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Silica, despite its name, has no relation to carbon. It derived its name because it exhibits similar reinforcing properties to carbon black in rubber and is white in color. Silica is obtained as a silicon dioxide powder through chemical extraction, with the chemical composition being "hydrated silicon dioxide."
Currently, there are two main preparation methods for silica:
One is the precipitation method. The process generally includes the following steps: Firstly, quartz sand and soda ash are reacted at high temperatures using fuel oil or gas to produce industrial water glass. Then, the industrial water glass is diluted to a certain concentration with water, and a certain acid is added under specific conditions to precipitate silicon dioxide. After that, the precipitate is washed, filtered, dried, and crushed to obtain the final silica product. The precipitation method is further subdivided into various specific methods such as the acid method, sol-gel method, and carbonation method. China mainly adopts the acid method. The specific process of the acid method involves reacting a soluble silicate with sulfuric acid (or another acid). When the reaction solution reaches a certain pH value, the acid addition is stopped, and aging takes place. Then, the precipitate is filtered and washed repeatedly with water to remove Na2SO4. After drying and crushing, the product is obtained.
The other method is the gas-phase method, also known as chemical vapor deposition (CAV) method, pyrolysis method, or dry method. The raw materials are generally silicon tetrachloride, oxygen (or air), and hydrogen, which react at high temperatures. The specific reaction formula is: SiCl4 + 2H2 + O2 → SiO2 + 4HCl. During the reaction, air and hydrogen are pressed, separated, cooled and dehydrated, dried with silica gel, filtered to remove dust, and then sent to the synthesis hydrolysis furnace. The silicon tetrachloride raw material is sent to a distillation column for distillation, heated and evaporated in an evaporator, and then sent to the synthesis hydrolysis furnace with dried and filtered air as the carrier. Silicon tetrachloride is vaporized at high temperatures (flame temperature of 10001800°C) and undergoes gas-phase hydrolysis with a certain amount of hydrogen and oxygen (or air) at around 1800°C. The resulting gas-phase silica particles are extremely fine and form an aerosol with the gas, which is difficult to capture. Therefore, they are first aggregated into larger particles in an aggregator, then collected by a cyclone separator, and finally sent to a deacidification furnace. The gas-phase silicon dioxide is blown with nitrogen-containing air until its pH value reaches 46, at which point it becomes the final product.
So, what are the differences between precipitation silica and gas-phase silica? The main differences lie in particle size and purity. Precipitation silicon dioxide is generally micrometer-sized, with a relatively high impurity content and a purity of around 93%. In contrast, gas-phase silicon dioxide is nanometer-sized, with very low impurity content and a purity of over 99%. Due to these significant differences in particle size and purity, gas-phase silicon dioxide outperforms precipitation silicon dioxide in thickening, thixotropy, anti-settling, reinforcing, matting, and friction properties.
Next, let's look at the specific applications of hydrated silicon dioxide:
In the rubber industry, silica is widely used as a reinforcing agent, especially for manufacturing white, colored, and light-colored rubber products due to its ease of coloring. For example, the tensile strength of unreinforced silicone rubber does not exceed 0.4 MPa, but after reinforcement with gas-phase silica, its strength can increase by 40 times.
As a matting agent, silica has a huge surface area and a refractive index of 1.45-1.50. When light strikes a paint film containing hydrated silicon dioxide, diffuse reflection occurs, resulting in a matte or dull appearance. Gas-phase silicon dioxide particles are more porous, with more particles per unit mass compared to precipitation silicon dioxide. Therefore, gas-phase silicon dioxide has a higher matting effect and better transparency at the same addition level.
In toothpaste, hydrated silicon dioxide is used as an abrasive due to its strong adsorption capacity. Gas-phase silicon dioxide, with its superior transparency and abrasiveness, is often found in crystal-clear toothpaste formulations.
In paper manufacturing, hydrated silicon dioxide enhances the whiteness, opacity, abrasion resistance, and strength of paper. The addition of gas-phase silicon dioxide can also reduce the weight of paper while increasing its strength and preventing ink penetration.
In adhesives and sealants, hydrated silicon dioxide helps control rheology, prevents settling and sagging, and reinforces the material. Gas-phase silicon dioxide, with its superior performance, is widely used as a rheology modifier, anti-settling agent, and dispersant in paints, inks, and coatings.
Furthermore, in cosmetics and pharmaceuticals, gas-phase silicon dioxide effectively reflects ultraviolet rays, has a matting effect, and does not decompose or change color upon exposure. As a carrier, it can also extend the efficacy and promote the absorption of drugs or skincare products, which precipitation silicon dioxide generally cannot achieve.