Showing papers on "Rheometer published in 2004"

Miniature universal testing platform: from extensional melt rheology to solid-state deformation behavior

Abstract: Described is a detachable fixture for a rotational rheometer, the Sentmanat Extensional Rheometer Universal Testing Platform, which incorporates dual wind-up drums to ensure a truly uniform extensional deformation during uniaxial extension experiments on polymers melts and elastomers. Although originally developed as an extensional rheometer, this highly versatile miniature test platform is capable of converting a conventional rotational rheometer host system into a single universal testing station able to characterize a host of physical properties on a variety of polymer melts and solid-state materials over a very wide range of temperatures and kinematic deformations and rates. Experimental results demonstrating these various testing capabilities are presented for a series of polymers of varying macrostructure and physical states.

Shear Properties of Brain Tissue over a Frequency Range Relevant for Automotive Impact Situations: New Experimental Results

Abstract: This research aims at improving the definition of the shear linear material properties of brain tissue. A comparison between human and porcine white and gray matter samples was carried out over a new large frequency range associated with both traffic road and non-penetrating ballistic impacts. Oscillatory experiments were performed by using an original custom-designed oscillatory shear testing device. The findings revealed that no significant difference occured between the linear viscoelastic behavior of the porcine and the human brain tissue. On the average, the storage modulus (G') and the loss modulus (G") of the white matter increased respectively from 2.1 +/- 0.9 kPa to 16.8 +/- 2.0 kPa and from 0.4 +/- 0.2 kPa to 18.7 +/- 2.3 kPa between 0.1 and 6300 Hz at 37 degrees C. In addition, the gray and white matter behaviors seemed to be similar at small strains. The reliability of the data and the robustness of the experimental protocol were checked using a standard rheometer (Bohlin C-VOR 150). A good agreement was found between the data obtained in the frequency and time field. As a result, the linear relaxation modulus was determined over an extensive time range (from 10(-5) s to 300 s). In a first approach, the nonlinear behavior of brain tissue was studied using stress relaxation tests. Brain tissue showed significant shear softening for strains above 1% and the time relaxation behavior was independent of the applied strain. On this basis, a visco-hyperelastic model was proposed using the generalized Maxwell model and the Ogden hyperelastic model. These models respectively describe the linear relaxation modulus and the strain dependence of the shear stress.

Shear and Dilational Surface Rheology of Oppositely Charged Polyelectrolyte/Surfactant Microgels Adsorbed at the Air−Water Interface. Influence on Foam Stability

Abstract: Using the oscillating drop method as well as the interfacial stress rheometer (ISR), we measure the surface dilational and shear properties of adsorbed layers of oppositely charged polymer−surfactant complexes at the air−water interface. These data are compared to foam volume data measured for the corresponding solutions. We show that the shear surface moduli G‘ and G‘ ‘ exhibit a maximum at a molar ratio of polymer monomers and surfactant in solution equal to 1. This maximum is strongly correlated to surface ellipticity measurements that indicate a strong adsorption of polymer−surfactant complexes at the air−water interface. These hydrophobic interfacial complexes generate a viscoelastic layer due to the polymer chain entanglements and hydrophobic interactions between bound surfactant. The viscous behavior of the surface layers is further characterized via creep, frequency sweep measurements, and surface aging experiments. Over a 10 h period we find that G‘ and G‘ ‘ increase logarithmically with time, re...

Rheological properties of short fiber filled polypropylene in transient shear flow

Abstract: The behavior of a commercial short glass fiber filled polypropylene has been studied. It is well known that a thermoplastic polymer filled with short fibers with an initial isotropic orientation state exhibits a viscosity overshoot when sheared in the parallel plate geometry of a rheometer in the molten state. In this work, following a first deformation, the sample has been sheared in the reverse direction and a small viscosity overshoot has been measured. Similarly under stress growth experiments a large normal stress overshoot was observed. When the flow was reversed the normal stresses were initially negative and then exhibited a small positive overshoot. A model has been used to simulate these viscosity and normal stress overshoots. It is based on the Folgar–Tucker equation for fiber motion and the Lipscomb constitutive equation. This equation has been empirically modified to correctly predict the overshoots.

Rheology of poly(propylene)/clay nanocomposites

Abstract: The linear and nonlinear shear rheological behaviors of poly(propylene) (PP)/clay (organophilic-montmorillonite) nanocomposites (PP/org-MMT) were investigated by an ARES rheometer. The materials were prepared by melt intercalation with maleic anhydride functionalized PP as a compatibilizer. The storage moduli (G′), loss moduli (G″), and dynamic viscosities of polymer/clay nanocomposites (PPCNs) increase monotonically with org-MMT content. The presence of org-MMT leads to pseudo-solid-like behaviors and slower relaxation behaviors of PPCN melts. For all samples, the dependence of G′ and G″ on ω shows nonterminal behaviors. At lower frequency, the steady shear viscosities of PPCNs increase with org-MMT content. However, the PPCN melts show a greater shear thinning tendency than pure PP melt because of the preferential orientation of the MMT layers. Therefore, PPCNs have higher moduli but better processibility compared with pure PP.© 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 2427–2434,2004

Estimation of the parameters of Herschel-Bulkley fluid under wall slip using a combination of capillary and squeeze flow viscometers

Abstract: The determination of the parameters of viscoplastic fluids subject to wall slip is a special challenge and accurate results are generally obtained only when a number of viscometers are utilized concomitantly. Here the characterization of the parameters of the Herschel-Bulkley fluid and its non-linear wall slip behavior is formulated as an inverse problem which utilizes the data emanating from capillary and squeeze flow rheometers. A finite element method of the squeeze flow problem is employed in conjunction with the analytical solution of the capillary data collected following Mooney’s procedure, which uses dies with differing surface to volume ratios. The uniqueness of the solution is recognized as a major problem which limits the accuracy of the solution, suggesting that the search methodology should be carefully selected.

Rheological analysis of vapor‐grown carbon nanofiber‐reinforced polyethylene composites

Abstract: The melt rheological analysis of high-density polyethylene reinforced with vapor-grown carbon nanofibers (VGCNFs) was performed on an oscillatory rheometer. The influence of frequency, temperature, and nanofiber concentration (up to 30 wt %) on the rheological properties of composites was investigated. Specifically, the viscosity increase is accompanied by an increase in the elastic melt properties, represented by the storage modulus G′, which is much higher than the increase in the loss modulus G″. The composites and pure PE exhibit a typical shear thinning behavior as complex viscosity decreases rapidly with the increase of shearing frequency. The shear thinning behavior is much more pronounced for the composites with high fiber concentration. The rheological threshold value for this system was found to be around 10 wt % of VGCNF. The damping factor was reduced significantly by the inclusion of nanofibers into the matrix. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 155–162, 2004

Gap-dependent microrheometry of complex liquids

Abstract: We review recent developments in the field of microrheometry and describe a new flexure-based microrheometer (FMR) that can make measurements of the steady shear viscosity of transparent or optically-opaque complex fluids using 1–10 μl samples. We document the complex microrheology that may arise in a typical consumer product such as a skin creme, when the microstructure of the multiphase material (in this case a oil–water emulsion containing microcrystalline wax particles) and the characteristic length scale of the flow device become comparable. The material is a highly filled emulsion and conventional cone-and-plate rheometry shows that the material exhibits an apparent yield stress. Analogous tests in the microrheometer show that a sequence of flow transitions occurs which lead to an apparent viscosity that is both gap– and shear-stress– (or shear-rate)–dependent. The microrheological measurements are combined with a slip analysis and optical microscopy to formulate a ‘phase diagram’ identifying the critical stress and corresponding slip velocity for each flow transition as a function of the gap. In the case of the skin creme investigated in the present study, the flow behavior in small gaps below 100 μm is dominated by the presence of micro-crystalline wax particles with mean diameters in the range of 50 μm. The sudden decrease in the apparent viscosity, or equivalently the sudden increase in the slip velocity between two surfaces with decreasing gap and increasing applied stress, is of importance in end-user perceptions of textural quantities such as ‘slipperiness’ or ‘lubricity’ which are typically used to characterize creams and other complex consumer products. The ability to progressively vary the gap between the two shearing surfaces means that the flexure-based microrheometer developed in the present work may find future applications in helping to bridge the gap between the traditionally-distinct fields of bulk rheological characterization and tribological testing.

Shear flow behavior of confined magnetorheological fluids at low magnetic field strengths

Abstract: The non-linear properties of iron based magneto-rheological (MR) fluids are investigated at low magnetic field strengths (0–1.7 kA/m) and different gap thickness (0–500 μm) in a plate-plate configuration. Single-width chain models qualitatively predict the low-shear flow behavior when plotting the field-specific viscosity, ηF, as a function of the Mason number, Mn: a slope close to −1 is observed in log-log representations. Wall depletion effects are observed when the suspensions are sheared under the presence of low external magnetic fields applied and/or large gap distances. These results are correlated to frictional yield stress measurements and chain length distribution calculations in the presence of the external magnetic field. Finally, an equivalent slip layer thickness is calculated using the method of Yoshimura and Prud’homme.

Tribo-Rheometry: From Gap-Dependent Rheology to Tribology

Abstract: We describe a new tribo-rheometry fixture that can be utilized with a commercial torsional rheometer in order to explore the coupled rheological and tribological properties of complex fluids and solid–liquid systems. The fixture is self-leveling and both the normal load and the sample gap can be monitored or controlled. At large gaps, the fixture imposes an approximately constant shear rate on the sample and the bulk viscometric properties of the fluid can be measured. However, as the gap between the plates is reduced, the measured viscosity function becomes gap-dependent. For gaps on the order of the surface roughness of the plates, the device is operated under a constant applied load and the tribological properties of the fluid–solid pair can then be measured. Using this new tribo-rheometer fixture it is possible to obtain tribological information over a wider range of sliding velocities than is typically possible using conventional devices such as pin-and-disk systems. The data can be represented in the form of a classical Stribeck diagram or, by using a dimensionless gap-dependent shift factor, it is possible to construct a more general “friction map” of the gap- and load-dependent effective viscosity (Luengo et al. Wear 200 (1996)). The capabilities of this system are illustrated using a number of different lubricant fluids, for a range of normal stresses and variations in surface properties such as the mean roughness.

New extensional rheometer for creep flow at high tensile stress. Part II. Flow induced nucleation for the crystallization of iPP

Abstract: A novel extensional rheometer for creep experiments has been presented in part I. The device can be used also for an investigation of the influence of short term elongational flow on the structure development in crystalline polymers. It enables the application of defined portions of mechanical work to the polymer sample in its state of undercooled melt. For the purpose short-term elongational creep is used. With increasing mechanical work the number density of nuclei in iPP increases by decades. Results, as previously obtained with shear flow, are introduced for comparison. Prospects of a theoretical description are discussed.

Nonlinear shear rheology of polystyrene melt with narrow molecular weight distribution—Experiment and theory

Abstract: Measurements of the shear viscosity and the first and second normal stress coefficients are shown at 175 °C for a nearly monodisperse polystyrene melt with Mw=200 kg/mol (PS 200 k). Tests are performed on a cone-partitioned plate shear rheometer and cover a range of Weissenberg numbers (τdγ) from 0.13 to 40. Experimental problems encountered are the axial compliance of the rheometer and the normal force capacity of the transducer. The later limits the maximum shear rate to τdγ=40. Experimental data are compared with the models of Ottinger [termed thermodynamically consistent reptation model (TCR), Ottinger, H. C., J. Rheol. 43, 1461–1493 (1999)] for the convective constraint release parameter δ2=0, 1, and 2 and Mead, Larson, and Doi [termed Mead, Larson, and Doi (MLD), Mead, D. W., R. G. Larson, and M. Doi, Macromolecules 31, 7895–7914 (1998)] for δ2=1. The steady state and transient values of p21, N2, and N1 agree qualitatively well between both models and the experiment. The predicted normal stress ra...

Development and characterization of hydrocarbon polyol polyurethane and silicone magnetorheological polymeric gels

Abstract: Magnetorheological polymeric gels (MRPG) have been developed for use in semi-active magnetorheological fluid (MRF) dampers and other magnetorheological (MR) devices The novel MRPGs are prepared by suspending iron particles in polymeric gels Off-state (ie, no applied magnetic field) viscosity and settling behavior can be controlled through the selection of polymeric gels In this study, tunable rheological properties were investigated with a piston-driven flow type rheometer with a shear rate varying from 20 s−1 to 6,000 s−1 Silicone MRPG (with 845 wt % iron particles) has controllable viscosity and a high shear yield stress over a wide range of shear rates Silicone MRPG (795 wt % iron particles) has the lowest viscosity of those studied Polyurethane MRPG has the lowest settling rate The order of addition of magnetic particles and polymer during the polymerization process affects the MRPG final off-state apparent viscosity (80% increase in apparent viscosity for silicone MRPG polymerized after adding iron particles) This indicates that polymer gels modify the surface properties of the magnetic particles, causing interaction among particles The dynamic shear yield stress is higher for fluids with better dispersion stability Polyurethane MRPG, which has the lowest settling rate, has a high dynamic yield stress (23 kPa at 350 mT) Both dynamic and static shear stress values of the MRPGs were found to be similar in magnitude (5–8 kPa at 120 mT for silicone MRPG with 845 wt % iron particles and polyurethane MRPG), indicating that MRPGs can provide consistent performance in devices © 2004 Wiley Periodicals, Inc J Appl Polym Sci 92: 1176–1182, 2004

Capillary rheometry for polymer melts revisited

Abstract: Capillary rheometry provides an efficient access to high shear rate flow properties relevant for processing. An automated gas driven capillary rheometer developed at BASF enables accurate measurements at imposed wall shear stress, thus supplementing instruments operating at imposed flow rate. A simplified treatment of dissipative heating based on the assumption of a radially flat temperature profile is outlined and justified by means of finite element simulations. The combined treatment of dissipation and pressure dependent viscosity yields relations to treat throttling experiments at imposed flow rate. Throttle pressure coefficients from a long die and an orifice agree for LDPE but significantly differ for PαMSAN. The effect is explained on the basis of identical pressure coefficients for shear and elongational flows, with regard to a constant stress, however. The effect of melt compressibility is negligible in practical capillary rheometry if the temperature and pressure coefficients of the melt density are by an order of magnitude smaller than those of the viscosity. Gas pressure driven instruments allow an effective determination of wall slip velocities from Mooney plots. This is of advantage for the investigation of the mechanism of additives or processing aids. Furthermore, imposed pressure experiments are pertinent to investigate the spurt effect of HDPE and to demonstrate that two different slip processes contribute to the apparent flow curve above spurt.

Dynamic behavior of MR suspensions at moderate flux densities

Abstract: In many proposed applications, magnetorheological (MR) fluids are subjected to a dynamic stimulus with finite deformation. Dynamic behavior of reduced iron based MR suspensions under oscillatory shear is experimentally investigated using a parallel-plate rheometer. The effects of strain amplitude, oscillation frequency, and magnetic field strength on the dynamic behavior are studied. MR fluids behave as linear viscoelastic materials for only sufficiently small strain amplitudes. At large strain amplitudes, MR fluids are nonlinear viscoelastic or viscoplastic, where the storage modulus shows a decreasing trend with the strain amplitude. At large enough strain rate amplitudes, the yield stress contribution is negligible and the suspension response is Newtonian. The dependence of the dynamic behavior on the strain amplitude and oscillation frequency is qualitatively summarized in the form of a Pipkin diagram.

Rheological Properties of Banana Puree at High Temperatures

Abstract: The rheological behavior of banana puree was determined using a dynamic stress rheometer with a pressure couette fixture, which allowed experiments to be conducted at high temperature. The pressure couette was pressurized with compressed air to 206.8 kPa (gage pressure) and experiments were carried out at temperatures ranging from 30 to 120°C. The shear stress values ranged from 10 to 170 Pa and the shear rate values from 10−5 to 103 s−1. The model that best fitted the experimental data at all temperatures was the Herschel-Bulkley model. There was a usual tendency for the apparent viscosity to decrease with increasing temperature but an increase in apparent viscosity with increasing from 50 to 60°C and from 110 to 120°C was found. This could be due to interaction of polysaccharides present in banana puree. There was a slight difference between the apparent viscosity values for increasing shear stress sweeps and those for the decreasing shear stress sweeps suggesting time dependency of the rheolog...

Measurements of vocal fold tissue viscoelasticity: Approaching the male phonatory frequency range

Abstract: Viscoelastic shear properties of human vocal fold tissues have been reported previously. However, data have only been obtained at very low frequencies (⩽15 Hz). This necessitates data extrapolation to the frequency range of phonation based on constitutive modeling and time-temperature superposition. This study attempted to obtain empirical measurements at higher frequencies with the use of a controlled strain torsional rheometer, with a design of directly controlling input strain that introduced significantly smaller system inertial errors compared to controlled stress rheometry. Linear viscoelastic shear properties of the vocal fold mucosa (cover) from 17 canine larynges were quantified at frequencies of up to 50 Hz. Consistent with previous data, results showed that the elastic shear modulus (G′), viscous shear modulus (G″), and damping ratio (ζ) of the vocal fold mucosa were relatively constant across 0.016–50 Hz, whereas the dynamic viscosity (η′) decreased monotonically with frequency. Constitutive c...

Process Rheology and Mechanical Property Correlationship of Wood Flour-Polypropylene Composites:

Abstract: The rheological properties of wood fiber reinforced thermoplastics blends are important to practical industry application, however few research is available regarding this field. In this study, a C. W. Brabender Plasti-Corder torque rheometer was used to examine the flow behavior of wood flour filled polypropylene (PP) composites. The effect of loading level of wood flour and an internal lubricant on the rheological properties was studied. The results show that the wood flour-PP blends behavior as pseudo-plastics or shear-thinning flow under the experimental conditions investigated. The addition of internal lubricant is effective in reducing the apparent viscosity that tends to increase with a higher loading of wood flour. The effect of shear rate on the efficiency of internal lubricant was also investigated. For practical application, it is recommended to use both internal lubricant and phase compatibilizer such as Maleic Anhydride grafted polypropylene (MAgPP), thus improving the processing operation while maintaining the strength properties of the wood flour-PP composites.

Elongational behavior of gelled propellant simulants

Abstract: An elongational rheometer is used to study the rheological behavior of gelled propellant simulants in uniaxial elongational flow. In simple shear such fluids typically exhibit a shear thinning behavior which could be described by a power-law constitutive equation. Knowledge of the elongational behavior of these fluids is important for understanding the processes of their atomization and spray formation. The results of the present work demonstrated that the three-dimensional power-law model permits description of uniaxial elongation of these fluids as well. Moreover, the values of the rheological parameters of the tested fluids measured in simple shear and in uniaxial elongation agree fairly closely. Therefore the elongational behavior of gelled propellant simulants can be inferred from their shear behavior.

Effects of stearic acid coated talc, CaCO3, and mixed talc/CaCO3 particles on the rheological properties of polypropylene compounds

Abstract: The effect of the stearic acid coated fillers and their geometry on the shear/dynamic viscosity and complex viscosity has been investigated using polypropylene (PP) compounds filled with stearic acid uncoated and coated talc, calcite, and mixed talc/calcite particles. The viscosity was measured over a wide range of shear rates (10−8 to 103) using a capillary, cone-plate and sandwich rheometer. Overall, the rheological properties of the compounds exhibited different behavior upon different filler systems, stearic acid involvement, shear stress or strain, and frequencies due to stearic acid involvement. This implies that the stearic acid lowers the interfacial force between the filler surface and the resin matrix, followed by a favorable processing. In addition, at very low shear stresses, the viscosity of talc(un) compounds was higher than calcite(un) ones; at very high shear stresses, on the other hand, talc compounds became lower than calcite(un) compounds. This is interpreted as due to the different geometry between talc and calcite. The yield value as a function of shear stress was observed for all filler systems and exhibited lower than that obtained from the extrapolation. Furthermore, the Cox–Merz relation between the complex and shear viscosity for both the stearic acid uncoated and coated compounds is found not valid. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 2105–2113, 2004

Fingerprinting the processing behavior of polyethylenes from transient extensional flow and peel experiments in the melt state

Abstract: The flow curves of linear (linear-low and high density) and branched polyethylenes are known to differ significantly. At increasing shear rates, the linear polymers exhibit a surface melt fracture or sharkskin region that is followed by an unstable oscillating or stick-slip flow regime when a constant piston speed capillary rheometer is used. At even higher shear rates, gross melt fracture appears. Unlike their linear counterparts, branched polyethylenes rarely exhibit sharkskin melt fracture and although gross melt fracture appears at high shear rates there is no discontinuity in their flow curve. The various flow regimes of these two types of polyethylenes are examined by performing experiments in the melt state using a unique extensional rheometer (the SER by Xpansion Instruments) that is capable of performing accurate extensional flow and peel experiments at very high rates not previously realized. The peel strength curves of these linear and branched polyethylenes exhibit all of the distinct flow regimes exhibited in their respective flow curves, thereby providing a fingerprint of their melt flow behavior. Moreover, these extensional flow and peel results in the melt state provide insight into the origins and mechanisms by which these melt flow phenomena may occur with regard to rapid tensile stress growth, melt rupture, and adhesive failure at the polymer wall interface.

Observations of instability, hysteresis, and oscillation in low-Reynolds-number flow past polymer gels.

Abstract: We perform a set of experiments to study the nonlinear nature of an instability that arises in low-Reynolds-number flow past polymer gels. A layer of a viscous liquid is placed on a polydimethylsiloxane (PDMS) gel in a parallel-plate rheometer which is operated in stress-controlled mode. As the shear stress on the top plate increases, the apparent viscosity stays relatively constant until a transition stress where it sharply increases. If the stress is held at a level slightly above the transition stress, the apparent viscosity oscillates with time. If the stress is increased to a value above the transition stress and then decreased back to zero, the apparent viscosity shows hysteretic behavior. If the stress is instead decreased to a constant value and held there, the apparent viscosity is different from its pretransition value and exhibits sustained oscillations. This can happen even if the stress is held at values below the transition stress. Our observations suggest that the instability studied here is subcritical and leads to a flow that is oscillatory and far from viscometric. The phenomena reported here may be useful in applications such as microfluidics, membrane separations, and polymer processing. They may also provide insight into the rheological behavior of complex fluids that undergo flow-induced gelation.

Methodology for rheological testing of engineered biomaterials at low audio frequencies.

Abstract: A commercial rheometer (Bohlin CVO120) was used to mechanically test materials that approximate vocal-fold tissues Application is to frequencies in the low audio range (20–150 Hz) Because commercial rheometers are not specifically designed for this frequency range, a primary problem is maintaining accuracy up to (and beyond) the mechanical resonance frequency of the rotating shaft assembly A standard viscoelastic material (NIST SRM 2490) has been used to calibrate the rheometric system for an expanded frequency range Mathematically predicted response curves are compared to measured response curves, and an error analysis is conducted to determine the accuracy to which the elastic modulus and the shear modulus can be determined in the 20–150-Hz region Results indicate that the inertia of the rotating assembly and the gap between the plates need to be known (or determined empirically) to a high precision when the measurement frequency exceeds the resonant frequency In addition, a phase correction is needed to account for the magnetic inertia (inductance) of the drag cup motor Uncorrected, the measured phase can go below the theoretical limit of −π This can produce large errors in the viscous modulus near and above the resonance frequency With appropriate inertia and phase corrections, ±10% accuracy can be obtained up to twice the resonance frequency

Characterization of i‐PP shear‐induced crystallization layers developed in a slit die

Abstract: In this work, the shear-induced crystalline layers of isotactic polypropylene (i-PP), developed in a slit die, were characterized by different techniques. Rheological studies made in a strain-controlled rheometer, at different crystallization temperatures, Tc, allowed us to observe that the induction time for the beginning of the shear-induced crystallization, ti, decreased as the shear rate increased, whereas at a given shear rate, the higher the Tc, the higher the ti. The thickness of the shear-induced crystallized layer, after extrusion through the slit die, was found to decrease with the increase of the die temperature, Td, at a given flow rate, Q, and to increase with the increase in Q, at a given Td. Regarding the die length, it was found that only at Td = 169°C, the thickness of this layer increased with the length. By polarized light optical microscopy (PLOM), five different crystalline layers were observed along the thickness of the sample. By scanning and transmission electron microscopy (SEM and TEM), respectively, and wide-angle X-rays (WAXS), it was found that layer 1, the nearest to the wall, was formed mainly by very small and oriented α-crystallites, while layer 2 was mainly composed of β-crystallites; also it was found that the amount of the β-phase decreased as the shear rate decreased. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 91: 3528–3541, 2004