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On the Behaviour of Spruce Thermomechanical Pulp Lignin during Hydrogen Peroxide Bleaching

Lignins were isolated from spruce TMP and hydrogen peroxide bleached TMP (BTMP) by means of enzymatic hydrolysis with cellulases in yields of 67 and 78%, respectively. In addition, the TMP lignin, TMP EL, was subjected to a treatment with bleaching agent to obtain a bleached lignin, TMP ELB. The three lignin samples were all characterized by thioacidolysis and C-NMR spectroscopy. Comparison of these lignin samples provided evidence about the behaviour of lignin towards alkaline hydrogen peroxide bleaching. It has been demonstrated that the bleaching does not affect the overall structure of lignin. The only one major change of the lignin moiety is elimination of coniferaldehyde end structures, which would be an important contribution to the brightening of pulp.