From the second half of the nineteenth century, when erecting transmission lines, the only insulating materials suitable for high voltages were 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. The physical properties, electrical properties, long-term reliability, and shape of electrical insulators were then investigated, and productivity was continuously improved.
Among the polymer materials that can replace ceramics and glass, silicone rubber has had practical performance since the 1960s and stood out among several polymers. Silicone rubber insulators have many advantages over ceramic insulators. First of all, it is lightweight, easy to handle and safe; in addition, ceramic insulators often suffer from cracking, and a shock will break it. Silicone rubber insulators, on the other hand, can resist mechanical shocks such as vehicles colliding with utility poles.
Although other polymer materials also have the advantages described above, only silicone rubber will not cause too much pollution to the environment. Polymer insulators are waterproof, so there is no leakage and surface arcing due to falling water droplets. The water resistance of silicone rubber insulators recovers faster than other polymer insulators, and is a durable material that can be used for a long time in harsh environments.
1 Characteristics of silicone rubber
1.1 Chemical characteristics of siloxane bonds
1.1.1 Chemical bonds with stable chemical properties
The main chain of silicone rubber is composed of siloxane (Si-O) chains. Since the electronegativities of Si and O on this bond are 1.8 and 3.5, respectively, which are very different, the polarized structure shown in Figure 1 (omitted) is formed, which has the property of ionic bonds. Thus, the bond energy of Si-O is higher than that of C-C (see Table 1). In addition, ①Due to the nature of the main chain ionic bond, the ionic properties of the side chain methyl C-H are weakened, and it is not easily attacked by other molecules, so its chemical stability is good; ②Since Si does not form double bonds and triple bonds, so It is difficult to form a starting point for the decomposition of the main chain (since the Si-C bond is quite stable for this reason), resulting in a more stable main chain of the silicone rubber.
1.1.2 High flexibility polymer
The bond angle (Si-O-Si) of siloxane is large (130°-160°), and its degree of freedom is higher than that of organic polymers (C-C, C: 110°). The Si-O bond distance (1.64A) is also larger than that of C-C (1.5A). That is, the overall movement of the polymer molecules is easy (easy to deform).
Since polysiloxane has a helical structure, the siloxane bond on the main chain is inward due to the attraction of ionic bonds, and the outside is the methyl group with weak side chain interaction, so the intermolecular attraction of polysiloxane becomes smaller.
1.2 Characteristics of silicone rubber
According to the chemical characteristics introduced in Section 1.1, silicone rubber has the following characteristics, which can be used in High-voltage electrical insulation applications.
1.2.1 Heat and cold resistance
Because of the high bond energy and good chemical stability of silicone rubber, its heat resistance is better than that of organic polymers. Furthermore, since the intermolecular interaction force is weak, the glass transition temperature is low and the cold resistance is also good. Therefore, its characteristics will not change when used in any region of the earth.
1.2.2 Water resistance
Because the surface of polysiloxane is methyl group, it is hydrophobic, so it can be used for waterproofing.
1.2.3 Electrical properties
The number of carbon atoms in the silicone rubber molecule is less than that of the organic polymer, so its arc resistance and leakage resistance are very good. In addition, even if it is burned, insulating silicon is formed, so its electrical insulating properties are excellent.
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