Effects Of PAN Fiber Diameters On Stableilization Process,Structurarl And Mechanical Properties Of Carbon Fibers

Recently, polyacrylonitrile (PAN) fibers are becoming one of the most important precursors for the preparation of high performance carbon fibers, and the quality of the PAN fibers is the key factor to influence the properties of the final carbon fibers. At present, the quality of domestic PAN fibers has been greatly improved, the performance of PAN based carbon fibers is equivalent to that of foreign general grade products, but the stableility of the carbon fibers products still needs to be improved. In this paper, the X-ray diffraction analyzer (WAXD), density gradient column and differential scanning calorimetry (DSC) were used to characterize the structure and thermal properties of five 12K PAN fibers with different diameters. We aimed to compare the effects of diameter differences on the microstructure and activation energy of PAN fibers. We explored the oxidative stableilization process of PAN fibers and characterized the structure and properties of PAN oxidative stableilization fibers and carbon fibers by DSC, density, infrared spectrum analysis (FTIR), element analysis (EA), WAXD, mechanical property, scanning electron microscopy (SEM) and Raman spectroscopy (Raman). We had researched the evolution of chemical structure of PAN fibers during oxidative stabilization process and the effect of oxidative stabilization temperatures on the mechanical properties of carbon fibers. Systematically, we studied the effects of changes in the time of oxidative stableilization middle and late period on the chemical reaction rate and the degree of oxidative stableilization of PAN fibers. Two kinds of PAN fibers with different diameters were treated by oxidative stableilization and carbonization under two different conditions. We were in order to explore the influence of PAN fibers'on chemical reaction rate in oxidative stableilization process, the degree of oxidative stableilization, the dependence of the oxidative stableilization time, the structure and properties of the final carbon fibers.The results showed, the difference of diameters had impacts on structure and properties of PAN fibers, the microstructure and activation energy changed regularly with the decreasing of the diameter. The crystallinity and the bulk density increased, the grain size decreased, and the activation energy of chemical reaction gradually became larger. The decrease of diameter was good for improving the regularity of PAN fibers' molecular chain, and the microstructure was more uniform. Relative to the larger diameter, PAN fibers with thinner diameter owned higher activation energy of chemical reaction, due to its relatively regular molecular chain. They need a higher oxidative stabilization starting temperature to initiate cyclization reaction.The termination temperatures were almost the same.The oxidative stableilization process of PAN fibers could be divided into two stages:the stage of cyclization reaction and the oxidation reaction stage. The cyclization reaction was more intense at 220-250?, and the relative cyclization degree increased rapidly. After 250 ?, cyclization reaction became slow and PAN fibers mainly happened oxidation reaction. Oxygen element took part in tautomerism process of functional groups. The initial and final oxidative stableilization temperatures had great influence on the chemical reaction degree of PAN fibers and the mechanical properties of the final carbon fibers. The effect of temperature change during the middle of oxidative stableilization process on the mechanical properties of carbon fibers was small. The cyclization reaction rate became slow and the aromatization reaction quickened of PAN fibers in the late stage of oxidative stableilization process. Long time oxidative stableilization would lead to excessive oxidation of PAN fibers, causing structural defects of carbon fibers. It was not conducive to the improvement of the structure and properties of carbon fibers. With the decrease of oxidative stableilization time, the d002 of carbon fibers decreased, the size of Lc and La increased, the porosity decreases, the structural regularity of carbon fiber and the degree of graphite were higher. For experimental PAN fibers, when the late oxidative stableilization time controlled in 30 minutes, the density of oxidized fibers was 1.3745g/cm3 and the content of oxygen element was 9.8%. After carbonization process, the carbon fibers' density reached 1.7913g/cm3. The tensile strength of carbon fibers had reached 5.12GPa.Under the same oxidative stableilization condition, the PAN fibers with smaller diameter possessed a larger rate of cyclization reaction, and the oxidative stableilization reaction was more adequate. The carbon fibers made by PAN fibers with smaller diameter owned smaller interplanar distance and larger crystal size. The regularity degree of the carbon network and graphitization were higher, the porosity caused by hole defects existing in the surface and internal of carbon fibers caused was lower, and the tensile strength was larger. The PAN fibers with larger diameter required a longer oxidative stableilization time. Reducing the PAN fibers' diameter could decrease the oxidative stableilization time, and was conducive to the preparation of PAN based carbon fibers with high performance.