. Examples of numerical solutions are presented and discussed.

    When a constant pressure is applied to a fibre slurry initially at rest, it under goes a continuously decreasing acceleration, reaching a maximum filtration speed, after which the speed decreases uniformly, corresponding to a constant pressure drop filtration process. The peak speed may be as much as eight times greater than the speed characterising the final constant pressure zone. Theoretical results for the density distribution in a forming mat illustrate the effect of relative compressibility, for which the more compressible material exhibits a rapidly changing density profile near the supporting septum. It is also found that the rat eat which the mat builds up after peak slurry speed decreases with increasing time to an extent depending on the mat compressibility. Filtration experiments with a bleached sulphite pulp yield results that agree satisfactorily with the calculations, confirming predicted formation times to within less than 10 percent. The experiments thus further corroborate the predicted inverse relationship of formation time with applied pressure, as well as an approximate proportionality of formation time and sheet substance.

  • 1965
    Cambridge
    pp 242-267Visco-elasticity and Consolidation of the Fibre Network during Free Water DrainageAbstractPDF

    Measurements of specific permeability and compressibility of pads of synthetic and wood pulp fibres have been made in an apparatus in which rate of flow of water through the pad, pressure drop across the pad, external applied load and pad thickness can be controlled and measured. For non-swelling fibres at high porosity, with no applied load, the Emersleben-lberall drag treatment was found to provide estimates of permeability in reasonable agreement with observed values. The relationships between solids concentration C and applied stress P and between C and pressure drop p have been studied separately in terms of the empirical compressibility equations C= MPN with p=0 and C=m(p)n with P=O. The connections between mean compacting pressure during flow and the total pressure drop, between the exponents N, n and between the coefficients M, m, are discussed. Factors influencing the compressibility and consolidation of the fibre network include the flexibility and lateral conformability of the wet fibres, which apparently affect not only their capacity to deform elastically, but also the extent to which irreversible relative movement can take place. These effects are illustrated by the properties of pads from two series of pulps, each covering a range of lignin content-Pinus radiata sulphate pulps at various stages of bleaching and Eucalyptus regnans NSSC pulps cooked to different degrees. When compacting pressures that are due to flow and external loads are applied simultaneously, the compressibility equations in Pand Ophold, within limits, for constant values of p and P,respectively.

  • 1965
    Cambridge
    pp 269-298Dependence of Sheet Properties on Formation and Forming VariablesAbstractPDF

    An investigation of the effect of forming variables on handsheet strength properties showed that stock dilution, shear gradients and controlled initial drainage are factors that have major effects on sheet structure and properties. A better understanding of the causes of changes in paper strength properties resulted from the introduction of a new concept of basic sheet properties.

    One basic property is the specific tensile strength. This represents the average tensile strength throughout a sheet,in contrast with the standard tensile strength, which is generally a measure of strength in the weakest part of the test samples. The well-known loss of tensile strength that occurs when handsheets are formed from stock at higher concentrations is shown to be caused mainly by small-scale substance variability, since the specific tensile strength is essentially constant over the same range of concentrations.

    The effect of substance variability on other strength properties was examined by means of uniform base layer sheets with superimposed substance spots. The spots were used to obtain a known and reproducible pattern of substance variability. Notwithstanding the increased substance of the spotted sheets, they were found to be physically weaker in all properties except tearing strength. Substance variability was found also to be responsible for the reciprocal dependence of tearing strength on bursting and tensile strengths.

    As  a test of the practical importance of dilution and shear gradients, Fourdrinier machine trials were run in which the water removal capacity was increased considerably by the application of fan-produced vacuum under the forming zone. Sheet properties were found to be still improving up to the maximum flow box dilution or speed set by other machine limits such as drying and stock pumping.

  • 1965
    Cambridge
    pp 299-304Dependence of Sheet Properties on Formation and Forming Variables – Prepared ContributionAbstractPDF

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  • 1965
    Cambridge
    pp 313-349Structure of Paper in Cross-sectionAbstractPDF

    Previous work on the sectioning of paper has not fully utilised the power of the light microscope. This paper describes techniques developed to enable the structure of paper to be seen in considerable detail in cross-section. The techniques are illustrated by sections of a wide range of types of paper. The consolidation of the structure of paper during manufacture is revealed by micrographs of the effects of beating, pressing, drying with and without restraint, super calendering and creping.

  • 1965
    Cambridge
    pp 353-367Structural Changes in Paper on Pressing and DryingAbstractPDF

    The scanning electron microscope has been used to observe changes in the structure of paper at different stages of pressing and drying.

    In the first experiments, beaten kraft pulp handsheets were subjected to various pressing and drying treatments. The structure at the solids content achieved was stabilised by freezing and drying by sublimation under vacuum. Photo micrographs show the collapse of the fibres and consolidation of the paper structure during processing.

    Samples of the web were obtained at positions from the wet end to the reel of operating kraft, bond and newsprint paper machines. As soon as the specimens were sampled, they were quickly frozen and later dried under vacuum in the laboratory.

    The influence of water removal on the web and fibre structure by pressing and drying is illustrated. The relative importance of fibre conformity and fine material differs for the three paper grades. Under pressure, fibres are deformed plastically, particularly at crossing points and asperities. Collapse of fibres on removal of water from the lumen and the fibre walls by drying can usually be distinguished from that produced by mechanical pressure.

     

  • 1965
    Cambridge
    pp 371-392Physical Processes Occurring during the Drying PhaseAbstractPDF

    A description is given of experiments designed to substantiate some facets of the authors’ comprehensive theory of paper shrinkage and structure that was presented at the Oxford symposium. In particular, considerable evidence is presented in support of the basic concept in the theory-that is, the hypothesis of `adhesion before shrinkage’ of the constituent fibres. Examples are shown of a phenomenon that is the direct result of the latter process, termed necking of the fibres. Other factors important in the drying and shrinkage process are discussed.

  • 1965
    Cambridge
    pp 397-409Fundamental Properties of High Stretch PaperAbstractPDF

    The studies upon which this contribution is based were made to investigate the structure and properties of high stretch papers. Webs produced by compaction and by creping, the two main commercial processes, were examined by light microscopy and physical testing.

    The micro photography shows a variety of web configurations found in crepe papers, including examples of wave formations, internal delamination and two sidedness. The characteristic fibre orientation and densification are illustrated by photo micrographs of webs taken before and after the compacting process.

    The mechanical behaviour of high stretch papers is illustrated by typical stress/strain curves and a discussion of their behaviour during the process of straining.

     

  • 1965
    Cambridge
    pp 410-434Extensible Paper by the Double-roll Compacting ProcessAbstractPDF

    After a review of the development of extensible papers, a description of the double-roll compacting process and its variables is given. Its principal feature is the venturi section formed in the nip between a rubber and a steel roll, between which the paper web passes in a semi-dry state. On running the rubber roll more slowly than the steel roll, the web will shrink in the machine-direction. Experiments on a pilot machine showed an increase in the compacting effect with increasing nip pressure and speed difference, though with certain limitations. When considering nip width and peripheral speed difference as primary variables, however, linear relationships with the paper properties were found. The nip width will vary with the nip pressure and rubber thickness and hardness.

    The mechanism of double-roll compacting is considered to involve tangential forces, which move the rubber towards the back side of the nip, where it contracts, thereby shrinking the web. The structure of the resulting extensible paper was examined by photo micrographs of surface and cross-sections, by measuring the thickness changes on stretching and by load elongation measurements. The fibres appear curved after the compacting operation. This will result in the breaking of bonds when stretching the paper and in an ultimate breaking load lower than for flat kraft. The total rupture energy, however, is considerably higher.

    An apparent increase in the rubber roll diameter on increasing nip pressure was observed. This will cause a decrease in the mean speed difference at the nip. At a limited set speed difference, the rubber roll was found to change from being driven to be driving on increasing the nip pressure. In an appendix, the nip width and the slip have been treated theoretically as well as experimentally.

  • 1965
    Cambridge
    pp 445-472Ultrasonic Impedometric Studies in the Cellulose Pulp/ water SystemAbstractPDF

    The high frequency shear mechanical behaviour of cellulose pulp/ watersystems during theprocess of drying from 3 percent solids to total dryness has been non-destructively and continuously monitored by the technique of ultrasonic impedometry. Unusual fibre/water interactions have been detected at both extremes of the concentration range studied. These interactions are given interpretation in molecular terms.

  • 1965
    Cambridge
    pp 473-474Ultrasonic Impedometric Studies in the Cellulose Pulp/water System – Prepared ContributionAbstractPDF

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  • 1965
    Cambridge
    pp 477-509Effects on Individual Fibres of Drying under TensionAbstractPDF

    Following a review of the effects of machine- and cross-direction forces in the web during drying on the stress/strain properties of the finished paper,the nature of the axial forces in the individual fibres during tension drying is discussed in the light of the theory of the structure. The effects on the mechanical properties and structure of individual holocellulose pulp fibres of tension drying have been carefully investigated. An unusual extensional behaviour was observed at the onset of drying. It was found that tension during drying promoted substantial increments in tensile strength, Young’s modulus and crystallite orientation; generally, the spring wood fibres under went larger changes than the summer wood ultimate elongation was reduced and crystallinity remained unchanged.

    Partial removal of the hemicelluloses resulted in large decrements in tensile strength and Young’s modulus, a phenomenon not attributable to degradation of the fibre or to such side effects as swelling; the levels of these mechanical properties were reduced to those of ordinary pulp and cotton fibres. The relative enhancement of tensile strength and Young’s modulus in the extracted fibres caused by tension drying was much greater than that observed in the holocellulose pulp fibres, the latter property rising almost to that of the holocellulose fibres dried under load as the drying load was increased. The crystallinity of the extracted fibres (as determined by the method of half-width of a diffraction peak) was higher than that of the original holocellulose pulp, suggesting enhanced cellulose/cellulose bonding within the fibre, which, in turn, seems to account for the tension drying behaviour.

    Theory and experimental data relating to the possible effects of tension drying on the zero-span tensile strength of machine-made paper are presented. It is indicated that more work needs to be done in this area and, more generally, on the effect of tension drying of individual fibres on all the mechanical properties of paper.

  • 1965
    Cambridge
    pp 507-528Load/elongation Properties of FibresAbstractPDF

    The goal of this study was to gain a greater understanding of the changes in the mechanical properties of spring wood fibres during the consolidation of the web. It was found that the elastic modulus of freely dried fibres E is essentially tripled by any axial load applied during drying. Their tensile strength t is increased and their stretch of is decreased in proportion to the loads applied during drying up a twofold change. The elastic modulus measured at 90°to the fibre axis E1, was found to be about 0.1E. These fibre properties are of paramount importance to the stress/strain properties of paper, which are described quantitatively in terms of the fibre properties and sheet structure in our second contribution to this symposium.

  • 1965
    Cambridge
    pp 529-537Effect of Drying on the Mechanical Properties of Pulp FibresAbstractPDF

    Changes in the axial dimension of chemical pulp fibres were studied, using various combinations of drying and wetting. It was found that the commonly experienced elongation, brought about as a result of wetting, was exchanged for a shrinkage when the applied load before wetting was considerably lower than that applied before the preceding drying. Dimensional stabilisation was found for certain combinations of drying and wetting loads.

    In addition, fibre stiffness was studied. During drying, the stiffness increased sharply within a dry solids content range of 15-35 percent. Upon further drying, the stiffness of the latewood fibres did not change, whereas that of the earlywood fibres was decreased. The former effect is most likely associated with an increased modulus of elasticity, whereas the latter is probably a result of changes in the fibre cross-section involving collapse.

     

  • 1965
    Cambridge
    pp 538-543Effect of Drying on the Flexural Rigidity of Single FibresAbstractPDF

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  • 1965
    Cambridge
    pp 544-549Effect of Drying on the Flexural Rigidity of Single Fibres – Prepared ContributionsAbstractPDF

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  • 1965
    Cambridge
    pp 555-568Thermal Softening, Adhesive Properties and Glass Transitions in Lignin, Hemicellulose and CelluloseAbstractPDF

    The thermal softening of isolated samples of lignin, hemicellulose and cellulose has been investigated by observation of the thermally induced collapse of a column of powder under constant gravitational load. Softening temperatures of lignins ranged from 127-193°C. Birch xylan and pine glucomannan softened at 167° and 181’C, respectively. Sorption of water by lignin and hemicellulose caused pronounced decrease of the softening temperature-in some cases,to as low as 54°C. Softening points of both dry and moist lignins or hemicelluloses have been shown to correlate with the temperature at which the sample develops adhesive properties. The softening and adhesive behaviour has been explained in terms of the concept of the glass transition for amorphous polymers. Sorbed water is considered to act as a low molecular weight diluent in plasticising the polymer chains and lowering the glass transition temperature.

    Celluloses were found to soften at temperatures greater than 230°C. In contrast to lignin and hemicellulose, sorption of water by the cellulose had negligible effect on the softening temperature. This difference was probably due to the crystalline nature of cellulose and indicated that water did not plasticise individual cellulose chains at the molecular level.

  • 1965
    Cambridge
    pp 569-572Thermal Softening, Adhesive Properties and Glass Transitions in Lignin, Hemicellulose and Cellulose – Prepared ContributionsAbstractPDF

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  • 1965
    Cambridge
    pp 576-626Dynamic Consolidation of Paper during Calendering: Dynamic Compressibility of PaperAbstractPDF

    Two dynamic compressibility testers are described and the accuracy of one of them is examined by means of an energy balance and found to be within 5 per cent. The relationship of dynamic and static compressibility is discussed, the effects of moisture and temperature briefly examined and the results of testing a number of papers on the dynamic tester are presented. The changes in compressibility through the calender stack of two papers are given as examples and the dwell time/pressure relationship for caliper reduction is given for three different papers. Printability is examined as a function of compressibility and the relative compression of press packing and various papers is examined. A graphical method for determining the specific pressure distribution in a calender nip is given and the implications of the results are discussed. Two extreme cases of materials in a calender nip, one completely elastic, the other completely plastic are briefly discussed and a rheological model for the calender action is presented in an appendix.

  • 1965
    Cambridge
    pp 627-630Dynamic Consolidation of Paper during Calendering: Dynamic Compressibility of Paper – Prepared ContributionsAbstractPDF

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  • 1965
    Cambridge
    pp 639-686Effects of Hydrophilic Colloids and Other Non-fibrous Materials on Fibre Flocculation and Network ConsolidationAbstractPDF

    Non-fibrous materials used in papermaking are considered under the headings of colloids (positive or negative, hydrophilic or hydrophobic), surfactants (cationic, anionic or non-ionic), electrolytes and other non-polymeric materials. Their effects on flocculation of fibres are discussed in terms of entanglement, bridging and electrokinetic theories of fibre interaction. Various methods of measuring the heterogeneity of pulp suspensions and papersheets are reviewed and it is concluded that the beta-ray scanning method offers several advantages for paper studies. In this work, the heterogeneity of handsheets made from short-fibred and long-fibred pulps has been determined with a beta-ray scanner (Pm14’7 source). The experimental results showed that flocculation was more pronounced in the long-fibred pulps, but that the reduction in paper strength with increasing heterogeneity is greater in the short-fibred paper. Scanning across the handsheet diameter revealed that in some sheets the observed heterogeneity is partly due to systematic variation. In the study of electrokinetic properties of fibre/water interfaces, two parameters-zeta-potential and ionic charge density in the double layer-have been evaluated for alum-treated and polyamide-treated pulps. The results indicate that the variation in zeta-potential with additive concentration is primarily related to charge on the fibre surface for the polyamide-treated pulps and to electrolyte concentration for the alum-treated pulps. The effect of nonfibrous materials on consolidation is considered in terms of surface tension, fibre collapse and hydrogen bonding. Preliminary work on the erect of cationic starch on fibre strength is reported and a method of obtaining lateral load/compression curves of single fibres is described. The curves can be compared to those observed with tubes of differing wall thickness and elastic modulus. Experiments on the effect of modified starches and starch fractions on inter fibre bonding are also described; these lead to the conclusion that the bonding efficiency depends on the state of dispersion of the colloid, the internal cohesion of the dried colloid film and the adhesion at the colloid/fibre interface.

  • 1965
    Cambridge
    pp 692-734Effects of Sizing, Adhesives and Fillers on the Formation and Consolidation of Paper WebsAbstractPDF

    The effect of additives of industrial importance in modern high speed papermaking is reviewed. These sizes, adhesives and fillers will be detrimental to formation, consolidation, physical and optical properties of paper or board, if they exist in a tightly aggregated condition and are poorly distributed in the fibrous product. Data are presented to illustrate suitable dimensions of the solid particles to supply the desired property with the least amount of addition. The retention of the additives without sacrifice of their efficiency is a difficult problem.

    Alumina precipitates can decrease paper strength to very low values, but can increase it again with additional aluminium sulphate. The absence of sulphate ions permits gain in strength above the starting value. Anionic polyacrylamide, carboxymethyl starch, urea-formaldehyde and cationic melamine-formaldehyde resins become considerably more effective with a closely controlled alumina compound. Recent studies on the degree of neutralisation of alum and the advantage of a moderate cationic charge in rosin sizing are reviewed, as well as an explanation for the improved efficiency of fortified rosin and synthetic sizes. Bridging by polyacrylamide molecules, the sorption and movement to fibre contact areas by molecules and macromolecular fragments from cationic starch are discussed.

    Evidence is presented for the agglomerated state of pigments in paper and electron micrographs establish that titanium dioxide can be retained at about 0.25 u. Modern criteria for pigment evaluation are given. Debonding reduces pick resistance in offset printings,but helps in the ability to run letterpress paper.

     

  • 1965
    Cambridge
    pp 735-736Effects of Sizing, Adhesives and Fillers on the Formation and Consolidation of Paper Webs – Prepared ContributionsAbstractPDF

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  • 1965
    Cambridge
    pp 741-772Effects of Soluble Non-fibrous Materials on Formation and Consolidation of Paper WebsAbstractPDF

    The effects of soluble electrolytes and certain polyelectrolytes on fibre swelling, beating rate, fibre flocculation, drainage and strength properties are discussed. Pulp fibres sorb electrolytes because of various acidic groups naturally present in hemicelluloses and lignin residues, also because of various oxidation reactions. Ion exchange reactions occur between the acidic groups and electrolytes on wood pulps, but purified pulps are much less acidic and sorb salts by means of surface reactions that are less well characterised. Dilute alkalis increase fibre swelling, beating rate and strength properties of pulps. The effects of cations vary with valency and concentration. Generally, rate of beating and strength properties are enhanced slightly by monovalent cations, retarded and reduced markedly by trivalent and quadrivalent cations, unaffected by divalent cations. Ion antagonism is observed. Electrolytes change the electrokinetic potential of fibres as expected and drainage rate of the pulp is a maximum at the isoelectric point, unless hydrous precipitates have formed by hydrolysis of the salt. The effects of surface-active agents are more complex.

    Low molecular weight electrolytes have discernible effects on fibre flocculation at low consistencies (0.01-0.05 per cent) and low rates of shear, but these effects are negligible at papermaking consistencies, unless hydrolysis of the salt occurs. The effects of polyelectrolytes are pronounced under papermaking conditions and may possibly be explained by La Mer’s theory of polymer flocculation.

    The factors that influence retention of wet strength polymers by fibres are discussed. The rate of retention appears to be governed by a diffusional transport process rather than a molecular segment adsorption step.

    The mechanism of development of wet strength apparently involves partial diffusion of the resin into the fibrous structure followed by curing of the resin, which subsequently restricts swelling of the bond region in water. Little evidence exists for the formation of chemical bonds between pulp fibres and the wet strength polymer. Very pure pulps may be an exception.

    Soluble gases become insoluble during papermaking and have profound effects upon stock preparation, drainage and formation on the machine and final sheet properties. Foam is discussed as a competition between two rate processes-the rate of introduction of gases and the rate of foam collapse. The rate of foam collapse may be increased substantially by anti-foams. A theory of anti-foam action is discussed.

  • 1965
    Cambridge
    pp 773Effects of Soluble Non-fibrous Materials on Formation and Consolidation of Paper Webs – Prepared ContributionAbstractPDF

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  • 1965
    Cambridge
    pp 779-800A New Theory for the Load/elongation Properties of PaperAbstractPDF

    The new theory of the load/elongation properties of paper, in essence, sums the loads developed in the fibres intersecting a line at right angles to the direction of straining. Fibre nonlinearity and the intermittent bond failure that occurs before the final gross rupture of the sheet are taken into account. The hypothesis is proposed that gross rupture is triggered by one of two mechanisms bond failure in sheets of strong fibres and fibre failure in sheets of weak fibres. The theory does not consider the phenomena occurring during gross rupture. Measured stress/strain curves of many handsheets of five pulps of springwood fibres were found to be in good agreement with calculated curves.

  • 1965
    Cambridge
    pp 801-803A New Theory for the Load/elongation Properties of Paper – Prepared ContributionsAbstractPDF

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  • 1965
    Cambridge
    pp 814-849Effect of Structure on the Mechanical Properties of PaperAbstractPDF

    Recent literature relating to the structure of paper and its load/ elongation behaviour is reviewed.

    Following an outline of the more important load/elongation features and the effects on them of testing conditions and sheet variables, paper structure is considered in terms of the fibre (its strength, conformability and response to drying tension), the interfibre bond (its structure, area, frequency, strength and energy) and the sheet geometry. The structural changes that occur in the sheet as a whole and in its various elements during elongation and rupture are also described.

    Theories relating mechanical properties to sheet structure are summarised in three categories-general theories, quantitative theories for the prediction of sheet elasticity and the application of the Griffith crack theory to paper. Some concluding remarks are offered about the significance, particularly in sheet elasticity theories, of the ability of a paper to distribute load evenly over all its structural elements.

     

  • 1965
    Cambridge
    pp 852-872Anisotropic Elasticity of Paper from Sonic Velocity MeasurementsAbstractPDF

    The theory of anisotropic elasticity for paper is reviewed. Sonic pulse velocity measurements were used to evaluate Young’s modulus, shear modulus and Poisson’s ratio of paper. The effects of fibre orientation and drying stresses on paper elasticity are readily measured by sonic velocity. A previously proposed relationship between the fourin-plane elastic constants is approximately true for well-bonded paper. Local modulus variations in a single specimen can be detected by sonic velocity measurements. Any dependence of sonic velocity on substance is due to real differences in mechanical properties of the sheet.

  • 1965
    Cambridge
    pp 875-895Effects of High Consistency Refining on the Properties of the Consolidated WebAbstractPDF

    The treatment of pulps at low consistencies in conventional beaters or refiners is partially a destructive process that severely damages and shortens the fibres. These disadvantages are over come in high consistency refining (HCR), a technique developed commercially in the USA, which treats a pulp at consistencies of 20-40 per cent by transmitting mechanical energy to a semi-solid fibre pad between the plates of a discrefiner. The refining action is brought about by strong inter fibres hearing actions, by rubbing and sliding of fibres on each other and by the intensive internal friction forces that result. This mechanism gives the fibres a unique appearance that makes it possible readily to detect the HCR pulp in a furnish.

    The preservation of the average fibre length while developing extensive fibrillation is the most important feature of the HCR process. The properties of webs formed from an HCR pulp and a jordan refined stock differ noticeably in every phase of sheet consolidation. HCR fibres lurries frequently have a poorer formation, but seem to drain better-at least, in the higher freeness range. The tensile strength of wet HCR fibre mats is not much different from that of conventionally refined pulps, but they have a higher stretch. This was confirmed incommercial newsprint runs. HCR results also in a lower water retention after wet pressing and a greater web shrinkage in the dryer section.

    Papers produced from HCR pulps show a very high tearing strength, a higher stretch and a somewhat lower tensile strength than papers obtained from a jordan refined stock. This is largely a result of the much lower fines content of HCR pulps.

     

     

  • 1965
    Cambridge
    pp 896Effects of High Consistency Refining on the Properties of the Consolidated Web – Prepared ContributionsAbstractPDF

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  • 1965
    Cambridge
    pp 909-927Relationships between Mechanical and Optical Properties of Paper Affected by Web ConsolidationAbstractPDF

    The relationships between bonding strength and web consolidation as affected by beating and wet pressing have been investigated. The effect of both the forces of external pressure and surface tension on the structure and on single fibre elements and crossings is discussed in the light of relevant publications. Wet pressing can exercise both direct effects and effects attributable to the induced liquid flow in the sheet. Various types of bonding strength measurement have been covered and attention has been devoted to the question whether data on the magnitude of the bonded area can be derived from scattering measurements. On introducing variations in the bonded area by means of variation in the wet pressure, investigations have shown that, for a given increase in bonded area, beating is more effective than wet pressing in increasing the rupture strength.

    In the present investigation, use has been made of a method developed previously to relate irreversibly expended work to bond breakage with a view to assessing the effect of beating and wet pressing on the bonding strength value. These tests indicate that, for some pulp qualities with a high hemicellulose content, the bonding strength rises with increases in the degree of beating and falls on the application of higher wet pressure. For bleached and unbleached pulp with a low hemicellulose content, the bonding strength is (as has been found earlier) virtually independent of both beating and wet pressing. The possible reasons for the beating and wet pressing effects are discussed and some alternatives proposed. Although no satisfactory explanation can be found, it is believed that beating increases the possibilities of intimate contact between the surfaces and gives rise to a structure that has larger radii of curvature between the building elements. On the other hand, wet pressing may permanently damage the cell walls and result in less entangled contact zones and a structure of more angular configuration.

  • 1965
    Cambridge
    pp 928-941Some Optical Consequences of the Consolidation of PaperAbstractPDF

    The Kubelka-Munk equations are presented and some consequences of these regarding the influence of brightness, formation and optical bleaching agents on opacity are discussed. The main part of the paper deals with the lights cattering properties of paper and the influence on the opacity of such different variables as fibre dimensions, pulping and bleaching processes, pulp drying,beating, fibre orientation, pressing and drying on the paper machine. It is shown how intimately opacity is affected by the degree of fibre-to-fibre bonding within the sheet.

  • 1965
    Cambridge
    pp 943-947Some Optical Consequences of the Consolidation of Paper – Prepared ContributionsAbstractPDF

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  • 1965
    Cambridge
    pp 959-976Effects of Dry Pressing on Printing Properties of Uncoated Paper WebsAbstractPDF

    Uncoated paper webs containing ground wood were pressed between surfaces of various hardnesses, using different combinations of pressure and moisture content. The physical and printing properties of the pressed webs were measured. For a given web, the properties that affect print-through such as opacity, scattering power and bulk were determined by the combination of moisture content and pressure that was used. In addition, roughness was influenced by the hardness of the surface used to press the paper. Some treatments gave results similar to super calendered paper in all respects but for gloss, even though no shear was used during pressing. It was concluded that an optimum degree of compacting existed for paper intended to be printed on both sides.

  • 1965
    Cambridge
    pp 981-1009Fluid Flow through Paper and Sheet StructureAbstractPDF

    A theory is presented to describe the relationship between the structure ofasheetofpaperandtheflowoffluidsthroughit.Basedonthemultiplanarconcept of paper, it defines a pore through the sheet in terms of structural and hydrodynamic variables. The effective pore size distribution thus depends on the type of flow,as well as on the structure of the sheet. It is in all cases approximately lognormal, with a standard deviation proportional to the mean.

    The theory is applied to the problem of the maximum pore size and to laminar flow, for which the connection with the Kozeny-Carman equation is established.

    The theory correctly predicts relationships between physical and structural variables and fibre/sheet properties. Numerical agreement with experiments is still limited by the lack of an appropriate definition of a layer in the multi-planar model.

  • 1965
    Cambridge
    pp 1013-1025Summary and AppraisalAbstractPDF

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  • 2013
    Cambridge
    pp 3-39Local Structural Orientation of Towel and Tissue Grades in Two and Three DimensionsAbstractPDF

    This paper describes the study of new methods for characterizing the orientation of fiber segments in low density paper towel from two- and three-dimensional X-radiographic data sets. The end use properties of the absorbent hygiene grades such as paper towels and tissues stem from an open porous structure where stochastically distributed fibers are contorted by post forming processes to increase bulk, stretch, flexibility and softness, while maintaining adequate strength. The orientation of free fiber segments that form the network are kinked and curved in three dimensions by processes including creping, through air drying and embossing. Providing a linkage between process conditions and the end use properties through the characterization of the network structure is the overarching goal of this investigation. A method is presented for mapping the 2D, in-plane orientation of fiber segments using soft (6kV) X-radiographs and an algorithm for calculating the image moments for circular sub-regions that surround each point. The eigenvectors form the major and minor axes of the inertial ellipse from which the principal orientation may be extracted. Colorized maps representing the local orientation are used to examine the effects of embossing and creping, as well as comparing different forming processes. A method for characterizing fiber segment orientation in three dimensions uses a similar approach applied to binarized X-ray micro-computed tomographic data sets. The inertial ellipsoid is determined by performing principal component analysis on the covariance matrix of the voxels contained within a spherical region surrounding each solid voxel within the structure. The eigenvectors are used to extract the shape and principal orientation of the ellipsoids which are plotted as colorized representations in 3D space. The 2D and 3D plots demonstrate the sensitivity of the method to orientation of fiber segment mass, while mean fiber orientation plots reveal differences between samples.

  • 2013
    Cambridge
    pp 41-53Revisiting the Random Disk Model: Decomposition of Paper Structure Using Randomly Deposited Disks with Arbitrary Size DistributionsAbstractPDF

    In this study, a numerical algorithm was developed to decompose the planar mass structure of paper into a random array of grey disks with a discrete size distribution. The optimum size and the frequency of these disks were determined such that the second order statistics of the corresponding random disk structure resembled that of the paper sample. Using this method, eighty two (82) commercial and laboratory-made samples were analyzed. It was found that; independent of the forming conditions, the average disk size was proportional to the standard deviation of the disk size distribution. The utility of this new tool in analyzing the effect of papermaking conditions on paper formation is illustrated.

  • 2013
    Cambridge
    pp 55-70Information Geometry and Dimensional Reduction for Statistical Structural Features of PaperAbstractPDF

    Information geometry provides a metric on spaces of probability density functions. Here we apply it to the space of trivariate Gaussian distributions of joint variation among the areal density variables for pixels and their first and second neighbours, from radiographs and simulations. At a pixel scale of one millimetre these distributions can pick up essential structural features including flocculation intensity and scale. We do this by applying the technique of dimensionality reduction to large mixed data sets of samples and the results show promise for classification, including extraction of groupings that represent different former types. This kind of analysis could be valuable in evaluating trials, comparing different installations of similar formers and for identifying anomalous behaviour.

  • 2013
    Cambridge
    pp 71-100The Effect of Moisture and Structure on Wet Web Strength and Its Variation – A Pilot Scale Approach Using Dry and Rewetted Mill Made PapersAbstractPDF

    In spite of extensive research on wet web strength properties and rheology, knowledge of the effect of web structure, e.g. formation and fibre orientation, on wet web strength properties has been limited. Therefore the topic was studied by running the re-wetted mill-made paper reels on a pilot runnability device at low dry solids contents of 56% and 68%. In addition, one trial was conducted by measuring the wet web strength properties in situ on the press section of a pilot Fourdrinier. The bene t of both these approaches is the ability to measure the strength properties in more realistic conditions compared with standard laboratory methods.

    In order to differentiate between the effect of formation and fibre orientation on strength properties, the variables should not be correlated. This requirement was met in the main mill trial by suitably selecting the headbox and wire section parameters. Formation was measured using a -radiographic method and local grammage variation was examined as standard deviation in different wavelength bands and size classes. In addition, formation was also measured with Ambertec formation tester. Fibre orientation was determined using layered fibre orientation measurements.

    It was shown that formation has an influence on the tensile strength variation and the effect depends on the scale of formation and dry solids content. In contrast to dry strength, the wet strength does not follow the Weibull distribution, but rather the Gaussian one. In addition, the distribution of wet strength is sensitive to centimetre-scale variability in paper structure instead of millimetre-scale in dry paper. When formation is good, as it typically is on modern paper machines, further improvement does not improve average wet strength properties. Only when large scale formation is poor, it has an influence on average wet web tensile strength and tensile stiffness. Presumably this would be the situation on a Fourdrinier type machine. Unlike formation, anisotropy does not affect the strength variation but it has an influence on the average tensile strength and tensile stiffness of wet and dry papers. The anisotropy profile in z-direction has no influence on the mentioned properties.

  • 2013
    Cambridge
    pp 227-259Achieving Maximum Filler Retention by Improving Chemical and Mechanical RetentionAbstractPDF

    Conditions for maximizing chemical and mechanical filler retention were studied through a combination of laboratory, semi-pilot and pilot scale experiments. In the first part of this work, we investigated the impact of particle size on the mechanical retention of particles in a fibre network using a modified laboratory hand sheet former. Quartz particles of well-de ned and narrow-size fractions were used to simulate pre-flocculated filler of different sizes. Five different size fractions were studied. The mechanical retention was found to increase linearly with both web fibre grammage for each quartz fraction and with particle size. These results were then validated through pilot- scale production trials where different filler floc sizes were created through pre-flocculation techniques. In the second part of this work, we studied the stability of pre-flocculated filler flocs through a set of semi-pilot scale ow loop trials. Trials were performed by pre-flocculating filler (PCC) with flocculating agents continuously and exposing the filler flocs to controlled levels of hydrodynamic shear created by ow through a partially closed gate valve. Changes in filler floc size were monitored continuously using Focused Beam Reflectance Measurements (FBRM). A clear reduction in the particle size was observed as the pressure drop increased. A major part of the floc degradation occurred at relatively low shear conditions while under the highest shear conditions, the pre-flocculated PCC floc size was reduced close to the unflocculated state. In the third part of this work, we investigated the effect of different forms of process related shear on retention polymer stability and its effect on chemical retention. A set of semi-pilot scale ow loop trials were performed to investigate the effect of elongational shear-strain and shear due to velocity differences created inside and outside a dosage nozzle respectively. We show that the effect of elongational strain created inside the dosage nozzle leads to significant retention polymer degradation while shear created outside the dosage nozzle due to velocity gradients has a smaller effect on polymer degradation. We investigate these results with a series of pilot scale production trials and show that high shear conditions created inside the dosage nozzle leads to significant reductions in chemical filler retention. However, pilot trials indicate that shear created outside the dosage nozzle can also have a significant effect on filler retention, although to a lesser extent.

  • 2013
    Cambridge
    pp 261-299Some Fundamental Aspects of Pulp Screen CapacityAbstractPDF

    Pulp screens remove contaminants from pulp suspensions and are critical to the production of high-quality paper products. Screen performance is determined by two internal components: a cylinder with apertures that pass acceptable fibres and block oversize contaminants, and a rotor that clears the apertures of any blockages. Capacity is an essential parameter of screen operation and a necessary consideration in evaluating changes made to enhance debris removal or reduce power consumption. The present study uses a pilot pulp screen to assess capacity limits, and a specialized laboratory screen with a-high-speed video camera to study what happens at a screen aperture.

    What results is an understanding of some mechanisms related to the deposition and removal of fibres at an aperture where there is a time- varying ow bifurcation, and which is proposed herein as the essence of screen capacity.

  • 2013
    Cambridge
    pp 301-319Stochastic analysis of the critical stable velocity of a moving paper web in the presence of a crackAbstractPDF

    In this study, we present a probabilistic approach for analysing runnability of a moving paper web with random defects. The paper web is modelled in the case of an open draw as an axially moving elastic plate that has an initial crack of random length. We derive a formula for the optimal velocity, at which the probability of fracture is limited. We study edge and central cracks perpendicular to machine direction (mode I cracks) and oblique central cracks (mixed mode). The crack length is modelled with the Weibull distribution. The effect of changing the value of distribution parameters and the probability of fracture on the optimal velocity is illustrated. It is found that the optimal velocity decreases when the expected value and variance of the crack length increase. The results also show the relation between the effect of edge and central cracks on the optimal velocity. The study is fundamental for rigorous analysis of the paper making process.

  • 2013
    Cambridge
    pp 321-357Review: Variability, Non-uniformity and Complexity: from Product to ProcessAbstractPDF

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  • 2013
    Cambridge
    pp 359-367Adaptation of Pressing Concepts to RefiningAbstractPDF

    The refining impulse is here de ned as the product of the normal loading force by the time. The refining impulse can be used in a beater for understanding and controlling the refining effects on fibres. The SR-degree and WRV evolutions depend only on the proposed refining impulse. For the shortening evolution of fibres, the normal loading force has to be introduced as a supplementary variable. By analogy with the variables controlling the pressing operation, namely press impulse and maximal applied pressure, the refining impulse alone, or complemented with the normal applied force have been experimentally shown to control the kinetics effects of refining on the fibrous suspension.

  • 2017
    Cambridge
    pp 369-401A Proposed Link between Machine Runnability and Stickies Distribution within the SheetAbstractPDF

    In recycled paper processes, stickies are at the origin of many production disturbances, such as machine breaks, defects in paper and converting problems. At the end of the recycling process, the most abundant and disturbing macro contaminants are fragments of pressure sensitive adhesives. These particles adhere to machines clothes, and clog the felts or even cause the break of the running paper web. The contamination is typically evaluated by measuring the total stickies concentration in the pulp after screening. However, industrial experience shows that it is difficult to correlate this stickies concentration with the occurrence of process disturbances. We suggest that only the amount of stickies that is effectively exposed at the surface of the sheet to the machine clothes is disturbing and is at the origin of runnability problems. In this work, we recall the definition of the stickies exposure, and use it to anticipate the effect of geometrical parameters on the fraction of stickies that are exposed at the surface of the sheet. Parameters such as stickies length and thickness, sheet thickness, or stickies orientation in the z-direction, are investigated. A new sensor is developed to characterise stickies in their 3 dimensions (without prior pressing), and discriminate them from other type of contaminants. Improvements compared to classical stickies measurements methods are discussed. The exposure of real stickies populations to machine clothes is measured in handsheets, and compared with results from the modelling.

  • 2013
    Cambridge
    pp 403-447Center Winding Versus Surface Winding: the Effect of Winder Type and Web Material Properties on Wound Roll StressesAbstractPDF

    The choice of winder type for various web materials has long been a qualitative discussion. Web materials are vast and hence the range of web material properties is also vast. Valid but conflicting opinions for an optimal winder type have been developed from experience bases that represent this vast range of web materials. The purpose of this publication is to quantify how the internal stresses in wound rolls are affected by winder type and web material properties.

  • 2013
    Cambridge
    pp 449-468On Travelling Web Stability Including Material Viscoelasticity and Surrounding AirAbstractPDF

    The aim of this research is to gain more understanding of the physics of the transportation of materials having viscoelastic characteristics, high transport speeds, a small thickness and a large surface area. This study introduces new models that take into account both material viscoelasticity and the fluid-structure interaction between the travel- ling material and the surrounding owing fluid. A web (continuum) travelling between two fixed supports is considered, modelling the web as a Kelvin–Voigt type viscoelastic panel and the air ow as a potential ow. Stability of the system is studied with the help of its eigenfrequencies (eigenvalues) for two different types of ow geometries. First, a ow inside an enclosure with a rectangular cross-direction, through which the panel is travelling, is added to the equations of out-of-plane motion of the panel with the help of added mass coefficients. Secondly, a free stream potential ow obstructed by the travelling panel is analyzed using the analytical solution for the aerodynamic reaction pressure. Some numerical examples are given for both models.

  • 2013
    Cambridge
    pp 469-492The Heat of Sorption in Paper Drying – An Investigation of Measurement Methods and Influence of Pulp ParametersAbstractPDF

    When the dry content during pulp drying reaches a level above 75% to 80% the free water, i.e. unbound water, has been evaporated. The remaining water is bonded to the surface due to physisorption, additional energy is necessary to overcome these bonding effects. This additional energy is called heat of sorption. At 80oC and a dry content of 95% for unbleached softwood kraft pulp the evaporation energy increases up to 2800 kJ per kg water compared to the latent heat of water of about 2300 kJ/kg at the same temperature.

    Different measuring methods to determine the heat of sorption HS are described in the literature, the reported values for HS of pulp show large differences. The first aim of this work is to compare the results of different measurement methods using the same sample pulp. We investigated calculation of HS from sorption isotherms collected with a conventional climate chamber (ESC) and differential vapor sorption (DVS) analysis. Furthermore we applied direct measurement of the heat flux generated by sorption using differential scanning calorimetry (DSC), differential scanning calorimetry combined with thermogravimetric analysis (DSC/TGA) and reaction calorimetry (RC). All but one measurement method delivered consistent results in the range of HS=40 kJ/kg to 70 kJ/kg additional energy due to surface sorption. The advantages and disadvantages of the different measurement techniques are discussed.

    In the second part of this work the impact of pulping and pulp treatment on the heat of sorption has been investigated. Refining and bleaching seems to have no impact. Pre drying reduces the overall heat of sorption by 15 %. The addition of inorganic fillers to paper reduces its heat of sorption due to a negligible HS of the filler.

    In conclusion the total additional energy caused by sorption effects is less than 2% of the overall energy necessary to fully dry the pulp. Additionally the heat of sorption is nearly unaffected by pulping, bleaching, refining or pre-drying of the pulp. Therefor the heat of sorption HS is playing only a minor role for industrial applications of paper drying.

  • 2013
    Cambridge
    pp 495-509Rheological Characterization of Micro-Fibrillated Cellulose Fibre Suspensions Using Multi scale Velocity Profile MeasurementsAbstractPDF

    A rheometric method based on velocity pro ling simultaneously by optical coherence tomography and the ultrasound velocity profilometry was introduced and used in a preliminary study of the rheological and boundary layer ow properties of micro fibrillated cellulose. The two velocity pro ling methods appear adequate and complementary for rheological characterization of opaque complex fluids. The ultrasound method is useful in measuring the velocity profile in the interior parts of the tube, while the optical technique is capable of high-resolution measurement of the boundary layer ow close to the tube wall.

    The preliminary results obtained for a 0.4% micro-fibrillated cellulose suspension show typical shear thinning behaviour in the interior part of the tube while the near wall behaviour shows existence of a slip layer of thickness ~200 m. Both the velocity profile measurement and the imaging mode data obtained by the optical coherence tomographic method indicate that the slip layer is related to a concentration gradient appearing near the tube wall. In a sublayer of thick- ness ~100 m, the fluid appears nearly Newtonian, and the viscosity value approaches that of pure water with decreasing distance from the wall.

  • 2013
    Cambridge
    pp 511-520Experimental Investigation of the Influence of Fibre Morphology on the Interrelation of Flocculation and Network StrengthAbstractPDF

    It is widely accepted and discussed in the literature that fiber morphological parameters like fiber length, fiber curl and fiber flexibility affect flocculation phenomena and the properties of the fiber network in suspension via mechanical entanglement and forces arising at fiber to fiber contact points. The focus of this work is the interrelation of the parameters floc size distribution, network strength and fiber morphology. Pulp samples of different length distribution, fiber curl, kink index and fiber flexibility are evaluated concerning their flocculation tendency and the strength of the fiber network using methods already established in the literature. The simultaneous measurement allows the investigation of the interrelation of these parameters. It is shown that floc size distribution and network strength are highly correlated. Still, depending on the morphological properties fiber curl, kink index and fiber flexibility, samples of comparable floc size distribution and different network strength (and vice versa) are evident.

  • 2013
    Cambridge
    pp 521-538Observation of the Turbulent Transition of A Fibre Suspension in Hagen-Poiseuille FlowAbstractPDF

    The focus of the present work is an experimental study of the transition to turbulent ow of papermaking fibre suspensions in a cylindrical pipe. The suspensions used in this study possess yield stress. With this class of fluid the axial profile in fully developed slow ow is characterized by an unyielded or plug zone. With increasing ow rates the size of the plug diminishes. One of the remaining open questions with these suspensions is the role of the plug during transition.

    In this work we characterize the size of the plug using ultrasound Doppler velocimetry (UDV) as a function of ow rate for dilute, i.e. less than 2% consistency, papermaking suspensions in a 50 mm diameter, 10 m long cylindrical pipe. The plug size was determined through analysis of local spatial and temporal variations of the velocity, strain- rate and the fluctuating component of velocity. With this, we were able to estimate the yield stress of the suspension through knowledge of the applied pressure gradient and find the yield stress to be in the range of 2–10 Pa, depending upon the consistency and Reynolds number Re. We observe complex behavior with the plug in which we see initially that with increasing velocity, the plug diminishes through a densification-type mechanism in a response to an increase frictional pressure drop. At higher Re, it diminishes through an erosion-type behavior. We estimate the critical Reynolds number Rec for the disappearance of the plug to be Rec ~ 105.

  • 2013
    Cambridge
    pp 541-583Review: Paper Microfluidics: Applications and PerspectivesAbstractPDF

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  • 2013
    Cambridge
    pp 585-598Detailed Insights to Liquid Absorption and Liquid-Paper InteractionAbstractPDF

    We present a method which provides detailed insights to the dynamics of the water absorption process and water- paper interaction, based on transmittance measurements of ultrasonic beams. We found that the water absorption process of an uncoated paper- sheet comprises two consecutive time regimes. The underlying mechanism that governs the regimes’ shift is the combination of fibre surface modification by water and the recreation of the fibres lumen after wetting. In the first regime, water advances along the dry pore surface, which is hydro neutral, and the water forms a solid column inside the capillary (pore), while in the second regime, moving along the primed (wetted) surface of the capillary is a more favourable path as the surface becomes hydrophilic when wetted. Consequently, the water may not necessarily ¿ ll the entire capillary when the capillary expands in volume due to hydro- expansion and hence forms a hollow water column. We propose a model that enables us to determine/predict the depth of water absorption by the dry pore structure of the paper which is often the case for ink- paper interaction during printing. The results of our studies suggest that the depth of water penetration along the dry pore surfaces can very well be described by the Bosanquet model.

  • 2013
    Cambridge
    pp 599-618Designing Microfabricated Paper Devices through Tailored Polymer AttachmentAbstractPDF

    In the present paper we show that polystyrene–based copolymers, which carry a defined amount of photo–reactive benzophenone moities can be transferred and immobilized to paper substrates via a simple dip coating approach and subsequent illumination of the paper substrates with UV-light. Non-bound macromolecules can be removed from the cellulose fibers by solvent extraction. Thereby, the amount of immobilized polymer can be adjusted over a wide range by changing the polymer concentration in the dip coating solution. The resulting polymer-modified paper substrates were characterized using IR spectroscopy, scanning electron microscopy (SEM), fluorescence microscopy and static contact angle measurements. The polymers are attached to cellulose fibers using a photo–chemical approach and stable chemical micro patterns, including paper-defined microchannels, can be designed inside model paper substrates by using conventional UV-lithography. These channels are capable to control the fluid penetration by capillary actions. An engineering of the paper substrate itself allows to modulate the speed of the fluid transport of an aqueous solution inside paper-defined microchannels. The latter will become important for a number of applications.

  • 2013
    Cambridge
    pp 619-634Infiltration and Dimensional Scaling of Inkjet Droplets on Thick Isotropic Porous MaterialsAbstractPDF

    We study the imbibition of picoliters (pL) sized inkjet droplets on controlled pore glass membranes (CPG). We do so using a variety of liquids, i.e., water, formamide and diiodomethane, as well as the CPG substrates, and measure the evolution of the imbibition process using high speed digital imaging. Here, experiments were conducted with a wide range of initial drop volume (100–600 pL) on 2–280 nm CPG membranes. We derive scaling laws through dimensional analysis of the equations of motion, and consider experimental parameters and liquid properties.

  • 2013
    Cambridge
    pp 637-672Utilization of Modified Linear Elastic Fracture Mechanics to Characterize the Fracture Resistance of PaperAbstractPDF

    Linear elastic fracture mechanics modified to account for an effective fracture process zone is sufficient to characterize and predict fracture resistance for a wide range of papers. The simplicity of the method, which only requires the tensile strength and a measure of the effective fracture process zone length, gives it great advantage over other existing approaches. The results presented here show that for a wide range of commercial papers, samples widths as narrow as 50 mm are sufficient to determine the effective process zone length, and that scaling holds well enough to allow prediction for fracture of wide webs. The results indicate that the tensile strength of paper is a result of a fracture process where the defect is most typically induced from cutting the network structure along the edges. As a consequence, the inherent tensile strength of the network can be significantly larger than the measured tensile strength. The effective fracture process zone length parameter is taken as a measure of the inability for the paper to concentrate load near the crack tip. This ability for network structures to concentrate load has significant impact on the fracture resistance of the sheet relative to its tensile strength.

  • 2013
    Cambridge
    pp 673-710Predictions of MD and CD Tensile Property ProfilesAbstractPDF

    To design new and optimize existing paper and board material an understanding of how the paper making process affects the final paper properties and how we can control them is neccesary. There is a link missing between pulp properties and machine made paper properties. The aim with this work is to close this gap by proposing an engineering model which, based on furnish properties, makes it possible to predict tensile property profiles in MD and CD.

    Two series of hand sheet trials were made to validate and formulate the model. The purpose with the first trial was to validate that the geometric mean of the studied properties in MD and CD is constant and equal to the isotropic value. The second trial was made to find relations between anisotropies of the studied properties and the fibre anisotropy.

    The model was applied on a press draw trial made on a production machine. The strain and tensile property profiles were measured and predicted based on laboratory measurements on the furnish. The predictive capability of the model was regarded as fairly good, especially since the general behaviour of the paper properties was correctly captured. The deviation of predictions compared to measurements were around 10% or less for most of the evaluated positions and properties, except for MD tensile energy absorption index that was poorly predicted.

  • 2013
    Cambridge
    pp 711-734Time-dependent, Statistical Failure of Paperboard in CompressionAbstractPDF

    This paper concerns the question of how to predict mechanical performance of box and paperboard subjected to fluctuating load/ environmental conditions encountered in end-use. Particularly such performance is notoriously variable (stochastic), and is known to be very difficult to predict.

    We have developed a theoretical framework for treating time- dependent, statistical failure based on the recent progresses in statistical physics of disordered materials. The main objective of this study is to experimentally determine the three key parameters that fully characterise the failure of component board subjected to general loading histories, namely the parameter c related to static strength and its uniformity, the load sensitivity/durability parameter W, and the uniformity parameter G of creep lifetime. Results showed that creep lifetime distribution is highly skewed with extreme scatters, but the distribution is still a class of Weibull distribution and can be handled without any problem. The durability parameter W also showed high values comparable with those for fibre-composites. These two results explained very well the variability and load sensitivity of box creep performance observed in the literature.

    This proposed approach offers a new set of material property parameters, other than traditional strength, that can be fully exploited in both materials and structural design to enhance end- use performance in the most resource- efficient manner.

  • 2013
    Cambridge
    pp 737-763Review: Preparation and Applications of Nanofibrillar CellulosesAbstractPDF

    Nanofibrillar celluloses are promising new bio-based nanomaterials that can be prepared from paper- grade chemical pulps and other plant celluloses by mechanical shearing in water, usually after pretreatments. For example, enzymatic hydrolysis, carboxymethylation, addition of cationic polymers, TEMPO-mediated oxidation and others have been applied as wood cellulose pretreatments to reduce the energy consumption of the mechanical shearing process and to improve nanofibrillation level. Nanofibrillated celluloses (NFCs) prepared from wood cellulose by either enzymatic hydrolysis or partial carboxymethylation and subsequent mechanical shearing in water are convertible to nanopaper films and aerogels using a filtration process like that used in papermaking, which is advantageous for efficient removal of water from the strongly swollen NFC/water dispersions. NFCs have high molecular weights and long fibrils and form fibril network structures both in aqueous dispersions and dried nanopaper films/aerogels. This makes them preferable for use as base materials for nanocomposites. Thus, various nanopaper/matrix composites have been prepared, some of which show remarkably high mechanical strength including high ductility. When TEMPO- mediated oxidation is used as the pretreatment, almost completely individualized TEMPO-o xidized cellulose nanofibrils (TOCNs) with homogeneous widths of ~3 nm dispersed in water can be prepared from oxidized wood celluloses with carboxylate contents >1.2 mmol/g by gentle mechanical disintegration treatment. Because TOCN elements form nematic-ordered structures due to their self- assembling behavior in water, TOCNs are able to be converted to dense films with plywood- like layered structures, stiff hydrogels by acid treatment, aerogels with extremely high specific surface areas, and other unique bulk materials. When TOCNs are used to make nanocomposite materials, high mechanical strengths and gas- barrier properties can be achieved even with low TOCN-loading ratios.

  • 2013
    Cambridge
    pp 765-784The Swelling and Pore Structure of Microfibrillated CelluloseAbstractPDF

    The aim of this work was to understand the particle level swelling and pore structure of microfibrillated cellulose (MFC). For this purpose, a new variant of the solute exclusion test was constructed which takes into account the adsorption of dextran onto the cellulosic material and the elastic response of the fiber material to external osmotic pressure. With the new method, two important properties, fiber saturation point at zero external osmotic pressure (FSP0) and isotropic elastic modulus could be obtained. The particle level swelling for MFC was found to be 1.6 ml/g which is about the same as the swelling of the parent pulp fibers. The MFC swelling was con¿ rmed with thermoporosimetry which yielded further insights into the development of pore structure and surface area when the fiber cell wall is defibrillated.

  • 2013
    Cambridge
    pp 785-801Investigation into the Structural and Thermal Behavior of Bacterial Cellulose Fbers after Biologically Relevant PurificationAbstractPDF

    Cellulose is the most abundant biopolymer on the planet. Historically rooted in the paper industry, advancements in colloidal chemistry, polymer chemistry, and the development of novel saccharification techniques have expanded the commercial applications of cellulose to include the production of liquid crystal displays, use in high strength composites, and biofuels. Despite this renewed interest in cellulosic products, the establishment of cellulose as a global commodity is significantly hindered by the inefficiencies in cellulose liberation and processing. The current model associated with cellulose liberation from lignin and hemicellulose relies on the use of highly basic reagents resulting in significant alterations to cellulose native structure. Laboratory techniques have been developed to attempt to isolate cellulose, while leaving it in its native structure. In this work, we demonstrate how even mild laboratory isolation techniques significantly influence cellulose structure in bacterial cellulose. Furthermore, we propose that bacteria cellulose serves as a model for cellulose as found in plants and animals.

  • 2013
    Cambridge
    pp 803-820Assessing Transverse Fibre properties: Fibre Compression and Artificial Hornification by Periodic CompressionAbstractPDF

    Assessing material properties on the micro and nanoscale requires appropriate tools and measurement systems. Micro- and nanorobotic systems allow for such investigations. This contribution represents a case study in this respect. A setup capable of compressing single fibres with force feedback under scanning electron microscopy observation is presented and used. Variations in the compression force during testing of single fibres are reported and analysed. It is shown that significant alterations of the fibre wall mechanical properties are already introduced during testing one fibre several times in the transverse direction. Although hornification is generally used in the context of changes on the fibre wall due to drying and rewetting cycles of fibres or paper, it is suggested to expand the term also on changes induced by mechanical treatment, as the outcome could be similar. Furthermore, a microrobotic method and a system are proposed which will allow simulating the mechanical hornification and which are based on a force generating apparatus.

  • 2013
    Cambridge
    pp 821-836Porous Structure of Never-Dried Pulp Fibers Analyzed by Nitrogen Adsorption MethodAbstractPDF

    Porous structures of never- dried pulp fibers were investigated by nitrogen adsorption method. A distinct bent-point in the desorption isotherms was observed at a 45% relative vapor pressure for both softwood and hardwood never- dried pulp fibers. The singular reduction in nitrogen adsorption volume was likely attributed to the presence of meso-pores formed via lignin removal in wood cell walls during pulping, which was indicated from the results of nitrogen adsorption/desorption isotherms for partially delignified wood powders. The specific surface area of a sufficiently delignified softwood powder was ~150 m2/g, slightly lower than 200 m2/g. These results indicate that 4×4=16 individual cellulose microfibrils (each of which is 3–4 nm wide) form one unit of cellulose fibril bundle and each bundle is surrounded by lignin thin layer in softwood cell walls. On the contrary, partially delignified hardwood powders had extremely small specific surface areas, whereas a hardwood pulp fiber showed a fairly large surface area. The specific surface areas of delignified hardwood powders were drastically increased after extraction with alkali to remove xylan. Thus, not only lignin but also xylan plays a significant role in the formation of pore structures for the hardwood pulp fiber and the delignified wood powders. The results of pore size distribution analysis using BJH and NLDFT techniques showed that the sizes of principal meso-pores present in unbeaten pulp fibers were 3.2–3.7 nm and were increased by disintegration process.

  • 2013
    Cambridge
    pp 837-850Benefits of Foam Forming Technology and Its Application in High MFC Addition StructuresAbstractPDF

    In paper and board industry there is a strong need for a radically more resource-efficient production technologies, which would also enable the manufacture of sustainable and value- added fibre products. In this paper we introduce foam forming method, where foam is used as a transporting media of furnishes. The technology enables to make structures with excellent formation in higher headbox consistencies and a very high bulk. When this is combined with the good water drainage properties, which allows the high addition levels of strengthening agents as micro fibrillar cellulose (MFC), one can make products with very big bulk and still have an adequate strength value. We studied different MFC grades and they seem to behave rather similarly in bulk vs. strength comparisons. However, some difference is obtained leaving room for optimization of best MFC grade on certain product.

  • 2013
    Cambridge
    pp 851-866Oxygen and Water Vapour Barrier Films with Low Moisture Sensitivity Fabricated from Self-Crosslinking Fibrillated CelluloseAbstractPDF

    To replace petroleum-based barriers used in, for example, packaging applications with a bio-based alternative, the sensitivity to moisture must be lowered. The present work describes the fabrication and characterisation of cellulose-based films with remarkably improved oxygen and water-vapour-barrier properties at 80% relative humidity. This was achieved by fabricating films of self- cross- linking fibrillated cellulose after partial periodate oxidation to dialdehyde cellulose. At a relative humidity of 80%, films made of 27% and 44% oxidised cellulose, respectively, showed less than half the water vapour permeability of the untreated reference; 3.8 g·mm/(m2·24 h·kPa) and 3.7 g·mm/(m2·24 h·kPa) compared to 8.0 g·mm/(m2·24 h·kPa). This was presumably due to a lower moisture uptake in the films, and consequently less swelling. In the absence of moisture, films from both unmodified and modified fibrillated cellulose were ideal oxygen barriers, but at a relative humidity of 80%, films based on 27% and 44% converted cellulose had an oxygen permeability of 2.2 ml·Rm/ (m2·24 h·kPa) and 1.8 ml·Rm/(m2·24 h·kPa), respectively, compared to 9.2 ml·Rm/(m2·24 h·kPa) for the non- oxidised material. The cross-linking resulted in an embrittlement of the films, but the 27% oxidised material still had a tensile strength of 148 MPa and a tensile strain at break of 2.0%, which is sufficient in, for example, many packaging applications.

  • 2013
    Cambridge
    pp 869-885Reactive Polyvinylamine-Graft-TEMPO/Laccase Complex Giving Wet Cellulose AdhesionAbstractPDF

    Cellulose surfaces are activated for wet adhesion, bioconjugation and other applications by the introduction of a “primer” layer consisting of a covalently bonded polyelectrolyte complex based on laccase and polyvinylamine with pendent TEMPO groups, PVAm-T . The laccase, in conjunction with dissolved oxygen, activates the TEMPO moieties on PVAm-T, facilitating the oxidation of primary hydroxyl groups on the cellulose surface. The resulting cellulosic aldehydes are free to couple covalently with amine groups on the PVAm-T . The generally accepted mechanism of TEMPO oxidation is that the primary oxidant converts TEMPO into a reactive oxoammonium ion that shuttles an electron to primary alcohols. Since the translational mobility of TEMPO will be limited when grafted to a polymer and present as a polyelectrolyte complex with laccase, it is proposed that the activated oxoammonium ions jump along the PVAm-T chain, from TEMPO to neighbouring TEMPO. Wet adhesion of laminated regenerated cellulose sheets was used as the primary assay indicating the presence of covalent bonding.

  • 2013
    Cambridge
    pp 887-906Improvement of Paper Strength by Increasing the Xylan ContentAbstractPDF

    Extracted xylan from beech dissolving pulp and eucalyptus kraft pulp was precipitated on unrefined, bleached, once-dried softwood kraft pulp and sulfite pulp. The temperature, pH, ionic strength, xylan concentration, pulp consistency, and the dwell time were analyzed regarding their influence on the adsorption of xylan. Furthermore, handsheets were made to investigate the impact of xylan on the tensile strength and the tearing resistance of the paper. The swelling behavior of the fibers was of interest as well as the determination of the total and surface charge of the pulp.

    The xylan content of the fibers could be significantly increased. The temperature, xylan concentration and ionic strength showed a large influence on precipitation. No significant change in the attachment of xylan between neutral and low alkaline level of the pH could be noticed. A higher pulp consistency, including a sufficient mixing during adsorption, is favorable. Xylan shows a large impact on the tensile strength of the softwood handsheets. The tensile index of the handsheets made of the softwood pulp and refined at 3.000 revolutions with a PFI mill could be increased by up to 38% compared to the reference sample. The xylan-modified samples also showed by trend a higher stretch at break of about 0.5%. This could be the result of a higher surface area and total charge of the fibers. Thus, enhanced swelling is caused leading to softer and more flexible fibers. These effects provoke a larger area in molecular contact. On the other hand, the trend of the tear index of the softwood handsheets with an increased amount of xylan declines stronger after a longer time period of refining.

     

  • 2013
    Cambridge
    pp 907-925Overview of Practical and Theoretical Aspects of Mineral Oil Contaminants in Mill Process and PaperboardsAbstractPDF

    The paperboard industry is committed to consumer protection in food packaging and has assigned top priority to the issue of mineral oil hydrocarbons (MOH) as early as spring 2010. In this work, we have developed practical methods, using gas chromatography with a flame ionization detector (GC-FID) laboratory equipment, to characterize the level of mineral oil saturated hydrocarbons (MOSH) and aromatic hydrocarbons (MOAH) in European and North American newspapers, paperboards, and inks. As part of our validation protocol, several samples were analyzed by an external laboratory (ISEGA, Aschaffenburg, Germany) using Biedermann’s protocol and an average deviation of 6% for the MOSH and 17% for the MOAH was observed between ISEGA and our method. Using the reference Tenax migration method (EN, 14338), the hexane or heptane vapour transmission rate (HVTR) method was developed to measure the barrier efficiency within one day, showing a very good correlation; R2 = 0.80 to 0.92.

    Much higher MOH concentrations in newspaper printed areas infer that printing inks constituents are the most likely source of MOSH/MOAH in recycled paperboard. When processing such raw material, the drying section is one of the paperboard making process steps that significantly reduces the MOSH and MOAH level. Although low or free mineral oil printing inks would be preferable, the use of functional barriers can significantly reduce the migration of MOSH/MOAH, whenever necessary. Mineral oil migration barrier efficiencies of about 90% were obtained using polymeric functional barriers applied at the mill with conventional coating equipment. Finally, a first attempt was made to theoretically model the migration of mineral oil through model polyacrylate functional barriers. The correlation between calculated (theoretical) and experimental hexane permeabilities seems reasonable and a predictive discrimination between good and bad barrier polymers appears possible for acrylate copolymers (R2=0.72) within foreseeable limitations with respect to chemical composition.

     

  • 2013
    Cambridge
    pp 929-944Paper-Based Blood Grouping; Exploring the Mechanisms of Red Blood Cell Agglutination in Antibody-Treated Paper via Confocal MicroscopyAbstractPDF

    The recent invention of paper-based blood typing devices which are inexpensive, but also accurate and easily interpreted, has shown great promise for the future. Despite the efficacy of these devices, the underlying mechanisms responsible for how they function have remained largely unknown. This work illuminates these mechanisms by using the technique of confocal microscopy to delve into the behaviour of red blood cells at the micro-scale and view exactly what is happening as blood samples interact with antibody treated paper substrates. The underlying mechanisms responsible for the phase separation of red blood cells and plasma from whole blood on paper are elucidated for the first time, opening the door to future enhancements to such devices. It was revealed that the dominant mechanism responsible for the separation of whole blood into its respective phases was the physical entrapment of large red blood cell aggregates following their agglutination. Understanding these mechanisms and the effects of the paper structure makes optimization of paper-based blood diagnostics possible. Further investigations of optimal pore sizes, tortuosity or fibre size may lead to significant improvements in the sensitivity and accuracy of this important diagnostic platform.

  • 2013
    Cambridge
    pp 945-966Paper Substrate for Printed FunctionalityAbstractPDF

    Requirements for paper to be used as substrate for printed functionality were investigated. A recyclable, multilayer-coated paper substrate that combines adequate barrier and printability properties for printed electronics and sensor applications was developed. In this multilayer structure, a thin top-coating consisting of mineral pigments is coated on top of a dispersion-coated barrier layer. The top-coating provides well-controlled sorption properties through controlled thickness and porosity, thus enabling optimizing the printability of functional materials. The optimum barrier layer structure was investigated by studying the influence of latex type and amount in blends with different size and shape factor kaolin pigments. Highly aligned high shape factor kaolin improved barrier properties in general, but was found especially useful against organic solvents, which may degrade the latex. Dimensional stability and its influence on substrate surface properties as well as on functionality of conductive tracks were studied by exposure to high/low humidity cycles. The barrier layer of the multilayer coated paper reduced the dimensional changes and surface roughness increase caused by humidity and helped maintain the conductivity of printed tracks. As proof of concept functional devices, hygroscopic insulator field effect transistors were printed on the multi- layer curtain coated paper using a custom-built roll-to-roll hybrid printer.

  • 2013
    Cambridge
    pp 967-989Gold Nanoparticles Paper as Surface Enhanced Raman Scattering (SERS) Platform for Bio-diagnostic ApplicationsAbstractPDF

    We explored the sensitivity and selectivity of gold nanoparticles (AuNPs) treated paper as a generic SERS diagnostic platform to identify and quantify low concentrations of a specific (bio)analyte in aqueous solutions. The effects of gold nanoparticles (AuNPs) concentration on their adsorption and aggregation states on paper were explored. The surface coverage of AuNPs on paper scaled linearly with their concentration profile in solutions. The SERS performances of the AuNPs-treated papers were evaluated with a model Raman molecule, 4-aminothiophenol (4-ATP), and their SERS intensities increased linearly with the density of AuNPs on paper. To increase the SERS sensitivity, the retention and aggregation state of nanoparticles on paper was controlled by pre-treating paper with a series of cationic polyacrylamide (CPAM) solutions. The CPAM pre-treated paper produced a more uniform distribution of AuNPs compared to untreated paper. Higher surface coverage and aggregation of AuNPs on paper were favoured by CPAM solutions of higher concentration, charge density and molecular weight. The optimized AuNPs-CPAM paper showed a higher sensitivity and Raman enhancement factor (EF), which was almost an order of magnitude higher than the untreated AuNPs paper. After the SERS sensitivity towards the detection of model Raman molecule (4-ATP) was proven, the SERS selectivity of AuNPs paper was demonstrated by functionalizing the AuNPs with a model biomolecule platform consisting of biotin/streptavidin assemblies for the detection of antibody-antigen binding. The modification of antibody local structure due to the interaction with antigen was detected. Evidence of antigen binding was elucidated from the SERS spectra, confirming the presence of antigen. Reproducible spectra features were observed for the functionalized AuNP papers which were exposed to different concentration of antigen; the spectra intensity increased as a function of antigen concentration. The sensitivity and selectivity of AuNPs paper substrates as a low-cost and generic SERS platform for bio-diagnostic application was demonstrated.

  • 2013
    Cambridge
    pp 995-1006Elastic Properties of Sheet and Materials from Vibration TestingAbstractPDF

    Many materials, including paper products, come in sheet form and exhibit orthotropic symmetry. Information about the elastic stiffnesses of such materials can often be obtained quickly and accurately using a measurement method based on the vibration modes and natural frequencies of rectangular panels. The method is outlined and illustrated, and some case studies discussed in which the method is applied to fibre-reinforced composite materials and to the selection of wood for musical instruments.

  • 2009
    Oxford
    pp 19-49Novel Fractionation Methods: Separation in A Viscoplastic FluidAbstractPDF

    In this work we propose a novel separation technique based upon the control of the threshold for motion of different classes of particles in yield stress fluids. The principle is demonstrated by observing the motion of particles under the influence of a centrifugal force in a weak gel. Here we develop calibration curves of the force required to initiate motion in a gel under numerous configurations of the particles. Demonstration separations of bidisperse suspensions are reported. Here we achieve complete separation of dilute suspensions based upon length, diameter, or density.

  • 2009
    Oxford
    pp 51-83The Net Normal Force Per Crossing Point: A Unified Concept for the Low Consistency Refining of Pulp SuspensionsAbstractPDF

    The objectives of this article are:

    – First, to theoretically propose a unified concept: the net normal force per crossing point,

    – Second, to experimentally undertake refining trials on a pilot disc refiner in order to compare all concepts for the refining intensity and to validate the chosen one.

    We will begin by re-visiting the old concepts of the refining intensity, in the low consistency regime. After a theoretical proof based upon the physics of the phenomena, applied to beaters and industrial refiners, a unified concept of the refining intensity is proposed and strengthened: the net normal force per crossing point.

    Then, experimentations are undertaken on a pilot refiner (single disc) in hydracycle (or batch) conditions. More precisely, the effects of the grinding codes and of the average crossing angle of the bars are analyzed in a set of 6 refining trials. For these experimentations, different engineering concepts of the refining intensity are compared (specific edge load Bs, specific surface load SSL, modified edge load MEL, net tangential force per crossing point and net normal force per crossing point). These refining intensities should allow to analysing the cutting kinetics of fibres.

    All the chosen engineering concepts reach this goal more or less however the net normal force per crossing point is the best tool. Indeed, through the range of the data concerned, it revealed a clear monotonous evolution with the cutting kinetics on fibres. The more is the net normal force per crossing point, the more is the cutting effect on fibres.

     

     

  • 2009
    Oxford
    pp 85-112Simulation of Surfactant Contribution to Ink Removal Selectivity in Flotation Deinking LinesAbstractPDF

    Transport coefficients and correlations recently used to describe surfactant contribution to particle and water transport in a laboratory flotation column were used to simulate the impact of surfactant contamination on the flotation selectivity of industrial two-stage deinking lines. Simulation results showed that surfactants are slightly removed in the first flotation stage and are concentrated in the second one, where they induce a drop in ink flotation and in fibre entrainment. Subsequently, flotation units in the second stage displayed lower ink removal than in the first stage. In the presence of a constant water reject flow, the increase in surfactant contamination in the pulp stock gave a general decrease in the removal of suspended solids. Surfactant removal increased from 5 to 50%, however, this increase was not sufficient to prevent surfactant accumulation in the deinking line. Simulation results were compared with data collected in an industrial deinking line running in similar conditions and pulp composition, ink and surfactant removal obtained with low surfactant contamination were in line with experimental data.

  • 2009
    Oxford
    pp 113-158Review: Biochemical Additives for PapermakingAbstractPDF

    Biochemical additives encompass materials added to the papermaking operation that are derived from biological origins. Other than starch, the majority of the biochemical additives currently used in the paper industry are enzymatic. Enzymes are protein structures that speed a particular chemical reaction. The enzymes are not consumed during the reaction and can be used repeatedly. The enzymes used in the paper industry typically target one of the four major components of wood: cellulose, hemicellulose, lignin or extractives. Enzymes have been used industrially to aid in bleaching, reduce pitch, enhance strength, alter pulp freeness, and aid in paper machine cleaning. This review focuses on the use of enzymes in the papermaking operation, but also addresses the use of enzymes in other areas of the pulp and paper mill. There has also been considerable work in the use of fungus for improving both mechanical and chemical pulping operations. This is considered a separate topic and is only briefly addressed in this review. The future of biochemical additives may extend well beyond the current use of enzymes and a few notes on potential application are given.

  • 2009
    Oxford
    pp 161-180New Experimental Results on the Flow Regimes in Closed Channel Flows of Wood Fibre SuspensionsAbstractPDF

    We consider here the behaviour of wood fibre suspension with fibre concentration above that of sedimentation in a pressure driven flow in a straight pipe with smooth walls. The flow behaviour can be roughly divided in two main regimes: the plug flow regime that occurs at low flow rates and the drag reduction regime that occurs at high flow rates. We utilized new experimental methods in order to gain more detailed understanding on the flow behaviour of wood fibre suspensions, and especially on the relevant physical phenomena inducing such behaviour. In addition to carrying out conventional loss experiment, the velocity profiles across the pipe were measured using pulsed ultrasound velocimetry (PUDV) techniques, and the thickness of the lubrication layer in fully developed flow was measured using a laseroptical device. Based on our direct measurements, we were able to indentify five different flow regimes in suspension flows. In addition, we refined the qualitative picture of these flows in relation to the forming of fibre plug and to the physical phenomena taking place in transition from one flow regime to another one.

  • 2009
    Oxford
    pp 181-206Near-wall Estimates of the Concentration and Orientation Distribution of Semi-dilute Rigid Fiber Suspensions in Poisieulle FlowAbstractPDF

    A model is presented to describe the orientation and concentration state of semi-dilute, rigid fiber suspensions in a rectangular channel flow. A probability distribution function is used to describe the local orientation and concentration state of the suspension and evolves according to a Fokker-Plank type equation. Long range hydrodynamic fiber-fiber interactions are modeled using the approach outlined by Folgar and Tucker (J. Reinforced Plast. Comp. 3 98–119 1984). Near the channel walls, we apply the no-flux boundary conditions proposed by Schiek and Shaqfeh (J. Fluid Mech. 296, 271–324, 1995). Geometric constraints are used to couple the fibers’ rotary motion with its translational motion. This eliminates physically unrealistic orientation states in the near-wall region. A two-way coupling between the fiber orientation state and the momentum equations of the suspending fluid is considered. Experiments are performed to validate the numerical model by visualizing the motion of tracer fibers in an index-of-refraction matched suspension. The orientation distribution function is determined experimentally as a function of channel height. The results indicate that at distances less than one half fiber length from the channel walls, the model accurately predicts the available fiber orientation states and the distribution of fibers amongst these states. The model further predicts a sharp concentration gradient in this region.

  • 2009
    Oxford
    pp 207-245Evolution of the Paper Structure Along the Length of a Twin-wire FormerAbstractPDF

    A particle-level numerical model is used to simulate forming with a twin-wire former configuration. The development of the paper structure along the length of the former is observed to explain the effects of the dewatering elements on the paper structure at different jet-to-wire speed ratios, consistencies, and target basis weights. The simulations indicate that most of the structure development takes place in the initial part of forming (forming roll) and, in some instances, at the drop to atmospheric pressure after the forming roll. Dramatic effects on the through-thickness fibre orientation anisotropy are observed when the consistency is varied by changing the jet thickness, while changes in basis weight had less impact. The through-thickness concentration gradient was almost uniform throughout the forming process, except in the lower range of typical papermaking consistencies. This indicates that the dewatering mechanism is normally thickening, rather than filtration.

  • 2009
    Oxford
    pp 247-271Single Fibre Flexibility Measurement in a Flow Cell Based DeviceAbstractPDF

    Single fibre flexibility is widely recognised as an important parameter in the papermaking process. A novel flow cell based method for its measurement was developed at the Institute for Paper, Pulp and Fibre Technology at Graz University of Technology. A special flow channel geometry is used to induce high shear forces in a laminar flow regime. The movement of single fibres passing the highly sheared region is recorded by means of a high speed image acquisition system. Based on the reactions of the fibres to the fluid forces a flexibility parameter is determined for each evaluated fibre.

  • 2009
    Oxford
    pp 273-352Optical Properties of Paper: Theory and PracticeAbstractPDF

    The perceived value of paper products depends not only upon their performance but also upon their visual appeal. The optical properties of paper, including whiteness, brightness, opacity, and gloss, affect its visual perception and appeal. From a practical point of view, it is important to quantify these optical properties by means of reliable and repeatable measurement methods, and furthermore, to relate these measured values to the structure of paper and characteristics of its constituents. This would allow papermakers to design new products with improved quality and reduced cost. In recent years, significant progress has been made in terms of the fundamental understanding of light-paper interaction and its effect on paper’s appearance. The introduction of digital imaging technology has led to the emergence of a new category of optical testing methods and has provided fresh insights into the relationship between paper’s structure and its optical properties. These developments were complemented by advances in the theoretical treatment of light propagation in paper. In particular, wave scattering theories in random media are finding increasing applicability in gaining a better understanding of the optical properties of paper. In this document, a review of these advancements is presented.

  • 2009
    Oxford
    pp 355-388The Influence of Grammage, Moisture Content, Fibre Furnish and Chemical Modifications on the Hygro-and Hydro-expansion of PaperAbstractPDF

    The conventional way to evaluate dimensional stability, regardless of end-use purpose, is to measure the change in dimensions when the moisture content is changed by changing the relative humidity. Sorption of moisture from moist air is a relatively slow process and for the evaluation of printing papers this may not be the most appropriate method.

    In the present work, data from conventional hygroexpansion measurements has been compared with data from hydroexpansion measurements, i.e. expansions caused by the sorption of liquid water, sprayed onto papers printed with a random speckle pattern, the expansion being monitored by electronic speckle photography.

    Sheets made from different pulps, with different fines contents and different modifications were studied at different grammages and water-transfer levels. The effect of drying-mode, i.e. restrained drying or free drying, was also studied. It was concluded that sheets expand less with a given amount of adsorbed water when it is sorbed in liquid form rather than from moist air. Chemical treatments known to increase both the dry and the wet strength, e.g. polyelectrolyte multilayers and cross-linking through periodate oxidation, did not significantly improve the dimensional stability when the papers were exposed to liquid water.

  • 2009
    Oxford
    pp 389-436On the Importance of In-plane Shrinkage and Through-thickness Moisture Gradient during Drying on Cockling and Curling PhenomenaAbstractPDF

    Cockling and curling are unwanted phenomena that occur already in the manufacturing process and continue through the end use of paper due to moisture content changes. This paper compares the effects of different in-plane tensions and throughthickness drying profiles during the drying process on cockling and curling by means of finite element simulations. In this study, paper is treated as a heterogeneous orthotropic elasto-plastic material. The results predict that finding the optimal way to dry paper is not straightforward; it also depends on fiber orientation structure. On the other hand, defects in the fiber orientation structure of paper can be redressed at some level by drying the paper appropriately.

  • 2009
    Oxford
    pp 437-454Flow Permeability of Fibrous Porous Materials. Micro-tomography and Numerical SimulationsAbstractPDF

    In this work we demonstrate the use of computerized x-ray micro-tomography and numerical simulations in evaluating flow permeability of fibrous porous materials. This ab-initio approach involves solving fluid flow through material samples in the actual pore space obtained by tomographic techniques. The procedure is applied here in three different materials, namely plastic nonwoven felt, newsprint and wet pressing felt. All numerical results presented are compared with experimental data for the same materials. The non-woven felt material, having a relatively simple structure, is first used as a test case for comparing two different numerical schemes, lattice-Boltzmann method and a finitedifference method. Here, values of both transverse and in-plane permeability are obtained. The transverse permeability of newsprint and wet pressing felt under varying degree of compression is then found using lattice-Boltzmann method. Finally, we apply the same approach in estimating permeability in different structural layers of the wet press felt material. These material parameters are laborious or even unfeasible to determine experimentally. The procedure is applicable e.g. in finding the relevant material parameters for macroscopic models describing calandering, drying and wet pressing processes.

  • 2009
    Oxford
    pp 455-473Multi-scale Moisture Transport in Paper: Impact of Pore and Fibre Tortuosity & AnisotropyAbstractPDF

    The diffusion of moisture in paper is a complex phenomenon with pore diffusion dominating at low moisture contents and diffusion through fibres dominating at high moisture contents. Vapor diffusion through the pore space depends on the topology of the pore connections. Recently available three dimensional digital reconstructions of the pore space using X Ray Micro computed tomography (XμCT) enable us to determine the impact of the pore and fibre tortuosities and connectivity to moisture diffusion in an explicit manner.

    In this study, moisture diffusion was simulated through XμCT reconstructions of paper structures using a hybrid random walk algorithm that was developed to allow simultaneous diffusion in both the pore and fibre spaces with differential ‘intrinsic diffusivities’. The algorithm is specifically applied to simulate simultaneous diffusion under low and high relative humidity conditions where diffusion occurs predominantly through one medium i.e. pore space and high humidity conditions where both media (i.e. fibre and pore spaces) are highly conductive. The ‘intrinsic diffusivity’ of moisture through fibres was determined by using numerical simulation and experimental results. This intrinsic diffusivity a fundamental fibre characteristics is found to be independent of refining level but depends only the fibre moisture content under the conditions studied.

    The algorithm also allowed the determination of the anisotropy in diffusivity. One interesting result is that the anisotropy in diffusion is most significant at low moisture contents when diffusion through the pore space dominates. At high relative humidities (i.e. at high moisture contents), fibre conduction provides an alternative diffusion path, homogenizing diffusion to a large extent. As a result, diffusion becomes more isotropic with increased moisture contents in paper.

  • 2009
    Oxford
    pp 475-490A Simple Model of the Air Permeability of PaperAbstractPDF

    We have developed a simple model of the air permeability of paper and shown that it is in reasonable agreement with experimental results for softwood chemical pulps. The permeability is given as a function of fibre external height and width and fibre volume fraction including the lumen volume if present. The most important conclusion from the model is that the average fibre thickness in the paper is the critical fibre property controlling air permeability. In fact, the theory predicts that the sheet permeability is roughly proportional to the fourth power of the fibre thickness.

  • 2009
    Oxford
    pp 491-513A Laboratory Investigation on the Origin of Machine Direction MicrostriationsAbstractPDF

    This paper is based on a laboratory-scale experimental study of machine direction microstriations (MDM) on board surfaces. We developed a pressing and drying simulator in which we can replicate some of the phenomena which are believed to be the origin of MDM: density stratification in wet pressing as well as in-plane restraining conditions during drying. Our laboratory experiments showed that we could generate surface features, visually similar to those classified as MDM in industrial paper production. In particular we could replicate the elongated appearance, the characteristic wavelength interval (1–4 mm) and the occurrence on one surface only.

    The most important parameter in respect to the absolute amount of surface roughness was the in-plane restraining conditions during drying. Biaxial restraining resulted in much lower surface roughness and prevented the occurrence of MDM. MDM started to appear as soon as uniaxial shrinkage was permitted. Interestingly, however, shrinkage perpendicular to the main direction of fibre orientation in an oriented paper sheet caused a less pronounced occurrence of MDM in spite of a larger absolute value of shrinkage. The surface presented topography features parallel to the restraining direction. The press felt surface, in our investigation the coarseness of the batt fibres, influenced the surface roughness of paper, however at a characteristic length that was much smaller than that of MDM.

  • 2009
    Oxford
    pp 517-556Instability of webs: the prediction of troughs and wrinklesAbstractPDF

    Web troughs are defined as an out-of-plane instability of a web in a free span between rollers. Troughs may be only an annoyance in some web process machines but are often detrimental to quality in coating or printing operations.

    Web wrinkles are defined as an out-of-plane instability of a web crossing a roller. Wrinkles are much more serious than troughs in that they result in creases, fold-overs, and bursts which can shut down a paper-making, coating, or printing operation or result in decreased quality or productivity.

    This paper will provide proof that troughs are a necessary precursor to wrinkles. Examples will show that linear buckling analyses can be used to predict web troughs. Further examples will show how nonlinear post buckling analyses can then be used to predict wrinkling.

     

  • 2009
    Oxford
    pp 557-575System Stability of the Open Draw Section and Paper Machine RunnabilityAbstractPDF

    The present work is concerned with the system dynamics and stability of the open draw sections of paper machines where web breaks occur most frequently. We have applied a novel particle based system dynamics model that allows the investigation of complex interactions between web property fluctuations and system parameters, without any constraints of a particular geometrical web shape or boundary conditions assumed a priori. The result shows that, at a given machine draw and web property parameters, the open draw section maintains its steady-state until it reaches a certain machine speed limit. At this speed the system looses its stability and the web strain starts growing without any limit, and thus leading to a web break. A similar instability can also be triggered when web properties suddenly fluctuate during steady-state operation. The parametric sensitivity studies indicate that, among the web property parameters studied, the elastic modulus of the wet web has the largest impact on the critical machine speed as well as on the detachment point where the web is released from the first roll. Further analysis shows that the decrease in dryness has a (negative) synergistic effect causing an increased risk of system instability. It is, therefore, most important to control, not only average dryness, but also its variations in order to enhance paper machine runnability.

  • 2009
    Oxford
    pp 577-595Wet Web Rheology on A Paper MachineAbstractPDF

    The runnability of a wet web is the sum of many factors, ranging from furnish variables to papermaking running parameters (speed, draws, distance from wet pressing etc). The relative importance of these factors was studied using several different experimental methods. The dynamic stress-strain relationship was determined in situ by measuring it on a wet web winder installed on a pilot paper machine. It was then compared with values obtained by testing the wet rolls on a separate running device. The comparison suggests that tensile strength is a more fundamental characteristic of the stress-strain curve than the dynamic stiffness affected by creep. Tensile strength is dominated by moisture content in a transition region where free water enters the fiber network. Its sensitivity to moisture content weakens as the paper becomes very wet. The location of the transition region depends on the fiber saturation point. This leads to complex changes in ranking when different pulps are compared at different moisture contents. The fines content of the furnish has a significant impact on wet web strength, whereas the fiber stiffness affects the measured dynamic stiffness but not tensile strength.

  • 2009
    Oxford
    pp 599-664Review: Three-dimensional Structural AnalysisAbstractPDF

    The literature related to the three-dimensional fibre networks is reviewed since 2000. Indeed, a review was presented in these symposia in 2001 [1]. Therefore, the updated articles are only considered here. The previous review concentrated on theory, whereas the focus here is on modern 3D analysis and comparisons of these with theory. Moreover, the focus is on paper structure. However, the general context of 3D structure is considered in the introduction in order to illustrate the main ideas that may be applied to paper. Then, the experimental methods are presented in order to show the potential of such techniques. In a third part, improvements of the description of the 3D structure will be presented. Namely, the quantitative description that completes the visualisation of the structures will be presented. The main 3D morphological properties will be presented and some examples will illustrate the existing developed tools. The Representative Elementary Volume (REV) dedicated to the structure properties will then be introduced. The theoretical models are briefly presented to prove the necessary development of both experimental tools and dedicated theories. Theoretical studies will be exemplified. Indeed, both theoretical and experimental demarches will enrich each other as will be shown. The influence of both deformation and humidity modifications on the 3D structure will illustrate the interest of the knowledge of the 3D structure, to tackle its influence on physical properties. The main perspectives and challenges related to the structure description will end up this presentation.

  • 2009
    Oxford
    pp 665-691Structural Invariance of Stochastic Fibrous NetworksAbstractPDF

    We use simulation and analytic modelling to probe the structural similarity reported in the literature for fibre networks with manifestly different degrees of uniformity. From simulations of point processes in the plane to represent random, clustered and disperse fibre centres, we show that the distribution of distances between pairs of centres is very insensitive to the extent of clustering. Further, we quantify the correlation between the lengths of adjacent polygon sides arising from a Poisson line process in the plane as being ρ = 0.616 ± 0.001 and show that this is very insensitive to fibre orientation and only weakly influenced by clustering. The relevance of this correlation to pore geometry is discussed.

    In the final part we analyze simulated areal density maps and show that their variance relative to that of a random fibre network of the same constituent fibres, as quantified by the formation number, depends at small scales on the flocculation intensity only and depends at large scales on the number of fibres per floc only.

  • 2009
    Oxford
    pp 693-719Spatial Partitioning of the Structural Properties of Tissue and Towel GradesAbstractPDF

    This paper describes an investigation conducted to examine the mesoscale structural properties of single-ply tissue and towel papers by mapping thickness, out-of-plane deformation, formation, and density. The uniformity of each of these properties was studied to a zone size of 0.1 mm in mapped regions of about 10 mm square. The sampling regions were partitioned into subsets that were separately analyzed. Regions of interest included wet pressing marks, embossed patterns and through air drying (TAD) pressed patterns. The relationship between thickness and grammage (basis weight) in the different regions of interest were compared. Differences were attributed to the indentation process that could affect the structure in different ways, depending on the process conditions. Six commercial tissue and towel products were tested.

    The regional differences were most apparent in the results for paper towels. Deformation of the structure to form textured patterns compressed regions and increased the local density. There was selected in-plane movement of fibers, in response to the indentation. The towel sample formed by wet pressing did not show the same extent of densification at the indentation site. The structure was deformed out of plane, but not significantly com- pressed. Tissue samples formed by conventional wet pressing showed increased densification at points of indentation. Differences in the out-of-plane response to the pressing process could be distinguished between different tissue samples.

  • 2009
    Oxford
    pp 721-747Introducing A Concept to Link 3D Paper Structure to 2D Paper PropertiesAbstractPDF

    This paper introduces a concept that spatially links local paper properties from a 3D paper dataset to a 2D paper property map of the same sample. The concept exploits the fact that paper is a planar structure. This permits successive conversion of 3D paper datasets first to a 3D surface (the paper center surface) and further to a plane. The flattened 3D paper data can then be linked to the 2D paper property map. Laser micro holes applied to the paper are employed as marks for alignment of the 2D maps. The key step of the procedure is flattening the paper data from a warped 3D surface to a 2D plane.

    Some exemplary applications for the introduced method are given. The effect of local coating layer thickness on local bright- ness and local print density is examined for wood free coated paper. Furthermore, the concept can be applied for quantitative comparison of data from different 3D-imaging techniques. We measured local coating layer thickness on the same paper specimen applying first fan-beam X-ray microtomography and then microtome serial sectioning. The results are compared using point-wise correlation.

  • 2009
    Oxford
    pp 749-838Review: The Surface Analysis of PaperAbstractPDF

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  • 2009
    Oxford
    pp 841-875Developing A Deeper Understanding of the Constitutive Behavior of PaperAbstractPDF

    A constitutive equation for the mechanical behavior of paper is presented. Since the initial work in the late 1940’s rheology studies of paper seem to come in waves and the results have not always been viewed favorably. With the benefit of fifty years of literature and the relative speed of modern computing, a more robust constitutive model is certainly within reach. The presented model captures the essence of the mechanical response and links the stress-strain curve to relaxation and creep in a reasonable manner. Experimental results are provided to illustrate the important components of the behavior and the model can easily be generalized to include more subtle features.

  • 2009
    Oxford
    pp 877-897Development of the Notched Shear TestAbstractPDF

    The notched shear test (NST) will be proposed to measure shear strength of paperboard by utilizing standard in-plane tensile testing equipment. The test is a further development of the double notch shear test specimen; where plastic lamination has been utilized. With the new test setup it becomes possible to measure shear strength profile in the thickness direction of paperboard. As a spin-off of the NST also the strip shear test (SST) was suggested. The SST test can be used as a quick measure of shear strength. It is shown that the SST measurement correlates well with the traditional rigid shear test (RST). In order to verify that the NST specimen failed due to shear stresses, finite element simulations were performed. The simulations showed that the shear zone had a uniform shear stress field at the time of failure. Moreover, with the finite element simulations it was also possible to predict the force-displacement curve.

  • 2009
    Oxford
    pp 899-930The Influence of Microfibrillated Cellulose, MFC, on Paper Strength and Surface PropertiesAbstractPDF

    This study demonstrates recent advances in microfibrillated cellulose (MFC) research. This novel material, which is mostly com- posed of nano-sized cellulose fibrils seems to have large potential within several applications. Three case studies are considered, i) MFC films that may be applied for packaging applications, ii) MFC applied in the paper bulk and iii) MFC applied as a coating layer on paper. It has been demonstrated that MFC forms films with high strength, high density and low permeability. The tensile strength of the TMP sheets assessed in this study have been increased by more than 20% by addition of 4% MFC to the furnish. Application of MFC as a coating layer on paper forms a dense continuous surface film. However, despite a considerable reduction of the surface roughness, the paper gloss was reduced. This contradictory phenomenon is most probably caused by the MFC layer covering the clay particles on the surface of the base paper.

  • 2009
    Oxford
    pp 931-946The Effect of Fibre Orientation on the Zero-span Testing of PaperAbstractPDF

    In this study, a new technique for producing almost fully aligned paper sheets was developed, and the resulting sheets were used to test the validity of the theory most commonly used to relate zero-span tensile strength to individual fibre strength. The standard theory predicts that a zero-span test of a sheet with randomly oriented fibres should yield a breaking load equal to 3/8 of the load that would be observed if all the fibres were aligned in the direction of loading. It is widely used, in spite of the fact that the underlying assumption of affine deformation is questionable under true zero-span conditions. The results obtained here suggest that the fibre strength may be overestimated because inclined fibres in a zero-span grip actually contribute more than the theory predicts. However, the results also suggest that this effect may be confounded because other factors lead to a variable contribution of individual fibres to the zero-span strength of the sheet.

  • 2009
    Oxford
    pp 947-1000Degradation Phenomenology and Life Modelling of PaperAbstractPDF

    This document reviews degradation phenomenology and life modelling of paper, illustrates different approaches, showing their advantages and applicability limits and raises problems relevant to practical applications of degradation indicators and degradation rate models and evaluation of paper permanence and durability. It deals with the fundamentals of paper degradation rate theory and life modelling, seeks to develop the degradation rate equations of paper that will closely track real degradation experimental results over extended period under practical service conditions, shows a collection of degradation rate and life data of different paper and paper insulation materials under a varied of in-service environmental conditions, develops continuous distribution kinetics theory of cellulose and paper degradation to form a united basis in deriving the traditional Ekenstam degradation rate equation and the alternative percentage DP (and TS) loss rate equations recently proposed by the present author and his coworker, and presents effective methodology for combining time temperature superposition method and paper degradation investigation towards a more reliable prediction of rate and life of paper degradation over a long time period under real in-service conditions.

  • 2009
    Oxford
    pp 1001-1017On the Mechanisms of Mechano-sorptive Creep Reduction by Chemical Cross-linkingAbstractPDF

    Despite the technical importance of mechano-sorptive creep in paper, the exact mechanism behind this phenomenon is still not fully understood. In this study it was shown that the mechanosorptive creep of paper sheets can be significantly reduced by chemical cross-linking through periodate oxidation. The mechanism behind this reduction has been examined through creep measurements of both sheets and individual fibres. For sheets the creep acceleration due to varying humidity was significantly reduced by the chemical cross-linking. For single fibres, however, the creep acceleration was not affected by the chemical crosslinking. In fact the absolute creep rate for the periodate oxidised fibres were higher than that of the reference fibres. This clearly showed that the improvement in mechano-sorptive creep found on a sheet level does not originate from an improved creep resistance for individual fibres but rather from mechanisms operating at the fibre network level. Hygroexpansion and moisture sorption of the sheets during the humidity cycling used for creep testing have also been measured, and the results showed that both was reduced by the periodate oxidation. Reduced moisture sorptivity and hygroexpansion probably minimises stress concentrations at the fibre network level and thereby also the creep acceleration.

  • 2009
    Oxford
    pp 1019-1037Effects of Furnish-related Factors on Tension and Relaxation of Wet WebsAbstractPDF

    A smooth web transfer in a paper machine requires sufficient tension. It is well known that excessive tension leads to web breaks. However, too low a tension can also be fatal. Particularly with wet web, the maintenance of the tension is challenging due to the fast relaxation of the tension. Some factors that affect the tension and relaxation of wet web were studied in this paper.

    The initial tension and the tension after constant relaxation time, called residual tension, was found to depend on factors such as the applied strain, straining rate, dry solids content, fibre and fines properties, substances in the white water, and the dry strength chemicals. The residual tension was reduced by increased straining rate, addition of TMP filtrate, and addition of cationic starch. The tension and residual tension seemed to be dependent on both the properties of the fibre fraction, such as in- and out-of plane stiffness, and on the factors affecting the stress transfer conditions at the inter-fibre contact areas.

  • 2009
    Oxford
    pp 1039-1078Prediction of Fibre Orientation and Stiffness Distributions in Paper – An Engineering ApproachAbstractPDF

    The orientation of the fibres in a paper directly influences many of its properties. The focus of this work was to predict the fibre orientation distribution and tensile stiffness distribution of a paper. The predictions were based on a proposed link between the two distributions and physical parameters measurable on the paper, no fitting parameters.

    The fibre orientation distribution in paper was approximated by a probability density function. Both curve fitting type of distribution functions earlier used in paper physics and physical based functions derived from Fluid mechanics, Orthotropic analysis and a simple Stress/strain analysis were evaluated. The physical based functions used one measurable physical parameter, the fibre orientation anisotropy. The tensile stiffness distribution was predicted with a distribution function from the literature and functions derived from the Fluid mechanics and Orthotropic analysis approach. The predictions needed two measurable physical parameters, the MD and CD tensile stiffness.

    Predictions of fibre orientation distribution and tensile stiffness distribution for restrained dried papers were compared with experimental data from restrained dried oriented handsheets with varying fibre orientation anisotropy. General approaches valid for all papers were compared with experimental data from pilot made papers with different drying restraint history. Both the predicted results for fibre orientation distribution and tensile stiffness distribution showed good agreement with experimental data.

  • 2009
    Oxford
    pp 1079-1094Effect of Drying Conditions on the Tensile Properties of PaperAbstractPDF

    Results in the literature disagree regarding the effect of drying temperature, final drying time, and drying constraint history, respectively, on the in-plane tensile properties of paper materials. Furthermore, it is debated whether the in-plane tensile properties are controlled by the final drying stress or by the total strain during drying.

    In this work, the drying mechanics of a pilot machine-made paper grade was studied. Wet paper sheets were collected after the wet press section. The sheets were dried in a laboratory dryer using different drying constraints. The supplied heating power, the ambient climate, and the ventilation of the paper sheets were controlled during the drying trials, which made it possible to independently alter the drying temperature and the final drying time. The dried sheets were conditioned in 23°C and 50% RH before tensile testing.

    The results showed that the tensile stiffness, tensile strength, strain at break, and tensile energy absorption of the dried sheets, respectively, were linearly related to the total strain during drying of the sheets. These linear relations were shown to be unaffected by drying temperature, final drying time, and drying constraint history. On the other hand, the corresponding relations between the in-plane tensile properties and final drying stress were found to be both non-linear and greatly dependent on the drying constraint history.

     

  • 2009
    Oxford
    pp 1095-1145Review: Bioactive Paper – A Paper Science PerspectiveAbstractPDF

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  • 2009
    Oxford
    pp 1149-1166Bioactive Enzymatic PapersAbstractPDF

    The thermal stability of Alkaline phosphatase enzyme (ALP) adsorbed on paper was measured using a colorimetric technique quantifying the intensity of the product complex. ALP adsorbed on paper retains its functionality and selectivity. Adsorption of ALP on paper increased the enzyme thermal stability by 2 to 4 orders of magnitude compared to the same enzyme in solution. Complex patterns of enzyme were also printed using a thermal inkjet printer on paper. Microfluidic channels were printed on paper to demonstrate the concept of paper-based bioassays as diagnostic devices. Paper is an ideal material for functional materials for functional bioactive surfaces.

  • 2009
    Oxford
    pp 1167-1191Adhesive and Long-range Capillary Forces Between Hydrophobic Surfaces in Water: Effects of Surface TopographyAbstractPDF

    Interactions between a hydrophobic probe particle and surfaces with nanoscopic surface features have been investigated. Such surfaces were prepared by polishing or by spin-coating of nanoparticles. The surface topography was characterized by AFM, using the methods of high-resolution imaging, low-resolution imaging using the probe particle, and by the rolling ball method. The polished surfaces display sharp nanoscopic peaks and hardly any crevices. In contrast, the spin-coated surfaces can be characterized as nanostructured, due to the high density of nanoparticles that on a short length scale provides a regular pattern of crevices and hills. On all surfaces a larger waviness is also distinguished. In all cases the dominant force at short separations was found to be a capillary attraction due to the formation of an air/vapour condensate. Our data show that the large-scale waviness of the surface does not significantly influence the range and magnitude of the capillary attraction, but large local variations in these quantities are found. The large variation in adhesion force corresponds to a small variation in local contact angle of the capillary condensate at the surfaces. The report discusses how the nature of the surface topographical features influences the capillary attraction by influencing the local contact angle and by pinning of the three phase contact line. The effect is clearly dependent on whether the surface features exist in the form of crevices or as extending ridges.

  • 2009
    Oxford
    pp 1193-1219The Structure and Strength of Flocs of Precipitated Calcium Carbonate Induced by Various Polymers Used in PapermakingAbstractPDF

    When the percentage of filler is increased in paper, the optical properties are improved and the production cost lowered. However, fillers weaken paper strength by decreasing the fibre-fibre bonded area. Little is known about the optimum filler floc size or filler floc properties to allow developing optimum paper characteristics. Consequently, the floc structure and strength of precipitated calcium carbonate (PCC) aggregates was studied using various polymers (flocculants and dry strength agents): by static light scattering/diffraction (SLS), real time fluorescent video imaging, image analysis, photometric dispersion analysis (PDA) and scanning electron microscopy (SEM).

    It was found that PEO/cofactor induced PCC aggregates were weaker at high shear and far more irreversible than those induced by the partially hydrolysed polyvinyl formamide copolymerised with acrylic acid (PVFA/NaAA) or C-starch. Flocs produced at low polymer dosages were smaller and weaker than those produced at higher dosages. The number of discrete PCC particles in aggregates was measured using real time fluorescent video imaging combined with image analysis.

     

  • 2009
    Oxford
    pp 1221-1252On Fundamentals and Applications of Superhydrophobicity in Papermaking and PackagingAbstractPDF

    Fundamental material science investigations of superhydrophobicity in recent years has evolved toward industrial applications and recently to papermaking and packaging. The present study concerns both fundamental and applied aspects of superhydrophobicity. An industrially viable process for a one-step waterborne superhydrophobic coating was developed. It is shown that different measures of the degree of superhydrophobicity are needed depending on the final application whether this may be self-cleaning or stain repellent action. Fundamental aspects of superhydrophobicity were investigated using silica wafers roughened by a particulate formulation containing nanosize silica particles, which were fixed to the substrate by calcination. After hydrophobization by silylation, the forces between a colloidal superhydrophobized silica probe, made according to a similar procedure, and these surfaces were measured by Atomic Force Colloidal Probe Microscopy. The results show an extremely long range interaction force and a large influence of surfactant and surfactant concentration. The results would prove useful in designing robust superhydrophobic application in the papermaking and packaging industry and also imply that coating and printing technique could be used for controlled deposition of superhydrophobized layers or areas.

  • 2009
    Oxford
    pp 1255-1292A Structural Analysis of the Thermal Conductivity of Paper Coatings: Application of Particle Deposition Simulation to a Lumped Parameter ModelAbstractPDF

    Thermal conductivity of paper coating structures can be regarded as an important property for many processes involving the application of thermal energy on coated papers. This work analyses the thermal conductivity of coatings in terms of their structure. A Monte Carlo simulation-based particle deposition was used to create idealised two-dimensional coating structures. They acted as a master template for the superimposed parameters of a Lumped Parameter Model for the calculation of thermal conductivity, in which pigment and binder are treated as separate solid phases within a fluid (air). Binder alone was initially assumed to provide the necessary thermal connectivity. Comparison of the numerically calculated conductivities with corresponding experimental results, obtained from ground calcium carbonate pigment structures, showed generally lower calculated conductivities and clear differences in the change of conductivity when increasing latex binder content. Two different mechanisms are suggested as the cause of this lack of correlation. Firstly, it is shown that both the simulation and the current Lumped Parameter Model do not account sufficiently for pigment connectivity. This is the reason for the underestimation, especially evident when no binder is present. The nature of pigment connectivity is related to polymer dispersant on the pigment surface and the surface crystallite planar structures, if present, mostly related to larger particles. Secondly, it is confirmed that surface and colloid chemistry factors cause binder to accumulate first at pigment nodal points, which causes a disruption of the pigment packing already at 6 w/w% binder. This creates in homogeneity in the real coating structure which is not accounted for by the homogeneous assumption of the model. It could be shown that an introduced parameter of pigment connectivity becomes lower for the binder concentrations for which pigment disruption occurs. It is shown that the method is sensitive enough in respect to refinement of both pigment and latex connectivity factors to allow identification and parameterisation of the subtleties occurring in real colloidally interactive particulate systems that are reflected in the thermal conductivity response of the dried coating structure.

  • 2009
    Oxford
    pp 1293-1323Probabilistic analysis of small-scale print defects with aligned 2D measurementsAbstractPDF

    We present an analysis of the pointwise relationship between the reflectance of print and the surface topography of the paper before printing. We have measured the surface topography and reflectance of paper before and after printing in a sheet-fed pilot offset printing press. The 2D measurement maps have been aligned to obtain local print reflectance and surface topography values for every spatial position on the samples. In contrast to the various deterministic modeling approaches, which imply an a priori defined underlying mathematical model, we apply probabilistic analysis. Therefore we first estimate joint probability density functions (pdfs) of local topography and print reflectance using Gaussian Mixture Models (GMMs). From these pdfs we select paper regions with unusual properties, i.e. regions from the tails of the pdfs. These anomaly maps are analyzed for interrelations between the print reflectance and surface topography, its gradient and local variance. The degree of interrelation is characterized by the mutual information (MI), a measure to quantify statistical dependence without making assumptions about the linear or nonlinear nature of the regression dependence. The significance of the MI values is confirmed by simulation based statistical hypothesis testing. The objective is to offer answers to the question: How does the observation of an exceptional topography point on the paper surface change our information about whether the print quality attainable at that point will be exceptional or not? The results suggest that topography in combination with its local variance have the most prominent interrelation to small scale print anomalies. Furthermore it is shown that regions with abnormal topography have at least ten-fold higher probability to exhibit exceptionally high print reflectance, compared to randomly selected regions.

  • 2009
    Oxford
    pp 1325-1350Fundamental Studies of Linting in Offset Printing of NewsprintAbstractPDF

    Linting is the removal of material from the surface of uncoated grades of paper during offset printing. Excessive linting reduces image quality and can reduce press productivity. In this paper, web-fed and sheet-fed linting trials have been used to investigate the effect of important press and paper variables on linting. Two of the most important printing parameters affecting linting were the take-off angle from the nip and the printing tone. From analysis of the effects of take-off angle and printing tone, two forces were identified as being especially important to linting: a film flow force in the nip and a tack force from the splitting of the ink film. A simple model was presented that could qualitatively explain why printing press speed, printing pressure and ink tack all had smaller effects on linting than would be expected from consideration of tack force alone. Laboratory printing tack tests and other measurements of paper properties were compared with lint measured in the sheet-fed trials. The tack force measured in laboratory printing was found to be lower for improved newsprint compared to newsprint, while the lint in both sheet-fed and web-fed trials was higher for the improved newsprint. Differences in the film flow in the nip were suggested to be responsible for both effects. The improved newsprint was also found to have a lower surface strength, as measured by delamination.

  • 2009
    Oxford
    pp 1355-1365Silks and Their CompositesAbstractPDF

    Silks make not only interesting natural materials but also, in the context of their use by the animals that produce them, fascinating natural composites. Importantly, the material properties of a silk depend not only on the chemistry and subsequent folding pattern of the silk protein themselves but also on the hierarchical structure of the poly-protein fibre. Both, in turn, depend to a large extend on the conditions under which a fibre has been spun and thus depend on the animal’s spinning behaviour. Not surprisingly, this gives the animal a high degree of flexibility in which to use its materials. And, if the materials (and typically silks are multi-faceted) are integrated into structures, then those too can have a range of ultimate properties, depending on the animal’s building behaviour. As both materials and structures have evolved over hundreds of millions of years, much can be gleaned and learned concerning highly adapted and often optimized structure-property-function relationships on the material level as well as on the composite level.

    Individual silk fibres can range in diameter from 20 to 7000 nm depending on species, animal size, silk type and spinning conditions. The hierarchical structure of a silk fibre can range from very simple to complex i.e. a singular filament consisting of its molecular chains folded into regions with differing degrees of order to, respectively, bundles of filaments aggregated and layered into fibre-ropes and covered with coatings of specialist compounds [1].

    Presented here are two very different silk fibre and composite types: the many silks and light-weight webs of spiders and the singular silk and solid cocoon of lepidopteran “silkworm” larvae.

  • 1989
    Cambridge
    pp 1-19Guest speaker’s address: “Change and the 21st Century”AbstractPDF

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  • 1989
    Cambridge
    pp 1–38The beating of chemical pulps – the action and the effectsAbstractPDF

    The article reviews the state of knowledge of the action and effect of beating on chemical pulps. The view is put forward that the problem is so complex that researchers have been tempted to oversimplify it, but some of these simplifications now may represent a barrier to progress. New approaches incorporating newly developed research techniques are proposed for future research efforts.

  • 1989
    Cambridge
    pp 39-47A comparison between industrial and laboratory beating with regard to fibre developmentAbstractPDF

    The effect of industrial beating on tensile strength properties has been compared with laboratory beating and the differences observed have been explained in terms of fibre properties.

    Tensile strength development is less pronounced for industrial beating than for laboratory PFI-mill beating. Beating in the laboratory Escher-Wyss conical refiner was found to develop strength to an level intermediate between those of the other two.

    The fibre characteristic causing these differences was found to be the ability of the fibres to transmit load in thesheet. Fibre deformations introduced during pulping and bleaching are to a large degree removed by PFI-mill beating. Industrial beating shows very little effect in this respect.Tensile strength development could be explained wholly by changes in fibre swelling (water retention value) and in the ability of the fibres to transmit load. The latter property was evaluated by zero-span measurements on rewetted sheets.

  • 1989
    Cambridge
    pp 39-65Deuterium NMR and water adsorption studies of the effect of beating on the water-cellulose interactionAbstractPDF

    The interaction between water and cellulose has been studied by deuterium NMR and water sorption isotherm measurements for a bleached kraft pulp and an unbleached liner board pulp beaten to various degrees. Measurements of 2H quadrupole splittings, spin lattice relaxation rates and half height line width shave been carried out at different relative humidities. Furthermore, the spin-lattice relaxation rates at 86%relative humidity were measured in the frequency range 2-62 MHz for the bleached kraft pulp, unbleached liner board pulp and for a microfibrillated cellulose sample. The relaxation rates show a strong dependence on the resonance frequency. The reorientation of the water molecules adsorbed on fibers can be described by two corrletion times ofthe order of 10 ns and 50ps respectively. Analysis of the experimental data does not show any effects of beating on the cellulose water interaction strength. All observations made can be explained by morphological changes of the fibers.

  • 1989
    Cambridge
    pp 73-149Overview of the physics of formingAbstractPDF

    In this overview forming covers all processes from the dilution of thick stock into a mix, using recirculated white water in the short circulation, to the dewatering in the wire section.

    Grammage non uniformity in the paper web is to a predominant degree generated by the forming process, and especially the small scale variations summarized in the term formation. The term mass formation is recommended when only grammage is considered and not the optical impressions there of. The forming process also generates the main part of the large scale variations, that is the MD-,CD-and residual grammage variance.

    Mass formation has traditionally been evaluated using beta radiography, combined with micro densitometry or image analysis. A new technique involving the direct recording of electron beam transmission is underdevelopment, with promises off aster processing, perhaps even on-line, andhighgeometricalresolution.Characterizationtechniquesbasedontheco-occurrence matrix, applicable to image analysis, can be a useful complement to the traditional power spectra techniques.

    It has recently been conclusively demonstrated that in flowing fibre suspensions, flocs are kept together by the bending forces of interlocked fibres. To study the dynamic behaviour off lowing fibre suspensions, modern video techniques, high speed movie pictures and image analysis are applied.

    To improve grammage uniformity, the mix should be fed to the headbox directly after the dilution of thick stock with white water. No processes like screening or cleaning, from which uncontrolled reject fibre flows are drawn should be allowed in the short circulation. Further the consistency of each material component in the recirculated white water should be controlled, if the content of the different components in the sheet produced is to be held constant.

    First pass retention is shown to be a badly defined retention value, and should only be used in comparisons for one paper machine when no changes of the material content in the long circulation occurs.

    In the headbox, the tapered manifoldis the commonly used means for achieving the coarse distribution across the machine width. From the cross distributor, a tube package can lead either to a stilling chamber, or directly into the out let nozzle. The nozzle has to be fed using maximum open area to avoid flow instability, and the nozzle contraction must be large enough to reduce velocity streaks and relative turbulence to an acceptable level. Mathematical models are now being applied to the calculation of water flow patterns in headboxes.

    Local slice lip adjustments, especially on headboxes without let nozzles of low convergence angle, can cause considerable sideways flow on a fourdrinier wire, and this will have a large effect on the CD grammage variations as well as on the final sheet anisotropy profiles.

    There are two basic headbox designs for stratified forming. In one of these, thin,pointed vanes separate the different furnishes. In the other, thicker separation walls generate air wedges, which may separate the furnishes up to the actual starting point for dewatering. In the first case, layer mixing can start already at the headbox, while in the latter four new surfaces between air and mix jets are created, all four being potential sources of disturbance generation.

    High consistency headboxes have been developed, with which paper is formed according to an extrusion process. To reduce mix flocculation, various channel shapes are used inside the headboxes.

    Sheet build-up generally takes place according to a filtration process, which has an inherent self healing effect. Therefore, the mass formation of a laboratory sheet is better than that of a random sheet. For a machine-made sheet, the comparatively high mix consistency causes floc generation, which may result in a worse large scale mass formation than that of the random sheet.

    When evaluating the mechanical and optical characteristics of a machine made paper sample, its properties relative to those of a laboratory sheet from the same furnish may be expressed as the Forming Efficiency.

    The Kozeny-Carman equation describing flow through porous beds can not be used to predict filtration dewatering rates during web forming. This is because of the gradual compression of the web by the dewatering forces, the closing of some pores, turbulent flow situations etc. Dewatering capacities must so far be predicted using empirical equations, and parameters evaluated on the basis of dewatering experiments.

    The development of forming wires has led to multi-layer designs where both the paper side and the wear side can be optimized simultaneously.

    Pressure pulses in hydraulic headboxes are detrimental to fourdrinier dewatering, since attenuation due to standing wave generation on the wire can create large MD grammage variations.

    In fourdrinier dewatering, several new dewatering elements have been developed, allowing a better control of the activity in the mix on the wire, and thus also of the mass formation of the web formed.

    Inconventional twin-wire forming,the dewatering pressure is generated by wire tension according to one of two basic principles: roll dewatering with constant or blade dewatering with pulsating dewatering pressure. A combination of these two principles may result in an improved combination of mass formation and retention. Recently a new method for the generation of dewatering pressure has been demonstrated, in which the pressure along the forming zone can be controlled freely, since it is generated by application of local forces and thus not by wire tension.

    Multiply products manufactured through simultaneous forming are now used for low grammage products. The problem is to achieve acceptable layer purity as well as layer mass formation. Controlled pressure pulse dewatering could provide the means to reach optimum dewatering conditions.

    The influence of forming conditons on product properties is a vast area within which two subjects are discussed: the interrelationship between mass formation and paper strength and finally fibre orientation anisotropy.

     

  • 1989
    Cambridge
    pp 67-86The effect of refining on wet fiber flexibility and its relationship to sheet propertiesAbstractPDF

    The wet fiber flexibilities of several softwood and hardwood species were measured by Steadman’s method (5). Softwoods showed a broader range of wet fiber flexibilities than hardwoods. Refining* decreased the spread as measured by the IQR and increased the median values. The wet fiber flexibility and its distribution was sensitive to the type of refiner, beating load and refiner consistency. Refining at high intensity by increasing the refiner load in a Valley Beater, resulted in the production of pulps with inferior strength properties. When the pulps were at constant WFF, those prepared under low intensity exhibited superior tensile strength. The main effect on the fibers beaten at high consistency was to sharply reduce their fiber length. Changing loads in the PFI mill had less of an impact on the WFF and paper properties . Refining with the low load resulted in maximizing fiber flexibility. However, the tensile strength was inferior. The tensile-density relationship did not change as a function of refining load in the PFI mill, indicating that the quality of refining did not change. The WRV was useful in understanding the relationship between fiber and sheet properties.

  • 1989
    Cambridge
    pp 87-104Response of pulp fibres to mechanical treatment during MC fluidizationAbstractPDF

    Medium consistency (MC) unit operations are becoming increasingly widespread in pulping and bleaching processes. Many MC devices, notably MC pumps, mixers andscreens,relyon creation of a fluid-like state in the pulp suspension. This fluidization requires the dissipation of considerable energy and exposes the pulp fibres to mechanical treatment. The treatment alters pulp physical properties.

    The changes in properties of a never-dried semi-bleached softwood kraft pulp with mechanical treatment were determined in a batch operated concentric-cylinder device. A fluid-like state was created in medium and low consistency pulp suspensions with treatment energy varying up to 10 MJ/kg pulp. The treatment energies encompassed the range used in mill and batch laboratory operations and the extensive exposure given during batch MC fluidization studies. MC treatment was found to beat, curl and microcompress the pulp fibres. The implications of MC treatment in mill devices are discussed.

  • 1989
    Cambridge
    pp 105-120Hydration of refined chemical pulps studied by microwave spectrometryAbstractPDF

    A bleached sulfate pulp has been refined in a disk refiner to different degrees : from 15 to 75° SR (720 to 70 CSF) . Different water contents ranging from 40 to 90 % by volume (70-90 % by weight) , have been obtained by centrifugation of small samples of ca.30 mg o.d. weight. The samples have been analyzed by microwave spectrometry performed in a cavity at 5 GHz. The theory of small perturbations of a resonant cavity allows the determination of the dielectric losses Es of the hydrated material. The slope of the lines : Es vs. moisture content is an index of the hydration of pulps. At the same water content (in the range 70 to 90 % by weight) , we have shown that refining increases the hydration, which can be defined as the transformation from free water to bound water.

  • 1989
    Cambridge
    pp 121-152Effects of pulp drying and refining on softwood fibre wall structural organisationsAbstractPDF

    Structural organisations of a range of softwood kraft and other pulps are described in terms of their response to pulp drying and refining. Fibre widths, thicknesses, wall thicknesses, and wall areas, as well as cross-sectional shapes can be very different depending on whether pulps are dried and/or refined. Fibres can also respond to refining in different ways depending on whether the fibres have thin or thick walls.

    Actual cross-sectional wall areas (including void space associated with delaminations) of undried kraft fibres normally remain unchanged with refining whereas those of dried and rewetted fibres increase. For refined fibres, delaminated walls are envisaged as consisting of several concentrically oriented lamellae aggregates or coarse lamellae (containingwall substance plus water) interdispersed with void space filled predominantly with water. Thus, effects of pulp refining could be to make the structural organisation of the coarse lamellae but not the walls of undried kraft fibres more dense, and the walls of dried and rewetted fibres less dense. Furthermore, the thicknesses of uncollapsed, undried fibres normally decrease with pulp refining while those of collapsed, dried and rewetted fibres increase. Finally, cross-sectional shapes and dimensions of dried and rewetted, thick- and thin-walled fibres are modified in different ways and at different rates.

    Wall organisations of kraft, soda-AQ and neutral sulphite-AQ (NSAQ) fibres are very different. Kraft fibre diameters (widths and thicknesses), wall thicknesses, wall areas and hemicellulose contents are substantially lower than those of NSAQ and soda-AQ fibres when compared at either the same lignin content or pulp yield. Effects of refining are to cause kraft fibre diameters to be decreased further, and walls to delaminate and expand into fibre lumens. The unique properties of kraft fibres are explained by a tightening and contraction of the fibrillar and lamella wall organisations.

    Observed changes in fibre dimensions and behaviours are discussed in terms of fibre wall structural organisations, compositions and behaviours .

  • 1989
    Cambridge
    pp 121–152Overview of physical forming – abstract onlyAbstractPDF

    In this overview, forming is defined’more generally to include all processes from thick stock dilution with recirculated white water (mix production), to the final dewatering of the wet web on the wire.

    Grammage variations in the finished paper are generated mainly during the forming process. These variations can be expressed as mass formation in the small scale range and MD-, CD- and residual variance for large scale variations.

    Mass formation can be evaluated off-line using beta radiography, combined with micro densitometry or image analysis . A new technique involving direct re cording of electron beam transmission is under development, with promises of faster processing, perhaps even on-line, and higher geometrical resolution. Characterization techniques based on the co-occurence matrix, especially suitable in image analysis, can be a useful complement to the traditional power spectra.

    It has recently been conclusively demonstrated that in flowing fibre suspensions, flocs are kept together by the bending forces of the fibres involved. To study the dynamic behaviour of flowing fibre suspensions, modern video techniques and image analysis are being applied.

    The build-up of the fibre mat is considered to be a filtration process under normal conditions. This process posesses an inherent “self healing” effect on local grammage variations . The mass formation of a laboratory sheet is for this reason superior to that of a random sheet. The higher mix consistencies used for machine made sheets cause floc generation, and usually results in a worse large scale mass formation than that of the random sheet.

    When evaluating the mechanical and optical characteristics of a machine made paper sample, its properties relative to those of a laboratory sheet formed from the same furnish may be expressed as the Forming Efficiency.

    To improve grammage uniformity, the mix should be fed to the headbox directly after the dilution of thick stock with white water. No processes like screening or cleaning from which uncontrolled reject flows are drawn from the measured fibre flow should be allowed . Further, the material content in the recirculated white water should be controlled to a constant level if the composition of the paper produced is to be held constant.

    In todays headboxes, the tapered manifold is the dominating method of distributing the mix flow into a tube bundle across the entire wire width. The distributing tubes can exhaust either into a stilling chamber, or directly into the outlet nozzle. In the latter case, the tube outlet area must be maximized to avoid excessive wake effects. Further, the nozzle contraction ratio must be large enough to reduce the degree of relative turbulence to an acceptable level.

    There are two basic headbox designs for stratified forming . In one of them, thin, pointed vanes separate the different furnishes. In the other, thicker separation walls generate “air wedges”, which may separate the furnishes all the way to the initial dewatering point . In the first case, layer mixing can begin be-fore dewatering; in the latter case, four new interfaces between air and mix are created, and all are potential sources for disturbance generation.

    Mathematical methods are now being applied to the calculation of water flow patterns in headboxes, which may eventually lead to designs with improved flow stability.

    High consistency headboxes have been developed to form paper according to an extrusion process. To obtain acceptable mass formation, various channel shapes, causing mix deflocculation, are used.

    Because of the complex interactions between dewatering forces, water flow, material movement and resulting web structure and elasticity, no models have so far been developed for the prediction of filtration dewatering rates. The Kozeny-Carman equation, describing flow through porous beds, is a too simplified model to be of any great value in this situation, and therefore empirical equations are applied instead.

    The development of forming wires have lead to multi-layer designs where the topography of the paper and the wear sides can be optimized simultaneously.

    For Fourdrinier dewatering, several new dewatering elements have been introduced, allowing a better control of the activity in the mix on the wire, and thus also of the mass formation.

    Fourdinier dewatering can be especially sensitive to pressure pulses from the hydraulic headboxes since amplification due to standing wave generation on the wire can create considerable MD grammage variations.

    Local slice lip adjustments, especially on hydraulic headboxes which normally have a low conver-gence nozzle, can cause considerable cross flows on the Fourdrinier wire, and this will have a large effect on the grammage profile. Further, local changes in fibre alignment will be generated, a problem which has not yet been given due consideration.

    In twin wire forming, the dewatering pressure is generated by wire tension according to either of two basic principles : roll dewatering with constant; or blade dewatering with pulsating dewatering pressure. A combination of these two principles has resulted in the best mass formation and retention. Recently a new method has been demonstrated, in which the dewatering pressure is not generated by wire tension but instead can be controlled to the desired pressure event along the forming zone.

     

     

  • 1989
    Cambridge
    pp 159–194Rapid imaging of mass distribution in paper by electron beamsAbstractPDF

    A rapid method to measure the distribution of mass density of paper and paper board was developed. Paper is irradiated with electron beams with energies from 50 to 200 keV and the attenuation of the electron beams by the paper is detected by a photographic film and a real-time TV monitor system. The image of the attenuation of the electron beams (electrograph) is converted into the distribution of mass density through a basis weight calibration. By this method, the basis weight distribution of paper and paper board from 0 to 400 g/m2 was obtained in less than 1 second with both detectors. It was found that the electrograph gives very similar information as beta-radiograph. Experiments to optimize the imaging of the distribution of mass density of paper by electron beams are described along with the relevant physics.

  • 1989
    Cambridge
    pp 211-218The physical properties of paper in relation to its micro- and macrostructure – prepared contributionAbstractPDF

    The properties of paper are defined on two sampling scales, depending on whether the micro- or the macroscopic physical characteristics of the sheet are of interest. Two models are presented for the structure of homogenized paper, based either on the orientational distribution of the fibres or on that of the interfaces of pores and fibres . Orientational anisotropy arises on the scale of the fibre aggregates through shearing between flocs. Accurate tests were performed of the floc anisotropy, which influences the properties of paper on the size scale of a few mm2.

  • 1989
    Cambridge
    pp 195–218Scaling, percolation and network theories: new insights into papermaking?AbstractPDF

    Recent advances in the theory of condensed matter physics have furnished us with powerful theoretical methods for understanding the structural and dynamical properties of in homogeneous materials, such as the fibre network which constitutes paper. We discuss the concepts of universality and scaling, on which all the new theoretical arguments are based. In understanding the properties of forming and of paper, the percolation transition, i.e. the unique state where an infinitely connected network of fibres is formed, is of particular interest. The physical aspects and the power of the percolation theory are discussed in the presentation. Applications of these new concepts to paper have so far been few.

    We show how the percolation theory yields practical qualitative results explaining the relationship between the consistency of the suspension in the headbox and the formation. The complicated structure of the turbulence on the wire and on the jet, which manifests itself in the residual variations of basis weight, is discussed using some scaling ideas related to the universality of the nonlinear dynamical systems. The effect of formation on the mechanical properties, especially ultimate strength, can be viewed theoretically with the aid of scaling, percolation and network theories. The reinforcing effect of chemical fibres in newsprint can be judged by a rather simple scaling argument,which,when developed further, gives insight into the nonlinear relationship between strength and the amount of chemical fibres in newsprint.

    Scaling and percolation are qualitative methods. When studied and applied properly, these concepts help us get a picture of the in homogeneous materials and understand the basic principles behind their properties. It is then easier to decide on the right quantities to measure when quantitative information is needed in papermaking.

  • 1989
    Cambridge
    pp 219–249Applications of flow visualization technique in wet end chemistryAbstractPDF

    A microcomputer–aided flow visualization system consisting of Strobovision analyzer(SVA) and image processing apparatus was applied to the analysis of the effect of a variety of parameters in wet end chemistry on the flocculation behavior of model stock systems. These are (a)single polymer addition systems and dual component additive systems,(b)low and high filler content suspensions,and (c)colloidal force and hydrodynamic shear force.

    Analysis by digital geometry was carried out to represent features of projected images of flocs and fiber network supporting flocs. The flocculation states were represented numerically. It was attempted to test visually and numerically a flocculation hypothesis in a dual component additive system.

    Within the scope of the present investigation,there is some limitations in that this system can be applied only for dilute suspensions with concentrations far lower than those of conventional paper stock in a headbox.

  • 1989
    Cambridge
    pp 251-274A concept for fibre flocculation in turbulent flowAbstractPDF

    A concept for the fibre flocculation process in turbulent flow is presented including both rupture and aggregation of fibre flocs. Based on results from the literature, a mechanism is proposed for the breaking-up and the building-up of flocs in interaction with turbulent eddies of different length scales in the turbulence spectrum. The mechanism has been formalized into a two-way hierachical concept with analogy to the decay of a scalar fluctuation. 

    Deformation and break-up of flocs by turbulent eddies with a length scale similar to the floc scale dominates the rupture process. Eddies of a smaller scale agitate the floc causing weakening and activation of the network. Floc aggregation happens due to floc collisions when the flocs are transported by turbulent eddies of a larger scale. The smaller scales activate the floc surface, and increase the probability of forming a floc. Erosion and deposition of single fibres takes place in the turbulent fine structure. 

    This concept finds support when tested and compared with the results in the literature. Experiments have been conducted in a vertical pipe flow with a varying flow rate, fibre length and consistency, taking measurements at various positions downstream of an orifice . Results were obtained using high speed movie film, image analysis of still photos and measurements of flocculation and turbulence spectra using a laser . Increasing the turbulent kinetic energy or decreasing the macro scale gives lower floc intensity at large length scales .

  • 1989
    Cambridge
    pp 275-308Distribution of fibre orientations in paperAbstractPDF

    During web formation, the difference between the speed of the pulp suspension and that of the wire gives rise to a smear force that orients fibres preferentially along the machine direction . This has been known for a long time. It is less clear, however, haw the distribution of fibre orientations is fly related to the conditions of web formation and to the properties of papermaking fibres. We have studied this question using simple theoretical models and experimental data inc1-tiding irk-We analysis results. 

    We find that fibre-to-fibre interactions determinehow much individual fibres are rotated by the oriented shear field. In this way the fibre orientation distribution obtained for a given speed cliff an the pulp properties. On the other hand, the turbulence of the pulp flow seems to be the mechanism through. which the conditions of web formation affect fibre orientation. In particular, we suggest that the on set of hear-induced turbulence in the suspension flow determine the maximum fibre orientation anisotropy that can be achieved. 

    The model calculations show that the distribution of fibre orientations cannot be represented in terns of a simple functional form with only one adjustable parameter. The shape of the distribution depends an the fibre p and especially the wet fibre flexibility. In paper the orientation distribution of curly fibres is quite different from that of straight fibres. On the other hand, no diff in the distribution is observed if long and short fibre are examined separately. 

  • 1989
    Cambridge
    pp 329-330The role of formation on soft nip and hard nip calendering – prepared contributionAbstractPDF

    This contribution is based on the work of Eerd Palokangas, a special student at the Institute of Paper Science and Technology. His investigation was concerned with the effects of formation on hard and soft nip calendering.

    Four levels of formation were produced namely poor, medium, good and excellent for a 45g/m’ newsprint furnish. The first three levels were obtained by varying the formation consistency on a pilot plant former, while the sheets with excellent formation were produced on a Formette Dynamique. The same method of wet pressing and drying was used for each set of sheets, however important differences between the web formed sheets and Formette Dynamique sheets were later discovered.

    The level of formation or type of calendering did not appear to significantly effect sheet roughness at various levels of sheet densification, as measured by Parker Print Surf. Losses in elastic and strength properties were found, and were higher for hard nip calendering. Interestingly, strength losses were higher for sheets having excellent formation when compared with those having poor formation.

    Investigation of this unusual result revealed that the poorly formed sheets produced on the web former, did not have the same initial level of internal stresses (drying stresses) as the sheets with excellent formation. This was determined by rewetting the poorly formed sheets, and drying them under full restraint (as was done with the sheets having excellent formation) . The elastic constants were then measured, and found to be in close agreement with those measured on sheets having excellent formation.

    From this limited study we conclude that: the higher the initial level of internal stresses, the greater the reduction of strength properties will be when the sheet is subjected to calendering. Therefore, attention has to be paid to the initial level of internal stress, when strength properties change as a result of calendering are being investigated.

  • 1989
    Cambridge
    pp 358-363Comments on the effects of chaos on “Controllability”AbstractPDF

    Below are appended some general comments on SESSION 8 “CONTROLLABILITY”, and some implications for the development of control systems, with particular reference to the phenomenon of chaos.

    There are many interesting aspects and the examples of process modelling and identification in the papers of this session. The complexity of the papermaking process and hence the requirements of an effective control system are very clearly illustrated. There is a vast useful literature on control theory and applications.

    The total process performance and its profitability must determine the role of the control system. The subject can be presented as a series of facets developed in an orderly manner. First there are some basic theoretical definitions such as controllability (in its specialised technical meaning), observability, sampled data systems, instability and chaos. After that, there is the process performance which needs to be considered: as a design or static system; as a dynamic operating system; and quality assurance. It is also important to encompass mill wide performance.

  • 1989
    Cambridge
    pp 363-366Ridges in the stock on Fourdrinier wires considered as wave motion – Prepared ContributionAbstractPDF

    NA

  • 1989
    Cambridge
    pp 367-382Product/process analysis as tools for selecting/optimising the papermaking processAbstractPDF

    NA

  • 1989
    Cambridge
    pp 383-388Summing up the symposium presentations.AbstractPDF

    Dr. D.A. Page mentioned in his review of the beating of chemical pulps: “If a unified simple theoretical treatment can be made from a complex phenomenon, it will then have the following three advantages:

    – professors can teach it

    – students can learn it,

    – engineers can practice it”.

    Against this statement I feel the Symposium has been succesful. The visionary evaluation of the opportunities and challenges of the future papermaking by Mr. R.C. Williams started the Symposium with high spirits. His call for increased cooperation between paper companies in supporting the development of new technology warrants full attention.

  • 1989
    Cambridge
    pp 311-412Some fundamental chemical aspects on paper formingAbstractPDF

    The fundamental physico-chemical aspects of retention chemistry are reviewed in the light of basic concepts in colloid chemistry.

    Special emphasis has been paid to the surface chemistry of cellulose and cellulosic materials, their origin of charge, dispersion force interactions as well as the implication of certain aspects of peculiar cellulosic surfaces, e.g. the influence of their porosity on polymer adsorption.

    Charge neutralization, patch flocculation, heterocoagulation, bridging and complex flocculation phenomena are discussed as well as polymer adsorption phenomena at the cellulose/water interface

  • 1989
    Cambridge
    pp 413-435Conformation of adsorbed polymers and flocculation of microcrystalline cellulose and pulp suspensionsAbstractPDF

    This paper consists of three parts. In the first part, the reconformation of polyelectrolytes adsorbed on fiber surfaces has been studied by measuring simultaneously the amount of adsorbed polymer and the number of released counterions. It was found that the time for reconformation from an initial, extended conformation to a more flat conformation was of the order of 60 seconds for a high molecular mass polyacrylamide. In the second and third parts, the flocculation of suspensions of microcrystalline cellulose and pulp fibers has been investigated. The results of these flocculation experiments can be related to the conformation and adsorbed amount of the polyelectrolyte.

  • 1989
    Cambridge
    pp 437-452The use of potentiometric titration and polyelectrolyte titration to measure the surface charge of cellulose fibreAbstractPDF

    The surface charge of cellulosic fibre was determined using two methods . The relative surface charge (PTC) of three bleached pulps was quantified using polyelectrol to titration and compared with the absolute surface charge (So) measured by potentiometric titration. It was established that the polyelectrolyte titration method was a less tedious, but valid method of determining the surface charge of fibre, providing the titration method is standardised and the effect of variables such as suspension, pH, fibre concentration, ionic content and polyelectrolyte molar mass are eliminated. The method was used to construct a charge/pH isotherm of a bleached hardwood fibre similar to those constructed using potentiometric titration. Furthermore, the effect of refining of bleached hardwood and softwood krafts on the PTC was investigated.

  • 1989
    Cambridge
    pp 453-470Exchange of cationic polymers adsorbed on cellulose fibers and on monodisperse polystyrene latexAbstractPDF

    Two series of cationic polyacrylamides (C-PAM) were prepared, with and without fluorescent labelling, one with constant charge density (CD:1.4 meq/g) and different molecular weights (MW:2×104, 4×105 and 8×106) and one with constant molecular weight (4×105) and different charge densities (CD:0.65 meq/g, 1.4 meq/g and 2.5 meq/g) .

    The exchange reactions of these C-PAMs on monodisperse polystyrene latex (PSL) and cellulose fibers have been studied using C-PAMs with the same MW and CD except that the pre adsorbed polymers were fluorescently labelled. The first series of experiments was carried out near the saturation level of the pre-adsorbed polymers. For C-PAMs with the highest MW(8×106) no significant exchange could be detected on PSL or on fibers. When C-PAM with the highest. CD (2.5 meq/g) was used a slight exchange occurred on both PSL and fibers . The exchange became more extensive with decreasing MW and CD. The exchange reactions initially proceeded quite rapidly and then slowed down. The initial exchange was more extensive on cellulose fibers than on PSL, probably because of the lower surface CD of the fibers, but it levelled off more rapidly than on PSL. This difference is most probably an effect of the porous structure of the fibers.

  • 1989
    Cambridge
    pp 471-494Physiocochemical and hydrodynamic aspects of fines and fillers retentionAbstractPDF

    From the results of model studies on well-characterized systems, valuable conclusions can be drawn regarding several phenomena occurring in papermaking suspensions relevant to fines and fillers retention. We have shown that long range hydrodynamic interactions are operating between small particles (such as fillers) and spheroids (such as fibers) subjected to simple shear, preventing small particles from approaching large ones to within distances where colloidal forces become important. We can expect similar effects in papermaking suspensions, resulting in very low efficiencies for the deposition of fillers or fines on fibers. The efficiency can be improved by high molecular weight polymers which adsorb on the particles and can reduce the minimum distance of approach between a filler and a fiber. From model experiments on the deposition of T102 particles on cellophane, it can be concluded that the electrostatic forces also play an important role in fines and fillers retention. Usually no deposition occurs far below a critical deposition concentration (CDC) of electrolyte, slow deposition occurs just before the CDC and fast deposition above the CDC. Charged polymers, such as cationic polyelectrolytes, are most effective in retention because they can reduce the gap between a filler and a fiber during an encounter, and they ensure that electrostatic repulsion is negligible. Resides increasing the efficiency of deposition, retention aids can also increase the bond strength between a filler and a fiber, thus preventing or minimizing the rupture of fiber-filler bonds.

  • 1989
    Cambridge
    pp 495-508Optimization of pigment performance in paperAbstractPDF

    The apparent light-scattering coefficient of a given pigment in filled papers depends on the state of pigment dispersion and the beating level of fibers. This effect is seen on handsheets containing pigments deliberately introduced either as individual particles or as aggregates; well dispersed T102 is optically superior and its contribution to the total light scattering increases with fiber beating. In order to achieve retention of dispersed pigment particles and to avoid their flocculation caused by polymeric retention aid, the pigment, prior to its addition to the fiber suspension, can be treated with a cationic polyelectrolyte. Since pigment introduction results in tensile strength loss, which is most pronounced with dispersed particles, a plot of optical properties versus tensile strength provides a means for evaluating the pigment effectiveness and for comparing different pigments.

  • 1989
    Cambridge
    pp 511-586Wet pressing research in 1989 – an historical perspective, analysis and commentaryAbstractPDF

    There is a large body of literature on wet pressing;almost all of it deals with water removal and much of it is empirical in nature. Though we have been forced to infer what happens inside a roll press nip by making observations from the outside, our qualitative and quantitative knowledge of the water removal process has improved greatly over the years. There have been some direct experimental measurements of several important variables inside the nip(applied pressure, fluid pressure, and mid nip roll separation), but only at the system boundaries (the roll surfaces). Direct data is still lacking on other important variables inside the nip such as localized pressure gradients, sheet thickness and sheet dryness, localized deformation, a good definition of the interfacial region, direct measurements of parameters in the thickness direction,and thermodynamic properties. So far, wet pressing models have had limited success in making a priori predictions of sheet water removal and have not begun to address paper properties, the next major thrust of wet pressing research. We have conjured up a mental picture of wet pressing which seems to fit well the observations made from the outside. It is quite possible that this picture is more inaccurate and incomplete than we imagine, but this state of affairs is actually exciting because it means much remains to be learned about the fundamental mechanisms of wet pressing. In this learning process, paper property development is expected to receive equal,if not greater,attention.

  • 1989
    Cambridge
    pp 587-624Compression behaviour of fibre mats in wet pressingAbstractPDF

    The dynamic compression behaviour of the wet fibre mat is the key factor to understanding the mechanisms of wet pressing. At present this behaviouris imperfectly understood. The main reason for this may be the lack of suitable measuring equipment. To fill this gap, laboratory devices (press simulators) were built at the Finnish Pulp and Paper Research Institute. The simulators permit controlled pressing to be performed within a wide speed range, the fastest pressure rise being equivalent to that encountered in the presses of the fastest paper machines. The web thickness and the hydraulic pressure created in the web during pressing can be measured acturately. Papers with grammages of 50 and 100 g/m2 and with various furnish compositions were tested with these devices.

  • 1989
    Cambridge
    pp 625-636Compression and expansion of CTMP-containing sheets in a wet press nipAbstractPDF

    The expansion in the Z-direction of individual layers in a wet sheet after leaving the nip of a double-felted press nip has been studied. In this investigation, pads of up to five wet sheets of CTMP, bleached kraft or mixtures of the two pulps, were used to simulate multilayer high-grammage sheets such as carton board. Using different lay-ups it was thus possible to study the expansion of both homogeneous and heterogeneous sheets.

    The expansion of 350 g/m2 sheets is illustrated by the air fraction in the wet sheet immediately after the nip. The results show that a sheet of CTMP can expand up to ten times as much as a sheet of bleached kraft. It is also shown that CTMP has the ability to expand even when sandwiched between bleached kraft as middle layer in a stratified sheet.

  • 1989
    Cambridge
    pp 639-678An integrated view of web consolidation processesAbstractPDF

    Historically, paper has been consolidated by wet pressing followed by drying on steam cylinders or by other low intensity evaporative processes. Although these time-tested pro cesses will continue to be the mainstay of papermaking for some time, they are now being challenged by new systems that exploit the interactive effects of increased pressures and temperatures. We want to review our understanding of these hybrid processes and the unprecedented performance potential they offer. At the same time, we want to integrate .them into a common context with the more conventional web consolidation processes. To do this, we define four classes of systems mechanical, thermal, thermal with restraint, and thermomechanical. Each class uses mechanical, thermal, and interactive effects in a unique way to determine water removal rates, energy efficiency, property development potential, and machine size and complexity. In a parallel and unifying fashion, each class also occupies a distinct region on coordinates of specific energy consumption and working temperature. This diagram is offered as a starting point for integrating wet pressing, hot pressing, cylinder drying, press drying, Condebelt drying, impulse drying, and others into four distinct classes of web consolidation systems.

  • 1989
    Cambridge
    pp 679-729The physics of impulse drying: new insights from numerical modellingAbstractPDF

    In order to better understand the physics of impulse drying, two numerical models have been developed to predict the transient heat transfer, vapor pressure development, and vapor liquid flow during impulse drying. The first model, MIPPS-I, examines impulse drying as a moving boundary problem in which a sharp front of steam displaces a saturated liquid phase. While several key insights were obtained with this approach, a comparison of predictions with experimental data suggested that the sharp-interface assumption should be abandoned in favor of a two-phase zone between the dry and saturated regions. A new model, MIPPS-II, was then developed which allows a two-phase zone to develop. Both models use finite-difference forms of the mass, momentum, and energy conservation equations adapted for porous media.

    Analysis of the numerical results in light of experimental data helps clarify some of the transport processes in impulse drying. In particular, it appears that the impulse drying process depends on the continued boiling of liquid near the hot surface with condensation occurring in the cooler, more saturated regions. The process of boiling and condensation is tied to sheet permeability and pore structure. The liquid for sustained boiling is available in saturated dead-end pores or is supplied by capillary flow.

    The numerical results show that the development of an internal vapor zone is critical to several features of the impulse drying process. The pressurized vapor zone enhances water removal through direct displacement and also possibly by reducing or eliminating rewet. Relationships between sheet properties and internal vapor pressure and water removal can now be better understood with the aid of the models.

    Several new pieces of experimental information are also presented which have guided recent model developments and, at the same time, can be interpreted in terms of results from the models. The new experimental data include flash x-ray visualization of interface motion in impulse drying and several measurements of thermal processes in both paper and model fibrous porous media.

  • 1989
    Cambridge
    pp 731-742Through flow across moist and dry paperAbstractPDF

    Pressure drop for air flow through dry and moist paper has previously been expressed in terms of permeability as determined by Darcy’s law, an approximation now demonstrated to be substantially in error at through flow rates relevant to through drying. The more fundamental, non dimensional treatment using, the Reynolds number-friction factor model has never been applied for paper because of the need for characteristic dimension in Reynolds number. The use of various assumptions for this characteristic dimension in terms of permeability, specific surface, and llagen-Poiseuille equivalent capillary diameter are now shown to be in substantial disagreement with the pore structure of paper as examined by scanning electron microscopy.

    A new characteristic dimension for flow through paper has been determined by application of fundamental principles of momentum transport. This characteristic dimension was determined for kraft paper over a wide range of basis weight, 25-250 g/m2 , and over the full range of moisture content from wet to dry. With this characteristic dimension, Reynolds number is rigorously the ratio of the inertial to the viscous contribution to momentum transport. With variation in .moisture content, the value of this characteristic dimension changes between two asymtotic limits which differ by a factor of about 2.5. The limits of these asymptotic regions correspond to known water-fiber relations. The values of the characteristic dimension agree with measurements by scanning electron microscopy.

    A theoretical relationship between Reynolds number and friction factor is shown to fit a set of about 3000 measurements of pressure drop taken with about 150 sheets of kraft paper over a wide range of air through flow rate, paper moisture content and basis weight. This successful treatment, based on momentum transport theory, not only eliminates the need for the Darcy law permeability approximation, which leads to errors up to 600% for through flow rates used industrially in through drying, but also provides the basis for theoretical analysis’of heat and mass transport phenomena during through drying.

     

  • 1989
    Cambridge
    pp 743-781Drying strategies and a new restraint technique to improve cross-directional properties of paperAbstractPDF

    Machine-made papers often have an unsatisfactory cross direction profile with regard to their mechanical properties. The edges often have a lower strength and stiffness than the middle of the web.

    Laboratory studies of the behaviour of paper during drying have shown the potential of improving CD properties without any deterioration in MD properties. This is achieved by preventing paper shrinkage in both directions during drying.

    After extensive trials on a full-size paper machine equipped with a new cross-direction restraining technique, it has been demonstrated that the laboratory results can be repeated in a continuous process. This invention has the potential of solving some old problems in papermaking such as evening out the cross-direction profile and improving strength and stiffness properties, dimensional stability and surface properties.

  • 1989
    Cambridge
    pp 783-830Mechanisms of fibre bond formationAbstractPDF

    The structure of interfibre bonding in hand sheets made of a hardwood BKP was investigated throughout the sheet formation stages using a transmission electron microscope (TEM), a scanning electron microscope (SEM) and a scanning laser microscope (SLM). The sample preparation techniques for TEM and SLM were newly developed for observation of the bond formation and bond structure.

    Observations were made on couched wet webs, pressed wet webs, drying stages of the wet webs and dry sheets . Structures of the bonded zone and bonded fibres were characterized based on these observations. In this paper we discussed how the structures are affected by beating, couching, pressing and drying. We also discussed the roles of these structures in the physical properties of the fibre bonding.

  • 1989
    Cambridge
    pp 833-870Review of surface applicationAbstractPDF

    Surface application has gained more and more popularity as a means of increasing the quality of paper and board. In its broadest sense, surface application covers everything from sur face-sizing to extrusion coating, but this review will concentrate on surface sizing and pigment coating, i.e. processes taking place as an integrated part of the paper manufacture. Because the goals of the surface treatmant are different and depending on the quality of the paper desired, the review will cover our present fundamental understanding of the processes themselves. The presentation will start with a description of the water or liquid penetration in paper. Then the mechanics of various blade coating processes and the liquid transport it blade coaters will be explained. Last will come a presentation of the size press and new equipment for surface sizing.

  • 1989
    Cambridge
    pp 871-893An evaluation of interactions between coating colour and base paper by coating profile analysisAbstractPDF

    Using techniques of coating thickness analysis and frequency transform procedures developed in the EM laboratories it has been possible to describe the basesheet surface profile changes caused by the coating application. Relaxation of the fibre network has been observed and found to depend not only on the base stock furnish, formation etc., but to a large extent on the coating pigment type, size distribution, coating colour rheology and most importantly the dewatering interactions between the coating colour and the base. Factors determining the dewatering characteristics of a coating colour will include the base absorbency, pigment particle packing and suspension fluid viscosity.

  • 1989
    Cambridge
    pp 895-920Influence of paper morphology on short term wetting and sorption phenomenaAbstractPDF

    Lucas Washburn theory has been used for many gears to model the penetration of liquids into paper where the rate of penetration is a function of the balance between surface tension forces and viscous drag. Interfacial contact angle is assumed to be constant and the pore morphology is reduced to an’ equivalent cylindrical pore radius. In reality, the pore morphology in paper is extremely complex. The interactions of liquids with paper are largely determine by local variation in Young-Laplace equilibria. Thus, the rate-determining factors for penetration of liquids in paper may be the distribution of divergence and convergence in pore wall geometry and the presence of discontinuities. In this paper, the penetration of liquids into paper coatings is examined as a function of pore morphology, pigment shape, and packing order. Sorption phenomena such as the “wetting delay” observed in uncoated but sized papers is explained in terms of the difference in morphology of surface and bulk pores. Fractal assembly rules are applied to morphological subunits in pigment coatings such that the rate of liquid penetration in a continuous coating could be predicted.

  • 1989
    Cambridge
    pp 921-950Factors in the blade coating process which influence the coating mass distributionAbstractPDF

    Many printing papers are pigment coated in order to obtain a paper surface with superior printing properties. To achieve this it may be assumed that a coating layer of uniform thickness or mass is extremely important.

    A soft X-ray technique has been developed in order to study the coating mass distribution on coated papers. In order to demonstrate the significance of this technique a wood-free and a wood-containing base paper were coated in a pilot coater. On the wood-containing base paper, blade coating with a flooded nip unit was compared with coating in a short dwell time (SDTA) unit. In the former unit, two blades of the same thickness but with different tip areas were used. The result indicated that the most uniform coating layer was obtained when the coating was performed in the flooded nip coater and with a blade of low tip area. This is interpreted as being due to a high specific load on the base paper when it passes under the blade tip, which smoothes out the irregularities in the base paper surface so that the coating film flowing out under the blade tip and the corresponding coating layer are of uniform thickness or mass.

    On the fine paper the effect of the strategy of drying the coating layer has been investigated. It was found that the coating layer is redistributed during its consolidation process, which is interpreted as being due to movements in the base paper, as a result of the water pick up from the wet coating layer which swells its fibers and break bonds between them.

    The coated fine paper was printed in a sheet-fed offset press and the mottle in the print was evaluated. The correlation between the print mottle and the coating mass distribution was very good.

  • 1989
    Cambridge
    pp 951-975Printing roughness and compressibility: a novel approach based on ink transferAbstractPDF

    A new approach to characterize the roughness of paper in contact with ink under compression in the printing nip is proposed. The printing roughness is calculated from the parameters of the ink coverage function contained in the ink transfer equations. The approach assumes an identity between the ink transfer coverage function and the pore shape function of the surface pores. Although the printing roughness correlates well with standard roughness/porosity tests, different regression lines result from different printing conditions . The printing roughness was found to be inversely linearly related to the logarithm of the printing pressure with the slope of the regression line representing a measure of the compressibility of the paper surface. The compressibility is independent of the printing pressure but, for rough papers, is a function of the nip dwell time.

  • 1989
    Cambridge
    pp 979-1025Recent developments in the technology and understanding of the calendering processesAbstractPDF

    Recent developments in industrial practice are briefly reviewed and then improvements of our understanding of calendering are reviewed under the headings: the assessment of surface properties, the compressibility of paper, the effects of calendering variables on paper properties (for hard (iron) roll calendering, soft roll calendering and temperature or moisture gradient calendering, and the effect of calendering on paper strength) and rolling contact phenomena. The Concluding Comments list the implications of these industrial and scientific developments for future technical development and research into the calendering processes.

    The last 8-9 years have seen further expansions and rationalisations within the paper industry so it is not surprising to read regular reports of new finishing lines on new or modernised machines. Particularly noticeable, however, have been the numerous installations of new supercalenders for LWC grades and the development of competing uncoated filled mechanical printing grades, also supercalendered. Also, there is a trend for makers of standard machine calendered newsprint to produce higher quality mechanical printing grades which require a supercalender finish.

    These developments have increased the already considerable interest in the possibility of reducing the costs of supercalendering compared in detail by Muller & Schmidt with machine calendering and with on-line soft roll calendering, using an 8 roll machine (1) . Although the analysis needs to be updated in the light of experience with modern 2 x 2 roll tandem soft calenders, not then in service, the relative importance of running costs, capital investment and production losses through down time are carefully compared. Supercalendering of newsprint produced at 1000 m/min was estimated to cost DM16/ton compared with about DM4 for conventional machine calendering, before down time losses adjust them to DM25 and 27 respectively. (At that time, the on-line alternative was apparently far more expensive) . The higher selling price of the supercalendered paper was not allowed for.

    Since 1980, there has been a rapid application of on-line soft roll calendering, in place of the conventional machine calendering with hard (iron) rolls. Excluding the gloss calenders developed about 1962, there are probably nearly two hundred installations, which, it may be said, give a supercalender type finish to papers which were usually not finished that way, or which were lightly supercalendered. Obviously, there has been considerable interest in seeing how far this on-line process can go to produce conventional supercalender grades.

    Before reviewing the scientific developments in these operations, it is useful to review briefly the practical improvements which have been made since about the time of the review by Peel, Kerekes and Baumgarten (2) .

  • 1989
    Cambridge
    pp 1027-1054The development of paper gloss in hot calendering and brushingAbstractPDF

    In bleached, white top and coated packaging paper and board, paper gloss is a desirable property, whereas in printing papers print gloss is most desirable. One section of this paper deals with gloss development of uncoated paper and board and another section deals with coated paper. A final section addresses print gloss.

    The enhanced effect of hot calendering on paper gloss and print gloss, as compared to “air leak” smoothness, is illustrated. This enhancement is due to a thermal softening con centrated at the outermost layer of the paper in contact with the highly polished hot steel roll acting preferentially on the topographical irregularities up to a few microns.

    Brushing of coated papers produces some densification of the outermost coating layer, thereby filling the surface voids. Brushing improves both paper gloss and print gloss, as well as the gloss uniformity with no significant effect on the ‘fair leak” smoothness.

  • 1989
    Cambridge
    pp 1055-1075Modelling the state of stress and strain in soft-nip calenderingAbstractPDF

    A brief review of pertinent works in the field of contact mechanics is made, with emphasis placed on works related to the calendering process, starting with Hertz, assumptions and ending with non-Hertzian effects, including thin-layer covered rolls, sliding, sticking, “micro-slip”, and friction effects.

    Emphasis is placed on the effect that changing Poisson’s ratio has on the behavior of stress and strain in the nip and how this is related to soft-nip calendering parameters (the relative rotational velocities of soft-covered and mating rolls, for example).

    High resolution color graphics of the state of strain in the nip zone are presented, as modelled by finite-element analysis of the contact problem in soft-nip calendering.

  • 1989
    Cambridge
    pp 1077-1101The calendering creep equation – a physical modelAbstractPDF

    The modelling of the calendering process has been largely empirical, resulting in “creep” equations which relate the finished paper properties to calender parameters. Such modelling has the utility of optimizing calendering configurations for the attainment of a desired paper finish. This approach demonstrates the limitations of machine calendering and other alternatives to reach higher levels of surface finish are suggested.

    This study endeavours to establish an understanding of the physical basis for the form of the calendering creep equation. A simple physical model of calendering has been developed which allows at least the qualitative prediction of various calendering effects. The physical model of paper compression is based on the non-Hookean behaviour of paper under compression which is known to arise from the statistical distribution of the number of fibers in a paper web. Elastic constants associated with the exponential stress-strain relation for paper determine the dependencies of caliper reduction on moisture, temperature and fiber processing. A simple viscoelastic model suggests that the dependencies on moisture and temperature may not be as autonomous as they appear in the usual forms of the creep equation. This observation is corroborated by experimental data obtained on pilot laboratory calenders.

  • 1989
    Cambridge
    pp 1105-1149Controllability of paper makingAbstractPDF

    This paper discusses the controllability of paper making. We have chosen to take a broad view on the subject . With such an approach, there are four major factors that can contribute to the over-all controllability, viz. : the process with its actuators, the measurement possibilities (sensors), the information technology and, not the least, the humans involved in the paper making operation.

    In the introduction to this paper, we try to put controllability in perspective . In order to make our following discussion less abstract, we then describe three cases related to controll ability in which STFI has been involved. The first deals with CD control ; the second with closed-loop quality control; the third with wet-end/retention control . Following these cases, we then discuss the four above-mentioned components that contribute to controllability and we conclude by indicating where we believe the major development needs and opportunities lie, viz. :

    – we still lack essential controllability due to inadequate process design .

    – new measurement possibilities are essential in order to improve the observability and thereby controllability.

    – the human organization has a key role in controllability . Full automation is utopia. The operators have an increasingly difficult task. Their motivation is important .

    – knowledge-based systems is an interesting area. Such systems can be of great value, but much development work is still needed .

    – it is important to increase the fundamental knowledge of paper making and to establish the knowledge in such a way that it can be used for better control .

  • 1989
    Cambridge
    pp 1151-1181System identification and adaptive control in papermakingAbstractPDF

    The concepts and methodology of system identification and adaptive control in papermaking an presented and discussed. In particular the crucial problems of dead time compensation and proper interpretation of scanning gauge measurements are reviewed. Two new approaches developed recently in our laboratory to overcome these problems are presented.

    The key points in extending the monovariable adaptive control schemes to the multivariable case are discussed. Successful applications of selftuning control to machine and cross-directional control of moisture and basis weight are outlined. The difficulties associated with multivariable adaptive control are illustrated by an example of colour control on a paper machine.

    Finally, novel approaches to adaptive control such as genetic algorithms, neural networks, and multimodel techniques are presented. The promise of these recent advances in adaptive control is illustrated by an example of automatic compensation for species changes in kraft pulping.

  • 1989
    Cambridge
    pp 1183-1207Instability and the papermaking processAbstractPDF

    Mechanisms causing instability and change are commonplace in nature. Similar phenomena exacerbate the tasks of devising, building and maintaining systems for efficiently manufacturing paper to a tight specification and high standard of uniformity.

    In the case of a machine calender stack, CD control can impart stability to an otherwise unstable system. Instability of the headbox or approach flow system can however seriously affect the MD control of basis weight. Some types of instability lie outside the reach of control, and must if possible be eliminated through improved equipment design. For this reason a good understanding of instability is necessary.

    This paper considers some specific examples of unstable behaviour, including eddy and vortex formation, waves and other amplification mechanisms on Fourdrinier wires, corrugation growth, uneven wear, self-excited vibration, and thermal deformation affecting calender stacks.

  • 1993
    Oxford
    pp 1-126Subfracture mechanical propertiesAbstractPDF

    Virtually all paper and board grades have one or more mechanical property specifications. These are typically fracture properties, but the subfracture mechanical properties are also important. In many situations raw material and papermaking variables impact subfracture and fracture properties in similar and predictable ways.

    In this review we discuss the impact of fiber and paper machine variables on the mechanical properties of paper and board up to the point of failure. As with any real material, the physical properties (mechanical, optical, electrical, etc.) are not independent but depend upon the constituents and structure of the material. We discuss these connections where appropriate.

    From a historical perspective, those “turning points” that led us to greater understanding of the mechanical properties are pointed out, as are situations where work is needed.

  • 1993
    Oxford
    pp 127-157Effect of refined softwood: eucalypt pulp mixtures on paper propertiesAbstractPDF

    Fibre property, refining requirement, and handsheet strength and optical property interrelations are examined for a eucalypt and several softwood market kraft pulps and blends. Market krafts included in the study are radiata pine pulps of low and medium coarseness, a benchmark pulp from the interior region of British Columbia, and a eucalypt pulp from Brazil. Eucalypt: softwood blends are in proportions of 100:0, 50:50, 80:20, and 0:100, and effects of separate and co-refining are assessed using -a laboratory scale Escher Wyssre finer which is considered to be indicative of commercial scale refining operations.

    For the softwood pulps, refining at the low 1 Ws/m specific edge load has minimal effects on fibre shortening, fibre collapse, and wall expansion and delamination. Under these conditions fibres are neither rapidly rewetted nor made flexible. The converse occurs with refining at 3 and 5 Ws/m.

    Tensile strengths are relatively high and softwood fibre walls are slow to respond with refining at low specific edge load. Such effects are consistent with the retention of fibre stiffness and length, and the development of high bonding potential. The high bonding potential is presumably developed through selective fibre surface disruption, wetting, and molecular and micro level fibrillation. Light-scattering coefficient/tensile index relations are independent of specific edge load and indicate mutual compensatory responses for these handsheet properties.

    For eucalypt: softwood blend proportions of about 80:20, tear/tensile relationships (reinforcement properties) and light-scattering coefficients (optical properties) are roughly the same and independent of the origin or type of softwood used in the investigation. Such results are to be expected since there are only 2-3% by number of softwood fibres included in the 80:20 eucalypt: softwood furnish blends. For 50:50 eucalypt: softwood blends the effects of using softwoods of different fibre quality are also relatively small.

    With co-refining reinforcement properties are decreased, and optical properties can be increased depending on specific edge load. It is envisaged with co-refining that the small number of softwood fibres present in the 80:20 eucalypt: softwood softwood blends (<3%) receive disproportionate levels of the refining, and tear strengths decrease forgiven tensile strengths and energy inputs. Also, such an explanation is consistent with the possibility that light scattering coefficients can increase with co-refining. Thus, softwood fibres can be expected to be more refined and hardwood fibres less refined for given energy inputs with co-refining than with separate refining before pulp blending.

  • 1993
    Oxford
    pp 159-182Relationships between properties of pulp-fibre and paperAbstractPDF

    In order to produce papers with differing qualities, papermakers use pulps with varying fibre and fines characteristics. Since the early 1900’s, numerous studies have been conducted to investigate the relationships between pulp-fibre and paper properties. However, a common weakness was that previous investigations focused on relationships within restricted ranges of properties of pulp-fibre and paper. Forth is reason,the main goal of this investigation was to examine and to characterize a broad scope of fibre-paper relationships. In order to achieve this goal, a statistical approach was taken. This technique explained much of the total variation in paper properties by investigating a large number of pulp-fibre types, including fines,with a broad range of characteristics. It was discovered that robust and orthogonal non-linear multiple regression models could be developed to predict various paper properties. The multiple curvi-linear regression models reported here (based on the PhD thesis of the first author of this paper) could explain, on the average, 85 ± 10% of the -variance (R2)in the paper properties. Although the models generated are empirical, and thus lack fundamental interpretative meaning, they do clearly rank those fibre properties most important fora given paper property and allow prediction of the “form” of the relationship. These relationships confirm the expectation that there are no universally optimum fibre paper properties. Instead, compromises must be made to achieve an acceptable balance of properties. Such interactions are described in more detail in the paper.

  • 1993
    Oxford
    pp 183-208Forming and formation of paperAbstractPDF

    Formation is a critical property for newsprint as well as printing and writing grades of paper. Measurements of the flow and fibre distribution properties in the forming section of an operating paper machine are related to the resulting paper structure. The 3-D M/K Analyzer is used to characterize the final paper structure in terms of the formation index and average floc size. A stochastic model is proposed to provide a basis for interpretation of the final paper formation.

  • 1993
    Oxford
    pp 209-236Surface characterization of communication papersAbstractPDF

    The characterization of surface properties of communication papers reflects both handling and printing requirements . Test methods recently applied in this area range from simple adhesive strength characterization and liquid penetration tests to more sophisticated gas chromatographic and spectroscopic procedures suitable for thermodynamic and chemical constituent analysis . The principles and results of such testing are reviewed .

  • 1993
    Oxford
    pp 237-262Finite element analysis of paper cocklingAbstractPDF

    A cockled paper sheet has lost its planarity because small (5 – 50 mm) randomly spaced areas have bent out of the plane of the paper. The variation in surface height is usually only of the order of one millimeter. Cockling is related to the hygroexpansivity and small-scale inhomogeneity of paper, and is a problem mainly with some lightweight papers and with copying papers.

    In this study, cockling is analysed theoretically using the finite element method. The results suggest that cockling is caused by local inhomogeneity in the two-sidedness of paper. Small-scale variation in the fiber orientation angle is especially detrimental in this respect. Large variations in in-plane contraction can also cause paper to buckle. The intensity of cockling increases with decreasing paper thickness. The cockles become more oval as the fibre orientation gets stronger.

    The conclusions of the theoretical analysis are supported by experiment. With high basis weight papers, the local fibre orientation of the top side was found to be almost independent of that of the bottom side. Thus, two-sidedness varies considerably on the small length scale, and this allows local curling to take place. At low basis weights the top and bottom sides are no longer independent, which offsets the amplifying effect of smaller thickness.

  • 1993
    Oxford
    pp 263-284The specular reflection of polarised light from coated paperAbstractPDF

    An experimental examination of the reflection of polarised light (s &p;k= 633 nm)from surfaces which have a significant specular component is presented. The reflected light intensity is measured using a detector with a narrow acceptance angle (0.50) and it is found, for a number of different surfaces based on coated paper, that as the detector scans through the angular distribution, the ratio of polarised light intensities (R~RS) changes in a systematic way which can be accurately predicted if the surface is modelled as an array of mirror-like facets each with the same refractive index, distributed in and around the mean plane ofthe coating. The change in the ratio R~RSwith detector position is independent of the angular distribution of the facets and depends on lyon the refractive index of the surface.

  • 1993
    Oxford
    pp 285-310Fibre collapse and sheet structureAbstractPDF

    A stylus profilometer has been used to evaluate the effect of wet pressing on the collapse of individual fibres from mechanical, chemimechanical and chemical pulps. The chemimechanical and chemical pulp fibres begin to collapse at low pressures and approach complete collapse at high pressures, while the mechanical pulp fibres do not exceed 80% collapse to 5000 kPa. The degree of collapse of southern pine TMP at a given pressure is about the same as that of northeastern spruce/balsam TMP. Since the thick-walled Southern pine fibres are less flexible, it is concluded that transverse collapsibility and flexibility are two independent fibre properties.

    On the basis of modelling results and the difference between fibre thickness-measurement from networks pressed in contact with smooth and with rough surfaces, it is suggested that wet pressure transferred locally at fibre contacts within a sheet leads to local collapse forces higher- than expected from nominal wet pressure vlues. Fibre contacts are initially present in the unpressed sheet.On pressing, free fibre segments will be deflected into contact with other segments above or below them, producing additional fibre crossings as pressing progresses . The effective pressing pressure will be highest at the initial fibre contacts, decreasing to zero at contacts just formed at the end of pressing. Because of the demonstrated effect of wet pressure on fibre collapse, it is the thickness at the initial fibre contacts or those formed early in the pressing process that is important in determining sheet density.

    Use of the more appropriate fibre thicknesses, substantially improves the prediction of sheet density by the Interactive Multiplanar Model of sheet structure for a range of pulp types.

  • 1993
    Oxford
    pp 311-349Characterization of paper as an information carrierAbstractPDF

    Printing on a paper carrier falls into the generic class of’ visual communication processes. This paper aims to provide a systematic overview, based on the literature, on current knowledge concerning the fundamental factors related to paper as an information carrier.

    Published work on informational aspects of imaging can be found in diverse fields. Yet it seems that research specifically from the starting point of paper and printing is needed if future advances are to be made in our understanding of how paper properties contribute to visually perceived information.

  • 1993
    Oxford
    pp 351-365Paper and print noise as limiting factors of information capacityAbstractPDF

    Print noise and its effect on information capacity and visual image quality are analyzed. Noise in prints originates from the signal (immaterial image information), the printing process and the materials. The frequency bands and orders of magnitude of the noise associated with the different sources are discussed with emphasis on general principles and limits in off set printing.

  • 1993
    Oxford
    pp 369-430Response of paper properties to changes in moisture content and temperatureAbstractPDF

    During production, converting and usage, paper and board products may be exposed to environmental conditions of both constant and variable nature. As these paper and board products are ultimately composites of the natural polymers cellulose, hemicelluloses and lignin, they are, due to their hydrophilic nature and also to the existence of thermal transitions, highly influenced by the surrounding environment. The paper properties are accordingly affected by moisture and temperature and it is these changes of a physical nature that are discussed in this paper.

    In this paper, an attempt is made to present a philosophy of how the total performance of paper products can be reduced to the question of how moisture and temperature interact with wood polymers on a molecular level. The multicomponent nature of the wood fibre and the consequences of the build up of dried-in stresses are particularly emphasised. Thus the sorptive properties of the wood polymers are described from the standpoint of molecular interaction and a plasticizing effect which reduces the glass transition temperatures of the wood polymers. The consequences that this softening have on mechanical properties and on hygroexpansivity are presented together with descriptions of the effect of drying stresses and the creep behaviour during moisture cycling.

    Practical examples are given, discussing high temperature processes such as hot calendering, corrugating and press drying. Effects related to printing and converting operations, such as dimensional stability, surface roughening and linting are also commented on. Finally some ideas regarding future research are presented.

  • 1993
    Oxford
    pp 431-497Odour and taste originating from food packaging boardAbstractPDF

    Four commercial grades of food packaging board were analysed by chromatographic techniques and sensory evaluation. Attention was paid to providing as similar conditions as possible for the sample preparation and sample pre-treatment in the two types of analysis. A mathematical function for the dependence of sensory data on instrumental data with a prediction error of 12.3 % was found. Hexanal and pentanal were to a significant degree responsible for the odour impact perceived from the board. The developed method has been used to estimate the odour level in boards with fibres from various sources.

  • 1993
    Oxford
    pp 499-519Optimal conditions for laminating and adhesion of wax-based hot melt coatings on paperAbstractPDF

    Wax based hot melts used for food packaging are blends of paraffin waxes, microcrystalline waxes and low density polyethylene whose characteristics are adapted to each specific application. The results of differential scanning calorimetry analysis at different cooling rates show that the crystallisation temperature of the polyethylene is a linear function of the logarithm of the rate of cooling. By extrapolating these results to very much greater cooling rates, we have assumed the existence of a critical rate of cooling for which the polyethylene and the paraffin wax crystallise simultaneously. By using finite element methods we have produced a heat transfer model that permits the rate of cooling at all points in the paper hot melt structure to be predicted throughout a manufacturing process

  • 1993
    Oxford
    pp 521-545Physical aging and glass transition: effects on the mechanical properties of paper and boardAbstractPDF

    Physical aging is a general process appearing in all amorphous or partially amorphous materials below the glass transition and is a fundamental characteristic of the glassy state. It is shown here that paper is no exception and the same behaviour is observable as in other materials. The aging process represents the slow movement of a glassy molecular network towards equilibrium and displays the ‘universal’ characteristics of strong age-dependence of some properties, scaling behaviour, reversibility, and relaxations which follow a stretched exponential or Kohlrausch law.

    Physical aging in paper is readily reversed by moisture sorption, which lends the aging effect particular practical importance. Small-strain mechanical viscoelastic properties such as creep and stress relaxation rates are strongly affected by age; this has very significant implications for the performance of paper webs or structures subject to short or long term endurance loading. A measurable but less important effect is also demonstrated for a very large-strain property, CD Ring Crush. The rate at which moisture is sorbed is strongly age dependent. Attempts to measure the glass-transition temperature in paper, and its dependence on moisture content, by dynamic mechanical thermal analysis are described; the results were inconclusive.

  • 1993
    Oxford
    pp 547-574Mechano-sorptive creep of paper – influence of drying restraint and fibre orientationAbstractPDF

    A four-point bending stiffness tester was used to study the creep behaviour of corrugated board panels under constant and cyclic relative humidity. By substituting one linerboard in a single-wall corrugated board with steel ribbons, creep tests on linerboard under compression and under tension were also carried out. The results were evaluated using is ochronous stress-straincurves relating to both constant and cyclic relative humidities. The is ochronous curves showed a straight line relationship at low stress levels, for both constant and cyclic relative humidities. This means that the linerboard could be described as a linear viscoelastic material in both tension and compression at constant relative humidity.

    The initial slope of the is ochronous curves, the creep stiffness index, was lower under the cyclic relative humidity than at the constant relative humidity equal to the highest used in the cyclic creep experiments, a phenomenon called “mechano-sorptive creep”. It is worth noting that, as opposed to many other investigations, mechano-sorptive creep was demonstrated for paper subjected to comparatively low strain levels. No difference was observed between creep stiffness indices in tension and compression under a constant climate, but under a cyclic relative humidity the compression creep stiffness index was foundtobeloweri.e.thecreepratewashigherincompression.Afreelydried paper was found to have a lower compression creep stiffness index both at constant and under cyclic relative humidity than a paper dried under restraint. In machine-made sheets, the compression creep stiffness index was higher in the MD than in the CD both at constant and under cyclic relative humidities. For panels of corrugated board, isochronous bending moment-curvature curves were used to describe creep behaviour under pure bending. The results are discussed in relation to a model previously proposed (1)to describe the cyclic creep behaviour of paper.

  • 1993
    Oxford
    pp 575-599Analysis of the strain field in a twisted sandwich panel with application to determining the shear stiffness of corrugated fibreboardAbstractPDF

    Accurate measurement of the shear stiffness of corrugated fibreboard and other high performance sandwich panels is difficult because most loading strategies required to introduce shear stresses also introduce secondary effects, which complicate the measurement.

    This paper analysis the strain field introduced by twisting a strip of material and demonstrates a simple relationship for accurately determining the shear stiffness of corrugated fibreboard and other high performance sandwich panels.

  • 1993
    Oxford
    pp 601-637The edgewise compression creep of paperboard: new principles of evaluationAbstractPDF

    A suggested method to describe the creep behaviour of paperboard in edgewise compression works for paperboard in the same way as for other polymers. The relation between stress, strain and time maybe determined by a simple equation involving two factors, a power function of time and a factor describing the non-linear behaviour of the stress-strain curve.

    For engineering purposes, it is an advantage to be able to design a given product in terms of stress and strain in isochronous curves. In some applications the critical design criteria maybe governed by a critical strain. For such applications isometric curves, i.e. stress versus time at different strains, are used.

    The strain at break in compression of the investigated paperboards was found to be independent of time. The strain to break in creep was equal to the strain at break for stress-strain testing under strain control. By inserting the strain at break in creep in the creep equation, the isometric curve at break could be derived which by definition gives the relation between stress and lifetime. The data in this investigation indicate that a rather small number of specimens and a reasonably short creep time are sufficient to provide a good prediction of the long-term creep behaviour. In other words, the creep of paper follows the fundamental behaviour of the material already established at short times, and described by the creep equation.

  • 1993
    Oxford
    pp 641-725Strength and fracture of paperAbstractPDF

    In spite of forty years of research, it is still unclear how the mechanical properties of paper, particularly strength, depend on the disordered geometry of the fibrous network. Most of our understanding of the fracture phenomena of paper is based on illustrative microscopic observations. Theoretical models have traditionally been focused on the behaviour of a typical element in the network. However, the failure process seldom starts from, or proceeds through, “typical” elements. Instead, the statistical distribution of local failures is crucial for the strength of paper. With the ever more powerful computers it is now possible to simulate numerically the behaviour of disordered systems such as paper. I believe that computer simulations, in combination with new measurements and effective data analysis will lead to a better understanding and more accurate microscopic characterization of the strength and fracture properties of paper.

  • 1993
    Oxford
    pp 727-769Effect of fines on the properties of fibre networksAbstractPDF

    The fines of mechanical and chemical pulps have a character very distinct from their respective fibre fractions. The fines can be regarded as a special furnish component and the optimum quality and percentage of fines in the sheet depends on the properties of the fibre fraction used. An addition of suitable fines can improve remarkably the properties of a printing paper.

    This paper deals with the structural, optical and strength properties of fibre networks, as influenced by fibre coarseness and the introduction of different types of fines. The response of these networks to calendering is also studied. Fibre fractions of different coarseness and lignin content are used. Confocal laser scanning microscopy (CLSM) is used to determine the changes in the structure and microscopic roughness of the sheet.

  • 1993
    Oxford
    pp 771-782Effect of fibre bonding on the tensile strength of printing papersAbstractPDF

    Printing papers contain both chemical pulp and mechanical pulp. Chemical pulp fibres are commonly used as reinforcement, while mechanical pulp maintains opacity and printability. Modelling was performed to provide hypothetical interrelationships between strength and bonding capacity of the fibres. It is suggested that the paper strength, within certain limits, is described by the number and type of bonds occurring in the fibre network. The number of possible bonds was determined as the optically active total area of the fibres, i.e. the light scattering coefficient. Actual bonds occur only when potential bonds are accessible, and accessibility should be improved with increased fibre flexibility and compressibility. It implies that paper sheet density has to be introduced into the model along with the light scattering coefficient. In addition, the tensile strength of the paper is supposed to depend on rheological conditions, i.e. shearing speed that is determined as the tensile speed, and viscoelasticity of the paper describing the type of bonds.

  • 1993
    Oxford
    pp 783-806The J-integral as a parameter for characterizing the fracture toughness of paperAbstractPDF

    In this study, the methods used by both industry and paper physicists for evaluating paper toughness (resistance to breakage) are critically reviewed, and a new method for determining the critical value of the J-integral, J, is presented. Difficulties arising from the use of tensile strength and tensile energy absorption in the evaluation of paper toughness are highlighted. J,, values obtained with the Leibowitz non-linear technique were relatively close to those obtained with a new method developed by the authors when the latter was used in conjunction with key-curve analysis for determining the critical point. This result suggests that the Liebowitz non-linear technique may give a relatively accurate J integral value at the crack initiation point with less experimental effort. Experimental results show that it may also be possible to use notched specimens in the standard tensile testing configuration for J-integral estimation, which would be an attractive method for industry.

  • 1993
    Oxford
    pp 807-823Fracture toughness as a pulp characterization method for runnability assessment of paperAbstractPDF

    For printing paper grades, runnability in the paper machine and in the printing press is partly attributed to the ability of the paper to tolerate flaws and defects in the paper. In these operations the paper fails due to forces applied in the plane of the sheet. It is important for the papermaker to have access to a relevant test method which can characterize those pulp properties applicable to this type of failure. Papermakers knew as early as the 1920’s that the strength of a cracked paper was a unique and useful paper property. This lead to the development of the out-of-plane tear strength test method. The tear test principle has, however, been criticized for many reasons. The most severe criticism lies in the mode in which the paper fails. The mode of fracture in most production and processing operations is seldom an out of-plane tearing, Mode III, but rather an in-plane crack propagation, Mode I.

  • 1993
    Oxford
    pp 825-847Observation of deforming and fracturing processes of paper by use of thermographyAbstractPDF

    Thermography has been found to be useful for detecting the local variations in temperature of paper sheet under strain. The changes in temperature images during the course of tensile straining could describe the local deforming and fracturing process of paper. A thermal deformation pattern of well formed paper was locally uneven, but uniform wholly throughout the specimen. On the other hand, the deformation pattern of poorly formed paper was uneven in micro and macro scale at the early stage of its plastic deformation region. However, the deformation pattern was fairly uniform through the specimen at the later stage of its plastic deformation.

  • 1993
    Oxford
    pp 849-850Effect of fines on the properties of fibre networks – colour pictures onlyAbstractPDF

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  • 1993
    Oxford
    pp 851-852Relative flow porosity in paper – colour pictures onlyAbstractPDF

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  • 1993
    Oxford
    pp 853Converting challenges to paper and board – title onlyAbstractPDF

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  • 1993
    Oxford
    pp 855-881The science of winding paper rollsAbstractPDF

    In many cases paper products are of a wound roll format. The wound roll product must have integrity such that it does not slip internally or telescope in printing or other converting operations or in the hands of a consumer if the wound roll is the final format of the product. Winding models which predict internal stresses within wound rolls begun development over twenty’ years ago. The purpose of this paper is to (1) show how the models can be used to insure roll integrity and (2) show how paper properties can affect the integrity of the wound roll and (3) show recent developments in wound roll models.

  • 1993
    Oxford
    pp 885-911On the interaction of liquids with paper under dynamic conditionsAbstractPDF

    Some recent advances in understanding the dynamic interactions of liquids with solid surfaces are reviewed. Specifically, acid-base theory is applied to wetting and adhesion, the meaning of precursor films for spreading is examined, the role of pore morphology in capillary penetration is explored theoretically and experimentally, adsorption and spreading of surfactant solutions is compared to that of aqueous alcohol solutions, three dimensional pressure penetration models are described along with new methods for characterizing penetration and pore size distribution, and means of reducing fiber swelling and the hygroreactivity of paper and board is discussed.

  • 1993
    Oxford
    pp 913-934A liquid absorption modelAbstractPDF

    The tortuosity factor is given as the ratio of the theoretical absorption coefficient calculated from the cylindrical model to the observed absorption coefficient by the Bristow method. A new formula to give mean pore radius of paper is derived by taking account of the liquid absorption into every pore size according to the Lucas-Washburn equation.

    The liquid absorption model is proposed to allow calculation of transferred liquid volume to paper in an arbitrary time from initial absorption through saturation. The results of the transfer tests of ethyl alcohol to four kinds of papers by the Bristow method show good agreement between the observed and calculated liquid volumes transferred to the papers.

    For other non-aqueous liquids which show the same V(t) vs. (7L tlrqY/2 relationship in the liquid transfer tests to a particular paper as ethyl alcohol, it is assumed that the liquid volume transferred to the paper in an arbitrary time can be estimated by means of the model.

  • 1993
    Oxford
    pp 935-968Relative flow porosity in paperAbstractPDF

    Relative flow porosity is defined as the fraction of the void volume in a porous medium through which fluid can flow under a macroscopic pressure gradient. In saturated paper, much of the void volume is occupied by water that cannot flow due to chemical or physical absorption and mechanical obstruction (isolated or dead-end pores). In partially saturated paper, surface tension effects further hinder fluid flow through the sheet. In this paper, we discuss various methods for examining relative flow porosity and present results of new experimental techniques based on in-plane flow measurements. The experimental approach involves radially injecting known volumes of aqueous, non-absorbing dye into the center of a compressed, saturated sheet restrained by solid surfaces. The volume of the sheet occupied by the dye is measured, as is the total porosity of the sheet. The ratio of injected dye volume to pore volume within the dyed region is an estimate of effective porosity.

    We show that in unrefined, filler-free paper, effective porosity values are on the order of 40% or more. The relative porosity may be as high as 90% of the extra fiber pore volume. Data for both initially dry and initially saturated sheets are presented. A geometric theory exists to predict relative porosity in fibrous structures, but we find that this model predicts values for relative porosity much lower than we observe here. Using simple measures of the volume occupied by the swollen-fibers in a compressed mat, we find that most(on the order of 90%) of the extra fiber pores pace is available to flow.

  • 1993
    Oxford
    pp 969-1021Surface force measurement in papermaking systemsAbstractPDF

    The surface force technique, whereby the forces acting between two solid surfaces immersed in liquids or in adhesive contact are directly measured, represents a novel approach for both fundamental and application-oriented studies of the surface and colloid science of papermaking. The nature and measurement of surface forces are briefly discussed, and some results reported for mica surfaces are reviewed in order to illustrate the surface chemical information obtainable using a conventional Israelachvili-type surface force apparatus. In the case of cellulose surfaces immersed in water and aqueous electrolyte solutions the measured force vs. distance profile is characterized by three regimes. Significantly, conventional DLVO theory cannot explain the interaction forces measured between cellulose surfaces. Electrostatic double-layer forces, as anticipated, dominate the long-range interactions. However, as the two cellulose surfaces begin to “contact” each other, there is an interplay of steric and electrostatic forces due to dangling tails of cellulose chains. The observed force curves, therefore, are interpreted in terms of a new model — the “dangling tail” model — of the cellulose surface, namely, the water-swollen cellulose surface has long and weakly charged cellulose chains or “molecularfibrils” which extend into the aqueous solution. In addition, the application of the surface force technique to basic problems in the adsorption of polymers, both cationic polyelectrolytes and hemicelluloses, and the colloidal stability of kaolin suspensions is illustrated. The advantages of using a new type of surface force apparatus in future studies of surface and physicochemical phenomena relevant to paper manufacturing, coating and recycling are also briefly discussed.

  • 1993
    Oxford
    pp 1023-1044An evaluation of the factors affecting perforation qualityAbstractPDF

    Samples of two types of carton board, a coated white line chip (WLC) and a folding boxboard (FBB), were perforated using an experimental cutting forme. Variations such as depth of cut and rule condition were introduced. These samples were then torn using an Elmendorf tear tester and assessed for their mode of failure. The results indicate that the detrimental effects of worn rules and poor cutting depth can be magnified by the size and relationship of certain board properties, particularly tear and plybond strength. The test results and micrographs suggest an unaided visual examination of the perforated line is insufficient to guarantee clean tearing, particularly when worn rules have been used.

    The effect of the pronounced directionality in the WLC has been demonstrated with regards to the perforation performance. The results have also indicated that plybond/tear ratio is significant in the mode of failure. The usefulness of transmitted light coupled with magnification has been shown with respect to the examination of perforations for quality control purposes. This holds possibilities within QCasa method of assessment.

  • 1993
    Oxford
    pp 1045-1072Wet reinforcing of paper and board by novel crosslinking chemicalsAbstractPDF

    Among the most extensively used chemicals in the paper industry are wet strength resins. They enhance the performance of paper products ranging from tissue to board. Environmental concerns are questioning the use of several of these additives and a strong effort is being made to replace these chemicals with benign ones. This paper reports one such study. Highly water soluble polyfunctional carboxylic acids
    succinic acid, citric acid, tricarballylic acid, and 1,2,3,4-butanetetracarboxylic acid (BTCA) when applied in a 1% solution to paper, dried, and heated (120 – 150 °C) with a catalyst develop wet strength as high as 55%. The effectiveness of these acids is in the ordertetrafunctionat>trifunctional >difunctional. This sequence is a reflection of their ability to form multiple anhydrides. Additional experiments indicate that the wet strength is the result of crosslinking of the hydroxyl groups in cellulose. The wet reinforced papers are easily dispersed under alkaline conditions. The crosslinking of the papers results in an increased dimensional stability.

    Most wet strength chemicals are added at the wet-end where they are substantively adsorbed on the pulp and cured during the drying process. In contrast, the chemicals used in this study are water soluble and therefore are applied to the paper by a saturation technique. Additional research will focus on the optimization of their delivery.

  • 1993
    Oxford
    pp 1073-1100Finite element modeling of the embossing of paper sheetsAbstractPDF

    Embossing processes are often used to increase the surface area of low basis weight papers giving them better absorbency properties. They also serve to add surface texture and increase ply bonding in multi-layer papers. The mechanical properties (e.g., stiffness and strength)of paper sheets are significantly effected in a detrimental manner by embossing. In this work, an initial study of the paper embossing process has been performed using finite element analysis. The paper was modeled as a nonlinear inelastic orthotropic solid and the rubber was taken to be an incompressible hyperelastic material. Interface elements were introduced to simulate the behavior of the contact surfaces. Since the measured mechanical properties of the paper under consideration were highly dependent on moisture content, analyses were performed at relative humidity levels of 50% and 90%. Permanent deformations, and residual stresses and strains in embossed paper sheets have been calculated. Also, the changes in the mechanical behavior of paper sheets due to embossing have been predicted.

  • 1993
    Oxford
    pp 1101-1138A new interfibre system for paper involving zwitterionsAbstractPDF

    This paper examines the effects of the natural ability of the fibres to bond and the introduction of moieties able to provide balanced ionic interactions, on the dry and wet mechanical properties of paper. Thus, the effects of the introduction of zwitterionic moieties (i.e. amino acids coupled with s-triazines) onto the surface of the fibers and their interaction are interpreted in the light of accepted theories of dry and wet paper strength and the topology of the location of the zwitterionic sites. The tensile strength and other mechanical properties of paper made with the zwitterionic fibers are reported. Paper made from zwitterionic fibers retain more than 35% of their dry strength after being immersed in water for weeks. The importance of these results is considerable when it is realized that no wet strength resins are being used. The benefits of zwitterionic bonding in papermaking are briefly discussed.

  • 1993
    Oxford
    pp 1141-1233Recycle, reuse and disposal of paper and boardAbstractPDF

    The current emphasis on the environment and its impact on recycling is placed into context with an introduction of how life cycle analysis and assessment techniques will play an increasing role in determining the direction of industrial development and legislation.

    The review is based around the subjects identified by European Paper industry experts on recycling and recycling research as being the most critical to the long term development of the industry. These are the characterisation of the changes which occur to fibres and their surfaces on recycling; the influence of chemicals on water quality and production efficiency; the fundamentals of flotation deinking and recycling models. In addition a section on alternative uses for waste paper is included.

    Recent reviews on the effects of recycling on paper properties are summarised and an explanation for the apparent anomalies and differences given. Results on post paper manufacturing treatment on recycle potential are also included.

    The information on the effect of chemicals on recycling is sparse and tends to be empirical. The greatest effects are o’n paper strength and brightness where bleaches or brightening agents are used. The chemicals which dissolve also have a detrimental effect on process additive efficiency and add to effluent treatment and discharge costs.

    The deinking section is subdivided into – a brief review of the chemistries and drying methods of inks and toners and their influence on particle size on repulping; the chemistry of the repulping process; the flotation process. The influences of chemicals on flotation and the fundamental phenomena which occur during the process are described.

    Models for assessing age distribution and product properties at various levels of recycling are explored. In addition the build up of materials within a recycling loop is examined along with the energy balance of recycling compared with manufacture from virgin materials.

    Alternative uses of wastepaper considered include energy generation, composting and as a raw material for a variety of applications. None of these are likely to become major uses, competitive with wastepaper used for paper and board production in the near future.

    It is concluded that there are various studies of a fundamental nature which need to be completed to advance the technology of recycling. Greater emphasis on the supply side of wastepaper would also yield industrial benefit.

  • 1993
    Oxford
    pp 1235-1260The mechanism of hornification of wood pulpsAbstractPDF

    Hornification is the loss of swelling of the fibre wall resulting from a drying-and-rewetting cycle: it is associated with a stiffening of the fibres which reduces their ability to form inter-fibre bonds. In this paper,factors affecting the extent of hornification are examined using the solute exclusion technique to measure swelling. Two major observations are that hornification is a feature of low-yield pulps and that it is primarily brought about by the removal of water from fibre walls rather than anyassociated heattreatments. The mechanism is proposed to be an increase in the degree of cross-linking between microfibrils due to additional hydrogen bonds formed during drying and not broken during rewetting. In high-yieldpulps, the presence of lignin and hemicellulose between the microfibrils prevents this bonding. In low-yield pulps, the absence of these materials permits hornification to occur. However, hornification can be prevented by interfering with hydrogen bond formation either by i), partially substituting the cellulose hydroxyls by groups that do not hydrogen bond, or ii), drying pulps in the presence of additives that”bulk” the fibre wall and so effectively replace the ligno-hemicellulose gel.

  • 1993
    Oxford
    pp 1261-1292The effect of recycling on the fines contribution to selected paper propertiesAbstractPDF

    The impact of fines from various sources on selected physical and mechanical properties of paper has been examined.

    In the first of two experiments, the influence of fines was determined by producing two fines free pulps from furnishes which had been refined to 600 ml and 290 ml CSF. Fines removal had a detrimental effect on most properties at a given level of densification including: formation, in-plane and out-of-plane elastic properties, and normal span tensile strength. Densification either by refining, wet pressing, or fines addition resulted in an increase in sheet roughness; this is tentatively attributed to an increase in nonuniform shrinkage in the thickness direction of the sheet. Fines removal gave a more porous sheet particularly at the higher level of refining. Zero span strength or the ultimate strength of the sheet increased with sheet densification, being largely independent of how that densification was produced.

    Fines type and addition level were investigated in the second set of experiments. Fines, up to a level of 30%, were added to a fines-free furnish 740ml CSF. Primary fines are those present in an unrefined virgin pulp, and secondary fines are those produced by refining. “Primary” fines are those fines present after repulping recycled paper, and include both primary and secondary fines. “Secondary”fines are the fines generated by refining a fines-free “primary” pulp.

    It was inferred, from drainage measurements, that the secondary fines had a greater hydrodynamic surface area and were, therefore, more effective than primary fines in enhancing sheet densification and some properties. Furthermore, “secondary”(H)fines, which had been produced from handsheets which had undergone more extensive wet pressing and drying, were, surprisingly, even more effective than the control fines and “secondary” fines. The behavior of newprint fines from preconsumer waste was similar to that produced by primary fines.

    It is clear that fines, defined as material passing a 200 mesh screen, are inadequate to characterize their impact on paper properties. This agrees with the findings of Hawes and Doshi (L6).

  • 1993
    Oxford
    pp 1293The effect of coating components and fillers in the recycling of paper – title onlyAbstractPDF

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  • 1993
    Oxford
    pp 1295-1314Characterization of deinked pulp for newsprintAbstractPDF

    Mill-made deinked pulp fibres for news print were compared with virgin single pulps in terms of pulp properties. The results showed that fundamental fibre properties ,which had been proposed by many researchers, were useful for the characterization of deinked pulp, provided their fractions were compared. Possible reason for this is that fractionation sorts the fibres, and in this way helps to characterize the complicated fibre composition of deinked pulp. Notable characteristics of long fibres of deinked pulp seemed to be fibre coarseness , wet fibre flexibility and curl or kink as well as fibre swelling. These results suggested that the characterization of mill-made deinked pulps with these fibre properties should be carried out by taking the fibre types into account. For this reason, a new method for the determination of fibre composition has been proposed. It provides good results within a certain range of pulps, but further studies are required to devise a more sophisticated method

  • 1993
    Oxford
    pp 1315-1353The permanence of papers containing modern high yield pulpsAbstractPDF

    Modern high yield pulps, typified by chemithermomechanical pulp (CTMP), are likely to be used increasingly for printing, writing and publication grades where longevity is a major consideration. This paper reports an investigation of the ageing of papers containing CTMP, and sized both with rosin/alum and alkyl ketene dimer(AKD) systems, using paper made specially for the study on a pilot papermaking machine. Emphasis was placed on mechanical properties, in view of the concern of librarians and archivists with the embrittlement of paper due to acid hydrolysis of the cellulose. The results show that use of CTMP has a negligible effect on rates of loss of strength; in a series of papers containing various proportions of CTMP with a bleached softwood sulphate pulp, sized with AKD, the rate of degradation was very similar, and acceptably low. Naturally, the papers containing CTMP are not initially as strong, and will become yellow on exposure to light; in judging whether or not such papers are acceptable for long term use, all these factors need to be considered with respect to particular applications.

  • 1993
    Oxford
    pp 1355Paper beyond 2000 – title onlyAbstractPDF

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  • 1993
    Oxford
    pp 1379-1383Forming and formation of paper – prepared contributionAbstractPDF

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  • 1993
    Oxford
    pp 1397-1427A structural approach to paper surface compressibility – relationship with printing characteristicsAbstractPDF

    Three-dimensional topographical maps of paper surfaces under load have been quantified using the confocal laser scanning microscopy. Distributions of the paper surface pores of the same area under different loads were evaluated by the Equivalent Surface Pore (ESP). The ESP roughness of the uncompressed and compressed surfaces of TMP and bleached kraft papers, calendered to the same Print-Surf roughness with different calendering processes, were used to evaluate the local static compressibility of these paper surfaces. Assuming an exponential decay of roughness with pressure, the local static compressibility is defined as the slope of the roughness as a function of the logarithm of the applied pressure. Upon calendering, the local compressibility of the paper surface decreases. The compressibility after calendering depends both on the calendering process and on the furnish. The stiffer TMP fibres present more residual compressibility than the kraft fibres, already pre-collapsed in the uncalendered sheet. The surface compressibility increases with the internal pore volume. The calendered papers were gravure printed at different printing pressures and the number of missing dots counted. A theory is developed which links roughness to ink coverage. It is proposed that roughness is linearly related to the logarithm of the number of missing dots where the slope represents the surface compressibility. Theoretical derivations have been experimentally verified. It was also found that the static roughness is linearly related to the dynamic roughness.

  • 1993
    Oxford
    pp 1449-1455Copy quality and readability of dip containing copy papers – prepared contributionAbstractPDF

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  • 1993
    Oxford
    pp 1501-1510Diagonal Curl in thin papers – prepared contributionAbstractPDF

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  • 1993
    Oxford
    pp 1511-1512Chiral curl in thin papersAbstractPDF

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  • 1993
    Oxford
    pp 1529-1552Plane stress fracture toughness and its measurement for paperAbstractPDF

    Fracture toughness is a material’s ability to resist propagation of a preexisting crack. In most end uses where fracture,toughness can be an important performance parameter for paper, stresses are applied in the plane of the sheet. Therefore, like tensile strength and elastic modulus, the fracture toughness of paper should be measured under in ‘ -plane loading. Our current industry practice of measuring the out-of plane tearing resistance by the Elmendor for Brecht-Imset tests seems inappropriate. Several techniques of fracture mechanics have been applied in recent years to characterize the plane stress fracture toughness of paper. An important consideration is whether a material property was measured particularly for tough papers.This contribution provides a background on these techniques.

  • 1993
    Oxford
    pp 1563-1568Computer simulations of paper strength – prepared contributionAbstractPDF

    We study the strength of paper using computer simulations. Our model is a stressed mechanical 2D random fibre network (RFN), the equilibrium of which is computed using a commercial FEM (Finite Element Method) solver Abaqus. The novelty in our approach lies in more detailed introduction of mechanical properties of fibres and bonds into the mechanical model, enabling the study of effects of microscopic (fibre/bond – level) properties.on relations between macroprocessor properties.

    The mechanical properties of fibres and bonds are defined by Young’s modulus (or moduli in anisotropic case) and stress displacement curve. In our video presentation, fibres are purely elastic whereas bonds are elastic-plastic. The video shows the behaviour of network being stressed by forcing increasing displacement on one boundary

  • 1993
    Oxford
    pp 1569-1633Converting challenges to paper and boardAbstractPDF

    The paper converting process forms part of that whole area of engineering known as conversion technology, which applies the rules and procedures acquired in converting to the manufacture of finished products from paper and board.

    The resulting conversion techniques consist of a sequence of sub processes in which certain changes in the state of the materials are brought about. The sum of these changes leads from the raw materials paper and board up to the final products.

    For the reshaping, separating and connecting processes covered by this report, the specific characteristics and physical principles will be described and the main options of technical realisation discussed. In a number of applied examples, details of the real physical mechanisms encountered in the individual processes will be gone into.

    The state of knowledge just set out is required for optimised designing of paper and board articles as well as of the necessary procedures and machinery. It is furthermore an absolute necessity with a view to realising the best possible manufacturing processes whilst taking ecological and work hygiene considerations into account and putting the means of production to economical use

  • 1993
    Oxford
    pp 1667Prepared contribution on the mechano-sorptive effectAbstractPDF

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  • 1993
    Oxford
    pp 1707-1731The effect of coating components and fillers in the deinking of paperAbstractPDF

    The effect of titanium dioxide, kaolin, calcium carbonate and talc on the properties of foams formed by sodium oleatelsodium stearate solutions and the flotation yield of these pigments in the presence of the same soaps have been investigated. In addition, the flotation of dispersed model coatings prepared from these pigments with styrene-butadiene and polyacrylate latexes was studied. The influence of coating colour composition and flotation chemicals (including silicate) was investigated using factorial experiments. The latexes strongly increase pigment floatability, but the effects are interrelated with the effects of other components in the coatings in a complexway. The flotation yield of the different pigments depend quite differently on the amounts of calcium and silicate added. This indicates that it may be possible to achieve flotation selectivity by proper design of flotation strategies. The factorial experiments also make it possible to draw some tentative conclusions with regard to mechanisms giving rise to the observed effects.

  • 1993
    Oxford
    pp 1749-1771Paper beyond 2000AbstractPDF

    The pulp and paper industry has a fantastic potential to become the first and possibly the only truly “evergreen” industry. Our raw material is renewable, we apply sustainable forest operations, within the next decade our mills will be ecologically balanced, and our products are non-toxic and recyclable.

    It is up to ourselves to “do-it-ourselves”, to take charge and to find new solutions which will prepare us for a future full of trend-breaks.

    The key is knowledge and competence. Our fundamental task is to build up the knowledge-base that will be necessary if we are to proceed and if we want success.

  • 1997
    Cambridge
    pp 1-82Physicochemical characterization of papermaking fibresAbstractPDF

    The purpose of this paper is to present as complete as possible, a picture of our present knowledge about papermaking fibres and their physico-chemical characteristics. The properties of the papermaking fibres are, in most cases, significantly influenced by the morphology of the wood fibres, but, from one and the same kind of wood fibres, paper can be produced with widely different properties as a result of different pulping and papermaking processes. Dissolution of material from the fibre wall and the middle lamella, structural changes of the polymeric material, and mechanical work on the fibrous material (defibration, refining, and to a certain extent undesired mechanical damage to the fibres in the pulp mill machinery) combine to produce the fibre properties required in the papermaking process.

    Starting from the structure of the wood, a survey is given of the pattern of dissolution of different important pulping processes and the resulting bulk composition of the pulps. Characterisation of papermaking properties should include effects of both bulk and surface of the fibres and for that reason they are both discussed. The bulk composition has been studied for many years and we have a fairly good knowledge of the main features, although there is still a need for more detailed knowledge. The properties of the fibre surface are less known, but they have been the subject of several recent studies. They are therefore dealt with in more detail, particularly the problem of making reliable and relevant measurements.

    Dissolution of lignin and other components in the pulping process is also important for the chemical composition of the surface. Mechanical removal of the remaining middle lamella and the outer layers of the fibre wall (the primary wall and SI of the secondary wall) substantially change the surface composition and create fines with a large surface area, which may interact with the wet-end chemicals in the paper mill and decrease the over-all effects of these chemicals. Removal of the outer layers will also change the fibre properties as a whole since, for instance, the SI layer restricts outward swelling of the main part of the secondary wall, S2, and preserves fibre rigidity.

    Swelling of the fibres influences there fining behaviour of a pulp. For lignin-containing pulps, swelling facilitates refining. For bleached pulps with a very low content of residual lignin, the effect of swelling on refining is rather difficult to as will be briefly discussed.

  • 1997
    Cambridge
    pp 83-137A review of the influence of pigments on papermaking and coatingAbstractPDF

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  • 1997
    Cambridge
    pp 139-207Binders for paper coating: starches, proteins and laticesAbstractPDF

    There are many chemicals, both natural and synthetic, which are used for surface application of paper and board. In this overview shall the use of starch and protein be discussed as representatives of products arriving from renewable resources and the use of latex as there presentative of products arriving from chemical synthesis. Starch is used for coating of paper both in combination of pigments- coating colours, and for surface application in size press, film press or other application units. As the influence of the starch properties -both from the chemical and from viscosity point of view is of great importance for the runnability of the machines and the paper quality, will starch quality be discussed to a great extend. The influence of starch quality on the rheological properties of coating colours of the different types of coating colours will there after be discussed. Protein and latex are mainly used as binders in coating colours. The influence of the chemical and physical properties of these binders on the pigment- binder interaction and the subsequent paper properties will be discussed as far it is possible from a general point of view.

  • 1997
    Cambridge
    pp 209-263A review of advances in internal sizing of paperAbstractPDF

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  • 1997
    Cambridge
    pp 265-350Some aspects of retention aidsAbstractPDF

    Fundamental aspects on polymer adsorption and flocculation phenomena are reviewed. Special emphasis is placed on recent developments. Examples are the mode of action of microparticle retention aid systems and of retention systems based on polyethylene oxide/phenolic resin.

    Fibre flocculation and the strength of fibrous networks are affected by retention aids and this is discussed in,connection with investigations on retention and formation in the laboratory and on pilot-scale paper machines.

  • 1997
    Cambridge
    pp 351-399The role of the fibre in the structural hierarchy of paperAbstractPDF

    In this review paper, the concept of structural hierarchy is applied to paper in an effort to establish the role of the fibre in the structure of paper . The structure of paper is partitioned according to the size or scale of the features of interest, and the basic nature of a generic paper physics research project is shown to be independent of the scale being considered. The literature dealing with paper structure and properties at each significant scale is briefly introduced, and the similarity of studies at quite different ends of the spectrum is highlighted. The prospects for an integrated model relating a mechanical property such as modulus to structural variables alone are discussed. Throughout the emphasis is on mechanical properties, and the central role of the fibre within the structural hierarchy.

  • 1997
    Cambridge
    pp 403-437Some fundamentals of paper and board engineeringAbstractPDF

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  • 1997
    Cambridge
    pp 439-472Genetic selection of trees with designer fibres for different paper and pulp gradesAbstractPDF

    Pinus radiata and eucalypts are fast-grown species, well suited to plantation forestry in New Zealand and elsewhere, and for the manufacture of a wide range of solid wood and reconstituted wood products, including pulp and paper.

    This paper examines the variation and end-use potential of the individual-tree kraft fibre and handsheet properties of 25 trees of 13-year-old P. radiata and 29 trees of 15-year-old Eucalyptus nitens. Individual-tree fibre property differences are assessed with reference to the fibre quality requirements of a range of wood-free paper grades. Strategies and procedures are also described which will enable parent trees with desired fibre properties to be identified, propagated and mass produced.

    In selecting fibre types for different paper and pulp grades, the apparent density of “unrefined” pulps(500 PFI mill rev) is the base against which other “unrefined” handsheet properties are compared. Apparent density is a direct measure of fibre packing density and arrangements in handsheets, and is determined by fibre length and cross-section dimensions, and the related morphological configurations of collapse and straightness. Although the individual-tree pulps of both species can normally be refined to the same tensile index, apparent density values can be very different depending on their fibre properties. Thus, minimal pulp refining is preferred for comparing individual-tree pulps for trees election.

    Apparent density is best predicted by the kraft fibre property combination of the fibre width/thickness ratio and length. The combination of chip basic density and kraft fibre length is also a good predictor of handsheet apparent density but not necessarily of the best fibres for the manufacture of particular products. Wood density is a measure of the ratio of wood substances to void space in each individual-tree chip sample and is not indicative of the numbers of fibers which make up a unit volume.

    Kraft handsheet propeties varied widely among trees for both species and were well-predicted by kraft-fibre dimensions. The high broad-sense heritabilities shown for these traits in P. radiata mean that clonal forestry could provide pulpwood of uniform and predictable pulping performance from monoclonal forest blocks. The high narrow-sense heritabilities shown so far for wood properties in P. rafiata (and for some wood properties in E. nitens) indicate that planting control-pollinated families of known characteristics could have a similar though less uniform result.

     

  • 1997
    Cambridge
    pp 473-503Transverse dimensions of wood pulp fibres and their implications for end useAbstractPDF

    The transverse dimensions of pulp fibres influence strongly their response to the papermaking process, and most end-use properties of products. However, fibre transverse dimensions are difficult to measure. Confocal microscopy combined with image analysis has been used for rapid and accurate measurement of fibre wall cross-sectional area,perimeter, and thickness. Results on kraft pulp fibres obtained from a variety of wood species are presented. They demonstrate how fibre transverse dimensions are distributed within a species, and can be described analytically. Comparison between different species shows that species with coarse or thick-walled fibres are likely to be more heterogeneous. Implications for pulp quality and fibre selection for end-use requirements are discussed.

  • 1997
    Cambridge
    pp 505-519Raman spectroscopic analysis of the microdeformation in cellulosic fibresAbstractPDF

    This paper addresses the application of Raman spectroscopic techniques to analyzing the deformation micromechanics of regenerated cellulosic fibres. In addition to information obtained on the macroscopic deformation, Raman spectroscopy enables the study of changes in internal strain associated with creep and recovery. The analysis of the mechanics of deformation of the fibres begins by following the response of the Raman-sensitive bands to external tensile loading. It is shown that the peak positions of the 895 cm-1 and 1095 cm-1 Raman bands shift to lower vibrational frequency under the action of tensile stress or strain due to the macroscopic deformation leading to direct stretching of the polymer molecules. Moreover, this approach makes possible the modelling of single fibre properties using simple viscoelastic dynamics; thereby relating the macromechanical properties of stress and strain to those obtained at the microscopic level via spectroscopy.

  • 1997
    Cambridge
    pp 521-545Fiber properties and paper fracture – fiber length and fiber strengthAbstractPDF

    Phenomenological theories on the effect of pulp fiber properties on the fracture energy of paper are discussed. The effect of fiber length and strength is clarified experimentally. Fiber length appears to affect fiber failure probability only slightly. When fiber strength is changed, the fracture energy decreases greatly with only a small increment in fiber failure probability. This suggests that the fracture energy contribution of a fiber may be correlated between fibers. The effect of fiber length and strength on the cohesive stress – crack widening relationship is clarified.

  • 1997
    Cambridge
    pp 547-605Some mechanical pulp fibre characteristics, their process relationships and papermaking significanceAbstractPDF

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  • 1997
    Cambridge
    pp 607-639Hydrodynamic modelling of the behaviour of the pulp suspensions during beating and its application to optimising the refining processAbstractPDF

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  • 1997
    Cambridge
    pp 641-662Changes in fibre wall structure during defibrationAbstractPDF

    Development of earlywood and latewood fibres was investigated to find out how morphologically different fibres undergo delamination. Fibre fractions rich in earlywood and latewood, were separated from mechanical pulps using a hydrocyclone and refined further in awing defibrator. Changes in fibre structure due to defibration were studied using microscopy techniques that included measurement of fibre stiffness, fibre wall thickness and external fibre surface. Before refining, the latewood fibres were stiff and their external fibre wallswere poorly developed. Refining reduced the stiffness of both fibre types. The stiffness of latewood fibres decreased to around that of unrefined earlywood fibres, andthe external walls of latewood fibres became fibrillated. The wall thickness of both earlywood and latewood fibres was reduced only slightly. Although the tensile and tear indices of sheets made of late wood fibres were improved by refining, the tensile index of flexible latewood fibres was only half of that measured for unrefined earlywood fibres. This indicates that there are fibre properties other than stiffness which must be changed in order to get latewood fibres to bond and conform properly.

  • 1997
    Cambridge
    pp 663-679Effects of fibre morphology on hygroexpansivity of paper – a micromechanics approachAbstractPDF

    Effects of fibre morphology on in-plane hygroexpansivity of paper have been studied using a general micromechanics formula and experimental results obtained for different fibre fractions. It was deduced from the micro-mechanics analysis that the major fibre geometry factors affecting paper hygroexpansivity are fibre width, wall thickness, and fibre length,all of which control the stress transfer ratio parameter defined in the formula. The microfibril angle was found to bean other major factor affecting the paper hygroexpansivity through the hygroexpansivity of a single fibre.

    The results from the cut-fibre experiment showed that decreasing fibre length increased the hygroexpansivity, but the effect was small within the range of fibre length more than 1mm. The results from Bauer-McNett classification, on the other hand, showed considerable increases of hygroexpansivity as the fraction became shorter. This large variation was attributed to the cell wall structure, in particular microfibril angle and cell wall thickness, both of which directly affect the key parameters in the formula. Fibre fractionation was found to be a very effective tool to consistently change all the fibre morphology factors for controlling dimensional stability of paper.

  • 1997
    Cambridge
    pp 683-701The swelling ability of pulp fibresAbstractPDF

    The swelling of fibres in water has a large impact on the consolidation of the web in papermaking and thus on the properties of the final paper. The water taken up by the fibre is held by many different mechanisms and it is not always clear from the techniques used which quantity of water is being measured. In this report, an attempt is made to obtain more knowledge regarding the water- holding mechanisms of fibres, by studying the amounts of bound water, of pore water and of the total water in the fibres. Effects of delignification, recycling, beating and ion exchange are examined. It is concluded that the amount of bound water is a reflection only of the wood polymer composition of the fibre, whereas the pore water is affected by physical changes of the fibre wall. The ionic charges mainly affect the surface water of the fibre.

  • 1997
    Cambridge
    pp 703-749Moisture sorption by paper materials under varying humidity conditionsAbstractPDF

    The uptake of moisture by paper sheets was analyzed as an adsorption process occurring in a porous medium. Water vapor was assumed to diffuse into the pore space and was subsequently adsorbed onto the surfaces of the fibers constituting the paper sheet. The response of the sheets to variations in relative humidity was investigated.

    Since the transient moisture profiles inside paper sheets depend upon the moisture sorption equilibria for paper, the equilibrium behavior was investigated with special emphasis on a description of sorption hysteresis. It is necessary to follow equilibrium trajectories inside the hysteresis loop for paper sheets. Sorption equilibria inside the hysteresis loop for paper sheets (bleached kraft linerboard, 290 gsm) were evaluated experimentally. Complete sets of desorption and adsorption scanning trajectories were obtained. Further higher order loops were obtained experimentally. The theory of independent domain complexions was applied to the hysteresis loop. By constructing a so called moisture distribution function for the hysteresis, arbitrary trajectories representing equilibrium sorption behavior under cyclic humidity changes could be predicted.

    An investigation of transient sorption was also undertaken. A model for moisture uptake based upon diffusion inside the pore space and the fibers in the sheet was set up. Experimental data on transient moisture uptake was obtained under ramped changes in humidity. It was found that the model for moisture uptake incorporating a linearized isotherm could describe the sorption response of paper sheets to ramped changes in humidity adequately. From the experimental data, a value for the intro-fiber mass transfer coefficient representing moisture diffusion through fibers was determined.

  • 1997
    Cambridge
    pp 751-769Surface energy characterization of surface modified cellulosic fibres by inverse gas chromatography (IGC)AbstractPDF

    In the present study, the surface properties of cellulosic fibres have been systematically varied and the dispersive and acid-base properties have been determined by inverse gas chromatography (IGC) at infinite dilution.

    Bleached kraft pulp fibres were carboxy methylated to different degrees. The results from the IGC measurements on the carboxymethylated fibres showed a linear relationship between the interaction energy with a basic probe (acidic properties) and the carboxylic acid group content. The extrapolation to zero degree of substitution ofcarboxylic acid groups indicates that,even in the absence of carboxylic groups, the fibres have acidic properties. The hydroxyl groups on the fibres obviously also contribute to the acidic properties. The interaction energy with an acidic probe (basic properties) was relatively constant with increasing carboxylic acid group content. An increase in the degree of carboxymethylation also seemed to slightly increase the dispersive part of the surface free energy. This could be a consequence of an increase in electron density, a more compact structure after drying the carboxymethylated fibres or removal of low molecular weight impurities. The dispersive as well as the acid-base properties are approximately the same for pulps in both their proton and sodium forms.

    The carboxymethylated fibres were peeled after the modification. The carboxylic group contents of the fibres and of the removed outer layers were determined by conductometric titration. The results showed that the carboxymethylation procedure is somewhat more effective in the outer layers of the fibres.

    IGC results for the peeled fibres pointed in the same direction. Diethylaminoethyl{DEAF}cellulose, which has a basic functional group was also characterised. The IGC results showed that the DEAE cellulose interacts more strongly with the acidic probe than the reference cellulose material

  • 1997
    Cambridge
    pp 771-788Porosity of swollen pulp fibers evaluated by polymer adsorptionAbstractPDF

    Adsorption of polyethylenimine (PEI) of different sizes on swollen delignified pulp fibers indicates that for PEI molecules of diameter smaller than 25 nm, the accessible internal surface area within the pores in the cell wall is independent of the size of the PEI molecule. This suggests that a minimum pore radius R. exists in the fiber wall (with the possible exception of very small pores of about 1 nm) through which all PEI molecules in the range 2-25 nn can pass freely. Since the molecules must be able to pass through pores with walls fully coated by PEI and since the thickness of an adsorbed PEI layer is comparable to the size of PEI in solution,the pore size must be at least 3 times the size of PEI, implying that R40 mn. A value of the pore radius in the range 45-50 nm is found from estimates of the area of pores accessible to PEI and the corresponding pore volume. No pores are found in the range 3-40 nm. These findings differ from the pore radii obtained by the solute exclusion technique which usually are around 10 nm. The difference might be due to the ease with which the pores contract and expand under different conditions. Non-adsorbing molecules could cause the pores to contract due to depletion effects, while adsorbing molecules might cause pores to expand.

  • 1997
    Cambridge
    pp 789-814Depth profile of paper additives within a pulp fibreAbstractPDF

    Paper sheets containing polyacrylamide(PAM)as a paper chemical additive have been analysed by attenuated total reflectance Fourier transform infrared (ATR/FT-IRS spectroscopy. Absorbance ratios of the selected band of PAM to that of cellulose were used to determine PAM content. In order to determine the distribution of the additive in the radial direction of pulp fibre, ATR/FT-IR analysis was carried out after successive etching of the paper sheet. From the relationship between etching time and the thickness of removed surface layer it is possible to follow the partial concentration profiles of the additive as a function of distance from the original surface. The obtained profiles are found to be consistent with those of variable-angle ATR/FT-IR depth profiling method qualitatively. Being distributed from fibre surface toward the centre of fibre wall, PAM exists, on the whole, close to fibre surface. Addition level does not affect the distribution curve, i.e. the concentration at every depth level is roughly proportional to the addition level. In the case of a paper sheet from heavily beaten pulp, this tendency is not so clear i.e. PAM shows broad distribution at near surface layer. PAM whose molecular weight is low is distributed widely toward the inner side of the fibre wall.

  • 1997
    Cambridge
    pp 815-836Diffusion controlled kinetics of electrolyte transfer within the pulp fiber wall: estimation of fiber wall pore structureAbstractPDF

    A kinetic model for diffusion of ions within the fiber wall is introduced. A good fit for the model was obtained with data from experiments, where diffusion of electrolyte through the fiber wall was studied by means of simple conductivity measurements. This model together with the respective experimental data makes it possible to study the pore structure of fibers.

    For the experiments done the pore structure of fibers was first controlled by precipitating calcium carbonate within the fiber walls. It was found that the diffusion coefficient of ions within unfilled and calcium carbonate filled fiber wall was about 1 .5 and 0.5 % of that in water, respectively, when experiments were done for fibers without pit apertures. Experiments were made for fibers with different degree of drying as well. The results showed that the diffusion of electrolyte reduced with replicate dryings as expected. After the dryings, the diffusion coefficient was less than half of that within newer dried fiber.

  • 1997
    Cambridge
    pp 837-858The exchange and removal of the metal cations in pulpsAbstractPDF

    Metal cations modify the optical and mechanical properties of pulps and in pulp mills can have adverse effects on such parameters as corrosion rate and recovery furnace temperature. With today’s emphasis on recycling effluents and on tighter control of all operations, there is a need for quantitative theory predicting the uptake of cations by pulps and the buildup of cations in process liquors.

    A variation of the Donnan Equilibrium Theory was recently developed to describe the partitioning of cations between the fibres and the liquor in a pulp suspension [1]. In this report we extend use of the theory to examine the ion-exchange of pulps from saturation with one cation to saturation with another. Theoretical predictions are closely duplicated by experiments using sodium, magnesium and lanthanum as model cations of different valencies. Pulps readily exchange cations even with very dilute solutions, preferentially taking up and most tenaciously holding the higher valency cations. Consequently, the replacement of high valency cations by monovalent cations requires higher concentrations of the replacement ion than for the reverse exchange. The differing tenacities by which cations are held are also shown during acid washing of pulps. Upon progressively lowering the pH of a suspension of pulp in mixed ionic form, monovalent, divalent and trivalent cations are successively released and replaced by hydrogen ions. The implication of these results for the “acid washing” of pulps is discussed.

  • 1997
    Cambridge
    pp 859-892The effect of surface chemical composition and charge on the fibre and paper properties of unbleached and bleached kraft pulpsAbstractPDF

    The objectives of this research were (i) to obtain a comprehensive description of the surface chemistry of cellulosic fibres by thermodynamic and spectroscopic methods, (ii) to use this description to clarify the effect of kraft cooking as well as ECF and TCF bleaching on the surface chemical and adhesive properties of cellulosic fibres, (iii) to clarify and understand the connection between the fibre surface properties and some key sheet properties (e.g. tensile strength). The charge, surface energy and hydrophobic material in the surface of cellulosic fibres (mainly pine, Pinus sylvestr is and birch, Betula verrucosa) were studied by potentiometric titration, determination of polyelectrolyte adsorption isotherms, determination of contact angles of single fibers and ESCA. Lignin (remnants of middle lamellae or reprecipitated during cooking) is enriched in the surface of unbleached kraft pulp fibres. The fraction of surface lignin removed by oxygen or hydrogen peroxide is much smaller than the total decrease in lignin content. Ozone removes both bulk and surface lignin, while the effect of chlorine dioxide depends on the number of treatments. The low reactivity of surface lignin is due to condensed lignin structures and/orto lignin-carbohydrate complexes.

  • 1997
    Cambridge
    pp 893-907Carboxylic acid groups and fibre bondingAbstractPDF

    It has been known for a long time that carboxylic acid groups in cellulosic fibres increase swelling in fibres- and hence this impacts flexibility, bonded area, and strength. In a few recent publications, attention has been drawn to the possibility that carboxyl groups may serve another function. Those located at the fibre surface have an effect on specific fibre-fibre bond strength . The evidence for this
    possibility is at present indirect . Using a technique developed for this work, pulps were prepared with uniform distribution of carboxyl groups across the fibre cell wall, and with carboxyl groups located primarily at the fibre surface. Using values of light scattering coefficient, tensile strength, and z-bond strength, it was possible to determine the relative importance of increased bonded area caused by swelling, and increased bond strength caused by localized surface effects. It was found that specific bond strength could be enhanced roughly 50% by surface enrichment, and this could exceed the strength increase attributable to enhanced bonded area. Previous work on the effect of carboxyl groups on strength needs reevaluation to determine the extent of the two effects . The mechanism of strength increase by surface carboxylic acid groups is not understood . It may be an ionic effect, but it is
    possible that the increased localized surface swelling allows more molecular flexibility and more intimate molecular contact and interdiffusion as found in other polymers . In practice, there are opportunities, especially in bleaching or other pulp treatments, for enhancing fibre surface carboxyl group concentration, and these may have practical value. The field looks ripe for exciting developments.

  • 1997
    Cambridge
    pp 911-953Surface energetics of calcium carbonate powders by inverse gas chromatography (IGC) and cleaved crystals by contact angle measurementAbstractPDF

    Inverse gas chromatography (IGC) and sessile drop contact angle measurement were used to study the surface energetics of calcium carbonates from different sources and the effects of surface conditioning . A compilation of the results using these two methods to determine the apolar and polar nature of the surface is given . The results of the two analytical methods are summarized individually . IGC analysis provided a sensitive tool for detecting differences in surface energy attributed to variation in surface water, or by contamination with other chemisorbed species . Basic probes were used to characterize surface acidic sites. Acidic probes, however, were found too reactive with the calcium carbonate to en-
    able a quantification of the surface basicity . Strong adsorption of acidic probes indicates the presence of a strong basic component at the surface . The susceptibility of the IGC method to misinterpretation retention data obtained from chemisorbed probes was also identified . Contact angle experiments were conducted on cleaved calcite samples using multiple wetting liquids and two techniques to separate the polar and apolar contributions of surface energy. The results demonstrate a strong interaction of water with the calcite surface and the reduction of total surface energy when the surface is treated with al-
    kylketene dimer. A comparison of results from both methods is provided . Differences in the application of the results of the two methods to access the surface energetics are addressed . The importance of distinguishing between the thermodynamics of gas/condensed and condensed/condensed phase interactions when comparing the results from IGC and wetting experiments is also discussed . For calcium carbonate, the trends observed by the two approaches are consistent, although a defining link between the two was not established.

  • 1997
    Cambridge
    pp 955-993Microbiological control of pigments and fillers in paper industryAbstractPDF

    Microbial contaminants will decrease the quality of pigments and fillers in paper and board industry . Microbe-caused spoilage of these raw materials as well as indirect deteriorative effects of these spoiled
    additives on the machine housekeeping, production and hygienic properties of paper products are well-known problems at the mills.

    The traditional analysis of colony forming units helps both the manufacturer ofraw material and his client to understand, what are the major contaminants and their densities in the raw material studied.
    These methods, unfortunately, are unsatisfying when rapid microbiological analyses are needed in urgent situations (trouble shooting, prevention of process contamination) . Their shortage as indicators of the effects of biocides on the activity of microbes is also evident . Three “rapid methods” were regarded as promising alternatives of colony count analyses in the control of paper and board
    machine’s processes: detection of microbial growth by impedance method or turbidity test and measurement of biomass by luminometric ATP assay . When tested for mineral additive control, difficulties were arisen in every case: inhibitive effect of sedimentation of the sample in direct impedance measurement, poor interpretation of slow growth responses in indirect impedance method and the adsorptive effects of turbidity on the light emission (ATP assay) or on the background
    absorbance (turbidometry) . For these reasons, strong dilutions of samples were needed and only high microbial densities (over 1000000 CFU/ml) were practically measurable by these methods. Integration of total impedance change may help to estimate low microbial densities by impedance methods, and more effective extraction and fractioning of microbial ATP may increase the sensitivity of ATP assay . Turbidometry seems to be a rather finished method, but its weak point is the long incubation period needed. Some efforts to develop all methods for the control of mineral additives seems thus to be necessary.

  • 1997
    Cambridge
    pp 997-1020On the mechanism of sizing with Alkyl Ketene Dimer (AKD): physico-chemical aspects of AKD retention and sizing efficiencyAbstractPDF

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  • 1997
    Cambridge
    pp 1021-1046Mechanism of internal sizing with Alkyl Ketene Dimers: the role of vapour depositionAbstractPDF

    The role played by AKD vapours during internal sizing was investigated using commercial AKD waxes and model surfaces. The model surfaces consist of cellulose and cellulose acetate films deposited on smooth glass slides. These cellulosic films were exposed to AKD vapours at temperatures ranging from 80’C to 175oC for different periods of time. The extent of sizing was followed by measuring the advancing contact angle of water over the treated surfaces. A simple model, considering both physisorption and chemical reaction, was developed and validated with experimental data. The energy of activation of 61 .4 kJ/mole was derived from the Arrhenius plot . From a series of indirect techniques, it is concluded that the establishment of a covalent bond between cellulose and AID is essential in order to introduce permanent hydrophobicity to cellulosic surfaces . The effect of “sizing promoters” on the reaction rate was also examined.

    Both NaHC03 and cationic PEI failed to catalyze sizing between cellulose and AKD vapours. The mechanism proposed and the model will shed new light on the phenomena of AKD sizing treatment and sizing migration.

  • 1997
    Cambridge
    pp 1047-1071Factors influencing on retention of Alkylketene dimerAbstractPDF

    Mechanisms of retention of AKD are studied in terms of effects of cationic polymer additions, and roles of fines and carboxyl groups of pulp fibers. Handsheets were prepared from normal pulp and fines-free pulp under various conditions, and AKD contents in the handsheets were determined by pyrolysis-gas chromatography (PY-GC). The addition of cationic polymer (PAE) to pulp suspension was clearly effective in promoting AKD sizing for both cured and noncured handsheets, resulting from higher
    retention of AKD in the sheet by cationic PAE molecules. Potential measurement indicated that the originally cationic AKD emulsions come to have amphoteric surface charges in pulp suspension . The effective AKD retention by PAE may, therefore, be due to ionic bond formation between anionic sites of AKD emulsion surfaces and anionic pulp fibers or fines through cationic PAE molecules. The experiments using fines-free pulp showed that most of the added AKD was adsorbed on fines of beaten pulp . Blocking of carboxyl groups in pulp with nonionic methylamide groups resulted in nearly no sizing degrees and quite low AKD contents. Therefore, it is clear that dissociated carboxyl groups in pulp fibers and fines are the actual retention sites of AKD emulsion particles at the usual addition levels of AM Among chitosan salts and PAE used at 0.1-0.4 addition levels, chitosan AcOH salt gave the highest effect on AKD retention as well as sizing degrees. Good correlation between AKD
    contents and sizing degrees was obtained ; sizing behavior of AKD-sized handsheets was explainable in terms of their AKD contents determined by PY-GC, and thus retention of AKD is the significant first step for AKD sizing. Solid-state ,3C-NMR analysis of cellulase-treated residues of 13C-labelled
    AKD-sized handsheets showed that size components were present in papersheet as structures of either the original AKD or ketones, hydrolyzed AKD, without forming ß-ketoesters . SEM observations of AKD-sized handsheets indicated that the effect of curing on AKD sizing is explained in terms of melting of size molecules and their spreading over pulp fiber surfaces .

  • 1997
    Cambridge
    pp 1075-1096Adsorption of cationic starch onto peroxide bleached TMPAbstractPDF

    An investigation concerning the interactions between cationic starch and peroxide bleached thermornechanical pulp (TMP) has been carried out. The influence of electrolytes, pH, temperature, a fixing agent and charge density of the starch is discussed. The adsorption of cationic starch onto dispersed and dissolved wood substances and unwashed fibres was reduced by an increase in NaCl concentration. The adsorption onto washed fibres, however, showed a maximum with increasing NaCl concentration. The adsorption onto dispersed and dissolved substances and unwashed fibres increased and reached a plateau level as function of increasing pH.

    Onto washed fibres, the adsorption was reduced as pH was increased from 5 to 8. An increase in temperature affected only the adsorption onto washed fibres . A fixing agent retained the dispersed and disolved wood substances more effectively than the ordinary cationic starch but the amount of adsorbed cationic starch remained unaffected by the presence of the fixing agent. Cationic starch of
    medium charge density (degree of substitution, D.S ., : 0.035) aggregated turbidsubstances more effectively than cationic starches of low charge density (D.S . 0.015) and of high charge density (D.S . 0.05) .

  • 1997
    Cambridge
    pp 1097-1119Cationic microparticle retention aids: the mechanism study and laboratory evaluationAbstractPDF

    The cationic polymeric microparticles (CPMP) with different charge densities and particle sizes were synthesized and characterized.The flocculation and retention of precipitated calcium carbonate {PCQ and titanium dioxide {TiO2) suspensions induced by a cationic polymeric microparticle retention system were studied in both the original and washed pulps. For the dispersion of PCC in the washed pulp, the CPMP alone is not an effective retention aid because both the CPMP and PCC are positively charged. However, the retention of fillers can be significantly enhanced by dispersing FCC in an unwashed pulp. The combination of CPMP and a high- molecular weight, water-  soluble polymer led to excellent retention of PCC. Although both anionic or cationic polymers can be used in conjunction with CPMP, different mechanisms are involved in these dual-retention systems. For the dual-retention system of CPMP-APAM (anionic polyacrylamide), it is believed that a bridge is formed between the CPMP patch and APAM . In contrast to the CPMP-APAM system, the increase in the filler retention efficiency by the dual-retention system of CPMP-CPAM (cationic polyacrylamide) is not through the bridging formation between CPMP and CPAM, but is caused by the configuration change of the cationic polymer in the presence of CPMP. It is believed that the pre-adsorption of CPMP can prevent CPAM from getting flat on the solid surface, resulting in an extended polymer conformation and high retention efficiency. It has also been found that the particle size of a cationic polymeric microparticle is a critical factor affecting the PCC retention.

  • 1997
    Cambridge
    pp 1121-1134Novel non-ionic polymeric flocculants for mechanical pulps: an overviewAbstractPDF

    Novel comb copolymer with long polyacrylamide backbones bearing very short polyethylene glycol (PEG) pendant chains were prepared by the free-radical copolymerisation of acrylamide (AM) and PEG acrylate macromonomers. The copolymers are effective retention aids for mechanical pulps in retaining fines and precipitated calcium carbonate (PCC). The optimum copolymer stricture had a molecular weight greater than 3 million and contained 0.5 – 1 .0 % of PEG pendant chains with 9 to 23 polyether repeat units. The retention of fines induced by copolymer/phenolic resin (PFR) dual-polymer system follows the mechanism called “complex bridging flocculation” . According to this mechanism, the copolymer molecules aggregate in the presence of PFR to form a colloidal dispersed polymer complex which hetero-flocculates with fine particles. The interpolymer complex formation were experimentally observed through precipitate isotherms measurement, dynamic viscosity measurement and fluorescent microscopy.

  • 1997
    Cambridge
    pp 1135-1160New information on the role of cofactor in PEO-type retention aid systemsAbstractPDF

    The flocculation of three colloidal dispersions (precipitated calcium carbonate (PCC), T’02 and a calcined clay) using a combination of polymeric flocculants and cofactors was investigated . The flocculants used were PEO (MW = 9×10′), CPAM (a cationic copolymer of acrylamide) and POLYPAM-CO-PEG (a non-ionic comb copolymer (MW = 5×100 consisting of a polyacrylamide backbone with – I mole % pendent PEG chains). The cofactors were based on polyvinyl phenol-co-sodium acrylate) and polyvinyl phenol-co-sodium styrene sulfonic acid). The amount of flocculation induced was dependent on the components present in each system. Cofactors containing sulfonic acid were more calcium ion
    tolerant than the cofactors containing carboxyl groups. These latter cofactors formed a precipitate when exposed to > 0.6 mM Ca”. Maximum flocculation of PCC was obtained by using POLYPAM-CO-PEG with a cofactor containing 23 mole % of sulfonated groups . TiO, was not flocculated when PEO was employed due to the adsorbed layer thickness of this flocculant being approximately equal to half the Debye screening length in 0.001 M NaCl. However, T’02 was flocculated by POLYPAM-CO-PEG and CPAM, the best flocculation being obtained when POLYPAM-CO-PEG was used with either a
    cofactor containing 19 mole % sulfonated groups, or a cofactor containing 14 mole % acrylic acid groups . Overall, the easiest colloid to flocculate was the calcined clay ; maximum flocculation being obtained when PEO was combined with a cofactor containing 19 mole % sulfonated groups.

    In general, the flocculation was most sensitive to the charge of the cofactors. It is proposed that the hydrophobic character of the vinyl phenol based cofactor is important. Hydrophobic interaction may be a part of the cofactor interaction with PEO and subsequent complex adsorption on surfaces.

  • 1997
    Cambridge
    pp 1161-1174A model for the retention of particles in evolving fibre networksAbstractPDF

    A model is presented for the entrapment of a distribution of particles by a distribution of pores. The fraction of particles retained and their distribution of sizes can be calculated for evolving and static porous structures. For evolving structures the change in distribution and fractional retention through the structure can be calculated. Also, the variance in these parameters between zones in the plane of the sheet can be calculated. The agreement between the theory and experimental data is good . The theory has relevance to the retention of fillers and fines in the papermaking process and to more general problems of stochastic porous media.

  • 1997
    Cambridge
    pp 1177-1213Performance of reinforcement fibres in paperAbstractPDF

    Paper properties can be controlled by mixing different furnishes . The outcome of the elastic, strength and toughness properties is analyzed in this work using results from other fields of material science . particularly from composites . We discuss the micromechanics of reinforcement fibres, their conformability to the background fibre web and the fracture processes in reinforced paper. Reinforcement fibres should have high ductility and they be similar to the mechanical furnish iii their
    micromechanical stiffness.

  • 1997
    Cambridge
    pp 1215-1247A heuristic model of paper ruptureAbstractPDF

    In-plane (Mode I) fracture of paper is tested at both cryogenic and standard temperatures . It is shown that newsprint tested at cryogenic temperatures is very nearly linear elastic but does not obey classical linear elastic fracture mechanics (LEFM). The discrepancy is traced to changes in the crack tip stress
    field due to the sheet’s fibrous structure.

    A new fracture model is proposed which integrates the Griffith energy method, Irwin’s correction for nonlinear material behavior, and a similar correction for the fibrous structure. The statistical distribution of mass and local fibre orientation (structural formation) are explicitly considered, and the model thereby links fracture and tensile strength of paper. This approach is consistent with Bazant’s
    theoretical treatment of quasi-brittle fracture.

    The separation of material nonlinearity and structural formation permits :

    • computing the Essential Work, linking the model to an established approach
    • estimating the fast fracture response that might occur in practice
    • normalizing strength for the paper machine dependent structural formation
  • 1997
    Cambridge
    pp 1249-1271Micromechanical modelling of tear strength in kraft and TMP papersAbstractPDF

    In this study, a mechanistic model was developed for the Elmendorf tear strength of paper based on the fundamental physics of the tear process . In the model, the tear strength was calculated as the sum of fibre fracture energy and fibre pull-out energy. The model also included statistical considerations, such as the distribution of fibre lengths. Through dimensional analysis, a “dimensionless tear index” was identified and was found to be a universal function of three dimensionless parameters. Using a bond strength obtained through non-linear regression analyses on a particular experimental data set, the model was validated with other sets of data. It was found that the model gave good quantitative predictions of tear strength of kraft papers without the need for adjustable fitting parameters. With a slight modification to account for the role of fines in the furnish, the model was also successfully applied to handsheets made from TMP pulps.

  • 1997
    Cambridge
    pp 1273-1291KCL-PAKKA: Simulation of the 3D structure of paperAbstractPDF

    We present a novel approach to study the three-dimensional network structure of paper. In the KCL-PAKKA simulation model, the porous sheet structure is compiled from the different papermaking raw materials, fibres, fillers and fines. The model geometry is simplified in order to enable effective numerical experiments with arbitrary composition and layering. The KCL-PAKKA model gives realistic predictions for many paper properties . In this paper we describe the cross-over that occurs with increasing grammage from a thin strictly two-dimensional network to a thick layered networks . According to our simulations the cross-over occurs at low grammages, around 20 – 30 g/m2, for papers made of stifffibres (e.g . mechanical pulp) and at higher grammages, 40 – 80 g/m2, for flexible fibres (e .g . beaten kraft) . We discuss the statistical properties of the three-dimensional but layered random fibre network, particularly the bonding degree and pore geometry . In thick networks the pore geometry . i s isomorphic and only the length scale of the pores depends on fibre properties.

  • 1997
    Cambridge
    pp 1293-1324Local orientation of flocs in paperAbstractPDF

    So far little attention has been paid to the orientation of flocs in paper sheets . It can be measured by means of an image analyser and the two-dimensional Fourier transform. The principal direction in the power spectrum of a paper in transmitted light is a measure of the floc orientation.

    It can be shown that the coefficient of correlation between fibre orientation (Lippke – Tester) and floc orientation amounts on average to 0.7, when profiles across the web are taken into account. Papers, particularly those which are made on hybrid formers, show a two-sidedness concerning floc orientation and anisotropy . The orientation has no significant effect on the uniformity of formation but determines the orientation of cockles. The power spectrum of the local orientation of a paper seems to be very useful in the detection of deterministic variations in orientation, which are invisible in light transmission images.

  • 1997
    Cambridge
    pp 1325-1327Determination of the friction of paper and board – colour pictures onlyAbstractPDF

    NA

  • 1997
    Cambridge
    pp 1329-1353Determination of the friction of paper and boardAbstractPDF

    Three test methods for the determination of the coefficient of friction of paper have been investigated : inclined-plane, horizontal-plane and strip-on-drum. Significantly different friction values were obtained for the same paper samples by each of the tests and the reasons for some of these differences were studied. Aspects such as contact deformation, contact pressure, testing procedures both environmental and mechanical and the presence of extractives were found to influence the frictional properties of paper and board.

  • 1997
    Cambridge
    pp 1355–1393Investigation of the pore structure of paper by novel porosimetric techniques: application to super and soft-nip finishingAbstractPDF

    Over the years, large research efforts have been spent on determining the pore volume distributions ofgraphic papers . The pore volume distributions determine the absorption properties and light scattering properties of these papers and can be modified by a variety of calendering conditions.

    In the present work we apply a variety of tests and void space modelling techniques to a series of five paper samples. The tests range from the standard to the novel, and all depend in some way on the void space structure of the samples. Three void space modelling techniques are presented. The first, traditional, method is based on the Laplace equation. The second model, developed by Yamasaki, implicitly assumes unconnected pores of a range of sizes, some of which saturate. The third, ‘Pore-Cor’,
    assumes a simplified three-dimensional structure. The samples all used the same 51 gM-2 SC grade paper, the first being uncalendered, and the other four involving combinations of soft- and super-nip , with and without the prior application of steam.

    Advanced imaging techniques such as optical microscopy and Environmental Scanning Electron Microscopy (ESEM) were used to obtain structural details of the paper cross-sections and paper surfaces, and with the ESEM it was also possible to investigate the effect of moisture on sheet structure.

    Absorption properties of the sheets were determined by using the well-known Bristow equipment and newly-developed equipment for determining liquid absorption by fibrous sheets based on Liquid / Air Displacement Analysis (LADA). The LADA equipment applies liquid in a well-defined way which enables valid comparisons to be made with common absorption theories.

    Results from the investigation with liquid porosiMetry using water and hexadecane probe liquids show that there is considerable sheet expansion when the sheets are exposed to water. Undoubtedly this will lead to a change in the absorption process when the sheets are in contact with moisture . The shape of the absorption curves in both the LADA and the Bristow test equipment also indicate that this expansion will affect the absorption process. A simple mathematical model was also used to take this into account.

    The mercury porosimetry data and the liquid porosimetry data were combined to yield a complete pore volume distribution curve for the sheet -structure. These data were then used in the Yamasaki absorption model to simulate absorption, and were compared with the measured absorption values. These results show that the absorption can be simulated with a knowledge of the pore volume distribution curve and basic properties of the absorbed liquid. Large pores dominate the liquid
    absorption at short contact times (<Is), whereas the smaller pores dominate the absorption at larger contact times, as simulated with Yamasaki model. The overall time limit for absorption is naturally dependent on the total pore volume in the paper.

    The combined porosimetry curves were also simulated using a recently developed three-dimensional void space modelling package, named Pore-Cor. This package generated three-dimensional structures with the same percolation characteristics and porosity as the experimental samples. The absolute gas permeabilities, of these structures showed the same trend as the permeabilities measured by a Gurley
    Densometer. The simulated structures facilitate the mathematical investigation of other effects, such as the trapping of non-wetting fluids and the effect on permeability of the inclusion of colloids, Matthews (1).

  • 1997
    Cambridge
    pp 1397-1424The Future of Paper: plus ça change ….. – Prepared contribution.AbstractPDF

    NA

  • 1997
    Cambridge
    pp 1469-1472Is fibre failure Griffithian? – Prepared contributionAbstractPDF

    NA

  • 1997
    Cambridge
    pp 1597-1601Fibril angle measurement using confocal microscopy – Prepared contributionAbstractPDF

    NA

  • 1997
    Cambridge
    pp 1617-1631The effect of AKD on paper friction – Prepared contributionAbstractPDF

    NA

  • 2001
    Oxford
    pp 19-80Jean-Claude RouxPulp Treatment ProcessesAbstractPDF

    The repulping, refining and hot dispersing processes are considered in this first part “Pulp Treatment Processes” of the review paper about “Stock Preparation”, which focuses on the process engineering aspects of the unit operations used in the production of virgin and recycled pulps. Chemical and physical-chemical aspects are beyond the scope of this paper, as are pulp dilution, transport and storage.

    The pulp treatment processes refer to the unit operations aim- ing at altering and/or upgrading the fibrous raw material and associated solid materials and contraries (inks and various contaminants). They include re-pulping or disintegration, refining or beating, hot dispersing and mixing. Pulp disintegration has curiously never been of great interest to the paper science community, and hence remains an area of investigation where quite substantial benefits could be gained through reductions in energy consumption. Some new approaches will be presented that get round the difficult concept of pulp apparent viscosity, which is really an aspect of rheology. One of the main operations in stock preparation is obviously pulp. While the effects of refining on fibres have been extensively studied in the past, its engineering parameters have not. For example, we do not yet know how to extrapolate refining results from the pilot to the industrial scale. It is even difficult to compare the effects of conical and disc refiners on the same pulp. It seems that only an integrated approach can improve our understanding of this process; one such will be proposed, building on fundamental engineering principles.

    Then, the paper goes on to consider hot dispersion, an important process step in the field of paper recycling and deinking, to complete the effects of pulping and/or refining in terms of ink detachment, alteration of contaminants and fibre conformability. Hot dispersion is generally combined with bleaching because of the high temperature, consistency and mixing effect.

  • 2001
    Oxford
    pp 81-191Particle Separation ProcessesAbstractPDF

    The screening, cleaning, flotation and washing processes are treated in this second part “Particle Separation Processes” of the review paper about “Stock Preparation”, which is focused on the process engineering aspects of the unit operations used in the production of virgin and recycled pulps. Chemical and physical- chemical aspects are out of the scope of this paper as well as the pulp dilution, transport and storage unit operations.

    Particle separation processes refer to the unit operations aiming at separating different pulp components in order to remove or concentrate some of them in different fractions. The particle separation processes are essential in recycling to remove various contraries in a very large particle size range. Separation is mainly based on particle dimensions in screening and washing, on particle density in centrifugal cleaning and on particle surface properties in flotation. Fractionation is normally performed as an intermediate process between screening and washing, though the centrifugal process can also be used. Particle size and shape and hydrodynamic phenomena are decisive in the separation processes.

    Fundamentals of centrifugal separation as well as flotation hydrodynamics have been extensively studied in the field of mineral processing, while fundamentals of pressure screening have not yet been investigated as much, since the technique is more recent and specific to the pulp and paper industry. The particle separation mechanisms are reviewed in this paper, with special emphasis placed on the description and analysis of physical mechanisms and on the theory developed about two main aspects:

    –  the large scale fibre suspension flows, which define particle transport and mixing;

    –  the particle separation micro-processes observed at the scale of the particles.

    Finally, the effects of machine, operating and material param- eters are briefly analysed with respect to the theoretical background.

  • 2001
    Oxford
    pp 193-210A Theory for the Transverse Collapse of Wood Pulp FibresAbstractPDF

    Theoretical considerations have led us to formulate a general equation for the collapse of wood pulp fibres in terms of trans verse dimensions, transverse elastic modulus of the fibre wall, and the collapse pressure. This equation is in agreement with experimental results on the effects of fibre transverse dimensions on collapse. We have also developed an equation for the transverse elastic modulus of the fibre wall in terms of fibril angle and the orthotropic elastic constants. We can therefore confirm the dependence of collapse on the transverse elastic modulus through the effect of fibril angle on the collapse of chemical pulp fibres.

  • 2001
    Oxford
    pp 211-224Conformability of Wet Pulp Fibres at Small Length ScalesAbstractPDF

    The conformability of wet pulp fibres has long been recognized as a key factor in the papermaking process. In order for fibres to bond in the sheet, their surfaces must be brought into contact.
    The conformability of wet pulp fibre surfaces at extremely low forces (nN) and displacements (nm) was investigated by micro-indentation measurements with a standard AFM tip and cantilever. Force-distance curves were measured as the tip impacted wet sulphate softwood fibre surfaces. The surface stiffness showed a large variability, even at different locations on the same fibre surface. Rough estimates of the local apparent elastic modulus of the wet fibres gave low values in the 0.01 GPa range.

  • 2001
    Oxford
    pp 225-254Characterizing the Mobility of Papermaking Fibres During SedimentationAbstractPDF

    The mobility of sedimenting fibre suspensions is characterized here in three different, yet complementary studies. In the first study we present a simple mathematical analysis to define more
    precisely the term sediment concentration. Through this analysis we correct the sediment concentration for compressibility effects and redefine this parameter as the gel concentration point. In the second study, we visualize the transient settling of radioactively labeled papermaking fibres using a new experimental technique, positron emission tomography (PET). In the third study, we measure the mass distribution of fibres (formation) in the sediment as a function of the initial suspension concentration. The results indicate that the gel concentration point occurs at a crowding number of approximately 16(±4). Two distinct regimes of settling were clearly identified with PET, depending upon the initial crowding number of the suspension (N). With N < 16, hindered settling was observed. With N > 16, fibres began to flocculate, starting with the long fibre fraction. Formation was found to be slightly dependent on N in the region N < 16 and then worsen significantly with N > 16. In summary, these findings indicate that within the suspension conditions found in papermaking 1 < N < 60, that there are two sub-regimes within these limits of differing levels of fibre mobility. These sub-regimes are delineated at N = 16.

  • 2001
    Oxford
    pp 255-286Liquid-Phase Turbulence in Pulp Fibre SuspensionsAbstractPDF

    Mixing-sensitive chemical reactions have been used to study liquid-phase turbulence in a number of dispersed two-phase systems, including pulp fibre suspensions. This technique has allowed the measurement and mapping of turbulence in a number of mixing configurations at pulp mass concentrations up to Cm = 0.10 (expressed as a fraction). Liquid-phase turbulence was found to
    decrease exponentially with suspension concentration, indicating that fibres are extremely efficient at dampening turbulence within a suspension. The magnitude of turbulence reduction correlates
    well with the reduction in gas-liquid mass transfer, an indirect measure of liquid-phase turbulence. The energy dissipated by the fibre network is compared to predictions made using fibre net-
    work theory.

  • 2001
    Oxford
    pp 287-338The Flow Resistance of Slotted Apertures in Pulp ScreensAbstractPDF

    Pulp screens remove fibre bundles, plastic specks and other oversize contaminants from pulp suspensions before the pulp is madeinto paper. Within the pulp screen is a screen cylinder that acceptable fibres pass through but oversize contaminants do not. Apertures in the screen cylinder are in the form of holes or slots, and their size is perhaps the most critical variable in screening. Smaller
    apertures increase the removal efficiency of contaminants, but also lead to a reduction in screen capacity.

    The development of screen plate “contours” in the early 1980s led to a revolution in pulp screen design. By locating apertures within recesses on the screen plate surface, smaller, more efficient, apertures could be used without a significant loss in capacity. Various theories have been proposed to explain the action of these contours. It may be that contours increase the turbulence at the aperture entry, which fluidizes the pulp and clears fibres from the aperture. It may be that the contours streamline the flow through the aperture to reduce hydraulic resistance. Alternatively, contours may alter the streamlines through the aperture to reduce the tendency for fibres to become immobilized at the slot entry, which
    is a precursor to blockage. What is clear is that understanding the action of the contours is critical to the full exploitation of this important development in screening technology.

    The objectives of the present study were: (1) to create a frame-work for assessing the flow resistance of screen plate apertures; (2) to learn how aperture size, screen plate contours, fibre blockages and other factors of practical importance affect resistance; and (3) to develop a more fundamental understanding of what determines resistance, and how this knowledge could be used to increase screen performance.

    Flow resistance was assessed using the non-dimensional pressure drop coefficient (K) across the screen plate, and K was studied by three methods. Computational fluid dynamics (CFD) was used to predict how aperture geometry and flow variables affect K in an idealized screening configuration. Experiments with a flow channel were used to confirm the theoretical CFD findings and explore how fibre accumulations at the screen aperture affect K. Finally, trials with an industrial pulp screen showed how industrial variables such as pulp consistency and pressure pulsations influence K.

    CFD analysis determined that the vortex at the slot entry has a dominant influence on K for water flow. The size of the vortex was reduced by increasing the ratio of slot velocity to upstream velocity, a quantity termed the “velocity ratio” (VN). The relationship between K and VN was defined by two regimes: a “descending regime”, where K decreased rapidly with increased VN, and a “constant regime” where K was relatively independent of VN. Examination of the flow patterns revealed that for smooth slots, the vortex on the upstream side of the slot diminished in size in the diminishing regime. The flow then approached a pattern that was relatively unaffected by further increases in VN (constant
    regime). The presence of a contour at the slot entry led to the expected reduction in K. This study showed that the effect of the contour is dependent on VN. At low values of VN, the contour actually caused K to exceed the value for a smooth slot. The precise dimensions of the contour are critical to its effect. For a step-step type of contour and VN = 0.5, the optimal contour for simple hydraulic resistance had a depth of 0.25 mm and step-width of 0.50 mm. An increase in the contour depth to 1.0 mm caused K to double and to exceed the value for when there was no contour at all.

    Experimental measurements of K were made for steady flow through slots in a plexiglas channel. Good agreement with the CFD findings was obtained for both smooth and contour slots. To assess the influence of fibre accumulations, the instantaneous value of K was monitored as a fibre accumulation grew. In one typical case, a fibre accumulation filled half of the slot width and caused K to increase from 4.3 to 9.8. This finding underlines that flow resistance is due to both the hydraulic resistance of the slot and the added resistance due to fibre accumulations within the slot.

    Pilot plant tests were conducted to assess K in an industrial-scale pulp screen. The screen was modelled as a series of resistances combined with the pumping effect of the rotor. One could thus infer the resistance of the screen apertures by measuring the overall pressure differential across the pulp screen, and then by discounting the influences of the screen rotor and housing. The findings for water flows were in agreement with the CFD and flow channel work: The descending-constant form of the K-VN relationship was again found, and the values of K were comparable to those in the CFD and flow channel work. The use of a 1.5% pulp suspension instead of water caused K to double in one typical case, indicating substantial accumulation of fibre within the slots.

    This study has defined K as the essential measure of flow resistance and shown how it can be measured through theoretical, flow channel or pilot plant tests. The value of K was found to depend on both hydraulic resistance and the degree of fibre blockage in the screen slot. Screen plate contours were found to reduce K by streamlining the flow, although it is recognized that they may also reduce the tendency for fibres to accumulate within the slot. Values of K can thus be used in the development of improved screening technology, and to compare the performance of screen cylinders from different suppliers. The use of K is also important for process control routines that estimate the extent of fibre
    blockages in screen plate apertures and act to prevent screen failure.

     

  • 2001
    Oxford
    pp 339-356Ink Release from Printed Surfaces – New Methodology and Initial Insights to the True Mechanisms Behind Ink DetachmentAbstractPDF

    The aim of this study was to develop and test a new equipment for evaluating the mechanism behind ink detachment from printed model surfaces. The equipment developed for this purpose consisted of an impinging jet cell, a printed model cellulose surface and a microscope equipped with a CCD camera for image collection. By applying image analysis to images of the printed surfaces at different time intervals, during the detachment studies, it was possible to quantify the ink detachment from the surface. Mechanistic studies of offset ink and flexographic ink detachment were also performed with the new equipment. Results show that the flexographic inks seem to be removed by a washing process in which the printed image is gradually removed from the surface. For the offset print the results are quite different. In order to remove the printed offset ink it is necessary to have a certain hydrodynamic shear in combination with a swelling of the cellulose surface. This swelling seems to create a relative movement between the ink and the cellulose surface. In solutions with higher ionic strength no offset ink is removed.

    These results are in line with earlier assumptions about the deinking mechanisms but in the present work these processes are actually shown for the first time.

     

  • 2001
    Oxford
    pp 359-392An Experimental Study by NMR and SANS of the Ambient Hydration of PaperAbstractPDF

    The structural changes in fibre polymers and dispersion of water in the polymer have been studied at length scales less than 400 Å with contrast variation small angle neutron scattering (SANS) and solid state nuclear magnetic resonance (NMR). The SANS of hydrating paper samples is discussed in different angular regions in terms of a scattering wavenumber vector, q (q = 4π/λ . sin θ/2
    where λ is the wavelength of the neutrons and θ is the scattering angle). At low q close to the neutron beam, the Guinier region, voids in the structure are found to disappear as the microfibrils
    swell with water. The lateral dimensions of the cellulose crystallite are calculated from x-ray diffraction data and there is a good qualitative correlation with relative size of the crystallites and the
    appearance of short range of order in the SANS in the mid-range of the q studied. The range of the length scale of the SANS feature is slightly larger than the elementary crystallite which is consistent with layers of swollen cellulose and water around the crystallite. In the high q region, the angular region furthermost from the beam, the scattering is discussed in terms of deviation from Porod scattering. According to this interpretation the interface between cellulose and water is not clearly defined and there is an increase in the amount of surface area for water to bind to.

    These results are consistent with water disrupting the hydrogen bonding in fibre polymers. The NMR spin diffusion experiment monitors the exchange of magnetisation between water and polymer protons. A simplistic model of this transfer process is justified and indicates that water is not uniformly dispersed in the polymer as a function of moisture content.

  • 2001
    Oxford
    pp 393-413Colloidal Stability of Alkyl Ketene Dimer (AKD) Dispersions. Infl uence of Shear, Electrolyte Concentration, Poly-electrolytes and Surfactants.AbstractPDF

    The influence of shear, electrolytes, polyelectrolytes (anionic trash), and surfactants on the colloidal stability of differently formulated AKD dispersions was investigated. The stability was tested under shear in a Britt Dynamic Drainage Jar by measuring particle size distributions and microelectrophoretic mobilities. Three cationic dispersions were used, stabilised with (1) starch with low charge density, (2) starch with medium charge density and a polyaluminum salt, (3) synthetic polymer with high charge density.

    All dispersions showed good to excellent stability to high levels of shear and electrolytes.

    All tested dispersions were influenced by carboxymethylcellulose, CMC, and flocculation was induced at a CMC addition giving a z-potential around zero. Dispersions with a higher charge density formed larger flocs and the floc size was a function of the ratio dispersion/CMC. The presence of xylan and sodium oleate only affected the dispersions with high charge density.

    Experimental results and theoretical calculations of the rate of AKD flocculation are in good agreement. The theoretical calculations are based on Smoluchowski’s equation for collisions between spherical particles in a uniform shear with a correction for hydrodynamic and van der Waals forces between the particles. Calculations indicate that AKD flocculation can compete with deposition on fines and fibres under papermaking conditions.

  • 2001
    Oxford
    pp 415-427Dissolution Behaviour of Calcium Carbonate in Mildly Acidic ConditionsAbstractPDF

    NA

  • 2001
    Oxford
    pp 431-558Overview of Forming Literature, 1990-2000AbstractPDF

    Paper structure characterisation has been extended to include 2-D formation and periodic marks as well as wavelet techniques. Local fibre orientation anisotropy and misalignment angle analysis, in
    combination with tape-splitting layering and image analysis have improved the understanding of the relationship between paper structure and properties as well as increased the possibilities to
    trace the forming history from final paper structure.

    The Crowding Factor has been introduced, to describe the tendency of fibre suspensions to form fibre flocs. It gives an improved predictability in comparison with fibre concentration, by also taking fibre slenderness into account. The dynamic characterisation of de-flocculating and re-flocculating mechanisms is getting increased attention, partly due to the potential of CCD-cameras and image analysis techniques. It is the opinion of the authors that floc stretching is a more promising way of de-flocculating than turbulent shear. It is doubtful whether turbulence in simple fluids can aim as a model for the flow irregularities in fibre suspensions, due to the damping effects of fibres and flocs.

    For headbox CD-profile control, dilution water injection has been introduced. Since this makes slice lip deformations un-needed, it has the potential to uncouple control of grammage and misalignment angle profiles and also to improve CD-control resolution.

    The importance of headbox nozzle design for fibre orientation anisotropy is now appreciated. A high nozzle contraction ratio will increase anisotropy and also introduce some deflocculation through floc stretching. Vane insertion helps to decrease fibre orientation anisotropy and is also applied for multi-layer forming. Although applied in tissue forming and in linerboard forming, further improvements have to be made for printing paper applications.

    In twin-wire forming, the roll-blade principle has been accepted by the main machine manufacturers. Since its introduction in the STFI-Former in 1991, the blade section design with stationary blades on one side and loadable blades on the opposite side is the main design for printing paper applications, and also increasingly used for different board qualities.

  • 2001
    Oxford
    pp 559-589Prediction of Paper Formation by Fluidisation and Reflocculation ExperimentsAbstractPDF

    Reference geometries consisting of a constriction block and a secondary pipe were used to provide reference information for estimation of the performance of more complex geometries resembling to real headbox designs. This information included the fluidisation ability curves, i.e. the minimum attained floc size in function of mechanical energy loss, the rate of fluidisation in sudden expansion, the rate of subsequent reflocculation and the level of saturation floc size. The functionality of this approach was illustrated with measured information of a particular complex geometry on several research environments of different scale. By paper samples produced with a similar geometry, the tight connection of the fluidised state of the suspension and of the attained formation was verified. Reduction of the residence time of suspension in the headbox resulted to lower floc size in slice lip area and to better level of formation in produced paper. In addition, both properties revealed a similar form for this dependency. By changes on tensile strength ratio of the produced paper, the operation of the forming section was shown to have an apparent but not controlling effect on the level of formation obtained.

  • 2001
    Oxford
    pp 591-604Application of Ultrasound Anemometry for Measuring Filtration of Fibre SuspensionAbstractPDF

    We introduce a novel method for measuring the properties of consolidating wood fibre network during filtration of liquid-fibre suspension. The device consists of a hand-sheet mould equipped with a pulsed ultrasound-Doppler anemometer for measuring the local time-dependent velocity field of the fibre phase during vertical filtration. Simultaneously, the total flux of the suspension and fluid pressure loss through the filtrated fibre layer are measured. Based on this experimental information other relevant flow quantities can be computed. We thus find the space-time evolution of velocity, volume fraction and pressure (stress) fields separately for the fluid phase and for the fibre phase. This method allows us to experimentally study the details of the consolidation process in dynamic conditions. The device can be applied as an advanced laboratory test instrument for measuring relevant physical fibre network properties. As a result we present, e.g., the measured local stress-strain history of the consolidating fibre layer during a filtration experiment.

  • 2001
    Oxford
    pp 605-618The Structure of Turbulence in the Near-wall Area of a Channel FlowAbstractPDF

    Fluid dynamics plays an essential role in the paper manufacturing process. The quality of paper is affected by the turbulence properties in the headbox and jet. In this paper the structure of turbulence in the wall area of a two-dimensional converging channel is studied experimentally. The measurements are performed with Particle Image Velocimetry, which provides instantaneous two-dimensional velocity fields. The structure of turbulence is studied by analysing both instantaneous and time-mean velocity fields. As a result several kind of flow structures can be identified close to the channel walls. The most prominent are streamwise elongated structures, which manifest themselves as a spatial modulation of the streamwise velocity component. Substantial activity in the near-wall region is related to the mean-shear close to the surface. The mechanisms of the wall-turbulence are discussed in a short
    review of the main concepts found in the literature. The results of this study are expected to improve the understanding of the significance of the headbox slice boundary layers in the paper-making process.

  • 2001
    Oxford
    pp 619-636Coherent Structures of Suspension Flow and their Inheritance in PaperAbstractPDF

    Normally, the main purpose of vanes installed in the slice chamber of a hydraulic headbox is to control the tensile strength ratio by affecting the mean fiber orientation of the suspension. The use
    of vanes inherently leads to the formation of peculiar vortex structures, referred to here as coherent flow structures (CS), in the downstream flow. These CS are believed to produce non-homogeneity in paper. Although the CS are geometrically three dimensional, their machine direction (MD) and cross direction (CD) components are dominant, cf. Kármán vortex street, and have distinctive characters of their own. The CD component of the vortex maintains its coherent nature better than the MD component and, therefore, its appearance in a form of MD spatial scale is used to express the inheritance of the flow structures in the paper. By using bluff vanes of different thicknesses, it is shown that the CD component of the vortex street maintains its characteristic appearance from the slice chamber to the slice jet. This made it possible to study the inheritance of flow structures in paper with more realistic vanes. By marking particular parts of the flow in the slice chamber with dye streamers the CS were also made visible in the paper. An analysis of these samples reveals that the average scale of ink spots is related to the MD spatial scale of flow structures. Finally, a correlation between the CS of flow and the structural cockling tendency was found. Thus, important complementary information has been created to serve the goal of finding a general linkage between structures in flow and paper. The knowledge of this link would not only allow research and analysis of wet end operations without actually producing paper, but would also provide a means of evaluating wet end conditions in a mill environment by paper analysis.

  • 2001
    Oxford
    pp 639-678Wet Pressing – Present Understanding and Future ChallengesAbstractPDF

    The essential scientific problems in wet pressing are concerned with water removal from the wet web, its runnability and the effect of pressing on the quality of the web and the paper produced from it. This paper briefly reviews the present understanding of the effect of wet pressing on the web and paper quality and discusses some questions concerning the runnability of the web through the press section. The main emphasis is placed on water removal. A short historical review of the development of our present understanding of wet pressing fundamentals is presented. The modelling of wet pressing is also discussed.

    The water removal from the fibre cell wall starts at fairly low solids contents of the web, in the range of 20–25%. In modern press sections, the solids content of the web after pressing is about 45–50%. At this solids content, most of the water is in the fibre wall. Thus, when trying to enhance water removal further, it is necessary to understand the mechanisms and controlling factors in cell wall dewatering. Present scientific efforts should therefore be focused on finding as invariant and quantitative  knowledge as possible on the behaviour of the cell wall under wet pressing conditions.

    Recent research on cell wall dewatering is reviewed in the paper. Advanced measuring methods such as NMR, solute exclusion, WRV(CCV) and DSC techniques have produced new and to a certain extent invariant information on the cell wall structure and dewatering. As a result, a clearer picture of the differences in the behaviour of mechanical and chemical pulps, softwood and hardwood pulps and different types of fines material has emerged. The effect of hornification and beating has also been clarified. Further development of measuring techniques such as DSC-based thermoporosimetry is most likely to improve our understanding in this area, helping to make it more accurate and invariant.

  • 2001
    Oxford
    pp 679-708Densification and Dewatering in High Temperature Wet Pressing.AbstractPDF

    A number of quantitative models of wet pressing have been proposed in the literature but none of them take into account the observed rate dependence of the structural pressure. This rate dependence, which is ascribed to the flow resistance of the water in the fibre walls, should be incorporated in wet pressing models.

    This paper presents a hydrodynamic model of wet pressing that includes the rate dependence of the structural pressure and thus takes into account the flow resistance of the intra-fibre water. The model is general in the sense that it can predict changes in the state of the fibre web due to a pressure pulse of arbitrary shape, under a wide range of web saturation and temperature conditions. The model can predict the effect of high press-roll temperatures.

    The predictive capability of the model has been studied with the help of data from the third press of a pilot paper machine. For wet pressing at ambient temperatures, the model gives good predictions of solids content over wide ranges of machine speeds, grammages and linear loads and its predictive capability is quite adequate for most engineering applications.

    For high press roll temperatures, the predictions are satisfactory over a wide range of press roll temperatures and linear loads but some significant systematic discrepancies occur. These predictions can probably be improved by using better estimates of the hardening curves at high temperatures. Once calibrated, the model can give considerable information about changes in the state of the web as it passes through the nip and some insights about the mechanisms that are active.

  • 2001
    Oxford
    pp 709-736Drying of Paper – An Overview, the State of Paper Drying KnowledgeAbstractPDF

    The paper concerns various present and projected future drying technologies employed to dry paper and paperboard products. The topics discussed include moisture distribution and mobility within the web and how this can effect important paper properties. It is suggested that dryer sections have a much higher potential than their present role of simply drying and transporting the web from press to reel and that their future active role will be made possible by sophisticated new drying models specifically developed to predict and control web defects such as curl, mottle and others. Also covered are economics of drying alternatives for various paper and board grades and an overall assessment of future drying technologies including impingement drying, Condebelt drying, impulse drying, direct steam drying, IR and induction and microwave drying. The paper concludes with a look at further needs in fundamental and applied research in drying.

  • 2001
    Oxford
    pp 737-758Drying Induced Tension Variation in Paper WebAbstractPDF

    Laboratory drying experiments were done with a standard tensile testing device to study basic mechanisms of moisture induced tension variation. Also, pilot machine experiments were done to compare with the laboratory tests. The results show that the tension variation of paper is generated by a combined effect of moisture variation and straining during drying. The laboratory experiments indicate that tension variation is induced by strain differences in paper web. Tensile stiffness, which is affected most by wet straining, is almost constant between areas of different initial moisture contents. On the other hand, pilot tests showed only small effect of the initial moisture variation. In rewet experiments it is found that dried-in strain is recovered when paper is rewetted.

  • 2001
    Oxford
    pp 759-781Modelling the Web Tension Profile in a Paper MachineAbstractPDF

    Poor cross-directional (CD) tension profiles of paper webs cause runnability problems in paper mills as well as in printing presses. This study explored the formation of the cross-directional web tension profile in paper machines. The research programme was implemented in a period of four years. Tension measurements were made during the production in both paper machines and printing machines.

    The finite element method (FEM) was applied to explore the build-up mechanisms of the web tension profile. In FEM modelling, the web was constructed from continuum elements and the physical interactions of the elements were defined, for example, by the Hookean law. The FEM analysis has numerous advantages and it is important to investigate the influences of the mechanical conditions and the material properties of the paper web. The simulations made by FEM could quantitatively predict the shape of the tension profile.

    The paper web is stretched in many stages through the drying section. Stretching in the machine direction (MD) causes a non-homogeneous stress field in the web because the paper is subjected to mechanical shrinkage which is defined by the Poisson’s ratio of the paper. This typically causes a situation where the edges of the web are slacker than the middle areas, the so-called crying shape. The uneven stress fields in the web also cause a lower MD and CD tensile stiffness because of lower frozen-in-stress in the paper sheet. Further on, the slacker edge areas are affected by higher CD drying shrinkage which further diminishes the tensile stiffness and also leads to higher relaxation in the machine direction.

    Several trials were made to control the web tension profile. These included moisture profiling, jet-wire speed, edge flow, strain rate and nip trials. The most effective control was by moisture profiling with the steam box in the press section. The drier sections of the web became tenser and vice versa. This was because the moisture content of the paper has a strong effect on the formation of the tensile stiffness of the paper. The principal shape of the tension profile is convex. According to this study it is impossible to reach an even tension profile with the existing dryer section configurations. However, the tension profile can be adjusted within certain limits by controlling, for example, the development of the moisture profile. These controls play an important role in improving the runnability of the paper machine, the winder and the printing press.

    Two different instruments were used to measure web tension. IQTension measures the pressure within an air film that is formed between the paper web and the curved metallic deflection plate. The curved deflection plate is perforated at intervals, so that the pressure sensors connected to the orifices in the plate measure the cross directional (CD) profile of the air pressure, which correlates with the web tension profile. The web tension is measured at a high sampling rate in several parallel positions across the whole web width. In addition, the tensile stiffness profile can be defined by measuring the tension at different draws.

    Scanning by Tenscan was used, when there was only a limited space for the measurement, or there was a need for measuring simultaneously with IQTension. Tenscan uses a laser beam to measure the passing time of a propagating membrane wave in the web. Other measurements included normal online measurements in the paper machines. The TSO and shrinkage profile measurements were made in the laboratory.

  • 2001
    Oxford
    pp 785-895On-line Papermaking Sensors: An Historical PerspectiveAbstractPDF

    As the title implies, this paper will discuss papermaking on-line instrumentation, concentrating on sensors for paper quality measurements. The review is intended to explore not just the techniques for measurement, but also the evolution of a particular measurement and the research behind it.

    On-line instrumentation will be the primary focus, covering not only the usual structural, mechanical and appearance properties of paper but also certain water system measurements around the papermaking thin/thick stock loop. These measurements will include a discussion of the sensors pertaining to “wet end chemistry” and consistency, with their incorporation into retention control schemes.

    Discussion of off-line, laboratory measurements will be included where pertinent to provide a prediction of future on-line sensor availability. The usual papermaking process measurements such as pH, temperature, level, pressure and flow will not be covered.

  • 2001
    Oxford
    pp 897-926Thermoporosimetry of Pulp FibersAbstractPDF

    This paper covers the use of thermoporosimetry to measure the pore size distribution (PSD) of pulp fibers. Thermoporosimetry is based on the melting temperature depression of an absorbate in a porous structure. A discreet or “step” melting procedure, rather than the usual continuous method, is used to melt the absorbate. This method eliminates thermal lag and gives the high temperature accuracy required for measuring large pores.

    Measurement of water-saturated chemical pulp fibers using this technique, combined with solute exclusion, indicates a bimodal distribution of cell wall pores. The interpretation of data from water-saturated fibers is complicated by several factors: 1) distortion of the cell wall by ice crystal growth; 2) the depression of water’s melting temperature by osmotic pressure; and 3) inadequate range to cover the larges pores. One way to correct these problems is by replacing the water with cyclohexane. The major disadvantage of this approach is that the cell wall contracts in cyclohexane and its pore structure may change in other ways which are not understood.

    Like water, the cyclohexane analysis shows a bimodal distribution of pores. The smaller pores, “micropores”, are less than about 5 nm in diameter, the “macropores” are about 15–700 nm. It was found that there is a quantity of cyclohexane in the cell wall which does not freeze. Analysis of nonfreezing cyclohexane indicates a surface area of about 400 m2/g for kraft pulp. The cyclohexane method is very suitable for studying beating, which primarily involves the opening of larger pores.

  • 2001
    Oxford
    pp 927-945Thermal Conductivity of Handsheets, Papers and Model Coating LayersAbstractPDF

    Thermal conductivity of paper is a property of importance in the understanding of conductive heat  transfer in the pulp and paper industry. For example, heat transfer between a paper web and a heated roll in a nip during calendering. Besides, the trends in the calendering area are to replace machine calenders by soft calenders and to replace cotton filled rolls by polymeric rolls in the supercalenders. As a result, temperatures of heated rolls in calenders are increasing constantly. There is a need for measuring thermal conductivity of paper. Some measurement methods and some values of thermal conductivity are available in the literature. But measurement methods are often reserved to specialists and values of conductivity show great variations. This drove us to develop a rapid method to measure the thermal conductivity of semi-insulated films. Our objective was to be able to measure the thermal conductivity of papers, of polymers and of model coating layers–with simplicity and rapidity, if necessary at the expense of the accuracy. Indeed heat transfer calculations often used a lot of restrictive hypotheses and a measurement of the thermal conductivity with an accuracy of less than 10% is quite sufficient.

  • 2001
    Oxford
    pp 947-974Permeability Measurements of Press FeltsAbstractPDF

    NA

  • 2001
    Oxford
    pp 975-998Experimental Technique for Tracking the Evolution of Local Moisture Nonuniformity in Moist Paper from Wet to DryAbstractPDF

    Local nonuniformity of moisture content, a basic characteristic of moist paper, affects efficiency and hence cost of paper drying, and may influence product quality. Such local nonuniformity may become even more of a problem with the current interest in combining higher intensity air convection drying and cylinder drying to produce the required higher capacity hybrid dryer sections of the future.

    Direct determination of local sheet moisture content under dynamic conditions during drying is unacceptable because the measuring instrument presence would change local moisture content. A novel indirect technique was developed for quantitative, precise determination of local nonuniformity of moist paper by monitoring continuously the local exit pore air temperature at many positions immediately below a moist sheet subjected to air through flow. This technique was used to investigate local non-uniformity for moist machine-formed papers of grammage 19–55 g/m2, and 20–100 g/m2 handsheets of variable formation. The effect of formation and basis weight on local nonuniformity was
    quantitatively documented. Formation was characterized using the new method of partitioning formation nonuniformity into its components as a function of scale of formation. The results provide some evidence that it is the components of formation nonuniformity in the range of larger scale of formation, 8 to 37 mm, which most affect moisture local nonuniformity while the formation components at 0.8 to 3 mm scale of formation appear less important. Such knowledge is relevant to the development of the improved drying processes of the future.

  • 2001
    Oxford
    pp 1001-1036Future Directions in Calendering ResearchAbstractPDF

    Calendering is the papermaker’s last chance to reduce thickness variations along the length and width of the finished sheet, and to improve the sheet smoothness. A smoother sheet results in improved print quality, while more uniform thickness profiles improve the winding process. The calendering operation thus improves the quality of the finished product. In recent years there has been an increase in the loads, speeds and temperatures at which soft-nip calenders, whether on or off line, can be operated without mechanical failure of the cover; the result has been an improvement in the surface and printing properties achievable with mechanical printing grades of paper, and an increase in the production rates which can be sustained. As a result, these calenders have slowly replaced traditional machine calenders in new and retrofit installations.

    The best available design and trouble-shooting tools for modern machine calenders are based on empirical models, whose coefficients have not been related to fundamental paper or fibre properties. New furnishes therefore require experimental determination of these coefficients, and extrapolation to new calendering conditions involves some risk. As well, there are no published models, empirical or otherwise, for the design and troubleshooting of soft-nip calenders, an unfortunate state of affairs given the increased number of installations of these machines. The purpose of this review is to outline the current understanding of the process, and to identify areas where further research could be useful to allow better prediction of paper properties arising from a change in the equipment or operating conditions.

  • 2001
    Oxford
    pp 1037-1042On the Distribution of Pore Heights in Random Layered Fibre NetworksAbstractPDF

    NA

  • 2001
    Oxford
    pp 1043-1066Reversible Compression of Sheet StructureAbstractPDF

    We study the compression behavior in the thickness direction of paper using quasi-static pressure cycles from 0–10 MPa. The reversible component of compression agreed reasonably well with
    the following equation:

    where is the volume fraction (porosity) of compressible pores, and E* is the effective elastic modulus characterizing the compression of the pores. The model can be derived both from the height distribution of pore space and from the general linear relationship between logarithmic strain and pressure. In handsheets, the fitted porosity parameter ranged from 0.13–0.22 depending on sheet density. The values of the elastic modulus E* varied between 4 and 5 MPa. They could even be set constant at ca. 4.5 MPa without a significant loss of model agreement. Our results suggest that the reversible compression behavior of paper depends primarily on the porosity of the fiber network and only a little on the furnish composition.

  • 2001
    Oxford
    pp 1069-1113Surface Treatment: Sizepress Tradition, Current Development and a Pigmented Chemical FutureAbstractPDF

    Evolution of surface improvements on paper are driven by tech- nical needs, in respect of printing and writing, economical drivers, such as mineral versus fibre cost and market niche exploitation, runnability and aesthetic requirements. From the simple anti- feathering absorption control of hydrophobisers, such as starch, to the sophistication of digital printing, the need to modify the surface of paper is an accepted criterion for investment in our industry. Simple pond and roll application has, through increases in speed and the application of pigmentising, developed in recent times to controlled film application processes. The demands throughout these developments for the scientific understanding of the interactional processes between fluids and substrate: absorption, hydraulic penetration, basepaper fibre debonding and roughening: and the complex rheology of pigment suspensions, based on stability, surface chemical and hydrodynamic criteria, have usually been met retrospectively. The machine concept or application is developed, installed, trouble-shooting achieves compatibility in the industrial environment and only latterly does the question of optimisation through a thorough scientific analysis come to the fore. By studying the models and accepted design

    criteria that have emerged throughout this evolution, it is pro- posed that advances through extrapolation of the models and by seeking new untried dimensions of the models can be made which can act to fuel the development further. Notwithstanding that current knowledge in multivariate applications inevitably remains inexact, the author attempts to demonstrate how the concepts available today could be extended to build potential for develop- ment in the future. Mechanics of suspension flow, dewatering and network absorption remain the primary controlling parameters and a critical review of past and present assumptions and new research is timely in order to re-fashion the directions that can be deduced. Rates of absorption as a function of available fluid vol- ume arising from the resistance dynamics of rapid dewatering provide a key opportunity for improving surface coating holdout especially in the challenging on-machine context where the time- scale for hydrophobising is too short and the wettability and moisture profiles are at their most critical. On the basis of this review, it is proposed that high rate absorption, through the use, for example, of ultra-high surface area filler pigments in the substrate, together with rapid low solids immobilisation of pig- mentised formulations with disparate coating and basepaper permeabilities, provides a means of maximising the potential for surface treatment applications both at high speed and in the con- text of sophisticated niche product development. The roles of basepaper and pigment structure are therefore also an integral part of the review.

  • 2001
    Oxford
    pp 1115-1147Influence of Coating Pigment Chemistry and Morphology on the Chromatographic Separation of Offset Ink ConstituentsAbstractPDF

    This work is aimed at characterising the chromatographic action of some typical paper coating pigments in relation to the separation and fractionation of offset printing ink components during absorption into the coating void structure, with particular emphasis given to the chemical and structural parameters of the pigments.

    The separation phenomenon was studied experimentally using primarily large-scale model systems based on a modified thin-layer chromatography method. The separation of the ink constituents was detected directly from the absorption path using Fourier Transform Infra-Red (FTIR) microscopy. The coatings were characterised for their physical properties using mercury porosimetry, and the absorbing fluids for their viscosity and surface energy. The ink-coated paper contact on the realistic scale was evaluated using the concept of ink-on-paper tack development. Coating pigments used in the work included typical paper coating grade ground calcium carbonates, fine clays of different origins, as well as talc and precipitated calcium carbonate. The choice of pigments provided the possibility to investigate independently the chemical and physical aspects of pigment properties on the constituent fluid separation. Ink chemicals included representatives of the most typical offset ink components; namely mineral oil, linseed oil, ink resins and ink pigment. A blend of mineral and linseed oils was used as the main test fluid.

    The differential interaction or adsorption-desorption on the pigments leads to separation of the mixture of mineral and vegetable (in this case linseed) oils as they are absorbed into the pigment coating structure, where the more polar linseed oil is preferentially retarded in the structure composed of the more polar pigments. The degree of the separation is directly proportional to the surface area of the pigment within a group of chemically and morphologically identical pigments. It is also affected by the surface chemistry of the pigment but more so in comparing dispersed and undispersed pigments, and polar versus non-polar rather than between the chemically different hydro-
    philic dispersed pigments. Pigment morphology strongly influences the macroscopic flow behaviour due to changes in physical porous structure (pore size, porosity, tortuosity). The macroscopic flow behaviour, which is affected by all of the above mentioned structural and chemical parameters of both the solid and liquid phases, proved to be a universal parameter determining the separation efficiency of the oils by affecting the retention time. Added ink resins in the oil mix retards the overall absorption rate of the oils, but does not affect the separation tendency of the oils in most of the cases studied, except in cases where less polar pigments or dispersions are involved. Adding latex into the coating structure, on the other hand, creates an additional gradient for separation of the oils as oil diffusion into the latex provides further selectivity.

  • 2001
    Oxford
    pp 1149-1181Binder Migration in Paper Coatings – A New PerspectiveAbstractPDF

    Clay or other minerals are coated onto fibrous paper to improve print and optical properties. The minerals are fixed to the fibrous substrate with a binder, either latex or starch. The coated paper is
    dried during manufacture, and if the binder migrates during this drying, there can be a deterioration in properties. Not only are there manufacturing considerations as to the choice of binder, but also environmental ones with regard to factors such as the energy used in drying. This work reveals new perspectives on binder migration which have been gained by a variety of approaches. Firstly, a critique is presented of the experimental methods which have been used by other workers. Then a comprehensive series of experiments is described. For ease of study, and because any effects are likely to be exaggerated, most of the experiments were carried out on samples which were 1.4 mm thick, some two orders of magnitude thicker than commercial coatings. However, the results were also replicated in less extensive tests on samples of thickness 55 μm. The experiments show that under a very wide range of conditions, including different coating thicknesses and drying temperatures, no latex migration is observed. Migration of starch was observed, however, and caused an increase in starch concentration at the surface during drying. If the sample was covered during the experiments, the system relaxed back and the concentration enhancement was reduced. The relationship between the particle size of the latex and starch, and the extent to which these particles might percolate through the void structures, was investigated by the application of the “Pore-Cor” software to mercury intrusion experiments. Also presented are the results of a mathematical continuum model, which accurately describes the migration of starch binder in terms of only two parameters, namely the evaporation efficiency and the diffusion coefficient of starch.

  • 2001
    Oxford
    pp 1183-1202Compressional Rheology of Model Paper CoatingsAbstractPDF

    The surface treatment of paper is commonly undertaken in order to improve a set of key end-use properties, including optical response and printability. These properties can be influenced, to a
    significant extent, by the sub-surface structure of the coating layer. The nature of coating dewatering also has strong implications for machine runnability. Thus, there is a clear need to understand in suitable detail the nature of the coating consolidation process. In this study, we have applied a novel approach to characterising the equilibrium consolidation state of calcium carbonate sediments, both with and without polymeric thickener. The aim is to provide a quantitative link between the structure of consolidated layers and their network strength, through the compressive yield stress, Py(). A suspension, prepared at a given volume fraction of solids 0, is centrifuged to produce a consolidated
    particle sediment (or gel). The solidity variation of that sediment as a function of depth is then measured using one-dimensional magnetic resonance imaging (MRI), and Py() calculated directly from the volume fraction profile. The results obtained are discussed in the light of particle network structure, the effect of polymer on particle consolidation, and the relation to viscoelastic properties of the suspensions. The link to the dewatering of coating suspensions, and structure formation in coating layers, is also considered.

  • 2001
    Oxford
    pp 1205-1288The Structural Characterisation of Fibre Networks in Papermaking Processes – A ReviewAbstractPDF

    The literature concerning the structure of two- and three- dimensional fibre networks is reviewed. Emphasis is placed on the literature concerning such networks in papermaking processes, though examples are drawn from other systems. The propensity of a suspension to flocculate is considered from a theoretical viewpoint. The experimental techniques and structural descriptors applied in the characterisation of fibre networks are discussed. Theoretical studies of random networks are presented along with their use as reference structures and comparison is made between the main techniques used in the structural characterisation of essentially two-dimensional networks such as paper. The relationships between the distributions of mass and voids are examined and the dependence of sheet nonuniformity on that of the suspension from which it is formed is reviewed.

  • 2001
    Oxford
    pp 1289-1311The 3D Structure of Paper and its Relationship to Moisture Transport in Liquid and Vapor FormsAbstractPDF

    The three dimensional structure of paper materials plays a critical role in the paper manufacturing process especially via its impact on the transport properties for fluids. Dewatering of the wet web, pressing and drying will benefit from knowledge of the relationships between the web structure and its transport coefficients. Among transport, moisture diffusion in paper is central to the understanding and optimal design of paper products for their performance in different environmental conditions. Our recent research of moisture sorption in paper has indicated that diffusion of water vapor through the pore space is an important mechanism for transport [1,2]. The effect of the three dimensional structure of the paper sheet on the diffusion of moisture is significant.

    The structure of the pore space within a paper sheet is imaged in serial sections using x-ray microtomography. The three dimensional structure is reconstructed from these sections using digital image processing techniques. The structure is then analyzed by measuring traditional descriptors for the pore space such as specific surface area and porosity. In addition, morphometric and quantitative stereological techniques are used to characterize the structure. Techniques of mathematical morphology [3] used include erosion, dilation, closing, opening and binarization with subsequent skeletonization.

    A sequence of microtomographs was imaged at approximately 2 μm intervals and the three-dimensional pore-fiber structure was reconstructed. The pore size distributions for both in-plane as
    well as transverse pores were measured. Significant differences in the in-plane (X-Y) and the transverse directions in pore characteristics are found and may help partly explain the different liquid and vapor transport properties in the in-plane and transverse directions. The results from the mathematical morphological study show that the pore space and the fiber space are bicontinuous. Some network measures of both these spaces are the network nodal density and bond co-ordination number distribution, both of which are determined. Significant transport properties for the pore space include the saturated water permeability and water vapor diffusivity. Due to the anisotropic nature of the structure, these are three-dimensional tensors in general.

  • 2001
    Oxford
    pp 1313-1342Characterization of Non-stationary Structural Non-uniformities in PaperAbstractPDF

    In this investigation, a novel approach to separating the static and stochastic components of paper variability data, such as mass formation or apparent density, was developed. Based on a discrete implementation of the continuous wavelet transform, the method provides information about the scale of features, e. g. flocs or streaks, as function of position within the data array in one direction. A non-rigorous, yet self-contained theoretical development of the method was given. The main discovery in this work was to mathematically show that, under conditions applying to a typical paper variability data, the distribution of energy among wavelets of different scale and at different positions, or simply, the local energy map, can be decomposed into two different parts that contain all the energy related to the static mean grammage profile or the local stochastic variability.

    In order to validate the approach and to justify its value, a set of simulated basis weight maps with different types of streaks were generated and analyzed successfully. This method was evenable to decompose overlapping grammage and formation streaks, which would have been impossible using traditional methods. As a final demonstration, data measured from real papers made in the laboratory and with a pilot machine were analyzed. Apparent density maps were determined using β-radiographic transmission imaging for mass formation and two-sided laser profilometry for local thickness maps. The method was able to reveal floc size variations buried into strong grammage streaks. The periodicity
    and the scale of the grammage streaks were also characterized by the decomposition of the wavelet map.

  • 2001
    Oxford
    pp 1343-1355Effect of Model and Fractionated TMP Fines on Sheet PropertiesAbstractPDF

    Two model fines, representing microfibrils and microgranules, and two fractions of TMP fines were used to demonstrate their effect on tensile strength and optical properties of handsheets formed from TMP and kraft fibers. The model microfibrils behave as a binder between fibers, thus improving tensile strength. The light scattering of kraft handsheets decreases but that of TMP handsheets increases. The model microgranules behave as a pigment by improving light scattering but preventing interfiber bonding. TMP fines, containing both types are capable of increasing tensile strength and light scattering simultaneously. The relation between tensile strength and light scattering depends on the proportion of fibrillated fines and granulated fines. The fractionated TMP fines of high surface area are shown to be very effective in improving the handsheet properties and the relation between tensile strength and light scattering is superior to that achieved by calcium carbonate filler.

  • 2001
    Oxford
    pp 1357-1370Sorption and Web Expansion MechanismsAbstractPDF

    The application of water to the surface of a paper or board web (in printing, coating, surface sizing etc.) causes a decrease in MD tension and a rapid increase in web width. Both processes have complex time-dependences whose details have direct practical relevance. For example, in color offset printing the amount of CD expansion between printing units determines fan-out and other misregister problems.

    No single traditional measurement can predict the amount of expansion or reveal its causes. We use KCL Vesikko and supplementary paper transmittance analysis to study the CD expansion and sorption dynamics for different papers and experimental conditions. With a new sorption model we can predict the experimentally observed movements of water inside paper.

  • 2001
    Oxford
    pp 1371-1383The Specific Pore Volume of Multi-planar Webs: The Role of the Short and Long Fibre Fractions.AbstractPDF

    Pore volume of paper sheets as measured by mercury intrusion porosimetry can provide an alternative structural description to solid phase-based measures such as density and may be used to examine sheet structure. We measured sheet densities and specific pore volume, i.e. the pore volume per unit sheet mass, for sheets made from whole pulp and the long fibre fraction of five different pulps. The coefficient of variation was 50% and 28% for specific pore volume and density of different pulps, respectively. Pore volume was more sensitive to structural differences than density. A theoretical measure of specific pore volume of the long fibre fraction (R48) was derived from the Interactive Multi-Planar Model (IMPM) of sheet structure. For the whole pulp we assumed that the shorter material (P48) filled voids and thus diminished the specific pore volume of the long fibre fraction (R48). Model predictions of specific pore volume agreed well with mercury porosimeter determinations for most of the samples. The effect of P48 fraction on sheet porosity was greater for newsprint than for hardwood kraft pulps.

  • 2001
    Oxford
    pp 1385-1401Stress-strain Performance of Paper and Fluff by Network ModellingAbstractPDF

    The stress-strain performance of 2D and 3D cellulose fibre networks was simulated using a network model. A model network consists of bonded curled fibres placed at random in a cell. The bonds show a stick-slip fracture behaviour. Results concerning the influence of network density, fibre orientation and ductility of bonds on the stress–strain behaviour of a network are given, and an example of fracture localization is provided.

  • 2001
    Oxford
    pp 1403–1421Fracture Properties in Filled Papers. In The science of papermakingAbstractPDF

    The mechanical properties of paper are impaired by the addition of filler. The beating of kraft pulp and addition of starch are possible remedies for this. However, beating also has negative effects because it reduces opacity and bulk, and starch effects are limited by retention. Optimal use of the kraft pulp and starch is therefore important. We show that in pure kraft sheets beating alone can compensate for most of the adverse effects on mechanical properties caused by kaolin addition. In TMP-based sheets with kaolin, the mechanical properties are fairly insensitive to the kraft content unless very high beating levels are used. The primary role of kraft is to improve tensile stiffness, not tensile strength of paper. Starch and beating both improve inter-fiber bonding but beating also raises fiber segment activation. The latter mechanism contributes to tensile stiffness but reduces damage width. The other mechanical properties of paper appear to be insensitive to fiber segment activation.

  • 2001
    Oxford
    pp 1423-1440Factors Controlling Pressroom Runnability of PaperAbstractPDF

    Web breaks in pressrooms have been modelled in terms of web strength variations and tension variations to identify principal factors controlling the pressroom performance. The web strength variations and their scaling law (the size dependence) have been formulated based on an extreme statistics approach, and the distribution parameters have been determined for a number of mechanical printing grades. By using the tension variation statistics data obtained from a pressroom trial, a parametric study has been conducted to examine the relative magnitude of the effects of key paper properties. The predictions have been compared with field study data. Among the conventional paper
    properties, tensile strength and elastic stretch consistently predicted the break frequency. The strength uniformity parameter (Weibull exponent) was shown to have the highest impact on the break frequency based on the parametric study.

  • 2001
    Oxford
    pp 1441-1465The Impact of Paper Defects on Paper Strength RequirementsAbstractPDF

    In this paper, the impacts of defect type, shape and position on the runnability related strength properties are discussed. As an example, a wood-containing coating base paper is studied. A typical break position on coated mechanical pulp-containing magazine paper production is the first coating unit on a blade coater. There the wetted paper web has to be strong enough and as uniform as possible in order to bear all the stresses under and after the blade.

    Very few systematic runnability studies in coated paper production have been reported. Only small step changes and no breaks are tolerated in commercial paper production. Consequently, opinions about relevant test methods for predicting paper runnability have been based on indirect studies. To bypass this obstacle, pilot runnability tests have been included in this study. These results are compared to laboratory sheet studies.

    Paper does not break because of its low average apparent strength, but because of a defect in the web of sufficient size, befitting shape, position and orientation. The type of defect is decisive on how reinforcement pulp must be treated and how much of it is needed. Defect type is also important in choosing proper measurement methods in order to predict the endurance of the paper web. Pilot coating trials were used to test base papers where two clearly different types of defects were intentionally made. Defect types were hole and plain cut. These were made at constant intervals into the web. During the coating trial of such defected web, the tension over the coating unit at the moment of a break was considered an indication of the actual strength of paper. In the results, one could clearly distinguish between different behaviour patterns with different defect shapes. Differences were noticed, e.g., as a different maximum tension at a break, and as a different behaviour under the coating blade.

    Additionally, handsheet studies using reinforcement pulps refined differently were carried out in order to evaluate their impact on tensile strength of notched samples. Holes and cuts were introduced into the test specimen. The effect of the addition of reinforcement pulp was dependent on the type of mechanical pulp used and on the level of refining of reinforcement pulp so that the effects obtained with notched samples were not predictable while testing undamaged sheets only. FEM (Finite Element Method) analyses simulating stress concentrations around defects gave compatible results with those from pilot coatings and improved our understanding why cut-type defects are so harmful.

    A fibre network study was included in order to study the effect of different fibre networks on paper strength. The importance of chemical pulp beating and interaction between mechanical and
    chemical pulp was emphasized in these experiments.

    The results seemed to be compatible with practical experiences in actual paper manufacturing processes, where paper is coated. The measurements based on the work needed to propagate a cut are not satisfactory in all respects. Fracture toughness may overestimate the benefits of chemical pulp addition and underestimate the benefit of chemical pulp beating. However, fracture toughness is clearly more suitable for predicting coating base paper runnability than the conventional (Elmendorf) tear strength measurement. Tear strength development of paper suggests that almost no beating of chemical pulp is needed, which is clearly not in accordance to our results. Instead, apparent tensile strength tests made on paper specimen with slits in it is a relatively well suited method for predicting the runnability of the base sheet in coating.

  • 2001
    Oxford
    pp 1467-1482Damage Width: A Measure of the Size of Fracture Process ZoneAbstractPDF

    The measurement of “damage width” from silicone-impregnated specimens reveals the area in which bond failures and other microscopic fractures take place. We demonstrate that damage width is a reasonable measure of the size of the fracture process zone in the sense of fracture mechanics. Firstly, the decay of cohesive stress against crack widening scales with damage width. Secondly, we can calculate the tensile strength of paper from fracture mechanics using damage width as the size of the fracture process zone. Armed with this interpretation, one can use damage width to evaluate, for example, the effective length and strength of fibers in paper.

  • 2001
    Oxford
    pp 1485-1490Stimulating Mathematics in the Paper IndustryAbstractPDF

    NA

  • 2005
    Cambridge
    pp 241-281Predictions of anisotropic multiply board properties based on isotropic ply properties and drying restraintsAbstractPDF

    There is a link missing between pulp properties and machine-made paper properties. The aim of this paper is to close a part of this gap by proposing an engineering model which, based on pulp or stock properties, makes it possible to predict the resulting anisotropic material behaviour of a multiply paper or board based on any given fibre anisotropy and drying restraint.

    An anisotropic model for the shrinkage and stiffness inter-action between the individual plies in a multiply structure is formulated. The input data to the model is the isotropic restrained dried and free dried stiffness, the free shrinkage strain and the density of each ply in the multiply structure. The basis weight, fibre anisotropy and total strain after drying are variables in the model. This means that besides the standard handsheet procedure only measurement of shrinkage and stiffness for one extra free dried handsheet is needed for the calibration of the model.

    The proposed model is validated with a series of anisotropic handsheet trials. Various combinations of single ply anisotropic handsheets were couched together into seven different multiply
    boards, which were dried freely and restrained. The isotropic input data of the individual plies were used to predict the free and restrained dried tensile stiffness index and bending stiffness of the multiply boards. The agreement between the experimental and predicted results showed good agreement. The model constitutes a useful tool in engineering predictions and parametric investigations of the mechanical behaviour of multiply boards. As a demonstration of the use of the model, the relation for tensile and bending stiffness versus total strain accumulated during drying was predicted for different board compositions. The basis weight and fibre anisotropy were varied in one of its plies.

    As another example the MD and CD stiffness profiles of a multiply board were simulated based on a given MD stretch and CD shrinkage profile.

  • 2005
    Cambridge
    pp 283-306The effect of machine conditions and furnish properties on paper CD shrinkage profileAbstractPDF

    All conventional paper machines exhibit a profile of cross-machine direction (CD) shrinkage which is developed during the drying stage of production. This paper suggests a new approach
    to the understanding of these profiles by suggesting that CD shrinkage at a point in the paper depends on the distance of that point from both edges of the sheet and also on the length of the unsupported draws in the dryer section. The nature of the function is unimportant as long as it can be made to fit data for one combination of machine width and effective draw length; substituting other values will then change the shape appropriately. In this paper, a simple exponential decay of shrinkage with distance from the edge is used successfully.

    This approach is initially demonstrated for laboratory results from the literature, where it successfully predicts changes produced by varying the length, width and by reversing the MD/CD orientation of samples. It is then shown to be consistent for changes of paper machine furnish, press section draw and sheet grammage produced in a series of trials on M-real, New Thames
    PM6. It is finally shown that theses ideas explain the effect on CD shrinkage results from the literature for splitting the sheet at the press section of a newsprint machine and for reduction of dryer section restraint by deactivation of the blow-boxes.

  • 2005
    Cambridge
    pp 307-406Review: Winding and unwinding webs: A review of the state of the science in 2005AbstractPDF

    A web is a material whose length is much in excess of the width and the width is much larger than the thickness. Various grades of paper, plastic films, metal foils and laminates can all be broadly categorized as webs. Web media are often stored for periods of time in the form of a wound roll. This form of storage is chosen because it is simply the most convenient and often the only form for storing vast lengths of web with minimal damage and loss. Winding can damage the web and has become a topic which has received considerable attention in the literature. Amongst operations in a web process line, the winder and the unwinder are often sites at which web defects appear that may result in a loss of quality or may lead to a break or burst of the web resulting in lost productivity. The purpose of this paper is to review the state of the science of winding and unwinding rolls. The threads of pertinent information in the literature focus on: the stresses that are wound into rolls, how those stresses are affected by the type of winder employed, how those stresses vary during roll storage, how the stresses predicted by models can be used to predict wound roll defects, and finally the measurements that are available to verify modeling efforts or help solve production problems. Each thread will be examined in this review from the perspective of what references exist but also giving enough detail that an appreciation for the significance of a topic can be developed.

  • 2005
    Cambridge
    pp 409–425Radial distribution of the pressure in an industrial refinerAbstractPDF

    This paper will consider the low consistency refining kinetics of a pulp suspension at a given level of applied specific energy. The mechanical treatment on fibres caught in the confined zones of gap clearance (bar crossings) reveals some heterogeneity. It can be experimentally verified that the physical effects on fibres are not the same whether the confined zones are close to the internal radius (input side) or close to the external radius (output side). One must also account for the dynamic sliding motion of the confined zones through the rotation of the rotor plate (or cone) in front of the stator. Hence, each bar crossing has its own sliding velocity.

    In order to predict the radial variability of the cutting effect on fibres, through engineering parameters that can be easily determined (radial coordinate; angular velocity; width of bars, width of grooves, average bar angle, both for rotor and stator patterns), a theoretical understanding of the radial distribution of the pressure must be undertaken. The pressure is locally applied on
    the pulp pads in confined zones of the gap clearance.

    The best homogeneous results are obtained either with a cylindrical or a conical refiner where the bars are parallel to the rotation axis. The degree of variability on the cutting effect on
    fibres is increased with an increase of the bar inclination versus the radial direction and a decrease of the ratio between the internal and external radius.

  • 2005
    Cambridge
    pp 427-456Refining characteristics of softwood fibre fractionsAbstractPDF

    The role of refining intensity and specific energy in refining of softwood kraft fibre fractions was studied. Several paper properties can be improved by selective refining of fractions. The tensile strength-dewatering resistance relationship benefits from low-intensity refining of the long-fibre fraction. The specific energy input determines the increase in fibre swelling which contributes to a higher sheet density and improved tensile strength. The apparent density-roughness relationship benefits from mild refining of the short-fibre fraction. Refining intensity has a strong effect on the magnitude of the gap between bar surfaces, on fibre shortening, and on the coarseness of fibres with high cell wall thickness. For the short-fibre fraction, which appeared to floccu-
    late less, the maximum intensity causing “pad collapse” and more severe fibre shortening was lower than for the long-fibre fraction and feed pulp. The fraction-specific intensity and gap behaviour are believed to relate to the compressibility of flocs under the stress applied by bar surfaces – a phenomenon discussed in recent studies concerning the forces acting on fibre flocs.

  • 2005
    Cambridge
    pp 457-562Review: On the nature of joint strength in paper – A review of dry and wet strength resins used in paper manufacturingAbstractPDF

    The properties of paper are largely dependent on the bonds between the fibres. This is, of course, primarily true of those strength properties that are directly related to the number of bonds in the paper. Other properties are also dependent on such bonds, properties such as the opacity of the paper, its smoothness, porosity, dimensional stability, pore size distribution, linting
    propensity, density, stiffness, formation, and compressibility to mention a few.

    The normal way of affecting the number of bonds in a paper is through the choice of fibre material and through a correct beating of the pulp. It is true that properties of paper may be manipulated through the choice of beater type, its specific edge load etc to expand the property or process space in paper manufacture. There are still many limitations as to what can be achieved by beating and other process tools, so the practical paper-maker is continuously looking for ways to expand property and process space to be able to manufacture new products or boost paper machine productivity.

    In this review the terms “bonding” and “joint strength” will be used interchangeably. “Joint strength” includes both the adhesion zone (2D zone of bonding) and the cohesion zone (3D zone of bonding).

    Despite massive efforts over the years, our understanding of the molecular mechanisms of bonding is still in its infancy. There is still the fundamental argument as to the relative contribution of hydrogen bonds, ionic bonds, dipolar interactions, induced polar interactions, long-range van der Waals forces, and covalent forces (for wet strength resins) in various situations. Taken to the extreme, it was once believed that lignin contributed little to bonding in lignin-rich pulps, because they were assumed to be poor hydrogen bonding agents. Not anymore, as it has been realised that strong bonding can be created between mechanically liberated pulp fibres. Though critical experiments still need to be formulated to examine such matters, this review will not focus on them.

    It is acknowledged, that hydrogen bond theories have been formulated by Corte and Shashek (1955), Nissan and Sternstein (1964) and others, but it has not been possible to further expand our knowledge from the initial formulations.

    This review will instead focus on the use of various dry and wet strength additives to improve bond strength. The authors have made efforts to relate the discussion to the historical and current context of dry and wet strength resins, and to discuss more recent developments in understanding adhesive and cohesive failure.

    Hence, after some general considerations and introduction to the concepts of process and property space in paper manufacture, a brief discussion of current paper strength theories will be made. A more detailed account of adhesive and cohesive failure mechanisms will follow, after which dry and wet strength resins will be reviewed. As far as wet strength agents are concerned,
    traditional wet strengthening will be given less emphasis; the focus of this later part will instead be on potential chemistries to alleviate tensile creep and compression creep under moist conditions.

  • 2005
    Cambridge
    pp 563-589Tensile strength of paper revisitedAbstractPDF

    Traditional micromechanical theories for the tensile strength of paper do not account for the tensile stiffness of paper, even though in practice tensile strength is closely coupled with tensile
    stiffness. Another problem is with the micromechanical input parameters, few of which have a precise meaning in real paper. Especially the interpretation of inter-fiber bonding is ambiguous.
    None of the existing theories connects tensile strength with an independently measurable value of bonding degree or bond strength. As a result, the conventional interpretation of tensile strength data is unreliable.

    In this paper we will present a “macromechanical” study that connects tensile strength with independently measured values of tensile stiffness and z-directional fracture energy (alternatively
    Scott bond or z-directional tensile index). The model expression agrees well with many – but not all – of the experimental datasets that we had available. The disagreements demonstrate that z-directional measurements do not capture some aspects of inter-fiber bonding that contribute to in-plane tensile strength.

  • 2005
    Cambridge
    pp 591-611Role of fiber deformations and damage from fiber strength to end userAbstractPDF

    Fiber deformations and damage have a considerable influence on both fiber strength and network properties. Through their influence on the fiber network, they can also affect the way paper behaves during converting. Another aspect is their influence on end-use properties, again via their effect on the fiber network.

    Separating the effects of fiber deformations and damage is often difficult. We prepared pulps in the laboratory and subjected them to different treatments that change the two relatively
    independently in a controlled manner. We found that wet/dry zero-span fiber strength is not dependent on the deformation method or on the extent of the deformations. Neither pulp sheet
    density nor Scott bond bonding was greatly affected by the type or method of deformation. In the case of deformed pulps, the pulp sheet tensile properties were dependent on the extent of fiber deformation via fiber segment activation. The pulp that was damaged instead of deformed had fiber deformations to about the same extent as the deformed pulp but lower single fiber strength measured with wet/dry zero-span. It also had lower bonding ability and sheet density. The tear index for the unbleached damaged pulp was 20–25% higher than that of the reference pulps. Fiber deformations (curl and kinks) affect fiber network properties via the lack of fiber segment activation. However, they do not significantly influence fiber shrinkage potential (WRV) or fiber strength.

  • 2005
    Cambridge
    pp 613-630Use of digital image correlation to study the local deformation field of paper and paperboardAbstractPDF

    Digital image correlation was used to measure the full-field deformation of paperboard and handsheet tensile specimens. The correlation technique was able to accurately measure strain in
    regions 0.6 by 0.6 mm. Results showed the variation of strain to be much larger than has been previously reported. For machine-made paperboard tested in the cross-direction, the variation of strain increased throughout the tensile test and became erratic near failure, indicating many local failures. The measured strain distribution can be characterized by a Weibull function in agreement with weak-link failure theories. The analysis of a handsheet tensile specimen with a low-grammage region, approximately 4mm wide, showed large negative strains near the region’s edge.

  • 2005
    Cambridge
    pp 631-647Effect of different degradation mechanisms on axial and z-directional fiber strengthAbstractPDF

    We have exposed pulp handsheets to two different degradative treatments, and compared their effects on strength properties. In accordance with earlier research, tensile stiffness and the shape of the stress-strain curve were independent of cellulose chain length and the fiber defects caused by the degradative treatments. When evaluated at the same viscosity, we found acid vapor treatment to be more detrimental to axial fiber strength than ageing treatment at elevated temperature and humidity. At the same mean fiber strength, acid vapor-treated handsheets show higher tensile strength. This is because acid vapor treatment is more heterogeneous than ageing treatment. The fiber network is able to compensate for the local defects, but not for the general degradation in fiber strength. In both treatments the mechanism for cellulose cleavage is assumed to be acid hydrolysis, the difference in the effect on fibers coming from the treatment conditions. Acid vapor treatment induces a fast reaction at defect sites and discontinuities in a fiber, while ageing treatment induces a slow, more homogeneous hydrolysis in fibers. Unlike axial strength, the Z-directional strength of softwood decreases only after harsh degradation when viscosity has dropped below 400 ml/g. The Z-directional strength of hardwood is already compromised at a viscosity of around 700 ml/g. The differences probably arise from differences in fiber ultrastructure.

  • 2005
    Cambridge
    pp 651-747The creep response of paperAbstractPDF

    This article examines the literature pertaining to the creep behavior of paper. The basic concept of creep, the terminology used to describe creep, and the various ways to present creep are introduced. This is followed by a historical overview of creep in paper. Using this framework, discussions centered on tensile, compressive, and accelerated creep are presented. For years,
    research efforts have focused on accelerated creep. Because of this diversion, an acknowledged fundamental understanding of paper creep is lacking. Using previous data for tensile creep in constant humidity conditions, a rudimentary model of creep in paper is developed. The model clearly demonstrates that the role of bonding is accounted for simply with an efficiency factor that acts to magnify the stress. In addition to the impact of inter-fiber changes, intra-fiber effects resulting from hardening and wet-straining are demonstrated. It is suggested that compressive creep differs from tensile creep due to material instability. Accelerated creep is taken to be the result of moisture-induced load cycling. The result of this discourse is that to increase understanding, fundamental studies of creep behavior in constant conditions are and will be more fruitful than studies in cyclic humidity.

  • 2005
    Cambridge
    pp 749-775Influence of fiber-fiber bonding on the tensile creep compliance of paperAbstractPDF

    In this study, two sets of sheets were made with differing levels of specific bond strength and relative bonded area. One set of sheets were wet pressed using a high press load and the other set of sheets were wet pressed using a low press load. Within each set, the sheets were treated with either a debonder or a bonder or received no treatment. Creep compliance data showed that creep curves for the debonder, bonder and untreated sheets were the same for the sheets wet pressed at the high press load and different for the sheets wet pressed at the low press load. Creep failure time was influenced by the treatments in both the high and low load wet pressed sheets; sheets treated with debonder failed first and the sheets treated with bonder failed last. It was concluded that at high levels of bonding as is the case with the high load wet pressed sheets, differences in specific bond strength due to the treatments do not influence creep deformation because fiber-fiber bonding is at a level where the sheets are efficiently loaded structures. The low load wet pressed sheets showed differences in creep deformation when specific bond strength was changed with treatments because fiber-fiber bonding was at a lower level where the sheets were inefficiently loaded. As the loading efficiency of the paper structure is improved through increased fiber-fiber bonding (either by increasing specific bond strength or relative bonded area), an efficiently loaded structure can be achieved where fiber-
    fiber bonding no longer affects deformation. This allows creep compliance to reach a minimum level which is dictated solely by the fibers. An efficiency factor can be used to describe deformation behavior where an efficiency of “1” indicates an efficiently loaded structure and lower values indicate a less than fully efficient structure, one in which fiber-fiber bonding influences deformation behavior. In this study, efficiency factors were used to scale the low load wet pressed sheet results and several sets of lesser refined and pressed sheets (thereby “removing” fiber-fiber bonding influence) and the data superimposed onto the high load wet pressed sheet results.

  • 2005
    Cambridge
    pp 777–796Effect of fibre shape and fibre distortions on creep properties of kraft paper in constant and cyclic humidityAbstractPDF

    The purpose of this investigation was to study the effect of fibre shape and fibre distortions on the creep properties in constant and cyclic humidity and compare these data to other standard paper properties. The fibre shape and magnitude of fibre distortions were varied by low consistency beating and high consistency treatment of the pulp. The term virgin were used for these pulps. Furthermore the effect of drying history was investigated where papers from straight fibres were dried under restrained- and freely-dried followed by reslushing. The term re-dried were used for these pulps.

    Fibre shape and fibre distortions of the virgin pulps had different effects on the creep stiffness. Straight fibres with few distortions had the highest stiffness values. By making the fibres curlier, the creep stiffness decreased significantly. The presence of fibre distortions in the straight fibres had no effect on the creep stiffness.

    The creep stiffness of papers from re-dried pulps depended on the drying method that produced the fibres. Without beating, fibres of the freely-dried paper gave a lower stiffness than fibres of the restrained-dried paper. The fibres had a memory of the distortions that were introduced by the free drying. After beating, the difference due to the drying method disappeared.

  • 2005
    Cambridge
    pp 797-825Review: Runnability, Fracture and Pressroom BreaksAbstractPDF

    Runnability of the paper web during production and converting is a topic which has always concerned the pulp and paper industry. Good runnability at the lowest possible production cost is of primary importance to paper producers, converters and printers. This paper will review the literature on runnability and fracture of dry printing paper webs.

    Several review articles have been written on this and related subjects. Niskanen [1] gave a thorough review of strength and fracture of paper. Niskanen reviewed the relationship between fibres, bonds and strength. He also discussed the relationship between web tension and fracture frequency before thoroughly describing the development of fracture mechanics methods. Kortshot [2] and Mäkelä [3] have also given excellent reviews of paper fracture and fracture mechanics. Roisum [4,5,6] has written reviews of the runnability of paper.

    It is important to remember that many causes for paper web breaks are quite trivial. Paper rolls are damaged by transport and handling. Direct contact with water or condensation due to rapid temperature changes may give damage. Poor tape gluing may give web breaks during the flying splice. For many such problems the best procedure for improving runnability is to keep
    the paper mill tidy, the floors clean and even. Further to follow and quality check the paper transport. Avoid gravel on the floor of transport containers, adjust the clamping pressure on the trucks used to handle paper rolls and so on. Yet even if the best precautions are taken, there will always be some damaged and weaker zones in the paper web. Thus, it is meaningful to use
    fracture mechanics as a tool to investigate if such defects will develop to a web fracture at the web tension conditions used.

    Much work on improving runnability of paper has been done based on the assumption that if the paper’s tensile strength, tear strength or fracture toughness is increased, then even the runnability will be improved. I will discuss this assumption and argue that the best way to improve runnability is to perform an engineering analysis of the converting or printing applica-
    tion where the fractures occur. The important factors in such an analysis are web stress, defect size distribution and mechanical properties of the paper.

  • 2005
    Cambridge
    pp 827-850Peeling pressure sensitive tape from paperAbstractPDF

    The ability of adhesives to bond paper and paperboard is critical for most packaging and converting operations. Despite the huge body of literature describing both paper and adhesives technologies, there are only a few research papers describing paper/adhesive interactions. Described herein are the results of a systematic investigation of pressure sensitive adhesive (PSA) peeling from paper. The peel force versus peel distance curve depends upon the failure mode. A constant force is observed when the PSA cleanly separates from paper (i.e. interfacial failure) at low peel rate. By contrast, at high peeling rates, in the paper failure domain, the peel force climbs to a maximum and then relaxes to a steady-state value. The maximum peel force, which we call the peak force, corresponds to the fracture of the top layer of fibres during the initiation of paper delamination whereas the steady-state peel force occurs during the propagation of paper delamination.

    To characterize the range of behaviors it is necessary to conduct a series of peeling experiments over an extended range of peel rates. The results are best analyzed by plotting the peak peel
    force versus the peel rate on logarithmic axes giving what we call a peel map. For a broad range of tape/paper combinations, peel maps have similar shapes. The interfacial failure domain consists of a linear segment with a positive slope. This line intersects with a horizontal line segment at higher peel rates, corresponding to the paper failure domain.

    Principal component analysis, a multivariate statistical analysis, of a large set of peel maps was used to reveal the influence of paper properties on peeling. The peak peel forces in the paper
    failure domain correlated with standard paper properties linked to z-directional strength. The slopes of the peel maps in the interfacial domain were independent of paper properties but were sensitive to adhesive rheology. The absolute location of the interfacial segment of the peel map mainly was sensitive to the chemical composition of the paper surface and secondarily related to surface roughness. Water contact angles on paper were not good predictors of adhesion. Finally, we illustrate the utility of peak peel force in the paper failure domain as a measure of paper surface strength.

  • 2005
    Cambridge
    pp 853-899A new slice-based concept for 3D paper structure analysis applied to spatial coating layer formationAbstractPDF

    This paper introduces a new concept for digitizing the three dimensional paper structure, based on a fully automated microtomy process and light microscopy. The microscope can be moved in all three directions of space with high accuracy in order to be able to digitize large samples with high spatial resolution. All components are controlled by a PC interface which enables an
    automated digitization process.

    The literature concerning 3D analysis of paper structure is reviewed. Non destructive and destructive techniques are compared.

    Image analysis algorithms for creation of a detailed digital representation are described. This digital data set is analyzed, to derive characteristics of the paper structure.

    As a first example of possible applications the analysis of the 3D coating layer formation is presented. The coating layer is detected by means of image analysis based on a 3D color segmentation concept. Initial experiments on analyzing coated paper samples prove the applicability of the concept.

    The correctness of the implemented sample digitization process and following image analysis was validated.

  • 2005
    Cambridge
    pp 901–920Characterisation of the 3D paper structure with X-ray synchrotron radiation microtomographyAbstractPDF

    Paper is a complex three-dimensional network of fibres, pores and often fillers. The main goal of this study is to characterise its structure in a non invasive and non destructive way. In order to overcome the limitations of 2D measurements, Synchrotron Radiation microtomography is used to visualise the samples. Coupled with appropriate processing tools, it allows a quantifica-
    tion of some structural characteristics on the samples. This is the main aim of this paper. Different articles relate the feasibility of such a study for paper samples: imaging at the European Synchrotron Radiation Facility (ESRF, France) in phase contrast [1] or in absorption mode [2] gives the opportunity to reach a pixel size smaller than a micron which can not be obtained with classical tomographs [3]. A first step is to visualise the structure then, structural parameters are extracted from these 3D data [4]. However, this requires a segmentation technique adapted for application to typical paper samples that are constituted of three phases. The first step consists in the segmentation of the different phases, namely, air, fibres and fillers. The amount of each component may be evaluated. This was validated for both the porosity and the filler content. Furthermore, structural parameters were calculated from the binarised volumes. The comparison with the published results validates the calculation.

  • 2005
    Cambridge
    pp 921-941Imaging fibre deformationsAbstractPDF

    A new experimental technique is presented that allows the direct observation of fibre deformation during wet pressing. Pulp fibres were wet pressed onto a glass slide and the region where two fibres crossed was examined microscopically through the glass. While the underlying fibre was in contact with the glass slide down its length, the overlying fibre must span from the top of the fibre to the glass slide. The geometry of the intersection is controlled by both the local conformability of the overlying fibre and the deformability of the underlying fibre. It is not primarily con-
    trolled by the longitudinal flexibility of a fibre. The method provides new opportunities to investigate the effect of mechanical and chemical treatment on the papermaking properties of pulp
    fibres in both the dry and wet state.

  • 2005
    Cambridge
    pp 943–960Effect of correlated free fibre lengths on pore size distribution in fibrous matsAbstractPDF

    We provide a simulator for a range of bivariate stochastic processes of various application in the physics of stochastic fibrous networks. We illustrate the effects of local correlation on the statistics of voids in the bulk and the surface of fibre mats in general and paper in particular. The reference case of random isotropy has an inherent ‘ground-state’ correlation of adjacent free-fibre-lengths; this explains the classical observation of Corte that pores seem mainly ‘roundish’ in real paper samples. In the isotropic case, the mean pore radius can be reduced from that in a random network by 20% through structural changes associated with increased flocculation. The mean eccentricity of pores seems to give a measure of the variability in free-fibre-length distributions that is not due to local correlation. We find a uniform effect of local correlation on mean pore eccentricity over a range of stochastic network structures; at a given correlation, increased flocculation increases mean eccentricity slightly.

  • 2005
    Cambridge
    pp 961–1007Applications of thickness and apparent density mapping by laser profilometryAbstractPDF

    This paper describes the development of a method for mapping the apparent density of paper, nonwovens and other fibrous webs based on non-contact laser profilometry and β-transmission radiographic imaging. The method is applied in three complimentary studies that examine the in-plane non-uniformity of thickness, grammage and apparent density in printing papers. The first section focuses on the development and verification of the analytical method for mapping thickness. Through simultaneous scanning of the topography of both sides of the specimen by opposing range sensors, the surface contour of each side, the local thickness, and the out of plane deformation were measured. It was demonstrated that the laser based method provides results that closely approximate the intrinsic thickness that is independent of paper formation. The method was then used to examine the structural differences in laboratory sheets pressed by soft and hard nip calenders. The well known difference in web densification mechanisms of the two was reaffirmed by mapping discrete changes in thickness and statistically comparing these with corresponding points on grammage maps. Densification was shown to be dependent on grammage for the hard calender, and independent of grammage for the soft calender. The final study used the thickness mapping method to monitor the hygroexpansivity of representative printing papers as equilibrium humidity was varied between 9% and 80%. Thickness maps were obtained for newsprint, SC-A (calendered and uncalendered), bulk offset and office copy. Differential thickness maps were used to compare the in-plane non-uniformity of hygroexpansion. The inplane hygroexpansion was characterized and corrected for using a recently developed algorithm known as Enhanced Digital Image Correlation (EDIC). Thickness change did not appear to correspond to grammage maps. The results suggest that a significant irreversible increase in thickness occurs for papers that are heavily calendered.

  • 2005
    Cambridge
    pp 1011-1073Review: Interactions between coating colour and base sheet in pigment coatingAbstractPDF

    The literature on the interactions between the coating colour and the base sheet during the pigment coating of paper is reviewed in the order to summarize the current knowledge within the area. The review is focused on the processes of forming and consolidation and on how the coating colour interacts with the base sheet during these processes, and how this interaction affects coating hold-out, roughness and coating mass distribution. A coating layer which stays on the surface of the base sheet and which has a uniform mass distribution is desired. The research world disagrees on whether coating hold-out is a relevant problem. The reason is that there is little direct evidence in the literature on coating penetration. However, there are numerous indications of an indirect character which suggest that coating penetration exists, both in blade coating and in coating with the metered size press. The pressure pulse to which the coating colour is subjected in the applicator nip during blade coating or in the transfer nip in metered size press coating and the permeability of the base sheet are factors which are said to control coating penetration. There is concordance in the opinion that the uptake of the aqueous phase of the coating layer is an extremely rapid process and that this uptake releases stresses in the sheet and plasticizes it. The release of stresses leads to roughening of the sheet whereas the plasticization makes it compressible and smooth in a compressed state beneath the blade tip during the forming of the coating layer. Much attention has been given to the roughening and a number of extensive studies have been published about that. The studies on plasticization and sheet compressibility and how they affect the mass distribution of the final coating layer are fewer in number but non-existent. The roughening and plasticization of the sheet are reported to be different for woodfree and wood containing sheets, due not only to the different types of fibres in the sheets, but also to their different densities. Woodfree sheets, which are generally the densest, are considered to be dimensionally the most stable. A number of researchers have reported that the pre-calendering has a great influence on the roughening. Studies have shown that the calendering builds in transverse stresses into the sheet and, in the case of wood- containing sheets, closes the lumen of thick-walled fibres. During the coating operation, when the sheet takes up the aqueous phase from the coating colour, these stresses are released and the lumen is opened. Several researchers have shown that all the smoothening effect of the pre-
    calendering can be lost during the coating process.

  • 2005
    Cambridge
    pp 1075-1099Tension wrinkling and fluting in heatset web offset printing process – Post-buckling analysesAbstractPDF

    Geometrically non-linear, large scale post-buckling analyses were carried out to investigate the influence of different parameters on residual waviness (fluting) after printing in a heat set web offset printing press. Mixed implicit-explicit finite element techniques were used in the analyses. The numerical procedure was verified by experimentally acquired data. Results show that when the paper web is perfectly flat before printing, fluting patterns after drying and moisture recovery generally have higher wavelength than those typically observed in fluted samples. Initial cockles of unprinted sheets were found to have impacts on the fluting patterns and amplitudes. Among the factors investigated, ink thickness and hygroexpansivity had significant influences on fluting: increasing these factors increased fluting amplitudes.

  • 2005
    Cambridge
    pp 1101-1137Review: Interaction between offset ink and coated paper – A review of the present understandingAbstractPDF

    The printing speed of offset presses has increased significantly during the last decade and this puts increasing demands on the interaction between the paper and ink. Poor interaction may
    result in runnability problems and low quality of the printed product. The research in this area has been intensive during many years and the knowledge of important phenomena has been significantly improved, although a full understanding is still missing.

    This review concentrate on offset printing of coated paper with special focus on sheet-fed offset. Properties of coatings and composition of offset inks are briefly discussed. Some data on ink film thickness and its lateral distribution is presented. Ink setting is reviewed in some detail and the effect of important coating properties (e.g. porosity, pore size, latex properties) and ink properties (ink oil viscosity and surface tension) are discussed. The interaction between latex binder and ink oil is given special attention.

    The impact of coating structure and ink setting on print quality is covered in some details. The importance of ink filament formation and levelling as well as coating topography and ink film thickness on print gloss is well established. Recent findings on relations between mottle tendency and non-uniformity of coating structure and ink setting are included.

  • 2005
    Cambridge
    pp 1139-1154A stochastic structure model for predicting sheet consolidation and print uniformityAbstractPDF

    The microscopic response of a paper sheet to compressive forces is of great importance in predicting print quality as well as the sheet structures developed in wet pressing and calendering. In this report, we propose a new compression model that preserves the entire three-dimensional, stochastic, fibre network structure. The model includes Z-directional deformation of fibres in both compressive and shear modes. Permanent deformation of each fibre (such as caused by fibre collapse) can be achieved by adjusting the stiffness of the fibre during compression or unloading. The stiffness of the plates can also be chosen to represent, for example, a hard printing plate, a blanket, or a soft-nip calendar cover. Although we still need to collect basic fibre stiffness data in order to perform quantitative comparisons between model predictions and experimental results, simulated structures already show typical features of consolidated paper sheets. As a first application, we study the contact mechanics between a printing plate and a model paper structure.

  • 2005
    Cambridge
    pp 1155–1189Quantitative mottle measurement based on a physical model of the spatial contrast sensitivity of the human visual systemAbstractPDF

    Print non-uniformity, or mottle, is an important factor in print quality. The ultimate judge of print quality is the printer or print buyer, so print quality measurement should be representative of human perception. Most methods that are currently available to systematically quantify print mottle do not consider eye response in the calculation. Instead, the user has to select appropriate scales for the analysis by comparing with separate visual ranking experiments for each new set of prints.

    We developed a method to process digital images of mottled black prints to provide a mottle index that takes into account eye response. The mottle indices obtained for a range of paper and
    board grades were compared with the results of separate visual rating experiments, and there was very good agreement between them. The mottle index outperformed other parameters also used for the quantification of mottle. Based on these results, the mottle index is deemed reliable enough to decrease the need for separate visual assessments by panels. The mottle index algorithm removes the need for the operator to make subjective choices on the appropriate analysis scales for sample sets where print uniformity is the dominant quality criterion.

    The proposed mottle measurement method allows systematic and objective quantification of mottle. The method can easily be implemented to analyze test prints using the analysis software we
    developed, and an appropriate desktop scanner that will require calibration to relate the greyscale to reflectance values.

  • 2005
    Cambridge
    pp 1225-1268Imaging of macromolecular events occurring during the manufacture of paperAbstractPDF

    A novel transmission electron microscopy (TEM) technique, developed to observe the nano-scale interactions of polymeric additives and cellulosic fibrils under idealized laboratory condi-
    tions, was applied for the first time in a comprehensive study of the colloidal interactions within a mill producing light-weight coated publication paper. The technique allows the observation of incremental changes in the nano-scale appearance of the paper-making slurry as successive additives are introduced to the system. Such changes include the coagulation of colloidal and
    dissolved substances present in thermomechanical (TMP) pulp after the addition of a low molecular weight, high charge density polymer, and the subsequent flocculation of the coagulated matter, hydrophobic materials, and fines following the introduction of talc, aluminum sulfate, a high mass cationic polyelectrolyte, and silica nanoparticles. The new results demonstrate that the TEM technique can be applied even in systems as complex as commercial papermaking, leading to a more accurate understanding of what happens on a macromolecular level.

  • 2005
    Cambridge
    pp 1269-1292Mechanisms of flocculation of microcrystalline cellulose by poly(ethylene oxide) and cofactor corilaginAbstractPDF

    Poly(ethylene oxide) (PEO), a widely known flocculation agent used primarily as a fines retention aid in mechanical grade papers, has its efficiency enhanced by various compounds, known as cofactors. These cofactors form a complex with PEO, which acts as an efficient bridging agent for fines flocculation. The nature of the PEO/cofactor complex is mainly unknown, and it was originally believed that the association was driven by hydrogen bonding. Therefore it was decided to investigate the complex formation in more detail. As a model system we studied PEO and a model cofactor, corilagin, a precursor of tannic acid, a known cofactor for PEO. We performed both Semi-Empirical Molecular Orbital (PM3) gas phase calculations and Molecular Dynamics (MD) calculations in the presence of water. Both methods lead to the same surprising conclusion: no association between PEO and corilagin occurs at room temperature. The reason is that the gain of association enthalpy is not large enough to overcome the loss in entropy. No correlation was found between the association enthalpy and the number of hydrogen bonds between PEO and corilagin. The absence of PEO/corilagin complexation was confirmed by NMR, isothermal titration calorimetry and the inability of PEO/corilagin to flocculate MCC (microcrystalline cellulose). In the presence of low concentrations of salt, complexation and MCC flocculation was observed.

  • 2005
    Cambridge
    pp 1293–1316The mechanism of polyvinylamine wet-strengtheningAbstractPDF

    Regenerated cellulose films were laminated with polyvinylamine, PVAm, and the wet peel delamination forces were used to explore the mechanism by which PVAm increases the wet strength of paper. Conventional wet strength resins contain highly reactive chemical groups which can crosslink the resin and graft it to fibre surfaces. By contrast, it is not obvious how PVAm provides wet strength.

    The delamination experiments revealed that PVAm gives strong adhesion which was approximately independent of drying temperature (23 to 110°C), pH 3 to 9, PVAm molecular weight (34,000 to 1,500,000Da), and PVAm coverage (monolayer to 70mg/m2 ). By contrast the adhesion increased with the amine content of PVAm and with the degree of oxidation of the cellulose films. It is proposed that the PVAm adhesion is a combination of electrostatic and covalent bonding. The electrostatic bonding is between protonated amines, which are positively charged, and carboxyl groups on the cellulose. Whereas the covalent bonds, aminal and imine linkages, are formed between amines and aldehyde groups on the oxidized films.

  • 2005
    Cambridge
    pp 1317–1335The effect of ketene dimer melting point on the rate of sizing developmentAbstractPDF

    Two studies were carried out in an effort to gain a better understanding of how ketene dimers develop sizing. In the first study, eight ketene dimers with a range of melting points, vapor pressures, and molecular weights were evaluated for rate of sizing development. Ketene dimer melting point had a clear effect on the rate of sizing development. High melting ketene dimers initiated sizing at higher sheet moistures than low melting dimers. High melting dimers also developed their ultimate level of sizing faster than low melting ketene dimers. These results suggest that solid and liquid ketene dimers have different mechanisms of sizing development. Ketene dimer vapor pressure and molecular weight had no consistent effects on the rate of sizing development.

    Pseudo first order rate constants for sizing development were then measured for a high melting solid ketene dimer and a liquid ketene dimer over dryer temperatures ranging from 55 °C to 85 °C. The rate constants measured for the liquid ketene dimer increased steadily as dryer temperature increased. An Arrhenius plot of the rate constants obtained for the liquid ketene dimer yielded an activation energy of 11 kcal per mole for sizing development. These results are consistent with the hypothesis that the chemical reaction of the lactone ring is the rate determining step in sizing development for liquid ketene dimers. The high melting solid ketene dimer followed more complex kinetics and probably developed sizing by a combination of mechanisms. It is likely that the differing sizing responses measured for the solid and liquid ketene dimers are due to differing sizing contributions from the unreacted and hydrolyzed ketene dimers.

  • 2005
    Cambridge
    pp 1337–1350Direct measurement of attractive van der Waals forces and repulsive electrostatic forces between regenerated cellulose surfaces in an aqueous environmentAbstractPDF

    Using the colloidal probe technique, the interaction between carboxymethylated cellulose films and a cellulose sphere was studied. At low pH (pH 3.5), the interaction was dominated by dispersion forces. This was due to the low dissociation of carboxyl groups within the film at pH 3.5. However, at pH greater than 5, the interaction was dominated by an electrostatic repulsion. The increase in pH had the two-fold effect of completely dissociating the charged groups as well as causing appreciable swelling of the film leading to a decrease in the van der Waals component of the interaction. From these results it can be concluded that these cellulose films are suitable for a range of surface forces measurements including electrostatic, van der Waals’, steric and adhesion forces. Furthermore, the measurement of forces using the colloidal probe technique can be extended to inorganic particles as well as for interactions between surfaces in the presence of for example wet and dry strength agents and other materials relevant to the paper-making industry.

  • 2005
    Cambridge
    pp 1351–1378On the origins of adhesion in papermaking systemsAbstractPDF

    The purpose of this study was to investigate the effect of polyelectrolytes on the adhesion forces between polyelectrolyte-coated surfaces and to relate the observed phenomena to the performance of dry strength additives in papermaking systems.

    The adhesive properties (pull-off forces) between polyelectrolyte-coated surfaces in air were determined as a function of the polymer charge density and the number of contacts. We found
    strong adhesion energy for model systems coated with polymers of the highest (cationic) charge density, which suggests that electrostatic surface-polyelectrolyte-surface bridges are the main cause of the adhesion. At low charge density another force contribution due to entanglement of polymer chains gives rise to a significant contribution to the adhesion forces. The disruption of the contact leads to irreversible changes in the adsorbed layers when either the number of electrostatic bridges formed is large or when entanglement effects are important for polymers with a relatively low binding strength to the surface. These results are discussed in terms of our current understanding of development of dry strength in fiber systems.

  • 2005
    Cambridge
    pp 1379–1391Molecular interactions between model cellulose surfaces and ink – Influence of surface energy and surface structure on adhesionAbstractPDF

    The influence of surface roughness and surface hydrophobicity on ink detachment of water-based flexographic ink was studied. It was shown that increased surface roughness and an increased surface hydrophobicity both had a negative effect on ink detachment. The increased surface roughness was suggested to increase the molecular contact area between ink and cellulose and thereby also to decrease ink detachment. Ink cellulose interaction was evaluated from interfacial energies and contact angle measurements. A new technique in which the adhesion properties between ink and the model cellulose surface were directly measured using a Micro Adhesion Measurement Apparatus (MAMA) was also used. Upon increasing the hydrophobicity of the model cellulose surface it was shown that the work of adhesion between ink and model cellulose surfaces decreased. At the same time the interfacial energy between cellulose and ink increased, as did the interfacial energy between cellulose and water resulting in a lower degree of ink detachment.

  • 2005
    Cambridge
    pp 1395–1396Composite micromechanics by synchrotron x-ray imaging and strain mappingAbstractPDF

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