Coextrusion melt flow instabilities

One interesting benefit that can be obtained with coextrusion is a more uniform temperature distribution in the material. This can be realized by coextruding a thin, low-viscosity outer layer over a high-viscosity inner layer. The highest shear rate and heat generation normally occurs at the wall. By having a low-viscosity material at the location of maximum shear rate, the heat generation is reduced at this point and a more even temperature profile is obtained. This is a useful technique for thermally unstable polymers. As discussed before in Section 7.5.3, this technique can also avoid the occurrence of shark skin and melt fracture type flow instabilities. 

Viscosities of non-Newtonian polymers are dependent on extrusion temperature and shear rate, both of which may vary within the coextrusion die. The shear rate dependence is further complicated in that it is determined by the position and thickness of a polymer layer in the melt stream. A polymer used as a thin surface layer in a coextruded product experiences higher shear rate than it would if it were positioned as a central core layer. There are several types of flow instabilities that have been observed in coextrusion. 

Coextrusion is promoted by the laminar flow of the melt in the feed block and die, which prevents the turbulent mixing of the various layers. The laminar flow is due to the low Reynolds numbers (low inertia between flow planes) that result from the high melt viscosities [10,11]. However, the generation of interfaces between the flowing materials requires that melt viscosities and melt elasticity between the layers be sufficiently matched to prevent the formation of flow instabilities [16-19]. Therefore, coextrusion requires superior equipment and process control throughout. 

The best designed die or feedblock does not necessarily ensure a commercially acceptable product. Layered melt streams flowing through a coextrusion die can become unstable leading to layer nonuniformities and even intermixing of layers under certain conditions. The causes of these instabilities are related to non-Newtonian flow properties of pol5uners and viscoelastic interactions. 

Figures 4 and 5 also indicate that as one would like to move to even higher heat sealing rates above 120 seals/min, the packaging film designer must consider the specification of sealant melt indices greater that 10 dg/min. While higher sealant melt indices yield shorter chain diffusion times at the seal interface with more rapid interface healing, at some point lower chain molecular weights will create dilemmas such as poorer seal mechanical strength, coextrusion instability in multilayer flows, excessive squeeze flow of the sealant, etc.

---