EFFECT OF BASESTOCK SURFACE-STRUCTURE AND CHEMISTRY ON COATING HOLDOUT AND COATED PAPER PROPERTIES

Water absorption by the basestock during coating affects coating picku p and coating mass distribution and, thus, the properties of the coate d paper. This work presents results of experiments that were designed to separate the effects of two important factors that determine sheet absorbency: hydrophobic sizing and porosity. Bleached kraft handsheets with a broad range of pore size, void fraction, and roughness were si zed to different levels with a solution of alkyl ketene dimer (AKD) in non-swelling hexane. A lightweight coated (LWC) basestock was treated similarly. Coating was performed at 762 m/min on a cylindrical labora tory coater. The effect of hydrophobic sizing on coating pickup depend ed greatly on sheet porosity and roughness. On dense-smooth sheets, si zing resulted in lower coating pickup. On rough sheets, including LWC basestock, the effect of sizing on pickup was concealed by the overwhe lming effect of roughness on the metering action. Coating uniformity a nd holdout were very good on the dense-smooth sheets (although sizing gave a slight reduction) and poor on the porous rough sheets and LWC b asestock (sizing had little effect). The results suggest that coating holdout and uniformity are determined mainly by sheet surface structur e (porosity and roughness) and not by sizing.