Two dies for polymer co‐extrusion layer multiplication are evaluated experimentally and computationally in terms of pressure drop and layer uniformity. The first design is that of the original die, is compact, and has successfully been used to co‐extrude low elasticity polymers with closely matched rheological properties. The second die design, the one that is being modified, achieves a more balanced flow path with constant cross‐sectional area. Flow visualization experiments and computational simulations show matched performance between the dies when layering similar viscosity materials and better layering performance of rheologically dissimilar materials with the improved dies compared to the original die design. Furthermore, the improved die has a much lower pressure drop. This facilitates decreased energy consumption or the allowance of additional multiplier dies to be added resulting in an increased total number of layers. POLYM. ENG. SCI., 54:636–645, 2014. © 2013 Society of Plastics Engineers

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.

Numerical simulations of co-extruded multilayered viscoelastic polymers flowing through afeed block were performed to analyze the effects of polymer rheology and process conditions on the distortion of interfaces between layers. Finite element simulations assuming Phan-Thien Tanner viscoelastic fluids were used to test the effects of increasing the number of layers introduced to the feed block and decreasing wall friction on interface distortion. It was found that increasing the number of layers introduced to the feed block decreased the overall distortion in the layers by reducing the overall magnitude of second normal stress differences and thus decreasing the normal in-plane velocities that cause distortion, which was validated by experiments. Decreasing wall friction was also found to decrease the amount of distortion in layer interfaces by decreasing the strength of these second normal stress differences. 1I ntroductionMultilayer coextrusion of polymers has been widely studied. Of particular interest is the deformation of the interfaces between polymers during coextrusion (Debbaut et al.). It is often the goal to have uniform layers with little or no distortion, especially for nanolayered materials (Andrade and Maia, 2011;Mueller et al., 1997; Ponting, 2010a; 2010b). Very finely layered polymer materials have al arge range of technological applications, including optical data storage, lenses, organic lasers (Mao, 2000), gas barriers (Li, 2011), and water purification membranes. These applications all require or are greatly enhanced by using materials with uniformly flat layers.Controlling the interface distortion in the feed block of this extruder is clearly vital, as the distortions that occur in the feed block are propagated throughout the layer multiplication process and can lead to the destruction of the layered structure of the material. Thus, it is important to understand what factors control the distortions and develop and operate the co-extrusion process to mitigate them.Here we use experimentally validated simulations, to understand the effects of rheology on the distortion of the interfaces and to explore two different methods of improving layer flatness and uniformity in the feed block. The first method is to increase the number of layers introduced into the feed block. The second is to reduce the wall friction in the channel. Both of these methods aim to reduce the magnitude of the second normal stress differences and decrease the in-plane velocities that are the primary cause of distortions of the interface. We evaluate these methods and determine their effect on controlling the shape of the interfaces in the multilayered polymer co-extrusion. 2M ethodsConsider amultilayered flow in the rectangular feedblock illustrated in Fig. 1. The cross-section of the feedblock is 13 mm by 13 mm and it has al ength of 35 mm. Two or more layers of two types of immiscible viscoelastic polymers are injected at the entrance of the channel, and we wish to determine the evolution of the shapes of the interfaces a...