Fabrication of durable flame retardant PAN fibers through bio-based ammonium phytate surface modification and highly efficient thermal oxidation

To address the flammable shortcoming of polyacrylonitrile (PAN) fibers, an ammonium phytate (AP) flame retardant was fabricated by using biomass phytic acid (PA) and urea as raw materials. PAN fibers were firstly treated with AP aqueous solution, which were then thermally oxidized to gain durable flame retardant PAN fibers (O-AP-PAN). The structure of fibers was characterized by X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy and X-ray diffraction. The results indicated that AP promoted the formation of a thermally stable cyclic conjugated structure, and the P element was incorporated into the PAN fibers through covalent bond. The limiting oxygen index (LOI) value of O-AP-PAN fibers improved from 17.0% of PAN fibers to 36.6%. Even after 50 laundering cycles, the LOI value of O-AP-PAN maintained as high as 30.5%, indicating exceptional washing durability. Additionally, the total heat release and heat release capacity of O-AP-PAN were reduced by 46.8% and 52.7%, respectively. Besides, the mechanical strength of O-AP-PAN was well maintained without affecting practical use. In this work, a simple and feasible durable flame retardant strategy for PAN fibers was established, which was convenient for large-scale preparation of flame retardant PAN fibers.