Preparation Of Naphthalene Pitch With Controllable Mesophase Content And Structure-property Research Of Large-diameter Carbon Fiber

Mesophase pitch-based carbon fiber is an indispensable engineering material in aerospace and defense industries owing to its high strength and modulus,good high-temperature corrosion resistance,and excellent thermoelectric conductivity.The microstructure of pitch-based carbon fibers is the determining factor of its physical properties,while the fiber microstructure is directly related to the structure and properties of pitch precursors.Thus the regulation of carbon fiber structure and properties can be achieved by modifying the properties of pitch raw materials.In this thesis,naphthalene pitch was used as a raw material,and spinnable pitches with different mesophase liquid crystal contents were prepared by thermal polycondensation method.Pitch-based carbon fiber with a large diameter was obtained by melt-spinning,oxidative stabilization and high temperature carbonization.The thermal polycondensation process of naphthalene pitch was systematically studied and the structure and properties of synthetic pitches were tested and characterized by various methods.In addition,the effects of mesophase liquid crystal content on the structure and properties?including spinnability?were analyzed,and the final structure and properties of the large-diameter carbon fiber were analyzed by microscopic observation,tensile strength and resistivity test,et al.The main results and conclusions are as follows:?1?The temperature,time and pressure of thermal polymerization showed great influence on the formation and transformation of mesophase liquid crystal.In the range of 400⁴40°C,the liquid mesophase crystal content increased with the increase of reaction temperature,and coking phenomenon occurred when the temperature rised to 440°C.Under the reaction conditions of 430^oC,the mesophase liquid crystal was gradually transformed from the small spherical phase to the bulk mesophase phase as the polymerization time prolonged.Furthermore,when the pressure of the reaction system increased,the polymerization rate of the naphthalene pitch was increased and the cracking of aromatic macromolecule was inhibited,which could result in a wide-area structural mesophase phase with better orientation than that done under a condition of normal pressure.The wide-area streamline mesophase pitch with an anisotropic content close to 100 vol.%and a moderate softening point of282°C could be prepared by polymerizing naphthalene pitch at a reaction temperature of 430^oC and a reaction pressure of 4 MPa for 8 h.Naphthalene pitch with different mesophase phase contents?5 vol.%?30 vol.%?50 vol.%?75 vol.%?98 vol.%?100vol.%?could be obtained by suitable preparation methods?direct thermal polycondensation and air oxidation?combined with an adjustment of reaction temperature,polymerization time,et al.And spinnable pitch with softening point ranging from 230 to 290^oC could also be obtained by further pretreatment of naphthalene pitch.?2?The mesophase liquid crystal content has an important influence on the structure and properties of the synthetic pith.As the polymerization degree increased,the softening point,corresponding molecular weight and QI component content as well as mesophase content of synthetic pitch also increased,the liquid crystal phase changed from diffused small spheres to large domain texture,finally formed a wide-area streamlined structure with mesophase content close to 100 vol.%.After proper pretreatment by removing some lighting component,the anisotropic and isotropiccomponents in the synthetic pitch with different mesophase contents could form a relatively stable coexistence system and no obvious phase-separation phenomenon occurred after a static hot settlement for 6 h or during the period of melt-spinning.For synthetic pitch with a lower mesophase content?⁵ vol.%?,the small spherical liquid crystal dispersed in the isotropic component and generated stress concentration during spinning,which led to obviously decreased spinnability than that of isotropic pitch.As for synthetic pitch with 30 vol.%mesophase content,the size of liquid crystal spheres increased and a small amount of coalesced spheresed generated,and the isotropic component was the main phase in green fibers,which showed bad mechanical properties and continuous fiber-collection could not be achieved.However,when the mesophase content exceeds 50 vol.%,the synthetic pitch showed a good spinning effect as the anisotropic component was the continuous phase.The higher the snisotropic content,the bettr the spinnability of the pitch.After sieve-filtration treatment,the spinnability of the synthetic pitch was further promoted,and the continuity and stability of spinning were significantly increased.?3?The liquid mesophase crystal content caused an important influence on the microstructure and physical properties of the prepared carbon fibers with a large diameter of²0?m.Carbon fibers prepared from a low mesophase content?⁵vol.%?had a non-uniform diameter,and the tensile strength was significantly lower than that of the carbon fibers produced from the isotropic spinnable pitch.For the spinnable pitches with mesophase content higher than 50 vol.%,as the mesophase liquid crystal content oincreased,the cross-sectional texture of the corresponding carbon fibers changed from random structure to radiation structure,and the inner molecular arrangement in the carbon fibers gradually became regular,the tensile strength gradually increased,while the axial electrical resistivity significantly reduced.However,the carbon fibers were more prone to generate axial splitting,and the splitting degree significantly increased with the increase of liquid crystal mesophase content in the parent pitch.The tensile strength and axial resistivity of carbon fibers prepared from the synthetic pitch with⁹8 vol.%mesophase content were 840 MPa and 2.61×10^-5?.m,respectively.The spinnability of purified synthetic pitch with a high mesophase content?¹00 vol.%?could be significantly improved owing to the obvious reduction of ash to 170 ppm in the precursor pitch,and the final tensile strength of prepared carbon fibers could be increased to 980 MPa.