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. 2024 Dec 23;16(24):3609.
doi: 10.3390/polym16243609.

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

Rubén García-Sobrino  1 , Alejandro Cortés  1 , José Ignacio Sevilla-García  1 , Marta Muñoz  1
Affiliations

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

Rubén García-Sobrino et al. Polymers (Basel). .

Abstract

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.

Keywords: Direct Ink Write; additive manufacturing; circular economy; expanded polystyrene; plastic waste recycling.

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Conflict of interest statement

The authors declare no conflicts of interest. The funders had no role in the design of the study, in the collection, analysis, or interpretation of data, in the writing of the manuscript, or in the decision to publish the results.

Figures

Figure 1
Figure 1
Scheme of the proposed multi-cycle physical recycling process. The as-received EPS waste (EPS-0) was dissolved in acetone, inducing the defoaming process, thus obtaining an EPS-ink (EPS-1). After manufacturing with this ink, the specimens were dissolved again in acetone, obtaining EPS-2, and so on, completing “n” cycles.
Figure 2
Figure 2
(a) CNC machining of the as-received EPS waste (EPS-0) (b) Molds obtained by SLA 3D printing technology and casting process using EPS-1, EPS-2, and EPS-3. (c) DIW 3D printing technology using a BCN3D Plus printer with a modified Paste Extruder Module.
Figure 3
Figure 3
Representative images of the specimens used for the infill percentage optimization study, manufactured in the longitudinal (a) and transverse (b) printing orientations.
Figure 4
Figure 4
Parts obtained for the tensile and the 3-point bending tests with the different manufacturing methods: CNC machining of as-received EPS waste (EPS-0), casting (EPS-1 to 3), and 3D printing (EPS-1 to 3) in the two proposed printing orientations, longitudinal and transverse.
Figure 5
Figure 5
Example curves of tensile (a) and three-point bending tests (b). Young’s (c) and flexural (d) modulus as a function of manufacturing technology, EPS-0 for the as-received material and EPS-1 for the casted and 3D-printed specimens. Molecular weight distribution obtained by GPC (e) and viscosity (f) as a function of the number of dissolution cycles in acetone. Young’s (g) and flexural (h) modulus as a function of manufacturing technology and the number of dissolution cycles in acetone.
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