HexA

Acid hydrolysis of commercial US HW kraft pulps demonstrated that between 20-55% of the kappa number of these pulps could be attributed to hexenuronic acids. For SW kraft pulps, the contribution of hexenuronic acids to the pulp kappa number was not as substantial. Examination of the xylan and hexenuronic acid content of the kraft pulps indicated that there was a correlation between these two components. Finally, analysis of HW kraft pulps for hexenuronic acid groups indicated that formation of this unsaturated hemicellulose could be influenced by the extent of delignification. EXECUTIVE STATEMENT: This study examines the role of hexenuronic acids in US HW and SW kraft pulps. The presence of hexenuronic acids was found to be a dominant component of the pulp kappa number for HW kraft pulps, whereas, for SW kraft pulps, it was found to be a minor component. Acid hydrolysis studies on HW kraft pulps suggest that it may be possible to reduce the apparent pulp kappa number by 35%-50% with a mild acid treatment prior to bleaching. The level of hexenuronic acids in a HW kraft pulp can be influenced by the extent of delignification. INTRODUCTION: Although fundamental bleaching research has largely focused on lignin chemistry for the past decade, recent studies have begun to readdress this issue and direct more attention toward carbohydrate chemistry. These studies have shown that hemicelluloses influence a variety of important properties throughout pulping, bleaching, and papermaking. The formation of hexenuronic acids during pulping (see Figure 1) is currently a very active research area that has been shown to have practical mill considerations. Early studies by Clayton examined the behavior of 4-O-methyl-α-D-glucuronoxylans from poplar, white birch, and white elm with alkali at 170 o C and found indirect evidence for the formation of hexenuronic acids(1).