Successive Mechanical Recycling of Poly(lactic Acid) by Injection Molding: Evolution of Molecular, Thermal, and Mechanical Properties

ABSTRACT This study evaluated the changes in the molecular structure, as well as the thermal and mechanical properties of poly‐lactic‐acid (PLA), during successive closed‐loop mechanical recycling through injection molding. Gel permeation chromatographic (GPC) analyses revealed a gradual decrease in molar mass (MM); after ten recycling steps, the MM of PLA was reduced to one‐third of its initial value. According to differential scanning calorimetric (DSC) measurements, both the glass transition temperature and the cold crystallization temperature of PLA decreased, while the crystallization was facilitated by the successive recycling steps. This was ascribed to the improved chain mobility resulting from the reduced MM. Quasi‐static mechanical properties were evaluated using tensile tests, while impact toughness was assessed through Charpy tests. The tensile strength of PLA was not significantly affected by recycling until the fifth recycling step, where it suffered a slight (∼5%) reduction. Meanwhile, Young's modulus, elongation at yield, and impact strength remained approximately the same as the freshly injected sample at that point. However, after ten recycling steps, all analyzed properties declined significantly: Young's modulus by 11%, tensile strength by 68%, elongation at yield by 63%, and impact strength by 30%.