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Article

Effect of Filler Concentration on Tracking Resistance of ATH-Filled Silicone Rubber Nanocomposites

by
Youngtaek Jeon
,
Shin-Ki Hong
and
Myungchin Kim
*
School of Electrical Engineering, Chungbuk National University, Cheongju 28644, Korea
*
Author to whom correspondence should be addressed.
Energies 2019, 12(12), 2401; https://doi.org/10.3390/en12122401
Submission received: 28 May 2019 / Revised: 15 June 2019 / Accepted: 20 June 2019 / Published: 22 June 2019
(This article belongs to the Section F: Electrical Engineering)
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Abstract

:
It is necessary for polymeric materials to have superior tracking resistance against various stress conditions for outdoor applications. In this study, the effect of nano-sized alumina tri-hydrate (ATH) particles on the tracking resistance of silicone rubber (SiR) is studied. Specimens with filler loadings of 1, 3, 5, 10, and 20 wt % are used for performance characterization. From the inclined plane test (IPT) results, apparent improvement in tracking resistance was achieved by mixing 3 wt % of nano-sized fillers, compared to unfilled specimens. ATH/SiR nanocomposites with 5 wt % loading showed comparable tracking performance to SiO2/SiR microcomposites with 20 wt % loading. For detailed analysis, measurements of surface contact angle (SCA) and surface leakage current, and thermo-gravimetric analysis (TGA) were performed. As the nano-ATH filler concentration increased, both thermal stability and leakage current characteristics were improved. Such results agreed with the tracking resistance performance by showing that thermal decomposition and surface charge transport is inhibited in ATH/SiR nanocomposites. Furthermore, performance improvement in nanocomposites was achieved, even at low filler loadings, compared to microcomposites. Meanwhile, the change in SCA was found to be rather limited, regardless of filler loading and filler size.

1. Introduction

The use of silicone rubber (SiR) has shown a continuous increase for outdoor insulation applications. Compared to conventional materials, such as glass and porcelain, SiR has desirable characteristics for outdoor insulation [1,2,3]. Examples of advantages include easier installation, lighter weight, and robust performance against pollution [1,2]. In particular, the hydrophobic surface characteristic prevents wetting and contributes to robustness against pollution [3].
For outdoor installations, the material should maintain its insulation performance against various stress conditions, such as ultra-violet radiation [4], acid rain [5], and dust [6]. As a result of being exposed to such stress, degradation or aging of insulation materials could occur. Representative failure modes that have been reported in outdoor insulation applications are surface tracking and erosion. The occurrence of surface tracking, which is characterized by the formation of an irreversible carbonized conduction pathway [2,7], involves various development stages and factors, such as surface wetting, leakage current flow, dry-band zone formation, and local heat accumulation. [7,8,9].
As a solution to address tracking failures, the addition of various fillers to base SiR has been considered [10,11,12,13,14,15]. It has been reported that such fillers could contribute to increased resistance against tracking by improving properties of polymeric materials, such as thermal conductivity [10] and thermal stability [11,12]. For tracking resistance improvement, the addition of micro-sized alumina tri-hydrate (ATH) or silica particles to SiR has been commonly considered [6,10,13]. The superior tracking resistance of such SiR microcomposites has been explained by the increased thermal conductivity, compared to pure SiR [6]. Considering that thermal decomposition by local heat accumulation is a critical mechanism for tracking failures, the addition of fillers with high thermal conductivity would mitigate thermal accumulation issues in polymeric materials with inherent low thermal conductivity (e.g., SiR and epoxy) [10,14]. In the case of ATH, the water that is released when ATH is exposed to high temperature also aids the heat removal process [11]. While [10] showed that thermal conductivity of silica-based composites tends to be lower than ATH added composites, experimental studies have shown that silica composites show a superior performance in thermal stability and mechanical properties, thanks to tighter molecular bonding [9,10]. In addition, tracking performance of ATH/SiR microcomposites highly depends on filler configuration, filler treatment, and filler concentration [1,15].
Similar to micro-sized particles, it has been reported that tracking resistance of polymeric materials could also be improved by adding nano-sized fillers. For example, it has been shown that higher tracking resistance is achieved in SiO2/SiR nanocomposites when the filler loadings have been increased [12,16]. Such a trend was also found for thermally aged specimens. The effect of nano-sized alumina and titanium dioxides on the tracking resistance of polymeric blend materials was also studied [17]. It was found that the optimal filler concentration value is different depending on the filler type. While the improved tracking resistance could be explained by increased thermal conductivity, it has been highlighted that interface regions of nanocomposites play a critical role [18]. As nano-fillers have a larger surface area compared to micro-sized particles, it is likely that properties of nanocomposites would be affected by the interaction between the added particles and the base material. For example, it has been found that apparent differences in the quantitative material characteristics could be discovered in nanocomposites [18]. In addition, further improvement could be achieved by adding both micro-sized and nano-sized fillers [19]. While only 3 wt % of nano-sized silica was co-filled with micro-sized AlN particles in a SiR composite in [19], considerable improvement in tracking resistance was achieved. The effectiveness of co-filled micro/nanocomposites has also been studied in terms of improved thermal stability and decreased dielectric losses [20].
Meanwhile, research on alternative filler types and the effect of filler configuration has also been performed. Considering the relatively high thermal conductivity of boron nitride (BN), in particular, studies on the application of BN as an additive for polymer composites has received recent interest [21,22,23]. Based on the comparison of various indicators, such as time to tracking failure, thermal images, and weight loss, it was reported that BN particles could effectively prevent local heat accumulation. Research on the effect of BN on mechanical properties was also performed [14]. Configuration of fillers also introduce a noticeable effect on tracking resistance [9]. For example, the effect of irregular and sphere-shaped SiO2 was compared in [9]. In addition, [1] demonstrated that the tracking resistance depends on whether the fillers have been treated during manufacturing.
In this paper, the effect of nano-sized ATH fillers on the tracking resistance of SiR nanocomposites was studied. From past research [1,10], water release during the dehydration reaction of ATH or increased thermal conductivity of ATH/SiR nanocomposites has been generally considered as the main mechanism for superior tracking resistance performance. However, such studies mostly considered micro-sized ATH particles or ATH/SiR nanocomposites with relatively large loadings (>10 wt%). Although it has been shown that nanocomposites could achieve tracking resistance performance similar to microcomposites, even with smaller filler loadings [24,25], detailed analysis on tracking resistance of ATH/SiR nanocomposites with low filler loadings seems to be limited. For filler loadings lower than 10 wt %, the tracking resistance of SiO2/SiR nanocomposites [12] and TiO2/epoxy composites [26] has been reported. While [11] has verified the effectiveness of nano-sized ATH particles for outdoor insulation, filler concentrations larger than 10 wt % have been considered. Compared to such previous research, this study performed a comprehensive analysis on how various factors relevant to tracking resistance (i.e., surface wettability, thermal stability, and surface leakage current) are affected by the addition of small loadings of nano-sized ATH particles. As polymeric nanocomposites with low filler loadings have shown comparable insulation performance to polymeric microcomposites with large filler loadings [12,24], detailed research on tracking failure mechanisms and material degradation of nanocomposites with small filler loadings is necessary [12]. While the performance of polymer composites shows an improvement as the filler loading increases in general, it has been reported that issues such as particle agglomeration [24] and need for specialized preparation methods [27] could occur in nanocomposites with high filler loadings. Depending on filler characteristics, mechanical properties (e.g., tensile strength) could also be affected by filler concentration [28]. In addition, several studies have reported that the relationship between filler loading and insulation performance (e.g., tracking resistance [26] and surface flashover [29]) of nanocomposites showed a drastic change with low filler loadings. Considering such uniqueness of nano-sized particles, it is necessary to perform a detailed analysis of the effect of low filler loadings on the tracking resistance of ATH/SiR nanocomposites.
In order to study the effect of different filler loadings, the tracking resistance was evaluated based on inclined plane test (IPT) results, using nanocomposite specimens. In addition, measurements on surface contact angle (SCA), surface leakage current, and thermo-gravimetric analysis (TGA) were also performed. For performance comparison, not only ATH/SiR nanocomposite but also pure SiR and SiR microcomposite specimens were manufactured to study how tracking resistance is affected by the filler loading and size. The effectiveness of SiR nanocomposites with low filler loading was also verified by comparing the performance with SiR microcomposites and SiR nanocomposites with high filler loading.

2. Material Preparation and Characterization Methods

2.1. Material Preparation

In this study, room temperature vulcanized (RTV) SiR (KE-1402, Shin-Etsu Chemical Co., Ltd., Japan) was used as the base material for the nanocomposites. Nano-sized ATH particles (N&A Materials Industry) with an average particle size of 15 nm and purity higher than 99% were added with different weight concentrations (i.e., 1, 3, 5, 10, and 20 wt %). The mixture of SiR and ATH was mixed using compound mixer THINKY ARM-310 from INTERTRONICS (Oxfordshire, United Kingdom). After adding curing agent (CAT-1402, Shin-Etsu Chemical Co., Ltd., Tokyo, Japan) and solvent (supplied by Samchun Pure Chemical Co., Ltd., Pyeongtaek-si, Korea), the mixture was stirred again. After being poured to a mold, the stirred mixture was degassed in vacuum for an hour. For curing, the mold was kept at room-temperature conditions for 24 hours. For comparison purposes, pure SiR and SiO2/SiR microcomposite specimens were also prepared. Micro-sized SiO2 particles with a particle size of 20 um were mixed with SiR. For microcomposites, filler loadings of 10 wt % and 20 wt % were considered. It is worth noting that SiO2, along with ATH, has been commonly considered as an additive for improving tracking resistance of SiR insulators, thanks to its contribution to enhanced thermal conductivity and molecular bonding [10]. Each specimen was prepared to have dimensions of 12 cm × 4.8 cm × 4 mm.

2.2. Transmission Electron Microscopy (TEM)

In order to explore the morphology of SiR composite specimens, JEM-2100F (JEOL Ltd., Tokyo, Japan) was used. Samples with a thickness of 100 nm were prepared using Tescan-RYRA3. The TEM image of the ATH/SiR nanocomposite with 5 wt % is shown in Figure 1. Similar to previous studies on polymeric nanocomposites [12,24], clusters of agglomerated nanoparticles can also be found. Although such agglomerated clusters could result in decreased interaction between nano-sized fillers and the SiR matrix, further experiments on tracking resistance and thermal stability were conducted in this study to verify the performance of considered ATH/SiR nanocomposites with such clusters.

2.3. Inclined Plane Test (IPT)

The tracking resistance of different samples was compared, based on results of the IPT. The overall test configuration, based on IEC 60587 [30], is shown in Figure 2. Each specimen was placed between two electrodes that were separated by 50 mm. An inclined angle of 45° was introduced to the specimen. Through the electrodes, an ac voltage of 4 kV was applied to the specimen. For contaminant fluid, 0.1 wt % NH4Cl and 0.02 wt % non-ionic Triton-X were added to deionized water. During the IPT, the contaminant fluid was applied to the specimen through a liquid pump at a rate of 0.6 ml/min. The tracking resistance of different specimens was evaluated by comparing the time required for a conduction track to grow longer than 25 mm [30,31].

2.4. Surface Contact Angle

As surface contamination is a main cause for tracking failures in polymeric insulators [7], quantitative characterization of surface hydrophobicity for different filler loadings is necessary. In this study, the effect of different filler concentrations on surface hydrophobicity of SiR micro- and nanocomposites was characterized based on measurement of the surface contact angle (SCA) [3,7]. In order to achieve repeatability, 9 droplets were applied to different areas for each specimen. Measurement of SCA between the specimen and the water droplets was measured using Phoenix 300 Touch equipment of Surface and Electro Optics Co. Ltd. (Suwon, Korea). After taking images of the drop, the corresponding SCA value of each droplet was retrieved using image analysis software.

2.5. Surface Leakage Current

Considering that heat generated from the surface leakage current plays an essential role for initiation of tracking failures [2,10], the effect of different filler loadings on the surface leakage current was also studied. In fact, leakage current has been reported to be an effective indicator for evaluating the progress and severity of tracking failures [32]. As charge transport dynamics along the surface are strongly affected by detailed properties of fillers that are added in polymeric composites [29,33], the characteristics of a surface leakage current with different filler loadings should not be overlooked. In this research, the effect of different filler loadings on the leakage current characteristics was studied using the experimental configuration shown in Figure 3. As shown in Figure 3, a high voltage source was applied between the main electrode and the guard electrode to measure the leakage current flow along the specimen surface. During the measurement, the counter electrode was grounded and current measurement was performed using the 6485 picoammeter of Keithly.

2.6. Thermal Stability

Tracking resistance is also affected by thermal stability characteristics of polymeric composites [11]. As the tracking failure process involves local thermal accumulation, characterization of thermal stability would contribute to detailed understanding of how different filler concentrations affect tracking resistance [11,12]. In this study, Discovery TGA of TA Instruments was used to perform thermo-gravimetric analysis (TGA). For TGA, 8.5 mg of each specimen was used and heat was introduced to each specimen in a nitrogen atmosphere. The temperature was increased from 25 °C to 900 °C at a rate of 20 °C/min.

3. Results

3.1. Tracking Time

Figure 4 shows the tracking time from the IPT results for the considered SiR micro- and nanocomposite samples. In addition, Figure 5 shows the track pattern left on each specimen after IPT. As shown in Figure 4, both micro- and nanocomposite SiR specimens showed longer tracking time compared to pure SiR. In the case of SiR nanocomposites, the tracking time showed a continuous increase as the concentration of nano-ATH fillers increased from 1 wt % to 20 wt %. While a slight increase in tracking time was measured in the 1 wt % nanocomposite specimen compared to pure SiR, a more apparent improvement was demonstrated as the filler loading value reached 3 wt %. The tracking time of the 20 wt % ATH/SiR nanocomposite specimen is nearly seven times longer than that of the pure SiR specimen. Hence, the tracking resistance seems to improve in ATH/SiR nanocomposites as the loading of nano-sized ATH fillers increases. While it has been reported that larger filler loadings would contribute to increased insulation performance of ATH/SiR nanocomposites [11], it is worth noting that the effectiveness of increasing filler concentration on tracking time improvement showed a decrease as the filler loading increased. Such an observation could be made by comparing the increment of tracking time between different filler loadings. Compared to the increment of tracking time observed when the filler loading was increased from 3 wt % to 5 wt % or from 5 wt % to 10 wt %, a rather limited increment of tracking time was observed when the loading increased from 10 wt % to 20 wt %. In fact, such a decreasing trend in effectiveness at higher filler concentrations would imply that an optimal level of filler loadings for performance improvement would exist in ATH/SiR nanocomposites, similar to other polymeric composites [17,29].
It is also worth noting that the tracking resistance performance of the SiR nanocomposite with 3 wt % is higher than that of the SiR microcomposite with 10 wt %. In addition, SiR nanocomposites with 5 wt % loading showed comparable tracking performance to SiR microcomposites with 20 wt % loading. Such comparison results demonstrate that less filler loading is required for performance improvement with nano-sized fillers, compared to micro-sized particles. The advantages of using nano-fillers could also be explained by comparing the tracking time between microcomposites and nanocomposites with the same level of filler loadings. For instance, the tracking time of the 10 wt % nanocomposite is more than four times longer than that of the 10 wt % microcomposite. Such a difference in the required level of filler loading to achieve equivalent performance between microcomposites and nanocomposites is aligned with other studies on nanodielectrics for high voltage applications [11,27,34]. While an agglomeration of nanoparticles was found in nanocomposite specimens during the TEM analysis, nanocomposites still showed superior tracking resistance performance, as shown in Figure 4.
The results of Figure 4 show that the tracking resistance increased with higher filler loadings for both SiR micro and nanocomposites. Such a relationship between tracking resistance and filler loadings has been reported in previous studies with micro-ATH fillers [10] or nano-ATH fillers with relatively high loadings [11]. Representative mechanisms that have been considered in previous studies to explain such improvement include effective heat removal by water released from ATH [10] and high thermal conductivity compared to pure SiR [1,10]. As studied in [10,11], the water released from the dehydration reaction process of ATH (i.e., 2Al(OH)3 → Al2O3 + 3H2O) introduces an effect similar to cooling. The addition of thermally conductive ATH particles contributes to the alleviation of local thermal accumulation so that improvement in tracking resistance could be achieved at higher filler concentrations. In addition to such mechanisms, measurement results of other material characteristics relevant to surface tracking are explored in the following sections to gain comprehensive insights on the effect of nano-sized ATH particles on SiR nanocomposites.

3.2. Surface Contact Angle

Measurement results of SCA are shown in Figure 6, while Figure 7 shows the water droplet pictures during the SCA measurement. As discussed in [35], materials with an SCA larger than 90° are considered hydrophobic and it can be seen from Figure 6 that all specimens have an SCA that is sufficiently larger than 90°. Hence, surfaces of the prepared micro- and nanocomposites could be considered hydrophobic. This hydrophobic nature contributes to improved tracking resistance performance by preventing the surface from being wet by contamination. For example, it has been shown that degradation of tracking resistance is related to the decrease in the SCA for MgO/epoxy nanocomposites [7]. The results of Figure 6 also demonstrate that the difference in the SCA caused by different filler loading seems to be negligible for the considered micro-sized SiO2 and nano-sized ATH fillers. Such a relationship between the SCA and the filler loading was also reported in [35], which reported SCA measurements for ATH/SiR composites with larger concentration values (up to 30 wt %).

3.3. Surface Leakage Current

The surface leakage current measurement results are shown in Figure 8. It can be seen that the leakage current decreases as the concentration of nano-ATH filler increases. This inverse relationship between the leakage current and the filler wt % value could be explained by the change in charge transport dynamics of polymer composites. According to [36], the addition of nano-sized fillers increases the density of deep traps in polymeric materials. From a perspective of charge transport, the existence of deep traps would affect the charge transport dynamics as deep traps contribute to an increased capture of mobile charge carriers [36]. That is, the decrease in the leakage current for the increased concentration of ATH nanoparticles could be explained by the increased density of deep traps in nanocomposites. As the concentration of nanoparticles increases, the effect of deep traps on charge transport properties would become more apparent. This trend could be explained by the continuous decrease in leakage current as the filler loading increases, as shown in Figure 8. It is worth noting that the effect of fillers on the properties of traps (e.g., type and distribution) in composite materials is different depending on various characteristics of fillers, such as filler type and size [29,34].

3.4. Thermal Stability

The TGA curves for the considered ATH/SiR nanocomposites are shown in Figure 9. In order to study how thermal stability characteristics are affected by different filler loadings, TGA was performed for all filler loadings, including pure SiR. Figure 9a represents the TGA for pure SiR (5 wt %, 10 wt %, and 20 wt %) nanocomposite specimens. Figure 9b shows the TGA for nanocomposites with relatively low filler loading (1 wt %, 3 wt %, and 5 wt %) and pure SiR. While the initial weight reduction that starts at around 200 °C could be explained by the water release of ATH fillers [2,35], significant weight decrease occurs as a result of thermal decomposition in the main chemical chain of SiR at temperatures above 350 °C [2]. From Figure 9, in general, it could be said that an increase in the residual weight occurs as the concentration of fillers increases. The increase in residual weight becomes more apparent at higher filler loadings, as shown in Figure 9a. From Figure 9b, the improvement in thermal stability could be clearly demonstrated by comparing the curve of the 5 wt % specimen and the pure specimen. The improvement in thermal stability could be explained by the stronger interaction between nano-sized fillers and the SiR matrix [11,34]. As discussed in previous studies on nanocomposites [18,34], material characteristics of nanocomposites are mostly affected by the interfacial region between nanofillers and base material. While sub-micron sized clusters of agglomerated nano-ATH fillers partly exist in the TEM image of the nanocomposite, an evident increase in interaction between fillers and the matrix would appear as the concentration of nano-sized fillers increased. Such an improvement in ATH particle/SiR matrix interaction could be explained by the increase in the effective surface area of fillers in nanocomposites [37].

4. Discussion

The results of Figure 4 illustrate that the tracking resistance performance of ATH/SiR nanocomposites improves as the concentration of nano-sized ATH fillers increases. As discussed in [7,8], the process of surface tracking failure involves various development stages, such as surface contamination, heat generation, heat accumulation, and thermal decomposition. The relationship between measurement results of material properties and factors that affect development stages of tracking failure is shown in Table 1.
In order to study how the tracking resistance of SiR nanocomposites is affected by the addition of nano-sized ATH fillers, it is necessary to discuss how major contributors of tracking failures are affected by the change in filler loadings, based on the measurement results of this research. From previous studies on SiR microcomposites and ATH/SiR nanocomposites with high filler concentration, the water release from the ATH at elevated temperatures and thermal conductivity have been considered for tracking resistance improvement by addressing heat accumulation issues [6,10]. Based on the experiment results on ATH/SiR nanocomposites with low filler loadings, a further discussion on the effect of nano-sized ATH particles on surface tracking resistance could be performed as follows.
The tracking resistance is affected by thermal properties of polymer composites, such as thermal stability and thermal conductivity [11,22]. Indeed, water release contributes to higher tracking resistance as the released water could hinder local heat accumulation. However, the effect of the interfacial region between the nano-sized particles and the SiR matrix on thermal properties of the overall SiR nanocomposite should also be highlighted. Regardless of analysis models that have been proposed for polymer nanocomposites [29,38,39], it is evident that the volume of the interfacial region would increase substantially as the size of the particle decreased to the range of nanometers. Hence, compared with microcomposites, the interaction between the base polymer and the fillers would become stronger. As a result, an increase in thermal stability was achieved, as shown in Figure 9, even at filler loadings lower than 10 wt %. Similar to studies on epoxy composites filled with highly conductive boron nitride (BN) particles [2], the increased thermal conductivity of ATH/SiR nanocomposites could also contribute to tracking resistance improvement. However, the effect of thermal conductivity would be rather limited for SiR nanocomposites with a relatively low filler loading [14,40]. Since it would be hard for thermal conductive pathways to be formed in polymer composites with a highly dispersed distribution of particles [40], only a limited increase of thermal conductivity could be achieved at such low filler concentrations. In fact, it has been reported that the effect of mixing ATH fillers on the thermal conductivity of epoxy microcomposites is rather limited, compared to BN particles for filler loadings lower than 30 wt % [14].
Considering that the severity of surface tracking is affected by the amount of heat that is generated, it is also necessary to study how the surface leakage current flow changes with different filler loadings. The heat generated during surface tracking is determined by the amount of surface leakage current [2,25]. As shown in Figure 8, the surface leakage current decreases as the filler concentration increases in the considered ATH/SiR nanocomposite. This change in charge motion dynamics along the surface could be explained by the deep traps formed in the energy band gap after mixing nano-sized particles [29,36]. With the addition of nanoparticles, as illustrated in Figure 10, deep traps are introduced and the charge movement along the surface would be restrained. As shown in Figure 9, this effect was apparently discovered even at low filler loadings. In fact, the effect of nano-sized fillers on the surface charge transport dynamics has also been demonstrated in previous studies on surface resistivity [7] and flashover voltage [29] of polymeric nanocomposites. In [7], an increase of surface resistivity with higher filler loadings was found for MgO/epoxy nanocomposites. A similar trend in the increase of surface resistivity was also reported in [7]. In addition, it has been shown that the surface flashover voltage of ATH/epoxy nanocomposites was higher than pure epoxy for filler loadings larger than 2 wt % [29]. Such a change in the flashover voltage was explained by the generation of deep traps. Although the difference in thermal stability was limited for specimens with loadings lower than 3 wt %, as shown in Figure 9b, the tracking time of the 3 wt % specimen increased by more than 100%, compared to that of the pure specimen. This increase could be explained, not only by thermal stability improvement, but also by the rapid decrease in surface leakage current, as shown in Figure 8. As discussed in [24], the surface tracking resistance is affected by characteristics of both the bulk material, which is characterized by TGA results, and the material surface (i.e., transport dynamics of surface charge).
Meanwhile, measurement results of SCA demonstrate that the difference is rather negligible for the considered concentrations, as shown in Figure 6, which is similar to previous studies that considered nanocomposites with higher loadings of ATH [35]. While previous studies on how the SCA is affected by small wt % values of ATH fillers are rather limited, the study of [7] demonstrated that an increase in the SCA could be achieved in MgO/epoxy nanocomposites with larger wt % values. Rather, it is worth noting that aging caused by thermal or water absorption was reported to affect the hydrophobic performance of surfaces of ATH/SiR composites [35].

5. Conclusions

For outdoor insulation applications, the surface tracking resistance of ATH/SiR nanocomposites with different concentrations of ATH fillers was studied. In particular, this research focused on material performance when relatively small filler loadings (<10 wt %) are considered. In order to perform a quantitative comparison, the tracking resistance was characterized using the IPT. In addition, measurement of SCA, leakage current, and TGA were also performed to study the effect of filler configuration (i.e., size and concentration) on tracking performance. From the IPT results, it was found that the tracking resistance improved as the filler loading of nano-sized ATH was increased. While the tracking resistance of both SiR micro- and nanocomposites was found to be higher than that of pure SiR, nanocomposites could achieve equivalent performance or even further improvement with smaller filler loading levels compared to microcomposites. From the experiments, it was found that a tracking performance similar to SiO2/SiR microcomposites with 20 wt % loading could be achieved in ATH/SiR nanocomposites with 5 wt % loading. For specimens with the same filler loadings, a longer tracking time was achieved with SiR nanocomposites than with SiR microcomposites. As the filler loadings of nano-sized ATH increased, the thermal stability improved and the surface leakage current showed a decrease. While improvement of thermal stability could be explained by the increase of the interfacial region volume in nanocomposites, the change in the leakage current could be explained by the additional deep traps that were introduced by the ATH nanofillers. The SCA did not show a noticeable difference with different filler loadings and particle size. In the case of ATH/SiR nanocomposites with small filler concentrations, the improvement of tracking resistance could be explained by an increased thermal stability and inhibited surface charge movement.

Author Contributions

Y.J. and S.-K.H. performed the experiment and analyzed the data; Y.J. prepared the manuscript draft and contributed to the literature review; M.K. proposed the idea, organized the supporting theory, and revised the manuscript.

Funding

This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Education (Grant Number: 2017R1D1A1B03031723).

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. TEM image of alumina tri-hydrate (ATH)/silicone rubber (SiR) nanocomposite with 5 wt % filler loading.
Figure 1. TEM image of alumina tri-hydrate (ATH)/silicone rubber (SiR) nanocomposite with 5 wt % filler loading.
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Figure 2. Diagram of experimental setup for inclined plane test (IPT).
Figure 2. Diagram of experimental setup for inclined plane test (IPT).
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Figure 3. Diagram of surface leakage current measurement.
Figure 3. Diagram of surface leakage current measurement.
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Figure 4. Plot of tracking time for different SiR micro- and nanocomposites.
Figure 4. Plot of tracking time for different SiR micro- and nanocomposites.
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Figure 5. Photograph of SiR micro and nanocomposite specimen after IPT.
Figure 5. Photograph of SiR micro and nanocomposite specimen after IPT.
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Figure 6. Plot of surface contact angle (SCA) for different SiR micro and nanocomposites.
Figure 6. Plot of surface contact angle (SCA) for different SiR micro and nanocomposites.
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Figure 7. Photograph of droplets on surface of SiR micro- and nanocomposites.
Figure 7. Photograph of droplets on surface of SiR micro- and nanocomposites.
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Figure 8. Plot of surface leakage current for ATH/SiR nanocomposites.
Figure 8. Plot of surface leakage current for ATH/SiR nanocomposites.
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Figure 9. Thermo-gravimetric analysis (TGA) curve of ATH/SiR nanocomposites: (a) Pure and filler loadings of 5, 10, and 20 wt %; (b) pure and filler loadings of 1, 3, and 5 wt %.
Figure 9. Thermo-gravimetric analysis (TGA) curve of ATH/SiR nanocomposites: (a) Pure and filler loadings of 5, 10, and 20 wt %; (b) pure and filler loadings of 1, 3, and 5 wt %.
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Figure 10. Deep traps and its effect on charge transport characteristics of SiR nanocomposites.
Figure 10. Deep traps and its effect on charge transport characteristics of SiR nanocomposites.
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Table
Table 1. Relationship between factors of tracking failures and material properties.
Table 1. Relationship between factors of tracking failures and material properties.
Relevant Material PropertiesFactors Affecting Development of Tracking Failure
Surface Contact Angle
(Section 3.2, Figure 6)		Contaminated surface
Surface Leakage Current
(Section 3.3, Figure 8)		Heat Generation
Thermal Stability
(Section 3.4, Figure 9)		Thermal decomposition

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Jeon, Y.; Hong, S.-K.; Kim, M. Effect of Filler Concentration on Tracking Resistance of ATH-Filled Silicone Rubber Nanocomposites. Energies 2019, 12, 2401. https://doi.org/10.3390/en12122401

AMA Style

Jeon Y, Hong S-K, Kim M. Effect of Filler Concentration on Tracking Resistance of ATH-Filled Silicone Rubber Nanocomposites. Energies. 2019; 12(12):2401. https://doi.org/10.3390/en12122401

Chicago/Turabian Style

Jeon, Youngtaek, Shin-Ki Hong, and Myungchin Kim. 2019. "Effect of Filler Concentration on Tracking Resistance of ATH-Filled Silicone Rubber Nanocomposites" Energies 12, no. 12: 2401. https://doi.org/10.3390/en12122401

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