Preparation Of Pitch-Based Carbon Fibers And Study On Properties

Pitch-based carbon fiber(PCF), obtained from pitch through refining, spinning, oxidation, carbonization or graphitization, owns so many excellent properties as high strength, high modulus, , thermal and electrical conductivity, heat-resistant, low thermal dilatability, low density etc.. PCF has been arousing considerable attention in recent years not only because of its abundant source and low cost but also its indispensable application in spaceflight and national defence fields.The properties of PCF are to a great extent determined by the internal structure of fiber related with the following factors: First, molecular structure of pitch; Second, the spinning process mainly including spinning temperature, shear force, elongate effect, spinneret structure etc.; Third, thermal-treatment steps, such as oxidation and carbonization program, stress along axis and so on.In this paper, three kinds of pitch as common petroleum pitch, coal tar pitch(isotropic pitch), naphthalene synthesized mesophase pitch(anisotropic) were used. First of all, the rheologic performance of pitch related to temperature and shear rate was investigated by a flat rheometer. The consequence was that the flow ability of pitch increases with higher temperature and stronger shear, and during which pseudoplastic characteristic appeared. The melt process was observed with heat-microscope. There was a wider flat area of low viscosity and stable flow ability for mesophase pitch, allowing it to be spun easily. Comparatively, the spinnability of petroleum pitch was poor, and coal tar pitch in mediate. Although petroleum pitch had the best flow ability, it could not be used to make fiber because of its unstable viscosity. The flow ability of isotropic pitch got worse above 350℃. The flow ability of the mesophase pitch was perfect at 330℃, and turned down over 370℃for self condensation.Mesophase pitch-based carbon fiber(MPCF) and isotropic pitch-based carbon fiber(IPCF) were gotten by melt spinning and post-treatment steps(i.e. oxidation, carbonization and further graphitization). Based on the flow ability analysis, it could be interpreted the consequence that thinner diameter and higher mechanical performance of carbon fibers were obtained from mesophase pitch.In order to investigate the influence of spinning process conditions on the structure and properties of MPCF, two kinds of spinneret with different path structure were chosen to carry out melt-spinning at 320℃and 330℃respectively. The as-spun fibers with different diameters were gained by means of adjusting pressure and winding speed turning out MPCF with various textures and crystallite preferred orientation(P.O.). It could be concluded that strong shear and haul are in favor of molecular orientation along fiber axis; Spinning at a reasonable higher temperature tends to get thinner fibers with lower orientation; Skin-core structure , which result in poorer mechanical property, is rendered to form in large diameter fibers; Tensile strength and modulus increase markedly with P.O. increasing. The tensile strength and modulus increased from 1.001GPa and 91GPa to 1.817GPa and 227GPa respectively when P.O. increased from 77.3% to 81.9%; Radial texture with crystallite layer planes arranging along diametral direction usually produced through shrinkage-path structured spinneret of.In this study, the effects of oxidation, carbonization, graphitization on the structure and properties of CF were investigated. TG analyses were operated in continuous heat-rising process and time-holding process at 250℃. The results indicated that the pitch fiber oxidized at temperature of 220℃～250℃and holding time from 3h to 5h, getting oxygen content of 9%～10% result in better performance carbon fiber.Graphitization of MPCF at 2600℃, promoted the tensile strength and modulus of the carbon fiber from 1GPa and 100GPa to 2.1GPa and 500GPa respectively. Moreover the carbon content reached 99.82%, the P.O. was also increased from 77.3% to 91.7%, the density was enhanced from 1.728g/cm~3 to 2.087g/cm~3 and the texture turned out to be more orderly arranged shown by the crystal layer space decreasing from 3.5618A to 3.3808A.The main effects of preparing high performance PCFs, such as melt-spinning, oxidation, carbonization, were studied in this paper with preparing PCFs and investigating their textures and properties. It was concluded that the structure could be improved and mechanical properties could also be elevated for PCFs by optimizing melt-spinning conditions and choosing reasonable oxidation and carbonization ways.