Product Development Manager
Fumed and precipitated silica powders are made by two different processes. Fumed silica is made by burning tetrachlorosilane in air, so the silicon dioxide forms in the combustion chamber like flakes of soot forming above a coal fire.
Precipitated silica is formed by treating a basic solution of sodium silicate with an acid such as concentrated sulphuric acid. The reaction produces a fine suspension of silica in the aqueous medium, which can be separated by filtration and dried. Both these processes yield feather-like particles, that is to say particles with very low bulk density and very high surface area to volume ratios. Surface areas of silicas can be as high as 600m2g-1. These physical characteristics allow silica to be used in a pigment dispersion to keep the pigment in suspension, and if fumed or precipitated silica is stirred into a mixture of other powders such as organic pigments, it ‘floats’ to the top of the mixture.
We routinely use fumed silica to increase the viscosity of polymer solutions. Even when thoroughly mixed into the solvent, the particles hang together and provide resistance to flow in the liquid. This structure is also effective in keeping high-density pigments such as antimony trioxide in suspension.
The chemically inert silica can also be modified to change its behaviour in various media. For instance, it can be treated with wax to make it hydrophobic. This material is very effective as a matting agent, as it will lie on the surface of a solvent-based paint film as it dries, and the rough texture disrupts the reflection of light from the outer layer. A different coating will make silica hydrophilic, allowing incorporation in water-borne coatings to achieve similar effects.