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.     

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 m 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.     

Oxygen distribution in the mesophase pitch fibre after oxidative stabilization

The oxygen distribution in the transverse section of 30m 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 10m showed no skin-core structure. Diffusion within 5m from the surface may be rapid under any conditions.     

Promoter action of sulphur on the stabilization of pitch spheres

Elemental sulphur was added into the starting pitch during the preparation of pitch-based spherical activated carbon in order to enhance the stabilization of pitch sphere. Pitch sphere (diameter 0.65–1.0 mm) without adding sulphur needs slow heating rate of 0.5 °C/min, high final temperature of 300 °C and long holding time of 20 h for the successful stabilization in air. While adding elemental sulphur with 2.5–10.0 wt % in total amount into starting pitch decreased the stabilization time significantly, pitch sphere containing 5.0 wt % of sulphur can be stabilized in air very easily at heating rate of 2.0 °C/min up to 270 °C without any holding time, and the successful stabilization time was only 3 h. Pitch molecules reacted with sulphur and some sulphur functional groups, such as C–SH, C–S–C, C=S, O=S=, O=S=O etc., were formed besides the oxygen functional groups under the stabilization condition. All of these sulphur functional groups acted as bridge bonds to make the pitch molecules polymerized so as to high up the softening point of pitch spheres, making the pitch spheres stabilized. Three kinds of sulfocompounds, i.e. H₂S, COS and CS₂ evolved in stabilization process.     

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.     

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 10m 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.     

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.     

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.     

同步氢化/热缩聚法制备中间相沥青的工艺研究

系统地研究了在氢化剂量固定情况下,反应温度与时间对同步氢化/热缩聚法所制得的中间相沥青(MP)性质的影响,并制得了可纺MP。研究表明反应时间同为4h时,MP的软化点和不溶分含量随反应温度的提高而升高;偏光结果显示,低温产物为中间相小球和各向同性基质的混合物,高温产物为连续中间相。反应温度同为410℃时,MP软化点和不溶分含量均随反应时间的延长而显著提高,经历了从中间相小球到小球发生融并,最后形成了马赛克织构的中间相。纺丝性能测试表明,反应温度为410或420℃,反应4h制得的中间相沥青,可以熔融纺丝,经氧化和碳化后制得两组碳纤维。     

Physical characteristics of baked carbon mixes employing coal tar and petroleum pitches

A study of the physical characteristics of baked carbon mixes employing coal tar and petroleum pitches as the binder has been made to explore their relative suitability in the fabrication of carbon products. The study reveals that coal tar pitch is the most suitable binder and a petroleum pitch of the same softening point leads to a carbon product of much inferior characteristics. Raising the softening point of the petroleum pitch from 78 to 150° C or adding 10 parts of carbon black in the lower softening point pitch, increases the density, strength and also the electrical resistivity of the baked carbons. The addition of 10 parts of carbon black to the higher softening point petroleum pitch leads to a carbon product with further improved values of density and strength, which are comparable to those obtained with the coal tar pitch. However, the electrical resistivity also is marginally increased. It appears that a petroleum pitch of high aromaticity may fulfill the requirements expected of a good binder.     

Preparation and structure of mesophase pitch-based thin carbon tape

A mesophase pitch of 100 vol % anisotropy prepared from methylnaphthalene using HF/BF₃ was spun through a slit-shaped nozzle, stabilized in air under strain and carbonized at 1300°C into a very thin slit-shaped carbon tape 1.6 m thick and 14 m wide. Better crystalline orientation of the carbon tape always provided a Bacon anisotropic factor higher by 2% than that of the circular carbon fibre prepared from the same pitch. Excellent mechanical properties of the present carbon tape were obtained. Factors influencing the shape and orientation of the carbon tape were examined in terms of properties of mesophase pitches, spinning temperature, and the extent and strain of stabilization.     

Diamagnetic characterization of carbon fibres from pitch mesophase,pitch and polyacrylonitrile

The room temperature diamagnetic characteristics of carbon fibres from pitch mesophase (PM) have been determined and compared with those of fibres from isotropic pitch (P) and polyacrylonitrile (PAN) both in the as-processed condition and as a function of heat-treatment temperature over the approximate range 1000 to 3000° C. These fibre types show quite different magnetic behaviours indicative of different graphitizabilities which may be ranked PM, PAN, P in decreasing order. Microstructural and X-ray diffraction observations are also consistent with this ranking. Magnetic measurements provide a useful tool for characterization of fibre type and/or processing history.     

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 10m diameter, as could the parent pitch. Fibrous polyphenyleneoxide of less than 1m 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.     

Tailoring structures and properties of mesophase pitch-based carbon fibers based on isotropic/mesophase incompatible blends

Blends composed of isotropic and mesophase pitches in various proportions were used as precursors for carbon fibers. The effects of composition on the morphology of blends, precursor spinnability, transversal texture, and mechanical properties of the final carbon fiber were studied. The blends are binary incompatible phase-separation systems. When the content of isotropic pitch is ≤30 %, it is distributed uniformly in the mesophase matrix as spheres; whereas, complete phase inversion occurs when the content is ≥35 %. Blends containing isotropic pitch in dispersion phase show good spinnability and small-diameter fibers can be continuously drawn. The transversal texture of carbon fibers is transformed from a radial type with a crack along the fiber axis to an intermediate morphology between radial and random type by blending 20–30 % isotropic pitch. The tensile strength of carbon fibers with 20–30 % IPc is 2.5 times as high as the AR-based CF without the reduction of Young’s modulus.     

Some factors for the high performances of mesophase pitch based carbon fibre

The tensile strength and Young's modulus of the carbon fibres prepared from naphthalene derived mesophase pitch were studied by varying the spinning temperature, heating rate and final temperature of the stabilization and graphitization temperatures to find the best properties obtainable from the particular pitch. The heating rate was very influential on the tensile strength of the fibre; a slow heating of 0.5° C min^–1, provided the highest strength, as high as 5 GPa at the optimum final temperature of stabilization. A higher or lower final temperature reduced the strength. Insufficient oxygen uptake or decomposition of oxygen groups at the surface of the fibre could induce defects, reducing the strength. In contrast, Young's modulus of the fibres was rather insensitive to preparation conditions. The carbonization and graphitization temperature influenced the mechanical properties as follows: the strength increased stepwise with the carbonization temperature, whereas the modulus increased sharply with the graphitization temperature. The structural factors most influential differed with these properties.     

Structure and stabilization reactivity of mesophase pitch derived from f c c-decant oils

Structure and stabilization reactivities of three mesophase pitches (MP-1-3) derived from fluid catalytic cracking (f c c)-decant oils were compared to reveal the structure-reactivity correlation in the stabilization reaction, which is the slowest step of the pitch-based carbon fibre production. Analyses of the toluene soluble fraction in mesophase pitches using ¹H-, ¹³C-nuclear magnetic resonance (NMR), infrared and FD-mass spectroscopy suggested that the most abundant molecules in MP-2 and MP-3, which should have the highest and lowest softening points, respectively, consisted of a 7-ring aromatic skeleton with two phenyl groups, molecular weight 478 and a 10-ring aromatic skeleton with a phenyl and four methyl groups, molecular weight 556, respectively. MP-3 exhibited much more rapid stabilization and oxygen up take than MP-2. The analysis of stabilized fibre with Fourier transform-infrared spectroscopy indicated that the oxidative stabilization consisted of the following three steps: (1) oxygen up take to produce functional groups such as carbonyl, phenolic and ether groups; (2) growth of the aromatic ring; and (3) molecular association through hydrogen bonding. The first step of oxidation occurred at substituent alkyl, naphthenic and isolated aromatic hydrogens in the mesogen molecules regardless of the mesophase pitches. The reactivity of the mesophase pitch appears to be correlated to the number of such groups present.     

Texture and graphitizability of carbons made from pitch and phenolic resin mixtures

Benzene-soluble pitch and two types of phenolic resin, resol- and novolac-type, were mixed by using pyridine. After removing pyridine, the mixtures were carbonized at 600° C and then heat-treated at 3000° C. By solvent-mixing, homogeneous optical textures within the carbons were found to be obtained, though mechanical-mixing led to heterogeneity. Changes in optical texture from coarse mosaic to isotropic through fine mosaic were observed in the carbons in the range of resin content between 40 and 60% for the pitch-resol-type-resin system, and between 30 and 40% for the pitch-novolac-type-resin system. The transitions from well graphitized to poorly graphitized carbons were observed on the 3000° C-treated samples in the same range of resin content. In the carbon fibre-carbon composites prepared by using these pitch-phenolic-resin mixtures as matrix, fine mosaic texture in the matrix appeared with a lower content of resin.     

Effect of pressure on the coking yields of coal tar pitches

Pyrolysis of five coal tar pitches with wide ranging characteristics, made from the same coal tar precursor, has been studied under nitrogen pressures of 10⁵, 50×10⁵, 90×10⁵ and 160×10⁵ Pa, at a temperature of 550 °C. The residues were further heat-treated to 900 °C to obtain the ultimate normal (10⁵ Pa) and pressure coking yields of these pitches. The literature states that for pitches with relatively lower softening points the carbonization pressure not only increases the coking yield but also lowers the temperature at which the pyrolysis is complete. This is seen to hold true for the present set of pitches, having a much wider range of softening points. Further, one of the pitches, earlier reported by us to be a good preforming pitch for carbon-carbon composites, gave an ultimate coking yield of 88% on subjection to a nitrogen pressure of 160×10⁵ Pa at 550 °C followed by ambient pressure carbonization to 900 °C. It thus appears that a carbonization pressure of 160×10⁵ Pa for a suitable preforming pitch can act as a reasonably good alternative to the expensive hot isostatic pressure impregnation carbonization technique employed in the production of carbon-carbon composites.     

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.     

Effect of sintering temperature on the characteristics of carbons based on mesocarbon microbeads

A study was conducted to determine the effect of sintering temperature, ranging between 350 and 2700°C, on the characteristics of carbons made from mesocarbon microbeads (MCMB) of size 4–16 m, formed in a coal tar pitch by heat treatment at 420 °C for 2.5 h, and separated by solvent extraction using a tar oil. It was found that the MCMB-based carbon plates heat treated to 1000 and 2700°C possess, respectively, an apparent density of 1.64 and 1.78 g cm-3, a weight loss of 11.5% and 15.3%, a volume shrinkage of 32.4% and 45.0%, an open porosity of 7.6% and 14.0%, a bending strength of 72 and 50 MPa, and an atomic C/H ratio of 30 and 417. The 2700°C heat-treated plates revealed homogeneity and fine isotropic structure.