Extrudate Distortion in the Capillary/Slit Extrusion of a Molten Polypropylene

Abstract

Experiments were carried out in both sliding plate and capillary rheometers with a polypropylene resin to determine the conditions for the onset of slip, surface and gross melt fracture. It was found that there was no distinction between surface and gross melt fracture, which is commonly observed in the case of polyethylenes. Furthermore, the flow curves determined by using capillaries having various diameters are diameter independent implying the absence of slip. However, performing experiments with slit dies having rough surfaces suggested the presence of wall slip. Further analysis has shown that the effect of viscous heating masks the detection of slip from the diameter-dependency of the flow curves. The effect of presence of a thin layer of fluoropolymer (Teflon® PA, Du Pont) on the critical shear stress for the onset of wall slip and melt fracture as well as on the relationship between the wall slip and the shear stress were also examined. It was found that the presence of such layers increases the slip velocity while it decreases the critical shear stress for the onset of slip.