On the Dry and Wet Surface Strengths of Coated Paper

Abstract

Lack of surface strength in coated paper can cause significant problems both in offset printing and in different converting operations. Several factors can influence this surface strength. The first part of this paper summarizes previous work concerning the relation between the mechanical properties of the coating film itself and the dry surface strength of coated paper. From studies including both clay and CaC03 pigments, it was concluded that both the in-plane mechanical properties of the coating films and the dry surface strength were to a large degree determined by the choice of pigments and the type of styrene-butadiene (SB) copolymer used as binder. The difference between clay-based and CaCO3-based coatings was of special interest in this study. The second part of this paper describes a laboratory method for measuring the wet strength of coated paper. The method has been applied to paper coated with either clay or CaC03, the coatings being bonded with different amounts of SB. The fracture in the coating in the wet state is shallow and, contrary to the fracture in a dry coating, does not penetrate down to the base paper. The particles removed are also rather small, typically 10-30 4m in diameter. Paper coated with CaC03 had a higher wet surface strength than paper coated with clay. This may be due to a higher degree of adhesion between CaC03 and SB than between clay and SB, which would yield a higher water resistance.