Studies on multilayer film coextrusion II. Interfacial instability in flat film coextrusion

Han, Chang Dae; Shetty, Rajesh

doi:10.1002/pen.760180303

Cited by 112 publications

(68 citation statements)

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“…Since these instabilities set severe limits to the industrial processes such as film or fiber fabrication, they have been extensively studied before [7,8]. Although previous experiments and theory agree reasonably well [7,9], a comprehensive description of the flow is still lacking [10].In this Letter we propose a quantitative explanation for various flow patterns observed in purely elastic interfacial instabilities. We perform a set of original experiments on co-flow of a polymer solution and water and map the full flow diagram.…”

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Elastic instability in stratified core annular flow

et al. 2011

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We study experimentally the interfacial instability between a layer of dilute polymer solution and water flowing in a thin capillary. The use of microfluidic devices allows us to observe and quantify in great detail the features of the flow. At low velocities, the flow takes the form of a straight jet, while at high velocities, steady or advected wavy jets are produced. We demonstrate that the transition between these flow regimes is purely elastic -it is caused by viscoelasticity of the polymer solution only. The linear stability analysis of the flow in the short-wave approximation captures quantitatively the flow diagram. Suprisingly, unstable flows are observed for strong velocities, whereas convected flows are observed for low velocities. We demonstrate that this instability can be used to measure rheological properties of dilute polymer solutions that are difficult to assess otherwise.PACS numbers: 83.80. Rs, 83.60.Wc, 83.85.Cg Polymer solutions exhibit purely elastic flow instabilities even in the absence of inertia [1]. The almost ubiquitous ingredient of such an elastic instability is the curvature of streamlines: polymers that have been extended along curved streamlines are taken by fluctuations across shear rate gradient in the unperturbed state which, in turn, couples the hoop stresses acting along the curved streamlines to the radial and axial flows and amplifies the perturbation [2,3]. Flat interfaces between two fluids with different viscoelastic properties can also become unstable [4-6] due to the normal stress imbalance across the interface. These instabilities often occur in coextrusion where different polymers are melted in separate screw extruders and then flown simultaneously in the extrusion nozzle. Undesirable wavy interfaces are sometimes observed between the adjacent polymer layers both during the flow and in the final product [7]. Since these instabilities set severe limits to the industrial processes such as film or fiber fabrication, they have been extensively studied before [7,8]. Although previous experiments and theory agree reasonably well [7,9], a comprehensive description of the flow is still lacking [10].In this Letter we propose a quantitative explanation for various flow patterns observed in purely elastic interfacial instabilities. We perform a set of original experiments on co-flow of a polymer solution and water and map the full flow diagram. In contrast to previous experimental studies dealing with macroscopic flows of molten polymers, we focus here on flows of dilute polymer solutions and water in microfluidic devices. The advantage of using dilute polymer solutions is that their elastic properties can easily be tuned by dilution and that most of the theoretical work has been performed for models representing dilute polymeric fluids. The use of microfluidic flow geometries offers simple control and visualisation of the flow.We observe that above some flow rate the interface between the polymer solution and water becomes wavy. Surprisingly, for relatively low velocities...

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Elastic instability in stratified core annular flow

et al. 2011

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“…When decreasing the temperature the viscosity of PET2 increases much more than the viscosity of PET1 and so the gap between viscosities of both polymers decreases and the shear rate for which the viscosities are equivalent decreases towards the range of processing conditions. Following the literature ( [1,2,3,4]) one may believe a strong influence of temperature and flow rate on the defect appearance. Figure 4 shows that the mean relaxation time is much more important for PET1 than for PET2 in the extrusion processing range.…”

Section: Presentation Of Coextrusion Process and Polymersmentioning

confidence: 99%

“…-In the first approach, the transition between stable and unstable processing conditions is investigated by examining the aspect of the extrudate obtained with different couples of commercial polymers (Han and coworkers [1,2,3], Antukar et al [4], Valette et al [5]). The authors related stable and unstable processing conditions to thickness ratio as well as viscosity and first normal stress difference ratios at the interface.…”

Section: Introductionmentioning

confidence: 99%

Numerical investigations of polyester coextrusion instabilities

Mahdaoui

Laure

Agassant

2013

Journal of Non-Newtonian Fluid Mechanics

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Interfacial instabilities occurring in the coextrusion process of molten polymers have been widely studied. The theoretical work based on the stability of 2D Poiseuille multilayer flows (invariant along the flow motion) have pointed out the convective nature of this instability (it is either amplified or damped in the flow direction). This behaviour has been confirmed by experimental observations. As this instability is purely elastic (the Reynold number in coextrusion process is very small), it is necessary to introduce a viscoelastic constitutive equation for the polymers. Comparisons between experimental observations and theoretical approaches based on the Giesekus or White-Metzner models give good agreements for laboratory simple devices. However, the applicability of this approach for industrial coextrusion process remains an important challenge, because one has to deal with the influence of complex geometries, varying temperature and more realistic constitutive equations. As it is still very costly in computational time to perform 3D instationary computations for viscoelastic fluids, a methodology to analyse the appearance of interfacial instabilities in a complex geometry is proposed within this paper. First, a stationary 3D computation is performed in the real geometry for a single polymer. This computation allows to find zones in which the flow motion is essentially 2D, isotherm and with maximum values of the shear rate and, thus, the Weissenberg number. Instationary 2D finite element computations for two layer flows and a multi-mode differential viscoelastic model are then performed in these zones. Spatio-temporal analyses give information on the spatial behaviour of interfacial disturbances. This methodology is applied to the coextrusion of two polyesters for which Preprint submitted to Journal of Non-Newtonian Fluid Mechanics December 7, 2012 experimental observations are available. The simulations predict well the experimental results and show that the outlet zone can amplify low frequencies perturbations. Finally, the influence of processing parameters (temperatures, flow rates, ...) and die geometries has been checked.

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“…The coextrusion process has been the subject of many experimental and numerical studies [5][6][7][8][9][10][11][12][13][14][15][16] over the past three decades. Such studies have been conducted to enhance our fundamental understanding of the mechanisms and kinematics involved in bringing together several melt streams with different rheological properties and temperature fields.…”

Section: Introductionmentioning

confidence: 99%

Visualisation and Analysis of Polyethylene Coextrusion Melt Flow

Martyn¹,

Coates²,

Zatloukal³

2009

AIP Conference Proceedings

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Abstract. Polymer melts experience complex, time variant, stress and deformation fields on their passage through fixed geometries in many conversion operations. Flow complexity is further increased in operations involving the co-joining of two or more melt streams where one confining boundary is moving and viscoelastic. Such a complex situation arises in coextrusion processes. This work covers experimental studies on polyethylene melt flows in complex coextrusion geometries with a view to understanding the stress fields involved and their effects on flow stability.A 30q coextrusion geometry is studied using two extrusion arrangements. In one arrangement a single extruder is used to feed a 'bifurcated' die design wherein the melt stream is split prior to, and rejoined after, a divider plate in the die. In the other design melt streams are delivered to, and converged at 30q, using two independent extruders. In a second die melt streams are brought together at 90q. In each die arrangement melt flow in the confluent region and die land to the die exit was observed through side windows of a visualisation cell. Velocity ratios of the two melt streams were varied and layer thickness ratios producing instability are determined for each melt for a variety of flow conditions. Stress and velocity fields in the coextrusion arrangements were quantified using stress birefringence and particle image velocimetry techniques.The study demonstrates conclusively that wave type interfacial instability occurred in the coextrusion geometries when the same low density polyethylene melt is used in each stream. This observation occurred at specific, repeatable, stream layer ratios in each die arrangement. The complex flows were numerical modelled using a modified Leonov model and Flow 2000 software. There was reasonable agreement between modelled at experimentally determined stress fields. Modelling however provided far more detailed stress gradient information than could be resolved from the optical techniques. A total normal stress difference (TNSD) sign criterion was used to predict the critical layer ratio for the onset of the interfacial instability in one die arrangement and good agreement between theory and experiment has been obtained.The study conclusively demonstrates wave type interfacial instability in the coextrusion process is not caused by process perturbations potentially introduced by extruder screw rotation but is associated with process-history dependant differences in melt elasticity.

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Studies on multilayer film coextrusion II. Interfacial instability in flat film coextrusion

Cited by 112 publications

References 12 publications

Elastic instability in stratified core annular flow

Elastic instability in stratified core annular flow

Numerical investigations of polyester coextrusion instabilities

Visualisation and Analysis of Polyethylene Coextrusion Melt Flow

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