Towards inhibition of yellowing of mechanical pulps. Part IV: Photostability of hydrogenated lignin model compounds

The effect of hydrogenation of aromatic moieties on the photostability of l ignin model compounds was studied by impregnating the compounds into Whatma n filter paper sheets and subjecting these to accelerated yellowing experim ents in a UV chamber: Hydrogenation of the aromatic rings of the model comp ounds 2-methoxy-4-propylphenol and 1,2-dimethoxy-4-propylbenzene gives a co mplete brightness stabilization (yellowing inhibition) within the model com pounds. Results of the yellowing experiments also suggest that methoxy-arom atic (nonphenolic) structures in lignin are susceptible to photooxidation, under short-wavelength UV irradiation, even in the absence of an alpha-carb onyl group, alpha-aryl ethers and beta-O-4 aryl ethers.