Hydrophobic Fumed Silica | HMDS, DDS & PDMS Treated Silica | SEMITECH
Advanced Surface Treatment

SEMISIL® Hydrophobic Fumed Silica

Chemically modified fumed silica engineered for extreme moisture resistance, superior anti-structuring in rubber, and powerful rheology control in highly polar resin systems.

Key Capabilities

  • Polar System Rheology: Provides stable, long-lasting thixotropy and anti-sagging properties in polar systems (epoxy, polyurethane, vinyl ester) where hydrophilic grades fail.
  • Anti-Structuring (Crepe Hardening): Effectively shields reactive silanols, preventing premature hardening of HCR silicone rubber during storage.
  • Ultimate Moisture Barrier: Highly water-repellent nature makes it essential for corrosion-resistant coatings, cable gels, and moisture-sensitive powders.
  • Extended Shelf-Life: Prevents viscosity drift and maintains stable extrusion rates in RTV silicone sealants over long periods.

Primary Applications

  • LSR, HTV & HCR Silicone Rubber
  • RTV Sealants & Structural Adhesives
  • Epoxy Resins & High-End Clear Coatings
  • Fiber Optic Cable Gels
  • Moisture-sensitive Technical Powders

Grade Selection Matrix

Compare our hydrophobic treatments (HMDS, DDS, PDMS) to find the exact chemical match for your formulation. Proper silane selection is critical for optimizing compatibility and reinforcement.

SEMISIL® R620 Treatment: HMDS
Primary Advantage: LSR clarity & tear strength.
Based on a 200 m²/g carrier, it provides superior mechanical reinforcement and high hydrophobicity. The gold standard for transparent LSR and HTV silicone rubber systems, as well as an anti-settling agent for high-end clear coats.
SEMISIL® R272 Treatment: DDS
Primary Advantage: RTV stability & shelf-life.
Optimized for moisture-sensitive formulations. Acts as a superior long shelf-life thixotropic agent, preventing viscosity drift in RTV sealants and ensuring optimal flow and pigment suspension in printing inks.
SEMISIL® R202 Treatment: PDMS
Primary Advantage: HCR anti-structuring & epoxy rheology.
Engineered for maximum moisture resistance. Eliminates the "structuring" effect in rubber and provides elite rheology control in polar resins (epoxy, vinyl ester), ensuring zero viscosity drop over time. Essential for cable gels.
💡 Formulator Tip: Matching the correct chemical treatment to your resin is crucial. Use PDMS (R202) for highly polar epoxy resins or HCR anti-structuring. Choose DDS (R272) for RTV sealants, while HMDS (R620) offers the ultimate optical clarity for LSR.

General Properties (Hydrophobic Series)

Typical physico-chemical data across the SEMISIL® hydrophobic portfolio. Each grade undergoes precise surface modification to deliver stable carbon content and target hydrophobicity.

Properties & Test Methods Typical Value Unit
Surface Treatment TypeHMDS / DDS / PDMS-
Specific Surface Area (BET)100 ~ 220m²/g
Carbon Content0.6% ~ 5.5%Wt%
CAS Number112945-52-5-
Primary Particle SizeNano-scalenm

* Exact BET and Carbon Content depend on the specific grade (R620, R272, R202). Refer to the TDS for precise batch ranges.

Frequently Asked Questions

Rheology and surface modification solutions from our R&D engineers.

Why use hydrophobic fumed silica instead of hydrophilic?

Hydrophilic silica relies on its surface silanol groups to form hydrogen bonds and build viscosity. However, in highly polar systems (like epoxy, polyurethane, or water-rich formulas), the liquid itself bonds with the silica, causing the thickening network to collapse. Hydrophobic silica solves this by using chemically bonded alkyl groups to form a stable, moisture-resistant 3D network that remains robust in polar environments.

How does SEMISIL R202 prevent "crepe hardening" in silicone rubber?

In High Consistency Rubber (HCR), untreated silica tends to react strongly with the silicone polymer chains over time, causing the rubber to become stiff and unworkable (crepe hardening or structuring). SEMISIL R202 is treated with PDMS, which heavily shields the reactive surface silanols. This acts as a powerful anti-structuring agent, keeping the uncured rubber soft and processable even after prolonged storage.

What is the difference between HMDS, DDS, and PDMS treatments?

HMDS (R620) replaces silanols with trimethylsilyl groups, offering the highest optical clarity and mechanical reinforcement, making it perfect for LSR. DDS (R272) provides dimethylsilyl groups, offering excellent viscosity stability and shelf-life for RTV sealants. PDMS (R202) grafts long silicone oil chains onto the silica, providing the most extreme moisture barrier, supreme anti-structuring, and heavy rheology control in epoxy.

Does hydrophobic fumed silica require different mixing equipment?

Because of its highly water-repellent nature and low bulk density, hydrophobic fumed silica can be more challenging to wet out than hydrophilic grades. It has a tendency to float on the liquid surface. Therefore, efficient high-shear mixing equipment (like rotor-stator mixers, planetary mixers, or three-roll mills) is strongly recommended to achieve full dispersion and activate its maximum thickening power.

Explore Related Products

SEMISIL® Hydrophilic For Non-Polar Systems
SEMISIL® H100 Fumed Matting Agent

Ready to Test the Hydrophobic Series?

Download the full Technical Data Sheet (TDS) and Safety Data Sheet (SDS), or request a complimentary lab sample for your formulation.

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