Supercritical carbon dioxide as a green solvent for processing polymer melts: Processing aspects and applications

Supercritical carbon dioxide (CO2) is well established for use as a processing solvent in polymer applications such as polymer modification, formation of polymer composites, polymer blending, microcellular foaming, particle production and polymerization. Its gas-like diffusivity and liquid-like density in the supercritical phase allow replacing conventional, often noxious, solvents with supercritical CO2. Though only a few polymers are soluble in supercritical CO2, it is quite soluble in many molten polymers. CO2 dissolution in a polymer has been interpreted physically but FT-IR studies lead to an explanation in terms of weak interactions between basic and acidic sites. Various experimental methods and equations of state are available to measure or predict the solubility of CO2. Dissolved CO2 causes a considerable reduction in the viscosity of molten polymer, a very important property for the applications stated above. CO2 mainly acts as a plasticizer or solvent when contacted with a polymer. Gas solubility and viscosity reduction can be predicted theoretically from pure-component properties. In this review, experimental and theoretical studies of solubility and viscosity of several polymer melts are discussed in detail. Detailed attention is also given to recently reported applications along with aspects related to polymer processing.