Moisture-Gradient Precalendering of Coated Board - A Case Study, 1999 Coating Conference Proceedings

Olle Steffner

Iggesund Paperboard AB

Mikael Rigdahl

Swedish Pulp and Paper Research Institute (STFI)

Paper and board are viscoelastic materials that are softened by an increase in temperature and/or moisture content. The moisture can have quite a significant influence on the softening behaviour. To take full advantage of this during calendering, it would in principle be favourable if a controlled moisture gradient could be created in the thickness direction. Some investigations indicate however that the moisture-induced softening is very fast and that any positive effects on the stiffness and smoothness of the sheets may be difficult to establish and control. In this study, a full-scale trial was performed where a waterbox-equipped precalender stack was used together with adjacent drying cylinders in order to produce different moisture distributions in the material during the calendering. Both coated and uncoated board were investigated. It was concluded that, at a given density, surface smoothness and to some extent also gloss were favoured by a moisture-gradient calendering operation. The moisture level and the moisture distribution had only a minor influence on the tensile mechanical properties of the board. The bending properties were affected by the moisture content during the calendering, but the moisture distribution appeared to be of less importance. The results obtained indicate that moisture-gradient calendering can produce or enhance non-uniform structural changes in the board.