High-temperature vulcanized silicone rubber is a high molecular weight (molecular weight is generally 400-800,000) polyorganosiloxane (ie raw rubber) added with reinforcing fillers and other various additives, using organic peroxide as the vulcanizing agent, and molded under pressure (Molding, extrusion, calendering) or injection molding, and cross-linked into rubber at high temperature. This kind of rubber is generally referred to as silicone rubber for short.
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 agents, additives, vulcanizing agents and structure control agents, and then add the mixture to the metal mold. Press heating molding and vulcanization, the pressure is about 50 kg/cm2, the temperature is 120-130°C, and the time is 10-30 minutes. The second stage is to remove the silicone rubber from the mold and put it in an oven at 200- Bake at 250°C for several hours to 24 hours. The rubber is further vulcanized and the organic peroxide is decomposed and volatilized.
The reinforcing filler of silicone rubber is various types of white carbon black, which can increase the strength of vulcanized rubber ten times. 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 retardant and electrical conductivity. The cross-linking agent is a variety of organic peroxides, such as benzoyl peroxide, 2,4-dichlorobenzoyl peroxide, dicumyl peroxide, 2,5-di-tert-butylperoxyhexane, etc. .Structural control agent is added in order to prevent the compounded rubber from being placed for a long time, resulting in "structuring", which makes the rubber harder and harder to process and mature. It can be added with methyl hydroxy silicone oil or diphenyl dihydroxy alkanes. As a structure control agent.
The pendant groups on the main chain of the silicone rubber can be methyl, ethyl, vinyl, phenyl, trifluoropropyl and the like. Commonly used is methyl, but other groups can also be introduced to improve processing performance and other properties. Therefore, according to the different side groups and rubber formulations, silicone rubbers for various purposes can be obtained. Generally, they can be divided into the following types: general type (containing methyl and vinyl), high temperature and low temperature type (containing Phenyl, methyl and vinyl), low compression set (including methyl and vinyl), low shrinkage (devolatization) and solvent resistance (fluorosilicone rubber), etc. Several important types of silicone rubber are introduced below.
one. Methyl silicone rubber
Dimethyl silicone rubber is a kind of silicone rubber that has been put into commercial production early. It can maintain good elasticity in the range of -60～200℃, good aging resistance, excellent electrical insulation, moisture resistance, shock resistance and physiological inertness, etc. characteristic.
Dimethyl silicone rubber is mainly used for coating of strong materials, and can also be made into various extruded and calendered products for electromechanical, aviation, automotive and medical industries. However, due to the low vulcanization activity of dimethyl silicone rubber, it is difficult to vulcanize when used in the manufacture of thick products, the inner layer is easy to foam and the high temperature compression permanent deformation 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 in the macromolecular structure of silicone rubber can greatly improve the vulcanization processability of silicone rubber, most of the currently used silicone rubbers contain vinyl. Methyl vinyl silicone rubber is a kind of general-purpose silicone rubber, currently occupying a leading position in the production of silicone rubber at home and abroad. Because it introduces partially unsaturated vinyl groups on the side chain, its processing performance and physical and mechanical properties are better than 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 within a range of 60 to 260 ℃. It is easier to vulcanize than dimethyl silicone rubber and has a smaller permanent compression. Deformation, good solvent resistance, high pressure steam resistance and excellent cold resistance, etc., and because of the use of less active peroxides for vulcanization, it reduces bubbles and poor rubber stability during vulcanization. weakness. Therefore, generally, methyl vinyl silicone rubber can be used to make products with a larger thickness. Methyl vinyl silicone rubber is a widely used variety in 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 Permanent deformation silicone rubber, silicone rubber without post-vulcanization, heat-resistant conductive silicone rubber and medical silicone rubber, etc. In the aviation industry, methyl vinyl silicone rubber is widely used as a protective layer for gaskets, sealing materials, and fragile and shock-proof parts; in the electrical industry, it can be used as high-level insulating materials such as electronic components, and 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 reaction to the human body and is non-toxic, it is used for surgical plastic surgery, artificial heart valves, blood vessels, etc.
3. Methyl phenyl vinyl silicone rubber
Methyl phenyl vinyl silicone rubber is a product obtained by introducing methyl phenyl silicone chain links or diphenyl silicone chain links into the molecular chain of methyl vinyl silicone rubber. The introduction of phenyl groups on the side groups of the polysiloxane destroys the regularity of the dimethylsiloxane structure, greatly reduces the crystallization temperature of the polymer, and expands the low-temperature application range of the polymer material. Therefore, methyl phenyl vinyl silicone rubber has all the characteristics of methyl vinyl silicone rubber, such as small compression set, wide temperature range, oxidation resistance, weather resistance, shock resistance, moisture resistance and good electrical insulation properties, but also has excellent electrical insulation properties. Low temperature resistance, ablation resistance and radiation resistance etc. performance... These properties vary with the content of phenyl in the molecular chain. Generally speaking, the content of phenyl (ratio of phenyl to silicon atom) is called low phenyl silicone rubber when it is 5-10%. It has unique cold resistance. Performance, it can maintain the elasticity of rubber at -70～-100℃. It is the best low-temperature performance of all rubbers. In addition, it has the advantages of methyl vinyl silicone rubber and is not high in cost, so it can replace methyl Trends in vinyl silicone rubber. When the phenyl content is 20-40%, it is said that the middle phenyl silicone rubber has the advantages of methyl vinyl silicone rubber and the cost is not high, so it has a tendency to replace methyl vinyl silicone rubber. When the phenyl content is 20-40%, 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. It has excellent radiation resistance and γ-ray resistance is 1xI08 roentgen. Generally speaking, as the phenyl content increases, the rigidity of the silicone rubber molecular chain gradually increases, and the low temperature resistance of silicone rubber gradually decreases. However, as the phenyl content increases, the flame resistance and radiation resistance of the vulcanizate are improved. According to sex. Content from Rubber Plantation
Methylphenyl vinyl silicone rubber is one of the important materials in the aerospace industry, cutting-edge technology and other sectors of the national economy. It can be used to make various molded and extruded products as cold-resistant rubber in the aviation industry and for ablation resistance. 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 low expansion rate in non-polar solvents, good cold resistance and thermal stability, and good fire resistance. , Rockets, missiles, space flight, petrochemicals, used as hoses, gaskets, seals, fuel tanks in contact with fuel oil and lubricants, etc., and can also be used to make corrosion-resistant clothes, gloves, coatings, adhesives, etc. .
5. Nitrile Silicone Rubber
Because the side chain of the polymer molecule contains β-nitrile ethyl or γ-nitrile propyl strong polar groups, the force between the molecular chains is greatly increased, and the oil and solvent resistance are improved. At the same time, due to the introduction of a certain amount of nitrile alkyl, 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 impact on performance. For example, silicone rubber containing 7.5 mol% γ-nitrile propyl 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 γ-cyanopropyl content increases to 33-50 mol%, the cold resistance decreases and the oil resistance improves. Using β-nitrile ethyl instead of γ-nitrile propyl can improve the heat resistance of nitrile silicone rubber and can withstand hot air aging at 250°C.
The main advantage of nitrile silicone rubber is that it has excellent oil resistance and bath agent 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; It can be used as an environmental sealant and fuel tank sealant for high-performance aircraft, and can maintain the seal at temperatures above -54℃ to 200℃. Nitrile silicone rubber can be processed with ordinary equipment.
Six. Phenylene Silicone Rubber
The main feature of phenylene silicone rubber is that it has excellent radiation resistance. Its radiation resistance is 10-15 times that of general methyl vinyl silicone rubber and 5-10 times that of high phenyl silicone rubber. Therefore, it can be used for In the aerospace industry, atomic energy industry, and nuclear reactors, it is used as high-energy radiation resistant cables, sheaths, gaskets, and heat shrinkable tubes.
7. Ethyl Silicone Rubber
The diethyl silicone rubber prepared by introducing ethyl groups into the side chain of polysiloxane has outstanding performance in that it has 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 the ethyl group is greater than that of the methyl group. Therefore, as the content of the ethyl group increases, the heat resistance decreases. As the ethyl silicone rubber used at low temperature, it is preferable that the polymer contains 8 mol% of diethyl silicone segments. The operating temperature of ethyl silicone rubber is generally -70-200°C. Content from Rubber Plantation
8. Silicon Nitrogen Rubber
The main advantage of silicone nitrogen rubber is that it has excellent thermal stability, does not decompose at 430～480℃, and some can even withstand high temperatures above 500℃. The outstanding weakness of silicone nitrogen rubber is poor hydrolytic stability, which was once considered to have no future for development. It was later discovered that the polymer with cyclodisilazane introduced into the main chain had good thermal stability. The elastomer containing ring disilazane modified with silicon arylene has no weight loss when heated to 425°C in air, and the weight loss is only 10% at 570°C, and has good hydrolytic stability.
