Improvement of thermo-oxidative stability of electrically insulating polymeric materials by organic-inorganic hybrid coating

The thermooxidative stability of poly(ethylene terephthalate) (PET) and low-d. polyethylene (LDPE) films, coated with org.-inorg. hybrid coatings of various compns., was investigated after accelerated ageing tests, in order to ascertain a possible beneficial effect of these coatings on the elec. performances of these insulating materials. The results have shown that the coating affects degrdn. mechanisms for both LDPE and PET. Thermooxidn. is slow in LDPE, leading to significantly better insulating characteristics after ageing: a strong redn. of the embrittlement time was also obsd. The strong increase of crystallinity upon ageing, obsd. for both coated and uncoated PET samples, probably dominates the effect of ageing on the elec. properties. As a consequence, only slight beneficial effects on PET elec. performances was obsd. (both on cond. and elec. strength). On the other hand, the coating has a strong effect on mol. wt. changes leading to a significant increase of mol. wt. for coated PET, while uncoated PET undergoes a significant decrease. This suggests that coated PET should present significantly better properties upon very long times of ageing.