Alkaline extraction and characterization of residual hemicellulose in dissolving pulp

Abstract

Residual hemicellulose in dissolving pulp is known as a reason for deterioration of final product quality and processability problems during cellulose derivative production. However, the issue caused by the residual hemicellulose is not fully understood and is primarily based on industrial experience. To better understand the effect of hemicellulose on cellulose derivative process, hemicellulose in prehydrolysis kraft hardwood dissolving pulp was extracted and its glycosidic linkage structure was thoroughly analysed. Two-stage alkaline extraction, consisting of 24 wt% KOH solution and 18 wt% NaOH with 4 wt% H3BO3 solution, was found effective for the sequential extraction of high purity xylan and glucomannan from the dissolving pulp. Considering the purity and yield of extracted xylan, room temperature for Stage-1 KOH extraction condition was preferred, while 1 °C was preferred for Stage-2 NaOH extraction. Mechanical refining treatment at a mild condition improved the extraction yield of hemicellulose, although the molecular weight of the cellulose was reduced with severe refining condition. Glycosidic linkage analysis showed that native 4-O-methylglucuronic acid side groups were removed during pulping process, resulting in a linearized xylan structure, while a high amount of glucuronic acid was detected in commercial beechwood xylan. The extracted hemicellulose from the second stage showed relatively high number of branched backbone unit e.g., 1,4,6-linked mannose and 1,4,6-linked glucose. The protocol developed in this study should be useful for hemicellulose-related applications.