Specific heats of a magnet wire varnish-enamel combination as a measure of thermal stability of the insulation system

In magnet wires the thermal stability of the insulated wire is gauged by tests of twisted-paired conductors (ASTM D2307-64T).⁵ This determines the thermal life rating of the wire, based upon the Arrhenius equation and the assumption of a first order degradation. S. D. Bruck⁶ determined the activation energies for decomposition of a polyimide film to be 33 kcal/mole in air and 72 kcal/mole in vacuum. It is seen that thermal stability is also a function of the environment (air, vacuum). If the polyimide magnet wire is exposed to a steam environment (120°C, 15 psig), the life of the film as determined by an 1100 volt breakdown test is of the order of 103 hours.² The life of the polyimide in air is of the order of 20,000 hours at 220 °C. It is evident that the activation energy of the polymide film in a steam environment would thus be far lower than its value of 33 kcal/mole in air.