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Views: 0 Author: Site Editor Publish Time: 2025-10-18 Origin: Site
It is becoming more common to combine multiple layers of different polymers to form products with properties that take advantage of the best properties of each component. For example, packaging film might be composed of several different types of polyethylene along with a layer of adhesive and a barrier polymer. There are basically three types of multiple-layer extrusion (or coextrusion) techniques:
1.Melt streams flow separately.
2.Melt streams flow separately and then together.
3.Melt streams flow together.
In type 1, polymer streams are extruded through separate flow channels and then joined outside the die. The advantage in this type of multilayer extrusion is that polymers with widely different processing temperatures and rheological properties can be used. The major problem is that of generating satisfactory adhesion between the components. Usually the technique is only used for two polymers. In the second technique the streams are brought together inside the die, and then they pass through a common land region. Because the streams are brought together under pressure, adhesion is improved. However, it is not possible to have the streams at widely different temperatures. Likewise the rheological properties cannot be too widely different or flow instabilities will arise. Furthermore, at the point where the streams converge, interfacial instability problems may arise. In the third method, which is not too dissimilar from the second method, the polymer streams are brought together in an adapter, and then they pass through a common die. In this case the same die as that used for single-component extrusion can be used. Again the melt rheological properties cannot be too dissimilar or an instability will arise that will disrupt the laminar nature of each stream. However, this is one of the most inexpensive and simplest methods for generating multiple-layer films and sheets.
Basically, two problems arise when trying to extrude multiple layers of different fluids through the same die. First, if there are distinct viscosity differences between the fluids, then the lower viscosity component will try to encapsulate the higher viscosity component. Second, there are situations when the viscosities of two polymers are closely matched, but the interface still becomes wavy and distorted. This type of instability is thought to be due to slip at the interface between the two polymers. However, the most probable region for the onset of the instability is at the die exit where large stresses arise as the velocity profile undergoes a rapid rearrangement.
