Date of Award
Doctor of Philosophy (PhD)
Dr, A.N. Hrymak
Dr. J. Vlachopoulos
Coextrusion is the simultaneous extrusion from a single die of two or more homogeneous melts which form a lamellar structure. Each resin needs its own extruder and a single extruder can supply more than one layer of the same resin. In recent years, coextrusion has gained importance because it is an economical and effective method to obtain plastic products that meet specific market requirements. From the fluid mechanics point of view, one of the main features of the coextrusion process is the presence of internal interfaces (that separate the fluid phases) and contact lines (which are formed when an interface intersects the die wall). Two types of problems exist in the coextrusion process; interfacial flow instability and non-uniform layer thickness distribution. Non-uniform layer distribution refers to the change in thickness of the distribution of the layers across the width of the sheet. Interfacial flow instability appears as an unsteady waviness of the interface between the two polymers. Materials with different theological properties such as apparent viscosity, elasticity and shear stress effects, give rise to irregular interfaces, which lead to high scrap rates and undesired mechanical and optical properties. This work focuses on the problem of interfacial instability. The study examines the main parameters involved in coextrusion operations, i.e., die design, processing conditions and rheological properties of polymer systems, in order to understand both their influence on the interfacial instability and suggest ways to control the instability. By means of a transient solution, the interfacial stability of a given coextrusion flow system can be analyzed in terms of the evolution of the interface in time in response to a external perturbation. Two-dimensional time-dependent finite element solutions of the bi-component flow of Newtonian, generalized Newtonian (Carreau model) and viscoelastic (Criminale-Ericksen-Filbey or CEF) fluids in complex geometries (parallel plates, abrupt and tapered expansions and contractions, and a coat-hanger die geometry), are presented. For the case of Newtonian and generalized Newtonian fluid systems through parallel plates, solutions have been successfully compared against experimental data from the literature and previous results provided by linear stability analysis (LSA). An industrial problem has been studied and the numerical simulation of a two layer homopolymer LDPE 1321(TM) (Dow Chemical Co.) through a tear drop coat-hanger die has been carried out. Finally, CEF flow solutions show that for given elasticity, viscosity and flowrate ratios, the interface between the two fluids has an irregular shape even in the steady state. This anomaly at the interface is qualitatively similar to that observed in experimental results from unstable coextrusion samples.
Rincon, Alberto, "Transient simulation of non-Newtonian coextrusion flows in complex geometrics" (1998). Open Access Dissertations and Theses. Paper 1955.