Interactions between base paper and coating color with double-coated paper

The aim of this study was to investigate the influence of base paper and film press roll cover hard-ness on the quality of the pre-coated paper. When the paper was pre-coated using hard rolls, coat-ing coverage depended mostly on the average pore size of the base paper. The coatweight was formed mainly through penetration, which resulted in a dense coating layer. With soft rolls, coat-weight was formed more through dewatering, especially on dense base paper, in which case good coverage was obtained, albeit accompanied by a porous coating layer. The average pore size of the base paper structure, base paper surface openness, and filler content at the surface are there-fore the main variables affecting coating coverage and coating layer porosity in film press coating.

The top sides of the base papers produced on a fourdrinier machine were dense and had higher surface filler contents than the bottom sides, and thus acquired a more uniform coating layer than the open bottom sides. The chemical properties of the base paper surface (hydrophobic / hydro-philic) had no effect on coating coverage or coating layer porosity. All pre-coated papers were top coated with a constant coatweight, which was controlled through on-line coatweight measurement. Decreased coating coverage and increased porosity and roughness of the pre-coated paper made setting of the top coating color slightly faster, as indicated by the greater blade pressure needed for constant coatweight. In terms of the gloss of the top-coated and calendered paper, pre-coating cov-erage was more important than pre-coating coatweight alone.