Sustainable Flame-Retardant PLA Composites Incorporating Raw Wood-Derived Biochar and Magnesium Hydroxide

The development of sustainable flame-retardant polymer composites is important for expanding the practical use of bio-based plastics while reducing reliance on petroleum-derived and halogenated materials. In this work, biodegradable polylactic acid (PLA) composites were prepared using raw wood-derived biochar as a degradable carbon-based filler and magnesium hydroxide (MH) as a halogen-free flame-retardant additive. PLA/Biochar/MH composites were prepared by melt compounding and compression molding, followed by systematic evaluation of their structural, thermal, flame-retardant, mechanical, and stability-related properties. The flame-retardant performance, evaluated by limiting oxygen index (LOI) and UL-94 (UL: Underwriters Laboratories) vertical burning tests, was significantly enhanced by the combined biochar/MH system. Biochar alone slightly increased the LOI of PLA, while MH-containing composites exceeded the practical 21% LOI threshold, with PLA/Biochar20/MH20 achieving the highest LOI value of 26.2%. This improvement was attributed to char formation, heat absorption, gas dilution, and magnesium oxide-supported barrier formation. The composites also maintained reasonable dimensional stability after accelerated aging with thickness changes below 1%. Overall, this study demonstrates that combining biodegradable PLA with degradable biochar and halogen-free MH provides a promising sustainable strategy for developing flame-retardant PLA-based composites with improved residue formation and dimensional stability.