Reaction retardancy and nonuniformity of polyacrylonitrile fibers by tow sizes during oxidative stabilization

Large-tow carbon fibers (CFs) possess more advantageous of cost than small-tow CFs, however, the uneven heat release during oxidative stabilization affects the chemical and physical performance of subsequent large-tow CFs. In this work, we prepared 12–48 K PAN oxidative stabilized fibers with continuous stabilization method, and the microstructural evolutions were investigated. The heat transfer and oxygen diffusion were inhibited and lagged in large-tow PAN fibers during oxidative stabilization, resulting in less cyclization reaction and oxygen-uptake reaction, and the small-tow stabilized fibers presented higher R_D, oxygen content, and more uniform degree of chemical reactions. Thermal behavior revealed that 48 K stabilized fibers demonstrated lower T_onset and higher Δ_H as they remained relatively more uncyclized linear structures. The cross-section morphologies were refined with the generation of oxidized structures, and the skin-core structures were induced, large-tow fibers demonstrated more radial heterogeneity related to the inhibited oxygen diffusion and uncompleted cyclization. Furthermore, less amorphous transformation occurred in large-tow fibers. Part of the heat generated by cyclization and oxidation reactions transferred into the inner layer and contribute to the chemical reactions, and some of it flew out, the ununiform extent of reactions made larger fluctuation in microvoid variations according to the SAXS results.