Aerosil 200 / HDK N20 — Hydrophilic Fumed Silica 200 m²/g

Aerosil 200 — The Industry-Standard Hydrophilic Fumed Silica

Aerosil 200 (Evonik) — and its Wacker counterpart HDK N20 and Cabot counterpart Cab-O-Sil M-5 — is the global benchmark grade for hydrophilic fumed silica at BET 200 ± 25 m²/g. For four decades it has been the default reinforcing filler for HTV silicone rubber compounding and the default thickener-of-record for solvent-borne paints, polyester gel-coats, and unsaturated polyester resins.

Specification (Evonik Aerosil 200 datasheet, typical):

The hydrophilic surface (≈4 SiOH groups per nm², about 800 μmol/g) gives Aerosil 200 strong hydrogen-bonding capability — the basis of both its reinforcing efficiency in silicone rubber and its thickening efficiency in polar systems.

Use in HTV Silicone Rubber Compounding

For high-temperature vulcanizing (HTV) silicone rubber, BET 200 m²/g represents the optimal trade-off between reinforcement potential and processing viscosity. At 25–35 phr loading, Aerosil 200 produces:

• Tensile strength 8–11 MPa
• Elongation at break 300–600%
• Tear resistance 15–30 kN/m
• Shore A hardness 30–60 (depending on loading and gum viscosity)

The principal compounding challenge with hydrophilic fumed silica in HTV is structuring (also called crepe-hardening): adsorbed silica-silica networks form via hydrogen bonding during storage, causing the uncured compound to harden and lose plasticity over weeks. Mitigation strategies include in-situ surface treatment with hexamethyldisilazane (HMDZ) or vinyltetramethyldisilazane during compounding (5–10 phr), or pre-treated alternatives like Aerosil R8200 (HMDZ-treated 200 m²/g grade).

Use in Paint, Adhesive, and Gel-Coat Thickening

For solvent-borne paint and unsaturated polyester gel-coat thickening, hydrophilic Aerosil 200 at 1.5–3.5 wt% provides:

• Sag resistance (rest viscosity 5×–20× higher than base resin)
• Thixotropic flow recovery (10–60 s rest time)
• Anti-settling for pigment and filler suspensions
• Surface texture control (matting at higher loadings)

Polar resin systems (epoxy, unsaturated polyester, alkyd) work well with hydrophilic Aerosil 200 because the resin's hydroxyl/carbonyl groups co-bond with surface silanols. Non-polar systems (PE, PP, hydrocarbon-solvent paints) require hydrophobic grades — see R972 or R805 for non-polar systems.

Aerosil 200 vs Aerosil 300

Aerosil 300 (BET 270–330 m²/g) provides higher reinforcement and thickening efficiency at the cost of dispersion difficulty and higher compound viscosity. The decision rule:

• Use Aerosil 200 for general HTV rubber, standard paint thickening, and any application where dispersion equipment is limited (single-screw extruders, low-shear mixers).
• Upgrade to Aerosil 300 for premium clear adhesives, high-clarity LSR, and electronic encapsulants where additional surface area justifies the dispersion overhead.

Equivalents and Alternatives

Aerosil 200 is the most widely cross-licensed and cloned fumed silica grade. Direct equivalents from other producers:

For most applications these grades are interchangeable on a 1:1 basis, but always verify with a small-batch trial before substitution — minor differences in tap density, structure, and pH can shift compound viscosity profiles.

Related Reading

Compare with Aerosil 300 (higher BET), R972 (DMDCS-treated hydrophobic), and R805 (octylsilane-treated). For the broader fumed-silica decision context, see the pillar guide.