Shaping of Polymers from Supercritical Solutions

One of the major potential advantages of supercritical carbon dioxide polymer processing, is the production of fine polymer materials without the use of organic solvents. It has been shown in the literature, that many possible shapes can be produced using a variety of methods. These include the production of elastomers, powder coatings, toners, foams and fibres. Additionally, when the polymerisation is also performed in supercritical carbon dioxide, it is possible to combine reaction and shaping in one single piece of equipment. A polymer solved or swollen in supercritical carbon dioxide can be shaped by forcing the polymer/CO2 mixture through a nozzle, while expanding to sub-critical conditions. Important process parameters thereby are pre- and post nozzle temperature and pressure, supercritical solution composition and nozzle dimensions. By controlling these parameters, virtually all types of morphologies can be obtained. However, no general concept exists linking these parameters, the dynamic phase transitions and the final morphology in a generalised manner. This project aims at understanding the effect of the various process parameters on the morphology of the final product. The key to successful application of this shaping technique lies in understanding the relation between the precipitation behaviour of polymers and the physical properties of the supercritical solvent. To establish a relation between physical properties of the solvent and nozzle geometry, the flow behaviour of the supercritical polymer solution before and after the nozzle is calculated using Computational Fluid Dynamics (CFD). The resulting expansion profiles are verified by flow profile measurements, using laser techniques. When the expansion profiles are related with the phase diagrams a model can be formed, predicting the product quality and morphology.