Solvent-free synthesis of silicon–nitrogen–phosphorus flame retardant for cotton fabrics

Combustible cotton fabric has limited applications particularly as a flame retardant. In this study, an efficient flame retardant ammonium of aminopropyl silanol methylene phosphate (AASMP) was synthesized using 3-aminopropylethoxysilane, phosphorous acid, urea and formaldehyde under mild and solvent-free conditions to solve cotton inflammable problems. AASMP was grafted onto the surface of cotton fabric, and new P–O–C covalent bonds, were generated using pad-curing treatment to obtain flame-retardant cotton fabrics. The treated cotton was characterized by scanning electron microscopy/energy-dispersive spectroscopy, Fourier-transform infrared spectroscopy and X-ray diffraction, and confirmed that AASMP was grafted well with slight damage to the cotton structure. Cotton treated with 30% AASMP achieved a 45.0% high limiting oxygen index (LOI). After 50 laundering cycles, a 31.0% LOI was retained, indicating that AASMP imparted remarkable flame retardancy and excellent washing durability to cotton. The results of cone calorimetry showed that the peak-heat release rate of the treated cotton was 6.57 kW/m², which was significantly lower than that of the control cotton (186.55 kW/m²). Thermogravimetric results demonstrated that treated cotton with 34.2 wt% residue at 800 °C exhibited significant thermal stability and thermal oxidation stability. The mechanical property of the treated cotton slightly decreased but retained more than 70%. Treated cotton can be used in clothing and interior items.