A new scale-up approach for dispersive mixing in twin-screw compounding

Abstract

Scale-up rules in polymer procebing are critical in ensuring consistency in product quality and properties when transitioning from low volume laboratory mixing procebes to high volume industrial compounding. The scale-up approach investigated in this study evaluates the procebes with respect to dispersive mixing. Demand of polymer composites with solid additives, such as carbon microfibers and nanotubes, has become increasingly popular. Dispersive mixing breaks down particles that agglomerate, which is paramount in procebing composites because solid additives tend to collect and clump. The amount of streb imparted on the material governs the degree of dispersive mixing. A methodology has been developed to characterize the Residence Streb Distribution (RSD) within a twin-screw extruder in real time through the use of polymeric streb beads. Through this technique, certain mixing scale-up rules can be analyzed. The following research investigated two different scale-up rules. The industry standard for mixing scale-up takes the ratio of outer diameters cubed to convert the volumetric flow rate from the smaller proceb to a flow rate appropriate in the larger machine. This procedure then resolves both operating conditions since shear rate remains constant. The second rule studied is based on percent drag flow, or the fraction of pumping potential, for different elements along the screw configuration. The percent drag flow rule aims to bring greater focus to operating conditions when scaling-up with respect to dispersive mixing. Through the use of the RSD methodology and a Design of Experiment (DOE) approach, rigorous statistical analysis was used to determine the validity between the scale-up rules of argument.