Toward Flame Retardancy, Antimelt Dripping, and UV Resistance Properties of Polylactic Acid Based on Eco-Friendly Core–Shell Flame Retardant

To enhance the flame retardancy of polylactic acid (PLA), the exploration of bioderived flame retardants has captured the focus of researchers globally. Herein, a core–shell bioderived flame retardant is prepared through electrostatic self-assembly using ammonium polyphosphate (APP) as the core and chitosan (CS)/tannic acid (TA) bilayer as the shell. In addition, the Fe³⁺ ion is introduced into the outermost TA shell through coordination with the phenolic hydroxyl group, which can reduce the droplets during combustion. The prepared flame retardant, APP@CS@TA-nBL-Fe³⁺, has core–shell structure (where “nBL” represents the number of coating layers of CS and TA bilayer) and excellent flame retardancy for PLA. With 5 wt % flame retardant, PLA/5% APP@CS@TA-2BL-Fe³⁺ attains the highest LOI value (31.6%) and achieves UL-94 V-0 rating in vertical combustion tests with light melt droplets. Furthermore, cone calorimetry results reveal that a reduction of 17.6% in the peak heat release rate and a 22.3% decrease in total heat release were achieved. Meanwhile, the Fe³⁺ catalyzes the matrix to form a micro-cross-linked char layer blocking the heat and oxygen exchange. Moreover, PLA/5% APP@CS@TA-2BL-Fe³⁺ not only has a 99.98% reduction in UV transmittance but also has better mechanical properties after UV aging than that of neat PLA. This study presents a convenient and environmentally friendly approach for preparing efficient biobased flame retardants for PLA, aiming to broaden the application of PLA.