Effect of biochar-derived ash on the thermal and processing stability of recycled PLA

Polylactic acid (PLA) is currently the most widely used biodegradable polymer, applied across diverse sectors and exhibiting significant ongoing market growth potential. However, its slow degradability under ambient conditions and vulnerability to thermomechanical degradation underscores the critical need for effective end-of-life management strategies. Mechanical recycling offers a practical solution, particularly when stabilizing agents are applied during reprocessing. In this context, biochar has emerged as an effective bio-based carbon filler that can improve PLA composite stability and performance, although variations in feedstock, and consequently ash composition, can significantly impact the polymer matrix. This study investigates the effect of biochars derived from different lignocellulosic feedstocks under the same operating conditions (slow pyrolysis at 600 °C), willow, hazelnut, and grape pruning at loadings of 1, and 2.5 wt%, on the mechanical properties, processing stability, and aging resistance of recycled PLA (r-PLA) composites. Particular attention is given to the role of biochar inorganic composition, especially alkali and alkaline-earth metals content, in influencing polymer stabilization and crystallization behavior. The results show that the type of biochar feedstock strongly influences composite performance. In particular, potassium, and its chemical form, plays a critical role, leading to a marked decrease in molecular weight and a substantial reduction in thermal stability, which indicates faster degradation during melt processing. Overall, the results highlight that biochar cannot be considered a chemically inert filler in PLA matrices evidencing the potential of tailoring biochar feedstock selection to optimize the properties of recycled PLA composites, contributing to more robust and sustainable bioplastic recycling strategies.