Aerosil 300 / HDK T30 — Hydrophilic Fumed Silica 300 m²/g

Aerosil 300 — Premium-Performance Hydrophilic Fumed Silica

Aerosil 300 (Evonik), HDK T30 (Wacker), and Cab-O-Sil EH-5 (Cabot) are the BET 300 m²/g hydrophilic fumed silica grades — a step up from the 200 m²/g standards in surface area, reinforcing efficiency, and thickening power. The 50% increase in surface area versus Aerosil 200 translates directly into:

• 30–40% higher tensile strength in HTV silicone rubber at equal loading
• 20–30% lower silica loading requirement to reach a target Shore-A hardness
• 30–50% higher thickening efficiency in clear resin systems
• Better optical clarity in cured silicone elastomers (smaller aggregate size scatters less light)

The trade-off is processing difficulty: BET 300 m²/g grades are noticeably harder to disperse than 200 m²/g grades, and uncured compounds are more prone to structuring (crepe-hardening) on storage.

Specification (Evonik Aerosil 300 datasheet):

When to Choose Aerosil 300 Over Aerosil 200

The decision rule is application-driven:

Choose Aerosil 300 when:

• Compounding LSR (liquid silicone rubber) for medical, optical, or precision-molded applications. The smaller aggregates produce better-flowing two-component systems with lower yield stress.
• Producing optically clear silicone elastomers for LED encapsulation, optical adhesives, or display gaskets. The reduced light scattering at higher BET preserves transparency.
• Formulating high-clarity adhesives where any haze is unacceptable.
• Reaching tear resistance > 30 kN/m in HTV — a target Aerosil 200 struggles to hit even at 35 phr.

Stick with Aerosil 200 when:

• Cost-driven mass-market HTV (industrial gaskets, cable insulation, general silicone rubber goods).
• Compounding equipment is limited to single-screw extruders or low-shear internal mixers.
• The downstream cure system is sensitive to acid pH (Aerosil 300 may slightly accelerate acid-sensitive systems due to its higher surface silanol concentration).

Use in LSR (Liquid Silicone Rubber)

LSR systems are pre-mixed two-component (Part A + Part B) injection-moldable silicones, typically platinum-cured. The filler must:

• Disperse fully into low-viscosity vinyl-PDMS gum without grit
• Maintain uniform distribution across both components after batching
• Deliver target Shore-A hardness (typically 30–70 A) without exceeding viscosity limits for injection (typically 10⁵–10⁶ cP at low shear)
• Not poison the platinum catalyst (no surface contaminants)

Aerosil 300 — particularly post-treated grades like R8200 (HMDZ-treated 300 m²/g) — meets all four criteria. Hydrophilic Aerosil 300 can be used directly but typically requires in-situ HMDZ treatment during compounding to achieve LSR-grade flow.

Use in High-Clarity Adhesive Systems

For acrylic adhesives, UV-curable resins, and unsaturated polyester gel-coats where any haze is rejected by the customer, Aerosil 300's smaller primary aggregates are essential. At 1.5–3.0 wt% loading in clear acrylic or epoxy systems, Aerosil 300 produces sag-resistant pastes that retain near-water clarity in a 1–2 mm bondline. The same loading of Aerosil 200 would produce visible haze.

Equivalents and Alternatives

Substitution requires more careful trial than for Aerosil 200 — the higher surface area amplifies any minor process differences between producers, particularly tap density and structure (DBP-equivalent).

Related Reading

For the standard alternative: Aerosil 200. For hydrophobic alternatives at similar BET: [R8200 (HMDZ-treated 300 m²/g)] in the pillar guide. For LSR formulation context: silicone rubber pillar.