Thermally vulcanized silicone rubber refers to silicone rubber with high relative molecular weight (400,000-600,000). Using organic peroxide as vulcanizing agent, the organic peroxide was decomposed into free radicals by heating, and crosslinked with the organic side radicals of rubber to obtain vulcanizates. Thermally vulcanized silicone rubber is the earliest type of rubber. There are many kinds of silicone rubber which can be divided into the following seven types according to their chemical composition.

(1) dimethyl silicone rubber
Dimethyl silicone rubber, short for methyl silicone rubber, is the oldest variety of silicone rubber. It can keep good elasticity in the temperature range of - 60 - 250 C. Due to the disadvantages of low vulcanization activity, poor technological performance, easy foaming of thick-walled products during secondary vulcanization and large compression deformation at high temperature, methyl vinyl silicone rubber has been replaced by a small amount of silicone rubber for fabric coating at present.

(2) Methyl vinyl silicone rubber
Methyl vinyl silicone rubber is short for vinyl silicone rubber. It is copolymerized by dimethyl siloxane and a small amount of vinyl siloxane. The vinyl molar fraction is generally 0.001-0.003. The introduction of a small amount of unsaturated vinyl group has greatly improved the vulcanization process and product properties, especially the heat aging resistance and high temperature compressive deformation resistance. In the production of silicone rubber, the production of methyl vinyl silicone rubber is the largest, the most widely used and the most varieties and brands. In addition to the widely used general-purpose rubber materials, various special silicone rubber and silicone rubber with processing characteristics (such as high strength silicone rubber, low compression permanent deformation silicone rubber, conductive silicone rubber, thermal conductive silicone rubber and no second stage) Vulcanized silicone rubber, granular silicone rubber, etc.) are also processed on the basis of it.

(3) Methyl vinyl phenyl silicone rubber
Methyl vinyl phenyl silicone rubber is called phenyl silicone rubber. It is made by introducing diphenyl siloxane chain (or methyl phenyl siloxane chain) into the molecular chain of vinyl silicone rubber. When the molar fraction of phenyl is 0.05-0.10, it is called low-phenyl silicone rubber. At this time, the hardening temperature of rubber decreases to the lowest value (-115 C), which makes it have the best low-temperature resistance and elasticity below-100 C. With the increase of phenyl molar fraction, the rigidity of molecular chain increases, and the crystallization temperature increases. When the molar fraction of phenyl is between 0.15 and 0.25, it is called medium phenyl silicone rubber, which has the characteristics of flame resistance. When the phenyl molar fraction is above 0130, it is collectively called high phenyl silicone rubber, which has excellent radiation resistance. The application of phenyl silicone rubber requires low temperature resistance, ablation resistance, high energy radiation resistance, heat insulation and other occasions. Medium-phenyl and high-phenyl silicone rubber is limited in production and application due to its difficult processing and poor physical properties.

(4) Methyl vinyl trifluoropropyl silicone rubber
Methyl vinyl trifluoropropyl silicone rubber is short for fluorosilicone rubber. It introduces fluoroalkyl (generally trifluoropropyl) into the molecular chain of vinyl silicone rubber. It has excellent oil resistance and solvent resistance. For example, for aliphatic, aromatic and chlorinated hydrocarbon solvents, various petroleum-based fuel oils, lubricants, hydraulic oils and some synthetic oils, their working temperature ranges from - 50 to 250 C, and their stability is good at room temperature and high temperature.

(5) Phenylene Silicone Rubber and Phenylene Ether Silicone Rubber
Phenylene silicone rubber and phenylene ether silicone rubber are new varieties of silicone rubber containing phenylene or phenylene ether chain segments in the molecular chain. They are developed to meet the requirements of nuclear power plant and navigation technology. Their main characteristics are high tensile strength, high temperature resistance (over 300 C), but cold resistance is inferior to that of low phenyl silicone rubber.

(6) Nitrile Silicone Rubber
Nitrile silicone rubber is mainly an elastomer containing methyl-beta-nitrile ethyl silicone oxide or methyl-gamma-nitrile propyl silicone oxide chain in the molecular chain. Its main characteristics are similar to fluorosilicone rubber, that is, oil resistance, solvent resistance and good low temperature resistance. However, due to the factors that cause the hydrolysis of nitrile groups under the conditions of polymerization, the reproducibility of raw rubber is poor, and its application and development are limited.

(7) Silicon boron rubber
Silicone-boron rubber is a new type of silicone rubber with decaborane cage structure on the molecular main chain. It has high heat aging resistance and can work for a long time at 400 C. It can work continuously for several hours at 420-480 C, while it can keep elasticity at - 54 C. It is suitable for sealing materials in high-speed aircraft and spacecraft. In the late 1960s, there were a series of products of silicone-boron rubber in the United States, but seldom reported after the 1970s. The main reason may be that the technological properties of rubber and the elasticity of vulcanizate are poor, and the synthesis of carbon-boron is very complex, toxic and expensive.