Fused-granular-fabrication 3D printing of recycled expanded polystyrene/polypropylene: Research on mechanical properties, thermal behavior, and morphology

Recycling of expanded polystyrene (EPS) waste plastic is costly and inefficient, leading to low recovery rates and exacerbating environmental pollution. This study introduces a custom-built Fused Granular Fabrication (FGF) 3D printer and designs printers with a mixing screw (MSP) and a conventional screw (CSP) for the direct recycling and modification of EPS. The mixing capabilities of the MSP and CSP were compared through fluid simulation. Experimental studies were conducted to investigate the effects of both printers and compatibilizer on the thermal and mechanical properties of recycled polystyrene/polypropylene (rPP/rEPS). The results showed that, MSP provided a higher proportion of stretching flow than CSP, facilitating polymer fragmentation and breaking down the compatibilizer and rEPS into smaller particles, thus enhancing mixing dispersion in single-screw extrusion. The addition of rPP and maleic anhydride-grafted polypropylene (PP-g-MAH) effectively improved the mechanical properties of rEPS. The rEPS/rPP/PP-g-MAH composites printed with MSP exhibited the best mechanical properties, with tensile strength and elongation at break increasing by 129.3% and 98.7%, respectively, compared to neat rEPS. The high shear effect of CSP caused the degradation of rPP and PP-g-MAH, with the degradation temperature of rEPS/rPP/PP-g-MAH printed by CSP being 7.2°C lower than that of MSP, thus deteriorating the thermal properties of the rEPS/rPP/PP-g-MAH composites. This study offers valuable insights into improving the mechanical properties of recycled rEPS and achieving a “one-stop” solution for waste plastic recycling.