Rheological investigation of coal-tar pitch during its transformation to mesophase

Rheological characteristics of two coal-tar pitches, during their transformation to mesophase, have been followed by rotational viscometry. In certain instances samples were also taken, enabling the mesophase microstructure to be correlated with the flow behaviour. Flow curves established that the pitches are Newtonian liquids at low temperatures, but non-Newtonian character appears at temperatures above 380 °C. Scanning the viscosity-temperature curve of a binder pitch showed that the viscosity began to change, as a result of polymerization processes, at about 240 °C. In both pitches the apparent viscosity, at all rates of shear, increased very rapidly at temperatures above 420 °C when the mesophase content was in excess of 25%. Shear-thinning behaviour was apparent and a peak in the apparent viscosity-temperature curves appeared at all the rates of shear investigated. It is suggested that the systems can be regarded as emulsions. Initially an emulsion of mesophase in isotropic liquid exists but this transforms, at higher temperatures, to one of isotropic liquid in mesophase. The maximum in the apparent viscosity appears to correspond to the phase inversion point.     

Modified characteristics of mesophase pitch prepared from coal tar pitch by adding waste polystyrene

The toluene soluble of coal tar pitch was carbonized with waste polystyrene. The properties of mesophase pitches were characterized using polarized light optical microscope, apparent viscometer, FT-IR, ¹H NMR and X-ray diffractometer. After adding the waste polystyrene into the toluene soluble of coal tar pitch, the soluble mesophase were was increased from about 9% to 52% and the mesophase pitches were changed from 74% with coarse mosaic texture to 100% with flow domain texture. The mesophase pitches were transformed from thixotropy to unthixotropy. By waste polystyrene added into the toluene soluble of coal tar pitch, the methylene group was increased a lot. The presence of more alkyl groups modified the characteristics of mesophase pitches and improved the assembly of mesophase pitch molecules.     

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.     

Effects of borane-pyridine complex on the mesophase pitch formation from decant oil

Mesophase pitches were prepared by adding 0.5 to 2 wt.% of a borane-pyridine complex to a decant oil in a tubing bomb reactor and heating in a fluidized-sand bath held at 500 °C for 30, 45, and 60 min, respectively. The mesophase pitches were analyzed using thermal gravimetric analysis, X-ray diffraction and polarized-light microscopy. Growth and coalescence of mesophase spheres were hindered, while the crystallinity of mesophase pitches was increased when the materials were heat treated with the boron compound. The addition of the borane-pyridine complex also accelerated the coke formation.     

Low shear rheological behaviour of two-phase mesophase pitch

The low shear rate rheology of two phase mesophase pitches derived from coal tar pitch has been investigated. Particulate quinoline insolubles (QI) stabilised the mesophase spheres against coalescence. Viscosity measurements over the range 10–10⁶ Pa s were made at appropriate temperature ranges. Increasing shear thinning behaviour was evident with increasing mesophase content. At low mesophase contents the dominant effect on the near Newtonian viscosity was temperature but at higher contents it was the shear rate; temperature dependence declined to near zero. The data indicated that agglomeration could be occurring at intermediate mesophase volume fractions, 0.2–0.3. The Krieger–Dougherty function and its emulsion analogue indicated that in this region the mesophase pitch emulsions actually behaved like ‘hard’ sphere systems and the effective volume fraction was estimated as a function of shear rate illustrating the change in extent of agglomeration. At the higher volume fractions approaching the maximum packing fraction, which could only be measured at higher temperatures, the shear thinning behaviour changed in character and it is considered that this is possibly due to shear induced deformation and breakup of dispersed drops in the shear field.     

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.     

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.     

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.     

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).     

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.     

中间相沥青流变参数的测定

本文采用XLY-2型毛细管流变仪和单孔纺丝机对中间相沥青的流变性和可纺性进行了研究。测定了一系列重要的流变学参数;并从分子量大小、族组成分布和显微结构等方面分析了沥青流变性的影响因素。研究表明,中间相沥青的流变特性与其可纺性之间有着密切的关系。最后,还根据Arrhenius方程和高聚物溶液理论提出了中间相沥青表观粘度的四元参数模型。     

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.     

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.     

Low-temperature solution synthesis of carbon nanoparticles, onions and nanoropes by the assembly of aromatic molecules

Graphitic carbon nanostructures were prepared in solution by two methods: solvothermal synthesis and hot injection. Small carbon nanoparticles with uniform diameters of 3-6 nm, carbon onion particles with larger diameters of 30-80 nm, and carbon nanoropes with a length of hundreds of nm and a width of 3-20 nm, were formed using commercial mesophase pitches as a carbon precursor through solution-phase synthesis below 200 °C. In the solvothermal synthesis, organic-organic assemblies of aromatic molecules from the pitches could be constrained into different stacking arrangements directed by varying the concentration of the block copolymer P123 template in toluene solution at 200 °C. In the hot injection method, when oleic acid was used as a solvent at 180 °C, the assemblies of the aromatic building blocks were controlled by varying the reaction time (5-30 min) or the concentration of H₂SO₄ catalyst (0.015-0.061 mol L⁻¹) in the nucleation and growth process.     

煤沥青基中间相沥青的制备研究

以纯化的煤焦油沥青为原料,考察了热聚合温度和恒温时间对中间相沥青的收率、光学显微形态、软化点和族组成的影响.结果表明:反应温度在420℃,恒温5 h时得到了软化点为312℃的流线体型中间相沥青,其收率为79.1%;热聚合反应在相对较低的温度400℃,反应时间为10 h时形成了软化点为305℃、收率为81.4%的优质广域型可纺性中间相沥青.对该原料煤沥青而言,通过控制热聚合反应温度和恒温时间可以达到制备优质中间相的目的.     

萘沥青及热转化产物的性质和结构表征

以萘为原料，5%、10%、20%质量分数AlCl₃为催化剂，采用自升压法制备了3种萘沥青（NP-5%、NP-10%、NP-20%），对其进行了工业分析、FTIR、GPC以及TG/DTG表征，并且利用PLM、SEM、Raman和XRD等手段对其中间相热转化产物（MNP-4h、MNP-4.5h、MNP-6h）和炭化产物（CNP-5%、CNP-10%、CNP-20%）的微观结构进行了研究。结果表明，3种萘沥青的收率都在80%以上，其分子量分布在147～3940范围内，质均分子量都为255左右；3种萘沥青的芳香性指数（I_ar）分别为0.05、0.27、0.36，支链化指数（CH₃/CH₂）分别为0.65、0.06、0.02，在750℃的残炭率分别为27%、31%、30%。PLM观察发现，3种中间相热转化产物的显微结构分别为中间相小球体、中间相纺锤状以及纤维状结构。从MNP-4h中可萃取得到产率为8%的中间相小球，其表面光滑、粒径均匀，直径在5～7μm。Raman分析表明，3种中间相热转化产物的I_G/I_all分别为15%、17%、30%，说明延长热转化时间，可增加产物碳微晶的规整性。3种炭化产物的显微结构都以镶嵌结构为主，其碳微晶层间距都为0.35nm，而CNP-5%具有最大的微晶尺寸。     

The correspondence between loss of H-NMR signal intensity and mesophase growth during the thermal maturation of a petroleum pitch

Samples of petroleum pitch, Ashland 240, were heated isothermally at 380, 400, 430 and 450°C and the hydrocarbon evolution and mesophase growth monitored simultaneously by the use of high temperature ¹H-nuclear magnetic resonance (NMR) analysis. For all four temperatures the NMR signal indicated no mesophase formation until the signal intensity had dropped by 30%, which corresponded to 20% weight loss from the pitch. The correlation between the onset and increase in mesophase formation with signal intensity loss was found to be independent of temperature. This temperature independence for the petroleum pitch agrees with the work presented previously by Honda et al. (Carbon 1970, 8, 181). who compared weight loss with the growth of quinoline-insoluble material for a coal-derived pitch. Given the correlation between signal loss and mesophase formation, it is suggested that a relative measure of NMR signal intensity could be used to estimate mesophase content of petroleum pitch, during maturation.     

Determining the anisotropic content in a petroleum pitch - Comparison of centrifugation and optical microscopy techniques

The anisotropic content of a pitch is one of the most important parameters for characterizing such materials. Polarized light optical microscopy is the technique most commonly employed (ASTM D 4616 standard procedure) to measure this pitch parameter. However, this standard procedure is limited to pitches with mesophase contents only up to 20%. An alternative technique for determining the anisotropic content of a pitch is high-temperature centrifugation, which can be used without limitation for pitches with up to 100% anisotropic content. In this work, the two techniques have been compared; samples of four pitches with mesophase contents lower than 20% have been analyzed by both techniques and the results have been compared. The high-temperature centrifugation technique showed good repeatability, and the results that it yielded matched those obtained from optical microscopy when the anisotropic content of the pitch was higher than around 5%. The centrifugation technique is always faster, simpler, and possibly more accurate than optical microscopy for pitches with mesophase contents higher than 20%.     

Formation of anisotropic mesophase from various carbonaceous materials in early stages of carbonization

The low-temperature carbonization of pitches, reduced crudes, synthetic polymers and various ranks of coals has been studied by polarized-light microscopy, X-ray diffraction and solvent extraction. Formation of anisotropic mesophase is a function of heat-treatment temperature and residence time for those materials that pass through a fusion stage in the temperature range from 350 to 500 °C. The nucleation of spherules from the pitches and reduced crude during low-temperature carbonization is proportional to the amount of β-resin, which is benzene-insoluble and quinoline-soluble, and the rate of its formation. Addition of sulphur to the pitch turns the texture of mesophase spherules to that of a fine mosaic when the mixtures are heat-treated in the temperature range of 350–500 °C. Methylation of the hydroxyl groups in 3,5-dimethyl phenol formaldehyde resin, by reacting with dimethyl sulphate, gives an anisotropic texture which consists actually of tiny spherules together with parent matrix; the original resin gives the typical mosaic texture. The molecular forces in the decomposing carbonaceous materials during low-temperature carbonization are among important factors governing the mesophase transformation. A common mechanism of formation of various anisotropic textures can explain the behaviour of coal-cokes, pitch, crudes and polymers.