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CARBON DIOXIDE TECHNOLOGY PLATFORM 137 can be viewed as a supercritical fluid extraction of condensate from the polymer particle. In addition, CO2 plasticizes the amorphous regions of the polycarbon- ate sample, providing increased mobility for chain extension reactions. Lower reaction temperatures should be feasible because the presence of CO2 depresses the glass transition temperature of the polymer. This is just one example of the benefits of using supercritical CO2 as a processing aide. Other examples could be envisioned if the mechanism of polymerization is compatible with the use of CO2. 5.3 Microcellular Foams Supercritical fluids (SCFs) in general and carbon dioxide in particular, provide a suitable medium for impregnating polymer matrices with metal nanoparticles. The impregnation of polymers and related porous solids has been monitored by Fourier transform infrared and UV-vis spectroscopy (72–77). In some cases, a metal-containing precursor has been incorporated into the polymer matrix and subsequently decomposed. The decomposed material is removed by venting CO2 and if necessary subsequently flushing with scCO2. These materials are being considered for electronic and optoelectronic devices and novel biomaterials for prosthetic replacements. (COD)-PtMe2 has been infused into polymer PMP and PTFE, respectively, to provide a composite material with <50 nm–sized Pt par- ticles (78, 79). By venting scCO2 in the preparation of composite material prior to hydrogenating the infused (COD)PtMe2 precursor, smaller particles were ob- tained (79). This work has been extended to include deposition of metal films on substrate and impregnation of catalyst supports (79). Infusion techniques in the pharmaceutical field has lead to controlled-release drug materials (80). Another area of interest has been SCF-assisted dyeing of textiles with mordant dyes (81). Spray coatings have also received interest; the UNICARB system (commercial- ized by Union Carbide) claims a 30%–70% reduction in VOCs by replacing the conventional organic solvents of the process with 10–50 wt% scCO2 (82). Powder coatings have been revolutionized by the VAMP process, which involves swelling a polymer with scCO2 and subsequently expanding the mixture through a valve. The foam breaks up as it is expanded, resulting in a fine powder that is at least 50% cheaper to manufacture than by conventional processing. The VAMP approach has been used to produce synthetic bone material for medical use (83). 5.4 Catalysis in Supercritical Fluids Several traditional chemical reactions are candidates for supercritical CO2. Cat- alytic reactions are important, and the field is rapidly expanding, with the opportu- nity for using scCO2 existing for both homogeneous and heterogeneous catalysis. Using SCFs avoids the potential hazards associated with volatile organic solvents and reduces the waste materials associated with isolating the hydrogenation prod- uct. In addition, reaction efficiency is improved because hydrogen is completely miscible with SCFs.PDF Image | OPPORTUNITIES FOR POLLUTION PREVENTION
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