Studies on Porous and Morphological Structures of Expanded PTFE Membrane through Biaxial Stretching Technique

Abstract

A porous expanded polytetrafluoroethylene (ePTFE) membrane was prepared from emulsion polymerized PTFE fine powders by a series of mechanical operations, which included extrusion, rolling, stretching and heat setting. Very small holes in the PTFE sheet were observed by SEM analyses after extrusion and rolling, and which were elongated and enlarged by longitudinal stretching. Fibrils between the slits are observed by SEM analyses. The second stretching operation, transverse stretching, provided a lattice-like porous structure. After heat setting, an island-like structure was formed, which is composed of billions of tiny inter-connected continuous fibrils and nodes. The porous structure was studied through SEM and the COULTER POROMETER tester. Results show that the mean pore size and porosity increase with an increase in longitudinal stretching ratio, transverse stretching ratio, and heat setting temperature. The mean pore size decreases and the porosity increases with an increase in transverse stretching rate. Remarkably, increasing transverse stretching rate increases porosity while the mean pore size decreases slightly, resulting in a membrane with a more uniform cell and denser cell structure. DSC, WAXD analysis and mechanical testing show that mechanical processing and heat setting decrease both the crystallinity and crystallite size.