Plasma & Corona Aging Properties Of Silicone Rubber For Composite Insulator

Nowadays, the silicone rubber composite insulator is widely used in electric power systems due to its high pollution flashover voltage. However, as the serving years increases, the aging problem of the silicone rubber composite insulator arises and, presently, attracts more and more attention among the researchers worldwide. Relatively, more efforts have been devoted to the macro properties of composite insulator, such as the mechanical property, the electrical property, the hydrophoicity, the pollution flashover performance, etc, whereas a systematic study on micro-structure is rare. Reasons for the aging of material and for becoming invalid depend, essentially, on the destruction of chemical structure and micro-structure and, hence, it is of great importance to develop techniques and methods for diagnosing and supervising insulators’ aging process based on the surface property and structure change.Following the above discussion, in this contribution, the author studied the effects of plasma aging and corona aging on the surface property and micro-structure of silicone rubber. The main contents are summarized as follows:Firstly, through an orthogonal test, a fundamental formula of a group of silicon rubber is determined, and, based on this derived formula, the author refined the specimens of silicone rubber with the contents of silica of 10,20,30,40 and 50 phr, as well as the contents of Aluminum hydroxide (ATH) of 70,90,110,130 and 150 phr. Then the author tested, for the silicon rubber samples made by ourselves and by the other factories, the hardness, the volume resistivity, the tensile strength, the elongation at break, the hydrophobicity, the flammability, and the breakdown strength. The results show that the hardness increases with the increase of the amount of fumed silica and ATH, and that the elongation at break reaches its minimum value when the fumed silica and ATH reaches to the highest content. Except for the hardness, no significant changes were observed for the other parameters. It is not sensitive to distinguish the effects introduced by the different components and by the origination of the raw material based on the macro performance parameters.The positron lifetime spectra are used to study the silicon rubber’s free volumes inside when the content of ATH and fumed silica is altered. Two sizes of free volume holes are discovered, of which the radiuses are, respectively,2.12 A and 4.05 A. When the amount of ATH and the fumed silica increase, the sizes of two free volume holes inside silicon rubber remain the same, but the number of free volumes, especially those with bigger holes, decreases. Combined with the macro test results, it reveals the regulatory mechanism of silica and ATH in the silicon rubber material.The aging testing of silicon rubber is launched by radio frequency capacitive coupled glow plasma. After the aging process with plasma treatment, most silicon rubber’s surface loses hydrophobicity after one minute’s work of 30W aging power. The hydrophobicity recovery curves of 10 kinds of silicone rubber with different contents of ATH and fumed silica are measured. After plasma treatment, the contact angle on silicon rubber’s surface cannot recover to its original level. The aging method is recommended.The micro properties of aging silicon rubber are studied through the SEM, FTIR, XPS and slow positron beam. Results of SEM show that after plasma treatment, organic elements on the surface of samples are slowly destroyed as the treating time increases, making the fillers exposed outside. Results of Fourier Transform Infrared Spectra (FTIR) reveal that the effects of plasma treatment on silicon rubber are of the depth level of the order of run. Results of XPS indicate that when the plasma ages, a chemical reaction occurs on the surface of silicon rubber, leading to the generation of inorganic SiOx(x=3-4). The thickness of such substances is measured by the slow positron beam, and it is found that the thickness is of the order of about tens of nm. According to the above study, a three-layer model (namely, silica-like layer-chain scission layer-bulk layer) was proposed.The needle-plate electrode corona aging test with a voltage of 8kV and a space of 6mm for silicone rubbers is also studied here. It is found that after a certain period of corona aging, obvious phenomenon of zoning of discoloration and damage occurs on the surface, which has a color range of ashen, black and aureole from center layer to outer layer. The black layer has water-absorbing quality and is easily erased. Radiuses of the black layer and the aureole layer become larger when the corona aging lasts. After 24 hours of corona aging, the ashen layer in the center does not feature with a decrease on hydrophobicity. Meanwhile, the contact angle becomes around 100°,but it can reach to 130° if going back to the stable state, in which the contact angle is much higher than the original one. Results of SEM show that as the time of corona aging increases, the surface of silicon rubber is slowly destroyed and the holes appear on the smooth surface and some parts of the surface meeting flaws and pulverization. Afterwards, the cracks become bigger, the surface turns into bulks, and holes and flaws become larger, when the destruction on surface comes, to some extent, to the inside part starting to be destroyed along the depth. Results of FTIR show that along with corona treatment, the peaks of Si-CH3 and C-H decrease and even disappear while the peak of -OH turns into a higher one. Results of XPS show that as the corona treatment time elapses, sample’s surface has less amount of C and more amount of O. Elaborate analyses of Si2P indicate that, after corona treatment, the silicon rubber’s surface has more SiOx(x=3,4), and that the inorganic silica layer is generated. Results of micro-FTIR show that after corona aging, the layer structure occurs from the surface to bulk. The affecting depths of corona treatment for 24,72,96,144,168h on silicon rubber are 20,40,50,90 and 160μm by measuring the areas of Si-CH3 along the vertical depth. According to the study above, a three-layer model (namely, silica-like layer-chain scission layer-bulk layer) with the aging of corona is presented. This model is different from the model treated by plasma due to the fact that, after corona aging, the inorganic level has a thickness of the order of μm, and that the thickness of this layer is not homogeneous. The closer it is to the center, the more serious it will be destroyed.The hydrophobic recovery equation is derived based on the above three-layer models, the Young’s equation, and the Fick diffusion law. The hydrophobic recovery angle can be represented by the recovery time, the thickness of inorganic layer, and the diffusion rate of LMW with mathematical formula.The diffusion coefficient of small molecular is solved based on the hydrophobic recovery model as treated by plasma for durations of 3 and 5 min.Our results show that the plasma intensity and the corona discharge will significantly impact the time-domain process of hydrophobicity loss and recovery about silicone rubber, as well as the physics process. The microscopic characterization technique reveals the variation of material structure sensitively. The three-layer model (namely, silica-like layer-chain scission layer-bulk layer) with the aging of plasma and corona is of great significance for understanding the aging mechanism of silicone rubber material.