Study Of The Effect Of PAN Precursor Fiber Pretreatment On The Stabilization Process

Polyacrylonitrile（PAN）-based carbon fiber has the advantages of high specific strength and modulus,good corrosion and fatigue resistance,low coefficient of thermal expansion,etc.It is a strategic new material indispensable for China’s economic development and national defense construction,and is widely used in aerospace,rail transportation,energy equipment and other fields.In the preparation process of PAN-based carbon fiber,the stabilization process is the longest time-consuming process,but also the PAN precursor fibers organic-inorganic structure transformation of the key link.Due to the uneven distribution of radial oxygen content in PAN fibers during stabilization,the stabilized fibers are prone to form a skin-core structure,which seriously affects the final structural properties of carbon fibers;in addition,the stabilization process starts at a high temperature,takes a long time to stabilize,and has low stabilization efficiency and high energy costs.It was found that the pretreatment of PAN precursor fibers with chemical modification before stabilization could improve the stabilization degree,reduce the skin-core structure,shorten the stabilization time and lower the stabilization temperature,but the oxidation of some of the modified reagents is too strong,which is easy to destroy the fiber structure causing fiber breakage,original fiber removal and more cracks,which adversely affects the final carbon fiber properties.In this paper,we systematically study the changes of chemical structure,aggregate state structure and morphological structure of fibers during the stabilization of dry sprayed wet spun PAN precursor fibers,and pretreat PAN fibers by KBr O₃ and Na IO₄ modification to investigate the effects of KBr O₃ and Na IO₄ modification on chemical structure and thermal behavior of PAN fibers,explore the reaction mechanism of their promotion of cyclization,and analyze their effects on stabilized fiber structure and properties.The results of the study showed that:（1）The cyclization and dehydrogenation reactions during the stabilization of dry sprayed wet spun PAN precursor fibers can occur slowly at a lower stabilization temperature,where the cyclization reaction mainly occurs above 200℃,the dehydrogenation reaction is more active above 240℃,while the oxidation reaction occurs at about 240℃and is more intense above260℃.With the increase of stabilization temperature,the original microcrystalline structure is gradually destroyed and transformed to amorphous structure above 260℃and reorganized into amorphous aromatized structure.With the extension of stabilization time,the sp³-hybridized carbon structure gradually transforms into sp²-hybridized C=C structure,and the fiber surface bump gradually decreases,but when the stabilization time exceeds 90 min,more cracks will appear on the surface.（2）KBr O₃ has strong oxidizing property,ionizes bromate ions in aqueous solution,has good nucleophilic property,can attack positively charged carbon atoms in C≡N,makes cyclization proceed by ionic mechanism,catalyzes C≡N cyclization,increases C-O-R and-C=O functional group content of PAN stabilized fibers,increases cyclization degree from 87.5%to 90.7%,and dehydrogenation index from 45 to 64.The KBr O₃ modification accelerated the destruction of the original microcrystalline structure and transformed it into a new aromatized structure,resulting in the conversion of more sp³-hybridized carbon structures into sp²-hybridized C=C structures.（3）The Na IO₄ modification can make PAN fiber undergo certain cyclization,oxidation and dehydrogenation reactions,and also promote the conversion of sp³ hybridized carbon structure to sp² hybridized C=C structure in the stabilization,so that the starting reaction temperature is advanced from 278.5℃to 275.6℃,and the peak temperature of cyclization reaction is also reduced from 285.9℃to 282.9℃,while the exothermic heat is reduced from 558.7 J/g to 444.4J/g.When the pretreatment temperature was 60℃,the overall PAN fiber surface was relatively smooth,while when the temperature was increased to 80℃,the PAN fiber surface showed longer grooves and structural features of partial detachment of the original fiber.