Tadayuki Matsumoto

A structural study on oxidative stabilization of mesophase pitch fibers derived from coaltar

Abstract The oxidized fiber of coal-tar based mesophase pitch showed the maximum weight gain at a particular oxidation temperature according to the heating rate to the temperature as the balance of oxygen uptake and decomposition of oxygen functional groups. The graphitized fiber showed the maximum tensile properties whenever the oxidized fiber gained the maximum weight, regardless of the heating rates of 0.5-2.0°C/min. Fourier transform infrared spectroscopy (FT-IR) and x-ray photoelectron spectroscopy (XPS) indicated the dominant presence of the conjugated carbonyl after oxidation at lower temperatures (below 350°C). NMR study revealed that these groups could be triggers for oxygen bridges such as ethers and esters and for dehydrogenative coupling to form the larger molecular weight components in successive carbonization at the elevated temperature. In contrast, more carboxylic acids were introduced after oxidation at higher temperatures (above 400°C) through the cleavage of the hexagonal ring. Such extensive oxidation may reduce the tensile properties of graphitized fibers.

Blending mesophase pitch to improve its properties as a precursor for carbon fibre Part 1 Blending of PVC pitch into coal tar and petroleum-derived mesophase pitches

The blending of mesophase pitch with isotropic PVC pitch was studied to improve their properties as a precursor for carbon fibre. PVC pitch prepared at 420° C which remained almost isotropic was found to be miscible with coal tar-derived mesophase pitch without reducing the anisotropic content and spinnability. The tensile strength of pitch fibres remained unchanged by the blending; however, the reactivity for stabilization was enhanced. The resultant carbon fibres from the blend exhibited slightly higher tensile strength. In contrast, petroleum-derived mesophase pitch failed to dissolve the PVC pitch, leaving a number of isotropic droplets. The structural factors of mesophase pitches with regard to their compatibility with PVC pitches are briefly discussed.

Oxygen distribution in oxidatively stabilized mesophase pitch fiber

Abstract The oxygen profile along the radial direction in mesophase pitch fiber of 10 μm diameter, stabilized by heating at a constant rate of 0.5°C/min in air, was measured with secondary ion mass spectrometry (SIMS), by correcting the atomic O C signal ratio according to that obtained by elemental analysis. The oxygen profile was found to be flat along the radial direction in fibers oxidized to 350°C which showed the maximum weight gain among the stabilized fibers, providing the maximum tensile strength and elongation after the carbonization. When stabilization was stopped at a lower temperature of 250°C, the oxygen profile indicated an unoxidized core, from which volatile components may diffuse to the surface, causing sticking of adjacent filaments together in the successive carbonization. The heating rate during stabilization was also found to influence the oxygen distribution, more rapid heating tending to increase the gradient. Such a situation indicates that a balance of oxygen diffusion and oxidation reactivity governs the oxygen distribution in the stabilized fiber. Hence, SIMS can be an appropriate tool to monitor the stabilization step in carbon fiber manufacture.

A structural study on the stabilization and enhancement of mesophase pitch fibre

The components of coal tar-derived mesophase pitch fibre and its blend with polyvinyl chloride (PVC) pitch were studied for chemical changes after the stabilization. Microanalyses, solubility and solid 13C NMR measurements were performed. The temperature was found to be very influential on the progress of the stabilization. At a temperature of 230° C, PVC pitch enchanced the oxygen uptake of both fusible pyridine soluble (PS) and non-fusible pyridine insoluble (PI) fractions in the pure mesophase pitch, so shortening the time required for complete stabilization and raising more rapidly the softening point of the PS fraction. More oxygen-containing functional groups, such as phenolic, ether, carboxylic and carbonyl groups, were formed in both fractions. It is noted that any increase in the aromatic ring size of the PI fraction is rather limited at this temperature. In contrast, stabilization of PVC pitch at a higher temperature of 300° C, accelerated the increase in PI without accelerating oxygen uptake of both fractions. Hence, the softening point of the remaining PS was unchanged or even lowered. An increase of aromatic ring size of the PI component by stabilization was marked at the higher temperature. Suggested stabilization schemes and the role of added PVC pitch in accelerating stabilization are discussed for each of these temperatures taking account of the above results.

A microscopic study on the oxidative stabilization of a coal-tar-based mesophase pitch and its blends with PVC pitch

Coal-tar-based mesophase pitch and its blends with PVC pitch at 5 or 10 wt% were oxidatively stabilized at 230, 270 and 300°C for variable periods to clarify the progress of stabilization and the effects of the blending with PVC pitch on the stabilization reactivity. PVC pitch which was prepared from PVC by heat-treatment at 420°C for 2h enhanced the stabilization reactivity of whole pitch fibres to shorten the stabilization time to a half of that for mesophase pitch alone. PVC pitch carrying considerable amounts of aliphatic components and large molecular weight may initiate, as a trigger, the stabilization reactions of mesophase constituent molecules. Carbonized fibres of 30μm diameter after stabilization at 270 and 300°C exhibited a skin-core structure, while fibres of 10μm diameter showed no skin-core structure, indicating a homogeneous progress of stabilization in the radial direction of the latter fibres. Lower stabilization temperatures provoked no skin-core structure even in the thick fibres. The rate of core diminishing became relatively slower in the later stage of the stabilization, even when the reactivity of the pitch fibres was enhanced by blending with PVC pitch and using a higher stabilization temperature. The diffusion of the oxidant and stabilization reactivity of the pitch fibres are discussed comparatively.

Control of molecular orientations in mesophase pitch-based carbon fibre by blending PVC pitch

Coal tar-derived mesophase pitch and its blends with PVC pitch in 5 or 10 wt% were spun at temperatures from 340 to 390° C by applying pressurized nitrogen. The parent mesophase pitch and the blended pitch showed an excellent spinnability at temperatures from 360 to 380° C and from 350 to 380° C, respectively, to give a thin pitch fibre of 10μm diameter. The transverse texture of the fibres from the parent mesophase pitch showed the radial orientation regardless of a higher spinning temperature of 390° C. In contrast, those from the blended pitches showed random orientation even at the lower spinning temperature of 350° C. The amounts of the blend extruded by spinning at each temperature under 0.2 kg cm−2 G−1 were always larger than those of the mesophase pitch. It is clarified in the present study that blending PVC pitch can realize stable spinning at lower temperatures, where the molecular orientation in the transverse section of the resultant carbon fibre was controlled through decreasing the viscosity of the whole mesophase pitch.

Extractive stabilization of mesophase pitch fiber

Abstract Extraction of coal tar pitch-based mesophase pitch fiber (diameter 30 μm) was studied with a view to improving efficiency of its stabilization step in carbon fiber production. The extraction with THF for 6 h in soxhlet stabilized the fiber sufficiently to maintain the fibrous form in the carbonization step without any oxidative stabilization, although every resultant fiber suffered some cracks parallel to the fiber axis. Extraction with benzene shortened the oxidation, which was required for sufficient stabilization with fewer cracks in the carbonization. In contrast, extraction with hexane exhibited no influence on the stabilization reactivity. The removal of soluble fractions in the mesophase pitch may eliminate its fusibility completely, decrease 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 thick fiber which was examined, a thinner fiber (10 μm) required a much shorter extraction time (with benzene 30 min at room temperature) and less oxidation for the complete stabilization.

Rapid stabilization of mesophase pitch-based carbon fibre by solvent extraction and successive oxidation

Pitch-based carbon fibre of high performance (PBCFHP) has been recognized as a strategic material because of its excellent strength and Youngs modulus per unit weight [1]. Although its manufacturing principle has been established [2-5], in practice several problems are still being encountered in reducing its cost to an acceptable point for wide application [6]. Stabilization of the mesophase pitch has been performed by the air-oxidation of pitch components [7-10]. The rate is very slow because of the low reactivity of highly aromatic constituents at a temperature which is restricted by the softening point of the pitch [10-14]. It was recognized that the mesophase pitch consisted of a complex mixture of aromatic hydrocarbons which exhibit thermotropic liquid crystal natures in a certain temperature range [15]. Blending of foreign substances or extraction of its fraction is expected to modify its properties [11-13]. In the present study it was intended to accelerate the stabilization of mesophase pitch fibre (diameter 10#m) derived from coal tar, by combining the solvent extraction with the oxidative stabilization. Extraction of some soluble fractions can raise the softening temperature of the fibre and/or leave heavier fractions of facile stabilization. Deep extraction alone could provide sufficient stabilization of the fibre, as reported in a previous paper [16], but it tended to introduce many defects in the resultant carbon fibre. Thus, the least extraction which raises the softening temperature and enhances the reactivity of the fibre skin can be followed by air-oxidation to complete the stabilization of the whole fibre in a shorter time without deterioration of the properties of the carbon fibre. Coal tar-based mesophase pitch was prepared from a QI free coal tar pitch after hydrogenation with a catalyst. Some properties of the mesophase pitch (C 94.6%, H 3.9%, N 0.8%, O (diff.) 0.7%, H/C 0.50) TABLE I Some properties of mesophase pitch