(81d) Rotational Rheometry of Polystyrene under High-Pressure Carbon Dioxide

Polymer based foams are used in many commercial applications, such as packaging, insulation, and even scaffolds for tissue engineering. Although refrigerants currently dominate carbon dioxide (CO2) in the blowing agent market, CO2 is an inexpensive, effective, and friendly blowing agent for thermoplastic foams. To expedite the introduction of CO2 to industrial foaming applications, it is important to understand its effects on viscosity, since viscosity controls both the extent of bubble growth and the fluid dynamics of flowing systems (e.g., foam extrusion). Several studies have measured the quantity of viscosity reduction of polymer melts under high pressure diluent (CO2), but almost all detect at high shear rates where correction factors and averaging of pressures convolute the experimental results. This study uses a high pressure couette rheometer to measure viscosity in a constant pressure environment to avoid these errors. It also examines viscosity in the low (Newtonian) shear rate region, where the magnitude of CO2 depression is larger. This again suggests greater accuracy when fully characterizing the viscosity-shear rate curve. Accuracy is especially important during the foam extrusion die design. If the viscosity at low shear rates is off by a factor of two (the approximate error of high shear rate experiments), simulations have predicted significant deviations in such parameters as pressure drop and viscous heating. The polymer used in this study is polystyrene (PS), an important foaming material. The viscosity of PS-CO2 was directly measured using the couette rheometer for a variety of CO2 concentrations and temperatures. To calculate the magnitude of the viscosity reduction, the value is compared with other results of PS viscosity without carbon dioxide. That method uses a parallel-plate geometry, then subsequently fitting the results to the Cross model. The resultant CO2-induced viscosity reduction is compared with both a WLF-Chow model and a free volume-equation of state model. Finally, the device is applied to a filled polymer system. Carbon nanofibers (CNFs) have been a very interesting additive for thermoplastic foams, allowing some control over bubble size. Thus, it is important to determine what effect CNFs have on the magnitude of CO2 viscosity reduction for these pre-foamed materials.