Impact of depressurizing rate on the porosity of aerogels

The effects of supercritical drying of diluted resorcinol–formaldehyde (RF) gels in CO2 and in acetone are compared. We show that, for both processes, depressurizing rate of the autoclave after drying has a significant influence on the resultant shrinkage, and hence on resultant bulk density, surface area and pore volumes. At depressurizing rates below 2 MPa/min, acetone leads to much lower shrinkage than does CO2, and is 1000 times cheaper. However, supercritical drying with CO2 remains interesting because it is faster, easier to control, less sensitive to the experimental conditions and cleaner. We indeed show that acetone is degraded and leads to various compounds that might partly remain in the porosity of the dried RF gels. Supercritical CO2 also clearly leads to the highest surface areas and micropore volumes, whereas supercritical acetone is in favour of higher mesopore volumes, especially at high depressurizing rates.

Depressurizing rate has a strong influence on shrinkage, whatever the supercritical fluid used for drying diluted gels. Volume shrinkage is lower when acetone is used below 2 MPa/min. Micropore and mesopore volumes also depend on depressurizing rate, especially when supercritical acetone is used.Figure optionsDownload as PowerPoint slideHighlights
► Supercritical drying of diluted RF gels in CO2 and in acetone has been compared.
► Depressurizing rate has a strong influence on the resultant shrinkage of the gels.
► Acetone leads to lower shrinkages at depressurizing rates below 2 MPa/min.
► Supercritical CO2 and acetone lead to higher micro and mesopore volumes, respectively.
► Depressurizing rate should always be fixed if reproducible materials are looked for.