Studies On The Microstructure And Hydrophobicity Recovery Of Aged Silicon Rubber Insulators By Positrons

In this paper, the degradation surface micro-structure and the influencing factors in hydrophobic recovery of silicon rubber insulators which have been widely used in electric power industry, were studied by a novel nuclear technology-positron annihilation spectroscopy and combined with SEM, contact angle measurement, and FTIR. The main results are as following:1. Using slow positron beam to investigate the degradation surface structure of silicon rubber insulators modified by UV exposing and Ar plasma treatment, respectively. It was found that UV-A (365nm) irradiation had no influence on the structure of the sample surface, while by0,90,120,360,840h UV-B (310nm) irradiation affected the sample significantly. When positron incident energy ranged between0to2.25keV(<100nm), S parameter shows a low level (-0.48), indicating a inorganic structure (silica-like layer) was formed, and its thickness was estimated as6-56nm. Hereafter, all the S parameter of irradiation samples is higher than virgin sample, indicate that a scission layer contains large and more free volume has formed. Contact angle measurement results show that the longer irradiation last, the slower hydrophobicity recover. This result could attribute to silica-like layer hinder the LWM diffused from bulk to surface. In addition, surface structure of silicon rubber treated by Ar plasma (0,1,2,3,5min) has also been studied. S parameter shows that, a6-18nm thick silica-like layer has not formed until3min treatment. As incident energy keep increasing, all the S parameter has undergone a same process:increase to a maximum then decline to a saturation value. This process showed that the degradation mechanism in Ar plasma treatment is different from UV radiation.2. The relationship between micro-structure of silicon rubber and diffusion of LWM has studied mainly by using positron annihilation lifetime and contact angle measurement. Positron annihilation lifetime results show that virgin silicon rubber have two longer lifetime component ?3and ?4, corresponding to two kind of free volume hole size:2.12A (small one) and4.05A (big one). With the increase of D4, the size and intensity of bigger holes keep the same level, and the size of smaller holes decrease but its intensity increase. These results means that LWM are more easily flows in bigger holes, while it may be stuck in smaller one, resulting smaller hole are divide by LWM. Hydrophobic recover property of silicon rubber treated with5min Ar plasma has obtained by contact angle measurement. It was found that with D4increase, the corresponding recover rate becomes faster. It could be attributed that the bigger hole can serve as transmission channel for LWM, and when the channel keep a constant, the hydrophobic recover rate is obviously parallel to the content of LWM. We also using slow positron beam and SEM to investigate the surface structure of hexane treated silicon rubber after modified by Ar plasma treatment. SEM showed that the longer the treating time, the more damage the sample surface. Slow positron beam suggested that more free volume have been generated in surface within the range of a few hundred nm, which may indicate that after LWM have been extracted from the topmost surface, longer or higher crosslinking molecular chains were directly suffered to plasma, resulting more scission reactions occurred.3. The effects of different levels of inorganic fillers on the micro-structure and hydrophobic recovery of silicone rubber were studied by using positron annihilation lifetime. As the content of silica increase from8to18%, ?3decrease while ?4and the total positron intensity have not change, indicating more positrons annihilated in silica. Based on reinforce mechanism of silica in rubbers, with the content of silica increase, physical crosslinking in rubber becomes more and make it easier to reunite the silica particles, which could result in more positrons annihilate in silica and decrease the transmission channel (free volume) in system. For the samples with different level of ATH, when ATH increase from40to55%, I3:6.1to5.2%and I4:24.8to20.8%. This phenomenon indicate that ATH particle may restrict mobile capacity of the molecular chains, resulting decrease the quantity of free volume holes. After5min Ar plasma treatment, the hydrophobic recovery obtained from contact angle suggested that the recovery rate was slow down as ATH content increase, and based on the suggestion from positron lifetime, this is because more ATH will occupy free volume space and thus prevent the transmission of LWM.