Extensional viscosity measurements of polyethylene using a melt flow indexer

Abstract

The extensional viscosity of polyethylene melts plays and important role in the processability of the resins. Extensional viscosity is important in processing operations such as film blowing, blow molding, and fiber spinning. The processability of the resin and extensional viscosity are also related to molecular weight architecture. Previous work has focused on obtaining extensional viscosity values using a capillary rheometer. This research focuses on developing a method for use with a melt flow indexer to determine extensional viscosity as a quality control test. Several methods for determining entrance pressure loss are examined, including the zero length die method, the two die length (2L) method, and the Bagley entrance correction method. The Cogswell and Darby methods of defining extension rate and extensional viscosity are examined and compared. Six polyethylene resins (A through F) have been selected for this study. They have different densities, molecular weights (MN and molecular weight distributions (MWD). Results indicate that the Cogswell and Darby methods of calculating extensional viscosity and extension rate can be related in a simple way, and are offset by constant values in most cases. No significant difference was found between 90- and 180-degree (flat) entrance angles in the melt flow indexer or the capillary rheometer for L/D < 10. The slope of the extensional viscosity curves transition well between the Bagley method at low extension rates and the zero-length die method at high extension rates. The magnitudes of the extensional viscosity also match up well between melt flow indexer methods. The extensional viscosity calculated using these methods can be used to distinguish between the different resins. The Trouton ratios at the high extension rates were predicted to be close to constant and separated the resins into three distinct groups. The zero-length die method using the standard melt flow indexer die and the zero-length die swell die was found to be the best for use as a quality control method.