Analysis of the PET P semicrystalline morphology and thermomechanical properties for long term physical aging research

This investigation aims at studying the mechanical and microscopic properties of PET P (polyethylene terephthalate) used for insulators in High Voltage Gas Insulated Substation before a long term physical ageing. For this reason, a basic study to understand the PET behavior in a reference state (before any thermal ageing) is definitely necessary. In a first step, the study focuses on revealing more detailed information about the PET morphology. For this purpose, PET samples have been subjected to special schemes of crystallization. In fact, after being quenched (amorphous state) from the semicrystalline state, the samples were annealed at different temperatures for several times. This controlled crystallization has shown a classical increase of the long period and the crystallinity ratio. The specimens have been characterized by different techniques, namely, differential scanning calorimetry (DSC), mechanical tests, and wide and small angles X scattering (WAXS, SAXS, respectively). The second step deals with the mechanical properties. We have studied closely the thermal effects below and above the glass transition (Tg), then the tensile rate effects during true stress-true strain tests. The mechanical measurements have shown that the Young modulus and the yield stress decrease when the temperature is increased. However, the material is slightly sensitive to the tensile rate below Tg or even unaffected above Tg.