A Comparison Study of the Synergistic Influence of Discrepancy in Exothermic Behavior of Polyacrylonitrile-Based Precursor Fibers and the Continuous Gradient Pre-Oxidation Condition on Stabilization Structure

Abstract

The synergistic influence of discrepancy in exothermic behavior of two types of polyacrylonitrile-based precursor fibers (labeled as fiber A and B) and the continuous gradient pre-oxidation conditions on stabilization structure were studied. The evolution and development process of skin-core structure at different stabilization stages and corresponding influencing factors was clarified. Results showed fiber A had gentle exothermic behavior, smaller exothermic rates, and weaker heat flow, while fiber B had faster exothermic rates and concentrated heat flow, especially at stabilization later stage. Stabilization behaviors revealed that the cyclization index (Ic), oxidation index (Io), dehydrogenation index (Id), aromatization index (AI), X-ray stabilization index, O elements, and bulk density increased with proceeds of pre-oxidation. The relationship between the aromatization cross-linked structure, bulk density, and chemical structure indicated that cyclization, dehydrogenation, and oxidation reactions jointly promoted the formation of aromatization cross-linked conjugated lamellar structure. Under the same pre-oxidation conditions, the stabilization degree of fiber B was much higher than that of fiber A. Meanwhile, a more serious inhomogeneous structure appeared at the early stage and further deteriorated at a later stage, which is related to its concentrated exothermic behavior, locally high temperature in the core and nonuniform oxygen distribution in the radial direction. These factors become the internal factors that affect the formation and development of skin-core structure. By adjusting temperature and gradient distribution as well as shortening time, the skin-core structure disappeared without compromising the stabilization degree. The formation of the skin-core structure was controlled by chemical reactions at early stage but by double diffusion and locally high temperature in the core at the later stage. In order to promoted oxygen diffusion, the Ic, Io, and AI values at the initial stage should be controlled below 0.8%, 0.36%, and 32%, respectively. The principles, reasons, and regulatory mechanisms for optimizing the stabilization structure and set proper pre-oxidation procedures were provided.