Silica Fume General
Silica fume is a completely different product to fumed silica and should not be mistaken for it. Silica fume is a by-product of the reduction of quartz with carbon in electric arc furnaces used for the manufacture of silicon and silicon alloys. Originally this by-product was allowed to discharge to the atmosphere, but environmental issues have prevented this since the 1970s. It is now collected and marketed for a number of applications. While polymer use is very small today, silica fume has been included because of the confusion that sometimes occurs with fumed silica.
Silica fume is an ultrafine powder consisting of spherical particles with an average particle diameter of 150 nm (0.15 pm). The specific surface area is about 20 m2/g by the BET nitrogen method. The primary component of silica fume is amorphous silicon dioxide (SiO2) which can vary from 85% to 98%. The main impurities include carbon, silicon carbide, and oxides of alkaline (earth) metals.
The particle size is of interest for several polymer applications, and in the early days of fumed silica recovery, there was interest in using it as a filler in polymers; but this has largely disappeared today. This is because it has found a higher added value application as a special additive (pozzolan) for concrete.
Respirable crystalline silica (RCS) of silica fume is very low at <0.1%, and the silicon carbide is in its safer nonfibrous form.
Being a by-product, silica fume can be regarded as a sustainable material. The finding of higher added value applications that has occurred illustrates one of the recurring issues with developing by-products for polymer applications.
The concrete market is expected to continue to dominate and limit any use in polymers.