IEEE Std 1523-2018 pdf download – IEEE Guide for the Application, Maintenance, and Evaluation of Room-Temperature Vulcanizing (RTV) Silicone Rubber Coatings for Outdoor Ceramic Insulators.
5. Types of RTV coatings Commercially available RTV insulator coating systems are of three types. Type I coating, and the longest in use, is a solvent based coating. This type typically consists of a base silicone polymer, extending fillers such as alumina tri-hydrate, ground quartz or a combination of the two fillers, or no extending fillers, a catalyst, reinforcing filler, pigment, an adhesion promoter, and a cross-linking agent. These systems are dispersed in a solvent such as naphtha, or a non-flammable solvent. The solvent merely acts as a carrier medium to transfer the RTV rubber to the insulator surface.As the solvent evaporates from the surface, moisture from the air triggers vulcanization forming a solid rubber coating. The speed at which this takes place depends on the type of solvent, the cure system chemistry, temperature, and humidity (see Gorur, et al. [B3]). Extending fillers displace the silicone polymer thereby lowering the cost of the composition. Normally, an appropriate loading of filler helps resist erosion under dry-band arcing, the degree of which is proportional to the amount and type of filler. Type II coating is a solvent-less coating which basically has the same ingredients as Type I coating without the carrier solvent. In most cases, reduced extending filler in a Type II coating compared to a Type I coating is necessary to render low viscosity for spray application, but this is not necessarily true in all cases. Type III coating is a water-based coating having entirely different cure chemistry. Essentially, the polymer is suspended in water, with little or no extending fillers, and the evaporation of the carrier water triggers the cure chemistry. This type of coating has very little experience as an insulator coating and will not be addressed in this guide. The electrical and physical properties of the coating systems vary depending on their formulation. These properties are the result of the amount of extending fillers, degree of cross linkage, and adhesion promotion.
6. Application guidelines 6.1 Test for adhesion Adhesion to the insulator or apparatus surface is of paramount importance and must be checked prior to application particularly if the insulator substrate is unknown or if a new coating system is being applied for the first time. Improperly adhered coatings can result in moisture developing between the coating and the insulator surface giving rise to blisters, cracks and eventually tearing of the coating by wind or during high- pressure water-washing maintenance. In the case of a composite substrate the efficiency of the test is still under investigation. To test for adhesion, a boiling water test is suggested. An insulator sample, prepared in the prescribed way, is first coated using the equipment that is at hand and the coating is allowed to properly cure. The sample is then immersed in water, boiled for 100 h and removed. Coating that is either not bonded or weakly bonded to the surface will show bubbles and blisters as the water migrates into the interface between the sample substrate and the coating.