Processing and structure-property behavior of microporous polyethylene: from resin to final film

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Virginia Tech

The three-stage (extrusion, annealing, stretching) method of producing microporous membranes from linear polyethylene (HDPE) was investigated in this dissertation. Two different HDPE resins with identical M̅n (14,600) values but different distributions (M̅w/M̅n=10.3, 15.1) were utilized for this study. In the extrusion process, the two HDPE resins were melt extruded into uniaxially oriented tubular films. Systematical changes were made in the process variables -- these being the melt temperature at the die exit, the quench height, the cooling rate, and the line speed. The melt relaxation time behavior of the HDPE resins was studied by a Carreau-Yasuda analysis. The orientation morphological features of the extruded films were examined by TEM, HSEM, birefringence, WAXS, SAXS, and linear IR dichroism. The molecular weight distribution of the raw resins as well as the specific processing variables of quench height and melt temperature were found to be important in determining the final structure of the HDPE extruded films.

Following the extrusion process, the effect of annealing on the structure and properties of the HDPE extruded films was investigated. The HDPE extruded films were annealed under different conditions. The annealing variables studied included the temperature, the line speed (or annealing time), and the amount of extension applied during annealing. The extruded films before and after annealing were characterized by DSC, WAXS, SAXS, birefringence, and TEM. The results suggest that upon annealing, perfection of the crystalline phase occurs by removal of the defects from the crystalline phase.

In the last step, the precursors (either the extruded films or the extruded films after being annealed) were uniaxially deformed along the extrusion direction. The variables of cold stretch ratio, hot stretch ratio, and total stretch percent were varied to alter the properties of the stretched microporous membranes. The pore structure, porosity, and permeability of the stretched microporous films were analyzed by TEM, HSEM, AFM, DSC, and Gurley number measurements. The importance of the orientation and morphological properties of the precursors, the annealing effects of the HDPE extruded films, and the stretching variables for influencing the microporosity behavior of the HDPE microporous membranes is clearly made evident in this dissertation.

polyethylene, orientation, microporous, extrusion, annealing, fibril nuclei