A simple empirical model describing the steady-state shear and extensional viscosities of polymer melts

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.

Section: Process Measurement Imaging Analysis and Modellingmentioning

confidence: 99%

Visualisation and Analysis of Polyethylene Coextrusion Melt Flow

Martyn¹,

Coates²,

Zatloukal³

2009

“…Although this modification improves the behaviour in steady shear flows, in elongational flows the model predicts unrealistic infinite elongational viscosity. This problem was overcome by Barnes and Roberts [24], who showed that, for specific functions of λ(II D ) and η(II D ) with (λ 0 /K 2 ) < (√3/2) (see Eqs. (3) and (7)), the model does not predict infinite elongational viscosity and can be used for a very good description of elongational viscosity of a wide range of real polymer melts:…”

Section: Modified White-metzner Modelmentioning

confidence: 99%

“…Modified White-Metzner model constitutive equation is a simple Maxwell model for which the viscosity and relaxation time are allowed to vary with the second invariant of the strain rate deformation tensor [24]. It takes the following form:…”

Section: Modified White-metzner Modelmentioning

confidence: 99%

“…Probably the most challenging rheological task is experimental determination of planar elongational viscosity as one can see from very small numbers of experimental data available in the open literature [1-2, 12-16, 21]. With the aim to understand this important rheological parameter in more detail, in this work, novel rectangle and circular orifice dies have been utilized for planar and uniaxial elongational viscosity measurements for branched LDPE by using standard twin bore capillary rheometer and Cogswell model [6,12] and the capability of three different constitutive equations [22][23][24][25][26] to describe the measured experimental data has been tested.…”

Section: Introductionmentioning

confidence: 99%

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Evaluation of temperature-strain rate dependent uniaxial and planar elongational viscosities for branched LDPE polymer melt

Zatloukal

2013

Abstract. In this work, novel rectangle and circular orifice dies have been utilized for temperature-strain rate dependent planar and uniaxial elongational viscosity measurements for the LDPE polymer melt by using standard twin bore capillary rheometer and Cogswell model and the capability of three different constitutive equations (novel generalized Newtonian model, modified White-Metzner model, modified Leonov model) to describe the measured experimental data has been tested. It has been shown that chain branching causes the strain hardening occurrence in both, unixial and planar elongational viscosities and its maximum is shifted to the higher strain rates if the temperature is increased. The level of uniaxial elongational strain hardening for the branched LDPE sample has been found to be higher in comparison with the planar elongational viscosity within wide range of temperatures.

“…In order to answer this question, virtual polymer melt M (having medium level of extensional strain hardening) has been chosen for FEM analysis for the case considering that downstream orifice die region is filled by the polymer melt. In this work, Compuplast software VEL 6.2 has been used together with the modified White-Metzner (mWM) constitutive equation according to Barnes and Roberts [10]. The sketch of the studied flow situation for conventional orifice die together with corresponding boundary conditions and FE mesh is provided in Figure 3 (grid consists of 7400 nodes and 3553 elements) whereas the Figure 4 describes the rheological behavior of the virtual polymer melt M. The problem has been solved for different flow rates and the obtained theoretical results were plotted in Figure 5 and compared with corresponding theoretical data considering that downstream orifice die region is not filled by the polymer melt.…”

Section: Theoretical Effect Of Orifice Die Design On the Measured Extmentioning

confidence: 99%

Development of Novel Orifice Die Design for Precise Entrance Pressure Drop Measurements

Zatloukal¹,

Musil²

2009

Abstract. In this work, novel patent pending orifice die design for precise extensional viscosity data determination from entrance pressure drop measurements has been developed and tested both, theoretically (through Finite Element Analysis) and experimentally. It has been demonstrated that the proposed novel orifice die allows much more precise extensional viscosity measurements for polymer melts in comparison with conventionally used orifice dies.