Photostabilizing milled wood lignin with benzotriazoles and hindered nitroxide

The inhibitory processes operating when a 2(-2'-hydroxyphenyl) benzotriazol e-based ultraviolet absorber (UVA) and a hindered nitroxide free radical ar e applied to high-yield pulps were studied using milled wood lignin and fil ter paper as a model. Using quantitative P-31 NMR it was determined that th e UVA is almost completely photostable during irradiation, suggesting that its protective mechanism is based primarily on ultraviolet absorption. Furt hermore, the nitroxide was found to protect the UVA from photodegradation. Analysis of irradiated lignin samples involved derivatization followed by r eductive cleavage of beta -arylether groups. The phenolic-OH groups thus re leased were quantified using P-31 NMR. The benzotriazole/nitroxide stabiliz ing system was found to cooperatively inhibit the cleavage of beta -aryleth er groups. Furthermore, this system was found to have a synergistic inhibit ory effect on the formation of catechol structures. These results suggest t hat the synergism observed between benzotriazole-based UV screens and nitro xyl radicals on the inhibition of yellowing could be a result of their abil ity to slow the formation of catechol structures and beta -O-4 cleavage in addition to the possibility of a UVA-regeneration mechanism.