Mechanistic Insights into the Influence of Nano-Biochar on the Enhancement of Mechanical and Flame Resistance of Recycled PLA Composites

Anbuchezhiyan G¹Vivek Anand A²Senthil Babu S³Madhubalan S⁴Vigneshwaran Shanmugam⁵Rhoda Afriyie Mensah^5*

• ¹Chitkara University, Rajpura, India
• ²MLR Institute of Technology, Hyderabad, India
• ³SRM Institute of Science and Technology (Deemed to be University) - Ramapuram Campus, Ramapuram, India
• ⁴Sona College of Technology, Salem, India
• ⁵Luleå University of Technology, Luleå, Sweden

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.