On the mechanisms of AKD retention and sizing

Retention and sizing mechanisms of AKD were studied using some new analytical techniques. Addition of cationic polymers to pulp suspensions was effective in promoting AKD sizing, because higher AKD retention was achieved thereby. The effective AKD retention by cationic polymers is due to formation of ionic bonds between anionic sites of AKD emulsion surfaces and anionic pulp fibers or fines through cationic polymer bridges. The dissociated carboxyl groups in pulp fibers and fines are the actual retention sites of AKD emulsion particles at the:usual addition levels of AKD. Solid-state C-13-NMR analysis of cellulase-treated residues of C-13- labelled AKD-sized handsheets showed that the size components had the structures of either the original AKD or ketones, hydrolyzed products of AKD, in the handsheets without forming covalent bonds with cellulose even after curing treatments. Therefore, the ketone molecules, i.e. hydrolyzed products of AKD, consequently contribute to sizing performance in AKD sizing. Furthermore, most size components were extracted from the AKD-sized handsheets by stirring the defibrated fibers with 1% Tween 80 at 60 degrees C for 4 h. SEM observations of surfaces of the AKD-sized handsheets indicated that the structure of AKD reactive with water is necessary for achieving evenly distributed states of the size emulsion particles on pulp fiber surfaces.