Viscoelastic behaviour of polystyrene/supercritical CO2 mixtures used for foaming applications

A study of polystyrene resins containing supercritical CO2 was made to understand their viscoelastic behavior for foaming applications. Three resins of polystyrene with different molecular weight distribution (MWD) were tested at three temperatures (170, 185 and 200ºC) and two pressures (6.89 MPa and 13.78 MPa) using CO2 as diluent. A testing methodology was developed to provide accuracy and repeatability. It reduced standard deviations by at least 52% for high pressure oscillatory rheology tests. Methodology included data correction using Sanchez-Lacombe EOS to describe the mixing phenomenon and polymer swelling. Effects of temperature, pressure and CO2 concentration were evaluated and isolated according to a time-temperature-pressure-concentration superposition. Another approach was accomplished using a high pressure rheology model developed by Total Petrochemicals & Refining USA, Inc. to identify differences in viscoelastic behavior of polystyrene resins. Overall, the model accurately described the shear-thinning behavior of PS and PS+CO2 systems. Finally, elongational viscosities were estimated to speculate about the polymer’s foaming behavior at different operating conditions. The analysis showed that apparently there were no significant differences between resins’ viscosities at high pressures. However, results were expected to differentiate according to resins’ MWD; the higher the molecular weight, the higher the elongational viscosity. Therefore, it is concluded that obtained rheological data is not suitable for this model and additional data adjustment is mandatory for this particular analysis.