Mesophase Pitch Fibers Research Articles

Spinning characteristics of mesophase pitches derived from naphthalene and methylnaphthalene with HF/BF 3

Transverse shape, texture, and the degree of preferred orientation along the fiber axis in mesophase pitch fibers were studied by varying spinning temperature and spinning nozzle to clarify the influences of the structure of mesophase pitch. The mesophase pitch derived from methylnaphthalene (mNP) showed much larger Lc(002) and smaller d 002 than that from naphthalene (NP), indicating larger and better ordered stacking of the aromatic planes in the former pitch, although their softening points, melt viscosities, spinning temperatures, and molecular weight distributions were much the same. MNP provided a higher degree of preferred orientation in the round shaped fiber, which led to an open wedge in the radial texture in a particular spinning temperature range, and larger width/ thickness ( W T ) ratio of tape spun through the slit-shaped spinning nozzle. Addition of NP to mNP in their melt states decreased Lc(002), lowering the degree of orientation and decreasing W T ratio according to the added amount. Such structural factors of fiber and tape are related to the stacking of aromatic molecules in the mesophase pitches. The higher stacking may emphasize the dimensional anisotropy of molecular assembly, influencing the viscoelastic flow of the mesophase pitch in the spinning nozzle. Larger anisotropy may also contribute to the better alignment of molecular assembly along the spinning nozzle wall.

Graphitization of a high-sulfur mesophase pitch-based fiber

Graphitization of high-sulfur content mesophase pitch fibers is complicated by the evolution of sulfur-bearing gases at high temperatures. Removal of these gases leads to appreciable strain and crystal misorientation if large structural units are present. The most obvious result of gas removal is “puffing,” an increase in the fiber cross-sectional area, accompanied by a decrease in lattice-dependent properties such as elastic modulus and conductivity. In this study, the effect of sulfur removal on fiber crystal structure is addressed. X-ray diffraction and differential scanning calorimetry are used to characterize the structure resulting from various heating rates, maximum temperatures, and dwell times. High heating rates during the sulfur removal period (from 1600 to 2000°C) were found to lessen the severity of fiber damage. When followed with a slow, high-temperature annealing treatment, a well-ordered graphitic structure was produced. Finally, a heat treatment strategy is outlined to improve the elastic properties of fibers produced from a high sulfur content mesophase.

Modelling of the isothermal oxidative stabilization of mesophase pitch fibre

Isothermal experiments were conducted to elucidate the rate process of stabilization for mesophase petroleum pitch fibre. The rate of oxygen consumption in the fibres was quantified by measuring the time course of the oxygen concentration in the effluent gas from the reactor. A mathematical model of the stabilization, in which the mass transfer of molecular oxygen and the autoxidation reaction with the active sites in the fibre were taken into account, was also constructed and a numerical simulation of the model equations was conducted. Physical parameters such as the effective diffusivity of molecular oxygen in the fibre, the reaction rate constant and the gas—solid equilibrium coefficient of molecular oxygen, which are included in the mathematical model, were determined by comparing the experimental values of oxygen concentration in the exit gas with theoretical values. Physicochemical processes of the stabilization were also discussed on the basis of the theoretical results. It was concluded that the stabilization is complete even when the reaction does not proceed deeply into the fibre.

Oxidation properties of mesophase pitch prepared by a heterogeneous nucleation method

Mesophase pitches prepared by a heterogeneous nucleation method from various mixtures of coal tar-derived isotropic pitch and petroleum-derived mesophase pitch (MP-P) were oxidatively stabilized, and the dependence of chemical reactivity and stabilization rate on their chemical structure was investigated. The rate and amount of oxygen uptake of the mesophase pitch fibres, revealed by thermogravimetry, increased under given conditions of oxidation with the amount of added MP-P, but the rate of stabilization showed a reverse trend. The lower chemical reactivity of coal tar-derived mesophase pitch (MP-C) appears to induce a slower rate of oxygen uptake, while the higher content of pyridine-insoluble fraction and higher aromaticity of MP-C may result in a lesser content of oxygen being required for stabilization, as compared with MP-P.

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

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.

Modelling and Simulation of the Oxidative Stabilization Process of Pitch-based Carbon Fiber

Oxidative stabilization of petroleum derived mesophase pitch fiber under increasing temperature conditions was simulated through the use of a kinetic model in which the mass transfer of the oxygen molecules and the autoxidation reaction with the active site in the fiber were taken into account. Temperature dependence of physicochemical parameters such as the effective diffusivity of oxygen molecules in the fiber, the reaction rate constant and the gas-solid equilibrium coefficient of oxygen molecules, were introduced into the mathematical model. The validity of the kinetic model and the numerical calculations was ascertained by comparison of the theoretical values forthe outlet concentration of oxygen gas and the rate of oxygen consumption by the fibers with experimental values. The stabilization reaction was completed prior to the entire consumption of activesites in the fiber, and fixed oxygen was considered to be non-uniformly distributed in the direction of the fiber radius.

Structure and properties of mesophase pitch carbon fibre with a skin-core structure carbonized under strain

Coal tar mesophase pitch fibres stabilized at 270° C to different extents were carbonized under strain by the constant load or constant length, using different heating rates, and further graphitized at 2500° C. Shallow and moderate stabilization provided a skin-core structure in the resultant fibres which exhibited higher orientation, tensile modulus, and better graphitizability after calcination at 1300° C and graphitization at 2500° C than deep stabilization. The tensile strength and modulus of the graphitized fibre was significantly improved through the strained carbonization when the stabilization was performed to a moderate extent. The strain tended to give an onion-like alignment in the fibre to improve the preferred orientation of carbon planes. Larger load and more rapid heating during carbonization modified the structure and properties of resultant fibres through a significant longitudinal elongation. The stabilization extent of pitch fibres governs the mobility or fusibility of mesogen molecules at the carbonization which allows their better alignment by the strain.

Domain structure in MP (mesophase pitch)-based fibres

The microstructure of a number of commercially available (Union Carbide, Thornel, or Amoco) mesophase pitch-based (MP) carbon fibres was investigated using light, scanning and transmission electron microscopy. Size, shape, and distribution of structure were examined in longitudinal and transverse sections. Because microtome sectioning methods produced considerable structural damage, a technique was developed for preparing ion-milled transverse sections for TEM. A domain structure is very common, perhaps ubiquitous, in MP fibres. Domains are long, thin needle-like or ribbon-like structural components of fibres that extend up to 100 μm parallel to the fibre axis and may be elongate in transverse section, the longer axis being 0.5 to 3 μm while the shorter axis is typically 0.2 μm. The fabric of domains is identified from textures in transverse polished and fractured sections. Many MP fibres examined have domains arranged in an oriented-core texture. This texture consists of polar zones where layering of domains is chaotic separated by an equatorial zone of layering continuous across each filament. PAC-man morphology can develop in some oriented-core filaments by radial failure in the flat equatorial regions. Not all domains are of the same kind. In dense domains, graphene layers are densely stacked with comparatively little void space, while microporous domains consist of thin walls of graphene sheets enclosing a high proportion of void space. Graphene layers within dense domains are oriented predominantly parallel both to the long axis of the domain in transverse section and to the fibre axis. Graphene layers in microporous domains have no preferred orientation. Dense domains represent former mesophase, while microporous domains represent a phase inferred to have been capable of forming at least some mesophase with generation of volatile substances. In some MP fibres, ordering in dense domains is turbostratic. In others, true graphite with three-dimensional crystal structure occurs, not as a separate domain type, but as a component of the domains described. All domains appear to originate at the time of pitch maturation (mesophase forming) and are considerably modified during spinning. Domains and their distribution influence mechanical properties. Factors such as the state of ordering within domains, volume fraction and distribution of microporous material and presence of folded and kinked layering and of flaws will modify processes like initiation and propagation of fracture and affect properties such as elastic modulus.

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% O 2, 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.

Characterization of Pitch-based Carbon Fibers. Part 1. Carbonization Behavior of Stabilized Mesophase Pitch Fibers.

Stabilized pitch fibers, precursors of high performance carbon fibers, have been prepared on a laboratory scale by spinning from mesophase coal tar pitch and then oxidizing in air at 310°C. Their physical and chemical behaviors during carbonization process (400-2, 300°C) have been investigated by using thermogravimetric analysis (TGA), gas adsorption technique and X-ray photoelectron spectroscopy (XPS). Weight losses of the stabilized pitch fibers were observed in the temperature range from 400 to 800°C, with the maximum rate of weight loss at 450-550°C. However, in the range 800-1, 000°C an increase in the specific surface area took place, which was accompanied with a structural transformation from a rheological state to infusible hydrocarbon polymers designated as coke. The surface functional groups produced by oxidization at 310°C in air disappeared below 600°C. However, the surface oxygen-containing functional groups appeared again when carbonization temperature was raised to 1, 000°C. The C-1s bearing hydroxyl group increased with increasing carbonization temperature from 1, 000 to 2, 000°C. Since such groups decreased their intensity by etching, it is concluded that the surface oxides are produced by oxygen and/or water both of which are contaminants of the stream during carbonization.

Solubility of heat-resisting polymers in mesophase pitches at their spinning temperatures

Solubilities of two thermoresisting polymers (polyethylene telephthalatep-hydroxybenzoate, a liquid crystal polymer and polyethylene naphthalate) in mesophase pitches (MP) derived from coal tar (C1) and petroleum residues (P1 and P2) were examined to prepare blended fibre as the better precursor for the high-performance carbon fibre. MP-C1 and MP-P1 of high aromaticity dissolved 5 wt% of both polymers on mixing at 360 °C for 3 h, maintaining 100 vol% anisotropy which the parent mesophase pitches exhibited, although only MP-C1 did so at 340 °C. In a marked contrast, a number of isotropic droplets from both polymers dispersed in MP-P2 after the blending under the same conditions. The blends of MP-C1 with polymers were spinnable at around 370 °C into fibres of 10 to 20 μm diameter although their spinnability was slightly inferior to that of the pitch alone. The blended fibre exhibited slightly higher stabilization reactivity at 270 to 300 °C than that of the parent mesophase pitch fibre. Structural factors of the mesophase pitch which influence their dissolving ability are discussed. Thermal stability of the polymers is also briefly examined.

The Creation of a Skin-Core Structure in Petroleum Derived Mesophase Pitch Based Carbon Fiber

Abstract Stabilization of a petroleum-derived mesophase pitch (P-MP) fibers was investigated in order to create a skin-core structure in the carbonized fibers. The pitch fibers were either oxidatively stabilized at 270°C in air or 10 vol% O2 for short periods, or oxidized in air for 15 min and successively extracted by tetrahydrofuran (THF) for 30 min. A skin-core structure could be observed in the carbonized fibers under a polarized-light microscope or SEM. The relationship between the oxidative reactivity of P-MP fiber and creation of a skin-core structure, and the effect of solvent extraction for the creation are discussed according to the results.

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

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 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.

Estimation of Fixed Oxygen and Gas Formation Amounts during Oxidation Stabilization Reaction of Spun Mesophase Pitch Fiber

Mesophase petroleum pitch fiber oxidation reaction was analyzed by applying the material balance equations to the time courses data relating to total amounts of oxygen consumed and elemental composition of carbon, hydrogen and oxygen in the fiber.Increase in fiber mass and amounts of water and carbonic gases formed during the reaction were estimated. The fixed oxygen amount in the fiber was in fair agreement with the value obtained previously from van Krevelen plot of elemental compositions.

Rate Analysis of Oxidation Stabilization Reaction of Spun Mesophase Pitch Fiber

Mesophase petroleum pitch fiber oxidation reaction in a gas phase continuous flow isothermal tank reactor was analyzed by a mathematical reaction model which includes the effect of diffusional resistance of oxygen molecule within a fiber as well as autoxidation reaction of oxygen with active site in the fiber.By fitting the calculated oxygen concentration in the reactor with experimental ones, optimal reaction rate constant, effective diffusion coefficient and gassolid distribution coefficient, which were introduced as parameters, could be estimated. Temperature dependence of these parameters was also elucidated.Radial distributions of oxygen and active site in the fiber were obtained by calculations. It can be concluded that stabilization is completed even the reaction does not proceed to the center of the fiber.

Structure of mesophase pitch fibers

The as-spun structure of mesophase pitch-based carbon fibers has been characterized by darkfield electron microscopy of microtomed fiber sections. Three levels of organization have been observed. At the molecular range, the pitch molecules are stacked together creating small coherent domains. At the intermediate range, microdomains are observed, consisting of molecular assemblies with long range orientational order. They are several μm long in the fiber direction, but their lateral extension is limited by wedge disclinations to a few hundred nm or less. Finally, the role of fluctuations of the microdomain density in controlling the degree of very long range statistical orientation over the fiber crosssection is described. The variety of observed textures encompasses the many types eventually observed in the carbonized fibers. The effect of pitch type on the microtexture and texture is demonstrated.

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.

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.

Extractive stabilization of mesophase pitch fiber

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.