Mechanisms of retention of AKD are studied in terms of effects of cationic polymer additions, and roles of fines and carboxyl groups of pulp fibers. Handsheets were prepared from normal pulp and fines-free pulp under various conditions, and AKD contents in the handsheets were determined by pyrolysis-gas chromatography (PY-GC). The addition of cationic polymer (PAE) to pulp suspension was clearly effective in promoting AKD sizing for both cured and noncured handsheets, resulting from higher
retention of AKD in the sheet by cationic PAE molecules. Potential measurement indicated that the originally cationic AKD emulsions come to have amphoteric surface charges in pulp suspension . The effective AKD retention by PAE may, therefore, be due to ionic bond formation between anionic sites of AKD emulsion surfaces and anionic pulp fibers or fines through cationic PAE molecules. The experiments using fines-free pulp showed that most of the added AKD was adsorbed on fines of beaten pulp . Blocking of carboxyl groups in pulp with nonionic methylamide groups resulted in nearly no sizing degrees and quite low AKD contents. Therefore, it is clear that dissociated carboxyl groups in pulp fibers and fines are the actual retention sites of AKD emulsion particles at the usual addition levels of AM Among chitosan salts and PAE used at 0.1-0.4 addition levels, chitosan AcOH salt gave the highest effect on AKD retention as well as sizing degrees. Good correlation between AKD
contents and sizing degrees was obtained ; sizing behavior of AKD-sized handsheets was explainable in terms of their AKD contents determined by PY-GC, and thus retention of AKD is the significant first step for AKD sizing. Solid-state ,3C-NMR analysis of cellulase-treated residues of 13C-labelled
AKD-sized handsheets showed that size components were present in papersheet as structures of either the original AKD or ketones, hydrolyzed AKD, without forming ß-ketoesters . SEM observations of AKD-sized handsheets indicated that the effect of curing on AKD sizing is explained in terms of melting of size molecules and their spreading over pulp fiber surfaces .