Rheology: AEROSIL® fumed silica promote structural viscosity and thixotropic behavior

AEROSIL® fumed silica allow you to adjust the rheological properties of a liquid to prevent dispersed solids from settling and to head off problems such as paints dripping down the wall. The liquid remains easy to stir and process, because the viscosity decreases with shear rate.

Used in countless applications, dispersions of finely divided solids need to remain stable when stored and should not separate out/settle, and paints and coatings should not run/flow. Achieving these goals requires more than high viscosity. The dispersion has to have a yield point and exhibit thixotropic behavior. In other words, the properties of the liquid at rest should be similar to those of a gel, in which dispersed particles remain stable for long periods of time. When subjected to shear forces, however, the liquid should become thinner and easy to process. Once the shear forces cease, the structure should recover fast enough and the dispersion should remain stable. AEROSIL® products from Evonik offer an ideal means of achieving that behavior. The broad portfolio of hydrophilic and hydrophobic AEROSIL® grades allows customers to find the right product for every formulation.


AEROSIL® fumed silica forms finely divided aggregates when dispersed in a liquid. With their branched structure, these aggregates are capable of forming a three-dimensional network with neighboring aggregates and other components of the liquid. When at rest, this network lends the liquid a gel-like structure in which dispersed solids are suspended. The liquid also does not drip when applied to vertical surfaces. Stirring or moving the liquid, however, causes the network to break apart and sets the AEROSIL® aggregates in motion. This process is reversible: once the stirring stops, the AEROSIL® aggregates regroup and the stabilization effect returns.


In order to optimize their effects, AEROSIL® products need to be well dispersed in the liquid phase. Dissolvers and rotor-stator homogenizers are very well suited to this task. Overprocessing AEROSIL® by leaving it in a pearl mill for long periods can, by contrast, lead to irreversible disintegration of the aggregates and to a decline in its rheological properties. The polarity of the liquid phase also has a major impact on the effect of AEROSIL® as a rheological control agent. Other additives, such as wetting agents, can combine with AEROSIL® to have an effect on rheology as well. Our application engineers will be happy to give you advice and to work with you to select the AEROSIL® grade that’s right for your system.


AEROSIL® fumed silica can be used for controlling rheology in applications such as adhesives and non-silicone sealants, agrochemicals, marinades, paints&coatings, silicon sealing compounds, lubricants, UPE resins, and many more.


Detailed information on this subject can be found in the following technical information sheets: Technical Overview AEROSIL®, TI 1125, TI 1209, TI 1253, TI 1373, TI 1380, and TI 1396, TB 28, TB 63. more