The Rheology and Processing of Molten Polyethylene
Karen conducted an experimental study and numerical modelling of the flow of molten polyolefins in typical polymer processing environment. A Multi-pass Rheometer (Fig. 1) was used in a capillary and a planar-slit geometry to observe the flow of commercial grades polyethylene melt. Optical techniques coupled with the MPR slit were setup to capture the field flow-birefringence of deforming melts. This device is capable of acquiring data over the range of stresses commonly encountered in industrial processing.
|Fig. 1: A schematic diagram of the Multi-pass Rheometer with a capillary test section.|
Simulating the flow in complex geometries that involve region of high stress gradient, mixed deformation kinematics and non-isothermal conditions was a challenging task. The equations of motion and continuity coupled with a constitutive equation had to be solved numerically for the field variables (e.g. pressure, velocity and stress), for which a commercial finite element solver, Polyflow, was used. Material parameters required were obtained from the rheological characterization using a Rheometrics Dynamic Spectrometer (RDSII), a controlled strain rotating rheometer with a parallel-plate fixture.
Measurements of pressure and global stress field were compared with numerical predictions. The quality or accuracy of the predictions was predominantly determined by the flow boundary conditions invoked, constitutive model and its corresponding material parameters used.