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152 E.J. Beckman / J. of Supercritical Fluids 28 (2004) 121–191 produced worldwide are produced in the complete ab- sence of solvent. Indeed, polyolefins (polyethylene), vinyl polymers (styrenics, acrylontrile, butadiene), polyamides (nylons) and polyesters are generated principally in bulk polymerization processes [123]. Further, for the most part, commercial polymers are poorly soluble (many, in effect, are insoluble) in CO2. However, owing to the asymmetry of amorphous polymer–CO2 phase envelopes, even polymers that are poorly soluble in CO2 will swell extensively under moderate CO2 pressure, allowing for a number of ap- plications using CO2 as reversible diluent/plasticizer. CO2 is used extensively in the foaming of polymers (both styrenics and polyurethanes), CO2 has been used as the solvent in coating processes (Union Car- bide’s UniCarb process) and CO2 is currently being explored at the pilot works level in fluoropolymer synthesis (DuPont) and powder coating processing (Ferro Industries). 3.2. Polymerizations: general background Polymerizations are typically classified by the mode of polymerization (ring-opening, free-radical, etc.), by the type of monomer used (styrenics, acrylates) or by the type of linkage formed during polymerization (polyamides, polyesters). In addition, polymerizations can be conducted in the bulk state, in solution, or in one of many so-called ‘heterogeneous modes’—namely precipitation, suspension, dispersion or emulsion. Because CO2 is typically proposed/employed as a benign solvent, the following discussion of polymer formation and processing in CO2 will focus on those applications where solvents are ordinarily used. How- ever, where examples can be found where use of CO2 in a formerly solvent-less process can provide sustain- able and other benefits, such applications will also be discussed. 3.3. CO2 as a solvent for polymer systems Polymers present special problems regarding disso- lution in any solvent—the very low entropy of mixing in polymer/solvent binaries (owing to the long chains of the polymer) requires a very favorable enthalpic interaction between polymer segments and solvent to ensure dissolution of substantial polymer concentra- tions [124]. This problem is magnified in the case of CO2 , given that CO2 ’s solvent power is admittedly weak. While a significant portion of academic polymer– SCF phase behavior work has considered solutions where the polymer is the minor component, it is im- portant to remember that the full phase diagram offers several interesting regimes with regards to possible green applications. In Fig. 5, we see a generic phase diagram of a polymer and a SCF [125], showing the various phase separation envelopes and the behavior both above and below the solvent critical temperature. As can be seen in Fig. 6, the liquid–liquid phase enve- lope is asymmetric (owing to the large disparity in size between polymer and solvent) with the liquid–liquid critical point shifted towards the 100% solvent axis. This is important—it means that solubilization of low concentrations of polymer in solvent will require the highest pressures. Swelling of the polymer by the sol- vent (moving to the right along the x-axis in Fig. 5) requires significantly lower pressures. Thus, in certain polymer–SCF mixtures, one can observe very high de- grees of swelling (>25% in polyacrylate–CO2 mix- tures, for example) at pressures of 100 bar and below [126]. The relatively low pressures required to elicit high degrees of swelling may be one reason why appli- cations where CO2 is the minor component have been successfully commercialized, while those employing dilute polymer solutions have not. High-pressure phase behavior studies of polymers and supercritical fluids have been conducted since the late 1940s; the early work was performed to sup- port the high-pressure polyethylene process. Ehrlich’s group performed some of the best early work on the phase behavior of polyolefins in supercritical alkanes and alkenes [127]; these studies have been followed by numerous others on polyethylene:alkane or polyethy- lene:alkene mixtures [128]. In the late 1960s, Giddings suggested a simple correlation between solubility parameter and criti- cal pressure that indicated that CO2’s solvent power should be similar to that of pyridine [4]. However, the strong quadrupole moment of carbon dioxide affects CO2’s pVT properties (including the critical pressure) without influencing its solvent strength. Consequently, early calculations of the solubility pa- rameter were invariably inflated. This was actually confirmed by the very study that proposed that CO2’s solubility parameter should approach that of pyridine;PDF Image | Supercritical and near-critical CO2 in green chemical synthesis and processing
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