The influence of different roughness scales of pigment coated papers on print gloss

Influence of different surface roughness scales and air permeability of coated paper on the print gloss was studied. The coatings were based on various pigments and their binary and tertiary mixtures commonly used in paper coating industry. The laboratory printability tests were done with a commercial offset cyan ink. Atomic force microscopy (AFM) and confocal optical microcopy (COM) were utilized to study the surface roughness of the coated surface at a wide length scale range. A first order exponential decaying relation between the root-mean-square (RMS) roughness (sigma) and correlation length (T) was empirically established. The sigma-T curves enabled the identification of two critical roughness points, describing the maximum roughness in the small and large T range. The former was coupled to fiber covering and pigment particle surface structure and the latter described the quality and nature of the surface smoothing. The maximum roughness (sigma > 1 mu m) at large T correlated linearly with the final print gloss for the fast-setting surfaces. On the other hand, the print gloss of slow-setting surfaces was mainly influenced by the ink-setting rate and ink amount. No correlation between print gloss and the maximum roughness at small T was found.