Faq

Fumed silica grades are defined primarily by BET surface area and surface chemistry. Hydrophilic grades (untreated, with surface silanol groups at ~2.5 OH/nm²) range from 130 to 400 m²/g BET. Hydrophobic grades are post-treated with dimethyldichlorosilane (DDS) or hexamethyldisilazane (HMDS) to replace silanols with methyl groups, yielding carbon contents of 0.8–3.5 wt%.

Higher BET means smaller primary particles (7 nm at 380 m²/g vs 40 nm at 50 m²/g), stronger hydrogen-bonding networks, and greater thickening efficiency — but also harder dispersion. Grade selection always trades rheological performance against processing ease.

• Hydrophilic 200 — Most versatile general-purpose grade: 200 ±25 m²/g BET, 12 nm primary particle, pH 3.7–4.7 in 4% dispersion.
• Hydrophobic R972 — DDS-treated 130 m²/g grade for moisture-sensitive systems — carbon content ~1.0%, effective in polyester and epoxy resins.
• High-BET 380 — Ultra-fine 380 m²/g grade for maximum thixotropy at low loading, but requires high-shear dispersion (tip speed ≥20 m/s).
• HMDS-treated — Hexamethyldisilazane surface gives higher carbon content (2.5–3.5%) and better compatibility with silicone and UV-cure formulations.

Effective fumed silica loading depends on the resin system, target viscosity profile, and chosen grade. In liquid coatings and adhesives, hydrophilic grades at 0.5–2.0 wt% deliver anti-settling and sag resistance. Hydrophobic grades typically require 1.0–3.0 wt% for equivalent rheological effect because methyl surface groups form weaker interparticle networks.

Dispersion quality is the single largest variable in performance. Under-dispersed fumed silica wastes material and creates grit; over-shearing in sensitive systems can break polymer chains. A dissolver disc at 15–25 m/s tip speed for 15–30 minutes is standard for most coatings. Pre-wetting the powder into a 50/50 paste with solvent or plasticizer before let-down reduces dust and accelerates incorporation.

• Anti-settling — 0.5–1.5% hydrophilic grade in solventborne coatings; yields yield stress of 2–8 Pa, sufficient to suspend pigments at SG ≤ 4.5.
• Thixotropy — 1.0–2.5% loading produces shear-thinning index (η₁/η₁₀₀) of 3–8× depending on BET and polarity match.
• Reinforcement — In silicone rubber, 15–40 phr of 200 m²/g hydrophilic grade raises tensile strength from 0.3 to 8+ MPa.
• Free-flow aid — 0.1–0.5% on powder weight prevents caking in hygroscopic dry blends; works by coating particle surfaces and interrupting moisture bridging.

Amorphous fumed silica (CAS 112945-52-5) is distinct from crystalline silica and carries no IARC Group 1 classification. Workplace exposure limits vary by jurisdiction: OSHA PEL is 6 mg/m³ (total dust) for amorphous silica, while ACGIH TLV is 2 mg/m³ (respirable fraction). European REACH registration covers tonnages above 1,000 t/year for major producers.

Food-contact and pharmaceutical applications require specific tested grades. USP/NF monograph ‘Colloidal Silicon Dioxide’ covers fumed silica for pharma excipient use. EU Regulation 231/2012 lists silicon dioxide (E 551) as a permitted food additive with purity ≥ 99.0% SiO₂ on dried basis and As ≤ 3 mg/kg.

• SDS classification — Not classified as hazardous under GHS. Dust may cause mechanical eye and respiratory irritation — PPE: N95 mask and safety goggles during handling.
• FDA status — 21 CFR 172.480 permits silicon dioxide as food additive at ≤ 2% by weight of food. Separate indirect food-contact clearances exist under 21 CFR 175–178.
• Nano-regulation — EU defines nanomaterial as ≥ 50% particles below 100 nm. Fumed silica primary particles (7–40 nm) qualify, but aggregates typically measure 100–500 nm — regulatory classification depends on method of measurement.

Fumed silica pricing reflects raw material costs (chlorosilane or silicon tetrachloride feedstock), energy intensity of the flame hydrolysis process (1,100–1,600°C hydrogen-oxygen burner), and the oligopolistic market structure. Three producers — Evonik, Cabot, Wacker — control roughly 70% of global capacity (~600 kt/year). Chinese producers (Tokuyama-Henan, Hubei Huifu, Guangzhou GBS) have added ~200 kt since 2018, compressing margins on standard 200 m²/g grades.

Hydrophobic grades command a 40–100% premium over equivalent hydrophilic base grades due to the additional surface-treatment step and lower production throughput. Specialty high-BET grades (≥300 m²/g) carry another 20–50% premium because of lower furnace yields and tighter quality control.

• Hydrophilic 200 benchmark — $2,800–4,200/t FOB China; $4,500–6,500/t delivered Europe/NA for Western brands. Spot prices fluctuate with SiCl₄ availability.
• Hydrophobic premium — DDS-treated grades add $1,500–2,500/t; HMDS-treated grades add $2,000–3,500/t over base hydrophilic equivalent.
• Volume breaks — 20-ft container loads (14–16 t) typically unlock 8–15% discount vs palletized LTL. Annual contracts with quarterly price adjustment are standard.