Silica gel can be divided into the following types: general-purpose silica gel, high temperature and low temperature silica gel, modified silica gel after low compression, fluorosilica gel, etc. Several types of silica gel are introduced below.
1. Dimethyl silicone is the earliest type of silicone produced by economic activity. It can maintain the elasticity of the heart in the range of -60 to 200 ° C, has good aging resistance, has particularly good electrical insulation properties, and is resistant to moisture and shock. And the body's inertia and other special properties. Dimethyl silicone is mainly used for fabric coating. It can also be made into various extruded and calendered products for mechanical and electrical, aviation, transportation and medical industries. However, due to the low vulcanization activity of dimethyl silica gel, it is difficult to vulcanize when used in the manufacture of thick silica gel products. The inner layer is easy to foam and has a large deformation after high temperature compression. So far, it has been replaced by methyl vinyl silica gel.
2. Methyl vinyl silica gel can greatly improve the vulcanization processing performance of silica gel due to the introduction of a small amount of vinyl groups in the macromolecular structure of silica gel. Because most of the silica gels that have been applied until now contain vinyl groups. Methyl vinyl silica gel is the most commonly used silica gel. Until now, it has dominated the production of silica gel at home and abroad. Because it introduces part of the vinyl group that does not reach the maximum limit on the side chain, its processing performance and physical and mechanical properties are better than two-base silica gel. In addition to its special properties like dimethyl silica gel, it also has a wide operating temperature range. It can maintain the elasticity of calling within a range of 60 to 260 ° C. It is easier to vulcanize than dimethyl silica gel and has a smaller Modified after compression, better resistance to solvent swelling, high pressure steam stability, and good cold resistance, etc., and the use of less active peroxides for vulcanization due to the appropriateness, thus reducing the vulcanization time Weakness of vitality foam and poor rubber stability. Therefore, the same thickness can be used to make silicone products with methyl vinyl silicone.
3. Methylphenyl vinyl silica gel Methylphenyl vinyl silica gel is a product obtained by introducing a methylphenylsilicone link or a diphenylsilicone link into the molecular chain of methylvinylsilicone. The introduction of a phenyl group on the side group of the polysiloxane, as the homeotropic properties of the dimethylsiloxane structure are destroyed, the temperature at which the polymer forms crystals is greatly reduced, and the low-temperature application range of the polymer material is expanded. Because of this, in addition to the small compression of methyl vinyl silicone, it has a wide range of operating temperature, oxidation resistance, weather resistance, shock resistance, moisture resistance, and electrical insulation properties. Low temperature resistance, ablation resistance and radiation resistance. These properties vary with the content of phenyl in the molecular chain. Similarly, the phenyl content (ratio of phenyl to silicon atom) is 5 ～ 10% is called low phenyl silica gel. It has unique cold resistance. It can still maintain the elasticity of rubber at -70 ～ -100 ℃. It is one of the best low temperature properties of the whole rubber. In addition, it has methyl group. Vinyl silicone is a bit cheaper and less costly, because this is a great alternative to methyl vinyl silicone.
4. Fluorosilica gel means γ-trifluoropropylmethylpolysiloxane. Fluorosilicone has good chemical resistance, solvent resistance, and lubricating oil resistance. This type of silicone has a small expansion rate in non-polar solvents, good cold resistance and thermal stability, and good fire resistance. Used in rockets, missiles, spaceflight, fuel chemical industry as hoses, gaskets, tightly closed closed loops, combustion materials tanks, etc. that are in contact with combustion materials oil and lubricants. Mixture and so on.
5. The nitrile silica gel has strong polar groups such as β-nitrileethyl or γ-nitrilepropyl in the side chain of the polymer molecule, which greatly increases the force between the molecular chains and improves the oil and solvent resistance. At the same time, due to the introduction of a certain amount of nitrile alkyl groups, the homeostasis 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, silica gel containing 7.5 mol% γ-nitrilepropyl has similar low temperature performance to low phenyl silica gel (its glass transition temperature is -114.5 ° C) and oil resistance. Better than phenyl silicone. As the content of γ-nitrilepropyl increases to 33-50 mol%, cold resistance decreases and oil resistance improves. Substituting β-nitrileethyl for γ-nitrilepropyl can improve the heat resistance of nitrile silica gel, and it can withstand hot air aging at 250 ℃. The main point of nitrile silica gel is that the performance of oil resistance and bath resistance is particularly good. It can maintain elasticity in the range of -60 to 180 ° C. Because of this, it can be made into oil resistant rubber products for the aviation industry, transportation industry and fuel industry. It can also be used as an environmentally tight sealant and a fuel tank tightly sealant for high-performance aircraft, and can maintain a tightly seal at -54 ° C to 200 ° C or more. Nitrile silica gel can be processed using ordinary equipment.
6. Ethyl silica Diethyl silica prepared by introducing ethyl group on the side chain of polysiloxane. Its outstanding performance is particularly cold resistance, which is better than dimethyl silica and the same methyl vinyl silica. . The higher the ethyl content, the better the cold resistance. However, the reactivity of ethyl is greater than that of methyl. Due to this, as the ethyl content increases, the heat resistance decreases. As the ethyl silica gel used at a low temperature, it is preferable that the polymer contains diethylsiloxane chain links at 8 mol%. The operating temperature of ethyl silica gel is also -70 ~ 200 ° C.
7. The main point of silicon nitrogen rubber is that it has excellent thermal fastness. It does not decompose at 430 ～ 480 ℃, and some even have high temperature above 500 ℃. The outstanding weakness of silicon nitrogen rubber is the poor stability of nutrient decomposition, which was once considered to have no development prospects. Later, it was discovered that the polymer introduced with cyclodisilazane in the main chain has good thermal fastness. Cyclic disilazane-containing elastomers modified with silarylene do not lose weight when heated to 425 ° C in air, and lose only 10% of weight at 570 ° C, and have good nutrient decomposition stability.