EFFECTS OF CELLULOSE, HEMICELLULOSE AND LIGNIN ON DISSOLUTION OF SOFT WOOD PULP IN AQUEOUS ALKALI/UREA SOLUTIONS

In this work, a series of softwood pulps with different lignin content were prepared by sodium chlorite delignification of unbleached kraft pulp. They were subjected to the dissolution treatment in alkali/urea solvent and the effects of lignin, hemicellulose, and cellulose content on the dissolution of wood pulp were surveyed. The insoluble fractions were isolated by centrifugation and filtration. The solubility of each wood pulp was obtained by weight method. Both the insoluble fraction and corresponding original wood pulp before dissolution treatment were subjected to chemical analyses. The glucose content of the insoluble fractions was higher, and the mannose and xylose content lower than those of corresponding original pulps. It indicated that at least some part of hemicellulose in wood pulps could easily dissolve in the alkali/urea solvent. The effect of lignin content of original pulps on their dissolution is complicated. With the decrease of the lignin content of the original pulp from 6.9 to 2.8%, the solubility of pulp increased from 14 to 26%. However, further reduction of lignin content from 2.8 to 0.3% led to the decrease in pulp solubility from 26 to 12%. The results indicated dissolution of wood pulp in alkali/urea solvent is much more difficult than that of cotton cellulose and the difficulty cannot be simply ascribed to the presence of lignin or hemicelluloses. A synergy of cellulose, hemicellulose and lignin, especially their interaction in wood pulp, must be one of the key among many factors to control the solubility.