In tuned by varying the temperature and
In order to obtain aerogels with low density and highspecific surface area, the solvent in the gel pores has to be removed withoutdestruction of the solid gel structure. Evaporation of the solvent generatescapillary pressure in the pores due to surface tension of the fluid causingshrinkage and even cracking of the material.
It has been found that undersupercritical conditions the fluid can be removed from the material withoutsurface tension, leaving the porous structure of the material unharmed. Hüvonen, 2008Water above its critical parameters (T?Tc, P?Pc) is veryreactive and direct supercritical drying decomposes the gel. Therefore, oneapproach is that after the gel has been formed, the water in the pores isreplaced with another solvent that has either lower critical parameters(acetone Anton, 2008, alcohol Volobujeva, 2008) to be dried directly viasupercritical state, or is miscible with the fluid that is used later forsupercritical drying of the gel. In the case of resorcinol-formaldehydehydrogel, the water is typically exchanged for acetone or alcohol, after whichthe solvent is removed using CO2 extraction, followed by venting theliquid CO2 from the pores of the gel via supercritical state. Carbon dioxide isoften the fluid used for supercritical drying because of its low criticalparameters (Tc=31.
13°C; Pc=73.75 bar). It is also readily available andinexpensive, non- flammable, non-toxic, reusable, environmentally friendly, noCO2 residues are left in the treated medium, and the properties ofthe supercritical fluid can be finely tuned by varying the temperature andpressure Jõgi, 2008. The polarity of CO2 is suitable for removingmost of the organic solvents used for the sol-gel preparation of organic polymersKadastik, 2008. However, for removing water, CO2 is not suitableand in order to use supercritical CO2 drying, water in the gel poresis replaced with another solvent miscible with CO2. The process ofsolvent exchange is time consuming, being limited by the speed of diffusion,and because it is crucial to remove the water to succeed in drying (resultingin low shrinkage), a considerable amount of replacement solvent is consumedwhich later requires energy for its recovery or disposal. Thus, an alternativestrategy is to prepare the gel directly in the suitable solvent.