Interactions between base paper and coating color in metered size press coating

The purpose of the thesis was to gain a greater understanding of the interactions between the base paper and coating color in the metered size press (MSP) nip. The effect of base paper in precoating with an MSP was investigated using mill woodfree base papers, laboratory sheets, pilot base papers and non-porous materials. The coating color formulations consisted of commercial raw materials, in most cases latex-CMC colors with a pigment mixture of CaCO3 and kaolin. Coating studies were carried out at 1000 m/min with a pilot-scale MSP coater. The coating layer formation on woodfree base paper in MSP precoating depended mainly on the base paper surface openness. Lowering the base paper openness by raising the filler content, adding mechanical pulp or through more intensive refining of the chemical pulps resulted in lower coatweights, but better coating coverage. Base paper roughness, small-scale basis weight variation (beta formation) and surface chemical properties of the fibers (hydrophilic / hydrophobic) had no effect on precoating layer formation or coverage. Coating color liquid phase seemed to penetrate mainly into the voids between the fibers, because of the short contact time in the film transfer nip. SEM-BSE and LIPS coverage measuring methods were found to give similar results for the precoated surfaces. The LIPS coverage results were lower than the SEM-BSE results at low coatweights. This indicated that the SEM-BSE method found thinner coating layers to be covered. Low base paper surface openness and high filler content on the surface of the sheet reduced coating color penetration into the base paper structure and the coating color stayed on the surface better. Dense base papers with high filler contents gave better small-scale uniformity than porous base papers. The coating coverage was sensitive to base paper shadow marking, which was clearly seen as large-scale coverage non-uniformity. With base papers with less filler on the surface, coverage could still be increased by increasing coating color viscosity. Using higher levels of kaolin in the coating formula improved coverage, especially at low coatweights. The machine speed and nip load did not affect coating coverage. The color solids content had only a minor impact on coating coverage. Smaller differences in coverage two-sidedness were observed when both paper sides were coated simultaneously compared to the situation where only one side was coated. When the paper was coated using hard rolls, coating coverage depended largely on the pore size of the base paper. Coatweight was formed mainly through penetration, the result being a dense coating layer. With soft rolls coating layer formation occurred more by dewatering, in which case better coverage was obtained, albeit accompanied by a porous coating layer. The porous coating layer with good coverage was the result of filter cake formation in the film transfer nip. The main mechanism by which coating layer formation took place was pressure-induced penetration of bulk coating color or liquid phase into the voids between fibers in the transfer nip. Water sorption into the fibers was slower than capillary penetration, and the movement of coating color liquid phase by diffusion and possibly by capillary absorption took place mainly after the nip.