Development of phase morphology in blending for modular co-rotating and counter-rotating twin screw extruders.

摘要

We studied the phase morphology development during blending in modular co-rotating and counter-rotating twin screw extruders. The system we investigated was polyethylene-polystyrene. Comparisons were made of various blend compositions, processing variables, and various screw configurations possessing different numbers and placement of kneading disc blocks. We also investigated the effect of the process material quenching method and the compatibilizer on blend morphology in the twin screw extruder.; Generally, the major morphological changes occur in the region of the first kneading disc block through pellet softening and subsequent melt mixing. Inspection of the blend material reveals the tremendous size reduction in the evolution of the pellets to droplets is associated with mechanisms including stretching pellets to sheets, sheet breakup to threads and thread breakup to droplets.; Generally, the HDPE melts first and encapsulates the PS, thus serving as the continuous phase.; The phenomena of phase inversion along the screw axis occurs in many PS/HDPE blends when the PS level is high.; Screw configuration has a major effect on the details of phase evolution. Generally the kneading disc blocks are fully filled and have a longer residence time. The introduction of the kneading disc block to the earlier position of the screw axis is effective for melt mixing.; The phase morphology evolution of the PS/HDPE blend in counter-rotating twin screw extruders appears similar to the co-rotating mode. However, pellet fusion and melt mixing seems more effective in the counter-rotating mode.; We established a model allowing the prediction of the down stream morphology development in a polymer blend along the screw axis of the intermeshing co-rotating twin screw extruder. This follows the phase size reduction of dispersed phase pellets to sheets. The stability and breakup of a viscous liquid sheet suspended in an immiscible viscous liquid in a quiescent flow was analyzed with equations of motion for both fluid phases.; Illustrative calculations of the dispersed phase size were made. The model only may be used for dilute blends of PS in HDPE where no coalescence occurs.