Light-Induced Yellowing of High-Yield Pulps. The Effect of Acetylation
Doctoral thesis, 1996
This thesis is a contribution to the understanding of the mechanism of photoyellowing of high-yield pulps. It also gives some general information about the reactivity of different functional groups in lignin towards acetic anhydride. The effect of acetylation on both optical and mechanical properties of unbleached and hydrogen-peroxide-bleached high-yield pulps is also discussed.
Acetylation was found strongly to improve the long-term photostability of both unbleached and bleached mechanical pulps at a low extent of derivatization. This was achieved with retained or even considerably improved strength properties (e.g., in the wet state) provided that the paper product instead of the pulp is acetylated. The improved stability towards light was closely related to the decrease in the phenolic hydroxyl content of the pulps as a result of the acetylation treatment.
The acetylation of model compounds of the .beta.-guaiacyl ether type and model compounds representative of chromophoric and leucochromophoric structures in lignin was studied to obtain a basis for judging how fast and in which order different functional groups in lignin are derivatized. The results showed that phenolic hydroxyl groups and .gamma.-hydroxyl groups (in both .beta.-O-4 and coniferyl alcohol structures) together with ortho-quinonoid units are easily acetylated. The benzylic hydroxyl groups (in .beta.-O-4 structures) are acetylated more slowly and most of these groups are still present at a moderate extent of derivatization. This suggests that reactions involving photooxidation of free phenolic hydroxyl groups and cleavage of 2-aryloxy-1-arylpropanone structures are important for the light-induced discoloration of lignin-containing materials. Cleavage of arylglycerol .beta.-aryl ether structures is a less probable reaction pathway for chromophore formation during irradiation, at least in the initial stage of yellowing.
The results of this work also demonstrate that the light-induced brightness reversion of unbleached and hydrogen-peroxide-bleached high-yield pulps proceeds in a rapid initial phase that is followed by a slower and less detrimental phase. Bleaching with hydrogen peroxide generates structures that make the bleached pulp more sensitive towards light exposure than the unbleached pulp.