Co-Extrusion Layer Multiplication of Rheologically Mismatched Polymers: A Novel Processing Route

Abstract

Co-extruded films with up to 65 layers of two rheologically mismatched polymer systems – polystyrene/poly(methylmethacrylate) (PS/PMMA) and hard/soft thermoplastic polyurethanes (TPUs) – were successfully produced using a combination of a 9-layer feedblock, low-pressure drop multiplier dies, and external lubricants. Formation of viscoelastic instabilities was studied using a custom visualization and by finite element method (FEM) simulations of a standard multiplier. The results showed that the flow inside the standard multiplier die is highly non-uniform, with severe gradients in shear and normal stresses and viscous encapsulation occurring mainly in the initial multiplication stages where there is enough material available in the low-viscosity layers to proceed with the encapsulation. To mitigate layer degradation the standard 2- or 3-layer feedblock was replaced with a 9-layer one, thereby decreasing the thickness of each layer at the end of the feedblock. Also, subsequent layering was performed using a low flow resistance die. This new multiplier die yields a more uniform flow profile and imparts a more homogeneous thermo-mechanical history on the melt which results in an improved layer stability. Simulations showed that in the standard die the second normal-stress difference (N2) responsible for elastic instabilities at the edges of the die are very high. These can be reduced by inducing slip at the wall resulting in be much improved layer uniformity and stability. This was accomplished experimentally via the use of external lubricants, and the resulting layered structure was indeed much better than was possible to achieve with the conventional multiplier dies.

Effect of Viscosity Ratio of Two Immiscible Polymers on Morphology in Bicomponent Melt Spinning Fibers

“…This interfacial instability was attributed to the non‐uniformity of the flow inside the die with severe gradients of shear rates. Thus, the possibility of reducing viscous encapsulation was carried out by changing the die geometry …”

Microconfinement effect on gas barrier and mechanical properties of multilayer rigid/soft thermoplastic polyurethane films

“…Both materials were dried at 80°C for 24 hours prior to processing. About 1.5 wt % external lubricants (TR 251) provided by Struktol Company, which mainly compose unsaturated primary amide, were added into TPU during co‐extrusion . Extensional rheometry experiments were performed in a SER device coupled to a Paar Physica MCR 501 rheometer at 175°C.…”

“…Since these two TPUs are rheologically mismatched, 205°C was chosen as the co‐extrusion temperature at which viscoelasticity ratio (the viscosity and elasticity ratio of two polymers at a certain shear rate and temperature) is minimum within the processing window. The combination of a 9‐layer feedblock and a second generation multiplier dies recently developed by the authors is used to produce 65‐layer TPU films . The flow rates of the TPU melts were the same and set by identical gear pumps.…”

Multilayer coextrusion of rubber compounds

“…Studies have shown layer nonuniformities in both material composition as well as cross-section [2,3]. Several material properties adversely affect the flow pattern that causes these nonuniformities.…”