Coating layer formation and liquid-phase penetration in metered size press coating

The aim of the study was to investigate how base paper characteristics influence coating layer formation and coverage in coating paper with a metered size press. Laboratory woodfree base sheets were produced on a dynamic sheet former. The sheet properties were changed by varying softwood pulp refining and alkyl ketene dimer sizing. The base sheets were taped on a supporting mil paper and coated on a pilot metered size press at 1000 m/min. Coating layer formation was controlled by base paper permeability, which dictated coating colour penetration into the surface voids. The coating coverage was measured with a scanning electron microscope in the backscattering mode. Coating coverage was higher on the denser paper, even though the coat weight was lower. The internal/hydrophobic sizing of the fibres had no effect on the coat weight and coating coverage. The main transport mechanism was pressure-induced penetration of the coating colour into the surface voids.