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Blending incompatible polymer blends
There are two main process issues when blending incompatible polymers: (1) creating the target morphology, and (2) maintaining it. Extensional and shear stresses are engineered into twin screw extruders for dispersive mixing. 
Wide kneaders, shaped lobal accelerators and related mixing elements are used to accomplish the required shear. This relationship is given by:
Where the stress rate in elongation or shear is the product of a controlling modulus (driven by viscosities) and a strain rate in shear and/or elongation. In the early 1990's Lim and White (Univ. of Akron) published on polyamide/PE blend morphology development in twin screw extruders. Very coarse blends resulted from only flighted corotating elements, which lack lobal capture (and therefore shear stresses). Morphological fineness improved with increasing lobal events and stress experiences from kneaders. The Canadian National Research Council and others have also extensively reported on the values of extensional stresses. When these shear stresses become small (such as in screw channels, gate adapters and dies) liquids and polymers that are immiscible with the host polymer seek to recombine with each other to coarsen the morphology. This process is called coalescence or de-mixing. Compatibilizers and other
techniques may be used to stabilize the morphology and to improve properties.
