The Role of Inorganic Fillers in Silicone Rubber for Outdoor Insulation Alumina Tri-Hydrate or Silica

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1 F E A T U R E A R T I C L E The Role of Inorganic Fillers in Silicone Rubber for Outdoor Insulation Alumina Tri-Hydrate or Silica Key Words: Silicone rubber, fillers, alumina tri-hydrate, silica, erosion resistance Introduction o improve the tracking and erosion resistance of silicone T rubber compositions for outdoor applications, fillers are added to the base composition. Sufficient filler is added to prevent tracking, but erosion still occurs in tests such as the ASTM D2303 inclined plane test. Although the ways in which fillers impart erosion resistance to silicone rubber vary with the type of filler [1], [2], [3], the improved thermal conductivity of the composite material is a common characteristic [4], [5]. Commonly used fillers in silicone compositions are alumina tri-hydrate (ATH) and silica. Although various studies have reported on the benefits of using either filler, the focus has generally been on the effects of filler particle size and concentration rather than comparative studies between the two types of fillers [6]. In addition, knowledge of the base composition in regard to additional fillers has generally been unknown [1], [7]. In general, a systematic approach to understand the way these fillers improve the erosion resistance of the composite material is lacking. Role of Fillers Silicone materials exhibit a low surface tension that renders them highly hydrophobic. This is generally considered as the main defense mechanism in preventing the development of leakage current that is a precursor to dry band arcing. The heat that is generated during dry band arcing is considered to be the primary source of material degradation [8], [9], [10]. The presence of fillers in the material then forms the second line of defense in helping to prevent tracking and to limit erosion of the material. To study the compositions, researchers have relied on various tests such as the inclined plane test, salt-fog test, and the tracking wheel. All of these tests attempt to simulate the outdoor conditions of wetting and contamination, both of which are required to initiate dry band arcing. Various studies have reported on the benefits of using ATH [11], [12] and silica [1], [7], [13] in silicone rubber formulations, although one controversial study has indicated that ATH is not L.H. Meyer University of Blumenau, SC, Brazil E.A. Cherney and S.H. Jayaram University of Waterloo, ON, Canada The industry perception that alumina tri-hydrate filler imparts better erosion resistance than silica in silicone rubber can be misleading. necessary to impart tracking and erosion resistance [14]. ATH filler (Al 2 O 3 3(H 2 O)) imparts a slightly better thermal conductivity to silicone rubber compositions than silica (SiO 2 ) does [4]. In addition, as the local hot spot temperature from dry band arcing exceeds 220 o C, ATH begins to release water of hydration, which is generally recognized as an efficient way to remove heat from the hot spots. However, the release of the hydrate also causes surface roughness, which leads to further wetting and dry band arcing [15]. On the other hand, silica, which is classified as a semi-reinforcing filler, improves the physical properties of silicone compositions through molecular bonding with the silicone polymer. For ATH to exhibit a similar bonding, the filler requires surface treatment, a process called silanization at higher cost. It is generally known that good dielectric properties stem from good physical properties in a material [16], and, therefore, the addition of silica filler is expected to be more beneficial than ATH. July/August 2004 Vol. 20, No. 4 13

2 Materials Used A) Silicone Rubber A two-part room temperature cure liquid silicone rubber (General Electric RTV 615) was selected as the base material in this study. The clear, low viscosity rubber is composed of about 70 % vinyl-polydimethylsiloxane resin with 30 % vinyl. This rubber was chosen as it contains no filler of any type, including fumed silica, which may influence the results. B) Silica Micronized Min-U-Sil grade silica was chosen from U.S. Silica for its high purity (>99% pure) and narrow particle size distributions. The types and physical properties applicable are listed in Table I. C) ATH Both SF2 and FRF grades of ATH from Alcan Chemicals were selected also for their high purity and narrow particle size distributions. Table I shows the applicable physical properties of the ATH grades. Sample Preparation The samples were prepared using 10, 30, and 50% concentrations by weight of the various fillers. To simplify the discussion in this article, the mean particle size applicable to both types of fillers is referred to as, 5, and 10 µm. Table II shows a summary and grouping of the prepared samples. The base silicone rubber and fillers were weighed accurately and mixed thoroughly. After the addition of the curing agent, the mixture was stirred again, poured into a mould, degassed, and allowed to cure at room temperature for at least 24 h. Six samples were prepared from each composition and were cut into the appropriate size according to ASTM D2303 [17]. The surfaces of all specimens were sanded with very fine sandpaper under tap water, washed with distilled water, and cleaned with isopropyl alcohol prior to testing. In addition, hot-pressed samples were prepared following the procedure described previously, but, instead of allowing the samples to cure at room temperature and pressure, the samples TABLE I. Specifications of the fillers. Filler Property Filler type Filler name Median particle size Density (µm) (g/cm3) ATH SF2E FRF FRF Silica Min-U-Sil Min-U-Sil Min-U-Sil TABLE II. Summary of prepared samples. Features RTV Hot pressed Filler mean particle size (µm), 5, and 10, 5, and 10 Filler mix ratio (wt%) 0, 10, 30, and 50 0, 30, and 50 Sample dimensions L W H (mm) were cured in a stainless steel mould at 150 C and a pressure of 0.5 kg/mm 2 for 30 min. Specimens were also cut from various locations and depths in the main mould for examination under a scanning electron microscope (SEM) with an attached Energy Dispersive X-Ray analyzer (EDAX) to check for uniformity of filler dispersions. The results have shown that filler dispersions were uniform throughout the samples. Experimental An inclined plane tracking and erosion test, in accordance with ASTM D2303, was setup to test six samples simultaneously. The test procedure followed the ASTM standard, with an initial voltage of 2.0 kv and a constant contaminant flow rate of 0.15 ml/ min for 4 h. At the end of the test, the samples were taken from the test bay, and the eroded mass was estimated by filling the eroded volume with a soft putty of known density. The weight of the putty was determined using a microbalance and was used in the eroded volume calculations. By knowing the density of the samples, the eroded mass was determined. A data acquisition system, composed of voltage dividers, overvoltage and overcurrent protection circuits, and a data acquisition card recorded the source voltage, voltage, and current for each sample. During the test, for every second, four 60-Hz cycles of data were acquired at a sampling rate of 7580 samples/s. These four cycles were processed by the Fast Fourier Transform (FFT) technique, and the RMS power of the fundamental and low frequency odd harmonics (3rd, 5th, 7th, 9th, and 11th) were computed. By integrating the fundamental and harmonic powers over time, the fundamental, harmonic, and total energy were calculated. The software for analyzing the data recorded by the data acquisition system was built on a LabVIEW platform [18]. The graphical interface allowed for the acquisition parameters setting, acquisition card channel selection and visualization, voltage control, visualization of the instantaneous, and the accumulated active power for the fundamental and harmonic components. Figure 1 shows the inclined plane setup and the data acquisition system, along with the video and thermal cameras to view the dry band arcing activity. Inclined Plane Results A) RTV Samples Fig. 2 shows the results of the first series of tests on RTV samples. It is clear that filler of either type and concentration is necessary to minimize erosion in the inclined plane test. At 10% filler concentration, the eroded mass does not appear to be de- 14 IEEE Electrical Insulation Magazine

3 120V Variac 120V/8 kv Transformer Ballast Resistor Top Electrode Sample Holder Bottom Electrode Sample Voltage Channel Current Channel Peristaltic Pump Contaminant Thermovision and Web Cameras Data Acquisition B) Hot-Pressed Samples As anticipated, the hot-pressed samples with either filler also showed that increased filler concentration improves the resistance to erosion (Figure 3). However, a large spread in the results is evident in the samples containing 30% ATH. ATH filler having a mean particle size of 5 µm showed better resistance to erosion than either - or 10-µm particles. At 50% filler concentration, no significant difference in the erosion resistance between samples filled with ATH or silica was seen, although the silica-filled samples tended to show a slightly better resistance to erosion. Figure 1. Inclined plane test set-up with infrared and web camera and data acquisition system. pendent on the size of the filler particles, and samples with ATH filler show a slightly better performance over samples with silica filler. However, at higher filler concentrations, the difference in the eroded mass between samples with the two fillers is not significant, even though greater dispersion in the silica-filled samples is shown. Laser-Based Thermal Conductivity As the details of this technique appear elsewhere [19], only a summary of the method is described here. A continuous mode semiconductor laser, of 7.5 W power and wavelength of 808 nm, was used to irradiate the silicone rubber samples through an optical fiber and a focusing lens irradiating a spot of about 2 mm diameter on the samples. An infrared camera recorded the thermal response of the samples, and thermovision acquisition software analyzed the temperature image. Two methods were used to estimate the thermal conductivity of the samples. In the first method, the samples were rapidly Filler Weight (%) and Filler Particle Size (µm) 10% 30% 50% 10% 30% 50% No Filler Silica Samples ATH Samples Eroded Mass (mg) Figure 2. Eroded mass of RTV samples subjected to the inclined plane test. Bars mark the 10 and 90 percentiles; extremities of the hatched box are at 25 and 75 percentiles, and the center line represents the statistical mean of the data. July/August 2004 Vol. 20, No. 4 15

4 Filler Weight (%) and Filler Particle Size (µm) 30% 50% 30% 50% Silica Samples ATH Samples Eroded Mass (mg) Figure 3. Eroded mass of hot-pressed samples subjected to the inclined plane test. Bars mark the 10 and 90 percentiles; extremities of the hatched box are at 25 and 75 percentiles, and the center line represents the statistical mean of the data. heated to 212 C and, after maintaining this temperature for 1 min, the laser beam was blocked, and the temperature cooling profile on the surface of the samples was recorded. The thermal conductivity was extracted from the fitted temperature decay curve, assuming conduction as the only mode of heat transfer. In the second method, the transient response model developed by Carslaw and Jaeger [20] for a continuous and constant point source of heat was applied. The recorded transient temperature response was curve fitted to the model from which the thermal diffusivity for each sample was obtained. From the density of the silicone and fillers given by the manufacturer s data sheets and applying the rule of mixtures, the heat capacity was obtained from which the thermal conductivity was extracted for each sample. For comparison, the thermal conductivity of several samples, with both types of filler, was determined following the ASTM E1225 [21] method. The measured values agreed well with the laser method. The thermal conductivities obtained by the laser method are shown in Tables III and IV. It is clear that increased filler level leads to higher thermal conductivity in all samples. For the samples containing ATH, the highest thermal conductivities were found in the samples having 5-mm mean particles. Agglomeration of the ATH particles in samples made from -µm mean particles has been shown to be the reason for the reduction in thermal conductivity for these samples. In general, thermal conductivity increased with decreasing particle size, and the silica-filled samples consistently showed higher thermal conductivity than the ATH-filled sample, despite the intrinsic higher thermal conductivity of ATH filler being approximately 15 times higher than that of silica. SEM Scanning electron microscopy studies were conducted to examine agglomeration and bonding of the filler particles. In Table III. Estimated thermal conductivities of RTV silicone rubber samples based on infrared laser. Filler Mean Thermal conductivity (W/m o C) particle size (µm) 10% 30% 50% ATH Silica IEEE Electrical Insulation Magazine

5 TABLE IV. Estimated thermal conductivities of hot-pressed silicone rubber samples based on infrared laser. Filler Mean Thermal conductivity (W/m oc) particle size (mm) 30% 50% ATH Silica samples made using -µm mean particles of ATH, it is visually evident that significant agglomeration of the smaller particles was present, in effect making the sample appear as though much larger particles were used in the sample preparation. This was not evident in samples that were made from silica. Furthermore, in samples made from ATH filler, bonding of ATH filler particles to the silicone polymer matrix core was not as extensive as found in samples that contained silica in which the silica particles showed good bonding to the silicone polymer matrix. Microphotographs showing agglomeration and poor bonding are shown in Figures 4 and 5. Thermal Conductivity vs. Erosion A silicone rubber sample filled with 10% ATH or silica has an average thermal conductivity of 0.31 W/m C, whereas a sample filled with 50% ATH or silica has an average thermal conductivity of about 0.56 W/m C, which is nearly twice that of the 10% sample. To illustrate the effect of thermal conductivity on erosion, the erosion of the various samples as determined from the inclined plane test (IPT) is shown in Figure 6 as a function of the thermal conductivity. A correlation between erosion and thermal conductivity is clearly evident for both types of fillers. At Figure 4. SEM of a sample containing -µm mean particles of ATH and 50% concentration showing agglomeration of smaller particles of ATH. Figure 5. SEM of a sample containing 10-µm mean particles and 50% concentration showing selected areas of poor bonding to the silicone matrix. 10% filler loading, silica is not as effective in minimizing erosion as ATH. However, at 30 and 50% filler loading, both fillers perform equally well in preventing erosion. Harmonic Power and Temperature The development of leakage current on the surface of the samples results in a continuous interruption of the leakage current because of the formation of dry bands. In fact, the current is not really interrupted; rather, it assumes a distorted sinusoidal waveform. The harmonic power produced by dry band arcing has been previously explored, and it has been shown that the third harmonic component contributes significantly to material erosion [22]. A subsequent study has shown the correlation between the dry band arcing temperature and the third harmonic power [23]. An example of the simultaneous recordings of both the dry band temperature and the third harmonic power is shown in Figure 7. Hot spots develop from the energy expended during dry band arcing, and the resulting heat gives rise to erosion of the material. Thermal Degradation Model Because of the diffusion of the heat that is generated by dry band arcing, the surrounding area of the dry band site will show an increased temperature, which aids in lowering the temperature of the dry band site. In this context, a silicone formulation with a high thermal conductivity diffuses heat much quicker than one with a lower thermal conductivity. The thermal conductivity of filled silicone rubber is dependent on filler type, concentration, particle size, and bonding between the filler and the silicone polymer matrix. To illustrate the effect of thermal conductivity on heat distribution during the inclined plane test, a FEMLAB [24] finite element simulation program was used to calculate the temperature distribution resulting from a hot spot produced during dry band arcing in which an arbitrary energy of 100 J is expended during dry band arcing. The temperature line profiles are used to illustrate the eroded mass, considering that the volume defined by temperatures >350 C (from thermogravimetric tests) is degraded, as illustrated in Figure 8. The simulation of the July/August 2004 Vol. 20, No. 4 17

6 Figure 6. Correlation between thermal conductivity and erosion rd Harmonic Temperature Power (W) Temperature ( o C) Time (min) Figure 7. Simultaneous plots of third harmonic power and temperature during an inclined plane test on silicone rubber samples filled with 30% ATH employing 5-µm mean particles. 18 IEEE Electrical Insulation Magazine

7 Temperature T = f{p,c,k} temperature profiles of samples filled with ATH and silica at 10, 30, and 50% concentrations is shown in Figure 9. Figure10 shows the correlation between the eroded mass in the IPT to the estimated eroded mass predicted by the thermal degradation model. At a filler concentration of 10%, the poorer correlation between the simulated and experimental results is thought to be due to the water of hydration in ATH having an effect on erosion. 350 o C r Radius Vol = 2/3 π r 3 Figure 8. Illustration of thermal degradation model showing the temperature profile and the extracted radius of the semisphere that corresponds to the estimated eroded volume. Conclusions A systematic study to understand how ATH and silica fillers improve the erosion resistance of silicone rubber during dry band arcing has demonstrated 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 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 filler No Filler 1000 Temperature ( o C) % ATH 30% silica 10% ATH 10% silica 50% silica 50% ATH Distance from the centre of the sample (mm) Figure 9. Simulation of the temperature profiles of samples filled with ATH and silica at 10, 30, and 50% concentrations. July/August 2004 Vol. 20, No. 4 19

8 Figure 10. Correlation between the eroded mass in the inclined plane test to the eroded mass predicted by the thermal degradation model. Acknowledgment This article has been extracted from a University of Waterloo Ph.D. thesis entitled, A Study of the role of Fillers in Silicone Rubber Compounds for Outdoor Insulation, by Luiz Henrique Meyer, October References [1] J. Mackevich and S. Simmons, Polymer outdoor insulating materials part II: Material considerations, IEEE Elect. Insul. Mag., vol. 13, no. 4, pp , Jul/Aug [2] R.S. Gorur, E.A. Cherney, R. Hackam, and T. Orbeck, The electrical performance of polymeric insulating materials under accelerated aging in a fog chamber, IEEE Trans. Power Delivery, vol. 3, pp , [3] R.S. Gorur, E.A. Cherney, and R. Hackam, Performance of polymeric insulating materials in salt-fog, IEEE Trans. Power Apparatus Syst., 1986, SM [4] R.S. Gorur, E.A. Cherney, and R. Hackam, The AC and DC performance of polymeric insulating materials under accelerated aging in a fog chamber, IEEE Trans. Power Delivery, vol. 3, pp , [5] L.H. Meyer, S.H. Jayaram, and E.A. Cherney, Thermal characteristics of RTV and hot pressed silicone rubber filled with ATH and silica under laser heating, in Proc. Conf. Elect. Insul. Dielect. Phenomena, Albuquerque, Oct , 2003, pp [6] R. Hackam, Outdoor HV composite polymeric insulators, IEEE Trans. Dielect. Elect. Insul., vol. 6, no. 5, pp , [7] J. Mackevich, S. Simmons, M. Shah, and R. J. Chang, Polymer outdoor insulating materials part III: Silicone elastomers considerations, IEEE Elect. Insul. Mag., vol. 13, no. 5, pp , Sep/Oct [8] S. Kumagai and N. Yoshimura, Tracking and erosion of HTV silicone rubber and suppression mechanism of ATH, IEEE Trans. Dielect. Elect. Insul., vol. 8, pp , [9] S. Kumagai and N. Yoshimura, Tracking and erosion of HTV silicone rubbers of different thickness, IEEE Trans. Dielect. Elect. Insul., vol. 8, pp , [10] T.G. Gustavsson, S.M. Gubanski, H. Hillborg, S. Karlson, and U.W. Gedde, Aging of silicone rubber under AC or DC voltages in a coastal environment, IEEE Trans. Dielect. Elect. Insul., vol. 8, pp , [11] E. Wendt and H. Jahn, Influence of chemical design of insulating silicone compounds on hydrophobic and electrical behavior, Insulator News and Market Rep. Conf. Proc., [12] S.H. Kim, E.A. Cherney, and R. Hackam, The loss and recovery of hydrophobicity of RTV silicone rubber insulating coatings, IEEE Trans. Power Delivery, vol. 5, no. 3, pp , [13] H.J. Kloes and D. Koenig, Multifactor-surface-tests of organic insulation materials in the early stage of degradation, IEEE Int. Symp. Elect. Insul., pp , IEEE Electrical Insulation Magazine

9 [14] R. Chang and L. Mazeika, Analysis of electrical activity associated with inclined plane tracking and erosion of insulating materials, IEEE Trans. Dielect. Elect. Insul., vol. 6, no. 3, pp , [15] S. Kim, E A. Cherney, and R. Hackam, Effects of filler level in RTV silicone rubber coatings used in HV insulators, IEEE Trans. Elect. Insul., vol. 27, pp , [16] L. E. Nielsen, Mechanical Properties of Polymers and Composites. NY, USA: Marcel Dekker, [17] ASTM-D2303, Standard test method for liquid-contaminant, inclined-plane tracking and erosion of insulating materials. [18] National Instruments, AT-MIO Data acquisition series user s guide, [19] L. Meyer, V. Grishko, S. Jayaram, E. Cherney, and W.W. Duley, Thermal characteristics of silicone rubber filled with ATH and silica under laser heating, in Proc. Conf. Elect. Insul. Dielect. Phenomena, Cancun, Mexico, Oct , 2002, pp [20] S. Carslaw and J.C. Jaeger, Conduction of Heat in Solids. London: Oxford Press, [21] ASTM E , Standard test method for thermal conductivity of solids by means of the guarded-comparative-longitudinal heat flow technique. [22] A. El-Hag, S. Jayaram, and E.A. Cherney, Fundamental and low frequency harmonic components of leakage current as a diagnostic tool to study aging of RTV and HTV silicone rubber in salt fog, IEEE Trans. Dielect. Elect. Insul., vol. 10, pp , [23] L. Meyer, S. Jayaram, and E.A. Cherney, Correlation of damage, dry band arcing energy, and temperature in inclined plane testing of silicone rubber for outdoor insulation, Reviewed manuscript accepted for publication in IEEE Trans. Dielect. Elect. Insul. [24] Comsol Group, Multiphysics Modeling with FEMLAB - User Guide, Luiz Henrique Meyer (S 93) is a professor in the Electrical Engineering Dept., University of Blumenau, SC, Brazil. He received his B.S. and M.S. in Electrical Engineering from the Universidade Federal de Santa Catarina, Florianópolis, Brazil, in 1991 and 1994, respectively, and his Ph.D. degree in Electrical Engineering from University of Waterloo in He joined the Electrical Engineering Dept. of the Universidade Regional de Blumenau, Blumenau, Brazil, in 1993, where he has been involved with academic and research activities. From 1998 to 2000 he was Director of the Technological Research Institute of the Universidade Regional de Blumenau, Blumenau, Brazil. His areas of research interest are high voltage engineering, polymeric insulating materials, and power apparatus. Edward A. Cherney (M 73, SM 83, F 97) received the B.Sc degree in Physics and Chemistry from the University of Waterloo, the M.Sc degree in Physics from McMaster University, and the Ph.D. degree in Electrical Engineering from the University of Waterloo in 1967, 1969, and 1974, respectively. In 1968, he joined the Research Division of Ontario Hydro, and, in 1988, he went into the insulator industry, first with a manufacturer of insulators and then later with a manufacturer of silicone materials. Since 1998, he has been an international consultant in the outdoor insulation field and an adjunct professor at the University of Waterloo. He has published extensively on outdoor insulation, holds several patents, co-authored one book on outdoor insulators, is involved in several IEEE working groups on insulators, is a registered engineer in the province of Ontario, and is a Fellow of the IEEE. Shesha Jayaram (S 88, M 91, SM 97) is a professor in the Electrical and Computer Engineering Dept., University of Waterloo, Waterloo, and an adjunct professor at the University of Western Ontario, London. She received the B. A. Sc. degree in Electrical Engineering from Bangalore University; the M. A. Sc. in High Voltage Engineering from Indian Institute of Science, Bangalore; and the Ph.D. degree in Electrical Engineering from the University of Waterloo in 1980, 1983, and 1990, respectively. Prof. Jayaram s research interests are developing diagnostics to analyze insulating materials, industrial applications of high voltage engineering, and applied electrostatics. Prof. Jayaram has been an active member of the IEEE Dielectric and Electrical Insulation Society and the Electrostatic Processes Committee (EPC) of the IEEE Industry Applications Society. In both, she has contributed as a board member, chair of EPC during , session organizer/chair, and as a member of the paper review process committee. She is a registered professional engineer in the province of Ontario, Canada. July/August 2004 Vol. 20, No. 4 21