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While energy reduction is an admirable part of green chemistry, the most significant targets for green chemistry in condensation polymers are probably not the polymerizations themselves, but rather the synthe- sis of the monomers. For example, diphenyl carbonate (monomer for polycarbonate) is synthesized from phosgene and phenol and a sizeable effort has been made by industry to optimize the catalytic production of DPC from phenol and CO [181]. Bisphenol A (also a precursor to polycarbonate) is under scrutiny for possible deleterious effects on humans. Tereph- thalic acid (precursor for polyesters) is generated via an oxidation of p-xylene that produces some prob- lematic waste streams [13]. DuPont has expended considerable effort in a joint venture with Genencor to create a biochemical route to propane diol, another precursor to aromatic polyesters. Pilot scale biologi- cal production of propane diol has been achieved and full-scale production is planned for the future [182]. Non-phosgene routes to di-isocyanates (precursors to polyurethanes) using CO2 as a raw material have been investigated by both industry and academia [183]. Finally, the oxidation route to adipic acid (precursor to nylon 6.6) and the synthesis of caprolactam (pre- cursor to nylon 6) are frequent targets of scientists involved in green chemistry, given the significant waste streams emitted by current processes [184]. Consequently, it would appear that real breakthroughs in green chemistry applied to condensation polymers will and should come in the area of more sustainable monomer synthesis. In some of these cases CO2 could play a significant role, but the primary research need appears to be more atom efficient synthetic routes. 3.5.2. Polyurethanes Polyurethanes are condensation polymers but rep- resent a special case, in that a small molecule is not produced during the primary polymerization reaction (where a hydroxyl group and an isocyanate react to form a urethane linkage). Whereas polyurethanes are applied as fibers, coatings and thermoplastics, their primary relevance to this report owes to their exten- sive use in foamed articles. Polyurethane flexible slabstock foam has been pro- duced via the ‘one-shot’ process since the late 1950s [185]. Here a stream of polyol (a multi-functional hydroxy-terminated oligomer, typically a polyether) is blended with water, catalysts, surfactants and ‘blowing agents’, then injected into a high-intensity mixing chamber with a multi-functional isocyanate. The resulting liquid blend is pumped evenly onto a moving belt, where polymerization occurs as hy- droxyl groups react with isocyanates to form urethane linkages. Further, water reacts with isocyanate to form an amine group plus CO2, where the amine subsequently reacts with another isocyanate to form a urea linkage. The heat of reaction boils the ‘blowing agent’; this plus the CO2 released during the poly- merization creates the foam, which is stabilized until cure by the added surfactant. For decades, the preferred blowing agent was either a chlorofluorocarbon or methylene chloride; note that these blowing agents were simply emitted to the atmo- sphere during foam formation. Following adaptation of the Montreal Protocols in 1986, foam producers searched for alternatives. Compounds such as pentane and hydrofluoropropane have been evaluated and ap- plied, yet these do not fully ameliorate the emissions problem (and, of course, hydrocarbons are flammable). In the late 1980s and early 1990s, Crain Industries created a CO2-based process (CarDio, [186]) where liquid CO2 (3–5% by weight) is injected into the polyol stream at pressures above the vapor pressure of CO2. The pressure is then gradually reduced, such that the pressure in the high intensity mixer is only 10–20 bar. The pressure is then reduced further via the use of a ‘gate-bar’ assembly that expands the mix- ture to one atmosphere and spreads it evenly onto the moving belt. The liquid mixture remains single phase through the mixing chamber because polyols absorb significant amounts of CO2, even at low pressures. Plants operate the CarDio process in both Europe and the US. Bayer Corporation has also commercialized a CO2 -based, continuous polyurethane process [187]. In both the CarDio and Bayer processes, CO2 directly replaces a large volume of organic solvent that would have been emitted to the atmosphere with little addi- tional energy input (cooling the liquid CO2). Conse- quently, polyurethane foam production using CO2 as the blowing agent is an excellent example of green chemistry using carbon dioxide. It is interesting to note that the first patent proposing the use of CO2 as the blowing agent for polyurethane foam was filed in 1959 [188]—it was only after perfection of the gate bar assembly in 1991 that Crain was able to success- fully scale up a CO2-based polyurethane foam line. E.J. Beckman / J. of Supercritical Fluids 28 (2004) 121–191 161PDF Image | Supercritical and near-critical CO2 in green chemical synthesis and processing
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