What is Silicone Rubber. How is Silicone Rubber Classified

Apr 27, 2022

What is silicone rubber. How is silicone rubber classified?
Silica gel is a kind of highly active adsorption material, which is an amorphous material. It contains polysiloxane, silicone oil, silica, coupling agent and filler. The main component is silica, and its chemical formula is msio2 nH2O. Insoluble in water and any solvent, non-toxic and tasteless, stable chemical properties, no reaction with any substance except strong base and hydrofluoric acid. Different types of silica gel have different microporous structures due to different manufacturing methods. The chemical composition and physical structure of silica gel determine that it has many characteristics that can not be replaced by other similar materials: high adsorption performance, good thermal stability, stable chemical properties, high mechanical strength, etc

Classification of silicone rubber:
According to its vulcanization characteristics, silicone rubber can be divided into heat vulcanized silicone rubber and room temperature vulcanized silicone rubber. According to the different performance and use, it can be divided into general-purpose type, ultra low temperature resistant type, ultra high temperature resistant type, high strength type, oil resistant type, medical type and so on. According to the different monomers used, it can be divided into methyl vinyl silicone rubber, methyl phenyl vinyl silicone rubber, fluorosilicone, nitrile silicone rubber, etc.
(1) Dimethyl silicone rubber (abbreviated as methyl silicone rubber)
In order to prepare linear dimethylpolysiloxane rubber with high molecular weight, it is necessary to have high purity raw materials, In industry, dimethyldichlorosilane with a content of more than 99.5% after distillation and purification is hydrolyzed and condensed in ethanol water medium under acid catalysis, and the bifunctional siloxane tetramer, namely octamethylcyclotetrasiloxane, is separated, and then the tetramer is made to form high molecular linear dimethylpolysiloxane under the action of catalyst. The formation reaction of dimethyl silicone rubber can be expressed as follows:
Dimethyl silicone rubber raw rubber is colorless and transparent elastomer, which is usually vulcanized with organic peroxide with high activity. The vulcanizate can be used in the range of - 60 ~ + 250C. The vulcanization activity of dimethyl silicone rubber is low, and the high temperature compression set is large, so it is not suitable for making thick products. It is difficult to vulcanize thick products, and the inner layer is easy to blister. Because the properties of methyl vinyl silicone rubber with a small amount of vinyl group are better than those of other silicone rubber, dimethyl silicone rubber has been gradually replaced by methyl vinyl silicone rubber. Other types of silicone rubber produced and applied nowadays contain not only the structural units of dimethylsiloxane, but also more or less the structural units of other bifunctional siloxanes. However, there is no essential difference between the preparation method of silicone rubber and that of dimethylsiloxane. The preparation method is to make the desired bifunctional silicone monomer into the ring under the conditions conducive to the formation of the ring After hydrolysis and condensation, the mixture is added according to the required proportion
Octamethylcyclotetrasiloxane was synthesized by the reaction of octamethylcyclotetrasiloxane with catalyst.

(2) Methyl vinyl silicone rubber:
Its structural formula can be expressed as follows:
This kind of rubber contains a small amount of vinyl side chain, so it is easier to cure than methyl silicone rubber, so it has more kinds of peroxides for curing, and can greatly reduce the amount of peroxides. Compared with dimethylsilicone rubber, silicone rubber containing a small amount of vinyl group can improve the compression set resistance significantly. The low compression set reflects that it has better supporting performance as a seal at high temperature, which is one of the requirements of O-ring and gasket. Methyl vinyl silicone rubber is one of the most commonly used silicone rubber because of its good process performance, easy operation, thick products and smooth surface of extruded and calendered semi-finished products.

(3) Methyl phenyl vinyl silicone rubber (abbreviated as phenyl silicone rubber):
The rubber is obtained by introducing diphenyl siloxane or methylphenyl siloxane into the molecular chain of vinyl silicone rubber. Its molecular structure can be expressed as follows:
According to the content of phenyl (phenyl: silicon atom) in silicone rubber, it can be divided into low phenyl, medium phenyl and high phenyl silicone rubber. When the rubber crystallizes or approaches the glass transition point, or the two conditions overlap, the rubber will appear rigid state. The introduction of appropriate amount of large volume group can destroy the regularity of polymer chain, and reduce the crystallization temperature of polymer. At the same time, the introduction of large volume group can change the intermolecular force of polymer, so it can also change the glass transition temperature. Low phenyl silicone rubber (C6H5 / Si = 6 ~ 11%) has excellent low temperature resistance due to the above reasons, and it has nothing to do with the type of phenyl monomer used. The brittleness temperature of vulcanizate is - 120c, which is the best rubber at low temperature. Low phenyl silicone rubber has the advantages of vinyl silicone rubber, and the cost is not very high, so it has a trend to replace vinyl silicone rubber. When the content of phenyl is greatly increased, the rigidity of molecular chain will be increased, resulting in the decrease of cold resistance and elasticity, but the ablation resistance and radiation resistance will be improved. When the content of phenyl reaches 20 ~ 34%, the medium phenyl silicone rubber has ablation resistance, while the high phenyl silicone rubber (C6H5 / Si = 35 ~ 50%) has excellent radiation resistance.

(4) Fluorosilicone, nitrile silicone rubber:
Fluorosilicone rubber: it is a kind of silicone rubber with side chain bow and fluorinated alkyl. The commonly used fluorosilicone rubber is the fluorosilicone rubber containing methyl, trifluoropropropyl and vinyl. Its structure can be expressed as follows:
Fluorosilicone not only has good heat resistance, but also has excellent oil and solvent resistance. For example, the stability of aliphatic hydrocarbon, aromatic hydrocarbon, chlorinated hydrocarbon, petroleum based fuel oil, lubricating oil, hydraulic oil and some synthetic oil at room temperature and high temperature is better than that of pure silicone rubber. Fluorosilicone rubber has good low temperature performance, which is a great improvement for pure fluorosilicone rubber. The elastic temperature range of fluorosilicone rubber containing trifluoropropyl group is generally - 50c - + 200C, and its high and low temperature resistance is worse than that of vinyl silicone rubber, and it will produce toxic gas when heated to above 300C. Compared with vinyl silicone rubber, its electrical insulation performance is much worse. Adding proper amount of low viscosity hydroxyl fluorosilicone oil into the compound, heat treatment of the compound, and adding a small amount of vinyl silicone rubber can significantly improve the process performance, solve the problems such as serious roller sticking and storage structure of the compound, and extend the effective service life of the compound. The introduction of methylphenyl siloxane into the fluorosilicone rubber can improve the low temperature resistance and good processability.
Nitrile silicone rubber is a kind of silicone rubber with side chain introducing nitrile alkyl group (usually β - cyanoethyl or γ - cyanopropyl). The introduction of polar nitrile group improved the oil resistance and solvent durability of silicone rubber, but its heat resistance, electrical insulation and processability decreased. The structural formula of silicone rubber containing methyl, nitrile, alkyl and vinyl groups can be expressed as follows:

The type and content of cyanoalkyl group have great influence on the properties of nitrile silicone rubber. For example, the cold resistance of silicone rubber containing 7.5% mole of y-cyanopropyl group is similar to that of low phenyl silicone rubber, and the oil resistance is better than that of low phenyl silicone rubber. When the content of y-cyanopropyl group is increased to 33 ~ 50% mole, the cold resistance is significantly reduced, the oil resistance is improved, and the heat resistance is 200 C. If β - cyanoethyl is used to replace y-cyanopropyl, the heat resistance of nitrile silicone rubber can be further improved.