What is fumed silica?
Fumed silica is produced by the high-temperature hydrolysis of silicon halides in a hydrogen-oxygen flame. It is amorphous, nanometer-sized particles with surface hydroxyl groups and adsorbed water. Normally, it appears as a white, flocculent powder and is a non-toxic, tasteless, odorless, and non-polluting non-metallic oxide. It is produced by the high-temperature hydrolysis of silicon tetrachloride in a hydrogen-oxygen flame. Its primary particles are spherical, pore-free particles.
The primary particles interact and aggregate within the flame, forming aggregates. The average particle size of the primary particles ranges from 7 to 40 nm, corresponding to a specific surface area of 150 to 400 m²/g. Siloxane and silanol groups are located on the surface of HDK fumed silica particles. The presence of the latter makes untreated HDK fumed silica hydrophilic. Figure 1 shows the surface groups of hydrophilic fumed silica.
Hydrophobic fumed silica
The surface of HDK fumed silica can be modified by reacting silanol groups with suitable compounds. Both hydrophilic and hydrophobic HDK fumed silicas have been successfully applied in many fields. Fumed silica can serve the following functions in customer products:
- Reinforcing filler
- Thickener and thixotropic agent
- Anti-settling agent
- Free-flowing agent
How to select fumed silica
- Select by system polarity
Differences in polarity can provide better dispersion of fumed silica, thereby fully realizing its properties.
- Select Based on Specific Grain Ratio
For the same type of silica gel: a larger specific gravity indicates greater thickening power, improved thixotropy, poorer extrudability, greater dispersion difficulty, and improved clarity. For products requiring cross-linking, such as adhesives, cross-linking increases the tensile and tear strength of the finished product; elongation at break and hardness are minimally affected.
- Select Based on Required System Viscosity
When adding silica gel, it’s common to encounter situations where low addition levels provide adequate viscosity but insufficient reinforcement. Increasing the addition level improves reinforcement but increases the system viscosity. In such cases, some of the existing hydrophilic silica gel can be replaced with a hydrophobic silica gel with a specific gravity Ratio. This is because higher hydrophobic silica gels exhibit poorer thickening and thixotropy, but better extrudability, transparency, and dispersion quality.
For products requiring cross-linking, such as adhesives, the mechanical properties of the cross-linked product are not affected by the degree of hydrophobization. Therefore, hydrophobic silica gels can act as a viscosity modifier within the system, providing a wider viscosity range without compromising the mechanical properties of the final product.
- Consider the impact of the amount of silica added on product performance.
For the same type of silica, a higher addition decreases transparency, increases thickening properties, reduces extrudability, and makes dispersion more difficult. After cross-linking, the finished product’s tensile strength and tear resistance are significantly enhanced, while hardness increases slightly. Elongation at break and resilience are not significantly affected.
