High-temperature vulcanized silicone rubber is a polyorganosiloxane (that is, raw rubber) with high molecular weight (molecular weight is generally 400,000 to 800,000), adding reinforcing fillers and other various additives, using organic peroxide as a vulcanizing agent, and press molding (Molding, extrusion, calendering) or injection molding, and cross-chained into rubber at high temperature. This rubber is generally referred to as silicone rubber.

The vulcanization of high-temperature vulcanized silicone rubber is generally divided into two stages. The first stage is to mix the raw silicone rubber, reinforcing agent, additive, vulcanizing agent and structure control agent, and then add the mixed material in the metal mold. Pressure heating molding and vulcanization, the pressure is about 50 kg/cm2, the temperature is 120 ~ 130 ° C, and the time is 10 ~ 30 minutes. Bake at 250°C for several hours to 24 hours. The rubber is further vulcanized, while the organic peroxide is decomposed and volatilized.

The reinforcing filler for silicone rubber is various types of silica, which can increase the strength of the vulcanizate tenfold. Adding various additives is mainly to reduce the cost of the rubber, improve the properties of the rubber, and give the vulcanized rubber various special properties such as flame retardancy and electrical conductivity. The cross-linking agent is various organic peroxides, such as benzoyl peroxide, 2,4-dichlorobenzoyl peroxide, dicumyl peroxide, 2,5-di-tert-butylperoxyhexane, etc. .Structure control agent is added in order to avoid the long time of mixing the rubber compound, resulting in "structure" and making the rubber compound hard and hard, and difficult to process and mature. Methyl hydroxy silicone oil or diphenyl dihydroxy radane can be used. as a structural control agent.

The pendant groups on the main chain of the silicone rubber can be methyl, ethyl, vinyl, phenyl, trifluoropropyl and the like. Methyl groups are most commonly used, but other groups can also be introduced to improve processability and other properties. Therefore, according to the different side groups and compound formulations, various silicone rubbers for different purposes can be obtained, which can be generally divided into the following types: general-purpose (containing methyl and vinyl), high-temperature and low-temperature (containing phenyl, methyl and vinyl), low compression set (including methyl and vinyl), low shrinkage (devolatilization) and solvent resistance (fluorosilicone rubber), etc. Several important types of silicone rubber are introduced below.

1. Methyl silicone rubber

Dimethyl silicone rubber is the earliest type of silicone rubber that has been put into commercial production. It can maintain good elasticity in the range of -60 to 200 °C, has good aging resistance, excellent electrical insulation properties, moisture resistance, shock resistance and physiological inertia. characteristic.

Dimethyl silicone rubber is mainly used for the coating of strong objects, and can also be made into various extrusion and calendering products for electromechanical, aviation, automobile and medical industries. However, due to the low vulcanization activity of dimethyl silicone rubber, when it is used to manufacture thick products, it is difficult to vulcanize, the inner layer is easy to foam and the permanent deformation of high temperature compression is large, so it has been replaced by methyl vinyl silicone rubber.

2. Methyl vinyl silicone rubber

Since the introduction of a small amount of vinyl groups into the macromolecular structure of silicone rubber can greatly improve the vulcanization processability of silicone rubber, most of the currently used silicone rubbers contain vinyl groups. Methyl vinyl silicone rubber is the most common type of silicone rubber and currently dominates the production of silicone rubber at home and abroad. Due to the introduction of a partially unsaturated vinyl group on the side chain, its processability and physical and mechanical properties are superior to those of two-base silicone rubber. In addition to the general characteristics of dimethyl silicone rubber, it also has a wide operating temperature range and can maintain good elasticity in the range of 60 to 260 ° C. It is easier to vulcanize than dimethyl silicone rubber and has a smaller compression permanent Deformation, better solvent expansion resistance, high pressure steam resistance stability, and excellent cold resistance, etc., and because of the use of peroxides with lower activity for vulcanization, it reduces the generation of bubbles during vulcanization and poor rubber stability. weakness. Therefore, methyl vinyl silicone rubber can generally be used to make products with larger thicknesses. Methyl vinyl silicone rubber is the most widely used variety of silicone rubber. In recent years, various high-performance and special-purpose silicone rubbers have emerged, mostly based on vinyl silicone rubber, such as high-strength silicone rubber, low Compression set silicone rubber, no post-vulcanization silicone rubber, heat-resistant conductive silicone rubber and medical silicone rubber, etc. In the aviation industry, methyl vinyl silicone rubber is widely used as gaskets, sealing materials and protective layers for fragile and shock-proof parts; in the electrical industry, it can be used as advanced insulating materials such as electronic components, dynamic sealing rings for high temperature potentiometers, Sealing ring for underground long-distance communication equipment; in medicine, because methyl vinyl silicone rubber has little physiological response to the human body and is non-toxic, it is used for surgical plastic surgery, artificial heart valves, blood vessels, etc.

3. Methylphenyl vinyl silicone rubber

Methyl phenyl vinyl silicone rubber is a product obtained by introducing methyl phenyl siloxane or diphenyl siloxane into the molecular chain of methyl vinyl silicone rubber.

The introduction of a phenyl group on the side group of the polysiloxane can greatly reduce the crystallization temperature of the polymer due to the destruction of the regularity of the dimethylsiloxane structure and expand the low-temperature application range of the polymer material. Therefore, in addition to all the properties of methyl vinyl silicone rubber, methyl phenyl vinyl silicone rubber has small compression set, wide operating temperature range, oxidation resistance, weather resistance, shock resistance, moisture resistance and good electrical insulation properties, it also has excellent electrical insulation properties. Low temperature resistance, ablation resistance and radiation resistance. These properties vary with the phenyl content in the molecular chain. Generally speaking, when the phenyl content (ratio of phenyl to silicon atoms) is 5-10%, it is called low phenyl silicone rubber, which has unique cold resistance. It can still maintain the elasticity of rubber at -70 ~ -100 ℃, which is the best low-temperature performance among all rubbers. In addition, it has the advantages of methyl vinyl silicone rubber and the cost is not high, so it has great potential to replace methyl vinyl silicone rubber. based vinyl silicone rubber trends. When the phenyl content is 20-40%, it is said that the medium phenyl silicone rubber has both the advantages of methyl vinyl silicone rubber and the cost is not high, so there is a great tendency to replace methyl vinyl silicone rubber. When the phenyl content is 20 to 40%, it is called medium phenyl silicone rubber. It has excellent flame resistance and can self-extinguish once it catches fire. When the phenyl content is 40-50%, it is called high phenyl silicone rubber, which has excellent radiation resistance, and its resistance to γ-ray is 1xI08 roentgen. Generally speaking, with the increase of phenyl content, the rigidity of the molecular chain of silicone rubber gradually increases, and the low temperature resistance of silicone rubber gradually decreases, but with the increase of phenyl content, the flame resistance and radiation resistance of vulcanizate are improved. photogenic.

Methyl phenyl vinyl silicone rubber is one of the important materials in the aerospace industry, cutting-edge technology and other sectors of the national economy. Sealing rings, gaskets, pipes and bars for heat-aging or radiation-resistant parts.

4. Fluorosilicone rubber

Fluorosilicone rubber refers to γ-trifluoropropyl methyl polysiloxane. Fluorosilicone rubber has excellent chemical resistance, solvent resistance and lubricating oil resistance. This kind of silicone rubber has a small expansion rate in non-polar solvents, good cold resistance and thermal stability, and good fire resistance, so it is used in aircraft. , rockets, missiles, space flight, petrochemical used as contact hoses, gaskets, seals, fuel tanks, etc. with fuel oil and lubricating oil, and can also be used to make corrosion-resistant clothes, gloves, coatings, adhesives, etc. .

5. Nitrile Silicone Silicone Rubber

Because the side chain of the polymer molecule contains β-nitrile ethyl or γ-nitrile propyl strong polar groups, the intermolecular force is greatly increased, and the oil and solvent resistance properties are improved. At the same time, due to the introduction of a certain amount of nitrile alkyl groups, the regularity of the polymer structure is destroyed, and the cold resistance is also greatly improved. The type and content of nitrile alkyl groups have a great influence on performance. For example, silicone rubber containing 7.5 mole% γ-nitrile propyl group has similar low temperature performance to low phenyl silicone rubber (its glass transition temperature is -114.5 ° C), Oil resistance is better than phenyl silicone rubber. As the content of γ-nitrile propyl group increased to 33-50 mole %, the cold resistance decreased and the oil resistance improved. Replacing γ-nitrile propyl with β-nitrile ethyl can improve the heat resistance of nitrile silicone rubber, which can withstand 250 ℃ hot air aging.

The main advantage of nitrile silicone rubber is that it has excellent oil resistance and bath resistance, and can maintain elasticity in the range of -60 to 180 ° C. Therefore, it can be made into oil-resistant rubber products for the aviation industry, automobile industry and petroleum industry; also It can be used as environmental sealant and fuel tank sealant for high-performance aircraft, and can maintain sealing at -54°C to above 200°C. Nitrile silicone rubber can be processed with ordinary equipment.

6. Phenylene silicone rubber

The main feature of phenylene silicone rubber is that it has excellent radiation resistance. Its radiation resistance is 10 to 15 times that of general methyl vinyl silicone rubber and 5 to 10 times that of high phenyl silicone rubber. Therefore, it can be used for Aerospace industry, atomic energy industry and nuclear reactors are used as cables, sheaths, gaskets and heat shrinkable tubes that are resistant to high-energy radiation.

7. Ethyl silicone rubber

The diethyl silicone rubber prepared by introducing an ethyl group into the side chain of polysiloxane has the outstanding performance of particularly good cold resistance, which is better than dimethyl silicone rubber and general methyl vinyl silicone rubber. The higher the ethyl content, the better the cold resistance. However, the reactivity of ethyl group is higher than that of methyl group, so the heat resistance decreases with the increase of ethyl group content. As the ethyl silicone rubber used at low temperature, it is preferable to contain 8 mol% of diethylsiloxane chain units in the polymer. The use temperature of ethyl silicone rubber is generally -70--200 ℃.

8. silicon nitrogen rubber

The main advantage of silicon nitrogen rubber is that it has excellent thermal stability, it does not decompose at 430-480 °C, and some can even withstand high temperatures above 500 °C. The outstanding weakness of silicon-nitrogen rubber is its poor hydrolytic stability, which was once considered to have no future. It was later found that the polymer with cyclodisilazane incorporated into the main chain has good thermal stability. The cyclodisilazane-containing elastomer modified with silicon arylene does not lose weight when heated to 425° C. in air, but only loses 10% at 570° C., and has good hydrolysis stability.