Proceeding Articles

Paper is a complex three-dimensional network of fibres, pores and often fillers. The main goal of this study is to characterise its structure in a non invasive and non destructive way. In order to overcome the limitations of 2D measurements, Synchrotron Radiation microtomography is used to visualise the samples. Coupled with appropriate processing tools, it allows a quantifica-
tion of some structural characteristics on the samples. This is the main aim of this paper. Different articles relate the feasibility of such a study for paper samples: imaging at the European Synchrotron Radiation Facility (ESRF, France) in phase contrast [1] or in absorption mode [2] gives the opportunity to reach a pixel size smaller than a micron which can not be obtained with classical tomographs [3]. A first step is to visualise the structure then, structural parameters are extracted from these 3D data [4]. However, this requires a segmentation technique adapted for application to typical paper samples that are constituted of three phases. The first step consists in the segmentation of the different phases, namely, air, fibres and fillers. The amount of each component may be evaluated. This was validated for both the porosity and the filler content. Furthermore, structural parameters were calculated from the binarised volumes. The comparison with the published results validates the calculation.

A new experimental technique is presented that allows the direct observation of fibre deformation during wet pressing. Pulp fibres were wet pressed onto a glass slide and the region where two fibres crossed was examined microscopically through the glass. While the underlying fibre was in contact with the glass slide down its length, the overlying fibre must span from the top of the fibre to the glass slide. The geometry of the intersection is controlled by both the local conformability of the overlying fibre and the deformability of the underlying fibre. It is not primarily con-
trolled by the longitudinal flexibility of a fibre. The method provides new opportunities to investigate the effect of mechanical and chemical treatment on the papermaking properties of pulp
fibres in both the dry and wet state.

We provide a simulator for a range of bivariate stochastic processes of various application in the physics of stochastic fibrous networks. We illustrate the effects of local correlation on the statistics of voids in the bulk and the surface of fibre mats in general and paper in particular. The reference case of random isotropy has an inherent ‘ground-state’ correlation of adjacent free-fibre-lengths; this explains the classical observation of Corte that pores seem mainly ‘roundish’ in real paper samples. In the isotropic case, the mean pore radius can be reduced from that in a random network by 20% through structural changes associated with increased flocculation. The mean eccentricity of pores seems to give a measure of the variability in free-fibre-length distributions that is not due to local correlation. We find a uniform effect of local correlation on mean pore eccentricity over a range of stochastic network structures; at a given correlation, increased flocculation increases mean eccentricity slightly.

This paper describes the development of a method for mapping the apparent density of paper, nonwovens and other fibrous webs based on non-contact laser profilometry and β-transmission radiographic imaging. The method is applied in three complimentary studies that examine the in-plane non-uniformity of thickness, grammage and apparent density in printing papers. The first section focuses on the development and verification of the analytical method for mapping thickness. Through simultaneous scanning of the topography of both sides of the specimen by opposing range sensors, the surface contour of each side, the local thickness, and the out of plane deformation were measured. It was demonstrated that the laser based method provides results that closely approximate the intrinsic thickness that is independent of paper formation. The method was then used to examine the structural differences in laboratory sheets pressed by soft and hard nip calenders. The well known difference in web densification mechanisms of the two was reaffirmed by mapping discrete changes in thickness and statistically comparing these with corresponding points on grammage maps. Densification was shown to be dependent on grammage for the hard calender, and independent of grammage for the soft calender. The final study used the thickness mapping method to monitor the hygroexpansivity of representative printing papers as equilibrium humidity was varied between 9% and 80%. Thickness maps were obtained for newsprint, SC-A (calendered and uncalendered), bulk offset and office copy. Differential thickness maps were used to compare the in-plane non-uniformity of hygroexpansion. The inplane hygroexpansion was characterized and corrected for using a recently developed algorithm known as Enhanced Digital Image Correlation (EDIC). Thickness change did not appear to correspond to grammage maps. The results suggest that a significant irreversible increase in thickness occurs for papers that are heavily calendered.

The literature on the interactions between the coating colour and the base sheet during the pigment coating of paper is reviewed in the order to summarize the current knowledge within the area. The review is focused on the processes of forming and consolidation and on how the coating colour interacts with the base sheet during these processes, and how this interaction affects coating hold-out, roughness and coating mass distribution. A coating layer which stays on the surface of the base sheet and which has a uniform mass distribution is desired. The research world disagrees on whether coating hold-out is a relevant problem. The reason is that there is little direct evidence in the literature on coating penetration. However, there are numerous indications of an indirect character which suggest that coating penetration exists, both in blade coating and in coating with the metered size press. The pressure pulse to which the coating colour is subjected in the applicator nip during blade coating or in the transfer nip in metered size press coating and the permeability of the base sheet are factors which are said to control coating penetration. There is concordance in the opinion that the uptake of the aqueous phase of the coating layer is an extremely rapid process and that this uptake releases stresses in the sheet and plasticizes it. The release of stresses leads to roughening of the sheet whereas the plasticization makes it compressible and smooth in a compressed state beneath the blade tip during the forming of the coating layer. Much attention has been given to the roughening and a number of extensive studies have been published about that. The studies on plasticization and sheet compressibility and how they affect the mass distribution of the final coating layer are fewer in number but non-existent. The roughening and plasticization of the sheet are reported to be different for woodfree and wood containing sheets, due not only to the different types of fibres in the sheets, but also to their different densities. Woodfree sheets, which are generally the densest, are considered to be dimensionally the most stable. A number of researchers have reported that the pre-calendering has a great influence on the roughening. Studies have shown that the calendering builds in transverse stresses into the sheet and, in the case of wood- containing sheets, closes the lumen of thick-walled fibres. During the coating operation, when the sheet takes up the aqueous phase from the coating colour, these stresses are released and the lumen is opened. Several researchers have shown that all the smoothening effect of the pre-
calendering can be lost during the coating process.

Geometrically non-linear, large scale post-buckling analyses were carried out to investigate the influence of different parameters on residual waviness (fluting) after printing in a heat set web offset printing press. Mixed implicit-explicit finite element techniques were used in the analyses. The numerical procedure was verified by experimentally acquired data. Results show that when the paper web is perfectly flat before printing, fluting patterns after drying and moisture recovery generally have higher wavelength than those typically observed in fluted samples. Initial cockles of unprinted sheets were found to have impacts on the fluting patterns and amplitudes. Among the factors investigated, ink thickness and hygroexpansivity had significant influences on fluting: increasing these factors increased fluting amplitudes.

The printing speed of offset presses has increased significantly during the last decade and this puts increasing demands on the interaction between the paper and ink. Poor interaction may
result in runnability problems and low quality of the printed product. The research in this area has been intensive during many years and the knowledge of important phenomena has been significantly improved, although a full understanding is still missing.

This review concentrate on offset printing of coated paper with special focus on sheet-fed offset. Properties of coatings and composition of offset inks are briefly discussed. Some data on ink film thickness and its lateral distribution is presented. Ink setting is reviewed in some detail and the effect of important coating properties (e.g. porosity, pore size, latex properties) and ink properties (ink oil viscosity and surface tension) are discussed. The interaction between latex binder and ink oil is given special attention.

The impact of coating structure and ink setting on print quality is covered in some details. The importance of ink filament formation and levelling as well as coating topography and ink film thickness on print gloss is well established. Recent findings on relations between mottle tendency and non-uniformity of coating structure and ink setting are included.

The microscopic response of a paper sheet to compressive forces is of great importance in predicting print quality as well as the sheet structures developed in wet pressing and calendering. In this report, we propose a new compression model that preserves the entire three-dimensional, stochastic, fibre network structure. The model includes Z-directional deformation of fibres in both compressive and shear modes. Permanent deformation of each fibre (such as caused by fibre collapse) can be achieved by adjusting the stiffness of the fibre during compression or unloading. The stiffness of the plates can also be chosen to represent, for example, a hard printing plate, a blanket, or a soft-nip calendar cover. Although we still need to collect basic fibre stiffness data in order to perform quantitative comparisons between model predictions and experimental results, simulated structures already show typical features of consolidated paper sheets. As a first application, we study the contact mechanics between a printing plate and a model paper structure.

Print non-uniformity, or mottle, is an important factor in print quality. The ultimate judge of print quality is the printer or print buyer, so print quality measurement should be representative of human perception. Most methods that are currently available to systematically quantify print mottle do not consider eye response in the calculation. Instead, the user has to select appropriate scales for the analysis by comparing with separate visual ranking experiments for each new set of prints.

We developed a method to process digital images of mottled black prints to provide a mottle index that takes into account eye response. The mottle indices obtained for a range of paper and
board grades were compared with the results of separate visual rating experiments, and there was very good agreement between them. The mottle index outperformed other parameters also used for the quantification of mottle. Based on these results, the mottle index is deemed reliable enough to decrease the need for separate visual assessments by panels. The mottle index algorithm removes the need for the operator to make subjective choices on the appropriate analysis scales for sample sets where print uniformity is the dominant quality criterion.

The proposed mottle measurement method allows systematic and objective quantification of mottle. The method can easily be implemented to analyze test prints using the analysis software we
developed, and an appropriate desktop scanner that will require calibration to relate the greyscale to reflectance values.

On a paper machine the retention of fillers and fines during the first pass can be very different from the retention during a subsequent pass, because fillers and fines have much more time to interact in the short circulation loop than during the period between the injection of a retention aid and the forming section of the machine. Also in the short circulation loop reconformation
of adsorbed polymer can occur, often reducing the flocculation efficiency of the polymer and interfering with subsequent adsorption. Thus for a meaningful modeling of retention on a paper  machine first pass retention and second pass retention should be distinguished. Retention of fillers and fines can result from deposition on fibers, either freely suspended in the papermaking
suspension or immobilized in the forming sheet, or by capturing fines and filler aggregates in the sheet by mechanical entrapment. Aggregates are more likely to be formed in the short circulation loop than on the paper machine. Effects of retention aids, detachment from fibers and the importance of polymer transfer are discussed.