Fumed silica is a fine and special amorphous powder material prepared from chlorosilane by high-temperature hydrolysis with oxyhydrogen flame. The average primary particle size is about 7-40 nm, the specific surface area is 50 ~ 380m2/g, the product purity is high, and the SiO2 content is not less than 99.8%. It is a multifunctional additive, which is widely used in the fields of silicone rubber, coating, rubber, chemical mechanical polishing, and can play the role of thickening, thixotropic, extinction, etc. Fumed silica generally has hydrophilic and hydrophobic products. Hydrophobic products are obtained by surface chemical treatment of hydrophilic products. 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 150000 tons, its production process is improving day by day, and its varieties have realized serialization and specialization. With the continuous expansion of the application field of fumed silica, the demand for fumed silica is increasing. This paper briefly describes the special properties and production methods of fumed silica, and summarizes some new application fields of fumed silica at home and abroad in recent years, except in the fields of silicone rubber, coating, rubber and chemical mechanical polishing.


Special structure of fumed silica


IR spectra show that there are a large number of polar hydroxyl groups on the surface of fumed silica. The main types of hydroxyl groups are: (1) adjacent hydroxyl groups: they can adsorb polar substances. Because they are close to each other, they generally exist in the form of hydrogen bonds. (2) Isolated hydroxyl: isolated, undisturbed free hydroxyl. (3) Siloxy: mainly exists in the surface layer of dehydrated white carbon black, which is not easy to be removed during heating.


These special structures of fumed silica make it have excellent thickening, dispersing, thixotropic and extinction properties.


Production technology of fumed silica


In industry, fumed silica is generally prepared by using silicon tetrachloride or methyltrichlorosilane as raw material, and burning and hydrolyzing silicon tetrachloride or methyltrichlorosilane at high temperature under gasification state.


3 new fields of application


3.1 application in pharmaceutical industry


Fumed silica is an amorphous structure with good physiological inertia, adsorption, dispersion and thickening, so it has been widely used in the pharmaceutical industry. Modern medical researchers have found that adding a small amount of fumed silica to ranitidine, pirenzepine and other drugs will change their fluidity; Adding a small amount of fumed silica to the drug containing griseofulvin can change its dissolution rate, that is, change the dispersibility and absorption of the drug in water; Adding a small amount of fumed silica to the powder containing aspirin will change the antistatic property of the drug; In skin medicine, the addition of fumed silica can use its high adsorption capacity to adsorb microorganisms and micro viruses, which exceeds the adsorption capacity of the corresponding adsorbents used in medicine. At the same time, the hydrophilicity of fumed silica can eliminate edema or reduce the secretion during wound inflammation.


The surface of fumed silica is coated with appropriate monolayer material, and then dispersed in polyvinylpyrrolidone colloidal solution containing a variety of cells. Through centrifugation, the required cells can be quickly separated without contaminating the cells. In addition, fumed silica can be used as a carrier of drug active ingredients, a stabilizer of emulsion, for the manufacture of tablets, pills, suppositories, etc., and can improve the flow performance of the powder.


3.2 application in agriculture


In agriculture, treating seeds with white carbon black mixed with trace elements, growth regulators and mineral fertilizers can protect seeds from mechanical damage and moisture, improve the germination rate of seeds, reduce the amount of fertilizer, shorten the maturity period and improve the yield of crops.


In the herbicides of ACOR / bensulfuron methyl and Propionamide / bensulfuron methyl, the dispersibility and adsorption of fumed silica can ensure the strong adsorption and long duration of herbicides. Adding a small amount of fumed silica to the mixture of dinitroaniline and urea can prevent the mixture from caking.


In insecticide powders such as polythiane and phoxim, the use of fumed silica as carrier and dispersant will facilitate the storage, transportation and use of pesticides, avoid the use of organic solvents, reduce environmental pollution, reduce production costs, and improve and prolong the efficacy.


Fumed silica can also be used as an absorbent to absorb pollutants in soil.


3.3 application in ink


In industrial (printing) inks, fumed silica is often used as thickener. Using hydrophilic fumed silica in offset printing inks for heat set printing can accelerate the drying speed of the ink and reduce the phenomenon of dirt and ink blur. 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 printing, aniline printing and silk printing, fumed silica can be used to adjust viscosity and prevent pigment sedimentation.


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 dispersant and flow control agent in toner cartridge of copier and laser printer.


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 bi-directional adjustment ability to 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 with good fresh-keeping effect.


In the production of liquor, adding a small amount of fumed silica can clarify beer and prolong the fresh-keeping period. In the production of sucrose, fumed silica can be used as defoamer. Fumed silica can be used as carrier and flow control agent in food.


3.5 application in plastics


Fumed silica has many other properties besides reinforcing in plastics. Fumed silica has good light transmittance and small particles, which can make plastics dense. Especially for the translucent plastic film, the addition of fumed silica not only improves the transparency, strength and toughness of the film, but also greatly improves the waterproof performance. The experimental results show that the water permeability of conventional PVC film is 1.5 times higher than that of PVC with fumed silica. This shows that the water resistance of PVC film with fumed silica is greatly improved. This kind of film can be used as advanced plastic film for special purposes.


3.6 application in other aspects


Adding fumed silica in the processing of vinyl fiber fabrics can change the properties of vinyl and play a role in extinction. On audio and video tapes, fumed silica is used as an anti sticking agent. As the filler of electronic packaging material, it can improve the performance of the material and reduce the curing time. In the bulb industry, lining the inner wall of the bulb with vapor phase white carbon black interference film can improve the luminous efficiency of the bulb and prolong the service life of the bulb. Adding fumed silica with surface treatment to PMMA can improve the UV radiation resistance and impact strength of the material. In battery industry, fumed silica is used as isolating agent. In the catalyst industry, fumed silica is used as a carrier. In optical fiber and optical cable filling materials, fumed silica can be used to thicken and thixotropic. In addition, the thermal insulation material with superior performance can be prepared by using the thermal insulation property of fumed silica.