DDS / DMDCS — Dimethyldichlorosilane

DDS is a bifunctional chlorosilane. It reacts directly with surface silanol groups via condensation:

≡Si–OH + Cl–Si(CH₃)₂–Cl + HO–Si≡  →  ≡Si–O–Si(CH₃)₂–O–Si≡ + 2 HCl

One DDS molecule bridges two adjacent Si–OH groups, forming a stable siloxane cross-link. Grafting density is the highest of the three agents — effectively every accessible Si–OH is replaced. The result is a pure dimethylsilyl surface with no residual Si–OH.

• Bond type: covalent Si–O–Si (stable to >300°C)
• Residual Si–OH: none
• Surface group: –Si(CH₃)₂–
• Methanol wettability: ≥ 40%
• Representative grades: Aerosil R972, CAB-O-SIL TS-530, SEMISIL D100

HMDS — Hexamethyldisilazane

HMDS reacts with surface Si–OH via silylation, releasing ammonia:

2 ≡Si–OH + (CH₃)₃Si–NH–Si(CH₃)₃  →  2 ≡Si–O–Si(CH₃)₃ + NH₃

Each HMDS molecule silylates two Si–OH groups. Grafting density is lower than DDS because the bulkier trimethylsilyl group causes steric hindrance — not every Si–OH is accessible. A small fraction of Si–OH remains unreacted, though the surface is still strongly hydrophobic.

• Bond type: covalent Si–O–Si
• Residual Si–OH: low (steric limitation)
• Surface group: –Si(CH₃)₃
• Methanol wettability: ≥ 50%
• Representative grades: Aerosil R805, CAB-O-SIL TS-610, SEMISIL M200

PDMS — Polydimethylsiloxane

PDMS is a polymer. Hydroxyl-terminated PDMS anchors to the silica surface at its chain ends:

HO–[Si(CH₃)₂–O]ₙ–H + HO–Si(surface) → chain-end covalent anchor

The chain ends bond covalently, but the interior of the polymer chain merely lies across the silica surface, physically covering — not chemically reacting with — the Si–OH groups beneath it. These underlying Si–OH groups are buried but intact.

• Bond type: covalent at chain ends; physical coverage in between
• Residual Si–OH: significant — buried, exposable
• Surface group: PDMS polymer layer
• Methanol wettability: ≥ 30%
• Representative grades: Aerosil R202, CAB-O-SIL TS-720, SEMISIL P100

Buried Si–OH under PDMS coverage has practical consequences in demanding applications. Four mechanisms expose these groups:

1. Mechanical pressure from dense fillers: In high-loading ATH systems (40–65 phr), ATH particles exert compressive forces on silica. PDMS chains are displaced laterally, exposing buried Si–OH, which immediately adsorbs onto ATH.
2. Cure exotherm: UPR cure peaks at 80–120°C. Elevated temperature increases PDMS chain mobility. At the critical liquid-to-gel transition, chains reorganize, transiently exposing buried Si–OH — network failure occurs precisely when stability is most needed.
3. Storage aging: At room temperature, PDMS chains slowly reorient over weeks to months. Stored formulations that initially showed good anti-settling behavior gradually develop hard settling as progressive Si–OH exposure allows silica–ATH bonding to accumulate.
4. High-shear dispersion: Intense mixing transiently lifts PDMS chains. If ATH is already present, exposed Si–OH binds to Al–OH irreversibly — the silica is permanently anchored to ATH, removed from the network.

• UPR/styrene + ATH (flame retardant FRP): DDS (SEMISIL D100). Maximum hydrophobicity, no residual Si–OH, zero ATH affinity.
• Epoxy or polyurethane systems: HMDS (SEMISIL M200). Trimethylsilyl surface offers excellent resin compatibility and strong thixotropy.
• Silicone RTV, silicone sealants, defoamers: PDMS (SEMISIL P100). The PDMS surface coating is chemically compatible with the silicone matrix backbone — this is the one application where PDMS is the correct choice.
• Toners, powder coatings, flow aids: DDS (SEMISIL D100). Inert methyl surface prevents moisture pickup and ensures consistent charge behavior.
• Systems with high-loading polar fillers (ATH, CaCO₃, BaSO₄): Avoid PDMS. Use DDS or HMDS.

DDS, HMDS, and PDMS produce fundamentally different surface chemistries despite all yielding "hydrophobic" fumed silica by wettability tests. The critical discriminator is residual Si–OH: DDS eliminates it completely; HMDS reduces it to a low level; PDMS covers it without reacting, leaving it exposable under demanding conditions. For ATH-filled UPR systems — where competitive adsorption, cure exotherm, and storage stability all converge — DDS-treated silica is the only treatment that eliminates all risk pathways simultaneously.