Preparation Of Fumed Silica And Its Application In Silicone Rubber

In the twentieth Century, in the 60's and 70's, fumed silica mainly used four silicon chloride as raw material, and the production process was easy to control, but the production cost was high.

With the development of organosilicon monomer industry, the treatment of its by-products, methyl three chlorosilane, has become the bottleneck of its development.

In general, they are used in silicone and waterproofing coatings, but they are limited in quantity.

Therefore, it is urgent to find a new way out.

By 80s, the process of preparing fumed silica by using silicone monomer by-product or by-product and four silicon chloride mixture as raw material has been developed. The cost of the process is low and the economic benefit is good.

The preparation principle of fumed silica is as follows:

Therefore, most manufacturers of gas-phase silicon dioxide choose to set up factories near large silicone monomer companies, and the two parties work closely together to promote development.

Fig. 1 is a schematic diagram of the recycling of gas-phase silicon dioxide industry and organic silicon industry. Fig. 1 is an organic silicon company and gas-phase silicon dioxide company, which are closely related to each other. They set up factories near each other to promote each other's development, and achieved excellent social and economic benefits.

Fumed silica is the main reinforcing factor in HvT silicone rubber. Because the molecular chain of silicone rubber is very compliant and the interaction force between the chains is weak, the strength of unreinforced silicone rubber is very low (not more than 0.4 M Pa), which has no practical value. It must be strengthened before it can be used. The strength of silicone rubber reinforced with fumed silica can be increased by 40 times.

In addition, the amount of fumed silica and its dispersion in rubber matrix have great influence on the properties of vulcanizates. Fig. 2 is the effect of the amount of fumed silica on the tensile strength of vulcanizates.

It can be seen from the graph that the strength of vulcanizate increases with the increase of fumed silica content, and the peak value can reach the peak when the general dosage is 35 or 50 parts.

There are many mechanisms and models for reinforcing silica rubber by gaseous silica. The more accepted explanation is that the free light radicals on the surface of gas-phase silica form a physical or chemical combination with silicone rubber molecules, and a silicon rubber molecular adsorption layer is formed on the surface of silica, forming a three-dimensional network structure of gas silica and silicone rubber molecules, thus effectively limiting the deformation of the molecular chain of silicone rubber and reinforcing it.

The change of tear strength of vulcanizate is similar to that of tensile strength. It increases with the increase of the performance of fumed silica, and increases initially with the increase of fumed silica.

The difference (see Fig. 3) indicates that the plasticity of the compound is related to the amount of silica, the surface properties and the structure.

The reason for the formation is the formation of hydrogen bonds on the surface of silicon dioxide and the oxygen atoms in the silicone rubber, and the adsorption of silicone rubber chains on the surface of silica rubber. As a result, the fluidity of the rubber decreases as the time goes on, and the rubber becomes hard and the processing performance is affected.

Therefore, in the process of processing, it is necessary to add structural control agents or choose surface treated fumed silica. The addition of structural control agents and surface treatment of fumed silica will react with silicon light base on silicon dioxide surface by means of structural control agents or surface treatment agents, thereby reducing the number of light radicals on the surface, reducing the number of hydrogen bonds formed with silicone rubber, shortening mixing time and increasing plasticity, reducing the structural effect, improving the processing performance and storage stability.

Different forms of RTV silicone rubber need to be filled with filler to have practical value.

At present, fumed silica is the most widely used and most effective RTV silicone rubber reinforcing filler.

RTv silicone rubber can be used as sealing materials such as pouring, seam and coating. In order to maintain the viscosity and fluidity before vulcanization, the addition of gas phase silica is generally much less than that of high temperature silicone rubber, and is often used with other reinforcing and semi reinforcing fillers for construction operation.

On the one hand, it is due to the small size effect and large specific surface area of gas-phase silica particles. On the other hand, the surface of silicon dioxide particles contains many silicon light radicals, and the particles can form a network structure by hydrogen bonding and van Edward force. At the same time, silica particles will interact strongly with polysiloxane molecules and improve the interface state.

Fig. 4 is the effect of the amount of fumed silica on the tensile strength and shore A hardness of RTV silicone rubber (the specific surface area of gaseous silica is 153m2/g). Fig. 5 is the effect of the specific surface area of gas-phase silica on the peel strength of RTv silicone rubber. Fig. 6 is the effect of the amount of fumed silica on the tear strength of RTv silicone rubber.

This is mainly because with the increase of the amount of fumed silica, it forms a relatively complete network in the whole rubber system, which can effectively restrict the movement of the molecular chain of silicone rubber and thus play a reinforcing effect.

With the increase of specific surface area, the particle size of silica decreases, and the interfacial interaction between silica and silicone rubber is enhanced, so the peel strength is increased.

Using this characteristic, fumed silica can be used as thickener and thixotropic agent in the sealant field, which can increase viscosity, ensure free flow of rubber, and prevent agglomeration, flow and collapse.

Fig. 7. 9 is the effect of fumed silica on the rheological properties of RTv silicone rubber.

It can be seen from the diagram that when the specific surface area is less than 200m2/g, with the increase of the specific surface area of fumed silica, the extrusion rate of RTv silicone rubber decreases, and tends to balance after 200m2/g. However, the yield value of RTv silicone rubber increases with the increase of fumed silica content.

The thickening and thixotropic mechanism of fumed silica mainly depends on the hydrogen bonding interaction of the surface silicon light group.

When it is dispersed in polysiloxane, a silicon dioxide network is formed between different particles through the silicon light group on the surface, forming a silica network, which limits the fluidity of the system, increases the viscosity, and plays a role of thickening. When the shear force is applied, the silicon dioxide network is damaged, resulting in the viscosity drop and thixotropic effect of the system, which is conducive to the construction.

Once the shear force is disappearing, the hydrogen bond reforms, the silica network can be restored, and the viscosity of the RTv silicone rubber material system is also gradually rising, which effectively prevents the rubber from flowing in the sulfuration process.

The anti flow characteristics of the system are closely related to the yield value and the network reduction rate of the material after shearing.

In practical applications, the higher the yield value, the better the anti flow performance of the compound.

The ideal compound should have high yield value, high shear dilution index and rapid reduction rate.

Fig. 10 is the effect of the amount of fumed silica on mixing time.

It can be seen from the diagram that the mixing time is prolonged with the increase of the amount and specific surface area of fumed silica.

When the shear force is applied, the viscosity of the rubber decreases greatly, showing a certain fluidity. After the shear force is released, the viscosity will recover quickly. If the dispersion is not enough or over dispersed, it will only form part of the gas-phase silica network, resulting in lower viscosity and poor contact characteristics.

In the pparent rubber system, the higher the pparency, the better the degree of dispersion of carbon black.

Under the same dispersion conditions, the pparency of the compound increased with the increase of specific surface area.