Extrusion of PS/LDPE blends with supercritical carbon dioxide: Morphology, viscosity and interfacial tension

Extrusion of PS/LDPE blends was carried out in the presence of supercritical carbon dioxide with a twin-screw extruder and the effects of supercritical carbon dioxide on the blend morphology were investigated. Supercritical carbon dioxide was found to reduce the dispersed phase size and narrow the phase co-continuity region of the blend system. The change of the blend morphology due to supercritical carbon dioxide was attributed to its effects on the shear viscosity of PS and LDPE as well as on the interfacial tension between PS and LDPE melts.; The shear viscosity of PS/CO₂ solutions was measured with an in-line slit die rheometer, while the shear viscosity of LDPE/CO₂ solutions was taken from the literature (Lee et al., 2000). A considerable reduction of shear viscosity was found when CO₂ was dissolved into the polymer melts. The shear viscosity of polymer/CO₂ solutions was found to be a sensitive function of shear rate, temperature, pressure and CO₂ concentration.; To study the effect of supercritical carbon dioxide, the system was pressurized with CO₂ and the interfacial tension measurements were carried out after CO₂ saturation. Supercritical carbon dioxide was shown to decrease the interfacial tension between PS and LDPE melts.; The effects of supercritical carbon dioxide on the morphology of PS/LDPE blends prepared with a twin-screw extruder were examined with scanning electron microscopy and solvent extraction. It was found that the injection of supercritical carbon dioxide decreased the dispersed phase size and narrowed the phase co-continuity region. Shear viscosity and interfacial tension were used to explain the effect of supercritical carbon dioxide on the morphology changes. The decrease of interfacial tension due to supercritical carbon dioxide seems to be the dominant factor in decreasing the dispersed phase size, while the decrease of the shear viscosity and the phase size was used to explain the narrower phase co-continuous region.