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.

The physical optics of iridescent multilayered plastic films are analyzed theoretically relating the layer thickness, layer arrangement, and refractive index difference between layers to the reflection spectra for light at normal incidence. Very vivid iridescent and metallic-appearing films may be made by proper control of layer distribution.

Some properties of multilayered films are summarized in relation to the properties of the component materials. Polymers may be selected for co‐extrusion into layered films to obtain various combinations of properties, such as barrier, stiffness, heat sealability, optics, and toughness. Properly chosen combinations of high and low modulus materials exhibit mutual interlayer reinforcement. Permeabilities of multilayer films can be calculated from layer geometry and individual component properties.

Coextruded plastic films containing 231 alternating layers of polystyrene and polymethylmethacrylate have been made with individual layer thickness in the range of .06–0.1μ. Due to the difference in refractive indices these unique films can be made to reflect portions of the visible spectrum, yielding a highly iridescent appearance. Reflectivity measured on films of known layer thickness distribution correlated with values predicted from optical theory.

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