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Analyzing the Role of Filler Interface on the Erosion Performance of Filled RTV Silicone Rubber under DC Dry-band Arcing
Abstract
This study investigates the role of filler interface on suppressing the erosion of room temperature vulcanized silicone rubber composites filled with fumed silica, nano alumina trihydrate and sub-micron hexagonal boron nitride fillers during DC dry-band arcing. Simultaneous thermogravimetric-differential thermal analyses indicate a superior effect for fumed silica in suppressing the depolymerization of silicone rubber and promoting radical based crosslinking. This can be attributed to favorable interactions at the fumed silica-silicone interface tethering the siloxane chains. At low filler loading of alumina trlhydrate, water of hydration has insignificant effects on suppressing depolymerization compared to that influenced by fumed silica's interface at equal filler loading. Similarly, incorporating thermally conductive boron nitride filler in silicone rubber does not show improvement in the depolymerization rate compared to that influenced by fumed silica. These findings correlate with the +DC inclined plane tracking and erosion test outcomes indicating superior erosion performance for the fumed silica filled composite. This accordingly supports the influential role of the filler interface over the water of hydration and thermal conductivity enhancement in suppressing the DC erosion of silicone rubber composites under the test conditions of this study. A statistical boxplot analysis technique is introduced to elucidate the inception of the stable dry-band arc in terms of the change in leakage current randomness during the +DC inclined plane tracking and erosion test. The boxplots reveal a slow inception of the stable dry-band arc with the fumed silica filled composite delaying the erosion of silicone rubber during the test. This finding confirms the influence of the filler interface over composite thermal conductivity in suppressing erosion of silicone rubber under DC dry-band arcing.
... The coherency and porosity of such residue could shield the SiR material against progressive erosion under DC dry-band arcing [11]. In an earlier study [12], fumed silica was found to have a significant effect in suppressing DC erosion as compared to nano ATH and sub-micron boron nitride (BN) in SiR composites, despite the comparable thermal conductivities reported for all of the composites. Accordingly, the literature suggests that the role of silica fillers in suppressing the DC erosion in SiR is more than simply improving the composite thermal conductivity. ...
... Computing the RMS value for every second of the test run would suffice for representing the change in the leakage current values during the test and would provide a practical approach for acquiring the data with smaller storage requirements instead of saving the entire current waveform of 468 samples per second. Following the analytical approach presented in an earlier study [12], a statistical boxplot method is used to observe the distribution of the RMS leakage current values acquired during the IPT in 20 min time intervals. This statistical analysis allows one to observe the evolution of the dry-band arc from the intermittent state to the stable severe state, which is reflected in the changing distribution of the RMS leakage current values between consecutive time intervals in the boxplot. ...
... It is important to note that despite having twice the thermal conductivity of the FS07 filled SiR, the GS10 filled SiR showed inferior erosion performance, which suggests that that the thermal conductivity is not the main governing factor in suppressing the erosion of SiR under DC voltage. In an earlier study [12], it was found that the favorable interaction of fumed silica with the SiR matrix was more decisive in determining the erosion performance of SiR under the +DC IPT than the improvement of the composite thermal conductivity using BN fillers. The difference in DC and AC erosion in silica filled SiR was thoroughly investigated and discussed in studies such as [11] and is not the subject of this work. ...
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.
... It is highly compatible with SR due to its structural resemblance, and the interaction between PDMS (polydimethylsiloxane) and silica is assumed to be established by the silanols on the silica surface and the oxygen atoms of the polymer chains resulting in higher molecular bonds [11][12]. Alqudsi et al. explored and found that fumed silica-doped SR composite significantly suppressed the depolymerization of SR through crosslinking in comparison to nano aluminium trihydrate or boron nitride fillers [13]. Similarly, the study conducted by Venkatesulu et al. revealed that nano-silica-doped SR composite can resist corona ageing. ...
... Further, the breakdown voltage increases as the filler wt.% increases from 1 to 3, but decreases as it further increases from 3 to 5. This increase in breakdown voltage for filler wt.% from 1 to 3 may be attributed to the uniform dispersion of filler particles as well as strong interfacial interaction between filler particles and base matrix [13]. The reduction in breakdown voltage from 3wt.% to 5wt.% might be due to agglomerations of nanoparticles in the base matrix due to the high surface energy. ...
Room temperature vulcanized silicone rubber (RTVSR) is widely used as a coating material for transformer bushings to provide anti-pollution resistance. However, mineral oil leakage from power transformers can significantly degrade the performance of these coatings. This study investigates the degradation dynamics of RTVSR coatings exposed to mineral oil and evaluates the potential of nano silica (SiO₂) doping to mitigate this issue, leveraging its molecular compatibility with silicone rubber. Silicone rubber (SR) doped with 1%, 3%, and 5% by weight of silica was subjected to thermal ageing in transformer mineral oil for 6 hours at 80°C. The physicochemical, surface morphological, and electrical properties of the aged specimens were analyzed using Fourier Transform Infrared Spectroscopy (FTIR), contact angle measurements, 3D microscopy, and breakdown voltage (BDV) testing. FTIR revealed molecular side-chain and backbone scission in all samples, with silica-doped specimens showing a less pronounced extent of degradation. This degradation caused reduced surface hydrophobicity, lower BDV, and increased surface roughness post-ageing. Thermal stability was further assessed using thermogravimetric analysis (TG-DTA) and infrared thermography. Residual weight, indicative of thermal stability, significantly decreased post-ageing for all specimens. Infrared thermography revealed lower heat accumulation in silica-doped specimens compared to undoped samples, although heat retention increased in all samples after ageing. Overall, the results confirm that mineral oil exposure has a detrimental effect on RTVSR coatings. However, 3 wt.% nano-SiO₂ doping significantly enhances degradation resistance, making it a promising composition for transformer bushings exposed to mineral oil leakage.
... On the other hand, cost is a critical factor taken into consideration in the selection of fillers for HTV SiR insulators. For example, silica filler was demonstrated to be as effective as ATH in suppressing tracking and erosion of SiR, but at a fraction of the cost of ATH [2][3][4]. ...
This paper investigates the erosion suppression mechanisms of Zinc Borate in high-temperature vulcanized silicone rubber using the IEC 60587 inclined plane test and simultaneous thermogravimetric-differential thermal analysis. Alumina tri-hydrate is employed in this study as a reference filler for comparison with Zinc Borate filler. The dehydration of Zinc Borate is reported to start around 350°C, whereas alumina tri-hydrate starts dehydration at lower temperatures around 230°C. An insignificant difference is shown in the erosion resistance between the alumina tri-hydrate and Zinc Borate-filled composites. Both fillers are shown viable in preventing the tracking and erosion failure in the IEC 60587 inclined plane test under the critical 4.5kV ACrms voltage. Zinc Borate is found to suppress failure with the formation of residue acting as a shield against the progression of erosion. Whereas, alumina tri-hydrate alleviates surface temperature by promoting an internal oxidation mechanism that suppresses combustion of silicone rubber. This study's findings highlight the potential application of Zinc Borate as a cost-effective filler in high-temperature vulcanized silicone rubber for outdoor insulation, particularly in regions where this filler is readily available.
... Numerous methods, including hot-line cleaning and greasing, can be used to lessen the problem of pollution-related outages [7]. While hot-line washing is thought to be an expensive procedure, the greasing technique is quite dirty [5,7,8]. The ability of solid electrical insulating materials to withstand electrical stress and contamination is assessed using the inclined plane tracking erosion test [9][10][11]. ...
... Silicone rubber consists mainly of rubber and silica, which is a straight-chain, high molecular-weight polysiloxane. Room temperature vulcanised silicone rubber can be vulcanised and moulded at room temperature [3,4]. RTV exhibits excellent hydrophobicity and hydrophobic migration properties, and it remains soft and flexible over time [5,6]. ...
Room temperature vulcanised (RTV) silicone rubber coatings effectively enhance the insulation properties of electrical equipment. However, RTV coatings are prone to internal defects caused by the coating process and the effects of aging during service, which can lead to debonding of the coatings. Internal debonding defects are challenging to detect and can ultimately lead to accidents due to a reduction in the insulation capacity of the equipment. To visualize the internal defect morphology of RTV coatings and quantify the defect size, an ultrasonic pulse-echo-based method for detecting and imaging debonding defects is proposed. The method involves the development of a finite element model to investigate how ultrasonic waves propagate in RTV coatings and the influence of ultrasonic probes and inspection conditions on defect echoes. Furthermore, an ultrasonic detection system specifically designed for RTV coating debonding defects is constructed. This system utilizes wavelet packets in the time-frequency domain to analyze the echo signals in both normal and defective regions. The three-dimensional reconstruction of the debonding defect morphology is accomplished by integrating ultrasonic echo amplitude and position information. Finally, the size of the debonding defects is quantified using an adaptive threshold segmentation method. The findings indicate that ultrasound waves reflected in RTV materials propagate as spherical waves, with the acoustic energy primarily concentrated near the acoustic axis. As the propagation distance increases, the sound beam disperses along the axis and extends beyond the transducer, resulting in a decrease in the sound field's directionality. The developed visual reconstruction method in this study offers the capability of three-dimensional visualization for defects present within RTV coatings, including their length, width, and depth. The accurate determination of defect size is achieved through the utilization of the adaptive threshold segmentation method, yielding an average error rate of 5.7 % across different defect types. In comparison, the maximal interclass variance method (OTSU) and the fuzzy C-means (FCM) method produced results with error rates of 9.8 % and 7.9 %, respectively. The research presented in this paper enables precise assessment of debonding defect severity and establishes a reliable foundation for on-site inspection, operation, and maintenance of RTV coatings.
... The unscheduled outages may result in seriously negative repercussions [1][2][3]. During the operation of the insulation equipment, pollution and moisture will accumulate on the surface of the coil and insulators [4][5][6][7][8]. Due to the existence of the electric field, large leakage current will appear on the surface of the coil and insulators, which causes moisture evaporation and forms dry bands. ...
The electrical erosion is a common problem in fields, such as motor insulation and gas-insulated substations. Electrical erosion resistance can be improved by enhanced nonlinear electrical conductivity or thermal conductivity. However, a problem yet to be overcome is the electrical erosion of polymeric structures under harsh environments with high humidity. This paper reports a novel electrical erosion resistance strategy by thin superhydrophobic layers on the SiC powder surface, with a static contact angle of 164° ± 2° and a slip angle of 7° ± 1°. The coating demonstrates excellent thermal conductivity of 1.62 W/(m·K) and nonlinear conductivity with aids of SiC fillers. The electrical erosion resistance of the coating was studied based on the DC inclined plane test and the arc ablation method. The results show that the electrical erosion resistance voltage of the coating reaches up to 4.5 kV. The presence of superhydrophobicity makes it difficult for the contaminant to form discharge channels on the coating surface. The nonlinear electrical conductivity of the surface-modified SiC particles can uniform the electric field distribution and inhibit the occurrence of arc discharge on the coating surface. Besides, the high thermal conductivity of superhydrophobic coatings inhibits the heat accumulation caused by arc ablation on the surface, which greatly improves the electrical erosion resistance of the coating. The research in this paper has promising potential for the safe operation of insulated equipments.
... Findings reveal that materials experienced significantly more loss of properties under DC electric stress as compared to AC. In addition, it has been reported that positive DC somehow attracts more contaminants and hence causes higher degradation of insulating materials compared to negative DC [23,24]. The effect of thermal conductivity and dielectric properties of silicone rubber in an inclined plane erosion and tracking test under DC voltage was studied in Ref. [25]. ...
The present work explores durability enhancement in various properties of room temperature vulcanized silicone rubber (RTV-SiR) coating with the incorporation of alumina trihydrate (ATH) and silica (SiO2) fillers. Test samples i.e., neat RTV-SiR, RTV-SiR +10% μSiO2, and RTV-SiR filled with micro-ATH/SiO2 (10% each) were chosen for experimentation. Long-term (10,000 hours) lab aging of composite coatings is performed under bipolar direct current (DC) voltage with simulated environmental stresses in a test chamber. The effect of fillers against stresses is quantified through characterization results comparison of filled composites with unfilled RTV-SiR samples. Moreover, the polarity effect on degradation is evaluated through the relative performance of each composite under the positive and negative polarity of the voltage. Findings revealed that samples filled with a mixture of additives bring enhanced improvement into the base matrix of RTV-SiR. Amongst unfilled and micro-silica filled specimens aged under positive DC, the samples S-3 with combined silica-ATH fillers offered superior hydrophobic behavior showing hydrophobicity class HC3 and demonstrated a minimum final leakage current of 6.13 μA. Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopic (SEM) results showed relatively lower variations in the chemical and morphological structure of cross-filed specimens (S-3). Furthermore, results of mechanical properties (tensile strength, elasticity, and weight) show a significant loss, whereas the hardness of composite increased with aging. Hybrid samples (S-3) exhibited the minimum deviation of 29.77% and 43.12% in tensile strength and elongation-at-break (EAB), respectively under positive DC voltage. Similarly, it shows enhanced resistance against weight loss and change in hardness due to aging. Moreover, thermogravimetric analysis (TGA) confirmed that filled composite gets enabled to offers the least alteration in thermal properties. In the context of the polarity, positive DC voltage incurred more devaluation of desirable properties as compared to negative polarity.
This paper proposes ultra-high frequency detection of the eroding dry band arcing as a non-intrusive and online detection technique of erosion failure during the DC inclined plane test. Tests are performed on silicone rubber composites, while conducting leakage current, surface temperature and ultra-high frequency measurements using a horn antenna. No ultra-high frequency detection is evident during mild dry-band arcing, which does not lead to erosion. Whereas, the detection is evident during stable dry-band arcing, particularly when the intensity becomes remarkable. Thermogravimetry, differential thermal analysis and Fourier Transform spectroscopy are conducted to elucidate temperature measurements and visual observations. The eroding dry-band arcing is shown leading to depolymerization of silicone rubber at 400°C, thereby leading to combustion and erosion failure, and stimulating a radical-based crosslinking reaction at temperatures exceeding 500°C. The intensity of the ultra-high frequency detection is also shown to be sensitive to the severity of the eroding dry-band arcing as it increased with time during the test or with higher applied test voltages.
High thermal conductive composites are widely used for heat dissipation, where suitable filling schemes are needed to meet the application requirements. In this study, a thermal conductivity prediction model with higher accuracy is developed to guide the preparation of composites. With experimental tests and simulations, the impacts of fillers on the overall thermal conductivity are investigated. Separation axis theorem is introduced to calculate the filler contact probability, with the aim of further explaining the impact mechanism of the filling scheme. It is found that increasing the filling volume fraction can promote the formation of inter-filler thermal conductivity pathways. The filler contact probability is approximately proportional to the composites thermal conductivity in our research, based on which the multiple connection mechanism prediction model is built. The average prediction accuracy of the new model considering the log-normal distributed size fillers is 96.8%.
Ensuring the fire safety of high voltage (HV) outdoor insulators is key to maintain the reliable operation of the electrical grid. This requires the careful selection of suitable polymeric composite materials with excellent electrical tracking, erosion, and fire resistance characteristics. To improve their performance against electrical tracking and potential combustion issues, inorganic additives are commonly integrated into these materials. The focus of this review article is on describing the progress and innovations in enhancing the electrical tracking, erosion, and fire performance of polymeric materials by incorporating inorganic additives. The main objective is to explore the development of these materials and showcase their evaluation through laboratory testing. This study highlights significant state-of-the-art advancements in the field, providing valuable insights into its current progress. Additionally, the research outlines prospects, offering a peek at how upcoming studies are expected to further advance the scientific knowledge in the field. By disseminating critical information about the development , testing, and future potential of polymeric materials containing inorganic additives, this work is expected to facilitate researchers in advancing their work in HV outdoor insulation, leading to more efficient electrical insulation solutions for safer and reliable electrical grids.
Hydrophobicity is an important property that enhances the insulating performance of polymeric insulators. It further protects the insulator from the formation of the dry band under wet conditions which causes leakage current, flashover and other surface effects. This work focuses on the enhancement of hydrophobicity by adding ZrO
2
nanofillers in different weight percentages (1wt%, 3wt%, and 5wt%) with Liquid Silicone Rubber (LSR). The diameter of the water droplet is measured by VMS (Video Measuring System) and the static contact angle is measured by ImageJ Analysis under clean conditions. As per ASTM D2303, IPT (Inclined Plane Test) is used to determine the tracking and erosion properties of the polymer insulator. IPT results are compared with surface roughness for all the prepared samples. The experimentation reveals that the 5wt% added nanocomposite sample shows improved hydrophobic and electrical characteristics when compared to all other samples.
Polysilazane (PSiN) and platinum (Pt) were used to enhance the tracking and erosion resistance of silicone rubber. The suppression effects of PSiN and Pt on tracking and erosion were investigated using an inclined plane test (IPT), thermogravimetry, thermogravimetry-Fourier transform infrared spectrometry, scanning electron microscopy and laser Raman spectroscopy. It was determined that the addition of 1.8-2.4 part per hundred parts of rubber (phr) of PSiN and 8 ppm of Pt significantly enhanced the tracking and erosion resistance of silicone rubber, whereby the test specimens passed a 4.5 kV IPT with a low eroded mass and undamaged surfaces. This was attributed to the synergistic effect of PSiN and Pt on silicone chains. At high temperatures and produced by arc discharge, PSiN/Pt-catalysed radical crosslinking suppressed the degradation of silicone chains. Furthermore, the formation of a tightly crosslinked network protected the inner materials from arc ablation. In addition, carbon deposition was prevented by PSiN/Pt, making it harder for tracking to develop.
Silicone rubber-based polymers are gradually replacing traditional insulators in outdoor applications due to their numerous advantages, in particular high hydrophobicity that is important for suppressing leakage current and hence surface flashover. However, the main problem with polymeric composites is their aging when exposed to environmental stresses causing loss of their desired properties and expected lifetime. In this work, aging behavior of three types of room temperature vulcanized silicon rubber (RTV-SiR) loaded with nano-silica and micro-ATH is studied. Samples of the materials with different specifications were fabricated and exposed to various environmental stresses in two specially designed weather chambers under both AC and bipolar DC voltages for a time period of 9000 hours. Thereafter, diagnosis was performed for identifying the degree of deterioration using several techniques. These include hydrophobicity classification, measurements of leakage current and mechanical properties, Fourier transform infrared spectroscopy, and Thermogravimetric analysis. Results of the conducted experiments indicated stronger degradation of the desired properties under positive DC stress as compared to the AC and negative DC voltages. Moreover, silicone rubber doped with nano-sized particles of silica filler demonstrated better anti-aging performance as compared to its micro-ATH filled counterpart.
This paper investigates the filler barrier effect on suppressing the erosion of silicone rubber composites during the DC dry-band arcing, by supplementing main micro fillers in silicon rubber, i.e. ground silica and micro-sized alumina tri-hydrate, with fumed silica and nano-sized alumina tri-hydrate fillers. A study framework employing simultaneous thermogravimetric-differential thermal analyses, the inclined plane tracking and erosion test and the dry-arc test is employed. Fumed silica in silicone rubber increases the amount of the crosslinked residue and suppresses silicone rubber depolymerization; whereas nano-sized alumina tri-hydrate releasing water of hydration may adversely impact the residue coherence and promote combustion. The viability of the filler barrier effect is synergistically achieved by adding fumed silica to the main ground silica filler, thereby maintaining the residue integrity. The inclined plane-tracking erosion test confirms the importance of the filler barrier effect of fumed silica in supplementing the volume effect of ground silica in the suppression of the DC dry-band arcing. These filler effects appear to mainly govern the erosion resistance, with insignificant effect shown for thermal conductivity under DC. The dry-arc resistance test is shown as a useful method to simulate a stable dry-band arcing and obtain reproducible surface erosion patterns that correlates with the outcomes of the DC inclined plane tracking and erosion test.
To improve the monitoring of the electrical power grid, it is necessary to evaluate the influence of contamination in relation to leakage current and its progression to a disruptive discharge. In this paper, insulators were tested in a saline chamber to simulate the increase of salt contamination on their surface. From the time series forecasting of the leakage current, it is possible to evaluate the development of the fault before a flashover occurs. In this paper, for a complete evaluation, the long short-term memory (LSTM), group method of data handling (GMDH), adaptive neuro-fuzzy inference system (ANFIS), bootstrap aggregation (bagging), sequential learning (boosting), random subspace, and stacked generalization (stacking) ensemble learning models are analyzed. From the results of the best structure of the models, the hyperparameters are evaluated and the wavelet transform is used to obtain an enhanced model. The contribution of this paper is related to the improvement of well-established models using the wavelet transform, thus obtaining hybrid models that can be used for several applications. The results showed that using the wavelet transform leads to an improvement in all the used models, especially the wavelet ANFIS model, which had a mean RMSE of 1.58 ×10−3, being the model that had the best result. Furthermore, the results for the standard deviation were 2.18 ×10−19, showing that the model is stable and robust for the application under study. Future work can be performed using other components of the distribution power grid susceptible to contamination because they are installed outdoors.
To improve the monitoring of the electrical power grid it is necessary to evaluate the influence of contamination in relation to leakage current and its progression to a disruptive discharge. In this paper, insulators were tested in a saline chamber to simulate the increase of salt contamination on their surface, and to evaluate the supportability of these components. From the time series forecasting of the leakage current, it is possible to evaluate the development of the fault before a flashover occurs. Choosing which method to use is always a difficult task since some models may have higher computational effort. In this paper, for a complete evaluation, the long short-term memory (LSTM), group method of data handling (GMDH), adaptive neuro-fuzzy inference system (ANFIS), bootstrap aggregation (bagging), sequential learning (boosting), random subspace, and stacked generalization (stacking) ensemble learning models are analyzed. A review and comparison of these well-established methods for time series forecasting is performed. From the results of the best structure of the model, the hyperparameters are evaluated and the Wavelet transform is used to obtain an enhanced model.
In last two decades a promising performance improvement in the electrical insulation materials has been reported by many researchers using different nano fillers (NFs). In recent years another type of NFs called core–shell type nano fillers have shown even more attention than ordinary NFs. The core–shell NFs are a combination of nano particle core coated with outer nano layer of different material. The core–shell NFs combine the beneficial properties of two material with in one NF. Recently a few studies have reported a considerable improvement in the dielectric properties of epoxy by utilising TiO 2 @SiO 2 core–shell NFs. The TiO 2 @SiO 2 particles have a core of TiO 2 coated with an out layer of SiO 2 at nano level. This study investigates the improvement in tracking performance of silicone rubber (SiR) using TiO 2 @SiO 2 core–shell nano particles which has not been reported previously. The tracking immunity of low (below 1%) and high (above 1%) filler concentrations of nano TiO 2 @SiO 2 incorporated into SiR was investigated according to IEC 60587. The results showed that an optimum percentage of 0.6 wt% of TiO 2 @SiO 2 imparts best immunity to silicone rubber against tracking.
Hexagonal boron nitride (h-BN) has been predicted to exhibit an in-plane thermal conductivity as high as ~ 550 W m−1K−1at room temperature, making it a promising thermal management material. However, current experimental results (220–420 W m−1K−1) have been well below the prediction. Here, we report on the modulation of h-BN thermal conductivity by controlling the B isotope concentration. For monoisotopic10B h-BN, an in-plane thermal conductivity as high as 585 W m−1K−1is measured at room temperature, ~ 80%higher than that of h-BN with a disordered isotope concentration (52%:48% mixture of10Band11B). The temperature-dependent thermal conductivities of monoisotopic h-BN agree well with first principles calculations including only intrinsic phonon-phonon scattering. Our results illustrate the potential to achieve high thermal conductivity in h-BN and control its thermal conductivity, opening avenues for the wide application of h-BN as a next-generation thin-film material for thermal management, metamaterials and metadevices.
This work explores the thermal behavior of silicone rubber filled at different concentrations with alumina trihydrate (ATH) and silica. Three particle sizes, 1.5 μm, 5 μm and 10 μm, were employed in the sample preparation. A near-infrared laser was used to heat the samples. The concentration of the filler plays a major role on improving thermal conductivity when compared to particle size. Nevertheless, better thermal conductivity could be distinguished at 5 μm for ATH and 1.5 μm for silica, at all concentrations.
The results of erosion resistance, tensile strength, elongation at break, hardness, and thermal stability measurements are presented for silicone nanocomposites prepared using various nano and micro silica and alumina fillers in a two-part silicone rubber (SiR) matrix. The fillers are used to improve the erosion resistance of SiR, which is necessary for outdoor insulation housing applications. Good dispersion of the fillers is achieved using Triton/sup TM/, a common surfactant. An optimal surfactant concentration imparts good erosion resistance to the nanocomposites in laser heating tests without adverse effects, but excess surfactant has a negative impact on the mechanical properties of the silicone. Thermal gravimetric analysis demonstrated that nano fumed silica imparts better heat resistance to silicone than natural nano silica or nano alumina fillers. Fourier transform infrared spectroscopy analysis of the nanofillers indicated a significantly higher concentration of silanol groups in the nano fumed silica filler than in micro silica. The influence of the increased number of silanol groups on the erosion resistance of the nanocomposites and their mechanical properties is discussed.
This review describes some recent works related to the development of the flame retardation of silicone elastomers and/or applications of silicones as flame retardant agents in other polymers. First, the thermal degradation of silicones themselves is discussed, focussing on depolymerization mechanisms, effect of structure, heating conditions, and effect of additives (i.e. less than 5 wt% fillers) on thermal degradation of silicones. Then, the influence of several types of mineral fillers (of up to 80 wt% content) as ceramization agents of silicones is presented. Finally, the introduction of (functionalized) silicones as flame retardants into other polymers is described.
The products of the thermal degradation of polydimethylsiloxane (PDMS) are determined by the heating conditions, since two competing mechanisms are involved.Cyclic oligomers are formed in the low degradation temperature range and during slow heating in programmed degradation. This involves molecular splitting of oligomers from loop conformations of the PDMS chain favoured by its flexibility, and assistance on the part of empty silicon d-orbitals.Methane and oligomers are formed in the high temperature range and during fast heating. This shows that homolytic scission of Si–CH3 also takes place and is followed by hydrogen abstraction.
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 μm 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.
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.
This paper investigates the erosion suppression effects of the water of hydration in alumina tri-hydrate and magnesium hydroxide fillers in silicone rubber during the AC and DC dry-bad arcing. Simultaneous thermogravimetric and differential thermal analyses show the hydrated filler type, level and size affecting erosion. Water of hydration in alumina tri-hydrate is shown to be released before the depolymerization of silicone rubber; whereas, the water of hydration in magnesium hydroxide is released during the depolymerization. As such, alumina tri-hydrate could promote a stronger shield of residue against dry-band arcing as compared to magnesium hydroxide, which could further suppress erosion. Increasing the hydrated filler loading raises the amount of the water of hydration released, which could dilute the silicone rubber in the gas phase and suppress combustion. However, insignificant effect on the suppression of depolymerization is obtained at higher levels of the hydrated filler, possibly due to the lack of filler-polymer interactions tethering silicone rubber chains. Internal oxidation promoted with lower sizes of alumina tri-hydrate could promote a radical-based crosslinking, leaving a stronger shield of residue against dry-band arcing. However, hydrolysis may take place and insignificant improvement is shown for the erosion performance with internal oxidation.
This paper investigates the residue formed on silicone rubber filled with natural silica in the inclined plane tracking and erosion test under AC and DC voltages. Simultaneous thermogravimetric and differential thermal analyses reveal residue formation dependent on volatilization and crosslinking in silicone rubber. Thermo-oxidation of volatiles yields silica-based residue protecting the surface during dry-band arcing; whereas, crosslinking leaves carbonaceous residue leading to more severe dry-band arcing. Volume fraction of natural silica replacing silicone rubber is shown to be a main source of silica residue suppressing carbonaceous residue, thus preventing failure in the inclined plane test. More severe erosion under the equivalent +DC voltages as compared to standard AC and equivalent — DC voltages of the inclined plane test is attributed to more severe dry-band arcing heat promoted by carbonaceous residue. Silicone rubber filled with lower particle sizes of natural silica is demonstrated to deteriorate erosion performance, possibly as result of filler-polymer interactions or lower particle size reducing the integrity of the protective silica residue. The following paper highlights the importance of using critical AC and DC voltages during the inclined plane tracking and erosion tests.
The increase in voltage level and compactness of electrical equipment result in demands for electrical insulations that have high breakdown strength, high thermal conductivity and high electrical resistivity. Use of dielectric polymer nanocomposites is a promising approach that has great advantages over the traditional materials. This paper explores the influence of alumina nanofillers, both treated and untreated, on properties of silicone rubber based nanocomposites. Composite samples made of 10 wt% and 20 wt% filler loading are prepared using high shear (HS) and electrostatic disperser (ES) techniques with the aim of achieving maximum dispersion of fillers in the silicone matrix. Effects of filler type, filler concentration and mixing method on morphological changes, thermal, erosion, and electrical properties are analyzed. Scanning electron micrographs (SEMs) showed better filler dispersion in composites prepared using ES than using HS mixer. Thermogravimetric Analysis (TGA) and thermal conductivity measurements revealed enhanced thermal stability and conductivity with increasing filler loadings. Additionally, ES composites showed high erosion resistance. Composites containing treated alumina performed better than those containing untreated alumina. In total, composites prepared with treated alumina using the ES method showed marked improvement in thermal properties and erosion resistance due to homogeneous filler dispersion imparting high number of filler-matrix interfaces and stronger bonding as visualized from SEMs and dielectric spectroscopy data.
This paper describes the suppression of dry-band arcing erosion of silicone rubber by alumina tri-hydrate and silica fillers in the DC inclined plane test, employing the wavelet based multiresolution analysis of leakage current. The third detail component of the leakage current as decomposed by the wavelet-based multiresolution analysis is shown to be an indicator of the effectiveness of the filler type in suppressing erosion by dry-band arcing. The addition of alumina tri-hydrate or silica filler to silicone rubber increases the thermal conductivity of the composites, retarding the development of the eroding temperature, and thus the evolution of the third detail. Additional effect is also obtained for the dehydration enthalpy, of alumina tri-hydrate in silicone rubber at a filler level of 30 wt%, in impeding the development of hot spots on the tested surface. A reduction in the magnitude of the third detail is evident with filler level, indicating that the increasing volume of silica or alumina tri-hydrate reduces the temperature of the dry-band arcing plasma. Comparable levels of the leakage current third detail is found between 30 wt% alumina tri-hydrate and silica filled composites which suggests the water of hydration plays a minor role in diluting the SiR but at 58 wt% an internal oxidation mechanism that produces gases diluting the arcing phase appears to suppress erosion.
The paper reviews previous work on the DC inclined plane test and suggests equivalent DC voltage levels in parallel to AC voltage in the ASTM inclined plane tracking and erosion test. The aim of this work is to provide a basis for standardizing the inclined plane test for DC voltage. Round robin tests done in five laboratories on five specimens of a silicone rubber material were done with the purpose of establishing appropriate ratios by which the equivalent DC voltages can be determined with respect to the corresponding AC voltages. These levels were determined as 67% and 87%, for +DC and -DC respectively, of the AC initial tracking voltage, and for practical purposes, these levels are rounded to 70 and 90%.
The paper describes a study into understanding the mechanism by which alumina tri-hydrate suppresses dry-band arcing erosion of silicone rubber under DC. Commercial silicone elastomers filled with alumina tri-hydrate to 25 and 58 wt% are exposed to the inclined plane tracking/erosion tests while simultaneously monitoring surface temperature. During the early stages of testing, a surface residue is formed, which promotes stable dry-band arcing. An earlier inception of the stable discharge, thus initiation of deep erosion at a temperature approaching 400 °C, is obtained with 25 wt% alumina tri-hydrate. A lower temperature rise is obtained for the 58 wt% filled specimens as compared to the 25 wt% filled specimens. Differential scanning calorimetry shows a larger enthalpy of dehydration for 58 wt% alumina tri-hydrate filled samples. To verify the role of dehydration on reducing the surface temperature and erosion, the erosion of liquid silicone rubber specimens is compared for samples filled with alumina tri-hydrate and silica, both at 30 wt%. As a measurement parameter, the time-to-erosion is compared between the specimens showing a reduced time for the silica filled specimens as compared to the alumina tri-hydrate filled specimens, thereby verifying the influence of the water of hydration to suppress surface temperature and erosion. The paper provides an experimental approach by which the suppression of erosion by alumina tri-hydrate can be investigated, employing simultaneous leakage current and temperature measurements.
Silicone rubber (SiR) is widely employed as an insulating material in transmission lines because of its excellent electrical properties and superior performance under wet and polluted conditions. However, the discharges that occur during operation can cause electrical erosion on the surface of silicone rubber insulators. The thermal conductivity of insulators has a correlation with the resistance to tracking and erosion. This study attempts to clarify whether the addition of boron nitride (BN) particles can improve the resistance to tracking and erosion of SiR by increasing its thermal conductivity. Before the tests, specimens were prepared by dispersing nano-BN particles into room temperature vulcanizing (RTV) silicone rubber at different loadings. In this paper, the dc test has been developed from the current IEC 60587 inclined plane tracking and erosion test to compare the phenomena occurring during the tests. Temperature distribution was observed by an infrared thermal imager. The experimental results indicate that the filled specimens have a lower degree of surface damage than the unfilled specimens. In addition, with the increase in content of fillers from 0 to 7 wt%, the thermal dissipation is improved and both the erosion depth and the weight loss show a decreasing trend, which proves the resistance of silicone rubber to tracking and erosion is improved.
In this paper the leakage current signature in the DC inclined plane test is examined for statistical indicators of the incipient erosion of silicone rubber. The mean, skewness and kurtosis are examined as useful indicators of the dry-band arcing current causing erosion. The moving window method is employed during the test to obtain an enhanced resolution in time. Simultaneous thermogravimetric-differential scanning calorimetry analysis is used to predetermine the temperature at which erosion begins, and temperature measurements taken during the inclined plane test are correlated to the leakage current leading to erosion. During erosion, more stable discharges as well as a recognizable change in the statistical distribution of the dry-band arcing current is recognizable through the analysis employed.
In order to improve the tracking and erosion performance of outdoor polymeric silicone rubber (SR) insulators used in HV power transmission lines, micron sized inorganic fillers are usually added to the base SR matrix. In addition, insulators used in high voltage dc transmission lines are designed to have increased creepage distance to mitigate the tracking and erosion problems. ASTM D2303 standard gives a procedure for finding the tracking and erosion resistance of outdoor polymeric insulator weathershed material samples under laboratory conditions for ac voltages. In this paper, inclined plane (IP) tracking and erosion tests similar to ASTM D2303 were conducted under both positive and negative dc voltages for silicone rubber samples filled with micron and nano sized particles to understand the phenomena occurring during such tests. Micron sized Alumina Trihydrate (ATH) and nano sized alumina fillers were added to silicone rubber matrix to improve the resistance to tracking and erosion. The leakage current during the tests and the eroded mass at the end of the tests were monitored. Scanning Electron Microscopy (SEM) and Energy dispersive Xray (EDX) studies were conducted to understand the filler dispersion and the changes in surface morphology in both nanocomposite and microcomposite samples. The results suggest that nanocomposites performed better than microcomposites even for a small filler loading (4%) for both positive and negative dc stresses. It was also seen that the tracking and erosion performance of silicone rubber is better under negative dc as compared to positive dc voltage. EDX studies showed migration of different ions onto the surface of the sample during the IP test under positive dc which has led to an inferior performance as compared to the performance under negative dc.
In the current study, the effects of loading level of micro or nano size BN particles on the thermal, mechanical, and morphological properties of silicone rubber are investigated. Three micron size and two nano size BNs with different particle sizes and shapes are used. All five types of BNs are found to be well-dispersed in silicone rubber matrix despite some local agglomerates. In general, the addition of BN particles in silicone matrix decreases the tensile strength and strain at break, coefficient of thermal expansion (CTE) values, on the other hand increases modulus, hardness and thermal conductivity. Nano size fillers have more pronounced effect on tensile properties of composites in comparison to micron size BNs at any given loading level. The aspect ratio of the filler is found to be very effective in achieving high thermal conductivity in composite systems. Dielectric constants of composites vary between dielectric constant of silicone and BN.
Some recent results of the research aimed to correlate properties of filled silicone rubber compounds with surface properties of fumed silica fillers are reported. It is shown that the specific interaction between silica surface silanol groups and the siloxane chain of silicone rubber plays the main role in “crepe hardening” and bound rubber formation in compounds. The silanol coverage of silica determines the tightness of polymer–filler network in bound rubber. The experimental data fit Meissner's theory of bound rubber quite well. Remilling of “crepe hardened” compounds was also studied, and the structure of the remilled compounds is proposed. The activation energy of bound rubber formation on mixing was found to be 16.8 kj/mole; this suggests that physical rather than chemical processes are involved in the interaction between silica and silicone rubber.
The requirements for a material to combust are heat, fuel, and oxygen. Limiting the interaction of a material with at least one of these requirements will impede the combustion process and impart a level of flame resistance. This is achieved by modifying the composition of a material to include chemicals that specifically target impedance of the combustion process. The level of flame resistance required from a material is specific to the end application and can be measured by one of the many test methods available. Silicone rubber is often selected as the design elastomer because of its excellent flame resistance and low smoke toxicity. Arlon has developed a number of calenderable silicone rubber compounds capable of meeting the flame resistance requirements for even the most stringent flame resistance requirements.
The inclined plane tracking and erosion test IEC-60587 is not specified for DC testing. A dc test has been developed from the current ac standard and three formulations of silicone rubber tested. These materials were tested under three voltage levels (2.3, 2.7 and 3.2 kV) for both polarities. Positive dc tests have the highest average and peak leakage current and exhibit a higher degree of surface damage. The observed surface degradation pattern is heavily dependant on polarity. Consistently higher levels of erosion have been observed in the higher voltage positive cases. Erosion of the surface always starts at the bottom electrode, and spreads toward the top electrode. Puncturing of the 6 mm thick samples or deep erosion over more than half the distance between electrodes has only been observed under 2.7 and 3.2 kV positive polarity tests. Results have been analysed using a variety of leakage current analysis techniques going beyond the criteria specified in the original ac standard. The low-frequency behaviour of the leakage current was monitored using a 15 sample per second current recorder. The leakage current magnitude is investigated in a case study and is shown to follow a normal distribution. Reversing the polarity of tests shows the leakage current seen in a test is largely independent of the surface degradation pattern present, but the surface degradation pattern over the first three hours may dictate the morphology of ensuing deep erosion.
The synergistic role of platinum and silica as a way to increase the final residue of pyrolized silicone was investigated and explained, giving new interpretations. Conditions were first set to study the thermal degradation of silicones in the presence of platinum based on the simplest silicone/silica/platinum formulation. Numerous parameters, e.g., platinum and silica content or silica surface modifications, were varied to track their influences on the final residues. A thorough DSC study, together with SEM/EDX and Pyrolysis/GC-MS analyses, led us to propose a three-stage process. The key parameter governing thermal stability and final content of the residue is the conjugated actions of immobilizing/cross-linking PDMS chains. Silica particles tether silicone chains through physical interactions, i.e., hydrogen bonding, facilitating a platinum radically catalyzed cross-linking reaction. Practical implications and possible improvements on LSR formulations are finally given.
It is argued that the behavior of dielectric particles as they shrink in size through micrometric to nanometric scales will be increasingly dominated by the properties of their interfaces with the environment. The various interatomic and intermolecular forces that determine the structure of these interfaces are reviewed with special emphasis on their electrical nature. A number of situations in which passive and dynamic dielectric properties are traceable to nanometric interfacial properties are considered. It is also demonstrated that such interfaces are nanometric electromechanical (NEM) systems which acting collectively also explain piezoelectricity in macroscopic systems. Interfaces are naturally nanometric entities and must have a major role in the future development of nanotechnology. Their ubiquitous employment in living systems is noted and comparison suggests synergistic opportunities.
Tracking and erosion of high temperature vulcanizing
(HTV)-silicone rubber (SIR) and the suppression mechanism of alumina
trihydrate (ATH) filler were investigated in the present study. The
tracking and erosion resistance of HTV-SIR filled with 0 to 60%wt ATH
was evaluated by employing an IEC 587 inclined plane (IP) tracking and
erosion test, during which leakage current pulses on HTV-SIR were
counted. Surface temperature distributions and the occurrences of
thermal spots >400°C also were observed by means of an infrared
thermovision study. We employed thermogravimetry (TG)-differential
thermal analysis (DTA)-mass spectrometer (MS) to observe the thermal
degradation of unfilled and filled HTV-SIR in both air and argon. The
test results indicate that 40%wt is a critical ATH level and whether
tracking and erosion is allowed in the IP tracking and erosion test.
Highly filling ATH (>40%wt) reduces the number of low unit silicone
oligomer precursors which promote dry-band arcing as well as the
presence of residual carbon which leads to carbonization. Chemical
modifications of water vapor liberated from heated ATH to methyl groups,
which occurs at the thermal decomposition temperature of silicone
rubber, were found to result in the above process. The protection
mechanisms of ATH for the tracking and erosion of HTV-SIR are formulated
herein