The reinforcing filler used in silicone rubber mainly refers to synthetic silicon dioxide, also known as white carbon black. White carbon black can be divided into vapor phase white carbon black and precipitated white carbon black.

a. The size, specific surface area, surface properties and structure of fumed silica particles are related to the ratio of feed gas, combustion rate and residence time of SiO2 nuclei in the combustion chamber.

The finer the fumed silica particles, the larger their specific surface area, the better the reinforcement effect, but the worse the operating performance. On the contrary, its particles are coarser 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 vulcanizate reinforced by fumed silica has high mechanical strength and good electrical properties. Fumed silica can be used together with other reinforcing agents or weak reinforcing agents to produce compounds with different application requirements.


b. Precipitated silica

The properties of precipitated silica are affected by precipitation conditions such as acidity and temperature.

Compared with the silicone rubber compound reinforced with fumed silica, the compound reinforced with precipitated silica has slightly lower mechanical strength, poor dielectric properties, especially after wetting, but better heat aging resistance, and much lower cost of compound. When the mechanical strength of the product is not high, precipitated silica or fumed silica can be used together.

White carbon black can be treated by suitable compounds to produce a surface hydrophobic substance. 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 and the treatment effect is good, but the process is complex and the solvent needs to be recovered; The gas phase treatment process is simple, but the product quality is not stable enough and the treatment effect is poor.

In principle, substances used as surface treatment agents are substances that can interact with hydroxyl groups on the surface of silica, including the following.

1) Alcohols 2) chlorosilane 3) alkoxysilane 4) hexamethyldisilane 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. The filler particles adsorb the polymer, so that the rubber molecular chain segments are directly fixed near the filler particles or oriented along the filler surface or retained by the filler aggregate.

b. The combination of rubber and filler particles the combination of filler particles and polymer segments produces effective cross-linking and polymer entanglement of filler particles.


Based on the above two functions, silica can reinforce silicone rubber.

White carbon black has different acidity and alkalinity due to its different production methods. Fumed silica is acidic and precipitated silica is alkaline. The pH value of the most pure HCl free fumed silica is 6, which is caused by 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 caused by high-temperature hydrolysis of the remaining HCl.

(3) Determination of physical and chemical properties of white carbon black

1. The physical and chemical properties of white carbon black directly reflect the quality, so accurate determination is very important for different use requirements. At present, the indicators of foreign manufacturers are different, but some recognized important indicators should be determined by each manufacturer. The most important indexes are: the indexes reflecting the primary structure, such as particle size, dispersion, and specific surface area; Indicators reflecting secondary structure, such as oil absorption value; Indicators reflecting the surface chemistry, such as the concentration of various hydroxyl groups on the surface.

a. The particle size and particle size distribution are different due to the formation conditions and particle growth, so the particle diameter of white carbon black is not uniform, and the commonly referred to particle diameter only has the meaning of statistical average.


b. Determination of specific surface area specific surface area is an index reflecting the size of the surface area of powder material. For a porous powder material, its specific surface area is the sum of the surface area in the pores and the surface area.


Generally speaking, the particle size of powder material is inversely proportional to its specific surface area, so the determination of specific surface product can qualitatively reflect the particle size of powder. Because the 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.


c. Determination of surface hydroxyl groups there are silanol groups on the surface of silica, and many applications of silica are directly related to this group. Therefore, it is very important to quantitatively determine the surface hydroxyl groups.


The data of surface hydroxyl groups of silica generally include total hydroxyl groups, adjacent hydroxyl groups, isolated hydroxyl groups, etc. 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 bound hydroxyl and the hydroxyl in the water molecules adsorbed on the surface of silica. These hydroxyl data can be determined under different conditions. The determination conditions are:

1) The hydroxyl group directly sampled from the silica bag is the total hydroxyl group;

2) The hydroxyl groups determined after drying the silica at 110 ℃ for 3 hours are bound hydroxyl groups;

3) The hydroxyl groups determined after drying the silica at 600 ℃ for 3 hours are isolated hydroxyl groups;

4) The difference between the binding hydroxyl group and the spacer group is the adjacent hydroxyl group.


d. Determination of secondary structure it is generally considered that the degree of secondary structure directly affects the reinforcement behavior of fillers abroad, so it is also very important to determine the secondary structure. But up to now, 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 second is to measure the oil absorption value.


2. Weak reinforcing filler

Weak reinforcing fillers can also be called inert fillers, which only play a small role in reinforcing silicone rubber. They are generally not used alone in silicone rubber, but act with silica to adjust the hardness of silicone rubber, improve the process performance of rubber and the oil resistance and solubility resistance of vulcanizate, and reduce the cost of rubber.

Commonly used weak reinforcing agents include diatomite, quartz powder, zinc oxide, titanium dioxide, zirconium silicate and calcium carbonate.