Hydrophilic vs Hydrophobic Fumed Silica in UPR/ATH Systems
Aluminum Trihydrate (ATH) is the dominant halogen-free flame retardant in UPR/FRP composites, used at 40–65 phr loading. Whether hydrophilic or hydrophobic fumed silica can suspend this ATH loading — without causing bridging flocculation or hard settling — is determined entirely by surface chemistry. This comparison explains why ZEMSIL FS-200 (hydrophilic) fails in ATH/UPR systems and why ZEMSIL FH-200D (DMDCS hydrophobic) is the only correct choice.
| Spec | ZEMSIL FS-200 | ZEMSIL FH-200D |
|---|---|---|
| Surface chemistry | Si-OH rich (hydrophilic) | Si-O-Si(CH₃)₂ (DMDCS-treated) |
| BET surface area | 200 m²/g | ~110 m²/g |
| Styrene wettability | Poor | Excellent |
| ATH surface affinity | Strong – causes bridging | None – stays in resin |
| Thixotropic network in UPR | Fails – migrates to ATH | Effective – builds 3D network |
| Hard settling prevention | No – worsens settling | Yes – soft settling, re-dispersible |
| Recommended dose in ATH system | Not recommended | 0.5–1.5% |
| Methanol wettability | Wets in water | ≥40% methanol |
| Best application fit | Silicone rubber, pharma, cosmetics | UPR/ATH FRP, flame retardant composites |
How to choose
- ▸Hydrophilic grades are the correct choice for aqueous, polar, or silicone rubber systems — not for styrene-diluted polyester.
- ▸In ATH-loaded UPR, the competitive Si-OH···HO-Al adsorption drives hydrophilic silica onto the ATH surface, removing it from the resin network and causing bridging flocculation.
- ▸ZEMSIL FH-200D's DDS-treated surface (pure –CH₃) ensures silica stays in the resin phase and builds the thixotropic network needed to suspend high ATH loadings (40–65 phr).
- ▸Dispersion sequence matters: pre-disperse FH-200D in styrene before adding UPR and ATH for optimal network formation.
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