Fundamentals of photobleaching lignin. Part I: Photobehaviours of acetylated softwood BCTMP lignin

Lignin isolated from spruce bleached chemithermomechanical pulp (BCTMP) by mild acid hydrolysis was acetylated at 110°C in dioxane/acetic anhydride. The photobehaviour of nonacetylated and acetylated lignins impregnated into filter paper handsheets were examined under irradiation with office and black light sources. Nuclear magnetic resonance studies suggest that aliphatic primary and phenolic hydroxyl groups in lignin structure are preferentially acetylated and acetylation leads to a reduction of quinone content in lignin. Handsheets treated with acetylated lignins yielded a higher brightness and lower post-colour number than those treated with nonacetylated lignin when irradiated. Acetylation efficiently inhibits the photoyellowing of spruce BCTMP lignin and the photostabilization effect is proportional to the degree of lignin acetylation. These effects are most likely attributable to the removal of quinoidal structures in the acetylated lignin and the acetylation of phenoxy and aliphatic hydroxyl groups. It has been established that acetylated lignin is the stabilizing component of acetylated BCTMP.