Mechanism And Kinetics During Thermal Stabilization Of Polyacrylonitrile

Thermal stabilization of polyacrylonitrile (PAN) is one of the critical processes for fabricating high performance carbon fiber. In order to optimize the process and improve the mechanical properties and carbon yield of carbon fiber, it is very meaningful to research the reaction mechanism and kinetics during stabilization of PAN in depth. In inert atmosphere, cyclization, dehydrogenation and pyrolysis take place in PAN molecular chains. In air, the situation becomes much more complicated due to the presence of oxygen. Overlap of some reactions can be observed. Until today, the mechanism during stabilization of PAN is not clearly.In this paper, on the basis of the literature, several test methods such as fourier transform infrared spectroscopy (FT-IR), solid state13C nuclear magnetic resonance (13C NMR), differential scanning calorimetry (DSC), thermomechanical analysis (TMA), thermogravimetry-mass spectrometer (TG-MS) and elemental analysis (EA) were implied to investigate the following areas:(1) structural evolution, thermal behavior and reaction mechanism during stabilization of PAN in inert atmosphere,(b) influence of oxygen on stabilization mechanism of PAN.(c) reaction kinetics during stabilization of PAN in inert atmosphere,(d) influence of oxygen on kinetics during stabilization of PAN. The main results are as follows.Cyclization is initiated by carboxylic acid copolymer in PAN molecular chains and-COOR group is formed. When heat treatment temperature is up to about220℃, cyclization becomes apparent and an imine-enamine tautomerism is generated. When temperature is higher than240℃, dehydrogenation and pyrolysis occur. The imine-enamine tautomerism is transformed into a fully conjugated structure owing to dehydrogenation. The pyrolysis products contain three main structural styles:micromolecule gases,uncyclized molecular chain and aromatic rings.The influence of oxygen on stabilization of PAN is significant. Oxygen can promotes cyclization and dehydrogenation. Oxygen in circumstance can combine with-C=N group in PAN molecular chains resulting in the formation of an amide group, which initiating the cyclization reaction. In addition, oxygen reacts with-CH2group and then H2O is liberated from PAN molecular chains. The conjugated structure is formed at lower temperature than that in inert atmosphere. It was found that an oxidation reaction generating-C=O group could take place at160℃in air. It is proposed that the cyclized structure in PAN molecule chains is a prerequisite for the oxidation. Cyclization and oxidation occur sequentially and the former is the rate determining step. The cyclized structure can react rapidly with oxygen once it has been generated during the stabilization. The-C=O structure can promote the subsequent stabilization of PAN by combining with the adjacent hydrogen containing groups and initiating the fasculation reaction.The total kinetic models for cyclization and pyrolysis of PAN in inert atmosphere were established by Kissinger and Ozawa methods based on the DSC curves. The results indicate that the apparent activation energy for pyrolysis is much higher than that for cyclization. This confirms that pyrolysis occurs at higher temperature than cyclization again. The "Three regions kinetic model" for stabilization of PAN is proposed by the "Improved Coats-Redfern method", which can describe the stabilization accurately. Inaddtion, the kinetic model for cyclization of PAN by cyclic index based on the FT-IR spectra was established. The apparent activation energy for cyclization caculated by this model is much closed to that by the "Three regions kinetic model".The influence of oxygen on stabilization kinetics of PAN is obvious. The kinetic parameters for cyclization were caculated by the TMA curves and the FT-IR spectra. Because of the promotion of oxygen, the apparent activation energy for cyclization in air is much lower than that in inert atmosphere. Based on the stabilization reaction feature of PAN in air, Levenberg-Marqardt method was used to establish a composite kinetic model by the DSC curves. The simulation curve and the experimental curve are matched quite well, so this model is feasible for describing the stabilization of PAN. The DSC curves of PAN precursor can not reflect the oxidation reaction which generating -C=O group accurately because of the effects of cyclization. The kinetic model for the oxidation was proposed by the DSC curves of PAN with maximum cyclized structure. This model is more reasonable due to eliminating the influence of the cyclized structure.