| Micromixing has been experimentally and theoretically investigated in neutral-kneading-disc sections of a co-rotating, self-wiping, twin-screw extruder using the Frey-Denson reactive tracer system. The reactive tracer measures the extent of micromixing in viscous polymer melts via the imidization reactions of p-phenylene diamine and phthalic anhydride. The experiments were designed to isolate the mixing effects of kneading discs from other factors such as melting and pumping, and to set up a known initial orientation of the two polymer flows entering the kneading discs. Experimental sensitivity was improved by developing an alternate equation to calculate selectivity that avoids some of the errors found by previous researchers using this competitive-consecutive reactive tracer, and by evaluating the local variations in selectivity which amplify segregation effects.; Micromixing was measured over the full range from complete segregation to complete mixing. The combined effects of screw speed and flow rate on micromixing were characterized by {dollar}Pi,{dollar} a dimensionless parameter proposed in this work, representing the ratio of characteristic times for axial to rotational flows in the extruder. The parameter {dollar}Pi{dollar} can be used to determine conditions causing complete mixing, while avoiding excessive mixing and degradation of polymer chains. Additionally, mixing inside the kneading discs was analyzed by cooling and removing samples from the extruder, and the results help to explain the macromixing and micromixing mechanisms in twin-screw extruders.; A semi-empirical model was developed based on a stretching and folding mechanism observed in flow-visualization studies conducted as part of this investigation and described in the literature. The model, which related the operating conditions and geometry to micromixing, agrees qualitatively with the experimental results. Reasonable values for the number of folds per revolution range from one to three times per revolution, for all the conditions studied. In addition, the effects on micromixing of mixing number, {dollar}Psisb{lcub}rm e{rcub},{dollar} the ratio of characteristic times for diffusion to extensional flow, were computed and it was found that the strain rate has a much stronger impact on micromixing than the initial scale of segregation, if the reaction proceeds to completion inside the extruder. |
