Multi-step heating strategy during thermal stabilization of polyacrylonitrile multi-filament bundle before carbonization

A polyacrylonitrile (PAN) multi-filament bundle underwent a multi-step heat treatment following impregnation with a solution of ammonium bromide, phosphoric acid and urea (A-P-U). The multi-step thermal stabilization process was conducted in an air atmosphere at temperatures ranging from 200 to 245 °C for various stabilization times spanning from 10 to 70 min. Comprehensive analysis of physical and mechanical properties, infrared (IR) spectroscopy, X-ray diffraction (XRD) and thermal analysis (DSC and TGA) revealed the significant influence of stabilization time on the structure and properties of thermally stabilized PAN multi-filament bundle. The fiber thickness and linear density of the stabilized fibers decreased by approximately 29.5% and 10.6%, respectively, after 70 min of heat treatment. However, the fiber density value increased from 1.18 to 1.38 g cm⁻³ during the same stabilization time. Additionally, the carbon yield value obtained using TGA increased from 31% to 73% at 850 °C. The presence of A-P-U markedly reduced the time needed for the conversion of the PAN polymer into a cyclized structure through its nitrile groups, thereby accelerating the stabilization reactions. Furthermore, the IR spectra exhibited the appearance of CC bonds, signaling the creation of a crosslinked ladder-like structure. The XRD traces confirmed the decrease in crystallinity with increasing stabilization time, consistent with the findings from IR spectra. The findings showed that the A-P-U integrated system is notably successful and proficient in promoting the cyclization of nitrile groups, thereby decreasing the time needed to establish a thermally stable structure capable of withstanding elevated carbonization temperatures. © 2024 The Author(s). Polymer International published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.