Fumed silica is an ultra-fine powder material obtained by hydrolysis and condensation of halosilane at high temperature in oxyhydrogen flame. Because of its unique preparation process, it has different structure and unique properties from other silica products.
So what are the unique structures and properties of fumed silica?
1 unique "three-dimensional dendritic" structure
In the production process of fumed silica, halosilane is first hydrolyzed and condensed to form a single silica particle, and then gradually grow into a 7-40 nm spherical particle, which is called the "primary particle" of silica.
"Primary particles" continue to move forward with the flame direction in the reaction furnace, and the particles collide with each other. At this time, due to the relatively high temperature in the reaction furnace, the particles are still close to the melting state. After the collision, the particles are fused together to form three-dimensional dendritic particles fused by multiple spherical particles, which is called the "aggregate particle" of silica. Because the particles in the aggregate are fused together, it is a stable structure and can hardly be separated.
Silica "aggregates" continue to move forward and collide with the air flow in the pipeline, and then connect with each other to form flocculent and fluffy powder, which is called silica "agglomerate particle". Because the temperature in the pipeline is low at this time, the connection between "aggregates" is only connected together through physical adsorption. It is an unstable structure and can be separated (dispersible) under certain mechanical force.
There is an obvious difference in the dispersion state between the two. It is precisely because after the gaseous silica is dispersed in the system, it can form a "nearly perfect" three-dimensional network structure of nanoparticles, which makes it have excellent properties such as reinforcement, thickening, thixotropy, anti settlement, drainage and hanging.
2 high surface activity
In the process of high-temperature hydrolysis and condensation of fumed silica, some silicon hydroxyl groups (Si oh) remain on the surface. The existence of surface silicon hydroxyl groups makes the surface polarity of fumed silica strong and the surface activity high.
There are "siloxy" and "silicon hydroxyl" on the surface of fumed silica, in which the silicon hydroxyl has high activity and can form hydrogen bonds or react with other groups, which also ensures that a stable network can be formed between silica particles, and has good interaction with other media, so that silica presents good reinforcement and toughening; Thickening thixotropy and anti settlement.
In addition, the existence of silicon hydroxyl groups also makes it possible for the surface modification of fumed silica. Surface modifiers with different structures are selected to react with silicon hydroxyl groups, and some functional groups are grafted onto the surface of fumed silica, so as to make the functions of fumed silica more diverse.
3. Small particle size and large specific surface area
The "primary particle size" of fumed silica is 7-40nm. What is the concept? We can understand from the data comparison in the following table. If we enlarge a gas-phase silica particle to the size of a standard football, we need to enlarge it about 30 million times; If you enlarge the football by about 30 million times in equal proportion, it will become about the size of Mars.
Due to the small particle size of fumed silica, its specific surface area is very large. Generally, the specific surface of fumed silica is 100-400m2 / g. Usually, if we have a house of 150m2, its indoor area is about 120m2, that is, using the gas-phase silica product with the lowest specific surface area in the current market (such as hl-150), its specific surface area can cover the indoor area of the whole house if it is less than 1G! If the product with high specific surface area (such as hl-380) is only 18.8g, its specific surface area is equivalent to the area of a standard football field!
Because of its small particle size and large specific surface area, gaseous silica has good adsorption performance. It can be widely used in the fields of catalyst, food, medicine, thermal insulation materials and so on.
In the field of powder industry, fumed silica is widely used as anti caking agent.
As we mentioned earlier, fumed silica itself has the characteristics of small particle size and large specific surface area, which makes it have good adsorption performance. Fumed silica can be widely used in the powder industry. The following analysis:
It has a large specific surface area. This characteristic enables it to wrap on the surface of powder material and play a role of spacing between powder particles, so as to prevent adhesion and promote the free flow of powder.
It has strong adsorption performance. The unique structure of fumed silica can promote it to absorb oil and water on the powder surface, so as to reduce the adhesion between the powders.
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 powder particles, so as to form a surface layer on the surface, improve the fluidity of powder, and greatly improve the efficiency of powder system in the process of production and transportation. 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.
Therefore, fumed silica has also become one of the most widely used additives in the powder industry. It is used in powder products such as powder food (milk powder, coffee, seasoning), powder coatings, pesticides, feed and so on.
In addition, fumed silica also has a series of unique properties such as low bulk 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 different from other silica.
