Thickening and thixotropy

However, not every type of AEROSIL^® product works best in every type of fluid!

What´s the mystery (magic) behind this?

If AEROSIL^® fumed silica is dispersed in a liquid, which possesses good wetting properties towards the surface of the AEROSIL^® products, then the particles absorb molecules from the liquid until the surface energy is minimized relative to the environment. Particles being densely covered by a molecular skin identical to the surrounding fluid tend to float in the medium rather independent of each other, resulting in only minor increase in viscosity of the dispersion compared to the pure liquid.

As a consequence the particles follow gravity and settle rather quickly. This can be observed with hydrophilic AEROSIL^® fumed silica in hydrophilic fluids e.g. alcohols, acetone, polar solvents.

Hydrophilic AEROSIL^® fumed silica, however, dispersed in non polar fluids like mineral spirit converts the fluid into a gel quite easily.
Hydrophobic AEROSIL^® fumed silica, accordingly, thickens polar fluids into gels.

In such cases the molecules of the liquid cannot adsorb onto the surface of the AEROSIL^® products and deprive it of its true nature. Consequently the AEROSIL^® fumed silica particles tend to cling to each other as close as possible in order to minimize the free energy of the surface. Thus a randomly distributed grid of solid forms inside the fluid medium.

This three-dimensional structure exhibits visco-elastic properties resulting in enhanced viscosity and , depending on concentration, more or less pronounced "yield values".

Sedimentation is inhibited, because the solid particles are no longer free to move downwards in the gravitational field. (Even larger, coarser particles with higher densities than AEROSIL^® products are held in suspension by the AEROSIL^® fumed silica grid, e.g. titanium dioxide pigments or zinc powder. AEROSIL^® fumed silica is ideal for use in Gel coats, adhesives and putties as a thickener to maintain uniformity of otherwise very heterogeneous mixtures.).

When shearing occurs, the resistance of the AEROSIL^® fumed silica grid is overcome and it breaks into smaller domains floating along in the streaming matter. The reduction in (apparent) viscosity is very large, but as the shear is reduced, immediately the AEROSIL^® fumed silica grid rearranges and the apparent viscosity rises again.

This thixotropic behaviour is of great importance wherever liquids are applied to substrates by brushing and spraying or on porous substrates.
The hydrophilic nature of AEROSIL^® products is characterized by the hydroxyl groups present on the surface while the hydrophobic types are characterized by a variety of organic groups permanently attached to the surface by Evonik’s proprietary technology.

Rheological concepts such as thickening, shear thinning, thixotropy and yield value can be achieved by using the right type and amount of AEROSIL^® fumed silica.

Recommended products for thickening and thixotropy in liquid resins

Unsaturated polyester resins are comprised of a polymeric resin and a monomer, which acts as a diluent during processing, and as a co-monomer during the hardening reaction. The resin generally is composed of glycols and phthalic acids with a certain percentage of unsaturated aliphatic acids like maleic or fumaric acid. In the NAFTA region, diclopentadiene may also be linked to the polymer chain.
As a reactive diluent styrene is chosen in most cases rendering the unsaturated polyester resin relatively unpolar.

For this reason, hydrophilic AEROSIL^® fumed silica is the thickening agent of choice. AEROSIL^® 200 is widely used for Laminating Resins, whereas for Gelcoats AEROSIL^® 300 and AEROSIL^® 380 may have advantages.

Some specialty polyester resins and vinylester resins, however, which have more polar character require hydrophobic AEROSIL^® products as a thixotrope. AEROSIL^® R 202 performs excellent in such formulations, whereas hydrophilic AEROSIL^® fumed silica will not give the desired effect.

Epoxy resins, being polar in nature, also need AEROSIL^® R 202 as a thixotrope.

During formulation of adhesives or coatings, there are numerous additives incorporated into the formulation. Depending on the amount and polarity of such additives, thickening efficiency of a given type of AEROSIL^® fumed silica may be altered significantly compared to the results found with the resin by itself. Therefore the grade and quantity of AEROSIL^® products must be selected by experiment in order to find the ideal formula.