Durable phosphorus/nitrogen flame retardant for cotton fabric

A new phosphorus/nitrogen flame retardant (FR) containing reactive –P–O−NH4+ groups was synthesized from glycerol, phosphoric acid, and urea. At high temperatures, the –P–O−NH4+ group decomposed into –P–O−H+ group, which produced phosphonic anhydride under the action of dicyandiamide catalyst. Phosphonic anhydride dehydrated and condensed with a hydroxyl group on the 6-position carbon atom in the glucose ring of cotton fiber, firmly binding the FR molecule to the fiber through a strong P–O–C bond. The structure of the FR was determined by Fourier-transform infrared spectroscopy (FT-IR) and nuclear magnetic resonance spectroscopy (1H NMR, 13C NMR, and 31P NMR). The effects of FR treatment on the flame retardancy, durability, and thermal stability of cotton fabrics were systematically investigated by measuring limiting oxygen index (LOI) and performing UL-94 vertical burning, cone calorimetry (CONE), thermogravimetric (TG), and differential scanning calorimetry tests. In addition, scanning electron microscopy-energy dispersive spectroscopy, FT-IR, and X-ray photoelectron spectrometry studies verified that FR molecules were grafted onto cotton fabrics. X-ray diffraction showed that FR did not affect their crystal structure. The treated cotton fabric with a weight gain of 25.3% exhibited an LOI of 40.5%, which was significantly higher than that obtained for the untreated cotton (17.0%); additionally, the treated cotton showed good self-extinguishing properties during the UL-94 vertical burning test. Furthermore, the results of CONE analysis indicated that the peak heat release rate of the untreated cotton reduced from 171.1 to 15.1 kW/m2 after FR treatment, and its total heat release decreased from 6.3 to 1.1 MJ/m2. TG data revealed that FR finishing reduced the initial thermal degradation temperature of the untreated cotton under heating conditions to 225.9 and 221.8 °C in N2 and air atmospheres, respectively, while the whiteness and mechanical properties of the treated cotton remained in the usable range. Analysis of the flame retardation mechanism of the FR showed that the dense phosphorus/nitrogen char layer that formed in the condensed phase effectively hindered the release of heat and diffusion of flammable volatile substances. This study provides new insights into the design and manufacturing of environmentally friendly FR-treated cotton fabrics with excellent flame retardancy and durability.Graphical abstract