Volume 35, Issue 3, April 1991
Index of content:
Flow behavior and exit pressures of corn meal under high‐shear–high‐temperature extrusion conditions using a slit diea)35(1991); http://dx.doi.org/10.1122/1.550217View Description Hide Description
Corn meal and commercial low density polyethylene (LDPE) were extruded through a slit die using a laboratory model single‐screw extruder at various extrusion conditions. Flow curves for the corn meal showed severe shear thinning behavior (n≲0) at lower moisture contents; molecular degradation, pressure dependence of viscosity,viscous dissipation, slip, and yield stress were identified as the possible factors. Survey of the available literature and the behavior observed in this study indicates that molecular degradation is the major factor responsible for n≲0. A modified viscosity model that includes the die entrance pressure (barrel pressure) for molecular degradation effects was developed; such a model also accounts for the pressure dependence of viscosity. Negative exit pressures for LDPE and both positive and negative exit pressures for corn meal were obtained. Statistical analysis indicated large errors in the exit pressures predicted by the linear extrapolation. These errors originate from several sources, thereby preventing the application of the exit pressure method as a simple method for estimating N 1 of complex materials such as corn meal.
35(1991); http://dx.doi.org/10.1122/1.550218View Description Hide Description
A printing press tackmeter was built by fitting a pressure sensor into the printing cylinder of a laboratory printing press. This tackmeter has been used to measurepressure profiles of thin (3–14 μm) ink films under printing conditions. We define the maximum tensile stress (or negative pressure) that the ink can withstand in the nip exit before splitting to be the tack of the ink. The tack of inks without dissolved polymer appears to be independent of film thickness. The tack of inks with dissolved polymer resin increases strongly with increased amount of ink in the nip, while pigment and dispersant content have a minor effect on ink tack. This suggests that at high speed, ink tack is mainly characterized by the response of polymer molecules toward rapidly applied stress. No universal correlation was found between the tack and viscous flow properties of the ink.
35(1991); http://dx.doi.org/10.1122/1.550219View Description Hide Description
Based on a simple analysis using the Larson model, a superposition procedure is proposed for constant stretch rate data from fiber spinning experiments. This procedure appears to be able to account for the shear prehistory undergone by the fluid in the spinneret and yields true extensional stress growth data. The effectiveness of this method is demonstrated with the help of previously published isothermal fiber spinning data on one polymer melt and new data on one polymer solution. For constant stretch rate spinning at constant force, it is further shown that all the data can be represented by a single equation with known coefficients. Comparison with literature data on the transient extensional viscosity suggests that this same equation may also hold for the high stretch‐rate filament stretching of molten polymers at constant stretch rate. This result should lead to a considerable reduction in the amount of experimental work needed to characterize polymeric fluids.
35(1991); http://dx.doi.org/10.1122/1.550220View Description Hide Description
Rheo‐optical studies on shear deformation of well‐aligned nematic solutions of the rodlike poly (1,4‐phenylene‐2,6‐benzobisthiazole), PBT, are reported. Conoscopic microscopy was used to follow the director during shear, and during relaxation on cessation of flow. The results show that the molecules are not homogeneously aligned in shearing flow. Rather, various complex director field distortions may occur, depending on the relative orientation of the initial director to the flow direction. The results are compared quantitatively with predictions based on the Ericksen–Leslie constitutive equation for nematic fluids.
35(1991); http://dx.doi.org/10.1122/1.550221View Description Hide Description
An ER material composed of alumino‐silicate particles in paraffin oil is studied for its response to sinusoidally oscillating shear strains at frequencies in the range of 10–50/s. The response of the material may be discussed in terms of three rheological regions; pre‐yield, yield, and post‐yield. Within each region there is a different mode of deformation;viscoelastic in the pre‐yield, viscoelasticplastic in the yield, and plastic in the post‐yield. The deformation modes are dependent on applied electric field strength, strain amplitude, and strain frequency. Furthermore, the energy dissipated by the ER material is analyzed as related to strain amplitude and electric field. The energy dissipated by the ER material is proportional to the strain raised to the second power when the material is deformed in the pre‐yield region, varies between the strain raised to the second power and raised to the first power during deformation in the yield region, and the energy is proportional to the strain raised to the first power when deformation occurs in the post‐yield region. Finally, a rheological model is introduced to account for the behavior of ER materials qualitatively.
35(1991); http://dx.doi.org/10.1122/1.550222View Description Hide Description
We calculate the linear frequency‐dependent modulus of a model electrorheological (ER) fluid at low and intermediate concentrations, assuming that particle strings are the dominant structures. At low concentrations hydrodynamic interactions along a single string act to reduce the dissipation of modes with low wave number (just as in the case of random‐coil polymers). At higher concentrations we expect hydrodynamic screening to be effective. This results in a dissipative part to the modulus.G ‘(ω) which has a broad plateau over a frequency bandwidth (L/a)2, where L is the characteristic string length and a of the order of the particle size. By contrast, the magnitude of G ‘(ω) is limited by the fraction of incomplete or broken strings. The real part of the dynamic modulusG ’(ω) is dominated by strings which span the electrodes, and shows very little structure with frequency when the majority of strings are complete.
35(1991); http://dx.doi.org/10.1122/1.550228View Description Hide Description
The Dinh–Armstrong constitutive equation is used to derive analytic expressions for the viscosity and the normal stress difference in shear and elongational deformation of nondilute, slender‐fiber suspensions in Newtonian media. This is achieved by approximating the fourth order orientation tensor of the integral constitutive equation with the components of the Finger strain tensor according to Currie’s expressions that relate the strain tensor to flowkinematics. The derived analytic expressions are in agreement with the numerical results given by Dinh–Armstrong in simple shear flow. In elongation, although the magnitude of the dimensionless viscosity at start up and steady state are identical, there is a quantitative disagreement in the values inbetween. Approximations to the fourth order orientation tensor are also derived for any two‐ or three‐dimensional mixed flow of the same suspensions.
35(1991); http://dx.doi.org/10.1122/1.550229View Description Hide Description