The Use of Thermal Analysis Methods for the Estimation of Thermal Life Ratings of Magnet Wire Enamels

An empirical correlation between thermal analysis data and ASTM D-2307 results for magnet wire enamels is presented. This correlation was sought as an aid in rapid screening of experimental wire enamels and was not intended as a replacement for the ASTM test. Correlation testing was carried out on a variety of commercial and experimental enamels on copper wire in air, nitrogen, oxygen, wet oxygen, and R-22 refrigerant via thermogravimetry (TG) and differential thermal analysis (DTA). Wet oxygen and R-22 damaged the test equipment and so were not used for the complete spectrum of enamels. The "best" correlations were obtained in air. Reproducibility studies were carried out on samples from six different production runs of each of two enamels. The reproducibility was sufficiently good to allow identification of serious changes in enamel formulation. Variations of the correlations versus wirecuring speed (factor of two) were found to be within the reproducibility limits. The "best" correlations were derived from least squares plots which exclude thermal analysis results for the highest temperature enamels (polyimides). Preliminary experiments on the effects of copper oxides on thermal analysis data for polyimides indicate that such testing yields data giving a better fit to the correlation curve.