Preparation of Dissolving Pulp by Totally Chlorine-free Bleaching：Roles of Hardwood Syringyl and Guaiacyl Lignins

Dissolving pulp was produced from four diff erent hardwoods by prehydrolysis-kraft cooking and totally chlorine-free （TCF） bleaching using oxygen （O），peroxymonosulfuric acid （Psa），and alkali extraction with hydrogen peroxide （Ep） in an O-Psa-Ep-Psa-Ep sequence. The hardwood lignin structures were characterized by the nitrobenzene oxidation method, which provided syringaldehyde （Sa） to vanillin （Va） molar ratios （S/V ratios） of the lignin. Eucalyptus globulus wood had the highest S/V ratio of 5.81 and a combined yield of Sa and Va of 3.04 mmol/g-lignin, while the Acacia hybrid wood exhibited the lowest S/V ratio and the combined yield. The E. globulus wood provided a fi nal pulp with an acceptable level of viscosity 7.0 mPa·s, a weight-average molecular weight （Mw） 3.04 × 105 g/mol, a number-average molecular weight （Mn） 5.56 × 104 g/mol, polydispersity （Mw/Mn） 5.47, a high brightness of 90.1% ISO, and an α-cellulose content of 94.2%. It is attributed to the high S/V ratio and the high combined Sa and Va yield, which is suggestive of a less-condensed lignin structure compared to the other hardwoods tested. No signifi cant diff erences in the cellulose crystallinities of the bleached pulp produced from the four hardwoods were observed.