Fumed silica produced by flame hydrolysis carries a surface rich in silanol groups (Si–OH) — polar, reactive, and hydrophilic. This is the baseline: hydrophilic fumed silica.

Hydrophobic fumed silica is produced by reacting this surface with silylating agents. DDS (dimethyldichlorosilane) converts Si–OH to Si–O–Si(CH₃)₂; HMDS yields Si–O–Si(CH₃)₃; PDMS deposits a polymer layer. The polar hydroxyl is replaced by non-polar organic groups.

This surface chemistry difference — polar Si–OH vs. non-polar –CH₃ — determines everything about how each grade behaves in a UPR/styrene/ATH system.

Non-Wetting by Styrene

Styrene monomer (η ≈ 0.7 mPa·s) is a low-polarity aromatic hydrocarbon. The large polarity mismatch with hydrophilic silica means aggregates resist breakdown during dispersion. No primary-particle network forms; what little structure exists collapses immediately.

Competitive Adsorption onto ATH

Aluminum trihydrate (ATH) has a surface densely covered with Al–OH groups. In a non-polar styrene medium, the thermodynamic driving force strongly favors polar-polar association. Hydrophilic silica (Si–OH) migrates from the resin phase to the ATH surface (Al–OH), driven by Si–OH···HO–Al hydrogen bonding. The silica abandons the resin phase entirely.

Bridging Flocculation

A single silica aggregate can simultaneously contact two ATH particles, bonding to both via Si–OH···HO–Al hydrogen bonds. This bridging flocculation creates large, dense ATH aggregates that settle faster and pack more tightly than ATH alone — the opposite of the intended effect.

DDS-treated hydrophobic silica has a surface of pure –CH₃ groups. The contrast across all three failure mechanisms is complete:

Hydrophobic silica particles in a non-polar medium form a three-dimensional network through van der Waals interactions between methyl-covered surfaces. This network is:

• Thixotropic: it has a yield stress that prevents sedimentation under gravity but breaks down under shear (mixing, pumping) and reforms when shear is removed
• Stable under storage: because the –CH₃ surface does not interact with ATH, the network remains intact regardless of ATH loading
• Effective at low dose: 0.8–1.2% SEMISIL D100 is typically sufficient to suspend 50 phr ATH in UPR/styrene for months without hard settling

The correct hydrophobic grade will underperform if added in the wrong order:

1. Styrene → SEMISIL D100: disperse at ≥ 3,000 rpm for 10–15 min — low viscosity allows full aggregate breakdown
2. Add UPR resin: mix thoroughly; network begins to set
3. Add ATH last: low-speed incorporation — ATH is now surrounded by an established silica network

Adding silica after ATH allows competitive adsorption before the network forms. The silica adsorbs onto ATH instead of into the resin, and the opportunity to build a protective network is lost.

• Use SEMISIL D100 (DMDCS-treated, BET ~110 m²/g) for UPR/styrene/ATH systems requiring hard-settling prevention
• Do not substitute hydrophilic grades in this application — they are counterproductive
• HMDS-treated grades (SEMISIL M200) are a viable alternative but slightly less effective in high-ATH systems due to lower grafting density
• PDMS-treated grades should be avoided: residual buried Si–OH groups are exposed under ATH pressure and cure exotherm conditions, progressively degrading anti-settling performance