Influence of processing aids on the thermal and photostabilisation of HDPE with antioxidant blends

The thermal and photostabilising performances of two commercial phenol/phosphite (Anox-Great Lakes) and three commercial phenol/phosphite/lactone (Ciba Specialty Chemicals) packages have been determined in HDPE in the absence and presence of two processing aids, calcium stearate and oleamide. Stabilities were assessed using FTIR, thermal analysis and mechanical property changes (tensile, elongation and impact). In terms of thermal stabilisation the phenol/phosphite blends are found to be superior to the Tris–phenol/phosphite/lactone blends. Solid-state (oven ageing) and melt (OIT) stabilities were found to follow a similar trend as does the yellowness index. The Anox blends were found to offer superior stability in terms of oxidative and mechanical performance. Colour inhibition was strong except in the case of calcium stearate. However, in the presence of calcium stearate strong synergism was observed with the Irganox blends and this may be associated with an exchange interaction/complexation between the calcium and the lactone/phosphite derivatives. These complexes appear to have high performance in thermal stabilization possibly through hydroperoxide decomposition and inhibition of carbonyl formation. In contrast, the effects of photostabilisation were significantly different with strong antagonistic effects in the presence of the calcium stearate due to the instabilities of any calcium complexes. Overall, the chemical and mechanical property changes followed a similar pattern.