Effect of basestock surface structure and chemistry on coating holdout and coated paper properties

Water absorption by the basestock during coating affects coating pickup and coating mass distribution and, thus, the properties of the coated 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 sized 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 laboratory coater. The effect of hydrophobic sizing on coating pickup depended greatly on sheet porosity and roughness. On dense-smooth sheets, sizing resulted in lower coating pickup. On rough sheets, including LWC basestock, the effect of sizing on pickup was concealed by the overwhelming effect of roughness on the metering action. Coating uniformity and holdout were very good on the dense-smooth sheets (although sizing gave a slight reduction) and poor on the porous rough sheets and LWC basestock (sizing had little effect). The results suggest that coating holdout and uniformity are determined mainly by sheet surface structure (porosity and roughness) and not by sizing.