Properties of Simulated Plastic Waste Mixtures in Upcycling Processes: An Experimental Evaluation

Current production of virgin plastics stands at approximately 0.4 billion tonnes annually, with significant applications in packaging, construction, and the automotive industry. Despite the utility of plastics, their waste management remains a challenge. This study focuses on upcycling plastic waste (PW) through mechanical recycling, which adds value to discarded plastics and aligns with sustainable practices. We have investigated the mechanical and thermal properties of ternary and quaternary blends of the most common polyolefin polymers, low-density polyethylene and polypropylene (PP), simulating typical PW mixtures. The blends were prepared using twin screw extrusion for melting, mixing, and strand extrusion. The strands from the extruder were guided through a water bath, optionally dried, and pelletized, to determine the processing conditions variations. Aliquots of the blends were directly taken from the extruder to form specimens from the melts. Additionally, the produced pellets were remolten to form specimens, resulting in a second heating cycle (HC). Thermal stability, calorimetric properties, surface morphology, and mechanical characteristics of the blends were analyzed. Results indicate that ternary blends exhibit higher thermal stability but lower mechanical strength compared with quaternary blends. The addition of PP slightly reduced the onset temperature in quaternary blends. The melting temperatures of the blends show minimal change with additional HCs or drying, suggesting the maintenance of thermal properties. The ternary blends exhibit consistent mechanical properties regardless of the processing conditions, but the breakup force and tensile modulus of the quaternary blends were lower with additional HCs. The surface morphological studies revealed increased cavitation with two HCs and improved surface smoothness with drying. These findings support the potential of mechanical recycling to produce marketable plastic grades from mixed PW.