The Effect of Base Paper and Coating Method on the Surface Roughness of Pigment Coatings

Ground calcium carbonate (GCC) and precipitated calcium carbonate (PCC) was applied on base paper with various coating amounts and two different coating methods. The surface topographical images were captured by atomic force microscopy and confocal optical microscopy. The surface roughness of pigment coatings was analyzed by using a two-point correlation method where both the amplitude and lateral spacing of surface heights are considered. Isotropy and periodicity of roughness were studied by using the angular distribution of the correlation length (T) and the autocorrelation function (ACF). All the studied pigment coatings gave a random distribution of surface heights. Both PCC and GCC considerably smoothed the base paper surface. However, the differences in roughness of PCC and GCC were quite small for similar coating parameters (e.g., amount, method). The increase of coating amount decreased the surface roughness. Rod coating yielded a somewhat more isotropic coating than blade coating. The best coating surface was achieved by using rod coating and GCC with coat weight of 23g/m2. PCC coatings yielded about 10units higher gloss (Ga) compared to GCC coatings with similar coating parameters. Ga was further found to correlate with /T, even if the correlation was not purely linear.