Paper coatings

Abstract

To produce a smooth ink receptive surface with improved optical properties, paper is finished with a thin mineral pigment coating. The aqueous coating formulation, which contains predominantly white mineral pigments and binders, are complex and varied. The coating ingredient selection depends on the raw material availability, cost, paper making process, paper grade, application and most importantly the printing process, which will be used. The aim is to produce a smooth surface, to maximise specular gloss with a pore size that optimises the printed ink gloss. The coating should also have an open packed internal structure with a void size to maximise brightness and opacity with sufficient binder to provide mechanical strength during printing and application. As no single pigment fulfils the entire coating property requirements, it is not uncommon for the coating formulation to contain more than ten ingredients with two or more pigments. While kaolin is the dominant coating pigment, kaolin producers are developing finer, brighter kaolin pigments to counter the growing calcium carbonate usage trend. Therefore, this article reviews paper coating and the major pigment used, with a focus on the issues associated with developing a new fine-bright kaolin coating pigment.