Brightness reversion of mechanical pulps part XIII: Photoinduced degradation of lignin on cellulose matrix

An acidic dioxaue extraction procedure was employed to isolate lignin from softwood bleached chemithermomechanical pulp (BCTMP). The isolated lignin w as characterized spectroscopically and applied on cellulose test sheets. Th e treated cellulose test sheets were shown to exhibit photoyellowing proper ties comparable to those of BCTMP. The photolyzed lignin was re-isolated an d characterized using spectroscopic techniques. UV-VIS and FT-IR studies im plied elimination of the guaiacyl structure of lignin and the formation of carboxyl and/or unconjugated carbonyl groups during the photoyellowing proc ess. The steady-state emission intensity of the excited states of guaiacyl structures decreased with increasing photolysis time, suggesting the format ion of quinones during photolysis. NMR spectroscopic analysis of the photol yzed lignin indicated that the formyl, methoxyl, and uncondensed phenolics decreased in concentration as the irradiation proceeds. In contrast, conden sed lignin and carboxylic acids increased as photolysis time was increased. These results are explained within the mechanism of lignin photodegradatio n.