The behavior of chromophoric structures in softwood mechanical pulp on bleaching with alkaline hydrogen peroxide

摘要

A commercial TMP as well as the corresponding isolated milled wood lignin were bleached/oxidized with alkaline hydrogen peroxide in order to explore the reasons for the limited brightness increase and the large consumption of hydrogen peroxide in the bleaching of mechanical pulp. Experiments with pulp showed that the elimination of phenolic hydroxyl groups in the lignin gave only a marginal increase in the bleaching response. It was, however, also observed that the bleaching reactions in the pulp could be accompanied by a color-forming reaction. This was further explored by peroxide oxidation of isolated lignin and subjecting the resulting product to different types of NMR analysis. It was found that the oxidative breakdown of coniferaldehyde and vanillin structures in lignin can give rise to a comprehensive consumption of hydrogen peroxide with the formation of new colored structures. This finding may well explain the brightness ceiling in mechanical pulp bleaching.