Interfacial flow instability in multilayer coextrusion

Abstract

In coextrusion of multilayer film and sheet, one type of flow instability which can occur is a wavelike distortion of the interface under certain coextrusion conditions. Severe instability can cause intermixing of the layers, particularly in thin skin coextrusion. Observations have shown that the onset of instability is associated with a "critical interfacial shear stress." This paper describes a mathematical model of non-Newtonian multilayer flow which was used to correlate experimental observations.

Effects of rheology on the interface of Pb(Zr, Ti)O3 monofilament composites obtained by co-extrusion

“…By increasing the shear rate, there is a breakdown of the agglomerates, which releases the absorbed polymer that will contribute to the inter-particle separation, therefore decreasing the viscosity. Nevertheless, when comparing Co-Ex 1 with CoEx 3, where the difference in viscosity between the materials was less than 10% and the tests were attained at lower shear rates, our results were quite similar to Schrenk et al 25 Co-Ex 1 was carried out with the inner part having a lower viscosity than the CB material, while for Co-Ex 3 the opposite was observed and as a result, interfacial flow instabilities were reduced, but not eliminated, due to the viscosity mismatch between the compounds.…”

“…The co-extrusion tests 3, 4 and 5 were carried out with a carbon black/LDPE composition revealing a smaller viscosity than the primary material as recommended by Schrenk et al 25 The aim was to minimize the velocity difference between the co-extruded layers by using an inner layer with a higher viscosity. However, in our study, the CB material indicates only lower viscosities at higher shear rates, which led to two drawbacks.…”

Measuring Slip at a Polymer/Polymer Interface during Three-Layer Flow

“…In addition, when two or more polymers with different shear-rate dependent viscosities are coextruded, irregularities occur at the polymer/polymer interface. [18][19][20] This irregular interface is particularly troublesome as it can interfere with the determination of the polymer/polymer interfacial slip velocity as a function of the interfacial stress.…”

Modeling viscoelastic flow in a multiflux static mixer

“…Van der Hoeven et al [4] performed 3D numerical simulations to identify layer inhomogeneities due to unequal pressure drops in the separating flows. Interfacial instabilities were investigated by Schrenk et al [5], who used a critical interfacial shear stress criterion to predict the onset. Also focusing on interfacial instability, Mavridis and Shroff [6] presented a computer simulation of multilayer flow of polymer melts in coextrusion.…”