Synergistic flame retardancy and mechanical reinforcement of bio-based PLA composites using polyphenol-assisted ammonium polyphosphate and sugarcane fiber

Poly (lactic acid) (PLA) is characterized by inherent flammability and limited mechanical performance, which restrict its engineering applications. In this study, ammonium polyphosphate (APP) was utilized as the primary flame retardant, while tannic acid (TA) and tea polyphenols (TP) acted as natural synergists. By combining these components with steam-exploded sugarcane fibers (SCF), we developed a fully bio-based intumescent flame-retardant PLA composite. The effects of polyphenol-assisted APP on thermal stability, flame retardancy, mechanical properties, and flame-retardant mechanisms were systematically investigated. The results indicate that polyphenols significantly enhance the charring ability of APP and improve the high-temperature thermal stability of the composites. Cone calorimetry demonstrates notable reductions in peak heat release rate and total heat release, along with an increased fire performance index. At a total flame-retardant loading of 13 wt%, the composites achieved a UL-94 V-0 rating with effective suppression of melt dripping. However, a moderate reduction in tensile strength and elongation at break was observed due to the incorporation of inorganic flame-retardant components. The addition of SCF partially mitigates the mechanical deterioration caused by the incorporation of flame-retardants. Char analysis indicates a condensed-phase-dominated mechanism, wherein polyphenols serve as efficient carbon sources and synergize with APP to form a dense, phosphorus-rich char layer. Notably, tea polyphenols exhibit slightly higher charring efficiency than tannic acid.