Fumed silica (commonly known as nano silica) is a synthetic amorphous white flowing powder. It is a loose, amorphous, non-toxic, tasteless, odorless and pollution-free non-metallic oxide nano material. Its primary particle size is between 7 ~ 80nm, and its specific surface area is generally greater than 100 ㎡ /g. It is one of the most important nano inorganic raw materials. Due to its nano effect, it shows excellent properties such as reinforcement, thickening, thixotropic, insulation, extinction, anti sagging, etc. in materials, so it is widely used in polymer industry fields such as rubber, plastics, coatings, adhesives, sealants, etc. As a thixotropic agent and thickener of liquid, it can prevent sedimentation and sagging; Improve the suspension and dispersion of pigment fillers in liquid system; Used as dispersing and grinding aids; Improve the corrosion resistance of the coating; Improve powder fluidity and storage stability; Improve the charge capacity and charge stability of powder; Improve water resistance; Improve scratch resistance of paint film; Improve the brightness of color; Fixed special effects; Improve the physical and mechanical properties of the paint film; Improve the adhesion and flexibility of the paint film; Improve the viscoelastic properties of rubber and elastomer and reinforce them; Defoaming effect in defoaming agent; Improve the surface hardness and scratch resistance of the coating; Prevent adhesion in film and elastomer; As adsorbent and carrier; For inkjet printing coating; As an advanced filler of dental materials; As a catalyst carrier, it has remarkable thermal insulation and is used for low-temperature and high-temperature insulation.


1、 Brief introduction of production method of fumed silica

Chemical vapor deposition (CAV) method, also known as pyrolysis method, dry method or combustion method. Its raw materials are generally silicon tetrachloride, oxygen (or air) and hydrogen, which are formed by reaction at high temperature. The reaction formula is: sicl4+ 2h2+ O2 - >sio2+4hcl. The air and hydrogen are respectively sent to the synthetic hydrolysis furnace after pressurization, separation, cooling and dehydration, silica gel drying, dust removal and filtration. The silicon tetrachloride raw material is sent to the rectification tower for rectification, heated and evaporated in the evaporator, and sent to the synthetic hydrolysis furnace with the dried and filtered air as the carrier. After silicon tetrachloride is gasified at high temperature (flame temperature 1000~1800 ℃), it is hydrolyzed with a certain amount of hydrogen and oxygen (or air) at a high temperature of about 1800 ℃; At this time, the generated gas-phase silica particles are very fine and form aerosol with the gas, which is difficult to capture. Therefore, it is first aggregated into larger particles in the aggregator, then collected by the cyclone separator, and then sent to the deacidification furnace. The gas-phase silica is purged with nitrogen-containing air to a pH value of 4 ~ 6, which is the finished product.



Fumed silica can be divided into two types: hydrophilic fumed silica and hydrophobic fumed silica.



Hydrophilic vapor phase silica: hydrophilic vapor phase silica is prepared by hydrolyzing volatile chlorosilane in a hydrogen oxygen flame. From a chemical point of view, these loose white powders are composed of high-purity amorphous silica. Hydrophilic silica can be wetted with water and dispersed in water. In addition to the application in traditional industrial fields, such as polyester, silicone, paint and coating, hydrophilic vapor phase silica products are more and more successfully applied in high-tech fields. The nano particle characteristics and high purity of fumed silica play a leading role in the application of electronic and optical fiber industries. Hydrophilic fumed silica products × X-ray analysis shows amorphous structure.



Hydrophobic fumed silica: hydrophobic fumed silica is prepared by chemical reaction between hydrophilic fumed silica and active silane (such as chlorosilane or hexamethyldisilazane). It is hydrophobic (hydrophobic) and cannot be dispersed in water. In order to solve some special technical problems in industry, various types of hydrophobic vapor phase silica have been developed. For example, hydrophobic fumed silica is produced by treating hydrophilic fumed silica with silane or siloxane. In the final product, the chemical treatment agent is bonded to the original hydrophilic oxide in the form of chemical bond. In addition to the above advantages of hydrophilic products, hydrophobic vapor phase silica products are characterized by low hygroscopicity, good dispersibility, and rheological ability even for polar systems. Some products, after structural modification on the basis of hydrophobic treatment, can provide further help for customers to develop new products and improve product performance. For example, in liquid system, hydrophobic fumed silica can reach a high addition amount, but has little effect on the viscosity of the system.



2、 Application of fumed silica in various fields:



1. Electronic packaging materials


For example, organic electroluminescent devices (OELD) is a new type of flat panel display device developed recently, but the service life of OELD devices can not meet the application requirements. One of the technical difficulties to be solved is the packaging materials and packaging technology of devices. By fully dispersing the fumed silica after surface activation treatment in the silicone modified epoxy resin packaging adhesive matrix, the curing time of the packaging material can be greatly shortened (2.0-2.5h), and the curing temperature can be reduced to room temperature, which significantly improves the sealing performance of OELD devices and increases the service life of OELD devices.


2. Resin composite

The purpose of improving the properties of resin based materials can be achieved by fully and evenly dispersing fumed silica particles into resin materials. The strength, elongation and wear resistance are improved, the surface finish of the material is improved and the aging resistance is enhanced.



3. Plastics

The use of fumed silica has light transmission and small particle size, which can make the plastic more compact. After adding fumed silica to polystyrene plastic film, it not only improves its transparency, strength and toughness, but also significantly improves its waterproof and anti-aging properties. The hardness, smoothness and anti-aging properties of plastic steel doors and windows produced by adding a small amount of fumed silica to common plastic PVC are greatly improved.


4. Paint

China is a large country of paint production and consumption, but the current domestic coatings generally have deficiencies in performance, 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 have successfully realized the application of fumed silica in coatings. This nano modified coating has changed the shortcomings of previous products. After testing, its main performance indexes have been greatly improved except that the contrast rate remains unchanged.


5. Rubber

Rubber is an elastomer with excellent elasticity. In the production of rubber products, carbon black is usually added to the rubber to improve the strength, wear resistance and aging resistance. However, the products are black due to the addition of carbon black. The fumed silica has laid a material foundation for the production of a new generation of rubber products with novel colors and excellent properties. 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 those of high-grade rubber products, and the color can remain unchanged for a long time. Nano modified color EPDM waterproof coiled material has significantly improved its wear resistance, tensile strength, bending resistance and aging resistance, and has bright colors and excellent color retention effect.


6. Ceramics

Adding fumed silica to ceramics instead of nano A1203 can not only play the role of nano particles, but also act as a second phase particle. It not only improves the strength and toughness of ceramic materials, but also improves the hardness and elastic modulus of materials. Its effect is more ideal than adding A1203.


7. Sealant, adhesive

Adding hydrophobic fumed silica to the sealant will quickly form a silica structure, that is, nano si0x small particles form a network structure to inhibit the colloid flow, accelerate the curing speed and improve the bonding effect. Due to the small size of fumed silica particles, the sealing and anti-seepage properties of the product are also increased.


8. Cosmetics

Fumed silica has strong UV reflection ability and good stability. It will not decompose or change color after being irradiated by UV light, nor will it react with other components in cosmetic formula. These outstanding characteristics of fumed silica have laid a good foundation for the upgrading of sunscreen cosmetics.

It can be seen from the above brief introduction of some applications of fumed silica that it has been applied in many fields of modern industry and has a very broad market prospect.