Grafted maleic acid copolymer giving thermosetting kraft pulp

Grafting 100–500 kDa hydrolyzed poly(ethylene-alt-maleic anhydride), PEMAc, onto northern softwood kraft pulp fibers has two impacts: (1) the density of surface charge groups increases reflecting the high carboxylic acid content of PEMAc; and, (2) the succinic acid moieties in PEMAc can be converted back to reactive succinic anhydride groups by high-temperature curing. PEMAc grafting did not influence pulp refining characteristics. Papers made from grafted pulp but with no other papermaking chemical additives have properties that depend on the heating/curing conditions. Papers dried at 23 °C and 50% relative humidity (CTH conditions) have the same dry strength and very low wet strength as papers made by non-grafted pulp. CTH drying conditions are too mild to regenerate succinic anhydride moieties on the PEMAc. Curing white papers containing 1.3 kg/tonne PEMAc, at 200 °C for 10 min gave five-fold wet strength improvements with little increase in dry paper strength. We propose that curing generates anhydride groups that increase the density of ester linkages to fiber surfaces. Curing brown papers made with PEMAc-grafted unbleached northern softwood grafted pulps gave greater wet-strength increases compared to papers from bleached pulps. The curing-activated reactivity of PEMAc-grafted pulps was demonstrated by impregnating the paper with fluorescently labeled dextran, followed by curing. The dextran was fixed to the paper presumably by ester linkages with PEMAc after curing. Based on anhydride reactivity, PEMAc grafting gives pulps thermosetting properties.Graphical abstract