2005 Volume 2

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.

A novel transmission electron microscopy (TEM) technique, developed to observe the nano-scale interactions of polymeric additives and cellulosic fibrils under idealized laboratory conditions, was applied for the first time in a comprehensive study of the colloidal interactions within a mill producing light-weight coated publication paper. The technique allows the observation of incremental changes in the nano-scale appearance of the paper-making slurry as successive additives are introduced to the system. Such changes include the coagulation of colloidal and dissolved substances present in thermomechanical (TMP) pulp after the addition of a low molecular weight, high charge density polymer, and the subsequent flocculation of the coagulated matter, hydrophobic materials, and fines following the introduction of talc, aluminum sulfate, a high mass cationic polyelectrolyte, and silica nanoparticles. The new results demonstrate that the TEM technique can be applied even in systems as complex as commercial papermaking, leading to a more accurate understanding of what happens on a macromolecular level.

Poly(ethylene oxide) (PEO), a widely known flocculation agent used primarily as a fines retention aid in mechanical grade papers, has its efficiency enhanced by various compounds, known as cofactors. These cofactors form a complex with PEO, which acts as an efficient bridging agent for fines flocculation. The nature of the PEO/cofactor complex is mainly unknown, and it was originally believed that the association was driven by hydrogen bonding. Therefore it was decided to investigate the complex formation in more detail. As a model system we studied PEO and a model cofactor, corilagin, a precursor of tannic acid, a known cofactor for PEO. We performed both Semi-Empirical Molecular Orbital (PM3) gas phase calculations and Molecular Dynamics (MD) calculations in the presence of water. Both methods lead to the same surprising conclusion: no association between PEO and corilagin occurs at room temperature. The reason is that the gain of association enthalpy is not large enough to overcome the loss in entropy. No correlation was found between the association enthalpy and the number of hydrogen bonds between PEO and corilagin. The absence of PEO/corilagin complexation was confirmed by NMR, isothermal titration calorimetry and the inability of PEO/corilagin to flocculate MCC (microcrystalline cellulose). In the presence of low concentrations of salt, complexation and MCC flocculation was observed.

Regenerated cellulose films were laminated with polyvinylamine, PVAm, and the wet peel delamination forces were used to explore the mechanism by which PVAm increases the wet strength of paper. Conventional wet strength resins contain highly reactive chemical groups which can crosslink the resin and graft it to fibre surfaces. By contrast, it is not obvious how PVAm provides wet strength.

The delamination experiments revealed that PVAm gives strong adhesion which was approximately independent of drying temperature (23 to 110°C), pH 3 to 9, PVAm molecular weight (34,000 to 1,500,000Da), and PVAm coverage (monolayer to 70mg/m2 ). By contrast the adhesion increased with the amine content of PVAm and with the degree of oxidation of the cellulose films. It is proposed that the PVAm adhesion is a combination of electrostatic and covalent bonding. The electrostatic bonding is between protonated amines, which are positively charged, and carboxyl groups on the cellulose. Whereas the covalent bonds, aminal and imine linkages, are formed between amines and aldehyde groups on the oxidized films.

Two studies were carried out in an effort to gain a better understanding of how ketene dimers develop sizing. In the first study, eight ketene dimers with a range of melting points, vapor pressures, and molecular weights were evaluated for rate of sizing development. Ketene dimer melting point had a clear effect on the rate of sizing development. High melting ketene dimers initiated sizing at higher sheet moistures than low melting dimers. High melting dimers also developed their ultimate level of sizing faster than low melting ketene dimers. These results suggest that solid and liquid ketene dimers have different mechanisms of sizing development. Ketene dimer vapor pressure and molecular weight had no consistent effects on the rate of sizing development.

Pseudo first order rate constants for sizing development were then measured for a high melting solid ketene dimer and a liquid ketene dimer over dryer temperatures ranging from 55 °C to 85 °C. The rate constants measured for the liquid ketene dimer increased steadily as dryer temperature increased. An Arrhenius plot of the rate constants obtained for the liquid ketene dimer yielded an activation energy of 11 kcal per mole for sizing development. These results are consistent with the hypothesis that the chemical reaction of the lactone ring is the rate determining step in sizing development for liquid ketene dimers. The high melting solid ketene dimer followed more complex kinetics and probably developed sizing by a combination of mechanisms. It is likely that the differing sizing responses measured for the solid and liquid ketene dimers are due to differing sizing contributions from the unreacted and hydrolyzed ketene dimers.