SYLGARD HIGH VOLTAGE INSULATOR COATINGANSWERS TO THE MOST FREQUENTLY ASKED QUESTIONS

Q. What is the fundamental difference between Sylgard HVIC and grease coatings?

A. Sylgard is an RTV (Room Temperature Vulcanising) silicone elastomer that cures to ahard rubber coating on the insulator surface.The most widely used grease coatings are silicone and hydrocarbon. These greases burythe pollutants as they fall on the insulators, leaving the surface free of conductivematerials that could cause an increase in leakage currents.Grease coatings need to be removed and replaced when they become saturated with thepollutants.

Q. Why does Sylgard have a longer service life than grease coating?

A. Because Sylgard cures to a hard rubber surface on the insulator, it does not bury thepollutants, does not become saturated and, as such, does not need to be removed.Pollutants can be removed by occasional wind or rain since they are not deeply imbeddedas in the grease.

Q. How does Sylgard restrict the rise in leakage currents and protect the insulatorsagainst pollution induced flashovers if it does not bury the pollutants?

A. The cured Sylgard coating has a high angle of hydrophobicity. This hydrophobiccapability is of prime importance because it is this factor that controls the degree ofwetting of the contaminants and thereby the amount of surface leakage current increase.Moisture on the insulator surface will form in droplets and by so doing will prevent thesurface pollution from becoming wet and producing a conductive layer of ionisablematerials that lead to increased leakage, dry band arcing and eventual flashovers.In addition, there are a certain percentage of polymer molecules that exist within thecured rubber as low molecular weight free fluid. These molecules are known as “cyclics”.The free fluids are easily able to migrate to the surface of the coating and, as pollutantsfall on the surface, they in turn are encapsulated and rendered non conductive andsomewhat hydrophobic.If leakage currents are controlled, there will be no arcing. If there is an extreme weatherevent then it may be that, for a time, the Sylgard coating cannot control the surfaceleakage currents. In this case Sylgard also provides a high degree of surface arcresistance. Incorporated into the formulation is an alumina trihydrate (ATH) filler, whichreleases H2O when it becomes hot and consequently resists the degradative effects ofhigh temperatures, resulting from exposure of the coating to arcing. This is really a lastline of defence.

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Q. For how long will a single coating of Sylgard provide protection?

A. The actual life expectancy of Sylgard is mainly dependant on the insulator design aswell as the type of contaminate and the rate of fall out on the insulator surface, howeverexperience shows us that a single application should provide excellent protection forperiods in excess of 10 years.The shortest life cycle we have observed is about seven years on a particular design of345kV bushing in heavy pollution. This particular design assures almost constant coronaon the high voltage end. A few users have noted wear and recoated the top sheds withnew Sylgard HVIC to assure continued trouble free performance in this type of situation.

Q. How is the coating applied to the insulators?

A. The most widely used application method is by spray coating.The insulator surface should be clean and free of dust, wax or grease. It is recommendedthat the surfaces be cleaned with a non-polar solvent followed by a wipe down withisopropyl alcohol. If the insulators have previously been coated with silicone compoundor other greases, the grease should be completely removed by hand or by high-pressurewater washing (if the “old” grease is difficult to remove it may be necessary to useparaffin or diesel diluted with an emulsifiable degreasing fluid). All residual greasedeposits or contaminates should then be removed with a non-polar solvent followed by awipe down with isopropyl alcohol prior to spraying the Sylgard.Diluting of the Sylgard may be necessary depending on the type of spray equipment to beused and also the temperature & humidity. Use a high flash point Naphtha Spirit Solventsuch as Shellsol D40 or White Spirit.Standard compressor operated air assisted/airless spraying equipment can be used andtwo to three coats would be required. To avoid unnecessary thinning of the Sylgard, thespray gun should have a nozzle diameter of approximately 1.8mm.For reasonably small applications or for repairs, it is also possible to apply Sylgard bypaintbrush.Detailed application procedures are available in the booklet, “Sylgard High VoltageInsulator Coating – A Guide To Proper Application”.

Q. How can it be determined if the surfaces are clean?

A. Simply wipe the insulator with a clean rag and check to see if the rag picks up

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anything. After removal of silicone compounds or greases, spray some water onto thesurface and see if the water beads up, if it does, there is still some grease remaining andfurther cleaning is necessary.

Q. What happens if Sylgard is applied to a dirty surface?

A. Two things. Firstly, you don’t get good adhesion, shortening the life of the product.Secondly, you don’t want to put Sylgard over a conductive contaminate. This conductivesurface could lead to interfacial tracking underneath the Sylgard.

Q. Can Sylgard be applied to a wet surface?

A. The surface should be dry for best adhesion.

Q. What time is required before applying second and subsequent coats?

A. Approximately 20 minutes is required before Sylgard becomes tack free resulting ingood adhesion to itself.

Q. When can the system be re-energised after coating?

A. Because the electrical properties are nearly the same for uncured as for cured material,the system can be energised immediately after coating is complete. Full cure will takearound 8 hours.

Q. To what thickness should it be applied?

A. To get the best optimum life, it is recommended that Sylgard HVIC be applied to acured thickness of 0.5mm +/- 0.14mm and to achieve this, the uncured (wet) thicknessshould be approximately 0.7mm.

Q. What effect will less than 0.5mm thickness have?

A. Dow Corning feels that 0.5mm thickness is optimal, based on our long-term successesat many application sites. End of life to Sylgard is by erosion, so you reduce the life if athinner coat is applied. It is also a function of the severity of the environment.

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Q. How can thickness be checked?

A. Precise measurement can be made during application by using an inexpensive wet filmthickness gauge. If using the wet film method, note that 0.7mm of wet film is required for0.5mm of cured Sylgard. Cured or dry Sylgard can be measured with ultrasonic thicknessdevice.

Q. Does the underside of insulator sheds require 0.5mm thickness?

A. It is not always possible to achieve a 0.5mm thickness under sheds, which have skirts.It is important to see that the “hard to get at” areas have at least one good coat of Sylgard(such that the insulator base colour is not visible).

Q. Do runs or drips cause problems?

A. No, but this should be avoided since it is a waste of material.

Q. What maintenance is required to be done to the coating during service?

A. Sylgard HVIC was designed to be essentially maintenance free. The coating willretain some dirt and discolouration on the surface but water repellency is fully maintainedwith encapsulation of the deposits by the low molecular silicone fluid.Washing or cleaning should not be considered purely on appearance (i.e. it is dirty) butonly if there is concern regarding increased electrical discharge or loss of hydrophobicity.

Q. Can Sylgard be applied energised?

A. Dow Corning recommends that Sylgard HVIC not be applied energised due to thesolvent. In addition it is best to shut down the system so that you can clean off theinsulators thoroughly for long-term performance.

Q. Is Sylgard HVIC effective against all types of contaminants?

A. Sylgard HVIC works well with practically all types of contaminants. Some highlyconductive industrial contaminates might require special attention, but flashovers causedby iron ore and graphite dust, for example, have been prevented by applying Sylgard.

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Q. How can we determine if the Sylgard coating is still good?

A. Normally an eye inspection is good enough to see the condition of the coating. Asimple hydrophobicity test (by a fine spray of water onto the coating) is a simple method4of judging the Sylgard. If the water beads up into small droplets it is an indication thatthe Sylgard is fine. If the droplets are large, the coating may need a clean.

More precise methods of judgement include measurement of the surface resistance and/orresistance.

Q. Can Sylgard be used for surface repair?

A. Yes. Simple repairs to a porcelain insulator where the glaze has been partiallyremoved by flashover, for example, can be effected by firstly priming the un-glazed partswith Dow Corning 1200 OS primer, Dow Corning HV1860/120 primer or low VOC 5200primer and then coating the whole insulator with Sylgard HVIC to the recommendedthickness of 0.5mm.For repair of an insulator, which has been chipped, the chip can be “built up” with epoxyresin, the epoxy treated with the primer (as above) and the whole insulator coated withSylgard.

Q. How is Sylgard removed after it’s effective service life?

A. The original coating should never have to be removed.If there are signs that the coating has become thin (e.g. the insulator can be seen throughthe coating), or there is a high degree of loss of hydrophobicity, then, provided theoriginal coating still has good adherence to the insulator, it should simply be a matter ofcleaning the original coating and applying a further coat over the top.A hand wash with warm water (no detergents) is usually sufficient but if thecontaminants are difficult to remove a solution of isopropyl alcohol mixed with 50%clean water is recommended.

Q. How does the cost of Sylgard compare with silicone grease?

A. Kilogram for kilogram silicone grease is substantially less costly than Sylgard HVICbut will be dependant on the quality of grease purchased. However the volume of Sylgardrequired per square metre of insulator surface is 30% (or less) of the volume of greaseneeded for the same area. [Sylgard thickness is 0.5mm - Grease thickness is 1.5 to 3.0mmdependant on pollution levels].

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With application costs for Sylgard and grease being about equal, the savings by usingSylgard would be realised at the time when the grease coating would need to be removedand replaced for the first time. In addition, the savings in labour and time to repeatedlyremove and replace the grease compounds these savings.

Q. What is the shelf life of Sylgard?

A. Sylgard has an expiry date, which is 12 months from date of manufacture, and eachpail is labelled with this date. However the product would only be kept in inventory for amaximum of 6 months so the actual minimum shelf life would be 6 months from date ofshipment. No maintenance is required to the pails during this shelf life period – unlikesome other RTV coatings, which need to be rolled and agitated to keep the polymerssuspended within the solvent.

Q. Can Sylgard be applied to non-porcelain and non-glass insulators?

A. Sylgard is designed to provide un-primed adherence to glass and porcelain insulatorsbut it is possible to coat polymer insulators (e.g. EPDM) by firstly applying a prime coat.The recommeneded prime coat is the Dow Corning 1200 OS Primer, Dow Corning HV1860/120 primer or low VOC 5200 primer that are specially used to improve andaccelerate adhesion build-up of silicone elastomers to various substrates.

Q. Will the Sylgard coating withstand wind blown particles such as sand?

A. Yes, because the coating is basically a rubber it can absorb the energy of a sandstorm.Tests have been carried out on a half coated insulator in a shot blast chamber, the resultsof which show the glaze was totally removed from the uncoated area of the insulatorwhereas the Sylgard coating was unaffected.

Q. Are all high voltage insulator coatings similar?

A. No. Sylgard HVIC has many unique features that are patented by Dow Corning and,as such are not included in other RTV formulations.e.g. The ATH “arc resistance” filler is a very important component of the coating and it isessential that the ATH particles be evenly distributed throughout the formulation. For thisreason Dow Corning uses the smallest possible particle size (one micron or less indiameter) and, because of the patent, all other RTV coatings use larger particle sizes ofATH.In general, smaller particle sizes have been shown to improve the ability of the

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coating to suppress leakage currents in salt fog chamber testing. In addition, larger andheavier particle sizes have difficulty remaining in suspension within the polymer andwhich could create two potential problems.

• One is that the ATH filler is not near the surface of the coating where it is neededto protect the polymer from degradation should leakage currents develop.

• Secondly, the larger particles have increased tendency to fall out of suspension inthe polymer while still in the storage containers. This can adversely affect theshelf life or, at least create a need to roll the pails and agitate the formulationduring the storage period. With the filler being manually stirred back into solutionby field personnel, the evenness of the filler dispersion would be of definitequestion. An even dispersion of the filler is of critical importance to the arcresistance of the coating.

Q. If insulators are pre-coated, how should they be handled?

A. The cured coating is reasonably resistant to damage. However to avoid abrasionduring erection, wrap the insulator in two sheets of plastic. For horizontal overheadinsulators be sure that the lifting rope is not rubbing on the coating. A lifting cradle couldbe made from carpet or other such material.