Structure-property relations in PP/HDPE blends: From processing to performance

Polypropylene (PP) and high-density polyethylene (HDPE) are commonly found together in waste streams and are difficult to separate due to their similar densities, resulting in recycled polypropylene often containing some polyethylene, creating a blend. PP and HDPE form an immiscible blend whose heterophasic and crystalline morphologies that contribute to the final mechanical properties are influenced by processing conditions. Although basic thermal and rheological characterizations of these blends are somewhat addressed in the literature, the impact of controlled processing conditions on various blend compositions remains largely unexplored. This study investigates the thermal and rheological properties of these blends, and utilizes for the first time extended dilatometry to simulate a range of realistic processing conditions such as pressure (0- 1200 bar) and shear rates (0-150 1/s) in such multi-component blends. The resulting mechanical properties and microstructure are evaluated through tensile testing and X-ray diffraction (XRD). Under quiescent and isobaric cooling conditions, increasing the polyethylene concentration results in reduced strength and elongation compared to the pure matrix. Conversely, when subjected to shear, the addition of polyethylene enhances the yield stress due to increased flow strength, provided the solidification of the melt happens rapidly, leading to higher oriented structures in polypropylene, as confirmed by XRD.