| Since the 1980s, high temperature vulcanized (HTV) silicone rubber composite insulator has been operated and widely used in electrical power system in the transmission lines due to its low weigh, high mechanical strength, good electrical insulation performance, and convenient operation maintenance. Compared with ceramic and glass insulator materials, HTV silicone rubber composite insulator used in ultra-high voltage power transmission lines in high altitude area are more susceptible to ultraviolet ray and aging. Large photon energy of ultraviolet can cut off parts of the chemical bonds in HTV silicone rubber, reducing its mechanical properties and electrical insulation performance, thus leading to safety accidents. Therefore, it is very important to study the effect of ultraviolet radiation on mechanical and electrical properties of HTV silicone rubber is of great significance for the safe operation on ultra-high voltage power transmission lines.In view of the composite insulator ultraviolet aging problems, the mechanical and electrical performance of HTV silicon rubber before and after ultraviolet aging is studied in this paper. The self-designed ultraviolet aging experimental box is used for the accelerated aging experiments (0-1000h) of HTV silicone rubber from two factories with the radiation wavelength from 320-750nm. The tensile test, shore hardness test and the measurements of volume resistivity, surface resistivity, breakdown strength, tracking and erosion resistance and hydrophobicity, are used to evaluate the mechanical and electrical properties of HTV silicone rubber. The surface and fracture morphology of samples were observated by optical microscope and scanning electron microscope (SEM). The change of the relative contents of functional groups on the surface of samples and the surface chemical element valence and content were analysed by fourier transform infrared spectrometer (FTIR) and X-ray photoelectron spectrometer (XPS). Combing with the structure of HTV silicone rubber, the paper analysed and discussed its ultraviolet aging mechanism, and explained the micromechanism of changing characteristics of mechanical and electrical properties preliminary. The result can be concluded as follows:With the increasing of ultraviolet irradiation time, the mechanical properties:tensile strength, elongation at break and residual deformation decreased in a fluctuating tendency, fracture angle increased, and fracture surface became smooth, shore hardness decreased at first and then increased; particles, holes and craze appeared on the sample surfaces, with exposed filler. The electrical properties:volume resistivity and surface resistivity decreases with the extension of ultraviolet irradiation time, breakdown strength, tracking and erosion resistance and hydrophobic had a decreasing trend.The micromechanism of changing characteristics of mechanical and electrical properties are as follows:some chemical bond such as Si-C, C-C, C-H have lower bond energy that easily damaged or cut off by ultraviolet radiation, and leading to the degradation of tensile strength, elongation at break, residual deformation and hardness. Volatile gas generated and overflowed from the material surface, this results in a decline of surface evenness, and holes appeared. Nonpolar methyl groups were oxidized or broken on the side chains, which lead to the shield of strongly polar of Si-O bonds had been abated, and hydrophobicity decline. In addition, part of the chemical bond rupture caused craze under tensile stress, also lead to hydrophobicity decline, and at the same time material is easy to absorb moisture, and the water itself has weak ionization, make the carrier concentration increase greatly, increased the conductivity of HTV silicon rubber and make the volume resistivity and surface resistivity decreased. Chemical bond fracture formed insulation weak areas, lead to the decline of breakdown strength and tracking and erosion resistance. Parts of the Si-C bond rupture caused the destroy of crosslinking network in HTV silicone rubber, with Al(OH)3 filler exposed and reached up to the surface of the material. Al(OH)3 decomposition and formed Al2O3 under the ultraviolet radiation, which made effective transmission of discharge energy, and resulting a small increasing of tracking and erosion resistance. The destruction of the crosslinking network is also an important reason for the declining mechanical properties. With the increase of ultraviolet irradiation time, the free radicals on the surface enable cross-linking reaction, which made silicone rubber surface strength, hardness increased, fracture angle increased, and fracture surface became smooth. And also lead to small increasing of breakdown strength and tracking and erosion resistance. The complex cracking, oxidation, crosslinking reaction happened in HTV silicone rubber material under the ultraviolet irradiation, and the molecular structure was destroyed or changed, which lead to the degradation of macroscopic mechanical properties and electrical performance. |
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