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The limiting oxygen index (LOI) of polyacrylonitrile (PAN) fiber is only around 18%, making it urgently needed to enhance flame retardancy. Treatment with hydrazine hydrate as cross-linking agents followed by adsorption of metal ions endows PAN fibers with excellent flame retardancy. The Fourier-transform infrared spectroscopy (FT-IR) was employed to investigate the evolution of the chemical structure of the fibers. The atomic adsorption spectrometer (AAS) was carried out to study the adsorption capacity for copper ions. The wide-angle X-ray diffraction (WAXD) and scanning electron microscope (SEM) were used to study the aggregate structure and morphology of the fibers. In addition, LOI, thermogravimetric analysis/infrared spectrometry (TG-FTIR), and thermogravimetric analysis (TGA) were employed to clarify the flame-retardancy and the mechanisms. The mechanical property was studied  via  a single fiber tensile tester. The results showed that more chelating groups arose in macromolecular chains of PAN fibers after the reaction with hydrazine hydrate, with increased adsorption capacity for Cu 2+ . The LOI reached 24.0% after the treatment of hydrazine hydrate and the fiber was still flammable in the air. After the treatment with metal ions, the LOI of the fibers dramatically rose to 45.2%. The thermal decomposition process differed after the treatment of hydrazine hydrate, resulting in a more violent release of ammonia and a sharp reduction of flammable gases. The introduction of copper ions made fibers form dense carbon residue during thermal decomposition, which was beneficial for the improvement of flame retardancy.

Flame retardant polyacrylonitrile fiber modified with hydrazine hydrate and copper ions