2008 Annual Report Conference on Electrical Insulation Dielectric Phenomena

Studies on the Tracking and Erosion Resistance ofRTV Silicone Rubber Nanocomposite

2B.Venkatesulul and M. Joy ThomasHigh Voltage Laboratory, Department of Electrical Engineering

Indian Institute of Science, Bangalore, India.'venkatesulub@ ee.iisc.ernet.in, 2Jtm@ee.iisc.ernet.in.

Abstract: One of the problems associated with outdoor polymericinsulators is tracking and erosion of the weathershed which candirectly influence the reliability of the power system. Flameretardants are added to the base material to enhance its trackingand erosion resistance. Hydroxide fillers are regarded as the bestflame retardants. This paper deals with studies related to nano -sized magnesium dihydroxide (MDH) and micron-sized AluminaTrihydrate (ATH) fillers as flame retardants in RTV siliconerubber. Tracking and erosion resistance studies were carried outon MDH and ATH silicone rubber composites using an inclinedplane tracking and erosion (IPT) resistance tester. The MDHfilled (5% by wt) composites performed much better than ATHcomposites in terms of eroded mass, depth of erosion, width andlength of erosion. The eroded mass of MDH composite is 49.8 %that of ATH composite which can be attributed to high surfacearea and higher thermal stability ofMDH nanofillers.

I. INTRODUCTION

With the advent of nanotechnology, and the progress madein processing technologies, outdoor nanocomposite polymericinsulators are emerging as promising candidates for use asoutdoor insulators in transmission and distribution of electricalpower. Composite insulators offer attractive advantages likelight weight (high strength to weight ratio), easy to transportand install and good resistance to vandalism. They alsoprovide much better performance under contaminatedenvironments during initial period of service as compared tothat of conventional insulators. One of the serious concernsabout polymeric insulators is aging [1]. Polymeric insulatorsare susceptible to aging due to arcing, corona and weathering.It is also possible that during service, considerable leakagecurrent can flow on the contaminated wet surface of aninsulator. Leakage current produces ohmic heating resulting inthe formation of dry bands. The sharp and subsequent increasein local field gradients across dry bands results in arcingacross them. Temperature generated due to repeated dry bandarcing can be beyond safe limits (> 4000 C) of organic material[2]. If the rise in temperature is beyond the safe limit that thepolymeric material can handle, then it can lead to trackingand/or erosion. To mitigate the deleterious effect oftemperature, flame retardants (micron-sized fillers) are addedto the base polymer in varying concentrations. Hence, eventhough silicone rubber (SR) is considered as a better materialfor outdoor insulator applications, it has become necessary toadd flame retardants to improve its performance with respectto tracking and erosion resistance. To improve different

properties of base polymer, it is compounded with variousfillers depending upon the property to be achieved.Improvements in various properties were observed recentlywith nano-sized filler of a very low concentration (weight %)as compared to micron-sized fillers [3].

Hydroxide fillers like ATH are very good flame retardantswhich are added to base polymers to improve the resistance totracking. Highly loaded SR performed better than SR withlower filler content in terms of tracking and erosion resistanceof [2, 4, 5]. Common practice to improve tracking and erosionresistance is by loading heavily (50 to 60 %) with micron-sized ATH filler in outdoor insulators. It has been reportedthat insulators in the field failed due to tracking and erosion onthe surface of the polymeric weathersheds in spite of theimproving tracking and erosion resistance by such heavyloading. Hence the current level of improved tracking anderosion resistance at this loading is also not sufficient. Alsothe processibility and flexibility of the end product getsaffected due to higher loadings of the micron-sized filler. As amatter of fact, improved level of resistance to tracking alone isnot sufficient but the filled material should have goodprocessibility and flexibility. Nanocomposites are promisingin terms of tracking and erosion resistance, processibility andend product flexibility at low concentrations [6]. As nano-ATH is commercially not available readily at this point oftime, alternate hydroxide filler (MDH) is chosen for the study.

II. EXPERIMENTAL DETAILS

A. MaterialTwo component Room Temperature Vulcanised Silicone

Rubber (RTV SR) supplied by Wacker Chemie, Germany wasused for making the samples. As per the manufacturers data itdoes not contain any added fillers. Also, its performance is asgood as the high temperature vulcanised silicone rubber usedfor outdoor insulators [7]. Silane treated micron-ATH of size3.5 Mm supplied by Huber corporation and untreated MDH ofsize < 100 nm supplied by Sigma Aldrich have been used formaking micro and nanocomposites.

B. Sample PreparationThe fillers were dried in an oven at 1300 C for at least 10 h

before their usage. Similarly RTV A and B components werekept under vacuum for few hours in order to remove trapped

978-1-4244-2549-5/$25.00 ©) 2008 IEEE 204

air and moisture. In this study two different types ofprocessing techniques (mechanical mixing andultrasonication) either singly or doubly are used to disperseparticles in the matrix. Microcomposites are made usingmechanical mixing. However, for making nanocomposites,first mechanical mixing followed by ultrasonication has beenused. Ratio of RTV components A: B for the compositedeveloped is 3: 1 which is appropriate for getting sampleswith very low tackiness, enough hardness and good curing.Sample has been cured in two stages i.e. primary andsecondary curing. During primary curing mould with thematerial is kept in the oven at 1500 C for 4 h and duringsecondary curing for 20 h at 1300 C. Rectangular samples ofsizes 130 mm (L) X 60 mm (B) X 5 mm (H) were casted forthe studies.

C. Inclined Plane Tracking and Erosion TestThe test has been conducted conforming to the ASTM

2303. Voltage has been fixed at 2.5 kV (r.m.s) as it createsaverage electric stress (50 kV/mm) close to the field levels (30to 40 kV/mm) existing on the insulators used for powertransmission [8]. Also it was noticed that scintillations athigher voltages (say 4.5 kV) has been very aggressive andhighly random w.r.t time and location on the surface of thesample. Conductivity of NH4Cl solution used is 2.5 mS/cm.All the samples were tested at 2.5 kV for 7 h. Theexperimental set up used is shown in Fig. 1. Eroded materialfrom the sample is removed completely with brush and thenthe eroded area is filled with putty. Eroded mass can beestimated by knowing the putty weight. The depth of erosionand length of erosion are measured using standard verniercalipers.

D. Scanning Electron Microscopy (SEM) StudiesFiller dispersion in the matrix has been obtained using

scanning electron microscope. Also the eroded areas wereobserved using microscope to understand the physical roleplayed by the particles in the matrix in enhancing the erosionperformance of the material. SEM machine (FEI make ESEMQuanta) at different accelerating voltages has been used toobtain the images.

E. Thermo Gravimetric Analysis (TGA)This study helps to understand the thermal stability of the

material. Here Perkin Elmer Pyris Diamond TG/DTAinstrument has been used and the temperature was raised to7000 C at a heat rate of 100 c from room temperature.

III. FLAME RETARDENCY MECHANISM OF HYDROXIDE FILLER

A. Alumina TrihydrateAs the temperature on the surface increases due to dry band

arcing, ATH decomposes at approximately 2200 C releasingwater of hydration. (In fact at 2200 C and 5000 C release ofwater takes place). The decomposition reaction is endothermicwith enthalpy of decomposition of -280 cal/g. Thedecomposition reaction is represented by the followingchemical equation.

230V!8kVFig. 1. Inclined Plane Tracking and Erosion Test Setup

2A1(OH)3 -----> A1203 +3H20

B. Magnesium DihydroxideMDH decomposes at approximately 3300 C releasing water

of hydration. The decomposition reaction is endothermic withenthalpy of decomposition of -328 cal/g. The chemicalequation for the reaction is as follows.

2Mg(OH)2 -----> 2MgO +2H20

It can be understood from the above that the decompositiontemperature ofMDH is on the higher side and also enthalpy ofdecomposition is more for MDH. Nano-MDH used in theseexperiments has a high surface area (94.8 m2/g as compared to7.5m2/g for ATH) and heat of decomposition than micron-ATH. Also the decomposition temperature of MDH is wellbelow the degradation temperature of SR polymer. For a givenfiller weight, the number of MDH nanoparticles is more ascompared to micron-ATH, which in turn leads to an increasedoverall surface area or volume occupied by nanoparticles.Hence during discharge, the particles coalesce togetherforming a protection barrier against the discharge for nano-filled SR composite

IV. RESULTS AND DISCUSSIONS

A. SEM StudiesFig. 2 shows the SEM images of the eroded regions of the

ATH and MDH composites and Fig. 3 shows the SEM imagesof ATH and MDH fillers used in the study. Visual observationof Figs. 2 a) and b) shows that less number of ATH particlesare present on the eroded portion of the SR composite ascompared to MDH particles. This is because for a givenweight, ATH systems contain less number of particles as that

a) Eroded region of ATH composite

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b) Eroded region ofMDH composite

Fig. 2. Eroded regions of ATH and MDH composites

of nano-MDH system as explained in the previous section.Moreover as shown in Figs. 3 a) and b), MDH particlesmorphology is platelet where as ATH particles are irregularlyshaped. Since MDH filler is flowery type, it is occupyingmajority of the matrix even at lower filler loadingoffering high resistance to discharges [9, 10].

B. Physical StudiesAs shown in Figs. 4 and 5 and Table I it is observed that

the performance of unfilled SR (UnF) is inferior to filled SR(ATH 1 &2 and MDH 1&2) composites in terms of erodedmass, eroded depth and length of erosion between electrodes.The eroded region of MDH composites is more of circular inshape while that of ATH is broader and also grows like afilament as the test proceeds. Another observation made is thatup to 6h of aging, the erosion performance of ATH system issame as that of MDH but after 6 h and before 7 h, itsdegradation rate is more. The eroded region of unfilled SR ischaotic, deeper, lengthy and wider. The length grownhorizontally at the ground electrode is more for the ATH ascompared to MDH SR composite. From Fig. 5, it can be seenthat the eroded mass of ATH composite is only 31.3 % whereas for MDH system it is only 15.6 % of unfilled RTV silicone

b) SEM image ofMDH filler

Fig. 3. SEM images of fillers used

rubber. Also the eroded mass percentage ofMDH is only 49.8% of ATH. From Table I, it is noticed that eroded depth, widthand length ofMDH are 85 %, 65.3 % and 78 % respectively asthat of ATH. Hence w.r.t above parameters, it is suggested thatnano-MDH filled silicone rubber performance is better thanmicron- ATH filled silicone rubber. Also it was observed thatthe spread in MDH filled samples data is much less comparedto unfilled and ATH filled samples.

C. TGA StudiesFig. 6 shows that the differential weight loss of MDH is

significantly less as compared to that of ATH composite [6].The slopes (dW/dT where dW and dT are the changes in thewt % and temperature of the sample respectively) of the TGA

Fig. 4. Photographs of RTV Silicone rubber composites of ATH ( ATH I and ATH2), MDH (MDH1 and MDH2) as well asunfilled (UnF) samples after 7h of IPT test

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2.5

(I)(I)cZ 1.5E

1a)0L 05

0UnF Filled ATH Filled MDH

Fig. 5. Eroded mass of micro and nanocomposite

TABLE IAverage Values of Depth of Erosion, Eroded Width along Ground Electrode, Eroded

Length between Electrodes of UnF, ATH and MDH after 7h of IPT Test.

Depth of erosion Horizontal width of Length of erosionSample between electrodes(mm) erosion (mm) (mm)

UnF SR 5.60 24.80 21.48

SR with ATH 4.75 16.17 15.15

SR with MDH 4.07 10.57 11.82

120 -

100 -

80 -

60 -

40 -

20 -

0-

MDH

0 200 400 600 800

Temperature ( C )Fig. 6. TGA curves of micron- filled ATH (5% wt) and nano-MDH

(5% wt) SR composite

curves have been calculated by drawing a tangent on the steepfalling portion of the curves. It was found that the slope ofthe ATH system's TGA curve is 0.85 and for MDH system itis 0.4 13. That is the rate of weight loss of MDH system is 51% that of ATH system. This informs that the deterioration ofMDH system is much slower compared to the ATH system atany given temperature. This is in good agreement to theeroded mass as observed in the tracking and erosion resistanceexperiment. So this result supports the better tracking anderosion performance of these composites as observed earlier.So it can be stated that the thermal stability of MDH (5 % wt)composite is better than ATH composite (5 % wt).

V. CONCLUSIONS

The inclined plane tracking and erosion performance ofmicron- filled ATH (5 % wt) and nano-filled MDH (5 % wt)

silicone rubber composites are compared. The resultsdemonstrate that at this low filler concentration, the MDHsystem has performed better in terms of eroded mass, depth,width and length. The eroded region of MDH system wascircular in shape where as for ATH system, it was broader andlike a filament. The eroded mass of MDH system was only49.8 % as that of ATH system. The rate of weight loss ofMDH system is 51 % that of ATH system (from TGA result)is in good agreement to the percentage of eroded mass asobserved in tracking and erosion resistance test. TGA resultsalso show that MDH system was more thermally stable.

ACKNOWLEDGMENT

The authors thank IEEE for awarding the DEIS graduatefellowship to the first author which has made it possible tocarry out a part of the work presented here. The authors wouldlike to thank the staff of Nano centre for the help inconducting SEM studies. The authors are also grateful toWacker Chemie AG, Germany for supplying RTV siliconerubber material for the studies. The authors thank Staff of theHigh Voltage Laboratory (Mr. Riaz and Mr. Sridharan) of ourInstitute for their help in developing the Inclined PlaneTracking and Erosion Resistance Test Setup.

REFERENCES

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[8] R. S. Gorur, J. Montesinos and L. Varadadesikan, "A Laboratory Test forTracking and Erosion Resistance of HV Outdoor Insulation", IEEETrans. on Dielectrics and Electrical insulation, Vol. 4, No. 6, pp. 767-774, December 1997.

[9] T. Tanaka, G.C. Montanari and R. Mulhapt, "Polymer Nano-compositesas Dielectrics and Electrical Insulation- Perspectives for processingTechnologies, Material Characteristics and Future Applications", IEEETrans. on Dielectrics and Electrical insulation, Vol. 11, No. 5, pp. 763-784, October 2004.

[10] M. Kozakao, N. Fuse, Y. Ohki, T. Okamoto and T. Tanaka, "SurfaceDegradation of Polyamide Nanocomposites caused by Partial Dischargeusing IEC (b) Electrode", IEEE Trans. on Dielectrics and Electricalinsulation, Vol. 11, No. 5, pp. 833-839, October 2004.

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