Effect Of Rigid Stretch During Stabilization Of Polyacrylonitrile Fiber

Although the performance of polyacrylonitrile(PAN) based carbon fibers is related to the property of their precursors,the preoxidization and carbonization technology are also the key factors.In order to manufacture high performance carbon fibers,how the structure factors influence their properties should be considered.There are many factors affecting the performance of PAN-based carbon fibers, such as the preferred orientation of the crystallites and the faults in the fibers.It is important to optimize the structure of PAN fibers.There maybe exist somehow a balance between oxidization and cyclization.Oxidation reaction may weaken the cyclization reaction and the preferred orientation when the fibers go in the forward process of the initial stabilization step at lower temperature.Gupta found that the thermal stress increased faster when heat treated in argon than in air,meaning that the PAN molecules cyclized slower with exist of oxygen.In order to improve the cyclization degree and the intactness of the ladder structure,the fiber was first heated and stretched in nitrogen atmosphere then oxidized in air.In the experiment,a high performance PAN precursor made by Mitsubishi Chemical Co.,Ltd was used as the raw precursor fiber.From FTIR and dynamic mechanical analysis we can found the molecules only occured intramolecular cyclization when the PAN fiber was heat treated in nitrogen.While the molecules occured both intramolecular cyclization and intermolecular cyclization when the fiber was heat treated in air.The cyclization degree of the fiber treated in nitrogen was higher than in air,resulting in an improved ladder structure.From DSC,density and element analysis,we found the exothermic peak shifted to lower temperature and the exothermic reaction became more acute.After the fiber oxydized in air,the weight gaining and oxygen content both increased,which showing the reactivity and structure of the molecules were both improved.During the heat treated process in nitrogen,the flexible molecular chains became rigid.The stretch applied after which means rigid stretch benefits to depress the disorientation.The more stretched,the higher orientation and crystallization the fiber would get.Moreover,the mechanical performance of the resulted carbon fiber could be improved:The fracture strength was enhanced and the modulus reached to the highest value in the range of 10%~20%.The result made out that the orientation and crystallization were both improved and the ladder structure turned out to be perfect after the PAN fiber was rigidly stretched in nitrogen in 170~250~.C.The method could be helpful to preparing high performance PAN-based carbon fiber.