Application Guide · Coatings & Paints
Fumed Silica for Paints & Coatings
Anti-settling agent, thixotrope, and flatting aid for waterborne, solventborne, and UV-cure systems. Five SEMISIL grades covering hydrophilic and hydrophobic requirements — with dosage guidance, grade comparison, and free samples.
Grade Selection — Which SEMISIL for Your Coating?
Fumed silica function in coatings depends on surface chemistry (hydrophilic vs. hydrophobic) and BET surface area. Higher BET = stronger thickening per unit dose. Hydrophobic grades disperse in non-polar and low-water-content systems without balling.
| Grade | Type | BET (m²/g) | Primary Function | Typical Coating System | Dosage |
|---|---|---|---|---|---|
| SEMISIL 150 | Hydrophilic | 150 ± 15 | Anti-settling, mild thixotropy | Architectural waterborne, primer | 0.5–1.5% |
| SEMISIL 200 | Hydrophilic | 200 ± 25 | Thixotropy, anti-sag, anti-settling | Waterborne decorative & industrial | 0.5–2.0% |
| SEMISIL 300 | Hydrophilic | 300 ± 30 | Strong thixotropy, reinforcement | High-build coatings, anti-corrosion | 0.3–1.5% |
| SEMISIL R202 | Hydrophobic (DDS) | 100 ± 20 | Anti-settling, free-flow in SB/UV | Solventborne, UV-cure, powder coat | 0.3–1.0% |
| SEMISIL R272 | Hydrophobic (HMDS) | 130 ± 25 | Thixotropy + water repellency | Solventborne automotive, marine | 0.5–2.0% |
For UV-cure flatting: GMATT U800 (synthetic amorphous silica, gel process) delivers controlled gloss reduction without the thickening side-effect of fumed silica.
Three Core Functions in Coating Formulation
1\. Anti-Settling Agent
Heavy pigments (TiO₂, iron oxide, barium sulfate) hard-settle within days if left unmodified. SEMISIL 200 at 0.5–1.0% loading forms a weak gel network that physically traps particles. Re-dispersion remains easy on stirring — no hard cake. Typical Hegman fineness impact: <0.5 units when properly dispersed at >12 m/s peripheral speed.
2\. Anti-Sag / Thixotropic Rheology Modifier
Vertical surfaces require coatings that resist gravity between application and cure. SEMISIL 200 / 300 increases low-shear viscosity (Brookfield) while leaving application viscosity (high-shear, brush/spray) unchanged. The thixotropic index (TI = η₁₀ / η₁₀₀) increases from ~1.2 (unmodified) to 3–6 with 1–2% fumed silica, sufficient to prevent sagging on 75–150 µm wet films.
3\. Flatting Agent (Secondary Function)
At concentrations above 2%, fumed silica generates surface micro-roughness that scatters light, reducing gloss (60° GU). For targeted flatting, dedicated silica matting agents such as GMATT series offer narrower PSD and more predictable gloss control.
Application Segments
| Coating Type | Recommended Grade | Key Benefit |
|---|---|---|
| Waterborne architectural | SEMISIL 150 / 200 | Anti-settling, low sheen |
| Solventborne industrial / anti-corrosion | SEMISIL R272 | Thixotropy + water repellency |
| UV-cure clear coat / flatting | SEMISIL R202 + GMATT U800 | Free-flow + gloss control |
| Automotive OEM / refinish | SEMISIL R272 | Anti-sag on complex shapes |
| Marine / protective epoxy | SEMISIL 300 / R272 | High TI + film reinforcement |
| Powder coatings | SEMISIL R202 | Free-flow, charge stabilization |
Formulation FAQ
+What dispersion speed is required for fumed silica in coatings?
Minimum peripheral speed 10–15 m/s (Cowles or saw-tooth blade). Below 10 m/s, agglomerates are not broken down and the thickening efficiency drops by 30–50%. Add silica to the resin/solvent phase before pigment addition for best results. Pre-dispersion in solvent as a 10–20% paste is an alternative for sensitive formulations.
+Hydrophilic vs. hydrophobic fumed silica — how do I choose for a waterborne system?
Hydrophilic (SEMISIL 150/200/300) is the default for waterborne coatings — the silanol surface wets readily in polar media and builds hydrogen bonds efficiently. Hydrophobic grades (R202/R272) are used when the waterborne system contains significant co-solvents or when the film must resist water spotting; they require a short pre-wetting step or surfactant addition to disperse properly.
+What dosage range prevents hard settling without over-thickening?
0.5–1.5% on total formulation weight is the practical window for anti-settling in most pigmented systems. At >2%, ICI viscosity starts to rise noticeably, affecting brush drag and leveling. Start at 0.8% and adjust based on Hegman grind gauge (target ≥ 5) and 6-month sedimentation test.
+Can SEMISIL 200 replace AEROSIL 200 in an existing formulation?
Yes — SEMISIL 200 is a direct equivalent to AEROSIL 200: BET 200 ± 25 m²/g, primary particle size ~12 nm, hydrophilic surface, tapped density ~50 g/L. Dosage and dispersion parameters are identical. A small-batch confirmation trial is recommended before full production switch; request a free 500 g sample and our comparison data sheet.
+Does fumed silica work in UV-cure coatings?
Yes, but use hydrophobic grades (R202/R272). Hydrophilic silica in acrylate systems can cause water absorption and haze. SEMISIL R202 is the standard for UV clear coats — it provides anti-sag without compromising cure speed or film clarity. For matting in UV coatings, combine R202 with GMATT U800 (0.5–1.0% R202 for rheology + 2–5% U800 for gloss control).
+Is fumed silica a matting agent or a rheology modifier?
Primarily a rheology modifier — fumed silica’s main value in coatings is thixotropy and anti-settling. At higher loadings (>2%) it reduces gloss via surface roughness, but control is less precise than dedicated synthetic amorphous silica matting agents. For formulations where flatting is the primary goal, use GMATT silica matting agents instead, which are engineered for narrow PSD and predictable gloss curves.