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  • 1
    ISSN: 0730-6679
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: A finite element simulation of viscoelastic flows in double-layer coextrusion of polymer melts has been performed. The constitutive equation used is a realistic integral model of the K-BKZ type which can describe the viscoelastic memory characteristics of polymer melts. Viscosity and first normal stress data of typical PS and HDPE melts at 200°C are employed along with a feed ratio used in actual experiments found in the literature. Our results reveal important viscoelastic effects that could not be predicted by previous inelastic simulations. In particular, when the outer layer is more elastic than the inner layer, an enhanced extrudate swell is observed relative to the opposite configuration at the same flow rate, even if the outer layer is less viscous than the inner layer. In terms of a dimensionless stress ratio SR measured on the die wall, the more elastic material at the outer layer results in a higher SR value than that of the opposite configuration at the same flow rate. On the other hand, when the outer layer is less elastic than the inner layer, a reduced extrudate swell is observed relative to the opposite configuration at the same flow rate even if the outer layer is more viscous than the inner layer. When compared at the same SR value, however, our results show that the inelastic swell mechanism proposed by Tanner still applies qualitatively, i.e., a more viscous outer layer will result in larger extrudate swell than the opposite configuration.
    Additional Material: 10 Ill.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 0730-6679
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Extrudate swell studies of a high-density polyethylene (HDPE) melt have been undertaken in flows through slit and capillary dies with the purpose of finding out the effect of die-length/diameter (gap) (L/D or L/2H) ratio on the viscoelastic behavior. Numerical solutions have been obtained by using the finite element method (FEM) and an integral constitutive equation of the K-BKZ type with a spectrum of relaxation times. The material parameters have been obtained by fitting experimental viscosity and normal stress data for the melt as measured in shear, and elongational viscosity data available in the literature. Different L/D (L/2H) ratios have been considered ranging from very short to infinitely long dies. The numerical simulations reveal that as the flow rate increases, viscoelastic effects exhibited by the HDPE melt become important and manifest themselves in an enhanced swelling behavior after the die exit, while small, Newtonian-like vortices exist in the contraction before entry to the die. Elastic recovery is also captured in an enhanced extrudate swell, which is always higher at the same apparent shear rate for the capillary than the slit dies and decreases drastically as the L/D (L/2H) ratio increases, reaching asymptotic values for very long dies. Such behavior is in agreement with experimental findings from flows through slit and capillary dies and in sharp contrast with purely viscous simulations which cannot predict such strong viscoelastic phenomena associated with the memory of the polymer melt. © 1993 John Wiley & Sons, Inc.
    Additional Material: 14 Ill.
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    Hoboken, NJ : Wiley-Blackwell
    AIChE Journal 31 (1985), S. 1736-1739 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Additional Material: 4 Ill.
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    Stamford, Conn. [u.a.] : Wiley-Blackwell
    Polymer Engineering and Science 24 (1984), S. 707-715 
    ISSN: 0032-3888
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: A general-purpose finite element program has been used to simulate the flow of Newtonian, power-law, and viscoelastic fluids in the entry and exit regions of a slit die. It was found that shear-thinning increases the entrance correction while it decreases the exit correction. Shear-thinning reduces the size of the small corner vortex that forms in the entry flow of a Newtonian fluid. The swelling ratio had a value of 1.196 for Newtonian fluids and decreased as the value of the power-law index decreased. Viscoelastic calculations were performed using the Criminale-Ericksen-Filbey (CEF) constitutive equation. Convergence of the iterative scheme was unattainable for Deborah numbers above 1.0. The results showed a decrease of the entrance correction and an increase of the exit correction with elasticity. Extrudate swell first decreased slightly and further increased with the Deborah number.
    Additional Material: 8 Ill.
    Type of Medium: Electronic Resource
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  • 5
    ISSN: 0032-3888
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: A general-purpose finite element program has been used to simulate the flow of a typical polystyrene melt in the entry and exit regions of a slit die. Instead of using a general viscoelastic constitutive equation, simplified models were used that include correlations based on experimental data available in the literature for the shear and elongational viscosities and the normal stresses. With such simple models convergence of the iterative scheme is extended to relatively high Deborah numbers (De ≈ 5). The models predict vortex growth in the entry region and an increase of extrudate swell at the exit in qualitative agreement with experimental observations. It was found that the normal stresses are primarily responsible for these phenomena, while the elongational viscosity tends to increase the end (Bagley) correction and decrease the swelling.
    Additional Material: 17 Ill.
    Type of Medium: Electronic Resource
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  • 6
    Electronic Resource
    Electronic Resource
    Stamford, Conn. [u.a.] : Wiley-Blackwell
    Polymer Engineering and Science 26 (1986), S. 1552-1562 
    ISSN: 0032-3888
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: A general-purpose finite element program has been used to simulate the flow of nonshear-thinning, highly elastic polymer solutions (Boger fluids). In particular, the creeping flow through an abrupt 4:1 circular and planar contraction is studied, as well as the flow at the exit of a capillary die for the determination of extrudate swell. Experimentally measured normal stress and viscosity data are included in a simple rheological model, based on the viscometric simplification of the CEF constitutive equation. Vortex size and intensity in the die entry and extrudate swell at the die exit increase rapidly, with elasticity level, in general agreement with experimental findings. It is shown that despite the limitations of the model, the viscometric approximation can be used to study the effect of normal stresses in cases where a main flow direction can unambiguously be defined. In die exit Flows, it can also provide an upper limit for the determination of extrudate swell, while Tanner's theory of elastic recovery provides the lower limit.
    Additional Material: 14 Ill.
    Type of Medium: Electronic Resource
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  • 7
    Electronic Resource
    Electronic Resource
    Stamford, Conn. [u.a.] : Wiley-Blackwell
    Polymer Engineering and Science 28 (1988), S. 291-310 
    ISSN: 0032-3888
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: The wire-coating process was analyzed numerically making use of a particular die design employed in highspeed industrial operations. Both the lubrication approximation theory and a fully two-dimensional finite element analysis were applied under isothermal and nonisothermal conditions, respectively. Particular emphasis has been given to the heat transfer effects between the melt arid the solid and free boundaries. A variety of thermal boundary conditions was studied, ranging from adiabatic to constant temperature walls. The influence of dimension less groups such as Peclet, Nahme, and Biot numbers is examined. Oscillation-free solutions are obtained for the temperature field by using a standard finite element Streamline-Upwind/Petrov-Galerkin technique. Rheological data for a wire-coating low-density polyethylene (LDPE) resin (Alathon-3535) were used in the analysis. The predictions include pressure and temperature distributions, shear stresses and shear rates both at the die wall and the wire, and wire tension for different wire speeds. The numerical results are compared with a set of experimental data for LDPE in a typical die used by Du Pont Co. It is found that the isothermal lubrication approximation for power-law fluids overestimates pressure distributions when applied at die operating temperature. The nonisothermal finite element analysis gives better predictions, especially when realistic thermal boundary conditions are imposed, with the experimental results lying between those found from simulations assuming isothermal walls (upper limit) and adiabatic walls (lower limit).
    Additional Material: 26 Ill.
    Type of Medium: Electronic Resource
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  • 8
    Electronic Resource
    Electronic Resource
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Applied Polymer Science 34 (1987), S. 1713-1725 
    ISSN: 0021-8995
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: A numerical simulation of coextrusion flow in a capillary die has been undertaken for polymer melts used in a previously reported experimental study of coextrusion. Viscosity data are used for PS, HDPE, and LDPE melts. A Newton-Raphson scheme is employed to solve the equations for a fully developed pressure-driven flow of two concentric layers in a capillary. A finite element method is used to simulate the full flow field behavior, including determination of the interface and free surface of the exiting stream. Double nodes have been used at the interface to ensure continuity of velocities and stresses and to capture the pressure discontinuities. Pressure gradients, extrudate swell, interface swell, and other relevent flow characteristics are presented and compared with the experimental findings. The finite element analysis revealed that satisfactory convergence of the interface location is found for the cases when the less viscous material wets the capillary walls, which is also the preferential configuration in coextrusion. For the opposite configuration, convergence proved either difficult or impossible depending upon the viscosity ratio. Discrepancies were found to exist between the theoretically predicted and experimentally measured pressure gradients.
    Additional Material: 11 Ill.
    Type of Medium: Electronic Resource
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  • 9
    ISSN: 0021-8995
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: Controlled-rheology polypropylene melts were prepared via molecular modification of a commercial polypropylene resin. A peroxide-initiated degradation was performed, resulting in materials with different molecular weight distributions. These resins were subjected to rheological characterization, and an integral constitutive equation of the K-BKZ type was used to study the effect of molecular weight characteristics on their rheological properties. Data for the linear viscoelastic spectrum and shear viscosities was used to obtain the model constants. The same constitutive equation has been used to predict the stress and Trouton ratios for simple shear and simple elongational flows, thus giving a quantitative assessment of the viscoelastic character of the melts. The results show the effect of the molecular modification on the rheological behavior of the melts. Polymers produced at higher peroxide concentrations exhibit reduced viscoelasticity manifested in less shear-and strain-thinning behavior. The present work clearly shows the potential of integral constitutive equations in fitting and interpreting experimental data and, thus, giving a much better understanding of the rheological behavior of commercial polymers. © 1996 John Wiley & Sons, Inc.
    Additional Material: 13 Ill.
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  • 10
    Electronic Resource
    Electronic Resource
    Hoboken, NJ : Wiley-Blackwell
    AIChE Journal 32 (1986), S. 497-500 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Additional Material: 5 Ill.
    Type of Medium: Electronic Resource
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