Showing papers on "Rheometer published in 1983"

Chemorheology of thermosetting resins. I. The chemorheology and curing kinetics of unsaturated polyester resin

Abstract: The rheological properties and curing kinetics of a general-purpose polyester resin have been determined during isothermal cure. Both steady and oscillatory shearing flow properties were determined using a cone-and-plate rheometer, and the curing kinetics were determined using a differential scanning calorimeter (DSC). It was found that, as cure progresses, the steady shear viscosity increases very rapidly with cure time at all shear rates investigated, and normal forces show negative values at low shear rates and positive values at high shear rates. The observed negative normal forces are believed to result from material shrinkage during cure, and positive normal forces from the deformation of large molecules, formed by crosslinking reactions during cure. Note that, in a cone-and-plate rheometer, the shrinkage force acts in the direction opposite to that of normal forces. It is, therefore, concluded that extreme caution is needed in the interpretation of normal force measurements with thermosetting resins, subjected to steady shearing flow. Dynamic measurements seem to offer some insight on the onset of gel formation. More specifically, we have found that, when the unsaturated polyester resin was cured at a fast rate, the time at which a maximum in the loss modulus G” occurs coincides reasonably well with the time tη∞ at which the steady shear viscosity η approaches infinity. However, at a slow rate of cure, the time at which tan δ equals unity agrees fairly well with tη∞. DSC measurement has permitted us to determine the degree of cure as a function of cure time and the kinetic parameters in an empirical expression for the curing kinetics advanced by Kamal and co-workers. By combining the rheological and DSC measurements, we have constructed plots describing how the viscosity increases with the degree of cure, at various values of isothermal curing temperature.

Shear Fracture in Cone‐Plate Rheometry

Abstract: When subject to moderate shear rates in a cone‐plate and parallel plate rheometer, viscoelastic samples tend to fracture at the edge of the sample thus preventing the measurement of viscosity and normal stresses at high shear rates. An explanation of this behavior has been given in terms of a critical elastic strain energy by Hutton and we offer an alternative explanation based on fracture mechanics. It is shown here that fracture occurs if |N2|>2Γ/3a, where N2 is the second normal stress difference, Γ is the surface tension coefficient, and a is the size of the fracture. The experimental data presented here show that this equation correctly predicts the occurrence of shear fracture and also the shear rate at which it occurs. At high shear rates, Newtonian liquids and some viscoelastic liquids are ejected from the cone‐plate and parallel plate rheometer due to the effect of centrifugal forces. For a Newtonian liquid a simple consideration of the centrifugal and surface tension forces at the sample edge pr...

Flow Properties of Time‐Dependent Foodstuffs

Abstract: In this work, a new model for viscosity decay at constant shear rate is tested and the thixotropic behavior of representative food products is experimentally analyzed. The equilibrium viscosity (or steady‐state viscosity) of some food products, obtained after a sufficiently long time of shear at a constant shear rate, is found to be well represented by the Herschel‐Bulkley model and by an exponential model in which a maximum of two terms of an infinite series are required. The model for viscosity decay, that is, the decrease in viscosity with time at constant shear rate, assumes nth order kinetics for the decay of a structural parameter λ. The rate constant k, for the decay of λ, is found to be a power law function of the shear rate. The equation for structure decay is combined with a scalar constitutive equation for the shear stress and the resulting model represents adequately the data for viscosity decay of foodstuffs in the range of shear rates 50<γ<5420 s−1. Data for suspensions such as tomato juice...

Polypropylene/nylon 6 blends: Phase distribution morphology, rheological measurements, and structure development in melt spinning

Abstract: A series of polypropylene (PP)/nylon 6 (N6) blends of composition 75/25, 50/50, and 25/75 have been prepared in a screw extruder combined with a Koch static mixer. The phase morphology was observed with a scanning electron microscope. The influence of heating in the reservoir of a rheometer followed by subsequent extrusion through a capillary on the phase morphology was investigated. Phase size growth as a function of time was observed under quiescent and mild deformation rate conditions. The discrete phase size was observed to decrease with increasing extrusion rate through dies. The shear viscosity and principal normal stress difference of the blends were measured as a function of composition. The crystalline orientation of both polypropylene and nylon 6 in blend melt spun fibers was characterized by wide angle X-ray diffraction and interpreted in terms of Hermans–Stein orientation factors. The orientation increases with drawdown ratio. The orientation factors for the polypropylene phase vary with spinline stress in a manner independent of composition and identical to that for pure polypropylene. Extracting melt spun blend fibers with formic acid has produced small-diameter polypropylene minifilaments with diameters of the order of microns.

A new elongational rheometer for polymer solutions

Abstract: Design and operation of a new elongational rheometer for low elastic polymer solutions are described. The free jet elongational rheometer is easy to operate and for suitable operating conditions a transient elongational flow with an approximately constant rate of strain can be realized. In any case, convenient comparative parameters can be obtained. The method of rheological curve fitting leads to a deformation dependent ralaxation time parameter of a modified upper convected Jeffreys-law.

From melt flow index to rheogram

Abstract: A knowledge of the complete flow curve or rheogram of a polymeric melt depicting the variation of the melt viscosity over industrially relevant range of shear rate and temperature is essential in the design of polymer processing equipment, process optimization and trouble-shooting. These data are generated on sophisticated rheometers that are beyond the financial and technical means of most plastics processors. The only flow parameter available to the processor is the melt flow index of the material. In the present work, a method has been proposed to estimate the rheograms of a melt at temperatures relevant to its processing conditions with the use of a master curve, knowing the melt flow index and glass transition temperature of the material. Master curves that coalesce rheograms of different grades at various temperatures have been generated and presented for low density polyethylene, high density polyethylene, polypropylene, polystyrene and styrene-acrylonitrile copolymer.

Positive hole pressures and negative exit pressures generated by molten polyethylene flowing through a slit die

Abstract: A new slit die rheometer has been constructed to measure elastic and viscous properties of molten polymers at low shear rates. The wall shear stress σ and the extrapolated exit pressurePx are determined by means of two pressure transducers mounted flush with a die wall. The hole pressureP* is obtained from one of the flush-mounted transducers and a third transducer mounted in a transverse slot opposite the flush-mounted transducer. The wall shear rate\(\dot s\) is obtained from a metering pump. Electrical heaters give melt temperatures that are uniform to within ±0.1 °C at 150°C. For two low-density polyethylene samples of Melt Index 2 and 50, at shear rates in the range 0.1 to 8 s−1, it is found that (a) viscosity values agree with those obtained using two Weissenberg Rheogoniometers (WRGs); (b) hole pressure data agree with the predictions of the “HPB” transverse-slot equationN1 = 2σdP*/dσ when WRG data are used for the first normal stress differenceN1; and (c) exit pressures are large and negative, in marked disagreement with certain published equations relatingPx,N1 andσ. An error analysis shows thatPx values can contain significant negative contributions arising from the pressure dependence of viscosity, even at low shear rates. As a means for in-line measurement of melt elasticity at low shear rates, the results favor the use of the hole pressure, but raise serious questions about the use of the exit pressure.

Viscoelastic fluid flow exhibiting hysteritic phase changes

Abstract: In the present paper a mathematical theory is given for a viscoelastic fluid exhibiting a hysteresis loop in the shear stress versus shear rate plane. The main rheological idea is to introduce a constitutive equation of rate type whose steady shear stress versus shear rate locus is nonmonotone. The main mathematical idea is to use a local shock structure theory to pick out the admissible solutions in loading and unloading of the applied driving force.

Rheology of unsaturated polyester resins. I. Effects of filler and low‐profile additive on the rheological behavior of unsaturated polyester resin

Abstract: Measurements were taken of the bulk rheological properties of concentrated suspensions of particulates in unsaturated polyester resins, using a cone-and-plate rheometer. The particulates used were clay, calcium carbonate, and milled glass fiber. With clay and milled glass fibers, shear-thinning behavior of suspensions was observed at low shear rates or low shear stresses as the concentration of particulates was increased, whereas concentrated suspensions of calcium carbonate exhibited Newtonian behavior over the range of shear stresses or shear rates investigated. The cone-and-plate rheometer was also used for measurements of the bulk rheological properties of various mixtures of polyester resin and low-profile additives. For low-profile additives, solutions, in styrene, of poly(vinyl acetate) (PVAc) and poly(methyl methacrylate) (PMMA) were used. It was found that the bulk viscosities of all mixtures of polyester resin and PVAc solution lie between those of the individual components, whereas the bulk viscosities of some mixtures of polyester resin and PMMA solution go through a minimum and a maximum, depending on the composition of the mixture. While all mixtures of polyester resin and PVAc solution exhibited negligible normal stress, some mixtures of polyester resin and PMMA solution exhibited noticeable normal stresses. It should be mentioned that polyester resin follows Newtonian behavior. It turned out that all mixtures of polyester resin and PVAc solution exhibited clear, homogeneous solutions, whereas mixtures of polyester resin and PMMA solution exhibited optical heterogeneity, i.e., turbidity. When polyethylene powders were used as low-profile additives, suspensions of polyester resin and polyethylene powders exhibited negative values of normal stress as the concentrations of suspension reached a critical value. When both filler and low-profile additive were put together in polyester resin, the rheological behavior became quite complex, indicating that some interactions exist between the filler and the low-profile additive.

Yield Stresses in Concentrated Dispersions of Closely Packed, Deformable Gel Particles

Abstract: Wheat starch granules dispersed in water and heated to 70°C swell extensively to form dispersions of irregularly shaped, deformable particles. Because the granules are deformable under shear, the rheological properties of the dispersions can be examined at volume fractions up to unity, which corresponds to a starch concentration of 14%. At concentrations of 10–13%, yield stresses are observed in couette flow at 23°. At concentrations of 14% and above, the dispersions show essentially solidlike behavior in shear. Shear stress‐shear strain diagrams for these gels were obtained in a parallel plate rheometer. The major experimental problem in this testing mode is slip between the gel and the parallel plates. This problem was resolved by using cyanoacrylate ester adhesive to attach the gel to the parallel plates. With slip eliminated, the shear stress‐shear strain diagrams show an initial linear region from which a small deformation modulus is obtained. As shear strain increases, a yield point is observed where the stress‐strain curve shows an inflection point. At higher strains, the curve rises again, and forces normal to the direction of shear are generated. Failure finally occurs at strains of 25–50%.

Centro‐symmetric technique for measuring shear modulus, viscosity, and surface tension of spread monolayers

Abstract: We have constructed an apparatus which enables one to measure, simultaneously, the shear modulus, the viscosity, and the surface pressure of a monomolecular film on water under a hydrostatic compression. It is particularly suitable for measurements of the static shear modulus of insoluble monolayers. A capillary wave generator/detector system was also incorporated to determine the surface pressure. The trough is a Teflon cup 12.7 cm (5.4 in.) in inner diameter with sloping interior sides. As the cup is raised, while maintaining the water elevation fixed, the film is uniformly compressed. A rotor suspended at the center from a fine torsion fiber is used to measure the mechanical properties (surface viscosity and/or surface shear modulus). To measure the quasistatic shear properties, the cup is rotated through a small angle and the residual deflection of the rotor determined. Dynamic shear response is probed by studying transient torsional oscillations following a sudden angular displacement of the torsion ...

Flow of a BKZ Fluid in an Orthogonal Rheometer

Abstract: Etude de l'ecoulement du fluide modele de Bernstein, Kearsley, Zapas dans un rheometre orthogonal

Adsorption of polyacrylamide on silica particles and its effect on the rheological properties of suspensions

Abstract: The adsorption of polyacrylamide (PAAm) on silica particles and its effect on the rheological properties of the suspensions were studied with a coaxial cylinder rheometer. The two unusual observations are reported. First, for suspensions prepared with PAAm solution in glycerin, the apparent viscosity of suspensions is lower than that of the medium. Adsorption experiments showed that a large amount of polymer was adsorsed on the particles. It is concluded that the decrease in polymer concentration in the liquid phase by adsorption is primarily responsible for the decrease in the apparent viscosity of the suspensions. Second, for suspensions prepared with PAAm solution in a glycerine/water mixture, the apparent viscosity irreversibly increases above the critical shear rate. This behavior may be originated from flocculation by shear-induced irreversible bridging.

Multiple solutions and hysteresis in steady parallel viscous flows

Abstract: A study is made of flows of viscous fluids when viscosity varies with temperature according to an Arrhenius law, and when viscous dissipation is taken into account. It is shown that for fluids with highly temperature‐sensitive viscosity the graph of shear rate against shear stress is an S‐shaped curve, with either one or three values of shear rate for given shear stress. In this situation hysteresis can occur if steady unidirectional flow is maintained. The velocity and temperature profiles are calculated, and the manner in which the temperature‐sensitivity parameter affects the conclusions is discussed. The calculations serve as criteria for evaluating the importance of viscous‐heating effects in applications.

The viscoelastic properties of concentrated suspensions

Abstract: The viscoelastic properties of concentrated, well dispersed, sterically stabilised polymethyl methacrylate particles were studied using a specially developed low frequency, controlled shear stress rheometer. The experimental results show a well defined viscoelastic transition over a narrow frequency range. Evidence is produced to show that the particles act as hard, non-interacting spheres. It is argued that the transition represents a change from a randomly packed assemblage with particle motion dominated by Browniandiffusion processes to a lower packing density shear flow regime.

Effect of Shear Degradation on the Rheological Properties of Dilute Drag‐Reducing Polymer Solutions

Abstract: Apparent viscosity over a range of 106 s−1 in shear rate and first normal stress difference functions for dilute drag‐reducing solutions of Separan AP‐30 in distilled water were measured for both freshly prepared and shear‐degraded solutions, for concentrations from 100 to 500 wppm. A nonlinear integral constitutive equation is proposed which accurately represents these data, and the effects of concentration and shear degradation on the properties (e.g., the model parameters) is shown. Shearing has a greater effect on the low shear viscosity of these solutions than it has on the elastic properties, resulting in a greater degree of drag reduction for the sheared than the fresh solutions.

Rheological Behavior of Concentrated Suspensions of Particulates in Unsaturated Polyester Resin

Abstract: Measurements were taken of the bulk rheological properties of concentrated suspensions of particulates in unsaturated polyester resins using a cone‐and‐plate rheometer. Over the range of shear rates investigated, suspensions of calcium carbonate showed Newtonian behavior, whereas suspensions of clay showed both yield behavior at low shear rates and shear‐thinning behavior at high shear rates. Suspensions of both calcium carbonate and clay together exhibited no normal stress effects. However, suspensions of polyethylene powders were found to exhibit positive values of normal stress at low concentrations, but negative values of normal stress at and above a certain critical concentration. In view of the fact that negative values of normal stress were observed at low shear rates (1–10 s−1), the possibility of inertial effects is ruled out because we observed inertia‐induced negative values of normal stress with three different Newtonian liquids at very high shear rates, say above 250 s−1. Having checked the p...

Rheological properties of branched low‐density polyethylene

Abstract: The shear flow properties of six commercially available long-chain branching low-density polyethylene resins were determined, using a cone-and-plate rheometer at low shear rates and a capillary rheometer at high shear rates. Also determined were the elongational viscosities of the resins, using an apparatus developed by Ide and White. Interpretation of the rheological measurements is given with the aid of the molecular parameters, namely, molecular weight and molecular weight distribution.

A parallel-plate rheometer for the measurement of steady-state and transient rheological properties

Abstract: A parallel-plate rheometer (PPR) has been constructed for use with the Materials Test System (MTS) for the measurement of fast transient and steady-state responses of polymeric liquids. This particular combination exploits the versatility and capability of the MTS programmable drive system, while the design of the PPR ensures the ideal simple shearing configuration. Instrument compliance, material inertia and mechanical lag in both the input and output components of the system are minimal, thus avoiding defects of most existing rheometers. Reproducible data on a 20% polyisobutylene-in-decalin solution have been obtained for various complicated flow programs. This test system is thus suitable for extensive rheological characterization of complex viscous liquids.

Rheology of poly(vinyl chloride) formulations from melt flow index measurements

Abstract: A knowledge of the complete flow curve, or rheogram, of a PVC formulation depicting the variation of the melt viscosity over industrially relevant ranges of shear rate and temperature is essential in the design of polyer-processing equipment, process optimization, and troubleshooting. These data are generated on sophisticated rheometers that are beyond the financial and technical means of most PVC processors. The only flow parameter that can be readily measured on an inexpensive apparatus is the melt flow index of the resin-mix. In the present work, a method has been proposed to estimate the rheograms of a PVC formulation at temperatures relevant to its processing conditions with the use of a master curve, knowing the melt flow index and glass-transition temperature of the formulation. A master curve that coalesces rheograms of different PVC formulations at various temperatures has been generated and proposed as a tool for estimating the flow curve.

Variable volume dual action rheometer

Abstract: The invention comprises a device which performs the functions of both a Mooney Viscometer and an Oscillating Disk Rheometer to measure the visco-elastic properties of rubber and rubber-like materials Rotational and oscillating motion are transmitted to a shaft by means of a sprocket and a hub connected to an eccentric Rubber samples having a wide range of stiffnesses can be accommodated by a provision for changing the thickness of the sample

Investigation of the oscillating flow phenomenon in high density polyethylene

Abstract: The nature and causes of the oscillating flow phenomenon in high density polyethylene are examined in this report. Empirical equations are developed describing the onset (both stress, SSOF, and rate, SROF) of oscillating flow in terms of molecular structure. The SSOF is not constant, but is found to vary slightly with the molecular weight distribution. The SROF is shown to be predictable in terms of molecular weight distribution. The physics of the oscillations is critically examined, and the simple model of Okubo and Hori describing these oscillations has been successfully tested. The model involves alternating compression of the polymer in the capillary rheometer barrel and decompression after a material breakdown in the die, in which flow from the capillary exceeds that expected from the rheometer output for an incompressible material. The point for initiation for oscillating flow is positively identified to be in the die, not, as some have suggested, in the die entrance region. Examination of the critical shear stress for random ethylene–propylene rubbers suggests that the reason why linear polyethylene appears unique is that the critical stress may be unattainable in other polymers and that the magnitude of the effect tends toward being vanishingly small. A critical shear strain criterion seems to describe the variations in critical stress with ethylene–propylene copolymer composition, but the critical strain criterion appears to be an oversimplification in general.

Determination of the weight-average molecular weight of polyamide-6 on the basis of melt viscosity

Abstract: A theory of a two-point rheometrical method of determination of the weight-average molecular weightM w of polyamide-6 is presented. The method is based on the measurement of the instantaneous values of zero-shear-rate viscosity of the degrading polymer melt, and a formula is derived which enables the calculation of the initial value ofM w (i.e. at zero-residence-time in molten state) of the investigated sample. The experimental verification of the method proves its applicability. The considerations carried out may be regarded as a first step towards developing a theory of an in-line rheometer for a continuous determination ofM w .

A New Method for Determining the Rheology of Crosslinked Fracturing Fluids using Shear History Simulation

Abstract: A new method which incorporates shear history simulation has been developed for measuring the rheological properties of crosslinked fracturing fluids. This proven method can provide the true rheology of the working fluid which enters the fracture. These data are essential when designing and conducting treatments in low-permeability formations, in order to obtain the desired fracture geometry. Recent research has shown that the rheological properties of crosslinked gels are dependent on the method of preparation and testing. A single formulation of a fluid prepared by two different methods can have resulting viscosity profiles which differ by several hundred percent. The new method of measuring rheology overcomes many of the previous discrepancies resulting from preparation and testing by incorporating a shear history into the procedure. This shear history is similar to the tubular shear history a fluid experiences before it enters a fracture. The shear history apparatus is a capillary viscometer consisting of a series of tubes and additive pumps for the addition of crosslinkers. The length of the tubing as well as the pump rate can be adjusted to match laminar shear rates and shear histories encountered in fracturing treatments. The paper describes a field scale-up model which verified the resultsmore » of the laboratory tests.« less

Temperature and shear history effects on fusion of PVC

Abstract: Standard PVC compounds containing various lubricant systems were fused in a Haake Torque Rheometer equipped with a torque integrator, holding the shear stress time value constant. The total work put into the system to fuse the compound was therefore held constant. The fused material was compression molded into sheeting for physical testing. The data will characterize the compounds for both rheological and physical properties at several processing conditions. Plotted curves will depict the lubricant systems and processing temperatures necessary to obtain maximum physical strength.

Elongational flow of polyvinyl chloride

Abstract: The elongational behaviour of both rigid and plasticised polyvinyl chloride were investigated using the prototype Rutherford Extensional Rheometer. Prior to this, the search for a suitable clamp design and sample shape, the tests on the compatibility of silicon oil as the heating medium for plasticised polyvinyl chloride and the reproducibility of measurements of the rheometer were carried out. [Continues.]

Torque self-balancing type rheometer

Abstract: PURPOSE:To enable measuring of a wide-ranged viscosity, by a method wherein, in a coaxial rotary viscometer, it is constituted such that a position of a force- receiving part and a force, generated at the force-receiving part, can be controlled in order to measure a shearing stress applied to an inner cylinder. CONSTITUTION:A space between an outer cylinder 1 and an inner cylinder 2, which are coaxial with each other, is filled with sample fluid, the outer cylinder 1 is spun in a direction of an arrow mark A to apply a shearing rate to the sample fluid. Then, a torque, proportioning a shearing stress, is generated at the inner cylinder 2 due to viscosity. A measurement is made on the torque by balancing it with an anti-torque at a force coil 5 through the working of an arm 3 secured to the center of the inner cylinder 2. Then, the length of the arm 3 is brought to a variable state by parallel movement of a moving base 6 over a slide shaft 11 through the working of a motor 7, a gear 8, a ball nut 10, and a screw 9. The length is electrically detected by a potentiometer 13, and from the output and a force, produced at the force coil 5, a wide-ranged viscosity can be measured.