Compared with general purpose rubber, all three types of silicone rubber have relatively simple compounding components, and the same is true for heat vulcanization type. In addition to raw rubber, the compounding agent mainly includes reinforcing agent, curing agent and some special additives. Generally, only 5-6 components are needed to form a practical formula. Silicone rubber formula design should consider the following points.

(1) Silicone rubber is a kind of raw rubber with high saturation. It can not be cured with sulfur, but heat cured. Thermal vulcanization uses organic peroxide as curing agent, so the compound must not contain active substances that can react with the decomposition products of peroxide, otherwise it will affect the vulcanization.

(2) Silicone rubber products are generally used at high temperature, and its mixture should be stable at high temperature. Therefore, inorganic oxide is usually used as reinforcing agent.

(3) Silicone rubber is easy to cause the cleavage and rearrangement of siloxane bonds under the action of a small amount of polar chemical reagents such as acid or alkali, which leads to the decrease of heat resistance of silicone rubber. Therefore, the acid-base properties of silicone rubber and the acid properties of peroxide decomposition products must be considered when selecting the compounding agents, so as not to affect the properties of vulcanizates.

（1） Selection of raw rubber

When designing the formula, raw rubber with different characteristics should be selected according to the product performance and service conditions. For general silicone rubber products, vinyl silicone rubber can be used when the service temperature is between - 70 ℃ and 250 ℃; low phenyl silicone rubber can be used when the service temperature of the products is between - 90 ℃ and 300 ℃; fluorosilicone rubber should be used when the products are required to be resistant to high and low temperature, fuel or solvent.

（2） Vulcanizing agent
The main curing agents used for heat vulcanized silicone rubber are organic peroxide, aliphatic azo compounds, inorganic compounds, high-energy rays, etc. This is because organic peroxides are generally stable at room temperature, but they can rapidly decompose to produce free radicals at higher vulcanization temperature, resulting in cross-linking of silicone rubber.

Peroxides can be divided into two types according to their activity. One is general type, which has high activity and can vulcanize all kinds of silicone rubber; the other is vinyl type, which has low activity and can only vulcanize silicone rubber containing vinyl.

In addition to the above general differences between the two types of peroxides, each peroxide has its own characteristics. Curing agent BP is the most commonly used curing agent for molded products. It has fast curing speed and high production efficiency, but it is not suitable for the production of thick products. The vulcanizing agent dCBP is suitable for hot air continuous vulcanization of extruded products because its product is not easy to volatilize and bubbles will be produced without pressure during vulcanization. However, it has low decomposition temperature, easy to cause scorch and short storage time. Vulcanizing agents BP and dCBP are crystalline powder, explosive, for safe operation and suitable for dispersion, they are usually dispersed in silicone oil or silicone rubber paste, the general content is 50%. The boiling point of the curing agent is 110 ℃, which is very volatile. When the compound is stored at room temperature, the curing agent will volatilize, and it is best to use molecular sieve as the carrier. The curing agent DTBP does not react with air or carbon black, so it can be used in the manufacture of conductive rubber and products with difficult molding operation. The curing agent dbpmh is similar to DTBP, but it does not volatilize at room temperature. The decomposition products of dbpmh are very volatile, which can shorten the second stage curing time. The curing agent DCP is non-volatile at room temperature, and has the characteristics of vinyl special type. At the same time, the volatile decomposition product is also low, so it can be used for curing under low external pressure. TbpB is used to make sponge products.

The amount of peroxide is affected by many factors. For example, raw rubber varieties, filler types and dosage, processing technology, etc. Generally speaking, as long as the required crosslinking can be achieved, the curing agent should be as little as possible. However, the actual dosage is much higher than the theoretical dosage, because the influence of various processing factors must be considered, such as uneven mixing, peroxide loss during storage, resistance of air and other compounding agents during vulcanization, etc. For various vulcanizates, the weight range of curing agent is as follows: tbbp2 ～ dbbp1 ～ 0.5 phr; tbbp1 ～ 0.5 phr. With the increase of vinyl content, the amount of peroxide should be reduced. The amount of peroxides in glue, extrudate and adhesive compound should be higher than that in molding compound. In some cases, the combination of two peroxides can reduce the amount of curing agent, reduce the curing temperature and improve the curing effect.