Carbon dioxide foaming and high-pressure rheology of polystyrene and polystyrene/organoclay nanocomposites

by 1979- Wingert, Maxwell J.

The polymer foam industry is slowly implementing carbon dioxide (CO2) as a low-cost, safe, and environmentally friendly blowing agent alternative to fluorocarbons and hydrocarbons. Progress is slow due to several obstacles, ranging from low blowing agent solubility to a lack of quantitative understanding of the influence of carbon dioxide on viscosity. A crucial property in foam extrusion is viscosity. Several research groups have published viscosity data of polymer melts under high pressure, using a variety of techniques. However, few studies assist in designing polymer processing equipment because most do not contain predictive scaling (e.g., WLF-analogous scaling factors) to apply to different operating conditions. A new high-pressure rotational rheometer has been applied to polystyrene and carbon dioxide at five concentrations. It provides direct measurement of the zero shear viscosity of the polymer under a high pressure diluent. The method allows many viscosity measurements to be performed on a single sample. Scaling factors are applied to the data and the WLF-Chow equation is found to describe the results when the appropriate parameter is selected. ii Due to an interest in using organoclay nanoparticles for foaming, the viscosity of the polystyrene-nanoclay-CO2 system is studied using the couette rheometer and an extruder slit die. At high shear rates (10 to 100 s-1), the viscosity of polystyrene-nanoclay- CO2 unexpectedly possesses a lower viscosity than polystyrene-CO2 at the same concentration. At low shear rates (10-3 to 1 s-1), this effect is not observed. It is suspected that interfacial slip is occurred at the interface at high shear rates. Polymer additives allow tuning of bubble morphology without changing operating conditions. In this study, either a second polymer or nanoparticles are studied. Poly (methyl methacrylate) (PMMA) has the ability to drastically reduce cell size of polystyrene (PS) foams. It is believed that heterogeneous nucleation occurs at the interface of PS/PMMA, but that the bubbles are able to grow out of both phases simultaneously. iii