Silicone rubber is mainly divided into room temperature vulcanized silicone rubber and high temperature vulcanized silicone rubber. Therefore, room temperature vulcanized silicone rubber can be divided into three types according to its composition, vulcanization mechanism and application process, namely, one component room temperature vulcanized silicone rubber, two component condensation room temperature vulcanized silicone rubber and two component addition molding room temperature vulcanized silicone rubber. These three series of room temperature vulcanized silicone rubber have their own characteristics: the advantages of one-component room temperature vulcanized silicone rubber are easy to use, but the deep curing speed is difficult; the advantages of two-component room temperature vulcanized silicone rubber are no heat release, small shrinkage, no expansion, no internal stress, curing can be carried out both inside and outside. The vulcanization time of addition room temperature vulcanized silicone rubber is mainly determined by temperature.

According to its vulcanization characteristics, silicone rubber can be divided into two types: hot vulcanized silicone rubber and room temperature vulcanized silicone rubber. According to the different performance and use, it can be divided into general type, ultra-low temperature type, ultra-high temperature type, high strength type, oil-resistant type, medical type and so on. According to the different monomers used, it can be divided into methyl vinyl silicone rubber, methyl phenyl vinyl silicone rubber, fluorosilicone, nitrile silicone rubber and so on.

1. Dimethyl Silicone Rubber

In order to prepare linear dimethylpolysiloxane rubber with high molecular weight, high purity raw materials must be used. In order to ensure the purity of raw materials, the dimethyldichlorosilane with 99.5% content is usually purified by refined neptunium in industry, hydrolyzed and condensed in ethanol-water medium under acid catalysis, and then the difunctional silicon is separated. Oxyalkane tetramer is octamethylcyclotetrasiloxane, and then tetracycle can form linear dimethylpolysiloxane with high molecular weight under the action of catalyst.

Dimethyl silicone rubber raw rubber is a colorless and transparent elastomer, which is vulcanized by organic peroxide with higher activity. Vulcanizate

When dimethyl silicone rubber is used in the range of - 60 ~+250 ~C, its vulcanization activity is low, its permanent deformation under high temperature compression is large, and it is not suitable for making thick products. The vulcanization of thick products is difficult and the inner layer is easy to foam. Because the properties of methyl vinyl silicone rubber containing a small amount of vinyl are better than others, dimethyl silicone rubber has been gradually replaced by methyl vinyl silicone rubber. Other types of silicone rubber produced and applied nowadays contain not only dimethylsiloxane structural units, but also more or less other bifunctional siloxane structural units. However, there is no essential difference between the preparation method of silicone rubber and that of dimethylsiloxane rubber. The preparation method of silicone rubber is generally conducive to the formation of rings. Under this condition, some bifunctional silicon monomers are hydrolyzed and condensated, then octamethylcyclotetrasiloxane is added in the proportion required, and then synthesized by the reaction of octamethylcyclotetrasiloxane with catalyst.

2. Methyl vinyl silicone rubber

This kind of rubber contains a small amount of vinyl side chain, so it is easier to vulcanize than methyl silicone rubber, so that it has more kinds of peroxides for vulcanization, and can greatly reduce the amount of peroxides. Comparing with dimethyl silicone rubber, silicone rubber with a small amount of vinyl group can significantly improve the compression permanent deformation resistance. Low compression deformation reflects that it has better support at high temperature as a seal, which is one of the requirements of O-ring and gasket. Methyl vinyl silicone rubber is a kind of silicone rubber with good technological properties and easy operation. It can be made into thick products and smooth surface of extruded and calendered semi-finished products.

3. Methyl phenyl vinyl silicone rubber

The rubber is obtained by introducing diphenylsiloxane or methylphenylsiloxane segments into the molecular chain of vinyl silicone rubber.

According to the different content of phenyl in silicone rubber (phenyl: silicon atom), it can be divided into low phenyl, medium phenyl and high phenyl silicone rubber. When the rubber crystallizes or approaches the glass transition point or the two conditions overlap, the rubber will be rigid. The regularity of polymer chains is destroyed by introducing a suitable amount of large groups, which can reduce the crystallization temperature of polymers. At the same time, the glass transition temperature can also be changed because the introduction of large groups changes the intermolecular force of polymers. Low phenyl silicone rubber (C6H5/Si=6-11%) has excellent low temperature resistance due to the above reasons, and has nothing to do with the type of phenyl monomer used. The brittleness temperature of vulcanizate is - 120 C, which is the best low temperature rubber nowadays. Low phenyl silicone rubber has both advantages of vinyl silicone rubber and low cost, so it has the tendency to replace vinyl silicone rubber. When the phenyl content is greatly increased, the rigidity of the molecular chain will increase, which will lead to the decrease of cold resistance and elasticity, but the ablation resistance and radiation resistance will be improved. The phenyl content of C6H5/Si = 20-34% is medium-phenyl silicone rubber with ablation resistance, while the high-phenyl silicone rubber (C6H5/Si = 35-50%) has excellent radiation resistance. Performance.

4. Silicone fluoride and silicone nitrile rubber

Fluorosilicone rubber is a kind of silicone rubber with side chain introducing fluoroalkyl. The commonly used fluorosilicone rubber is fluorosilicone rubber containing methyl, trifluoropropyl and vinyl groups.

Fluorosilica gel has good heat resistance and excellent oil and solvent resistance, such as the stability of aliphatic hydrocarbons, aromatic hydrocarbons, chlorinated hydrocarbons, various petroleum-based fuel oils, lubricants, hydraulic oils and some synthetic oils at room temperature and high temperature, which is better than that of pure silicone rubber. Fluorosilicone rubber has good low temperature performance, and it is a great improvement for pure fluorosilicone rubber. The temperature range of fluorosilicone rubber containing trifluoropropyl is generally - 50 ~200 C. The high and low temperature resistance of fluorosilicone rubber is worse than that of vinyl silicone rubber, and toxic gases will be produced when heated to above 300 C. The electrical insulation performance is much worse than that of vinyl silicone rubber. Adding proper amount of low-viscosity hydroxyl fluorosilicone oil, heat treatment of the compound and a small amount of vinyl silicone rubber into the compound of fluorosilicone rubber can significantly improve the process performance, which is conducive to solving the serious problems of roll sticking and storage structure of the compound, and can prolong the effective service life of the compound. The introduction of methyl phenyl silicone oxide chains into the fluorosilicone rubber mentioned above will help to improve the low temperature resistance and good processing performance.

Nitrile silicone rubber is a kind of silicone rubber with side chain introducing nitrile alkyl (generally beta-nitrile ethyl or gamma-nitrile propyl). The introduction of polar nitrile group improved the oil resistance and solvent resistance of silicone rubber, but its heat resistance, electrical insulation and processability decreased.

The type and content of nitrile alkyl have great influence on the properties of nitrile silicone rubber. For example, the cold resistance of silicone rubber containing 7.5% molecule of gamma-acrylonitrile propyl group is similar to that of low-phenyl silicone rubber, and the oil resistance of phenyl silicone rubber is better than that of low-phenyl silicone rubber. When the content of gamma-nitrile propyl group increases to 33-50% molecule, the cold resistance decreases significantly and oil resistance decreases significantly. The heat resistance is 200 C. If the beta-nitrile ethyl group is used instead of the gamma-nitrile propyl group, the heat resistance of the nitrile silicone rubber can be further improved.

5. Silicone Rubber of Phenylene and Phenylene Ether

1. Phenylene silicone rubber is a kind of silicone rubber with phenylene group introduced into the main chain of polysiloxane. Because of the introduction of phenylene group, the radiation resistance of silicone rubber is greatly improved. At the same time, the rigidity of molecular chain is increased, the flexibility is reduced, the glass transition temperature is increased, the cold resistance is decreased, and the tensile strength is increased due to the presence of aromatic rings. Polystyrene silicone rubber has excellent heat resistance, radiation resistance, high temperature resistance up to 250 ~ 300 C, and has good dielectric properties, moisture resistance, mildew resistance, water vapor resistance and other characteristics. When the content of phenylene is 60%, phenyl is 30%, methyl is 10% (vinyl content is 0.6%) in the raw rubber of styrene silicone rubber, the vulcanizate has good comprehensive properties.

The disadvantage of phenylene silicone rubber is that its low temperature performance is not good, brittle temperature is - 25 C, which affects its application in some aspects. The low temperature performance of phenylene ether silicone rubber is far better than that of phenylene silicone rubber, brittle temperature is - 64 ~ 70 C.

Phenylene ether braided silicone rubber is a polysiloxane with phenylene ether braided and phenylene braided groups introduced into the molecular backbone.

Phenylene ether braided silicone rubber has good mechanical properties. Generally, its tensile strength can reach 150-180 kg/cm (i.e. 14.7-17.7 Mpa is much higher than that of vinyl silicone rubber, at the same time, it has excellent radiation resistance and is superior to that of phenylene silicone rubber. It can withstand the aging of hot air at 250 C for a long time, and still has high strength after aging. Although the low temperature properties of phenylene ether silicone rubber are worse than that of vinyl silicone rubber, it is much better than that of phenylene silicone rubber. Its dielectric properties are similar to those of vinyl silicone rubber, but the oil resistance of phenylene ether-supported silicone rubber is poor. It can neither resist non-polar petroleum base oil nor polar synthetic oil (such as 4109 diester synthetic lubricant and phosphate hydraulic oil). In conclusion, compared with vinyl silicone rubber, phenylene ether braided silicone rubber has higher strength and radiation resistance, similar high temperature resistance and dielectric properties, poor low temperature performance, oil resistance and elasticity. Phenylene ether braided silicone rubber has good processing properties and can be used to manufacture model products and extruded products with special requirements.

6. Room temperature vulcanized silicone rubber

Room temperature vulcanized silicone rubber (RTV) is a kind of silicone rubber that can be vulcanized at room temperature without heating. Room temperature vulcanized silicone rubber (RTVR) is a kind of silicone rubber with hydroxyl (or acetoxyl) end groups. Its molecular weight is low and it is usually a viscous fluid. This kind of rubber can be vulcanized to elastomer at room temperature by adding proper reinforcing filler, vulcanizing agent and catalyst (or by the action of moisture in air). After curing, it has good heat resistance, cold resistance and dielectric properties, but its mechanical strength is lower, so it can be used for casting and coating compound. Room temperature vulcanized silicone rubber can be divided into two types: single component type and double component type.

Two-component room temperature vulcanized silicone rubber is composed of hydroxyl-terminated silicone rubber, reinforcing filler and vulcanizing agent. Catalyst is added when used. Organotin salts, such as dibutyltin dilaurate, are commonly used as curing agents. The dosage is usually 0.5-5 phr or stannous octoate is used as curing agent. It has better catalytic ability than dibutyltin dilaurate. When vulcanized, the cross-linking structure was formed by dealcoholization and condensation reaction between the silicone rubber containing hydroxyl end group and the vulcanizing agent under the action of catalyst. The curing rate can be adjusted by changing the amount of curing agent and catalyst. When the amount of curing agent is large, the curing speed is fast, and vice versa, the curing speed is slow. During the curing process, the alcohols gradually diffused from the vulcanizate.

One-component room temperature vulcanized silicone rubber is composed of silicone rubber containing acetoxyl end group, reinforcing filler and other auxiliaries. It can be vulcanized into elastomer by removing water from sealed packaging without adding catalyst. The silicone rubber has good adhesion to metals, glass and plastics. Its disadvantage is that acetic acid is formed during vulcanization. Although it can diffuse and escape from vulcanizate, it can corrode contact objects, especially metal. The single component type has convenient function and is especially suitable for sealing, seaming and other purposes.

7. Liquid Silicone Rubber

According to the position of functional groups (i.e. crosslinking points) in the molecular structure, liquid rubber with functional groups is often divided into two categories: one is called teleclaw liquid rubber whose functional groups are at both ends of the molecular structure; the other is called teleclaw liquid rubber whose active functional groups are randomly distributed in the main chain, that is, those with functional groups in the molecular structure. It is called non-teleclaw liquid rubber. Of course, there are also those with both intermediate and terminal functional groups. At present, the focus is on the study of teleclaw liquid rubber. For liquid rubber, chain growth agents or crosslinking agents with appropriate functional groups should be selected according to the active functional groups contained in the liquid rubber.

Liquid silicone rubber can be used for coating, impregnation and filling. For example, hydroxyl-terminated polydimethylsiloxane with viscosity of 0.07-50 Pa.s/25 C, methyl vinyl bispyrrolidone silane as chain growth agent and organic peroxides such as dibenzoyl peroxide and 25-dimethyl 5-di-tert-butyl peroxyhexane as vulcanizing agent have good fluidity and low viscosity and have been vulcanized. The chain growth reaction occurs at the same time in the process, so high molecular weight elastomer can be obtained, which has good physical and mechanical properties.

The chain growth agent, methyl vinyl bispyrrolidone silane, can be obtained by the reaction of pyrrolidone with methyl vinyl dichlorosilane in the presence of triethylamine. The product is easy to hydrolyze, so it needs to be stored in a dry and airtight container. The pyrrolidone group of this compound can react slowly with hydroxyl end group in polydimethylsiloxane at room temperature. The reaction rate increases with the increase of temperature.

In theory, the reaction can proceed continuously until an infinite molecular weight is obtained. Vinyl group in methyl vinyl pyrrolidone silane can also be used as the activation point of vulcanization reaction, which can promote the crosslinking of polydimethylsiloxane to form high molecular weight elastomers.

Because the reaction between pyrrolidone group and hydroxyl group is very slow at room temperature, the compound added with each component after mixing has a long life, the viscosity of the compound remains basically unchanged within an hour, but it still maintains good fluidity and can be injected into tiny pores. The blended rubber can be vulcanized into elastomer by heating at 150 C for 10 minutes.