What is the use for Silicon dioxide?

Silicon dioxide is also called silica, with the chemical formula SiO2. There are two kinds of crystalline silica and amorphous silica in nature.

Crystalline silica is divided into three types: quartz, tridymite and cristobalite due to different crystal structures. Pure quartz is a colorless crystal, and the large and transparent prismatic quartz is called crystal. If the crystals containing trace impurities have different colors, there are amethyst, tea crystal, black crystal and so on. Ordinary sand is fine quartz crystal, there are yellow sand (more iron impurities) and white sand (less impurities, more pure). In the silicon dioxide crystal, the 4 valence electrons of the silicon atom form 4 covalent bonds with the 4 oxygen atoms. The silicon atom is located in the center of the regular tetrahedron, and the 4 oxygen atoms are located on the 4 corners of the regular tetrahedron. Many Two such tetrahedra are connected by oxygen atoms at the top corners, and each oxygen atom is shared by two tetrahedra, that is, each oxygen atom is combined with two silicon atoms. SiO2 is the simplest formula to express the composition, and it only expresses the ratio of the number of atoms of silicon and oxygen in the silicon dioxide crystal. Silica is an atomic crystal.

The bond energy of Si—O bond in SiO₂ is very high, and its melting point and boiling point are relatively high (melting point 1723°C, boiling point 2230°C). The refractive index is about 1.6. The refractive index of various silica products: quartz sand is 1.547; powder quartz is 1.544; vein quartz is 1.542; diatomaceous earth is 1.42~1.48; fumed white carbon black is 1.46; precipitated white carbon black is 1.46. Diatomaceous earth existing in nature is amorphous silicon dioxide, which is the remains of diatoms of lower aquatic plants. It is white solid or powdery, porous, light, soft solid, and has strong adsorption.

Application of silica

Fumed silica is one of the extremely important high-tech ultra-fine inorganic new materials. Because of its small particle size, it has large specific surface area, strong surface adsorption force, large surface energy, high chemical purity, good dispersion performance, thermal resistance, Resistance and other aspects have specific properties. With its superior stability, reinforcement, thickening and thixotropy, it has unique characteristics in many disciplines and fields and plays an irreplaceable role. Fumed silica, commonly known as "ultrafine silica", is widely used in various industries as additives, catalyst carriers, petrochemicals, decolorizers, matting agents, rubber reinforcing agents, plastic fillers, ink thickeners, metal softness Polishing agent, insulating filler, high-grade daily cosmetic filler and spraying material, medicine, environmental protection and other fields. It also provides a new material foundation and technical guarantee for the development of related industrial fields. Because it shows special functions compared with conventional materials in terms of magnetism, catalysis, light absorption, thermal resistance and melting point, it has attracted great attention.

1. Electronic packaging materials

Organic electroluminescent device (OELD) is a new type of flat display device newly developed at present. It has low turn-on and driving voltage, and can be driven by DC voltage. High (>105cd/m2) and other advantages, but the service life of OELD devices can not meet the application requirements, one of the technical difficulties that need to be solved is the packaging material and packaging technology of the device. At present, silicone-modified epoxy resins are widely used in foreign countries (Japan, the United States, Europe, etc.), that is, through the blending, copolymerization or grafting reaction between the two, the internal stress of the epoxy resin can be reduced and the molecular structure can be formed. Internal toughening improves the high temperature resistance, and also improves the waterproof, oil-proof, and oxygen-resistant properties of silicone, but it requires a long curing time (several hours to several days), and to speed up the curing reaction, it needs to be at a higher temperature. Temperature (60°C to above 100°C) or increase the amount of curing agent used, which not only increases the cost, but also makes it difficult to meet the requirements of large-scale device production lines for packaging materials (short time, room temperature packaging). Fully dispersing the surface-active treated fumed silica in the silicone-modified epoxy resin encapsulation matrix can greatly shorten the curing time of the encapsulation material (2.0-2.5h), and the curing temperature can be lowered to room temperature , so that the sealing performance of the OELD device is significantly improved, and the service life of the OELD device is increased.

2. Resin composite materials

Resin-based composite materials have the characteristics of light weight, high strength, and corrosion resistance. However, in recent years, the material industry and the pillar industries of the national economy have increasingly high requirements for the performance of resin-based materials. How to synthesize high-performance resin-based composite materials has become a Important current topics in the materials and business world. The advent of fumed silica provides new opportunities for the synthesis of resin-based composite materials and a new way for the modification of traditional resin-based materials, as long as the fumed silica particles can be fully and uniformly dispersed in the resin Among the materials, the purpose of improving the properties of resin-based materials can be fully achieved.

1). Improve strength and elongation. Epoxy resin is the basic resin material. Adding fumed silica to epoxy resin is completely different in structure from epoxy resin-based composite materials added by coarse crystalline silica (white carbon black, etc.), coarse crystalline SiO2 It is generally added as a reinforcing agent, and it is mainly distributed in the chains of polymer materials. Due to the serious lack of coordination on the surface, the large specific surface area and the lack of oxygen on the surface, it shows a very strong Active, it is easy to bond with the oxygen of epoxy ring molecules, which improves the bond force between molecules. In comparison, it shows a high flow rate, so that the strength, toughness and ductility of the epoxy resin material added with fumed silica are greatly improved.

2). Improve the wear resistance and improve the surface finish of the material. Fumed silica particles are 100-1000 times smaller than SiO2, adding it to epoxy resin is beneficial for drawing into wire. Due to the high fluidity and small size effect of fumed silica, the surface of the material is denser and finer, and the friction coefficient becomes smaller. In addition, the high strength of nanoparticles greatly enhances the wear resistance of the material.

3). Anti-aging performance. A fatal weakness in the use of epoxy resin-based composite materials is poor anti-aging performance. The reason is mainly due to the ultraviolet medium and long-wave effects of solar radiation in the 280-400nm band, which has a very serious destructive effect on resin-based composite materials. , the degradation of polymer chains leads to rapid aging of resin-based composites. Fumed silica can strongly reflect ultraviolet rays, and adding it to epoxy resin can greatly reduce the degradation effect of ultraviolet rays on epoxy resin, thereby achieving the purpose of delaying the aging of materials.

3. Plastic

The light transmission and small particle size of fumed silica can make the plastic more dense. After adding silica to polystyrene plastic film, it not only improves its transparency, strength, toughness, but also has obvious waterproof performance and anti-aging performance. improve. The hardness, smoothness and anti-aging properties of plastic-steel doors and windows produced by adding a small amount of fumed silica to ordinary plastic polyvinyl chloride are greatly improved. The ordinary plastic polypropylene is modified by fumed silica, and the main technical indicators (water absorption, insulation resistance, compression residual deformation, flexural strength, etc.) all meet or exceed the performance indicators of engineering plastic nylon 6, realizing the polypropylene railway Parts are used instead of nylon 6, the product cost is greatly reduced, and its economic and social benefits are very significant.

4. Coating

My country is a big country in the production and consumption of coatings, but the current domestic coatings generally have performance deficiencies, such as poor suspension stability, poor thixotropy, poor weather resistance, poor scrub resistance, etc., resulting in the need to import a large number of high-quality coatings every year. Some coating manufacturers in Shanghai, Beijing, Hangzhou, Ningbo and other places dared to innovate and successfully realized the application of fumed silica in coatings. This nano-modified coating changed the shortcomings of previous products. After testing its main performance indicators Except for the contrast ratio unchanged, the rest have been greatly improved. For example, the scrub resistance of the exterior wall paint has been increased from more than 1,000 times to more than 10,000 times, and the artificial accelerated weather aging and artificial radiation exposure aging time have been increased from the original 250 hours ( Chalking level 1, discoloration level 2) increased to 600 hours (no pulverization, no discoloration of the paint film, color difference value 4.8), in addition, the bonding strength between the coating film and the wall is greatly improved, the hardness of the coating film is significantly increased, and the surface self-cleaning ability also improved.

5. Rubber

Rubber is an elastomer with excellent stretchability, but its overall performance is not satisfactory. In the process of producing rubber products, it is usually necessary to add carbon black to the rubber compound to improve strength, wear resistance and aging resistance, but due to carbon The addition of black makes the products all black, and the grade is not high. The advent of fumed silica in my country has laid a material foundation for the production of a new generation of rubber products with novel colors and excellent performance. After adding a small amount of fumed silica to ordinary rubber, the strength, wear resistance and aging resistance of the product can reach or exceed that of high-grade rubber products, and the color can be kept unchanged for a long time. Nano-modified colored EPDM waterproofing membrane, its abrasion resistance, tensile strength, flexural resistance, and anti-aging performance are all significantly improved, and the color is bright and the color retention effect is excellent. The development of colored tires has also made some progress. For example, the flexural performance of the tire side rubber has increased from the original 100,000 times to more than 500,000 times. It is expected that in the near future, the colorization of domestic automobile and motorcycle tires will be realized.

6. Color (dye) material

Although organic pigments (dye) have bright colors and strong tinting strength, their general light resistance, heat resistance, solvent resistance and migration resistance are often inferior to inorganic pigments. The surface modification treatment of organic pigments (dye) by adding fumed silica not only greatly improves the anti-aging performance of the pigment (dye), but also improves the indicators such as brightness, hue and saturation to a certain extent. Comparable to imported high-end products, it greatly expands the grade and application range of organic pigments (dye).

7. Ceramics

Adding fumed silica instead of fumed aluminum oxide to 95 ceramics can not only play the role of nanoparticles, but also the second phase particles, which not only improves the strength and toughness of ceramic materials, but also improves the hardness of materials. Hardness and elastic modulus and other properties, the effect is more ideal than adding A1203. The use of fumed silica to compound ceramic substrates not only improves the compactness, toughness and smoothness of the substrates, but also greatly reduces the sintering temperature. In addition, the application effect of fumed silica in ceramic products such as ceramic filters and corundum balls is also very significant.

8. Sealant, adhesive

Sealants and adhesives are important products with large volume, wide range and wide application range. It requires product viscosity, fluidity, and curing speed to reach the best conditions. Our country's products in this field are relatively backward, and high-grade sealants and adhesives are all dependent on imports. Foreign products in this field have used nano-materials as modifiers, and fumed silica is the preferred material. It mainly coats a layer of organic materials on the surface of fumed silica to make it hydrophobic. Add it to A silica structure is quickly formed in the sealant, that is, the small particles of fumed silica form a network structure to inhibit the flow of the colloid, accelerate the curing speed, and improve the bonding effect. Due to the small size of the fumed silica particles, the sealing performance of the product is also increased. and impermeability.

9. FRP products

Although FRP products have the advantages of light weight, high strength, and corrosion resistance, they have low hardness and poor wear resistance. Relevant experts added fumed silica to the gel coat resin by ultrasonic dispersion method, and conducted a performance comparison experiment with the gel coat without fumed silica, and found that its Mohs hardness was changed from the original 2.2 (equivalent to the hardness of gypsum). Increase to level 2.8~2.9 (level 3 is the hardness of natural marble), and the wear resistance is increased by 1~2 times. Due to the grafting and bonding of nanoparticles and organic polymers, the toughness of the material is increased, so the tensile strength and resistance The impact strength is more than doubled, and the heat resistance is also greatly improved.

10. Drug carrier

With the rapid increase of urban domestic garbage and the increasingly serious environmental pollution, it is very urgent to increase the efforts to eliminate the "four pests" and prevent the spread of diseases. Brushing lime on tree trunks and spraying liquid medicine on dustbins have little effect. Now big cities have adopted central nervous anesthetic insecticides to eliminate mosquitoes, flies, cockroaches and other insect pests, but these insecticides are mostly from Imported from abroad, the price is higher, and the validity period after spraying is shorter (only one month). The use of fumed silica as a carrier to adsorb this type of insecticide has a very good slow-release effect. According to the measurement, the effective period after spraying is more than one year.

11. Cosmetics

For cosmetics, it is required to have a strong ability to shield ultraviolet rays. It is best to protect the human body from the harmful effects of ultraviolet mid-wave (UVB) and long-wave ultraviolet (UVA). In essence, ultraviolet shielding includes two aspects, one is the absorption of ultraviolet rays mentioned above, and the other is the reflection of ultraviolet rays. At present, no anti-ultraviolet agent developed from the perspective of ultraviolet reflection performance has been reported in the world. In the past, organic compounds were mostly used as ultraviolet absorbers in sunscreen products, but there were problems such as increasing the amount of addition in order to protect the skin from ultraviolet rays as much as possible, which would increase the occurrence of skin cancer and chemical allergies, while fumed silica is Inorganic components, easy to be compatible with other components of cosmetics, non-toxic, tasteless, do not have the above-mentioned problems, and they are white in color, can be easily colored, especially valuable is the strong ultraviolet reflection ability and good stability of fumed silica, which is used as It will not decompose, change color after ultraviolet irradiation, and will not chemically react with other components in the formula. These outstanding characteristics of fumed silica have laid a good foundation for the upgrading of sunscreen cosmetics.

12. Antibacterial materials

Utilizing the huge specific surface area, surface multi-mesoporous structure, super adsorption capacity and unique physical and chemical properties of fumed silica, functional ions such as silver ions are evenly designed into the mespores on the surface of fumed silica, and stabilized , and successfully developed high-efficiency, long-lasting, high-temperature-resistant, broad-spectrum antibacterial nano-antibacterial powder (the particle size is only about 70 nanometers), which not only fills the domestic blank, but also the main technical indicators have reached or exceeded similar products in Japan. After testing, when the concentration of nano antibacterial powder in water is only 0.315%, the antibacterial ability against representative Gram-positive bacteria and Gram-negative representative bacteria can be clearly revealed, and the inhibition zone appears 2— 3mm, and with the increase of the concentration of nano antibacterial powder in water, the inhibition zone increases obviously. 

According to the measurement, when the Ag+ in the water is 0.01mg/1, it can completely kill the Escherichia coli in the water, and it can keep no new bacterial flora for up to 90 days. Apply nano-antibacterial powder to enamel glaze to produce a drum washing machine with anti-mildew and anti-bacterial functions, and its anti-bacterial rate is as high as 99%. It should be pointed out that nano antibacterial powder is used in enamel glazes under relatively harsh conditions. It must maintain strong antibacterial properties in strong alkaline liquids and high temperature (about 900°C) after firing porcelain. Unmatched. The nano-antibacterial powder is added to the interior wall paint to produce an interior wall paint with long-term antibacterial and anti-mildew functions. Nano antibacterial powder is used in women's underwear detergent, wool, cashmere detergent, detergent, and hand sanitizer. The antibacterial performance is very significant after testing by the health and epidemic prevention department. It is foreseeable that with the enhancement of people's health awareness, nano antibacterial powder will gradually be accepted by the general public of related application companies, and will emerge in the bills, medical and health care, chemical building materials, home appliances, functional fibers, plastic products and other industries.

13. Other

1). Applications in the field of optics

Nanoparticles used in infrared reflective materials are mainly made into thin films and multilayer films. The film material of nano particles has a good application prospect in the light bulb industry. High-pressure sodium lamps and various iodine arc lamps used for photography and photography require strong lighting, but 69% of the energy of the filament is converted into infrared rays after being heated, which shows that a considerable amount of electrical energy is converted into heat energy and consumed, and only a small amount is consumed. Part of it is converted into light energy for lighting. At the same time, the heating of the lamp tube will also affect the life of the lamp. How to improve the luminous efficiency and increase the illuminance has always been a key problem to be solved urgently. The birth of nanoparticles provides a new way to solve this problem. Since the 1980s, scientific research and technical personnel have made a multi-layer interference film with fumed silica and nano-titanium dioxide particles, with a total thickness of micron, lining the inner wall of the bulb cover. The result is not only good light transmittance, but also strong infrared rays reflexes. According to experts' calculations, under the same light brightness, this kind of lamp can save 15% of electric energy compared with traditional halogen lamps.

2). New plexiglass additive

The window material of aircraft is usually plexiglass (PMMA). When the aircraft is flying at high altitude, the window material is easily aged by ultraviolet radiation, resulting in a decrease in transparency. In order to solve this problem, using the strong ultraviolet reflection performance of fumed silica, adding surface-modified fumed silica in the production process of plexiglass, the anti-ultraviolet radiation ability of the produced product is more than doubled, and the impact strength is improved 80%.