Influence of the Processes Parameters on the Properties of The ...

Chapter 2.
<strong>Processes</strong> to Manufacture Foams and to Functionalize <strong>the</strong> Surface
In Table 2.2, <strong>the</strong> critical properties <strong>of</strong> some compounds which are commonly used as supercritical
fluids are shown. Of <strong>the</strong>se, carbon dioxide and water are <strong>the</strong> most frequently used in a wide range <strong>of</strong>
applications.
Table 2.2: Critical conditions <strong>of</strong> several substances.
[Leitner and Jessop, 1999]
Recently, various supercritical fluid processing methods have been developed for <strong>the</strong> production
<strong>of</strong> polymer-based materials such as foams, micro-particles, and fibres. Microcellular polymers can be
formed with no use <strong>of</strong> organic solvents through <strong>the</strong> gas foaming technique. In this process <strong>the</strong> polymer is
saturated, first, with carbon dioxide (CO 2 ) at high pressure. Then, <strong>the</strong> system is quenched in supersaturated
state ei<strong>the</strong>r by reducing pressure or by increasing temperature resulting in <strong>the</strong> nucleation and growth <strong>of</strong>
pores—cells inside <strong>the</strong> polymer matrix [Goel and Beckman, 1994a]. This technique is adapted to PLGA
because it is an amorphous polymer. In <strong>the</strong> case <strong>of</strong> semi-crystalline polymers, <strong>the</strong> final porous structure
obtained by this technique is non-uniform, since <strong>the</strong> diffusion <strong>of</strong> <strong>the</strong> fluid is different in <strong>the</strong> crystallites and
in <strong>the</strong> amorphous zones [Fages et al., 2003; Lucien and Foster, 2000; Doroudiani et al., 1996]. With respect
to dense phase fluids, supercritical water has been shown to be a very effective reaction medium for
oxidation reactions [Thomason and Modell, 1984; Modell, 1982].
3.2 Basic Techniques in Supercritical Fluids Technology
Classification <strong>of</strong> SCFs based techniques can be proposed according to <strong>the</strong> role played by <strong>the</strong> SCFs
in <strong>the</strong> process. Various SCFs processes include [Sekhon, 2010]:
1. Rapid expansion <strong>of</strong> supercritical solutions,
2. Supercritical anti-solvent precipitation technique,
3. Particles from gas saturated solutions,
4. Gas antisolvent system,
5. Precipitation using compressed antisolvent,
6. Aerosol solvent extraction system,
7. Solution enhanced dispersion by supercritical fluids,
8. Supercritical antisolvent system with enhanced mass transfer,
9. Impregnation or infusion <strong>of</strong> polymers with bioactive materials.
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