Preparation And Properties Of Ethylene Tar Derived Pitch Based Carbon Fibers

Carbon fibers are widely used in military,industrial and sporting goods due to their high mechanical properties,low density,good electrical and thermal conductivity,as well as high temperature and corrosion resistance.Isotropic pitch has obvious advantages in achieving low-cost manufacturing and application of carbon fibers due to its simple modulation process and low price.Therefore,in this paper,ethylene tar was selected as raw materials because of its high-purity and low-cost,and simple atmospheric distillation and air blowing methods were performed to prepare isotropic pitch-based carbon fibers,which is expected to provide a certain theoretical basis and technical support for the low cost and high properties carbon fibers.Based on this,this dissertation focused on the different molecular structure characteristics of the pitch precursors prepared by two different pitch preperation methods,atmospheric distillation and air blowing.The influence mechanism of the difference in physical properties of pitch precursors on the structure and performance of carbon fibers were explored.The evolution of the oxygen content and functionality in the pitch precursors and its effect on the fiber properties was described.The main contents and conclusions of this thesis were as follows:?1?Isotropic pitch-based carbon fibers were prepared through melt spinning,stabilization and carbonization of ethylene tar-derived pitch obtained by atmospheric distillation.The effects of atmospheric distillation conditions on the properties of the prepared ethylene tar derived isotropic pitches were investigated,and suitable distillation conditions were established.The results showed that the spinnability of pitch was poor and fusion of fibers occurred during carbonization if there was much light component in pitch.Meanwhile,excessive distillation would lead to the formation of mesophase,which could deteriorate the spinnability of pitch drastically.Therefore,the ethylene tar-derived isotropic pitch with high softening point and excellent spinnability should be prepared under the condition of atmospheric distillation at 380oC for 6 h.The tensile strength and modulus of the resultant carbon fibers were up to 670MPa and 42 GPa,respectively.?2?Three isotropic spinnable pitches?P200AT,P250AT and P300AT?and carbon fibers were prepared through air-blowing of different basic pitches from the distillated ethylene tar at 200,250 and 300oC,respectively.The oxidation behavior of basic pitches and the influence of oxygen composition in spinnable pitches on their spinnability and structural properties of their carbon fibers were investigated.The results showed that the basic pitches performed different physical characteristics and oxidative reactivity with increasing distillation temperature.P250AT showed higher softening point,carbon yield,oxygen content and toluene insoluble fraction than P200AT and P300AT.The higher oxygen content in P250AT was mainly in the form of C=O group than that of P200AT and P300AT,which would contribute to more cross-linked molecular structure in P250AT.Moreover,the introduced oxygen in pitch precursor could decrease the oxygen uptake of pitch fiber during stabilization process and reduce the gas release of stabilized fiber during the carbonization process at low temperature,which would lead to acquire the higher tensile strength of carbon fiber.Thus,the tensile strength of P250AT-CF was increased to about 980 MPa.?3?Two isotropic oxidized and unoxidized pitches were prepared by optimized air blowing and nitrogen distillation methods using ethylene tar as a raw material.The differences in structures and properties of the resultant pitches and the stabilization behavior of their pitch fibers were examined.The results showed that the introduction of oxygen by the air blowing method could quickly increase the yield and the softening point of the pitch.Moreover,the air-blown pitch?ABP?was composed of aromatic molecules with linear methylene chains,while the nitrogen-distilled pitch?NDP?mainly contained polycondensed aromatic rings.This is because the oxygen-containing functional groups in the ABP could impede ordered stack of pitch molecules and led to a methylene bridge structure instead of an aromatic condensed structure as in the NDP.Meanwhile,the spinnability of the ABP did not decrease even though it contained2.31wt%oxygen.In contrast,the ABP had narrower molecular weight distribution,which contributed to better stabilization properties and higher tensile strength of the carbon fiber.The tensile strength of carbon fibers from the ABP reached 860 MPa with fiber diameter of about 10?m,which was higher than the tensile strength of 640 MPa for the NDP-derived carbon fibers.?4?Two ethylene tar-derived isotropic pitch-based fibers were prepared from nitrogen distilled pitch?NDP?and air blown pitch?ABP?,respectively.The resultant fibers were subsequently carbonized at different temperature in order to investigate the effect of oxygen-introduced pitch precursor on the properties and structure evolution of fibers during carbonization and graphitization process.The structure and properties of NDP-derived carbon fibers?N-CFs?and ABP-derived carbon fibers?A-CFs?were characterized and their differences were discussed in this work.The results showed that the tensile strength of A-CFs was higher than that of N-CFs below 1200oC because more gas was released for N-CFs during carbonization at lower temperature.However,the mechanical properties of both A-CFs and N-CFs decreased drastically at 1400oC,this resulted from the defects produced by the removal of oxygen in the form of gas.Above 1400oC,the mechanical properties increased slightly,the interlayer spacing d₀₀₂ decreased with increasing heat treatment temperature for both two series fibers as a result of the rearrangement and development of microcrystalline,and A-CFs performed lower Young's modulus and higher electrical resistivity than N-CFs,because the cross-linked structure in oxygen-introduced pitch precursor impeded the orientation of its fiber during carbonization.Therefore,A-CFs exhibited higher strength than N-CFs at lower carbonization temperature and maintained tensile strength of 350 MPa as well as higher electrical resistivity of about 40??·m at high temperature.In a word,low cost and high performance isotropic pitch-based carbon fibers could be prepared from ethylene tar,which was of great significance for expanding the application field of carbon fiber.