The dried structure of paints and coatings are important to understand. In paper coatings, pigments and binder types are known to influence this structure. The influence of a new additive such as cellulose nanofibrils (CNF) on the interaction between the coating components is not thoroughly examined. In this study, the effect of CNF on the rheology of coating color and structure formation of the coating layer was investigated and compared to that of carboxymethyl cellulose (CMC). The addition of these two rheological modifiers made the dried coating layer porous, but the working mechanisms associated with these two additives were different. CMC, which flocculated coating components, limited the rearrangement of the components, resulting in a loosely packed coating structure in wet state. CNF, which did not significantly influence the interactions between the coating components, increased effective volume fraction by absorbing water. The water absorbing characteristics of CNF expanded the dried coating structure.

We have developed a new method for characterizing paper coating structure using FIB milling technique and FE-SEM. A new image analysis method combining smart blur and locally adaptive thresholding technique was applied. Compared to conventional method, this method preserved the edge of target and gave better automatic image segmentation of pores. We applied this method for the measurement of the pore structure in pigment coating layers. Staining with OsO4 and resin-embedding treatments were used for the sample preparation of coated papers for image analysis. A control sample without any treatment was also used in analysis. BSE images of osmium stained and resin-embedded coated paper gave the clearest image segmentation of coating components. The results showed that three sample preparation methods gave similar z-directional porosities. Lower porosity and smaller pore size was obtained from the top surface layer with the osmium stained and resin embedded samples.

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The pore structure of the coating layer is one of the most important factors in determining the printability of coated papers. The coating pigment and binder are two principal components in paper coating, and their characteristics have a critical influence on the coating structure. The glass transition temperature (Tg) of latex binders affects the mechanical strength and pore structure of the pigment coating layer because the latex Tg influences the binding ability of latex and the shrinkage of the coating layer during the drying process. In this study, styrene-acrylate (S/A) core-shell structure latexes with different monomer compositions in the core and shell layers were designed, and their properties were compared with those of a conventional latex. These core-shell latexes were prepared using the same monomers in the same proportion and were used to investigate the effect of the core-shell structure on the structural and mechanical properties of the coating layer. The hard-shell latex with a high styrene content in the shell part yielded paper that was glossier and less rough and formed finer pores, resulting in an increased ink absorption rate into the coated paper compared to the other types of latex. The hard-shell structure showed better performance in printing uniformity and had less mottling.

The glass transition temperature (T g) of an S/B latex binder, which is highly related to the film-forming properties of latex, has long been of interest, and its effects on the properties of the coating layer are well-known. Measurements of the bulk and surface properties of coating layers or the properties of coated papers have been used to investigate the effect of T g on the coating structure. However, these approaches cannot provide a detailed analysis of the coating structure because they give an average value for the whole coating layer. We applied a newly developed image analysis method, consisting of smart blur and locally adaptive thresholding techniques, for evaluation of the coating structure depending on the T g of the S/B latex and the calendering conditions. This approach allowed us to obtain the average pore structure, pore shape, pore size, and Z-directional distribution. The changes in the coated paper properties by calendering or latex types were compared and analyzed based on the coating layer structure.