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Due to their pronounced hygroscopicity, salts tend to absorb moisture from the environment. At increasing temperature and humidity the outer layer of the salt crystals tends to dissolve in the water layer on the surface and recrystallise again at lower ambient temperature. Thus the crystals may grow together over a longer time of storage resulting in a solid block.
The addition of 0.5 to 2 percent of a hydrophilic AEROSIL® grade or 0.1 to 0.5 percent of a hydrophobic AEROSIL® grade avoids the build-up of salt blocks.
Electron microscopy of a salt crystal without and with silica
Polymer powders tend to cake due to their soft structure. Especially at higher temperatures they build agglomerates finally leading to big lumps. Small amounts of hydrophilic or hydrophobic AEROSIL® types are able to prevent these powders from caking.
Especially PVC-powders show electrostatic charging, when conveyed through metal or plastic pipes or moved in a mixer. Besides some AEROSIL® fumed silica grades there is one special product within the range of Degussa fumed oxides, which already neutralises the charge at an extremely low level. AEROXIDE® Alu C is already highly efficient at 0.05 to 0.3 percent.
PVC powder without and with 0.1 percent AEROXIDE® ALU C
Once a fire breaks out, fire extinguishers have to empty 100 percent. This requires an absolutely free flowing powder, which does not show any reagglomeration even after years of storage. E.g. ABC-powders consist of mono ammonium phosphate (MAP) as active ingredient, which usually is diluted with ammonium sulphate. Typical ABC-powders contain 40—90 percent MAP. Depending on the pureness of the MAP this ingredient shows high hygroscopicity. Most of the powders are siliconised after grinding them down to optimally <45µm. The silicon layer prevents the powder from taking up moisture during storage and leads to an initially good flow. However, after some time of storage in the fire extinguisher, the particles start reagglomerating. 0.5—1 percent of a hydrophobic AEROSIL® grade, like AEROSIL® R 972 or AEROSIL® R 812 avoids this leading to a powder that is easy to fluidise even after a long period of time.
Outstanding flowability of a fire extinguishing powder treated with a hydrophobic AEROSIL® grade
AEROSIL® fumed silica is also used as a flow aid and anticaking agent in many other industries, such as: