Effects of Polylactic Acid Content, Reprocessing Cycles, and Aging Conditions on the Mechanical Properties of Recycled High‐Density Polyethylene

The increasing incorporation of biodegradable plastics into conventional recycling streams has raised concerns regarding their influence on the performance of recycled polymers. In particular, the presence of polylactic acid (PLA) in recycled high‐density polyethylene (rHDPE) may significantly affect mechanical stability during reprocessing and environmental exposure. This study systematically evaluates the combined effects of PLA content (0–50 wt%), recycling cycles, exposure time, and aging conditions (natural and accelerated) on the mechanical performance of rHDPE/PLA blends using a multilevel full factorial experimental design comprising 96 experimental runs. Tensile stress, elongation at break, and impact resistance were measured according to ASTM standards. Statistical analyses including ANOVA, regression modeling, and interaction analysis were applied to identify significant factors and combined effects, followed by multiresponse optimization using a desirability‐function approach. PLA incorporation produced a drastic reduction in ductility and toughness, with elongation at break decreasing by more than 95% and impact resistance by approximately 80%–85% as PLA content increased from 0 to 50 wt%. Accelerated aging generated the most pronounced reduction in tensile strength, with decreases on the order of 20%–30% compared with natural aging conditions. Recycling cycles further contributed to mechanical degradation, although to a lesser extent. Significant interaction effects among composition, recycling cycle, and aging environment demonstrated that mechanical degradation results from synergistic thermo‐oxidative, processing‐induced, and interfacial incompatibility mechanisms. The results provide a comprehensive statistical framework for understanding the degradation behavior of rHDPE systems contaminated with biodegradable polymers and support the identification of optimal operating conditions for maximizing the mechanical performance of recycled polymer blends.