The structuralization of silicone rubber is a common problem. Silica results in the structuring of silicone rubber, primarily based on the massive hydrogen bonding of silica and high molecular weight polysiloxanes. The reinforcing filler for silicone rubber can be divided into reinforcing filler and non-reinforcing filler according to the reinforcing effect. The former has a diameter of 10~50nm and a specific surface area of 70~400m2/g. The effect is better; the latter is usually 300~10000nm, the specific surface area is below 30m2/g, and the reinforcing effect is poor.
The reinforcing filler used in silicone rubber mainly refers to synthetic silica, also known as white carbon black. The white carbon black is divided into gas phase white carbon and precipitated white carbon black.

Vapor white carbon black
The size, specific surface area, surface properties, structure, etc. of the gas phase white carbon black particles are related to the ratio of the raw material gas, the burning speed, and the residence time of the SiO2 core in the combustion chamber.
The finer the gas phase white carbon black particles, the larger the specific surface area, the better the reinforcing effect, but the worse the handling performance. On the contrary, its particles are coarser, the specific surface area is smaller, and the reinforcing effect is worse, but the operability is better.
Gas phase white carbon black is one of the most commonly used reinforcing agents for silicone rubber. The rubber compound which is reinforced by it has high mechanical strength and good electrical properties. The fumed silica can be used in combination with other reinforcing agents or weak reinforcing agents to prepare the compound for different use requirements.

Precipitated silica
Compared with the silicone rubber compound reinforced with fumed silica, the rubber reinforced with precipitated silica is slightly lower in mechanical strength, and the dielectric properties, especially the dielectric properties after damp, are poor, but heat aging The performance is better and the cost of the rubber compound is much lower. When the mechanical strength of the article is not critical, precipitated silica may be used or used in combination with fumed silica. The performance of precipitated silica is affected by precipitation conditions such as acidity, temperature, and the like.
Silica can be treated with a suitable compound to form a surface hydrophobic material. The treatment methods mainly include liquid phase method and gas phase method. The conditions of the liquid phase method are easy to control, the product quality is stable, the treatment effect is good, but the process is complicated, the solvent needs to be recovered; the gas phase treatment process is simple, but the quality of the product is not stable enough, and the treatment effect is poor.
Surface treatment agent
The substance used as the surface treatment agent can in principle interact with the hydroxyl group on the surface of the silica, and there are the following.
1) Alcohol 2) Chlorosilane 3) Alkoxysilane 4) Hexamethyldisiloxane 5) Silazane.

The reinforcing mechanism of silica to silicone rubber is considered to be the following two.
a. The rubber is adsorbed by the filler particles to adsorb the polymer, so that the rubber molecular segment is directly fixed in the vicinity of the filler particles or oriented along the surface of the filler or retained by the filler aggregate.
b. Rubber and filler particle bonding The filler particles combine with the polymer segment to produce effective crosslinking and the polymer entangles the filler particles.
Based on the above two effects, the silica is reinforced by the silicone rubber.
Silica is different in acidity and alkalinity due to its different production methods. The fumed silica is acidic and the precipitated silica is alkaline. The purest HCl-free fumed silica has a pH of 6, which is due to the H+ generated by the dissociation of the hydroxyl groups on the surface of the silica. When the pH is lower than 4.6, it is caused by HCl remaining in high temperature hydrolysis.
Method for determining physical and chemical properties of silica
The physicochemical properties of silica directly reflect the quality, so it is important to accurately measure different usage requirements. At present, the indicators of foreign manufacturers are not the same, but some recognized important indicators must be measured.
The most important indicators are: indicators that reflect their primary structure, such as particle size and dispersion, specific surface area; indicators that reflect their secondary structure, such as oil absorption values; indicators that reflect their surface chemistry, such as various hydroxyl groups on the surface The concentration and so on.
1. Particle size and particle size distribution There are differences in the conditions of formation and particle growth. Therefore, the particle diameter of silica is not uniform. The particle diameter is usually only statistically significant.

2. Determination of Specific Surface Area The specific surface area is an index reflecting the size of the external surface area of the powder material. For a porous powder material, the specific surface area is the sum of the surface area and the external surface area within the pore. In general, the particle size of the powder material is inversely proportional to its specific surface area, so the measurement of the specific surface product can qualitatively reflect the particle size of the powder. Since the electron microscope is not available in all industrial units, the particle size of the powder cannot be obtained, and thus the measurement of the specific surface area has important practical application value.

3. Determination of surface hydroxyl groups There are silanol groups on the surface of silica, and many applications of silica are directly related to such groups. Therefore, it is important to quantitatively determine the surface hydroxyl groups. The data of the hydroxyl group on the surface of silica often includes total hydroxyl groups, adjacent hydroxyl groups, and isolated hydroxyl groups. The latter two are bound to the surface of silica in the form of Si-OH, collectively referred to as bound hydroxyl; the total hydroxyl is the sum of the hydroxyl groups bound to the hydroxyl molecules adsorbed on the surface of the silica, these hydroxyl groups The data can be determined under different conditions. The measurement conditions are:
1) The hydroxyl group measured by direct sampling in the white carbon black bag is the total hydroxyl amount;
2) The hydroxyl group measured after drying the silica at 110 ° C for 3 hours is a binding hydroxyl group;
3) The hydroxyl group measured after drying the silica at 600 ° C for 3 hours is an isolated hydroxyl group;
4) The difference between the combined hydroxyl group and the spacer is an adjacent hydroxyl group.
Determination of secondary structure It is generally believed that the degree of secondary structure directly affects the reinforcing behavior of the filler, so it is also important to determine the secondary structure. However, there is still no good measurement method. At present, there are two widely used methods: one is to determine the apparent specific volume under compression; the other is to determine the oil absorption value.

In addition to silica, there is also a class of weak reinforcing fillers, also known as inert fillers, which have only a small reinforcing effect on silicone rubber. They are generally not used alone in silicone rubber, but rather in silica. The function is to adjust the hardness of the silicone rubber, improve the process performance of the rubber compound and the oil resistance and solvent resistance of the vulcanized rubber, and reduce the cost of the rubber compound. Commonly used weak reinforcing agents are diatomaceous earth, quartz powder, zinc oxide, titanium dioxide, zirconium silicate and calcium carbonate.