Thermal aging processes in polyethylene terephthalate film (PETP) were investigated in three laboratories in France, Finland and Norway, using both sensitive analytical methods to measure the aging reaction rates and physical aging tests of conventional nature. The known non-linearity  of the thermal endurance graph of this material was confirmed at ca. 180°C. In spite of this, the behavior of the material was predictable, especially when the EAP test method  was applied. Analytical methods combined with suitable physical aging tests were shown to be rapid tools for investigating the long term thermal endurance of materials in short time.
Paloniemi, P., Fallou, B., Herstad, K., Sletbak, J., & Lindström, P. (1986). Rapid determination of thermal endurance of PETP film combining sensitive reaction rate measurements and conventional aging tests. IEEE Transactions on Electrical Insulation, EI-21(4), 599 - 607. https://doi.org/10.1109/TEI.1986.348965