Showing papers on "Rheometer published in 1972"

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

Abstract: 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.

Development of a Universal Extensional Rheometer for the Uniaxial Extension of Polymer Melts

Abstract: A recent development in extensional rheometry of molten polymers is based on the use of a pair of gears as a ‘rotational clamp’. In a tensile tester for polymer melts, the rotational clamp provides the draw‐down of the melt being extruded out of the die of a melt indexer. The result is a quick characterization of extensibility and tensile strength. In a new extensional rheometer, two rotational clamps are used which extend homogeneously a molten polymer rod floating on a silicone oil bath. For constant rotational speeds of the clamps, the (Hencky) strain rate is constant. The creep test with constant tensile stress can be performed in the same rheometer making use of a servo motor of quick response with an appropriate servo control. A further modification of the servo control is described by means of which any uniaxial stress or strain history can be imposed on the melt under test, with stress σ>0 the only constraint. In this rheometer, the extensional behavior of a LDPE melt was measured at 150°C under a variety of mechanical conditions. The data presented demonstrate the versatility of the apparatus.

Time Dependent Viscoelastic Properties of Concentrated Polymer Solutions

Abstract: Measurements of stress growth and stress relaxation after onset and cessation of steady simple shear flow in concentrated polymer solutions were carried out with a Weissenberg rheogoniometer R‐17 having a gap servo system. By using monodisperse polymers and their blends, the effect of molecular weight distribution on those transient phenomena is discussed. The so‐called stress‐overshoot was observed in both experiments of shear and normal stress growths. Ratio of the time of the maximum normal stress difference and that of the maximum shear stress is close to 2 at the limit of low shear rate for both monodisperse and polydisperse polymers. In the range of finite shear rate, the ratio is remarkably dependent on shear rate for polydisperse samples, whereas it is almost independent of shear rate for monodisperse polymers. Shear rate dependence of the ratio of the apparent relaxation time of normal stress difference and that of shear stress in stress relaxation experiments is also found to be remarkably affected by molecular weight distribution. From a comparison between these experiments and theories so far published, it is concluded that these transient phenomena may be explained by assuming that the relaxation spectrum is a function of shear rate at least if the shear rates are not too high. The comparison between theory and experiments is carried out without assuming explicit forms for relaxation spectrum.

Experimental investigation of the concept of molecular migration within sheared polystyrene

Abstract: Several important aspects of the flow in polymer melts through capillaries remain unexplored. This paper examines experimentally one such effect associated with the radial shear-stress gradient in capillaries. During capillary melt flow of a polymer with a wide molecular weight distribution, migration of the large molecules away from the region of highest shear stress, i.e., at the capillary wall, has been predicted but only modestly investigated. This effect has the potential to produce a molecular weight spectrum over the cross section of extruded polymer. Studies of distribution in shear were conducted on a well-characterized wide-distribution polystyrene (Mw = 234,000). An Instron Rheometer equipped with a long capillary (length/diameter ratio of 66.7) was used to perform the extrusion at temperatures of 160–250°C. A solvent coring procedure was used to dissolve away concentric layers of polymer from the extrudate for molecular weight analyses. The method has been shown to cut clean sections without selective extraction. Values of Mw, Mn and Mw/Mn were calculated from complete molecular weight distribution data obtained by calibrated gel permeation chromatography. For a wide range of shear rates and temperatures, no evidence for molecular fractionation was observed. Shear degradation of this polymer was found to be small. However, at high shear rates at 250°C, evidence indicating extensive shear-induced thermal degradation was found. No evidence for oxidative degradation at the extrudate surface was found at either low or high shear rates at this temperature.

The Limit of Linear Viscoelastic Response in Polymer Melts as Measured in the Maxwell Orthogonal Rheometer

Abstract: The Maxwell orthogonal rheometer was used to measure the limit of linear viscoelastic response (LLVER) as a function of frequency for three linear polyethylenes at 170°C, for six atactic polystyrenes at 170°C, and for Phenoxy A at 212°C. For the elastic component of the stress, linear viscoelastic response was found to exist in all cases up to finite strains of approximately 50%. The viscous component of the stress exhibited linear viscoelastic response up to approximately 100% strain and sometimes as high as 120%, the maximum strain tested. Two constitutive equations (Bird and Carreau's theory with Gordon and Schowalter's generalization and Tanner's network rupture theory) are noted for the dependence they predict of the strain LLVER as a function of frequency. Bird's theory, with Gordon and Schowalter's generalization, predicts that the elastic strain limit times the frequency will be a constant as the frequency is varied; Tanner's theory predicts that the elastic strain limit will be a constant with va...

Electron microscope studies of polymer degradation

Abstract: Degradation studies have been conducted on a dilute solution of a polyacrylamide, Polyhall 402, in distilled water with the aid of a simple rheometer, employing a capillary tube operating in the turbulent flow regime. The degraded samples, collected after each pass through the rheometer were analyzed with the aid of an electron microscope. The analysis showed how the polymer molecules fracture under turbulent shear with the fragments in solution approaching a symptotically an average molecular weight of approx. 1.0 x 10U6D after a number of passes. (13 refs.)

Thixotropy of filled-polyethylene melts subjected to periodic shear deformation

Abstract: As a result of the action of periodic finite-amplitude shear deformations filled high-pressure polyethylene melts exhibit thixotropic properties. The effect of the previous deformation history on the frequency dependence of the complex dynamic shear modulus is investigated.

Rheology of Oil Films at High Contact Pressures

Abstract: WE report here a new approach to studying the shear behaviour of thin films of oil under conditions of elastohydrodynamic lubrication such as arise at points of contact in ball bearings or between gear teeth. During its passage through the contact the oil is rapidly compressed to pressures in the region of 104 bar and then sheared by the sliding motion of the metal surfaces. Hitherto this behaviour has been studied using rolling contact disk machines in which rollers having parallel axes are pressed into contact. Rolling motion generates an oil film of known thickness (∼1 µm) between the hard steel surfaces; superimposed sliding shears the film. At very small sliding speeds—shear rates—the behaviour is linear: the mean shear stress required to shear the film τ is proportional to the sliding speed. At higher sliding speeds the mean shear stress reaches a maximum (critical) value τc1. It has been suggested2 that the observed behaviour can be explained by the oil exhibiting viscoelastic properties at these high pressures, when the viscosity is known to increase by many orders of magnitude. Unfortunately this hypothesis cannot be tested with certainty by conventional disk machine tests, which do not distinguish unequivocally between viscous and elastic response of the fluid in the contact zone. We report here a different rolling contact experiment which reveals viscoelastic behaviour directly by separating elastic and viscous response of an oil film under elastohydrodynamic conditions.

Extrusion Rheology of Lubricated Polytetrafluoroethylene

Abstract: Polytetrafluoroethylene is probably the only polymer which is commercially extruded in solid state. The polymer is provided in powder form which is lubricated by an oil. The samples of this study were powder of about 500 μ size, coagulated from latex particles of average .2–.4 μ. Naphtha was used as the lubricant at 16.5 weight percent of the composition. The pressure‐output rate relation was examined with two types of capillary rheometers. A constant pressure rheometer was used to characterize the pressure range of 1000 3000 psi. The output rate at a given pressure decreased with time. A constant drive speed rheometer was used in the pressure range of 4000–14,000 psi. The recording of the force was very irregular but had an oscillating pattern. With both rheometers the extrudate diameters were equal or smaller than the die diameter. The compression experiment showed that the material behavior was dependent on not only the available free volume but the nature of powder itself. The compressive stress relax...

Shear-dependent relaxation spectrum: Conversion from viscosity to stress relaxation after cessation of steady-state flow for polyethylene melts

Abstract: Interrelations were established, in a limited range of shear rate γ, among the non-Newtonian shear stress relaxation following cessation of steady-state flow σ(γ, t), the steady-flow viscosity η(γ), and the dynamic viscosity η(ω) or the storage modulus G′(ω) for three linear polyethylene melts (190°C). The data for these melts were obtained using the Weissenberg rheogoniometer and the Instron rheometer. Agreement between the experimental σ(γ, t) and those calculated from the relaxation spectrum H(ln τ) was good and involved no coordinate shift. The calculated values were obtained through the relation where θ = γτ/2, τ is the relaxation time, ω is the radian frequency, h(θ) = (2/π)[cot−1 θ + θ(1 - θ2)/(1 + θ2)2], andg(θ) = (2/π) [cot−1 θ + θ)/(1 + θ2)]. The relaxation spectrum was obtained either from the linear viscoelastic data [η(ω)] using the iterative method of Roesler and Twyman or from the nonlinear steady-flow viscosity using the iterative procedure based on the equation

Total applied shear in capillary extrusion

Abstract: Viscoelastic properties of polymer fluids are single-valued functions of shear stress or shear rate only at high total applied shear. These parameters may vary with applied shear under milder shear histories. The mean total shear in capillary extrusion is shown to be a function of orifice geometry. Apparent flow curves can be measured at various total shear values by changing the length/radius ratio of the capillary. The true shear stress and true shear rate at the orifice wall correspond to infinite total shear conditions. The true flow curve and elastic parameters like die swell are not measured at equivalent total shear unless the capillary is extremely long.

Rheological properties of polymers in the fluid state

Abstract: The possibilities and conditions of correlation are determined for the principal rheological characteristics of single-phase polymer systems measured for one-dimensional shear deformation in steady-state flow regimes, on transition from rest to steady-state flow, and in harmonic vibration regimes. Special significance attaches to the quantitative results of measuring the high-elastic properties of the polymer systems. It is shown that the Lodge theory, describing the flow behavior of high-elastic media, is well-founded in the linear region of deformation, i.e., for the limiting case of shear rates and shear stresses tending to zero, whereas the Mooney-Rivlin-Weissenberg theories are not in accord with the experimental data even in this limiting deformation regime.

Viscoelastic Response in Shear

Abstract: In the classical linearized theory of elasticity, the stress in a sheared body is taken to be proportional to the amount of shear. The Navier-Stokes theory of viscosity takes the shearing stress to be proportional to the rate of shear. In most materials, under appropriate circumstances effects of both elasticity and viscosity are noticeable. If these effects are not further complicated by behavior that is unlike either elasticity or viscosity, we call the material viscoelastic.

An automatic Zimm-Crothers type viscometer

Abstract: An automatic Zimm-Crothers type viscometer is described which permits very accurate measurements at various values of the shear stress, using only one rotor.

Dynamic properties of certain unvulcanized rubbers in the high-elastic and fluid states

Abstract: The dynamic properties of a series of unvulcanized rubbers (cis-polybutadienes, Na-polybutadiene, cis-polyisoprene, butyl rubber, polyisobutylene, and ethylene-propylene copolymer) have been investigated in a low-frequency dynamic testing machine and a frequency rheometer on the frequency range from 5 · 10−2 to 4.5 · 103 Hz at temperatures of 25 and 60°C. At these temperatures the mechanical loss factor is the most sensitive criterion of transitions from one physical state of the polymer to another. For all the specimens investigated on the experimental range of angular frequencies the modulus of the complex dynamic viscosity and the effective viscosity determined under static conditions coincide, assuming the equivalence of angular frequencies and shear rates.

Relationship between the normal and shear stresses in elastico-viscous liquid flow

Abstract: A class of elastico-viscous liquids is considered. On the basis of an analysis of the experimental data of a considerable number of authors it is shown that the Weissenberg relation between the shear stresses and the first difference of the normal stresses is limited and a more general expression of the exponential type is proposed.