Using Fumed Silica for Thixotropy

by Peter Collins - Sales Representative for Texas


Fumed silica (also known as pyrogenic silica because it is produced in a flame) consists of microscopic droplets of amorphous silica fused into branched, chainlike, three-dimensional secondary particles, which then agglomerate into tertiary particles. The resulting powder has an extremely low bulk density and a high surface area. Its three-dimensional structure results in viscosity-increasing, thixotropic behavior when used as a thickener or reinforcing filler. – Source: wikipedia

Fumed silica is sold under trade names like Cabosil, Aerosil and Orisil. It’s used as a
lightweight filler (a 10-gallon unit weighs only 4 pounds) to increase the thixotropic (anti-sag) properties of an epoxy coating system. However, it can be difficult to incorporate this material into liquid epoxy systems on a jobsite because the microscopically fine fumed silica particles don’t easily and fully wet when using portable, low shear and slower jobsite-type mixing equipment.  
Another downside is that some types of fumed silica can absorb moisture, which can cause clear epoxy binders (the binder in this case being the combination of an epoxy resin and a corresponding catalyst or part A + part B) to appear hazy or amber and pigmented systems to discolor. For this reason, we recommend that caution be taken when deciding to add fumed silica to epoxies in the field.  

Two specific fumed silica products that lend themselves to easier onsite mixing in the field are CABOSIL M-5 and Aerosil 200. However, they are both hydrophilic (attracted to water) materials. As such, both of these products have the capability of absorbing moisture from the surrounding environment. In turn, this may affect the curing an application finish of the epoxy as described above.
Hydrophobic (meaning repelled by water) fumed silica should be mixed into epoxy materials at the
formulator’s plant using high-shear mixers in order to assure the proper integration and
wetting of the silica particles by the liquid resin. Hydrophobic fumed silica hinders moisture integration into the epoxy binder system and, in so doing, eliminates one possible source of moisture that can have a negative effect during the curing process of an epoxy coating system.

Arizona Polymer Flooring offers two standard thixotropic products. One of which is a wall
coating (Epoxy 400 Wall Coating) and the other (Epoxy 400P Thixotropic) is used as a
pigmented grout coat over troweled epoxy mortars. Both of these  APF materials are also available in fast cure (FC) versions and in our 16 standard colors.
Fumed Silica Manufacturers:
 Evonik (Degussa):  AEROSIL
 Waker:  HDK
 Cabot:  Cabosil
 Orisil:  Orisil
 DC    Konasil
 Tokuyama:  Reolosil
 Xunyu:  Xysil