From the second half of the 19th century, when erecting transmission lines, the only insulating materials suitable for high voltage are ceramics and glass. In the 1940s, due to the advent of polymer materials, ceramics and glass were no longer the preferred insulator materials, and European and American countries began to study polymer insulators. Then the physical properties, electrical properties, long-term reliability and optimal shape of the electrical insulator are studied, and the productivity has also been continuously improved.

Among the polymer materials that can replace ceramics and glass, silicone rubber has made practical achievements since the 1960s and stands out among several polymers. Silicon rubber insulators have more advantages than ceramic insulators. First, it is light, easy to operate and safe; In addition, ceramic insulators often suffer from cracks, and they will be damaged under one impact. Silicone rubber insulator can resist mechanical impact such as vehicle collision with electric pole.

Although other polymer materials also have the advantages described above, only silicone rubber will not cause too much pollution to the environment. The polymer insulator can be waterproof, so there will be no leakage and surface arc due to water drop. The waterproof performance of silicone rubber insulator is faster than that of other polymer insulators. It is a durable material that can be used for a long time in harsh environment.

Characteristics of silicone rubber:
The main chain of silicone rubber is composed of siloxane (Si-O) chain. Because the negativity of Si and O on this bond is 1.8 and 3.5 respectively, which are very different, it has ionic bond properties. In this way, the bond energy of Si-O is higher than that of C-C (see Table 1). In addition, ① because of the nature of the main chain ionic bond, the ionic property of the side chain methyl C-H is weakened, and it is not easy to be attacked by other molecules, so its chemical stability is good; ② Since Si does not form double and triple bonds, it is difficult to form the starting point of the main chain decomposition (due to this reason, the Si-C bond is quite stable), and thus the main chain of silicone rubber is more stable.
Characteristics of silicone rubber:
Heat and cold resistance; Water resistance; Electrical performance; Weather resistance; Permanent deformation.

Classification of silicone rubber:
Silicone rubber can be divided into solid and liquid according to the characteristics before vulcanization, and can also be divided into peroxide vulcanization, addition reaction vulcanization and condensation reaction vulcanization according to the vulcanization mechanism. The difference between solid and liquid silicone rubber is the molecular weight of polysiloxane. Solid silicone rubber can be vulcanized by either peroxide vulcanization or addition reaction, which is usually referred to as high-temperature vulcanized rubber (HTV) and heat vulcanized rubber (HCR). Although the liquid silicone rubber material vulcanized by addition reaction can also be vulcanized at room temperature, it is called liquid silicone rubber (LSR), low-temperature vulcanized rubber (LTV) and two-component room temperature vulcanized rubber (RTV) due to different molding methods and curing temperatures. Injection molding method and pouring molding method can usually be used to manufacture polymer insulator.

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