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In tuned by varying the temperature and

In order to obtain aerogels with low density and high
specific surface area, the solvent in the gel pores has to be removed without
destruction of the solid gel structure. Evaporation of the solvent generates
capillary pressure in the pores due to surface tension of the fluid causing
shrinkage and even cracking of the material. It has been found that under
supercritical conditions the fluid can be removed from the material without
surface tension, leaving the porous structure of the material unharmed.  Hüvonen, 2008

Water above its critical parameters (T?Tc, P?Pc) is very
reactive and direct supercritical drying decomposes the gel. Therefore, one
approach is that after the gel has been formed, the water in the pores is
replaced with another solvent that has either lower critical parameters
(acetone Anton, 2008, alcohol Volobujeva, 2008) to be dried directly via
supercritical state, or is miscible with the fluid that is used later for
supercritical drying of the gel. In the case of resorcinol-formaldehyde
hydrogel, the water is typically exchanged for acetone or alcohol, after which
the solvent is removed using CO2 extraction, followed by venting the
liquid CO2 from the pores of the gel via supercritical state. Carbon dioxide is
often the fluid used for supercritical drying because of its low critical
parameters (Tc=31.13°C; Pc=73.75 bar). It is also readily available and
inexpensive, non- flammable, non-toxic, reusable, environmentally friendly, no
CO2 residues are left in the treated medium, and the properties of
the supercritical fluid can be finely tuned by varying the temperature and
pressure Jõgi, 2008. The polarity of CO2 is suitable for removing
most of the organic solvents used for the sol-gel preparation of organic polymers
Kadastik, 2008. However, for removing water, CO2 is not suitable
and in order to use supercritical CO2 drying, water in the gel pores
is replaced with another solvent miscible with CO2. The process of
solvent exchange is time consuming, being limited by the speed of diffusion,
and because it is crucial to remove the water to succeed in drying (resulting
in low shrinkage), a considerable amount of replacement solvent is consumed
which later requires energy for its recovery or disposal. Thus, an alternative
strategy is to prepare the gel directly in the suitable solvent. Peikolainen,
2011.

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