Innovative P/N flame retardant as filler for enhancing flame retardancy in lyocell fiber

A phosphorus‑nitrogen synergistic flame retardant polymer (FR-P) was synthesized by condensation reaction of tetrakis(hydroxymethyl)phosphonium chloride-urea (THPC-Urea) with ammonia, resulting in a polymer containing 17.71% phosphorus and 18.43% nitrogen. The FR-P was characterized using thermogravimetry (TG), Fourier-transform infrared spectroscopy (FTIR), inductively coupled plasma (ICP), and X-ray photoelectron spectroscopy (XPS). Flame retardant lyocell fibers (FR-L) were subsequently prepared using dry–wet spinning technology, with FR-P incorporated as a filler. The flame-retardant properties were evaluated using the limiting oxygen index (LOI), thermogravimetric analysis coupled with FTIR (TG-FTIR), and TG. The results indicated that the incorporation of 26 wt % FR-P brought a 31.6% LOI, which remained at 29.3% even after 30 laundering cycles (LCs). The pyrolysis mechanism of cellulose was changed by FR-P which played a crucial role in the condensed phase during combustion by catalyzing the dehydration of cellulose to form a compact char layer structure. Under a nitrogen atmosphere at 700 °C, the char residue of FR-L was 37.42%. The char residue was characterized using raman spectra, and scanning electron microscopy (SEM). The results indicated that the char residue exhibited a higher degree of graphitization. Furthermore, the mechanical properties were tested by fiber strength and elongation tester. The results showed that the draw ratio increased from 4.11 to 6.88, while the tensile strength of FR-L remained at 3 cN/dtex. The crystal structure of the fibers was characterized by X-ray diffraction (XRD), revealing that the crystallinity of the fibers was cellulose II. At a draw ratio of 5.27, the crystallinity was measured at 61.0%.