Quick Search
CAS no: 68611-44-9
EC / List no: 271-893-4
Chemical formula: SiO2
Synonyms: Silane, dichlorodimethyl-, reaction products with silica, Hydrophobic Amorphous Fumed Silica, Hydrophobic Fumed Silica, Reaction Products of Silica and Dichlorodimethylsilane, Silicon Dioxide, Reaction Product with Dimethyldichlorosilane, Silica, Reaction Product with Dimethyldichlorosilane, Dimethyldichlorosilane-Treated Silica, Surface-Treated Fumed Silica, Hydrophobically Modified Fumed Silica, Treated Filler
Hydrophobic Fumed Silica is an advanced engineered amorphous nanomaterial (SiO2) produced via the flame hydrolysis of silicon chlorosilanes, followed by a precise post-treatment surface modification. By chemically reacting hydrophilic fumed silica with reactive silanes or siloxanes—such as Hexamethyldisilazane (HMDS), Dimethyldichlorosilane (DDS), or Polydimethylsiloxane (PDMS)—surface reactive silanol groups are successfully capped with organic methyl groups.
This post-treatment transforms the ultrafine powder into a highly water-repellent, low-polarity material that resists wetting, lowers surface moisture absorption, and exhibits excellent dispersibility in non-polar, medium-polarity, and highly polar organic solvent systems.
Key Properties of Hydrophobic Fumed Silica
Appearance: Fluffy, ultra-fine, white amorphous powder
Odor: Odorless
Tamped Density: ~40–60 g/L (highly lightweight and fluffy)
True Density: ~2.2 g/cm³
Boiling Point: ~2230 °C
Melting Point: ~1700 °C
Solubility: Insoluble in water (completely hydrophobic; floats on the surface); dispersible in organic solvents and polymer matrices
Surface Area (BET): ~90–300 m^2/g (depending on the specific grade)
Carbon Content: > 2.7 wt% chemically bound carbon (indicates high hydrophobicity)
Methanol Wettability: ~50–65% (index of water repellency)
pH Value: ~3.5–5.5 (in a 4% dispersion of water-methanol mixture)
Chemical Structure: Synthetic amorphous silicon dioxide treated with organic silyl groups (e.g., from HMDS, PDMS, or DDS)
Polarity Compatibility: Medium to high polarity systems (epoxies, polyurethanes) and certain low polarity systems (MS polymers)
Stability: Highly stable chemically and physically under dry storage conditions; thermal stability up to ~300°C (depending on the organic surface treatment)
Reactivity: Chemically inert to most substances; surface methyl groups provide stability against moisture degradation
Main Function: Rheology modifier, thixotropic thickener, structural reinforcement filler, anti-settling/anti-sagging agent, and powder free-flow aid
Structure and Chemical Behavior
The specialized performance profile of hydrophobic fumed silica stems from its unique three-dimensional network structure combined with custom surface modifications:
Water-Repellent Surface Chemistry: Chemically bonded alkyl or polydimethylsiloxane chains permanently lower the surface energy of the nano-silica particles. Even though its true density is greater than water, the powder completely repels water molecules and floats on the surface.
Branched Nanostructure: The pyrogenic process builds complex, branched, three-dimensional particle networks. When dispersed, these aggregates form reversible hydrogen bonds combined with hydrophobic interactions, establishing highly efficient physical barriers.
Low Polarity Matrix Compatibility: The organic surface treatment ensures exceptional miscibility and high loading capacities within organic resins, polyols, polymers, and non-polar oils without inducing unwanted particle agglomeration.
Key Advantages of Hydrophobic Fumed Silica
Integrating hydrophobic fumed silica into manufacturing and formulation processes delivers critical performance upgrades:
Exceptional Moisture Resistance: Exhibiting extremely low hygroscopicity, it acts as a highly effective desiccant and moisture barrier, ensuring long-term rheological and storage stability in water-sensitive formulations.
Advanced Rheology & Thixotropy Control: Delivers powerful shear-thinning (thixotropic) properties. It structure-directs liquid systems to increase viscosity at rest while reversing thickening easily under minimum processing energy.
Structural Reinforcement: Enhances mechanical cross-linking in elastomers and polymers, boosting tensile strength, tear resistance, and elongation at break without excessively increasing baseline system viscosity.
Anti-Caking & Free-Flow Promotion: Acts as a physical spacer between particles, eliminating undesirable electrostatic charge effects and moisture pickup to preserve the bulkiness and fluidity of powders.
Industrial Applications
Due to its multifunctional particle dynamics, hydrophobic fumed silica is deployed across a vast array of global industrial, commercial, and high-tech sectors:
Paints, Coatings, and Printing Inks
Hydrophobic fumed silica is widely used in solvent-borne, waterborne, and powder coatings to optimize processing and film durability:
Rheology Modifier: Functions as an anti-settling and anti-sagging agent for pigments and heavy fillers, preventing dripping on vertical surfaces while maintaining smooth leveling.
Surface Enhancement: Drastically hardens dried films, maximizing scratch, abrasion, UV, and long-term weather resistance.
Heavy-Duty Protective Systems: Crucial for marine anti-corrosion coatings and automotive OEM topcoats, preventing blistering, peeling, and moisture degradation under continuous salt spray exposure.
Adhesives and Sealants
It provides specialized structural enhancement in demanding assembly applications:
Resin Thickening: Imparts strong thixotropic effects in polar and non-polar resin matrices, including epoxy, polyurethane, vinyl ester, silicone, cyanoacrylate, and MS polymer systems.
Slump Prevention: Eliminates slump or sag on vertical joints in structural curtain wall RTV sealants and automotive engine bay sealing compounds.
Shelf-Life Extension: Minimizes premature moisture-induced curing inside the packaging, ensuring reliable storage stability.
Silicone Rubber and Elastomers
Reinforcing Filler: Serves as an indispensable reinforcing agent in Room-Temperature Vulcanizing (RTV-2C), High-Temperature Vulcanizing (HTV), and Liquid Silicone Rubber (LSR) systems.
Mechanical Fortification: Impedes crack propagation and enhances resistance to deformation under extreme temperatures, which is highly beneficial for industrial gaskets, molded parts, and foaming materials.
Cosmetics and Personal Care
Texture & Matte Finish: Enhances spreadability, improves sensory texture, and provides excellent light-diffusing properties in makeup and skincare formulations.
Anhydrous Formulations: Boosts water resistance and counteracts oiliness in anhydrous products like lipsticks, sunscreens, protective creams, and pressed powders.
Powder Processing, Agriculture, and Food Additives
Glidant & Flow Aid: Added in tiny percentages to hygroscopic powders to ensure uniform flowability during industrial handling and dosing.
Industrial and Commercial Powders: Prevents lump formation and caking in agricultural plant protection compounds, pesticide carriers (offering a controlled slow-release effect), fire extinguishing powders, and table salt or nutritional powders (utilizing specialized food-grade E551 compliance).
Electronics, Cable Compounds, and Greases
Electronic Packaging: Its high bulk density, low solid-state thermal conductivity, and electrical resistance make it ideal for electronic potting compounds and thermal insulation materials.
Dielectric Stability: Acts as an anti-blocking agent and moisture barrier in cable insulation gels, boosting dielectric properties.
Functional Fluids: Provides structural thickening, thermal stability, and separation resistance in high-performance industrial greases and lubricants.
Specialized Environmental Technologies
Pollutant Adsorption: Added to advanced aerogels and composite structures to exponentially increase their ability to adsorb organic pollutants and oil spills from aqueous solutions without disrupting the underlying network structure.
Manufacturing Process
Hydrophobic fumed silica is manufactured via a multi-stage continuous process. First, high-purity amorphous hydrophilic silica is synthesized via flame hydrolysis, reacting silicon tetrachloride (SiCl4) in an oxygen-hydrogen flame at elevated temperatures to yield nano-sized SiO2 particles.
In a subsequent phase, typically within a specialized fluid bed reactor, the hydrophilic silica is subjected to a surface chemical reaction with reactive silane agents (such as hexamethyldisilazane, dimethyldichlorosilane, or polydimethylsiloxane). The treating agents covalently bond with the surface silanol groups (Si-OH), capping them with hydrophobic methyl functionalities. The final product undergoes precise fractional separation and de-acidification to ensure extreme batch-to-batch consistency and high chemical purity.
Hydrophobic fumed silica stands as an indispensable, high-performance nanomaterial that solves critical processing challenges across multiple global industries. By perfectly balancing permanent water repellency with a versatile three-dimensional nanostructure, it delivers unmatched rheology control, anti-settling stability, and structural reinforcement. Whether optimizing heavy-duty industrial coatings or maintaining the free-flow properties of fine powders, hydrophobic fumed silica remains a vital driving component for modern formulation efficiency and long-term product durability.
