Supercritical fluid assisted polymer processing: Plasticization, swelling and rheology

The use of supercritical carbon dioxide, scCO₂, as a transient plasticizer eliminates the disadvantages associated with many other industrial plasticizers. Because CO₂ is a gas under atmospheric conditions, it can be used as a processing aid and then easily removed from a polymer through evaporation to obtain the original physical properties of the unplasticized polymer matrix. In this work, a three-step approach is used to investigate and quantify the physical phenomena associated with CO₂-induced plasticization of polymer melts.; First, a novel experimental apparatus was designed and constructed to measure equilibrium swelling, swelling kinetics and diffusion of CO₂ into a polymer melt. It was found that diffusion of CO₂ into PDMS exhibited Fickian behavior up until two-thirds of the equilibrium swelling value was obtained. The CO₂ pressure had a negligible effect on the diffusion coefficient; however, the system temperature directly affected the diffusion coefficient. Increased pressure was found to enhance the extent of swelling whereas a maximum was observed with increasing temperature, at pressures above 15 MPa.; Secondly, a high pressure extrusion slit die rheometer was constructed to measure the viscosity of polymer melts plasticized with low concentrations of CO₂. Polystyrene, poly(methyl methacrylate), polypropylene, low density polyethylene, and poly(vinylidene fluoride) were investigated. CO₂ was found to be an efficient plasticizer for all of these polymer materials, generally lowering the viscosity of the melt 30–80%, depending on processing conditions. Predictive viscoelastic scaling models based on free-volume principles and a prediction of T_g depression from a diluent were developed to quantify the effects of CO ₂ concentration, pressure and temperature on viscosity. This unique free-volume approach allows the high pressure polymer/CO₂ rheology to be predicted based solely on physical parameters of the polymer melt and CO₂.; Finally, a novel high pressure magnetically levitated sphere rheometer (MLSR) was developed to further investigate the effects of CO₂ on the viscosity of polymer melts. This rheometer was used to measure the viscosity reduction of poly(dimethyl siloxane) by CO₂. The viscosity of the polymer melt could be lowered in excess of 97% of its original value at atmospheric pressure by adding a CO₂ concentration of approximately 30wt%.