Effect of kraft cooking conditions on efficiency of oxygen delignification .2. The structural characteristics of residual lignin in oxygen delignified pulp

It has been reported that the structural characteristics of residual lignin in kraft pulp were different depending on kraft cooking conditions, especially, the cooking temperatures. To study how these residual lignins behave differently during oxygen-alkali delignification, residual lignins in the O-2-delignified pulps were isolated and analyzed by C-13 NMR (Carbon 13 nuclear magnetic resonance) and gel-permeation chromatography (GPC). For the residual lignins in kraft pulps cooked at 180 degrees C,the side chain cleavage and benzenoid ring-open reaction were easier than those at 170 degrees C and 160 degrees C during O-2-delignification. These reactions lead to the fact that the residual lignin in pulp cooked at 180 degrees C was easier to remove than those cooked at 170 degrees C and 160 degrees C. The results of analyzing the residual lignins in O-2-delignified pulps showed that the beta-beta' bonds, the beta-O-4 without alpha-CO, and the lignin-carbohydrate complex (LCC) were difficult to oxidize by oxygen-alkali. Therefore, the existence of these alkali-resistant structures were the main reason why the pulp was difficult to be delignified further by oxygen-alkali.