Surface degradation of HTV silicone rubber and EPDM used for outdoor insulators under accelerated ultraviolet weathering condition

The chemistry of surface degradation by ultraviolet light (UV) of high temperature vulcanized dimethyl silicone rubber (HTV SIR) and ethylene propylene diene ter-polymer (EPDM), has been studied by many researchers. Insulators fabricated from these materials are normally "qualified" for outdoor use by accelerated life tests that typically include salt-fog and high AC voltages but with insignificant levels of UV. The manufacturer usually counters this by subjecting samples of the material to UV in a weathering tester. However, the nature of the interaction of the UV with the other stress factors present in the accelerated life test remains unknown. This paper focuses on the effects of prolonged UV exposure (5000 h) on electrical and chemical surface properties. Periodic tests of contact angle, SEM, XPS, surface resistivity and surface voltage decay analyses were made. The contact angle of silicone rubber dropped slightly (5° after 1000 h of UV; 10° after 5000 h of UV) and little damage was indicated by SEM; but EPDM suffered a severe drop in contact angle (46° after 1000 h of UV; 68° after 5000 h of UV) accompanied by surface cracking and chalking. Surface resistivity (ρ_s) of HTV SIR increased by 50% to 2 ×10¹⁴ Ω/square after 5000 h of UV. This indicates unexpected lack of ionic character of the degraded surface, but may be partly due to UV generated SiO₂ between the specimen and the electrode. Surface voltage decay after corona charging, a non-contacting technique with appeal because it is non-damaging and easy to apply to a variety of geometry. The accuracy of the Cresci theoretical equation for the geometry used was confirmed. We feel that these trends in surface resistivity correlate well with observations in the field where a tendency toward surface tracking in EPDM controls life while for HTV SIR life may depend more on the degree of contamination of the environment that can lead to surface erosion. The surface abundance trends of O-C = O and SiOx (x=3∼4) with time indicate that O-C=O and SiO_x have increased rapidly at short treatment times and then level off at about 10% and 50%, respectively, until the end of the test time (5000 h). We su- spect that these trends are important in the initiation of tracking in EPDM and of erosion rate increase in HTV SIR. It might also be indicative of the time when the SiO_x functional groups start to degrade to SiO₂+O_x which causes HTV SIR to undergo mainly a cross-linking reaction.