Shear viscosity, extensional viscosity, and die swell of polypropylene in capillary flow with pressure dependency

摘要

The shear and extensional viscosities of a polypropylene resin were studied using a capillary rheometer and capillary dies of 1-mm diameter and length of 10, 20, and 30 mm. Melt temperatures at 190, 205, and 220degreesC and shear rates between 100 and 5000 s(-1) were used. At the highest shear rate a visible melt fracture was observed. An equation relating the pressure drop and die length was derived with consideration of pressure effects on melt viscosities and the end effect. After the correction for pressure effects the true wall shear stress and end effect at zero pressure were calculated. The end effect showed a critical stress of melt fracture around 10(5) Pa, and increased rapidly when shear stress increased above the critical stress. From shear stress the shear viscosity was calculated, and a power law behavior was observed. Extensional viscosity was calculated from the end effect and showed a decreasing trend when strain rate increased. After time-temperature superposition shift shear viscosity data correlated well, but an upward trend was observed in extensional viscosity when melt fracture occurred. Die swell ratio at different temperatures can be plotted as a function of wall shear stress and was higher for shorter dies. (C) 2002 Wiley Periodicals, Inc. [References: 22]