The role of base paper porosity in MSP coating

The effects of base paper permeability and pore size on coating coverage in metered size press (MSP) coating are presented. The intensive refining of softwood and hardwood pulps reduced both the permeability and the pore size of the base paper, which resulted in lower coatweights. Neither base paper roughness nor small-scale basis weight variation (beta formation) at the same porosity had any effect on coating color transfer. Reducing the pore size improved the coating coverage of the pre-coated paper, especially at low coatweights. A dense base paper gave better light scattering properties than a porous base paper because of the higher coating coverage. The results showed that the liquid phase of the color penetrated deeper into the base paper structure as base paper porosity was increased or the coating color solids content was reduced. In conclusion, we recommend that attention be given to the physical structure of the base paper. Any means of reducing base paper permeability increases the average coverage and improves its uniformity in MSP coating.