Structure And Property Of Microporous Polytetrafluoroethylene Membrane Prepared By Stretching

Microporous Polytetrafluoroethylene(PTFE) membrane was widely used for filtration membrane, industrial sealant , substitute materials for biologic organs and clothing with water-resistant due to its unique structure and outstanding combined properties. But the membrane was mainly imported from abroad with high price, and there was few report about preparing of the membrane. So it was beneficial not only in theories but also in industrial practice to thoroughly explore the fabrication process of porous PTFE membrane.Microporous PTFE membranes were prepared from PTFE fine power paste by a series of mechanical manipulation. The detailed fabrication method included extruding the paste that mixed PTFE dispersion resin with lubricants through a die with a cone entrance, rolling the extrudates into tapes with about 250μm through thickness, stretching the tapes which were pre-heated to the temperature of 220-320℃ . at different highly stretching rates in longitudinal direction and finally heat-treating the membranes at temperature of 220-360℃.The effects of preforming duration, preforming pressure, extrusion die's geometry size and content of lubricant on the extrudate's tensile strength were studied. The microfibril which created during extruding were characterized through Differential Scanning Calorimetry(DSC) and Raman spectroscopy. PTFE fine power resin paste flow mechanism and fibrillation mechanism were investigated briefly. The morphology changes of PTFE particles during extruding and stretching and the microporous membrane's unique node-fibril micro-stucture were tracked through Scanning Electron Microscopy (SEM) analysis. The effects of stretching process on the membrane's porosity and pore size distribution were researched through mercury porosimetry. Crystallinity of the PTFE were characterized by DSC and X-Ray diffraction (XRD). The effects of stretching processes on the membrane's structure and pore property were mainly discussed. The result show that stretching temperature, stretching ratio and stretching rate is the crucial factors affecting the morphology of the membrane, so it can be controlling the morphology by adjusting the above three parameters. The effects of heat treatment on microporous membrane's shrinkage,sinter degree and crystallinity were discussed additional.The structure of the prepared microporous membrane was well characterized by a spatial periodicity of island like nodes(domain of agglomerated PTFE particles) and microfibril domain that interconnected the nodes. In case of uniaxial stretching the nodes were elongated , the longer axes of nodes being oriented perpendicular to the direction of stretching. The microfibrils which interconnected the nodes are oriented parallel to the stretching direction(perpendicular to the longer axes of nodes). The membranes were white", soft and having the appearance resembling silk. Its maximal tensile strength was 8.5MPa, porosity in the range of 26-85%, pore size distribution mainly in 4.8-56.1 urn.