Biodegradation of 3D-printed biodegradable/non-biodegradable plastic blends

Abstract

Thermoplastic blends are applied for three-dimensional (3D) printing to obtain improved functionality. While thermal, chemical, and mechanical properties of 3D-printed blends were typically examined, biodegradability was seldom evaluated. In this study, biodegradable plastics (polylactic acid (PLA), polyhydroxybutyrate (PHB)) were blended with non-biodegradable plastics (high-density polyethylene (HDPE), polypropylene (PP)), and their biodegradation was observed. Letter-shaped specimens were prototyped using a commercial fused deposition modeling (FDM) printer with blend filaments (PLA/HDPE, PLA/PP, PHB/HDPE, PHB/PP, and PLA/PHB). Biodegradation tests on these prototypes were performed employing a systemically designed respirometry reactor by simulating controlled composting and aerobic aqueous environment. FDM printing of polyolefin thermoplastics (HDPE and PP) have been problematic due to thermal shrinkage, warping deformation, void formation, and adhesion. We show that PLA/HDPE, PLA/PP blends are printable, but the increasing ratio of HDPE and PP in the blends severely exacerbated the warping deformation, while PLA/PHB blends exhibited outstanding printing performance. The immiscible biodegradable/non-biodegradable polymer blends showed no evidence of synergistic improvement in biodegradation, likely due to phase separation and incompatibility in the blend system. The 3D-printed prototypes exhibited significant biodegradation in composting test (PHB100 and PLA50/PHB50 biodegraded 81.3, 67.3 % over 16 days). In the aerobic aqueous test utilizing municipal wastewater sludge as an inoculum, only PHB100 and PLA50/PHB50 exhibited significant degradation (91.7, 55.9% after 50 days). Taken together, considerable printing performance, mechanical properties, and biodegradability suggest the potential of PLA/HDPE, PLA/PP, and PLA/PHB blends as novel FDM printing filaments, providing a novel option for recycling and valorizing HDPE and PP.