New challenges in polymer foaming: A review of extrusion processes assisted by supercritical carbon dioxide

It is well known that supercritical carbon dioxide (sc-CO(2)) is soluble in molten polymers and acts as a plasticizer. The dissolution of sc-CO(2) leads to a decrease in the viscosity of the liquid polymer, the melting point and the glass transition temperature. These properties have been used in several particle generation processes such as PGSS (particles from gas saturated solutions). It is therefore highly likely that extrusion processes would benefit from the use of sc-CO(2) since the rationale of the extrusion processes is to formulate, texture and shape molten polymers by forcing them through a die. Combining these two technologies, extrusion and supercritical fluids, could open up new applications in extrusion. The main advantage of introducing sc-CO(2) in the barrel of an extruder is its function as a plasticizer, which allows the processing of molecules which would otherwise be too fragile to withstand the mechanical stresses and the operating temperatures of a standard extrusion process. In addition, the dissolved CO(2) acts as a foaming agent during expansion through the die. It is therefore possible to control pore generation and growth by controlling the operating conditions. This review focuses on experimental work carried out using continuous extrusion. A continuous process is more economically favourable than batch foaming processes because it is easier to control, has a higher throughput and is very versatile in the properties and shapes of the products obtained. The coupling of extrusion and supercritical CO(2) technologies has already broadened the range of application of extrusion processes. The first applications were developed for the agro-food industry 20 years ago. However, most thermoplastics could potentially be submitted to sc-CO(2)-assisted extrusion, opening new challenging opportunities, particularly in the field of pharmaceutical applications. This coupled technology is however still very new and further developments of both experimental and modelling studies will be necessary to gain better theoretical understanding and technical expertise prior to industrial use, especially in the pharmaceutical field. (C) 2010 Elsevier Ltd. All rights reserved.