Fumed (pyrogenic) silica
A feather-light thickener that keeps epoxies and polyureas from sagging on vertical surfaces while letting them flow under the trowel or spray gun.
Fumed (pyrogenic) silica is amorphous silicon dioxide made by burning silicon tetrachloride in a hydrogen–oxygen flame. It forms branched aggregates of tiny primary particles with a very high surface area and extremely low bulk density. Hydrophilic and hydrophobic (surface-treated) grades exist.
It is used as a thixotrope and anti-settling agent in epoxies, polyurethanes, polyureas, gelcoats and sealants, as a matting agent in some clears, and as a free-flow aid in powders.
Surface silanol groups hydrogen-bond between aggregates, building a three-dimensional network that thickens the liquid at rest. Shear breaks the network so the coating flows; it then rebuilds, preventing sags. Hydrophobic grades are used where moisture sensitivity or polarity matter.
Very dusty; use dust control and respiratory protection. Fumed silica is amorphous, not crystalline, but fine dust exposure should still be minimized.
Typically small amounts, often around 0.5–3% depending on the system and the grade.
No. Fumed silica is amorphous. Crystalline silica, such as quartz sand, poses a recognized silicosis hazard when respirable.
