Handbook of Solvents - George Wypych - ChemTech - Ventech!

21.1 Supercritical solvents 1455
Figure 21.1.20. Supercritical fluid impregnation processes.
Figure 21.1.21. Sol-gel drying methods.
ure 21.1.20 involves homogenization, impregnation, deposition, and optional post
treatment steps such as curing reactions. The mechanism of transport in the porous matrix is
permeation and diffusion while the primary mechanisms for deposition are pressure reduction,
temperature swing, sorption, and reaction (i.e., polymerization).
This process is a simplified version of impregnation process. 100 The impregnating solvent
does not contain any material to be deposited and the pressure release causes disintegration
of the impregnated material. 6,72
21.1.4.11 Parts cleaning - generic application
In essence, the part cleaning process is basically extraction/leaching with or without surfactants.
The basic steps are shown in Figure 21.1.13. Drying in the absence of capillary forces
and solvent residue free substrate makes the technology attractive. The technology is comprehensive
covered in a monograph 65 and there are many existing commercial applications.
21.1.4.12 Drying - generic application
A pilot plant used in drying is shown in Figures 21.1.15 and 21.1.17. The supercritical drying
routes are particularly attractive in their ability to eliminate or at least minimize the capillary
effect that cause non-uniformities in films as well as shrinkage and collapsing in pore
structure. These supercritical avenues permit successful creation of highly porous structures
such as foams, aerogels, coatings, and films.
The solvent can be removed from the wet gel using different methods. These methods
and resulting gels are shown in Figure 21.1.21. If solvent is evaporated slowly from the gel,
a xerogel is obtained. During evaporation, large capillary forces are exerted as the liquid-vapor
interface moves through the gel. These forces cause shrinkage of the pores within the
gel. Removal of the solvent (alcohol) from the gel under supercritical conditions results in
the formation of the aerogel. Since this drying procedure eliminates the liquid-vapor interface,
aerogels are formed in the absence of capillary forces. Aerogels retain the morphology
of the original alcogel.
There are several methods developed for removing the solvent from the gel under
supercritical conditions. The first one is the one suggested in the pioneering work by
Kistler, 101 in which the solvent is brought to supercritical conditions in an autoclave and
evacuated under these conditions. In order to pressurize the autoclave to a pressure above
the critical value for the alcohol, more alcohol is added to the autoclave. Supercritical conditions
of the solvent are reached by supplying heat to the autoclave. After the pressure