The reinforcing filler used in silicone rubber mainly refers to synthetic silica, also known as white carbon black. Silica is divided into fumed silica and precipitated silica.
a. The size, specific surface area, surface properties, structure, etc. of fumed silica particles are related to factors such as the ratio of raw gas, burning speed, and residence time of SiO2 nuclei in the combustion chamber.
The finer the particles of fumed silica, the larger its specific surface area, the better the reinforcing effect, but the worse the handling performance. On the contrary, its particles are thicker, the specific surface area is also smaller, and the reinforcing effect is poor, but the operability is better.
Fumed silica is one of the most commonly used reinforcing agents for silicone rubber. The vulcanized rubber reinforced by it has high mechanical strength and good electrical properties. Fumed silica can also be used together with other reinforcing agents or weak reinforcing agents to prepare rubber materials with different requirements.
b. Precipitated silica
The performance of precipitated silica is affected by precipitation conditions such as acidity, temperature, etc.
Compared with the silicone rubber compound reinforced with fumed silica, the mechanical strength of the compound reinforced with precipitated silica is slightly lower, and the dielectric properties, especially the dielectric properties after being exposed to moisture, are poor, but heat aging resistance The performance is better, and the cost of the compound is much lower. When the mechanical strength of the product is not high, precipitated silica can be used or used in combination with fumed silica.
Silica can be made a surface hydrophobic material by treating it with suitable compounds. 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 complicated and the solvent needs to be recovered; the gas phase method has a simple treatment process, but the quality of the product 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 the hydroxyl groups on the surface of silica, and there are the following types:
1) Alcohols 2) Chlorosilanes 3) Alkoxysilanes 4) Hexamethyldisiloxane 5) Silazanes.
(2) The reinforcement mechanism of silica and surface chemistry The reinforcement mechanism of silica to silicone rubber is considered to have the following two types:
a. The rubber is adsorbed by the filler particles. The filler particles adsorb the polymer, so that the rubber molecular segments are directly fixed near the filler particles or oriented along the filler surface or retained by filler aggregates.
b. Combination of rubber and filler particles The filler particles combine with polymer segments to create effective crosslinking and polymer entanglement of the filler particles.
Based on the above two effects, the silica can reinforce the silicone rubber.
Due to the different production methods of white carbon black, its acidity and alkalinity are different. Fumed silica is acidic, and precipitated silica is alkaline. The pH value of the purest HCL-free fumed silica is 6, which is due to the dissociation of hydroxyl groups on the surface of silica in water to generate H+. When the pH value is lower than 4.6, it is caused by the remaining HCL from high temperature hydrolysis.
(3) Determination of physical and chemical properties of white carbon black:
1. The physical and chemical properties of silica directly reflect the quality, so accurate determination is very important for different use requirements. At present, the indicators of foreign manufacturers are not the same, but some important indicators recognized by each manufacturer must 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, etc.; indicators reflecting its surface chemistry, such as various hydroxyl groups on the surface concentration etc.
a. Particle size and particle size distribution are different due to the production conditions and particle growth conditions, so the particle diameter of white carbon black is not uniform, and the particle diameter usually referred to only has the meaning of statistical average.
b. Measurement of specific surface area The specific surface area is an index reflecting the size of the external 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 area.
Generally speaking, the particle size of a 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 electron microscopes are 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, it is very important to quantitatively measure surface hydroxyl groups.
The 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; the total hydroxyl group is the sum of bound hydroxyl groups and hydroxyl groups in water molecules adsorbed on the surface of silica. The data can be measured under different conditions respectively. The measurement conditions are:
1) The hydroxyl group measured directly from the silica bag is the total hydroxyl group;
2) The hydroxyl group measured after drying the white carbon black at 110°C for 3 hours is the bound hydroxyl group;
3) The hydroxyl group determined after drying the white carbon black at 600°C for 3 hours is an 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 Foreign countries generally believe that the degree of secondary structure directly affects the reinforcing behavior of fillers, so the determination of secondary structure is also very important. But so far there is no good measurement method, and there are two most widely used methods: 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 have a small reinforcing effect on silicone rubber. They are generally not used alone in silicone rubber, but act with white carbon black to adjust the hardness of silicone rubber and improve the hardness of silicone rubber. The process performance of the rubber compound and the oil resistance and solvent resistance of the vulcanized rubber reduce the cost of the rubber compound.
Commonly used weak reinforcing agents include diatomaceous earth, quartz powder, zinc oxide, titanium dioxide, zirconium silicate and calcium carbonate.