Fabrication of a high phosphorus–nitrogen content modifier with star structure for effectively enhancing flame retardancy of lyocell fibers

A novel phosphoramidate salt fire retardant (FR) with high phosphorus–nitrogen (P–N) content, tris(2-aminoethyl)amine phosphoramidate salt (TAEAPA), was synthesized by a simple design philosophy. A three-arm star-like structure FR with a high level of P or N content was obtained. Phosphorus and nitrogen contents of TAEAPA increased up to 8.6 at% and 14.5 at%, respectively. Subsequently, TAEAPA was employed to finish lyocell fibers through the classical dip-dry-cure technique. Consequently, the thermal stability and fire retardancy of finished lyocell fibers were improved significantly, as evidenced by thermogravimetric and vertical flammability test. Finished lyocell fibers exhibited higher char residues (39.2 wt%) and lower degradation rate (0.6%/°C) under nitrogen atmosphere. Besides, lyocell fabrics finished by 300 g/L of TAEAPA finishing solution only had a char length of 53 mm when exposed to flame more than 60 s, and immediately extinguished after the removal of flame. Thermogravimetry-infrared and pyrolysis gas chromatography/mass spectrometry coupled techniques confirmed that the high contents of P and N included in TAEAPA played a good synergistic effect on both condensed and gaseous phase during the thermal pyrolysis process of lyocell fibers.