The Effects Of Tension On Stabilization Of PAN Fibers

Stabilization is a very important process in manufacture of PAN carbon fiber. Tension should be applied during stabilization to restrain disorientation of molecule chains. Most researchers focused on the effects of tension during stabilization in a constant temperature range, the opinions of how tension affects stabilization reactions are not consistent, or even opposite to each other. Only in gradually increasing temperature ranges can the stabilization reactions be totally completed, there are also interactions between adjacent stages and interactions between oxidization and cyclization. Thus only through continuous treatment, changing the tensions during different stages, and separating oxidization and cyclization in different atmosphere, we can determine the effects of tension on structures and reactions in stabilization clearly.In this work, various tensions were applied on PAN precursor fibers in different stages of stabilization, where the PAN precursor fibers were both heated and stretched in either air or nitrogen atmosphere respectively, for clarifying the effects of tension on stabilization. FT-IR, DSC, WAXD and other analyses were used to analyze the structures and properties of stabilized fibers. The effects of tension on orientation and crystallinity of stabilized fibers, and effects of tension on cyclization and oxygen diffusion were all studied in this article. The results show that:When stabilization fibers were treated at175-218℃, it is considered that most chemical reactions had not largely reacted in this temperature range, therefore the crystalline regions were mostly preserved at this time. The applied tension can restrained the entropic shrinkage of molecular chains and induced some PAN chains near the crystallite boundary to adjust and align orderly, thereforeLc, crystallinity, orientation of overall fiber and crystalline regions all increase with the increase of tension. The effects of tension applied to amorphous regions made degree of cyclization changing, due to spatial distance between adjacent groups changing. The oxygen content and gradient of oxygen content decrease with the increase of tension in air.For the treated fibers up to226-232℃, in nitrogen, the overall orientation increase at first then turn to decrease with tension increasing, orientation of crystalline regions and crystallinity decrease with tension increasing. When reactions extended to crystalline regions, the mobility of PAN chains became higher, so stretching would made crystalline regions collapsed. When PAN chains were arranged in suitable comformations, degree of cyclization could reach a relatively higher value. The orientation and degree of cyclization change tendencies were similar with the previous stage, a similar but slower adjusting process happened in air. When the degree of cyclization is higher, the oxygen content and the gradient of oxygen content is lower.For the treated fibers up to238-270℃, crystalline regions became smaller with temperature rising, in nitrogen atmosphere degree of cyclization increased gradually, because the increase of tension facilitated the longer ladder structure with more aromatic repeated units to form. In air atmosphere, the overall orientation increase with tension increasing, the oxygen content and the gradient of oxygen content increase with the increase of tensionThrough applied high tension restrain the entropic shrinkage of molecular chains in the initial stage, for the treated fibers up to higher temperature, applied the tension facilitate the cyclization and also considering the effects of tension on oxygen content, orientation and defects, which is a useful method to make high quality carbon fiber.