Retention and paper-strength characteristics of anionic polyacrylamides conjugated with carbohydrate-binding modules

The retention behavior of polymers having the specific affinities of glycohydrolases for pulp fibers was investigated with regard to paper-strength enhancement in contaminated papermaking systems. Carbohydratebinding modules (CBMs) of cellulases derived from Trichoderma viride and T. reesei, and of xylanase from Thermomyces lanuginosus, were obtained by site-directed digestion with papain, then introduced into anionic polyacrylamide (A-PAM) via a peptide condensation reaction. Three types of CBM-conjugated A-PAMs (CBM-A-PAMs) displayed different retention behavior, depending on the kind of pulp substrates, i.e. hardwood and softwood fibers. The CBM-A-PAM from T. viride demonstrated good additive retention for hardwood pulp fibers, resulting in high tensile strength of paper sheets, even under contaminated conditions in the presence of Ca²⁺ ions and ligninsulfonate. The CBM-A-PAM from T. reesei showed better performance for softwood than for hardwood sheets. The xylanase CBM-A-PAM was preferentially retained on hardwood fibers in which hemicelluloses might be present. Such an additive retention system, with inherent affinities of enzymes for pulp fibers, is expected to expand the application range of CBM-polymers in practical wet-end processes.