Improving cellulose viscosity protection and delignification in ozone bleaching of low-consistency pulp with water-soluble chitosan

The effect of water-soluble chitosan (WSC) with molecular weights (Mw) from 1.0x104 to 1.0x106 on cellulose viscosity protection was explored during the treatment of low-consistency pulp with the ozone bleaching process (OBP). Through determination of the change in crystallinity, carboxyl content, and physical strength of ozonebleached refined dissolving pulp, WSC was shown to effectively protect cellulose viscosity by scavenging free radicals and forming a protective film deposited on the fiber surface to prevent the destruction of cellulose by oxidants. During the OBP of sulfate hardwood pulp (SHP), application of a 0.5 % dose of WSC with a Mw of 1.0x105 increased not only the cellulose viscosity but also the selectivity of ozone delignification. The brightness, viscosity, and delignification selectivity of the bleached SHP were enhanced by 3.88 +/- 0.05 %, 13.0 +/- 0.10 %, and 76.6 +/- 0.50 %, respectively. Accordingly, the indexes of tensile strength, burst strength and tear strength of the resulting SHP hand sheet were increased by 38.6 +/- 1.0 %, 74.8 +/- 1.5 %, and 117.3 +/- 2.5 %, respectively. WSC had the best performance among the reported additives of equal weights for protection of cellulose viscosity in improving the pulp qualities after the OBP.