Recovery of Hydrophobicity of a NewNano-composite Coating for Ceramic

Insulators

Igor Gutman, IgorDjurdjevic

HV Technology and TestingSTRI AB

Ludvika, Swedenigor.gutman@stri.se

igor.djurdjevic@stri.se

Jens Seifert R&D and Technical

Applications Lapp Insulators Europe

GmbH Wunsiedel, Germany

jseifert@lappinsulators.de

Corinne Greyling Chief Technology Officer

NanoNCICape Town, South Africa

cg@nanonci.com

Abstract—The mechanism of hydrophobicityreduction and recovery is not known for a newnano-composite material, at the same time thismight be a promising idea to use this coatingto increase pollution flashover performance(similar as RTV-coating provides). Thus thegoal was to evaluate a speed of the reductionand recovery of hydrophobicity of nano-coated“super-hydrophobic” material in polluted andwet conditions and to compare it with thestandard HTV silicone rubber material. Thelaboratory program included investigation ofrecovery after pollution application and afterwater immersion. The recovery ofhydrophobicity was in both cases much fasterfor nano-coating than for HTV silicone rubberindependently of pollution level. The polymer-non-ceramic insulators coated with this nano-composite coating were also subjected to harshenvironment over a period of more than a yearat Koeberg Insulator Test Station in SouthAfrica. The insulators passed the testallowing their usage in both inland andcoastal areas.

Keywords-nano-composite; coating; insulators;hydrophobicity

I. INTRODUCTION For two years STRI has tested a new

type of “super-hydrophobic” nano-compositecoating on conductors in iced conditions

and found it also promising for furtherinvestigation as a new coating forinsulators. It was thus decided toinvestigate, as the first step, thespecific hydrophobic properties of “super-hydrophobic” nano-coating insulationmaterial in polluted and wet conditions.The main goal was to evaluate a speed ofthe reduction and recovery ofhydrophobicity of nano-coated “super-hydrophobic” material in polluted and wetconditions and to compare it with thestandard HTV silicone rubber material. Theprogram includes investigation of recoveryafter pollution application and afterwater immersion. Tracking and erosionparameters (i.e.) ageing were investigatedduring one year filed tests at KoebergInsulator Pollution Test Station (KIPTS)in South Africa located in extremely harshenvironment.

II. DETAILS ON NANO-COMPOSITE MATERIALA coating was developed by NanoNCI and

it can be spray coated from a “one-pot”formulation using environmentally friendlysolvents [1] as can be seen in Figure 1.The biomimetic coating, based on thestructure of the Lotus leaf is super-hydrophobic (SH), exhibiting a contactangle of more than 160 when new (SH is

defined for contact angles > 150 [2]),due to the 3-dimensional hierarchicalstructure at the outer surface, as can beseen in Figure 2. The coating is a highlyfilled siloxane hydrocarbon resin whichundergoes phase-separation upon curing tocreate a hard, SH surface, thus it can bedefined as inorganic-organic according to[3].

Figure 1. “One-pot” spray application of thenano-composite.

Figure 2. SEM micrograph at 30000 Xmagnification showing 3D structure.

III. LABORATORY TEST PROGRAMA. Test samples

Samples of two materials in the form ofrectangular plates were used in the tests.

The first 10 samples of super-hydrophobicnano-composite coating on porcelain plateswere approximately 70x40x15 mm (LxWxH).The second 10 samples of standard HTVsilicone rubber were used as a referenceand originated from [4], where they aredescribed in detail. These plates haddimensions approximately 120x50x10 mm.B. Pollution testing

This part of the program consisted ofresistance measurements of the pollutedand wetted test samples in time, this isin order to check a speed of recovery ofhydrophobicity as was earlier performed in[4]-[5]. It was decided to test thesamples at two different pollution levels.

The pre-testing was performed in orderto obtain a correct solution of kaolin,water and salt in order to give the targetpollution levels on the samples. As statedpreviously two different pollution levelswere decided for testing. The first targetpollution level was Equivalent SaltDeposit Density (ESDD) of about of 0.15mg/cm2. This level corresponds to earlierresults from [4]. The second targetpollution level was decided as more severewith an ESDD of about 0.45 mg/cm2. Aftersome trials during pre-testing it wasdecided to apply the pollution layers tothe samples by spraying. This method gavea more even distribution of pollutionacross the sample than dipping. Once acorrect concentration of kaolin, salt andwater was found, the pre-testing wasrepeated three times to ensure therepeatability, i.e. that the same ESDD andNon-soluble Deposit Density (NSDD) valueswere obtained each time. The repeatabilityof application was very good (deviationwas less than 5%, while 15-20% is normallyrecommended by pollution standards, e.g.IEC 69507). The samples were firstlyprepared using the similar to ArizonaState University method for pre-conditioning [6] and subsequently sprayedby the different solutions. They were leftto dry naturally to avoid the loss of anyparticles through forced air drying. Afternatural drying at room temperature the

testing of the samples began.

The testing was performed as follows.Each of the samples was gently wetted byspraying of distilled water until theentire surface was visibly wet. The samplewas then placed between electrodes and avoltage of 500 V was applied across theelectrodes. The resistance of the pollutedand wet sample was measured, see Figure 3.These measurements were repeated once in24 hours for a period of 10 days.

Figure 3. Test setup for resistancemeasurements.

C. Water immersionTwo samples of nano-composite coating

and silicone rubber were inserted inseparate containers filled with tap waterwith the conductivity 240 µS/cm. Thetemperature of the water was 25˚C in bothcontainers. The samples were left in thewater for a period of 10 days. After 10days of water immersion these conditionswere basically un-changed (i.e. theconductivity of the water was 254 µS/cm inboth of containers and the temperature inboth of containers was 21˚C). After 10days the samples were removed from thecontainers and left to dry naturally. Oncecompletely dry they were sprayed by tapwater and their Wetting Class wasevaluated. This process was repeated oncein 24 hours for a period of 11 days.

IV. LABORATORY TEST RESULTSA. Pollution testing

The results of the change of resistanceover the period of 10 days are shown inFigure 4. The three HTV samples forsolution 1 and solution 2 were averagedeach day. The three individual samples ofnano-composite coated porcelain were alsoaveraged each day for solution 1 andsolution 2, i.e. the upper curvecorresponded to the ESDD level 0.15mg/cm2, while the lower curve correspondedto ESDD 0.45 mg/cm2. Due to the largedifference in scales between the HTV- andnano-composite material coated samples therecovery of HTV samples is not clearlyevident. Thus the recovery for HTV isvisualized separately in Figure 5.

The ESDD measurements were taken after10 days when resistance measurements werecompleted. The results of a comparisonbetween samples measured after 10 days andat the start of testing are shown in TABLEI. This data is important to confirm thatthe change of resistance is explained notby washing off part of salt (40-70%according to TABLE I. ), but by otherfactors actually changing the resistance7-10 times during the same 10 days period(Figure 4. Figure 5. ).

Figure 4. Resistance measurement over time(averaged values).

Figure 5. Resistance measurement over time for HTV silicone rubber samples.

TABLE I. ESDD MEASUREMENTS COMPARISON

SampleESDD

(mg/cm2)initially

ESDD(mg/cm2)after 10days of

measurementHTV, low pollution level 0.12 0.07

Nano-composite,low pollution level

0.07 0.02

B. Water immersionThe wetting class evaluation over the

period of 10 days is shown in Figure 6. Aphotograph of each of the samples has alsobeen captured on each of the days. Thephotographs of the samples are presentedfor three occasions during testing toillustrate the comparative results in thebeginning, at the middle and at the end ofthe testing period, see Figure 7. - Figure9.

0

1

2

3

4

5

6

7

0 2 4 6 8 10 12

W ettability class

Days

W ettability class changes over tim e

HTV 1

HTV 2

Nano 1

Nano 2

HTV avg

Nano avg

Figure 6. Wettability Class measurements.

Figure 7. Hydrophobicity directly after water immersion test: left - HTV silicone rubber; right – nano-composite.

Figure 8. Hydrophobicity after 6 days afterfinishing of water immersion test: left - HTV

silicone rubber; right – nano-composite.

Figure 9. Hydrophobicity after 11 days afterfinishing of water immersion test: left - HTV

silicone rubber; right – nano-composite.

V. FIELD TEST PROGRAM AND RESULTSA. Test samples

The nano-composite coatings wereapplied to polymer based non-ceramic line-post insulators. The units were specifiedfor voltage class 22 kV; the test set-upis shown in Figure 10.

Figure 10. Test prototypes installed at KIPTSfor one year test.

B. Field test programKIPTS station located at the Atlantic

Ocean coast close to Cape Town is known asthe most severe place for testing frominsulation and ageing points of view andactually is considered by experts as“natural accelerating chamber”.Insulators, which pass the complete oneyear test at Koeberg are recommended forservice at South African power utility

ESKOM. Examples of evaluation of SitePollution Severity (SPS) according to IEC60815-1 are shown in Figure 11. and Figure12. Maximum ESDD is 0.55 mg/cm2. TypicalNSDD value for Koeberg is 0.6 mg/cm2 [7].These values are plotted as a circle inthe chart for pollution severity ofreference cap and pin insulator adoptedfrom IEC 60815-1 (Figure 12. ). This showsthat the pollution severity at Koebergtest station can be classified as “Veryheavy”.

Figure 11. Example of average ESDD on top and bottom side of standard porcelain insulator during the complete test period in 2009/2010.

Figure 12. Pollution severity from IEC 60815-1 –Relation between ESDD/NSDD and SPS for reference capand pin insulator (adopted from IEC 608125-1). Typical ESDD/NSDD data from Koeberg is included (seecircle pointed by the arrow).

C. Field test resultsInsulators covered by nano-composite

coating passed complete test sequence fromMay 2009 to April 2010 without flashoversand with only beginning of slight erosionand thus were allowed to be used in allenvironments including heavy coastalenvironment of South Africa. This was oneof the first compositions of the coatingwhich is significantly improved atpresent.

VI. DISCUSSIONA. Pollution testing

It is evident that recovery ofhydrophobicity measured as resistance ofthe pollution layer exists both for HTVsilicone rubber and nano-coated porcelainsamples. The recovery of hydrophobicity ofthe nano-composite coated samples appearsto be much faster than the recovery ofhydrophobicity of the HTV samples.

The ESDD measurements taken from thesamples in the beginning and at the end of10-days testing confirmed that the changein resistance cannot be explained bywashing of salts and is thus attributed tothe recovery process. This confirms thatthe method for recovery evaluation issimple and reliable, which was earlierconfirmed for other materials [4]-[5].B. Water immersion

The nano-coated samples were much morehydrophobic than HTV silicone rubber afterthe water immersion. Although thehydrophobicity of nano-coating diddecrease after the water immersion, itrecovered their hydrophobicity quickerthan the HTV samples.

Although the mechanism of recovery ofnano-coating after water immersion isstill unclear it is surmised that thefollowing are contributing factors. Theglass transition temperature is higher andthe free volume is lower for the nano-coating in comparison to the HTV siliconerubber. These inhibit the flipping oflabile methyl groups into the bulk of the

material and exposure of the partiallyionic Si-O backbone of the siloxanepolymer when placed under stress by waterimmersion. The concentration of the methylgroups can therefore recover faster at theouter surface and reach equilibrium in thelowest energy conformation. In additionthe siloxane moieties are copolymerizedwith the hydrocarbon oligomers and thuscannot undergo the “back-biting” reactionwhich initiates the degradation of thestandard silicone rubber.

Some specific observation was that thewater immersion method did not appear toremove any of the nano-coating off thesample, which seemed to be the case whenthe ASU method of pre-conditioning wasused in pollution trials.C. Field testing

Based on positive experience, the nano-composite coating has been furtherimproved and applied onto cap-and-pinglass and porcelain insulators, as shownin Figure 13. and their evaluation isunderway at KIPTS in 2011-2012. There isno flashovers/deterioration so far.

Figure 13. Example of new test prototype installed at KIPTS for one year test during 2011-2012 (test to be finished in April 2012).

VII. SUMMARYIn polluted conditions both nano-

composite coating and HTV silicone rubbersamples were initially put in hydrophilicconditions by ASU pre-conditioning methodwith dry kaolin. After this the recoveryof hydrophobicity, measured in terms ofcomparative resistance of the pollutedlayer, was much faster for nano-compositecoating than for HTV silicone rubberindependently of pollution level.

Reduction of hydrophobicity measured interms of Wettability Class (WC) was lowerat nano-composite coating than at HTVsilicone rubber after 10 days of waterimmersion at room temperature. Recovery ofhydrophobicity measured in the same termsof WC was faster for nano-compositecoating than for HTV silicone rubber andafter 10 days the nano-composite coatingwas recovered completely.

The nano-composite coated samples thatwere affected by pollution and repeatedwetting in pollution trials seems to losea small part of their coating. Even itappears to be still an extremely super-hydrophobic surface, it is recommended tocheck the ageing performance of thecoating by e.g. some laboratory trackingand erosion test and also by field ageingunder UV-radiation and corona stress.

The precise mechanism of recovery ofthe NanoNCI coating is not properlyunderstood yet. It would be interestingto investigate the reduction and recoveryover an extended period of time underdifferent conditions recommended above.

REFERENCES

[1] C. J. Greyling, “A Polymeric High VoltageInsulator with a Hard, Hydrophobic Surface”,WO2009/073901 A2.

[2] Z. Guo, W. Liu and B.-L. Su, “Superhydrophobicsurfaces: From natural to biomimetic tofunctional”, Jnl of Colloid & Interface Science,353, 2011, pp 335-355.

[3] Working Group D1.24, “POLYMER NANOCOMPOSITES.Fundamentals and possible applications to powersectors”, CIGRE Technical Brochure 451,February 2011.

[4] I. Gutman, A. Dernfalk, A. Nefedov and J.Seifert, “Pollution test methods for compositeinsulators: simulation of coastal environmentand recovery of hydrophobicity“, 16th ISH-2009,Cape Town, South Africa, 24-28 August, 2009,paper E-4.

[5] I. Gutman, I. Djurdjevic, S. Berlijn, Á.J.Elíasson, P. Söderström and L. Wallin,“Influence of air-borne volcano ashes on outdoorinsulation”, 17th ISH-2011, Hannover, Germany,22-26 August, 2011, paper C-014.

[6] I. Gutman, K. Kondo, R. Matsuoka and R. W.Garcia, “CIGRE Round Robin Pollution Test forpolymeric insulators: results in four high-voltage laboratories”, 17th ISH-2011, Hannover,Germany, 22-26 August, 2011, paper C-013.

[7] W.L. Vosloo, ”A comparison of the performance ofhigh-voltage insulator materials in a severelypolluted coastal environment”, PhD dissertation,University of Stellenbosch, South Africa, March2002.