STUDY ON THE ENHANCEMENT OF WET STRENGTH OF UNBLEACHED KRAFT PULP BY ENZYMATIC CO-POLYMERIZATION WITH FERULIC ACID

摘要

The mechanism of enhancement of wet strength of paper from unbleached kraft pulp with laccase treatment in existence of ferulic acid was investigated. The combination of chemical analysis, FT-IR and CP/MAS 13C-NMR analysis were used to analyze the mechanism of graft co-polymerization and crosslinking of unbleached kraft pulp through ferulic acid catalyzed by laccase. It is found that kappa number of unbleached kraft pulp increased when the increase of dosage of ferulic acid. FT-IR analysis of cellulolytic enzyme lignin (CEL) of treated and untreated unbleached kraft pulp revealed an increase in carboxyl group after enzymatic co-polymerization. The results of CP/MAS analysis show that the CEL of treated unbleached kraft pulp has more ferulic acid substructures as compared with that from untreated unbleached kraft pulp. These results strongly suggest that laccase-catalyzed grafting of ferulic acid into lignin moieties on the surface of fiber. Furthermore, the wet strength of paper prepared from unbleached kraft pulp treated with laccase and ferulic acid showed significant increase from 2.15 N·m·g-1 to 4.42 Nm-g-1. The wet strength is enhanced by 105.6% as compared to untreated papers. However, there is little change in the dry strength of paper. From these results, it is found that the reason of improvement of wet strength of paper is that ferulic acid is copolymerized to lignin on the fiber surface so that covalent bonds between fibers is formed during sheets formation.