The market for composite insulators is growing steadily, both in long-rod line applications and hollow core station insulators.Composite designs typically use engineered polymers which offer higher mechanical strength, greater design flexibility,reduced weight, and lower breakage rates than ceramic components. The emerging shift to composites adds new importance to the debate over which polymeric material should be used for the housing. Insulator field experience and extensive, multi-stress lab testing of different elastomer formulations have shown silicone’s unique surface behavior to be an advantage in these applications.

  Composite insulators designed with silicone rubber have been found to reduce the need for maintenance in areas of moderate to high contamination. The surface of a silicone elastomer has a unique ability to interact with the contaminant and control leakagecurrents, interrupting the normal process that leads to contamination-induced flashover. Improved control of leakage current helps prevent dry band arcing and subsequent flashover.
Silicone has demonstrated better hydrophobicity and lower surface energy
than most organic polymers. The surface properties of silicone are such that the material recovers its hydrophobicity between contamination and/or corona episodes, while other materials progressively deteriorate.
Corona exposure temporarily increases the‘wettability’ of silicone rubber, a phenomenon associated with the increase in surface oxygen content. After a rest period, the material’s water-repellency returns. This hydrophobic recovery is thought to result at least in part from the diffusion of low molecular weight PDMS (polydimethylsiloxane) fluid to the surface, as well as a unique ability of the silicone surface to reorient after exposure. This free fluid in silicone rubber is also the key to another important surface characteristic of silicone rubber. The microscopic diffusion of fluid serves to encapsulate contaminant particles and prevent moisture absorption. Migration of the hydrophobic fluid helps maintain a high surface resistance and water-shedding ability.