Low-molecular weight silicone fluid in RTV silicone rubber coatings

The author comments on a recent paper by H. Deng and R. Hackam (see ibid., vol.6, no.1, pp.84-94, 1999), which studied several factors affecting the level of low molecular weight (LMW) silicone fluid which is present in room temperature vulcanized (RTV) silicone rubber (SIR) coatings. A reply by the original authors is included     

Electrical performance of RTV silicone rubber coatings

The role played by two major constituents in a room temperature vulcanized (RTV) silicone rubber coating, namely, the inorganic alumina trihydrate (ATH) filler and the silicone polymer, on the coating's electrical performance was examined. The principal variable in the RTV silicone rubber coatings evaluated was the different weight fraction of the silicone polymer and ATH inorganic filler normally used for improving the tracking and erosion resistance. Other formulation details were essentially the same in the cured rubber. The coatings were spray coated on glazed porcelain rods and subjected to accelerated aging in a salt fog chamber. The changes produced by accelerated aging on the contamination withstand capability, leakage current suppression and erosion resistance were determined and correlated with the ratio of polymer to inorganic filler in the coating. It is shown that critical aspects of electrical performance necessary for satisfactory service operation such as contamination withstand capability before flashover and leakage current suppression are superior for the coating with a relatively high ratio of polymer to inorganic filler in comparison to the coatings with a relatively low ratio of polymer to inorganic filler. While all the coatings showed a high erosion resistance under mild discharge activity, the erosion resistance was found to be superior for the formulations with high inorganic filler under conditions of intensive surface discharge activity     

RTV silicone rubber coatings for outdoor insulators

The paper presents a review of room temperature vulcanized (RTV) silicone rubber high voltage insulator coatings (HVIC). These coatings are designed to replace greasing and water washing of insulators. Present day coatings are the result of nearly thirty years of development and use. In all but a very few dirty environments, these coatings have lasted ten or more years without maintenance and where maintenance has been found to be necessary, water washing is done at a significantly reduced schedule. The paper focuses on the important characteristics of these coatings and on the rigorous attention that is required in their application in order to ensure good performance     

Performance of RTV silicone rubber coatings installed in coastal systems

RTV SIR coatings can improve the performance of ceramic insulators, in cases where pollution problems are experienced and the use of composite housings is not technically or financially in profit for the electrical utility. In this paper the performance of coatings installed in a coastal high voltage substation is investigated by measurements of leakage current. The measurements indicated the importance of the wetting mechanism, for the specific environmental conditions. In the case of condensation the surface activity on the coated insulators has been remarkably suppressed. On the other hand, in the case of light rain the same levels of activity have been recorded for both coated and non-coated insulators. The overall performance of the coated insulators, for the specific conditions, is superior in comparison to the non-coated insulators.     

Chemical changes at the surface of RTV silicone rubber coatings oninsulators during dry-band arcing

Under wet and contaminated conditions dry band arcing occurs on the surface of HV outdoor insulators coated with room temperature vulcanizing (RTV) silicone rubber. The thermal impact of the arcing on the surface of the RTV causes chemical changes. The gradual loss of hydrophobicity due to dry band arcing is of considerable concern because it results in a reduction of the withstand voltage and subsequent failure of the insulator and power outage. In this study the RTV coatings were subjected simultaneously to salt-fog and electrical stress. Attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy and electron spectroscopy for chemical analysis (ESCA) were then employed to investigate the chemical changes. Possible chemical reactions and processes occurring under dry band arcing are suggested. The results of ATR-FTIR and ESCA indicate the formation of low molecular weight polydimethylsiloxane (PDMS), the reduction in the surface density of CH3 groups and crosslinking of the PDMS backbone chains at the surface. Chemical derivative analysis was also carried out to further elucidate the chemical changes on the surface of RTV coating     

RTV硅橡胶涂料防污闪技术及其在天津电网中的应用

介绍RTV硅橡胶防污闪涂料的作用及在天津电网中的应用，阐述其实际推广应用的巨大成效。此项应用技术为电网防污闪专业工作由传统落后的多维护、短时效、高成本、低可靠性向先进的少维护、长时效、低成本、高可靠性转变，提供了成功的技术途径，将在国内发、供、 用电各系统得到更广泛的推广应用。     

Solid residue formation of RTV silicone rubber due to dry-bandarcing and thermal decomposition

In this study, the solid residue of a RTVSR (room temperature vulcanized silicone rubber) prepared in a tracking test are analyzed. By using the techniques of KBr pellet FTIR (Fourier transformed infra-red) spectroscopy and XRD (X-ray diffractometry), cristobalite SiO₂, amorphous SiO₂, amorphous SiO₂.xH ₂O (namely, silica gel) and moissanite SiC are identified in the solid residue. The TG (thermogravimetry) and TG-DTA (differential thermal analysis) are applied to study the thermal characteristics of RTVSR and the concentration of carbon which contributes to tracking of polymers is evaluated. By comparison of the solid residue of the tracking test to thermally decomposed RTVSR in air and nitrogen, models of chemical changes during the formation of the solid residue are proposed. Furthermore, it is found that RTVSR has a excellent property not to form conductive carbon     

Influence of single and multiple environmental stresses on trackingand erosion of RTV silicone rubber

Single and multiple effects of UV, corona, thermal, water absorption and acid rain stresses on the tracking and erosion characteristics of room temperature vulcanized silicone rubber (RTV) are investigated. It is shown that corona stress and water absorption stress decrease the tracking and erosion resistance of RTV, while thermal stress and UV stress improve it. Synthetic acid rain, 500× stronger than actual rain in Japan, cannot induce the change in the tracking and erosion characteristics. RTV subjected to simultaneous multiple stresses also is evaluated. Significant variations in the tracking and erosion resistance appear. Chemical and morphological analysis for assessing the aging level is carried out by scanning electron microscope (SEM)-energy dispersive X-ray (EDX), attenuated total reflection (ATR)-Fourier transform infrared (FTIR) and differential scanning calorimetry (DSC). In this study, DSC plays an important role to detect boiling and combustion temperatures of byproducts affecting tracking and erosion. Noteworthy results are obtained in that the decrease in the resistance to tracking and erosion is related to the formation of byproducts that burn at low temperature (170 to 275°C)     

Role of the size of particles of alumina trihydrate filler on thelife of RTV silicone rubber coating

The paper reports on a study of the influence of the size of the particles of alumina trihydrate (ATH-Al₂O₃.3H₂O) filler on the life of room temperature vulcanised (RTV) silicone rubber coating in a salt-fog chamber. The particle sizes examined include 1.0, 4.5, 13, 17 and 75 μm. The optimum size to give the lowest leakage current and the longest time to failure of the coating is determined. The particle size affects the roughness of the coating. This is determined by a high resolution surface roughness tester and a scanning electron microscope (SEM) examination. The roughness is enhanced after prolonged testing in salt-fog. The leakage current affects the amount of silicone fluid on the surface. The amount of silicone fluid present on the surface after exposure to dry-band arcing in salt-fog is a function of the particle size. Measurements of surface roughness, the amount of silicone fluid on the surface and the leakage current combined with theoretical analysis of the heat of conduction lead to identification of the mechanisms by which the size of the ATH particle imparts resistance to tracking and erosion     

Properties of interfaces in silicone rubber

A salt fog test and an inclined plane test have been combined in order to trace changes in electrical and mechanical properties as well as in chemical stability of high temperature cured silicone rubber (HTV) samples containing interfaces. The interfaces were created in two different ways. The first technique, named adding/curing, was based on pressing together an uncured HTV silicone plate towards a piece of already cured one, followed by curing at atmospheric pressure and high temperature. The second type of interface was made by joining two pieces of cured HTV rubber by silicone glue (RTV-1). Thereafter both types of samples were divided in two parts, one of which was additionally exposed to UV-radiation before testing. The combined salt fog inclined plane test did not reveal significant differences in the development of leakage currents, in the change of surface roughness, or in the degree of surface oxidation in comparison to reference samples of cured HTV rubber without interfaces. At the same time the leakage currents became lower on the UV-exposed samples. It is suggested that the reduction in leakage currents resulted from a combination of increased surface roughness and increased hydrophobicity. Reduction of mechanical properties of UV-exposed samples was also observed. The tensile testing showed that samples containing the adding/curing interfaces maintained their mechanical strength after ageing while the strength of the glued samples was significantly reduced, indicating that the adding/curing system has better long-term mechanical stability compared to the glued system     

双组分室温硫化硅橡胶防污闪涂料的研制

研制了适合电力设备绝缘子使用的双组分室温硫化硅橡胶防污闪涂料,讨论了树脂基料、填料、助剂对涂料性能的影响,结果表明,双组分室温硫化硅橡胶涂料具有良好的耐电弧性、防污闪性和耐磨性。     

Fundamental and low frequency harmonic components of leakage current as a diagnostic tool to study aging of RTV and HTV silicone rubber in salt-fog

The paper presents the results of using the fundamental and the low frequency harmonic components of leakage current to study aging of silicone rubber in salt-fog. Experiments have been conducted on RTV and HTV coated rods at different fields (0.25-0.6 kV/cm) and conductivities (1000 to 2500 /spl mu/S/cm). The onset of dry-band arcing on samples could be determined by measuring the low frequency harmonic components. A correlation has been found between the fundamental and low harmonic components of leakage current and different forms of aging. Where erosion could be associated with an increase in the level of both the fundamental and low frequency harmonic components of leakage current. For example, surface damage for HTV rods occurred when the fundamental component of leakage current was greater than 2 mA. On the other hand, when the samples approached failure, the fundamental component of leakage current reached relatively high values ( > 6 mA for HTV rods and > 2 mA for RTV rods) and the low frequency harmonic components of the leakage current tended to decrease. The results suggest that both the fundamental and low frequency harmonics of leakage current can be used as a tool to determine both the beginning of aging and end of life of silicone rubber in salt-fog.     

纳米材料在硅橡胶改性中的应用

在26篇文献的基础上，综述了Al203、CaC03、TiO2、Si02等纳米粒子对硅橡胶的力学性能、阻隔性能、抗老化性能、电气性能等方面的影响。评价了硅橡胶纳米复合材料的工业价值，并指出了其发展趋势。     

Erosion resistance of nano-filled silicone rubber

The paper presents the experimental results obtained on the erosion resistance of silicone rubber (SIR) filled with 12 nm size fumed silica (nano filler) to those filled with 5 /spl mu/m size silica filler (micro filler). The ASTM 2303 inclined plane tracking and erosion test was used in the comparison as well as an infrared laser as the source of heat to erode the SIR samples. The erosion resistance of the SIR materials increased with increasing percentage of the fillers, and it was observed that 10% by weight of nano-filled SIR gives a performance that is similar to that obtained with 50% by weight of micro-filled SIR. The low frequency components of leakage current and the eroded mass are used to evaluate the relative erosion resistance of the composites and the third harmonic component of the leakage current shows good correlation to the measured eroded mass. The paper discusses the possible reasons for the improvement in the erosion resistance of nano-filled silicone composites.     

Aging in silicone rubber used for outdoor insulation

A hypothesis for aging produced by dry band arcing in silicone rubber material used for outdoor insulation is presented and experimentally proven. Aging is indicated by permanent changes. The analytical techniques used to study the permanent changes are Fourier transform infrared (FTIR) spectroscopy, energy dispersive X-ray (EDX) analysis, X-ray diffraction (XRD), and surface roughness measurement. The authors highlight the fact that there are permanent changes occurring in the material that lead to progressive degradation in the long run, even though there can be a complete recovery of surface hydrophobicity in a short time     

硅脂环境下硅橡胶的吸收特性及其关键电气性能研究

电缆附件在长期运行过程中,其界面硅脂与绝缘硅橡胶直接接触使其逐渐被硅橡胶吸收,导致硅橡胶性能下降,甚至引发电缆附件界面问题。文中以附件绝缘硅橡胶吸收硅脂的增重试验为基础,研究不同温度下硅脂/硅油在硅橡胶中的扩散规律并获得相应的扩散参数;研究硅脂对硅橡胶体积电阻率、相对介电常数、介质损耗角正切值和工频击穿场强等电气性能的影响规律。结果表明,不同温度下硅脂/硅油在硅橡胶中的扩散规律服从Langmuir扩散模型;硅橡胶对硅脂/硅油的吸收能力随温度升高而增强,且硅橡胶内硅油分子的脱附过程更强;随着硅橡胶对硅脂吸收量的增加,硅橡胶的体积电阻率逐渐降低,相对介电常数和介质损耗角正切值逐渐增加,但其工频击穿场强呈先增大后减小的趋势。     

On the breakdown strength of silicone rubber

Polymeric materials are an alternative to porcelain and glass regarding outdoor HV insulators. Silicone rubber appears to be one of the main candidates to replace porcelain and glass. Although much work has already been done regarding the behavior of complete polymeric insulators in the laboratory and in the field, relatively little is known with respect to the breakdown strength of silicone rubber. In this paper the breakdown strength of silicone rubber with respect to gap spacing and the electrical behavior of built-in interfaces inside the silicone rubber are investigated     

Effects of added silicone oils on the surface characteristics of silicone rubber

This paper reports on the effects of silicone fluids, used as processing agents, on the loss and recovery of hydrophobicity. The recovery of hydrophobicity was evaluated by the measuring the contact angle, the surface electrical resistance and the leakage current on a silicone rubber (SIR) surface. Two types of silicone fluids (A and B) having different chemical structures were selected from consideration of hydrophobicity and processability. SIR specimens were exposed to corona discharges in air and were analyzed using contact angle and surface resistance measurements. It was observed that the contact angle and the surface resistance increased gradually with increasing test time. The recovery of the hydrophobicity expressed as the contact angle and the surface resistance increased with increasing ratio of fluids A/B. The tracking and erosion resistances of SIR were investigated using the rotating wheel dip test. The leakage current of SIR decreased with increasing ratio of fluids A/B.     

Contamination performance of silicone rubber cable terminations

The authors describe the results of a systematic study of the performance of silicone rubber terminations under contaminated conditions. The terminations evaluated were of the 15 kV class cold shrink type for concentric neutral cables. They were evaluated in a fog chamber where the electric stress, water conductivity, water flow rate, and the deenergization time between subsequent fog chamber exposures were varied in order to alter the test severity. Changes in the surface hydrophobicity were monitored periodically by analyzing the samples using ESCA (electron spectroscopy for chemical analysis), SEM (scanning electron microscopy), and contact angle measuring techniques. The results indicate that the experimental conditions play a significant role in the outcome of the tests. It was demonstrated that the recovery of hydrophobicity can be studied by using several techniques     

抗静电硅橡胶泡沫材料的研制

通过向硅橡胶中加入15％处理过的乙炔碳黑,在一定的工艺条件下,制备出了新型抗静电硅橡胶泡沫材料,并对其微观结构、电学性能和力学性能进行了表征。研究结果表明,所制备的抗静电硅橡胶泡沫材料的承力结构完整,泡孔呈规则的球形,孔径分布均匀。与未添加乙炔碳黑的硅橡胶泡沫材料相比,抗静电硅橡胶泡沫材料的密度有所增大,拉伸性能有所下降,但压缩性能变化不大,其表面电阻率已经达到消除静电的技术指标。