FIBER MODIFICATION OF UNBLEACHED KRAFT PULP WITH LACCASE AND HISTIDINE

The unbleached softwood kraft pulps were treated by laccase and histidine with the aim of improving paper strength. The paper and pulp properties were determined. The treated pulp fibers were analyzed by fiber quality analyzer (FQA), x-ray photoelectron spectroscopy (XPS) and atomic force microscope (AFM). The results showed that the dry tensile strength of paper was improved and the wet tensile index increased by around 20% by laccase-histidine treatment compared with control pulp. The bulk carboxyl group content and water value retention of the treated fibers significantly increased. The opacity of handsheet increased while the brightness decreased. The FQA results showed that the kink index and curl index of fibers decreased while the fiber coarseness increased compared with the control samples. The reduced granular lignin on the fiber surface could be observed by AFM. The XPS results showed that the surface lignin coverage of fibers decreased and the surface carboxyl content increased. It indicated that the improvement mechanism of dry- and wet- strength of paper by laccase-histidine treatment can be related to the increases in carboxyl group caused by graft of histidine, contact joint between fibers, covalent bonding area and the strength enhancement of fiber itself, as well as removal of surface lignin. The ability to use laccase selectively to graft amino acids on pulp fibers provides a new and unique fiber modification technology which will have many future opportunities.