Silicone rubber is a kind of linear high molecular weight polysiloxane, the molecular weight is generally above 654.38+0.5 million, and its structural form is similar to silicone oil. According to the different organic groups attached to silicon atoms, there are many kinds of silicone rubber, such as dimethyl silicone rubber, methyl vinyl silicone rubber, methyl phenyl silicone rubber, fluorine silicone rubber, nitrile silicone rubber, ethyl silicone rubber and ethyl phenyl silicone rubber. According to the different vulcanization methods, silicone rubber can be divided into two categories: high temperature vulcanization (thermal vulcanization) silicone rubber and room temperature vulcanization (including low temperature vulcanization) silicone rubber. No matter which type of silicone rubber is vulcanized, there will be no exothermic phenomenon. High temperature vulcanized silicone rubber is a high molecular weight polysiloxane (molecular weight is generally 400,000 ~ 800,000), while room temperature vulcanized silicone rubber has a low molecular weight (30,000 ~ 60,000), and there are one or two functional groups at both ends (sometimes in the middle) of the molecular chain. Under certain conditions (moisture in the air or suitable catalyst), these functional groups can react to form high molecular weight cross-linked structures. Room temperature vulcanized silicone rubber can be divided into condensation type and addition type according to its vulcanization mechanism; According to its packaging method, it can be divided into two kinds: two-component and one-component. The nature of the silicon-oxygen bond that constitutes the main chain of silicone rubber determines that silicone rubber has advantages that natural rubber and other rubbers do not have. It has the widest working temperature range (-100 ~ 350 oc) and excellent high and low temperature resistance. In addition, it has excellent thermal stability, electrical insulation, weather resistance, ozone resistance, air permeability, high transparency, tear strength, excellent heat dissipation and excellent adhesion. In the service temperature range, silicone rubber can not only maintain certain softness, resilience and surface hardness, but also have no obvious change in mechanical properties, and can resist long-term thermal aging. Because of the special properties of silicone rubber, it can be used to mold electronic components such as high-voltage wheels, which makes the products have excellent anti-leakage performance and excellent demoulding performance. Used in the production of televisions, computers, photocopiers, etc. , with good heat dissipation and insulation performance. It is also used as a molding gasket requiring weatherability and durability, a packaging material for electronic components and a protective material for automobile electrical components. Silicone rubber can be used in construction and maintenance, joint sealing in expressway and joint sealing in reservoirs and bridges. Silicone rubber can also be used in coatings with strong adhesion, weather resistance, alkali resistance and water resistance. Because of its high temperature resistance, silicone rubber is widely used in manufacturing automobile parts. In addition, there are special purpose silicone rubber, such as conductive silicone rubber, medical silicone rubber, foamed silicone rubber, molded silicone rubber, heat shrinkable silicone rubber and so on. High temperature vulcanized silicone rubber is a kind of high molecular weight polysiloxane (i.e. raw rubber) with reinforcing fillers and other additives. Using organic peroxide as vulcanizing agent, rubber is crosslinked at high temperature by molding (molding, extrusion and calendering) or injection molding. This kind of rubber is generally referred to as silicone rubber for short.
Brief introduction of high temperature vulcanized silicone rubber
High temperature vulcanized silicone rubber is a kind of high molecular weight polysiloxane (i.e. raw rubber) with reinforcing fillers and other additives. Using organic peroxide as vulcanizing agent, rubber is crosslinked at high temperature by molding (molding, extrusion and calendering) or injection molding. 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. In the first stage, raw silicone rubber, reinforcing agent, additive, vulcanizing agent and structure control agent are mixed, and then the mixed materials are pressurized, heated, molded and vulcanized in a metal mold at a pressure of about 50kg/cm2, a temperature of 120 ~ 130℃ and a time of10 ~ 30min.
The reinforcing filler of silicone rubber is various types of white carbon black, which can improve the strength of vulcanized rubber ten times. The main purpose of adding various additives is to reduce the cost of rubber, improve the properties of rubber and give vulcanized rubber various special properties such as flame retardancy and electrical conductivity. The crosslinking agent is various organic peroxides, such as benzoyl peroxide, 2,4-dichlorobenzoyl peroxide, diisopropylbenzene peroxide, 2,5-dimethyl-2,5-di-tert-butylperoxyhexane, etc. The structure control agent is added to prevent the rubber mixture from being left for too long, resulting in "structure" and making it difficult to process and cure. Methyl hydroxy silicone oil or diphenyl dihydroxy silane can be used as structure control agent.
The side groups on the main chain of silicone rubber can be methyl, ethyl, vinyl, phenyl, trifluoropropyl, etc. Methyl is the most commonly used, but other groups can also be introduced to improve processability and other properties. Therefore, according to the different side groups and rubber formulations, silicone rubber with different uses can be obtained, which can be generally divided into the following types: universal type (containing methyl and vinyl), high temperature and low temperature type (containing phenyl, methyl and vinyl), low compression set (containing methyl and vinyl), low shrinkage (volatilization) and solvent-resistant type (fluorosilicone rubber). Several important silicone rubbers are introduced below.
1. dimethyl silicone rubber
Dimethyl silicone rubber is the earliest silicone rubber put into commercial production, which can maintain good elasticity in the range of -60 ~ 200℃, good aging resistance, excellent electrical insulation, moisture-proof, shock-proof and physiological inertia.
Dimethyl silicone rubber is mainly used for fabric coating, and can also be made into various extruded calendered products, which are used in electromechanical, aviation, automobile, medical and other industries. However, dimethyl silicone rubber was replaced by methyl vinyl silicone rubber because of its low vulcanization activity, difficult vulcanization, easy foaming of the inner layer and large permanent deformation under high temperature compression.
2. Methyl vinyl silicone rubber
Because introducing a small amount of vinyl into the macromolecular structure of silicone rubber can greatly improve the vulcanization performance of silicone rubber, most of the silicone rubber used at present contains vinyl. Methyl vinyl silicone rubber is the most common type of silicone rubber, which is currently dominant in the production of silicone rubber at home and abroad. Because it introduces partially unsaturated vinyl groups into the side chain, its processability and physical and mechanical properties are better than those of double-base silicone rubber. Besides the general characteristics of dimethyl silicone rubber, it also has a wide application temperature range and can maintain good elasticity in the range of 60-260℃. It is easier to vulcanize than dimethyl silicone rubber, and has small compression set, good solvent swelling resistance, high pressure steam stability and excellent cold resistance. Moreover, due to the use of peroxide with low activity for vulcanization, the shortcomings of bubble generation and poor rubber stability during vulcanization are reduced. Therefore, methyl vinyl silicone rubber can generally be used to make products with large thickness.
Methyl vinyl silicone rubber is the most widely used silicone rubber. In recent years, various kinds of high-performance and special-purpose silicone rubber have emerged, mostly vinyl silicone rubber, such as high-strength silicone rubber, low-compression permanent deformation silicone rubber, post-vulcanized silicone rubber, heat-resistant conductive silicone rubber, medical silicone rubber and so on. Methyl vinyl silicone rubber is widely used as a protective layer for gaskets, sealing materials and fragile shock-proof parts in aviation industry. In the electrical industry, it can be used as advanced insulating materials such as electronic components, dynamic sealing rings of high-temperature potentiometers and sealing rings of underground long-distance communication equipment; In medicine, methyl vinyl silicone rubber is used in plastic surgery, artificial heart valves, blood vessels and so on. Because it has little physiological reaction to human body and is non-toxic.
3. Methyl phenyl vinyl silicone rubber
Methyl phenyl vinyl silicone rubber is a product obtained by introducing methyl phenyl siloxane chain or diphenyl siloxane chain into the molecular chain of methyl vinyl silicone rubber.
The introduction of phenyl into the side group of polysiloxane destroyed the regularity of dimethylsiloxane structure, which greatly reduced the crystallization temperature of polymer and expanded the low-temperature application range of this polymer material. Therefore, methyl phenyl vinyl silicone rubber not only has all the properties of methyl vinyl silicone rubber, such as small compression set, wide temperature range, oxidation resistance, weather resistance, impact resistance, moisture resistance and good electrical insulation, but also has excellent low temperature resistance, ablation resistance and radiation resistance. These properties change with the change of phenyl content in molecular chain. Generally speaking, when the phenyl content (the ratio of phenyl to silicon atom) is 5 ~ 10%, it is called low phenyl silicone rubber. It has unique cold resistance, and can still maintain the elasticity of rubber at -70 ~- 100℃. It has the best low temperature performance of all rubbers. In addition, it also has the advantages of methyl vinyl silicone rubber, and the cost is not high, so it is likely to replace methyl vinyl silicone rubber. When the phenyl content is 20 ~ 40%, it is called middle phenyl silicone rubber, which has excellent flame retardancy and can extinguish itself once it catches fire. When the phenyl content is 40 ~ 50%, it is called high phenyl silicone rubber, which has excellent radiation resistance and gamma ray resistance of 1xI08 roentgen. Generally speaking, with the increase of phenyl content, the rigidity of silicone rubber molecular chain increases gradually, and the low temperature resistance of silicone rubber decreases gradually, but with the increase of phenyl content, the flame retardant and radiation resistance of vulcanized rubber improve.
Methyl phenyl vinyl silicone rubber is one of the important materials in aerospace industry, cutting-edge technology and other sectors of national economy. It can be used to manufacture all kinds of molded and extruded products of cold-resistant rubber in aviation industry and sealing rings, pads, tubes and rods of parts with ablation resistance, heat resistance and radiation resistance.
4. Fluorosilicone rubber
Fluorosilicone rubber refers to γ-trifluoropropyl methyl polysiloxane. Fluorosilicone rubber has excellent chemical resistance, solvent resistance and lubricating oil resistance. This silicone rubber has small expansion rate in nonpolar solvents, good cold resistance and thermal stability, and good flame retardancy. Therefore, it is used as rubber hose, gasket, sealing ring, oil tank village, etc. It can also be used to make corrosion-resistant clothes, gloves, paints and adhesives in aircraft, rockets, missiles, aerospace and petrochemical industries.
5. Nitrile silicone rubber
Because the side chain of polymer molecules contains strong polar groups of β -nitrile ethyl or γ -nitrile propyl, the intermolecular force is greatly increased, and the oil resistance and solvent resistance are improved. At the same time, due to the introduction of a certain amount of nitrile alkyl, the regularity of polymer structure was destroyed and the cold resistance was greatly improved. The type and content of alkyl nitriles have great influence on the properties. For example, the silicone rubber containing 7.5mol %γ- nitrile propyl has similar low-temperature performance to that of low-phenyl silicone rubber (its glass transition temperature is-1 14.5℃), and its oil resistance is better than that of phenyl silicone rubber. With the increase of γ -nitrile propyl content to 33 ~ 50 mol%, the cold resistance decreased and the oil resistance increased. Using β-nitrile ethyl instead of γ-nitrile propyl can improve the heat resistance of NBR silicone rubber and resist hot air aging at 250℃.
The main advantages of NBR are excellent oil resistance and bath resistance, which can keep elasticity in the range of -60 ~ 180℃, so it can be made into oil-resistant rubber products, which are used in aviation industry, automobile industry and petroleum industry. It can also be used as environmental sealant and fuel tank sealant for high-performance aircraft, and can keep sealing at -54℃ to above 200℃. Nitrile-butadiene silicone rubber can be processed by common equipment.
6. Phenylene silicone rubber
The main feature of styrene silicone rubber is its excellent radiation resistance, which 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 as cable, sheath, gasket and heat shrinkable tube with high energy radiation resistance in aerospace industry, atomic energy industry and nuclear reactor.
7. Ethyl silicone rubber
Diethyl silicone rubber prepared by introducing ethyl into polysiloxane side chain has excellent 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 is greater than that of methyl, so the heat resistance decreases with the increase of ethyl content. As ethyl silicone rubber used at low temperature, it is recommended to contain 8 mol% diethyl siloxane chain in the polymer. The use temperature of ethyl silicone rubber is generally -70 ~ 200℃.
8. Silicone rubber and nitrogen rubber
The main advantage of silicon-nitrogen rubber is its excellent thermal stability, which does not decompose at 430 ~ 480℃, and some can even withstand high temperatures above 500℃. The outstanding weakness of silicone rubber is its poor hydrolysis stability, which was once considered to have no development prospect. Later, it was found that the polymer containing cyclodisilazane in the main chain had good thermal stability. The elastomer containing siloxane-modified cyclodisilazane does not lose weight when heated to 425℃ in air, and the weight loss at 570℃ is only 65438 00%, which has good hydrolytic stability.