Characterization of autohydrolysis aspen (P. tremuloides) lignins. Part 3. Infrared and ultraviolet studies of extracted autohydrolysis lignin

Abstract

IR and UV studies of the lignins extracted from extractive-free aspen wood meal after autohydrolysis at 195.degree. C for periods varying from 5 min to 2 h indicated that these lignins were functionally modified and different from aspen milled wood lignin. The extracted lignins changed from a guaiacyl-syringyl type lignin to a syringyl-deficient type lignin with increasing autohydrolysis time. Extracted lignins also contained unconjugated .beta.-ketone groups and conjugated carboxylic acid groups. The unconjugated .beta.-ketone groups probably resulted from the depolymerization reactions of the lignin macromolecule under acidic conditions which formed monomeric, dimeric and oligomeric/polymeric lignin fragments with Hibbert''s ketone side chains. The conjugated carboxylic acid group present was attributed to p-hydroxybenzoic acid. p-Hydroxybenzoic acid was postulated as contributing to the extractability of aspen lignin by acting as a blocking agent in the repolymerization of lignin fragments to form insoluble lignin.