Compression of Coating Structures During Calendering, 2006 TAPPI Advanced Coating Fundamentals Symposium

Calendering is widely used to enhance the smoothness and gloss of coated papers. However, in addition to the smoothening effect, calendering can also cause compression of the coating layer, and this affects the optical and ink-setting properties. The objective of this study was to determine how, and to what extent, calendering affects the structure of pigmented paper coatings. As expected, the gloss increased with increasing temperature or increasing load, but calendering also caused a significant reduction in the pore volume (up to 45%) and in the average pore size (up to 40%). Clay coatings collapsed more than calcium carbonate coatings, because of the collapse of pigment bridges and the re-orientation of clay particles. Higher latex content led to greater compression during calendering. To explain these observed effects, a mechanistic explanation is proposed which takes into account pigment re-arrangement and the lubricating role of latex under high temperature and high load.