Although other polymer materials also have the advantages described above, but only silicone rubber will not cause too much pollution to the environment. Polymer insulators are waterproof, so there is no leakage or surface arcing caused by falling water droplets. Silicone rubber insulators recover water resistance faster than other polymer insulators, which is a durable material that can be used in harsh environment for a long time.
1. Characteristics of silicone rubber
1.1 Chemical characteristics of siloxane
1.1.1 Chemical bonds with stable chemical properties
The main chain of silicone rubber is made up of a siloxane (Si-O) chain. As the electronegativity of Si and O on this bond is 1.8 and 3.5 respectively, which are very different, the polarization structure as shown in FIG. 1(omitted) is formed, which has the property of ionic bond. Thus, the bond energy of Si-O is higher than that of C-C (see Table 1). In addition, due to the properties of the main chain ionic bond, the ionic properties of the side chain methyl C-H are weakened, and it is not easy to be attacked by other molecules, so its chemical stability is good. ② Because Si does not form double and triple bonds, it is difficult to form the starting point of main chain decomposition (because of this reason, Si-C bond is quite stable), which leads to the main chain of silicone rubber is more stable.
1.1.2 High flexibility polymer
The siloxane has a large bond Angle (Si-O-Si) (130°-160°) and a higher degree of freedom than the organic polymer (C-C, C: 110°). The Si-O bond distance (1.64A) is also larger than that of C-C (1.5A). In other words, polymer molecules move easily (easily deformed) as a whole.
(3) Spiral structure (FIG. 2(omitted))
Due to the helical structure of polysiloxane, the siloxane bond on the main chain gravitation inward due to ionic bond, and the lateral is the methyl group with weak side chain interaction, so the polysiloxane intermolecular gravitation becomes smaller.
1.2 Characteristics of silicone rubber
According to the chemical characteristics described in Section 1.1, silicone rubber has the following characteristics, which can be used for high voltage insulation.
1.2.1 Heat and cold resistance
Because of the high bond energy and chemical stability of silicone rubber, its heat resistance is better than that of organic polymer. Moreover, due to the weak interaction between molecules, the glass transition temperature is low and the cold resistance is good. As a result, its properties will not change when used anywhere on the planet.
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1.2.2 Water resistance
Because the surface of polysiloxane is methyl group, so it is hydrophobic, and can be used for waterproof.
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, it will form insulating silicon, so it has excellent electrical insulation.
1.2.4 Weather tolerance
1.2.5 Permanent deformation
Silicone rubber has better permanent deformation characteristics (permanent elongation and compressive permanent deformation) at room temperature/high temperature than organic polymers.
2 Classification of silicone rubber
According to the characteristics before vulcanization, silicone rubber can be divided into solid and liquid two types, according to the vulcanization mechanism can be divided into peroxide vulcanization, addition reaction vulcanization and condensation reaction vulcanization three types. The difference between solid and liquid silicone rubber is the molecular weight of polysiloxane. Solid silicone rubber can be vulcanized using either of the peroxide vulcanization and addition reactions, commonly known as high temperature vulcanized rubber (HTV) and hot vulcanized rubber (HCR)(see Table 5, 6). 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) because of the different molding methods and vulcanized temperature. In the manufacture of polymer insulators, injection molding and pouring molding are commonly used.
Single component condensation reaction type (wet air vulcanization) silicone rubber, can be used for building sealant and electrical and electronic products, useful in the use of electric power solvent dilution vulcanized silicone rubber coating at room temperature, by spraying as a protective material for ceramic insulators.
Silicone rubber for polymer insulators
According to the use of silicone rubber can be divided into several kinds.
3.1 Silicone rubber containing aluminum hydroxide
Silicone rubber with good leakage trace resistance and arc resistance can be obtained by adding high filling amount of aluminum hydroxide (ATH). Silicone rubber filled with 50 mass parts of aluminum hydroxide has qualified leakage trace resistance to high voltage (4.5kV), and has good arc resistance, weather resistance, salt spray resistance and acid rain resistance, can be used as insulation materials in heavy salt spray areas. However, due to the high filling ATH of this silicone rubber, its disadvantages are high viscosity (plasticity), low mechanical strength and so on.
3.2 Silicone rubber without aluminum hydroxide
In areas such as the interior of Europe where salt spray is not present, silicone rubber that is not filled with ATH can be used because of its low pollution level. In this case, by selecting the appropriate silicone rubber, white carbon black surface treatment, adding can improve the leakage trace resistance of the compound to improve its hydrophobicity, to meet the high pressure leakage trace resistance requirements. Compared with silicone rubber containing ATH, it has lower viscosity, better physical mechanical properties and electrical properties.
3.3 Outdoor cable accessories
As it is an outdoor cable fitting, it must be resistant to leakage traces. Materials with low permanent elongation characteristics can be obtained by using polymers with an adjusted crosslinked density for use in products that shrink at room temperature (low temperature shrinkage).
3.4 For indoor cable accessories
Because it is an indoor cable fitting, the possibility of being affected by salt spray is small, so it is often not necessary to have the performance of leakage trace resistance. It is still necessary to have low permanent deformation characteristics when used for room temperature shrinkage (low temperature shrinkage).
3.5 Application of Coating
If the heavily polluted part is sprayed with silicone rubber coating, it can maintain good hydrophobicity for a long time. The insulators can also be coated according to the pollution level, so as to achieve the purpose of continued use and cost saving. It has been reported that the hydrophobicity of silicon-rubber insulators can be further maintained if they are coated. There are currently two types of coated insulators and rubber type insulators.
4 Closing Remarks
Silicone rubber materials for polymer insulators are described above. At present, various research institutions and manufacturers are also constantly conducting research and testing. If the durability test can prove its high reliability, it can be predicted that the application of silicone rubber insulators will be further expanded.