A supercritical fluid (SCF) is a substance that exists above its critical temperature and critical pressure.In theory any fluid can under the correct conditions become a supercritical fluid.In this state the liquid and gaseous phases are in equilibrium, and the fluid has the properties of both phases. The supercritical fluid state has the added benefit of increased density and therefore such a fluid has the ability to dissolve, diffuse and carry materials. Carbon dioxide is at present the most popular fluid for this technology as its critical temperature is 31oC and its critical pressure is 1050 psi. Supercritical fluids have been used extensively as solvent substitutes and extraction aids in both the pharmaceutical and food industries as they provide a clean, efficient and an environmentally friendly solution to both industries. Drug delivery and the decaffeination of coffee are two commercially successful examples of this technology.
Recently supercritical fluids in particular supercritical carbon dioxide (scCO2) have been used as foaming aids in melt processing of commodity plastics.As a result of this work, it has been reported that scCO2 has the added benefit of reducing plastic viscosity during melt processing, enabling enhancement of process performance.There are are number of excellent reviews on supercritical fluid technology such as Cooper’s review ‘Polymer synthesis and processing using supercritical carbon dioxide’ (A. Cooper, Journal of Materials Chemistry, 10 (2000) 207-234)& Kazarian’s review ‘Polymer Processing with Supercritical Fluids’ (Polymer Science, Ser. C, Vol. 42, No. 1, 2000, pp. 78–101).
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