Environmental Degradation of Aromatic Polyimide-Insulated Electrical Wire

The continued interest in the survivability, reliability, and maintainability of aerospace systems has led to studies of the long-term stability of polymeric materials. Of particular interest is the long-term stability of the aromatic polyimide (Kapton, an E. I. du Pont Co. trade name) used extensively for the insulation of electrical wires. This material is subjected to a harsh complex service environment of high temperature, mechanical stress, and high humidity. We have investigated the effect of these paramaters on the insulating properties of aromatic polyimide-insulated electrical wire. Three types of Kapton insulated wires were studied; all failed a standard dielectric test and developed cracks when immersed in water at 70°C while stressed. The failure rate is a strong function of the strain. However, wires treated in an identical manner, but thermally annealed at 210°C for 8 h, did not fail even after 26 weeks immersion. The annealing reaction is first-order in the range 140 to 180°C and has an activation energy of 107 kJ/mole (26 kcal/mole).