Abstracts Chemistry

Protection mechanisms of novel photoyellowing inhibitors

by Patrick. Halevy

The long wave aerobic photochemistry of two frequently used lignin model compounds ie. I (3,4-dimethoxy-alpha-(4-methyl-2-methoxyphenoxy)-beta-hydroxypropriophenone) and II (4-hydroxy-3-methoxyphenol)-2-(2-methoxyphenoxy)propan-1,3-diol) were studied in the presence of three UV absorbers 2,4-dihydroxybenzophenone (DHB), an aminated DHB (MDHB), a benzotriazole derivative (Tinuvin P) and an antioxidant; the additives being incorporated into thin films of I and II were applied on filter paper. Several products were identified and quantified using a combination of GC-MS and GC-FID techniques. The photodegradation mechanisms of I and II were found to be consistent with the phenacyl and ketyl pathways from which products were generated via oxidation, heterocoupling, H-abstraction and homocoupling reactions. In the presence of the inhibitors, some of these reaction pathways were minimized and others eliminated. In decreasing order, the Tinuvin P/Tempo pair were the most effective in protecting the models against photodegradation followed by MDHB and DHB. A synergistic photostabilization effect was also observed between Tinuvin P and the antioxidant, but was not retained during irradiation, probably due to the low regeneration rate of Tempo on the matrix. The photostabilization effectiveness of MDHB can be related to the cooperative action of its UV absorbing, hydrogen donating and oxygen scavenging functionalities. However, the separation of product 12 from MDHB was not possible by GC and the photo-chemical fate of MDHB remains undetermined. No synergism was found between MDHB and Tempo. DHB displayed good UV absorbing characteristics but no other photostabilization properties were found to cooperate with it.