Gas-phase flame retardant DNPMI for polylactic acid fibers: Achieving high flame retardancy with significant smoke...

Polylactic acid (PLA), as a bio-based and environmentally friendly material, is attracting increasingly widespread attention, and its fiber materials are being used more extensively in textiles and other fields. However, due to its inherent flammability, the application of PLA remains limited in certain specific scenarios. This study reports the fabrication of flame-retardant PLA fibers via the melt spinning of PLA with a synthesized phosphorus‑nitrogen flame retardant 3-(6-oxidodibenzo [c, e] [1, 2] oxaphosphinin-6-yl)-1-phenylpyrrolidine-2,5-dione (DNPMI). The modified fiber materials achieved a limiting oxygen index (LOI) of 32.2 ± 0.2% and a UL-94 V-0 rating with only 2.5 wt% flame retardant loading. Thermogravimetric analysis (TGA) showed that DNPMI did not cause a drastic reduction in the initial decomposition temperature. Cone calorimetry indicated a moderate increase in peak heat release rate (pHRR) relative to neat PLA, but a significant reduction in total smoke production (TSP). A marked increase in CO₂ release suggesting a modified gas-phase combustion behavior. X-ray photoelectron spectroscopy (XPS) analysis detected nitrogen on the char surface but absent in energy-dispersive X-ray spectroscopy (EDS), suggesting that nitrogen species act primarily in the gas phase or enrich the surface. Although scanning electron microscopy (SEM) imaging revealed micro-cracks and interfacial defects that reduced tensile strength to 1.95 cN/dtex, the mechanical properties remain suitable for fire-safety applications. Combined thermogravimetric analysis coupled with fourier transform infrared spectroscopy (TG-FTIR), pyrolysis gas chromatography-mass spectrometry (Py-GC/MS), and char-residue analyses suggest that DNPMI mainly acts through a gas-phase-dominated flame-retardant mechanism, accompanied by a weak condensed-phase contribution.