Formaldehyde-free and durable phosphorus-containing cotton flame retardant with -N=P-(N)- and reactive ammonium phosphoric acid groups.

A flame retardant (FR) hexachlorocyclotriphosphazene diethylenetriamine ammonium phosphoric acid (HDAPA) was synthesized. Vertical flammability test and limiting oxygen index (LOI) results showed that cotton samples finished with HDAPA solutions (15 % and 20 %) could pass vertical flame retardancy test, and LOIs reached 30.1 % and 35.4 % even after 50 laundering cycles according to AATCC 61-2013 3A washing standard (3A), performing flame retardancy and washing durability. Meanwhile, Fourier transform infrared and X-ray photoelectron spectroscopy analyses suggested that HDAPA was grafted on cotton fibers through -P(=O)-O-C covalent bond. Total heat release (1.98 MJ/m) and char residue (16.2 %) of HDAPA treated cotton were much lower than those (4.26 MJ/m, 3.2 %) of untreated cotton. Thermogravimetry results showed HDAPA changed thermal decomposition pathway of cotton fabric, which was further supported by thermogravimetric-Fourier infrared spectrometer results, revealing HDAPA performed a condensed phase flame retardancy mechanism. Scanning electron microscopy implied HDAPA entered amorphous region of cotton fibers to react with cellulose. Mechanical properties of HDAPA treated cotton decreased a little. Although the synthesis process used formaldehyde but no free formaldehyde released. In consequence, the aforementioned results indicated that the introduction of -N=P-(N)- and -P(=O)(ONH) groups to FR was an viable method to improve flame retardancy and durability.