A reliable and simple test for determining the relative hydrolytic and thermal stability of polyester ground insulation

The commonly accepted procedures for assessing the relative hydrolytic and/or thermal stability between several polyester (PET) films to be used as ground insulation involves following the changes in either tensile strength or elongation as the film is aged at elevated temperature(s) in sealed tubes or in open air ovens. We describe a test which avoids the hazards involved with sealed tubes as well as the errors induced by inadequate accounting of moisture and the temperature variation which can attend these previously accepted procedures. A variety of failure criteria are also examined in conjunction with this test. It is found that the relative ranking or performance of a film is the same regardless of the failure criteria chosen. Our results convincingly reaffirm the validity of employing intrinsic viscosity as a failure criteria for the differentiation of the relative performance of PET films under hydrolytic and thermal stress. The utility of the procedure is demonstrated by examining the relative functional life of three different polyester films. Two of the films are employed in compressors requiring improved hydrolytic/thermal performance and/or lower extractable residues. The remaining film is characterized as a standard product.