Abstract
Fundamental mechanisms of paper sizing by internal sizes are reviewed on the basis of results obtained in our laboratory. Factors influencing paper sizing are explained in terms of basic theories of contact angles, changes in free energies, wettability, liquid-penetration and others. Recently, pyrolysis-gas chromatography techniques have been applied to determination of size content in paper, and thus sizing behavior can be examined in terms of size retention values. Pulp carboxyl groups and cationic compounds added at the wet-end have significant roles in size retention ; electrostatic interactions among sizes, pulp fibers and fines, anionic trashes, fillers and cationic compounds in pulp suspensions are the driving force of size retention, as well as physical filtration effects. Not only size retention but also distribution of size components and changes in chemical structures of size components affect sizing behavior. SEM obsevation of AKD-sized handsheets dried at 20°C, which had sufficient sizing levels, showed that most pulp fiber surfaces were occupied by hydrophilic cellulose and hemicellulose. Thus, complete coverage of pulp fiber surfaces with size molecules is not necessary for paper sizing, and distribution of hydrophobic size components on hydrophilic pulp fiber surfaces seems to govern the resultant sizing behavior. Solid-state 13C-NMR analysis of AKD-sized handsheets, extractions with either chloroform or 1% Tween 80 and SEM observations of handsheets prepared with AKD and ketone emulsions suggested that most size components were present as hydrolyzed structures without forming covalent bonds with hydroxyl groups of cellulose in paper. The structures of AKD and ASA, which can react with water and thus are somewhat hydrophilic, are necessary for paper sizing in order to avoid flocculation of the size components due to hydrophobic interactions on hydrophilic pulp fiber surfaces in papermaking process. This hypothesis of sizing mechanisms of AKD and ASA is applicable to other non-reactive sizes such as rosin soap, rosin acid emulsion and rosin-ester sizes.
