The Use of Fumed Silica

Fumed silica with trade names such as Cabosil and Aerosil are used to increase the thixotropic properties of an epoxy system. Care must be taken as to what type of fumed silica is used and the manner in which it is incorporated. If the wrong type of fumed silica is used, moisture absorption can occur resulting in discoloration of pigmented systems and yellowing and/or cloudiness in clear materials.

Based on field, lab and production experience, General Polymers does NOT recommend adding fumed silica to epoxies in the field. CABOSIL M-5 and Aerosil 200 are hydrophilic which allows them to be mixed into epoxies with relative ease in the field. Their hydrophobic nature, however, also renders these products susceptible to absorbing moisture from the air, which may interfere with the epoxy cure. Excess moisture in an uncured epoxy will result in a cloudy or yellowing appearance or may discolor pigmented epoxies. Hydrophobic fumed silica must be ground into the resin with high shear and is used in the plant to produce highly thixotropic products, such as 3561V. The hydrophobic nature of this fumed silica prevents problems with moisture interactions during installation. General Polymers recommends the use of 3561V for cove base and other vertical applications.