Rhodia' ability to tightly control its production process enables tailoring of product characteristics to exact customer needs.
Primary physical characteristics of precipitated silicas:
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.
The purity of precipitated silica is greater than 98 percent silicon dioxide on a “dry” basis. Normal impurities include iron oxyde and water soluble sodium sulfate. Any other trace elements remaining in the precipitates come either from the original sand or from the method of manufacture. These impurities are controlled by Atomic Absorption (AA) or Ion Chromatography (IC).
Specific Surface Area (SSA)
SSA is the main factor determining the level of reinforcement offfered by the silica and also determines processing parameters such as mix viscosity. 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 an extension of SSA possibilities. In other applications, the SSA is also responsible for easy adsorption of liquids.
Generally, a precipitated silica can adsorb two to three times its weight of liquid. Porosity, particle size and surface characteristics determine exactly how much liquid a silica can absorb.
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.
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).