Thermal ageing of a silane-crosslinked polyethylene stabilised with an excess of Irganox 1076Ⓡ

Abstract

This work focuses on the thermal ageing in air of a silane-crosslinked polyethylene stabilised with an excess of phenolic antioxidant (Irganox 1076®) in the temperature range between 87°C and 130°C. In such case, it was shown in previous works that the antioxidant is present in two physical states: dissolved in the polymer matrix and exuded at the sample surface under the form of crystals. The purpose was, in particular, to investigate the role of an antioxidant excess in the stabilisation process of the polymer. On this purpose, the pure antioxidant crystals (i.e. the commercial powder) and a polymer sample stabilised with only dissolved antioxidants (i.e. in a concentration lower than the solubility threshold) were investigated at 130°C to isolate and elucidate the thermal ageing behaviour of each antioxidant phase. For all the three studied samples, quinone methide and cinnamate species were formed through the chemical consumption of antioxidants. For both stabilised samples (with an excess, or not, of antioxidants), similar behaviours were observed in term of antioxidant depletion. In particular, it was shown that although some chemical consumption was also detected, the initial antioxidant depletion was essentially due to its physical loss through evaporation. In addition, in both cases, the polymer oxidation was observed after the almost total depletion of antioxidants and without any additional oxidation induction period. The comparison of the oxidation induction periods obtained at 130°C for these two samples showed that the antioxidant excess efficiently participates to polymer stabilisation during thermal ageing, which could be due to a further solubilisation of antioxidants during thermal exposure.