High Temperature Vulcanized Silicone Rubber Research Articles

Cover Image, Volume 139, Issue 17

Parambalath Narendran Ashitha designed the cover image and, along with colleagues at the Central Power Research Institute, wrote the accompanying article on how the successful deployment of nanofillers in hightemperature vulcanizing silicone rubber matrix requires chemical surface treatments. The authors utilized a facile technique employing the use of an in-situ modification of filler surfaces. In this work, the importance of judicious selection of dispersing aid is shown through the comparison of properties of silane coupling agent KH-570 and non-ionic surfactant Triton-X-100 dispersed composites. The enhanced interaction in the presence of KH-570 and the subsequent reduced arching, generation of plasma, and erosion during the dry arc resistance test at 12.50 kV a.c. is illustrated by the formation of thermal pathways or bridges. DOI: 10.1002/app.52102

Study on aging index of mechanical and electrical properties of high temperature vulcanized silicone rubber

Composite insulators are affected by various factors in operation. With the increase of operation time, the shed of silicone rubber composite insulators will become hard and brittle. In this paper, it is found that the hardness of long-term silicone rubber shed meets the standard requirements. The tear strength, tensile strength, elongation at break, resistance to electric tracking and corrosion loss decrease significantly after 5-10 years of operation. The decline of silicone content on the micro surface and the escape of inorganic fillers are the reasons for the deterioration of macro performance. Finally, the evaluation system for aging performance of insulators is preliminarily established, according to the aging degree of insulator, operation and maintenance suggestions are proposed.

Mechanical Properties of High-Temperature Vulcanized Silicone Rubber Under Acid-Fog With AC Energized

Mechanical Properties of High-Temperature Vulcanized Silicone Rubber Under Acid-Fog With AC Energized

On the icephobicity of damage-tolerant superhydrophobic bulk nanocomposites.

To address the increase in demand for superhydrophobic and icephobic surfaces with greater mechanical robustness, we fabricated damage-tolerant, abrasion-insensitive, and icephobic superhydrophobic bulk nanocomposites using a facile, cost-effective, industrially applicable, and environmentally benign strategy. We prepared nanocomposites composed of high-temperature vulcanized silicone rubber through the highly controlled incorporation of nanosized fumed silica and microsized aluminum trihydrate particles. The produced nanocomposites did not require additional processing, such as sand abrasion or plasma treatment, to acquire their superhydrophobic properties. The extended roughness throughout the whole bulk of the nanocomposites imparted the volumetric superhydrophobicity and resistance to mechanical damage. The presence of micro-nanoparticles also enhanced the thermal stability and icephobic properties of the silicone rubber. The icephobic behavior of the developed nanocomposites was assessed based on freezing delay and push-off tests both of which denoted improved icephobic properties, i.e., high freezing delay time and low ice adhesion strength. We verified the extended duration of superhydrophobicity within the bulk nanocomposite using sandpaper abrasion, severe cutter scratching, tape peeling, and water-jet impacts. This study represents the first evaluation, to the best of our knowledge, of the icephobic properties of both the surface and bulk of the produced nanocomposite subjected to several cycles of sandpaper abrasion. Interestingly, even after multiple abrasion cycles, the samples demonstrated considerably low ice adhesion strength confirming their bulk icephobicity. In a nutshell, our findings are very promising for the fabrication of mechanically robust icephobic materials.

Investigation of high temperature vulcanized and room temperature vulcanized silicone rubber based on flexible piezo‐electric energy harvesting applications with multi‐walled carbon nanotube reinforced composites

AbstractFlexible energy harvester can be able to produce continuous voltage output under the mechanical deformations. High temperature vulcanized (HTV) and room temperature vulcanized (RTV) silicone rubbers are used for making the flexible piezo‐electric energy harvester. A total of 1 and 2 phr of multi‐walled carbon nanotube (MWCNT) are used as the electrode material mixed with the rubber solution for the preparation of flexible electrode. Cyclic loading machine is used for the application of 2 Hz of bi‐axial load and uniaxial compression load in the specimen. One centimeter depth of displacement is applied during the bi‐axial cyclic loading and 4 mm amplitude of displacement is applied during the compression loading. Tensile and compression tests are performed to determine the engineering properties of the material. Tensile load withstanding value of the silicone rubber is increasing as increasing the MWCNT filler in the rubber. Tensile value of the pure HTV silicone rubber is 10.020 N and for 2 phr MWCNT reinforced rubber is 22.106 N. Compressive withstanding load of pure HTV rubber is 189.565 N and for 2 phr MWCNT reinforced rubber is 287.066 N. HTV silicone rubber is the highly stiffness material on compares with RTV rubber based on the tensile and the compressive value. Transmission electron microscopy, optical micrographs, and scanning electron microscope show the morphology of MWCNT and surface image of specimen. A total of 2 mm substrate based 2 phr MWCNT reinforced HTV flexible energy harvester undergo decrease in voltage during the cyclic loading due to breakage of electrode. A total of 2 phr MWCNT electrode‐based sample can be able to produce more output voltage than 1 phr MWCNT electrode‐based sample during bi‐axial and compression loading. More voltage is produced in the compression specimen than bi‐axial one.

Terahertz spectroscopic study of optical and dielectric properties of typical electrical insulation materials

Researchers and engineers are constantly developing novel techniques to accurately measure the electromagnetic properties of electrical insulators. These properties provide useful information about the insulators' performance and aging degree, which may help users to take necessary actions to prevent potential risks to the safety and stability of electric power systems. In this study, terahertz (THz) time-domain spectroscopy (THz-TDS) is used to analyze the spectral absorption and dielectric dispersion of five electrical insulation materials viz. epoxy resin (PER), E-glass fiber-reinforced polymer-matrix composite (GFRP), cross-linked polyethylene (XLPE), electrical porcelain (E-PRL), and high temperature vulcanized silicone rubber (HTVSR), in the THz frequency range (0.1 THz-1.8 THz). The refractive index, absorption coefficient, and complex relative permittivity of each sample are experimentally measured using the THz-TDS system and theoretically calculated using the original Debye model (i.e., based on the relaxation process of the individual samples’ dipoles). Experimental and theoretical results of the refractive indices and dielectric constants of the individual samples are highly correlated with 0.22% and 0.17% aggregate maximum errors, respectively. The values of dielectric constants and imaginary permittivities are found to be the highest and the lowest for E-PRL and XLPE, respectively. In terms of dielectric performance, XLPE exhibits the highest and most stable dielectric performance, followed by the E-PRL, whereas PER, HTVSR, and GFRP exhibit fluctuating and slightly less stable dielectric performance. This study shows the great potential of THz nondestructive testing (NDT) and structural health monitoring (SHM) for the defect detection and service life prediction of electrical insulators.

Viability of Fillers in HTV Silicone Rubber in the AC and DC Inclined Plane Tests

The viability of fillers for inhibiting the dry-band arcing damage on high temperature vulcanized silicone rubber under the critical AC and the equivalent DC voltages of inclined plane test is investigated. Alumina tri-hydrate and ground silica are employed in the study as common fillers employed in silicone rubber formulation for outdoor high voltage insulation, and thermal analysis is applied in order to understand the protective mechanisms of the fillers. Filler volume effect plays a role in improving the erosion performance, and the dilution effect of water of hydration is shown advantageous for preventing progressive (directed) erosion. Dilution effect promoted with internal oxidation is found only notable at 50 wt% alumina tri-hydrate in this study. On the other hand, alumina tri-hydrate at 10 or 30 wt% may yield a porous residue, which is conducive to thermo-oxidation (combustion) and consequently erosion. The erosion suppression effects reported for the fillers in the composites employed in this study are found less viable under the equivalent DC as compared to the critical AC voltages of the inclined plane test. This study clarifies common perceptions about the viability of fillers in silicone rubber for AC and DC outdoor insulation applications.

Effect of Acidic-fog on performances of HTV silicone rubber with AC energized

The performance of high temperature vulcanized (HTV) silicone rubber used in composite insulator will be worse in harsh environment. The acid-base pollutants in the environment have a great influence on the performance of silicone rubber. As one of the aging factors of composite insulators, acid corrosion has not been studied deeply. In this paper, the AC charged aging test of the silicone rubber sample in an acid environment was carried out, and then its physicochemical and electrical properties were tested, including microscopic morphology, hydrophobicity, infrared spectroscopy, and electrical strength. The effect trends of duration time, charged voltage and acid pH value on material properties are studied. The results show that under the combined action of acid-fog and voltage, the performance of silicone rubber will be degraded. The smooth structure of the surface of the material is destroyed, the side chains of the polymer are severely cut, and the hydrophobicity becomes worse. The AC flashover voltage along the surface has a power function relationship with the pH value. Continuous deterioration of composite insulators may occur in acid polluted areas for a long time.

Understanding Variations in the Tracking and Erosion Performance of HTV-SR-Based Composites due to AC-Stressed Aging

Among the polymeric family, high-temperature-vulcanized silicone rubber (HTV-SR) is the most deployed material for high voltage insulation applications. However, in an outdoor environment, due to contamination and wetting-induced dry band arcing, consequently SR experiences surface tracking and erosion. From a practical standpoint, the tracking and erosion performance under multi-stress aging is required to be known. It is in that context that the present study was undertaken to measure and analyze the effect of multi-stress aging on tracking and erosion performance. Composite samples of SR having different filler concentrations of silica and alumina trihydroxide (ATH) were aged in a multi-stress chamber for a period of 5000 h, and after that their electrical tracking performance was studied. Simultaneously, unaged samples were also exposed to tracking test for comparison. To conduct this test, the inclined plane testing technique was used in accordance with IEC-60587. All samples exposed to tracking test were analyzed using different diagnostic and measuring techniques involving surface leakage current measurement, Fourier transform infrared spectroscopy (FTIR), thermal stability and hydrophobicity classification. Experimental results shown that the tracking lifetime increased through incorporation of silica and ATH fillers in the SR. Amongst all test samples, two samples designated as filled with 2% nano silica and 20% micro silica/ATH exhibited greater resistance to tracking. This was attributed to the optimum loading as well as better dispersion of the fillers in the polymer matrix. The presence of nano-silica enhanced time-to-tracking failure, owing to both improved thermal stability and enhanced shielding effect on the surface of nanocomposite insulators.

A Proposal for the Evaluation of HTV Silicone Rubber Composite Insulators.

This paper describes in detail a step-by-step methodology for obtaining specimens from the housing material (high temperature vulcanized (HTV) silicone rubber with aluminum trihydrate (ATH) filler) of composite insulators (finished products), for five well known and commonly used tests. The aim of the paper is to render practical clarifications and additions to the instructions for five tests on composite insulators provided by international standards. Additionally, the ranges of the results of these tests are presented. More specifically, shore A hardness measurement, tensile strength and elongation at break test, tear strength test, density measurement and inclined plane test were conducted on the housing material of ten, new, unaged medium voltage composite insulators made by six different manufacturers. The results of these tests are presented as a contribution to the existing knowledge and a comparative study of corresponding results of previous investigations is performed. The presented procedures for specimens’ preparation, as well as the results (arithmetical ranges), could be used as guidelines for future testing on the housing material (HTV silicone rubber with ATH filler) of composite insulators, either by researchers and manufacturers, during laboratory testing and material development processes, or by customers (distribution and transmission networks owners), during batch acceptance tests.

Effect of ketjenblack and barium titanate on the piezoresistive behaviour of silicone rubber particulate composites

In the present study, silicone rubber reinforced with ketjenblack and barium titanate were fabricated using high temperature compression molding method. These particulate composites are characterized for piezoresistive sensitivity. Ketjenblack and barium titanate are the conducting and dielectric fillers in the high temperature vulcanized silicone rubber matrix. The effects of ketjenblack and barium titanate fillers, filler loading, amount of curing agent, curing temperature and mixing time on the piezoresistive properties of these composites were investigated. The piezoresistive sensitivity recorded maximum values of 3.7(10–3) (kPa)−1 for dielectric and dielectric-conductive composites. The linear variation of normalized resistance change was observed for change in pressure up to 20 kPa. The dielectric composite sensitivity increases with reduction in dielectric filler and curing agent loadings, increasing with mixing time and curing temperature. The piezoresistive sensitivity for dielectric-conductive composites depends on ketjenblack and barium titanate filler loading. These composites demonstrate interaction effects among the factors.

Spectroscopic, Mechanical and Electrical Studies on Surfactant Modified HTV-SiR/Nano Alumina Composites

Using an experimental approach, the effectiveness of nonionic surfactant, Triton™-X-100 in dispersing nano Alumina in high temperature vulcanized silicone rubber are studied in detail. Nanocomposites are prepared in varying concentrations of 0, 0.01, 0.05, 0.2, 1, and 5 phr levels by weight of the surfactant. Evidence to moderate dispersion of nano Alumina for composites at 0.01 phr and 0.05 phr weight loadings is gained from SEM pictures. Structural changes observed for the composites in FTIR spectra are examined and peaks identified. Thermogravimetric analysis shows some improvement in thermal resistance. However, an adverse effect on properties, such as volume resistivity, surface resistivity and dissipation factor are observed even at low surfactant loadings. A plausible role of surfactant as a plasticizer is explained with respect to the results of tensile strength. From this work, it can be inferred that surfactant at 0.01 phr may be taken as an optimum loading concentration for a desirable level of electrical properties, without compromising on insulation characteristics of volume resistivity and dissipation factor. Nonetheless, utmost caution needs to be exercised while employing Triton™-X-100 in compositions prepared for insulation applications because of inadequacies in values of volume resistivity, surface resistivity, dissipation factor and dry arc resistance even at low loadings.

Investigation of 9000 hours multi-stress aging effects on High-Temperature Vulcanized Silicone Rubber with silica (nano/micro) filler hybrid composite insulator.

Degradation in the polymeric insulators is caused due to the environmental stresses. The main aim of this paper is to explore the improved aging characteristics of hybrid samples by adding nano/micro silica in High Temperature Vulcanized Silicone Rubber (HTV-SiR) under long term accelerated aging conditions for 9000 hours. As HTV-SiR is unable to sustain environmental stresses for a long time, thus a long term accelerated aging behavior is an important phenomenon to be considered for field application. The aging characteristics of nano/micro filled HTV-SiR are analyzed by using techniques such as Scanning Electron Microscopy (SEM), Leakage Current (LC), Fourier Transform Infrared Microscopy (FTIR), Hydrophobicity Classification (HC), and breakdown strength for the aging time of 9000 hours. FTIR and leakage currents are measured after every cycle. All the co-filled samples revealed escalated aging characteristics as compared to the neat sample except the SN8 sample (8% nano-silica+20% micro-silica) after 9000 hours of aging. The highest loading of 6% and 8% nano-silica with 20% micro-silica do not contribute to the improved performance when compared with the neat and hybrid samples. However, from the critical experimental analysis, it is deduced that SN2 sample (2% nano-silica+20% micro-silica) is highly resistant to the long term accelerated aging conditions. SN2 has no cracks, lower loss percentages in the important FTIR absorption peaks, higher breakdown strength and superior HC after aging as compared to the unfilled and hybrid samples.

Design a new thermoelectric module with high practicability based on experimental measurement

In practical applications, the thermoelectric module (TEM) conducts continuous heat exchange with the external environment. This causes uneven temperature distribution inside the module and some PN legs do not work effectively. The internal heat transfer direction of the TEM changes under the influence of the environment, resulting in energy loss and an inability to transfer heat in a consistent direction. In this study, the actual working environment of the TEM was simulated by a constant temperature heating platform, and we analyzed the heat transfer characteristics of the module using numerical calculation methods. Then, according to the results of this analysis, a sealed thermoelectric module (STEM) filled with high-temperature vulcanized silicone rubber (HTV) was designed. Besides, to improve the practicality of the STEM, we designed an integrated thermoelectric (TE) plate. The TE plate was equipped with a controllable water-cooling device, which allowed adjustments of the TEM cold end temperature according to the heat source to ensure maximum output power. Results indicated that the STEM can reduce heat exchange behavior at the edges and ensure uniform temperature distribution throughout the module. The maximum temperature gradient at the cold end of the STEM is 4.39 °C, which is 5.27 °C lower than the equivalent value of the TEM. Therefore, the temperature gradient in each of the PN legs in the STEM is similar, which can reduce the electrical loss generated by itself. We implemented a closed structure on the TE plate, and the internal circuit was installed on the contact surface of the water-cooling device to ensure that the circuit was not damaged due to the influence of the thermal environment. When the heat source temperature was 200 °C, the TE plate adopted a two-stage water-cooling mode to deliver a maximum output power of 78.64 W, which equates to a power generation per unit area of 0.86 kW·h/m2. The TEM structure optimization and TE plate design scheme proposed in this study provides an efficient solution for the practical application of TE technology.

Cracking mechanism and degradation performances of HTV silicone rubber with interfacial defects under acid and thermal stress

High temperature vulcanized (HTV) silicone rubber (SIR), as the sheath of composite insulators, is widely used as the insulation device in transmission lines due to their excellent hydrophobicity and pollution flashover resistance. However, the interfacial defects of insulators and severe operating environment will accelerate the aging of HTV SIR sheath, seriously affecting the operation reliability of electrical equipment. Herein, the samples designed with different sizes of interfacial defects are aged under water, sulfuric acid and nitric acid at 80 °C, respectively. The results show that a large number of cracks appear on the surface of HTV SIR samples with interfacial defects after nitric acid treatment at 80 °C for more than 2 days, while samples with no interfacial defects aged under same condition for 8 days maintain unchanged. According to the results of the micro morphology of the sample, the crack is caused by the interfacial stress of the sample. In addition, after nitric acid treatment, the thermal stability, tensile and dielectric properties of HTV SIR with interfacial defects decreased significantly. The initial decomposition temperature decreases by about 30 °C, while the residual weight increases by about 2.5%. Meanwhile, the tensile strength and fracture strain decrease from 4.58 MPa and 470% to 2.07 MPa and 130% respectively, proving that cracks caused by interfacial defects can critically avianize the mechanical properties and thermal stability of HTV SIR. Also, there is an extra dielectric loss peak appearing in the frequency range of 102–104 Hz, which attributed to water invading and the MWS polarization at cracks. By aging HTV SIR with artificial interfacial defects under thermal and acid stress, the HTV SIR cracking phenomenon of composite insulators under severe operating environment is successfully simulated. This work can provide theoretical support ulteriorly for studying the cracking mechanism and the corresponding aging characteristics of HTV SIR used in composite insulators.

Comparative Properties of HTV Silicone Rubber for Composite Insulators—ATH and Silica Fillers

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 (SiO2) 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.

Effect of AC stressed aging on partial discharge, thermal and tensile performance of silicone rubber-based composites

In this paper, partial discharge characteristics of the multi-stressed aged and un-aged high temperature vulcanized silicone rubber (HTV-SR) samples with several concentrations of alumina trihydrate (ATH) and silica are studied. This is to elucidate whether the addition of nano/micro fillers of silica and ATH in silicone rubber enhances the resistance against partial discharges and aging behavior under different weathering conditions. Further, performance assessment is made by thermogravimetric analysis, differential scanning calorimetry and through measurement of tensile strength. Using IEC (b) electrode system, partial discharge measurements are performed according to IEC 60270. From these experimental measurements, it has been observed that by adding filler contents, the multi-stress aging performance improved and partial discharge resistance is enhanced up to 40% as compared to the HTV-SR without any filler. Moreover, through analysis of data obtained by thermogravimetric, indicate that nano doping increases the physical and chemical cross-linking points which confine the motion at the particle-matrix interface. As a result, the enhanced thermal stability of the composites. Considerable improvement in the stress-strain behavior of test samples is also observed with the addition of silica and ATH.

Influence of non ionic surfactant on electrical and hydrophobic properties of binary composites of silicone rubber

In this study, effects of a non ionic surfactant, Triton TM -X-100 on various properties of binary composites formulated with nano Alumina and high temperature vulcanised silicone rubber is studied. Triton TM -X-100 was added in varying concentration by weight, in ranges of 0 phr, 0.2 phr, 1 phr and 5 phr. Analysis of properties of dielectric constant and dissipation factor at power frequency showed stark increments in magnitues. Changes to hydrophobic behaviour was analysed by measuring static contact angle variations. A reduction in static contact angle measurement by 36 0 was observed at an addition of 0.2 phr of the surfactant. Spectroscopic analysis using FTIR was also studied. Results indicates to a modification of interaction between nano fillers and base silicone rubber upon addition of Triton TM -X-100. The changes in property values, points to an infirm relation between the nano fillers and base matrix.

Wettability behavior and anti-icing property of superhydrophobic coating on HTV silicone rubber

Composite insulators, which use high temperature vulcanized (HTV) silicone rubber as shed material, are widely applied in transmission lines. Ice accumulation on their surfaces may inflict flashover accident and even massive power outage. In this study, a superhydrophobic (SH) coating on HTV silicone rubber was fabricated by the sol–gel process combined with the plasma jet treatment method. It was found that the as-prepared SH coating exhibited prominent superhydrophobicity and an excellent self-cleaning property with a water contact angle of 160.15°, a contact angle hysteresis of 0.60°, a sliding angle of 1.8°, and a surface free energy of 0.1421 mN/m. The anti-icing behavior of water droplets on the as-prepared SH coating surface was investigated at a low temperature of −30 °C and compared with that of the HTV silicone rubber surface. The results indicated that the freezing time on the SH coating was postponed obviously and was as long as 150 s. The SH coating surface exhibited about 5.6 times delay in freezing at −30 °C compared with the HTV silicone rubber surface. Furthermore, heterogeneous nucleation theory and heat transfer theory were introduced to explain the difference in freezing time between the as-prepared SH coating and HTV silicone rubber. It could be concluded that the SH coating had a large nucleation free energy barrier and a low heat transfer rate between the droplet and the surface and, thus, was able to effectively delay the freezing time.

Effect of mineral oil on flashover voltage of HTV silicone rubber under AC corona ageing: from the view of trap level distribution

The flashover voltage of high temperature vulcanised (HTV) silicone rubber will be decreased after corona ageing, and it further decreases under the effect of mineral oil. The mechanisms for the phenomena can be explained by considering the role of trap level distribution. In this study, the corona ageing of silicone rubber was conducted and the flashover voltage was tested. The isothermal surface potential decay was tested to explore the internal relationship between trap distribution and flashover voltage. Besides, the surface microtopography and chemical structure of silicone rubber were gained by scanning electron microscopy and Fourier infrared spectroscopy to explain the change of trap distribution. The test results show that the flashover voltage of HTV silicone rubber decreases with AC corona ageing time, the reduction of flashover voltage for an oil-impregnated sample is more serious. The density of shallow electron traps increases at first and then decreases, while the density of shallow hole traps increases with ageing time. The density of deep traps decreases after corona ageing. These trap level distribution characteristics are beneficial to promote the development of flashover, leading to lower flashover voltage. The effect of mineral oil can be regarded as an accelerated ageing effect.