The Structural Evolution And Control Of Polyacrylonitrile Fibers During Heat Treatment

Carbon fibers are now effectively used as a reinforcing component in the preparation of lightweight composite materials because of their high strength, high elastic modulus, low density and good heat resistance. The theoretical strength of carbon fibers is180GPa, however, the highest strength of carbon fibers manufactured in laboratory is9.03GPa, only5%of the theoretical value. The mechanical properties of carbon fibers are determined by the structural defects, such as misorientation and dislocation of crystallites, microvoids, skin-core structure and surface defects. In order to enhance the mechanical properties of carbon fibers, the formation and control of microstructures and defects should be further investigated. Focusing on the formation and control of microstructures for polyacrylonitrile (PAN)-based carbon fibers, and the effect of microstructures on mechanical properties, this paper investigated the structural evolution and development of mechanical properties for PAN-based carbon fibers during stabilization, carbonization and graphitization, and the influence of manufacture conditions on the structures and properties was also discussed.(1) The PAN precursor fibers were heated and stretched in nitrogen atmosphere before oxidation. The effect of this pretreatment on the structure and properties of the resulting stabilized and carbonized fibers was investigated. The stabilization of PAN precursor fibers was divided into two stages. In the former stage, precursor fibers were heated in nitrogen in the temperature of180-230℃with suitable stretching ratios. The intramolecular cyclization was improved and the intermolecular cross-link was suppressed. The misorientation of molecules was effectively inhibited due to the formation of rigid ladder polymers. In the latter stage, the fibers pretreated in nitrogen was heated and oxidized in air to obtain completely aromatic structures. A further reaction in air made the fibers fully stabilized, the resultant stabilized fibers showing a higher preferred orientation and larger cyclization degree than those of conventionally stabilized fibers. The data of thermal and elemental analysis showed that the fibers subjected to the pretreatment in nitrogen were more easily oxidized. Thermal gravimetric analysis of the resultant stabilized fibers showed a higher carbon yield. The resultant carbonized and graphitized fibers had a smaller interlayer spacing and microporosity, and a higher preferred orientation, which gave rise to considerable increases in tensile strength and Young’s modulus. The graphitization seemed to be accelerated by this pretreatment.(2) PAN precursor fibers were heated and stretched in a steam bath mixed with nitrogen in the early stage of stabilization. During this process, the flexible chains of PAN molecules became rigid by intramolecular cyclization. The water molecules here acted as a plasticizer for stretching. As a result, the tension along the fiber was decreased during stabilization and the preferred orientation of ladder molecules in the stabilized fibers was enhanced. Consequently, the preferred orientation, crystallite dimensions and mechanical properties of the resultant carbon fibers and graphite fibers were improved, comparing with the corresponding carbon and graphite fibers without pretreatment in the steam bath.(3) The structural evolution and property development of PAN-based carbon fibers during carbonization were investigated. Based on the thermal gravimetric behavior, it was noted that the carbon yield increased as heating rate increased from5to60℃/min. The results of stress development indicated that there were three stages from50to600℃, i.e., increased before330℃, decreased from330to470℃, and increased again after470℃. This trend was attributed to various chemical reactions during this process:cyclization and cross-linking reactions of nitrile groups, pyrolysis and degradation of linear molecular chains, and solid cross-linking and cohesion reactions between neighboring aromatic polymers to form the carbon basal planes, respectively. The influence of velocity and tension during carbonization on the microstructure and mechanical properties of carbon fibers was also investigated. Carbon fibers with higher strength were obtained with faster velocities, i.e.20-40m/h; but the modulus was enhanced with slower velocities, i.e.2.0-7.5m/h. The results of tension effect suggested that the modulus was strongly depended on the tension carried out during low-temperature carbonization, and the strength was influenced significantly by the tension applied during high-temperature carbonization.(4) The microstructures and mechanical properties of PAN and mesophase pitch (MPP)-based carbon fibers were investigated during graphitization in the temperature range of1300-2700℃. With temperature increasing, the Young’s modulus for both PAN and MPP-based carbon fibers increased due to the growth of crystallites and the increment of preferred orientation. The tensile strength of PAN-based carbon fibers decreased as temperature increased. This happened because entanglements of carbon ribbons and cross-links of covalent bonds, which provided high shear modulus between layer-planes, were greatly reduced and the microvoid defects were increased with noncarbon elements escaping, crystallites twisting and dislocating. For MPP-based carbon fibers, the tensile strength was improved with temperature increasing. The reasons were that the shear stress between layer-planes was enhanced with the crystallite dimensions and preferred orientation increasing, the microporosity monotonously decreased, and fewer additional crystallite defects derived from the breaks of physical entanglements and chemical cross-links were introduced into fibers during graphitization.