Mechanical Recycling of Biochar‐Filled Poly(lactic acid): A Study of Mechanical and Thermal Properties Supported by Meta‐Analysis

Recent life cycle assessments show that poly(lactic acid) (PLA) should be mechanically recycled as it conserves energy, retains material value, and supports a circular economy more effectively than composting or chemical recycling. Fillers can reduce recycling-induced deterioration of mechanical properties. Among them, biochar, a renewable additive, has attracted growing interest. In this study, we investigated the effect of biochar and repeated mechanical recycling on the structural, thermal, and mechanical properties of injection-molded PLA. To this end, PLA composites with 5 wt.% biochar were subjected to 0, 1, and 5 reprocessing cycles. Rheological results indicate that each processing step decreased the weight-average molecular weight of PLA by approximately 9%. 5 reprocessing cycles caused a significant decrease in tensile strength and strain at break, while a single recycling step had no adverse effect. Our meta-analysis of the literature confirmed that biochar generally causes only minor changes in the tensile properties of PLA (median: Δσ≈ -2.9%, ΔE≈+9.7%, Δε≈−16.7%). Neither the addition of biochar nor recycling affected its glass transition or melting temperatures. Overall, the results indicate that mechanical recycling is feasible as an end-of-life solution for PLA with biochar as a filler, but only with a limited number of recycling cycles.