silicone comparison

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2004 Annual Report Conference on Electr ical Insulat ion and Dielec tr ic Phenome naComparison between Silicone Rubber containing Micro- and Nano- Size Silica

FillersA. H. El-Hag S.H. Jayaram E.A. Cherney

Electrical and Comp uter Engineering Departme ntUniversity of WaterlooWaterloo. Ontario, Canada

Abstract: In this study the influence of nan es ize silicafiller is compared with micro-size filler in siliconerubber using the ASTM 2303 inclined plane trackingand erosion test. Silicone rubber samples filled with 5pm mean size ground silica are compared with siliconerubber filled with 12 nm average size fumed silica. Boththe low frequency components of leakage current anderoded volume are used to evaluate th e performance ofthe two types of composites. While the fundamentalcomponent of leakage current does not correlate withthe erosion, the third harmonic component of theleakage current shows good correlation to the erosion,in terms of volume, and for both composites. Based onthe results of the eroded volume, it is observed that 10%by weight of nano-size filler in the silicone rubber givesa performance that is similar to that obtained with 50%by weight of micro-size filler. The paper discusses themerits of n ano-size filler in silicone composites.IntroductionSilicone tubber (SIR) has been used as an insulatingmaterial for outdoor applications for more than 30years. Compared to ceramic insulators, SIR insulatorshave advantages of g o d pollution performance,reduced weight and lower cost [I]. However, aging isstill considered the main problem facing SIR insulators.To improve the aging performance of silicone rubberinsulators various types of fillers are needed.Fillers are added to silicone rubber to reduce cost,improve dry band arcing resistance and to improvemechanical properties. Extensive research has beenconducted to study the influence of filler size,concentration and type on the aging performance of SIRinsulators f3-51. However, much of the previous effortshave con centrated on micro-size fillers.With the growing interest in using nano-size tillersin polymeric materials, the use of these fdlers couldhave a strong influence on the performance of SIRinsulators. Although fumed silica has been extensivelyused for reinforcemen t of silicone rubber, its role on theerosion and tracking resistance has not been studied.This paper presents a comparison between nano-sizesilica filled SIR with micro-size silica filled SIR

samples using the ASTM 2303 standard inclined planetracking and erosion test (IPT).A companion paperdiscusses the underlying physicochemical properties offumed silica filled SIR composites [7].Materials and MethodsThe test procedure followed the ASTM 2303 standard,with an initial voltage of 2.0 kV and a constantcontaminan t flow rate of 0.15 mVmin for four hours [SI.The voltage was increased at a rate of 250Vho ur. At theend of the test, the samples were taken from the test bayand the eroded volume was estimated by filling theeroded volume with a soft putty of known density. Theweight of the putty was determined using amicrobalance and used in the eroded volumecalculations. The base rubber (elastomer) used was atwo-component room temperature vulcanized (RTV)SIR material which contained no inorganic filler of anytype. The filler typelconcentration used is presented inTable 1.

Table I: Chemical composition of the tested samples in IP I test.

A PC based data acquisition system, composed ofvoltage dividers, overvoltage and overcurrent protectioncircuits, and a 100 IcHz, 16-channel data acquisitioncard, recorded the source voltage, and voltage andcurrent for each sample. During the test, for everysecond, four 60 Hz cycles of data were acquired at asampling rate of 7580 samples per second. These fourcycles were processed by the Fast Fourier Transform(EFT)technique for the leakage current (LC). Then, theLC current data were averaged and smoothed using themoving average technique. The software for analyzing

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the data recorded by the data acquisition system wasbuilt on a LabVIEWC3 platform. The results presente d inthe following section are average values from threesamples for each test case. Figure 1 shows the schem aticof inclined plane setup and the data acquisition system.

T cFigure I Experimenel setup [6 ]

Results and DiscussionBoth the funda menta l and third harmoniccomponents of LC were monitored continuously for thenano- and micro- size silica filled SIR as shown inFigures 2 and 3. Also, the eroded volumes for all thetested samples were measured and are depicted inFigure 4. It is evident from Figure 4 that as the fillerconcen tration increased, the eroded volume dec reased.Also, the weight loss for a 10% nano-size silica filledSIR is comparable to a 50% micro-size silica filled SIR.

15% I

0 50 rw 150 203 250r m c rn,"ubn)(b)Figure 2 Fundamental compOnent of leakage current during the Fl'

test, (a) Nano-sire f i e d SIR, (b)Micro-size filed SIR.

- 2.5

0 50 1w 15 0 2m 250Time (minutes]

0 50 100 150 200 250Time (minutes)(b )Figure 3 Third harmonic component of leakage cUrren1 during the U T

t e ~ f a) (a) Nano-size filled SIR, (b) Micro-sire filled SIR.

70 ,

En

0lOXm*m mnd sm Wm*m 5Xnano 1 O X m

Figure 4 Comparison between micro- nan-size silica filledcompo sites in terms of ercded volume.It is evident from Figures 2an d 3 that the fundamentalcomponent of leakage current does not show anydifference between the tested samples regardless of theeroded volume. Also, the average value of fundamentalcomponent saturates around 6 mA. However, thelevels of both the fundam ental compo nent of the LC, aswell as the third harmonic component, are correlatedwith the degree of the surface erosion in a salt-fog test[9]. This could be attributed to the controlled flow of

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contaminant on the material surface in the P T , whichresults in a controlled surface resistance.On the other hand, the third harmonic component ofthe LC shows a better correlation with the erodedvolume, Figure 3. This is because dry band arcing iswell correlated to both surface damage and the thudharmonic of the LC [91.As reported in the companion paper, a TGA analysiswas conducted on nano-size silica filled SIR andcompared to unfilled SIR [7]. Also, the TGA analysiswas conducted on micro-size filled silica samples asshown in Figure 5.Comparing the TGA results of nano-and micro- size silica filled SIR, Figures 5and [7], asthe filler concentration increases, the weight lossdecreases in both composites. However, comparing theTGA results of nano- and micro- size silica filled SIRFigure 6 do not agree with the ET esults. The TGAanalysis show that both the 10% nano- and micro- sizesilica filled SIR materials show a reduction in weight ofabout 30%. On the other hand, the eroded volume for10%nano-size silica filled SIR is about one sixth of itsweight compared to micro-size silica filled SIR.

..60 I

to 0 233 3W 40 0 500 SW 700 8MTemperature (deg. C. )

Figure 5 E A h l y s i s for different micro filled silicone rubbaSamples.

EPEm-

im100806040

m

tm m YKI m 5m ma0 7m amTemperature (deg.C )Figme 6 TG A Analysis fM 10%nano and micro f i e d sdi cone rubbaSampleS.

To further understand the difference between nano-and micro-size silica filled SIR, a detailed scanningelectron microscopy (SEM) investigation wasconducted o n both the damaged and undamaged 10%nano- and m icro-size silica filled SIR samples as shown

in Figure 7.There is a significant difference between thesurfaces of the damaged nano- and micro-size silicafilled SIR. While it was not possible to see thedispersion of the nano-size filler in the undamagedsample, Figure 7 d , the fumed silica tended toagglomerate during the dry hand arcing, forming asilica-like layer, Figure 7-c. The formation of this typeof a layer was not appa rent in the micro-size silica filledsamples, as shown in Figure 7-a. It can therefore hestated that the formation of silica like layer with nano-size silica filled composites aids to resist furtherdegradation of SIR during dry hand arcing.

Figure 7 SEM Images for different SIR filled mterials: (a) Damaged10% micro-size silica fdled, (b) Undamaged 10% micro-size silicafdled, (c) Damaged 10%nano-sire silica filled, (d) Undamaged 10%nano-size silica filled.

ConclusionsThe third harmonic component of LC is well correlatedwith the surface damage of both micro-size silica filledand nano-size silica filled silicone rubber samples testedin IF'T test, although the fundamental component of LCis defined by the contaminant flow. Also, weight lossmeasurements using IF'T test results confirm that thenano-size silica filled SIR composites depict significantresistance to erosion comp ared to micro-size silica filledSIR composites with comparable amounts of fillers, byweight, in SIR.References[I] R.S. GMU, E.A. Cherney and I.T.Burnham Outdoor Insulators,Ravi S. Gorur, Inc .,199 9. Chapter 8.

S. S i m n s . M . Shah, 1. Mackevich. and R. I. Chang. "PolymerOutdoor Insulating Materials Part III - Silicone Elastomer121

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Considerations. IEEE Electrical Insulation Magazine. vol. 13,pp. 25-32, Sep/Oct 1997.Deng, H.: C h e y , E.A .: Hackam R. Effects of panicles sizeof ATH fillers on the performance of RTV rubber coatingsIEEE Electrical h u l a t i o n and Dielectric PhenomenaConference, pp. 598 604,993.Kim S.-H.; Cherney, E.A.: Hackam R. The rml characteristicsof RTV silicone rubber coatings as a function of f i e r levelIEEE CEDP -19 92, pp. 713-718, 18-21/Octobed1992.Meyer, L.; Grishko, V.; Jayaram S .; Chemey, E.; Duley, W.W.Thermal characteristics of silicone rubber filled with ATH andsilica under laser heating IEEE CEIDP-2002, 20-24/October/2002 .pp. 848-852.

I61 R. Omranipour. L. Meyer, S. H. layaram E.A. ChemeyInclined plane wcking and erosion evaluation of filled andunfilled silicone rubbei IEEE C EID P 2001, 14-17 Oct. 2M)I.pp. 632 - 63 5A. H. -Hag, S . Jayaram and E. A. Chemey. Physicochemicalproperties of silicone rubber nanc+composites for outdoarapplications paper accepted and to be presented at CEIDP2004.

181 ASTW D230 3 Slandard Test Method for Liquid-Contaminan4Inclined-Plane Tracking and Erosion of Insulating Materials.[9] A.H. El-Hag, S . Jayararn & E.A. Chemey Fundamental an dLow Frequency Harmonic Compo nents of Leakage Current as aDiagnostic Tool to Study Aging of RTV and HTV SiliconeRubber in Salt-Fog IEEE Transaction on Dielectrics andElectrical Insulation. Vol. IO, No. I . February 2W3, pp. 128-136.

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silicone comparison

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