Each day, we are confronted with a large amount of more or less important information that we have to consider, and even in our digital society we need paper for communication, documentation and education. Much of the paper we use or are confronted by in our daily life, such as newspapers, books and packages, contains printed images or texts, and the appearance of both the print and the supporting surface is important. A good contrast between a printed text and the paper makes it easier to read, a detailed print of an illustration makes it more informative, and clear and evenly distributed colours on a package or on a poster make it more appealing. All of these qualities depend on the optical properties of the paper product and the the behavior of light illuminating the different materials. The aim of the work described in this thesis is to characterize the structure of coatings and prints, and to validate models for the optical response and interaction of ink and coating based on optical measurements of physical samples. It is the interactions between the printing ink and the porous structure of the coating layers that are subject to investigation. Experiments have been employed to relate the physical conditions in a flexographic printing nip to the ink setting, affected by the physical and chemical properties of the coating, to the resulting optical response of the printed paperboard. / The aim of the work described in this thesis is to characterize the structure of coatings and prints, and to validate models for the optical response and interaction of ink and coating based on optical measurements of physical samples. It is the interactions between the printing ink and the porous structure of the coating layers that are subject to investigation. Experiments have been employed to relate the physical conditions in a flexographic printing nip to the ink setting and the resulting optical response. By comparing simulated and measured results, it was shown that modifications of the surface properties account for the brightness decrease when substrates are calendered. Light scattering simulations, taking into account the surface micro-roughness and the increase in the effective refractive index, showed that surface modifications accounted for most of the observed brightness decrease, whereas the bulk light scattering and light absorption coefficients were not affected by calendering. Ink penetration affects the print density, mottling and dot gain. Results show that ink distribution is strongly affected by surface roughness, differences in pore size and pore size distribution. For samples having different latex amounts and different latex particle sizes, a higher print force did not increase the depth of penetrated ink to any great extent, but rather allowed the wetting to act more efficiently with a more evenly distributed ink film, a higher print density and fewer uncovered areas as a result. Uncovered areas could be linked both to local roughness variations and to local wettability variations on the surface. Samples with different ratios of calcium carbonate/kaolin clay pigment showed an increased porosity and an increase in print density with increasing amount of kaolin in the coating layer.

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One of the problems encountered in paper converting is caused by the occurrence of "baggy webs", which essentially is when the tension profile of the paper web is uneven. In an area with low tension the paper is longer, which results in bagginess. The baggy parts can not usually be stretched to even out the tension of the paper web in a converting machine, with the result that runnability problems are likely to occur. The aim of the work described in this thesis was to investigate three particular stages in papermaking, namely drying, calendering and storage, and rank them according to their propensity for inducing baggy webs. The focus was placed on investigating the effects of uneven moisture and grammage profiles on the machine-direction strain difference profile. The largest strain difference occurred when there were systematic thick streaks throughout a reel that formed ridges. Stress relaxation during storage then gave rise to a difference in strain of 0.14% when the ridge height was around 2-3 mm. Thickness variations due to variations in grammage is also a source of moisture variation. A difference in moisture of 5% in the calendering stage resulted in strain differences of about 0.05-0.08%. These strain differences resulted in creases being formed as early on as in the calender nip when differences in both grammage and moisture content were present. Most creases appeared when the moisture difference was 2-8%. The difference in grammage could be large without creases being formed when no differences in moisture content were present. A moisture difference of about 5-6% during drying resulted in a strain difference of 0.1% measured on isotropic samples. The moist area turned into a tight streak when the moisture difference appeared at moisture contents higher than 25%. At moisture contents lower than 20%, on the contrary, the moist area turned into a slack streak. The conclusion drawn is that papermakers should concentrate first and foremost on eliminating variations in grammage, especially if these are systematic. This would also eliminate some variations in moisture content, which would solve more problems.

Starch in paper coatings is known to increase the risk of print mottle in lithographic offset printing. The objective of this study was to increase the understanding of this behaviour. Four phenomena that could lead to print mottle, where the presence of starch might be important, have been identified: uneven binder migration, uneven coating mass distribution, uneven deformation during calendering and differential shrinkage. The latter three were investigated in this project. Starch-containing coating colours often have high water retention. A relationship between the water retention of the coating colours and the distribution of coating thickness was found in a pilot trial. A theory is proposed, where the surface profile of the base paper beneath the blade, that governs the coat weight distribution in blade coating, is affected by moisture from the dewatering coating colours and the compressive force exerted by the blade. Drying strategies were studied to see whether they would induce porosity variations in the coating layers. There is a strong connection between the rate of evaporation and the shrinkage of the coating layer, but no porosity variations due to the choice of drying strategy were found. Shrinkage is governed by the capillary forces. At the same capillary pressure, the coating shrinks more for some binder systems, which is suggested to be due to a weaker chemical interaction between the binder and the pigment. Oxidized starch/latex coatings, stained with a fluorescent marker, had a greater standard deviation in fluorescence intensity than CMC/latex coatings caused by a difference in either porosity or latex distribution. It was shown that calendering introduces porosity variations into the coating layer that are larger for starch-containing coatings. The drying strategies appeared to have a significant effect on these porosity variations and they correlated positively with print mottle in some cases and in another case negatively. In the case of the negatively correlated, the mottle was probably caused by variations in surface porosity existing prior to the calendering.