Dielectric response of PTFE and ETFE wiring insulation to thermal exposure

The change in dielectric properties of wire insulation due to various degradation processes is significant for health management of air- and spacecraft. Polytetrafluoroethylene (PTFE) and ethylene-tetrafluoroethylene (ETFE) are two fluoropolymers which have been widely applied as wiring insulation materials. This paper investigates the dielectric response of PTFE and ETFE to thermal exposure for up to 96 hours at 340°C and 160°C, respectively. The initial dielectric properties of PTFE and ETFE were measured using a Novocontrol dielectric spectrometer over the frequency range 1 Hz to 1 MHz and as a function of temperature. The real permittivity of PTFE decreases with temperature from -150°C to 300°C, and reveals step-like changes near 19°C and 31°C due to crystal phase transitions. The dissipation factor of ETFE, measured from -150°C to 210°C, shows two peaks at -80°C and 92°C, which are associated with molecular relaxations. After thermally exposing the samples, permittivity measurements using an Agilent E4980A LCR meter over the frequency range 1 kHz to 2 MHz at room temperature (25°C) reveals a significant increase in the real permittivity of both PTFE and ETFE as function of thermal exposure time. These observations are explained in terms of an increase in crystallinity of PTFE after thermal exposure above the melting temperature followed by slow cooling, and polar group formation in ETFE following oxidant thermal exposure.