The Preparation And The Mechanism Of Stabilization Of Mesophase Pitch Fibers

High-performance mesophase pitch-based carbon fibers have attracted more and more attentions and research because of its high strength, high modulus, low density, corrosion resistance, high thermal conductivity, low thermal expansion coefficient and so on, mainly used in the aerospace, automotive industry and other high-tech fields. The oxidative stabilization process is the key step in the preparation of high-performance mesophase pitch-based carbon fiber, during which the oxidation kinetics is very significant. Some studies have shown that the oxidation weight gain process among appropriate temperatures obeyed first order reaction, and the better oxygen content of oxidized fibers was in the range of 8% to 12%. However, the oxidation of pitch fiber and the removal of functional groups is a dynamic process, therefore, it is very limited to study the oxidation mechanism by only focusing on the weight of oxidized fiber and oxygen content.Therefore, in this work, mesophase pitch fibers were prepared from naphthalene-derived mesophase pitch using the single screw extruder by the method of melt spinning. During the winding process, the spinning oil produced in our laboratory was used to improve the processing performance of the fibers. A series of different oxidative stabilized fibers were made by determining the optimal range of oxidation temperature according to the thermal properties, such as differential scanning calorimetry(DSC), thermogravimetry(TG) and dynamic thermal mechanical analysis(DMA). Based on the mechanism of consecutive reaction, the stabilization mechanism of mesophase pitch fibers was investigated by dividing the oxidation process into the generation and removal of oxygen containing functional groups to form crosslinking structure combining with DMA, Fourier transform infrared spectroscopy(FT-IR) and solid state nuclear magnetic resonance technique(13C-NMR). The results are shown as follows:The pitch fibers with appropriate diameter have been obtained by controlling the spinning temperature, the pump flow and winding speed during the process of spinning. The diameter of melt-spinning mesophase pitch fibers possessed a consistency between measured and calculated values. The spinning oil had an excellent stability and the contact angle on pitch was 20 o lower than on water contributing to removing electrostatic interaction, which was beneficial to the cluster of pitch fiber.For the process of oxidative stabilization, we detailedly studied the oxidation behavior of stabilized fibers treated in different time at 230 and 290 oC respectively, because an optimal temperature range of 230 to 290 oC was determined according to DSC, TG and DMA. The oxidation reaction of pitch fibers was a reflection of weight gain and loss in TG curves and an increase of oxygen content in elemental analysis(EA). It indicated that oxidation was not only a process of the generation of oxygen containing functional groups, but also a process of condensation and removal of the functional groups. Combining with the consecutive reaction mechanism, reaction rate constants of weight gain and loss in oxidation reaction at different temperatures could be obtained and named as k1 and k2. From our results, k1 had a higher value than k2 at lower temperature, with increasing temperature, k2 increased faster than k1. When k2 was higher than k1, the rate of oxidation reaction of pitch fibers was fast but slower than that of the removal of functional groups and cross-linkage, resulting in weight loss. Therefore, the temperature was not suitable to oxidize the pitch fibers. What’s more, with the increase of oxidation temperature, the types and structure of functional groups were changed. Based on the results of FT-IR, 13C-NMR and TG, hydroxyl had transformed into carboxyl gradually and the thermal stability was improved with the increase of oxidation temperature. DMA showed the degree of cross-linkage was increased along with the removal of functional groups. Although the cross-linkage was enhanced further with the increase of temperature and the prolongation of thermostatic time, the mechanical properties of carbon fibers firstly increased up to a maximum value then decreased. It indicated that the purpose of oxidative stabilization was to form the structure of cross-linkage, but could not pursuit the high degree of cross-linkage by excessive oxidation.