Investigation of the Effect of Reprocessing on Thermal and Mechanical Properties of Polymers and Polymer Nanocomposites

This study explored the impact of multiple reprocessing cycles on the thermomechanical properties of polystyrene (PS) and low-density polyethylene (LDPE), simulated through reprocessing with a twin-screw extruder. Additionally, it compared the thermal and mechanical properties of graphene nanoplatelets (1% w/w) reinforced polypropylene (PP-GNP) nanocomposites with PP. The materials undergo seven consecutive extrusion cycles at varying screw speeds (100 and 150 rpm) and temperatures (180 and 200 °C). Increasing the screw speed from 100 to 150 rpm raised LDPE's screw torque by about 40% at the first reprocessing cycle. Processing PS at 200 °C reduced screw torque by ≈20% compared to 180 °C at 1–5 reprocessing cycles. Both torque and power decrease for PP and PP-GNP with each reprocessing cycle. LDPE's tensile modulus decreases with more cycles at 200 °C, while PS shows no consistent variation. PP's tensile modulus and ultimate tensile strength drop by 24% and 12%, respectively, from the first to the fifth cycle, while PP-GNP exhibits no consistent variation. Differential scanning calorimetry shows no clear change in LDPE's melting point, but an increase in PP and PP-GNP's melting points up to the fifth cycle. This research provides crucial insights to advance the recycling of polymers reducing environmental impact.