Stabilisation of LDPE cross-linked in the presence of peroxides I. Kinetic study of the oxidation

Abstract

The oxidation resistance of LDPE, cross-linked in the presence of dicumyl peroxide and stabilised with a commercial antioxidant, bis[(3- t-butyl-2-hydroxy-5-metyl)phenyl] sulphide (Irganox 1081) was investigated. Oxidation induction time, t i, as found by isothermal DTA, was used as the criterion of evaluation of LDPE resistance to oxidation. Without the peroxide, Irganox 1081, at concentrations in the range 0.1–0.5% w/w, was found to inhibit LDPE oxidation. Its depletion rate at temperatures in the range 195–225°C, as measured by oxidation induction time, t i, may be described as a zero-order reaction with activation energy E=141.4 kJ/mol. Simultaneous addition of 2.0% w/w dicumyl peroxide and 0.1–0.4% w/w Irganox 1081 to LDPE by rolling at 130°C was observed to reduce oxidation induction time to t i=0. After cross-linking at 180°C and 5.0 MPa for 20 min, the same compositions recovered their resistance to oxidation over the whole concentration range of Irganox 1081.