A sliding plate normal thrust rheometer for molten plastics

A sliding plate rheometer has been developed to measure the normal thrust of a molten plastic in large amplitude oscillatory shear. The normal thrust of a molten plastic in large amplitude oscillatory shear is a significant nonlinear effect that has previously been unobtainable. Normal thrust measurements may now be used to help understand and characterize nonlinear viscoelastic behavior inherent in most molten plastics. The new rhemoeter incorporates a piezoelectric pressure transducer in a very stiff plate that minimizes compliance. Normal thrust measurements with cone and plate rheometers are made by measuring the total force on the plate (or cone) and are subject to error due to edge effects. The new rheometer measures the local pressure in the sample and is therefore unaffected by sample size or edge effects. Normal thrust measurements in large amplitude oscillatory shear are reported for both molten Phillips TR480 high density polyethylene pipe resin which contains 2% by weight of carbon black filler and IUPAC LDPE X. Crosstalk due to shear stress on the active face of the pressure transducer causes signal error which for oscillatory shear is filtered out using a discrete Fourier transform.     

Steady‐state viscoelastic flow simulation of polymer melts in a rotational‐type rheometer

Polymer samples in the jigs begins to protrude when the heat is turned up when we measure the rheological characteristics of polymer melts using rotational‐type rheometers, such as parallel and cone‐and‐plate types. To clarify the effects of this protruding part on the obtained rheological data, we tried to evaluate the rotational‐type rheometer by a non‐isothermal viscoelastic flow simulation using the finite element method. The multiple mode Phan‐Thien Tanner (PTT) model was employed as the constitutive equation. As a result, the shear viscosity in the steady state increases with the size of the protruding part of the polymer melt specimen at the same shear rate in case with a parallel plate and a cone‐and‐plate type rheomters. In contrast, the deviation of the primary normal stress difference between the estimated value from the simulation results and the data from the PTT model is almost independent of the size of the protruding part with the cone‐and‐plate type rheometer. In addition, the deviations of the primary normal stress difference with a parallel plate rheometer increase with the protruding part size. However, these deviations are smaller than those of shear viscosity. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers     

Dynamic slip and nonlinear viscoelasticity

Several significant problems arise when film is fabricated on a large scale. One of these is the appearance of irregularities on the extrudate surfaces when the polymer melt is extruded at high rates. These irregularities vary in intensity and form and are generally known as sharkskin melt fracture. This phenomenon, which occurs when the wall shear stress exceeds a critical value, is a limiting factor for production rates in many industrial extrusion operations such as film blowing of polyethylene. We used a sliding plate rheometer incorporating a shear stress transducer to study slip in both steady and unsteady flows. By combining a dynamic slip model with a nonlinear viscoelastic constitutive model, we determined the slip model parameters for LLDPE film resin with and without a fluoropolymer sharkskin suppressant. The models give good prediction of our slip data in steady shear but show insufficient gap dependence in exponential shear. Our own film blowing studies demonstrated the efficiency of the sharkskin suppressant; it has more than doubled the throughput in our laboratory setup. The fluoropolymer additive was found to profoundly affect both the steady and dynamic slip parameters. Hence, the sharkskin suppressant alters how the LLDPE remembers its past slipping motions.     

Transient modeling of viscosity

Capillary and parallel plate rheological characterization was conducted for a low‐density polyethylene. In contrast with conventional rheological analysis, steady conditions were not assumed. Transient data, with time steps between 0.0001 and 0.2 s, were analyzed with a nonlinear, viscoelastic constitutive model in which the relaxation time was modeled as a function of the applied stress. The fit model explained more than 99% of the observed transient variation in the capillary and parallel plate rheometers. The model coefficients for the capillary and parallel plate were compared directly to conventional linear viscoelastic analysis of the same parallel plate data. The results indicate that the described constitutive model closely predicts the observed viscoelastic behavior of the polymer melt tested in the parallel plate rheometer. Furthermore, the results indicate that the relaxation spectrum modeled with the transient analysis of the capillary rheological data correlate closely to the results predicted by the same transient analysis of parallel plate rheological data. The conclusion is that described constitutive modeling describes the viscoelastic behavior in both capillary and parallel plate rheometers. Moreover, the analysis and results suggest that the viscoelastic behavior of the polymer melt is a significant factor during the rheological characterization and the modeling of the transient response should be taken into consideration during rheological analysis to provide high fidelity models. POLYM. ENG. SCI., 57:1110–1118, 2017. © 2017 Society of Plastics Engineers     

Shear and extensional properties of short glass fiber reinforced polypropylene

Shear and extensional properties of a commercial short glass fiber reinforced polypropylene were carefully investigated using commercial rheometers and a novel on‐line rheometer. This on‐line slit rheometer, installed on an injection molding press, has been designed to measure the steady shear viscosity, the first normal stress difference, and the apparent extensional viscosity of polymer melts and composites for high strain rates up to 10⁵ s⁻¹ in shear and 200 s⁻¹ in extension. Our results show that the steady‐state viscosity measurements using the on‐line rheometer are in excellent agreement with those obtained using commercial rheometers. The steady‐state and the complex viscosities of the composites were found to be fairly close to that of the matrix, but the Cox‐Merz rule was not verified for the composites at high rates. The elasticity of the composites was found to be equal to that of the polypropylene matrix. The apparent extensional viscosity was obtained from the pressure drop in the planar converging die of the slit rheometer using the analyses proposed by Cogswell [1] and Binding [2]. The extensional viscosity of the polypropylene was found to be much larger than the shear viscosity at low strain rates with a Trouton ratio of about 40 that decreased rapidly with increasing strain rate down to the value of 4 at 200 s⁻¹. The extensional viscosity of the composites was also found to be close to that of the matrix, with values 35 and 5% larger for the 30 and 10 wt% reinforced polypropylenes, respectively. These results are compared with the predictions of the Goddard model [3], which are shown to overpredict our experimental results. POLYM. COMPOS. 26:247–264, 2005. © 2005 Society of Plastics Engineers.     

Superposed flow properties of ceramic powder‐filled polymer melts

Complicated shape products from ceramic composite materials are nowadays intensively processed via flow molding technologies. Rheological properties of these materials are essential for the clarification of the deformation behavior through channels under various conditions. In this article, ceramic powder (zirconia) was mixed (10–50 vol%) with polypropylene, paraffin, and stearic acid in an elastic extruder. Parallel superposed steady and oscillatory shear flows were measured on a cone‐plate rheometer. Flow properties at high shear rates were evaluated on a capillary rheometer. The effects of powder content, shear rate/angular frequency, and temperature were clarified. The studied filled systems showed highly non‐Newtonian behavior and apparent yield stress; their viscoelastic properties were influenced remarkably under the superposed shear flow at low shear rate and angular frequency, and they showed significantly different behavior from unfilled and fiber‐filled systems. POLYM. COMPOS., 2009. © 2008 Society of Plastics Engineers     

A comparison of measurements of the viscoelastic properties of polymer melts by means of the Han slit/capillary rheometer and the Weissenberg rheogoniometer

Measurements were taken of the viscoelastic properties of six polymer melts by mean of the Weissenberg rheogoniometer and the Han slit/capillary rheometer. Polymers in vestigated were three high-density polyethylenes of different polydispersity, a low-density polyethylene, a polypropylene, and a polystyrene. The range of shear rates tested was from about 5.0 × 10^?3 to 10 sec^?1 with the Weissenberg rheogoniometer, from about 10 to 10² sec^?1 with the slit rheometer, and from about 10² and 10³ with the capillary rheometer: the temperature of measurement was 200°C. The three different apparatuses give consistent results over almost six decades of shear rates, yielding satisfactory correlations of shear viscosity to shear rate and of normal stress difference to shear rate.     

Shear‐induced crystallization of isotactic polypropylene studied by simultaneous light intensity and rheological measurements

We investigate the effects of high shear rate and a melt‐insensitive, organophosphate nucleating agent on shear‐induced crystallization of four isotactic polypropylenes using a sliding plate rheometer under isothermal, low‐supercooling conditions. We used a bifurcated optical fiber probe to measure light intensity and a shear stress transducer to monitor the simultaneous viscoelastic response to small‐amplitude oscillatory shear. The two techniques complement each other; at early times of crystallization, large attenuation in the light intensity is observed, whereas during the later stages, a major change in the viscoelastic response occurs due to the growing volume fraction of spherulites. In contrast to quiescent crystallization, the nucleation pathway of nucleated polymers after a brief, strong shear is little influenced by the nucleating agent but strongly affected by molecular weight. The early kinetics of non‐nucleated polymers is more strongly enhanced by shear than that of nucleated polymers. Increasing either shear rate or strain accelerates crystallization, and we found the product of shear rate and strain to be useful for correlating our data. POLYM. ENG. SCI., 2011. © 2011 Society of Plastics Engineers     

Rheology to understand and optimize processibility, structures and properties of starch polymeric materials

This paper reviews the state of the art in the field of the rheology of starch polymers, including specially designed rheometric techniques and complex rheology as influenced by different conditions. In terms of rheometric techniques, off-line extruder-type capillary/slit rheometers are commonly used but subsequent changes during measurement often occur as starch structures are highly sensitive to thermomechanical treatment. An in-line rheometer set-up with a double-channel die incorporated to the processing extruder is a direct and effective method to minimize the processing history change at different testing shear rates. In addition, pre-shearing, multipass, and mixer-type rheometers are also suitable for starch polymers. The rheological behavior of starch polymeric materials can be greatly impacted by their formulation (botanical source, plasticizer and additive type and content, and the structure related to blend or composite) and processing conditions (temperature, mechanical energy, etc.). Starch polymer melts exhibit shear-thinning and extension-thinning behaviors, and shows strong elastic properties. A wide range of rheological models, considering formulation and processing conditions, have been reviewed for different multiphase systems. The rheological behavior can also be related to the compatibility (blends, composites), expansion/foaming properties, film blowing properties, etc. The significance of processing rheology of starch polymers lies in characterizing the complex melting and flow behaviors, characterizing the viscoelastic properties, determining optimal processing method and conditions, and better controlling the quality of the final products.     

Boron nitride as a processing aid for the extrusion of polyolefins and fluoropolymers

The influence of a new processing additive (fine particles of boron nitride) on the processability of polyolefins and fluoropolymers in extrusion is studied. The equipment used includes an Instron capillary rheometer with two types of dies, namely capillary dies and special annular dies (Nokia Maillefer wire coating crosshead) attached to the rheometer, and an extruder. Two metallocene polyethylenes and several Teflon® fluoropolymers were tested using these two pieces of equipment. The additive had a significant effect on the extrudate appearance of polyethylene and fluoropolymer particularly in the crosshead dies. It was found to eliminate surface melt fracture and to postpone the critical shear rate for the onset of gross melt fracture to significantly higher values depending on resin type, temperature, and additive concentration (typically 0.005% to 0.5%). To explain the possible mechanism for the effect of the additive on the processability of the resins, rheological measurements using both parallel‐plate and sliding‐plate rheometers were carried out. The rheology of the resins did not seem to change significantly with the addition of boron nitride except for the low‐shear‐rate (low‐frequency) range, where the behavior of the filled resin was found to be similar to that of a crosslinked polymer or a phase‐separated entangled blend. Practical wire coating and tubing extrusion studies for these resins were also carried out.     

Glass bead-filled polypropylene part I: Rheological and mechanical properties

Five glass bead-filled polypropylene composites were rheologically characterized at 240°C using two rotational rheometers to obtain low shear-rate data and a capillary rheometer to obtain high shear-rate data. Both steady and dynamic properties were measured at low shear rates. Each composite was also injection molded into tensile and flexural test bars for a mechanical properties profile at 25°C. The tensile modulus was determined from a simple extensional deformation whereas the flexural modulus was determined from a three-point-bend test. The relative shear viscosity and relative loss modulus are different nonlinear functions of the volume fraction of beads at a constant shear rate, while the relative storage modulus appears to be a linear function of bead fraction. The relative viscosity decreases with increasing shear rate and the zero shear-rate data are in very good agreement with the Guth-Gold equation. The relative tensile modulus and relative flexural modulus are each linear functions of bead fraction over the entire range of filler concentration, 0-29 vol percent. From these data it is concluded that a simple correspondence between slow viscous flow and small strain elasticity does not exist for these composites.     

A new rheometer measuring infrared dichroism in molten polymers subjected to transient and steady shear flow

A rotational parallel plate rheometer that enables simultaneous measurement of the transient or steady-state rheological properties and infrared dichroism was designed and constructed to study orientation in molten polymers. Measurements can be carried out at shear rates between 0.05 and 300 s⁻¹ and at temperatures between 20 and 300°C. Both shear stress and axial normal force together with dichroism are continuously measured during shear flow. Infrared dichroism data for polypropylene showed excellent agreement with data obtained with a FTIR-instrument. The Hermans orientation function for molten poly(dimethyl siloxane) at steady state showed a strong shear-rate dependence in the region 0.1−20 s⁻¹. Rheological data for molten poly(dimethyl siloxane) agrees with data obtained from a conventional rheometer.     

Measurement of the viscosity of guar gum solutions to 50,000 s−1 using a parallel plate rheometer

It is frequently necessary to measure the viscosity of polymer solutions at high shear rates to obtain data under the conditions encountered in industrial processes. Such measurements are most often made on a capillary viscometer. This paper presents a method of determining solution viscosities at shear rates up to 50,000 s^?1 in a rotational rheometer using a parallel plate geometry. The two keys to performing these measurements are very small gaps between the parallel plates (on the order of 50 microns) to eliminate inertial secondary flows, and the ability to increase and decrease the shear rate quickly to minimize viscous heating. A technique for setting and measuring small gaps is presented. Possible sources of error including inertia, axial compliance, and viscous heating are analyzed. A comparison Is made between the viscosity of a 0.7 percent hydroxypropyl guar (HPG) solution measured on the parallel plate rheometer and the viscosity measured in a capillary viscometer. Viscosities of HPG solutions having concentrations of 0.25, 0.50, 1.00, and 1.45 percent are presented over the shear rate range 100 to 50,000 s^?1.     

The Rheological Behavior of Salt Tolerant Polyacrylamide Solutions

The rheological behavior of viscoelastic fluids was investigated with regard to the specialty of tertiary oil recovery. Aqueous polyacrylamide solutions at different concentrations were selected to simulate viscoelastic polymer systems, and the Haake RS 150 type rheometer was used to measure the rheological behavior. The experimental results showed that the viscoelasticity was positively influenced by polyacrylamide solution concentrations and negatively affected by solution temperatures. The coefficient of first normal stress deference decreased with increasing shear rate. In addition, the relationship between viscosity and the coefficient of first normal stress with shear rate obeyed a power law model. The viscosity decreased with increasing metal ion concentration and time.     

Rod‐climbing characteristics of α‐Fe2O3 suspended polyisobutylene/polybutene solution

From rod‐climbing rheometer measurements, we systematically investigated the normal stress values of suspended particles in polymeric liquids at low shear rates using the second‐order fluid constitutional relationship. The climbing constants β of the suspended α‐Fe₂O₃ particles in polyisobutylene/polybutene solutions, which exhibit Boger fluid characteristics (highly elastic, but no shear‐thinning), were estimated from the rod‐climbing experiment, showing that β increased with polymer concentration and polymer molecular weight. The first normal stress difference coefficient of the suspended α‐Fe₂O₃ in polymeric liquids obtained from the rod‐climbing rheometer was well correlated with the rheological properties measured by rotational rheometers. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 1548–1552, 2004     

Process viscometry of complex fluids and suspensions with helical ribbon agitators

The viscosity of highly inelastic shear thinning fluids and aqueous suspensions of kaolin clay particles has been investigated using a helical ribbon impeller fitted to a rheometer. Viscosity data for the single phase non-Newtonian fluids adequately processed with a generalized Reynolds number based on the impeller tip speed are shown to superimpose very well to the results obtained with a cone and plate rheometer. In the case of the two-phase system, it is shown that the data treatment for single phase system can be extended. The helical ribbon impeller yields more stable viscosity values than with the traditional geometries and no spurious flow phenomena (i.e., sedimentation, slip at the wall, etc.) was observed, making this system a superior device for suspension rheology over cone and plate and Couette flow rheometers.     

Rheological properties of calcium carbonate-filled polypropylene melts

An experimental study was carried out to investigate the viscoelastic behavior of calcium carbonate-filled polypropylene melts, using the Han slit rheometer. In the analysis of the experimental data, the pressure gradient was used to determine the wall shear stress, and the exit pressure to determine the elasticity of the filled polymers. The study shows that the materials studied follow a power law in viscous behavior over the range of shear rates investigated and that the viscosity increases and elasticity decreases as filler concentration is increased. Also investigated was the effect of temperature on the viscoelastic properties of filled polypropylene melts.     

Measurement of the rheological properties of thermoplastic elastomers

Using the Han slit/capillary rheometer, measurements were taken of the rheological properties of commercially available thermoplastic elastomers, namely, styrene–butadiene–styrene (SBS) block copolymer (Shell, Kraton), ethylene–propylene copolymer (Exxon, Vista), olefinic-type thermoplastic rubbers (UniRoyal, TPR 1600, TPR 1900, and TPR 2800), and urethane thermoplastic elastomers (UniRoyal, Roylar A863 and Roylar E9). The rheological properties determined were shear viscosity and first normal stress difference at various melt temperatures. Depending on the material and the melt temperature tested, the range of shear rates tested was 50 to 700 sec^?1, and the range of shear stresses tested was 10⁵ to 10⁶ dyn/cm². For comparison purposes, rheological measurements were also taken for a few materials using the Weissenberg rheogoniometer although its use was limited to low shear rates (or shear stresses). It was demonstrated clearly that the Han slit/capillary rheometer is a unique instrument for determining the rheological properties of thermoplastic elastomers in the range of high shear rates (or high shear stresses) often encountered in various polymer processing operations.     

A study of rheological behavior of a polypyrrole modified UHMWPE gel using a parallel plate rheometer

Polypyrrole powder was added to a 6% UHMWPE decalin solution as an interfacial modifier. Rheological properties and dynamic viscoelastic properties, including shear viscosity, shear storage modulus (G′), and shear loss modulus (G″), were measured with a parallel plate rheometer using rotation and oscillation modes. The results show that these properties increase with the PPy content. In addition, the gelation temperature decreased with increasing PPy content. This suggests that the presence of PPy enhanced the entanglement network in the solution, which is similar to the role of filler in rubber.     

Modifications of the weissenberg rheogoniometer for measurement of transient rheological properties of molten polyethylene under shear. Comparison with tensile data

Improvements to the Weissenberg rheogoniometer are necessary in order to measure the transient rheological properties of polymer melts correctly. The improvements reported concern the mechanical design, a new heating system, a new normal force measuring system, and additional equipment for the relaxation test. Reliable short-time results require sufficiently stiff torque and normal force springs, and a small radius and relatively large angles of the cone-and-plate gap. The behavior of the LDPE melt under test is “linear viscoelastic,” if shear rate or total shear are small: The relaxation modulus, the stress growth at the onset of constant shear rate, the stress relaxation after cessation of steady shear flow, and, in addition, dynamic shear data (from an oscillation viscometer) all show consistent results when correlated by means of formulae from the theory of linear viscoelasticity. Shearing in the nonlinear range with constant shear rate leads to pronounced maxima of the shear stress p₁₂ and of the first normal stress difference p₁₁ ? p₂₂ which occur at constant total shear, almost independent of shear rate. Comparison of shear and tensile data (from extensional rheometer) confirms the Trouton relation in the linear-viscoelastic case. In the nonlinear case, there is a “work softening” in shear and a “work hardening” in extension.