Theory of Stratified Bicomponent Flow of Polymer Melts. II. Interface Motion in Transient Flow

Abstract

An approximate theoretical analysis of transient bicomponent flow and interface motion in the entrance region of infinite parallel plates is presented, and an estimate of the entry length required to attain equilibrium flow is obtained. The theoretical predictions are compared with experimental measurements of interface configuration and extrudate bending in the bicomponent tube flow of a nylon/nylon system. The results indicate that interface motion takes place at two distinct rates in the tube flow case. Initially a very rapid interface movement occurs which is qualitatively described by the theoretical parallel plate analysis. This is followed by a very slowly occurring interface curvature which has no apparent counterpart in the parallel plate case considered here.

Three-dimensional studies on bicomponent extrusion

“…Southern and Ballman [2,3] reported an almost flat interface shape when the shear rate was at the crossover point for the circular die geometry used in their experimental studies. However, in this study the square geometry results in a shearing distribution along the die walls.…”

“…The swelling behaviour of the system is extremely complicated. The extrudate not only swells but it also bends because of the viscosity difference of the two fluids [2,3,6] and distorts because of the 3-D geometry and the unsymmetric velocity profile inside the die. The finite element grid deforms significantly, especially in elements that…”

“…Generally, a very long die is required for the less viscous fluid to totally encapsulate the more viscous fluid depending on the magnitude of the viscosity ratio. For example, Everage [3] demonstrated that for stratified flow through a tube, a tube length of more than 20 characteristic lengths (tube diameters) is required before appreciable encapsulation is noted, while full encapsulation occurs at the order of 100 characteristic lengths. Everage's investigation proves that there are cross-flow velocities which result in the interface shape change, even 100 characteristic lengths downstream.…”

“…One of the polymers was coloured to facilitate the identification of the extrudate shape. This experimental procedure was designed to study the interface deformation and the resulting degree of encapsulation in addition to previous experimental reports [1][2][3] which examined the interface shape in the extrudate. The interface shape is frozen and studied inside the die instead of the extrudate, and extrudate swelling and bending effects which may cause further interface and also free surface distortion are therefore avoided.…”

Three-dimensional Viscoelastic Analysis of Multi-Layer Polymer Flow by Numerical Simulation

“…4) A "minimum viscous dissipation principle" has been invoked to explain the interface shape found in experiment, 6,7) although such a variational principle is not rigorously justifiable for flows with interfaces, and is known to fail under certain conditions. 8) Everage 9) carried out both experimental and theoretical studies on the transient interface motion in a bicomponent tube flow of a nylon/nylon system, and identified two stages in viscous encapsulation.…”