Viscoelastic analysis of multilayer polymer melt flow in die

In multi-layer coextrusion of polymer melts, various kinds of interfacial instabilities occur due to processing conditions, die geometry, and the rheological properties of each layer. Suppressing the interfacial instabilities is a crucial task in manufacturing a multi-layer polymer film of high functionality. However, multi-layer flow of viscoelastic fluids has been poorly understood. We performed a course of viscoelastic simulations of multi-layer flow to clarify the flow and stress states, and investigated the effect of channel geometry and the layer arrangement of viscoelastic fluids with different rheological properties on it. It was found that by arranging the fluid with higher viscosity at the outermost layer, large strain rate developed in the second layer of a fluid with lower viscosity. Furthermore, change of thickness of layers caused by die channel enhanced the jump of the normal stress difference at interfaces. This behavior was most enhanced in the arrangement of the high-viscosity fluids at the outer-most layer. It is supposed that this elasticity inhomogeneity might be an important factor controlling the interfacial instabilities in viscoelastic multi-layer co-extrusion.