AEROSIL® fumed silica – advantageous for rheological properties
With AEROSIL® fumed silica it's guaranteed to flow perfectly—or not at all
Silicone rubber and AEROSIL®—without thickening?
In addition to their reinforcing effect, fumed silicas such as AEROSIL® fumed silica in particular have thickening and thixotropic properties that are disadvantageous in a number of applications and must therefore be suppressed. This is especially important in very low viscosity and free-flowing systems such as Liquid Silicone Rubber (LSR) and RTV two-component systems (casting and molding compounds). Low viscosities are essential here, and not only for ease of processing: fairly long storage periods, without significant changes in the rheological properties, must be guaranteed for these sensitive systems. By modification of hydrophilic starting products in special processes, Evonik can offer highly hydrophobic (e.g. AEROSIL® R 812 S) and even highly hydrophobic structurally modified products (e.g. AEROSIL® R 8200) that satisfy these exacting requirements.
Structurally modified, hydrophobic AEROSIL® R 8200
for low-viscosity, free-flowing RTV two-component Silicone Rubber
Silicone rubber deliberately thickened—thanks to AEROSIL®!
Other silicone systems, on the other hand, require certain levels of viscosity, which can be set selectively by the use of a fumed silica. When processing RTV one-component silicone sealants, for example, a particular extrusion rate (extrudability) is absolutely essential. The attainment of a particular viscosity level (yield point) is also essential so that the sealant remains in the desired location in the application (e.g. in a wall joint) and is fully cured at exactly that location.This effect that AEROSIL® fumed silica provides is known as thixotropy.
In general, hydrophilic AEROSIL® products of moderate active surface area such as AEROSIL® 150 are used for RTV one-component silicone sealants. Hydrophobic products like AEROSIL® R 106 and AEROSIL® R 812 S with higher active surface areas are excellently suited for highly transparent systems.
RTV one-component silicone sealant before
hardening in a seal: (left) without AEROSIL® products
(right) with AEROSIL® fumed silica as thixotropic agent