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When selecting fumed silica for a sealant or adhesive, BET surface area is not the only important parameter.
The surface chemistry of the silica can also influence compatibility, moisture interaction, dispersion, rheology, and processing behavior.
This is why hydrophilic and hydrophobic fumed silica with similar particle size or BET values may behave differently in the same formulation.
Hydrophilic fumed silica contains silanol groups on its untreated surface.
Hydrophobic grades are produced by chemically modifying part of the silica surface with organosilane or siloxane chemistry.
This treatment reduces the affinity of the surface for water and changes how the silica interacts with polymers and formulation ingredients.
The result is not simply “better silica.”
It is different surface chemistry for different formulation requirements.
Sealant formulations need to balance several properties:
The silica surface can influence how strongly particles interact with each other and with the polymer matrix.
For this reason, changing from a hydrophilic to a hydrophobic grade may change viscosity and processing even when the BET value appears similar.
Hydrophobic fumed silica may be worth evaluating when:
However, hydrophobic silica is not automatically the best choice for every sealant.
The curing chemistry, polymer type, filler loading, and required rheology should all be considered.
Two grades can have similar BET surface areas but different surface treatments.
As a result, they may produce different:
When changing grades, a parallel formulation trial is more reliable than direct replacement based only on the data sheet.
IOTA HB7135 is a hydrophobic fumed silica produced by post-treating a hydrophilic silica with HMDS.
The current IOTASILICA product page lists a BET surface area of approximately 210 m²/g and applications including silicone rubber, adhesives, and sealants. It also highlights processability and reinforcement at higher filler levels.
For an existing sealant formulation, a practical evaluation is:
Compare specifications → Run equal-dosage trials → Compare rheology and extrusion → Adjust processing if needed → Verify storage and finished-product performance
No. The appropriate choice depends on polymer chemistry, moisture sensitivity, rheology, and processing requirements.
Not necessarily. Surface treatment can significantly change formulation behavior.
Surface chemistry changes particle-particle and particle-polymer interactions, which can change rheology and dispersion.
Viscosity, thixotropy, extrusion, dispersion, storage stability, and relevant finished-product properties should be compared.
HB7135 is an HMDS-treated hydrophobic fumed silica intended for applications including silicone rubber, adhesives, and sealants.
Product Reference: IOTA HB7135 Hydrophobic Fumed Silica
Final material selection should be confirmed in the customer's own form
