| Elongational flow fields are of immense importance in the polymer processing industry as the elongational flow used in processes such as fiber spinning, film blowing, melt drawing, etc., produces the desired degree of polymer orientation so that the final products have the target properties of toughness and durability. Although these processes are widely used, it is very difficult to characterize the extensional rheological properties of polymers, such as elongational viscosity. This can be attributed to the difficulty in developing the experimental methods for measuring the elongational flow properties.; In late 1990s, Collier and co-workers began work on a project aimed at developing an inexpensive, reliable, accurate, and simple to use device to measure the elongational viscosity of polymer melts in a uniaxial elongational field. They have developed the semi-hyperbolically converging dies, the shape of which follows the natural pathways of the streamlines required to produce a purely uniaxial elongational field, provided that the assumptions used in the theoretical development are satisfied. We have continued the work with the semi-hyperbolically converging dies developed by Collier and co-workers. The research focused on analyzing the assumptions used in the development of these dies.; The semi-hyperbolically convergent dies are used to measure the effective elongational viscosities of polymer melts and solutions at the industrial processing conditions. Based on a recent study by Feigl et al., it was confirmed that when full slip was assumed at the die walls in the semi-hyperbolically convergent dies, one can obtain an essentially purely uniaxial elongational flow. The degree of slippage taking place in these dies was scrutinized experimentally in our research based on the knowledge that small amounts of carboxylic acid additives can greatly reduce the shear viscosity in many polymer melts. We used the Mooney analysis to quantify the slip in capillary dies for various thermoplastics such as polypropylene, low-density polyethylene, and high density polyethylene. The Mooney equation was modified to fit the semi-hyperbolically convergent dies. Experiments were then performed to quantify the degree of slip in both the capillary and the semi-hyperbolically convergent dies. |
