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.