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How to Compare the Hydrophobicity of Hydrophobic Fumed Silica
The strength of hydrophobicity of hydrophobic fumed silica is primarily determined by the surface silane modification coverage, the number of residual silanol groups, and the carbon content. It can be accurately compared through three methods: simple and rapid on-site testing, quantitative laboratory detection, and horizontal comparison of key indicators. This method is suitable for all scenarios including workshop screening, formulation adjustment, and quality inspection.
I. Simple and Rapid On-Site Comparison Method (No Instruments Required, Directly for Workshop Use)
Water Flotation Method (Most Commonly Used): Take an equal amount of the hydrophobic silica to be tested and gently sprinkle it on the surface of room temperature pure water. Complete floating, no wetting, no sinking, and no agglomeration indicate the best hydrophobicity; partial sinking and slow dispersion after stirring indicate moderate hydrophobicity; rapid wetting and direct sinking indicate poor hydrophobicity.
Pressure Flotation Method: Gently press the powder into water and release it. If it floats quickly and the particle surface is not wetted or clinging to water, it has strong hydrophobicity; if the powder surface is wetted and difficult to float, it has weak hydrophobicity.
Solvent Dispersion Comparison Method: The powder is dispersed in both polar solvents (water, ethanol) and non-polar solvents (white oil, toluene, solvent oil). Stronger hydrophobicity results in more uniform dispersion and higher transparency in non-polar solvents, while in polar solvents, it is more difficult to wet and more prone to aggregation.
II. Laboratory Quantitative Detection Method (Precise Comparison, R&D/Quality Control Standard):
* Methanol Titration Method (Industry Standard): Methanol is slowly added dropwise to pure water until the hydrophobic fumed silica is completely wetted and suspended. The higher the required methanol volume percentage, the stronger the hydrophobicity. Generally, a methanol value <30% indicates low hydrophobicity, 30%–50% indicates moderate hydrophobicity, and >50% indicates high hydrophobicity.
Water Contact Angle Test: The water contact angle is measured after pressing the powder into a sheet. A larger contact angle indicates stronger hydrophobicity. A contact angle <90° indicates weak hydrophobicity, 90°–120° indicates moderate hydrophobicity, and >120° indicates ultra-high hydrophobicity.
Carbon Content Testing (Most Direct Judgment Basis): Carbon content is determined through elemental analysis. Hydrophobic fumed silica relies on silane modification to introduce organic carbon chains. Under the same modification type, higher carbon content indicates more thorough surface modification and stronger hydrophobicity.
Surface Silanin Hydroxyl Group (Si-OH) Determination: Residual silanin hydroxyl groups on the surface are detected by infrared spectroscopy or chemical titration. Fewer residual hydroxyl groups indicate more complete modification and better hydrophobicity.
III. Horizontal Comparison of Key Physicochemical Indicators: Under the same specific surface area conditions, products with stronger hydrophobicity generally have lower oil absorption values; in high-temperature and high-humidity environments, products with stronger hydrophobicity are more stable, less prone to reverting to hydrophilicity, and less prone to agglomeration and thickening; in moisture-sensitive systems (such as polyurethane and epoxy adhesives), stronger hydrophobicity results in better system storage stability.
IV. Summary of Quick Judgment of Hydrophobicity Level: For hydrophobic fumed silica modified by the same type of modifier (such as dimethyldichlorosilane, silazane, polysiloxane), the higher the carbon content, the higher the methanol value, the larger the water contact angle, and the better the water buoyancy, the stronger the hydrophobicity. This can be directly used to compare the advantages and disadvantages between different models.
