Effect Of Modification With H₂O₂ On Chemical Structure And Pre-oxidation Process Of Polyacrylonitrile Precursor

Pre-oxidation is one of the most important step to manufacture polyacrylonitrile(PAN)-based carbon fiber.The pre-oxidation process of PAN fiber is accompanied by severe heat release.Therefore,reduction in the activation energy of the pre-oxidation reaction not only alleviates concentrated heat release effectively,but also provides more space to regulate the pre?oxidation parameters.Hydrogen peroxide(H2O2)is the cleanest modifier and has obvious advantages in the modification process for the PAN precursor.In my research,chemical structure of the precursor was influenced by PAN post-spinning modification with H2O2,so as to optimize structure of the pre-oxidized fiber and improve efficiency of the pre-oxidation.Chemical structure of the PAN fiber was analyzed by FT-IR and XPS,et al;exothermic and thermal weight loss behavior of the PAN fiber was analyzed by DSC and TG;sections of the pre-oxidized fiber was obtained by OM and radial difference of the pre-oxidized fiber was analyzed by optical density(OD)method.The results show that:1.During the modification period,elemental oxygen of H2O2 aqueous solution induces nitrile(C=N)of PAN precursor to cyclize.The imine(C=N)at the terminal of cyclization structure tautomerizes to enamine which exists more stably,so that it brings about quasi-aromaticity primary amine.The amount and conjugation degree of C=N of the modified precursor increases with the increment of modification temperature,while the content of quasi-aromaticity primary amine increases and then decreases abruptly at 120?.2.Cyclization during the pre-oxidation period can be catalyzed by quasi-aromaticity primary amine of the modified precursor and the apparent activation energy of cyclization is reduced by 30%approximately,while dehydrogenation is catalyzed slightly.Moreover,modification of precursor with H2O2 makes cyclization become well-distributed during the whole pre-oxidation period so that it ensures cyclization efficiency in the later period of the pre-oxidation.With non-cyclization section and oxygen-containing structure as reference,the cyclization degree of the pre-oxidized fiber obtained from the modified precursor was higher than that without modification of precursor,and it is proportional to the content of quasi-aromaticity primary amine of the modified precursor.3.During the modification period,if ammonia is utilized to improve hydrophilicity of the PAN precursor,it can deepen the dipping depth of H2O2 in the radial direction of precursor which determines the modification depth.Compared with the fiber obtained by heating at 230? for 90min from the unmodified precursor,the one obtained by heating for 60min from the precursor modified with H2O2 and NH3 · H2O has the similar pre-oxidation degree.Moreover,its skin area ratio is higher and ?OD and ?OD in the radial direction is reduced from 0.082 and 0.029 to 0.071 and 0.026.Therefore,pre-oxidation efficiency and radial homogeneity of the pre-oxidized fiber are both optimized and the carbon yield at 1000? is increased by 3.7%.Concerned about PAN post-spinning modification with H2O2,my research investigates transformation of chemical structure of the modified precursor and its evolution in the pre-oxidation process deeply,as well as reveals the mechanism that cyclization was facilitated by H2O2 during modification of precursor period and quasi-aromaticity primary amine during the pre-oxidation period explicitly.PAN post-spinning modification with H2O2 has an effect on improving cyclization degree and reaction efficiency of the pre-oxidized fiber,and modification with H2O2 and additive can optimize radial homogeneity of the pre-oxidized fiber obviously.