The flow stability of linear low-density polyethlene at polymer and metal interfaces

Abstract

The role of the single component instability of surface melt fracture on the interface behavior in stratified bicomponent flow has been examined. First, the factors and conditions leading to the onset of surface melt fracture in linear low-density polyethylene (LLDPE) were identified using fluoro-elastomer (FE) as a blending additive and as a die coating in two visualization dies. A visualization die was constructed so that subsequent experiments examining the joining flow behavior of two stratified flows could be examined. Experiments were conducted in the joining flow die over a range of upstream conditions corresponding to surface melt fracture behavior and the resulting flow birefringence patterns and the interface of the extrudate were examined. It was determined from the: single component studies that the role of FE in eliminating surface melt fracture behavior for LLDPE was to introduce slip at the melt/metal interface in the dies. Additionally, it was determined that the coupling of a critical stress with a critical acceleration of the melt as it exits the die, suggested by Kurtz [19], was an accurate description of the behavior observed experimentally. Under upstream conditions corresponding to surface melt fracture behavior, no irregular distortions were observed in the bicomponent interface. It was therefore concluded that the single component instability of surface melt fracture does not play a role in irregular distortions of the interface.

Numerical simulations employing the Phan-Thien Tanner (PTT) constitutive model and the finite element method (FEM) were conducted to examine the influence of relaxation times and extensional viscosity on the developing flow region in joining flow die. Numerical predictions employing material constants fit to the rheological properties of LLDPE were compared with the ex- perimental results to establish the reliability of the numerical method. Qualitative agreement between the predictions and the experimental behavior was observed. However, the magnitude of the stresses predicted by the model were not quantitatively accurate. It was concluded that the numerical method was capable of predicting trends in behavior, but was not quantitatively accurate. Given this limitation, it was suggested by the results of the numerical studies that the relaxation behavior has a pronounced effect on the developing stress field, while the impact of the extensional viscosity is minimal. Simulations were also performed to evaluate the ‘stick-slip’ behavior of LLDPE. The results provided additional support to the supposition of the role of FE in eliminating surface melt fracture behavior in LLDPE.