Fumed silica (fumed silicon dioxide) achieves precise thixotropic and rheological control in silicone sealants by constructing a dynamic hydrogen bond network, making it a core functional additive for improving application performance and storage stability.

Core Mechanism of Thixotropic and Rheological Control
Three-dimensional Hydrogen Bond Network Formation: The surface of fumed silica nanoparticles is rich in silanol groups (Si–OH). In a non-polar siloxane polymer matrix, these groups interconnect through hydrogen bonds, forming a reversible three-dimensional network structure. This structure maintains high viscosity when at rest, preventing filler sedimentation and sealant sagging; under the shear force of application, the hydrogen bonds break, and the system viscosity drops sharply, facilitating extrusion and leveling; after the shear force is removed, the network quickly rebuilds, achieving "thixotropy"—that is, shear thinning and viscosity recovery upon standing.
Yield Stress Regulation: This network gives the sealant a significant yield stress, allowing it to resist gravity during application on vertical surfaces, preventing sagging, while maintaining a flat, non-flowing surface on horizontal surfaces, achieving "anti-sagging" performance. Hydrophilic vs. Hydrophobic Fumed Silica Performance Comparison
Performance Indicator | Hydrophilic Fumed Silica | Hydrophobic Fumed Silica
Surface Groups | Abundant free Si–OH | Si–OH covered by methyl/polydimethylsiloxane after silanization
Dispersibility in Silicone Systems | Poor, prone to agglomeration, requires strong shear | Excellent, high compatibility with siloxanes, easily dispersed uniformly
Initial Viscosity | Lower (e.g., HL-200 approximately 200,000 cp) | Extremely high (e.g., HB-139B can reach 700,000 cp)
Thixotropic Recovery Rate | Slower, incomplete structural recovery | Fast, network quickly rebuilt after shearing
Anti-sagging Performance | Fair, requires higher addition amount | Excellent, low addition amount achieves no dripping on vertical surfaces
Long-term Stability | Easily absorbs moisture, large viscosity fluctuations during storage | Strong chemical inertness, stable during storage, high batch consistency
In industrial applications, hydrophobic fumed silica (such as HB-139B) has become the preferred choice for high-end silicone sealants due to its excellent rheological control capabilities.  Its addition amount is typically 0.5%–3% (by weight) to achieve ideal application performance.

Key Role of Surface Silanization Modification
Modification Mechanism: Through dry or wet processes, trimethylchlorosilane, hexamethyldisilazane, or polydimethylsiloxane (PDMS) reacts with the hydroxyl groups on the fumed silica surface via condensation, eliminating polar sites and forming a hydrophobic organic layer.
Functional Improvements:
Enhanced Dispersibility: Reduces surface energy, prevents nanoparticle agglomeration, and improves filler uniformity.
Improved Stability: Inhibits moisture absorption, preventing viscosity drift or scorching of the colloid during storage.
Optimized Rheological Curve: Makes the viscosity-shear rate curve smoother, the thixotropic loop narrower, and the recovery more predictable. Industrial Application Parameters Reference
Typical Product Parameters: Commercially available hydrophobic fumed silica for silicone sealants typically has a specific surface area concentrated between 150–200 m²/g and a particle size of approximately 7–40 nm, meeting the requirements for high thickening and reinforcement.
Recommended Addition Amount: 0.8%–2.0% (adjust according to sealant type and performance requirements). Excessive addition will lead to excessively high viscosity, affecting workability and flow properties.