Coextrusion of polymers: interfacial instabilities

Extrusion is the most widely used thermoplastic forming process. Its applications cover a wide range, from the manufacture of films to profiles. When several fluids are extruded simultaneously from several extruders into a single die, this is known as coextrusion: a multi-material flow is created, and the product obtained at the die exit is said to be multi-layered.

The advantage of such a process is that it enables the product to benefit from the specific properties of each of its component layers. Combining the qualities of the various polymers in this way makes it possible to achieve significant material savings (and therefore reduce cost and/or weight) compared to a single-layer product with the same characteristics.

In general, three types of defects can be observed on the solidified product:

• poor adhesion between layers;

• poor distribution of products in the direction transverse to the flow, due to one product being bypassed by another (coating phenomenon);

• interface irregularity, ranging from oscillating to chaotic.

Variations in the thickness of the various layers can alter the product's aesthetic or optical properties. If one of the interfaces disappears locally, the product may also lose its barrier properties and become unusable.

These irregularities are the consequence of interface instabilities that develop in the final part of the die, where the various molten polymers flow together in stratified form.

This dossier describes the convective nature of interface instability in coextrusion, with stratified flow transporting and amplifying (or attenuating, in stable cases) disturbances intrinsic to the process, which will appear as more or less visible at the die exit.

A model of this type of defect is then proposed, showing the determining role of the viscoelastic behavior of coextruded materials. Emphasis is placed on linear stability analysis of the isothermal planar bilayer case of immiscible polymers, then extensions to more complex cases are proposed.