Thermal and photooxidative stabilisation behaviour of 4-naphthyl esters of 2-hydroxybenzophenone and 3,5-di-tert-butylphenol in polyolefin films

Two novel naphthyl ester derivatives have been synthesised and characterised based on a 2-hydroxybenzophenone and a 3,5-di-tert-butylphenol. Thermal and photochemical ageing studies have been carried out to determine their potential as stabilisers in polypropylene (PP). Whilst both compounds exhibit no oven ageing antioxidant properties alone they are somewhat effective melt stabilisers. Differential thermal analysis and mass spectrometry show both compounds fragment at the carbon-oxygen bond (CO) of the ester group. Whereas both compounds exhibit good oven ageing stability for PP with a thermal hindered phenolic antioxidant, Irganox 1010, antagonism is observed when combined with a hindered piperidine light stabilizer, Chimassorb 944. In the latter case the antagonism is significant in the presence of the benzophenone derivative. The benzophenone derivative is not an effective light stabiliser alone in PP despite its high absorptivity in the near-UV region 300–350 nm and its ability to act as an excited state quencher. However, it does exhibit strong synergism with both the hindered phenolic antioxidant and the hindered piperidine stabilisers. The naphthyl benzoate ester on the other hand is an extremely effective light stabiliser alone in PP, has weak absorptivity in the near-UV region and is unable to act as an excited state quencher. Strong synergism is observed with a hindered piperidine light stabiliser, Chimassorb 944 but not with a hindered phenolic antioxidant. Spectroscopic evidence indicates that the naphthylbenzoate ester undergoes a rapid initial photo-Fries rearrangement to produce a more strongly absorbing hydroxynaphthylphenyl methanone product.