Thermo oxidative stability of polyamide 6 films I. Mechanical and chemicalcharacterisation

Unstabilised and stabilised polyamide 6 films were subjected to long-term o ven ageing in ambient atmosphere at temperatures in the range of 100-120 de grees C. The strain at break of film strips showed a drop at a point of oxi dation corresponding to a critical molar mass range of 15000-19000 g mol(-1 ). Embrittlement occurred at a molar mass of 40-50% of the original value o f 27000 g mol(-1). The efficiency of a phenolic antioxidant such as Irganox 1098 was poor, extending the drop in strain at break only by a factor of 2 . All investigated polyamide films showed a high rate of oxygen uptake at t he beginning of oxidation followed by a gradual decline. The oxidation was accompanied by an autoretardant accumulation of peroxides as well as of car bonyl groups consistent with the oxygen uptake data. This behaviour is diff erent from that reported for polyolefins, which show a well-defined inducti on period followed by a sigmoidal increase in the oxygen uptake, also in th e presence of a phenolic antioxidant. It is unlikely that the rate of oxida tion solely depends on the concentration of peroxides since, throughout the oxidation, peroxides were accumulating, while the rate of oxygen uptake wa s decreasing. (C) 1999 Elsevier Science Ltd. All rights reserved.