Study Of PAN-based Carbon Fiber Pre-oxidation Processing

Pre-oxidation is a key element in manufacture of carbon fiber, setting of processing parameters during pre-oxidation is reasonable or not, will make a great impact on the performance of carbon fiber. For different conditions of the precursor fiber and equipment, series of comparative tests are necessary to determine the optimal processing parameters. In this paper, simulation experiments of continuous pre-oxidation are made with pre-oxidation furnace designed and manufactured by our laboratory. The effect of residence time and temperature gradient on composition and structure are also studied by FTIR, XRD, SEM and other characterization methods. The relationship between processing parameters and performance of fibers is also discussed.It is found that, in pre-oxidation process, C= N, CH2 and other structures of PAN molecules consumed by cyclization and dehydrogenation reactions, meanwhile, ladder structure consist of conjugated imine (C=N) and C=C appears. Accompanying with chemical reactions, diffraction peaks of the fibers at 29≈17°and 29≈29°increase at the beginning and then decrease, indicating both stretching and pre-oxidation reactions affect the fiber's crystalline morphology. At low temperature, the stretching is dominant, making the crystallinity increase; At high temperature, the reaction of PAN has spread to the crystal region, and destroy the crystal structure. At the end of pre-oxidation, the two diffraction peak almost disappear, and a new peak at 2 9≈25°shows the formation of new structure similar to graphite. The bulk density of fiber increases with the pre-oxidation and the rate of increasing shows an increasing trend. Tensile strength is increasing before 220℃, and then following by rapid decrease, which is caused by low-temperature phase of drafting molecular orientation, crystallization due.Study of residence time of the pre-oxidation indicates that cyclization degree of fiber is increasing with pre-oxidation time and then remains stable. Pre-oxidation of crystalline regions and amorphous area is different, crystalline region is more slowly than amorphous. But the extension of pre-oxidation time will lead to the diffraction peak at 29≈25°increase continually. It indicates that after the complete of cyclization, crosslinking reaction, continue to heat the fiber can keep the crystal structure's transformation, which also results in increase of density and decrease of strength. Pre-oxidation for very short time can cause significant skin-core structure; skin-core structure disappears when time is long.