Inhibition of phosphite antioxidant hydrolysis via synergistic blends for the thermal processing of polyolefins

Ortuoste, Nerea (2004) Inhibition of phosphite antioxidant hydrolysis via synergistic blends for the thermal processing of polyolefins. Doctoral thesis (PhD), Manchester Metropolitan University.

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Abstract

Secondary phosphite antioxidants are widely used as polymer additives. They are good processing stabilisers and form good synergistic blends with primary phenolic antioxidants. However, some phosphites suffer from poor hydrolytic stability that leads to storage and handling problems. The hydrolytic stability of eight commercial phosphites under controlled temperature and relative humidity conditions were examined. The excellent processing stability afforded by Alkanox P-24 is well known in the literature. However, it was found that Alkanox P-24, (Great Lakes Chemical Corporation) is hydrolytically unstable. Enhancement of its hydrolytic stability is therefore a key objective and some binary and ternary blends were developed using other additives that are often used polymer stabilisation, including the primary antioxidant Tetrakis [methylene 3-(3’,5’- di-ter/-butyl-4-hydroxyhyphenyl)propionate] methane (Anox 20, Great Lakes Chemical Corporation), acid scavengers calcium stearate (DW, Faci SpA) and the hydrotalcite-like compound (DHT-4A, Kyowa Chemical Industry). An improvement in the hydrolytic stability of Alkanox P-24 was found when it was blended with these additives. A comparison with different physical forms of blends (traditional powders versus recently introduced No Dust Blends) was undertaken. Better performance was observed with NDB relative to powders. Spectroscopic studies (FTIR, and Mass Spectrometry) were also undertaken to elucidate the hydrolysis mechanism of the phosphite antioxidant Alkanox P- 24. Mechanistic schemes were devised and interpreted. Hydrolysis products of Alkanox P- 24 are believed to be involved in the mechanism of stabilisation. In this programme of work, the role of the hydrolysis products was investigated by controlled thermomechanical degradation in an extruder and stabilisation activity evaluated by following the yellowness index and the melt flow rate. The influence of partially hydrolysed Alkanox P-24 on polymer processing was studied. It was found that some active hydrolysis products showed significant antioxidant activity and retarded polymer degradation during processing. Finally, the statistical experimental design proved to be a very powerful tool for studying the impact of each additive in the stabilisation of HDPE. Also the possible interactions between additives present in the additive system were investigated.