Rheo-optic flow-induced crystallization of polyethylene and Polypropylene within confined flow geometries

Experimental observations on the way High Density Polyethylene (HDPE) and Polypropylene (PP) can crystallize under flow in a Multi-pass Rheometer (MPR) are reported. Both deep and shallow flow geometries were chosen for the rheo-optical study. Preliminary linear viscoelastic theological tests enabled the temperature window for quiescent crystallization to be established. Flow-induced crystallization (FIC) studies were performed in a temperature regime above the normal quiescent crystallization conditions. In the case of HDPE in a contraction-expansion cell, FIC occurred during flow at the sidewalls of the slit and in localized regions downstream and the processing pressure increased during the piston movement. In the case of PP, flow-induced crystallization was generally observed after flow cessation and the processing pressure did not change during flow. For PP, FIC also occurred preferentially at the walls in the form of elongated crystallites but the fibres gradually emerged after flow cessation. The difference in the FIC behavior was attributed to differences in the crystal growth kinetics of the two materials at the particular super-cooling used. 2D numerical simulations of the polymers flowing in a deep slit geometry were per-formed using Flowsolve, an Arbitrary Lagrangian Eulerian solver developed at Leeds University (Harlen OG et al., 1995 [10]). The local principal stress difference, orientation and stretch of the molecules in the flow at the onset of crystallization were determined for a range of conditions.