Silica Flour and Silica Fume
What is the difference between silica
flour and silica fume?
Both materials are pure silica (silicon dioxide). Silica
flour is simply made by grinding pure silica sand to a very
fine powder. It is used as a cheap filler in plaster and some
plastics to add strength, or just to reduce the amount of
(expensive) resin needed to fill a mold.
Fumed silica is made in a very unusual way, by reacting
silicon tetrachloride in an oxy-hydrogen flame that has an
excess of oxygen. A very light, fluffy, pourable solid
results. Fascinating stuff, actually! Fumed silica is mainly
used as a thixotropic additive to plastic resins and other
liquids. For example, when added to an epoxy resin, the resin
will not "run" when left to stand, but will still pour if it
is agitated. Both forms of silica are essentially inert.
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Update: June 2012