Formation and degradation of hexenuronic acid in kraft cooking of some wood species

摘要

The formation and degradation of hexenuronic acid in kraft cooking was studied with Eucalyptus camaldulensis, Acacia mangium and Scandinavian softwood mixture. Hexenuronic acid is formed rapidly during the initial phase of cooking. It is relatively stable in kraft cooking and its content drops markedly only in late residual delignification phase. The content of hexenuronic acid in pulp depends on the xylan content of raw material, uronic acid content in xylan and finally cooking conditions. About 50% of 4-O-Me-glucuronic acid units in pulp seem to convert to hexenuronic acid; there are other degradation routes. Very clearly hexenuronic acid stabilizes xylan towards alkaline peeling reaction. The wood species studied had different xylan contents as well as different uronic acid contents resulting in clearly different hexenuronic acid contents in unbleached pulps. Normal ECF bleaching removes 95% of hexenuronic acid.