Proceeding Articles

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.

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 [1].

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

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

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

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

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

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

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

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

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

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

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