Fumed silicas also belong to the group of inorganic, powdered rheology additives which, depending on their modification, are used in solvent-free and solvent-borne as well as in aqueous systems.
Fumed silicas are produced in a complex manufacturing process by pyrolysis of silicon tetrachloride. In this process, the particle size and morphology as well as the polarity of the surface (hydrophilic/hydrophobic) and thus the technical properties can be specifically controlled.
Fumed silicas are characterized by a very large surface area and low density
Since they generate a lot of dust during processing, inhalation should be avoided (risk of silicosis). Thus they are often used in closed systems. High shear forces are required for optimum activation. Depending on the polarity of the system, different wetting and dispersing additives can be used to improve wetting and stabilization of the silica particles. It is also possible to improve the effectiveness by using rheological synergists.
Hydrophilic fumed silca
Organically modified phyllosilicate
Characteristic for fumed silicas is a thixotropic flow behavior. Depending on the type of silica used (hydrophilic/hydrophobic), the structure is built up via hydrogen bonds (silanol groups) as well as via van der Waals forces. With increasing dosage and filler loading, the flow behavior becomes increasingly pseudoplastic. Even though gloss is reduced at higher dosages and mechanical properties are increasingly affected, fumed silicas are widely used in the market. In addition to their use as rheology additives, they are used as matting agents, insulating materials, and as flow aids.
