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.     

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.     

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.     

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.     

Modification of precursor pitch for general performance carbon fibre by blending naphthalene-derived isotropic and mesophase pitches

Modification of an isotropic precursor pitch (IMP) for general performance carbon fibre (GPCF) was carried out by blending isotropic and mesophase pitches (IP and MP, respectively), both of which were synthesized from naphthalene using HF/BF₃ as catalysts because they carried a large amount of naphthenic hydrogens. Both IP and MP were miscible with IMP at temperatures higher than their softening points. The softening point of the blended pitch dropped remarkably, being lower than 200° C with 40% IP. Such lower softening points made spinning much smoother. Modification also enhanced the stabilization reactivities of pitch fibre, the time required for the sufficient stabilization at 270° C decreasing by 20 and 30 min by blending with 20% IP and 20% MP, respectively. The mechanical properties of the resultant carbon fibres were also improved.     

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.     

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.     

Preparation of anisotropic mesophase pitch by carbonization under vacuum

Anisotropic mesophase pitch of high solubility and low melting temperature was prepared from petroleum pitch by carbonizing under vacuum conditions at 430° C for 5 h. The solubility and melting temperature were 55 wt% in quinoline and 270° C respectively, at complete development of anisotropic mesophase pitch. This solubility is much higher than that of mesophase pitch from the same feedstock using refluxing and gas blowing conditions. Yields of the mesophase pitch of complete anisotropic development were 60 wt% under vacuum. The structure of these mesophase pitches were analysed in terms of preparative procedures which create high solubility of a completly anisotropic material.     

Interaction among fractions in mesophase pitches derived from AlCl3-modified ethylene tar

The anisotropic development is studied of mesophase pitches prepared from modified ethylene tar (ETP) using AlCl₃, of fractions separated by benzene, tetrahydrofuran (THF) and pyridine, and of mixed fractions, to find structural factors affecting fusibility and optical anisotropy of mesophase pitch. Annealing was carried out at 360° C for 10 min. Each fraction developed a unique optical anisotropy quite different from that of the parent mesophase pitch. The lightest fraction (soluble in benzene) was highly fusible with small numbers of small anisotropic spheres. The heaviest fraction (insoluble in pyridine) was infusible and exhibited a total mosaic anisotropy. In contrast, mixed fractions behaved like the parent mesophase pitch in terms of liquid crystal behaviour. The extent of anisotropy and fusibility after annealing were strongly dependent on the preparatory conditions of the parent mesophase pitch. The fusibility of mixed fractions is ascribed to the dissolving ability of the fusible fraction and the solubility of the infusible fraction at the annealing temperature. Small molecules in the lighter fractions also contribute to anisotropy when they are located in interlayer positions between the larger aromatic molecules which constitute liquid crystals. Such co-operative properties of constituent molecules of the mesophase pitch can be described in terms of a practical compatibility.     

Carbonization and graphitization of meso-carbon microbeads prepared by the emulsion method

Meso-carbon microbeads (MCB) prepared from coal-tar and fluid catalytic cracking (FCC)-decant oil pitches by an emulsion method were carbonized and graphitized. When using the thermosetting process as a pretreatment, the weight gain of MCB by oxidation was approximately 5 wt%, and Fourier transform-infrared measurement showed that the oxidation occurred mainly at aliphatic components in MCB. In the course of carbonization, a heat treatment at 700°C eliminated the C-H groups and the oxygen-containing functional groups, which had remained partly at 500°C. MCB were fairly graphitizable, considering the high values of obtained from X-ray diffraction measurement. The surface of MCB, which had been originally smooth, became slightly irregular during carbonization, and deformation in the shape of MCB was observed after the graphitization process. These are due to the anisotropic growth and shrinkage of the mesophase structure.     

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.     

The effects of pressure on the carbonization of pitch and pitch/carbon fibre composites

Three pitches which give carbons of varying optical texture have been carbonized singly and with different carbon fibres at pressures in the range 0.1 to 200 MPa. The effect of pressure on the carbonization system is to retard growth and coalescence of the growth units of mesophase, thus reducing the size of the optical texture of the resultant carbon. With increasing pressure botryoidal (spherical) structures are formed. On co-carbonization of pitches with carbon fibres the alignment of the basal planes of the matrix carbon parallel to the fibre length at the fibre/matrix interface is improved within a given pressure range. This range is dependent upon the parent pitch used and is experimentally determined. This effect is seen for all fibre types.     

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.     

Structural changes during pitch-based carbon granular composites carbonisation

This article deals with the study of carbon composites behavior during their carbonization. Composites were prepared using four granular carbons (graphite, anthracite, green petroleum coke, and foundry coke) and four pitches (a commercial impregnating coal-tar pitch, an air-blown and two thermally treated pitches). The evolution of the optical microstructure, porosity, volume, and weight of carbon composites was monitored at different intermediate carbonization temperatures (350, 500, 700, and finally 1000 °C). The porosity of composites increases with carbonization due to volume changes and weight loss of pitches. Weight loss of carbon composites during their carbonization mainly depends on the pitch characteristics and it was slightly influenced by the presence of granular carbon. On the other hand, carbon composites with the commercial coal-tar pitch and foundry coke, anthracite, or graphite deform in the initial stages of carbonization (<350 °C) probably due to the lower porosity of the green pellets and the high amount of low-molecular weight compounds of the pitch. Carbon composites with green petroleum coke underwent important dimensional changes during their carbonization, expanding initially and then shrinking at temperatures above 700 °C. The type of granular carbon strongly influenced the microstructure of the final carbon composite, as a result of its effect on the development of mesophase. Graphite, anthracite and foundry coke delays mesophase development, whereas green petroleum coke accelerates mesophase formation.     

Formation of mesophase spherules in low-Ql coal tar pitches and development of monolithic carbons therefrom

Three coal tar pitches having different and low contents of primary quinoline insolubles (Ql) were subjected to a series of thermal treatments at a predetermined temperature for different periods of soaking. The development of mesophase, in terms of its size and content in the heat-treated pitches, was studied as a function of the soaking time and content of primary quinoline insolubles in the original pitches. Mesophase spherules, with an average size of about 5 m, formed in one of the coal tar pitches were separated using solvent extraction employing a suitable tar oil. These spherules, also called mesocarbon microbeads (MCMB), after being moulded into small rectangular plates, were carbonized at temperatures of 950 and 2700 °C to obtain fine textured monolithic carbons possessing apparent densities of 1.66 and 1.85 g cm^–3, respectively, at these heat-treatment temperatures.     

The hot compaction of 2-dimensional woven melt spun high modulus polyethylene fibres

In this paper we describe the production, properties and morphology of hot compacted 2-dimensional woven high modulus polyethylene fibres. The aims of the work were to establish the optimum conditions for production of the compacted woven PE sheets using a combination of mechanical measurements at Leeds and morphological investigations at Reading. This joint approach had proved very successful in a previous study on the compaction of unidirectional arranged PE fibres, where the optimum compaction temperature was established as 138°C, where 10% of the original fibres were melted. Morphological studies clearly showed that the melted material had recrystallised, epitaxially, onto the original fibre backbones, forming a coherent network to bind fibres into a continuous structure. The current studies, using the woven PE material, showed that a higher temperature was needed to fill all the space between the woven polyethylene fibres, and so produce a coherent material. Peel tests, where two layers of cloth are compacted together and then pulled apart, were carried out over a range of compaction temperatures to measure the interlayer bond strength; this increased with increasing compaction temperature. Significantly, reasonable bond strengths were established at the optimum temperature established for the unidirectional samples (138°C measured on the mould or 136°C in the centre of the fibre assembly) which produces 10% melted and recrystallised material, although a higher interlayer strength was measured at higher temperatures where more of the melted phase was produced. Morphological investigations of woven samples with 10% melted material, showed that while the individual fibre bundles were well bonded, not all of the complicated junctions between the fibre bundles in the woven network were completely filled with melted and recrystallised material, and that a temperature 2°C higher than for 1D compactions was probably optimum. The optimum temperature was found to fall very close to the temperature at which complete melting of the fibre occurred.     

Diels-Alder reaction of mesophase pitch insoluble fractions with maleic anhydride

Pyridine insoluble fractions in the mesophase pitches derived from a decant oil (PMP-PI) and naphthalene (NMP-PI) were found to be rendered almost completely soluble in pyridine by the Diels-Alder reaction with maleic anhydride, maintaining their optical anisotropy and molecular association. Their solubility in pyridine reached up to 95 wt % by the reaction at 175–200 °C. Solubilized fractions were analysed to obtain their average structures, suggesting that PMP-PI consists of aromatic nuclei of peri-condensed rings connected through a small number of methylene and aryl-aryl linkages, having a molecular weight (MW) of 1800, and that NMP-PI consists of oligomeric naphthalene with a large number of naphthenic groups (MW 1000). Such structures are basically much the same as those of their soluble fractions, although the molecules in Pls were much larger. The reaction sites in such structures for the Diels-Alder reaction are 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.     

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.