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Surface Potential Decay and Hydrophobicity of Three Liquid Silicone Rubbers Before and After Direct Fluorination
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Superhydrophobic materials have great application potential in the insulation equipment of power systems because their excellent water-repelling and self-cleaning abilities maintain clean and dry surfaces. However, the electrical performance of superhydrophobic materials has been ignored for a long time; surface charge accumulation is a significant problem when they are used in direct-current power systems. A scheme that concurrently optimizes the surface wettability and electrical characteristics of insulating materials is urgently needed. Herein, a simple method for designing superhydrophobic coatings using ZnO and polydimethylsiloxane (PDMS) is proposed. Silicone rubber is chosen as the experimental material because of its wide application in insulators. Contact angles greater than 160° and sliding angles smaller than 2° are achieved. In addition, the coating exhibits excellent optimization of the surface charge distribution. Compared with pristine silicone rubber, the coated silicone rubber has a more uniform charge distribution, lower initial charge accumulation, and faster charge dissipation. The trap parameters and conductivity are used to explain the mechanism responsible for this phenomenon. The combination of superhydrophilicity and surface charge optimization can provide new insights for improving the performance of insulation materials in power systems.
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This paper investigates the effect of ground and fumed silica fillers on suppressing DC erosion in silicone rubber. Fumed silica and ground silica fillers are incorporated in silicone rubber at different loading levels and comparatively analyzed in this study. Outcomes of the +DC inclined plane tracking erosion test indicate a better erosion performance for the fumed silica filled composite despite having a lower thermal conductivity compared to the ground silica composite. Results of the simultaneous thermogravimetric and thermal differential analyses are correlated with inclined plane tracking erosion test outcomes suggesting that fumed silica suppresses depolymerization and promotes radical based crosslinking in silicone rubber. This finding is evident as higher residue is obtained with the fumed silica filler despite being filled at a significantly lower loading level compared to ground silica. The surface residue morphology obtained, and the roughness determined for the tested samples of the composites in the dry-arc resistance test indicate the formation of a coherent residue with the fumed silica filled composite. Such coherent residue could act as a barrier to shield the unaffected material underneath the damaged surface during dry-band arcing, thereby preventing progressive erosion. The outcomes of this study suggest a significant role for fumed silica promoting more interactions with silicone rubber to suppress DC erosion compared to ground silica fillers.
Silicone rubber (SIR) samples are fluorinated using a F2/N2 mixture at different temperatures. Physicochemical characteristics are analyzed by attenuated total reflection infrared spectroscopy, X‐ray photoelectron spectroscopy, and scanning electron microscopy. Basic properties of the fluorinated surface layers are investigated, and the intrinsic alternating current (AC) flashover performance and tracking resistance of the surface fluorinated samples are evaluated. The flashover test results indicate that the fluorination leads to an improvement in AC flashover performance of the SIR material, as a result of the competition between the favorable effect of the increase in surface conduction and the adverse effect of a likely increase in surface layer permittivity. The increase in AC flashover voltage is associated with the voltage‐increasing mode of the flashover test. The flashover test results also show an improved tracking resistance of the SIR material by the fluorination, and the improvement is especially significant for the fluorinated surface layer that has less cleavage of SiC bonds or a good compactness as well as a large thickness. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020, 137, 48556.
In order to investigate the effect of fluorination temperature on the resistance to corona discharge of HTV silicone rubber, HTV silicone rubber samples were fluorinated using a F
2
/N
2
mixture at temperatures of 25, 55, and 85 °C for 30 min. The surface fluorinated and unfluorinated (virgin) samples were simultaneously exposed to corona discharge generated by a multi-needle-to-plate electrode system. ATR-IR analysis and SEM observation indicate that the virgin surface layer is severely oxidized and degraded by corona discharge. In contrast with this, the surface layer fluorinated at 85 °C does not show obvious changes in the chemistry, surface morphology and thickness after corona exposure. However, the corona exposure led to definite changes in both the chemistry and surface morphology of the surface layer fluorinated at 55 °C, and its corona-resistance property appears to be worse than the surface layer fluorinated at 25 °C. The abnormal effect of fluorination temperature on corona resistance is explained by unusual changes of surface physicochemical characteristics with fluorination temperature. Measurements of contact angle show a much higher water contact angle on the fluorinated surfaces than the virgin surface before corona exposure, an initial significant decrease in water contact angle for all the surfaces just after the end of the corona exposure, and the subsequent recovery of contact angle only for the exposed virgin surface. Measurements of surface potential reveal that the potential decay rate obviously increases for the virgin sample after corona exposure, while the decay rate decreases rather than increases for the fluorinated samples after the same exposure. There is no significant difference in surface properties between the fluorinated samples before or after corona exposure. This is considered to be due to the combined effect of compositional and structural changes and the fact that the surface properties are very sensitive to the top several molecular layers.
A systematic study to understand how alumina tri-hydrate (ATH) and silica fillers improve the erosion resistance of silicone rubber during dry band arcing showed that the thermal conductivity of the resulting composite material is the main criterion governing material erosion. The thermal conductivity of the composite material is dependent on the thermal conductivity, concentration, particle size, and bonding of the filler particles to the silicone matrix. In this context, either filler can be shown to perform better than the other, depending on the formulation, in the ASTM inclined plane tracking and erosion test. Therefore, the industry perception that ATH filler imports better erosion resistance than silica in silicone rubber can be misleading. The release of water of hydration from ATH appears to have a secondary effect that may be more relevant in silicone compositions having a low concentration of a filler.
In order to provide further evidence for the improvement in tracking and erosion resistance of alumina trihydrate (ATH)-free silicone rubbers (SRs) by direct fluorination and to explore the improvement mechanisms, the ATH-free SR samples containing 15-parts per hundred parts of rubber by weight (phr) melamine cyanurate and 18-phr fumed silica were prepared, and then they were surface fluorinated using a
mixture at 0.1 MPa and 85 °C for 120 min. Inclined plane tracking and erosion tests indicate that at 4.5-kV rms ac, all 15 fluorinated samples passed the test, while 7 of the 15 unfluorinated samples failed the test due to combustion. Analyses of erosion depth, length, and area show that the fluorinated sample, compared to the unfluorinated sample, has a larger average erosion depth but a smaller average erosion length or area. Attenuated total reflection infrared spectroscopy (ATR-IR) and X-ray photoelectron spectroscopy (XPS) analyses indicate that both the SR matrix and the fillers of melamine cyanurate and fumed silica in the surface layer were fluorinated, forming the Si–F, C–F, and N–F bonds. Scanning electron microscopy (SEM) observations show that the fluorinated layer has a thickness of
and a rough surface, and the fillers are uniformly dispersed in the matrix. The resistance improvement by the fluorination is attributed to these changes of the surface layer in composition, structure, and interface of the SR matrix and the fillers and is also influenced by the change in surface morphology.
This paper presents a comparative evaluation of high temperature vulcanizing (HTV) silicone rubber consisting of ATH and silica fillers for use as the housing of outdoor composite insulators. Various volume filler fractions (vol%) of alumina trihydrate (ATH) and silica (SiO
2
) and several median particle sizes are incorporated into HTV silicone rubber and evaluated for erosion and tracking resistances using the inclined-plane tracking and erosion tests (IPT), in accordance with IEC 60587 at several AC test voltages. Test durations, eroded masses, erosion depths, and leakage currents are reported. The dry-band arcing temperatures of the various samples are recorded during the tests. The results show that at 30 vol% filler fractions, ATH filled samples show lower erosion than the corresponding silica filled samples; however, at 15 vol% fraction, the silica filled samples show lower erosion. For most tested conditions, the maximum dry-band temperatures of the silica filled samples are lower than the corresponding ATH filled samples.
Instability of SiF bonds in fluorinated silicon oxide (SiOF) films is studied. Al wiring corrosion and underlayer SiO2 etching problems are the major issues for the use of SiOF interlayer dielectric films. To clarify the mechanism, three kinds of SiOF films have been used for this study. They are: (i) a fluorinated silicon oxide (SiOF) film prepared by room-temperature chemical vapour deposition (RTCVD) using fluorotriethoxysilane and pure water as gas sources; (ii) a fluorinated spin-on-glass (SOG) film prepared by fluorotrialkoxysilane vapor treatment (FAST); and (iii) a room-temperature liquid phase deposition (LPD) SiOF film. The initial refractive indices for the RTCVD-SiOF, FAST-SOG and LPD-SiOF films are 1.400, 1.398 and 1.433, respectively. After conducting a pressure cooker test (PCT) at 125 °C for 520 h, the refractive indices for the RTCVD-SiOF, FAST-SOG and LPD-SiOF films increase to 1.450,1.440 and 1.436, respectively. The SiO bond peak absorption coefficient for the LPD-SiOF film decreases at the early stage of PCT, but those for the RTCVD-SiOF and FAST-SOG films increase at the early stage of PCT. The initial SiF bond peak absorption coefficient for the RTCVD-SiOF film is much higher than those for the LPD-SiOF and FAST-SOG films. It decreases drastically in the PCT time ranging from 0 to 140 h. The SiF bond peak absorption coefficients for the FAST-SOG and LPD-SiOF films show a slow reduction, as compared with that for the RTCVD-SiOF film at the early stage of PCT. Although the OH peak absorption coefficients for the RTCVD-SiOF and FAST-SOG films increase at the early stage of PCT and level off at 50 h, that for the LPD-SiOF film increases at 306 h. After conducting 520 h PCT, concentrations of fluorine atoms for the RTCVD-SiOF and FAST-SOG films decrease by three orders and two orders of magnitudes, respectively. However, the LPD-SiOF film has a limited change in the fluorine concentration, as compared with those for the RTCVD-SiOF and FAST-SOG films. The thicknesses for all of the films remain almost unchanged after PCT for 520 h.
Surface modification of poly(dimethylsiloxane) (PDMS) was carried out via CF4 plasma treatment. The test PDMS used contains significant amounts of quartz and silica fillers, while the control material is the same PDMS with quartz removed by centrifugation. Fluorination accompanied with roughening was produced on both PDMS surfaces. With short plasma times (15 min or less), a macromolecular fluorocarbon layer was formed on the PDMS surfaces because of the dominant fluorination, leading to significant increase in F concentration, decrease of surface energy, and some roughening. With intermediate plasma times (15-30 min), dynamic balance between fluorination and ablation was achieved, leading to a plateau of the surface roughness, fluorine content, and [F-Si]/[F-C] ratio. At our longest investigated plasma time of 45 min, the plasma ablated the fluorinated covering layer on the PDMS surfaces, leading to significant increase in roughness and [F-Si]/[F-C] ratio and decrease of surface F concentration. The effect of additional quartz in the test PDMS on surface F concentration, [F-Si]/[F-C] ratio, and roughness was dramatic only when ablation was significant (i.e., 45 min). The obtained Teflon-like surface displays long-term stability as opposed to hydrophobic recovery of other plasma-treated PDMS surfaces to increase hydrophilicity. On the basis of the optimized plasma treatment time of 15 min, a microstructured PDMS mold was plasma treated and successfully used for multiple high-aspect-ratio (about 8) UV embossing of nonpolar polypropylene glycol diacrylate (PPGDA) resin.