Degree Of Shear Thinning Research Articles

Rheology of linear and branched styrene–acrylonitrile copolymers. Similarities and differences

A comprehensive investigation of rheological properties of linear and branched styrene-acrylonitrile copolymer specimens with similar molecular characteristics has been carried out. During the steady-state shear flow, the viscosity properties of both specimens are described by the Cross equation. In this case, the branched copolymer is characterized by a higher viscosity and shear thinning degree as well as by substantially lower shear rate values corresponding to transition to the non-Newtonian flow region. The elasticity of the branched copolymer melt (estimated from the value of the first normal stress difference) is considerably higher than that of the linear. This is reflected on the characteristics of occurrence of unstable flow at high shear rates. Rougher extrudate surface distortions are characteristic for the branched copolymer, and the shear rate corresponding to their occurrence is noticeably lower than for the linear copolymer. The dynamic characteristics of the copolymers being compared also attest to a greater elasticity of the branched specimen. An investigation of the viscoelastic properties in a wide temperature range allowed constructing a generalized frequency dependence of dynamic moduli encompassing various regions of the relaxation states of the copolymer specimens. Continuous relaxation spectra were calculated by means of the Mellin transform. It is shown that relaxation phenomena caused by segmental mobility doesn’t depend on the presence of branchings, whereas branching of the chain has a substantial effect on translation mobility of the chain as a whole. Branching leads to a noticeable increase of transient elongation viscosity but has almost no effect of strain hardening of the melt.

Novel acrylic nanocomposites containing in-situ formed calcium phosphate/layered silicate hybrid nanoparticles for photochemical rapid prototyping, rapid tooling and rapid manufacturing processes

Novel acrylic nanocomposites containing calcium phosphate/layered silicate hybrid nanoparticles have been developed for use in photochemical Rapid Prototyping processes like Structural Light Modulation (SLM) and Stereolithography (SL). When tertiary alkyl amines, protonated with phosphoric acid, were added to an acrylic suspension of calcium bentonite, the cation exchange of Ca2+ rendered bentonite organophilic, caused swelling, intercalation and dispersion of silicate nanoplatelets in the monomer. The simultaneous precipitation of calcium phosphate onto the silicate nanoplatelets accounted for the in-situ formation of hybrid nanoparticles. The uniform dispersions of such hybrid nanoparticles afforded a high degree of shear thinning, reflecting the presence of anisotropic filler particles, and increased photosensitivity in SLM with respect to the unfilled resin. Young’s modulus of green and postcured parts increased by 30% at a filler content of 15 wt.% with respect to that of the unfilled benchmark material. This enhanced stiffness was paralleled by 30% increased fracture toughness. As evidenced by fracture surface analysis using Environmental Electron Microscopy (ESEM) and optical microscopy, the improved energy dissipation at the crack tip correlated with roughness of the fracture surfaces, increasing with increasing filler content. Moreover, the examination of the volumetric polymerization shrinkage and the fabrication of H-shaped diagnostic specimens revealed that the nanocomposites were processed with high accuracy, increasing with increasing filler content. Nanocomposite morphologies, examined by means of Transmission Electron Microscopy (TEM), demonstrated that the large primary bentonite particles with average diameters >10 μm fragmented into much smaller particles with average diameters in the range of 1 μm. According to TEM and Wide Angle X-Ray Scattering (WAXS), such in-situ formed nanoparticles were composed of both stacks of organoclay nanoplatelets and also isolated nanoplatelets typical for fully exfoliated organoclays.

Electrospun Hybrid Soy Protein/PVA Fibers

AbstractRheological behavior and spinnability of biodegradable materials based on SPI and PVA were studied for the production of electrospun fibers. pH level, processing temperature, and heating time were adjusted to investigate the effects of denaturing of soy protein on the rheology of SPI/PVA solutions. The results show that zero shear viscosity and degree of shear thinning of the SPI solution can be controlled by adjusting pH level and thermal treatment. The continuous production of uniform SPI/PVA fibers was achieved by electrospinning. The presence and amount of soy protein in the electrospun fibers was determined by EMPA and elemental analysis, confirming that the SPI was well incorporated into the PVA and remained in the electrospun fibers.magnified image

Viscoelastic flow in a two-dimensional collapsible channel

We compute the flow of three viscoelastic fluids (Oldroyd-B, FENE-P, and Owens blood model) in a two-dimensional channel partly bounded by a tensioned membrane, a benchmark geometry for fluid–structure interactions. The predicted flow patterns are compared to those of a Newtonian liquid. We find that computations fail beyond a limiting Weissenberg number. Flow fields and membrane shape differ significantly because of the different degree of shear thinning and molecular extensibility underlying the three different microstructural models.

Formation of viscoplastic drops by capillary breakup

The process of growth and detachment of drops from a capillary nozzle is studied experimentally by high-speed imaging. Newtonian drops are compared to shear-thinning and viscoplastic drops. Both Newtonian and shear-thinning fluid drops grow on the end of the capillary until a maximum supportable tensile stress is reached in the drop neck, after which they become unstable and detach. The critical stress is not influenced by variations in viscosity or in the degree of shear thinning. Viscoplastic fluids show a different behavior: at low values of the yield stress, the critical stability behavior is similar to that of Newtonian and shear-thinning drops. Above a threshold value, characterized in terms of the drop size, surface tension and tensile yield-stress magnitude, yield-stress forces are larger than surface forces, and the maximum tensile stress achievable in the drop neck at the point of critical stability is governed by the von Mises criterion.

Tension thickening, molecular shape, and flow birefringence of an H-shaped polymer melt in steady shear and planar extension

Despite recent advances in the design of extensional rheometers optimized for strain and stress controlled operation in steady, dynamic, and transient modes, obtaining reliable steady-state elongational data for macromolecular systems is still a formidable task, limiting today's approach to trial-and-error efforts rather than based on a deep understanding of the deformation processes occurring under elongation. Guided, in particular, by the need to understand the special rheology of branched polymers, we studied a model, unentangled H-shaped polyethylene melt using nonequilibrium molecular dynamics simulations based on a recently developed rigorous statistical mechanics algorithm. The melt has been simulated under steady shear and steady planar extension, over a wide range of deformation rates. In shear, the steady-state shear viscosity is observed to decrease monotonically as the shear rate increases; furthermore, the degree of shear thinning of the viscosity and of the first- and second-normal stress coefficients is observed to be similar to that of a linear analog of the same total chain length. By contrast, in planar extension, the primary steady-state elongational viscosity eta(1) is observed to exhibit a tension-thickening behavior as the elongation rate epsilon increases, which we analyze here in terms of (a) perturbations in the instantaneous intrinsic chain shape and (b) differences in the stress distribution along chain contour. The maximum in the plot of eta(1) with epsilon occurs when the arm-stretching mode becomes active and is followed by a rather abrupt tension-thinning behavior. In contrast, the second elongational viscosity eta(2) shows only a tension-thinning behavior. As an interesting point, the simulations predict the same value for the stress optical coefficient in the two flows, revealing an important rheo-optical characteristic. In agreement with experimental indications on significantly longer systems, our results confirm the importance of chain branching on the unique rheological properties of polymer melts in extension.

Rheology and thermal properties of polypropylene modified by reactive extrusion with dicumyl peroxide and trimethylol propane triacrylate

AbstractTrimethylol propane triacrylate (TMPTA) and dicumyl peroxide (DCP) were used to modify polypropylene (PP) by reactive extrusion in a twin‐screw extruder. The effects of TMPTA concentration on oscillatory shear rheology, melt elongational rheology, and thermal properties were comparatively evaluated. Fourier transform infrared spectroscopy indicated that the grafting reaction took place and TMPTA had been grafted onto the PP backbone. Differential scanning calorimetric results showed that the crystallization temperatures of modified PPs were higher than those of the initial and degraded PPs. The rheological characteristics such as higher storage modulus (G′) at low frequency, increased degree of shear thinning, a plateau in tan δ–ω plot, and upturning at high viscosity in the Cole–Cole plots proved that the long‐chain branches have been added to the linear PP molecule. The melt elongational rheology showed that the modified PPs exhibit improved melt strength and increased elongational viscosity in the presence of TMPTA and DCP, which further confirmed the existence of long‐chain branching (LCB) in their backbone. According to the analytical results from oscillatory shear rheology and elongational rheology, it can be inferred that the LCB level in modified samples increases with an increase in TMPTA concentration. © 2009 Wiley Periodicals, Inc. Adv Polym Techn 28:16–25, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/adv.20146

Material functions of liquid n-hexadecane under steady shear via nonequilibrium molecular dynamics simulations: Temperature, pressure, and density effects

Computer experiments of rheology regarding the effects of temperature (T), pressure (P), and density (rho) on steady shear flow material functions, which include viscosity (eta) and first and second normal stress coefficients (psi(1) and psi(2)) depending on shear rate (gamma), have been conducted via nonequilibrium molecular dynamics simulations for liquid n-hexadecane. Straightforwardly, using both characteristic values of a zero-shear-rate viscosity and critical shear rate, eta-gamma flow curves are well normalized to achieve the temperature-, pressure-, and density-invariant master curves, which can be formulary described by the Carreau-Yasuda rheological constitutive equation. Variations in the rate of shear thinning, obviously exhibiting in eta-gamma, psi(1)-gamma, and -psi(2)-gamma relationships, under different T, P, and rho values, are concretely revealed through the power-law model's exponent. More importantly, at low shear rates, the fluid explicitly possesses Newtonian fluidic characteristics according to both manifestations; first and second normal stress differences decay to near zero, while nonequilibrium states are close to equilibrium ones. Significantly, the tendency to vary of the degree of shear thinning in rheology is qualitatively contrary to that of shear dilatancy in thermodynamics. In addition, a convergent transition point is evidently observed in the -psi(2)/psi(1)-gamma curves undergoing dramatic variations, which should be associated with shear dilatancy, as addressed analytically.

Weakly nonlinear viscoplastic convection

The development of thermal convection is studied for a viscoplastic fluid. If the viscosity is finite at zero shear rate, the critical Rayleigh number for linear instability takes the value given by a Newtonian fluid with that viscosity. The subsequent weakly nonlinear behaviour depends on the degree of shear thinning: with moderate shear thinning, convective overturning for a given temperature difference is amplified relative to the Newtonian case. If the reduction in viscosity is sufficiently sharp the transition becomes subcritical (the relevant situation for many regularized constitutive laws). For an infinite viscosity at zero shear rate, or a yield-stress, the critical Rayleigh number for linear instability is infinite. Nonlinear convective overturning, however, is still possible; we trace out how the finite-amplitude solution branches develop from their Newtonian counterparts as the yield stress is increased from zero for the Bingham fluid. Laboratory experiments with a layer of Carbopol fluid heated from below confirm that yield strength inhibits convection but a sufficiently strong perturbation can initiate overturning.

A numerical study for the cessation of Couette flow of non-Newtonian fluids with a yield stress

We performed a numerical study for the cessation of Couette flow of non-Newtonian fluids with a yield stress. We solved this problem by employing our recently developed constitutive model for such fluids [H. Zhu, Y.D. Kim, D. De Kee, Non-Newtonian fluids with a yield stress, J. Non-Newton. Fluid Mech. 129 (2005) 177]. In the numerical simulation we adopted the finite volume method with an implicit scheme in time. We computed the finite stopping times for fluids with different yield stresses and shear thinning properties. Our numerical results show that the yield stress play a important role in determining the finite stopping times for non-Newtonian fluids with a yield stress; on the other hand, the shear thinning property does affect the finite stopping times in that the larger the degree of shear thinning, the longer the finite stopping time. The present study is an extension of the previous work by Chatzimina et al. [M. Chatzimina, G.C. Georgiou, I. Argyropaidas, E. Mitsoulis, R.R. Huilgol, Cessation of Couette and Poiseuille flows of a Bingham plastic and finite stopping times, J. Non-Newton. Fluid Mech. 129 (2005) 117], in which only Bingham fluids are considered.

Thermal and rheological properties of highly concentrated PET composites with ferrite nanoparticles

Ferrite nanoparticles were introduced into poly(ethylene terephthalate) (PET) in a melt state at 270 °C upto 20 wt%, and the thermal and rheological properties of the nanocomposites were investigated. The introduction of ferrite nanoparticles increased a little the crystallization temperature ( T c) of PET by ca. 3 °C, while it had little effect on the melting temperature ( T m). In addition, it increased both heat of crystallization (Δ H c) and heat of fusion (Δ H m) with ferrite content. PET nanocomposites with ferrite 5 wt% and above exhibited an increased thermal stability and a two-stage degradation. The dynamic viscosity of PET nanocomposites was increased with ferrite content. However, ferrite loading of 5 wt% and above produced a high degree of shear thinning leading to even lower viscosity in a high frequency range than that of pure PET. The nanocomposites gave a non-zero positive value of yield stress, which was notably increased particularly from 5 wt% loading. In the Cole–Cole plot, at contents 1 wt% and above, ferrite nanoparticles caused the deviation from the master curve and a reduced slope. In addition, the relaxation time was increased with ferrite content and an increasing degree was more notable at a lower frequency.

Rheological properties of ferrite nanocomposites based on nylon‐66

AbstractEffects of ferrite nanoparticles (0.1–20 wt %) on the rheological and other physical properties of nylon‐66 were investigated. The presence of ferrite nanoparticles less than 1 wt % increased the crystallization temperature (Tc) by 4.2 °C with ferrite content, but further addition decreased Tc. The onset temperature of degradation was increased by 7.3 °C at only 0.1 wt % loading of ferrite, after which the thermal stability of nylon‐66 was decreased with ferrite content. The incorporation of ferrite nanoparticles more than 5 wt % increased the dynamic viscosity (η′) with the loading level. Further, it produced notably shear thickening behavior in the low frequency, after which high degree of shear thinning was followed with ferrite content. In the Cole–Cole plot, the nanocomposites with ferrite lower than 5 wt % presented a single master curve, while further addition gave rise to a deviation from the curve. The relaxation time (λ) was increased with ferrite content and the difference of λ between nylon‐66 and its nanocomposite was greater at lower frequency. The tensile strength was a little increased up to 1 wt % loading, after which it was decreased with increasing the loading level. In addition, the introduction of the nanoparticles increased tensile modulus and decreased the ductility with ferrite content. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 371–377, 2006

Flow restrictor design for extrusion slit dies for a range of materials: Simulation and comparison of optimization techniques

Based on a finite element simulation of slit die performance that takes into account the coupling of melt flow and die body deflection due to melt pressure, a genetic algorithm, a multipoint approximation optimization technique, and a gradient-based optimization technique are applied to determine choker bar profiles to give optimum melt flow distribution. The performances of the optimization methods are compared with respect to efficiency and ability to obtain a global optimum. This work involves three materials of varying degrees of shear thinning, each at a high and a low flow rate. The developed software makes it possible to predict whether restrictor adjustment can produce acceptably uniform flow distribution in a given application, using a given die, and shows what restrictor profile should be set up, thereby eliminating or reducing the need for on-line trial and adjustment, and establishing if the same die can be used for a range of materials to reduce costs.

Temperature dependence of rheological behavior of a metallic automotive waterborne basecoat

The temperature dependence of the rheological behavior of a metallic automotive waterborne basecoat was studied with the temperature ranging from 10 to 45 °C. It is found that the paint exhibits both shear thinning and thixotropic behavior. The degree of shear thinning and thixotropy increases as the temperature increases. Small amplitude oscillatory shear tests show that the basecoat displays more elasticity at higher temperature. Both steady shear viscosity and dynamic properties indicate that the paint gradually develops a network structure as the temperature increases, which leads to increased viscosity. The viscosity at very low shear rate at 45 °C is about 500 times greater than that at 20 °C. The high shear viscosity decreases with the increase of temperature in the range of 10–35 °C, and then increases with temperature up to 45 °C. The Ford cup #4 measurements show good agreement with the steady-state viscosity at high shear rate.

Gas displacing liquids from non-circular tubes: high capillary number flow of a shear-thinning liquid

Transient, three-dimensional finite element analysis has been used to investigate the displacement of a shear thinning liquid from prismatic channels of square, rectangular and trapezoidal cross sections. Inertia, gravity and surface tension effects are neglected and the results therefore apply in the limits of low Reynolds and high capillary numbers. The analysis is carried out in a fixed frame of reference and gas penetration is modelled as the bubble moves down the tube, which is long relative to its transverse dimension. Results are provided for the thickness of the layers left on the channel walls under developed conditions, and the fraction of the cross section occupied by liquid, as a function of the channel cross-sectional geometry and the degree of shear thinning, modelled using the power law. Interface contours on the channel cross sections are displayed. It is found for the Newtonian liquid that the fingering instability arises in the rectangular channel when the aspect ratio reaches about five. Shear thinning delays the onset of the instability to higher aspect ratios. The results are systematized, and insights gained into the influence of channel geometry and shear thinning, by noting a qualitative, inverse relationship between the deposited layer thickness and the shear rate at the wall in the flow ahead of the bubble.

Polymer/organoclay nanocomposites with biodegradable aliphatic polyester and its blends: preparation and characterization

Abstract Synthetic biodegradable aliphatic polyester (BAP) and its blends with polyepichlorohydrin (PECH) were processed with organophilic montmorillonite (OMMT) to make polymer/organoclay nanocomposites via a solvent casting method. Interlayer spacings of each system, measured by X-ray diffraction analysis, suggested that BAP had better affinity to clay than PECH in a competitive intercalation mechanism. The internal structures were visualized using transmission electron microscopy and were discussed in conjunction with the result of other experimental studies, such as thermogravimetric analysis and rheometry. In rheological measurements, the degree of shear-thinning increased drastically with OMMT loading, and both the storage modulus and loss modulus were found to increase with OMMT loading at all frequencies. Throughout this study, we compared the behaviours of polymer/clay nanocomposites in homo and blend systems, and examined the role of various parameters on the material characteristics of polymer/clay nanocomposites.

Rheology properties of glucopyranoside stabilized oil–water emulsions: effect of alkyl chain length and bulk concentration of the surfactant

Effect of the alkyl chain length and concentration of the surfactant on Rheology properties of O/W emulsions stabilized with homologues series of glucopyranoside was studied. The Rheology properties in turn are being used to describe emulsion structure, emulsifying effect and emulsion stability. The emulsifying property was also explored through interfacial tension measurement. The homologues series of glucopyranoside possess identical polar head group structure, whereas the alkyl chain length varied from C 7 to C 10. The O/W emulsions were prepared by mixing 0.8 volume fractions of oil phase into a much smaller volume fraction of water phase; in this case 0.2 volume fractions of aqueous surfactant solution. Given energy input during emulsification and bulk surfactant concentration kept constant, the given oil to water mixing ratio would give high internal phase ratio concentrated O/W emulsions with dispersed phase volume fraction of ≈0.8, whereas the average size of the droplets was varied with the variation of both the alkyl length and the surfactant concentration. The average droplet size decreased with increasing both the length of the alkyl tail and the bulk concentration of the surfactant, so did interfacial free energy and cmc. As expected the Rheology properties of these emulsion samples depended strongly on average droplet size of the dispersed phase. The steady shear Rheology properties, such as shear viscosity, degree of shear thinning and σ y enhanced with the decrease in average droplet size. This implies that not only the size of the emulsion droplets decreased with increasing both the length of the alkyl tail and bulk concentration of the surfactant, but also the emulsion and its storage stability were improved and enhanced with the increase in these two factors. Likewise, in the oscillatory shear mode the degree of viscoelasticity also depended on average droplet size. The fact that the G ′ ≫ G ″ , it is evident that the elastic property is dominant in these emulsions over viscous property. As expected the degree of elasticity enhanced with decreasing average droplet size of the dispersed phase. The G ′ ( ω ) ≫ G ″ ( ω ) at all measured ω, but the G′( ω 2) function was found to be linear whereas the G″( ω) functions was independent, indicating that the oscillatory shear property of these emulsions can not be adequately described by Maxwell type viscoelastic model. Also, the fact that, δ of these emulsions was much smaller than 45° at all measured oscillatory ω, it is evident that elastic property of these emulsions dominated their viscous property. δ of these emulsions decreased strongly with decreasing droplet size, further implying that the degree of elasticity of these emulsions enhanced and improved with decreasing average droplets size.

Gas displacing liquids from tubes: high capillary number flow of a power law liquid including inertia effects

Computer simulation has been used as a virtual experimental tool to investigate the displacement of a shear thinning Power Law liquid from a cylindrical tube by a gas, in the limit of high capillary number and in the absence of gravity effects. Two scenarios have been considered. In the first, gas enters at a steady rate, and the gas penetration velocity and residual wall layer thickness attain steady values. In the second, a constant gas pressure is applied at the inlet, and the gas penetration rate accelerates as the column of liquid ahead of it becomes shorter. The first set of experiments confirm that the developed wall layer thickness falls with increased degrees of shear thinning and with increased Reynolds Number, and quantifies the latter effect for the first time. The relationship is summarized by a correlation formula for dimensionless layer thickness as a function of Power Law index, n, and an appropriately defined Reynolds group in the range 0.1⩽ n⩽1.0,0.001⩽ Re⩽100. The flow pattern ahead of the gas bubble throughout the range of these experiments was always of the ‘by-pass’ type, consistent with a generalized criterion for the transition between by-pass and re-circulating flow which is derived for a Power Law liquid. In the second set of experiments, where a constant gas inlet pressure is applied, giving accelerating gas penetration, a comparison of layer thickness values at various axial positions, with those obtained at corresponding Reynolds number in steady flow, showed close agreement, though a small discrepancy for the highest Reynolds numbers could indicate some influence of inertia in the accelerating liquid column. At higher Reynolds number, in both steady and accelerating flow, the gas bubble near to the inlet shows a concave region on the axis, with re-circulation in the liquid ahead of it. As the bubble moves down the tube, the radius of this concavity decreases and a steady convex profile is eventually attained, with reversion of the flow to by-pass type. We show that the origin of this is inertial. The results have applications in a number of technologies, including gas-assisted injection moulding of plastics and certain gas liquid reactors.

Effect of the viscoelasticity of the suspending fluid on structure formation in suspensions

Particles suspended in viscoelastic media are known to develop string-like structures when being sheared at sufficiently high shear rates. The occurrence of these strings has been attributed to normal stress effects, and it has been suggested that a critical Weissenberg number of 10 governs their appearance. This assumption is tested here, using suspensions of monodisperse polystyrene spheres. The level of viscosity and elasticity, as well as the degree of shear thinning of the suspending fluid, have been varied over a wide range of values. The critical Weissenberg number for the onset of string formation, as detected by microscopy, was not constant but ranged from 0.5 to 16.5 in contrast with literature results. Suspensions in highly elastic Boger fluids did not display any alignment at all, even at Weissenberg numbers as high as 260. Small angle light scattering (SALS) was used to calculate an alignment factor. It provides an objective and quantitative measure for the degree of alignment of the particles. It also makes it possible, in some cases, to determine precisely the interparticle spacing in the strings. The kinetics of string formation can be studied as well, they seem to be strain-controlled. SALS experiments with different gap spacings have been used to demonstrate that the string formation occurs in the bulk of the liquid rather than at the walls of the cell.

Preparation of alumina by aqueous gelcasting

The alumina ceramic was prepared by aqueous gelcasting. The effects of zeta potentials, solid loading, dispersant content and milling time on the alumina suspension were studied systematically. The dispersant content has remarkable effects on the viscosity of the suspension. The appropriate dispersant concentration for alumina aqueous slurry with the solid loading of 55 vol.% is 0.6 wt.%. It can be seen that all suspensions (50–56 vol.% solid loading) exhibited a shear-thinning behavior and relatively low viscosity, which was suitable for casting. The degree of shear thinning and the viscosity at high shear rates increased with increasing volume fraction of solid. As the milling time prolongs, viscosity of the suspension decreases first, then the plateau appears and the average diameter keeps changeless. When the milling time was shorter than 20 h, the viscosity of slurries decreased gradually as the time of milling increased. After 20 h milling, the viscosity of the slurry tended to be consistent. Therefore, the ball milling time should be equal to or more than 20 h to obtain a stable suspension at equilibrium. The time available for casting the slurry (idle time) can be controlled by the amounts of initiator and catalyst added to the slurry as well as by the processing temperature. Micrograph of the gelcast green body was homogeneous.