Common matching of silicone rubber materials

1. Vulcanization reaction of dimethylsilicone rubber:
Dimethylsilicone rubber molecules do not contain vinyl, is saturated rubber, usually use highly active peroxides as vulcanizing agent, peroxide free radicals to seize the hydrogen on the silicone rubber methyl to form macromolecular free radicals, and then the macromolecular free radicals recombine to form cross-linked bonds, such as dibenzoyl peroxide as vulcanizing agent.
It can be seen from the above reaction that silicone rubber containing vinyl can regenerate free radicals that can continue to react in the vulcanized parts, so introducing a small amount of vinyl into the silicone rubber can greatly improve the vulcanization activity, improve the cross-linking efficiency of the vulcanizing agent, reduce the amount of peroxide and improve the performance of the product. Since the initial free group triggering the crosslinking reaction is obtained from the decomposition of peroxides, increasing the amount of peroxides within a certain range can significantly improve the crosslinking degree of silicone rubber vulcanizate, which will lead to the improvement of the fixed elongation strength of the rubber, and improve the dynamic performance and compression deformation, but the tear resistance will be reduced.
According to the sulfurization activity of peroxides, the commonly used peroxides can be divided into general type and vinyl specific type. The general type has high activity and can vulcanize all kinds of silicone rubber, such as dibenzoyl peroxide (BP), 2· 4-dichlorodibenzoyl peroxide (DCBP), tert-butyl benzoate peroxide (TBPB), which belongs to this type. Vinyl-specific activity is low, and can only vulcanize silicone rubber containing vinyl, such as di-tert-butyl peroxide (DTBP), diisopropylbenzene peroxide (DCP), 2· 5-dimethyl - 2· 5-di-tert-butyl hexane peroxide (DBPMH), etc., are of this type. Dibenzoyl peroxide is often made into a silicone oil paste with an active component of 50% to ensure production safety and improve its dispersion in the compound. Its decomposition products are benzene, benzoic acid and carbon dioxide, is volatile, in a stage of vulcanization must be pressurized, and because the decomposition products contain acidic substances, so the amount should not be too much to avoid reducing the heat resistance of the product, this product is not suitable for the manufacture of thick wall model products. General 100 parts of dimethyl silicone rubber with dibenzoyl peroxide silicone oil paste 4-6 parts, the amount of vinyl silicone rubber is 0.5 to 2 parts.
2.4-dichlorodibenzoyl peroxide compared with dibenzoyl peroxide, its decomposition temperature is lower than dibenzoyl peroxide, and the decomposition rate is higher, because of the low decomposition temperature, so the coke performance is not good, therefore, the amount of this substance should be as little as possible. Its decomposition products are 2.4-dichlorobenzoic acid and 2.4-dichlorobenzene, which are less volatile, so bubbles can be avoided when vulcanized without pressure, especially suitable for continuous vulcanization of pressed products by atmospheric pressure hot air. The dosage is similar to that of dibenzoyl peroxide.
Dibenzoyl peroxide, 2.4 - dichlorodibenzoyl peroxide is an aryl peroxide and cannot be used in carbon black materials because carbon black interferes with the vulcanization of the peroxide. Light-colored rubber has a strong tendency to scorch, and the acidic substances in its decomposition products will damage the heat resistance of silicone rubber products in the sealing system.
Di-tert-butyl peroxide is effective to silicone rubber containing vinyl, is not easy to scorch and vulcanized rubber compression deformation is small, physical and mechanical properties are good. The disadvantage is that the vapor pressure is high, so the volatilization is also high, and it is very volatile during the storage of the rubber material. This product can be used for model products. And can be used for molding thick products and rubber materials containing carbon black. The dosage is generally 0.5~1.
Diisopropylbenzene peroxide and 2.5-dimethyl - 2.5-di-tert-butyl hexane peroxide are not volatile, easy to use, low compression deformation of vulcanizate, and because they do not decompose products with carboxyl group, they are particularly stable in vulcanizate of sealed products. They are suitable for thick molded products, products bonded with metal and injection products, and are also suitable for rubber materials containing carbon black, and have a wide range of applications. When 2.5-dimethyl-2.5-di-tert-butylhexane peroxide is used, the vulcanized rubber has higher elongation. When using diisopropylbenzene peroxide, its decomposition products have a bad smell, and this smell will be stored in the product for a long time, when using 2.5-dimethyl - 2.5-di-tert-butyl hexane peroxide, this drawback can be avoided. The tear strength of the vulcanizates obtained by these two peroxides is low. Its dosage is generally 0.5~1.
The amount of peroxide should not be too large, when more than the appropriate amount will make the vulcanized rubber elongation, tear strength and other properties decreased. This downward trend is especially in the use of 2.4 - dichlorodibenzoyl peroxide, dibenzoyl peroxide.
The silicone rubber containing vinyl can be vulcanized by addition besides peroxide. The vulcanizing agents used in addition vulcanization nowadays are mainly peroxides or polymers containing silyl. The Si-H group is vulcanized by addition to the vinyl group, in which a complex of platinum is usually used as a catalyst.
2. Reinforcing and filling system
The strength of unreinforced silicone rubber vulcanizate is very low, the tensile strength is low by 1 mpa, and the elongation is about 50~80%. The strength and elongation of such a fragile rubber can be greatly improved after being combined. Although it can not be said that the quality of rubber only depends on the filler, of course, the quality of raw rubber and the improvement of its production process also play a certain role, but when we compare the tensile strength of the initial rubber is 2.7~2.9 mpa, and now if needed, this figure can reach 98~12 mpa. This difference is largely due to the result of the study of fillers with good reinforcing properties. At present, methyl vinyl silicone rubber or methyl phenyl vinyl silicone rubber as a base rubber, although the chemical structure also takes into account the strength of the problem, but mainly by filler to reinforce. Commonly used reinforcing fillers are mainly white carbon black, its varieties are precipitated white carbon black, vapor phase white carbon black and surface treated white carbon black (with silicone compounds or organic alcohols to treat the surface of white carbon black, or in the process of white carbon black is added to these substances) and so on. The adhesive material using fumed white carbon black has good physical and mechanical properties and dielectric properties, and excellent water resistance, but special special additives (structure control agent) must be used in the adhesive material at the same time to obtain good process properties. The rubber material using precipitation method has slightly lower physical and mechanical properties and poor dielectric properties, especially after moisture exposure, but the cost is lower. The adhesive material with surface treatment of white carbon black has good process performance and high strength. The reinforcing effect of silica is related to particle size, surface structure, activity (number of hydroxyl group) and dispersion degree of silica in rubber. Generally speaking, the smaller the particle size, the larger the surface area, the greater the reinforcement, and the higher the dispersion, the greater the reinforcement. When the particle size is less than 50 mm, its reinforcing performance is higher, which is called reinforcing filler. In silicone rubber, silica black with a particle size of 8 to 30 nm and a specific surface area of 150 to 400 meters 2/ g is used as a reinforcing filler. It is generally accepted that reinforcement usually results in improvement. Although there have been many studies on the surface structure, activity and the relationship between silica and reinforcement, it has not been completely clear so far. Some data point out that the reason why the surface activity increases and the reinforcement effect is probably through the establishment of some bond or the formation of condensation to improve the strength. The vulcanized high-strength silicone rubber not only has intermolecular crosslinking, but also forms a large number of condensation points, causing a certain crosslinking between the filler and the rubber interface, which is higher than the crosslinking degree of other rubber parts. It can be imagined that the white carbon black dispersed in amorphous silicone rubber is closely surrounded by rubber molecules, and the particles of white carbon black play a role of microcrystals. It can be seen that it is of great significance to improve the dispersity of the filler and form uniformly dispersed silica particles in the rubber molecules. Because the higher the activity of the filler particles, the worse the dispersion, the choice of dispersion additives and matching conditions is also very important.
With the increase of the amount of white carbon black, the hardness of vulcanized rubber increases, and the tensile strength and tearing strength of vulcanized rubber within a certain range are increased with the increase of the amount of white carbon black, and the general dosage is about 40 to 60 parts. The appropriate amount of white carbon black is related to its specific surface area, such as the specific surface area of 140~300 meters 2/ g, the amount of white carbon black can be low limit, when the particle size increases the specific surface area of 70~110 meters 2/ g or less, the upper limit or more fillers are desirable.
There are many kinds of semi-reinforcing fillers, such as diatomite, precipitated calcium carbonate, zirconium silicate, titanium dioxide and so on. Some colorants are often used in silicone rubber compound, these colorants should be heat stable, and should not affect the vulcanization process with peroxide, generally with inorganic pigments, such as titanium dioxide, chromium trioxide, iron trioxide, cadmium dioxide and so on. These pigments are also fillers and have a certain reinforcing effect. Red iron oxide can accelerate vulcanization and improve the heat aging performance of vulcanized rubber, the dosage is generally 2 to 5 parts. Ordinary carbon black is not used as a filler in silicone rubber, only acetylene carbon black is used to make conductive rubber products, and in some cases, a small amount of carbon black is used as a black pigment.
3. Other compounding agents
In order to improve some process properties or performance of silicone rubber, some components can be added when preparing the compound. Like the active silanol group on the surface of fumed carbon black particles, it can condense with the end silanol group of silicone rubber molecules at room temperature, so that the silicone rubber rubber material becomes hard during storage, the plasticity is reduced, and gradually loses the return and processing performance, which is called structuring. A compound added to prevent or weaken the tendency to structure is called a structure control agent. The commonly used structural control agents are diphenylsiladiol, methylphenyldiethoxysilane, tetramethylethyldioxy dimethylsilane and low molecular weight hydroxyl silicone oil. The structural formula of tetramethylethylene dioxy dimethylsilane is:
In order to reduce the compression permanent deformation of vulcanized rubber, anti-shrinkage agent mercurous oxide or cadmium oxide can be added, which can form additional bridge structure between rubber macromolecules and reduce the compression permanent deformation. For example, in order to improve the tear strength of vulcanized rubber, a small amount of oily phosphate-cyanogen chloride polymer can be added under the condition of adding hydroxyl silicone oil. The phosphate-cyanide polymer can promote the interaction between silica filler and silicone oil containing hydroxyl groups and silicone rubber polymers, so that the intermolecular force in the vulcanized rubber is strengthened, which can improve the physical and mechanical properties of silicone rubber, especially the tear strength. Adding a small amount of polytetrafluoroethylene into silicone rubber compound can improve the calendering process performance and increase the tear strength of vulcanized rubber. In addition, adding an appropriate amount of n-butyl borate to the mixture of phenyl silicone rubber can overcome the phenomenon of sticky roll and is conducive to operation.