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to the extruder is probably the most common means to induce foaming, ‘chemical’ blowing agents, i.e. compounds that thermally decompose to form gases, are also employed. The extrusion based foaming of polymers is concep- tually simple, yet requires complex analysis to fully understand the system. In the case of polystyrene, a fluid is injected into the extruder, where the pressure and temperature are sufficient (ostensibly) to create a single-phase mixture of blowing agent and polymer. Mixing is enhanced through strategic screw design. Following mixing, the melt is cooled (in some cases in a second, tandem extruder) to build melt strength, as the addition of the fluid greatly lowers the melt vis- cosity. The die is cooler still. Upon exiting the die, the rapid pressure drop creates a supersaturated solu- tion, where small pores containing CO2 nucleate and grow (nucleating agents are often added to stimulate this process). The pores grow until the rapidly rising viscosity of the polymer (owing to cooling and loss of blowing agent) restricts further expansion. In conven- tional extruded foam, the cells are of order 100–1000 microns in diameter. Microcellular foam [208], formed in much the same way albeit with higher concentration of CO2 in the polymer melt, exhibits cells 50 microns and below in size. The generation of foamed thermoplastics using CO2 as the sole blowing agent is most definitely ‘green’ processing, as the CO2 replaces either organic or hy- drofluorocarbon agents that would otherwise directly enter the atmosphere. A number of researchers have investigated the fundamentals of foam formation using high pressure CO2, and several important conclusions have arisen [209]: • The number of cells nucleated during a pressure quench in a CO2–swollen polymer depends di- rectly upon the degree of swelling of the polymer. Swelling, in turn, rises as pressure rises and as tem- perature falls. To create more cells one must adjust conditions to ensure higher degrees of swelling. • The growth of cells is dependent upon the degree to which CO2 diffuses into the nuclei and also the degree to which CO2 expands as pressure drops. At the same time, growth is inhibited by the retractive force of the polymer melt, which increases as the temperature drops and CO2 diffuses from the melt. Hence, to make smaller cells, one must restrict growth soon after nucleation, by vitrifying the sys- tem before the pressure drops to the point where CO2 begins to expand significantly. If one desires to make a large number of very small cells, then in the- ory one should start with a high degree of swelling of the polymer by CO2 and vitrify the material as soon as possible after nucleation of pores. Unfor- tunately, very high degrees of swelling lower the melt strength (related to viscosity) significantly and hence pores tend to coalesce during growth [210]. • Our understanding of the fundamental processes that control foam morphology derives in large part to fundamental studies performed in academia and industry during the late 1980s and early 1990s. For example, early studies of the effect of pressure on the swelling of polymers by CO2 by Berens and Huvard [211], Liao and McHugh [212] and Wissinger and Paulaitis [126] paved the way for fu- ture work on polymer foaming. Wang and Kramer [213] first explored the behavior of the glass tran- sition of a polymer versus CO2 pressure in 1983; this was followed by a seminal study by Condo and Johnston [66]. Fundamental studies of the viscosity of polymer–CO2 melts, for example, were per- formed by Manke and also by Khan [214]. These studies provided the data that made later studies of foam formation more tractable. While it is likely that similar work was performed in industry, little of it can be found in the open literature and hence the academic work has been vital in providing a basis for recent foam research. Foam formed using CO2 as the sole blowing agent has been commercialized in a number of cases, yet the process is non-optimal, as foam properties using CO2 still do not approach those when CFCs are employed as blowing agents. While the foam-forming process is understood from an academic sense, a number of sci- entific/technical challenges remain before optimiza- tion can occur. These include: • Shear effects on phase behavior: The phase behavior of CO2 –polymer mixtures is generally measured (in academia) under static conditions; there have been reports that the phase behavior of CO2 –polystyrene, for example, depends sig- nificantly on shear [215]. Measurement of high pressure phase behavior under shear presents a significant experimental challenge, yet one which E.J. Beckman / J. of Supercritical Fluids 28 (2004) 121–191 165PDF Image | Supercritical and near-critical CO2 in green chemical synthesis and processing
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