Thermo-oxidative resistance of crosslinked polyethylene (XLPE) coated by hybrid coatings containing graphene oxide

Crosslinked polyethylene (XLPE) has been coated with hybrid organic-inorganic thin films obtained via sol-gel reactions and containing small amounts (from 02 to 0.8 wt%) of graphene oxide (GO), with the aim to improve durability of electrical insulators and devices. Accelerated aging in an air-circulating oven at 120 degrees C, slightly above the typical on-service temperature of insulators for high-voltage electrical cables, has been carried out over times up to 2000 h. The thermo-oxidation of XLPE substrate has been assessed by isothermal differential scanning calorimetry (DSC) and infrared analysis (FTIR) to determine the oxidative induction time (DSC-OIT) and the oxidation index, respectively. Indirect evidence of the damages induced by thermo-oxidation has been achieved by tensile tests on aged samples. Data collected by DSC, FTIR and tensile tests outline a remarkable contribution of GO to the improvement of the thermo-oxidative resistance of the coated XLPE. The effect of GO is attributable to an improved barrier against oxygen diffusion, which in turn results in a lower damage of XLPE with significant improvement of mechanical properties (durability) over long aging times. Additional measurements of the surface resistivity have been also carried out before and after the chemical reduction of GO in the coatings, by exposition to hydrazine hydrate vapours. A strong decrease of surface resistivity has been observed after GO reduction with hydrazine, leading, for the sample containing 0.8 wt% of GO in the coating, to an almost antistatic behaviour of the resulting coated XLPE.