Pigmentation of paper coatings

Paper is coated mainly to improve its aesthetic appeal and its printability. The pigmentation of these paper coatings is just one of several interlocking factors that influence the final print quality. Other main features include optical uniformity which is related to gloss, and geometrical uniformity which is related to smoothness and printability. Apart from the actual pigments used, print quality is also influenced by the coat weight, see Fig. 1. In general the pigments that print the best are those with the worst flow properties in suspension. This in turn is related to pigment packing since bad packing gives poor flow properties (Fig. 2). Good printability is related to good compressibility. Softer lattices are more compressible and consequently give better printability. There is also a relationship between pigment type and size with coating porosity and uniformity, which in turn is related to other factors such as ink scuff, mottle and blistering, etc. which give poor print quality.