Silicone rubber is one of the most important silicone products. The raw material for preparing silicone rubber is usually composed of linear polysiloxane [silicone rubber rubber, referred to as silicone rubber], reinforcing filler, crosslinking agent, catalyst, modification additive, etc. All the raw materials can be processed into rubber by mixing (or mixing). Under certain conditions, the mixed rubber can be cross-linked vulcanized, and it can be transformed from a viscous liquid into a high elastic - vulcanized rubber.
1. Classification
   Silicone rubber According to the organic groups connected to the silicon atoms, silicone rubber can be divided into dimethyl silicone rubber, methyl vinyl silicone rubber, methyl phenyl rubber, fluorine silicone rubber, nitrile silicone rubber, ethyl silicone rubber and phenylene silicone rubber and many other varieties.
   According to the different vulcanization method and vulcanization conditions, silicone rubber can generally be divided into two types of high temperature vulcanized silicone rubber and room temperature vulcanized silicone rubber.
According to different product forms and mixing methods, high temperature vulcanized silicone rubber is divided into high viscosity silicone rubber and liquid silicone rubber.
2. Performance
  Silicone rubber has advantages that natural rubber and other synthetic rubber do not have, and these characteristics are determined by the nature of the silicon-oxygen bond that forms the main chain of silicone rubber. Silicone rubber has high thermal stability and excellent low temperature performance, and can maintain softness, resilience, surface hardness and mechanical properties in the temperature range of -60 ~ 260 ° C. In addition, they also have excellent electrical insulation, weather resistance, ozone resistance and breathability, and are non-toxic and odorless. Some special structure of silicone rubber also has excellent cream, solvent resistance, radiation resistance and other characteristics.
(1) High and low temperature resistance
   The most remarkable characteristic of silicone rubber is their high temperature stability, which can be used for a long time in an environment with 180 ° C and the presence of air. If the appropriate filler and high temperature additives are selected, the use temperature can be as high as 375%, and can withstand the instantaneous high temperature of thousands of degrees. It is estimated that the service life of ordinary silicone rubber can reach 20 years at 120 ° C and 5 years at 150 ° C.
The thermal aging of silicone rubber is mainly carried out through the oxidation of side chain and the breaking of main chain oxane bond. So on the one hand, the heat resistance of silicone rubber depends on its side chain. On the other hand, the fracture of the main chain can also be called sealing heat resistance, which is different from oxidative aging, and it is related to the softness of the main chain and the existence of the end group, and the presence of water, acid, alkali, etc., has a significant promotion effect on the fracture of the chain.
(2) Electrical properties
   Silicone rubber has excellent electrical properties. Its outstanding advantage is that dielectric strength, power factor and insulation properties are subject to little temperature and frequency variation. In a wide temperature range, the dielectric strength is basically unchanged. The dielectric constant and dielectric loss tangent values are also almost constant over a large frequency range. The corona resistance and arc resistance of silicone rubber are also very good, and its corona resistance life is about one thousand times that of PTFE; The arc resistance life is about 20 times that of fluorine rubber. Silicone rubber is not easy to burn, that is, in case of combustion, the generated silica is also insulating.
Therefore, it is used as a safe and reliable wire and cable skin material.
   The arc resistance of silicone rubber is much better than that of ordinary rubber, and it is generally believed that the reason is that its carbon content is lower than that of ordinary organic rubber, so the amount of conductive carbon precipitated by the arc is very small, and it is replaced by insulating silicon dioxide.
(3) Chemical and weather resistance
    In general, silicone rubber has good chemical resistance, melt resistance and oil properties. The effect of solvent on silicone rubber is mainly expansion and softening, and once the solvent evaporates, most of the original properties of silicone rubber will be restored. Silicone rubber has good resistance to ethanol, acetone and other polar solvents and edible oils, they generally only cause a small expansion, and the mechanical properties are basically unchanged.
   The ability of silicone rubber to withstand the action of low concentration of acid, alkali and salt is also good, such as immersion in 10% sulfuric acid at room temperature for 7d, its volume and weight change is less than 1%, and the mechanical properties are basically unchanged. Silicone rubber has excellent oxygen resistance, ozone resistance and ultraviolet radiation resistance, so long-term outdoor use will not occur cracking phenomenon. It is generally believed that silicone rubber can be used outdoors for more than 20 years.
(4) Breathability
   The helical structure of polydimethylsiloxane, large free space and its good solubility to gas make silicone rubber have excellent permeability. At room temperature, its permeability to air, nitrogen, oxygen, carbon dioxide and other gases is 30 to 50 times higher than that of natural rubber. According to these characteristics of silicone rubber, it can be made into artificial strontium breathable in water, it can also be used to concentrate or separate gases, and it can also be used for the preservation of agricultural and sideline products. Of course, due to its high permeability, it cannot be used as a conduit for inert gases.
(5) Other characteristics
   After full vulcanization, the silicone rubber is odorless, tasteless and non-toxic, and has good biocompatibility. It can be disinfected with epoxy ethylene in gamma ray boxes and rarely causes adverse tissue reactions when buried in animals, so it is widely used in food and medical health. In addition, silicone rubber also has excellent fire resistance, and its flash point is above 300 ° C, so it is difficult to burn. In particular, the addition of molded silicone rubber, even without adding flame retardants such as halides and antimony oxide, and only adding general metal oxides, can be prepared into UL-94 grade flame retardant silicone rubber. Therefore, it can be used not only in household appliances such as television sets, but also in atomic energy devices and fireproof materials for various buildings.