Effect Of PAN Precursor Modification On The Structure And Performance Of Preoxidized Fibers

Preoxidation is a crucial step during the transition process of polyacrylonitrile (PAN) precursors to carbon fibers, which always accompanies with high exothermic reaction and complicated physical and chemical conversion. The skin-core structure caused in the preoxidation process greatly influences the performance of carbon fiber. Thus, traditional preoxidation usually adopt slow heating method to control the release of reaction heat. The great consumption of time and energy during traditional preoxidation process leads to high costs and greatly restrains its application in civilian areas. Therefore, shortening preoxidation time and reducing the cost of carbon fiber in the premise of high fiber performance have become a pressing problem in the industrial production of carbon fiber.In order to resolve the above problems, three modification methods of PAN precursor have been adopted, such as irradiation modification, chemical modification and complex irradiation-chemical modification. The effect of polyacryonitrile precursor modification on the preoxidation process and the properties of preoxidized fiber had been studied.First, PAN precursors were modified using special irradiation source. The results show that partial dehydrogenation reactions happened after irradiation modification for PAN fibers, which reduce the temperature of exothermic peak and the energy of exothermic reaction. The thermogravimetic rate is relatively gentle and the carbon yield is increased. This helps to slow up the centralized exothermic reaction in the preoxidation process, reduce preoxidation temperature and save energy cost. In addition, the irradiation modification is helpful for oxygen diffusion during preoxidation process, because of decreased crystallinity of the modified fiber. After continuous preoxidation and carbonization treatment, it was found that the irradiation modification succeed in shortening preoxidation time to 60min which is half of the traditional technology. The mechanics properties of preoxidation fiber and finally formed and carbon fiber is also improved.Second, dyeing machine was used in the chemical modification of PAN precursor by KMnO4. The complex of Mn7+ and the C atom in C=N group of PAN fibers reduces the force between C=N groups on molecular chains. After chemical modification, the exothermic reaction becomes gentle and the released energy is decreased. By IR and XRD analysis, it is found that C=C structure has generated in modified fiber. This indicates that KMnO4 catalyzes cyclization reaction. In addition, crystallinity decreases of the modified fiber and oxygen element content increases with the modification time. The results show that KMnO4 modification can accelerate preoxidation reaction rate to a certain extent and reduce preoxidation time to 75min～90min. EDXA analysis indicates that skin-core structure is prohibited after KMnO4 modification and oxygen elements on fiber section is uniform. Mechanical performance of the preoxidation fibers is relative higher, especially for precursor modified for 5min. The finally carbon fiber has a mechanical property close to T300 carbon fiber.Finally, the preoxidation of the precursor modified by the complex modification method has been discussed. The preoxidation time is reduced further compared with the former two methods.The detailed experiments are under way.