EXTRUDATE DISTORTION IN THE CAPILLARY SLIT EXTRUSION OF A MOLTEN POLYPROPYLENE/

Experiments were carried out in both sliding plate and capillary rheom eters with a polypropylene resin to determine the conditions for the o nset of slip, surface, and gross melt fracture. It was found that ther e 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 d iameter independent implying the absence of slip, However, experiments with slit dies having rough surfaces suggest wall slip. Further analy sis has shown that the effect of viscous heating masks the detection o f slip from the diameter-dependency of the flow curves. The effect of a thin layer of fluoropolymer (Teflon PA, DuPont) on the critical shea r stress for the onset of wall slip and melt fracture, as well as on t he 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 on set of slip.