Synthesis of an effective halogen-free flame retardant rich in phosphorus and nitrogen for lyocell fabric

A novel effective halogen-free flame retardant (FR) rich in phosphorus (P) and nitrogen (N) was synthesized via Mannich and esterification reaction of ethanolamine, glyoxal, phosphoric acid and urea, and used for the preparation of durable flame retardant lyocell fabric (FR-lyocell). Its chemical structure was characterized by Fourier transform infrared spectra, 1H and 31P nuclear magnetic resonance spectra. Inductively coupled plasma emission spectrometry and elemental analysis were conducted to test the phosphorous, carbon and nitrogen contents. The results confirmed that the FR was successfully synthesized and grafted on the lyocell fabric via firm P–O–C covalent bonds. The limiting oxygen index of the FR-lyocell treated with 30 wt% flame retardant finishing solution is 44.6%, demonstrating high efficiency flame retardancy. The thermo-gravimetric analysis indicated that FR-lyocell degraded preferentially than that of the control sample and the char residue was 38.8 wt% at 800 °C in N2 atmosphere. In addition, the cone calorimeter test revealed that FR-lyocell could not be ignited and the peak of heat release rate was much lower than that of original specimen. Furthermore, the char residue of FR-lyocell is compact and the ID/IG value is 0.771, exhibiting a super degree of carbonization. Moreover, TG-IR result revealed that FR played dual flame-retardant effect in the gaseous and condensed phases. The flame retardant finishing has a negligible impact on the mechanical properties of the fibers. In summary, FR-lyocell exhibits excellent flame retardancy and superior char forming capability, which is suitable for the preparation of flame retardant lyocell fibers.