Comparison of Fumed Silica and Precipitated Silica

Silica is a general term for white amorphous powdered silicic acid and silicate products, mainly referring to precipitated silica, fumed silica, ultrafine silica gel and aerogel, as well as powdered synthetic Aluminum silicate and calcium silicate, etc. Silica is roughly divided into precipitated silica and fumed silica (scientific name: fumed silica) according to the production method.

Under normal conditions, fumed silica is white amorphous flocculent translucent solid colloidal nanoparticles, non-toxic, and has a huge specific surface area (100-400m2/g). The fumed silica is all nano-silica, the product purity is over 99.8%, and the primary particle size is 10-40nm; the precipitation silica is divided into traditional precipitation silica and special precipitation silica. The former refers to silica produced with acid (sulfuric acid, hydrochloric acid, etc.), CO2 and water glass as the basic raw materials, and the latter refers to the use of hypergravity technology, sol-gel method, chemical crystallization method, secondary crystallization method or reverse crystallization method. Silica produced by special methods such as phase micellar microemulsion method.

Comparison of Fumed Silica and Precipitated Silica

1. The difference in structure between fumed silica and precipitation silica
Fumed silica is neither produced by simple pulverization nor by special drying. In any case, the smallest particle is the primary particle, but it will more or less agglomerate (because of the existence of a large number of silanols on the surface), and the special surface is its most important property. Silica gel has an extremely large internal surface area, which is the reason for its extremely strong adsorption capacity. In contrast, the primary particles of fumed silica are flame hydrolyzed and have only an external surface area. This also explains why the rheology of many systems combined with fumed silica has been greatly improved, such as the application of fumed silica in powder coatings.

2. The difference in the number of silanols between fumed silica and precipitation silica

The difference in the drying loss of silica has a great influence on the product characteristics and product application performance. The lower the drying loss requirement of silica, the better. For example, the lower the drying loss, the better the insulation of the silicone rubber used to make the cable. Low drying loss increases the stability of adhesives and sealants incorporating silica monocomponents during storage. In fact, the main difference between silicas is that their silanol numbers are different (that is, SiOH/nm2). The number of silanols in the hydrophilic fumed silica is between 2 and 3, compared to about 6 in the precipitation silica, and the surface-modified silica (hydrophobic silica) ) with a silanol content of less than 1.

3. The difference in silicon purity between fumed silica and precipitation silica

The difference in purity of silica is also noteworthy. In terms of anions, fumed silica contains only trace amounts of chloride ions and metal oxide impurities. The anions of precipitated silica include acid ions, basic ions and alkaline earth metal ions (about 1000ppm). Differences in process technology lead to differences in product purity.