Fumed silica is a kind of ultra-fine powder material obtained by hydrolysis and condensation of halosilane at high temperature in an oxyhydrogen flame (Figure 1 is a schematic diagram of the synthesis principle of fumed silica). Due to its unique preparation process, it has a different structure and unique properties from other silica products.
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So what are the unique structures and properties of fumed silica?
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1
Unique "three-dimensional dendritic" structure
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Because in the production process of fumed silica, the halosilane is first hydrolyzed and condensed into a single silica Particle, and then gradually grows into a spherical particle of 7-40 nanometers, which is called the "Primary Particle" of silica.
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The "primary particles" continue to move forward in the direction of the flame in the reactor, and the particles collide with each other. At this time, due to the relatively high temperature in the reactor, the particles are still close to the molten state. After the collision, the particles are fused together to form a three-dimensional dendritic structure of particles fused together by multiple spherical particles. It is called an "Aggregate Particle" of silica. Because the particles in aggregates are fused together, they are stable structures that are almost impossible to separate.
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The silica "aggregates" continue to move forward with the air flow in the pipe, collide, and then join together to form a flocculent fluffy powder called the silica "Agglomerate Particle" (Agglomerate Particle). Due to the low temperature in the pipeline at this time, the connection between the "aggregates" is only connected by physical adsorption, which is an unstable structure, and under a certain mechanical force, it is separable (dispersible).
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FIG. 2 is a transmission electron microscope image of fumed silica. Figure 3 is a transmission electron microscope image of precipitated silica. It can be seen from the photos that there is a significant difference in the dispersion state between the two. It is precisely because the dispersed fumed silica in the system can form a "nearly perfect" three-dimensional network structure of nanoparticles, which makes it have excellent reinforcement, thickening, thixotropic, anti-settlement, and release and hanging properties.
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2
High surface activity
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In the process of hydrolysis and condensation of fumed silica at high temperature, there are still some silica hydroxyl group (Si-OH) on the surface, which makes the surface polarity of fumed silica stronger and surface activity higher. FIG. 4 is a schematic diagram of the surface structure of fumed silica
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FIG. 4 Surface structure diagram of fumed silica
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As can be seen from the figure, there are "silicoxy group" and "silicon hydroxyl group" on the surface of fumed silica, in which the silicon hydroxyl group has a high activity, can form hydrogen bonds, or react with other groups, which also ensures the formation of a stable network between silicon dioxide particles, and has a good interaction with other media, so that silicon dioxide presents a good strengthening and toughening; Thickening thixotropy and anti-settling properties.
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In addition, the presence of silicon hydroxyl group also provides the possibility for the surface modification of fumed silicon dioxide. Surface modifiers with different structures are selected to react with silicon hydroxyl group, and some functional groups are grafted to the surface of fumed silicon dioxide, thus making the function of fumed silicon dioxide more diversified.
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3
The particle size is small and the specific surface area is large
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The "primary particle size" of fumed silica is 7-40nm, which is a concept? We can understand this by comparing the data in the following table. If we were to enlarge a fumed silica particle to the size of a standard football, we would need to enlarge it by about 30 million times; If you were to scale a soccer ball up 30 million times, it would be about the size of Mars.
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Due to the small particle size of fumed silica, its specific surface area is very large, usually the specific surface of fumed silica is 100-400m2/g. Usually we have a 150m2 house, its indoor area is about 120m2, that is, the use of the lowest specific surface area of the market common fumed silica products (such as HL-150), less than 1g, its specific surface area can cover the entire house indoor area! And if it is like a product with a high specific surface area (such as HL-380), only 18.8g of products, its specific surface area is equivalent to the area of a standard football field!
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It is precisely because of the characteristics of small particle size and large specific surface area of fumed silica that it has good adsorption performance, and can be widely used in the fields of catalyst, food, medicine, thermal insulation materials, and plays the functions of adsorption, anti-caking, heat insulation and so on.
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In the powder industry, fumed silica is mainly used as an anti-caking agent.
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As we said earlier, fumed silica itself has the characteristics of small particle size and large specific surface area, which makes it have good adsorption performance, and fumed silica can be widely used in the powder industry, which is also due to this. The analysis is as follows:
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• Has a large specific surface area. This characteristic enables it to be wrapped on the surface of the powder material, and plays the role of spacing between the powder particles, so as to prevent adhesion and promote the free flow of the powder.
• ?It has strong adsorption properties. The unique structure of fumed silica can promote it to absorb oil and water on the surface of the powder, thereby reducing the adhesion between the powder.
• ?In the powder system, due to the small particle size and high surface energy of white carbon black, they can be adsorbed on the surface of the powder particles, thus forming a surface layer on the surface, improving the fluidity of the powder, thus greatly improving the efficiency of the powder system in the production and transportation process. At the same time, fumed silica can also be used as a dispersant in systems such as environmentally friendly high solid content paints and powder coatings.

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Therefore, fumed silica has also become one of the most widely used additives in the powder industry, used in powder food (milk powder, coffee, seasoning), powder coatings, pesticides, feed and other powder products.
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In addition, fumed silica also has a series of unique properties of low packing density and high porosity, which are inseparable from the unique production process of fumed silica. These unique structures and properties also bring unique application properties of fumed silica that are different from other silica.