Degradation and stabilisation of poly(ethylene- stat-vinyl acetate): 1 – Spectroscopic and rheological examination of thermal and thermo-oxidative degradation mechanisms

Abstract

The degradation of ethylene vinyl acetate (EVA) copolymers was compared with low density polyethylene (LDPE), poly(vinyl acetate) (PVAc) and poly(vinyl chloride) (PVC) using FTIR, UV–visible and fluorescence spectroscopy as well as thermal and rheological analyses. Thermal, thermo-oxidative and photo-oxidative studies were conducted. Thermo-oxidation below 180 °C shows more similarities between EVA and LDPE. The luminescence spectra of degraded EVA and LDPE were almost identical but very different to that of PVAc. UV–vis analysis showed that the polyenes present in aged PVC were unlikely to be the same species responsible for the observed colour formation in aged EVA. It is suggested that they are polyconjugated carbonyl products. Rheological analysis also showed the evolution of crosslinking reactions during thermo-oxidation. FTIR studies after thermal degradation in inert conditions 290 °C showed complete loss of the ester functionality and associated lactone formation along with some evidence for ketonic and unsaturated carbonyl groups. Degradation in air at 180 °C, however, revealed that loss of the ester group was not so marked, with PVAc exhibiting the greatest stability. This was in line with the induction time to onset of autocatalytic carbonyl growth at 180 °C; the latter showed an apparent exponential decrease with increasing vinyl acetate content up to 28% w/w. Fluorescence analysis produced trends that complemented those of carbonyl index; the time to decomposition of initial fluorescent α,β-unsaturated carbonyl species coincided with the time to onset of carbonyl growth. Furthermore, the rate of formation of the new fluorescent species produced in EVA, and LDPE was similar to that of carbonyl growth. These new fluorescent species are therefore likely to be di- or tri-carbonyl products.