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Structurally modified, hydrophobic AEROSIL® R 8200 for low-viscosity, free-flowing RTV two-component Silicone Rubber
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. 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 specific requirements.
RTV one-component silicone sealant before hardening in a seal: (left) without AEROSIL® products (right) with AEROSIL® fumed silica as thixotropic agent
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.
Rheological properties of silicone systems can also be improved significantly by modifying the silica with functional silanes. By treating the silica with functional silanes strongly hydrophobic products emerge, which enable e.g. a low viscosity in special silicone rubber applications.
Industry Information 2247: Materials for the Silicone Rubber Industry
Technical Information 1148: AEROSIL® R 104 and AEROSIL® R 106 for silicone rubber applications
Technical Information 1209: AEROSIL® R 8200 for Silicone Rubber
Technical Information 1253: AEROSIL® and AEROXIDE® for Liquid Silicone Rubber (LSR/LIMS)
Technical Information 1374: Products for 1-component RTV silicone sealant compounds
Product- as well as safety data sheets (SDS) of our various AEROSIL® fumed silica grades, can be found at our product recommendationsmore