Thermo-oxidative degradation of irradiated ethylene tetrafluoroethylene

The thermal oxidative degradation of the semicrystalline thermoplastic polymer, ethylene tetrafluoroethylene (ETFE), used for electrical insulation was studied as a function of radiation (1.5 MeV electrons), temperature, and contact with a metal surface. The radiation, whose total dose varied from 0 to 48 MRads, produces two effects: (1) cross-linking which enhances the high-temperature mechanical properties of the polymer and (2) degradation which reduces its high-temperature lifetime. Modified wires were constructed by replacing the original silver-plated copper conductor with tin-plated copper to ascertain the effect of metal surface on the degradation process. The rates of degradation of insulation (no conductor), original wires, and modified wires were measured by weight loss and by thermogravimetric analysis (TGA). The primary degradation products were identified by evolved gas mass spectrometry. A simple model in which the rate of decomposition as a function of temperature, degree of conversion, and radiation dose was developed. The lifetime of the insulation in a thermal oxidative environment is a strong function of the composition of the electrical conductor it surrounds. Silver over copper was a much more effective catalyst in promoting the thermal oxidative degradation of the ETFE than tin over copper. The rate of degradation was a strong function of the total radiation dose regardless of conductor