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Mixing Polymers under Rheo-Fluidification Conditions



Summary: Shear-thinning is known to decrease the viscosity of polymer melts and is commonly used in processing. It is due to the influence of strain rate on viscosity. Rheo-Fluidification conditions are realized when shear-thinning and strain softening combine to further reduce melt viscosity (to simplify, we have called this “disentanglement”). The combination of extrusion flow (pressure flow) and drag flow, produced by the cross-lateral shearing of the melt (in a Couette type of configuration) is not new, has been described, for instance, by Cogswell1 and results in the vectorial increase of the strain rate, hence producing more shear-thinning and further decrease of viscosity. The addition of a low frequency vibration component (0.25 to 80 Hz) to the lateral shear rate induced by the rotation of the inner shaft introduces the possibility to control the amount of non-linearity triggered by the amplitude of the oscillating shear rate, producing strain softening and new states for the melt, in particular new meta-stable oriented states that modify the rheology of pellets manufactured from such “Rheo-fluidified” melts.2–4 Under these specific processing conditions, the viscosity reduction of the melt obtained by combining strain softening and shear-thinning can be preserved in the pellets granulated at the exit of the line. These “treated” pellets display a lower viscosity when they are reheated in a melt flow indexer or in a dynamic rheometer after being compressed into disks. The Rheo-Fluidizer can either treat a melt “statically” (as a batch) or continuously (in conjunction with an extruder feeding melt into it). The on-line reduction and control of the melt viscosity during extrusion allows the Rheo-fluidizer to be used to mix, disperse and blend polymer mixtures and additives. The purpose of this paper is to report results obtained with the Rheo-Fluidizer, used continuously to blend two immiscible polymers, polycarbonate and polymethylmethacrylate (PC, PMMA) in proportions that have been challenging other continuous feed dispersing devices such as a twin-screw. In other words, the homogeneous PC/PMMA mixtures described in this paper were not processed successfully by other classical processing methods. We show that the use of a Rheo-fluidification treatment during melt extrusion of polymer blends/alloys yields a battery of novel characteristics, often large improvements, for the pellets and the products created by such a process.

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