Determination of pressure drop for concentrated suspension in a capillary flow

The rheological behavior of concentrated suspension melts in a capillary die is investigated. Particle migration and wall slip are two major factors affecting the flow behavior. A numerical model is proposed to describe the coupling effect of particle migration and wall slip in a capillary tube flow, incorporating a power‐law model for binder viscosity and a concentrated suspension viscosity model proposed by Krieger. Wall slip of a non‐Newtonian concentrated suspension is characterized by a modified Mooney method for which the conventional Mooney method is not applicable. We characterized the flow behavior of a concentrated suspension of a non‐Newtonian binder, EVA 460 (ethylene vinyl acetate), mixed with spherical glass beads of 40% by volume. Predicted results were compared with experimental observations, with good agreement. POLYM. COMPOS., 2010. © 2009 Society of Plastics Engineers     

A Flow Time Model for Melt‐Cast Insensitive Explosive Process

Diphasic flows of concentrated suspensions of melt‐cast insensitive explosives exhibit specific rheological properties. In order to limit the handling of pyrotechnical products presenting a risk with respect to the mechanical and thermal shocks, a lot of work has been undertaken for many years in the civil engineering sector. The objective of this study is to propose a predictive model of the flow time of a concentrated suspension through a nozzle located at the bottom of a tank. Similar to our industrial process, the suspension is made out of insensitive energetic materials and flows under gravity. Experimental results are compared to three models (Quemada, Krieger‐Dougherty, and Mooney) predicting the viscosity μ of a suspension as a function of the solid volume fraction ϕ, the maximum packing density ϕ_m and the viscosity μ₀ of the interstitial liquid. De Larrard's model is used to calculate ϕ_m. The value of viscosity measured for the pure liquid is close to the one predicted by the Bernoulli theorem, where liquids are considered as incompressible and inviscid. Finally, it was found that the Quemada's model gives a fair agreement between predictions and experiments.     

3D printing ceramic cores for investment casting of turbine blades,using LCD screen printers: The mixture design and characterisation

The primary objective of this study is to demonstrate the possibility of developing silica, alumina, and zircon-based photocurable ceramic suspensions that can be used for visible light photopolymerization (> 450 nm) and to optimise the binder formulations for the purpose of LCD-based ceramic 3D printing applications. Reference ceramic components for this work are ceramic cores employed in the investment casting of high-pressure turbine blades and vanes. Arguably, one of the most critical steps in photoinduced ceramic 3D printing is developing suitable ceramic suspensions, having high ceramic loading, low viscosity, and short curing times. Ceramic suspensions with four different novel binder formulations and commercial ceramic powders used in core manufacturing (SiO₂, Al₂O₃ and ZrSiO₄) were investigated to achieve the best trade-off between: (1) their curing performance (cure depth and curing speed), (2) rheological properties of the binder mixtures at the solid loadings of 60 vol.% for SiO₂, 55 vol.% for ZrSiO₄, and 45 vol.% for Al₂O₃; and (3) the green body mechanical properties of the mixtures after printing. The effect of ceramic particles on the selected binders was examined individually, and the correlation between cure depth (C_d), volumetric loading, and curing speed are evaluated. The results show all binders designed in this study provide an adequate cure depth, even at high ceramic loadings. When the curing behaviour of all unloaded binder mixtures from the previous study [1] compared with the 10 vol.% SiO₂ loaded mixtures, the cure depth of all formulated binder mixtures increased 50–55 % and the curing thickness of 60 vol.% SiO₂ loaded suspensions were still slightly higher than their unloaded counterparts. The rheology outcomes indicate that lower viscosity binders always result in lower viscosity of the ceramic loaded inks, even without taking the effect of dispersants into account. Besides, the addition of N-Vinyl-2-Pyrrolidone (NVP) monofunctional monomer to the binder mixtures significantly reduces the viscosity and changes the normally linear relationship of the mix viscosity and its silica loading content. Among the binder formulations loaded with 60 vol.% of SiO₂, the formulation providing the lowest viscosity and highest mechanical property consists of 5 wt.% of NVP, 45 wt.% of HDDA and 50 wt.% of Photocentric 34 resin. Although this binder mixture showed the highest green flexural strength when loaded by 55 vol.% ZrSiO₄, all other mixtures loaded with zircon flour also demonstrated a near-fluid behaviour, below 200 s^?1. In Al₂O₃ loaded mixtures, the HDDA di-functional binder formulations present lowest viscosity and the di- and multifunctional monomer blends (HDDA-Photocentric27) showed the highest mechanical properties when used in a 50/50 ratio. This work summarises the best binder choices for silica, alumina and zircon based ceramic suspensions used in core printing for investment casting applications through LCD screen printing.     

Preparation of high solid loading titania suspension in gelcasting using modified boiling rice extract (MBRE) as binder

Preparation of high solid loading homogeneous titania suspension using modified boiling rice extract (BRE) as consolidator (network-former)/binder for gelcasting application has been investigated. To achieve in situ consolidation forming of TiO₂ ceramic, the gel network formed by swelling and gelatinization of the modified BRE (MBRE) with 2-hydroxyethylmethacrylate was studied. The dispersion behaviour of the titania powder and rheology of the suspension under the influence of binder content, dispersant (Darvan 821A) concentration and pH of the dispersing media have been discussed. The present process of gel casting deals with 50-80 weight% solid loading of titania particles with MBRE (2-10 weight% to that of total solid loading) in presence of dispersant (ammonium salt of polyacrylic acid [(C₄H₅O₂-NH₄⁺)n]). The influence of BRE concentration and solid particle loading on rheological properties of aqueous titania suspensions has been analyzed under steady and oscillatory shear conditions. Thermogravimetry (TG) and differential thermal analysis (DTA) on gelcast green body has been evaluated and analyzed. The characterization of green and sintered body has been done with respect to density, porosity and microstructure.     

YAG ceramic processed by slip casting via aqueous slurries

Stable YAG (Y₃Al₅O₁₂) aqueous slurry with ammonium polyacrylate (NH₄PAA) polyelectrolytes as dispersant was prepared by ball mill method. The effects of polyelectrolyte concentration and pH value on the stability of the suspension is described here, and the stability maps are constructed at different pH value and polymer concentration. The rheological behavior of YAG slips of different solid loading (60–70%) has been studied by measuring their viscosity and shear stress as a function of shear rate and pH of the slurry. An optimal amount of dispersant and pH value for the suspension was found. YAG suspension displays a maximum in zeta potential values and a minimum viscosity in pH range of 9–11. Slips behaved as near Newtonian at the pH value up to a solid loading of 60 wt% and as non-Newtonian with thixotropic behaviors above this solid loading value. The density and the green as well as sintered microstructure of the cast products bear a direct relationship to the state of this slips induced by the alternation in the pH and the concentration of the dispersant as well as solid loading.     

Rheology and processing of BaSO4‐filled medical‐grade thermoplastic polyurethane

The rheological and processing behavior of BaSO₄‐fllled medical‐grade (additive‐free) thermoplastic polyurethanes (TPUs) was investigated. The rheology and the single‐screw and twin‐screw extrusion processing of filled TPU were found to be complicated by the moisture and the air entrained into the suspension upon the incorporation of the filler BaSO₄. It was observed that the 20 vol% BaSO₄‐filled TPU exhibits decreased, rather than increased, shear viscosity and elasticity (as manifested through smaller storage modulus values in small‐amplitude oscillatory shear flow) in comparison to unfilled TPU in the temperature range of 190°C to 200°C at which these materials are usually processed. The moisture remaining in the BaSO₄ will hydrolyze the polymer, leading to a decrease in the molecular weight of the TPU. However, the moisture contained in the filler itself is not sufficient to explain the significant reductions in viscosity and elasticity of the suspension upon the compounding of the BaSO₄. It is shown that the major factor giving rise to the reductions in elasticity and the viscosity of the suspension of TPU and BaSO₄ is the air (and the moisture air contains) entrained into the extruder during the feeding of the BaSO₄. Air, carried with the filler into the extruder, is entrained into the suspension to hydrolyze the TPU and to further impart a foamy structure under typical processing conditions to significantly reduce the shear viscosity and the elasticity of the suspension in comparison to unfilled TPU. Polym. Eng. Sci. 44:1941–1948, 2004. © 2004 Society of Plastics Engineers.     

Rheological properties of sepiolite ground in acid and alkaline media

Effects of different acids (HCl, H₂SO₄ and citric acid) on rheological properties of brown sepiolite from the Eskisehir region of Turkey have been determined. The optimum apparent viscosity value was obtained at the natural pH of the suspension. Below the natural pH, partial collapse of the structure due to the release of Mg ions causes a significant decrease in viscosity values. However, below pH 1, there is a substantial increase in viscosity values owing to gel formation. On the other hand, above the natural pH of suspension, increased amounts of OH ions lead to a decrease in viscosity values and inhibit gel formation. The reversible nature of sepiolite was tested by changing the pH of acid and alkaline treated sepiolite suspensions back to its natural pH by washing with water and acid, respectively. In summary, from a practical point of view, there is no favorable effect of acid treatment of sepiolite on its rheological properties. On the contrary, reducing the pH to natural pH after grinding in a basic environment led to an improvement in rheological properties.     

A new empirical viscosity model for ceramic suspensions

This paper presents a new predictive viscosity model for ceramic suspensions. Starting from Einstein's model (1906), various theoretical, empirical, and phenomenological models have been proposed for different suspension systems. However, there is still a lack of reliable model for ceramic suspensions used in colloidal ceramic shape-forming methods. Here, the rheological properties of ceramic suspensions comprising NiO/YSZ (nickel oxide/yttria stabilized zirconia) as the ceramic powder, and furfuryl alcohol as the suspending media were measured over a range of shear-rates (between 1 and 1000 s⁻¹) and different solid volume fractions from 0 to 0.4010. An empirical equation was then developed for the ceramic suspensions using the mobility parameter (?/(?_m−?)), which links Einstein's model with the more recent relative viscosity models. The proposed model was used to predict the relative viscosity data, showing excellent agreement to the experimental data from this study and with reported data in literature for other ceramic systems. The model was also used to estimate the maximum solid volume fraction for the ceramic suspensions (?_m=0.571), with better accuracy than those estimated by existing models.     

RDX/PEG悬浮液的流变性能

为获取RDX对固体火箭推进剂药浆流变性能的影响规律,采用稳态和动态流变学方法研究了RDX在硝酸酯增塑的聚乙二醇黏合剂体系中的流变特性、黏弹性及温度对悬浮液流变性能的影响.结果表明,随RDX含量的增加,颗粒间相互作用增强,从而导致悬浮液体系假塑性程度升高和损耗模量频率谱偏离线性程度增大.在RDX(I)悬浮液中RDX存在临界体积分数,且在硝酸酯增塑的聚乙二醇黏合剂体系中的表观黏度随温度的升高呈增加趋势,这与RDX的粒度和固-液界面相关.     

Rheological and sedimentation properties of ceramic suspensions containing granular aggregates

Conclusions A study was made of the rheological properties and sedimentation stability of kaolin and quartz suspensions with granular fillers. The rheological nature of the behavior of the suspension with the granular filler is determined by the structure of the original suspension.The relationship between the effective and relative viscosities of the suspension and the volume concentration of the aggregates is demonstrated.The rate of settling of the coarse particles is determined by the density, the rheological characteristics of the suspension, and the viscosity with the minimum shear stresses.Conditions are formulated for the complete sedimentation stability of the suspensions with different types of rheological behavior.A concept is proposed for the coefficient of deposit formation K_dep, and its relationship with the density (concentration) of the suspension is demonstrated.Translated from Ogneupory, No. 4, pp. 52–57, April, 1972.     

A rheological behavior of various polyvinylidene difluoride binders for high capacity LiNi0.6Mn0.2Co0.2O2

Various polyvinylidene difluorides (PVDF) as polymeric binding agent in lithium‐ion battery were rheologically evaluated on high capacity and excess lithium‐LiNi_0.6Mn_0.2Co_0.2O₂ (Ni‐rich NMC). We found that the suspension polymerized PVDF binder is more stable upon gelation for long duration while other high molecular weight PVDF binders (MW > 900,000) show irreversible rheological transformation that can dramatically compromise the coating step during lithium‐ion battery electrode slurry preparation. POLYM. ENG. SCI., 56:760–764, 2016. © 2016 Society of Plastics Engineers     

Fabrication of Ti3SiC2-based pastes for screen printing on paper-derived Al2O3 substrates

This paper describes the development and fabrication of pastes suitable for screen printing process using Ti₃SiC₂ as the ceramic filler and ethyl cellulose as the binder. With the aim of obtaining high quality screen printed films, the influence of different amounts of Ti₃SiC₂ filler (20–40?vol%) and binder (0–5?vol%) on the rheological properties of the pastes was investigated. Samples with higher viscosity, such as pastes containing 30?vol% and 40?vol% Ti₃SiC₂ filler, regardless of the amount of ethyl cellulose, showed a higher printing quality compared to the samples with other compositions. The different paste compositions were screen printed onto paper-derived Al₂O₃ substrates containing 28.6 ± 4.8% open porosity and sintered for 1?h under an argon atmosphere at 1600?°C. X-ray diffraction (XRD) measurements and scanning electron microscopy (SEM) analysis showed that the sintered films contained TiC as a primary phase and Ti₃SiC₂ as a secondary phase. The partial decomposition of Ti₃SiC₂ after sintering can be attributed to residual carbon from the organic additives, which decreases the thermal stability of this material.     

Optimization of feedstock properties for reaction-bonded net-shape zircon ceramics by design of experiments

For the manufacturing of reaction-bonded ceramic microparts by low-pressure injection moulding, feedstocks with an optimized rheological behaviour are required. In order to evaluate the main influences on the rheological properties of feedstocks and their possible interactions, various compositions were systematically tested in the frame of design of experiments (DoE). For this purpose, ZrSi₂, ZrO₂, Al₂O₃, and MgO, and two different paraffins were used as starting materials. The influences of powder volume content, Zr/Si ratio, binder composition and processing temperature on the flowability of the feedstocks were observed in this line of experiments. A four-factorial fully fractional CCC-model was used. Finally the reliability of the computed statistical model was experimentally verified by means of two compositions, whose rheological behaviour has been predicted by the software.     

Spray-drying of ceramics for plasma-spray coating

The spray-drying process of ceramics which are candidate materials for thermal barrier coatings (TBCs), i.e. 3YSZ+0, 2, 4, 6 wt.% Al₂O₃, is discussed in this paper. The two most important properties of spray-dried powders to determine the coating quality are density and particle size. Polyethyleneimine (PEI) acts as both an organic binder and a dispersant giving low viscosity in the suspension. The optimised suspension composition is: ⩾ 33.6 vol.% powder+1.8 wt.% PEI+ethanol, and operational parameters of the spray-dryer: drying temperature 175°C, feeding rate 55 cm³/min, feeding pressure 1.013×10⁴ Pa.     

The rheological properties of paper coating suspensions and their application

Conclusions  The experimental results lead to the following observations:

In‐line Rheological Behaviors of Gun Propellant Substitute Assisted with Supercritical CO2 in Extrusion Processing

To improve the rheological behaviors of gun propellants, SC‐CO₂ was injected into the gun propellant substitute in extrusion processing. A slit die rheometer was used to investigate the in‐line rheological behaviors of CA solution. A Power model was applied to describe the rheological behaviors of CA/SC‐CO₂ mixtures. The viscosity and pressure of CA solution obviously decrease with the assistance of SC‐CO₂. The viscosity of CA solution reduces by 16.64 % at 55 °C and 10 s⁻¹ with the presence of SC‐CO₂. Increasing the processing temperature makes the viscosity of CA/SC‐CO₂ mixture decrease remarkably, but it weakens the plasticization of SC‐CO₂ to CA. Although the increasing solvent content improves the flow of the CA/SC‐CO₂ mixture, it lowers the strength of CA/SC‐CO₂ mixture, which is not in favor of the quality of product. The investigation of the in‐line rheological behaviors of CA/SC‐CO₂ mixture is fundamental and important for the safe extrusion of gun propellants assisted with SC‐CO₂.     

Viscoelastic properties of suspensions with weakly interacting particles

Rheological characterization of a model suspension containing hydroxyl-terminated polybutadiene and glass beads with filler concentration up to 30% by volume was performed by using a Haake parallel disk rheometer. The rheological tests conducted were the measurement of the storage modulus, G′, loss modulus, G′, and complex viscosity, η*, as functions of the frequency and the steady shear viscosity as a function of the shear rate. The linear viscoelastic region was determined to extend up to 50% strain by measuring G′, G′, and η* as functions of strain amplitude. By using multiple gap separations between the disks, it was found that the suspension did not exhibit slip at the walls of the rheometer. G′ and G′ were used to determine the relaxation times distribution, G_i(λ_i, ⊘) as functions of the relaxation time, λ_i, and the filler content, ⊘. The relaxation moduli, G_i(λ_i, ⊘), decreased with the relaxation time, but increased with the filler content. The Cox–Merz rule was also observed to be valid for these suspensions. © 1998 John Wiley & Sons, Inc. J. Appl. Polym. Sci. 70: 507–514, 1998     

Effect of the addition of silica sol on layered extrusion forming of Al2O3-based cores

Layered extrusion forming of ceramic cores with a nanoceramic suspension as a binder was conducted to explore a novel method to produce complex-shaped ceramic cores. Green bodies were prepared using Al₂O₃ particles as precursor materials and silica sol combined with aqueous polyvinyl alcohol solution as a binder. Increasing the silica sol content increased the viscosity of the slurry, enhanced the green bending strength, and decreased the green linear shrinkage. The green microstructure showed the nanosized silica particles were deposited on the surface of the Al₂O₃ particles and among the pores formed by Al₂O₃ particles irregular packing. In addition, increasing the silica sol content increased the bending strength, however, decreased linear shrinkage and open porosity of the sintered bodies. During sintering, the nanosized silica particles converted to the melting phase and reacted with Al₂O₃ and the microstructure of sintered bodies indicated the existence of sintering neck with silica sol addition.     

Changes in rheological properties of polyvinyl chloride plastisols with storage time

In this study, the changes in the rheological curves of polyvinyl chloride (PVC) plastisols with increasing storage time and the factors affecting these changes were studied. The results show that with increasing storage time, all the “viscosity–temperature” and “viscosity–time” rheological curves of PVC plastisols exhibit nonnormal distribution change trends, that is, the viscosity first decreases, and then changes from slow increasing to rapid increasing, forming a shoulder peak, reaches to the maximum value and gradually decreases. With increasing storage time, the complex viscosities of PVC plastisols increased generally in the first, the second, and the fourth stages, and the gelation process shortened in the third stage. The first and second stages of the viscosity changes reflect the “time–temperature” equivalence principle of PVC plastisol in suspension stage. However, the maximum viscosity of PVC plastisol corresponding to temperature Tη_max does not change with increasing storage time.     

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).