Dielectric response of polyimide to thermal and saline degradation

Polyimide (PI) is a widely used wiring insulation material for aircraft. Given the harsh environment of its employment, wiring insulation material for aircraft suffers from various degradation processes which may cause failure of electrical insulation. In this paper, the dielectric response of PI to thermal degradation and to saline exposure is investigated. To investigate the effect of thermal exposure, PI samples were isothermally heated at 400, 425, 450 and 475°C for up to 5 hr in an isotemp muffle furnace. After heating, the samples were removed from the furnace and allowed to cool at room temperature. To investigate the effect of saline degradation, PI samples were immersed in both distilled water and salt water with salinity of 80 g/l for up to 4 days at room temperature. Measurements of permittivity over the frequency range 1 kHz to 2 MHz at room temperature, obtained using an Agilent E4980A LCR meter, revealed an increase of up to 14% in the measured real relative permittivity of the thermally-exposed samples, compared with a sample completely dried by heating at 200°C for 1 hr. The frequency-dependence of the dissipation factor was also changed significantly after thermal exposure at 475°C. The relative permittivity and dissipation factor of Kapton samples immersed in distilled water were increased by approximately 10% and 40%, respectively. The influence of dissolved salt had only a small additional effect, however.