Comparative Analysis of Mechanical Recycling Simulation of Poly(Hydroxybutyrate-Co-Hydroxyvalerate) (PHBV): Injection Molding Vs. Extrusion Processes

This study investigates the effects of simulated mechanical recycling cycles on poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV), a biobased and biodegradable polymer, processed by twin-screw extrusion and injection molding. A decrease in melt flow index and an increase in melt viscosity and molar mass after the first cycle indicate branching and recombination reactions altering the polymer structure. Fourier-transform infrared spectroscopy reveals pronounced degradation in injection-molded samples, with carbonyl loss, while extruded samples show limited spectral changes, suggesting different degradation mechanisms. After the first injection cycle, thermal stability improves temporarily, with a higher degradation temperature than the neat polymer, but declines in subsequent cycles. Extruded samples show greater stability, with minimal variation in degradation temperature. Mechanically, extruded samples develop higher stiffness, indicated by increased Young's modulus, while stress at break remains stable across both methods. Impact toughness decreases after the first cycle, though injection-molded samples maintain higher impact resistance. Biodegradation is faster in injection-molded samples due to lower crystallinity and greater molecular mobility. Differential scanning calorimetry of degraded samples reveals two melting points, suggesting chain rearrangement and phase separation during microbial attack. The study highlights how processing methods influence PHBV's structure, stability, mechanical performance, and biodegradability, offering valuable insights for optimizing its recyclability and functionality in sustainable material applications.