| Abstract: | SUMMARY. In almost fifty years research has shown that light-induced yellowing of mechanical and ultra-high yield pulps occurs by photooxidation of lignin in the fibre wall. The phenoxyl free-radical believed to be the precursor to light-induced yellowing occurs through at least four reaction pathways. The first is absorption of uv light by conjugated phenolic groups, the second is abstraction of hydrogen from the neighbouring phenolic hydroxyl group by the aromatic carbonyl triplet excited state and the third is cleavage of non-phenolic phenacyl-a-O-arylethers. Light-induced yellowing of mechanical and ultra-high yield pulps has been delayed for about 2 hours by the complete reduction of carbonyl groups by sodium borohydride followed by alkylation of phenolic hydroxyl groups. After 2 hours, a fourth reaction pathway occurs through abstraction of benzylic hydrogen by oxygen based free-radicals such as peroxyl and alkoxyl to produce ketyl free-radicals that undergo facile -O-4-aryl ether cleavage to phenoxy radicals and the corresponding guaiacyl-1-propanone. There is general agreement that the phenoxyl free radical reacts with oxygen and/or with functional groups in the fibre wall to form quinones, the coloured chromophores. Although there has been much progress both in elucidating the reaction pathways leading to yellowing of bleached mechanical and ultra-high yield pulps, and in identifying the coloured chromophores, more needs to be known about the key oxidation step of phenoxyl free-radicals to quinones. |
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