Specific uses of fumed silica Fumed silica is a fine and special amorphous powder material obtained by high-temperature hydrolysis of chlorosilane through oxyhydrogen flame. The average primary particle size is about 7-40 nanometers, and the specific surface area is 50-380 m2/g. High purity, SiO2 content not less than 99.8%, is a multi-functional additive, widely used in the fields of silicone rubber, paint, rubber, chemical mechanical polishing, can play the role of thickening, thixotropy, extinction and so on. Fumed silica generally has two types of products: hydrophilic and hydrophobic, among which the hydrophobic product is obtained by using the hydrophilic product through surface chemical treatment. Since the German Degussa Company successfully developed the production technology of fumed silica in 1941, the total annual output of fumed silica in the world has exceeded 150,000 tons. change. With the continuous expansion of the application field of fumed silica, the demand for fumed silica is increasing. In this paper, the special properties and production methods of fumed silica are briefly described, and some new application fields of fumed silica at home and abroad in recent years are reviewed, except in the fields of silicone rubber, paint, rubber, and chemical mechanical polishing.
1 The special structure of fumed silica
Infrared spectroscopy research shows that the surface of fumed silica contains a large number of hydroxyl polar groups. The main types of hydroxyl groups are: (1) Adjacent hydroxyl groups: they have adsorption effects on polar substances. Due to the close distance, they are generally in the form of hydrogen bonds. exist. (2) Isolated hydroxyl groups: isolated, undisturbed free hydroxyl groups. (3) Siloxy: mainly exists in the surface layer of dehydrated silica, and it is not easy to be removed during heating.
These special structures of fumed silica molecules make it have excellent thickening, dispersibility, thixotropy and matting properties.
2. Production technology of fumed silica
In industry, fumed silica is generally prepared by using silicon tetrachloride or methyltrichlorosilane as raw materials, and by burning and hydrolyzing silicon tetrachloride or methyltrichlorosilane at high temperature in a gasified state.
3 New areas of application
3.1 Application in the pharmaceutical industry
Fumed silica is an amorphous structure with good physiological inertness, adsorption, dispersibility and thickening properties, so it is widely used in the pharmaceutical industry. Modern medical researchers have found that adding a small amount of fumed silica in Ranitidine, Pirenzepine and other drugs will change their fluidity; in drugs containing griseofulvin, adding A small amount of fumed silica can change its dissolution rate, that is, to change the dispersibility and absorption of the dissolved drug in water; adding a small amount of fumed silica to the aspirin-containing powder will change the antistatic properties of the drug; Adding fumed silica into the medicine can take advantage of its high adsorption capacity to adsorb microorganisms and microviruses, and its adsorption capacity exceeds that of the corresponding adsorbents used in medicine. Sex, can eliminate edema or reduce the secretion of wound inflammation.
The surface of the fumed silica is coated with a suitable monolayer material, and then dispersed in a polyvinylpyrrolidone colloidal solution containing a variety of cells. The required cells can be quickly separated by centrifugation technology without stain cells. In addition, fumed silica can be used as a carrier for active pharmaceutical ingredients, a stabilizer for emulsions, used in the manufacture of tablets, pills, suppositories, etc., and can improve the flow properties of powders.
3.2 Application in agriculture
In agriculture, treating seeds with silica mixed with trace elements, growth regulators and mineral fertilizers can protect seeds from mechanical damage and moisture, improve seed germination rate and reduce fertilizer usage, shorten maturity period and improve crop yield.
In herbicides such as moxa/bensulfuron-methyl, pretochlor/bensulfuron-methyl, the dispersibility and adsorption of fumed silica can be used to ensure strong adsorption of herbicides and long duration of effect. In two common herbicides, dinitroaniline and urea, adding a small amount of fumed silica can prevent the mixture from caking.
In insecticide powders such as polythiane and phoxim, the use of fumed silica as a carrier and dispersant will facilitate the storage, transportation and use of insecticides, avoid the use of organic solvents, reduce environmental pollution, and reduce It reduces the production cost and improves and prolongs the efficacy.
Fumed silica can also be used as an absorbent for pollutants in the soil to absorb pollutants in the soil.
3.3 Application in ink
In industrial (printing) inks, fumed silica is often used as a thickener for inks. In heat-set offset printing inks, the use of hydrophilic fumed silica can speed up the drying speed of the ink and reduce the phenomenon of smearing and blurring of the ink. In offset printing ink, the use of hydrophobic fumed silica can reduce the water absorption of the ink, eliminate foam, and improve the depth of the ink without affecting its surface gloss. In gravure, flexographic and silk printing, fumed silica is used to adjust viscosity and prevent pigment settling.
Fumed silica can also be used to control the flow of printer ink and the fluidity of ink to obtain clear printing; it is used as a dispersant and flow control agent in the toning of ink cartridges for copiers and laser printers.
3.4 Application in daily necessities
The special fresh-keeping film for fruits and vegetables added with fumed silica has stable air-conditioning index, strong bidirectional adjustment ability for O2 and CO2, good moisture permeability, and is not easy to condense inside. It can be used to store fruits and vegetables for a long time and has a good fresh-keeping effect.
In the production of alcohol, adding a small amount of fumed silica can play a role in clarifying beer and prolonging the preservation period. In the production of sucrose, fumed silica can be used as a defoamer. Fumed silica can be used as a carrier and flow control agent in food.
3.5 Application in plastics
In addition to reinforcement in plastics, fumed silica has many other properties. The good light transmittance and small particles of fumed silica can make plastics denser. Especially for translucent plastic films, adding fumed silica not only improves the transparency, strength and toughness of the film, but more importantly, greatly improves the waterproof performance. Experiments show that the water permeability of conventional PVC (polyvinyl chloride) film is 1.5 times higher than that of PVC added with fumed silica. This shows that the water repellency of the PVC film added with fumed silica is greatly improved. Such films can be used as high-grade plastic films for special purposes.
3.6 Other applications
Adding fumed silica during the processing of vinyl fiber fabrics can change the properties of vinyl and play a role in matting. On audio and video tapes, fumed silica is used as a release agent. As a filler for electronic packaging materials, it can improve the performance of the material and reduce the curing time. In the light bulb industry, the fumed silica interference film is lined on the inner wall of the light bulb, which can improve the luminous efficiency of the light bulb and prolong the service life of the light bulb. Adding surface-treated fumed silica to plexiglass can improve the material's resistance to ultraviolet radiation and impact strength. In the battery industry, fumed silica is used as a release agent. In the catalyst industry, fumed silica is used as a carrier. In the filling materials of optical fibers and optical cables, fumed silica can be used for thickening and thixotropic effects. In addition, by utilizing the thermal insulation properties of fumed silica, a thermal insulation material with superior performance can be prepared.
4 Conclusion
In recent years, my country has greatly improved the application of fumed silica, but there is still a gap compared with the international advanced level. We should speed up the application research of fumed silica, and move towards functionalization, compounding and The direction of serialization is to expand the application field of fumed silica and further promote the development of fumed silica industry to meet the needs of the rapidly developing national economy.