High-temperature vulcanized silicone rubber is a high molecular weight (generally 40-800,000) polyorganosiloxane (ie, raw rubber) added to reinforcing fillers and other various additives, using organic peroxide as a vulcanizing agent, and pressure molded (molding, extrusion, calendering) or injection molding, and cross-linking into rubber at high temperatures. This rubber is generally referred to as silicone rubber.
The vulcanization of high temperature vulcanized silicone rubber is generally carried out in two stages. The first stage is to mix the silicone raw rubber, reinforcing agent, additive, vulcanizing agent and structure control agent, and then add the mixed material in the metal mold. Pressure heating and vulcanization, the pressure is about 50 kg / cm2, the temperature is 120 ~ 130 ° C, the time is 10 ~ 30 minutes, the second stage is to remove the silicone rubber from the mold, into the oven, at 200 ~ Bake at 250 ° C for several hours to 24 hours. The rubber is further vulcanized while the organic peroxide is decomposed and volatilized.
Silicone rubber reinforcing fillers are various types of silica, which can increase the strength of vulcanized rubber by a factor of ten. The addition of various additives mainly reduces the cost of the glue, improves the properties of the rubber compound, and imparts various special properties such as flame retardancy and electrical conductivity to the vulcanizate. The cross-linking agent is various organic peroxides such as benzoyl peroxide, 2,4-dichlorobenzoyl peroxide, dicumyl peroxide, 2,5-di-tert-butylperoxyhexane, etc. The structure control agent is used to prevent the rubber compound from being placed for too long, resulting in "structuring" to make the rubber hard and hard, and difficult to process and mature. It can be added with methyl hydroxy silicone oil or diphenyl dihydroxy condensate. As a structure control agent.
The pendant groups on the silicone rubber backbone can be methyl, ethyl, vinyl, phenyl, trifluoropropyl, and the like. The most commonly used are methyl groups, and other groups may be introduced to improve processability and other properties. Therefore, depending on the side group group and the compound formulation, silicone rubbers of various uses can be obtained, which can be generally classified into the following types: general type (including methyl and vinyl), high temperature and low temperature type (including Phenyl, methyl and vinyl), low compression set (including methyl and vinyl), low shrinkage (devolatilization) and solvent resistant (fluorosilicone rubber). Several important types of silicone rubber are described below.
First, methyl silicone rubber
Dimethyl silicone rubber is the first silicone rubber to be put into commercial production. It can maintain good elasticity in the range of -60 to 200 °C, good aging resistance, excellent electrical insulation properties, moisture resistance, shock resistance and physiological inertia. characteristic.
Dimethyl silicone rubber is mainly used for strong material coating, and can also be made into various extruded and calendered products for the electromechanical, aerospace, automotive and medical industries. However, since the dimethyl silicone rubber has low vulcanization activity, it is difficult to vulcanize when used for manufacturing thick products, and the inner layer is easy to foam and has high compression and permanent deformation at a high temperature, so it has been replaced by methyl vinyl silicone rubber.
Second, methyl vinyl silicone rubber
Since the introduction of a small amount of vinyl in the macromolecular structure of the silicone rubber can greatly improve the vulcanization processability of the silicone rubber, most of the silicone rubbers currently used contain vinyl groups. Methyl vinyl silicone rubber is the most versatile silicone rubber and currently dominates the production of silicone rubber at home and abroad. Since it introduces a partially unsaturated vinyl group on the side chain, its processability and physical and mechanical properties are superior to those of the two base silicone rubbers. In addition to the general characteristics of dimethyl silicone rubber, it also has a wide temperature range of use, can maintain good elasticity in a range of 60 ~ 260 ° C, it is easier to vulcanize than dimethyl silicone rubber, with a small compression permanent Deformation, better solvent-resistant expansion and high-pressure steam resistance, excellent cold resistance, etc., and vulcanization due to the use of less active peroxide, thereby reducing the occurrence of bubbles during vulcanization and poor rubber stability. weakness. Therefore, methyl vinyl silicone rubber is generally used to make a product having a relatively large thickness. Methyl vinyl silicone rubber is the most widely used variety of silicone rubber. In recent years, various high-performance and special-purpose silicone rubber products have been developed, most of which are based on vinyl silicone rubber, such as high-strength silicone rubber. Low compression and permanent deformation silicone rubber, no need for post-vulcanized silicone rubber, heat-resistant conductive silicone rubber and medical silica gel.
In the aviation industry, methyl vinyl silicone rubber is widely used as a protective layer for gaskets, sealing materials and fragile and shock-proof components. It can be used as an advanced insulating material for electronic components and a dynamic sealing ring for high temperature potentiometers in the electrical industry. The sealing ring of underground long-distance communication equipment; in medicine, because methyl vinyl silicone rubber has little physiological reaction to human body, it is used as surgical plastic surgery, artificial heart valve, blood vessel and the like.
Third, methyl phenyl vinyl silicone rubber
Methylphenylvinyl silicone rubber is a product obtained by introducing a methylphenyl silicon oxy-chain or a diphenyl siloxane chain into a molecular chain of a methyl vinyl silicone rubber.
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 silicone products and extruded silicone products for cold rubber in the aviation industry and for resistance. Seals, pads, tubes and rods for ablation, heat aging or radiation resistant parts.
Four, fluorosilicone rubber
The fluorosilicone rubber means γ-trifluoropropylmethylpolysiloxane. Fluorosilicone rubber has excellent chemical resistance, solvent resistance and lubricating oil resistance. This silicone rubber has a small expansion ratio in non-polar solvents, good cold resistance and thermal stability, and good fire resistance. , rockets, missiles, space flight, petrochemicals used in contact with fuel oil and lubricating oil hoses, gaskets, seals, fuel tanks, etc., can also be used to manufacture corrosion-resistant clothes, gloves, paints, adhesives, etc. .
Five, nitrile silicone rubber
The main advantage of nitrile silicone rubber is its excellent resistance to oil and bathing agents. It can maintain elasticity in the range of -60 to 180 °C. Therefore, it can be made into oil-resistant rubber products for aviation industry, automobile industry and petroleum industry. It can be used as an environmental sealant and tank sealant for high performance aircraft, and can be sealed from -54 ° C to 200 ° C. Nitrile silicone rubber can be processed by ordinary equipment.
Sixth, phenylene silicone rubber
The main feature of phenylene silicone rubber is its excellent radiation resistance. Its radiation resistance is 10 to 15 times that of general-purpose methyl vinyl silicone rubber and 5 to 10 times that of high phenyl silicone rubber. Therefore, it can be used. Astronautics, atomic energy, nuclear reactors, etc., as high-energy radiation cables, sheaths, gaskets and heat shrinkable tubes.
Seven, ethyl silicone rubber
The diethyl silicone rubber obtained by introducing ethyl group on the polysiloxane side chain has outstanding performance in that cold resistance is particularly good, and cold resistance is superior to that of 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 larger than that of the methyl group, and therefore, as the ethyl content is increased, the heat resistance is lowered. As the ethyl silicone rubber used at a low temperature, it is preferred that the polymer contains diethyl siloxane segments in an amount of 8 mol%. The use temperature of ethyl silicone rubber is generally -70 to 200 °C.
Eight, silicon nitride rubber
The main advantage of silicon-nitrogen rubber is its 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 thought to have no future. It was later found that the polymer in which the cyclodisilazane was introduced in the main chain has good thermal stability. The silicon-arylene-modified elastomer containing cyclodispilazane is heated to 425 ° C in air without loss of weight, and has a weight loss of only 10% at 570 ° C, and has good hydrolytic stability.