Brightness Reversion of Mechanical Pulps Part XIII: Photoinduced Degradation of Lignin on Cellulose Matrix

Abstract

An acidic dioxane extraction procedure was employed to isolate lignin from softwood bleached chemithermomechanical pulp (BCTMP). The isolated lignin was characterized spectroscopically and applied on cellulose test sheets. The treated cellulose test sheets were shown to exhibit photoyellowing properties comparable to those of BCTMP. The photolyzed lignin was re-isolated and characterized using spectroscopic techniques. UV-VIS and FT-IR studies implied elimination of the guaiacyl structure of lignin and the formation of carboxyl and/or unconjugated carbonyl groups during the photoyellowing process. The steady-state emission intensity of the excited states of guaiacyl structures decreased with increasing photolysis time, suggesting the formation of quinones during photolysis. NMR spectroscopic analysis of the photolyzed lignin indicated that the formyl, methoxyl, and uncondensed phenolics decreased in concentration as the irradiation proceeds. In contrast, condensed lignin and carboxylic acids increased as photolysis time was increased. These results are explained within the mechanism of lignin photodegradation.