Thickening and thixotropy
AEROSIL® fumed silica is an excellent thickening agent and thixotrope for liquid systems and even a synonym for a thixotrope.
Excellent thixotropy is very important for a good processability for many adhesive and sealant systems. Many adhesives and sealants must show a low viscosity during the application at high shear rates and a high viscosity after the application at rest or low shear for good sag resistance.
If AEROSIL® fumed silica is dispersed in a liquid, the surface silanol groups interact with each other directly or indirectly via the molecules in the liquid. This affinity must be attributed to the hydrogen bridge linkages, and results in a temporary, three-dimensional lattice structure becoming macroscopically "visible" as thickening. Hydrophobic AEROSIL® grades create especially Van-der-Waals-linkages among the silica particles. Under a mechanical stress, for example due to intensive stirring or shaking, the structur is broken down again. The system becomes more fluid, in other word the viscosity drops. In the static state AEROSIL® fumed silica particles join again, and the viscosity regains its original value. This process is reversible and termed thixotropy.
Thixotropy can be measured with modern rheometers measured by the 3-interval-time-test (3ITT). The first interval simulates the state before the adhesives and sealants are applied. The second interval simulates the application, and the last interval measures the viscosity recovery after the application. The more effectively and quickly the original viscosity rebounds, the greater is the thixotropy.
The diagram shows the excellent performance of AEROSIL® R 202 as thixotrope in an epoxy resin.