Chemical features of hardwood unbleached kraft pulps and their ECF bleachability

Eucalyptus globulus kraft pulps with kappa number 14 (uncorrected for hexenuronic acid (He-A)) were produced with variations in the main pulping parameters and bleached by a conventional DEDED bleaching sequence. Bleachability was affected strongly by the conditions used in pulping. The unbleached pulps were submitted to detailed chemical analysis. The content of residual lignin was shown to be the determining factor in the bleaching response. At constant kappa number, in general, bleachability increased with a decrease in the residual lignin/HexA content ratio in the unbleached pulps. Unsaturated extractives did not play a significant role in the bleaching response of pulp. No clear correlation could be established between bleachability and the content of beta-O-4 structures, phenolic OH groups, the ratio of guaiacyl/syringyl units or the degree of condensation of residual lignins in unbleached pulps. In the series of pulps investigated, the bleaching response was shown to be independent of the brightness of the unbleached pulp.