Tracking and Erosion Resistance - Engineering a wet or rain season, followed by sustained periods of

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  • were comparable or better than thecommercially available peroxide curedsilicone compound which is highly filledwith alumina trihydrate (E). For surfacestreated with longer corona exposuretimes (e.g. 150 sec.), recovery stilloccurs and is complete after a 24 hourperiod.Beyond laboratory data which do notallow one to predict product performanceunder different and varyingenvironmental conditions, Raychem hasmonitored the leakage currents ofvarious hybrid insulator designs andceramic insulators, pairing them inservice under different types ofenvironments. We have been conductingtests at different locations around theworld. Figure 4 shows the typicalleakage current results from the PenghuDistrict in Taiwan. The pattern of leakagecurrent shows clearly that leakagecurrent increases during early morningtime and decreases during the middle ofthe day (The rising sun reduces dewformation). It can be inferred from theseresults and those taken at other sitesthat leakage current is higher on ceramicinsulators than the RayBowl silicone rubber insulator. The resultsdemonstrated the superior performanceof Raychem RayBowl hybrid insulator inthe severe winter season. These resultsalso confirm the results we haveobserved in other parts of the world.Average leakage currents on ceramicinsulators with complex designs aretypically six to eight times higher than onthe Raychem hybrid insulator. This ratioincreases with higher pollution levels.

    Tracking and Erosion ResistanceWhile in a hydrophobic state, siliconeswill limit leakage current and surfaceactivity during wetting conditions withcontamination present. As hydrophobicproperties change, surface activity canoccur and become concentrated,rooting at a specific area. As locally, veryhigh temperatures are generated(>1000C), the silicone polymer will startto degrade. Under these conditions,most neat silicones and certain siliconeformulations will generate a resistivetrack when exposed to rooted arcingactivity on their surfaces. The trackforms from the carbon that is present(for PDMS, there are at least 2 carbonatoms for every silicon atom), so itfollows that some silicones are moreresistant than others based on theconstituents which make up the side-groups and crosslink sites.Track resistant additives, such asalumina trihydrate (ATH), can be blendedinto the formulation. However, as part ofthe optimization process, the additiveloading level must be carefully evaluated.The addition of ATH enhances the trackresistance, but adversely affectsprocessing and dielectric strength.

    Additional filler may also lessenhydrophobicity. The literature indicatesthat work by others with siliconeelastomer products found the need toreformulate and to increase the filler levelto improve weathering characteristics,demonstrating that the originaloptimization to facilitate manufacturingprocessing was inadequate. [8,11] A saltfog chamber study of different cabletermination technologies indicated thatof the samples tested, the siliconerubber product with the intentionally lowfiller loading performed poorly, [12]which may help explain the need for thereformulation discussed in Ref. 11.

    While the literature devotes a great dealof discussion to hydrophobicity and itsrecovery, little attention is paid to erosion

    resistance. It has been suggested thataccelerated tests need to include restperiods which permit hydrophobicityrecovery and various unsupportedclaims are made to justify the position.However, contamination does not occuron a regular schedule, nor has any effortbeen made to provide an engineeringbasis for the introduction of real-timerest periods into an accelerated test.Rest periods clearly seek to exploithydrophobicity recovery properties ofsilicone, yet such test data may misleadusers whose service environment mayconsists of little or no contamination

    Figure 3. Advancing contact angle of thesurfaces of various silicone formulationsas a function of time after coronatreatment. Contact angles of untreatedsurfaces ranged from 105 to 112degrees. Samples A, B, C, and D areexperimental formulations which employthe same silicone grade but differentfillers. Sample E is a commerciallyavailable HTV silicone formulation.

    Figure 4. Leakage current results fromTaiwan Power Company (PenghuDistrict)-Comparison of ceramic andpolymeric insulators. Average Current (inmA) is monitored with time for theRayBowl Protected Creepage, HybridInsulator (triangle) versus a ceramic fogbowl type insulator (square). Variations inleakage current correspond to dailyenvironmental cycles (i.e. highest peakoccurs during early morning fogperiods). The leakage current isapproximately 6 to 8 times lower usingthe silicone shed compared with theceramic insulator control.

    Contact Angle(Degrees)

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    00 0,5 1 2 3 4 6 8Recovery Time (Hours)

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    0242 258 9PM 290 306 9PM 338 354 9PM 386 402 9PM 434 450 9PM 482

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    Introduction 14.12.1999 21:24 Uhr Seite 5

  • during a wet or rain season, followed bysustained periods of contaminationduring a dry season. In order to assurethemselves of adequate performance,users must also evaluate the erosionresistance of silicone products. It isknown that silicones used in conven-tional weathershed designs can losehydrophobicity during contaminatedconditions, and if the material has poorerosion resistance, it may suffer extremedamage during periods of sustainedcontamination. Users must take abalanced view and not make productselection based only on the positivefeatures without consideration of theinherent limitations that may exist. Ref.12 clearlyindicates what can happen to a materialwith poor erosion resistance withrelatively short periods of contaminationexposure.

    The ASTM D2303 and IEC 587 trackingand erosion test (TERT) methods areused during development of high voltagesilicone compounds to evaluate theirrelative tracking and erosion resistance.Although the TERT test may be severe, it gives an indication of the materialsresistance to arcing under extreme environments (and where hydrophobicrecovery is not taken into account). AtRaychem, samples are tested accordingto the step-voltage method (increasingfrom 2.5 to 4 kV and holding) forextended periods (up to 14 hours) oruntil failure, as defined by ASTM D2303tracking or erosion criteria. In this way,samples are subjected to a bandwidth ofstress conditions, since damage for aselect sample may only occur for anarrow set of conditions. TERT resultsare shown for the four experimentalsilicones (A, B, C, and D) in Figure 5.Compound D represents a slight

    formulation variation of D. The figureillustrates significant improvements intracking and erosion resistance asformulations are optimized.

    The erosion resistance of the Raychemsilicone material used in the HybridInsulator has been demonstrated inseveral ways. In the ASTM D2303Tracking and Erosion Test (TERT),Raychem silicone materials consistentlywithstand in excess of 7 hours followingthe step voltage method to 4 kV and holding.While the standard only defines the testmethod and not pass/fail criteria,withstand performance in excess of 400minutes represents superiorperformance. Raychem products havealso been extensively tested in salt fogchambers as well as in field trials inextreme contamination conditions.

    Moisture Vapor TransmissionEven though the surface is very hydro-phobic, silicones have a higher moisturevapor transmission rate than Raychemspolyolefin co-polymer insulationmaterials. This is primarily attributed tothe very high free volume associatedwith an amorphous polymer (lowmolecular density) and the presence ofoxygen groups on the siloxanebackbone. The choice of fillers toimprove electrical, mechanical, andweathering properties can influence boththe hydrophobicity and moisture vaportransmission within silicone-basedformulations. Some fillers can lower themoisture vapor transmission, but cannoteliminate it. The semi-crystallinepolyolefins, with lower free volume, tendto have much lower moisture vaportransmission rates. The hybrid insulatorpart, with a very resistant ceramic at itscore, is not sensitive to this property.

    WeatherabilityThe claim is commonly made thatsilicones cannot be damaged by UVexposure, as the bond energy of the Si-O backbone is higher than the energylevel in photons. While this is true in neatsystems, it is a simplistic statement thatfails to consider the overall situation. Aspreviously shown, silicone contains otherside-groups and crosslinks whichcontain organic bonds that aresusceptible to UV attack. Sunlight incombination with surface chemicalattack may also result in damage. Somefillers may also worsen the UV resistanceof the polymer itself by acting ascatalysts, lowering the energy requiredto break the Si-O bond. With the highraw materials cost of silicones,formulations range from 20%-80% byweight of silicone, leaving a considerableamount of the material that can alsosuffer UV degradation.

    Figure 5. ASTM D2303, tracking anderosion resistance test results for variousexperimental silicone formulations(samples A, B, C, and D correspond tothe same formulations shown in Figure3). Sample D represents an optimizedsilicone formulation. Test conditions areshown on the right of the graph (sampleA was only tested for 420 minutes).

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    4.0 kV, 0.6 ml/min

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    3.0 kV, 0.30 ml/min

    2.75 kV

    2.5 kV, 0.15 ml/min

    Introduction 14.12.199