Hiroshi Toshima

Mesophase pitch catalytically prepared from anthracene with HF/BF3

Abstract Structure and properties of mesophase pitches prepared catalytically from anthracene by the aid of HF/BF 3 were studied to reveal the influences of starting aromatic hydrocarbons. In comparison to naphthalene mesophase pitch, the anthracene pitch exhibited higher yield and slightly higher softening point under milder conditions of preparation. More naphthenic and aryl-aryl bonds with fewer methyl groups were characteristic of the anthracene pitch. Such characterizations are ascribed to the high reactivity of 9,10 positions in the anthracene ring. Although difficulty of spinning reduced the tensile strength of its resultant carbon fiber, its larger molecular units provided higher young modulus, especially at lower graphitization temperature. Lower spinning viscosity was found to give higher modulus of the carbon fiber from the same mesophase pitch at the same graphitization temperature. Molecular alignment at the spinning is briefly discussed.

Oxygen distribution in the mesophase pitch fibre after oxidative stabilization

The oxygen distribution in the transverse section of 30μm diameter mesophase pitch fibres after oxidative stabilization was measured by using EPMA (electron probe X-ray microanalyser) to clarify the progress of the oxidative reaction and diffusion of the oxidant during the stabilization. Oxygen was distributed in shallow gradients regardless of the stabilization time from the surface to the centre of the mesophase pitch (MP) fibres stabilized at 230° C, suggesting sufficient diffusion of the oxidant to the centre of the fibre at this temperature. In contrast, steeper gradients of distribution were observed in the MP fibres stabilized at 270° C although oxygen up-take of the centre increased steadily with the longer stabilization time to decrease the gradient. Much steeper gradients of the oxygen distribution were observed in the cross-sectioned surface of the fibres stabilized at 300° C for 15 and 30min. The gradient became much steeper with longer stabilization, suggesting some barriers in the deeply oxidized zone which may block the oxygen diffusion. The PVC-10 fibres, whose reactivity was enhanced by blending PVC pitch of 10wt%, showed steeper distributions of oxygen after the stabilization at 270° C comparing to those of the MP fibres stabilized under the same conditions. It showed steeper gradient with the longer stabilization time. In conclusion, stabilization at a lower temperature (230° C) allows relatively rapid diffusion of the oxidant into the centre of the MP fibre during rather slow stabilization but, a higher temperature of stabilization (at 300° C) and/or higher reactivity of the mesophase pitch accelerates the oxidation much more rapidly than the diffusion, providing a blockade zone for the oxygen diffusion near the fibre surface. The extensive oxidation may cross-link three dimensionally the mesophase molecules thus allowing no diffusion of oxygen among the molecules. Such diffusion control tends to provide skin-core structure in the carbonized fibre.It should be noted that fibre thinner than 10μm showed no skin-core structure. Diffusion within 5μm from the surface may be rapid under any conditions.

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.

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.

Modification of mesophase pitch by blending Part 2 Modification of mesophase pitch fibre precursor with thermoresisting polyphenyleneoxide (PPO)

Polyphenyleneoxide was blended in amounts of 5 or 10 wt% into petroleum-derived mesophase pitch to reinforce the pitch fibre before the oxidative stabilization to achieve better handling properties. Although polyphenyleneoxide was fusible but hardly soluble in the mesophase pitch even at a spinning temperature of ∼ 350° C, blended pitch could be smoothly spun into pitch fibre 10μm diameter, as could the parent pitch. Fibrous polyphenyleneoxide of less than 1μm diameter was homogeneously dispersed in the pitch fibre, being arranged along the fibre axis. Such fibrous polyp henyleneoxide reinforced the pitch fibre considerably. The fibrous substances at the centre of the fibre disappeared in the carbonized fibre at 1300° C after oxidation at 250° C, although some short ones were observed in the skin region of the fibre, suggesting that polyphenyleneoxide was co-carbonized to be assimilated with mesophase pitch at the centre of the fibre, where the effects of oxidation may be rather limited. The oxidation reactivity and its mechanical strength after carbonization were slightly lower in comparison with those of the parent mesophase pitch.

Comparative evaluation of mesophase pitches derived from coal tar and FCC-DO

Three kinds of mesophase pitches (MPs) derived from FCC-DO (P) and hydrogenated QI free coal tar (QIF) were comparatively evaluated in terms of their spinnability and stabilization reactivity based upon their structural characterizations. MP-P, which is meso-phase pitch from FCC-DO, preserved considerable amount of aliphatic and naphthenic hydrogens to show higher solubility, fusibility and softening temperature of as low as 245 °C in spite of its complete anisotropy. MP-C1 derived from catalytically hydrogenated QIF carried less hydrogen content and smaller molecular weight although its solubility and softening temperature were almost the same to those of MP-P. MP-C2 which was prepared from QIF treated with tetrahydro-quinoline (THQ) showed the least hydrogen content, the lowest solubility and the highest softening temperature of 290 °C. MP-P allowed smooth spinning for much longer time at the temperature from 320 to 350 °C. MP-C1 could be spun at the temperature from 340 to 370 °C, which was much higher than that of MP-P in spite of their similar softening temperatures. MP-C2 showed spinnability at the temperature from 340 to 390 °C, although evolved gases disturbed its smooth spinning at the higher temperature.MP-P showed the highest stabilization reactivity to require the shortest time (120 min) for the sufficient stabilization at 250 °C. Although much longer time of 180 min was necessary for the MP-C1 at 250 °C, a higher temperature of 270 °C accelerated the stabilization reactions to shorten the time to 60 min. MP-C2 showed the least reactivity, requiring 120 min at 270 °C. More aliphatic and naphthenic structure of FCC-DO derived mesophase pitch is related to its superiority as the pitch fibre precursor. The catalytic hydrogenation which can produce naphthenic or aliphatic structure is a better pre-treatment to modify the coal tar as the mesophase pitch precursor.

The introduction of a skin-core structure in mesophase pitch fibers by oxidative stabilization

Abstract It is definite that stabilization conditions such as stabilization temperature, time, and heating rate, as well as partial pressure of oxygen, are very influential in the formation of a skin-core structure in carbonized fibers about 10 μm in diameter, defining the thickness of the skin as well as the orientation degree of the carbon sheets in the core. The higher stabilization temperature and faster heating to stabilization were responsible for the formation of a distinct skin-core structure and wider sheets in the core. A lower partial pressure of oxygen in the stabilization appears to be a key for the introduction of a skin-core structure at conventional stabilization temperatures (230–270°C). Such results suggest that the formation of a skin-core structure originates from the oxygen gradient formed in the stabilized fibers by the competition between the oxidation progress and restricted supply of the oxidant along the radius of the pitch fiber. Such stabilized fibers were carbonized, using a heating rates from 1 to 30°C/ min, with or without strain. The growth of mesophase domains in the core was enhanced by increasing the heating rates to carbonization. Layers of unfolded or onionskin sheets were introduced in the core of carbonized fibers by carbonization under strain.

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.

The introduction of a skin-core structure in mesophase pitch fibers through a successive stabilization by oxidation and solvent extraction

Two-step stabilization, air oxidation, and solvent extraction were applied to introduce a skin-core structure in a coal tar mesophase pitch based carbon fiber. Pitch fiber was first oxidized at 270°C for short periods in air or 10% O2, successively extracted in a Soxhlet for 15 to 300 min with tetrahydrofuran (THF) or benzene, and then carbonized to 600°C at a selected heating rate (from 1 to 10dgC/min). A distinct skin-core structure was introduced in carbonized fibers of about 10 μm diameter. By adjusting the processing variables, one could sensitively control the skin-core structure without any adhesion remaining between the filaments. The extent of oxidation governed the thickness of the skin and the domain size of the core, whereas the extent of extraction prohibited the adhesion by removing the soluble or fusible fractions in the surface layers, fixing the thickness of the skin that had been oxidatively stabilized. The heating rate to carbonization should be selected according to the extent of stabilization, to prevent adhesion and deformation of the carbonized fibers.