Influence of heat and pressure treatment on the rheological behavior of petroleum pitches

Pitch rheological properties are extremely important during the manufacturing process of carbon materials, in mesophase formation, and with regard to the final properties of the carbon products. In this work, pitch samples have been prepared from three different FCC decant oils by heat-treatment, under 0.9 MPa pressure, in a reactor at 390 °C, 410 °C, and 430 °C. These samples were analyzed in a rotational rheometer using a parallel-plate sensor. The rheometric softening points matched the results obtained using conventional equipment and exponential relationships were found to exist between these softening points and the pitch cosity when the former approached 180 °C. The quinoline-insoluble content (QI) has been shown to be more important in increasing the pitch viscosity than the toluene-insoluble content (TI). Oscillatory rheometry analysis has shown that an elastic response is not always found in creep and recovery tests, even when the elastic modulus G′ is dominant over the viscous modulus G′′. Pitch elasticity was found to be independent of the mesophase, and this pitch property was either only observed when the cross-over point occurred at very high frequencies or did not occur at all within the frequency range studied.     

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.     

镁碳砖用中间相沥青-酚醛树脂结合剂的研制

研究了由煤焦油经蒸馏制得的软沥青在(430±1)℃下的热转化时间对转化产物中间相含量、残碳率和软化点的影响,并选择残碳率为71.4%~83.7%,软化点为212~273℃的5种中间相沥青与酚醛树脂进行复合,研究了复合比例对其残碳率的影响。结果表明:中间相沥青的软化点和残碳率与其中间相含量密切相关,并且均随着热转化时间的增加而增高;中间相沥青的残碳率明显高于酚醛树脂,随着中间相沥青配入量的增加,中间相沥青-酚醛树脂复合物的残碳率提高;中间相沥青与酚醛树脂二者存在协同作用,中间相沥青-酚醛树脂复合物的实际残碳率高于二者按比例计算得到的理论残碳率,因而有望成为镁碳砖用新型结合剂。     

Carbonization behaviour of modified synthetic mesophase pitches

Stabilization of naphthalene-derived synthetic mesophase pitch was achieved by controlled oxidation or reaction with phenanthrenequinone (PHQ) in order to suppress the swelling during high-temperature treatments. The modified pitches were characterized and their carbonization behaviour was studied by thermogravimetry-mass spectrometry. The results show that naphthenic groups are involved in the stabilization process. PHQ reacts via cycloaddition reactions, yielding oxygen-containing non-planar structures, a feature that accounts for the loss of anisotropic properties of the final material.     

MALDI-TOF mass spectrometry: Obtaining reliable mass spectra for insoluble carbonaceous pitches

The insolubility of high molecular weight (MW) carbonaceous pitches (e.g. mesophases) in even aggressive solvents has always been a significant obstacle to their MW characterization. By using matrix-assisted, laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry with the matrix 7,7,8,8-tetracyanoquinodimethane (TCNQ), we are now able to obtain reproducible, highly resolved mass spectra for insoluble pitches. A solvent-free sample preparation technique was developed in which the matrix and pitch sample are combined and ground together to form a fine powder. We then use the high surface tension of water to create a thin uniform coating of the matrix-pitch mixture on the target cell. Laser power and pulsed ion extraction delay were both found to significantly alter the spectra obtained, and a procedure for optimizing each is described. Three carbonaceous pitches were evaluated: an anthracene pitch produced by thermal polymerization, a petroleum pitch similar to A-240 and produced by heat-soaking decant oil, and a high MW cut of this pitch produced by supercritical extraction. The anthracene pitch contained enough lower MW species to serve as a self-matrix, so no TCNQ addition was required. For the high MW cut of petroleum pitch, the use of TCNQ or dithranol revealed the presence of trimers and tetramers at MWs up to 1250 Da.     

3D printable ceramic pastes design: Correlating rheology & printability

Binder-free ceramic pastes based on TiO₂ nanoparticles (64 wt%) with different solvent:co-solvent volumetric ratios (water:glycol, W:G) are disclosed for the first time. Ceramic pastes were prepared without binders (polymers or gel-precursors) using a straightforward, and environmentally respectful method. Two criteria to achieve three-dimensional and complex structures were proposed. 1.- shear-thinning ability and 2.- G' = G'' > 1 × 10³ Pa. Pseudoplastic behaviors were found and were suitably modeled for all pastes analyzed, fulfilling the first criterion. Nonetheless, only those pastes with 60:40, 50:50, and 40:60 (W:G) fulfilled the second criterion. These latter exhibited a good balance between rigidity to form long and horizontal bridges but fluid enough to avoid flow cuts or clogs. Thus, 3D structures (20 layers of stacking) were obtained with outstanding self-supporting and excellent shape-retention. The complex structures were dried at 400 °C, thus obtaining binder-free and mechanically stable ceramic structures preserving their printed shape.     

Injection and stabilization of mesophase pitch in the fabrication of carbon-carbon composites. Part III: Mesophase stabilization at low temperatures and elevated oxidation pressures

Micrography and infrared spectroscopy were applied to explore microstructural stabilization of mesophase pitch at oxidation temperatures as low as 130 °C. AR mesophase pitch synthesized from naphthalene was drawn to rods and thick filaments with fine fibrous microstructures that coarsen upon carbonization without adequate stabilization. At 270 °C, the stabilization front advances rapidly to a depth of about 7 μm, after which no further growth is perceptible. At low temperatures, there is a time lag before a stabilized layer can be observed near the surface, but thereafter the stabilization front advances relatively rapidly. For longer oxidation times, the low-temperature stabilization depths can exceed those attained at higher temperatures. FTIR spectra confirmed chemical reactivity and the formation of oxygen-bearing functional groups even at the low oxidation temperatures. Oxidation under a moderate pressure of 0.7 MPa can be effective in raising the oxygen uptake and increasing the stabilization depths significantly. All evidences point to the advantageous use of lower oxidative temperatures: more effective penetration of stabilization depth, lessen destructive effects of over-oxidation, and potentially shorter processing times.     

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%.     

Synthesis of mesoporous carbon and its adsorption property to biomolecules

Mesoporous carbon (MC) with high surface area and large pore volume was synthesized using mesophase pitch as a carbon precursor and nanosized MgO as an additive. The maximum surface area, largest pore volume and highest mesoporous ratio of as-prepared MC were up to 1400 m²/g, 2.8 cm³/g and 89%, respectively. The mesoporous structures (3–40 nm) of MC were directly observed under SEM and TEM. The adsorption capacity and adsorption rate of MC to vitamin B₁₂ (VB), chicken egg white albumin (CEWA) and bovine serum albumin (BSA) were proportional to the mesopore volume and average pore size. MC (PM4-OC) exhibited the maximum adsorption capacity to the typical biomolecules, 486, 140 and 176 mg/g for VB, CEWA and BSA, respectively. In contrast, Maxsorbs (commercial activated carbons) with a surprising surface area gave a very low adsorption to such biomolecules. The research indicates that MC may be potential in the selective adsorption and separation of biomolecules, based on a molecule sieve effect.     

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.     

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.     

Supercritical carbon dioxide-assisted reactive extrusion for preparation long-chain branching polypropylene and its rheology

An environmental benign process, which uses supercritical carbon dioxide (ScCO₂) as a processing aid, is developed in this work to prepare long chain branching polypropylene (LCB-PP). Results from the oscillatory shear rheology, melt elongational behavior and Fourier transformed infrared spectroscopy (FTIR) show that long chains have been linked as branches to the original linear PP chains using scCO₂-assisted reactive extrusion in the presence of cumene hydroperoxide and 1,6-hexanediol diacrylate. Compared to the initial linear PP, the branched samples show higher storage modulus (G′) at low frequency, distinct strain hardening of elongational viscosity, lower melt flow rate, increased crystallization temperature and improvement of the melt strength. ScCO₂ can improve the branching efficiency of modified PPs. The elastic response, melt strength and strain hardening parameter of the modified PPs increase with increasing scCO₂ concentration, which is ascribed to scCO₂ acting as a plasticizer for reducing PP viscosity and a carrier for active chemical species.     

Fine-woven punctured felt preform advanced ceramic composite for long duration hybrid fuel firing testing

A three-dimensional fine-woven punctured felt green body was used as a preform to generate an advanced ceramic. After chemical vapor infiltration and several cycles of precursor infiltration and pyrolysis, the pyrolysis carbon, as well as the silicon carbide and zirconium carbide ceramic matrix were formed. The advanced ceramic composite was tested under a hybrid fuel firing environment (hydrogen peroxide/ hydroxyl-terminated polybutadiene, H₂O₂-HTPB) for a relatively long time (240 seconds) as the puncture fibers head to the firing direction. After the test, beaded and bald fibers could be identified, and the sphere particles of firing residuals sizing from 10 μm to 1 mm were found. The test results show that the material is capable of withstanding a severe firing environment up to 2 MPa with an average linear erosion rate of 0.082 mm/s.     

Binder stabilization and rheology optimization for vat-photopolymerization 3D printing of silica-based ceramic mixtures

Enhancing inlet gas temperature in aero/gas turbines to reduce their carbon-footprint, has led to a strive for better performing inlet cooling mechanism of the turbine blades. The internal cooling of the blades is made by ceramic cores in their casting process, but conventional ceramic molding has long reached its maximum possible geometrical complexity, hence shedding light on 3D printing of these cores. The objective of this study is to develop low-viscous, fully stabilized, commercially viable ink for vat-photopolymerization of silica-based ceramics. This paper investigates the best dispersion type and amount for different formulated monomer mixtures, and explains the best correlation between viscosity, solid loading, binders, dispersants, peeling forces and mechanical properties, and offers an optimized mixture to avoid the common ceramic printing issue, namely crack propagation of cores during sintering. Among five dispersant agents, the SOL20, SOL24 and FA4611 exhibited better performance than other dispersion agents, and the optimum concentration level for each binder and dispersant agent was ensured through sedimentation test. Their dispersion capability and long-term stability were further investigated to designate the best dispersion agent for each binder system. Further verification was made by sedimentation study of the samples at 40 °C for 40 days and reducing the superficial area of the used powder mixture. According to the result of the rheology analysis, the best dispersions were achieved using SOL20 for the loaded binder mixtures of M1 and M4, SOL24 for M3and FA4611 for M2. The instability of M1 and M2 with their respective dispersant agent was coordinated through the thixotropic agent of TX/2, and complete stabilization and near-Newtonian behavior were achieved. However, the research showed that the addition of TX/2 to fully stabilized M4 and M2 suspensions negatively impacts the mixtures’ rheological behavior from near-Newtonian to shear-thickening. In the final stage of this study, peeling forces, sintering and three-point bending tests were conducted to determine the final formulated suspension to print ceramic core components. M4 and SOL20 combination was selected for SiO₂-ZrSiO₄ loading and dispersing, respectively. The impact of solid loading between the range of 58 and 65 vol% on the rheological behavior of the final suspension and the mechanical properties of sintered bodies were investigated to assign an optimum solid rate. The adequate strength on sintered and degree of viscosity for ceramic vat-polymerization processing was achieved at 58 vol%. Lastly, a validation study is conducted by printing a complex ceramic core model by a commercial LCD hobby printer. This validation shows the significance of this study to scale up the manufacturing of complex-shaped ceramic cores and to revolutionize the sector, by printing inexpensive and readily available irregular-shaped (non-atomized) ceramic powder, using the most cost-effective LCD printers (non-specialized expensive ceramic printers).     

Experimental study on mix proportion and fresh properties of fly ash based geopolymer for 3D concrete printing

This paper presents the material design and fresh properties of geopolymer mortar developed for 3D concrete printing application. Unlike traditional casting, in 3D printing, extruded materials are deposited layer-by-layer to build complex architectural and structural components without the need of any formwork and human intervention. Extrudability, shape retention, buildability and thixotropic open time (TOT) are identified as critical early-age properties to characterize the 3D printable geopolymer material. Five different mix designs of geopolymer are tested in a systematic experimental approach to obtain a best printable mix and later it is used to print a 60-centimeter-tall freeform structure using a concrete gantry printer to validate the formulation.     

Extrusion-based 3D printing alumina-silica inks: Adjusting rheology and sinterability incorporating waste derived nanoparticles

The nanoparticle (NP) exhibits numerous distinctive and extraordinary properties than micron level and up. The inclusion of NP effects in the rheological and densification behavior of extrusion-based (direct ink writing (DIW)) inks has been extensively investigated. The aqueous-based alumina-silica inks were first designed using waste rice husk ash (RHA) derived nano-silica (NS) (0–10 wt%) and found that the solid-to-liquid ratio reduces continuously with NS addition for printable rheology. For functionalization of NS, dispersant requirement is increased that improve the solids loading of inks. Second, the optimized inks are printed via DIW technique and sintered at a temperature of 1400–1650 °C. The NS has remarkably enhanced the shrinkage, density, and morphology of sintered DIW specimens and 7.5 wt% RHA NS reduces the sintering temperature ∼150 °C. Incorporating NP in the 3D printing ink is a clean approach to filling pores generated by binder-burnout and fabricating a dense ceramic at a low temperature.     

PU elastomers comprising spherical nanosilicas: Balancing rheology and properties

For polyurethane (PU) engineering elastomers, although urethane chemistry allows tailoring of elastomer properties, there is an inherent correlation between increasing strength and decreasing elongation-to-break. The ability to break this correlation provides great potential for creating high-performance materials that possess attractive combined properties. The incorporation of nanofillers is one of the promising approaches. However, while property enhancement is achieved, nanocomposite systems often experience significant increase in viscosity leading to process difficulty. In this paper, the use of nanospherical particles to mitigate the rheology was explored. We were able to achieve significant property enhancement in the ultimate mechanical properties and thermal stability while maintaining low viscosity during process. By first forming well dispersed nanosilica in polyol, followed by elastomer casting, material with significantly higher strength, elongation-to-break, thus, toughness, and degradation temperature than one without nanosilica was obtained. The performance improvement is attributed to the ability to form a low viscosity, stable nanofluid where the nanosilicas are present as primary nanoparticles in the polyol. The morphology of the formed elastomers was studied. It is hypothesized that nanoparticles reduce defects that contribute to premature failure of the elastomer, thus improving ultimate properties.     

3D printing of bioactive macro/microporous continuous carbon fibre reinforced hydroxyapatite composite scaffolds with synchronously enhanced strength and toughness

Continuous carbon fibre (CF) reinforced HA (CF/HA) composite scaffolds were prepared using a self-designed and manufactured 3D printer. The optimised design of nozzle structure and the tailored viscoelastic property of HA inks ensured compound extrusion of monofilament and multifilament CF with HA rod. The composite scaffolds designed using the CAD programme and sintered via a suitable process exhibited a hierarchical macro/microporous structure and contained approximately 50% HA and 50% β-TCP. The continuous CF synchronously enhanced the strength and toughness of the scaffolds. The compressive strengths of 1CF/HA and 5CF/HA were 11.4 ± 1.7 MPa and 16.3 ± 2.6 MPa, respectively, which were approximately double and triple compared with that of HA scaffolds. The fracture toughness of 1CF/HA was approximately double that of HA scaffolds and close to that of cortical bone. In vitro and in vivo studies demonstrated that 1CF/HA also had apatite formation capability and adequate bone regeneration capacity.     

Improving processability for in-mold coating formulations: Part II: Two-reinforcement formulations

The in-mold coating (IMC) process nowadays is well accepted by the sheet molding compound industry. The currently used IMC contains 2.8 wt% carbon black (CB) to provide enough electrical conductivity for maximum paint transfer efficiency (PTE) for electrostatic painting. Due to its relatively large viscosity, this formulation makes use of more than one injection gate for coating some large parts necessary. Our previous research investigated the possibility to replace the CB with higher conductivity carbon-based nanoparticles, namely carbon nanofibers (CNFs), multi-wall carbon nanotubes (MWCNTs), industrial graphene (grapheneblack [G]), and single-wall carbon nanotubes (SWCNTs) and found that the IMC with 11.3 wt% G has the best processability among all IMC formulations. To improve this formulation, herein, we study the use of a second reinforcement in combination with G, i.e., CB, CNF, and MWCNT. Results from this study suggest that most G/CB-reinforced IMC formulations have a better performance than the G-reinforced IMC formulations, and IMC with 1 wt% CB and 6 wt% G is the best among all G/CB-reinforced IMC formulations. To be specific, the new formulation allows parts to be painted to have a 300% increase in size when compared with the standard IMC.