What is Fumed Silica?
Fumed silica, also known as pyrogenic silica, is an ultra-fine, high-purity silicon dioxide produced by flame hydrolysis of silicium tetrachloride gas in an oxygen-hydrogen flame. The result is aggregates of crystalline silica, whose very fine particle size gives it a number of distinctive properties: fumed silica has a large specific surface area, up to 400 m2/g; it is white, fluffy, and non-abrasive; and it is chemically inert and permanent.

Formation and Properties
The flame hydrolysis process produces primary particles in the range of 7-40 nanometers that cluster into aggregates 50-200 nanometers in size. The large specific surface area of fumed silica gives it unique behavior as a rheology modifier, thickening agent, reinforcing filler, absorbent, and anticaking agent. The aggregates are highly porous, leading to low bulk density around 30 kg/m3. Fumed silica is chemically similar to naturally occurring crystalline silica but distinct in its method of formation and properties due to its much finer structure and larger specific surface area.

Uses for Fumed Silica
The largest application for Fumed Silica is as a reinforcing filler in silicone rubber. Its large surface area allows it to form many interactions with the polymer, enhancing mechanical properties such as tensile strength, tear resistance and compression set. Other major uses of fumed silica include:

- Personal care products: As a thickening agent, fumed silica provides structure to creams, gels, and other formulations. It enhances spreadability and suspends other ingredients.

- Paints and coatings: Used as a thickener, reinforcing filler and rheology control agent. Improves gloss, hardness and durability of paint films.

-Food applications: Acts as a free-flow agent in powders, prevents caking and agglomeration. Also used in baked goods to improve volume, texture and shelf life.

-Printing inks: Improves the rheology, reinforces pigments and prevents settling in liquid inks.

- grease and lubricants: Absorbs oil while maintaining a smooth texture. Used in pastes, greases and other products resistant to dripping or running.

-Pharmaceuticals: Used as a glidant and lubricant in tablet formulations to improve flow properties and machinery performance.

-Other industrial uses: Includes chemicals, plastics, sealants, thermoplastics, foundry cores and molds, sealants, insulators.

Microscopic Structure and Properties
Under an electron microscope, the fine structure of fumed silica can be clearly seen. The primary silica particles are joined into branched, three-dimensional aggregates around 100-200 nm in size. These aggregates then come together to form larger agglomerates anywhere from 50 nm to over 1000 nm depending on conditions.

The porous, high surface area structure gives fumed silica excellent absorption properties for both liquids and gases. A single gram can absorb over 100 ml of liquid due to capillary condensation in the vast internal surface area. Its high absorbency paired with low bulk density make it an effective thickening and reinforcing agent. The purity of fumed silica also makes it effective for applications requiring a chemically inert filler.

While light and fluffy, fumed silica has sufficient abrasiveness to act as a polishing or lapping agent given its hard, crystalline silica composition. This abrasiveness must be balanced for uses in personal care. Overall, the unique combination of high surface area, porosity and nanoscale structure provide a versatile reinforcing and rheology-modifying material.

Production and Applications
Fumed silica is produced through a multistep pyrolysis process. First, silicium tetrachloride gas is produced by reacting high purity quartz sand with coke and chlorine in an electric furnace at over 1200°C. The resulting silicon tetrachloride vapor is then recovered.

Next, this vapor is injected into an oxhydrogen flame with oxygen and hydrogen fuel gases. Within the flame, which can reach temperatures up to 2000°C, the gas hydrolyzes into silicon dioxide and hydrogen chloride. The silicon dioxide forms as ultra-fine primary particles of 3-100 nm.

The hot particle-laden gases are quenched and passed through fabric filters to separate and collect the dry silica product. The fumed silica aggregates have a extremely low bulk density of around 30 kg/m3. Further processing may include classification and surface treatment to achieve the desired properties for various applications.

Global production capacity for fumed silica exceeds 500,000 metric tons annually. The market is highly consolidated among a few major producers. Key regions for fumed silica manufacture include Europe, North America, China and Japan. Major applications leveraging the high surface area properties include silicone rubber reinforcement, personal care, coatings, food, printing inks and greases. With its purity, inertness and ability to modify rheology and reinforce solids and polymers, fumed silica continues to find new uses in advanced materials. Proper quality control is important to maintain consistency across different production batches for various performance-critical applications.

The versatility of fumed silica stems from its nanoscale structure. This article detailed the flame hydrolysis process used to produce fumed silica aggregates with surface areas over 400 m2/g. Key properties and applications utilizing this ultra-fine silica's thickening, reinforcing, absorbent and anti-caking behavior were explored across sectors like personal care, food, printing and grease manufacture. Fumed silica remains an important industrial material facilitating technological and product advancements through its ability to modify rheology and reinforce materials at the microscopic level.

Get more insights on this topic: Fumed Silica