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Vulcanizing agent and mechanism of heat vulcanized silicone rubber

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(1) vulcanizing agent

The vulcanizing agents used for thermal vulcanization of silicone rubber mainly include organic peroxides, aliphatic azo compounds, inorganic compounds and high energy radiation, among which organic peroxides are the most commonly used. This is because organic peroxides are generally stable at room temperature, but they can decompose rapidly to produce free radicals at higher vulcanization temperature, which results in cross-linking of silicone rubber.

These peroxides can be divided into two categories according to their activity: one is general type, with high activity, which can vulcanize all kinds of silicone rubber; the other is vinyl-specific type, which can only vulcanize silicone rubber containing vinyl because of its low activity.

(2) vulcanization mechanism
When silicone rubber is vulcanized by peroxide, the crosslinking of silicone rubber by peroxide is carried out by free radical reaction between two activated methyl or vinyl groups.

In addition to the peroxide vulcanization mentioned above, silicone rubber can also be vulcanized by radiation with high-potential radiation. Radiation vulcanization is also carried out according to the free radical mechanism. When the vinyl molar fraction of raw rubber is high (0.01) or used with other rubber, sulfur can also be used to vulcanize, but the performance is very poor.

3. Reinforcement Agents and Related Mechanisms
The strength of silicone rubber vulcanizate without reinforcement is very low, only about 0.3 MPa, which has no practical use value. The strength of silicone rubber vulcanizate can reach 3.9-9.8 MPa by adding proper reinforcing agent, which is very important for improving the properties of silicone rubber and prolonging the service life of products. The selection of reinforcing filler for silicone rubber should take into account the adverse effects of high temperature use of silicone rubber and vulcanization with peroxide (especially acid-base substances) on silicone rubber.

The reinforcing filler used for silicone rubber can be divided into reinforcing filler and weak reinforcing filler according to their reinforcing effect. The former has a particle size of 10-50 nm, a specific surface area of 70-400 m2?G-1, and the latter has a particle size of 300-1000 nm and a specific surface area of less than 30 m2?G-1. The reinforcing effect is poor.

Silicone rubber reinforcing filler mainly refers to the synthesis of silicon dioxide, also known as silica. Silica can be divided into fumed silica and precipitated silica. The fumed silica is one of the most commonly used reinforcing agents for silicone rubber. The vulcanizate reinforced by fumed silica has high mechanical strength and good electrical properties. It can also be used with other reinforcing agents or weak reinforcing agents to prepare the compound with different application requirements.

Compared with the silicone rubber compound reinforced with fumed silica, the silicone rubber compound reinforced with precipitated silica has lower mechanical strength, lower dielectric properties (especially after moisture exposure), better heat aging resistance and lower cost. When the mechanical strength of products is not high, precipitation silica or fumed silica can be used alone.

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