Although called “white carbon black,” it actually has no direct relation to carbon. It earned this name because it exhibits reinforcing properties in rubber similar to carbon black, but is white in color. It is a silica powder obtained by chemical extraction, with the chemical composition being “hydrated silica”.
Currently, there are two main methods for producing white carbon black. One is the precipitation method. Its general process route is roughly: first, quartz sand and soda ash are reacted at high temperatures using fuel oil or gas to produce industrial sodium silicate. The industrial sodium silicate is dissolved in water to form a dilute solution of a certain concentration. Then, under specific conditions, an acid is added to precipitate the silica. This is followed by washing, filtration, drying, crushing, and milling to obtain the finished white carbon black product. The precipitation method includes various specific techniques like the acid method, sol-gel method, and carbonation method. In China we primarily uses the acid method. The acid method involves reacting a soluble silicate with sulfuric acid (or another acid). When the reaction mixture reaches a certain pH value, acid addition is stopped, and the mixture is aged. It is then filtered and washed repeatedly with water to remove Na₂SO₄ before being sent for drying and milling to yield the product.
The other method is the vapor phase method, also known as chemical vapor deposition (CVD), pyrolysis, dry process, or combustion method. Its raw materials are typically silicon tetrachloride (SiCl₄), oxygen (or air), and hydrogen (H₂), reacted at high temperatures. Air and hydrogen are pressurized, separated, cooled and dehydrated, dried with silica gel, and filtered to remove dust before being fed into the synthesis hydrolysis furnace. The silicon tetrachloride feedstock is sent to a rectification column for purification, then heated and vaporized in an evaporator. Using dried, filtered air as a carrier gas, it is transported to the synthesis hydrolysis furnace. The vaporized silicon tetrachloride (flame temperature 1000~1800°C) undergoes gas-phase hydrolysis with a certain amount of hydrogen and oxygen (or air) at around 1800°C. The resulting fumed silica particles are extremely fine and form an aerosol with the gas, making them difficult to collect. Therefore, they are first aggregated into larger particles in an aggregator. They are then collected by a cyclone separator and sent to a deacidification furnace. The fumed silica is flushed with nitrogen-enriched air until its pH reaches 4~6 to obtain the final product.
So, what are the differences between precipitated silica and fumed silica?
The main differences between precipitated silica and fumed silica lie in particle size and purity. Precipitated silica is generally micron-sized, while fumed silica is generally nano-sized. Precipitated silica has a higher impurity content, with purity typically around 93%. Fumed silica has very low impurity content, achieving purity above 99%. Due to these significant differences in particle size and purity, fumed silica outperforms precipitated silica in functions such as thickening, thixotropy, anti-settling, reinforcement, matting, and friction. Below, let’s look at the specific applications of hydrated silica:
1.Rubber Reinforcement: Silica is widely used as a reinforcing agent in the rubber industry, accounting for 70% of its total usage. It is particularly suitable for manufacturing white, colored, and light-colored rubber products because its white color facilitates coloring. For example, the tensile strength of unreinforced silicone rubber does not exceed 0.4 MPa. After reinforcement with fumed silica, the strength can increase by 40 times.
2.Matting Agent: Silica has a huge surface area and a refractive index between 1.45 and 1.50. Paint films containing hydrated silica matting agents cause diffuse reflection of incoming light, resulting in a matte or flat appearance. Fumed silica particles are more porous, and the number of particles per unit mass is greater than that of precipitated silica. Therefore, at the same addition level, fumed silica offers higher matting efficiency and better transparency.
3.Abrasive Agent: Besides its large surface area, the adsorption capacity of hydrated silica is also very strong. Consequently, hydrated silica is one of the primary abrasives used in toothpaste. Due to its superior transparency, abrasiveness, etc., fumed silica is found in the ingredients of many crystal-clear toothpastes.
4.Paper Sizing and Strengthening: Hydrated silica possesses excellent abrasion resistance and strength. As mentioned earlier, it is very “white.” Therefore, when used in paper, it can increase the whiteness and opacity of the paper, making it more resistant to wear and oil, with better gloss. For instance, adding 1-2% fumed silica to newsprint can reduce the paper’s weight by 10% while also increasing strength and preventing ink penetration.
5.Anti-Settling and Flow Aid: When used in adhesives and sealants, hydrated silica controls rheology, prevents settling, prevents sagging, and reinforces. Furthermore, fumed silica, with its superior performance, is widely used as a rheological additive, anti-settling agent, and dispersant in paints, inks, and coatings.
6.Protection and Carrier Function in Cosmetics and Pharmaceuticals: In cosmetics, fumed silica can effectively reflect ultraviolet light and has a matting effect. It does not decompose or discolor after irradiation. As a carrier, it can also prolong the efficacy of drugs or skincare products and promote their absorption. Precipitated silica generally cannot meet these requirements.
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Post time: Jul-16-2025