THE EFFECT OF DIE MATERIALS AND PRESSURE-DEPENDENT SLIP ON THE EXTRUSION OF LINEAR LOW-DENSITY POLYETHYLENE

The flow of linear low-density polyethylene through stainless-steel sl it dies occurred at shear rates approximately 12% higher than in ident ical alpha-brass dies at the same wall shear stresses, indicating near -wall slip. The flow curves were independent of gap spacing, We show t hrough the slip theory of Hill and co-workers [J. Rheol. 34, 891-918 ( 1990)] that a measurable gap dependence of the flow curve is not a nec essary consequence of wall slip; the flow curves for both stainless st eel and alpha-brass dies can be fit with the same rheological paramete rs, with a difference in the work of adhesion accounting for the diffe rences in the flow curves. X-ray photoelectron spectroscopy revealed d ifferences in the chemistry of brass surfaces with different pretreati ng, corresponding to small differences in flow curves. Fluorocarbon-co ated die surfaces showed no more slip than stainless steel, while the flow curve with gold-coated surfaces followed stainless steel at inter mediate stress and brass at high stress. (C) 1997 The Society of Rheol ogy.