Understanding Particle Size in Fumed Silica: The Role of BET Surface Area

Fumed silica is a widely used additive in industries such as coatings, adhesives, sealants, elastomers, composites, and construction chemicals. It provides excellent benefits in terms of rheology control, thixotropy, anti-sagging, reinforcement, and stability.
One of the most common questions we receive from customers is:
"What is the particle size of your CISIL fumed silica grades?"
The answer is more nuanced than it seems.

Why We Use BET Surface Area Instead of D50

Unlike pigments or mineral fillers, fumed silica is produced via flame hydrolysis, creating primary particles that form three-dimensional aggregates and agglomerates. These aggregates are extremely low density and non-spherical, making standard particle size measurements (such as D50, laser diffraction) not very meaningful.
Instead, the industry standard for characterizing particle size is:
👉 Specific Surface Area (BET method), measured in m²/g.
What does BET tell us?
- The higher the BET value, the smaller the primary particles.
- Typical correlations exist between BET and primary particle size in nanometers (nm).

CISIL Fumed Silica Grades: Overview of Particle Size

Product	Type	BET Surface Area (m²/g)	Estimated Primary Particle Size
CISIL 150	Hydrophilic	150 ± 15	~12–14 nm
CISIL 200	Hydrophilic	200 ± 25	~8–10 nm
CISIL 300	Hydrophilic	300 ± 25	~7–8 nm
CISIL R 138	Hydrophobic	110–150	~14–16 nm
CISIL R 139	Hydrophobic	200 ± 25	~8–10 nm
CISIL R 151	Hydrophobic	120 ± 30	~12–15 nm
CISIL R 152	Hydrophobic	150 ± 15	~12–14 nm

Visual Comparison

Below is a visual comparison of the CISIL grades based on BET surface area:
(Insert chart here)

Key Takeaways for Formulators

✅ BET is the most reliable parameter to compare fumed silica grades
✅ Higher BET → smaller primary particles → stronger thixotropy/reinforcement
✅ CISIL offers a broad range of grades for different application needs
✅ We can support customers with technical advice, data sheets, and application guidance

Typical Applications of CISIL Fumed Silica

✔ Paints and Coatings
✔ Adhesives & Sealants
✔ Silicone Rubbers (RTV, HTV)
✔ Unsaturated Polyester Resins
✔ Thermal Insulation Panels
✔ Pigments & Inks
✔ Powder Coatings
✔ Greases and Lubricants

Conclusion

Understanding how BET surface area relates to particle size helps formulators select the right CISIL fumed silica for their applications — whether for viscosity control, anti-sagging, reinforcement, or flow enhancement.
At Chemion AG, we are ready to provide technical support and help you select the optimal CISIL grade for your specific needs.
👉 Contact us today to receive detailed technical data or samples!