LIGNIN REMOVAL DURING KRAFT PULPING - AN INVESTIGATION BY THIOACIDOLYSIS

A series of Pinus radiata kraft pulps with yields between 96% and 47% were prepared and examined by thioacidolysis. The levels of uncondense d beta-ether units in the residual pulp lignins dropped almost linearl y with the decreasing lignin content of the pulp. The drop in the leve ls of uncondensed beta-ether units in the spent liquor lignins was muc h less rapid than in the fibre lignin. Enol ether units were formed in the dissolved lignin during the bulk phase of cooking, and also to a small extent in the residual fibre lignin. By examining the molecular weight distribution of the pulp lignin thioacidolysis products using g el permeation chromatography, it was found that the proportion of unre active linkages increased in the residual fibre lignin as kraft pulpin g proceeded. This was due both to breakage of reactive interunit linka ges and to condensation.