Torsten Gottschalk-Gaudig

Fumed Silica — Rheological Additive for Adhesives, Resins, and Paints

Fumed silica, a synthetic silicon dioxide, is a powerful rheological additive for resins and paints to introduce thixotropy or even a yield point. The rheological effectiveness of fumed silica is based on its ability to form percolating networks which immobilize large volumes of liquid. By a combination of advanced rheological experiments, spectroscopical investigations, and quantum chemical calculations it could be demonstrated that the formation and stability of the silica network is strongly influenced by particle-resin interactions. The results can be used to develop comprehensive models, which explain the rheological performance of different grades of fumed silica in different resins.

Influence of specific surface area of pyrogenic silicas on their heat of immersion in water and on their surface properties assessed using inverse gas chromatography.

The heat of immersion and the surface properties of pyrogenic silicas were examined using microcalorimetry and inverse gas chromatography at finite concentration (FC-IGC). The microcalorimetric measurements showed a regular decrease of the heat of immersion, in water, per area unit with an increase in the surface areas. The desorption isotherms were established using FC-IGC. It is observed that BET constant value goes through a minimum, whereas the shapes of distribution function of the adsorption energies of isopropanol are changing. All results could be interpreted using a model of formation of the pyrogenic silica in the flame, which explains the change of surface functionality and geometry occurring around 200 m2/g.

Evolution of the surface polar character of pyrogenic silicas, with their grafting ratios by dimethylchlorosilane, studied by microcalorimetry.

The interactions of water, hexamethyldisiloxane, and dodecane with pyrogenic silica samples, modified by a controlled partial silylation with dimethyldichlorosilane, were studied by microcalorimetry and wettability measurements. The samples, having a coverage ratio lower than dimethylsilyl (DMS) monolayer capacity ( approximately 2.6 DMS/nm(2)), show a regular and linear decrease of their heat of immersion into water with the coverage ratio and correlate with the increase of residual silanol groups. Two critical coverage ratios were evidenced at about 25 and 50% of the DMS monolayer capacity, the grafted silica remaining hydrophilic, below 25% being strongly hydrophobic beyond. The heat of immersion into hexamethyldisiloxane decreases until 50% of the DMS monolayer whereas that of dodecane remains independent of the grafting ratio. This study demonstrates that the water/residual free silica surface plays the main role in the stabilization of the W/O Pickerings emulsions.