2007 International Conference on Solid Dielectrics, Winchester, UK, July 8-13, 2007

Aging Performance of ATH based RTV Insulator Coatings

Ayman H El-Hag', Ali Naderian Jahromi2 and Shesha Jayaram21- Electrical Engineering Department, American University of Sharjah

Sharjah, United Arab Emirate,2- Faculty of Engineering, University of Waterloo

Waterloo, Canada

Abstract: The paper presents the results of aninvestigation about the aging performance of three differenttypes ofroom temperature vulcanized (RTV) silicone rubbercoatings for high voltage insulators when subjected toaccelerated tests in acid water. Although all coatings haveATH as the main filler, different performance was observedunder simulated acid rain conditions. Erosion area, length oferosion and the development of the 3rd harmonic in theleakage current during inclined plane tests are employed toevaluate the tracking and erosion of acid aged samples.Thermo-gravimetric analysis (TGA) and thermalconductivity was employed to investigate the thermalperformance of the coatings. Scanning electron microscopephotographs are also used to assess surface changes afteraging. Factors that have significant influence on the agingperformance of RTV coatings include filler bonding to thepolymer matrix and the existence of other fillers, which canimprove the performance of RTVs in severe ambientconditions.

INTRODUCTION

Outdoor insulators are subjected to different natural andpolluted environmental contaminants which may include seasalt, cement dust, fly ash, bird droppings, acid rain etc.Porcelain insulator has a high surface energy which allowswater to make a continuous film. With introduction ofmoisture, salt fog or rain the contaminants form aconductive film on the surface of insulators, which results inincrease of leakage current that may lead to a flash overacross the insulator [1].

The maintenance of outdoor ceramic insulators hasbeen an art learned through experience. Severaltechnologies have been employed with varying degree ofsuccess like insulator washing, application of grease androom temperature vulcanized (RTV) coating. Althoughinitially water washing was an economical method, this isnot the case today in many parts of the world. Since nomeans are available for determining accurately whenwashing should be done, past experience on periodsbetween flashover has been used to estimate the frequencyof washing. As a result washing is either done muchfrequently or after flash over.

Hydrocarbon and silicone greases are used asprotective coatings on porcelain insulator that are able toabsorb contaminants and act to prevent flash over. Theyreduce the tendency for water drops to coalesce into acontinuous film and tend to encapsulate particles of

1-4244-0750-8/07/$20.00 ©2007 IEEE.

contaminants to prevent from adding to surface layer. Theneed to reapply the grease every 1-2 years is the majorlimitation of using this technology.

In order to reduce the incidence of insulatorflashover, room temperature vulcanizing (RTV) siliconerubber is widely used on outdoor insulator. RTV siliconecoating typically incorporate a base silicone rubber, a fillerto enhance the resistance to tracking and erosion, a cross-linking agent, a catalyst, a reinforcing filler, a pigment andan adhesion promoter. The RTV coating due to its lowsurface energy maintains hydrophobic surface and helps inpreventing of continuous water filming on the surface. Thissuppresses the development of leakage current andconsequently the flash over.

The major problem with silicone rubber is itssusceptibility to aging. The combined effect of electricalstresses (electric field and leakage current), environmentalstresses (UV, temperature, pollution and acid rain) willaccelerate the aging process. The effects of acid rain onRTV silicone rubber have been recorded in various parts ofthe world [2,3]. The ensuing degradation is anelectrochemical process of de-polymerization that is moresevere in the presence of voltage [1]. Acid de-polymerization occurs when OH radicals begin to severpolymer chains resulting in shorter chains that result in adrastic physical change in material properties such as a lossof elasticity.

In our earlier study, it was observed that the influence ofacid on RTV silicone rubber degradation was more severewith ATH filled coatings than with silica filled coatings [4].In this paper, three commercial RTV coatings, with ATH asthe primary filler, are examined for acid-water resistance.

MATERIALS AND METHODS

Three commercially available coatings have been selectedfor the study. All three coatings use ATH as main filler;however, detailed analysis showed little differences inviscosity, cure time, and specific gravity. For the study, allsamples were prepared under the same conditions, after athorough mixing the samples were left to air cure, for 7 daysprior to characterization and acid-water immersion. Thespecific gravity of the coatings was measured by accuratelyweighing a 10 mL sample of each coating before curing.The primary filler percentage was approximately calculatedusing a thermo-gravimetric analysis (TGA). Figure 1compares the TGA results of un-aged samples. The curvetrend begins to change at around 200 °C, which coincideswith the release of the water of hydration from filler

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particles. The concentration ofATH in the coatings can beestimated from the weight loss at 350 °C, which is due to thecomplete release of the water of hydration from the ATHparticles. Summary of the estimated ATH percentage alongwith the specific gravity is depicted in Table 1.

110

100

90

80

70

60

50

40

30200 400 600 800

Temperature (OC)Figure 1. Thermal gravimetric analysis of the coatings inTable 1

Table 1: Characteristics of Cured coatingsType Primary Estimated Specific

Filler Filler Wt % Gravity

A ATH 54 1.48B ATH 51 1.54C ATH 47 1.45

A wide range of pH values between 2 and 5.6 has beenused in accelerated aging tests; thus various rates ofacceleration have been reported [2-5]. According to a fieldmeasurement along the Connecticut coast, the acid rain wasmeasured as having a pH of 3.9 [3]. The same studyestimated that samples immersed in acid water of pH=3.9 at90 °C in the laboratory for one month is equivalent to 90months of aging the samples in the field. The accelerationfactor in this study was determined by comparing the weightloss of coatings immersed in nitric acid with time which was

compared to the weight loss of field aged coatings.Based on the literature [3], nitric acid was selected for the

aging process in this work. Several samples of each coatingwere aged for two weeks in a temperature controlled oven at90 °C. The pH was monitored every 4 hours and keptconstant at 4 ± IO%. Assuming a linear relationship betweenlaboratory aging and natural aging, an aging time of2 weekscorresponds to about 4 years in the field.

RESULTS

Inclined Plane Test (IPT) was used as the basicevaluation tool of the three coatings. The development of 3rdharmonic of leakage current in the IPT is a signal ofdegradation. Other tests like thermo-gravimetric analysis(TGA), thermal conductivity, and SEM photographs were

employed to investigate the effect of acid in details. In

addition, a standard mechanical test was used to investigatethe mechanical changes on RTV specimens. All these testswere conducted before and after aging.

A. Inclined Plane Test

The test was conducted using the apparatus outlined inASTM D 2303 on six samples of each coating. The sampleswere prepared by coating 50 mm x 120 mm ceramic slabswith about 1 mm thickness of coating. The leakage currenton each sample was limited using a series resistance of 10kQ, and the initial voltage was 2.0 kV. The contaminant was0. I% ammonium chloride with a flow rate of 0.15 mL/minand the test continued over four hours. At each hour duringthe test, the voltage was raised by 250 V. The measuringinstrumentation consisted of a National Instruments TM PCI6111 data acquisition card, shunt resistors to measure thecurrent in the 6 channels and resistor dividers to monitor theapplied voltage on each channel.

In addition to estimating the extent of the erosion on thesamples' surface after the inclined plane test, thedevelopment of the 3rd harmonic component of the leakagecurrent is used to evaluate the effect of acid immersion onthe coating samples. The leakage current was recorded onthe six samples of each coating, before and after acid aging,and the average of the current was calculated.

Figures 2 and 3 compare the leakage current of the threecoatings before and after acid aging. Before acid aging thetrends with leakage current for all three samples are similarexcept the early development of leakage current for RTV-Bcoating. After acid aging, leakage current for RTV-Cshowed considerably different trends compared to recordedcurrents for the other two samples. For RTV-C coating, thefinal value of 3rd harmonic of the leakage current of the un-aged samples was around 0.8 mA which increased to 1.6mA for aged samples, indicating the severity of acid-wateraging on surface degradation.

E

I-O

3

Li

0

._

50 100 150 200 250time, minute

Figure 2. Development of leakage current for non-agedsamples.

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RTV-C

~~~~~~~RTV-C} 1.2 kRTV-A

RTV-B

IL

00.

so 100 1sQ 200 250Time, minute

Figure 3. Development of leakage curr ent for aged samples.

After the inclined plane test on the samples, the length

and area of the eroded paths was measured and the results of

these measurements are shown in Table 2. None of samples

failed according to the ASTM D 2303 standard.

Table 2: Effect of aging on erosion length and area

Based on erosion parameters, RTV-A has less ratio oflength and area of erosion after aging. RTV-C exhibitedconsiderably more erosion area after aging, which is 4.5times than the new samples. The ratio of length of erosionof RTV-B is equal to RTV-C but acid immersion has lesseffect on the erosion area ofRTV-B compared with RTV-C

B. Thermo-gravimetric Analysis

Thermo-gravimetric analysis was performed for agedsamples of all three coatings using a TA Instrumentsanalyzer model STD 2860. The temperature was increasedat a rate of 20 °C/min from 100 to 800 °C. The derivativesofweight loss versus temperature are illustrated in Figure 4.

As mentioned before, the first peak in Figure 4 is due tothe release of water of hydration. On the other hand, thesecond peak of the curve is due to silicone rubberdecomposition that occurs above 400 °C. It is evident fromFigure 4 that RTV-C showed almost double the weight losscompared to the other two samples which confirm thefindings of the IPT test.

n 1

.-- U.o fU,

C -0.

O.ThraO onutvt

-0.310

-0.4

-0.53

system. The thermalconductivitiesof RTV-A,BandTC-0.6

20-0 400 600- 800

Temperature (r)Figure 4 Comparison ofTGA of acid rain aged coatings

C. Thermal Conductivity

Thernal conductivity measurement was performed usinga thermal contact resistance rig. The test apparatus is

enclosed in a Pyrex bell jar connected to a 10g torr vacuumsystem. The thermal conductivities of RTV-A, B and C

were measured to be 0.60, 0.46 and 0.46 W m.0K

respectively.

D. Mechanical Test

Standard mechanical tests such as tensile strength andelongation at break outlined in ASTM D3039, are used todetect changes that takes place in the physical structure of a

RTV coating after acid-water aging. A tensile tester,

MINIMlAT 2000, was used to measure these parameters.

The speed of tensioning was set to 2 mm/min for all

samples and the number of samples varied from 5 to 8

depending on the variance. Initially, five samples of each

coating were tested and if the variance was greater than 10

, an additional sample was tested and the procedure was

repeated until the variance was within The results are

depicted in Table 3. Both RTV-A and RTV-C showed

significant reduction in tensile strength at break compared to

RTV-B.

Table 3: Summary of mechanical tests' results

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Relative erosion parameters CoatingA B C

Length of erosion aged 1.3 1.7 1.7unaged

Erosion area aged 1.6 2.7 4.5unaged

Coating

Property A B C

Aged/New Aged/New Aged/NewTensile strength at

break, (Mpa) 0.7/2.4 2.2/2.7 0.6/1.2

break, (%o) 86/156 53/81 112/88Change in tensile

strength, (%) - 70 - 19 - 50

Change inelongation at - 45 - 35 + 27break, (%)

DISCUSSION

The IPT results indicate that although all the three coatingshave ATH as the main filler; the influence of acid aging onthem is different. Both the eroded area and 3rd harmoniccomponent of leakage current for RTV-C is much largerthan the other two coatings. Examining the three coatingsusing TGA confirm the findings of the IPT test, Figure 4.

The superior aging performance ofRTV-A compared toRTV-C could be attributed to the high thermal conductivity(3500 more than the other two coatings). The thermalconductivity, k, can be roughly calculated using thefollowing formula:

K = - 2K X 2 + K2 X I

(1)

Where:K1 = thermal conductivity of pure silicone rubberK2= thermal conductivity of ATHX1 = the fraction volume of silicone rubberX2= the fraction volume ofATH

Although the fraction volume ofATH is slightly higherin RTV-A compared to the other two coatings, Table 1, suchdifference can't explain the larger change in the thermalconductivity. A closer examination of an RTV-A sampleusing SEM showed uniform bonding between the filler andthe bulk ofRTV, Figure 5. Such strong bonding between thefiller and the polymer was not noticed for the other twocoatings which could be due to the use of chemically treatedATH. It has been previously reported that such bonding cansignificantly increase the thermal conductivity [7].

On the other hand, the improved aging performance ofRTV-B is due to the improved mechanical properties.Mechanical test results have showed that the tensile strengthat break of RTV-B has only changed by 19%, which isconsiderably lower than the other two samples, Table 3.Such improvement in the mechanical performance could bedue to the presence of fiber-like filler as depicted in Figure6. EDAX analysis revealed that the fiber filler is most likelyto be Silicate.

Figure 6 SEM ofRTV-B showing fiber like fillers

CONCLUSIONS

In this study the aging performance under acid conditionfor three different commercial ATH filled RTV coatingswas investigated. Based on the test results, the three samplesare ranked with RTV-A giving best performance, thenRTV_B and RTV-C. While the improvement in the RTV-Aaging performance is due to the stronger bonding of thefiller with the polymeric matrix and hence enhancing thethermal conductivity, the improvement of RTV-B is due tothe availability of fiber-like filler which tremendouslyimproved mechanical properties. This study confirms oneimportant fact that is not enough to know the main filler andits percentage to pre-judge the RTV coating, more analysisis needed.

REFERENCES

[1] R.S. Gorur, E.A. Cherney, and J.T. Burnham, Outdoor Insulators,Ravi S. Gorur, Inc., 1999

[2] X. Wang, S. Kumagai, N. Yoshimura, "Contamination Performancesof Silicone Rubber Insulator Subjected to Acid Rain ", IEEETransaction on Dielectrics and Electrical Insulation, Vol. 5, No. 6,December 1998, pp. 909-916

[3] H. Homma, C. L. Mirley, J. Ronzello, S. A. Boggs, "Field andLaboratory Ageing ofRTV Silicone Insulator Coatings", IEEE Trans.on Power Delivery, Vol. 15, No. 4, October 2000, pp. 1298-1303

[4] A. Naderian Jahromi, E. A. Cherney, Ayman El-Hag, SheshaJayaram, "Effect of Acid on Different RTV Silicone Rubber Coatingsin Inclined Plane Test", CEIDP 2005, Nashville, USA, pp. 313-316

[5] J. Montesinos, R. S. Gorur, J. Goudie, "Electrical Performance ofRTV Silicone Rubber Coatings After Exposure to an AcidicEnvironment", IEEE CEIDP, October 1998, vol. 1, pp. 39 - 42

[6] Nancy E. Frost, Paul B. McGrath, C. W. Burns, "Accelerated Agingof Insulators Under Acid Rain Conditions", IEEE InternationalSymposium on Electrical Insulation, California, USA, April 2000, pp.197-200.

[7] L. Meyer, S. Jayaram, and E.A. Cherney, "Thermal conductivity offilled silicone rubber and its relationship to erosion resistance in theinclined plane test" IEEE Transaction on Dielectrics and ElectricalInsulation, Vol. 11, No. 4, August 2004, pp. 620-630.

Figure 5 SEM ofRTV-A, with magnification of 10000

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