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A silicone sealant manufacturer faces a frustrating situation:
The sealant is difficult to extrude from the cartridge, but the applied bead still sags on a vertical surface.
At first, this may seem contradictory.
If the sealant is already difficult to extrude, shouldn't it also resist sagging?
Not necessarily.
The reason is that extrusion behavior and sag resistance describe different stages of the application process.
During extrusion, the sealant is forced through a cartridge or dispensing nozzle.
The material must flow under the applied pressure and shear conditions.
After application, the sealant needs to retain its shape under relatively low stress.
Therefore, an effective formulation needs an appropriate balance between:
Flow during application and shape retention after application.
A formulation that performs well in one stage may still perform poorly in the other.
Increasing the overall viscosity may make the sealant harder to dispense.
However, this does not guarantee that the material will maintain the required bead shape after application.
The result also depends on its rheological structure and recovery behavior.
For this reason, sealant manufacturers should evaluate:
Extrusion force
Flow under application conditions
Shape retention after extrusion
Rheological recovery
Finished sealant properties
Rather than relying only on one viscosity measurement.
Fumed silica is widely evaluated in silicone formulations for rheology modification and reinforcement.
Its influence depends on several factors:
Surface treatment
Specific surface area
Dispersion
Filler concentration
Polymer system
Other formulation components
Changing the silica grade can therefore change processing behavior without necessarily producing the expected improvement in every property.
IOTA HB7135 Hydrophobic Fumed Silica is an HMDS-treated fumed silica.
Its published product specifications include:
|
Property |
Published specification |
|---|---|
|
BET surface area |
210 m²/g |
|
pH, 4% dispersion |
7.0–8.5 |
|
Loss on drying, 105°C for 2 hours |
≤0.5% |
|
Silica content, ignited basis |
≥99.8% |
|
Surface treatment |
HMDS |
The product is positioned for applications including silicone rubber, adhesives and sealants, with emphasis on low thickening effect, processability and reinforcement in silicone rubber.
It may therefore be considered when a manufacturer is trying to improve the processing balance of a silicone formulation.
However, low thickening does not automatically guarantee improved sag resistance.
Both properties must be tested.
A useful trial should separate the two problems.
First: Evaluate extrusion.
Measure or compare extrusion force under consistent cartridge, nozzle and temperature conditions.
Second: Evaluate sag resistance.
Apply the sealant under consistent conditions and observe whether the bead maintains its required shape.
Third: Evaluate finished performance.
Check curing behavior, mechanical properties, adhesion and other application-specific requirements.
For the first comparison, keep the base formulation and processing conditions as consistent as possible.
Then adjust filler loading or processing variables separately.
Why can a sealant be hard to extrude but still sag?
Because extrusion under pressure and shape retention after application are different rheological requirements.
Does increasing fumed silica always improve sag resistance?
No. The result depends on the grade, dosage, dispersion and complete formulation.
What type of silica is IOTA HB7135?
It is an HMDS-treated hydrophobic fumed silica.
What is its published BET surface area?
210 m²/g.
Can IOTA HB7135 be directly substituted for another silica grade?
A direct replacement should not be assumed. Controlled formulation trials are recommended.
What information should a buyer provide?
Sealant type, one- or two-component system, curing chemistry, current silica grade, dosage, extrusion problem, sagging problem and target finished properties.
Related Product: IOTA HB7135 Hydrophobic Fumed Silica
Product link: IOTA HB7135 product information
Application Boundary: Extrusion, sag resistance, curing and finished properties depend on the complete sealant formulation. Final grade and dosage should be confirmed through controlled production and application testing.
