A study of the extensional flow behaviour of low density polyethylenes

Extensional flow is an important component of the major commercial polymer shaping operations. In processes such as fibre spinning and film blowing, it is believed to be the dominating influence. Extensional flow theory is as well advanced as that for shear flow. However, it is only in the last 15 years that instruments capable of measuring extensional flow characteristics have been developed by various workers. A review of current theory and practice has been carried out. The Rutherford Extensional Rheometer is the latest of this family of instruments designed to measure flow behaviour in the uniaxial extension mode. Various modifications have been made to the instrument either to meet the original specification or in response to experience gained during use. Appropriate experimental techniques and specimen preparation methods have been developed. A novel method of holding the sample during testing is used which is shown to be superior in practical terms to those developed by other workers for similar instruments. The result of this development is an instrument which provides a viable means of ascertaining some extensional flow characteristics of polymer melts. Several commercial low density polyethylene resins have been studied using the Extensional Rheometer. The polyethylene grades were chosen to provide a range of melt flow indices, processing behaviour and molecular characteristics Results from the Extensional Rheometer were produced at a range of extensional strain rates and temperatures, and analysed to determine the extensional flow characteristics of each polyethylene grade. The results were found to be in agreement with those of some other workers and in conflict with others. Most of the differences can be explained by differences in test geometry, but inflections in the curves of strain dependent extensional viscosity were found which have not previously been reported. Extensional flow data was compared with available data on shear flow behaviour, processing behaviour and material characteristics, and the correlations found have been used in an attempt to explain observed extensional behaviour in molecular terms.