The vulcanization reaction of peroxide high-temperature vulcanized silicone rubber proceeds according to the free radical reaction mechanism. Under heating conditions, organic peroxides decompose to produce free radicals, which then react with organic groups in polysiloxane molecules such as methyl and ethyl groups to form high-molecular free radicals. When these high-molecular free radicals couple with each other, cross-linking occurs.
There are six commonly used peroxides in high-temperature vulcanized silicone rubber. The vulcanization conditions and applicable scopes of different organic peroxides are not the same.
Users can choose the suitable compounds according to their needs. For instance, the low-temperature decomposition of peroxides such as di peroxide (2, 4-dichlorobenzene) can provide a relatively fast vulcanization rate, and the vapor pressure of its decomposed compounds is very low, so no additional pressure is required to prevent the formation of pores. Therefore, this compound is often used in hot air vulcanization extrusion and molding processes. However, due to the relatively fast vulcanization speed, charring is prone to occur when casting thinner products or transferring molds. In such cases, peroxides with a slower vulcanization speed, such as benzoyl peroxide, are suitable.
However, this compound will produce gas during the vulcanization process, so it cannot be vulcanized with hot air. Instead, continuous vulcanization with steam is commonly used. Another drawback of the above-mentioned low-temperature vulcanizing agents is that when casting thinner products, their acidic decomposition products will have a negative impact on the thermal stability of the products.
Although high-temperature vulcanizing agents such as diisopropylbenzene peroxide and di-tert-butyl peroxide are only suitable for highly reactive methyl vinyl raw rubber, they are the most ideal vulcanizing agents for thick products and can also be used in carbon black-containing compound rubber