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
Super critical fluid Technologies, although environmentally friendly and suitable for mass
production, requires specially designed equipment and is more expensive. In <strong>the</strong> early days, supercritical
fluids were mainly used in extraction and chromatography applications [Smith, 1999; Dean, 1998;
Vandenburg et al., 1997; McNally, 1995; Brunner, 1994; Hedrick et al., 1992]. A well-known example <strong>of</strong>
supercritical fluid extraction is caffeine extraction from tea and c<strong>of</strong>fee [McHugh and Krukonis, 1994].
Supercritical chromatography was frequently used to separate polar compounds [Berger, 1997; Cantrell and
Blackwell, 1997]. Nowadays, an increasing interest is being shown in supercritical fluid applications for
reaction, catalysis, polymerization, polymer processing, and polymer modification [Eckert et al., 1996]. SCF
technologies are now emerging as an alternative to conventional materials processing methods in <strong>the</strong> area <strong>of</strong>
tissue engineering [Duarte et al., 2009a; Duarte et al., 2009b]. ScCO 2 processing may be used to form
foamed scaffolds in which <strong>the</strong> escape <strong>of</strong> CO 2 from a plasticized polymer melt generates gas bubbles that
shape <strong>the</strong> developing pores.
3.3 Scaffolds Prepared by Phase Inversion using scCO 2 as Anti-solvent
Phase inversion using supercritical CO 2 as antis-olvent is analogous to traditional phase inversion
with immersion precipitation. This technique consists <strong>of</strong> immersing a thin film <strong>of</strong> <strong>the</strong> polymer solution in a
bath containing a non-solvent (with respect to <strong>the</strong> polymer). The properties <strong>of</strong> <strong>the</strong> final porous structure are
mainly controlled by <strong>the</strong> precipitation temperature, <strong>the</strong> strength <strong>of</strong> <strong>the</strong> non-solvent bath and <strong>the</strong> composition
<strong>of</strong> <strong>the</strong> casting solution. The use <strong>of</strong> a supercritical fluid as an antisolvent allows for <strong>the</strong> tuning <strong>of</strong> <strong>the</strong>
antisolvent strength simply by regulating <strong>the</strong> pressure. As a consequence, <strong>the</strong> pressure is an additional
parameter for tailoring <strong>the</strong> final structure [Tsivintzelis et al., 2007a].
The use <strong>of</strong> CO 2 as an antisolvent for <strong>the</strong> production <strong>of</strong> porous structures with polymers has not
been thoroughly investigated. Since <strong>the</strong> majority <strong>of</strong> foaming methods applied in <strong>the</strong> semicrystalline
polymers involve <strong>the</strong> use <strong>of</strong> organic solvents, <strong>the</strong>re is an important advantage <strong>of</strong> using phase inversion in <strong>the</strong>
presence <strong>of</strong> supercritical CO 2 . With this technique, it is possible to dry <strong>the</strong> final polymer structure simply by
flashing <strong>the</strong> pressure vessel with fresh CO 2 . Thus, <strong>the</strong>re is no need for additional post-treatment in order to
remove <strong>the</strong> residual organic solvent [Tsivintzelis et al., 2007a]. Dichloromethane can be selected as solvent
since it is completely miscible with CO 2 at pressures higher than 95 bars and temperatures up to 55 o C
[Tsivintzelis et al., 2004]. Additionally, <strong>the</strong> solubility <strong>of</strong> P L LA in CO 2 at <strong>the</strong>se conditions is negligible,
making <strong>the</strong> CO 2 an appropriate antisolvent for this system. Figure 2.17 represents a schematic diagram <strong>of</strong> <strong>the</strong>
system.
Figure 2.17: ScCO 2 experimental apparatus (A) CO 2 tank, (B) syringe pump and (C) pressure vessel.
[Tsivintzelis et al., 2007a]
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