CHANGES IN THE LIGNIN STRUCTURE DURING A TOTALLY CHLORINE-FREE BLEACHING SEQUENCE

A bleaching sequence involving oxygen bleaching (O), treatment with a chelating agent EDTA (Q), and an alkaline hydrogen peroxide stage (P) was studied in order to understand how the residual fibre lignin in a kraft pulp is attacked and degraded during the bleaching process. Resi dual lignin from the different pulps and lignin dissolved in the Vario us bleaching stages in-the sequence were isolated and analyzed by diff erent means. It was found that bleaching according to this sequence le ads to a partial removal of the residual lignin accompanied by an exte nsive removal of chromophoric groups. It appears that the chemical str ucture of lignin remaining in the fibres after the OQP sequence is mai nly unaffected by the treatment. The oxidation has resulted mainly in an increase in the number of hydrophilic groups but the lignin is stil l phenolic to a certain extent and the aromatic structure is preserved . These results suggest that the residual lignin in the OQP-bleached p ulp is still reactive towards oxidative bleaching agents and further o ptimization of the bleaching conditions should be possible.