Classification and Main Characteristics of Silicone Rubber
Silicone rubber is a kind of rubber whose main chain is composed of silicon and oxygen atoms alternately. Silicon atoms are usually connected with two organic groups. Ordinary silicone rubber is mainly composed of silicone oxygen chains containing methyl group and a small amount of vinyl group. The introduction of phenyl can improve the high and low temperature resistance of silicone rubber, while the introduction of trifluoropropyl and cyanogen can improve the temperature and oil resistance of silicone rubber. Silicone rubber has good low temperature resistance and can work at - 55 C. When phenyl is introduced, the temperature can reach - 73 C. The thermal resistance of silicone rubber is also outstanding. It can work for a long time at 180 C, and can withstand several weeks or longer elasticity at slightly higher than 200 C. It can withstand high temperature above 300 C instantaneously. Silicone rubber has good air permeability, and oxygen permeability is the highest in synthetic polymers. In addition, silicone rubber has the outstanding characteristics of physiological inertia and does not cause coagulation, so it is widely used in the medical field.

classification
According to its vulcanization mechanism, silicone rubber can be divided into heat vulcanization type (HTV) and room temperature vulcanization type (RTV), of which room temperature vulcanization type can be divided into condensation reaction type and addition reaction type. High temperature silicone rubber is mainly used to manufacture various silicone rubber products, while room temperature silicone rubber is mainly used as adhesives, filling materials or moulds. Thermal vulcanization type is the most widely used type. Thermal vulcanization type can be divided into methyl silicone rubber (MQ), methyl vinyl silicone rubber (VMQ), methyl vinyl phenyl silicone rubber (PVMQ) (low temperature and radiation resistance), and other kinds of silicone rubber, such as eye silicone rubber and fluorosilicone rubber.
Main characteristics

(1) High and Low Temperature Resistance
Among all the rubbers, silicone rubber has the widest working temperature range (-100-350 C). For example, vinyl silicone rubber or low-phenyl silicone rubber can remain elastic after aging in hot air at 250 C for thousands of hours or 300 C for hundreds of hours; low-phenyl silicone rubber vulcanizate can remain elastic after aging in hot air at 350 C for tens of hours, its glass transition temperature is - 140 C, and its vulcanizate is between - 70 and 100 C. It is still elastic at temperature. Silicone rubber can withstand thousands of degrees of high temperature instantaneously when it is used as thermal protection coating on the inner wall of rocket nozzle.
(2) Ozone aging, oxygen aging, light aging and weathering aging resistance
The properties of silicone rubber vulcanizate did not change significantly after being exposed to the sun for several years in a free state.
(3) The electrical insulation properties of silicone rubber vulcanizates change little when damp, frequency change or temperature rise, and the silicon dioxide produced after combustion is still an insulator. In addition, the molecular structure of silicone rubber has few carbon atoms, and it does not need carbon black as filler, so it is not easy to burn in arc discharge, and it is very reliable in high voltage applications.  Its corona resistance and arc resistance are excellent. Its corona resistance life is 1000 times that of PTFE and 20 times that of fluororubber.
(4) Special surface properties and physiological inertia
The surface energy of silicone rubber is smaller than that of most organic materials, and it has low hygroscopicity. The water absorption rate of silicone rubber immersed in water for a long time is only about 1%. The physical properties of silicone rubber do not decline. It has good anti-mildew performance. It does not bond with many materials and can play an isolation role. Silicone rubber is odorless, non-toxic, has no adverse effects on human body, has slight reaction with body tissues, and has excellent physiological inertia and physiological aging.
(5) High permeability
Compared with other macromolecule materials, silicone rubber has good air permeability, the permeability of nitrogen, oxygen and air at room temperature is 30-40 times higher than that of NR, and the permeability of gas is selective, such as about 5 times as much as that of carbon dioxide.
(6) Biomedical performance
Silicone rubber has excellent biomedical properties because of its molecular structure. Successful application of a large number of animal and human experiments has proved this point.