Relationship Between Residual Lignin Structure and Pulp Bleachability, 1996 International Pulp Bleaching Conference Proceedings

Efficient use of ClO2 during ECF chemical pulp bleaching is an important concern for mills employing such bleaching technology. Efficient use requires a fundamental understanding about the reactions of ClO2 with residual lignin. The objective of this study was to define a relationship between unbleached residual lignin structure and pulp bleachability during a D(EO) sequence. This has been accomplished by extensively characterizing the residual lignin structure in a series of unbleached conventional kraft (CK) and EMC ® kraft (EK) pulps and relating this structure to measures of bleachability. Additionally, functional group changes in residual lignins isolated from these chemical pulps and treated with chlorine dioxide have been related to the measures of bleachability. Results from this study suggest that although free phenolic structures are believed to be the main site of attack by ClO2 their content in the unbleached residual lignin cannot simply be related to the observed bleaching response. Lower kappa number pulps were found to contain a greater content of free phenolic structures yet the bleaching experiments, in general, found that as the kappa number is decreased there is a slight increase in TAC consumption per unit kappa number reduction in the D(EO) sequence. Reactivity studies of ClO2 with residual lignin suggest that residual lignin in higher kappa number pulp is more reactive towards ClO2 , which is consistent with the observed bleaching response. Additional structural differences between these unbleached residual lignins include the content of aryl ether linkages and the amount of condensed aromatic units. How these structural features may relate to the observed bleaching response is discussed.