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6 1 Supercritical Carbon Dioxide for Sustainable Polymer Processes perature. Above this temperature, the distinction between the liquid phase and the gas phase disappears, resulting in a single supercritical fluid phase behavior. In 1875, Andrews discovered the critical conditions of CO2 [39]. The reported values were a critical temperature of 304.05 K and a critical pressure of 7.40 MPa, which are in close agreement with today’s accepted values of 304.1 K and 7.38 MPa. In the early days, supercritical fluids were mainly used in extraction and chromatography applications. A well-known example of supercritical fluid extraction is caffeine extraction from tea and coffee [40]. Supercritical chroma- tography was frequently used to separate polar compounds [41, 42]. Nowadays, an increasing interest is being shown in supercritical fluid applications for reac- tion, catalysis, polymerization, polymer processing, and polymer modification [43]. More detailed historical overviews are given by Jessop and Leitner [12] and by McHugh and Krukonis [40]. A supercritical fluid is defined as a substance for which the temperature and pressure are above their critical values and which has a density close to or high- er than its critical density [44–46]. Above the critical temperature, the vapor-liq- uid coexistence line no longer exists. Therefore, supercritical fluids can be re- garded as “hybrid solvents” because the properties can be tuned from liquid-like to gas-like without crossing a phase boundary by simply changing the pressure or the temperature. Although this definition gives the boundary values of the supercritical state, it does not describe all the physical or thermodynamic prop- erties. Baldyga [47] explains the supercritical state differently by stating that on a characteristic microscale of approximately 10–100 Å, statistical clusters of aug- mented density define the supercritical state, with a structure resembling that of liquids, surrounded by less dense and more chaotic regions of compressed gas. The number and dimensions of these clusters vary significantly with pres- sure and temperature, resulting in high compressibility near the critical point. To illustrate the “hybrid” properties of supercritical fluids, Table 1.2 gives some characteristic values for density, viscosity, and diffusivity. The unique properties of supercritical fluids as compared to liquids and gases provide op- portunities for a variety of industrial processes. In Fig. 1.5, two projections of the phase behavior of carbon dioxide are shown. In the pressure-temperature phase diagram (Fig.1.5a), the boiling line is ob- served, which separates the vapor and liquid regions and ends in the critical point. At the critical point, the densities of the equilibrium liquid phase and the saturat- Table 1.2 Comparison of typical values of physical properties of gases, supercritical fluids and liquids [48], where q, g and D stand for density, viscosity and diffusivity, respectively. Properties q (kg m–3) g (Pa s) D (m2 s–1) Gas Supercritical fluid 1 100–800 0.001 0.005–0.01 1·10–5 1·10–7 Liquid 1000 0.05–0.1 1·10–9PDF Image | Supercritical Carbon Dioxide for Sustainable Polymer Processes
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