Synthesis of polysaccharide-based macromolecular flame retardants containing ammonium phosphonate and phosphonic acid ester for cotton fabrics

A novel phosphorus-containing cotton flame retardant 2, 6-dimethoxy polysaccharose phosphonic acid ester ammonium phosphonate (DOPSPAP) was synthesized and its chemical structure was confirmed by nuclear magnetic resonance (1H, 13C and 31P NMR) and Fourier infrared spectroscopy (FT-IR). The damage length of the cotton fabric treated with 25wt% DOPSPAP was only 4.2 cm in the vertical flame test (VFT), and the char residue was observed to maintain the intact fiber structure by scanning electron microscopy (SEM). The limiting oxygen index (LOI) of 25wt% DOPSPAP treated cotton fabric was as high as 44.5%, and the LOI was still 34.5% after 50 LCs according to AATCC 61–2013 3A standard. SEM observed the presence of DOPSPAP flame retardant on the surface of cotton fiber. Durability, FT-IR and energy dispersive X-ray spectroscopy (EDS) confirmed the grafting of DOPSPAP on cotton fibers through P(= O)-O-C covalent bonds. The cone calorimetric results showed that DOPSPAP effectively inhibited the heat release during combustion of cotton fabrics. The thermogravimetric (TG) and TG-FTIR results suggested DOPSPAP changed the decomposition pathway of cotton fibers and promoted their dehydration into char. The bending length and tensile strength of DOPSPAP-treated cotton fabrics are well maintained. Compared with 2, 6-dimethoxy polysaccharide ammonium phosphonate (DOPSAP), DOPSPAP-treated cotton fabric showed better flame retardant durability and tensile strength. The EDS results showed that the metal ion content of DOPSPAP-treated cotton was significantly lower than that of DOPSAP-treated cotton after 50 LCs. These results indicate that the simultaneous introduction of phosphonic acid ester and ammonium phosphonate groups into flame retardants can not only improve the durability of flame retardants, but also reduce the loss of strength.