Upcycling of Post-Consumer Polystyrene into Size-Tunable Lightweight Foamed Beads via Supercritical CO ₂ Foaming

Upgrading and converting postconsumer polystyrene (PCR–PS) is crucial for mitigating its environmental impact, yet achieving efficient processing and high-value utilization remains a significant challenge. This work proposes an innovative strategy: employing green and efficient supercritical CO2 extrusion foaming coupled with air-cooled die-face pelletizing technology for producing PCR–PS foamed beads. Remarkably, this approach enables continuous processing of PCR–PS foamed beads across multiple grades and densities without requiring additional equipment investment or water consumption. Furthermore, the lightweight application possibilities of the new materials, recycled materials, and their composite products were comprehensively evaluated. A series of extruded foam beads with varying dimensions (6.4, 5.0, 3.6 mm) and expansion ratios (19–23) were successfully produced through pressure drop and secondary foaming. Besides, it was found that the incorporation of virgin polystyrene (PS) can optimize the melt viscoelasticity and secondary foaming property of PCR–PS, which could improve its interfacial bonding effect during steam-chest molding. Further evaluations of the mechanical properties and characteristics of PCR–PS were conducted through compression and bending tests. The green, efficient processability and tunable interfacial bonding behavior provide a practical, controllable technical solution to environmental challenges posed by postconsumer polystyrene. This research pioneers a sustainable recycling pathway for postconsumer plastic waste.