Stabilisation of LDPE cross-linked in the presence of peroxidesII. FTIR study of chemical changes taking place in the LDPE–dicumyl peroxide–Irganox 1081 system

Abstract

The chemical changes taking place in the systems: LDPE–Irganox 1081, LDPE–dicumyl peroxide and LDPE–Irganox 1081–dicumyl peroxide, after homogenisation (in a rolling mill at 130°C) and after homogenisation and cross-linking (at 180°C and 5.0 MPa for 20 min) were investigated by FTIR. It was found that the OH groups in Irganox 1081 were in a bound form, and were most probably linked to the sulphur co-ordination centre. Further, the presence of dicumyl peroxide without Irganox 1081 in LDPE resulted in the formation of hydroperoxide groups at a temperature of 130°C, which were replaced by free hydroxy and carbonyl groups of a ketonic type after cross-linking. Simultaneous addition of dicumyl peroxide and Irganox 1081 to LDPE prevented the formation of hydroperoxides, and the free OH group band was no longer observed in the spectrum after cross-linking. Carbonyl group bands were formed after cross-linking and their absorbances were about three times as low as that of the LDPE–dicumyl peroxide system. The presence of the peroxide resulted in depletion of the hydroxy groups of Irganox 1081 in the homogenisation process and further in the cross-linking process. A new form of hydroxy group, most probably linked to the –S(O)–acceptor centre appeared after cross-linking.