Overview of the production method and application of fumed silica
Fumed silica (commonly known as nano-silica) is a synthetic amorphous white flowable powder. It is a loose, amorphous, non-toxic, tasteless, odorless and non-polluting non-metal oxide nanomaterial. Its primary particle size is between 7 and 80 nm, and its specific surface area is generally greater than 100 m2/g. It is one of the extremely important nano-scale inorganic raw materials. Due to its nano-effect, it exhibits excellent properties such as reinforcement, thickening, thixotropy, insulation, matting, and anti-sagging in materials, so it is widely used in rubber, plastics, coatings, adhesives, sealants and other polymer industries. field. As a thixotropic agent and thickener for liquids, preventing sedimentation and sagging; improving the suspension and dispersibility of pigment fillers in liquid systems; used as dispersing and grinding aids; improving coating corrosion resistance; improving powder flowability , storage stability; improve powder charge and charge stability; improve water resistance; improve scratch resistance of paint film; improve color vividness; fix special effects; improve physical and mechanical properties of paint film; ;Improve the viscoelastic properties and reinforcement of rubber and elastomers; Defoaming effect in defoaming agents; Improve coating surface hardness and scratch resistance; Prevent adhesion in films and elastomers; As adsorbents and carriers; Used in inkjet Printed coating; as a premium filler for dental materials; as a catalyst carrier, significant thermal insulation for low and high temperature insulation.

1. Introduction to the production method of fumed silica

Chemical vapor deposition (CAV) method, also known as pyrolysis, dry or combustion. The raw materials are generally silicon tetrachloride, oxygen (or air) and hydrogen, which are reacted at high temperature. The reaction formula is: SiCl4+ 2H2+ O2—>SiO2+4HCl. Air and hydrogen are respectively pressurized, separated, cooled and dehydrated, dried on silica gel, filtered by dust removal, and sent to the synthesis hydrolysis furnace. After the silicon tetrachloride raw material is sent to the rectification tower for rectification, it is heated and evaporated in the evaporator, and is sent to the synthesis hydrolysis furnace with the dried and filtered air as the carrier. After silicon tetrachloride is gasified at high temperature (flame temperature 1000~1800°C), gas phase hydrolysis is carried out with a certain amount of hydrogen and oxygen (or air) at a high temperature of about 1800°C; the fumed silica particles generated at this time It is extremely fine and forms aerosol with gas, which is not easy to be captured, so it is first aggregated into larger particles in the agglomerator, then collected by the cyclone separator, and then sent to the deacidification furnace, and the fumed silica is purged with nitrogen-containing air When the pH value is 4 to 6, it is the finished product.

Fumed silica is roughly divided into two categories, one is hydrophilic fumed silica and the other is hydrophobic fumed silica.

Hydrophilic fumed silica: Hydrophilic fumed silica is produced by hydrolysis of volatile chlorosilanes in an oxyhydrogen flame. Chemically, these loose white powders consist of high-purity amorphous silica. Hydrophilic silica is wettable with water and dispersible in water. In addition to traditional industrial applications such as polyesters, silicones, paints and coatings, hydrophilic fumed silica products are increasingly used successfully in high-tech applications. The nanoparticle nature and high purity of fumed silica have led to its use in the electronics and optical fiber industries. The hydrophilic fumed silica product has an amorphous structure by X-ray analysis.

Hydrophobic fumed silica: Hydrophobic fumed silica is produced by chemically reacting hydrophilic fumed silica with reactive silanes such as chlorosilane or hexamethyldisilazane. It is hydrophobic (hydrophobic) and cannot be dispersed in water. In order to solve some special technical problems in the industry, various types of hydrophobic fumed silica have been developed. For example, hydrophobic fumed silica is produced by treating modified hydrophilic grade fumed silica with silane or siloxane. In the final product, the chemical treatment agent is chemically bonded to the original hydrophilic oxide. superior. In addition to the above-mentioned advantages of hydrophilic products, hydrophobic fumed silica products are characterized by: low hygroscopicity, good dispersibility, and rheological adjustment even for polar systems. Some products, on the basis of hydrophobic treatment, undergo structural modification, which can provide further help for customers to develop new products and improve product performance. For example, in liquid systems, hydrophobic fumed silica can be added in high amounts with little effect on the viscosity of the system.