Fiber modification of kraft pulp with laccase in presence of methyl syringate

An approach aiming at improving paper properties is to use laccase to copolymerize low-molecular weight phenols with the pulp before papermaking. The addition of methyl syringate (MS) gave a twice wet tensile index of unbleached kraft pulp with laccase treatment but had little effect on the dry tensile index and substantially decreased the brightness of pulp. The radical concentration in laccase-treated fibers increased up to 2.5 times of that in control sample after 60 min. The radical concentration in laccase/MS-treated fibers increased up to 20 times of that in control sample within 2 min, and a highest radical concentration (increased by near 65 times) was obtained after 40 min. A strong agglutination of large-area fibers in handsheet was observed after laccase/MS treatment. The surface lignin coverage of the laccase-treated fibers increased from 54.96% (control) to 59.36%, while that of the laccase/MS-treated fibers increased only up to 55.22%. It is suggested that the graft of MS on fiber and degradation of surface lignin occurred simultaneously. The addition of MS can enhance the activation of fibers and extend the enzymatic oxidation of lignin within the cell wall. An increased bonding area of fibers resulting from interaction of laccase, MS and fibers via radical-coupling reaction maybe account for the significantly improved wet strength of pulp.