Relationship between gloss and surface texture of coated papers

Gloss strongly affects visual perception and print quality of coated papers. In this study, we investigated standard TAPPI gloss and microscale gloss of several commercial and laboratory-made coated samples. The laboratory coated samples contained seven different kaolin clays and four ground calcium carbonates with a wide range of pigment morphology, particle size, and particle size distributions. The samples were calendered at various conditions. We found that coated paper samples were optically rough surfaces with Rayleigh parameter much larger than 1. A semi-empirical approach using the root-mean-square roughness, sh, and correlation length of surface heights, LC , showed that gloss of coated samples is proportional to (LC /sh2 )n. The exponent n was found to depend on the angle of illumination. For the TAPPI 20°, TAPPI 75°, and 60° standard gloss, as well as average microgloss, n was 0.41, 0.31, 0.52, and 0.23, respectively. Despite large variations in the pigment size, morphology, and calendering conditions, all data points closely followed the same regression line, suggesting that gloss is predominantly governed by the surface texture of paper.