Volume 20, Issue 4, December 1976
Index of content:

Processability Difference of Elastomers Observed by Rate Dependence of Viscoelastic Properties
View Description Hide DescriptionTwo similar elastomers of known difference in processability are examined by means of the MTS high speed tensile tester and the Mooney rheometer at a low speed. The samples are butadiene‐acrylonitrile copolymers having 42% acrylonitrile content. Measurements on the MTS tester are made at −35, 24, and 85°C and cross‐head speeds of 2.5, 25, and 250 in./sec. These speeds correspond to strain rates of 468, 4680, and 46,800%/sec, respectively. In addition, measurements are made at 100°C and 2.5 in./sec. A Mooney rheometer is used to obtain shear stress‐strain curves at 100°C and 0.05 rpm. Both tensile and shear stress‐strain curves are used to construct master curves, which cover 17 decades of the deformation rate. A major difference between the samples is found in the low rate range of the observation. The ultimate properties are also examined. Rather small sample‐to‐sample differences are observed in the stresses at break.

Rheological Properties of Glass Fiber‐Reinforced Dough Molding Compounds
View Description Hide DescriptionRheological properties of thermosetting glass fiber‐filled Dough Molding Compounds (DMC) are useful in understanding the fiber orientation in moldings caused by flow patterns in the mold as well as in assessing the moldability of the material. A rheometer utilizing the squeeze flow between two parallel circular discs was shown to be successful in characterizing the viscousproperties of DMC. The viscometric data indicate that addition of glass fibers makes the DMC slurry more viscous and shear thinning and that the extent of these effects increases with the weight fraction of glass fibers. The length‐to‐diameter ratio of the fibers was found to be an important variable. At the same volume fraction, glass fibers with a larger length‐to‐diameter ratio make the DMC more viscous and shear thinning.

Velocity Descriptions in the Die Exit Region for a Polymer Solution: Influence of Die Entry Instabilities
View Description Hide DescriptionA Laser Doppler Anemometer and a capillary rheometer are used to study the nature of emerging jets of liquid paraffin and a highly elasticsolution of Separan Ap30. In particular, the effect of die entry instabilities reported earlier for the polymer solution is studied. Velocity profile measurements are made in the ranges for liquid paraffin and for the polymer solution.Surfacevelocity measurements for the polymer solution in the above range show that the propagation of spiralling instabilities results in a transverse wave‐like phenomenon which causes distortion of the jet diameter at die swell. The distortion increases with increasing shear rates and decreasing tube ratios. Data for are presented and the results for the polymer solution are discussed in relation to the reported extrudate distortions in polymer melts. A mechanism for the occurrence of matte and sharkshin extrudates is proposed.

The Rheology of Solid Polymers Subjected to Large Deformations
View Description Hide DescriptionExperimental data on the deformation of polyethylene terephthalate over a range of temperatures below its melting point are presented. The observed behavior is of the type usually considered as “plastic”; in spite of this, it is described by a modification of a constitutive description initially proposed for molten polymers by Tanner and Simmons. The constitutive description proposed here correctly portrays the observed behavior without reference to classical plasticity theory, and is of a type which allows in principle to predict the behavior of the material for every conceivable deformation history.

The Strain‐Energy Function for Elastomers
View Description Hide DescriptionIt has been shown that the strain‐energy function for an incompressible isotropic elasticmaterial must be expressible as a function of two strain invariants which are elementary symmetric functions of the squares of the principal extension ratios Valanis and Landel have proposed a strain‐energy function of the form It is shown how such a strain‐energy function can be expressed explicitly as a function of and Also, necessary and sufficient conditions are derived for any function of and to be expressible in the form proposed by Valinis and Landel. It is shown how the results of problems which have been solved using the general form of the strain‐energy function can be conveniently specialized to the case when the Valanis‐Landel strain‐energy function is valid.

An Empirical Constitutive Model for Polymer Solutions
View Description Hide DescriptionConstitutive relations generally require a large set of fluid relaxation times to provide an accurate fluid description. In situations involving the solution of the dynamic equations of flow simultaneously with the constitutive relation, the use of a single relaxation timemodel is desirable because of the time consuming nature of the numerical solution schemes. This work presents a single relaxation time fluid model, obtained by empirically modifying the existing form of the Meister model. The empiricisms take the form of two functions of deformation rate, chosen so the model will predict quantitatively the time to maximum overshoot and the magnitude of the maximum overshoot in transient shear stress for a “start‐up” experiment. The modified model compares very well to transient and steady data for five polymer solutions and to the predictions of the Carreau Model B.

A Network Rupture Constitutive Equation Based on Transient Stress Experiments
View Description Hide DescriptionConstitutive equations of the so‐called rate‐dependent type were found to be in disagreement with the experimental data of stress development after onset of steady shear flow, observed with a Weissenberg Rheogoniometer. A discussion on the reason for the disagreement is given. It is reported that such transient phenomena can be quantitatively explained if we assume that a transition from the equilibrium entangled structure to the steady structure at the shear rate occurs when the strain in the material reaches a critical value.

On the Interface Deformation in the Stratified Two‐Phase Flow of Viscoelastic Fluids
View Description Hide DescriptionExperimental and theoretical studies were carried out to investigate the mechanism of the interface deformation in the stratified two‐phase flow of viscoelastic fluids through a rectangular duct. For the experimental study, a transparent test channel was constructed to permit visual observations, from the direction perpendicular to flow, on the interface formed when two incompatible molten polymers flow between two parallel planes. Extrudate samples were also collected, and the interface shape of the extrudate cross section was examined. For the theoretical study, using the Coleman‐Noll second‐order fluid as the constitutive equation, an approximate solution of the equations of motion was obtained numerically. The movement of the interface from the initially flat position was determined with the use of the normal stress continuity condition across the interface. The computed results show that the more viscous component tends to push into the less viscous component, consistent with experimental observation. It has been shown that the viscosity difference between the two components predominates over the elasticity ratio in determining the interface shape in stratified two‐phase flow.

Experimental Tests of Some Integral Rheological Relations
View Description Hide DescriptionSome new rheological relations are derived for the BKZ fluid in shear. These relations all involve integrals rather than derivatives of stress measurements. For suddenly applied constant rate of shear, several relations are found connecting (a) time‐dependent normal‐stress at one shear rate to integrals of time‐dependent shear‐stress over shear‐rate and time, (b) time‐dependent shear‐stress to integrals of time‐dependent normal‐stress over shear‐rate and time, (c) an integral over time of normal‐stress to an integral over shear rate of steady‐state shear‐stress, and (d) an integral over time of shear‐stress to an integral over shear‐rate of steady‐state normal‐stress. For sudden cessation of steady shear a rheological relation is found connecting an integral over time of shear‐stress to an integral over shear‐rate of normal‐stress. These relations are tested with data from measurements on PIB solutions. The results suggest that time‐dependent normal‐stress data are not reliable. Within this limitation, the results are consistent with the requirements of a BKZ fluid. A rheological relation appropriate to single‐integral rate‐type constitutive equations is examined also and found to be inconsistent with the data, as is the van Es and Christensen test.

The Rheology of Hydroxyethyl Cellulose Solutions
View Description Hide DescriptionA systematic and detailed study of the viscous behavior of aqueous hydroxyethyl cellulose solutions has been made in a cone‐and‐plate viscometer over very wide ranges of shear rate and temperature. High shear stability and high degrees of pseudoplasticity have been observed. The rheograms have followed the Ostwald‐de Waele model. Simple and accurate expressions have been formulated relating temperature, solution‐concentration and power law exponent to consistency coefficient; also the solution‐concentration to activation energy.