A novel ε-polylysine-derived durable phosphorus‐nitrogen‐based flame retardant for cotton fabrics

A novel biomass-based, phosphorus-nitrogen-based, halogen-free flame retardant, ammonium salts of polylysine methylphosphonate glycerol ester (PLUEG), was successfully synthesized based on ε-polylysine and glycerol, used to prepare highly efficient and durable flame-retardant cotton fabrics. The large number of –P=O(O⁻NH₄⁺)₂ reactive groups improved the binding between flame retardants and cotton fabrics, making it harder for flame retardants to be washed off from cotton fabrics. During the synthesis process, some of the –P=O(OH) groups on flame retardants were esterified in advance with glycerol, to reduce the effect of metal ions on the flame retardancy of cotton fabrics during washing process. And cotton fabrics were pretreated with sodium hydroxide to partly break the crystalline region of cellulose and increase the number of reacting sites in cellulose for flame retardants. After the pretreatment with sodium hydroxide and treatment with PLUEG, cotton fabrics with high flame retardancy and durability was prepared. The limiting oxygen index value of cotton fabrics increased to 42.7%, which remained at 28.6% after 50 laundering cycles (LCs) of home machine washes. Analysis by Fourier-transform infrared spectroscopy (FTIR) and energy-dispersive X-ray spectroscopy revealed that PLUEG were bound to cotton fabrics by –P(=O)–O–C bonds, and the flame retardancy of treated cotton fabrics decreased during washing process because of the replacement of NH₄⁺ in unreacted –P=O(O⁻NH₄⁺)₂ groups by the metal ions in washing liquid. Also, analysis by thermogravimetric (TG), TG-FTIR spectroscopy and scanning electron microscopy indicated that PLUEG was mainly effective in the condensed phase. The whiteness and mechanical properties of treated cotton fabrics were retained well.