The potential benefits of composite insulators have interested utilities and equipment manufacturers worldwide since polymeric designs were first introduced in the 1960s. Lighter in weight and less susceptible to breakage than glass or porcelain materials, they showed immediate promise for lowering transportation costs, easing installation and reducing maintenance. The savings could be especially dramatic in areas with difficult accessibility. Typically smaller than ceramic insulators, composites can also reduce tower heights and right-of-way space requirements.

In particular, polymeric materials have been found to drastically reduce the effects of vandalism on high-voltage insulator. Polymers typically resist mechanical shock much better than ceramics or glass. The inherent flexibility of these materials helps an insulator absorb an impact when struck by a bullet or other projectile, offering a less appealing target for vandalism.

Initial materials testing for HV insulator applications focused on a number of candidates, including ethylene propylene rubbers (such as EPDM), epoxies, polyolefins, polyurethanes, polyethylene, silicones and PTFE. Water repellency has been a fundamental design parameter, regardless of the material or specific insulator design.

 

Silicone rubber

Silicone materials of various types have been used in electrical service for over 50 years. In fact, one of the first applications for a silicone was electrical insulation on aircraft in World War II. In the 1970s, Dow Coming developed a room-temperature curing elastomer, designed for spray application to porcelain insulators to reduce insulator maintenance and resist flashovers, particularly in salt fog environments. The product is still used today, allowing utilities to improve the electrical performance of porcelain arrestors without replacing them.

Insulators molded from silicone rubber are also gaining increased recognition as an effective alternative to traditional porcelain and glass designs. In fact, nearly every major manufacturer of high-voltage electrical insulators has now introduced a silicone product

among its offerings. The primary advantages of silicone insulators include light weight (as little as 1/10 of the electrical equivalent in porcelain), impact resistance, and good performance in contaminated environments. Unlike EPDM and other organic materials for high voltage applications, silicone elastomers resist degradation from UV exposure, salt fog and extreme temperatures.

Silicone elastomers for high-voltage insulator applications are generally high-consistency rubber (HCR) compounds. Two types of filler are commonly used: silica is the reinforcement that lends physical strength to the polymer, while alumina trihydrate (ATH) improves arc resistance. Filler treatments, pigments and cure agents may also be part of the formulation in small amounts.

The polymer-filler combination is important in silicone insulators. Processing, physical properties and electrical performance are all affected by the molecular weight and structure of the polymer, as well as filler type, size, shape, surface treatment and residual catalyst or contaminants. In determining the optimum formulation for specific applications, device manufacturers and silicone suppliers must determine the best balance of properties, processing characteristics and economic considerations.

Silicone has demonstrated better hydrophobicity and lower surface energy than most organic polymers. The surface properties of silicone are unique, in that it recovers its hydrophobicity between contamination and/or corona episodes, while other materials progressively deteriorate. Corona exposure does temporarily increase the wettability of silicone rubber, a phenomenon associated with an increase in surface oxygen content, but the water-repellency returns after a period of rest. The material ability to recover its hydrophobicity is thought to result at least in part from the diffusion of low molecular weight PDMS(polydimethyl-siloxane) fluid to the surface.

Another force which affects the hydrophobic hydrophobic recovery demonstrated by silicone insulators is surface reorientation. The extreme flexibility of the siloxane chain and the low molecular forces between methyl groups produce a low glass transition temperature and a high free volume of PDMS. These conditions readily permit surface reorientation of silicone rubber, which has the most mobile surface of all common polymers for HV applications.