EFFECT OF KRAFT COOKING CONDITIONS ON THE EFFICIENCY OF OXYGEN DELIGNIFICATION .1. THE STRUCTURAL CHARACTERISTICS OF KRAFT AND RESIDUAL LIGNINS

It has been reported that the cooking temperatures greatly affected the degree of the alkali-oxygen delignification for krait pulp; the delignification of the pulp cooked at higher temperatures was much more effective than that cooked at lower temperatures. To find the reasons for this phenomenon, the lignins in the pulps and waste liquors, which were cooked under different conditions were isolated, and their characteristics mere analyzed and compared by Gel-permeation Chromatography (GPC) and Carbon 13 Nuclear Magnetic Resonance (C-13 NMR). It was found that the lignin was degraded drastically at the bonds of beta-O-4 and alpha-OR at cooking temperatures of up to 180 degrees C, Therefore, the residual lignin cooked at 180 degrees C was degraded into a smaller size than at 170 degrees C or 160 degrees C, and it was easier to be degraded and dissolved in the alkali oxygen stage, Furthermore, splitting of beta-O-4 and alpha-OR produced active groups for oxygen delignification, such as hydroxyphenyl, C-OH, and C=O at the C-alpha position, which could improve the reaction activities of the residual lignin. These results showed that the cooking temperature at 180 degrees C improved the efficiency of the oxygen delignification. Considering the losses of yield and strength, however, we are trying in further studies to develop a new cooking method by controlling the heating history.