Environmental Stress Cracking of Thermoplastic Polyimide Insulated Wires

Polyimide is often the first choice to insulate instrumentation and magnet wires working under demanding environmental and operating conditions since it shows good mechanical properties at low temperatures and high radiation resistance. Nevertheless, we have recently discovered that thermoplastic polyimide (TPI) insulated wires may suffer an accelerated brittle failure from a combination of environmental and mechanical stress, summarized as environmental stress cracking (ESC). Amorphous plastics immersed in an aggressive liquid and under a certain stress level, may develop crazes below the stress that would normally cause crazing in air. Certain alkaline hardeners can be considered aggressive toward polyimide. This study reports the causes and effects of environmental stress cracking of thermoplastic polyimide insulated wires subjected to resin systems commonly used in superconducting magnet technologies, and tentatively identifies those that seem more benign. We suspect that ESC can be behind some of the failures (Paschen breakdown under test voltages or even in operation) seen in some superconducting coils.