Investigation of the photo-oxidative chemistry of acetylated softwood lignin

Lignin was isolated from a spruce bleached chemithermomechanical pulp (BCTMP) with a mild acid hydrolysis and acetylated. Nonacetylated and acetylated lignins were impregnated onto cellulosic testsheets and photolyzed by two different light sources. Optical reflective studies indicated that the irradiated lignin underwent a two-phase photodiscoloration process with the acetylated lignin exhibiting reduced color formation. To examine the chemical processes initiated by irradiation, the photolyzed nonacetylated and acetylated lignins were extracted from the cellulosic testsheets and analyzed by using nuclear magnetic resonance (NMR). The results indicated that the photobehavior of the acetylated lignin exhibited substantially decreased reactivity during the light irradiation. The extent of lignin acetylation prior to photolysis was shown to be a very important parameter in controlling the overall photoreactivity of the acetylated materials. Aliphatic hydroxyl groups in lignin were observed to extensively degrade during photolysis, whereas the amounts of C5 condensed phenolics, guaiacyl phenolics, acids, and p-hydroxyphenyl increased.