High temperature reactions of an aryl-alkyl phosphine, an exceptionally efficient melt stabiliser for polyethylene

Bis(diphenylphosphino)-2,2-dimethylpropane (PMP) is a highly efficient melt stabiliser of polyethylene. This aryl-alkyl phosphine hinders the degradation of the polymer during processing even in small concentrations and in combination with a phenolic antioxidant its consumption rate is considerably slower than that of phosphites and phosphonites. In this study the reactions of PMP were studied at temperatures corresponding to those used for the processing of polyethylene in order to explore the processing stabilisation mechanism of this additive. Thermal and thermo-oxidative stability were determined by DSC and TGA, respectively by heating PMP in argon and oxygen at 200 and 240 °C. Reactions with peroxy, carbon-centred and oxy radicals, as well as with hydroperoxide were investigated at 200 °C. Reaction products were identified by FT-IR and solution-state NMR spectroscopy. The results revealed that the phosphine studied has sufficient thermal- and thermo-oxidative stability under the processing conditions of polyethylene. It oxidises easily with any oxidising agent including molecular oxygen of air. Consequently, PMP does not only decompose hydroperoxide groups and react with oxy macroradicals during the processing of polyethylene, as claimed by most references on phosphorous antioxidants, but it can also hinder the formation of peroxy macroradicals, i.e., the initiation reaction of thermo-oxidative degradation.