Differential behavior between acacia and Japanese larch woods in the formation and decomposition of hexenuronic acid during alkaline cooking

Abstract

The effects of anthraquinone (AQ) and polysulfide (PS) on the hexenuronic acid (HexA) content of pulp during kraft cooking were studied using Acacia mearnsii (acacia) and Larix leptolepis (Japanese larch) sapwood. In contrast to the results of cooking Japanese larch at an H-factor of 1200, the HexA contents of acacia pulp with a kappa number of 20 at an H-factor of 291 did not differ greatly between the kraft, kraft-AQ, and PS-AQ cooking methods, although the hydroxide ion concentration in the acacia cooking liquor decreased on the addition of AQ or sulfur. To explain this difference, we studied the behavior of the formation and degradation of HexA during alkaline cooking of glucuronoxylan from cotton linter, which was cooked with 1.0 and 2.0 mol/l NaOH. The relationship between HexA content and H-factor during alkaline cooking of glucuronoxylan was clarified. The amount of HexA and its rate of decomposition were higher in the 2.0 mol/l solution than in the 1.0 mol/l solution. At a low H-factor similar to that for hardwood cooking, HexA content increased to a maximum level and then started to decrease at high hydroxide ion concentrations such as 2.0 mol/l, whereas it slowly decreased at low hydroxide concentrations such as 1.0 mol/l. At an H-factor of around 450, the HexA formation/degradation curve for 1.0 mol/l of hydroxide crossed the decomposition curve for 2.0 mol/l of hydroxide. Therefore, it was shown that at a low H-factor, a decrease in hydroxide ion concentration during acacia wood cooking had little effect on the HexA content of pulp.