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A number of studies have reported on the mechanical properties of paper coatings and the influence of various parameters on cracking at the fold. Failure at the pigment-binder interface is important to understand because it likely controls how coating layers fail or crack in different applications and processes. In this study, a model system of a calcium carbonate surface in contact with latex and starch binders was used to evaluate the interfacial bonding performance affecting failure in tensile mode. Samples of calcium carbonate marbles with binder films between them were tested in tensile tests and the results were compared with those of pure binder films. In addition, contact angles of calcium carbonate, latex and starch were measured. The failed surfaces were analyzed using Raman spectroscopy and scanning electron microscopy (SEM). All sample involving the calcium carbonate surface failed at a maximum stress of around 25% compared to that of pure films. Samples that contained starch exhibited larger maximum stress values than pure latex system. The strain at failure for these samples did not depend on the starch content to a large extent. While Raman results did not indicate any residual latex particles on the adhesively failed surfaces, SEM microscopy confirmed that at least part of the failure was still cohesive because regions of binder were present on the surface.

The article focuses on the use of confocal laser scanning microscope in the three-dimensional observation of coated paper. The three-dimensional structure of a paper coating layer is observed through a laser beam which scans a specimen and a photodetector which collects the fluorescent light emitted. The results of the process can be used to characterize the depth of the coating layer penetration into the base paper, the occurrence of bubbles, and the possible distribution of latex and optical brightener.

The composition of the top surface of a coating layer can influence its functional properties or subsequent processing steps. The effect of the substrate absorbency on the coating surface chemistry is reported. Different coating systems containing a kaolin clay pigment, fine or coarse precipitated calcium carbonates, and a common latex binder were examined. The influence of a soluble polymer added into the coating was characterized. The surface chemistry was measured with attenuated total internal reflectance (ATR) and X-ray photoelectron spectroscopy (XPS).Absorbent substrates generate bulky coatings with high voids and low gloss. Rapid dewatering by the absorbent substrate pulls the small particles, like latex binder, away from the top layers causing a low latex concentration at the surface. On non-absorbent substrates, the addition of the soluble polymer generates coating layers with higher void volume, lower gloss, and lower latex concentrations at the coating surface. However, on absorbent substrates, polymer addition causes coatings with lower void volumes and higher gloss. In this case, the rapid dewatering and mobility of particles is reduced by the polymer, which helps to retain the small particles at the surface. As a result, latex concentration at the surface increases with polymer addition on absorbent substrates. [Copyright &y& Elsevier]

The mechanical properties of paper coating layers are important in various converting processes such as printing and folding of the paper. A method was developed to produce free-standing pigmented coating layers thick enough to be tested in bending as well as tension. The mechanical properties of these coating layers were characterized. Free-standing films with two types of binder formulations, pure latex or latex-starch combinations, with different binder content were prepared through an innovative film forming method that allows uniform drying of the coating layer while minimizing cracking. Tensile and flexural samples were cut out of these films. Tensile and flexural tests were carried out on samples and the results were compared. Tensile and flexural moduli were found to be similar. However, both stress at failure and strain at failure were higher in flexural tests compared to those in tensile tests. Three different types of paper were coated with these formulations and their failure during printing was evaluated through a standard ink picking test. Papers with different basis weights and porosities were found to have different picking resistance values. An interesting result was that the picking resistance did not correlate with the elastic modulus of the coating, but with the strain at failure behavior. This result indicates that the stiffness of the coating layer is not as critical during printing compared to its flexibility. [ABSTRACT FROM AUTHOR]