The significance of paper machine calendering on coating coverage

Machine calendering is conventionally used to smoothen and densify the base paper surface, prior to coating application. The intention is to improve t he coating coverage of a machine calendered base paper. However, several st udies of base paper calendering claim that the induced during machine calen dering relax during coating and increase the roughness to nearly the initia l level. Mechanical pulp is especially prone to such a stress relaxation of the fiber network, whereas woodfree pulp seems to retain a compressed stat e better. Other detrimental effects on the paper, besides roughening during wetting, are bulk reduction and an uneven z-directional densification of t he sheet (i.e. fiber flocs and basis weight variations). Certain aspects of machine efficiency, however, advocate the use of a machine calender. Fiber s and loose components on the paper surface need to be attached or pressed onto the surface of the sheet, which can be done, for example, in a calende r nip. Without a calender nip, particles caught at the blade tip mey cause streaks in the coating layer, blade lines and even web breaks.