High temperature vulcanized silicone rubber is a high molecular weight (molecular weight is generally 400,000 to 800,000) of polyorganosiloxane (that is, raw rubber) adding reinforcing fillers and other additives, using organic peroxides as vulcanizing agents, by pressure molding (molding, extrusion, calender) or injection molding, and at high temperature cross-chain into rubber. This rubber is generally referred to as silicone rubber.
The vulcanization of high temperature vulcanized silicone rubber is generally divided into two stages, and the stages are the mixing of silicone rubber, reinforcing agent, additive, vulcanizing agent and structure control agent, and then the mixing material is heated and vulcanized under pressure in the gold mold, the pressure is about 50 kg /cm2, the temperature is 120 ~ 130℃, and the time is 10 ~ 30 minutes. The second stage is to remove the silicon rubber from the mold, put it in the oven, and bake it at 200 ~ 250℃ for several hours to 24 hours. The rubber is further vulcanized, while 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 by ten times. The addition of various additives is mainly to reduce the cost of the rubber, improve the performance of the rubber and give the vulcanized rubber various special properties such as flame retardant, conductive and so on. Cross-linking agents are various organic peroxides, such as benzoyl peroxide,2, 4-dichloroyl benzoyl peroxide, dicumyl peroxide,2, 5-di-tert-butylperoxyhexane, etc. The structure control agent is added to avoid the mixing material being placed for too long, resulting in "structure" that makes the rubber material hard and hard, and difficult to process and finish. Methyl hydroxyl silicone oil or diphenyl dihydroxyl dehydrane can be used as the structure control agent.
The side groups on the main chain of silicone rubber can be methyl, ethyl, vinyl, phenyl, trifluoropropyl, etc. 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, you can get a variety of different uses of silicone rubber, generally can be divided into the following types: general type (including methyl and vinyl), high and low temperature type (including phenyl, methyl and vinyl), low compression deformation (including methyl and vinyl), low shrinkage (volatilization) and solvent resistance (fluorine silicone rubber). Several important types of silicone rubber are described below.
First, methyl silicone rubber
Dimethylsilicone rubber is the first kind of silicone rubber put into commercial production, which can maintain good elasticity in the range of -60 ~ 200 ° C, good aging resistance, excellent electrical insulation properties, moisture resistance, shock resistance and physiological inertia.
Dimethylsilicone rubber is mainly used for strong material coating, and can also be made into various extrusion and calendering 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 to make thick products, and the inner layer is easy to foam and the high temperature compression deformation is large, so it has been replaced by methyl vinyl silicone rubber.
Second, methyl vinyl silicone rubber
Because the introduction of a small amount of vinyl into the macromolecular structure of silicone rubber can greatly improve the vulcanization processing performance of silicone rubber, most of the silicone rubber used at present contains vinyl. Methyl vinyl silicone rubber is the most common kind of silicone rubber, and currently plays a leading role in the production of silicone rubber at home and abroad. Due to the introduction of partially unsaturated vinyl in 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 use temperature range, can maintain good elasticity in a range of 60 ~ 260 ° C, it is easier to vulcanize than dimethyl silicone rubber, has smaller compression deformation, better solvent expansion resistance and high pressure steam resistance stability and excellent cold resistance, etc. Moreover, due to the use of peroxides with lower activity for vulcanization, the weakness of bubbles and poor rubber stability during vulcanization is reduced. Therefore, generally using methyl vinyl silicone rubber can be made of larger thickness products. Methyl vinyl silicone rubber is the most widely used variety of silicone rubber, and in recent years, a variety of high-performance and special purpose silicone rubber products have emerged, mostly based on vinyl silicone rubber, such as high-strength silicone rubber, low compression deformation silicone rubber, no post-vulcanized silicone rubber, heat-resistant conductive silicone rubber and medical silicone rubber.
Methyl vinyl silicone rubber is widely used in the aviation industry as a protective layer for gaskets, sealing materials and fragile and shock-proof parts. In the electrical industry, it can be used as insulating materials such as electronic components, dynamic sealing ring of high temperature resistant potentiometer, sealing ring of underground long-distance communication equipment; In medicine, because of the small physiological reaction of methylvinyl silicone rubber to the human body, non-toxic, it is used as surgery, artificial heart valves, blood vessels and so on.
Three, methyl phenyl vinyl silicone rubber
Methylphenylvinylsilicone rubber is a product obtained by introducing methylphenylsiloxane link or diphenylsiloxane link into the molecular chain of methylvinylsilicone rubber.
Methyl phenyl vinyl silicone rubber is one of the important materials in the aerospace industry, technology and other sectors of the national economy, which can be used to make a variety of molded silicone products and extruded silicone products as cold-resistant rubber in the aviation industry and seals, pads, pipes and bars for ablative, heat-aging or radiation-resistant parts.
Four, fluorine silicone rubber
Fluorosilicone rubber refers to γ-trifluoropropyl methyl polysiloxane. Fluorosilicone rubber has excellent chemical resistance, solvent resistance and lubricating oil resistance, such silicone rubber in non-polar solvent expansion rate is small, cold resistance and thermal stability is good, fire resistance is good, so in aircraft, rockets, missiles, space flight, petrochemical industry used as contact hose with fuel oil and lubricating oil, gaskets, seals, fuel tank village, etc. It can also be used to make corrosion-resistant clothes, gloves, coatings, adhesives, etc.
Five, nitrile silicone rubber
The main advantage of nitrile silicone rubber is the excellent performance of oil resistance and bath resistance agent, which can remain elastic in the range of -60 ~ 180 ° C, therefore, it can be made into oil-resistant rubber products for the aviation industry, automotive industry and petroleum industry; It can also be used as an environmental sealant and fuel tank sealant for high-performance aircraft, and can maintain a seal above -54 ° C to 200 ° C. Nitrile silicone rubber can be processed with ordinary equipment.
Six, benzylene silicone rubber
The main feature of benzoic silicone rubber is that it has excellent radiation resistance, and 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 in aerospace industry, atomic energy industry and nuclear reactors as high-energy radiation resistant cables, sheaths, gaskets and heat shrinkable tubes.
Seven, ethyl silicone rubber
The outstanding performance of diethyl silicone rubber prepared by the introduction of ethyl on the polysiloxane side chain is that the cold resistance is particularly good, and the cold resistance 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 is larger than that of methyl, so the heat resistance decreases with the increase of ethyl content. As a low temperature use of ethyl silicone rubber, it is appropriate to contain 8 grams of diethyl silicoxy chain in the polymer. The use temperature of ethyl silicone rubber is generally -70--200℃.
Eight, silicon nitrogen rubber
The main advantage of silicon nitrogen rubber is that it has excellent thermal stability, does not decompose at 430 ~ 480 ° C, and some even can withstand high temperatures above 500 ° C. The outstanding weakness of silicon-nitrogen rubber is poor hydrolytic stability, which was once considered to have no future. It was later found that the polymer with the introduction of cyclodisilazane into the main chain has good thermal stability. The elastomer containing cyclodisilazane modified with silicon arylene has no weight loss when heated to 425℃ in air, and the weight loss is only 10% at 570℃, and has good hydrolysis stability.
