Stabilisation of low softening point petroleum pitch fibres by HNO3

This paper deals with the stabilisation of low softening point pitch fibres obtained from petroleum pitches using HNO₃ as oxidising agent. This method presents some advantages compared with conventional methods: pitches with low softening point (SP) can be used to prepare carbon fibres (CF), the stabilisation time has been reduced, the CF yields are similar to those obtained after general methods of stabilisation, and the initial treatments to increase SP when low SP pitches are used to prepare CF, are avoided. The parent pitches were characterised by different techniques such as diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), elemental analysis and solvent extraction with toluene and quinoline. The interaction between HNO₃ and the pitch fibres, as well as the changes occurring during the heat treatment, have been followed by DRIFTS.     

Molten storage and carbonization characteristics of premium grade coal-tar pitch: Influence of crude tar type and distillation conditions

A growing trend towards the transport and use of molten rather than solidified pitch has caused tar distillers to examine the relative stability of pitches while held in storage tanks. The scope of laboratory storage-stability tests can be extended to higher temperatures as a method of studying the earliest stages of pitch polymerization/carbonization. Some indications of the ultimate performance of the pitch in the end-use process can be gained. Crude tar has a strong influence on the ultimate properties and performance of a pitch. Seven tars have been made into 120 °C Mettler softening-point pitches by two distillation methods to study their subsequent storage and carbonization behaviour. The behaviour exhibited by each pitch allows a judgement on overall ‘reactivity’ and on the sensitivity to thermal conditions in the distillation of each tar. Two distillation schemes were adopted: batch distillation followed by heat treatment; and batch distillation at a pressure which was progressively reduced. The results show that time at elevated temperature has a major effect on the properties and performance of the resulting pitch. Vacuum distillation allows lower processing temperatures. Larger amounts of secondary quinoline-insolubles were formed under the ‘distillation with heat treatment’ scheme. Complete wetting of coke by pitch took place at lower temperatures for the vacuum-distilled pitches. Heat-treated pitches increased more rapidly in softening point and lost more mass when thermally soaked. Distillation under vacuum conditions resulted in significantly modified pitch carbonization characteristics.     

Supercritical fluid extraction of coal-tar pitch: Part 2. Extraction of the lower molecular weight species with aliphatic solvents

Extraction of coal-tar pitches has been carried out with a number of aliphatic hydrocarbons at temperatures between 210°C and 300°C and pressure of 10 or 15 MPa resulting in extraction yields of up to 50%. The toluene-insoluble (TI) content, molecular weight and the rate of mesophase formation of the residual (refined) pitches increase as the extraction yield increases. The molecular weight of the extract also increases with extraction yield. The volatiles produced during mesophase formation from a supercritical hexane-extracted pitch were analysed by NMR and mass spectrometry and were similar to the hexane extract.     

Structural modification of coal-tar pitch fractions during mild oxidation—relevance to carbonization behavior

Modified pitches with softening points of about 175 °C were prepared by air-blowing at 300 °C of coal-tar pitches from different commercial coke-oven tars. The modifications induced by the mild oxidation were monitored using solvent and extrographic fractionation, elemental analysis, ¹H NMR and HPLC. Optical microscopy was used to follow the effect of air-blowing on carbonization behaviour. Low molecular weight cata-condensed PAHs and those with basic nitrogen and hydroxylic functionalities present in extrographic fractions F2 and F4, respectively, are preferentially polymerized pitch constituents. In contrast, peri-condensed PAHs in extrographic fractions F2 and F3, are practically unreactive under the oxidation conditions used. The mild oxidation enhances the tendency of quinoline insoluble (QI) particles to form aggregates in an early stage of thermal treatment, modifying the mode of mesophase development and leading to a non-homogeneous optical texture. The enhanced propensity of QI to aggregation is discussed in terms of structural peculiarities of the parent pitch and possible oxidative polymerization reactions.     

Effect of different oxidation treatments on the chemical structure and properties of commercial coal tar pitch

Different oxidation treatment was used for the increase of the softening point of a commercial coal tar pitch. H₂SO₄, HNO₃, H₂O₂ and air are selected as treatment reagents. These preliminary investigations show that the oxidation treatment of commercial coal tar pitch with different reagents at 160 °C and heat treatment to 250 °C causes considerable changes in the chemical composition of obtained pitches. This leads to increase of TI and QI fraction, and results in considerable increase in the softening point of the pitches. The yield of modified pitches is considerable in the case of treatment with H₂SO₄, H₂O₂, and HNO₃ and lesser in the case of air blowing. The data obtained also indicate some differences in the composition and softening point of pitches obtained after modification with different reagents. These differences could influence the applicability of the obtained pitches in the various areas of carbon material production.     

Carbon-boron-nitrogen alloys from borazarene-derived mesophase pitches

Synthetic routes to new C-B-N-containing mesophase materials from borazarene-type precursors have been explored. Such mesophases have the potential for forming C/BN alloys which could be processed in analogous ways to carbonaceous mesophases, e.g. into cokes, fibres and composites. Carbon-boron-nitrogen-based mesophase pitches have been generated at ambient pressure by pyrolysis of borazarenes in the presence of AlCl₃. The AlCl₃ is essential for mesophase formation in these systems. Pyrolyses of 10-chloro-9-aza-10-boraphenanthrene and 2,2′-bis(dichloroborylamino)biphenyl have both yielded black, optically anisotropic pitches. These precursors form C/BN ceramic alloys which are turbostratic following heat treatment to 1000 °C. The alloys show a clear advantage in terms of their oxidation resistance, compared with that of carbon derived from naphthalene mesophase pitch, despite contamination with AlCl₃, which probably catalyses their oxidation.     

The stabilisation of carbon fibres studied by micro-thermal analysis

Micro-thermal analysis, or local thermal analysis, is a relatively new technique that combines the imaging capabilities of atomic force microscopy with the physical characterisation capabilities of thermal analysis. This paper reports the application of this technique in a study of the stabilisation process of pitch-based carbon fibres. The technique allowed the determination of softening temperature profiles across model large diameter fibres, as a function of the stabilisation degree. The changes in the softening temperatures promoted by the stabilisation were related to the local oxygen content of the fibre measured by electron probe micro-analysis (EPMA). The depth of the stabilised region was also determined. The results obtained indicate that the higher temperatures studied (180-200 °C) promoted a rapid oxidation of the outer region of the fibre, which acts as a surface diffusion barrier for oxygen, longer times being required for full-stabilisation of the fibres.     

Primary carbonization of an anisotropic ‘mesophase’ pitch compared to conventional isotropic pitch

The evolution during carbonization treatments of a 100% anisotropic pitch (pitch C) was compared to that of Ashland 240 (100% γ resins). The anisotropic pitch C results from a gas-sparge preparation leading to a composition of 93% β resins (QS-TI) and 7% γ resins (QS). It is made of a major component (QS-TI), in which droplets (100-300 nm in size) partially toluene soluble are distributed. The physicochemical, textural and microtextural evolutions of the two pitches were studied. During pitch C primary carbonization, anisotropic droplets grow by coalescence, then decompose into Brooks and Taylor mesophase spheres suspended in isotropic drops. These drops develop at the expense of the anisotropic matrix by a continuous regeneration of the small anisotropic droplets which feed the isotropic drops by diffusion process. Then inside these drops, mesophase spheres grow then coalesce and the behaviour of a conventional pitch is restored. These various molecular associations are due to absence of chemical events below 450 °C, leading to the global mass spectrum being constant. At 500 °C the material is homogeneously anisotropic though plastic, the metastable system is destroyed and the evolution of conventional pitches is recovered, i.e. above 550 °C macropores develop up to solidification at 600 °C (semi-coke stage).     

Microstructure of carbon/carbon composites reinforced with pitch-based ribbon-shape carbon fibers

Carbon/carbon composites were prepared with ribbon-shape pitch-based carbon fibers serving as reinforcement and thermosetting PFA resin and thermoplastic pitch as matrix precursors. The composites were heat treated to 1000, 1600 and 2700 °C. Microstructural transformations taking place in the reinforcement, carbon matrix, and the interface were studied using polarized optical and scanning electron microscopy. The fiber/matrix bond and ordering of the carbon matrix in heat-treated composites was found to vary depending on the heat treatment temperature of the fibers. Stabilized fiber cleaved during carbonization of resin-derived composites. In contrast, fibers retain their shape during carbonization of pitch matrix composites. Optical activity was observed in composites made with carbonized fibers; the extent decreases with increased heat treatment of the fibers. Studies at various heat treatment temperatures indicate that ribbon-shape fibers developed ordered structure at 1600 °C when co-carbonized with thermosetting resin or thermoplastic pitches.     

Biopitch-based general purpose carbon fibers: Processing and properties

Eucalyptus tar pitches are generated on a large scale in Brazil as by-products of the charcoal manufacturing industry. They present a macromolecular structure constituted mainly of phenolic, guaiacyl, and siringyl units common to lignin. The low aromaticity (60-70%), high O/C atomic ratios (0.20-0.27%), and large molar mass distribution are peculiar features which make biopitches behave far differently from fossil pitches. In the present work, eucalyptus tar pitches are evaluated as precursors of general purpose carbon fibers (GPCF) through a four-step process: pitch pre-treatment and melt spinning, and fiber stabilization and carbonization. Homogeneous isotropic fibers with a diameter of 27 μm were obtained. The fibers had an apparent density of 1.84 g/cm³, an electrical resistivity of 2 × 10⁻⁴ Ω m, a tensile strength of 130 MPa, and a tensile modulus of 14 GPa. Although the tensile properties advise against using the produced fibers as structural reinforcement, other properties give rise to different potential applications, as for example in the manufacture of activated carbon fibers or felts for electrical insulation.     

A study of pitch-based precursors for general purpose carbon fibres

The isotropic phase isolated from a thermally treated coal-tar pitch was studied as a possible precursor for carbon fibres. Extraction with different solvents was performed in order to increase its softening point and so enable higher stabilisation temperatures to be used, with a significant reduction in time. The extraction conditions were selected studying the softening temperatures of the residues, the results of their thermogravimetric analysis and reactivity in air studied by means of differential scanning calorimetry. The residue obtained with a mixture of 40% acetone-60% acetonitrile was found to be the most suitable precursor for the fibres. The carbonised fibres showed a homogeneous surface and diameter, and had tensile properties comparable to other isotropic fibres described in the literature.     

The influence of primary QI on the oxidation behaviour of pitch-based C/C composites

Three unidirectional C/C composites were prepared from PAN-based carbon fibres and three coal-tar pitches obtained from the same parent coal-tar pitch but differing in their primary QI content. The effect of the amount of primary QI present in pitches on the air reactivity of pitch-cokes and C/C composites was studied by thermogravimetric analysis. The oxidation of pitch-cokes was performed at 10°C min⁻¹ up to 1000°C. The air reactivity of C/C composites was assessed from the weight loss curves obtained under isothermal conditions in the temperature range of 500 to 575°C for 4 h. The topography of the oxidized composites was examined by scanning electron microscopy. The results showed that pitch-cokes were more reactive than carbon fibres because the presence of QI particles increased their reactivity. With the increase of primary QI content in the pitches, C/C composites showed a higher percentage of carbon matrix, with a finer optical texture, fewer microcracks and less closed porosity. The air reactivity of the composites was affected by both chemical and physical factors, the extent of oxidation determining which of these exerted the greater influence. At low conversions the reactivity was decided by both QI content and the optical texture of the matrix. At high conversions the porosity of the composite was the determining factor.     

Preparation of carbon fibers with excellent mechanical properties from isotropic pitches

A novel isotropic pitch composed of linear methylene chains of polycondensed aromatic molecules was synthesized from naphtha-cracked oil through bromination and subsequent dehydrobromination (NB). Isotropic pitch-based carbon fiber obtained from the prepared NB yielded an unprecedentedly high tensile strength and elongation at break of 1500 MPa and 3.2%, respectively, following carbonization at only 800 °C for 5 min. The aromatic components of NB were primarily condensed cyclic compounds containing three and four aromatic rings. In contrast, a pitch prepared by simple distillation (ND), was composed of compounds containing three to six aromatic rings, which carried the tensile strength of carbon fiber by only 700 MPa with the similar fiber diameter. Interestingly, TOF-Mass analysis indicated that the molecular weight of the NB was higher than that of the ND. ¹³C-NMR analyses revealed that the NB pitch contained up to 18.8% aliphatic and naphthenic components compared to the 2.8% found in the ND pitch. Both isotropic pitches exhibited Bingham behavior above their softening temperatures. However, the linear chains of the NB pitch resulted in a higher degree of shear-thinning than was observed with the nonlinear ND pitch. This could result in a greater degree of molecular orientation during spinning.     

KOH activation of pitch-derived carbonaceous materials—Effect of carbonization degree

Two series of mesophase pitches and semi-cokes of different carbonization degree were produced by heat treatment of anthracene oil derived pitches P1 and P4 in the temperature range of 460-700 °C. These carbonaceous materials were activated with potassium hydroxide at 700 °C using 1:3 reagents ratio to assess the effects of the precursor optical texture and carbonization degree on the activation behavior. The results show that the increase in the pitch pretreatment temperature suppresses propensity to the pore generation while enhancing particle breaking. The effect can be illustrated by decreases in the BET surface area S_BET from ~ 2700 to ~ 1500 m² g⁻¹ and the micropore volume V_DR from ~ 0.85 to ~ 0.45 cm³ g⁻¹. These parameters are inversely related with the H/C atomic ratio of precursor. In contrast, the anisotropic development of pitch coke, varying from flow type to mosaics, has a slight effect on the activation behaviour. The mechanism of porosity generation, that is proposed, stresses the role of hydrogen occurring at the edges of graphene layers and potassium metal insertion/deinsertion on the porosity development and particle disintegration during KOH activation of pitch-derived carbons.     

Mesophase pitches for 3D-carbon fibre preform densification: rheology and processability

Synthetic mesophase pitches, produced from naphthalene (ARA24, ARA24r), were examined in terms of fluidity and thermal stability in comparison with other mesophase, isotropic pitches, or mesophase pitch/softening molecules blends. Their interest for carbon/carbon (C/C) composites processing by liquid impregnation or vacuum transfer inside three-dimensional carbon preform is discussed. Viscosity versus reduced temperature plots is reported on the same graph to compare the studied pitches and to define a suitable C/C processing window. Methylnaphthalene-based pitch is a good candidate with viscosity and stability adapted to the processing temperature and high carbon yield.     

Controlling the Oligomeric Composition of Carbon-Fiber Precursors by Dense-Gas Extraction

Dense-gas extraction of M-50 petroleum pitch with the solvent toluene was carried out in a continuous, countercurrent-flow, packed column at temperatures from 330° to 380°C, pressures from 42 to 84 bar, and a solvent-to-pitch ratio of 5:1. Both one- and two-column configurations were used to fractionate raw pitches into top and bottom cuts of controlled oligomeric distribution. Using matrix-assisted laser desorption–ionization mass spectroscopy for absolute molecular weight determination, the effect of changes in pitch oligomeric distribution on both the softening point and the degree of anisotropy (i.e., isotropic–mesophase equilibrium) was established. For example, pitch fractions rich in dimer and trimer species, and with essentially all monomer removed, exhibit the anisotropic microstructure typically found in precursors for high-performance carbon fibers.     

Preparation,modification, and characterization of pitches from apricot stones

Apricot stones steam pyrolysis tar and mixtures with petroleum pitch were modified by sulfuric acid and heat treatment at various temperatures for the purpose of obtaining a raw material for the production of carbon-related materials. The products obtained were characterized by elemental analysis, total carbon contents, FTIR spectroscopic analysis, ¹H and ¹³C NMR spectroscopic analyses, softening point measurement, and oxygen functional group content. During the acid and heat treatment, the reactions of polycondensation and polymerization occur. The heat-treatment modification of the pitches decreases the oxygen-containing groups and sulfur content and increases the content of aromatic structures. Chemical transformations and the removal of light components increase the softening point of the pitches. The results show that apricot stones tar with some additional modifications can be used as a raw material for the production of advanced carbon materials.     

Needle coke formation derived from co-carbonization of ethylene tar pitch and polystyrene

Ethylene tar pitch was co-carbonized with waste polystyrene to prepare needle coke. The modified properties of mesophase, which were greatly improved due to increasing naphthenic and other alkyl content, availed the formation of needle coke with high quality. The coefficient of thermal expansion value was decreased from 3.2 × 10⁻⁶/°C to 0.3 × 10⁻⁶/°C and the optical texture of the coke was changed from coarse mosaic texture to flow domain of high uniaxial orientation after adding waste polystyrene into ethylene tar pitch. The low viscosity of the mesophase pitches favored the development of mesophase and highly uniaxial arrangement. The increase in alkyl group content greatly improved characteristics of the needle coke.     

Improving graphitization degree of mesophase pitch-derived carbon fiber by solid-phase annealing of spun fiber

The solid-phase annealing of the mesophase pitch spun fiber was examined between the glass transition (T_g) and softening (T_s) temperatures of the pitch to improve the graphitization degree of the graphitized fiber through recovering or further improving the stacking height of the mesogen molecules in the spun fiber, since the rapid spinning reduced markedly stacking height in the as-spun fiber. A naphthalene mesophase pitch as received carried stacking height of 2.9 nm which was markedly reduced to 1.7 nm by spinning at 230 m/min, giving Lc=40 nm for its graphitized fiber. Annealing at 206 °C improved the stacking height of the spun fiber to 2.4 nm and Lc(002) of the graphitized fiber to 54 nm. Annealing of the methylnaphthalene mesophase pitch fiber at 200 °C was much more effective in improving the stacking height from 3.5 to 5.0 nm and its graphitized fiber to Lc=91 from 40 nm. Such an improved graphitization degree led to improved thermal conductivity and tensile modulus of the graphitized fiber. It must be noted that the annealing of the spun fiber reduced its stabilization rate, indicating densification of molecular stacking in the fiber. The transformation scheme of mesophase pitch into graphite fibers is discussed to clarify the roles of molecular stacking in the clusters and their arrangement in the mesophase pitch fiber during the carbon manufacturing process.     

Fractionation of pitches by molecular weight using continuous and semibatch dense-gas extraction

A countercurrent, multistage, dense-gas extraction technique with reflux was investigated for the fractionation of carbonaceous pitches. Two modes of operation were investigated: continuous-stripping and semibatch operation. For example, continuous stripping with dense-gas toluene in the supercritical state, a positive column temperature gradient from 330 to 380 °C, and a pressure of 49 bar was used to strip the monomer and dimer species from an A-240 petroleum pitch feed, yielding a high molecular weight (mol wt) bottoms product rich in trimer and higher oligomers. Afterwards, semibatch operation was used with supercritical, dense-gas toluene, a temperature gradient of 330 to 380 °C, and pressures from 84 to 111 bar to fractionate the above bottoms product, yielding a trimer-rich overhead (average mol wt (M_w) = 800) and a tetramer and higher residue with M_w ∼ 1000. Considering the two operations as a unit, a combined selectivity factor of ∼350 was obtained. Not only is this at least an order of magnitude better than what can in principle be accomplished by conventional, single-stage solvent extraction, but such extraction is inapplicable to our system because of the insolubility of the pitch fractions of interest in typical liquid solvents. Matrix-assisted, laser desorption/ionization time-of-flight mass spectrometry (MALDI) was used to verify that separation was indeed occurring by mol wt and to study the relationship between the M_w, softening point, and C/H ratio of the fractions produced.