Polypropylene membranes, application

While several niche applications for OD have been identified, the commercial acceptance of the technology has been hampered by the nonavailability of a suitable membrane-membrane module combination. Fluoropolymer membranes, such as PTFE and PVDF, have been shown to provide superior flux performance, but are still unavailable in hollow fiber form with a suitable thickness for use in OD applications. The inherently low flux of OD requires fhaf membranepacking density be maximized for effective operation, and hence the available flat-sheet form of perfluoro-carbon membranes is unsuitable for commercial use. Four-port hollow fiber modules that provide excellent fluid dynamics are currently available, but only low-flux polypropylene membranes are utilized. 

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. 

The thermal process is perhaps the most universally applicable of all the phase inversion processes because it can be utilized over the widest range of both polar and nonpolar polymers. However, its commercial use for membrane applications will probably be restricted to polyolefins, particularly polypropylene. A large number of the substances can function as latent solvents (Table X). They usually consist of one or two hydrocarbon chains terminated by a polar hydrophilic end group. Therefore, they exhibit surface activity which may explain their ability to form the emulsion-like Sol 2 micelles at elevated temperatures. One latent solvent which is worthy of special mention because of its broad applicability is N-Tallowdiethanolamlne (TDEA). 

Polypropylene membranes of this type can be used where the chemical resistance of polypropylene and tight control of pore size are needed. These polypropylene membranes, known as ACCUREL, are more chemically resistant than the remaining polypropylene filter cartridge components due to a proprietary process used during membrane manufacture and have replaced poly(tetrafluoroethylene) membranes In many applications. 

HoUow-fiber SLMs have been used in the removal of phenol from aqueous matrices. Kujawski et al. [142, 143] studied polypropylene membranes impregnated with methyl-terbutyl ether, cumene, and/or a mixture of hydrocarbons. With Cyanex 923 (a mixture of trialkylphosphine oxides), the recoveries of phenol reached of 98% into the stripping phase from the 0.2 mol.dm solution of caustic soda [144, 145]. Carriers for phenol removal from wastewaters have included hnear monoalkyl cyclohexane [146], N,N-di(l-methyl heptyl) acetamide [147], dibenzo-18-crown-6 [148], dodecane [149], trioctylamine [150], and N-octanoylpyr-rolidine [151]. Many diluents and carriers are of synthetic origin, and so their application carries with issues of flammabUity, volatility, toxicity, and potential detrimental effects to the environment and the health of the human population [152]. 

A similar approach has been used by Celanese Corp. to make Celegard . an expanded polypropylene membrane (see Figure 2.4). The stretching creates elongated pores measuring 0.02 by 0.20 ju or 0.04 by 0.40 ju. The low flow rates of this membrane limit applications primarily to battery separators and airvents. 

The commercially available transdernal systen of clonidine consists of an outer layer of pigmented polyester a drug reservoir of clonidine, mineral oil, polyisobutylene, and colloidal silicon dioxide a microporous polypropylene membrane that controls the rate of diffusion of the drug and a final adhesive layer that provides an initial release of drug and contains those ingredients found in the reservoir. The adhesive layer is covered by a protective strip which is removed prior to application (1). 

Membrane characterization by CSLM has been rather limited when compared with other microscopic techniques such as SEM and atomic force microscopy (AFM). The earliest work found in the literature [13] records how van den Berg et al. used a combination of AFM and CSLM to study qualitative differences in the pore geometry of different brands of polypropylene membranes. The first reported applications that used only CSLM for membrane characterization [14,15] were by Charcosset et al. who used CSLM to characterize microporous membrane morphologies and to obtain values of surface porosity and pore size. The conclusions of those studies were that CSLM gave some characteristics on membrane morphology that SEM, which views only surfaces, cannot provide. However, as also mentioned previously in this chapter, they pointed out low resolution for membrane characterization as the main drawback of CSLM. This restricts the use of CSLM to the characterization of microfiltration membranes if measurements on pore size and surface porosity have to be performed. 

The Institute of Environmental and Energy Technology (TNO) in Netherlands have developed macroporous polypropylene membrane contactors and used it with alkali aminoacid salt as absorbant. No wetting or degradation of polypropylene membrane surface was observed and hence stable membrane performance was reported. The Kvaerner Process is used for Teflon membranes with aqueous amine solutions of MEA, DEA, MDEA and DIP A. The membrane system was used to recover acid gases from natural gas for offshore platform applications. No performance data are available for the flue gas treatment. 

The most common material used is cellophane, which is a cellulose film, which acts as a membrane and is capable of resisting zinc penetration. The cycle life of cells utilizing this material is severely limited due to the hydrolysis of the cellophane in alkaline solution. Various methods have been tried to stabilize cellulose materials, such as chemical treatment and radiation grafting to other polymers, but none have, as of now proved economically feasible. The most successful zinc migration barrier material yet developed for the nickel—zinc battery is Celgard microporous polypropylene film. It is inherently hydrophobic so it is typically treated with a wetting agent for aqueous applications. 

The most current method of nitroglycerin application is a transdermal device or skin patch. A cross section of such a patch is illustrated in Figure 6. The patch is actually a multi-layered polymer stack. The semipermeable membrane which comes in contact with the skin is usually composed of an ethylene-vinyl acetate copolymer or polypropylene. The reservoir contains the drug in a hydrogel or polymer matrix or solvent (the material must be chosen to insure uniform delivery). Examples of some solvents used include dimethyl sulfoxide (DMSO), sodium lauryl sulfate (SDS - a detergent) and propylene glycol/oleic acid. 

The design of the first commercial modules has allowed the commercial application of membrane contactors for some specific operations. This is the case of the Membrana-Charlotte Company (USA) that developed the LiquiCel modules, equipped with polypropylene hollow fibers, for the water deoxygenation for the semiconductor industry. LiquiCel modules have been also applied to the bubble-free carbonation of Pepsi, in the bottling plant of West Virginia [18], and to the concentrations of fruit and vegetable juices in an osmotic distillation pilot plant at Melbourne [19]. Other commercial applications of LiquiCel are the dissolved-gases removal from water, the decarbonation and nitrogenation in breweries, and the ammonia removal from wastewater [20]. 

Membrane polymeric materials for separation applications are made of polyamide, polypropylene, polyvinylidene fluoride, polysulfone, polyethersulfone, cellulose acetate, cellulose diacetate, polystyrene resins cross-linked with divinylbenzene, and others (see Section 2.9) [59-61], The use of polyamide membrane filters is suggested for particle-removing filtration of water, aqueous solutions and solvents, as well as for the sterile filtration of liquids. The polysulfone and polyethersulfone membranes are widely applied in the biotechnological and pharmaceutical industries for the purification of enzymes and peptides. Cellulose acetate membrane filters are hydrophilic, and consequently, are suitable as a filtering membrane for aqueous and alcoholic media. 

Hollow membrane fibers are required for many medical application, e.g. for disposable dialysis. Such fibers are made by usmg an appropriate fiber spinning technique with a special inlet in the center of the spinneret through which the fiber core forming medium (liquid or gas) is injected. The membrane material may be made by melt-spinning, chemical activated spinning or phase separation. The thin wall (15-500 xm thickness) acts as a semi-permeable membrane. Commonly, such fibers are made of cellulose-based membrane materials such as cellulose nitrate, or polyacrylonitrile, polymethylmethacrylate, polyamide and polypropylene (van Stone, 1985). 

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