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A formulator adds fumed silica and notices a new problem:
“There are many bubbles in the compound after mixing. Is the silica causing them?”
Fumed silica can make air incorporation more noticeable during processing, but bubbles should not automatically be treated as a silica quality problem.
The complete mixing process needs to be evaluated.
Fumed silica is a very light, low-bulk-density powder.
During incorporation, air can enter the formulation together with the powder.
If the formulation becomes more viscous as silica is added, that trapped air may also become more difficult to remove.
This can make bubbles more visible after mixing.
Adding a large quantity of fumed silica at once can make incorporation more difficult.
Depending on the formulation and equipment, more controlled addition may help improve wetting and reduce excessive air incorporation.
The correct addition method should be determined for the actual production process.
Higher mixing speed may improve incorporation and dispersion in some processes.
However, the mixing pattern can also pull air into the formulation.
Therefore, the objective is not simply to use the highest possible speed.
Mixer design, speed, batch volume and viscosity should be considered together.
As fumed silica builds structure in a formulation, viscosity can increase significantly.
Once air becomes trapped in a highly viscous material, bubbles may rise and escape much more slowly.
This is why a formulation can show acceptable dispersion but still require an appropriate deaeration step.
Silica grade and surface characteristics can influence incorporation, rheology and processing.
For example, IOTA HL7200 hydrophilic fumed silica can be evaluated for reinforcement and rheology control in silicone materials, coatings, adhesives and sealants.
IOTA HL7200 Hydrophilic Fumed Silica
Hydrophobic fumed silica grades can also be considered where their surface characteristics better match the formulation.
However, changing silica alone should not be expected to solve every air-entrapment problem.
When bubbles appear after adding fumed silica, review:
This helps separate a formulation problem from a processing problem.
Air bubbles observed during normal incorporation should not automatically be interpreted as gas generation by the silica. Air entrapment during powder addition and mixing should first be investigated.
Not automatically. Mixing speed needs to provide sufficient incorporation and dispersion without unnecessarily increasing air entrainment.
It may reduce viscosity, but it can also change reinforcement and rheology. The complete formulation should be considered.
Vacuum processing can be useful in formulations where entrapped air must be removed, provided the process and equipment are suitable.
Provide the formulation type, silica grade and dosage, base-polymer viscosity, mixer type, addition method, mixing conditions and when the bubbles appear.
When bubbles appear during fumed silica incorporation, look at powder addition, mixing, viscosity and deaeration together—not the silica specification alone.
