Transformations of glucuronoxylan during acid sulphite pulping of eucalyptus dissolving pulp

Abstract

Xylan is the main contaminant of hardwood sulphite dissolving pulp used for the production of regenerated cellulose. The amount of xylan in pulp should not exceed the minimum allowable values and which is not easy to achieve. To control the pulping process, a basic knowledge of xylan removal is essential. In this study, Eucalyptus globulus glucuronoxylan (GX) conversion during acidic Mg-base acid sulphite pulping of dissolving pulp was assessed for the first time in pilot experiments simulating the industrial process. The kinetics of GX degradation and changes in its structure and molecular weight along all pulping steps were evaluated. According to pentosans analysis, more than 60% of wood xylan was removed already in the heating-up step. A pseudo-first-order reaction for xylan degradation at the final pulping temperature of 140–148 °C was obtained with an activation energy of 133.4 kJ/mol. Removal of residual GX from pulp was hampered by its entrapment in cellulose fibril aggregates. The dynamics of xylan deacetylation and depolymerization during pulping in isolated samples was revealed by 2D NMR and SEC techniques, respectively. Overall, the residual xylan structure in pulp is predetermined by its structural heterogeneity in the cell wall, rather than by structural changes occurring during pulping.