Study On Preoxidization Technology And Physico-Chemical Behavior Of Polyacrylonitrile Fibers

Preoxidation treatment plays a bridge role and is a crucial step during the conversion from polyacrylonitrile (PAN) fibers to carbon fibers, so it is worth studying the preoxidation of PAN fibers. Some advanced methods for sample preparation and measurement were used in the paper. At first, the comparative investigations were concerned with different kinds of PAN fibers. Then, the technological parameters for preoxidation of PAN fibers were exploited, and the physico-chemical behaviors were studied during preoxidation, while systemic analysis was performed on the structure, element content and density of PAN fibers, and the mechanical properties of carbon fibers were tested and evaluated. Finally, the defects and their origins of the preoxidized fibers were discussed, the aim is to optimize technology and further improve the performances of carbon fibers.Various PAN fibers were studied for comparing the difference of their structure and properties, accordingly, some measurement methods, including x-ray diffraction (XRD), Fourier Transform infrared spectra (FT-IR), elemental analysis, Transmit electron microscope (TEM), high-temperature differential scanning calorimetry (DSC) and tension testing, were used. It was found that high-quality PAN fibers should have the following characteristics: low titer, high strength, relative less heat energy releasing during the cyclization reaction and broader exothermic range, good compactness, high orientation degree, appropriate crystallinity, small crystalline size and uniform dispersion, and defects as little as possible.DSC analysis of PAN fibers was conducted from ambient temperature to 1400℃ using high-temperature DSC. The endothermic and exothermic peaks in the DSC curve imply that several structural transformations occurring during fabricating PAN-based carbon fibers. The DSC results of PAN fibers can be referred to as guide for preoxidation and carbonization technology. The preoxidized fibers and carbon fibers were also measured under the same DSC testing condition as PAN fibers, and the exothermal peak at 335℃ is associated with the cyclization reaction of the residual nitrile groups in the preoxidized fibers. The fibers at different preoxidation stages were tested from ambient temperature to 400℃ by an ordinanry DSC, and the corresponding elemental contents were obtained by elemental analyzer. It was shown that heating history has great effect on DSC curves and changes of element contents.The exothermic range become broader with increasing temperature and prolonging time in the range from 190°C to 270°C, while the exothermic peak shifts to the higher temperature, the maximum temperature may reach 329°C for the preoxidized fibers. Thus the fibers can be avoided from burning out due to concentrative heat energy releasing during preoxidation. It is appropriate that the initial carbonization temperature is selected to be not less than 350°C.Complicated physical change and chemical reaction may take place during preoxidation of PAN fibers. Both the accumulative shrinkage and the structural conversion are resulted from the phyico-chemical behaviors. The inner stress is originated from fiber shrinkage. The tension force is the extrinsic representation of the inner stress in the fibers and the outcome of the accumulative shrinkage, and its changes reflect the thermal stability of PAN fibers. The tension force was measured by a tensiometer when various PAN fibers were treated by conducting the different preoxidation technology. The distinct tension force-temperature relations indicate that the following factors influence the tension force: the varieties of PAN fibers, filament quantity in a fiber tow, filament titre, preoxidation temperature, preoxidation time and stretching rate. Therefore, the tension force is a comprehensive index, and its changes are attributed to collective action of many factors. It is a key technique to exert stretch in the whole course of manufacturing carbon fibers. Therefore, it is very significant to attain an optimum stretching rate during preoxidazation, and it happens that tension force-temperature relation is a new approach to optimize the stretching rate.The physico-chemical behaviors of PAN fibers are responsible for the structural and component changes during preoxidation. The structural changes include the chemical structure and microstructure. For this reason, the structures of both PAN fibers and the preoxidized fibers were examined or characterized by a variety of methods such as high-resolution Transmit electron microscope (HRTEM), XRD and FT-IR. There are both new peak and nitrile groups' peak on the FT-IR spectra of the preoxidized fibers, which mean that the given technology should be improved. It is also convinced that the structures are not converted sufficiently by comparing the XRD patterns of PAN fibers with the preoxidized fibers, and crystallinity and orientation degree reduce for the latter.It is an innovative investigation that characterizes the fine structure of the PAN fibers and the preoxidized fibers by HRTEM, because no literatures are available. Considering PAN fibers, the structure information in the HRTEM images of thetransverse section matches well with the XRD testing results, and the former is more intuitionistic, but the structure information in the HRTEM images along the longitudinal section is very different from the XRD results, indicating that higher orientation of the PAN fibers causes an important structure discrepancy between the transverse section and the longitudinal section. The structure models established by predecessors are not the same as the HRTEM images of PAN fibers. Besides new lattice spacings in the HRTEM images of the preoxidized fibers, there are still original ones in PAN fibers, demonstrating that the structural conversion from PAN fibers to the preoxidizd fibers is not completed and is still underway. The larger discrepancy of fine structure is also observed by comparing the HRTEM images of the transverse section with the images of the longitudinal section in view of the preoxidized fibers. HRTEM method provides some significant structure information concerned with the conversion of fine structure in PAN fibers: the preoxidation treatment is an evolution course of the microcrystalline structure. The formation of new crystalline structure begins at different preferential sites inside the original crystals in PAN fibers, where plane (110) transforms in priority. As stated above, HRTEM method is a cogent tool to study the fine structures in PAN fibers and the preoxidized fibers, and it is superior to other testing methods in studying the difference of various sections of fibers and elucidating the mechanism of the microstructure transformation during preoxidation.The skin-core morphology in the preoxidized fibers is adverse to improve the properties of the final carbon fibers. Oxidation reaction occurs at higher temperature while oxygen in air diffuses into fibers during preoxidation treatment, and meanwhile, the gaseous nitrides diffuse outward. Therefore, the development of the radial morphology in the fibers associates with the different oxygen and nitrogen content between the interior and the exterior. The characterization of PAN fibers, the middle preoxidized fibers and the final preoxidized fibers were performed by using different methods such as elemental analyzer, optical microscope (OM), FT-IR and electron microprobe analysis (EMPA). With temperature increment and time prolongation, the fiber density becomes larger, oxygen content increases, but nitrogen content does not decrease evidently. The larger density implies the better compactness, oxygen is responsible for this. FT-IR testing results indicated that there are oxygen-functionality-grouping and nitrogen-functionality-grouping in the preoxidized fibers. The change of skin-core morphology was observed by OM. The radial linear distribution of elements oxygen and nitrogen was measured for various fibers byEMPA, respectively, and oxygen content in the preoxidized fibers reduces from the external skin to the inner region, but nitrogen content hardly changes. It was demonstrated that the radial oxygen distribution in the fibers is associated necessarily with skin-core morphology feature. In addition, the oxygen diffusion mechanism was also discussed.The systemic measurements on the fibers at the different preoxidation stages were carried out by various methods such as density, FT-IR, XRD and element analysis, so as to further understand their modification during continuous preoxidation. The results of the final preoxidized fibers were confirmed as follows, density 1.357xlO3 kg/m3, oxygen content 12.151%, a small quantity of nitrile groups and lower crystallinity. The mechanical properties of the resulting carbon fibers are very stable, i.e. relative low standard deviation and variance coefficient, and their average tensile strength are measured up to 3.76GPa.The defects were studied due to their worse effect in improving the tensile strength of carbon fibers. OM, TEM and EMPA were used to investigate the defects in the preoxidized fibers and their origins, and it was found that the formation of defects is dominantly at the preoxidation stages and the defects heredity from PAN fibers. The approach reducing defects was also discussed. The intention is to lessen defects in the preoxidized fibers and further enhance the tensile strength of the carbon fibers by taking some effective measures.