Thermal-oxidative degradation of polyamide 6,6 containing metal salts

The thermal-oxidative stability of oven aged polyamide 6,6 (PA6,6) doped with Co, Cu, Ni and Zn chlorides combined with KI was examined. Aging caused a depression in melting temperature and an increase in enthalpy of fusion of PA6,6 films due to the formation of a strongly degraded crystalline fraction with a lower molecular weight. A build-up of carbonyl absorption in the range 1700-1780 cm⁻¹ due to primary and secondary photo-oxidation products was detected. The kinetics of carbonyl accumulation was affected by the morphology of the samples, and it was observed that at a later stage of aging the crystalline phase was also involved in the oxidation process. The above mentioned changes were greatest in the case of neat, Co and Ni doped polymer, suggesting that these metal salts acted as pro-oxidants. On the other hand, the use of Cu and Zn chlorides brought about a substantial increase in long-term polymer stabilization.Tensile tests revealed a large reduction in ductility as a result of aging for neat, Co and Ni doped polymer, whereas long-term retention of tensile properties was found for the polymer stabilized with Cu and Zn. The presence of the metal salts combined with KI led to increased stabilization for chlorides of Ni, and Co, owing to the participation of KI in non-radical decomposition of peroxides. No effect due to KI was observed for ZnCl₂.