Investigation of distributed recycling of polylactic acid over multiple generations via the granule-based material extrusion process

Plastic production has experienced exponential growth in recent decades, resulting in a significant increase in plastic waste worldwide due to its durability and non-biodegradable nature. With the rapid development and worldwide utilization of material extrusion (ME) 3D printers, the generation of 3D printed thermoplastic waste has also increased rapidly. However, ME 3D printers themselves can offer a promising solution for implementing a novel, practical, and cost-effective approach to distributed recycling of 3D printed waste. While previous studies have predominantly focused on validating and evaluating the distributed recycling using the filamentbased material extrusion (FME) method, research on distributed recycling over multiple cycles (more than two) using the granule-based material extrusion (GME) method is limited. This can be attributed to the fact that the GME process is relatively immature compared to the more established FME method, and recent studies on the GME method have mainly focused on the viability, optimization of processing parameters, and mechanical performance of GME printed specimens. To address this research gap, this study aimed to comprehensively investigate the GME-based distributed recycling of PLA waste over multiple recycling generations using a multiscale characterization framework, including tensile tests, hardness tests, mass measurements, optical microscopy, scanning electron microscopy (SEM), color measurements, gel permeation chromatography (GPC), differential scanning calorimetry (DSC), and rheometer tests. The results of this study demonstrated consistent decreases in the tensile strength, melting temperature, and rheological properties of the recycled PLA samples with the increase of recycling cycles. These changes were consistent with the gradual decrease in the average molecular weights of the PLA samples. Notably, the recycling process was terminated after the third recycling cycle due to the distinct color exhibited by the third recycling printed specimens compared to the initially printed PLA specimens, as indicated by chromaticity measurements.