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166 E.J. Beckman / J. of Supercritical Fluids 28 (2004) 121–191 may have to ultimately be conquered if a full understanding of extrusion foaming is to be found. • Pressure limitations in conventional extruders: While extruders can theoretically be operated at very high pressures (300 bar+), the typical oper- ating pressure for a polystyrene foam extruder is ≈100 bar at temperatures in excess of 470 K. At the same time, the swelling of polymers such as polystyrene is not sufficient under these conditions to produce foam of the same quality as can be produced with liquid blowing agents. While rais- ing the pressure is the usual remedy for insuffi- cient swelling, it is not a viable one in this case, and hence additives must be developed that will al- low enhanced swelling of ‘CO2 -phobic’ polymers by CO2 [216]. Further, these additives must be de- signed in order to be effective at low loadings (or else foam physical properties and cost will be ad- versely impacted). • Rapid diffusion of CO2: Compared to conventional blowing agents, CO2 diffuses rapidly from foam pores—this rapid diffusion in practice contributes to foam collapse [217]. Consequently, there is a need to develop additives that will partition to the CO2–polymer interface, then set up a barrier against CO2 diffusion. • High thermal conductivity of CO2: Insulation is a prime application for foamed polymeric materi- als. Further, the effective thermal conductivity of a polymer foam, at low foam density, is a strong function of the thermal conductivity of the gas in- side the pores. Because CO2 exhibits a significantly higher thermal conductivity than CFCs [218], one may have to employ larger quantities of foam to accomplish the same insulation job if CO2 is em- ployed as the blowing agent. The blowing agent, although originally entrapped within the foamed polymer, will eventually diffuse out and be replaced by air diffusing in—the high diffusion coefficient of CO2 renders this exchange faster with CO2 than with chlorofluorocarbons. Thus, an additional chal- lenge is to achieve high insulating value while em- ploying CO2. Finally, a general conclusion that one can draw from the extensive previous work on foaming is that, using the ‘swell-quench’ method, one can generate a foam with either small pores (<10 microns) or low bulk density (<0.05 g/cc), but not both. Low bulk density requires the generation of very large numbers of small pores, and hence high swelling (and hence high nu- cleation density) but limited growth. Unfortunately, as mentioned previously, high swelling also leads to low melt strength and hence pore coalescence. The lower limit for cell size in extruded foam with low bulk density (<0.1 g/cc) appears to be approximately 50 microns. Consequently, researchers have explored new strategies for forming low bulk density, fine-celled foams. For example, Enick et al. [219] have generated molecules that will dissolve in CO2, then self assemble to form gels. Removal of the CO2 (via depressuriza- tion) leaves behind a porous structure with submicron cell size and bulk density below 0.05 g/cc. In summary, the foaming of thermoplastics using CO2 as the sole blowing agent is undeniably green polymer processing, in that use of CO2 directly re- places organic solvent that would ultimately enter the atmosphere. The challenges to efficient use of CO2 in foam production are given above—it should be noted that these are entirely technical and hence would pro- vide excellent targets for future research. 3.10. Industrial activity: post-polymerization processing As mentioned above, a large number of patents have been issued for both the foaming of polymers with CO2 and the use of CO2 to dye textiles. For the case of polymer foaming, the technology has achieved commercial status, both macrocellular foam formation (Dow, for example) and microcellular foam formation (Trexel has licensed technology developed at MIT by Nam Suh et al. [220]). The textile work has been ad- vanced to the pilot stage in Germany and in the US. 3.11. Use of CO2 in polymer science applied to the microelectronics industry The preparation of an eight-inch silicone wafer requires hundreds of individual process steps, of which approximately half involve washing [221]. It has been estimated that a single fabrication line will use over one million gallons of solvent each year. In photolithography, the technique used to create pat- terned microelectronic components, a polymer layer is applied to an inorganic substrate by spin coatingPDF Image | Supercritical and near-critical CO2 in green chemical synthesis and processing
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