The fumed silica is a fine and special amorphous powder material prepared by high temperature hydrolysis of chlorosilane by oxyhydrogen flame. The average primary particle size is about 7-40 nm, and the specific surface area is 50-380 m2/g. It has high purity and SiO2 content of not less than 99.8%. It is a multifunctional additive widely used in the fields of silicone rubber, paint, rubber and chemical mechanical polishing. It can thicken, thixotropic and extinction. The fumed silica generally has both hydrophilic and hydrophobic products, wherein the hydrophobic product is obtained by surface chemical treatment using a hydrophilic product. 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. The production process is improving day by day, and the varieties have been serialized and specialized. Chemical. With the continuous expansion of the field of fumed silica, the demand for fumed silica has increased. In this paper, the special properties and production methods of fumed silica are briefly described. In recent years, some new application fields of fumed silica in the field of silicone rubber, coating, rubber and chemical mechanical polishing have been reviewed.
1 Special structure of fumed silica
Infrared spectroscopy studies show that the surface of fumed silica contains a large number of polar groups of hydroxyl groups. The main types of hydroxyl groups are: (1) adjacent hydroxyl groups: adsorption on polar substances, due to the close distance, generally in the form of hydrogen bonds presence. (2) Isolation of hydroxyl groups: isolated, undisturbed free hydroxyl groups. (3) Siloxy group: mainly present in the surface layer of dehydrated white carbon black, which is not easily removed at the time of heating and heating.
These special structures of fumed silica have excellent thickening, dispersibility, thixotropic and matting properties.
2 Production technology of fumed silica
Industrially, fumed silica is generally produced by using silicon tetrachloride or methyltrichlorosilane as a raw material, and hydrolyzing a reaction of silicon tetrachloride or methyltrichlorosilane in a gasified state at a high temperature.
3 new areas of application
3.1 Application in the pharmaceutical industry
The fumed silica is an amorphous structure with good physiological inertness, adsorption, dispersibility and thickening, and thus has been widely used in the pharmaceutical industry. Modern medical researchers have found that in the treatment of Ranitidine and Pirenzepine, a small amount of fumed silica will change its fluidity; in drugs containing griseofulvin, A small amount of fumed silica can change its dissolution rate, that is, change the dispersibility and absorption of the dissolved drug in water; in the powder containing aspirin, adding a small amount of fumed silica will change the antistatic property of the drug; In the medicine, the addition of fumed silica can utilize its high adsorption property, adsorbing microorganisms and micro viruses, and its adsorption capacity exceeds the adsorption capacity of the corresponding adsorbent applied in medicine, and the hydrophilicity of the fumed silica is utilized. Sexuality can eliminate edema or reduce secretions when the wound is inflamed.
The surface of the fumed silica is coated with a suitable monolayer material and dispersed in a colloidal solution of polyvinylpyrrolidone containing various cells. By centrifugation, the desired cells can be quickly separated without Stain cells. In addition, fumed silica can be used as a carrier for pharmaceutically active ingredients, a stabilizer for emulsions, for the manufacture of tablets, pills, suppositories, etc., and can improve the flow properties of the powder.
3.2 Application in agriculture
In agriculture, the treatment of seeds with white carbon black mixed with trace elements, growth regulators and mineral fertilizers can protect the seeds from mechanical damage and moisture, increase seed germination rate and reduce fertilizer use, shorten maturity and increase Yield of crops.
In the herbicides such as acesulfame/bensulfuron-methyl, chlorfenapyr/bensulfuron-methyl, etc., the dispersibility and adsorption of the fumed silica can ensure the strong adsorption of the herbicide and the long-lasting effect. In a mixture of two common herbicides, dinitroaniline and urea, a small amount of fumed silica is added to prevent agglomeration of the mixture.
In insecticidal powders such as polythiane and phoxim, the use of fumed silica as a carrier and dispersant will facilitate the storage, transportation and use of pesticides, avoid the use of organic solvents, reduce environmental pollution and reduce The production cost increases and prolongs the efficacy.
The fumed silica is also used as an absorbent for soil contaminants to absorb contaminants in the soil.
3.3 Application in ink
In industrial (printing) inks, fumed silica is often used as a thickener for inks. In hot-set offset printing inks, the use of hydrophilic fumed silica can speed up the drying of the ink and reduce the smudging and blurring of the ink. In offset printing inks, the use of hydrophobic fumed silica can reduce the water absorption of the ink, eliminate the foam, and increase the depth of the ink without affecting the surface gloss. In gravure, flexographic and silk printing, fumed silica is used to adjust the viscosity and prevent pigment settling.
The fumed silica can also be used to control the flow of printer inks, control the fluidity of the ink for clear printing, and act as a dispersant and flow control agent in the copier color of copiers and laser printers.
3.4 Application in daily necessities
The special plastic wrap for fruits and vegetables added with fumed silica has stable gas-adjusting index, strong two-way regulation ability for O2 and CO2, good moisture permeability, and low internal condensation. It has a long shelf life of storage of fruits and vegetables and good preservation effect.
In the production of alcohol, the addition of a small amount of fumed silica can play a role in clarifying beer and prolonging the shelf life. In the production of sucrose, fumed silica can be used as a defoaming agent. Fumed silica can be used as a carrier and flow control agent in foods.
3.5 Application in plastics
The fumed silica has many other properties in addition to reinforcement in plastics. The good light transmittance and small particles of fumed silica can make the plastic dense. Especially for translucent plastic films, the addition of fumed silica not only improves the transparency, strength and toughness of the film, but more importantly, it greatly improves the waterproof performance. Experiments have shown that the conventional PVC (polyvinyl chloride) film has a water permeability 1.5 times higher than that of the fumed silica. This shows that the waterproofness of the PVC film to which the fumed silica is added is greatly improved. Such films can be used as advanced plastic films for special applications.
3.6 Applications in other areas
The addition of fumed silica during the processing of vinyl fiber fabrics can change the properties of the vinyl and act as a matte. On an audiovisual tape, fumed silica is used as a release agent. As a filler for electronic packaging materials, it can improve the performance of materials and reduce the curing time. In the bulb industry, the inner wall of the bulb is lined with a fumed silica interference film, which can improve the luminous efficiency of the bulb and prolong the service life of the bulb. The addition of surface treated fumed silica to plexiglass improves the UV radiation resistance and impact strength of the material. In the battery industry, fumed silica is used as a release 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 for thickening and thixotropic action. In addition, by using the heat insulating property of the fumed silica, it is possible to prepare a heat insulating material having excellent properties.