Coatweight formation and coating color liquid phase penetration in film coating

A film transfer coating trial was used to investigate how base paper characteristics influence coatweight and coating coverage. The mechanism by which liquid phase enters into the base paper was also investigated. Laboratory base paper sheets (woodfree) with different air permeabilities and surface chemistries were used. The sheets were coated on a pilot coater with the help of a supporting paper web. Coatweight formation was determined by base paper permeability, which dictated coating color penetration into the surface pores. The coating coverage achieved with high base paper permeability was lower than that with dense base paper, even if the coatweight was higher. The surface chemistry of the fibers (hydrophilic/hydrophobic) had no effect on the coatweight formed or on the positioning of coating color in the base sheet. The physical structure of the sheet determined coverage and was the only factor affecting penetration of the coating color's liquid phase into the paper. The main mechanism by which the latter takes place is pressure-induced penetration into pores between fibers. Coating color liquid phase entered the walls of fibers located in the surface layers, irrespective of fiber surface chemistry, though this took place mainly after the nip.