Flow through a convergence : Part 2. Mixing of high viscosity ratio polymer blends

Dispersive mixing of high viscosity ratio blends was studied in a converging flow using a capillary rheometer equipped with dies having different entry profiles. Three inhomogeneous bimodal polyolefin blends, with stress-dependent viscosity ratios ranging from 8 to 450, were used in this work. Such magnitudes of viscosity ratio indicate that the dispersed droplets can be mixed only in an elongational flow field. The mixing efficiency was found to be dependent on both the profile of the convergence and flow rate. At the lowest flow rates, the dispersive mixing efficiency was very low, but it increased with an increasing flow rate until a profile-dependent maximum. This maximum mixing efficiency was observed prior to fracture of the matrix material, after which the efficiency decreased. Stress and deformation fields within different profiles were estimated by numerical simulation using the K-BKZ equation, and the results were used to interpret experimental results. The dispersive mixing efficiency was found to be proportional to the maximum elongational stress within the converging section, and to the length of the region where the critical conditions for elastic fracture of droplet material were met. It is shown that the dispersive mixing mechanism in high viscosity ratio blends is mainly dictated by elastic fracture of the droplet, and hence is applicable over a wide range of polymer blending processes