Silicone rubber refers to the rubber whose main chain is composed of alternating silicon and oxygen atoms, and two organic groups are usually connected to the silicon atom. Ordinary silicone rubber is mainly composed of silicone chain units containing methyl groups and a small amount of vinyl groups. The introduction of phenyl can improve the high and low temperature resistance of silicone rubber, and the introduction of trifluoropropyl and cyano groups can improve the temperature and oil resistance of silicone rubber. Silicone rubber has good low temperature resistance and can generally still work at -55 degree . After the introduction of phenyl, it can reach -73 degree . The heat resistance of silicone rubber is also very outstanding. It can work for a long time at 180 degree , and can withstand a few weeks or longer even if it is slightly higher than 200 degree . It can withstand high temperatures above 300 degree instantaneously. Silicone rubber has good air permeability and oxygen transmission rate is the highest among synthetic polymers. In addition, silicone rubber has outstanding properties of being physiologically inert and not causing coagulation, so it is widely used in the medical field.
Silicone rubber is divided into thermal vulcanization type (high temperature vulcanization silica gel HTV) and room temperature vulcanization type (RTV). Among them, the room temperature vulcanization type is divided into polycondensation reaction type and addition reaction type. High temperature silicone rubber is mainly used to manufacture various silicone rubber products, while room temperature silicone rubber is mainly used as adhesive, potting material or mold. The thermal vulcanization type has the largest amount, and the thermal vulcanization type is divided into methyl silicone rubber (MQ), methyl vinyl silicone rubber (VMQ, the largest amount and product brand), and methyl vinyl phenyl silicone rubber PVMQ (low temperature resistance, radiation resistance). ), and others include silicone rubber, fluorosilicone rubber, etc.
Development history:
The highest silicone rubber is synthesized by the United States using ferric chloride as a catalyst. In 1945, silicone rubber products came out. In 1948, silicone rubber reinforced with high specific surface area fumed silica was successfully developed, which made the performance of silicone rubber jump to the practical stage and laid the foundation of modern silicone rubber production technology. The United States is the country that produces silicone rubber from the synthesis of dimethyldichlorosilane. Russia, Germany, Japan, South Korea and China, etc. The industrialization research of silicone rubber in China began in 1957, and many research institutes and enterprises have successively developed various silicone rubbers. By the end of 2003, China's silicone rubber production capacity was 135,000 tons, of which 100,000 tons of high-temperature rubber
Silicone rubber manufacturers generally have dozens or even hundreds of varieties and brands, most of which are classified according to product characteristics and use numbers, and also classified according to vulcanization methods and chemical structures. According to the vulcanization temperature, silicone rubber can be divided into high temperature vulcanized silicone rubber and room temperature vulcanized silicone rubber. From the packaging form, it can be divided into one-component and two-component RTV silicone rubber. According to the degree of polymerization, silicone rubber can also be divided into kneading silicone rubber and liquid silicone rubber
High temperature vulcanized silicone rubber
High temperature vulcanized silicone rubber means that the process of polysiloxane into an elastomer is vulcanized at high temperature (110-170 degree C). It mainly uses high molecular weight polymethyl vinyl siloxane as raw rubber, mixed with reinforcing filler, vulcanizing agent, etc., and vulcanized into elastomer under heating and pressure. The reinforcement of silicone rubber is mainly various types of silica, which can increase the strength of the vulcanizate dozens of times. Sometimes in order to reduce the cost or improve the performance of the rubber compound and give the vulcanized rubber various special properties, various additives are also added. Sulfiding agents are various organic peroxides or addition reaction catalysts. 1
Raw rubber types and preparation
The main types of raw rubber are methyl silicone rubber, methyl vinyl silicone rubber, methyl phenyl vinyl silicone rubber and fluorosilicone rubber, as shown in the figure. In addition, a phenylene group can also be introduced on the main chain. The largest amount is methyl vinyl silicone raw rubber. Due to the introduction of vinyl groups, which are active groups for cross-linking, the cross-linking efficiency of peroxide cross-linking of raw silicone rubber is greatly improved. Almost all commercial silicone rubbers contain some amount of vinyl.
Silicone rubber is mainly produced by alkali-catalyzed polymerization and acid-catalyzed polymerization in industry. More use of KOH and temporary catalysts (CH3)4NOH, (n-C4H9)4POH. 1
Processing and forming
The processing method is shown in the figure. The purpose of the primary vulcanization is to carry out the cross-linking reaction of the polymer chain; the purpose of the secondary vulcanization is to supplement the cross-linking, drive off the decomposition products of the vulcanizing agent and other volatile compounds to stabilize the properties of the vulcanizate. Commonly used equipment are open rubber mixer, kneader and vacuum mixer. 1
Main performance
(1) High temperature performance. The remarkable feature of silicone rubber is high temperature stability. Although the strength of silicone rubber at room temperature is only half of that of natural rubber or some synthetic rubbers, it can still maintain a certain degree of flexibility and recovery at high temperatures above 200 degree . Elasticity and surface hardness, and no significant change in mechanical properties. (2) Low temperature performance The glass transition temperature of silicone rubber is generally -70-50 degree , and the special formula can reach -100 degree , indicating that its low temperature performance is excellent. This is of great significance to the aviation and aerospace industries. (3) The Si-O-Si bond in weather-resistant silicone rubber is very stable to oxygen, ozone and ultraviolet rays, and has excellent weather resistance without any additives. (4) Electrical properties. Silicone rubber has excellent insulating properties, corona resistance and arc resistance are also very good. (5) Physical and mechanical properties. The physical and mechanical properties of silicone rubber at room temperature are worse than that of general-purpose rubber, but at a high temperature of 150 degree and a low temperature of -50 degree , its physical and mechanical properties are better than general-purpose rubber. (6) Resistance to oil and chemical reagents. Ordinary silicone rubber has moderate oil and solvent resistance. (7) Gas permeability. At room temperature, the air permeability of silicone rubber to air, nitrogen, oxygen, carbon dioxide and other gases is 30-50 times higher than that of natural rubber. (8) Physiological inertia. Silicone rubber is non-toxic, tasteless, odorless, non-adhesive to human tissue, has anticoagulant effect, and has very little reactivity to human tissue. Especially suitable as medical material. 1
4 RTV silicone rubber
The main difference between room temperature vulcanized silicone rubber and high temperature vulcanized silicone rubber is that it is based on polysiloxane with a smaller molecular weight. RTV silicone rubber is composed of base rubber, crosslinking agent, catalyst, filler, etc. From the packaging form, it can be divided into one-component and two-component. 1
base glue
The base rubber is preferably polydimethylsiloxane terminated with hydroxyl group, and polymethylsiloxane containing alkoxy or other active groups can be selected for special purposes. ,ω-Dihydroxypolysiloxane is the base glue of two-component and one-component condensation type silicone rubber, commonly known as 107 glue in the market. 1
One-component RTV silicone rubber
One-component RTV silicone rubber is one of the main products of condensation type silicone rubber. It is composed of base glue, filler, crosslinking agent, catalyst, plasticizer, pigment, etc. The base gum is ,ω-dihydroxypolysiloxane. Fillers act to reinforce, compatibilize and impart special properties. There are many types of crosslinkers and catalysts and other fillers, depending on market demand and product performance. A typical recipe is shown in the figure. 1
Two-component RTV silicone rubber
The two-component RTV silicone rubber has the same ingredients as the one-component except the crosslinking agent. Two-component room temperature vulcanized silicone rubber is commonly used as a crosslinking agent of ethyl orthosilicate or polyethyl silicate, and the latter has a high crosslinking speed. The vulcanization reaction is shown in the figure. 1
Application of RTV Silicone Rubber
(1) Construction industry. It is used for the bonding of glass and metal curtain walls, roof embedding, door and window sealing, bonding and sealing of various pools and tiles.
(2) Electronics industry. Encapsulation and potting material for electrical and electronic components, resistant to moisture, shock and impact, temperature shocks and chemicals.
(3) Mold. The excellent simulation and good release properties of silicone rubber make it widely used in the soft mold industry.
(4) Automobiles, ships and aviation. It is used for automotive in-situ forming gaskets, window seals, and electrical and electronic connectors to prevent corona, etc. 1
5 Addition type liquid silicone rubber
Addition type liquid silicone rubber is mainly based on polydimethylsiloxane containing terminal vinyl group, polymethylsiloxane containing dimethyl link and methyl hydrogen link as cross-linking agent, with The platinum complex is a compound made of a cross-linking catalyst. Under the action of a catalyst, the rubber compound undergoes an addition reaction at room temperature or under heating to obtain a silicone rubber with a network structure. 1
6 main varieties
There are many ways to classify silicone rubber, usually by curing
The former form is divided into solid silicone rubber and liquid silicone rubber; according to the vulcanization temperature, it can be divided into room temperature vulcanized silicone rubber and high temperature vulcanized silicone rubber; according to the different monomers used, it can be divided into methyl vinyl silicone rubber, methyl phenyl vinyl Base silicone rubber, fluorosilicone, nitrile silicone rubber, etc.; according to different properties and uses, it can be divided into general type, ultra-low temperature resistance type, ultra high temperature resistance type, high strength type, oil resistance type, medical type and so on. The following is a brief introduction to the commonly used silicone rubber:
Dimethyl Silicone Rubber
Dimethyl silicone rubber is referred to as methyl silicone rubber:
The preparation of high molecular weight linear dimethyl polysiloxane rubber requires high-purity raw materials. In order to ensure the purity of the raw materials, the industry is usually first to purify the dimethyl dimethyl polysiloxane with a content of more than 99.5 percent . Chlorosilane in ethanol-water medium, in
Silicone Rubber
Silicone Rubber
Hydrolysis and condensation are carried out under the catalysis of acid, and the bifunctional siloxane tetramer, namely octamethylcyclotetrasiloxane, is separated, and then the tetracyclic body is formed under the action of a catalyst to form a high molecular linear dimethylpolysiloxane. Siloxane.
Dimethyl silicone rubber is a colorless and transparent elastomer, which is usually vulcanized with organic peroxides with high activity. Vulcanized rubber can
When used in the range of -60 plus 250 degree , the vulcanization activity of dimethyl silicone rubber is low, and the permanent deformation of high temperature compression is large, so it is not suitable for thick products. Since methyl vinyl silicone rubber containing a small amount of vinyl has better properties, dimethyl silicone rubber has been gradually replaced by methyl vinyl silicone rubber. Other types of silicone rubbers produced and used today, in addition to containing dimethylsiloxane structural units, also contain more or less other bifunctional siloxane structural units, but their preparation methods are different from those of dimethylsiloxane. There is no essential difference in the preparation method of rubber. The preparation method is generally to hydrolyze and condense a desired difunctional silicon monomer under the conditions conducive to the formation of the ring body, and then add octamethacrylate in the required proportion. base cyclotetrasiloxane, and then co-reacted under the action of a catalyst.
Methyl Vinyl Silicone Rubber
Methyl vinyl silicone rubber is referred to as vinyl silicone rubber.
Because this kind of rubber contains a small amount of vinyl side chains, it is easier to vulcanize than methyl silicone rubber, so that it has more kinds of peroxides for vulcanization and can greatly reduce the amount of peroxides. Compared with dimethyl silicone rubber, the use of silicone rubber containing a small amount of vinyl can significantly improve the resistance to compression set. The low compression set reflects its better support as a seal at high temperatures. It is one of the requirements that O-rings and gaskets must have. Methyl vinyl silicone rubber has good technological performance and is easy to operate. It can be made into thick products, and the surface of the semi-finished products that are extruded and calendered is smooth. It is a commonly used silicone rubber.
Methyl phenyl vinyl silicone rubber
Methyl phenyl vinyl silicone rubber is referred to as phenyl silicone rubber. This kind of rubber is obtained by introducing diphenylsiloxane link or methylphenylsiloxane link in the molecular chain of vinyl silicone rubber.
According to the phenyl content (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 is close to the glass transition point, or when the two conditions overlap, the rubber becomes stiff. Introducing an appropriate amount of bulky groups to destroy the regularity of the polymer chain can reduce the crystallization temperature of the polymer. At the same time, because the introduction of bulky groups changes the intermolecular force of the polymer, it can also change the temperature. Low phenyl silicone rubber (C6H5/Si=611 percent ) has excellent low temperature resistance due to the above reasons, and has nothing to do with the type of phenyl monomer used. The brittleness temperature of vulcanizate is -120 degree , which is the best rubber with low temperature performance. Low phenyl silicone rubber has the advantages of vinyl silicone rubber, and the cost is not very high, so there is a trend to replace vinyl silicone rubber. When the phenyl content is greatly increased, the rigidity of the molecular chain will increase, resulting in the reduction of cold resistance and elasticity, but the ablation resistance and radiation resistance will be improved, and the phenyl content will reach C6H5/Si=2034 percent is medium phenyl silicone rubber has the characteristics of ablation resistance, high phenyl silicone rubber (C6H5/Si=3550 percent ) has excellent radiation resistance.
Nitrile Silicone Rubber
Fluorosilicone rubber is a type of silicone rubber with a fluoroalkyl group introduced into the side chain. Commonly used fluorosilicone rubbers are fluorosilicone rubbers containing methyl, trifluoropropyl and vinyl groups.
Fluorosilicone has good heat resistance and excellent oil and solvent resistance, such as aliphatic hydrocarbons, aromatic hydrocarbons, chlorinated hydrocarbons, various petroleum-based fuel oils, lubricating oils, hydraulic oils and some synthetic oils at room temperature. The stability at high temperature is better, which is beyond the reach of pure silicone rubber. Fluorosilicone rubber has good low temperature performance, which is a great improvement for pure fluororubber. The temperature range of fluorosilicone rubber containing trifluoropropyl group to maintain elasticity is generally -50 degree plus 200 degree , and its high and low temperature resistance is worse than that of vinyl silicone rubber, and toxic gas will be generated when heated to above 300 degree . It is much worse than vinyl silicone rubber in terms of electrical insulating properties. Adding an appropriate amount of low-viscosity hydroxy fluorosilicone oil to the fluorosilicone rubber compound, heat treatment of the compound, and then adding a small amount of vinyl silicone rubber can significantly improve the process performance, which is beneficial to solve the problems of sticky roller and serious storage structure. , can prolong the effective service life of the rubber compound. When the methylphenylsiloxane chain segment is introduced into the above-mentioned fluorosilicone rubber, it will contribute to the improvement of low temperature resistance, and the processing performance is good.
Nitrile silicone rubber is a type of silicone rubber in which a nitrile alkyl group (usually -nitrile ethyl or -nitrile propyl) is introduced into the side chain. The introduction of polar nitrile groups improves the oil resistance and durable solvent properties of silicone rubber, but its heat resistance, electrical insulation and processability are reduced.
The type and content of nitrile alkyl groups have a great influence on the performance of nitrile silicone rubber. For example, silicone rubber containing 7.5 percent -nitrile propyl group has a similar cold resistance to low phenyl silicone rubber and lower oil resistance. Base silicone rubber is better. When the content of -nitrile propyl group increases to 3350 percent molar, the cold resistance is significantly reduced, the oil resistance is improved, and the heat resistance is 200 degree . If -nitrile ethyl is used instead of -nitrile propyl, the heat resistance of nitrile silicone rubber can be further improved.
Phenylene and phenylene ether silicone rubber
Phenylene silicone rubber is a type of silicone rubber in which a phenylene group is introduced into the main chain of polysiloxane.
Due to the introduction of phenylene groups, the radiation resistance of silicone rubber is greatly improved. At the same time, due to the presence of aromatic rings, the rigidity of the molecular chain is increased, the flexibility is reduced, the glass transition temperature is increased, the cold resistance is decreased, and the tensile strength is reduced. increased. Phenylene silicone rubber has excellent high temperature resistance, radiation resistance, high temperature resistance up to 250300 degree , and has good dielectric properties, moisture resistance, mildew resistance and water vapor resistance. In the raw rubber composition of phenylene silicone rubber, it is suitable when the phenylene content is 60 percent , the phenyl content is 30 percent , and the methyl content is 10 percent (the vinyl content is 0.6 percent ). Has good overall performance.
The disadvantage of phenylene silicone rubber is that the low temperature performance is not good, and the brittle temperature is -25 degree , which affects its application in some aspects. The temperature is -6470 degree .
Phenylene ether silicone rubber is a polysiloxane with phenylene and phenylene groups introduced into the main chain of the molecule.
Phenylene oxide silicone rubber has good mechanical properties
