Sustainable Multi-Cycle Physical Recycling of Expanded Polystyrene Waste for Direct Ink Write 3D Printing and Casting: Analysis of Mechanical Properties

This work investigates the sustainable reuse of expanded polystyrene (EPS) waste through a multi-cycle physical recycling process involving dissolution in acetone and subsequent manufacturing via Direct Ink Write (DIW) 3D printing and casting. Morphology and mechanical properties were evaluated as a function of the manufacturing technique and number of dissolution cycles. Morphological analysis revealed that casted specimens better replicated the target geometry, while voids in 3D-printed specimens aligned with the printing direction due to rapid solvent evaporation. These voids contributed to slightly reduced stiffness in 3D-printed specimens compared to casted ones, particularly for transverse printing orientation. The defoaming process during dissolution significantly increased the density of the material, as well as removed low molecular weight additives like plasticizers, leading to a notable enhancement in stiffness. Successive dissolution cycles led to increased removal of plasticizers, enhancing stiffness up to 52 times (cast), 42 times (longitudinally printed), and 35 times (transversely printed) relative to as-received EPS waste. The glass transition temperature remained unchanged, confirming the preservation of polymer integrity. This work highlights the potential of EPS inks for sustainable, multi-cycle recycling, combining enhanced mechanical performance with the flexibility of 3D printing for complex, cost-effective designs, aligning with circular economy principles.