Silicone rubber is generally highly saturated structure, vulcanization activity is low, so it is usually not vulcanized with sulfur, the most common and most commonly used vulcanizing agent is organic peroxide. Silicone rubber can also use high-energy radiation vulcanization, radiation vulcanization and peroxide vulcanization mechanism is the same, both of the free group reaction and crosslinking.
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 universal type has high activity, which can vulcanize all kinds of silicone rubber, such as dibenzoyl peroxide (BP), 2? 4-dichlorodibenzoyl peroxide (DCBP), tert-butyl benzoate peroxide (TBPB), belong 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 and odorless DCP), 2? 5-dimethyl-2? 5-di-tert-butylhexane peroxide (DBPMH) and others belong to 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 DCP and odourless DCP, 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.
Silicone rubber containing vinyl can be vulcanized by addition to 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.
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