Efficiency and mechanism of phosphorous antioxidants in Phillips type polyethylene

The stabilising efficiency of three phosphorous secondary antioxidants of different chemical structures (phosphonite, phosphite and phosphine) was compared in a Phillips type polyethylene. The polymer was processed by six consecutive extrusions in the presence of 700 ppm primary antioxidant and 700 ppm phosphorous compound. The consumption of the secondary antioxidant was followed quantitatively by Fourier transform infrared (FTIR) spectroscopy. The properties of the polymer were characterised by FTIR spectroscopy, colour and rheological measurements, as well as by the determination of its residual thermo-oxidative stability. The results of the experiments proved that the chemical reactions occurring in the first extrusion of the polymer are different from those taking place in the further processing operations. The rate of antioxidant consumption and the chemical reactions of the polymer are strongly affected by the type of the phosphorous secondary antioxidant. The analysis of the results indicated that the three stabilisers must act according to different mechanisms. The investigated phosphine showed the best melt stabilising efficiency, while phosphonite was found to protect the polymer most effectively from discoloration.