Silicone rubber has excellent heat resistance, cold resistance, dielectric properties, ozone resistance and atmospheric aging resistance. The outstanding performance of silicone rubber is that it has a wide operating temperature range and can be used for a long time at -60℃ (or lower temperature) to +250℃ (or higher temperature). However, the mechanical properties of silicone rubber such as tensile strength and tear strength are poor. Its physical and mechanical properties are not as good as most synthetic rubbers at room temperature. In addition, except for nitrile silicone and fluorosilicone rubber, general silicone rubber has poor oil resistance and solvent resistance. Therefore, silicone rubber is not suitable for occasions under ordinary conditions, but it is very suitable for many specific occasions. It is worth mentioning that in biomedical engineering, polymer materials play a very important role, and silicone rubber is a particularly important type of medical polymer material. It has excellent physiological inertness, is non-toxic, odorless, non-corrosive, anti-coagulation, has good compatibility with the body, and can withstand harsh disinfection conditions. It can be processed into pipes, sheets, films and special-shaped components as needed, and can be used as medical devices, artificial organs, etc. Nowadays, there are specialized medical-grade silicone rubbers at home and abroad.

Overview

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 use process, namely, single-component room temperature vulcanized silicone rubber, two-component condensation room temperature vulcanized silicone rubber and two-component addition 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 advantage of two-component room temperature vulcanized silicone rubber is that it does not release heat during curing, the shrinkage rate is very small, it does not expand, there is no internal stress, the curing can be carried out simultaneously inside and on the surface, and it can be deeply vulcanized; the vulcanization time of addition room temperature vulcanized silicone rubber is mainly determined by temperature.

Silicone rubber can be divided into two categories according to its vulcanization characteristics: heat-vulcanized silicone rubber and room temperature vulcanized silicone rubber. According to different performance and uses, it can be divided into general 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

(Methyl silicone rubber for short): To prepare high molecular weight linear dimethyl polysiloxane rubber, high purity raw materials are required. To ensure the purity of the raw materials, the industry usually first hydrolyzes and condenses dimethyl dichlorosilane with a content of more than 99.5% in an ethanol-water medium under acid catalysis, and separates the difunctional siloxane tetramer, namely octamethylcyclotetrasiloxane, and then the tetracyclic body is used under the action of a catalyst to form a high molecular linear dimethyl polysiloxane. Dimethyl silicone rubber raw rubber is a colorless and transparent elastomer, which is usually vulcanized with highly active organic peroxides. Vulcanized rubber can be used in the range of -60~+250℃. Dimethyl silicone rubber has low vulcanization activity and large high temperature compression permanent deformation. It is not suitable for making thick products. Thick products are difficult to vulcanize, and the inner layer is also prone to blistering. Since the performance of methyl vinyl silicone rubber containing a small amount of vinyl is better than that of dimethyl silicone rubber, dimethyl silicone rubber has been gradually replaced by methyl vinyl silicone rubber. Other types of silicone rubber currently produced and used contain more or less other difunctional siloxane structural units in addition to dimethyl siloxane structural units, but their preparation methods are essentially the same as those of dimethyl silicone rubber. Their preparation methods are generally to hydrolyze and condense a certain difunctional silicon monomer under conditions that are conducive to the formation of a ring body, and then add octamethylcyclotetrasiloxane in the required proportion, and then react together under the action of a catalyst to obtain the obtained product.

2. Methyl vinyl silicone rubber

(abbreviated as vinyl silicone rubber): This type of rubber contains a small amount of vinyl side chains, so it is easier to vulcanize than methyl silicone rubber, so that it has more types of peroxides available for vulcanization and can greatly reduce the amount of peroxide used. Compared with dimethyl silicone rubber, the use of silicone rubber containing a small amount of vinyl can significantly improve the compression set resistance. The low compression set reflects that it has better support at high temperature as a seal, which is one of the requirements for O-rings and gaskets. Methyl vinyl silicone rubber has good process performance and is easy to operate. It can be made into thick products and the surface of extruded and calendered semi-finished products is smooth. It is currently a more commonly used silicone rubber.

3. Methyl phenyl vinyl silicone rubber

(Phenyl silicone rubber for short): This type of rubber is obtained by introducing diphenyl siloxane chain segments or methyl phenyl siloxane chain segments 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 the rubber crystallizes or approaches the glass transition point or these two situations overlap, the rubber will become rigid. The introduction of an appropriate amount of large-volume groups to destroy the regularity of the polymer chain can reduce the crystallization temperature of the polymer. At the same time, since the introduction of large-volume groups changes the interaction between polymer molecules, the glass transition temperature can also be changed. 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 brittle temperature of the vulcanized rubber is -120℃, which is the rubber with the best low temperature performance today. Low phenyl silicone rubber has the advantages of vinyl silicone rubber, and the cost is not very high, so it has a tendency to replace vinyl silicone rubber. When the phenyl content is greatly increased, the rigidity of the molecular chain will increase, resulting in a decrease in 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, and 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 fluoroalkyl introduced into the side chain. Commonly used fluorosilicone rubbers are fluorosilicone rubbers containing methyl, trifluoropropyl and vinyl. Fluorosilicone has good heat resistance and excellent oil and solvent resistance. For example, it has good stability at room temperature and high temperature for aliphatic hydrocarbons, aromatic hydrocarbons, chlorinated hydrocarbons, various petroleum-based fuel oils, lubricating oils, hydraulic oils and some synthetic oils, which are beyond the reach of pure silicone rubber. Fluorosilicone rubber has good low-temperature performance, which is a great improvement over pure fluororubber. The temperature range for trifluoropropyl-containing fluorosilicone rubber to maintain elasticity is generally -50℃~+200℃. Its high and low temperature resistance is worse than that of vinyl silicone rubber, and it will produce toxic gases when heated to above 300℃. It is much worse than vinyl silicone rubber in terms of electrical insulation performance. Adding an appropriate amount of low-viscosity hydroxyl fluorosilicone oil to the rubber compound of fluorosilicone rubber, heat-treating the rubber compound, 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 compound sticking to the roller and serious storage structure, and can extend the effective service life of the rubber compound. When methylphenylsiloxane chain segments are introduced into the above-mentioned fluorosilicone rubber, it will help improve the low temperature resistance and have good processing performance. Nitrile silicone rubber is a type of silicone rubber with nitrile alkyl (generally 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 its heat resistance, electrical insulation and processability are reduced. The type and content of nitrile alkyl have a great influence on the performance of nitrile silicone rubber. For example, silicone rubber containing 7.5% γ-nitrile propyl has similar cold resistance to low phenyl silicone rubber, and its oil resistance is better than low phenyl silicone rubber. When the γ-nitrile propyl content increases to 33~50% gram molecule, the cold resistance is significantly reduced, the oil resistance is improved, and the heat resistance is 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

Phenylene silicone rubber is a type of silicone rubber with phenylene groups introduced into the main chain of polysiloxane. Due to the introduction of phenylene groups, the radiation resistance of silicone rubber is greatly improved. At the same time, due to the presence of aromatic rings, the rigidity of the molecular chain is increased, the flexibility is reduced, the glass transition temperature is increased, the cold resistance is reduced, and the tensile strength is increased.

Phenylene silicone rubber has excellent high temperature resistance and radiation resistance, and can withstand high temperatures of 250~300℃. It also has good dielectric properties and moisture-proof, mildew-proof and water vapor-proof properties. 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 properties.

The disadvantage of phenylene silicone rubber is that its low temperature performance is poor and its brittle temperature is -25℃, which affects its application in some aspects. The low temperature performance of phenylene ether silicone rubber is much better than that of phenylene silicone rubber, and its brittle temperature is -64~70℃. Phenylene ether silicone rubber is a polysiloxane with phenylene ether and phenylene groups introduced into the main chain of the molecule. Phenyl ether silicone rubber has good mechanical properties. The general tensile strength can reach 150~180 kg/cm2 (i.e. 14.7~17.7Mpa), which is much higher than the strength of vinyl silicone rubber. It also has excellent radiation resistance and is better than phenylene silicone rubber. It can withstand long-term 250℃ hot air aging and still has high strength after aging. Although the low-temperature performance of phenylene ether silicone rubber is worse than that of vinyl silicone rubber, it is much better than phenylene silicone rubber. Its dielectric properties are close to those of vinyl silicone rubber, but phenylene ether silicone rubber has poor oil resistance, and is not resistant to non-polar petroleum-based oils, nor to polar synthetic oils (such as 4109 diester synthetic lubricants and phosphate ester hydraulic oils.

In short, phenylene ether silicone rubber has higher strength and radiation resistance than vinyl silicone rubber, similar high temperature resistance and Dielectric properties, poor low temperature performance, oil resistance and elasticity. Phenyl ether silicone rubber has good processing performance and can be used to manufacture special model products and extruded products.

6. Room temperature vulcanized silicone rubber

Room temperature vulcanized silicone rubber (RTV for short) refers to a type of silicone rubber that can be vulcanized at room temperature without heating. Room temperature vulcanized silicone rubber is a silicone rubber with hydroxyl (or acetoxy) end groups, low molecular weight, and usually a viscous fluid. This type of rubber can be vulcanized at room temperature to form an elastomer after adding an appropriate amount of reinforcing filler, vulcanizer and catalyst (or under the action of moisture in the air). After complete vulcanization, it has good heat resistance, cold resistance, dielectric properties, etc., but its mechanical strength is lower, and it can be used for casting and coating rubber.

Room temperature vulcanized silicone rubber can be divided into single group There are two types: one-component and two-component. Two-component room temperature vulcanized silicone rubber is made of silicone rubber containing terminal hydroxyl groups, reinforcing fillers, vulcanizers, etc., and a catalyst is added when used. Commonly used vulcanizers are organic tin salts, such as dibutyltin dilaurate, and the dosage is generally 0.5~5 parts or stannous octoate, which has stronger catalytic ability than dibutyltin dilaurate. During vulcanization, under the action of the catalyst, a dealcoholization condensation reaction occurs between the silicone rubber containing terminal hydroxyl groups and the vulcanizer to form a cross-linked structure.

The vulcanization speed can be adjusted by changing the dosage of the vulcanizer and the catalyst. Generally, when the dosage is large, the vulcanization speed is fast, and vice versa. During the vulcanization process, the generated alcohol substances gradually diffuse and escape from the vulcanized rubber. One-component room temperature vulcanized silicone rubber is made of silicone rubber containing acetoxy groups at the end groups, reinforcing fillers and other It is made of additives and does not require the addition of catalysts when used. After being taken out of the sealed package, it can be vulcanized into an elastomer by reacting with moisture in the air. This type of silicone rubber has good adhesion to metals, glass, and plastics. 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 the contact objects, especially metals. The single-component type is convenient to use and is particularly suitable for sealing, caulking, and other purposes.

7. Liquid silicone rubber

According to the position of the functional groups (i.e., cross-linking points) contained in the molecular structure, liquid rubbers with functional groups are often divided into two categories: one is that the functional groups are at both ends of the molecular structure, which is called telechelic liquid rubber; the other is that the active functional groups are randomly distributed in the main chain, that is, those with functional groups in the molecular structure, which are called non-telechelic liquid rubbers. Of course, there are also those with both intermediate functional groups and terminal functional groups. At present, the focus is on the study of telechelic liquid rubbers.

For liquid rubber, chain extenders or crosslinkers with appropriate functional groups should be selected according to the active functional groups it contains. 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℃ uses methyl vinyl bispyrrolidone silane as a chain extender and organic peroxides such as dibenzoyl peroxide and 25-dimethyl-5-di-tert-butyl peroxyhexane as a vulcanizer. This type of rubber has good fluidity and low viscosity. Chain extension reaction occurs simultaneously during its vulcanization process, so a high molecular weight elastomer with good physical and mechanical properties can be obtained.

Chain extender methyl vinyl bispyrrolidone silane can be obtained by the reaction of pyrrolidone and methyl vinyl dichlorosilane in the presence of triethylamine. The product is easily hydrolyzed, so it needs to be stored in a dry and closed container. The pyrrolidone group of this compound can slowly react with the terminal hydroxyl group in polydimethylsiloxane at room temperature, and its reaction rate increases with increasing temperature. Theoretically, this reaction can be continued until an infinite molecular weight is obtained. The vinyl group in methyl vinyl pyrrolidone silane can also serve as an activation point for the vulcanization reaction, which can promote the cross-linking of polydimethylsiloxane to form a high molecular weight elastomer.

Since the pyrrolidone group and the hydroxyl group react very slowly at room temperature, the rubber compound after adding the components and mixing has a long applicability period. The viscosity of the rubber compound remains basically unchanged within 1 hour, but it still maintains good fluidity and can be injected into tiny pores. The mixed rubber compound can be vulcanized into an elastomer by heating at 150°C for 10 minutes.