aging characteristics of rtv silicone rubber insulator coatings

Aging Characteristics of RTV Silicone Rubber Insulator Coatings

A. Naderian Jahromi, Edward A. Cherney, Shesha H. Jayaram

Department of Electrical and Computer Engineering, University of Waterloo 200 University Avenue West, Waterloo, ON, N2L 3G1, Canada

and Leonardo C. Simon

Department of Chemical Engineering, University of Waterloo

ABSTRACT The paper reports on tests done on four commercial room temperature vulcanized (RTV) silicone rubber high voltage insulator coatings aimed at achieving information on the relative performance of the coatings with respect to aging. The initial characterization of the coatings includes the identification of the primary filler type, concentration of filler, the amount of free fluid, and the molecular species of the free fluid. Soxhlet extraction and gas chromatography tests show considerable depletion of fluid after accelerated aging tests in acid-water and the associated changes to the physical properties is assessed by standard mechanical tests on coupon size specimens. The effect of acid-water aging on the hydrophobicity is assessed by measurements of the static contact angle and the onset of leakage current in an incline plane test. Several field aged coatings are assessed for residual LMW fluid which is compared to accelerated aging tests.

Index Terms - RTV silicone rubber coating, HVIC, accelerated aging, depletion of fluid, hydrophobicity, de-polymerization, Soxhlet extraction, lifetime, gas chromatography, LMW fluid.

1 INTRODUCTION

OF the concerns that users have today in using room temperature vulcanized (RTV) silicone rubber coatings for high voltage insulators, surely the life expectancy of the coatings has been the main one. Lifetime is not easy to assess as many factors play a role in the life of a coating. However, it is generally considered that the end of life is associated with the depletion of the low molecular weight fluid resulting in permanent changes to the hydrophobicity of the surface layer. More specifically, a loss of fluid leads to a greatly reduced rate of return of hydrophobicity after experiencing a temporary loss for example, during rain washing. Ultimately, a permanent loss of hydrophobicity may occur but this has not yet been observed in the field.

Considerable research has been reported on the performance of high voltage insulator coatings (HVIC) dealing with the type and amount of inorganic fillers as documented in a review article by Hackam [1]. These studies have generally been performed in the laboratory by exposing samples to salt-fog or in an inclined plane test. The time in which a temporary loss of hydrophobicity as gauged by the onset of leakage current and the ensuing physical erosion due to dry band arcing of the various coatings have been compared. However, in all of this work, the main

emphasis has been on the degree at which the various fillers are capable of reducing the mass loss during dry band arcing. Of course this assumes that the temporary loss of hydrophobicity inevitably leads to the development of leakage current and dry band arcing on insulators in the field. After many years of HVIC use, this observation has yet to be reported. What has been observed by users is a loss of hydrophobicity but only on the unprotected parts of the leakage path., for example on the upper surfaces of insulators as evident in Figure 1. Flashover has also been reported without the usual dry band activity that is normally associated with uncoated insulators. This type of flashover is somewhat analogous to a wet flashover of a non-ceramic insulator but at normal line-to-ground voltage and the mechanism has been referred to as a “sudden flashover” which has been previously discussed by Gorur et al [2].

The loss of hydrophobicity of a RTV coating occurs with time through natural washing and this process takes a very long time before the coating performance is affected. In addition, as fluid diffuses into surface contaminant, the removal of contaminant speeds up the depletion of the fluid responsible for hydrophobicity. Furthermore, in coastal regions where wetting is often constant, the loss of hydrophobicity occurs very rapidly. In other regions the loss of hydrophobicity is accelerated by acid-rain which often causes depolymerization of the RTV coatings [3, 4].

Manuscript received on 10 January 2007, in final form 24 April 2007.

444 A. Naderian et al.: Aging Characteristics of RTV Silicone Rubber Insulator Coatings

1070-9878/08/$25.00 © 2008 IEEE

Figure 1. Photograph of the top end of a RTV coated 500 kV station post insulator taken at night showing corona discharge from wetted parts of the insulator without dry band arcing.

The definition of acid rain is not very clear; generally

rainwater having a pH below 5.6 is called acid-rain. The main acids in rain are sulfuric and nitric acids [5] and the root cause of acid rain stems from the burning of coal and hydrocarbon fuels. When the atmosphere is polluted with sulfur dioxide (SO2) or nitrogen oxides (NOX), rain-water goes through oxidation with ozone (O3) or hydrogen peroxide (H2O2) forming H2SO4 or HNO3 before reaching the earth. The most severe influence of acid rain on RTV coatings is depolymerization [4] and in the extreme cases reported, the coating becomes very soft somewhat similar to a putty. Acid-depolymerization of RTV coatings occurs when OH radicals begin to sever polymer chains resulting in shorter chains in which the manifestation is a drastic change in material physical properties such as a loss of tensile strength, elongation, and hardness. Previous studies have shown that nitrous oxides are more harmful and corrosive than sulfuric acid [6].

Various rates of aging have been reported in different types of acids and a variation in pH between 2 and 5.6 has been used for this purpose [3-5, 7, 10, 11]. According to a field measurement in a 230 kV high voltage substation, the acid rain has a pH of 3.9 [3]. By comparing the weight loss of field aged RTV samples to laboratory aged samples, this study suggested that one month of immersion in nitric acid of pH=3.9 at 90 0C in the laboratory is equivalent to 90 months of field aging.

In this work, four commercial RTV coatings are examined for performance characteristics with respect to aging. The loss of hydrophobicity is assessed by the ease in which leakage current develops in an inclined plane test, measurement of the static contact angle, loss of LMW fluid, and the loss of LMW species. The associated effect of

depolymerization due to acid-water is assessed by investigation of physical properties, i.e. standard mechanical tests of hardness and elongation of coupon specimens.

2 CHARACTERISTICS OF COATINGS EVALUATED

Considerable variation in viscosity, skin-over and cure times, and specific gravity of the four commercial coatings was observed in the as-received condition. Therefore, all samples were made under the same conditions after thorough mixing and left to air cure for 7 days prior to characterization and acid-water immersion. The specific gravity, primary filler type and concentration, porosity, the amount of free fluid available, were the main properties used in the characterization of the coatings. The method by which each property was determined is briefly outlined below. Table 1 summarizes sample composition and the data obtained in the initial characterization.

Table 1. Characteristics of cured coatings.

RTV Coating

Primary Filler

Filler Wt %

Specific Gravity

Water Absorption

Wt %

Fluid Wt %

A ATH 54 1.48 13.6 2.26 B ATH 51 1.54 4.20 1.13 C ATH 47 1.45 17.4 3.47 D Silica 40* 1.61 4.10 1.14

* Estimated from specific gravity

2.1 SPECIFIC GRAVITY The specific gravity of each coating was measured before

and after curing by accurately measuring the weight and volume of the specimens.

2.2 PRIMARY FILLER TYPE Specimens of the coatings were studied in a scanning

electron microscope (SEM) with an energy dispersive X-ray attachment (EDAX). The particles that were dispersed throughout the rubber matrix were scanned and X-ray counts of the elements were noted. The presence of aluminum (Al) or silicon (Si) X-ray counts from the particles are attributed to alumina and silica fillers, respectively. Thermo-gravimetric analysis (TGA) confirmed that Al in three of the coatings was from alumina tri-hydrate (ATH).

2.3 PRIMARY FILLER CONCENTRATION Thermo-gravimetric analysis was used to analyze the

coatings thermally. In this measurement, the temperature was increased at a rate of 20 0C/min from 100 to 800 0C. The weight loss versus temperature for the RTV coatings is shown in Figure 2. For coatings A, B and C, the curve trend changes at around 200 0C which is correlated to the release of the water of hydration and confirms ATH as the primary filler type. The concentration of ATH in these coatings can be calculated from the percentage weight loss

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 15, No. 2; April 2008 445

at 350 0C being due to the release of the water of hydration from the ATH filler. For coating D, the silica filled coating, the specific gravity is used to estimate the concentration of filler.

Figure 2. Thermo-gravimetric plots of cured coating specimens.

Once again, RTV-D has not ATH, as no water of

hydration was extracted in TGA test. Apart from the filler concentration, the thermal stability of coatings was observed from the TGA, which is outlined in Table 2. The definition of the parameters and evaluation shown in the table is based on reference [7]. The initial degradation temperature (Ti) corresponds to a 1% thermal decomposition and this temperature is about 100 oC higher for RTV-D. Also, T50 is more than 500 oC for this coating. The comparison of thermal decomposition of the coatings clearly shows that RTV-D is more thermally stable than the others considering Ti and T50.

Table 2. Parameters evaluated from TGA.

RTV Coating Ti (oC) TATH (oC) T50 (oC) Tf (oC)

A 250 343 410 560 B 222 350 463 600 C 240 349 490 560 D 345 - 523 570

Definition of Parameters [7]: Ti: Initial decomposition temperature, TATH: Water released ending temperature, T50: 50% weight loss temperature, Tf: Final decomposition temperature.

2.4 WATER ABSORPTION Water absorption of a RTV coating is an indicator of

porosity and in filled materials this is normally associated with poor bonding between the filler particles and the silicone matrix and spaces between the particles. Moreover, soaking in water has been used previously to evaluate the loss of hydrophobicity of polymer surfaces [8]. In these tests, two samples of each coating were immersed in 80 0C tap water, of 350 μS/cm conductivity, for 15 days. At 5 day intervals, the samples were removed, air dried, and the weight gain was recorded using a microbalance having a sensitivity of 0.1 mg. The static contact angle, as recorded with a digital camera from which the angle was measured.

The average contact angle was calculated from ten droplets; each having a volume of about 20 Lμ , and these results are shown in Figures 3 and 4.

The water immersion test indicates that the increment of water absorption for RTV-D and RTV-B is an order of magnitude smaller than RTV-A and RTV-C. RTV-C lost hydropobicity after 15 days whereas the other coatings still exhibited a static contact angle of 85 degrees. Scanning electron microphotographs show considerable agglomeration of micron size filler particles in RTV-A and this may be the reason for increased voids while in RTV-C, the relatively large particles give rise to large voids in the silicone matrix and the microphotographs suggest poor bonding between the particles and the matrix.

Figure 3. Weight gain of samples measured after water immersion.

65

75

85

95

105

115

0 5 10 15Number of Days

Con

tact

Ang

le, D

egre

e RTV-ARTV-BRTV-CRTV-D

Figure 4. Static contact angle measured as a function of immersion time in water.

2.5 LMW FLUID CONCENTRATION The fluid content of the silicone rubber was measured

using the Soxhlet extraction technique with analytical grade n-hexane [9]. Extraction was carried out for 72 hours on two specimens of each coating. The dimensions of specimens were 2 cm x 2 cm, and 4 cm x 2 cm respectively. A vacuum oven was employed to dry the samples for 10


hours at 80 oC and at 0.1 kPa pressure. The difference in weight of the specimens before and after immersion in n-hexane after drying in an oven is assumed to be a measure of the low molecular weight (LMW) content. A microbalance having a sensitivity of 0.1 mg was used in this measurement. RTV-C showed the highest percentage of free fluid while RTV-B and RTV-D had the lowest and nearly the same percentage of free fluid.

2.6 GAS CHROMATOGRAPHY ANALYSIS OF LMW FLUID

Gas chromatography (GC) was employed to investigate the cyclic species of the LMW extracted from the bulk and the surface layer of the RTV coatings. The gas chromatographs indicate the distribution of polydimethylsiloxane, [(CH3)2Si-O]n, in which n= 3, 4, 5,…., where n=3 to 7 corresponds to low molecular weight, n=8 to 12 mid-weight and n is higher than 12 to high-weight siloxanes [3, 9, 12]. As a note, the viscosity of the free fluid increases with increasing molecular weight, or with n.

An Agilent 6890 GC system utilizing HPGC ChemStation software was used for the analysis. A solution of 20 mL n-hexane plus 1000 ppm of each cyclic group of D4, D5, D6, and D9 was employed as the standard to compare the siloxane chromatograms of the coatings. The standard elements were supplied with a minimum capillary GC purity specification of 98.0 % by Ohio Valley Specialty Chemicals. In the analysis, the fluid was extracted in a similar way as in the Soxhlet test using 100 mL of hexane. The extracted fluid samples were injected into the GC and the oven was heated to 40 oC, held for one minute, then the temperature was increased to 300 oC at 20 oC/min and held at this temperature for 6 minutes. The distributions of the molecular species of the coatings evaluated are shown in Fig. 5. All samples showed an abundance of low-weight cyclics, D4, D5, D6, and D7 followed by a lesser amount of mid-weight cycles D8 to D12. The amount of higher-weight cyclics (D13 to D18) is relatively higher in RTV-D. Clearly, RTV-C shows a higher abundance of mixed cyclics and confirming the Soxhlet result of higher wt % fluid shown in Table 1.

3 ACID-WATER IMMERSION TESTS As mentioned in introduction, an earlier work [3] formed

the basis for the present study using nitric acid for aging. Samples were aged by immersion in nitric acid solution for two weeks in a temperature controlled oven at 80 oC and the acidity was adjusted every 5 hours to a pH of 4 ± 0.4. A slightly lower temperature was selected for the aging due to limitations in the laboratory. However, unlike the earlier study, the present study does not attempt to develop an acceleration factor for aging.

3.1 LOSS OF HYDROPHOBICITY Contact angle is normally used as a means of estimating

the loss of hydrophobicity after aging. However, as the loss of hydrophobicity leads to the development of leakage current and dry band arcing, a test such as the inclined plane test provides a means of evaluating the effect of acid-water aging.

Figure 5. Gas chromatographs of new RTV samples.


Figure 6. Development of the 3rd harmonic of the leakage current for sample RTV-D before and after acid-water aging.

The test was conducted using the apparatus outlined in

ASTM D 2303 using six samples of each coating. The samples were prepared by applying the coating to ceramic slabs having the dimensions of 50 mm x 120 mm and the thickness of coated material was about 1 mm.

The test method generally followed the standard using 0.1% ammonium chloride as the contaminant with a flow rate of 0.15 mL/min and the initial test voltage was 2.0 kV. At each hour during the test, the voltage was increased by 250 V and the test continued for 4 hours. The measurement instrumentation consisted of a National Instruments TM PCI 6111 data acquisition card, shunt resistors for current measurement and resistor voltage dividers for monitoring of the voltage on each of the channels. The 3rd harmonic of the leakage current, which stems from dry band arcing, was recorded with the time of the test. A forward averaging technique was used to smooth out the plots of leakage current plots to show the trends.

Figure 7. Development of the 3rd harmonic of the leakage current for sample RTV-C before and after acid-water aging.

Two sets of six samples were evaluated, one before aging and the other after aging. Samples RTV-A, RTV-B and RTV-D did not show a significant change in the development of leakage current after aging, and one example is shown for RTV-D in Figure 6. However, RTV-C showed a significant change in the leakage current development, as shown in Figure 7 which is indicative of a complete loss of hydrophobicity. These results correspond to the rapid loss of hydrophobicity after water immersion as evident in Figure 4. All the other three coating showed a very similar rate of leakage current development which is indicative of a similar rate of loss of hydrophobicity. Once again, this can be noted in Figure 4.

3.2 LOSS OF PHYSICAL PROPERTIES It has been observed that changes to the chemical

structure of RTV rubber occurs in the presence of moisture, acids and other severe ambient conditions and correlate to changes in physical properties such as hardness, tensile strength, and elongation at break [12, 13, 14].

Table 3. Summary of mechanical tests.

Coating

A B C D

Property Initial/Aged Initial/Aged Initial/Aged Initial/Aged

Tensile strength at break, Mpa

2.4/0.7 2.7/2.2 1.2/0.6 2.5/1.6

Elongation at break, % 156/86 81/53 88/51 86/66

Hardness, Shore A

Durometer 62/70 63/73 57/77 55/60

Change in tensile

strength, % - 70 - 19 - 50 - 36

Change in elongation at break, %

- 45 - 35 -42 -23

Change in hardness, % +13 +16 +35 +9.5

A MINIMAT 2000 tensile tester was used to measure the

tensile strength and elongation at break and these tests were performed in accordance with ASTM D3039. For each test, the number of samples was 5 to 8 depending on the variance in the results. Initially, 5 samples were tested and if the variance was more than 10 %, an additional sample was tested and this procedure was continued until the variance was below 10 %. These results are shown in Table 3.

Hardness gives an indication of the change that occurs in RTV rubber that results from oxidation [11]. A type-A durometer tester with an accuracy of ±1 point was used following the procedure in ASTM D2240. The thickness of samples was 2 mm and five measurements were carried out at least 0.6 cm apart on the surface of the samples and the average value is reported in Table 3.


RTV-A and RTV-C exhibited the largest reductions in the tensile strength and elongation at break and after acid-water aging. The degradation of RTV-C was visible even without microscope (Figure 8). Such deep cracks resembling mud crack was not observed in the other coatings.

All samples showed increased hardness with RTV-C showing the greatest increase of 35% after 336 hours in nitric acid. The brittleness of RTV-C is noted in the surface cracks evident in Figure 8.

Figure 8. Surface deterioration of RTV-C observed after 336 hours of acid-water aging.

3.3 LOSS OF LMW FLUID The polydimethylsiloxane (PDMS) chains in silicone

rubber have molecular weights in the range of 104 to 105 g/mol, of which only a very small fraction of these are cyclic groups and these are commonly referred to as LMW fluid [7, 9].

To analyze the LMW fraction in the samples, 1 g of each sample with the same thickness was extracted using 250 mL of analytical grade n-hexane following the general method outlined in section 2.5 above. The concentration of fluid was calculated from the weight difference before and after extraction and these values are listed in Table 4. RTV-C showed the highest loss of LMW fluid after acid-water aging followed by RTV-A. Coatings RTV-B and RTV-D showed a considerably lower loss of LMW fluid.

Table 4. Soxhlet extraction of LMW fluid in un-aged and acid-water aged RTV coatings.

RTV Coating A B C D

Un-aged, LMW fluid, % 2.26 1.13 3.47 1.14 Acid-water aged, LMW fluid,

% 1.11 1.03 1.25 0.97

Loss of LMW fluid after acid-water aging, % 51 9 64 15

3.4 GAS CHROMATOGRAPHS OF ACID-WATER AGED COATINGS

Gas chromatographs of the coatings after acid-water aging were determined for the coatings according to the method outlined in section 2.6, to observe the reduction in polydimethylsiloxane cyclics as compared to new coatings. These results are shown in Figure 9.

Figure 9. Gas chromatographs of acid-water aged coatings.

All coatings showed a marked decrease in all cyclic groups after acid-water aging with RTV-C showing the greatest decrease which corresponds to the Soxhlet extraction results shown in Table 4. The least change occurred in RTV-B followed by RTV-D. The reduction in the cyclic groups following acid-water aging is mainly due to opening of the PDMS structure and reduction of molecular weight allowing the LMW fluid to be easily extracted.


4 FIELD AGED COATINGS Field aged coatings of known length of time in service

and type is always difficult to obtain with any certainty as record keeping is not always reliable. However, two field aged coatings RTV-A and RTV-C were obtained from insulators having 6 and 10 years of exposure, respectively, and are identified as RTV-AFA and RTV-CFA. RTV-AFA was exposed to industrial contaminant in a region where natural rain washing occurs infrequently while RTV-CFA was exposed to a coastal environment. Both were covered with a light deposit and the ESDD was not estimated. Soxhlet extraction tests were done on the samples with and without surface contaminant following the procedure outlined above in section 2.5. The samples were in the weight range of ~ 2.5 grams to reduce possible error. The samples without contaminant were cleaned using distilled water and a soft brush. These results are shown in Table 5.

Table 5. Soxhlet extraction of LMW fluid in field aged RTV coatings.

In the case of RTV-A, that was in the field for 6 years,

the LMW of the sample, RTV-AFA, is very close to the loss of LMW of the laboratory acid-water aged sample shown in Table 4. Un-aged RTV-C has a 3.47% LMW free fluid according to Table 4 and this value is 1.73% based on Table 5. Therefore, RTV that was in the field for 10 years, showed a loss of about 50 % of the LMW fluid. Of course this assumes that no changes in the coating formulations occurred over this time. As evident in Table 5, a part of the LMW is trapped in the contaminant layer which may not participate effectively to help the recovery of hydrophobicity.

Figure 10. Return of hydrophobicity after a temporary loss as determined by measurement of the static contact angle on new and field-aged samples of RTV-A and RTV-C.

The return of hydrophobicity, after a temporary loss, was investigated by measuring the static contact angle as a function of time. This was done on new and field aged specimens of RTV-A and RTV-C. Before the test, the samples were washed in water, which simulates a heavy rain condition, to temporarily destroy the hydrophobicity. Then, the contact angle was measured at intervals over a 36 hour period by averaging the contact angle of 10 drops, each about 20 Lμ volume, of distilled water and these results are shown plotted in Figure 10.

According to Figure 10, field aged RTV-C, identified as RTV-CFA, showed an initial contact angle more than 30 degrees lower than the un-aged sample, and again, this assumes the manufacturer has not changed the formulation over this time. Also, the sample did not show hydrophobicity recovery in the 36 hour test period. But in the case of RTV-A, RTV-AFA showed a similar trend of recovery of hydrophobicity compared with a new sample of RTV-A.

There is a notable difference on the amount of low and medium molecular weight of polydimethylsiloxane cyclics of field aged samples: RTV-AFA and RTV-CFA, according to Fig.11. To quantify the difference, Table 6 compares the relative abundance of PDMS cyclics for new, acid-water and field aged samples of RTV-A and RTV-C. Once again it must be assumed that the coating formulations have not changed and under this assumption, the field aged samples show a marked reduction in the D6 to D12 cyclic groups. Inconsistencies in the relative abundance of the cyclic groups below and above this range are believed to be due to error in the detected values. In addition, the relative abundance of cyclic groups in field aged samples are similar to those detected in laboratory acid-water aged samples although with only two samples, one can only speculate on the effect of field exposure.

Figure 11. Gas chromatographs of field aged coatings, RTV-AFA and RTV-CFA.

Sample RTV-AFA RTV-CFA

With contaminant, % 1.18 1.73 Without contaminant, % 1.05 1.22


Table 6. Comparison of the relative abundance of cyclic groups for new,

acid-water and filed aged coatings.

5 CONCLUSIONS

A comprehensive study of the performance characteristics of four commercial RTV silicone rubber coatings for high voltage insulators has shown considerable variation in the physical properties of the coatings affecting life. Accelerated aging in acid-water has shown considerable modification to the physical properties of the various coatings evaluated, notably more so to RTV-C, which is likely due to greater porosity as evidenced by higher water absorption as compared to the other coatings evaluated. Severe acid de-polymerization of RTV-C has also been shown to occur, albeit under harsh conditions of acid-water exposure. The largest percentage of LMW fluid was found in RTV-C, albeit occurring in the mid-weight range of cyclics, but the coating showed the largest depletion of LMW fluid under the aging conditions reported, and is likely due to the higher porosity allowing a more rapid diffusion of the mixed cyclics from the bulk to the surface.

Depletion of the low molecular weight cyclics, that is responsible for the hydrophobicity, is considered to be the most important property affecting life. Gas chromatography is a useful tool for determining the residual amount of mixed cyclics remaining in a field aged coating. However, the rate of recovery of hydrophobicity after producing a temporary loss of hydrophobicity is certainly a more direct way of establishing the condition of a field aged coating. To this end, what remains to be established is

rate of return of hydrophobicity that constitutes the end of useful life on an insulator.

There were no field aged samples of RTV-B and RTV-D available for this study, but it may be possible to extend the conclusion that the acid-water laboratory aged samples shows a similar loss of LMW % as the field aged samples. Based on this observation and assumption, and considering Table 4, it is predicted that RTV-B and RTV-D show a better performance in the field and have longer life

ACKNOWLEDGEMENT The financial support provided by Natural Sciences and

Research Council of Canada is appreciated. The authors gratefully acknowledge Dr. Ayman El-Hag for helping in the TGA work.

REFERENCES [1] R. Hackam, “Outdoor HV composite polymeric insulators”, IEEE

Trans. Dielectr. Electr. Insul., Vol.6, pp. 557-585, 1999. [2] R.S. Gorur, A. De La, H. El-Kishky, M. Chowdhary, H. Mukherjee,

and R. Sundarjan, “Sudden flashover of non-ceramic insulators in artificial contamination tests”, IEEE Trans. Dielectr. Electr. Insul., Vol. 4, pp. 79-87, 1997.

[3] K. Eldridge, J. Xu, Wl Yin, A. Jeffery, J. Ponzello and S.A. Boggs, “Degradation of a silicone-based coating in a substation application”, IEEE Trans. Power Del., Vol. 14, pp. 188-193, 1999.

[4] H. Homma, C. L. Mirley, J. Ronzello and S. A. Boggs, “Field and laboratory aging of RTV silicone insulator coatings”, IEEE Trans. Power Del., Vol. 15, pp. 1298-1303, 2000.

[5] J. Wisniewski and E. L. Keitz, “Acid rain deposition patterns in the continental United States”, Water, Air, Soil Pollution, Springer, No. 4, pp. 327-339, 1983.

[6] J. Montesinos, R. S. Gorur and J. Goudie, “Electrical performance of RTV silicone rubber coatings after exposure to an acidic environment”, IEEE Conf. Electr. Insul. Dielectr. Phenomena (CEIDP), Vol.1, pp.39-42, 1998.

[7] S. J. Clarkson, J. J. Fitzgerald, M. J. Owen, S. D. Smith and M. Van Dyke, Synthesis and properties of silicones and silicone-modified materials, Published by American Chemical Society, Washington, DC, USA, 2003.

[8] H. Hillborgl and U.W. Gedde, “Hydrophobicity changes in silicone rubbers”, IEEE Trans. Dielectr. Electr. Insul., Vol. 6, pp 703-717, 1999.

[9] R. J. Hill, “Laboratory Analysis of Naturally Aged Silicone Rubber Polymer Insulators from Contaminated Environments, 138 to 765 kV”, IEEE T&D Conf., Texas, USA, pp. 488–493,1994.

[10] X Wang, L Chen and N Yoshimura, “Erosion by acid rain, accelerating the tracking of polystyrene insulating material”, J. Phys., D: Appl. Phys., Vol. 33, pp. 1117-1127, 2000.

[11] N. Frost, “Acid rain aging of silicone rubber materials in a fog chamber”, IEEE Electr. Insul. Conf. and Electr. Manufacturing & Coil Winding Technology, pp. 17-20, 2003.

[12] Z. Rappoport and Y. Apeloig, The Chemistry of Organic Silicon Compounds, Chapter 38, Recent advances in the chemistry of siloxane polymers and copolymers, John Wiley & Sons Ltd., 1998.

[13] R. N. Jana, G. B. Nando, and D. Khastgir, “Compatibilised blends of LDPE and PDMS rubber as effective cable insulants”, Plastics, Rubber and Composites, Vol. 32, pp 11-20, 2003.

[14] Y. Koshino, I. Umeda and M. Ishiwan, “Deterioration of silicone rubber for polymer insulators by corona discharge and effect of fillers”, IEEE Conf. Electr. Insul. Dielectr. Phenomena (CEIDP), Atlanta, USA, pp. 72-79, 1998.

Coatings RTV-A RTV-C

Cyclic group

New

Acid-water aged

Field aged 6 y

New

Acid-water aged

Field aged 10 y

D3 12 11 8.5

D4 5 7 6.5 5 8 6

D5 6 6.5 9 27 2 1.5

D6 30 22 17 94 12.5 7.5

D7 25 19.5 19 70 13 15

D8 13 11 9 40 8.5 10

D9 16 8 6.5 28 6.6 8

D10 8 5.4 4.5 22 5 9

D11 6 9 5 18 4.8 6

D12 10 6 6 16 6 8

D13 5.8 5 4.5 15 4.5 7

D14 6 4.5 5 13 6 9

D15 4.7 3.5 4 11 3.5 5.5

D16 3.5 3 3 8 3 5.4

D17 2 1 1 5 1.5 2.5

D18 1.5 1.2 1.3 4 3.5 3


Ali Naderian Jahromi (S’04-M’06) received the B.Sc. and M.Sc. degrees from Sharif University of Technology, Iran in 1998 and the University of Tehran, Iran in 2000, respectively. He received the Ph.D. degree in 2005 from the University of Tehran after a two-year leave of absence as a research student at the high voltage laboratory of the University of Waterloo. His employment experience includes Iran-Switch Company (1997-1999) testing of switchgear and circuit breakers, Iran-Transfo Company (1999-

2001) designing and manufacturing of HV transformers, and Iran Power Generation and Transmission Organization (TAVANIR) in the HV substations planning division (2001-2004). Currently he is a Post Doctoral Fellow at the high voltage laboratory of the University of Waterloo. His research interests include high voltage test techniques, power transformers, XLPE cables, outdoor insulators and diagnostics of power system apparatus such as on-line PD measurement.

Edward A. Cherney (M’73-SM’83-F’97) received the B.Sc. degree from the University of Waterloo, the M.Sc. degree from McMaster University and the Ph.D. degree 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 manufacturing of polymer insulators and of silicone materials. Since 1999 he has been involved in international projects in outdoor insulation. He has been an adjunct

professor for 22 years, first at the University of Windsor and currently at the University of Waterloo. He has published extensively, holds several patents, co-authored a book on outdoor insulators, actively involved in IEEE working groups and a registered engineer in the Province of Ontario.

Shesha H. Jayaram (M’87-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 the Bangalore University, M.A.Sc. in high voltage engineering from Indian Institute of Science, Bangalore, and the Ph.D.

degree in electrical engineering from 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. She is a registered professional engineer in the Province of Ontario, Canada

Leonardo C. Simon received the B.S. M.S. and Ph.D. degree from Federal University of Rio Grande do Sul, Porto Alegre, Brazil in 1995, 1998 and 2001, respectively. From April to December 2001, he worked as a Postdoctoral Fellow at University of Waterloo. Currently Dr. Simon is an assistant professor at chemical engineering department of University of Waterloo and his main areas of interest are

correlation of synthesis-structure-properties of polymers, development of polymer nano-composites and mathematical modeling of polymerization mechanisms.


aging characteristics of rtv silicone rubber insulator coatings

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