The reinforcing filler used for silicone rubber mainly refers to the synthetic silicon dioxide, also known as white carbon black. Silica is divided into gaseous silica and precipitated silica.

a. The size, specific surface area, surface property and structure of fumed silica particles are related to the proportion of raw gas, combustion rate, residence time of SiO2 core in the combustion chamber and other factors.

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

Vapor silica is one of the most commonly used reinforcing agents for silicone rubber. The vulcanizate reinforced by it has high mechanical strength and good electrical properties. Vapor phase white carbon black can be used together with other reinforcing agents or weak reinforcing agents to prepare rubber materials with different use requirements.

b. Precipitated white carbon black

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

Compared with silicone rubber compound reinforced with fumed silica, the mechanical strength of the compound reinforced with precipitated silica is slightly lower, and the dielectric property, especially after being damped, is poor, but the heat aging property is better, and the cost of the compound is much lower. When the mechanical strength of the product is not required, precipitated white carbon black can be used or mixed with fumed white carbon black.

Silica can be treated with appropriate 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 process is simple, but the product quality is not stable enough and the treatment effect is poor.

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

1) Alcohols 2) chlorosilanes 3) alkoxysilane 4) hexamethyldisilane 5) silazane.


(2) The reinforcing mechanism of white carbon black and the reinforcing mechanism of surface chemical white carbon black 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 chain segments are directly fixed near the filler particles or oriented along the filler surface or detained by the filler aggregates.


b. The combination of rubber and filler particles and the combination of filler particles and polymer chain segments produce effective crosslinking and the polymer entangles filler particles.

Based on the above two functions, white carbon black plays a reinforcing role in silicone rubber.


Due to different production methods, the acid and alkali of white carbon black are different. The fumed silica is acidic, and the 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 HCL retained by high-temperature hydrolysis.

(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 need to be measured. The most important indicators are: indicators reflecting its primary structure, such as particle size and dispersion, specific surface area; Indicators reflecting its 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 differences in generation conditions and particle growth, so the particle diameter of white carbon black is not uniform. The particle diameter commonly referred to is only statistically average.


b. Determination of specific surface area Specific surface area is an index reflecting the size of the external surface area of the powder material. For a porous powder material, its specific surface area is the sum of the surface area in the pores and the external 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 cannot be obtained, so the measurement of specific surface area has important practical application value.

c. The determination of surface hydroxyl groups There are silanol groups on the surface of white carbon black, and many applications of white carbon black are directly related to this group. Therefore, it is very important to quantitatively determine the surface hydroxyl groups.


The data of hydroxyl groups on the surface of silica generally include total hydroxyl groups, adjacent hydroxyl groups, isolated hydroxyl groups, etc. The latter two are combined on the surface of white carbon black in the form of Si-OH, which are collectively referred to as combined hydroxyl groups; The total hydroxyl group is the sum of the bound hydroxyl group and the hydroxyl group in the water molecule adsorbed on the surface of white carbon black. The data of these hydroxyl groups can be determined under different conditions. The measurement conditions are:


1) The hydroxyl measured directly from the silica bag is the total hydroxyl content;

2) The hydroxyl determined after baking white carbon black at 110 ℃ for 3 hours is the bound hydroxyl;

3) The hydroxyl determined after drying white carbon black at 600 ℃ for 3 hours is isolated hydroxyl;

4) The difference between bound hydroxyl and spacer is adjacent hydroxyl.


d. Determination of secondary structure It is generally believed abroad that the degree of secondary structure directly affects the reinforcement behavior of filler, so it is also important to determine the secondary structure. However, there is not a good method to determine it. 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 white carbon black to adjust the hardness of silicone rubber, improve the processing performance of rubber compound, oil resistance and solubility resistance of vulcanizate, and reduce the cost of rubber compound.


Common weak reinforcing agents include diatomite, quartz powder, zinc oxide, titanium dioxide, zirconium silicate, calcium carbonate, etc.