AN EXPERIMENTAL-STUDY ON THE EFFECT OF POLYMER VISCOELASTICITY ON LAYER REARRANGEMENT IN COEXTRUDED STRUCTURES

Previous work on the layer thickness uniformity of coextruded structur es has centered primarily on the effects of differing polymer viscosit ies in the individual layers. These differences in viscosities result in a phenomenon known as 'viscous encapsulation'' in which the less vi scous layer tends to encapsulate the more viscous layer producing nonu niform layer thicknesses. In this study, the effect of polymer viscoel asticity on the layer thickness uniformity of multilayer coextruded st ructures was investigated by coextruding multilayer structures through die channels of different cross-sectional shapes and observing the lo cation of the interface. In order to minimize the viscosity effects, t he coextrusion experiments were conducted with identical materials in each layer that were pigmented to allow observation of the layer inter face. It was shown experimentally that coextruding identical polymers through channels of various geometries can cause the layers to rearran ge. This layer rearrangement appears to be caused by secondary flows t hat occur in different channel geometries due to the viscoelastic char acteristics of the polymer. Layer rearrangement of identical polymers implies that even resins with viscosities well matched for coextrusion may experience nonuniform layer thicknesses when coextruded through l arge dies.