Theory of Life Testing and Use of Thermogravimetric Analysis to Predict the Thermal Life of Wire Enamels

The treatment of the deterioration of insulation as a chemical rate phenomenon has been extended to cover complex multiple reactions. The relationships between the parameters goveming the thermal stability of a polymer in a given environment are calculated first for isothermal thermal endurance test conditions and second for thermogravimetric analysis (TGA) conditions. The two equations can be combined to give a rigorous mathematical solution to the problem of predicting thermal ratings of wire enamels by TGA and this is demonstrated for a Formvar ® enamel (a simple degradation reaction). For complex reactions (recognized by certain features of thermograms) life lines are shown to be curved rather than dually linear, and by examining the case of simultaneous competing reactions, the method is shown to be inapplicable to complex cases. Both the life theory and the TGA theory proposed avoid the usual assumptions concerning order of reaction, apparent activation energy, and extent of reaction. It is suggested that TGA, a very rapid technique, could also be used to investigate environmental variables in life test procedures. This approach is considered to be valid for insulations other than wire enamels, provided that electrical or mechanical failure is the result of chemical reactions.