| Abstract: | CONCLUSION. Our study of the photoreactivity of hydroquinone and quinone lignin models in solid carbohydrate matrix has revealed the key role probably played by hydroquinones and the solid carbohydrate framework in the discolouration process of high-yield pulps. For bleached pulps, the rapid yellowing step is probably correlated with the amount of methoxyhydroquinone type structures that are produced during the peroxide bleaching, which are not eliminated when sodium borohydride reduction and catalytic hydrogenation are performed on lignin. Interconversion between both forms of these types of redox pairs in oxygen-containing media is very easy and the redox state of the medium determines the balance between the oxidized and the reduced forms. Irradiation changes at least partly this balance, which is observed by photoreduction of quinone generating hydroquinone. Other compounds, both organic and inorganic such as degraded carbohydrates and metal ions can react similarly. The involvement in the discolouration process of carbohydrates, probably hemicelluloses which are non-crystalline and strongly linked to lignin, is consistent with the huge amounts of alkylating agents needed to stabilize bleached high-yield pulps [16, 17]. Further studies are being pursued in our groups for a better understanding of the brigtness reversion mechanism of high-yield pulps. |
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