Development of microporous membranes from PP/HDPE films through cast extrusion and stretching

One of the techniques to make porous membranes from polymers without using solvent and/or particles is based on the stretching a polymer film containing a row-nucleated lamellar structure. Resin characteristics such as molecular weight, molecular weight distribution, and amount of long chain branching as well as process conditions are the key factors for the production of the precursor films with appropriate crystalline morphology and orientation, which in turn control the final membrane properties. We have developed microporous membranes with high performances from monolayer as well as multilayer polypropylene/high density polyethylene (PP/HDPE) films through cast extrusion followed by uniaxial stretching of row-nucleated lamellar structure involving industrial scale capabilities. We also investigated the drawing of polypropylene films with coexisting rows of lamellae and spherulites and only spherulites using a machine direction orientation (MDO) unit. We have investigated changes in morphology in relation with the barrier properties as well as tear and puncture properties. A complete disruption of the initial structure was observed at high draw ratios and a correlation between orientation of both crystalline and amorphous phases and barrier properties was found.;;In addition to the membrane fabrication by stretching, comprehensive investigations on the rheological and thermal properties of blends of different linear polypropylenes (L-PP) and a long chain branched polypropylene (LCB-PP) were performed. This provides valuable information concerning the resin characteristics particularly the effect of molecular weight and long chain branching on the rheological and thermal properties, which was helpful in the membrane development.