Extrudate distortion studies of polystyrene using an extrusion rheometer

Abstract

Uncorrected and corrected logarithmic flow-curves for a general purpose polystyrene (MW=261000 and MW/MN=4.4) obtained using a Davenport Extrusion Rheometer are shown for the range 160 to 200° C. The uncorrected flow curves show a change in slope, but at the lower extrusion temperatures this change occurs after the appearance of distorted extrudates. The onset of extrudate distortion obtained from observation does not coincide with the change in slope of the graph. The corrected logarithmic flow curves show no change in slope. Values of $$\dot \gamma _c$$ and η c from both sets of graphs show that $$\dot \gamma _c$$ is inversely proportional to η c, and for the higher melt temperatures the corrected τ c values increase with temperature. The high value of critical wall stress at 160° C is attributed to the increase in melt elasticity with decreasing temperature being a greater effect than the decrease in elasticity due to a decrease in $$\dot \gamma _c$$ .