Numerical investigations of polyester coextrusion instabilities

Abstract

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.

Three‐dimensional isothermal simulation of PP melt flow in laminating‐multiplying elements based on the finite element method

“…The figure shows a kind of secondary flow that is a reflection of velocity X and velocity Y shown in Fig. a and b. Dooely studied the secondary flow caused by polymeric viscoelasticity in a rectangle channel with constant cross‐section and discovered that the velocity component perpendicular to the flow direction was approximately two orders of magnitude lower than that along the flow direction in this secondary flow process. However, in Fig.…”

“…Until now, most studies concerning layer interface deformation concentrate on traditional coextrusion processing, which shows relatively simple flow behavior . Experimental and numerical analysis methods were widely used to study these problems in the first 20 years of development; however, these methods were gradually replaced by the finite element method (FEM), which is a powerful tool for easily and quickly studying the rheology of polymer melts in a mold.…”

Investigation of the Polymer Coextrusion Process: A Review

“…The coextrusion of two polyesters in the final section of a coat-hanger die with a convergent gap and a constant width was considered. The two polyesters had different behaviors, one was Newtonian and the other one viscoelastic and shear-thinning, described by a multi-mode Oldroyd-B model ([ 38 ]). A small amplitude perturbation is imposed on one of the two inlet flows and the propagation of this disturbance along the flow is computed.…”

Multilayer Coextrusion of Polymer Composites to Develop Organic Capacitors

“…Because of the complex rheology of the polymers being coextruded several type of instabilities exist, including viscous instabilities from viscosity mismatch, elastic instabilities from time constant mismatch between layers exacerbated by the sec-ondary flows present in viscoelastic fluids, and temperature and surface tension gradient driven instabilities. Alarge body of literature has been devoted to understanding instabilities associated with both viscosity (see Alba et al, 2011 for as ummary of the literature) and elastic effects (see Mahdaoui et al, 2013 for as ummary of this literature.) It has been found that these instabilities are convective in nature meaning that they either grow or decay in the flow direction.…”