Comparative study of the influence of thermal ageing on PET and PEN polymers

The present paper deals with the study of poly(ethylene-2,6-naphtalenedicarboxylate) (PEN) and poly(ethylene terephtalate) (PET) which are polymers widely employed in Electrical Engineering. Up to now, the researches have been mainly devoted to the study of their synthesis and physico-chemical characterisation. However, in some applications such as stator insulation in rotating machines, new working procedures impose more constraints to the polymers than in the past. Indeed, the latest power inverters supply an input voltage to the motors whose waveform presents short rising times, high frequencies and high overvoltages. Thus, these higher constraints may accelerate considerably the ageing processes of the material. One reason is the increase of the material temperature. Therefore, the study of the thermal properties of those polymers seems to be of prime interest. We present in the following, the investigation that was implemented on specific samples which were aged at different temperatures (from 120/spl deg/C to 210/spl deg/C) and for different time durations (up to 1000 hours). This thermal characterisation was achieved employing dielectric spectroscopy, breakdown voltage determination and Differential Scanning Calorimetry.