The structuring of silicone rubber is a common problem. Silica causes structure of silicone rubber, which is mainly based on the large amount of hydrogen bonding between silica and high molecular weight polysiloxane. Reinforcing fillers for silicone rubber can be divided into reinforcing fillers and non-reinforcing fillers according to their reinforcing effect. The former has a diameter of 10 to 50 nm and a specific surface area of 70 to 400 m2 / g. The effect is better; the latter is usually 300 ~ 10000nm, the specific surface area is below 30m2 / g, and the reinforcement effect is poor.

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.

Fumed silica
The size, specific surface area, surface properties, structure, etc. of the fumed silica particles are related to the ratio of the raw material gas, the combustion speed, and the residence time of the SiO2 core in the combustion chamber.

The finer the fumed silica particles, the larger their specific surface area, the better the reinforcement effect, but the worse the performance. On the other hand, its particles are coarser and its specific surface area is smaller, so the reinforcing effect is poor, but the operability is better.

Vapor white carbon is one of the most commonly used reinforcing agents for silicone rubber. The rubber reinforced by it has high mechanical strength and good electrical properties after vulcanization. Gaseous white carbon black can be used in combination with other reinforcing agents or weak reinforcing agents to prepare rubbers with different requirements.

Precipitated silica
Compared with silicon rubber compound reinforced with fumed silica, the mechanical strength of the compound reinforced with precipitated white carbon is slightly lower, and the dielectric properties, especially the dielectric properties after moisture, are poor, but the heat-resistant aging 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 or fumed silica can be used together. The performance of precipitated silica is affected by precipitation conditions such as acidity, temperature, and the like.

White carbon black can be treated with suitable compounds to make it a surface hydrophobic substance. There are two main treatment methods: liquid phase and gas phase. 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 is simple, but the product quality is not stable enough, and the treatment effect is poor.

Surface treatment agent
In principle, the substances used as surface treatment agents can interact with the hydroxyl groups on the surface of silica, and there are the following.

1) Alcohols 2) Chlorosilanes 3) Alkoxysilane 4) Hexamethyldisilazide 5) Silazane.

The reinforcement mechanism of silica by silica 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 segment is directly fixed near the filler particles or oriented along the filler surface or retained by the filler aggregates.

b. Combination of rubber and filler particles. The combination of filler particles and polymer segments results in effective cross-linking and the polymer entangles the filler particles.

Based on the two effects mentioned above, silica is reinforced by silica.

Due to different production methods of white carbon black, its acidity and alkalinity are different. Gaseous silica is acidic and precipitated silica is alkaline. The pH value of the purest HCl-free gas phase silica is 6, which is due to the dissociation of hydroxyl groups on the surface of the silica to generate H +. When the pH value is lower than 4.6, it is caused by the remaining HCl at high temperature hydrolysis.

Method for measuring the physical and chemical properties of white carbon black
The physical and chemical properties of white carbon directly reflect the quality situation, so it is very important to accurately measure the different use requirements. At present, the indicators of foreign manufacturers are not the same, but some of the important indicators that are recognized must be measured.

The most important indicators are: indicators that reflect its primary structure, such as particle size and dispersion, and specific surface area; indicators that reflect its secondary structure, such as oil absorption values; indicators that reflect its surface chemical conditions, such as various hydroxyl groups on the surface Concentration.

1. The particle size and particle size distribution are different due to the generation conditions and particle growth. Therefore, the particle diameter of silica is not uniform. The particle diameter usually referred to only has statistical significance.

2. Determination of specific surface area The specific surface area is an index reflecting the external surface area of the powder material. For a porous material, the specific surface area is the sum of the surface area in the pores and the external surface area. In general, the particle size of a powder material has an inverse relationship with its specific surface area, so the measurement of 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 measurement of specific surface area has important practical application value.

3. Determination of surface hydroxyl groups Silanol groups exist on the surface of silica, and many applications of silica are directly related to this group. Therefore, it is very important to determine the surface hydroxyl groups quantitatively. The fixed data of hydroxyl groups on the surface of white carbon black generally include total hydroxyl groups, adjacent hydroxyl groups, and isolated hydroxyl groups. The latter two are bonded to the surface of silica in the form of Si-OH, collectively referred to as bonded hydroxyl groups; the total hydroxyl group is the sum of the combined hydroxyl groups and the hydroxyl groups in the 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 white carbon bag is the total hydroxyl amount;
2) The hydroxyl group measured after drying white carbon black at 110 ° C for 3 hours is a bound hydroxyl group;
3) The hydroxyl group measured after drying white carbon black at 600 ° C for 3 hours is a isolated hydroxyl group;
4) The difference between the bound hydroxyl group and the spacer is the adjacent hydroxyl group.

Measurement of the secondary structure It is generally believed that the degree of the secondary structure directly affects the reinforcing behavior of the filler, so it is also important to determine the secondary structure. But so far there is not a good measurement method. At present, there are two most 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 white carbon black, there is also a class of weakly reinforcing fillers, which can also be called inert fillers. They only have a small reinforcing effect on silicone rubber. They are generally not used alone in silicone rubber, but are used in combination with white carbon black. Function to adjust the hardness of silicone rubber, improve the technical properties of rubber compounds and the oil resistance and solvent resistance of vulcanizates, and reduce the cost of rubber compounds. Common weak reinforcements include diatomaceous earth, quartz powder, zinc oxide, titanium dioxide, zirconium silicate, and calcium carbonate.

(Source: Silicone Forum)