Aerosil R805 — Hydrophobic Fumed Silica (Octylsilane-treated)

Aerosil R805 — Octylsilane-Treated Hydrophobic Fumed Silica

Aerosil R805 (Evonik), HDK H30 (Wacker), and TS-720 (Cabot) are octylsilane-treated hydrophobic fumed silica grades. The starting material is hydrophilic Aerosil 200 (BET ≈ 200 m²/g), post-treated with n-octyltriethoxysilane to graft eight-carbon (C8) alkyl chains onto the surface. The resulting carbon content is 4.5–6.5% — significantly higher than the 0.6–1.2% of DMDCS-treated R972.

The longer alkyl chain produces:

• Stronger hydrophobicity (water contact angle >150°)
• Better wetting and dispersion in aliphatic hydrocarbon solvents (mineral oil, kerosene, hexane)
• FDA 21 CFR 175.300 compliance for indirect food-contact applications
• Lower thickening efficiency in polar systems compared to R972 (the alkyl chains shield residual silanols)

Specification (Evonik R805 datasheet):

When R805 Beats R972

R805 is the preferred choice when any of these requirements applies:

• Aliphatic hydrocarbon system: R972's DMDCS treatment is too short to wet out fully into mineral spirits, kerosene, or paraffin oil; R805's C8 chains do.
• Food-contact compliance: R805's octylsilane treatment is FDA 21 CFR 175.300 compliant for indirect food contact (cured silicone gaskets, mold-release coatings on food-contact surfaces).
• Maximum moisture barrier: R805 has the lowest water uptake of any commercial fumed silica grade, ideal for moisture-sensitive electronics potting and aerospace coatings.
• Long storage in HV cable terminations: high-voltage silicone insulating compounds with R805 maintain dielectric properties through years of warehouse storage in tropical climates.

For polar resin systems (epoxy, unsaturated polyester) and cost-driven thickening applications, R972 typically wins on cost-performance.

Primary Applications

Personal Care and Cosmetics — Food-Contact Variants

R805's FDA 21 CFR 175.300 compliance makes it the preferred fumed silica for:

• Lipstick and lip-balm rheology control
• Nail-polish thickening
• Mascara and eye-liner viscosity-building
• Antiperspirant and deodorant flow conditioning (at higher humidity than R972 can handle)
• Food-contact silicone moldings (baking molds, ice trays, food storage seals)

Aliphatic Solvent-Borne Adhesives and Sealants

In one-component PU sealants based on aliphatic plasticizers, R805 disperses uniformly without the polar-solvent assistance R972 sometimes requires. Typical loadings 5–10 wt% give non-sag rheology with cartridge storage stability >18 months at 30 °C.

High-Voltage Electrical Insulating Compounds

For HV cable joints, transformer bushings, and outdoor insulator coatings, the combination of low water uptake, dielectric stability, and arc resistance makes R805 the standard filler. Loadings of 10–20 phr in HTV silicone grease or RTV compound provide tracking resistance to ASTM D2303 / IEC 60587 Class 2A or higher.

Fumed-Silica-Modified Lithium Battery Electrolytes

A growing application is in semi-solid and gel-polymer lithium battery electrolytes, where R805's hydrophobicity prevents water contamination of the electrolyte while its branched aggregates mechanically suspend lithium-salt-containing solvents.

R805 vs R974 — Disambiguation

Confusingly, R974 and R805 sit at similar BET ranges (170 m²/g and 150 m²/g respectively) but use different chemistries:

If the system contains mineral oil, paraffin, or any lipophilic plasticizer, choose R805. If it's a polar paint or PU adhesive, R974 (or R972) is the cost-performance choice.

Equivalents

Related Reading

For polar-system hydrophobic alternative: R972 (DMDCS-treated). For hydrophilic baseline: Aerosil 200. Pillar context: fumed silica overview.