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.     

Injection and stabilization of mesophase pitch in the fabrication of carbon-carbon composites. Part I. Injection process

The fabrication of carbon-carbon composites by injection of low viscosity mesophase pitch through a fiber preform followed by stabilization and carbonization was examined. The fully transformed mesophase MOMP and AR pitches were injected through either soft or rigidized disk preforms 35 mm thick and 68 mm in diameter. Injection provided good even filling of major flow channels and fiber bundles. Flow-induced fibrous microstructures were retained by quenching and preserved by stabilization upon carbonization. A second injection cycle was effective in filling voidage created by thermal densification. A third cycle was applied, but required severe injection conditions and provided only incremental improvement. The carbon-carbon composite reached a density of 1.8 g/cm³ after three injection cycles.     

Rheological behavior of a semirigid liquid crystal polymer

The rheological behavior of liquid crystal polymers is still far from completely clarified from both experimental and theoretical points of view. In this paper, the shear flow and the non-isothermal elongation flow behavior of a semirigid liquid crystal copolyester is discussed. The viscosity strongly decreases when the test temperature is increased above the crystal-nematic transition temperature; below this temperature the viscosity is very high. The thermal history strongly affects the shear viscosity. The elongational flow behavior depends also on temperature. Above the crystal-nematic transition temperature and the mesophase is easily spinnable, whereas below this temperature, the spinnability is very poor.     

Anomalous capillary flow of coal tar pitches

Capillary flow of liquid coal tar pitch into a coke bed was studied. Anomalies in the flow could not be attributed to a plugging effect for mesophase content lower than 20 wt%. The flow behaviour of small pitch droplets can be correlated with the change in physicochemical properties, as measured by the glass transition temperature, on penetration into the coke bed.     

Rheostructural studies on a synthetic mesophase pitch during transient shear flow

The steady, shear viscosities of a synthetic mesophase pitch (Mitsubishi AR-HP) obtained from rate-sweep experiments at 0.1-10 s⁻¹ exhibited shear-thinning (region I) and plateau responses (region II), but displayed a hysteresis during the decreasing rate sweep. Transient tests revealed that the shear stress (and consequently the shear viscosity) displayed a local maximum and a minimum before approaching a steady state. Following steady flow at 1 s⁻¹, a reversal of flow direction or a very short interruption in flow did not lead to the maxima or minima in the transient shear stress, but the maxima and the minima reappeared in the transient stress after a rest time of ∼1000 s. An experimental protocol was developed to preserve the rheological samples, and their microstructure was characterized in three orthogonal planes for the initial and final states. The initial microstructure was found to have a weak, but preferred, orientation of mesophase layers in the radial direction of the rheometer cone-plate (due to the initial squeezing flow). The initial microstructure changed to a flow-aligned fibrous structure after shearing in the viscometric flow.     

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.     

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.     

反应温度和压力对制备中间相沥青的影响

以中温煤沥青为原料,于高压反应釜中进行热转化,在不同的反应温度和压力条件下制备中间相沥青。采用偏光显微镜、红外光谱仪、x射线衍射仪等测试仪器对所得中间相沥青进行分析和表征。结果表明,温度对中间相沥青的产率、形貌和结构影响显著,随着温度升高,中间相含量增加;中间相小球体的尺寸增大,逐渐出现融并现象,最终形成流域型体中间相。压力对中间相的含量和结构也有一定影响,实验结果显示施加3MPa压力,有利于中间相的形成和其含量的提高。420℃、3MPa条件下形成的中间相含量高达81．0％,并形成流线域状体中间相。     

Carbon fibres from mesophase pitch

Union Carbide Corporation has recently developed a process for preparing high-modulus carbon fibres from mesophase pitch. These high-modulus carbon fibres can be prepared from low-cost carbonaceous pitches which have been converted to a liquid crystal state. The liquid crystal or mesophase state results when a proper size distribution of planar aromatic molecules is produced by dehydrogenative condensation reactions. Mesophase pitches can be melt-spun into fibres possessing a high degree of axial preferred orientation which can be preserved and enhanced through carbonizing and graphitizing. Mesophase pitch-derived carbon and graphite fibres can have several transverse structures and are capable of achieving various combinations of high strength and high modulus. Their electronic properties are extremely sensitive to structure and further confirm the graphitizability of carbon fibres derived from mesophase pitch. A number of strength-limiting defects have been identified and their incidence reduced. The number of applications of carbon fibres continues to increase. The unique structure and properties of mesophase pitch-based carbon fibres make them particularly suitable for those applications requiring high stiffness, high electrical conductivity, high thermal conductivity, and low coefficient of thermal expansion.     

Computational modeling in processing flows of carbonaceous mesophases

Carbonaceous mesophases are liquid crystalline precursor materials that can be spun into high performance carbon fibers using the melt spinning process, which is a flow sequence consisting of capillary, diverging, porous media, converging, and extensional flows that modifies the precursor molecular orientation structure. Carbon fiber property optimization requires a better understanding of the principles that control the structure development during the fiber formation processes and the rheological processing properties. This paper presents the elastic and continuum theory of liquid crystals and computer simulations of structure formation for pressure-driven capillary flow of carbonaceous mesophase precursors used in the industrial carbon fiber spinning process. The simulation results capture the non-Newtonian rheology of mesophase and the formation of characteristic fiber macro-textures.     

The role of liquid crystals in the formation of graphitizable carbons (cokes)

The formation of cokes and graphites proceeds via the creation from the isotropic fluid phase of carbonization of pitch and coal, of lamellar nematic liquid crystals or mesophase. This anisotropic fluid, deformable mesophase, develops as spheres within which constituent molecules are stacked parallel to an equatiorial plane. This type of structure facilitates coalescence to a coherent mass which eventually forms a graphitisable carbon. The ‘onion-skin’ structure of mesophase spheres cannot so coalesce. Different optical textures of cokes and graphites owe their origin to different chemical reactivities and fluidities of mesophase, the lower the fluidity the smaller the size of the optical texture. Mesophase from lameller molecules is compared with conventional rod-like nematic liquid crystals. Structures in needle-cokes, metallurgical coke, coke from solvent refiend coal and carbon fibre from pitch are discussed in terms of formation and properties of lamellar nematic liquid crystals.     

Structure of carbon fiber obtained from nanotube-reinforced mesophase pitch

Flow-induced orientation of discotic mesophase precursor typically leads to a center-origin, radial microstructure in the resulting carbon fibers. In the present study, we report the microstructure of carbon fibers that were derived from mesophase pitch that was modified using carbon multi-wall nanotubes (MWNTs). For an MWNT content as low as 0.1 wt.%, the microstructure was found to be fairly random (rather than radial).     

Computational rheology of carbonaceous mesophases

Mesophase pitches are multicomponent discotic nematic liquid crystals (DNLCs), whose characteristic molecular weight is intermediate between low molar mass and polymeric nematic liquid crystals. Flow modelling of these fluids is performed using a previously formulated mesoscopic viscoelastic rheological theory [J. Non-Newtonian Fluid Mech. 94 (2000) 87] that takes into account flow-induced texture transformations. A complete extra stress tensor equation is developed from first principles for liquid crystal materials under non-homogeneous arbitrary flow. This mesoscopic viscoelastic model has been adapted to describe the rheology of flow-aligning thermotropic DNLCs as models of mesophase pitches. We develop a fundamental understanding of the relations between rheology and flow of carbonaceous mesophases using theory and simulation by characterizing the steady and transient shear rheological material functions of flow-aligning DNLCs. Predictions for simple shear flow (under non-homogeneous conditions) for the apparent shear viscosity and first normal stress differences are presented. The predicted relations among rheological properties, shear-induced microstructure, processing conditions and material parameters of discotic mesophases are characterized and discussed.     

Quantitative determination of anisotropic domain size in mesophase pitch

The optical texture of the carbonaceous mesophase is related to the nature of the pitch precursor, the mesophase viscosity and mode of preparation. The anisotropic domain size, represented by the distance between extinction lines, has been a useful parameter for characterizing mesophase pitches. A procedure for quantitative determination of mesophase domain size, utilizing annealing techniques and image analysis, is described and applied to several different mesophase pitch systems.     

Injection and stabilization of mesophase pitch in the fabrication of carbon-carbon composites: Part II. Stabilization process

In the fabrication of carbon-carbon composites by mesophase injection through a fiber preform, it is essential to stabilize the flow-induced microstructure in the flow channels and to prevent relaxation and exudation of the mesophase. Oxidation stabilization studies were conducted on preforms injected with the naphthalene-based AR mesophase pitch. Oxidation mass gain (OMG) curves at 170, 222, and 270 °C were generated for 60°-wedges cut from full size composite disks. The rates of OMG at 170 °C of first- and second-cycle injection wedges and full-size disks were comparable to those using as-spun filaments 30 μm in diameter, and particles sieved to 200 to 340 μm. The results suggest that oxygen is accessible deep into a mesophase matrix and the transport is facilitated by connected array of shrinkage cracks. Oxidation at 170 °C has strong advantage over higher oxidation temperatures by having a higher carbon yield and lower OMG threshold and thus oxidation time required for stabilization. The 60°-wedges could be stabilized at 170 °C after a 25 h oxidation with a 7.2% OMG and attaining a carbon yield above 85%.     

On the flow‐phase diagram for nematic liquid crystalline polymer under magnetic field

The effect of magnetic fields on molecular configuration of liquid crystalline polymers under shear flows are numerically analyzed using the extended Doi theory in which a molecular shape parameter is admitted. The evolution equation for the probability density function of the LCP molecules is directly solved without any closure approximations. One case is considered that the magnetic field makes 45° with respect to the flow direction. We can find that the magnetic fields strongly affect on the transition among flow‐orientation modes, such as tumbling, wagging, and aligning modes. And a new aligning flow‐orientation mode emerges at low shear rate, which is macroscopically same as the ordinary aligning mode, but is microscopically quite different from the ordinary one. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

热聚合温度对煤液化沥青制备中间相沥青的影响

为制备优质的中间相沥青,以煤液化沥青为原料,在不同热聚合温度下制备中间相沥青,采用偏光显微镜、红外光谱仪、XRD、热分析等测试仪器对所得中间相沥青进行分析和表征。结果表明,温度对中间相沥青的收率、形貌和结构影响显著。随着温度升高,中间相沥青的收率降至86.2%,H含量降至3.96%,S含量有所下降,残炭率增大;中间相小球体的尺寸增大,逐渐出现融并现象,最终形成广域型中间相;煤液化沥青中的稠环芳烃、芳香烃的含量明显增加,烷烃成分则明显减少;煤液化沥青中的无定型区含量减少,分子的排列与取向性变好。选择低的热缩聚温度(410~420℃),适当延长反应时间有利于反应方向的控制,从而达到制备优质中间相的目的。     

Photoinitiated crosslinking of hydroxypropyl cellulose in the isotropic and liquid crystal states

The influence of temperature, concentration, and shear rate on the formation of liquid crystal solutions of hydroxypropyl cellulose (HPC) in DMAc and the preservation of the structure by photoinitiated crosslinking were investigated. The rate of changes in the relaxation pattern for the characteristic band texture of HPC liquid crystals after cessation of shear is inversely proportional to the extent of the crosslinking. This shows that the liquid crystalline structure is preserved upon crosslinking. The molecular orientation of the crosslinked films as a function of mesophase organization and concentration during crosslinking was studied.     

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.