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 based on its composition, vulcanization mechanism, and usage process, namely single component room temperature vulcanized silicone rubber, two-component condensation type room temperature vulcanized silicone rubber, and two-component addition type room temperature vulcanized silicone rubber. These three series of room temperature vulcanized silicone rubber each have their own characteristics: the advantage of single component room temperature vulcanized silicone rubber is that it is easy to use, but the deep curing speed is difficult; The advantages of two-component room temperature vulcanized silicone rubber are that it does not release heat during curing, has a small shrinkage rate, does not expand, has no internal stress, and can be cured simultaneously on the inside and surface, allowing for deep vulcanization; The vulcanization time of room temperature vulcanized silicone rubber is mainly determined by temperature.

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

1. Dimethyl silicone rubber

To prepare high molecular weight linear dimethyl polysiloxane rubber, high-purity raw materials are necessary. To ensure the purity of the raw materials, in industry, dimethyl dichlorosilane with a content of 99.5% or more is first hydrolyzed and condensed in an ethanol water medium under acid catalysis, and the bifunctional siloxane tetramer, namely octamethylcyclotetrasiloxane, is separated. Then, the tetramer is catalyzed to form high molecular weight linear dimethyl polysiloxane.

Dimethyl silicone rubber raw rubber is a colorless and transparent elastomer, usually vulcanized with highly active organic peroxides. Vulcanized rubber can

When used within the range of -60~+250 ℃, dimethyl silicone rubber has low vulcanization activity, large permanent deformation under high temperature compression, and is not suitable for making thick products. Thick products are difficult to cure, and the inner layer is also prone to foaming. Due to the superior performance of methyl vinyl silicone rubber containing a small amount of vinyl groups, dimethyl silicone rubber has gradually been replaced by methyl vinyl silicone rubber. The other types of silicone rubber currently produced and applied contain not only dimethyl siloxane structural units, but also more or less other bifunctional siloxane structural units. However, their preparation methods are not fundamentally different from those of dimethyl silicone rubber. The preparation method is generally to hydrolyze and condense a certain bifunctional silicone monomer under favorable conditions for ring formation, and then add octamethylcyclotetrasiloxane in the required proportion, and then react together under the action of a catalyst to obtain it.

2. Methyl vinyl silicone rubber

Due to the presence of a small amount of vinyl side chains, this type of rubber is easier to cure than methyl silicone rubber, providing more types of peroxides for vulcanization and greatly reducing the amount of peroxides used. Compared with dimethyl silicone rubber, the use of silicone rubber containing a small amount of vinyl group can significantly improve its resistance to compression permanent deformation. The low compression deformation reflects its better support as a seal at high temperatures, which is one of the necessary requirements for O-rings and gaskets. Methyl vinyl silicone rubber has good processability and is easy to operate. It can be made into thick products and has a smooth surface when pressed or rolled into semi-finished products. It is currently a commonly used type of silicone rubber.

3. Methyl phenyl vinyl silicone rubber

This type of rubber is obtained by introducing diphenylsiloxane or methylphenylsiloxane units into the molecular chain of vinyl silicone rubber.

According to the different phenyl content (phenyl: silicon atom) in silicone rubber, it can be divided into low phenyl, medium phenyl, and high phenyl silicone rubber. When rubber crystallizes or approaches the glass transition point, or when these two situations overlap, it will cause the rubber to become stiff. Introducing an appropriate amount of large volume functional groups can disrupt the regularity of polymer chains, thereby reducing the crystallization temperature of the polymer. Additionally, the introduction of large volume functional groups can alter the intermolecular forces between polymer molecules, thereby changing the glass transition temperature. Low phenyl silicone rubber (C6H5/Si=6-11%) has excellent low temperature resistance due to the above reasons, and is independent of the type of phenyl monomer used. The brittleness temperature of vulcanized rubber is -120 ℃, making it the best rubber with low-temperature performance today. Low phenyl silicone rubber combines the advantages of vinyl silicone rubber, and its cost is not very high, so there is a trend to replace vinyl silicone rubber. When the benzene content is greatly increased, the rigidity of the molecular chain will increase, resulting in a decrease in cold resistance and elasticity, but the resistance to erosion and radiation will be improved. A benzene content of C6H5/Si=20-34% is the characteristic of medium phenyl silicone rubber with erosion resistance, while high phenyl silicone rubber (C6H5/Si=35-50%) has excellent radiation resistance.

4. Fluorosilicone and nitrile silicone rubber

Fluorosilicone rubber is a type of silicone rubber with fluorinated alkyl groups introduced into its side chains. The commonly used fluorosilicone rubber is one containing methyl, trifluoropropyl, and vinyl groups.

Fluorosilicone has good heat resistance and excellent oil and solvent resistance, such as good stability against various fuel oils, lubricants, hydraulic oils, and certain synthetic oils based on fatty hydrocarbons, aromatic hydrocarbons, chlorinated hydrocarbons, and petroleum at room temperature and high temperature, which is not comparable to pure silicone rubber. Fluorosilicone rubber has good low-temperature performance, which is a great improvement for pure fluororubber. The temperature range for maintaining elasticity of fluorosilicone rubber containing trifluoropropyl groups is generally -50 ℃~+200 ℃. Its resistance to high and low temperatures is inferior to that of vinyl silicone rubber, and it will produce toxic gases when heated above 300 ℃. In terms of electrical insulation performance, it is much worse than vinyl silicone rubber. Adding an appropriate amount of low viscosity hydroxy fluorosilicone oil to the rubber material of fluorosilicone rubber, heat treating the rubber material, and then adding a small amount of vinyl silicone rubber can significantly improve the process performance, which is conducive to solving the problems of rubber material sticking to rollers and serious structural storage, and can extend the effective service life of the rubber material. When introducing methyl phenyl siloxane chains into the above-mentioned fluorosilicone rubber, it will help improve the low-temperature resistance and good processability.

Nitrile silicone rubber is a type of silicone rubber with nitrile alkyl groups (usually β - nitrile ethyl or γ - nitrile propyl) introduced into the side chain. The introduction of polar nitrile groups improves the oil resistance and solvent durability of silicone rubber, but reduces its heat resistance, electrical insulation, and processability.

The type and content of nitrile alkyl groups have a significant impact on the properties of nitrile silicone rubber. For example, silicone rubber containing 7.5% molecular weight of γ - nitrile propyl groups has similar cold resistance to low phenyl silicone rubber and better oil resistance than phenyl silicone rubber. When the content of γ - nitrile propyl groups increases to 33-50% molecular weight, the cold resistance significantly decreases and the oil resistance improves, with a heat resistance of 200 ℃. If β - nitrile ethyl is used instead of γ - nitrile propyl, the heat resistance of nitrile silicone rubber can be further improved.

5. Phenylene and phenylene ether silicone rubber

1. Phenylene silicone rubber is a type of silicone rubber that introduces a phenylene group onto the main chain of polydimethylsiloxane. Due to the introduction of phenylene groups, the radiation resistance of silicone rubber is greatly improved. At the same time, the presence of aromatic rings increases the rigidity of molecular chains, reduces flexibility, increases the glass transition temperature, reduces cold resistance, and increases tensile strength. Phenylene silicone rubber has excellent high temperature resistance and radiation resistance, with a high temperature resistance of up to 250-300 ℃. It also has good dielectric properties, moisture resistance, mold resistance, and water vapor resistance. In the raw rubber composition of phenylene silicone rubber, it is suitable when the phenylene content is 60%, the phenyl content is 30%, and the methyl content is 10% (vinyl content is 0.6%). In this case, the vulcanized rubber has good comprehensive performance.

The disadvantage of phenylene silicone rubber is poor low-temperature performance, with a brittle temperature of -25 ℃, which affects its application in certain aspects. The low-temperature performance of phenylene ether silicone rubber is much better than that of phenylene silicone rubber, with a brittle temperature of -64~70 ℃.

Phenyl ether silicone rubber is a polydimethylsiloxane with phenyl ether and phenyl groups introduced into the molecular main chain.

Phenyl ether silicone rubber has good mechanical properties, with a general tensile strength of 150-180 kg/cm2 (14.7-17.7 Mpa), which is much higher than the strength of vinyl silicone rubber. It also has excellent radiation resistance and is superior to phenylene silicone rubber. It can withstand long-term 250 ℃ hot air aging and still has high strength after aging. Although its low-temperature performance is worse than that of vinyl silicone rubber, it is far superior to phenylene silicone rubber. Its dielectric properties are similar to those of vinyl silicone rubber, but its oil resistance is poor. It is not resistant to non-polar petroleum based oils or polar synthetic oils (such as 4109 diester synthetic lubricants and phosphate hydraulic oils). The performance of oil. In summary, compared with vinyl silicone rubber, phenyl ether silicone rubber has higher strength and radiation resistance, similar high temperature resistance and dielectric properties, but poorer low temperature performance, oil resistance, and elasticity. Phenyl ether silicone rubber has good processing performance 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) refers to a type of silicone rubber that can be vulcanized at room temperature without the need for heating. Room temperature vulcanized silicone rubber is a type of silicone rubber with hydroxyl (or acetoxy) end groups, low molecular weight, and typically a viscous fluid. After adding an appropriate amount of reinforcing fillers, vulcanizing agents, and catalysts (or being affected by moisture in the air) to this type of rubber, it can be vulcanized into an elastic body at room temperature. After complete vulcanization, it has good heat resistance, cold resistance, dielectric properties, etc., but its mechanical strength is lower and can be used for casting and coating rubber materials. Room temperature vulcanized silicone rubber can be divided into two types: single component type and two-component type.

Two component room temperature vulcanized silicone rubber is composed of silicone rubber containing terminal hydroxyl groups, reinforcing fillers, vulcanizing agents, etc., and a catalyst is added during use. The commonly used vulcanizing agents are organotin salts, such as dibutyltin dilaurate, with a dosage of generally 0.5-5 parts or stannous octoate, which has stronger catalytic ability than dibutyltin dilaurate. During vulcanization, under the action of a catalyst, a dealcoholization condensation reaction occurs between the silicone rubber containing terminal hydroxyl groups and the vulcanizing agent to form a cross-linked structure. By changing the dosage of vulcanizing agent and catalyst, the vulcanization rate can be adjusted. Generally, when the dosage is high, the vulcanization rate is fast, while when it is low, it is slow. During the vulcanization process, the generated alcohol substances gradually diffuse and escape from the vulcanized rubber.

Single component room temperature vulcanized silicone rubber is composed of silicone rubber with acetoxy groups at the end groups, reinforcing fillers, and other additives. It does not require the addition of catalysts during use and can be vulcanized into an elastomer by reacting with moisture in the air after being removed from the sealed packaging. This type of silicone rubber has good adhesion to metals, glass, plastics, etc. Its disadvantage is that acetic acid is generated during the vulcanization process. Although it can diffuse and escape from the vulcanized rubber, it has a corrosive effect on contact objects, especially metals. The single component type is convenient to use and is particularly suitable for sealing, jointing, 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 remote claw type liquid rubber, which has functional groups at both ends of the molecular structure; Another type is the random distribution of active functional groups in the main chain, which refers to the presence of functional groups within the molecular structure, known as non telechelic liquid rubber. Of course, there are also those with both intermediate functional groups and end functional groups, and the current focus is on the research of remote claw type liquid rubber. For liquid rubber, chain extenders or crosslinkers with appropriate functional groups should be selected based on the active functional groups they contain.

Liquid silicone rubber can be used for coating, impregnation, and infusion. For example, hydroxyl terminated polydimethylsiloxane with a viscosity of 0.07~50 Pa · s/25 ℃, using methyl vinyl dipyrrolidone silane as a chain extender and organic peroxides such as benzoyl peroxide and 25 dimethyldi tert butyl peroxyhexane as vulcanizing agents, has good fluidity and low viscosity. During its vulcanization process, chain growth reactions occur simultaneously, resulting in high molecular weight elastomers with good physical and mechanical properties.

Chain extender methyl vinyl dipyrrolidone silane can be obtained by reacting pyrrolidone with methyl vinyl dichlorosilane in the presence of triethylamine. The product is easily hydrolyzed and needs to be stored in a dry and sealed container. The pyrrolidone group of this compound can slowly react with the terminal hydroxyl group in polydimethylsiloxane at room temperature, and the reaction rate increases with temperature.

In theory, this reaction can continue until an infinitely large molecular weight is obtained. The vinyl group in methyl vinyl pyrrolidone silane can also serve as an activation point for sulfurization reaction, which can promote the crosslinking of polydimethylsiloxane and generate high molecular weight elastomers.

Due to the slow reaction between pyrrolidone groups and hydroxyl groups at room temperature, the rubber material mixed with various components has a long shelf life. The viscosity of the rubber material remains basically unchanged within 1 hour, but it still maintains good fluidity and can be injected into small pores. The mixed rubber material can be vulcanized into an elastomer by heating it at 150 ℃ for 10 minutes.