Structure and Reactivity of Spruce Mechanical Pulp Lignins Part III. Bleaching and Photoyellowing of Isolated Lignin Fractions

Abstract

Organosolv lignin fractions were used as substrate for further investigations on the effect of bleaching and photoyellowing on lignin. These lignins were extracted from spruce TMP, bleached TMP (BTMP) and bleached and yellowed TMP (YBTMP). Furthermore, lignin fractions from TMP were also directly submitted to bleaching conditions and lignin fractions from BTMP to irradiation in the solid state. These lignin samples were all analyzed by thioacidolysis and FT-IR spectroscopy. The bleaching did not substantially change the lignin polymer but considerably destroyed some chromophoric structures such as coniferaldehyde end-groups. On the other hand, the photoyellowing resulted in marked degradations of lignin, shown by a decrease in the amount of β-O-4 and β-1 interunit bonds and a substantial increase in vanillin and vanillic acid end-groups and in catechol units.