Selecting the right fumed silica grade for transparent formulations depends on refractive index matching, primary particle size, and dispersion quality.
Selecting the right fumed silica grade for transparent formulations depends on refractive index matching, primary particle size, and dispersion quality.
Haze in clear systems originates from light scattering at interfaces between the filler and the resin matrix. Amorphous fumed silica has a refractive index of approximately 1.46, which closely matches many acrylate, epoxy, and silicone resins (1.45–1.52). When the refractive index gap stays below 0.02, visible-light scattering drops to near zero at typical loading levels of 1–3 wt%. Polyester and some urethane systems with indices above 1.53 require either lower loadings or surface-treated grades to keep haze below the 0.5% threshold most OEM specs demand. For a deeper look at the physics behind this tradeoff, see our guide on optical clarity vs. haze in fumed silica systems.
Primary particle size is the single most important grade parameter for clear formulations. Fumed silica grades with primary particles in the 7–14 nm range (BET surface area 150–380 m²/g) scatter far less visible light than coarser grades. SEMISIL 150 (BET ~150 m²/g, ~14 nm primary particles) delivers effective thixotropy with minimal haze in solventborne clears. For UV-curable systems requiring maximum transparency, SEMISIL 380 (BET ~380 m²/g, ~7 nm) provides superior clarity because its particles are well below the 40 nm scattering threshold — see our UV curable systems guide for formulation details. Avoid grades with BET below 130 m²/g in any clear application; their 16+ nm particles introduce measurable haze above 1.5 wt% loading.
Even the correct grade will cause haze if poorly dispersed. Agglomerates of 0.5–5 µm act as scattering centers regardless of primary particle size. High-shear dispersion (tip speed ≥15 m/s, or three-roll mill with gap
Clear automotive topcoats demand 90 GU gloss at 20°; use SEMISIL 150 at 1.0–1.5 wt% with HMDS treatment. LED encapsulants require optical transmission \>95% at 450 nm — SEMISIL 380 at 0.5–1.0 wt% meets this while providing enough thixotropy to prevent wire-sweep during cure at 150 °C. Wood coating clearcoats tolerate slightly higher haze (
The table below summarizes the critical specifications for selecting between fumed silica grades in transparent formulations. All values represent untreated (hydrophilic) base grades; surface-treated variants share the same particle size and BET but differ in carbon content and wetting behavior.
ParameterSEMISIL 150SEMISIL 200SEMISIL 380
BET Surface Area (m²/g)150 ± 15200 ± 25380 ± 30Primary Particle Size (nm)14127Tamped Density (g/L)50pH (4% dispersion)3.7–4.53.7–4.53.7–4.5Haze at 2 wt% in Acrylate (%)Max Recommended Loading (wt%)3.02.52.0Relative Cost Index1.0×1.2×1.8×Best FitSolventborne clears, wood coatingsGeneral UV & encapsulantsHigh-clarity LED, optical UV5050
For most clear coating and UV applications, start with SEMISIL 150 at 1–2 wt% — it delivers reliable thixotropy with sub-0.5% haze at the lowest cost per kilogram. Move to SEMISIL 380 only when optical specs demand transmission above 95% or haze below 0.2%.
Haze results from light scattering at refractive index boundaries between silica particles and the resin matrix. Larger primary particles (\>16 nm), poor dispersion leaving agglomerates above 0.5 µm, and refractive index mismatch greater than 0.02 are the three main contributors.
SEMISIL 150 with HMDS surface treatment at 1.0–1.5 wt% is the standard choice. Its 14 nm primary particles keep haze below 0.3% while providing sag resistance at film builds of 40–50 µm, meeting OEM 20° gloss requirements above 90 GU.
Higher BET means smaller primary particles, which scatter less visible light. A 380 m²/g grade (~7 nm particles) produces roughly half the haze of a 150 m²/g grade (~14 nm) at the same loading, but costs approximately 1.8× more per kilogram.
Hydrophilic (untreated) grades are generally preferred for acrylate-based UV systems because surface silanols interact well with polar monomers. Use hydrophobic grades only in non-polar UV silicone or thiol-ene systems where untreated silica causes wetting problems.
For SEMISIL 150, haze stays below 0.5% up to about 2 wt% in most acrylate resins. SEMISIL 380 can reach 2 wt% with haze still under 0.2%. Exceeding these thresholds without adjusting dispersion intensity typically pushes haze above specification limits.
Fumed silica agglomerates of 0.5–5 µm scatter light regardless of primary particle size. High-shear mixing at tip speeds above 15 m/s or three-roll milling below 10 µm gap breaks agglomerates to the primary particle level, which is essential for achieving sub-0.5% haze.
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