Thermal degradation and flame retardant mechanism of sulfonated polyoxadiazole fibers modified by metal ions

The sulfonated polyoxadiazole (SPOD) flame retardant fibers are prepared by wet spinning and modified by metal ions (Na+, Ca2+, Ba2+, and Mg2+). The structures of SPOD fibers are characterized by Fourier transform infrared spectrometry (FTIR), elements analysis (EA), inductively coupled plasma emission spectrometry (ICP), scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS). Results show that sulfonate groups were successfully grafted on the POD fiber polymer, and metal ions are adsorbed in the fiber by physical and chemical adsorption. The micro-scale combustion calorimeter (MCC), limiting oxygen index (LOI), and UL-94 vertical burning tests are used to test the flame retardant properties of the fibers. Results show that the Mg-SPOD had the best flame retardant performance with LOI of 40% and the lowest heat release rate (HRR). The thermal degradation mechanism of the fiber is analyzed by thermogravimetric analysis coupled with infrared spectrometry (TG-FTIR), and the char residues were investigated by X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). The rapid and sharp emission of non-flammable SO2 and high content char residue covered with dense MgO and MgSO4 improve the flame retardant properties of Mg-SPOD fibers significantly.