What are the structural characteristics of silica?（2）
Determination method of physical and chemical properties of silica
1. The physical and chemical properties of silica directly reflect the quality, so accurate measurement is very important for different usage requirements. At present, the indicators of foreign manufacturers are not the same, but some recognized important indicators must be measured by each company. 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, etc.; indicators reflecting its surface chemistry, such as various hydroxyl groups on the surface concentration, etc.

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

b. 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 in the pores and the external surface area.

Generally speaking, the particle size of the powder material is inversely proportional to its specific surface area, so the measurement of the specific surface area 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, so the determination of the specific surface area has important practical application 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, quantitative determination of surface hydroxyl groups is very important.

The predetermined data of the 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 silica in the form of Si-OH, which are collectively referred to as bound hydroxyl groups; total hydroxyl groups are the sum of bound hydroxyl groups and hydroxyl groups in water molecules adsorbed on the surface of silica. Data can be measured separately under different conditions. The measurement conditions are:

1) The hydroxyl group measured by direct sampling in the silica bag is the total amount of hydroxyl groups;

2) The hydroxyl group measured after drying the silica at 110°C for 3 hours is the bound hydroxyl group;

3) The hydroxyl group measured after drying the silica at 600°C for 3 hours is the isolated hydroxyl group;

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 believed that the degree of secondary structure directly affects the reinforcing behavior of fillers, so it is also very important to measure the secondary structure. But so far there is no good measurement method. There are two methods that are most widely used at present: one is to measure the apparent specific volume under compression; the other 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 reinforcing effect on silicone rubber. They are generally not used alone in silicone rubber, but act with silica to adjust the hardness of silicone rubber and improve The process performance of the rubber compound and the oil resistance and solvent resistance of the vulcanizate reduce the cost of the rubber compound.

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