Abstract
The influence of molecular weight distribution and extrusion processing variables on the morphological features and orientation of high density polyethylene (HDPE) uniaxially extruded tubular films was investigated. In order to gain a better understanding of the orientation–crystallization behavior occurring during extrusion processing, the melt flow properties of the two HDPE resins with identical M̄n (14 600) values but different molecular weight distributions (M̄w/M̄n=10.3, 15.1), utilized in our previous study, were characterized by dynamic rheological experiments over the temperature range from 150 °C to 230 °C within the angular frequency range from 0.1 to 100 rad/s. The experimental data were shifted to produce master flow curves. The flow activation energy calculated from the shifting process was found to be 25.9 kJ/mol for resin 1 and 29.1 kJ/mol for resin 2. The characteristic relaxation time at 190 °C obtained by use of a Carreau–Yasuda analysis for resin 2 having the broader molecular weight distribution was found to be 6.5 times greater than that of resin 1. This observation further supports our previous conjecture that the prominence of the fibril nuclei in resin 2 is due to its longer melt relaxation time behavior. The extrusion processing variables of melt temperature at the die exit, quench height (which is the distance from the exit of the die to the cooling ring), flow rate of the air through the cooling ring, film line speed, and die gap were varied to control the melt relaxation time of HDPE resins and the processing time frame for cooling. The results show that a longer melt relaxation time and a shorter cooling processing time can enhance the formation of fibril nuclei. The importance of melt relaxation behavior in influencing the final morphological structure in HDPE extruded films and their associated properties is clearly made evident in this paper.
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