FRC Symposium Proceedings
FRC Symposium Proceedings
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.
Oxfordpp 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.
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.
Oxfordpp 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.
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.
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.
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.
Oxfordpp 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.
Oxfordpp 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.
Oxfordpp 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.
Oxfordpp 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.
Oxfordpp 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.
Oxfordpp 323-343The Effect of Press Nip Geometry on Dryness, Density and Paper PropertiesAbstractPDF
Oxfordpp 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.
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.
Oxfordpp 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.
Oxfordpp 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.
Oxfordpp 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.
Oxfordpp 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.
Oxfordpp 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.
Oxfordpp 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.
Oxfordpp 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.
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.
Oxfordpp 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.
Oxfordpp 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.
Oxfordpp 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.
Oxfordpp 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.
Oxfordpp 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
Oxfordpp 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.
Oxfordpp 801-811Determination of Length and Width of Nanocelluloses from Their Dilute DispersionsAbstractPDFLength/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.
Oxfordpp 813-821Preparation and Utilization of Highly Transparent and Viscous Dispersion of Phosphorylated Cellulose NanoﬁbersAbstractPDF
Oxfordpp 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.
Oxfordpp 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.
Oxfordpp 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.
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.
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.
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.
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.
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.
Cambridgepp 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.
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.
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.
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.
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.
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.
Cambridgepp 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.
Cambridgepp 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.
Cambridgepp 299-304Dependence of Sheet Properties on Formation and Forming Variables – Prepared ContributionAbstractPDF
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.
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.
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.
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.
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.
Cambridgepp 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.
Cambridgepp 473-474Ultrasonic Impedometric Studies in the Cellulose Pulp/water System – Prepared ContributionAbstractPDF
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.
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.
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.
Cambridgepp 544-549Effect of Drying on the Flexural Rigidity of Single Fibres – Prepared ContributionsAbstractPDF
Cambridgepp 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.
Cambridgepp 569-572Thermal Softening, Adhesive Properties and Glass Transitions in Lignin, Hemicellulose and Cellulose – Prepared ContributionsAbstractPDF
Cambridgepp 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.
Cambridgepp 627-630Dynamic Consolidation of Paper during Calendering: Dynamic Compressibility of Paper – Prepared ContributionsAbstractPDF
Cambridgepp 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.
Cambridgepp 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.
Cambridgepp 735-736Effects of Sizing, Adhesives and Fillers on the Formation and Consolidation of Paper Webs – Prepared ContributionsAbstractPDF
Cambridgepp 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.
Cambridgepp 773Effects of Soluble Non-fibrous Materials on Formation and Consolidation of Paper Webs – Prepared ContributionAbstractPDF
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.
Cambridgepp 801-803A New Theory for the Load/elongation Properties of Paper – Prepared ContributionsAbstractPDF
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.
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.
Cambridgepp 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.
Cambridgepp 896Effects of High Consistency Refining on the Properties of the Consolidated Web – Prepared ContributionsAbstractPDF
Cambridgepp 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.
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.
Cambridgepp 943-947Some Optical Consequences of the Consolidation of Paper – Prepared ContributionsAbstractPDF
Cambridgepp 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.
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.
Cambridgepp 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.
Cambridgepp 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.
Cambridgepp 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.
Cambridgepp 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.
Cambridgepp 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.
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.
Cambridgepp 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.
Cambridgepp 321-357Review: Variability, Non-uniformity and Complexity: from Product to ProcessAbstractPDF
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.
Cambridgepp 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.
Cambridgepp 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.
Cambridgepp 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.
Cambridgepp 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.
Cambridgepp 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.
Cambridgepp 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.
Cambridgepp 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.
Cambridgepp 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.
Cambridgepp 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.
Cambridgepp 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.
Cambridgepp 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.
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.
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.
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.
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.
Cambridgepp 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.
Cambridgepp 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.
Cambridgepp 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.
Cambridgepp 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.
Cambridgepp 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.
Cambridgepp 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.
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.
Cambridgepp 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.
Cambridgepp 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.
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.
Cambridgepp 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.
Cambridgepp 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.
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.
Oxfordpp 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.
Oxfordpp 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.
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.
Oxfordpp 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.
Oxfordpp 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.
Oxfordpp 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.
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.
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.
Oxfordpp 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.
Oxfordpp 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.
Oxfordpp 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.
Oxfordpp 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.
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.
Oxfordpp 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.
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.
Oxfordpp 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.
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.
The literature related to the three-dimensional ﬁbre networks is reviewed since 2000. Indeed, a review was presented in these symposia in 2001 . 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.
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.
Oxfordpp 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.
Oxfordpp 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.
Oxfordpp 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.
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.
Oxfordpp 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.
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.
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.
Oxfordpp 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.
Oxfordpp 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.
Oxfordpp 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.
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.
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.
Oxfordpp 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.
Oxfordpp 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.
Oxfordpp 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.
Oxfordpp 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.
Oxfordpp 1293-1323M. Mettänen, U. Hirn, M. Lauri and R. Ritala. Probabilistic analysis of small-scale print defects with aligned 2D measurements. In Advances in Pulp and Paper Research, Oxford 2009, Trans. of the XIVth Fund. Res. Symp. Oxford, 2009, (S.J. I’Anson, ed.), pp 1293–1323, FRC, Manchester, 2018.AbstractPDF
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.
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.
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 .
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.