Fumed Silica Surface Treatment Comparison: DDS vs HMDS vs PDMS
Hydrophobic fumed silica grades differ not only in BET surface area but in the chemistry of surface treatment. DDS (DMDCS), HMDS, and PDMS treatments each create a distinct surface structure with different residual Si-OH levels, hydrophobicity, and compatibility with polymer/filler systems. Choosing the wrong treatment can undermine rheology control in ATH-filled composites or cause compatibility failures in silicone matrices.
| Spec | ZEMSIL FH-200D | ZEMSIL FH-200M | ZEMSIL FH-200V |
|---|---|---|---|
| Treatment agent | DMDCS (dimethyldichlorosilane) | HMDS (hexamethyldisilazane) | Polydimethylsiloxane (PDMS) |
| Reaction type | Covalent substitution / both ends | Covalent substitution / one end | Covalent anchor + physical coverage |
| Residual surface Si-OH | None – fully replaced | Low – partially replaced | Yes – buried under PDMS chain |
| Hydrophobicity | ★★★★★ | ★★★★ | ★★★ |
| Methanol wettability | ≥40% | ≥50% | ≥30% |
| BET after treatment | ~110 m²/g | ~150 m²/g | ~100 m²/g |
| Styrene/UPR compatibility | Excellent | Good | Moderate |
| Performance in ATH-filled UPR | Best – zero Si-OH affinity | Good | Poor – residual Si-OH risk |
| Best application | UPR/ATH FRP, toners, hydrophobic coatings | UPR, epoxy, PU, inks | Silicone RTV, defoamers, silicone sealants |
| Equivalent Aerosil | R972 | R805 | R202 |
How to choose
- ▸Choose DDS (FH-200D) when formulating ATH-filled UPR or any system where competitive adsorption with polar fillers must be avoided.
- ▸Choose HMDS (FH-200M) for organic resin systems (epoxy, polyurethane) where moderate hydrophobicity and good resin compatibility are the priority.
- ▸Choose PDMS (FH-200V) exclusively for silicone matrices (RTV, defoamers, silicone sealants) where PDMS chain compatibility with the silicone backbone is an advantage.
- ▸PDMS-treated grades carry residual buried Si-OH: avoid them in systems with polar competing surfaces (ATH, CaCO₃, BaSO₄) at high loadings.
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