The reinforcing filler used in silicone rubber mainly refers to synthetic silica, also known as white carbon black. White carbon black is divided into gas-phase white carbon black and precipitated white carbon black.
a、 The size, specific surface area, surface properties, and structure of gas-phase silica particles are related to factors such as the proportion of raw gas, combustion rate, and the residence time of SiO2 nuclei in the combustion chamber.
The finer the particles of gas-phase white carbon black, the larger its specific surface area, and the better the reinforcement effect, but the worse the operational performance. On the contrary, if its particles are thicker and have a smaller surface area, the reinforcement effect will be poor, but the operability will be better.
Gas phase silica is one of the most commonly used reinforcing agents for silicone rubber, and the vulcanized rubber reinforced by it has high mechanical strength and good electrical properties. Gas phase white carbon black can be used in combination with other reinforcing agents or weak reinforcing agents to produce rubber materials with different usage requirements.
b、 precipitated silica 
The performance of precipitated silica is influenced by precipitation conditions such as acidity and temperature.
Compared with silicone rubber reinforced with gas-phase white carbon black, the rubber reinforced with precipitated white carbon black has slightly lower mechanical strength, poorer dielectric performance, especially after being damp, but better heat aging performance, and much lower cost of mixed rubber. When the mechanical strength requirements for the product are not high, precipitated white carbon black can be used or combined with gas-phase white carbon black.
White carbon black can be treated with suitable compounds to create a surface hydrophobic substance. There are two main methods for processing: liquid phase method and gas phase method. The conditions of 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.
The substances used as surface treatment agents are generally substances that can interact with the hydroxyl groups on the surface of white carbon black, and there are several types.
1) Alcohols 2) Chlorosilanes 3) Alkoxysilanes 4) Hexamethyldisilane 5) Silazanes.
(2) The reinforcement mechanism of white carbon black and the reinforcement mechanism of surface chemical white carbon black on silicone rubber are considered to have the following two types.
a. Rubber is adsorbed by filler particles to adsorb polymers, causing the rubber molecular chain segments to be directly fixed near the filler particles or oriented along the surface of the filler or retained by the filler aggregates.
b. The combination of rubber and filler particles results in effective crosslinking and polymer entanglement of filler particles with polymer chain segments.
Based on the above two effects, make white carbon black have a reinforcing effect on silicone rubber.
Due to different production methods, the acidity and alkalinity of white carbon black vary. Gaseous silica is acidic, while precipitated silica is alkaline. The pH value of the purest HCL free gas-phase white carbon black is 6, which is due to the dissociation of hydroxyl groups on the surface of white carbon black in water to produce H+. When the pH value is below 4.6, it is caused by the residual HCL from high-temperature hydrolysis.
(3) Method for determining the physical and chemical properties of white carbon black
1. The physical and chemical properties of white carbon black directly reflect the quality situation, so accurate measurement is crucial for different usage requirements. At present, the indicators of various foreign manufacturers are not the same, but it is recognized that some important indicators need to be measured by each company. The most important indicators are: indicators that reflect its primary structure, such as particle size and dispersion, specific surface area; Indicators that reflect its secondary structure, such as oil absorption value; Indicators that reflect the surface chemical condition, such as the concentration of various hydroxyl groups on the surface.
a、 Due to differences in generation conditions and particle growth, the particle size and distribution of white carbon black are not uniform. The commonly referred particle diameter only has a statistical mean.
b、 The measurement of specific surface area is an indicator that reflects the size of the outer surface area of a powder material. For a porous powder material, its specific surface area is the sum of the surface area inside the pores and the outer surface area.
Generally speaking, the particle size of powder material is inversely proportional to its specific surface area, so the determination of specific surface area can qualitatively reflect the particle size of powder. Due to the fact that electron microscopes are not available in all industrial units, the particle size of powders cannot be obtained. Therefore, the determination of specific surface area has important practical application value.
c、 The determination of surface hydroxyl groups is crucial as there are silanol groups on the surface of silica, and many applications of silica are directly related to these groups.
The data for determining the hydroxyl groups on the surface of white carbon black generally includes 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 groups; The total hydroxyl group is the sum of hydroxyl groups in the water molecules adsorbed on the surface of silica, which can be measured under different conditions. The measurement conditions are:
1) The hydroxyl groups measured directly from the white carbon black bag are the total hydroxyl groups;
2) The hydroxyl group determined after drying the white carbon black at 110 ℃ for 3 hours is the bound hydroxyl group;
3) The hydroxyl group measured after drying the white carbon black at 600 ℃ for 3 hours is the isolated hydroxyl group;
4) The difference between the binding hydroxyl group and the isolating group is the adjacent hydroxyl group.
d、 It is generally believed in foreign countries that the degree of secondary structure directly affects the reinforcement behavior of fillers, so measuring secondary structure is also important. But so far, there is no good measurement method, and there are currently two most widely used methods: one is to measure the apparent specific volume under compression; The second is to measure the oil absorption value.
2. Weak reinforcement filler
Weak reinforcing fillers, also known as inert fillers, only have a small reinforcing effect on silicone rubber. They are generally not used alone in silicone rubber, but interact with white carbon black to regulate the hardness of silicone rubber, improve the process performance of rubber materials, and the oil and solution resistance of vulcanized rubber, reducing the cost of rubber materials.
Common weak reinforcing agents include diatomaceous earth, quartz powder, zinc oxide, titanium dioxide, zirconium silicate, and calcium carbonate.