Match fumed silica grade and loading level to your exact viscosity target — from light anti-settling at 1% to structural anti-sag putty at 12%.
Match fumed silica grade and loading level to your exact viscosity target — from light anti-settling at 1% to structural anti-sag putty at 12%.
At 1–2 wt% loading, fumed silica provides enough hydrogen-bonded network to prevent pigment settling without significantly raising application viscosity. A grade like SEMISIL 200 (BET 200 ± 25 m²/g, primary particle ~12 nm) delivers 500–2,000 mPa·s viscosity build in epoxy systems — sufficient for storage stability while preserving sprayability. In polyester gelcoats, 1.5% of a 200 m²/g hydrophilic grade raises Brookfield viscosity from ~800 to ~3,000 mPa·s at 20 rpm. The network is fully shear-thinning: viscosity drops 80–90% under spray shear (\>10,000 s⁻¹), then rebuilds within 30–90 seconds at rest.
Between 3–5 wt%, fumed silica transforms liquid resins into structured thixotropic pastes suitable for gap-filling adhesives and brush-applied coatings. Higher surface area grades (300–380 m²/g, primary particle ~7 nm) achieve the same viscosity build at lower loading than 200 m²/g grades — typically 3% vs 4.5% to reach 25,000 mPa·s in a bisphenol-A epoxy. This matters for formulation cost and filler space. In silicone sealants, 4% of a hydrophobic (dimethylsilyl-treated) grade at 300 m²/g BET yields a non-slump bead with yield stress above 200 Pa. See our viscosity build curves for grade-by-grade comparisons across binder families.
Loading fumed silica to 6–8 wt% creates strong anti-sag behavior for vertical-surface coatings and thick-film systems that must resist flow before cure. At 7% loading of a 200 m²/g hydrophilic grade in unsaturated polyester, Brookfield viscosity exceeds 80,000 mPa·s at 20 rpm while remaining trowelable under moderate shear. The silanol-driven network creates a yield stress of 400–800 Pa — enough to hold 3–5 mm wet film thickness on vertical surfaces. Hydrophobic grades require ~15% higher loading to match the same yield stress because surface methyl groups reduce inter-particle hydrogen bonding density.
Above 8 wt%, fumed silica produces non-flowing putties and body fillers with yield stresses exceeding 1,500 Pa. At 10% of a 200 m²/g grade in epoxy, the system behaves as a soft solid at rest (G′ \> 50 kPa) but flows under spatula pressure. Dispersion becomes critical at these loadings — incomplete wetting leaves agglomerates that act as stress concentrators, reducing final mechanical strength by 20–40%. High-speed dissolvers (tip speed ≥18 m/s) or three-roll mills are mandatory. For formulators needing extreme thixotropy at lower loading, switching to a 380 m²/g grade achieves putty-like rheology at 7–8% instead of 10–12%, freeing volume for functional fillers.
The table below maps common fumed silica grades to the loading range needed for each viscosity target in a standard liquid epoxy (EEW 185–192). Actual values vary with resin chemistry, dispersion quality, and temperature — always validate with your specific system. Higher BET grades deliver more viscosity per percent loading but cost more per kilogram; total formulation cost often favors the higher grade because you use less.
| Grade (BET m²/g) | Anti-Settle (mPa·s) | Thixo Paste (mPa·s) | Anti-Sag (mPa·s) | Putty (mPa·s) |
|---|---|---|---|---|
| 150 m²/g | 2.0–2.5%: ~2,000 | 5–6%: ~20,000 | 8–9%: ~70,000 | 11–12%: >150,000 |
| 200 m²/g | 1.5–2.0%: ~3,000 | 4–5%: ~25,000 | 6–7%: ~80,000 | 10–11%: >150,000 |
| 300 m²/g | 1.0–1.5%: ~3,500 | 3–4%: ~30,000 | 5–6%: ~90,000 | 8–9%: >150,000 |
| 380 m²/g | 0.8–1.2%: ~4,000 | 2.5–3.5%: ~35,000 | 4–5%: ~100,000 | 7–8%: >150,000 |
For most industrial formulations, start with a 200 m²/g hydrophilic grade at the midpoint of the loading range for your viscosity target, then adjust ±0.5% based on actual dispersion results — switching to a 300 m²/g grade only when formulation volume constraints demand lower silica loading.
Higher BET surface area means more silanol groups per gram, producing a denser hydrogen-bonded network at lower loading. A 300 m²/g grade typically needs 25–35% less loading than a 200 m²/g grade to reach the same viscosity in the same resin system. The relationship is not linear — doubling BET does not halve the required loading.
Hydrophobic surface treatment replaces silanol (Si-OH) groups with methyl or dimethylsilyl groups, reducing inter-particle hydrogen bonding. This weakens the thixotropic network, requiring 10–20% more loading to match the yield stress of an untreated hydrophilic grade at the same BET surface area.
Above 6 wt%, high-speed dissolvers with tip speeds of 18–25 m/s are the minimum. For loadings above 8%, three-roll mills or high-pressure homogenizers deliver better deagglomeration. Incomplete dispersion at high loading wastes material and creates weak points in cured films.
Recovery to 80% of rest viscosity typically takes 30–120 seconds depending on grade and loading. Higher surface area grades and higher loadings recover faster because the denser silanol network re-forms hydrogen bonds more quickly. Full recovery to 95%+ can take 5–15 minutes.
Yes, blending a 200 m²/g and a 380 m²/g grade is a practical way to hit an intermediate viscosity target without changing total loading. A 50:50 blend behaves roughly like a 280 m²/g single grade. Pre-blend the powders dry before adding to the resin for uniform distribution.
Hydrogen bonding weakens with rising temperature, so viscosity at rest drops 5–15% per 10°C increase between 20–60°C. The shear-thinning ratio remains largely unchanged. Formulators targeting hot-climate anti-sag performance should validate at 40–50°C, not just at 25°C lab conditions.
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