Correlation of properties of polyethylene terephthalate with aging in refrigerants

Our department frequently receives, through our customers, motors which have failed prematurely in the field. Invariably, we are asked to determine the reason for the early failure. Often a visual inspection of the motor will yield the answers. If this fails, the condition of the motor insulation, especially the polyester film, is often the next area of investigation. The conditions under which the motor was run can frequently be estimated from the physical properties of the aged polyester film. However, physical property measurements are often impractical because of the required sample size and shape. Thus, an attempt was made to correlate chemical properties, which we could measure from as little as 0.5 grams of powdered film, with physical properties in order to ultimately be able to estimate the history of failed motors.