Shear Effects on Crystallization in Polyolefin Blends

Polyolefins, e.g., polyethylene (PE) and polypropylene (PP), constitute over half of the worldwide plastics production, but recycling rates for these materials remains low. Post-consumer PE and PP typically enter the recycling stream in mixture form and are processed together. PE/PP blends are often brittle due to thermodynamic incompatibilities that result micro-separated domains which crystallize separately upon cooling. The production of useful products from these materials requires an understanding of the fundamental parameters that affect crystallization during processing of polyolefin blends. In this work, we employ a multi-modal approach using a combination of rheo-Raman spectroscopy, differential scanning calorimetry, and optical and electron microscopy to explore the effects of shear on the morphology, crystallization kinetics, and rheological properties of blends of high-density PE and isotactic PP. Results show that flow-induced crystallization in the PP phase has a surprising composition dependence, which we attribute to the domain structure and micro-flow fields in the blends. These results underscore the importance crystallization and processing on the structure and properties of mixed recyclates.