Aerosil R972 — Hydrophobic Fumed Silica (DMDCS-treated)

Aerosil R972 — DMDCS-Treated Hydrophobic Fumed Silica

Aerosil R972 (Evonik) — alongside Wacker HDK H17 and Cabot M-5K — is the workhorse hydrophobic fumed silica grade. The starting material is hydrophilic Aerosil 130 (BET ≈ 130 m²/g), which is post-treated with dimethyldichlorosilane (DMDCS) to replace approximately 50% of the surface silanols with dimethylsilyl groups.

The DMDCS treatment converts the silica from hydrophilic to moderately hydrophobic — the carbon content sits at 0.6–1.2% — and shifts the formulation behavior dramatically:

• Water contact angle increases from ~0° (hydrophilic Aerosil 130) to >130°
• BET surface area drops from ~130 to ~110 m²/g (DMDCS occupies surface area)
• pH shifts from 3.7–4.5 (acidic hydrophilic) to 3.6–4.4 (essentially unchanged)
• Tap density increases from ~50 to ~50–70 g/L (compaction during the post-treatment step)
• Thickening efficiency in non-polar systems increases 2–3×

Specification (Evonik R972 datasheet):

Why Hydrophobic Treatment Matters

Many systems are destabilized by water absorption from the air during storage:

• One-component PU adhesives: hydrophilic silica's surface water reacts with isocyanates, generating CO₂ bubbles and reducing pot life.
• RTV-1 silicone sealants: hydrophilic silica brings adsorbed water into the system, prematurely activating the moisture-cure mechanism in the cartridge.
• Solvent-borne hydrocarbon coatings: hydrophilic silica fails to disperse in non-polar solvents (mineral spirits, xylene, toluene), forming clumps.
• Anhydrous epoxy systems: hydrophilic silica's adsorbed moisture inhibits amine cure or causes blistering in cured films.

R972 solves all four problems while preserving most of the thickening capability of hydrophilic fumed silica.

Primary Applications

Solvent-Borne Coatings & Inks

In hydrocarbon-solvent systems (alkyd paints, polyester gel-coats, screen-printing inks), R972 at 1.5–4.0 wt% provides sag resistance and pigment anti-settling without the dispersion problems of hydrophilic silica. The DMDCS-treated surface hydrogen-bonds via residual silanols (still ~50% of surface) but does not absorb moisture, so paint storage stability extends from weeks to years.

One-Component Polyurethane

Hydrophobic R972 is the standard thickener for moisture-cured one-pack PU adhesives and sealants. Loadings of 4–8 wt% provide non-sag rheology while contributing zero free water that would prematurely cure the isocyanate. R972 is preferred over R805 (octylsilane-treated) when the system needs higher stiffness and more anchoring force into the resin matrix.

Cosmetics and Personal Care

R972 is widely used as a flow conditioner and matting agent in pressed-powder cosmetics, deodorants, and antiperspirants — its hydrophobic surface prevents caking under high-humidity storage.

Toner and Powder Flow

A long-running application is as an external additive on electrophotographic toner powders, where 0.5–1.5 wt% R972 dressing on the toner surface provides flow, charge control, and developer life — most office laser-printer toners contain R972 or its competitors.

R972 vs R805 vs R974

R972 is the most general-purpose hydrophobic grade; the alternatives serve specific niches:

• R972 (DMDCS-treated, BET 110 m²/g): balanced hydrophobicity and reactivity; the universal answer.
• R805 (octylsilane-treated, BET 150 m²/g): stronger hydrophobicity, longer alkyl chains; better in low-polarity systems and food-contact compliance.
• R974 (DMDCS-treated, BET 170 m²/g): same chemistry as R972 but on a higher-BET base; for stronger thickening efficiency at lower loading.
• R8200 (HMDZ-treated, BET 200 m²/g): silicone-rubber-grade hydrophobic; the standard filler for storage-stable HTV compounds.

Equivalents

Related Reading

For the highest hydrophobicity treatment: R805 (octylsilane-treated). For the hydrophilic alternative: Aerosil 200. Pillar context: fumed silica overview.