Silica characteristics, physical characteristics
Precipitated silica has both internal and external surface area, which allows for easy adsorption of liquids, among other qualities. Primary characteristics include:
Average Agglomerate Size (morphology)
The precipitated silica manufacturing process results in the formation
of distinct agglomerates based on primary particles. These agglomerates
range in size from 50 to 100 microns. They can be milled by various methods
to reduce agglomerate size to the range of 2 to 15 microns. Particle size
of the milled products can be measured by the Coulter Counter or laser
diffraction.
pH
The pH of precipitated silica is normally slightly acidic (5.5-7.0). But
for defoamer applications, the pH is adjusted to the alkaline range to
ease the hydrophobizing process which most defoamer manufacturers employ.
Moisture or Ignition Loss
Precipitated silicas contain both physically and chemically bound moisture.
The physically bound moisture is driven off of heating to 105 degrees
centigrade for two hours to determine Loss on Drying (LOD), which is usually
in the range of 5 percent.. Chemically bound moisture, in the form of
surface silanol groups, is removed by igniting the dried silica for two
hours at 1000 degrees centigrade. Loss on Ignition is also in the range
of 5 percent.
Tapped Density and Specific Gravity
The specific gravity of precipitated silicas is approximately 2.1. This
is a chemical property of silicon dioxide manufactured by this type of
process. The resulting solvated weight per gallon (bulking value) is about
17.5 pounds. The density of precipitated silicas can range from about
70 g/l to 300 g/l depending on the type of silica and at what point the
material is sampled. Unmilled grades of precipitated silicas will have
higher densities than milled products, since milling reduces both particle
size and density.
Absorptivity/Porosity
Dibutyl Phthlate (DBP) absorption is a method used to determine the absorptive
capability of precipitated silica. Another is DOP, which is the number
of ml of dioctylphthlate that 100 g of precipitate can absorb before becoming
pasty. Generally, a precipitated silica can adsorb two to three times
its weight depending on the density of the liquid being absorbed. Particle
size and surface characteristics determine how much DBP or DOP a silica
can absorb. This characteristic is directly linked with morphology. In
practice, it means the ability of the conglomerates to store appreciable
quantities of liquids. Absobency corresponds essentially to the filling
of the micropores by liquids.
Purity
The purity of precipitated silica is greater than 98 percent silicon dioxide
on an ignited basis. Normal impurities include Fe2O3 and water soluble
Na2O3. Any foreign elements remaining, at acceptable levels, in the precipitates
come either from the original sand or from the method of manufacture.
These impurities can be determined by Atomic Absorption (AA) or Ion Chromatography
(IC).
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The use of silica :
Tire & Rubber Goods (Manufactured Rubber Goods & Tire)
Polymer Performance (Organic Rubber For Footwear, Plastics and TPE, Polymer
for Membranes
Silicone Elastomers), Nutritions & Industrial (Specialties, Nutritions,
Industrial Specialties), Performance Silicas (Dentifrice)
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