Fluoropolymers membranes

The anode and cathode chambers are separated by a cation-permeable fluoropolymer-based membrane (see Membrane technology). Platinum-electroplated high surface area electrodes sold under the trade name of TySAR (Olin) (85,86) were used as the anode the cathode was formed from a two-layer HasteUoy (Cabot Corp.) C-22-mesh stmcture having a fine outer 60-mesh stmcture supported on a coarse inner mesh layer welded to a backplate. The cell voltage was 3.3 V at 8 kA/m, resulting ia a 40% current efficiency. The steady-state perchloric acid concentration was about 21% by weight. 

The reactors are cylindrical in shape and can carry up to 30 mg of resin. Polymer sieves at the top and bottom of the cylinders serve for liquid feed and withdrawal. The array of reactors is attached to a capillary system allowing feed to either columns or rows. This distribution system is said to provide uniform charges to the various reactors. A specific detail of the reaction system is that mixing is achieved by pneumatic actuation using a fluoropolymer membrane (Figure 4.36). 

Membranes UF membranes consist primarily of polymeric structures (polyethersulfone, regenerated cellulose, polysulfone, polyamide, polyacrylonitrile, or various fluoropolymers) formed by immersion casting on a web or as a composite on a MF membrane. Hydrophobic polymers are surface-modified to render them hydrophilic and thereby reduce fouling, reduce product losses, and increase flux [Cabasso in Vltrafiltration Membranes and Applications, Cooper (ed.). Plenum Press, New York, 1980]. Some inorganic UF membranes (alumina, glass, zirconia) are available but only find use in corrosive applications due to their high cost. 

In contrast to past environmental problems associated with fluorocarbon refrigerants, the exceptional properties of fluorine in polymers have great environmental value. Some fluoropolymers are enabling green technologies such as hydrogen fuel cells for automobiles and oxygen-selective membranes for cleaner diesel combustion. 

One of the first fictional fluoropolymers was poly-1,2,2-trifluorostyrene. On one hand, it has much better oxidation and chemical resistance in comparison with common hydrocarbon polymers and, on the other hand, a wide range of functional groups can be attached to the aromatic ring. A sulfonated polymer was successfully used as a membrane for fuel cells by General Electric Co.3... 

As was noted above, functional fluoropolymers produced by copolymerization of fluoroolefins with functional PFAVE have several unique properties, with the main disadvantage of these materials being the extremely high cost of functional monomers and the resulting high cost of the functional polymers produced from them. The fact that they are so expensive limits their wider industrial application in other fields such as catalysis and membrane separation, except for chlorine-alkali electrolysis and fuel cells, where the only suitable materials are fully fluorinated polymers because of the extreme conditions associated with those processes. 

Modem membranes usually consist of at least two layers one from sulfonic type copolymer and another from carboxylic type copolymer. The membranes are usually reinforced by fluoropolymer fabric to provide better mechanical properties and long lifetimes. The most important properties are considered in detail in the reviews mentioned above10,11 and in a basic text by Seko et al.6... 

Grafting of functional monomers onto fluoropolymers produced a wide variety of permselective membranes. Grafting of styrene (with the following sulfonation), (meth)acrylic acids, 4-vinylpyridine, A-vinylpyrrolidone onto PTFE films gave membranes for reverse omosis,32-34 ion-exchange membrane,35-39 membranes for separating water from organic solvents by pervaporation,49-42 as well as other kinds of valuable membranes. 

Perpall, M. W., Smith, D. W., Jr., DesMarteau, D. D. and Greager, S. E. 2006. Alternative trifluorovinyl ether derived fluoropolymer membranes and functionalized carbon composite electrodes for fuel cells. Journal of Macromolecular Science Part C Polymer Reviews 46 297-313. 

For instance, the Dow experimental membrane and the recently introduced Hyflon Ion E83 membrane by Solvay-Solexis are "short side chain" (SSC) fluoropolymers, which exhibit increased water uptake, significantly enhanced proton conductivity, and better stability at T > 100°C due to higher glass transition temperatures in comparison to Nafion. The membrane morphology and the basic mechanisms of proton transport are, however, similar for all PFSA ionomers mentioned. The base polymer of Nation, depicted schematically in Figure 6.3, consists of a copolymer of tetrafluoro-ethylene, forming the backbone, and randomly attached pendant side chains of perfluorinated vinyl ethers, terminated by sulfonic acid head groups. °... 

Like many other fluoropolymers, Nafion is quite resistant to chemical attack, but the presence of its strong perfluorosulfonic acid groups imparts many of its desirable properties as a proton exchange membrane. Fine dispersions (sometimes incorrectly called solutions) can be generated with alcohol/water treatments. Such dispersions are often critical for the generation of the catalyst electrode structure and the MEAs. Films prepared by simply drying these dispersions are often called recast Nafion, and it is often not realized that its morphology and physical behavior are much different from those of the extruded, more crystalline form. 

The great value of the unique characteristics of fluorinated polymers in the development of modern industries has ensured an increasing technological interest since the discovery of the first fluoropolymer, poly(chlorotrifluoro-ethylene) in 1934. Hence, their fields of applications are numerous paints and coatings [10] (for metals [11], wood and leather [12], stone and optical fibers [13, 14]), textile finishings [15], novel elastomers [5, 6, 8], high performance resins, membranes [16, 17], functional materials (for photoresists and optical fibers), biomaterials [18], and thermostable polymers for aerospace. 

We will continue platform extensions, taking Nafion fluoropolymer membranes from chloralkali cells and catalysts to advanced fuel cells, and Kapton polyimide film from flat electronic computer circuits to complex flexible circuits in cell... 

Continuous mns were carried out in a stirred cell ultrafiltration module using the fluoropolymer membrane FS61PP with a nominal molecular weight cut-off of 20kDa. The reactor, loaded with an appropriate amount of resting cells, was fed with a buffered substrate solution by a peristaltic pump with the flow rate set at... 

Yi et al. reported a new type of PVDF membrane prepared by blending two very different polymers, a PVDF fluoropolymer such as Kynar with a sulfonated poly-electrolyte. The new membrane is inexpensive and displayed good performance and durability based on 1,000-h test data. 

Souzy, R. and Ameduri, B., Functional fluoropolymers for fuel cell membranes. Prog. Polym. Sci., 30, 644, 2005. 

Membrane and Membrane Design Most membranes are polymers in nature, but some inorganic membranes have become available. The most common membranes are based on polysulfone, cellulose acetate, polyamide, fluoropolymers, and other compounds. Formation of a symmetric membrane structure is an important element in the success of UF/NF membrane separation (16). The other considerations for membrane separation are as follows (1) separation capabilities (retention or selectivity), (2) separation rate (flux), (3) chemical and mechanical stabilities, and (4) membrane material cost. 

A new development reported by Li and Sirkar [141] for MD-based desalination makes use of polypropylene hollow fibers coated with a plasma polymerized sUicone-fluoropolymer. This ultrathin coating on the outside of the fiber was water vapor permeable and was instrumental in decreasing the susceptibility of the composite membrane to wetting and fouling. They reported stable water vapor fluxes between 41 and 79 kg h for runs lasting up to 400 h. 

OD applications are those fabricated from nonpolar polymers with low surface free energies. The most commonly used OD membrane materials are polyolefins, such as polyethylene and polypropylene, and fluoropolymers, such as polytetrafiuoroethylene (PTFE) and polyvinylidine difiuoride (PVDF). ... 

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. 

PEMFC (proton exchange membrane) uses fluoropolymer or similar type membranes such as SPEEK. Microbial and direct methanol fuel cells fall into this category also. 

PEMFCs have a solid ion exchange membrane made of sulfonated fluoropolymer, or a sulfonated polyetheretherketone (SPEEK) which is the electrolyte, and for the most part uses platinum catalysts. There are other materials in use for combination type membranes. It should be mentioned that currently SPEEK membranes do not hold up as well as the fluoropolymers such as Nafion , but research is ongoing to produce a more reliable and longer lasting membrane. 

At the present time, the Nogoya Institute in Japan is making progress developing a new glass based electrolyte that is much less expensive than fluoropolymer membranes, but just as durable. In the near future, these membranes may replace Nafion and SPEEK in PEM cells. 

Virtually the entire membrane manufacture today is based on laminate structures comprising a thin barrier layer deployed upon a much thicker, highly permeable support. Most are formed of compositionaUy homogeneous polysulfone, cellulose acetate, polyamides, and various fluoropolymers by phase inversion techniques in which ultrathin films of suitably permselective material are deposited on prefabricated porous support structures. Hydrophobic polymers as polyethylene, polypropylene, or polysulfone are often used as supports. A fairly comprehensive hst of microporous and ultrafiltration commercial membranes and produced companies are presented in Refs [107-109]. A review on inorganic membranes has been given in Ref. [110]. 

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