Reinforcing Mechanism of Fumed Silica
Fumed silica in RTV silicone rubber mainly achieves its reinforcing effect through the following mechanisms:

Hydrogen Bonding: The surface of fumed silica contains a large number of hydroxyl groups (-OH), which can form hydrogen bonds with the silicone rubber molecular chains, significantly enhancing the interfacial bonding force. This effect allows the tensile strength to reach up to 9.43 MPa (when 50 parts are added).

Nanostructure Effect: The average particle size of fumed silica primary particles is approximately 7-12 nm, and its high specific surface area (130-380 m²/g) enables it to form an "invisible skeleton" structure, greatly improving mechanical strength.

Hydrophobic Modification: Fumed silica treated with DDS or D4 hydrophobic agents (such as R106) can improve dispersibility, reduce agglomeration, and further enhance the reinforcing effect. Improved thermal stability: The addition of fumed silica increases the oxygen index (LOI) of the material from 19.4% to 25.0%, significantly improving its flame retardant properties.

The Influence of Fumed Silica Content on the Rheological Properties of RTV Silicone Rubber:

Viscosity Changes:
Low content (approx. 5-10%): Moderate viscosity, good flowability
Medium content (10-20%): Significant thixotropy, viscosity decreases during application for easier handling
High content (>20%): Viscosity increases sharply, potentially affecting processability

Thixotropic Properties:
Forms a three-dimensional network structure; high viscosity at rest prevents sagging
Network destruction under shear forces (e.g., stirring, injection), resulting in viscosity decreases
Network rapid recovery after shear force disappears

Curing Rate:
Hydrophobic fumed silica (e.g., A380) cures faster than hydrophilic fumed silica
Increasing the content of fumed fumed silica accelerates the curing rate; the opposite is true for precipitated fumed silica.

Rheological Control Methods for Application:
1. Formulation Design Control
Control Parameters | Recommended Range/Method | Effect
Fumed Silica Content | 5-20% (mass ratio) | Balances reinforcement and processability
Fumed Silica Type Hydrophobic (A380/R106) improves dispersibility
Surface Treatment: Silane coupling agent treatment enhances interfacial bonding
Dispersant: Superdispersant (0.5-2%) prevents agglomeration

2. Process Parameter Control
Mixing Process:
Temperature: 40-60℃ (optimal dispersion temperature)
Shear Rate: 3000-21000rpm (depending on viscosity)
Time: 30-60 minutes (ensure thorough dispersion)

Application Conditions:
Electronic Seals: Low viscosity (15,000-50,000cps), rapid surface drying (10-30 minutes)
Construction Seals: High thixotropy, anti-sagging properties

Equipment Selection:
Low viscosity systems: High-speed shear disperser
High viscosity systems: Kneader or open mill