1, raw rubber:
Silicone rubber raw rubber types are: dimethyl silicone rubber (referred to as methyl silicone rubber), methyl vinyl silicone rubber (referred to as vinyl silicone rubber), methyl phenyl vinyl silicone rubber (referred to as phenyl silicone rubber) and Fluorosilicon, nitrile silicone rubber, etc. Most of the hot vulcanized silicone rubber is methyl vinyl silicone rubber, especially the type of end of the biopolymer molecular chain end with vinyl dimethyl siloxane (vinyl double head) chain link is the main product type of the market. .

Commercially available vinyl rubber is generally 110 methyl vinyl silicone rubber, and is classified into three types, 110-1, 110-2 and 110-3, depending on the vinyl content. Each model is divided into A and B grades according to the molecular weight.


2, filler:
Adding filler can improve the mechanical properties of silicone rubber, and the amount of filler has a great influence on the electrical properties and mechanical properties of silicone rubber. The filler is generally an inorganic substance, mainly white carbon black, titanium dioxide, quartz powder, kaolin, micronized talc, precipitated calcium carbonate and the like. The reinforcing fillers used in silicone rubber are mainly white carbon black, including gas phase and precipitated silica. Due to the low intermolecular cohesive energy density of silicone rubber, the tensile strength of vulcanizate is only about 0.35 MPa, which has almost no use value. It needs to be reinforced by gas phase method or precipitation method to improve the mechanical properties of vulcanized silicone rubber. With the increase of the amount of silica, the tearing and tensile strength of the vulcanized silicone rubber increased, the hardness gradually increased and then decreased, and the volume resistivity gradually decreased.
The organosilicon compound itself has good insulation, and the addition of the filler affects the molecular chain, so that the resistivity of the vulcanized silicone rubber decreases. However, due to the reinforcing effect of the filler, the mechanical properties of the silicone rubber can be improved, and the hardness of the silicone rubber is improved. However, if the filler is added too much, the crosslinking between the polymer molecular chains is affected, so that the hardness of the silicone rubber is lowered. 

3. Structure control agent
The silicone rubber rubber reinforced with gas phase white hair black will harden during storage and reduce plasticity, thus gradually losing the processing performance. This phenomenon is called "structural" effect. Experiments have shown that the degree of hardening of the compound is related to the specific surface area of the silica and the isolated hydroxyl groups on the surface of the silica.
The reason is that the white carbon black particles have a large specific surface area and a large amount of Si-OH groups on the surface (the cohesive force between the particles is quite strong, and it is easy to agglomerate to make it difficult to disperse in the silicone rubber raw rubber, which is not conducive to reinforcement), and the activity is The Si-OH group and its adsorbed water are condensed with the silanol group and the residual alkoxy group at the end of the raw rubber molecule at room temperature, and some of the active hydroxyl groups can form a hydrogen bond type chemical adsorption with the silicone rubber molecular chain to form a stereoscopic shape. Network structure.
     
A compounding agent added to prevent and attenuate this "structural" tendency is referred to as a "structural control agent." The structure control agent prevents or retards the structuring because the reactive groups of these materials condense with the active hydroxyl groups on the surface of the silica, thereby preventing the interaction between the silica and the green rubber.
Two methods of treating reinforcing filler silica and adding a structuring agent during mixing can be generally employed. However, the cost of treating white carbon black is too high, and most of the methods of treating silica in situ by adding a structured control agent during mixing are now used. The addition of a suitable structured control agent not only solves the processability and storage stability of the silicone rubber compound, but also significantly improves the overall performance of the rubber compound such as tear resistance, elongation, rheology, and the like. In the case of performance, the amount of filler is increased to reduce the cost.
The structure control agent is usually a low molecular organosilicon compound containing Si-OH, Si-NH-Si or a boron-containing atom. Can be summarized into the following four categories:
 (1) Hydroxy-containing silanes The higher the hydroxyl content, the greater the activity and the better the anti-structuring effect;
 (2) alkoxysilanes including various alkoxysilanes and low molecular polysiloxanes;
 (3) Silico-nitrogen (nitrogen) alkanes These structural control agents are relatively efficient, expensive, odorous and require heat treatment to remove them;
 (4) Boron-containing siloxanes Condensed from boric acid and various chlorosilanes, it is characterized by an effective period of up to 6 months.
 
The most common anti-structurant currently used is (terminal) dihydroxy oligodimethylsiloxane, a low molecular weight hydroxy silicone oil. The hydroxyl content is the main performance index. The higher the hydroxyl content, the better the anti-structural property and the less the amount. The use of hydroxy silicone oil as a structural control agent has the advantages of good structural resistance, convenient use, and transparency of the obtained rubber mixture, but has a single function and poor stability (often accompanied by dehydration and shrinkage reaction), and the amount is large, and the glue is easy to be used. The material is sticky, odorous when heated or vulcanized, and the amount used is slightly lower, which will reduce the tear strength, increase the tear permanent deformation and resilience.
In addition to hydroxy silicone oil, diphenylsilyl diol, methylphenyldiethoxysilane, tetramethylethylenedioxydimethylsilane, and silazane (hexamethyldisilazane and hexamethylcyclohexane) A mixture of trisilazanol and octamethylcyclotetrasilazane can also be used as an anti-structurant, wherein diphenyldihydroxysilane can also improve the physical and mechanical properties and heat aging resistance of the rubber compound, especially The permanent deformation at high temperatures is small. The rest of the structure control agent has a single function.
However, diphenylsilyl glycol is a solid crystalline material, and its amount is generally 5 to 10% of the amount of fumed silica. The compound using diphenylsilyl glycol must be subjected to heat treatment (160 to 200 ° C × 0.5 to 1 h) after kneading (before adding the vulcanizing agent) to quickly and fully exert its structural control effect, which is inconvenient to use. Moreover, its products are less transparent than hydroxy silicone oil.
Other structural control agents are liquids, and the rubber can be used without heat treatment (except for silazane, the silazane rubber should also be heat treated to enhance its structural control and make ammonia in the rubber compound). Can be completely evaporated. The heat treatment conditions are generally 180 ~ 200 ° C × lh), after mixing for 2 to 3 days, the plasticity of the rubber is significantly increased. Some structural control agents have delayed vulcanization, and the amount of vulcanizing agent should be appropriately increased when used. The amount of the structure control agent is related to its own activity and the type of silica and the properties required for the compound. When a gas phase white carbon having a large specific surface area is used in the rubber compound, the amount of the structure control agent should be appropriately increased. Or preferably a more active structural control agent such as silazane.
The method of compounding two structural control agents is better than using a certain structure control agent. Such as: hydroxy silicone oil + silazane, diphenyl silicon diol + silazane and so on.
In addition, the special structure control agent used for fluorosilicone rubber should be fluorine-containing low molecular organic silicon compounds, such as hydroxyfluorosilicone oil and fluorine-containing silazane, so as not to make the solvent resistance and oil resistance of the vulcanized rubber. Performance is declining.