Coating layer uniformity as affected by base paper characteristics and coating method

The print quality of coated papers is to a large extent determined by the uniformity of the coating layer. Here a systematic laboratory scale study of the influence of the interaction between a wood-containing LWC base paper, the coating method (blade or film coating) and the precalendering strategy on the uniformity of the coating layer was performed. The uniformity was evaluated as the coating mass distribution and the gloss variations of the coating surface. Six important variables were considered: the type of mechanical pulp used, filler content, addition of retention aids, precalendering of the base papers, solids content of the coating colour and the coating method. Multivariate analysis was used in order to assess the importance of the variables with regard to the coating mass distribution and the gloss variations. A good formation was in general noted to be prerequisite for achieving a high degree of uniformity, and papers containing pressurised groundwood pulp (PGW) performed better in this respect than those containing thermomechanical pulp (TMP). The effect of precalendering was not entirely clear but seemed in many cases to have a negative influence on the coating mass uniformity. A higher filler content reduced the gloss variations, but was less favourable for the mass distribution of the layer. The mass uniformity of the layers was evaluated in different size regions and it was only in the larger size regions (corresponding to the size of fibre floes or larger) that an effect of the coating method could be ascertained. Using the film coating technique, a contour-type of coating was noted in this size region.