Ncdtc-artigo-2013-2946

EXTRACTIVE STABILIZATION OF MESOPHASE PITCH FIBER\nYANG DUK PARK and ISAO MOCHIDA\nInstitute of Advanced Material Study, Kyushu University, Kasuga Fukuoka 816, Japan\nand\nTADAYUKI MATSUMOTO\nToray Gisal Co. Sonyokusa, Oita, Shiga 520, Japan\n(Received 8 September 1987; accepted 12 November 1987)\n\nAbstract—Extraction of coal or pitch-based mesophase pitch fiber (diameter 30 μm) was studied with a view to improving efficiency of its stabilization in pitch fiber production. The extraction with THF for 6 h solvate stabilized the fiber sufficiently to maintain the fibrous form in the carbonization stage with an extractive stabilization, although very certain fiber surfaces formed cracks parallel to the fiber axis. Extraction with hexane shortened the oxidation, which was required for sufficient stabilization with fewer cracks in the carbonization. In contrast, extraction with hexane showed influence on the stabilization reactivity. The removal of soluble fractions in the mesophase pitch may eliminate its finishing temperature, decreasing the extent of oxidation required for the stabilization, or raise its softening temperature, enabling stabilization at a higher temperature. Although the resultant carbon fiber was not excellent in its properties and the extraction took a long time at present as far as the fiber thickness 30 μm is concerned, a thinner fiber (10 μm) required a much shorter extraction time at the same temperature and less oxidation for the complete stabilization.\n\nKey Words—Mesophase, pitch fiber, stabilization, coal tar pitch. Table 1. Some properties of mesophase pitch\n\nH C N Ash BS THF S P(C) Anisotropic content (vol%)\n3.9 94.1 1.0 0.95 15 34 42 290 >98\n\nThe spun fiber was extracted in a socket for 6 h. The extracted pitch fiber was, if necessary, oxidized up to 337°C in atmosphere at a heating rate of 5°C/min to the oxidation temperature.\n\nThin fiber (10 μm) was also obtainable by selecting the spinning conditions. It was extracted for 30 min at room temperature. The extracted fiber was, if necessary, further treated oxidatively.\n\nThe extractor or oxidized fiber was further carbonized at 1200°C for 0.5 under nitrogen in a horizontal electric furnace. The heating rate to the carbonization temperature was 10°C/min. 1.3.1 Scanning electron microscopy of pitch fiber\n\nScanning electron micrographs of mesophase pitch fiber are shown in Fig. 1. The diameters of the fibers ranged from 10 μm. Both longitudinal and transverse (after fractured) surfaces of the mesophase pitch fiber were very smooth, although some striations running parallel to the fiber axis were observable in the fiber surface.\n\nFig. 2 shows scanning electron micrographs of the pitch fiber filaments extracted with benzene for 6 h. Most filaments showed a smooth surface (2a), whereas in few filaments a crack was found running parallel to the fiber axis (2b). A probe-type congealed parallel to the axis (2c) was left on the surfaces of another few filaments. The diameter of the extracted pitch fiber shrank by 5 μm from that of the as-spun fiber. Fig. Optical and SEM micrographs of carbonized fibers after extraction with benzene (A) and THF (B).\n\nprocess time for the stabilization before carbonization are the most critical problems to be solved. 1, 26) There are several approaches to solve the problem such as chemical modification of the mesophase pitch components, utilization of stronger oxidants, and activation of oxidants, although the diffusion of oxidant may also influence the rate of stabilization. 27-29).\n\nThe present study proposed a solvent-extraction procedure of the pitch fiber to eliminate or shorten the oxidative stabilization step with the least deterioration of properties of resultant carbon fibers. The mesophase pitch has been revealed to consist of condensed amounts of solubles in hexane, benzene, THF, and pyridine-insoluble (PI) fraction of large molecules. 30) Such soluble fractions tend to fuse to give a thermotropic nature to the whole mesophase pitch by being accommodated among the layers of PI. Thus, the mesophase pitch is defined as a cooperative liquid crystal. 30) The oxidative stabilization should modify, through the oxidative condensation or oxi-\n\nFig. Scanning electron microphotographs of stabilized fibers after the extraction with hexane (A) and benzene (B). Table 3. Mechanical properties of carbon fibers\n\nTensile Young's\nstrength modulus\n(kg/mm2) (m/m2)\n\nHexane-treated 145 16.9\nCF\n\nBenzene-treated 197 20.3\n\nTHF-treated CF Nonmeasurable Nonmeasurable\n\nto satisfy the stabilization left a considerable amount of PS in the stabilized fiber. 34, 35) The extracted fiber was dense and the orientation of its large aromatic molecules is well maintained parallel to the fiber axis with or without the least oxidative disruption. Thus, the mechanical strength of the extracted fiber after carbonization was reduced, or was even improved over that of the original mesophase pitch carbon fiber produced conventionally through extensive oxidation, if the extraction is properly controlled.\n\nThe morphology of the extracted fiber attracts another interest of microscopic change during the extraction. The diameter of most filaments shrank considerably, a few small pores being observable, although some filaments contained some large pores, producing new and large cracks as the evaporation and carbonization stages. The shrinkage according to the radial orientation of those molecules has been reported to be observable by SEM when the soluble fraction distributes homogeneously and disappears by the shrinkage. Nevertheless, such pores may enhance the oxidation through the easy diffusion of oxidant.\n\nFinally, the mesophase pitch fiber principally used in the present study was rather thick and was not particularly suitable for the extractive stabilization. Hence, it is noted that the extraction and the properties of the carbonized fiber afterward are not satisfactory. The radial structure of the present fiber causes some disadvantages such as the formation of cracks. Nevertheless, extractive stabilization looks promising as a means to improving the efficiency of the stabilization step. In fact the thinner fiber (10 μm) was found to require a shorter time for the extraction with benzene and oxidative stabilization. More specified properties suitable for the extractive stabilization can be designed for the mesophase pitch.