The concept of silicone
Silicone rubber is a linear high molecular weight polysiloxane with a molecular weight generally above 150,000. Its structural form is similar to silicone oil. Depending on the organic groups linked to the silicon atoms, silicone rubber includes dimethyl silicone rubber, methylvinyl silicone rubber, methylphenyl silicone rubber, fluorosilicone rubber, nitrile silicone rubber, ethyl silicone rubber and ethylbenzene Support silicone rubber and many other varieties. According to their vulcanization methods, silicone rubber can be divided into two categories: high-temperature vulcanization (heat vulcanization) silicone rubber and room temperature vulcanization (including low-temperature vulcanization) silicone rubber. No matter which type of silicone rubber, no heat will be released during vulcanization. High temperature vulcanized silicone rubber is a high molecular weight polysiloxane (the molecular weight is generally 400,000 to 800,000). Room temperature vulcanized silicone rubber generally has a lower molecular weight (30,000 to 60,000), with bands at both ends of the molecular chain (sometimes also in the middle). There are one or two functional groups that can react under certain conditions (moisture in the air or appropriate catalysts) to form a high molecular weight cross-linked structure. Room temperature vulcanized silicone rubber can be divided into condensation type and addition type according to its vulcanization mechanism; it can be divided into two types according to its packaging method: two-component and one-component. The nature of the silicon-oxygen bonds that make up 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 operating temperature range (-100~350oC) and excellent high and low temperature resistance. In addition, It also has excellent thermal stability, electrical insulation, weather resistance, ozone resistance, air permeability, high transparency, tear strength, excellent heat dissipation and excellent adhesion, fluidity and release properties. Some Special silicone rubber also has excellent properties such as oil resistance, solvent resistance, radiation resistance and use at ultra-high and low temperatures. Within the operating temperature range, silicone rubber can not only maintain a certain degree of softness, resilience and surface hardness, but also have no significant change in mechanical properties, and can resist long-term thermal aging. Due to the special properties of silicone rubber, it can be used to mold high-voltage insulators and other electronic components, so that the products have excellent tracking resistance and excellent release properties; it is used in the production of televisions, computers, copiers, etc., and has good heat dissipation and insulation properties. It is also used as molded gaskets requiring weather resistance and durability, packaging materials for electronic parts, and protective materials for automotive electrical parts. Silicone rubber can be used in house construction and repair, highway joint sealing, and reservoir and bridge caulking sealing. Silicone rubber can also be used in coatings with strong adhesion, weather resistance, alkali resistance, and water resistance. Because silicone rubber is resistant to high temperatures, it is widely used in the production of automobile parts. In addition, there are also special-purpose silicone rubber, such as conductive silicone rubber, medical silicone rubber, foam silicone rubber, molding silicone rubber, heat shrinkable silicone rubber, etc. High temperature vulcanized silicone rubber is a high molecular weight (generally 400,000 to 800,000 molecular weight) polyorganosiloxane (i.e., raw rubber) added with reinforcing fillers and various other additives. It uses organic peroxide as the vulcanizing agent and is molded under pressure. (molding, extrusion, calendering) or injection molding, and cross-linked into rubber at high temperatures. This kind of rubber is generally referred to as silicone rubber
Introduction to high-temperature vulcanized silicone rubber
High-temperature vulcanized silicone rubber is a high molecular weight (molecular weight is generally 400,000 to 800,000) polyorganosiloxane ( Raw rubber) is added with reinforcing fillers and various other additives, uses organic peroxide as the vulcanizing agent, is pressure molded (molded, extruded, calendered) or injection molded, and cross-linked at high temperatures to form rubber. This kind of 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 silicone raw rubber, reinforcing agents, additives, vulcanizing agents and structure control agents, and then mix the The material is pressurized, heated, formed and vulcanized in a metal mold. 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 take the silicone rubber out of the mold and put it in. In the oven, bake at 200-250°C for several hours to 24 hours to further vulcanize the rubber and at the same time decompose and volatilize the organic peroxide.
The reinforcing filler of silicone rubber is various types of silica, which can increase the strength of the vulcanized rubber ten times.
The main purpose of adding various additives is 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 conductivity. Cross-linking agents are various organic peroxides, such as benzoyl peroxide, 2,4-dichlorobenzoyl peroxide, dicumyl peroxide, 2,5-dimethyl-2,5-di Tert-butylperoxyhexane, etc. The structure control agent is added to prevent the mixed rubber material from being left for too long and causing "structuring" that makes the rubber material hard and difficult to process and mature. Methylhydroxy silicone oil or diphenyldihydroxysilane 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. The most commonly used is methyl, but other groups can also be introduced to improve processing properties and other properties. Therefore, according to the different side groups and rubber compound formulas, silicone rubber can be obtained for various purposes, which can generally be divided into the following types: general-purpose 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 (devolatilization) and solvent resistance (fluorosilicone rubber), etc. Several important types of silicone rubber are introduced below.
1. Dimethyl silicone rubber
Dimethyl silicone rubber is the earliest silicone rubber put into commercial production. It can maintain good elasticity in the range of -60~200℃, has good aging resistance and has excellent Excellent electrical insulation properties as well as moisture-proof, shock-proof and physiological inertness properties.
Dimethyl silicone rubber is mainly used for fabric coating, and can also be made into various extruded and calendered products for use in electromechanical, aviation, automotive, medical and other industries. However, due to the low vulcanization activity of dimethyl silicone rubber, it is difficult to vulcanize when used to manufacture thick products. The inner layer is prone to blistering and 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 into the macromolecular structure of silicone rubber can greatly improve the vulcanization processing performance of silicone rubber, most of the silicone rubber currently used contains vinyl. Methyl vinyl silicone rubber is the most versatile silicone rubber and currently dominates the production of silicone rubber at home and abroad. Because it introduces partially unsaturated vinyl groups on the side chains, its processing performance and physical and mechanical properties are better than those of two-based 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, good resistance to solvent expansion, stability to high-pressure steam, and excellent cold resistance. Moreover, the use of less active peroxide for vulcanization reduces the generation of bubbles and poor rubber stability during vulcanization. weakness. Therefore, methyl vinyl silicone rubber can generally be used to make thicker products.
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, most of which are based on vinyl silicone rubber. For example, high-strength silicone rubber, low compression permanent deformation 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 for electronic components and dynamic sealing rings for high-temperature potentiometers. Sealing rings 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 in plastic surgery, artificial heart valves, blood vessels, etc.
3． Methyl phenyl vinyl silicone rubber
Methyl phenyl vinyl silicone rubber is introduced into the molecular chain of methyl vinyl silicone rubber with methyl phenyl silicone segments or diphenyl silicone Products derived from chain links.
The introduction of phenyl groups into the side groups of polysiloxane destroys the regularity of the dimethylsiloxane structure, greatly reduces the crystallization temperature of the polymer, and expands the potential of the polymer material. Low temperature application range.
Therefore, in addition to the small compression permanent deformation, wide operating temperature range, antioxidant, weather resistance, shockproof, moisture-proof and good electrical insulation properties of methylphenyl vinyl silicone rubber, methylphenyl vinyl silicone rubber also has excellent Low temperature resistance, ablation resistance and radiation resistance. These properties vary with the phenyl content in the molecular chain. Generally speaking, the phenyl content (ratio of phenyl to silicon atoms) is between 5 and 10%. It is sometimes called low phenyl silicone rubber. It has unique cold resistance and can still maintain the elasticity of rubber at -70~-100℃. It is the best low-temperature performance among all rubbers. In addition, it also contains methyl vinyl silicone. Due to the advantages of rubber and its low cost, there is a strong trend 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 to 50%, it is called high phenyl silicone rubber. It has excellent radiation resistance, and its gamma-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 vulcanized rubber are improved. Illumination.
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. It can be used to make various molded and extruded products and is used as cold-resistant rubber in the aviation industry. And sealing rings, gaskets, pipes and rods used in ablation-resistant, heat-aging-resistant or radiation-resistant parts.
4. Fluorosilicone rubber
Fluorosilicone rubber refers to γ-trifluoropropylmethylpolysiloxane. Fluorosilicone rubber has excellent resistance to chemicals, solvents and lubricants. 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, spaceflight, petrochemical industry, used as hoses, gaskets, sealing rings, fuel tanks, etc. in contact with fuel oil and lubricating oil. It 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, it greatly increases the inter-molecular force and improves oil and solvent resistance. performance. 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 greatly improved. The type and content of nitrile alkyl groups have a great impact on performance. For example, silicone rubber containing 7.5 mol% of γ-nitrile propyl group has similar low-temperature properties to low phenyl silicone rubber (its glass transition temperature is -114.5°C). Oil resistance is better than phenyl silicone rubber. As the γ-nitrile propyl content increases to 33 to 50 mol%, the cold resistance decreases and the oil resistance improves. Replacing γ-nitrile propyl group with β-nitrile ethyl group 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 use in the aviation industry and automobiles. In the industrial and petroleum industries; it can also be used as an environmental sealant and fuel tank sealant for high-performance aircraft, and can maintain sealing at temperatures from -54°C to over 200°C. Nitrile silicone rubber can be processed with ordinary equipment.
6． Phylene 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 methyl vinyl silicone rubber. It is a high phenyl silicone rubber. It is 5 to 10 times that of rubber. Therefore, it can be used in the aerospace industry, atomic energy industry, and nuclear reactors as high-energy radiation-resistant cables, sheaths, gaskets, and heat-shrinkable tubes.
7. Ethyl silicone rubber
Diethyl silicone rubber is made by introducing ethyl groups into the polysiloxane side chain. Its outstanding performance is that it has particularly good cold resistance, and its cold resistance is better than that of dimethyl silicone rubber. and methyl vinyl silicone rubber in general. The higher the ethyl content, the better the cold resistance. However, the reactivity of ethyl group is greater than that of methyl group. Therefore, as the ethyl group content increases, the heat resistance decreases. As an ethyl silicone rubber for low-temperature use, it is appropriate to contain 8 mol% of diethyl silicone chain segments in the polymer. The service temperature of ethyl silicone rubber is generally -70~200℃.
8. Silicone rubber
The main advantage of silicone rubber is its excellent thermal stability. It does not decompose at 430~480℃, and some can even withstand high temperatures above 500℃. The outstanding weakness of silicone rubber is its poor hydrolytic stability, and it was once considered to have no future. Later, it was discovered that polymers with cyclodisilazane introduced into the main chain have good thermal stability. The cyclodisilazane-containing elastomer modified with silarylene does not lose weight when heated to 425°C in air, and loses only 10% at 570°C, and has good hydrolysis stability.