The influence of nano-biochar on the mechanical and flame resistance of recycled PLA composites

The growing accumulation of plastic and agricultural waste highlights the urgent need for sustainable material alternatives. This study investigates the incorporation of nano-biochar derived from cashew nut shell biomass to enhance the mechanical and thermal performance of recycled polylactic acid (rPLA). Nano-biochar produced via controlled pyrolysis and high-energy ball milling was incorporated into rPLA at 0–2 wt% loadings through melt compounding and injection moulding. The resulting composites were evaluated for tensile, flexural, impact, and interlaminar shear strength (ILSS), alongside UL-94 flammability testing. A one-way ANOVA followed by Tukey’s HSD post-hoc analysis confirmed statistically significant improvements (p < 0.05) across all mechanical properties. The tensile strength of virgin PLA (32.23 MPa) decreased to 25.92 MPa in recycled PLA due to polymer chain scission; however, the addition of 1.5 wt% nano-biochar increased tensile strength to 49.54 MPa and ILSS from 21.37 MPa to 36.31 MPa. Flexural and impact strengths also rose by 34.19% and 45.85%, respectively, compared to unfilled rPLA. In UL-94 testing, the rPLA1.5 composite achieved a V-0 rating with no dripping, indicating excellent flame retardancy. Overall, nano-biochar reinforcement not only restored but substantially enhanced the mechanical integrity and fire resistance of rPLA, with ANOVA validating the statistical robustness of these improvements. This work demonstrates a viable circular-economy pathway for converting biomass waste into functional nano-reinforcements for sustainable polymer composites. These composites are particularly suitable for automotive interiors, building materials, and consumer goods where improved flame resistance and mechanical durability are required.