3） Reinforcing agent

The strength of silicone rubber vulcanizate without reinforcement is very low, only about 0.3MPa, which has no practical value. The strength of silicone rubber vulcanizate can reach 3.9 ~ 9.8MPa by using proper reinforcing agent, which is very important to improve the properties of silicone rubber and prolong the service life of products. The selection of silicone rubber reinforcing filler should take into account the high temperature use of silicone rubber and peroxide curing, especially the adverse effects of acid-base substances on silicone rubber.

Reinforcing fillers for silicone rubber can be divided into reinforcing fillers and non reinforcing fillers according to their reinforcing effects. The former has a diameter of 10-50nm and a specific surface area of 70-400m2 / g, and the latter has a diameter of 300-10000nm and a specific surface area of less than 30m2 / g.

1 reinforcing filler
(1) The types and characteristics of silica, silicone rubber used as reinforcing filler mainly refers to the synthesis of silica, also known as silica. The precipitates are silica and silica.
The size, specific surface area, surface properties and structure of fumed silica particles are related to the ratio of raw gas, combustion speed and residence time of SiO2 core in the combustion chamber.

The finer the fumed silica particles are, the larger the specific surface area is, the better the reinforcement effect is, but the worse the operation performance is. On the contrary, its particles are thicker and its specific surface area is smaller, so the reinforcement effect is poor, but the operability is better.

Fumed silica is one of the most commonly used reinforcing agents for silicone rubber. The vulcanizates reinforced by fumed silica have high mechanical strength and good electrical properties. Fumed silica can be used together with other reinforcing agents or weak reinforcing agents to prepare compounds with different requirements.

b. Precipitated silica
The properties of precipitated silica are affected by the precipitation conditions such as acidity and temperature.
Compared with the silicone rubber compound reinforced with fumed silica, the mechanical strength of the compound reinforced with precipitated silica is slightly lower, the dielectric properties, especially the dielectric properties after damp, are poor, but the heat aging resistance is better, and the cost of the compound is much lower. When the mechanical strength of products is not high, precipitated silica or fumed silica can be used.

Silica can be treated with suitable compounds to make a hydrophobic material. There are two kinds of treatment methods: liquid phase method and gas phase method. The conditions of liquid phase method are easy to control, the product quality is stable, and the treatment effect is good, but the process is complex, and the solvent needs to be recovered; the gas phase method is simple, but the product quality is not stable, and the treatment effect is poor.

In principle, the substances used as surface treatment agents are those that can interact with hydroxyl groups on the surface of silica, including the following.
1) Alcohols 2) chlorosilanes 3) alkoxysilanes 4) hexamethyldisiloether 5) silazane.
(2) The reinforcing mechanism of silica and the reinforcing mechanism of surface chemical silica on silicone rubber are considered as follows.

a. The rubber is adsorbed by the filler particles, and the filler particles adsorb the polymer, so that the rubber molecular segments are directly fixed near the filler particles or are oriented along the filler surface or detained by the filler aggregates.
b. Rubber and filler particles combine, filler particles combine with polymer segments to produce effective crosslinking and polymer entanglement of filler particles.

Based on the above two effects, silica can play a reinforcing role in silicone rubber.

Because of the different production methods, the acid and alkali of silica are different. Fumed silica is acidic and precipitated silica is alkaline. The pH value of the most pure HCl free fumed silica is 6, which is due to the dissociation of hydroxyl groups on the surface of silica in water to produce H +. When the pH value is lower than 4.6, it is due to the high temperature hydrolysis of retained HCl.
(3) Determination of physical and chemical properties of silica
The physical and chemical properties of silica directly reflect the quality situation, so it is very important for different requirements to accurately determine. At present, the indicators of foreign manufacturers are not the same, but some of the recognized important indicators need to be measured. The most important indexes are: the indexes reflecting the primary structure, such as particle size, dispersion and specific surface area; the indexes reflecting the secondary structure, such as oil absorption value; the indexes reflecting the surface chemistry, such as the concentration of various hydroxyl groups on the surface.

A particle size and particle size distribution due to the difference of formation conditions and particle growth, the particle diameter of white carbon black is not uniform. Generally speaking, the particle diameter only has the meaning of statistical average.

B specific surface area is an index reflecting the surface area of powder material. For a kind of porous powder material, the specific surface area is the sum of the surface area in the pores and the surface area.

Generally speaking, the particle size of powder is inversely proportional to its specific surface area, so the measurement of specific surface product can qualitatively reflect the particle size of powder. Because electron microscope is not available in all industrial units, the particle size of powder can not be obtained, so the determination of specific surface area has important practical value.

There are silanol groups on the surface of silica, and many applications of silica are directly related to this group. Therefore, quantitative determination of surface hydroxyl groups is very important.

The data of hydroxyl groups on the surface of silica generally include total hydroxyl groups, adjacent hydroxyl groups and isolated hydroxyl groups. The latter two are combined on the surface of silica in the form of Si Oh, collectively referred to as bound hydroxyl; the total hydroxyl is the sum of bound hydroxyl and hydroxyl in water molecules adsorbed on the surface of silica, and these hydroxyl data can be determined under different conditions. The determination conditions were as follows

1) The total hydroxyl content was determined by direct sampling from silica bag;
2) After drying the silica at 110 ℃ for 3 hours, the hydroxyl determined is the binding hydroxyl;
3) After drying the silica at 600 ℃ for 3 hours, the hydroxyl determined was isolated hydroxyll；
4) The difference between the binding hydroxyl group and the spacer group is the adjacent hydroxyl group.

It is generally believed that the degree of secondary structure directly affects the reinforcement behavior of fillers abroad, so the determination of secondary structure is also very important. But so far, there is no good method for determination. At present, there are two most widely used methods: one is to determine the apparent specific volume under compression; the other is to determine the oil absorption value.
2 weak reinforcing filler
Weak reinforcing fillers, also known as inert fillers, only play a very small reinforcing role in silicone rubber. They are generally not used alone in silicone rubber, but interact with silica to adjust the hardness of silicone rubber, improve the process performance of rubber compound, oil resistance and solution resistance of vulcanizate, and reduce the cost of rubber compound.

The commonly used weak reinforcing agents are diatomite, quartz powder, zinc oxide, titanium dioxide, zirconium silicate and calcium carbonate.