RHEOLOGY AND EXTRUDABILITY OF VERY CONCENTRATED SUSPENSIONS - EFFECTSOF VACUUM IMPOSITION

Earlier studies have indicated that consequential amounts of air can b e incorporated into concentrated suspensions during their processing. In this study the effects of air entrainment on the theological behavi or and extrudability of two concentrated suspensions with 57 and 63% v olume of solids were investigated. The air content was altered by the imposition of vacuum in a twin screw extruder. The theological behavio r of the suspensions was characterized by employing steady torsional f low in a Rheometrics System IV in conjunction with a flow visualizatio n technique for the characterization of the yield stress, shear viscos ity and wall slip behavior of the suspensions. The removal of air from the suspension was indeed found to modify the theological behavior an d extrudability of the suspension. Furthermore, it was demonstrated th at when the solid concentration approaches the maximum packing fractio n the sudden decrease of the air concentration of the suspension resul ts in severe instabilities which are manifested by results in signific ant oscillations in operating parameters during twin screw extrusion p rocessing.