Abstract
The interaction of benzoate-type ultraviolet absorbers (UVA) with hindered amine light stabilizers (HALS) was investigated. Benzoate-type UVA showed antagonism or synergism with HALS, depending on the chemical structures of the UVA. p-Aminobenzoates exhibited antagonism with HALS to accelerate the photo-oxidation, despite absorbing UV rays with considerably high absorption coefficients. In contrast, p-hydroxybenzoates showed synergistic photo-antioxidant activity with HALS, despite no or little absorption of ultraviolet rays. The synergism has been ascribed to conversion into benzophenone-type UVA by a photo-Fries rearrangement. This mechanism can explain the photo-antioxidant ability of substituted phenyl p-hydroxybenzoates, but not that of alkyl p-hydroxybenzoates not undergoing the photo-Fries rearrangement. This study proposes a new and comprehensive synergism in which the photo-antioxidant activity of p-hydroxybenzoate is due to the formation of a UV-absorbing intermediate and a photo-antioxidant compound with a catechol structure from the reaction of the benzoate with HALS nitrosonium.
