Aspects of the thermal oxidation of ethylene vinyl acetate copolymer

The thermal oxidation of ethylene-vinyl acetate copolymer [EVA-17 and 28% w /w VA (vinyl acetate) units] has been examined by thermo-gravimetric and hy droperoxide analysis, FTIR (Fourier transform infra-red) fluorescence spect roscopy and yellowness index. Thermal analysis indicates the initial loss o f acetic acid followed by oxidation and breakdown of the main chain. The de gradation rate is greater in an oxygen atmosphere as is the formation of co loured products. FTIR spectroscopic analysis of the oxidised EVA shows evid ence for de-acetylation followed by the concurrent formation of hydroxyl/hy droperoxide species, ketone groups, alpha,beta-unsaturated carbonyl groups, conjugated dienes, lactones and various substituted vinyl types. Hydropero xide evolution follows typical autooxidation kinetics forming ketonic speci es. In severely oxidised EVA evidence is given for the subsequent formation of anhydride groups. The initial fluorescence excitation and emission spec tra of EVA is not unlike that reported for polyolefins confirming the prese nce of low levels of unsaturated carbonyl species. There are however, signi ficant differences in a long wavelength component in the fluorescence emiss ion indicating the presence of other active chromophores. These long wavele ngth emitting components grow in intensity and shift to longer wavelengths with ageing time. However, unlike studies on PVC these emission spectra are limited due to the vinyl polyconjugation lengths and tend to be consistent with the formation of specific degraded units, possibly polyunsaturated ca rbonyl species of a limited length confined to the EVA blocks. During oxida tion of EVA the original unsaturated carbonyl species remain as distinct em itting chromophores. This suggests that the growth and decay of these chrom phores is virtually constant indicating that they could be an integral part of the EVA polymer that are responsible for inducing degradation. Degradat ion is limited to the vinyl acetate moieties where hydroperoxides can lead to the formation of polyconjugated carbonyl groups. The EVA degradation is therefore, different from that of PVC where in the latter case polyconjugat ed vinyl groups are evident through conjugated absorption bands in the UV s pectrum. In the case of degraded EVA no such bands are observed. Also, degr aded coloured EVA is not bleached by treatment with bromine, maleic anhydri de or peracetic acid. Primary phenolic antioxidants exhibit variable activi ty in inhibiting the yellowing of EVA while combinations with phosphites ge nerally display powerful synergism. (C) 2000 Elsevier Science Ltd. All righ ts reserved.