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Request QuoteBET Surface Area and Fumed Silica Grade Selection: A Buyer’s Guide
BET surface area is the single most important specification when selecting a fumed silica grade. It determines thickening efficiency, dispersibility, and cost-in-use. This guide explains what the BET number means, how it translates to real-world formulation performance, and how to choose between the 90–400 m²/g range available from major suppliers.
What Is BET Surface Area?
BET (Brunauer–Emmett–Teller) surface area is measured by nitrogen adsorption at cryogenic temperatures. The nitrogen molecules form a monolayer on the particle surface; the amount adsorbed allows calculation of total surface area per gram (m²/g). For fumed silica, this ranges from about 90 m²/g (coarse grades) to 400 m²/g (ultra-fine grades).
BET vs. Primary Particle Size
| BET Surface Area (m²/g) | Approx. Primary Particle Size (nm) | Representative Grade |
|---|---|---|
| 90 ± 15 | ~30 nm | Aerosil 90 / CAB-O-SIL L-90 |
| 150 ± 15 | ~16 nm | Aerosil 150 / HDK N20 |
| 200 ± 25 | ~12 nm | Aerosil 200 / CAB-O-SIL M-5 / HDK N20P |
| 300 ± 30 | ~8 nm | Aerosil 300 / CAB-O-SIL H-5 |
| 380 ± 30 | ~7 nm | Aerosil 380 / CAB-O-SIL HS-5 |
Primary particle size is determined during flame synthesis — shorter residence time in the flame produces smaller particles and higher surface area. This cannot be changed post-synthesis; it is a fixed characteristic of each grade.
How Surface Area Affects Formulation Performance
Thickening Efficiency
Higher BET = more silanol groups per gram = stronger inter-particle hydrogen bonding = higher viscosity at the same loading. The relationship is approximately linear within each surface type family:
Aerosil 200 (200 m²/g)
At 3 wt% in epoxy resin: viscosity ~15,000 mPa·s, TI ~3.5. Most widely used general-purpose grade — good balance of efficiency and dispersibility.
Aerosil 380 (380 m²/g)
At 3 wt% in the same epoxy: viscosity ~35,000 mPa·s, TI ~5.5. Nearly double the thickening power — but significantly harder to disperse and produces more foam.
Dispersibility (Inverse of BET)
Higher BET grades are harder to disperse because they have smaller primary particles, tighter aggregate packing, and stronger inter-particle forces. An Aerosil 380 requires 2–3× more dispersion energy than Aerosil 90 to achieve equivalent grind fineness. This means longer mixing times, higher tip speeds, or additional bead mill passes.
Bulk Density
Higher BET grades have lower bulk density — Aerosil 380 has a bulk density of ~50 g/L vs. ~80 g/L for Aerosil 90. This makes higher BET grades more voluminous and harder to handle in powder form, increasing dust generation and requiring more careful addition protocols.
Grade Comparison: Major Suppliers
The following table cross-references equivalent grades across Evonik (Aerosil), Cabot (CAB-O-SIL), and Wacker (HDK) for hydrophilic grades. Note: exact BET and properties may vary slightly between suppliers — always verify against the current product data sheet.
| BET (m²/g) | Evonik (Aerosil) | Cabot (CAB-O-SIL) | Wacker (HDK) | Typical Application |
|---|---|---|---|---|
| 90 ± 15 | Aerosil 90 | L-90 | HDK N90 | Powder flow aid, mild anti-caking, low-demand thickening |
| 150 ± 15 | Aerosil 150 | EH-5 (alt.) | HDK N15 | Mid-range thickening, food contact applications |
| 200 ± 25 | Aerosil 200 | CAB-O-SIL M-5 | HDK N20 / N20P | General-purpose: coatings, adhesives, sealants, pharma |
| 300 ± 30 | Aerosil 300 | CAB-O-SIL H-5 | HDK N30 | High-efficiency thickening, waterborne coatings, demanding sag control |
| 380 ± 30 | Aerosil 380 | CAB-O-SIL HS-5 | HDK N40 | Maximum thickening, specialty waterborne, pharmaceutical gels |
Highlighted rows (200 m²/g) are the most widely used grades in industrial formulation. Equivalent hydrophobic grades follow the same BET-to-performance relationship but with surface-treated surfaces — see our Surface Chemistry Guide.
Cost-Performance Analysis
Higher BET grades cost more per kilogram but deliver more thickening per gram — so cost-in-use must be compared on a per-unit-viscosity basis, not raw price per kg.
When Higher BET Is Worth the Premium
- Low-loading requirement: formulations where silica adds to viscosity but filler loading is constrained
- Transparency-critical: clear coats where lower loading reduces haze at equivalent thickening
- High-solids coatings: less room for filler, so efficiency per gram matters more
- Weight-sensitive applications: aerospace, electronic potting, lightweight composites
When Lower BET Is Preferred
- High-volume commodity coatings: raw material cost dominates; dispersion cost (energy, time) is also a factor
- Opaque filled systems: transparency irrelevant; lower BET disperses more easily, saves processing cost
- Pharmaceutical free-flow: lower BET (Aerosil 90–200) is preferred for powder flow; higher BET contributes excess thickening
- Matting applications: surface roughness matters, not thickening — lower BET gives coarser texture for comparable surface area loading
Quick Selection Guide
| Application Requirement | Recommended BET Range | Starting Grade |
|---|---|---|
| Anti-settling in primers / basecoats | 150–200 m²/g | Aerosil 200 / M-5 |
| Sag resistance in brush/roller applied coatings | 200–300 m²/g | Aerosil 200 / H-5 |
| Clear coat transparency + mild thickening | 200 m²/g | Aerosil 200 |
| High-solid waterborne thickening | 300–380 m²/g | Aerosil 380 |
| Structural adhesive / sealant | 110–200 m²/g (hydrophobic) | Aerosil R974 |
| Silicone rubber reinforcement | 90–150 m²/g (hydrophobic, PDMS) | Aerosil R202 |
| Powder flow / anti-caking | 90–200 m²/g | Aerosil 200 / 90 |
| Pharmaceutical oral dosage | 200 m²/g (pharma grade) | Aerosil 200 Pharma |
| Ink thickening (offset, gravure) | 200–300 m²/g | Aerosil 300 |