Polypropylene Porosity

Cells consist of porous sintered silver electrodes and high rate iron electrodes. The latter are enclosed with a seven-layered, controUed-porosity polypropylene bag which serves as the separator. The electrolyte contains 30% KOH and 1.5% LiOH. 

Equation 19.5 is often utilized in MD, while Equations 19.6 and 19.7 are the alternate expressions to calculate the membrane thermal conductivity, where e is the porosity, k and kg are thermal conductivities for solid (polymer) and gases in the pores (air and water vapor), respectively. The values of thermal conductivity of polyvinylidenedifluoride (PVDF), Polytetrafluoroethylene (PTFE), and Polypropylene (PP) have been reported in a narrow range 0.17-0.29 W m [48]. The thermal conductivities... 

A novel idea for the production of water is by the combination of MD and membrane crystallization [ 139], where the salt is concentrated on the feed side to a point close to super-samration, thereby inducing nucleation of crystals. Recently Gryta and Morawski [140] performed experiments using polypropylene capillary membranes with pore diameters ranging between 0.2 and 0.6 p.m, and 70% porosity. They found crystallization to occur at the membrane surface, but by increasing the distillate temperature to 328 K, the problem was eliminated and stable flux restored. 

Most UF membranes are made from polymeric materials, such as, polysulfone, polypropylene, nylon 6, PTFE, polyvinyl chloride, and acryhc copolymer. Inorganic materials such as ceramics, carbon-based membranes, and zirconia, have been commercialized by several vendors. The important characteristics for membrane materials are porosity, morphology, surface properties, mechanical strength, and chemical resistance. The membrane is tested with dilute solutions of well-characterized macromolecules, such as proteins, polysaccharides, and surfactants of known molecular weight and size, to determine the MWCO. 

As new membranes are developed, methods for characterization of these new materials are needed. Sarada et al. (34) describe techniques for measuring the thickness of and characterizing the structure of thin microporous polypropylene films commonly used as liquid membrane supports. Methods for measuring pore size distribution, porosity, and pore shape were reviewed. The authors employed transmission and scanning electron microscopy to map the three-dimensional pore structure of polypropylene films produced by stretching extended polypropylene. Although Sarada et al. discuss only the application of these characterization techniques to polypropylene membranes, the methods could be extended to other microporous polymer films. Chaiko and Osseo-Asare (25) describe the measurement of pore size distributions for microporous polypropylene liquid membrane supports using mercury intrusion porosimetry. 

Anhydrous ammonia was purchased from Matheson Canada Ltd., Whitby, Ontario. The substrate materials used were polypropylene membrane (Celgard-2400, Celanese Corp., Summit, NJ) and polypropylene beads (Hercules Canada Ltd., Montreal, Quebec). The Celgard-2400 was a 25.4 pm thick porous membrane with an effective pore size of 0.02 pm with a 38% porosity. The beads as purchased were slightly flattened spheres having a diameter of about 2.5-3 mm and being about 3.5 ran thick. The membranes and the beads were washed with distilled water, then with absolute ethanol in an ultrasonic cleaner and were finally dried in a vacuum oven, at about 70° C. These substrate materials were ready for use as a control or for further treatment. The further treatment involved the attachment of NH2 groups onto the surface of the substrate material in an ammonia plasma reactor, perhaps by free-radical reaction, as reported in our previous report (2). 

McHugh and Krukonis ( ) have demonstrated with the aid of SEM, that polypropylene preforms can be rendered porous by extraction with supercritical CO2 and propylene. However, these investigators noted that SCF extraction of the preformed polymer sheets at higher pressures and temperatures deformed the polymer matrix leading to a fused appearance when examined by SEM and a concomitant decrease in porosity. Similar interaction between porous polyurethane foam and supercritical CO2 reported by Smith and coworkers (J) lead to a physical alteration of the exposed sorbent. Such pressure-induced structural changes lead to a decrease in available surface area for sorbate adsorption. 

During sintering, a powder of particles of a given size is pressurized at elevated temperatures in a preformed shape so that the interface between the particles disappears. Microfiltration membranes can thus be obtained from PTFE (polytetra-fluoroethylene), PE (polyethylene), PP (polypropylene), metals, ceramics, graphite and glass, with pore sizes depending on the particle size and the particle-size distribution. Porosities up to 80% for metals and 10-20% for polymeric membranes can be reached with pore sizes varying between 0.1 and 10 pm. Most of these materials have excellent solvent and thermal stability. 

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