Drying of resorcinol–formaldehyde gels with CO2 medium

The effect of using real supercritical conditions in the CO2 drying process on the structure and texture of resorcinol–formaldehyde networks is investigated by low temperature nitrogen adsorption, scanning electron microscopy and by small and wide angle X-ray scattering. If supercritical conditions are maintained throughout the whole extraction process the resulting networks exhibit much more developed porosity. The surface area of the supercritically dried gel, in excess of 500 m2/g, is more than twice that of the sample dried with liquid CO2. Pore volumes are also significantly higher in all pore classes. In the supercritical region the applied pressure strongly affects the porosity, while the effect of temperature is limited. Drying time also influences the total pore volume of the samples, but not the mesopore and micropore volumes. The volume filling character of the molecular adsorption process in this system is illustrated by the difference in surface areas measured by small angle X-ray scattering and that by nitrogen adsorption.

Schematic drawing of the structures of resorcinol–formaldehyde gels dried with supercritical (a) or liquid (b) CO2 extraction.Figure optionsDownload as PowerPoint slideHighlights
► Influence of liquid and supercritical CO2 extraction on morphology of RF aerogels.
► Supercritical CO2 more efficient than liquid CO2 extraction.
► Above critical point, pressure has strongest effect, but only on macroporosity.
► Elementary building units much larger in liquid CO2.