Enhanced Dispersive Mixing in Twin-Screw Extrusion via Extension-Dominated Static Mixing Elements of Varying Contraction Ratios

Twin screw extruders (TSE) are widely used in polymer blending and compounding operations, since the relatively high stresses they impart on melts and controllable residence times make them highly suitable for mixing operations. However, the mixing action in the industry-standard kneading blocks (KB) is shear-dominated and shear flows are inefficient for dispersive mixing compared with extensional flows. A novel static extensional mixing element (EME) was developed recently by our group with the objective of providing extension-dominated flow during blending and compounding. Dispersive mixing in the EME is provided by flow through stationary hyperbolic converging-diverging (C-D) channels placed along the screw. The improvements in dispersive mixing for immiscible blends using EMEs with wide hyperbolically channels, i.e., low contraction ratios, has been proved. In this work, we begin by performing computational investigations of more extreme contraction geometries to determine the design flexibility limits for the EME. The contractions again showed more extension-dominated flow patterns than the original EME, the drawback being the obvious increase in the pressure drop. However, at extremely high contraction ratios, the entry angle is so shallow that recirculation zones appear at the entrance, which are undesirable, implying that there are geometrical limits above which the EME ceases to be an effective dispersive mixer. Next, experimental validation on the EMEs with different degrees of aggressiveness of the C-D channel were performed and dramatic improvement in droplet breakup (dispersive mixing) in case of immiscible blends is clearly observed at all viscosity ratios and with increasing degree of EME aggressiveness.