Study of the high temperature reactions of a hindered aryl phosphite (Hostanox PAR 24) used as a processing stabiliser in polyolefins

The processing stabilising performance of various phosphorous antioxidants in polyolefins is affected significantly by their chemical composition. In order to explore the mechanism of stabilisation, the reactions of a hindered aryl phosphite [tris(2,4-di-tert-butylphenyl)phosphite (DTBPP), Hostanox PAR 24] were investigated at temperatures corresponding to polyethylene processing. The thermal and thermo-oxidative stability of the additive was determined by differential scanning calorimetric (DSC) and thermogravimetric methods. DTBPP was heat treated under argon and oxygen at 200 and 240 °C. The stabiliser was reacted at 200 °C with azobisisobutyronitrile (AIBN) in oxygen-free environment (carbon centred radicals) and under oxygen (peroxy radicals), with dicumyl peroxide (DCP) in oxygen atmosphere (oxy radicals), and with cumene hydroperoxide (CHP) under argon. The reaction products were identified by FT-IR, HPLC and HPLC-MS. The results revealed that besides the known reactions of hindered aryl phosphites, thermal decomposition and recombination reactions also take place above the melting point of the antioxidant. DTBPP does not react with molecular oxygen, but its decomposition is accelerated by oxygen and especially by radicals. Accordingly, the heat-stability of phosphorous stabilisers also has to be taken into account in their application, as it is one of the factors which influence the processing stabilisation of polyolefins.