Study On The Relationship Between Element Content And Structure Properties In The Preparation Of Carbon Fiber

The physical and chemical changes of fibers during preoxidation and carbonization were analyzed by means of different measurement methods including X-ray diffraction (XRD), differential scanning calorimetry (DSC), Fourier infrared spectra (FT-IR), elemental analysis, density and tension testing. The changes of element content were analyzed in the whole process, and the relationship were explored between processing parameter in each stage, fiber structure and the changes of element content.It includes four main parts. First, study the element content changes in the whole process and approach the relationship of the changes and the performance of fibers in each process. Second, study the structural conversion of precursor fiber, preoxidated fiber and carbon fiber and explore the relationship of the structural change and the fiber performance, such as fiber tensile strength, linear density, bulk density, tension and so on. Third, study the effect of the main processing parameters of preoxidation on oxygen element content of preoxidated fibers as well as the structure and performance of the final carbon fibers. Fourth, systematically study the changes of fibers performance during preoxidation and carbonization and discuss the main influential factors of fiber performance.The test results of element content in each stage indicated that the content of S element showed a tendency to descend, especially during the coagulation bath, washing bath, boil stretch bath. Oxygen content was increasing while carbon, nitrogen, hydrogen content decreasing during preoxidation. The carbon content increasing while other elements (O, H, N) contents decreasing during pre-carbonization and carbonization.The change of S element content reflects the change tendency of remnant solvent (DMSO). When DMSO remains too much, the tensile strength of carbon fiber would be impacted. DMSO's residual content of spinning must be controlled under 0.1%. During the stabilization and carbonization, oxygen content increases first and then decreases, which plays an important role just like a bridge. At first oxygen element is combined into fiber to form a thermoresistence-ladder structure during preoxidation, and then released out during carbonization. C content is increasing gradually and up to more than 90% at the end during carbonization. Carbon content of final product is the mainest factor that affects the performance of the final carbon fiber.PAN's linear macromolecular structure converted to a thermoresistence ladder structure in the preoxidation process and fiber structures were almost noncrystal after preoxidation. After carbonization process, fibers with turbostratic structure were obtained. In this conversion process, linear density of fibers had few change during preoxidation, showing a total tendency to descend. But it descended quickly during pre-carbonization and carbonization. The bulk density increased constantly, which meant that fibers compaction get better and better. Fibers still had fairly large tension even without stretch, which said that chemical reaction was severe and fibers contracted constantly in this process. Fiber tensile strength decreased during preoxidation and increased during pre-carbonization and carbonization. It was weakest during preoxidation.Processing parameters of each process have quite important effect on the elment content, structure and performance of fibers. Oxygen content can be influenced by the temperature and time of the preoxidation, and also by precursor's native characteristics such as copolymer component and titre. It also can be impacted easily by fiber's water absorbability. The precursor fibers with different copolymer component were preoxidized and the result showed that improper parameter of preoxidation induced PAN fiber not to be preoxidized thoroughly and impacted the performance of the final carbon fibers. Both the temperature, the time of preoxidation and the titre of PAN fiber could impact the oxygen content of the preoxidation fiber: the higher the temperature, the longer the time and the thinner the fiber, the higher the oxygen content of preoxidation fiber. At the end, we approached the effect of preoxidation fiber's water absorption on the oxygen content.In the preoxidation process, oxygen content can reflect the preoxidation level of PAN fibers in some degree. Improper oxygen content can affect the structure as well as the performance and quality of final carbon fibers. When the oxygen content is too low, preoxidation cannot be carried out thoroughly and no high-strength carbon fibers will be produced. But if it's too high, some oxygen will be released by means of CO or CO₂ remaining molecular-degree space, which will decrease the tensile strengtch of carbon fiber.To observe the change of fiber's structure and performance in different preoxidation phase and approach the main influential factors, fibers of 10 temperature zone at different preoxidation level have been studied. The main performances of fiber during preoxidation were analyzed, including the linear density, bulk density, oxygen content, the change of mechanical property, degree of crosslinking and also the change of chemical structure and thermal property. During preoxidation, density, oxygen content, degree of crosslinking were increasing while fiber strength decreasing. Two diffraction peaks of PAN fiber at 20=17°and 29°get decreasing gradually to disappear at the end, and a new, identical but wider diffraction peak appeared at 29=25.5°. Exothermic peak changed from sharper to milder gradually, and the peak move to the higher temperature.Finally, two experiments were designed to study the effect of temperature and stretch multiple on the fiber performance in the pre-carbonization process. In this process, as the temperature was getting higher, fiber becomed shrinking, the linear density decreasing and the bulk density increasing. There were also changes of element content that nitrogen, hydrogen and oxygen contents get decreasing gradually while carbon content increasing. Stretch multiple had little effect on the linear and bulk density of fiber in this process. In some extent, tension increased with the stretch multiple and the carbon fiber's tensile strength also increased correspondingly. But over stretch would decrease carbon fiber's tensile strength.