Durable flame retardant cotton fabrics modified with a novel silicon–phosphorus–nitrogen synergistic flame retardant

A new silicon–phosphorus–nitrogen flame retardant, di-(trimethoxysilylpropyl) spirocyclic pentaerythritol bisphosphorate (SPDP-PTMS) was synthesized and the SPDP-PTMS treated cotton fabrics were prepared using sol–gel technology. The chemical structure of SPDP-PTMS was characterized by FT-IR and ¹H NMR. The surface morphologies and elemental changes of SPDP-PTMS treated cotton fabrics and char residues were characterized by scanning electron microscopy (SEM) and energy dispersive spectrometer. After treating with 500 g/L SPDP-PTMS, the LOI value reached 29.5% and maintained 26.2% after 20 washing times. The char length of cotton fabrics treated with 500 g/L SPDP-PTMS was 8.8 cm with no afterflame and afterglow in the vertical burning test. The thermal stability of SPDP-PTMS treated cotton fabrics was improved and measured by thermogravimetric (TG) analysis. In the cone calorimetry test, the peak heat release rate and total heat release decreased from 232.1 to 179.4 kW/m² and from 6.5 to 4.5 MJ/m², and the char residue reached 23.5%. Thermogravimetric analysis coupled with Fourier transform infrared analysis (TG-FTIR) results indicated that SPDP-PTMS treated cotton fabrics released nonflammable gases and less flammable gases than the untreated cotton fabrics. The analysis of the data proved that SPDP-PTMS might have a flame retardant effect by releasing non-flammable gases and forming a stable char layer.