2001 Volume 2

Clay or other minerals are coated onto fibrous paper to improve print and optical properties. The minerals are fixed to the fibrous substrate with a binder, either latex or starch. The coated paper is dried during manufacture, and if the binder migrates during this drying, there can be a deterioration in properties. Not only are there manufacturing considerations as to the choice of binder, but also environmental ones with regard to factors such as the energy used in drying. This work reveals new perspectives on binder migration which have been gained by a variety of approaches. Firstly, a critique is presented of the experimental methods which have been used by other workers. Then a comprehensive series of experiments is described. For ease of study, and because any effects are likely to be exaggerated, most of the experiments were carried out on samples which were 1.4 mm thick, some two orders of magnitude thicker than commercial coatings. However, the results were also replicated in less extensive tests on samples of thickness 55 μm. The experiments show that under a very wide range of conditions, including different coating thicknesses and drying temperatures, no latex migration is observed. Migration of starch was observed, however, and caused an increase in starch concentration at the surface during drying. If the sample was covered during the experiments, the system relaxed back and the concentration enhancement was reduced. The relationship between the particle size of the latex and starch, and the extent to which these particles might percolate through the void structures, was investigated by the application of the “Pore-Cor” software to mercury intrusion experiments. Also presented are the results of a mathematical continuum model, which accurately describes the migration of starch binder in terms of only two parameters, namely the evaporation efficiency and the diffusion coefficient of starch.

The surface treatment of paper is commonly undertaken in order to improve a set of key end-use properties, including optical response and printability. These properties can be influenced, to a significant extent, by the sub-surface structure of the coating layer. The nature of coating dewatering also has strong implications for machine runnability. Thus, there is a clear need to understand in suitable detail the nature of the coating consolidation process. In this study, we have applied a novel approach to characterising the equilibrium consolidation state of calcium carbonate sediments, both with and without polymeric thickener. The aim is to provide a quantitative link between the structure of consolidated layers and their network strength, through the compressive yield stress, Py(). A suspension, prepared at a given volume fraction of solids 0, is centrifuged to produce a consolidated particle sediment (or gel). The solidity variation of that sediment as a function of depth is then measured using one-dimensional magnetic resonance imaging (MRI), and Py() calculated directly from the volume fraction profile. The results obtained are discussed in the light of particle network structure, the effect of polymer on particle consolidation, and the relation to viscoelastic properties of the suspensions. The link to the dewatering of coating suspensions, and structure formation in coating layers, is also considered.

The literature concerning the structure of two- and three- dimensional fibre networks is reviewed. Emphasis is placed on the literature concerning such networks in papermaking processes, though examples are drawn from other systems. The propensity of a suspension to flocculate is considered from a theoretical viewpoint. The experimental techniques and structural descriptors applied in the characterisation of fibre networks are discussed. Theoretical studies of random networks are presented along with their use as reference structures and comparison is made between the main techniques used in the structural characterisation of essentially two-dimensional networks such as paper. The relationships between the distributions of mass and voids are examined and the dependence of sheet nonuniformity on that of the suspension from which it is formed is reviewed.

The three dimensional structure of paper materials plays a critical role in the paper manufacturing process especially via its impact on the transport properties for fluids. Dewatering of the wet web, pressing and drying will benefit from knowledge of the relationships between the web structure and its transport coefficients. Among transport, moisture diffusion in paper is central to the understanding and optimal design of paper products for their performance in different environmental conditions. Our recent research of moisture sorption in paper has indicated that diffusion of water vapor through the pore space is an important mechanism for transport [1,2]. The effect of the three dimensional structure of the paper sheet on the diffusion of moisture is significant.

The structure of the pore space within a paper sheet is imaged in serial sections using x-ray microtomography. The three dimensional structure is reconstructed from these sections using digital image processing techniques. The structure is then analyzed by measuring traditional descriptors for the pore space such as specific surface area and porosity. In addition, morphometric and quantitative stereological techniques are used to characterize the structure. Techniques of mathematical morphology [3] used include erosion, dilation, closing, opening and binarization with subsequent skeletonization.

A sequence of microtomographs was imaged at approximately 2 μm intervals and the three-dimensional pore-fiber structure was reconstructed. The pore size distributions for both in-plane as
well as transverse pores were measured. Significant differences in the in-plane (X-Y) and the transverse directions in pore characteristics are found and may help partly explain the different liquid and vapor transport properties in the in-plane and transverse directions. The results from the mathematical morphological study show that the pore space and the fiber space are bicontinuous. Some network measures of both these spaces are the network nodal density and bond co-ordination number distribution, both of which are determined. Significant transport properties for the pore space include the saturated water permeability and water vapor diffusivity. Due to the anisotropic nature of the structure, these are three-dimensional tensors in general.

In this investigation, a novel approach to separating the static and stochastic components of paper variability data, such as mass formation or apparent density, was developed. Based on a discrete implementation of the continuous wavelet transform, the method provides information about the scale of features, e. g. flocs or streaks, as function of position within the data array in one direction. A non-rigorous, yet self-contained theoretical development of the method was given. The main discovery in this work was to mathematically show that, under conditions applying to a typical paper variability data, the distribution of energy among wavelets of different scale and at different positions, or simply, the local energy map, can be decomposed into two different parts that contain all the energy related to the static mean grammage profile or the local stochastic variability.

In order to validate the approach and to justify its value, a set of simulated basis weight maps with different types of streaks were generated and analyzed successfully. This method was evenable to decompose overlapping grammage and formation streaks, which would have been impossible using traditional methods. As a final demonstration, data measured from real papers made in the laboratory and with a pilot machine were analyzed. Apparent density maps were determined using β-radiographic transmission imaging for mass formation and two-sided laser profilometry for local thickness maps. The method was able to reveal floc size variations buried into strong grammage streaks. The periodicity and the scale of the grammage streaks were also characterized by the decomposition of the wavelet map.

Two model fines, representing microfibrils and microgranules, and two fractions of TMP fines were used to demonstrate their effect on tensile strength and optical properties of handsheets formed from TMP and kraft fibers. The model microfibrils behave as a binder between fibers, thus improving tensile strength. The light scattering of kraft handsheets decreases but that of TMP handsheets increases. The model microgranules behave as a pigment by improving light scattering but preventing interfiber bonding. TMP fines, containing both types are capable of increasing tensile strength and light scattering simultaneously. The relation between tensile strength and light scattering depends on the proportion of fibrillated fines and granulated fines. The fractionated TMP fines of high surface area are shown to be very effective in improving the handsheet properties and the relation between tensile strength and light scattering is superior to that achieved by calcium carbonate filler.

The application of water to the surface of a paper or board web (in printing, coating, surface sizing etc.) causes a decrease in MD tension and a rapid increase in web width. Both processes have complex time-dependences whose details have direct practical relevance. For example, in color offset printing the amount of CD expansion between printing units determines fan-out and other misregister problems.

No single traditional measurement can predict the amount of expansion or reveal its causes. We use KCL Vesikko and supplementary paper transmittance analysis to study the CD expansion and sorption dynamics for different papers and experimental conditions. With a new sorption model we can predict the experimentally observed movements of water inside paper.

Pore volume of paper sheets as measured by mercury intrusion porosimetry can provide an alternative structural description to solid phase-based measures such as density and may be used to examine sheet structure. We measured sheet densities and specific pore volume, i.e. the pore volume per unit sheet mass, for sheets made from whole pulp and the long fibre fraction of five different pulps. The coefficient of variation was 50% and 28% for specific pore volume and density of different pulps, respectively. Pore volume was more sensitive to structural differences than density. A theoretical measure of specific pore volume of the long fibre fraction (R48) was derived from the Interactive Multi-Planar Model (IMPM) of sheet structure. For the whole pulp we assumed that the shorter material (P48) filled voids and thus diminished the specific pore volume of the long fibre fraction (R48). Model predictions of specific pore volume agreed well with mercury porosimeter determinations for most of the samples. The effect of P48 fraction on sheet porosity was greater for newsprint than for hardwood kraft pulps.

The stress-strain performance of 2D and 3D cellulose fibre networks was simulated using a network model. A model network consists of bonded curled fibres placed at random in a cell. The bonds show a stick-slip fracture behaviour. Results concerning the influence of network density, fibre orientation and ductility of bonds on the stress–strain behaviour of a network are given, and an example of fracture localization is provided.

The mechanical properties of paper are impaired by the addition of filler. The beating of kraft pulp and addition of starch are possible remedies for this. However, beating also has negative effects because it reduces opacity and bulk, and starch effects are limited by retention. Optimal use of the kraft pulp and starch is therefore important. We show that in pure kraft sheets beating alone can compensate for most of the adverse effects on mechanical properties caused by kaolin addition. In TMP-based sheets with kaolin, the mechanical properties are fairly insensitive to the kraft content unless very high beating levels are used. The primary role of kraft is to improve tensile stiffness, not tensile strength of paper. Starch and beating both improve inter-fiber bonding but beating also raises fiber segment activation. The latter mechanism contributes to tensile stiffness but reduces damage width. The other mechanical properties of paper appear to be insensitive to fiber segment activation.