NUCLEAR-MAGNETIC-RESONANCE STUDIES - 4 - ANALYSIS OF RESIDUAL LIGNIN AFTER KRAFT PULPING

Residual lignin from a series of softwood kraft pulps was isolated and structurally characterized by nuclear magnetic resonance, Results of these analyses showed that residual lignin structure differs depending upon the lignin content of the pulp and the pulping technology employ ed. The lower lignin content pulps had reduced contents of aliphatic h ydroxyl groups and beta-O-4 structures and higher contents of phenolic hydroxyl groups, carboxylic acid groups, and condensed structures com pared to higher kappa number pulps. Analysis of residual lignins isola ted from conventional kraft pulps (CR) and extended modified kraft pul ps (EK) indicated that the CK pulps had lower contents of beta-O-4 str uctures and higher contents of condensed structures. These differences potentially explain the improved delignification selectivity of EK kr aft pulping because residual lignin with a higher content of beta-O-4 structures and a lower content of condensed structures would be more r eactive under pulping conditions.