Rhodia Silica Systems’ ability to tightly control its production process leads to excellent silica stability and enables tailoring of product characteristics to exactly meet customer needs.

Physical Characteristics of Precipitated Silicas

Primary characteristics include:

Average Agglomerate Size (morphology)
The precipitated silica manufacturing process results in the formation of distinct agglomerates based on aggregates of silica primary particles. For typical silica powders these agglomerates range in size from 50 to 100µm. They can be milled by various methods to reduce agglomerate size to the range of 2 to 15µm. In the case of Micropearl silicas, the average size is around 250µm.

Purity
The purity of precipitated silica is greater than 98 percent silicon dioxide on a “dry” basis. Normal impurities include Fe2O3 and water soluble Na2O. Any exogenous 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).

Specific Surface Area
The SSA for precipitated silica is a function of both internal and external surface area. In rubber applications, the SSA mainly determines the level of reinforcing properties as well as processing parameters such as viscosity. In other applications, the SSA is also responsible for easy adsorption of liquids.
Our standard range offers an SSA of 100 to 200 m2/g but we have the research and development facilities to work on projects requiring a greater SSA.

Absorptivity/Porosity
Oil absorption is a method used to determine the absorptive capability of precipitated silica. 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 oil a silica can absorb. These characteristics are directly linked with morphology. In practice, it means the ability of the conglomerates to store appreciable quantities of liquids. Absorbency corresponds essentially to the filling of the micropores by liquids.

pH
The pH of precipitated silica is normally slightly acidic (5.5-7.0). But for specific applications, the pH can be adjusted according to customer application needs.

Specific Gravity and Tapped Density
The specific gravity of precipitated silicas is approximately 2.1.
Unmilled grades of precipitated silicas have higher densities than milled products, since milling reduces both particle size and density.
The tapped density is:
• between 0.26 and 0.36 (DRT) for Micropearl silicas
• between 0.1 and 0.3 (DRT) for Powder silicas
• between 0.26 and 0.36 (DRT) for Milled powder
• between 0.05 and 0.1 (DRT) for Micronised powder

Moisture and Ignition Loss
Precipitated silica contains both physically and chemically bound water. The physically bound water is released by heating to 105°C for two hours (usually in the range of 5 to 8 wt.%) whereas dehydration of surface silanol groups occurs at high temperatures (between 200 and 1000°C).