Green synthesis of a novel P/N/S containing bio-based flame retardant and its applications in poly(lactic acid): rapid self-extinguish, anti-dripping, and excellent mechanical performance

Poly(lactic acid) (PLA), which is recognized as a potential biodegradable material, brings serious fire concern because of its flammability with heavily molten drips. In this investigation, a bio-based flame retardant named CS@ATMP@MI (CAM) consisting of chitosan (CS), amino trimethylene phosphonic acid (ATMP), and methionine (MI) was developed through a simple and environmental-friendly synthesis approach. CAM provided effective flame retardancy and anti-dripping ability without compromising mechanical properties. PLA/CAM composites achieved a UL-94 V-0 rating with 1.0 wt% CAM and a limited oxygen index value of 28.4 % with 3.0 wt% CAM. The high efficiency is due to the high phosphate content in ATMP and the strong coordination between CS, ATMP, and MI. The cone calorimetry tests showed that adding 2.0 wt% CAM increased the total smoke production from 0.2 to 0.6 m², and thermogravimetric analysis/infrared spectrum analysis indicated that combustible gas production was reduced over 50 % with 3.0 wt% CAM, demonstrating that phosphorus-nitrogen synergy in CAM could effectively capture free radicals and released inert gases. Char analysis showed that the PLA/CAM composites could form a thicker and denser char layer during combustion due to phosphorus-sulfur synergy, which promoted char formation to shield mass transfer. PLA/CAM composites maintained excellent mechanical performance, with a 3.0 wt% CAM increasing tensile strength from 53.9 to 57.3 MPa and Young's modulus from 1782 to 2262 MPa. This study presented a sustainable method for synthesizing an effective bio-based flame retardant and a strategy for producing eco-friendly, fire-resistant PLA composites for industrial applications.