5. the Science Behind SiCoat

8/9/2019 5. the Science Behind SiCoat

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The Science behind Si-COAT RTV Silicone High Voltage Insulator Coating

With its introduction to the market nearly 20 years ago, Si-COAT was the first to offer the user a number of previouslyunavailable benefits.

The greatest of these benefits was Si-COATs superior adhesion to theinsulator surface. Thus, in the extreme case where infrequent waterwashing was still required to keep the coating performing optimally,Si-COAT would not come away from the insulator. This was not thecase with Si-COATs leading competitor.

But since those early days the competition has learned, andoutstanding adhesion of room temperature vulcanizing (RTV)coatings to insulators remains an innovation and benefit first broughtto the market by Si-COAT.

Perhaps less obvious but more significant an advantage, Si-COAToffered the user superior electrical characteristics. Such performance

yielded enhanced hydrophobicity, quicker hydrophobic recovery afterwater immersion (such as after heavy rainfall) and far greaterleakage current suppression than could be found in the competitorsproduct. These superior attributes were achieved through theidentification and patenting of the optimum particle size of a keyingredient called alumina trihydrate (ATH).

The attributes of Si-COAT are based on two decades of R&D by CSLSilicones Inc. in the field of RTV technology. Besides its patentedformulation, Si-COAT RTV derives its characteristics from aproprietary process of manufacturing the silicone polymer. Allengineering behind Si-COAT was undertaken independently and done in-house.

Even as the competition introduces PLUS or Next Generation versions of their RTV coatings, they still cannot

match the superior performance of Si-COAT. The reason for this lies in the patented discovery and the finelytuned manufacturing processes behind Si-COAT.

How Si-COAT RTV Silicone HVIC Functions

The purpose of an RTV coating for high voltage insulators is to suppress the evolution of leakage current.Leakage current develops along contaminated or dirty insulators. As the contaminant (typically carbon deposits,desert sand, industrial pollution or, most notoriously, salt deposits from coastal exposure) settles on theinsulator surface and combines with environmental moisture (rain, fog or dew), an electrically conductivesolution is formed.

Obviously, the primary function of an insulator is to prevent the flow of electricity from the conductor to thetower and down to ground. One can imagine, then, if there exists an electrically conductive fluid on the surfaceof an insulator, that insulators function is compromised.

It is very common for insulators to become contaminated in the manner described above. When this happens,small amounts of electricity leak out of the conductor and along the body of the insulator. This is what is calledleakage current. If leakage current becomes too great, there is a short circuit as electricity flows directly out othe conductor and straight to ground. What ensues is a power outage and extensive equipment damage.

The development of leakage current is reduced by coating the insulator. In the case of Si-COAT, leakage currentis nearly eliminated entirely. When the insulator has been coated and the RTV silicone has fully cured, thereremains within the body of the coating small chain, low molecular weight silicone (LMWS) polymer that is free tomove around within the body of the coating. Different manufacturers will have varying amounts of this LMWS in

Tested in the Lab and in the Field


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the coating. Studies under electron microscopes have revealed Si-COAT to have the greatest concentrations oLMWS.

Figure 1: Electron microscopic scan A depicting the very rich and even concentrationof LMWS in Si-COAT versus scan Bfor the leading competitor


It is the LMWS in the coating that plays a very essential role in the function of the coating. Two key properties oLMWS to bear in mind are its very high dielectric strength (i.e. high electrical insulation properties) and very lowsurface free energy. It is a common phenomenon that microscopic matter existing in a high concentration in onearea but in a low concentration in another area will migrate from the area of higher concentration to that olower concentration. This is termed diffusion. Because the concentration of LMWS is greater within the body o

the coating than at the surface of the coating, there is a natural tendency for the microscopic LMWS to diffuse tothe surface of the coating. But, since the surface free energy of LMWS is so low, it exists only in a monolayer onthe coating surface. That is to say, when the LMWS migrates to the coating surface, it stops migrating when thesurface is covered in a one molecule thick layer of LMWS.

Figure 2:A microscopic monolayer of LMWScovers the surface of Si-COAT


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When an insulator has been coated with RTV silicone, contaminants from the surroundings deposit directly ontothe surface of the coating and rest on the monolayer of LMWS. Again, because of its very low surface freeenergy, the LMWS begins to creep over the contaminant particle until it is fully encapsulated in a monolayer oLMWS.

Figure 3:Contaminant particles that land on the coating surface are quickly microencapsulated by the monolayer ofLMWSin Si-COAT

At this point, when moisture from the environment collects on the surface of the contaminated insulator, thecontamination particles are isolated from the water droplets. This fact, combined with the very high dielectricstrength of LMWS means an electrically conductive solution cannot form along the surface of the insulator.

Figure 4:Water droplets are isolated and electrically insulated from contaminants by virtue of Si-COATs LMWS

If rainfall is heavy, the contaminants will be washed away. In some instances, the monolayer of LMWS will alsobe washed away. In this situation, the LMWS from within the body of the Si-COAT layer again diffuses to thesurface to regenerate the monolayer.


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Silicones Weakness Under High Voltage Applications

Despite the very positive electrical characteristics of silicones and LMWS in general, when they are put intoservice under high voltage circumstances, they do show one key weakness. It is common in high voltageapplications for a phenomenon called dry band arcing to take place. Dry band arcing occurs when the body oan insulator is covered in moisture, say, by dew or fog formation. By virtue of the electrical current runningthrough the conductor on a transmission or distribution line, the end of the insulator connected to the conductor

is at a temperature greater than the ambient temperature. This situation creates a dry band along the insulator.Across this dry band, small sparks will jump, or arc, giving rise to the phenomenon called dry band arcing.

The sparks formed in dry band arcing are at an exceptionally high temperature. Although this temperature spikeis of a very short duration, it is enough to damage any RTV silicone rubber. The type of damage that occurs isscarring and pitting of the silicone. Such damage can act detrimentally to the coatings performance.

To overcome the damage, silicones for high voltage applicationsmust contain an ingredient called alumina trihydrate (ATH). ATHis a very small particle that is incorporated into the siliconeformulation. The granules of ATH are composed of numerousatoms of aluminum each attached to three hydrate (OH)

molecules. On heating to 200C, a temperature surpassed by theheat of dry band arcing, the granules of ATH will decompose into

66% alumina and 34% water. However, this water is generatedonly from the microscopic ATH particles at the surface of theoverall ATH particle. Of course, within the depth of the particle,hydrate molecules also exist. However, it is not always possiblefor these hydrate molecules to escape when subjected to hightemperatures. The greater the size of the ATH granule, thegreater is this likelihood.

It is the generation of water molecules from ATH that protectssilicone from the damaging effects of dry band arcing. The heatenergy of dry band arcing is first, partially absorbed by the ATH ingenerating the water molecule and second, further absorbed bythe water molecules themselves. Water has a very high capacityfor absorbing heat.

Now that the mechanics of how the RTV coating functions and theinherent weakness of silicones in high voltage applications havebeen described, it is pertinent to discuss ATH in greater detail.

ATH: An Essential Ingredient in High Voltage Silicones

The secret of ATH is actually not so secret at all. All high grade silicones for high voltage applications contain ATHin varying amounts and of various particle sizes. But, because a manufacturer can formulate their silicone withvarying loadings of ATH or choose a specific ATH particle size, not all high voltage RTV coatings are createdequal.

Through their research, the engineers and chemists at CSL discovered there, in fact, exists an optimum size oATH particle and an optimum loading of ATH in the silicone formulation.

A solid particle of ATH has a specific gravity of 2.42. Imagine 1 kg of ATH. If this ATH existed in one abnormallylarge spherical particle, that particle would have a volume of 413 cm3 and a corresponding surface area of 268cm2.

Imagine, instead, that 1 kg of ATH existing in 10 equally sized smaller spheres. Each of those spheres wouldthen have an individual volume of 41.3 cm3 and a corresponding individual surface area of 57.8cm2. What isinteresting to note, however, are the collective properties of the 10 spheres. All 10 spheres taken together stillhave a volume of 413 cm3 as did the single large sphere. However, the 10 spheres have a collective surface area

Figure 5:A dry band is formed at the endof the insulator closest to the conductor.

Across this dry band sparks will jump thatcan damage a silicone without ATH in itsformulation


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of 578 cm2. This is substantially greater than the surface area of the one large sphere! As you can imagine, thesmaller and more numerous that the ATH particles become, the greater the overall surface area becomes eventhough the collective volume (or mass) remains constant.

Figure 6:An equal mass or volume of ATHin a smaller particle size has a greater surface area

A greater surface area of ATH is beneficial, in part. Recall from above, the water generated by ATH comesessentially from the surface of the particle. Thus, a primary positive effect of employing very fine ATH particles is

that greater quantities of water will be generated in order to keep the silicone protected. Yet, at the same time,as the ATH particle gets finer it has a secondary negative effect.

13 Microns: The Optimum ATH Particle Size

As was previously described, the finer the ATH particle, the greater total surface area. Although this has anapparent benefit, it also has a detrimental effect when incorporated into high voltage RTV silicone coatings. As

the LMWS floats freely within the body of the coating, it requires a free and clear path in order to quickly migrateto the surface of the coating where it can perform its essential functions. By employing too fine an ATH particle

in the RTV formulation, these pathways for the LMWS become blocked and tortuous, making it rather difficult forthe LMWS to migrate to the coating surface. However, if the particle size of the ATH is increased, the LMWSpathways remain relatively straight. Dont forget, though, with an increased ATH particle size what is being

traded off is the ATHs capacity to generate water to protect the silicone in the event of dry band arcing.

Figure 7: The optimized and patented ATH particle size in Si-COAT imparts far superior performancecharacteristics when compared to the leading competitors products

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5. the Science Behind SiCoat