Mottling-Phänomene bei holzfreien, gestrichenen Papieren

Mottling, or in other words the unevenness of a printed image, is a very important criterion for the quality of coated printing papers. According to a recent publication, mottling accounts for 42% ofall quality complaints for sheet-fed offset printing papers. The reasons for uneven appearance of printing products are manifold. Mottling is affected by a number of influencing factors, not only binder migration as suggested in the majority of studies. Basically, all mottling phenomena are related to interactions between liquids (printing inks, fountain solution et cetera), frequently containing dissolved polymers and salts, and the coating structure. This paper aims to summarise some of the crucial influencing parameters relevant for mottling, with the main focus on base paper properties, critical interactions in coating colour formulations, consolidation and drying of the coating as well as calendering. It can be shown that a highly uniform print quality can only be obtained if the coat weight distribution ensures an optimum physical coverage of fibre structures whilst maintaining the product target of surface smoothness. At the same time the coating structure has to be homogeneous in terms of wettng, absorption and adsorption properties. This homogeneity of the coating structure can be obtained by the choice of compatible, well-dispersed pigments, binders and additives, the avoidance of flocculation effects along with a well-controlled consolidation and immobilisation process of the wet coating film in the coating machine.