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Hyflon Ion Membranes for Fuel Cells Vincenzo Arcella, Claudio Troglia, and Alessandro Ghielmi* Solvay Solexis S.p.A., viale Lombardia 20-20021, Bollate (MI), Italy
Processes based on membranes are attracting growing interest in the scientific community and in industry, because, in many cases, they offer a favorable alternative that is not easily achievable using conventional routes. In particular, membranes made with perfluorinated polymers are very interesting, because they exhibit unique features. Hydrophilic highly conductive proton exchange membranes have been developed from the copolymer of tetrafluoroethylene (TFE) and a short-side-chain (SSC) perfluorosulfonylfluoridevinyl ether (Hyflon Ion); they have found interesting application in the field of fuel cells, especially in view of the current tendency to move to high-temperature operation. The advantages given by these hydrophilic perfluorinated materials for use in membrane technology are discussed. The properties and performance of Hyflon Ion membranes are compared with other perfluorinated membranes present on the market. 1. Introduction Fluoropolymer materials are capturing greater and greater interest in industrial applications, because of the remarkable combination of properties that they exhibit, when compared to other polymeric materials. The most well-known property for which fluoropolymers are used in high-demanding applications is their outstanding thermal and chemical resistance. However, the peculiar nature of the C-F bond confers to these materials other unique physical properties (e.g., electrical, optical, superficial, etc.) that can be valuably exploited in the most different fields of utilization. Perfluoropolymers represent the ultimate resistance to hostile chemical environments and high service temperature. In many applications, fluoropolymers offer the greatest protection given by any polymer available today from a huge variety of chemicals, such as acids and alkalis, fuels and oils, low-molecular-weight esters, ethers and ketones, aliphatic and aromatic amines, and strong oxidizing substances. Monomers used for the synthesis of fluorinated polymers can be briefly subdivided into two categories, i.e., base monomers and special monomers, with the former being represented by those monomers that constitute the basic structure of modern fluoropolymers and the latter being represented by those other monomers that add special desired characteristics for matching specialty application requirements. Within this scheme, the base fluoromonomers are tetrafluoroethylene (TFE), hexafluoropropylene (HFP), vinylidenefluoride (VDF), and chlorotrifluoroethylene (CTFE). Proper combination of these monomers yields homopolymers or copolymers with the most diverse characteristics: polytetrafluoroethylene (PTFE), FEP, fluoroelastomers, polyvinylidene difluoride (PVDF), poly(chlorotrifluoroethylene) (PCTFE), and THV (Figure 1). Special monomers produced at the industrial scale by Solvay Solexis are reported in Figure 2. These are used to modify the polymer characteristics, such as the degree * To whom correspondence should be addressed. E-mail:
[email protected].
Figure 1. Fluoromonomers and derived homopolymers and copolymers.
Figure 2. Fluoromonomers produced by Solvay Solexis on an industrial scale.
of crystallinity, thermal transition temperatures, and surface properties. In particular, perfluorosufonylfluoridevinyl ether (SFVE) has been used in copolymers with TFE for the preparation of perfluorinated high-ionic-conductance hydrophilic membranes. This last membrane typology, which is based on TFE-SFVE copolymers, is discussed in the following pages. 2. Ion-Exchange Perfluoropolymer Membranes TFE and SFVE are copolymerized by free-radical polymerization to obtain the polymer depicted in Figure 3a (Hyflon Ion).
10.1021/ie058008a CCC: $30.25 © 2005 American Chemical Society Published on Web 08/30/2005
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Figure 3. Structure of (a) Hyflon Ion and (b) Hyflon Ion H.
Figure 4. Thermogravimetric analysis (TGA) curve for Hyflon Ion.
Amorphous polymers are obtained when m/n < 4, i.e., when the SFVE molar content is higher than ∼20%. The glass-transition temperature (Tg) of the polymer is a function of polymer composition but increases typically from ∼5 °C to 50 °C for SFVE contents that decrease from 30% to 10% (molar). SFVE contents yielding amorphous polymers correspond to Tg values below ambient temperature; therefore, amorphous Hyflon Ion polymers are rubbers at room temperature. These can be dissolved in a variety of perfluorinated or partially fluorinated solvents. On the other hand, when a crystalline phase appears, TFE-SFVE copolymers become scarcely soluble in any solvent. In this case, films can be prepared, by taking advantage of the melt processability of the polymer. The thermal stability of the Hyflon Ion polymers is very high, according to its perfluorinated nature. Thermogravimetric analysis (TGA) shows weight losses of 1% at temperatures as high as 420 °C (Figure 4). After synthesis of the polymer in the sulfonyl fluoride form shown in Figure 3a, the polymer is transformed to an ionomer (i.e., an ion-containing polymer) via conversion of the -SO2F group to -SO3X, where X is a metal or H atom. This conversion is typically performed in alkaline aqueous solutions at medium temperature (e.g., 80 °C). The polymer is finally treated with a strong acid solution if the -SO3H form of the functional group is required in the application. Therefore, the sulfonyl fluoride form of the polymer can be considered the precursor form of the ionomer in the salt or acid form (Figure 3b, Hyflon Ion H). Transformation of the precursor to an ionomer dramatically changes its properties.1 This is due to strong Coulombic associations,
which lead to the formation of ionic regions that are commonly called clusters. Therefore, TFE-rich ionomers can be considered to consist of three different phases: an amorphous phase, a crystalline phase, and an ionic phase. Perfluorinated sulfonate ionomers find effective or potential application in the form of membranes in a wide variety of fields, ranging from electrochemical electrolyzers (chlor-alkali,2 HCl electrolysis3), to fuel cells (proton exchange membrane fuel cells,2 direct methanol fuel cells4), energy storage and delivery devices (lithium ion batteries,4 Br2/S batteries5), microfiltration,6 reverse osmosis and ultrafiltration,4 pervaporation,4 electrodialysis,7 diffusion dialysis,7 and membrane catalytic reactors.8 Short-Side-Chain (SSC) Ionomers and Related Membranes. Conventional ion-exchange perfluoropolymer membranes, such as Nafion (Du Pont), Aciplex (Asahi Chemical), Flemion (Asahi Glass) and GoreSelect (Gore and Associates), are based on the so-called “long-side-chain” (LSC) polymers.3,9,10 Compared to the Hyflon Ion (SSC) polymers, these LSC polymers have a longer pendant group carrying the ionic functionality. The most extensively used and studied LSC ionomer is Du Pont’s Nafion, which was developed in the late 1960s as a polymer electrolyte for a GE fuel cell that was designed for NASA spacecraft missions. Since then, Nafion and polymers of the same family have found wide application in the chlor-alkali industry, because of their very high chemical inertness. The very high chemical stability of Nafion has been demonstrated in fuel cell application by operating lifetimes in excess of 57 000 hours. In the mid 1980s, Ballard Power Systems showed significant improvements in fuel cell performance using SSC ionomer membranes obtained from Dow Chemical Company.11 The chemical structure of the Dow membrane was the same as that of Hyflon Ion polymer (Figure 3); i.e., the Dow polymer was obtained by copolymerizing TFE with SFVE (shown in Figure 2). Demonstration of six-cell stacks, giving four times the power obtained with a standard Nafion membrane, was given; however, because different membrane thicknesses of nondisclosed equivalent weight (EW) (in the case of the Dow membranes) were compared, it is difficult from this work to assess the real advantage of the Dow polymer. Besides improved power output, another important aspect of SSC ionomers, compared to LSC ones, is their different behavior, relative to temperature. SSC ionomers in the protonic form (-SO3H) present a primary transition, defined as the “R” transition, at ∼160 °C, whereas LSC ionomers show this transition at ∼110 °C.12 This difference is very important when the use of the membrane in the fuel cell system is considered. First of all, the fact that SSC ionomers present the R transition at 160 °C implies that one necessary condition for the membrane to operate up to such a high temperature is ensured. An increase of the fuel cell temperature is highly desirable, because this means a reduction of the complexity of the system, both in terms of cooling and fuel preprocessing (CO content reduction) and consequent lower cost. In direct methanol fuel cells, higher temperatures are also required to improve the fuel oxidation kinetics. Second, if the fuel cell system, for any reason, goes out of control and the temperature of the stack increases locally or overall, the SSC ionomer
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Figure 5. Flow chart of the process developed by Dow Chemical Company for the synthesis of the short-side chain (SSC) monomer.
Figure 6. Flow chart of the Solvay Solexis fluorovinyl ether process for the production of the SSC monomer.
membrane has a good capacity for recovery. In fact, if the temperature exceeds the R transition temperature, a catastrophic change of the membrane structure can result.13 The higher the transition temperature, the lower the probability for this to happen. Last, but not least, SSC ionomers, as a consequence of their lowermolecular-weight pendant group, show a crystallinity content that is higher than correspondent LSC ionomers of the same EW.14 Therefore, lower-EW (i.e., higherionic-content) membranes can be prepared with the same crystallinity (i.e., similar mechanical properties) or the same EW membranes with higher crystallinity (i.e., higher mechanical properties). This last possibility is quite important, because higher mechanical properties allow membranes with lower thicknesses to be attained, which, in turn, means higher membrane conductance and peak power. These are highly desirable features, especially in automotive applications. A New Technology for SSC Ionomers and Membranes. Many patents and papers on the Dow polymer and membrane appeared during the 1980s and the years immediately following.1,11,12,14-21 Since then, the interest in SSC membranes has seemed to have been reduced. No industrial development and commercialization of these very promising experimental membranes followed. The process developed by Dow22 for the synthesis of the SSC monomer is reported in Figure 5. The scheme shown involves a large number of steps. This complexity (and the resulting high cost) was probably one of the main obstacles to the industrial development of these ionomers and membranes, although very interesting fuel cell membrane properties were envisaged. Recently, Solvay Solexis has applied its proprietary fluorovinyl ether process to the production of the SSC monomer at the industrial scale. This process, which is extremely simple compared to the old Dow process, is outlined in Figure 6. Within this process, the fluorination and fluoro-olefine addition steps are, in practice, a single reaction step. Hyflon Ion Nonextruded Membranes. Based on its capability of producing this monomer at the industrial scale, Solvay Solexis has started a research and
Figure 7. Conductivity of nonextruded Hyflon Ion H membranes.
development (R&D) project to develop new ionomer membranes for fuel cells and other different applications. Starting from the monomers, ionomers are synthesized by taking advantage of a proprietary microemulsion polymerization process.23 This technology, which has been broadly applied to the polymerization of other fluoropolymers, is able to give very high polymerization kinetics and high-molecular-weight polymers with accurate control of the molecular structure. Many Hyflon Ion ionomers were polymerized at the pilot scale in a broad range of molecular weights and EW values, i.e., from amorphous soluble ionomers to highly crystalline ones. Many different membranes have been prepared with these ionomers. Self-supported cross-linked membranes have been prepared with EW values of 500-700 g/mol and thicknesses of 100-300 µm.24 These membranes have shown very high average conductivities (up to 10-1 S/cm) under fuel cell operation, which imply high conductances (>3 S/cm2), despite their thickness. However, the mechanical properties are insufficient, probably because of their excessive hydration. In addition, the compression molding process, which is used at the laboratory scale for cross-linked membrane formation, appears to be not easily industrially viable. Composite membranes have been also prepared with EW values of 750-1100 g/mol and thicknesses of 2080 µm. The ionomer in the acid form has been dissolved at ambient temperature25 or high temperature26 in water-ethanol mixtures, and these mixtures have been used to impregnate perfluorinated porous supports.27 Dissolution at low temperature is assisted by the addition of a small amount of fluoropolyether in the solvent mixture.25 These membranes also have shown high conductance (up to 4 S/cm2), as expected, considering the low thickness. They also possess good mechanical properties. However, for the more-crystalline higher-EW polymers, the limited gain in mechanical stability that is due to the support is outweighed by a loss in conductivity that is due to the presence of a fraction of volume that is occupied by the nonconducting material (Figure 7). Hyflon Ion Extruded Membranes. Finally, extruded semicrystalline membranes have been prepared with EW values of >750 g/mol and thickness down to 15 µm. These membranes have shown very good conductance and also very good mechanical properties and easy handling, even at extremely low thicknesses. In addition, the ionomer, when tuned with the appropriate molecular weight distribution, gave, via film extrusion, high-quality and consistent membranes. An activity was
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Figure 8. Water uptake in liquid water at 100 °C of extruded Hyflon Ion H membranes as a function of equivalent weight (EW).
Figure 9. Tensile properties (ASTM D1708) of extruded Hyflon Ion H membranes in terms of stress at break, as a function of EW in the range of 700-900 g/equiv.
performed to select the ionomer that gave the best combination of processability, conductance, mechanical properties, and dimensional stability, while being able to guarantee extremely low membrane thickness and durability. Figure 8 shows the water uptake in liquid water at 100 °C of extruded Hyflon Ion membranes (acid form), as a function of EW. These data are compared to the water uptake of Nafion extruded membranes, as reported in the literature19 and measured by the authors. The water absorption is significantly lower for the Hyflon Ion membranes at a given EW. This implies that the same water uptake is obtained with a lower EW Hyflon Ion ionomer. For example, a water uptake of 35%, which is typical of the commonly available 1100 EW Nafion membrane, is obtained with a Hyflon Ion of EW ≈ 900. From the water uptake curve for Hyflon Ion ionomers, it can be observed that, for EW moving downward to 850, the water uptake curve becomes progressively steeper. On one hand, this means that proper compositional control in polymerization is necessary to obtain membranes that have reproducible water uptake behavior. On the other hand, to restrain the water uptake to acceptable values (e50%), EW values of g850 must be selected. Figure 9 shows the tensile properties (ASTM D1708) of extruded Hyflon Ion membranes (acid form) in terms of stress at break, as a function of EW in the range of 700-900 g/equiv. Measurements were performed both on dry and wet membranes. Here, “dry” represents the membrane under laboratory conditions (i.e., 23 °C and 50% relative humidity), whereas “wet” refers to the membrane soaked in water at 100 °C. In all cases, the
Figure 10. Fuel cell performance (polarization curves) of 50-µmthick extruded membranes of different equivalent weights.
tensile test was conducted under laboratory conditions (23 °C and 50% RH). In the case of wet membranes, these were extracted from the water and the measurement was performed immediately. The data in Figure 9 refer to the measurement performed in the machine direction (MD) of the extrusion. It can first be observed that the stress at break decreases when the membrane is hydrated: this is due to swelling and plasticization of the film. It can also be observed that the stress at break increases with EW. This can be related to higher crystallinities and molecular weights of the higher-EW ionomers, because of lower molar contents of the modifier low-reactive SFVE monomer. Higher molecular weights of the higher-EW ionomers were confirmed by the decreasing melt indexes (i.e., increasing melt viscosities) of the polymers in the precursor (SO2F) form. Stresses at break of >25 MPa are obtained on the dry form for ionomers with EW values greater than ∼800. Figure 10 shows the fuel cell performance (polarization curves) of 50-µm-thick extruded membranes of different equivalent weights. The single cell (active area of 10 cm2) was operated in air and hydrogen at 2.5 atm abs and 75 °C; both gases were humidified by bubbling in water at 80 °C. Commercial electrodes from E-Tek (single-sided Elat) were used, with a catalyst loading of 0.6 mg Pt/cm2 and treated with Nafion (0.7 mg Nafion/cm2). The samples were assembled in the cell without previous hot pressing to the electrodes. The membrane-electrode assemblies were initially conditioned in the cell at the temperature, pressure, and relative humidity conditions of the measurement and at a fixed voltage (0.4 V), until the current was constant for at least 3 h. The polarization curve measurements then were taken. It can be observed that the fuel cell performance is strongly dependent on the EW. Figure 11 reports a comparison of fuel cell performance, in terms of power density (obtained as the product of voltage and current density). Passing from EW ) 1350 to EW ) 850, the peak power is approximately doubled. Comparison with Commercially Available Extruded Nafion Membranes. Extruded Hyflon Ion membranes with EW ≈ 850 show mechanical properties that are similar to those of Nafion extruded membranes with EW ) 1100 at comparable thicknesses, both in terms of tensile properties and of tear resistance (resistance to tear initiation and propagation). This also holds true for the hydrated state, although hydration is somewhat higher for the Hyflon Ion membranes, as
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Figure 11. Comparison of fuel cell performance in terms of power density (obtained as voltage × current density) for different EW values.
have also been used in fuel cells, which were developed in the 1960s for the NASA space program. The chemistry that leads to such membranes is accessible to few companies in the world. A process that has been developed by Solvay Solexis is particularly suitable to produce special fluoromonomers of crucial importance for the development of advanced fluoropolymer membranes. A special sulfonyl fluoride monomer and a proprietary microemulsion polymerization process available from Solvay Solexis technology are the basis for the production of short-side-chain (SSC) ionic perfluoropolymers called Hyflon Ion polymers. Based on these new ionomers, self-supported and composite sulfonic acid membranes for fuel cells are currently under development. Compared to presently commercial membranes, selfsupported Hyflon Ion membranes show higher conductivity, similar mechanical properties, and a higher ionic glass-transition temperature, which is a necessary condition for operating the cell at a higher temperature without damaging the membrane. Tests are currently being performed to prove the long-term stability of Hyflon Ion perfluorinated membranes in long-life tests. Literature Cited
Figure 12. Comparison of the fuel cell performance of a 50-µm Hyflon Ion H EW ) 850 membrane, relative to that of Nafion N112.
shown in Figure 8, which compares the water uptake of an EW ) 850 Hyflon Ion polymer with that of an EW)1100 Nafion polymer. Equivalent mechanical properties can, in first approximation, be related to an equivalent level of crystallinity for these two ionomers (e.g, as determined by differential scanning calorimetry (DSC), from the heat of fusion). A comparison of the fuel cell performance of a 50-µm Hyflon Ion EW ) 850 membrane with that of Nafion N112 is reported in Figure 12. Measurement was performed in a single cell (active area of 25 cm2) that was operated in air and hydrogen at 2.5 atm abs and 90 °C (both gases were humidified at 95 °C). Commercial electrodes by E-Tek (single-sided Elat) were used, with a catalyst loading of 0.5 mg Pt/cm2. The polarization curve, after membrane-electrode assembly (MEA) conditioning, can be observed to be higher for the SSC ionomer membrane. This can be related to a lower EW (i.e., higher proton-exchange capacity) and to higher hydration, because the cell is operated at full humidification and conductivity is well-known to be also a function of the state of hydration of the membrane.13 3. Conclusions Fluoropolymer membranes offer, in many cases, the key to processes that are not easily achievable via conventional routes. Hydrophilic membranes have been used for many years in the chlorine and caustic electrolytic process as the sole ecologic alternative to the mercury and the asbestos diaphragm processes; they
(1) Tant, M. R.; Darst, K. P.; Lee, K. D.; Martin, C. W. Structure and Properties of Short-Side-Chain Perfluorosulfonate Ionomers. In Multiphase Polymers: Blends and Ionomers; Utracki, L. A., Weiss, R. A., Eds.; ACS Symposium Series 395; American Chemical Society: Washington, DC, 1989; pp 370-400. (2) Eisenberg, A., Yaeger, H. L., Eds. Perfluorinated Ionomer Membranes; ACS Symposium Series 180; American Chemical Society: Washington, DC, 1982. (3) Trainham, J. A., III; Law, C. G., Jr.; Newman, J. S.; Keating, K. B.; Eames, D. J. (E. I. Du Pont de Nemours and Company). Electrochemical Conversion of Anhydrous Hydrogen Halide to Halogen Gas Using a Cation-Transporting Membrane, U.S. Patent No. 5,411,641, May 2, 1995. (4) Yeager, H. L.; Gronowski, A. A. Membrane applications. In Ionomers. Synthesis, Structure, Properties, and Applications; Tant, M. R., Mauritz, K. A., Wilkes, G. L., Eds.; Blackie Academic & Professional: London, 1997; pp 333-364. (5) Cooley, G. E.; D’Agostino, V. F. (National Power PLC). Cation Exchange Membranes and Method for the Preparation of Such Membranes, U.S. Patent No. 5,626,731, May 6, 1997. (6) Moya, W. (Millipore Corporation). Surface Modified Polymeric Substrate and Process, U.S. Patent No. 6,179,132, January 30, 2001. (7) Naylor, T. deV. Polymer Membranes: Materials, Structures and Separation Performance. Rapra Rev. Rep. 1996, 8 (5), 3945. (8) Olah, G. A.; Iyer, P. S.; Prakash, G. K. S. Perfluorinated Resinsulfonic Acid (Nafion-H) Catalysis in Synthesis. Synthesis 1986, 7, 513-531. (9) Fielding, H. C. Fluoropolymer Membranes. In Fluoropolymers ’92. Synthesis, Properties & Commercial Applications, UMIST, Manchester, U.K., January 6-8, 1992; Paper No. 5. (10) Heitner-Wirguin, C. Recent advances in perfluorinated ionomer membranes: structure, properties and applications. J. Membr. Sci. 1996, 120, 1-33. (11) Prater, K. The Renaissance of the Solid Polymer Fuel Cell. J. Power Sources 1990, 29, 239-250. (12) Eisman, G. A. The Physical and Mechanical Properties of a New Perfluorosulfonic Acid Ionomer for Use as a Separator/ Membrane in Proton Exchange Processes. In Proceedings of the 168th Electrochemical Society Meeting, Vol. 83-13; Electrochemical Society: Pennington, NJ, 1986; pp 156-171. (13) Zawodzinski, T. A., Jr. Water Uptake by and Transport Through Nafion 117 Membranes. J. Electrochem. Soc. 1993, 140 (4), 1041-1047. (14) Moore, R. B., III, Martin, C. R. Morphology and Chemical Properties of the Dow Perfluorosulfonate Ionomers. Macromolecules 1989, 22, 3594-3599.
Ind. Eng. Chem. Res., Vol. 44, No. 20, 2005 7651 (15) Ezzell, B. R.; Carl, W. P.; Mod, W. A. (The Dow Chemical Company). Novel Polymers Having Acid Functionality, U.S. No. Patent 4,330,654, May 18, 1982. (16) Ezzell, B. R.; Carl, W. P.; Mod, W. A. (The Dow Chemical Company). Sulfonic Acid Electrolytic Cell Having Fluorinated Polymer Membrane with Hydration Product Less than 22,000, U.S. Patent No. 4,358,545, November 9, 1982. (17) Alexander, L. E.; Eisman, G. A. (The Dow Chemical Company). European Patent Specification No. 0 221 178 B1, 1993. (Date of application: May 15, 1986.) (18) Ezzell, B. R.; Carl, W. P.; Mod, W. A. Ion Exchange Membranes for the Chlor-Alkali Industry. In Industrial Membrane Processes; White, R. E., Pintauro, P. N., Eds.; AIChE Symposium Series 248; American Institute for Chemical Engineers (AIChE): New York, 1986; Vol. 82, pp 45-50. (19) Ezzell, B. R.; Carl, W. P. (The Dow Chemical Company). Low Equivalent Weight Sulfonic Fluoropolymers, U.S. Patent No. 4,940,525, July 10, 1990. (20) Carl, W. P., et al. (The Dow Chemical Company). European Patent No. 0 498 076 A1, 1992. (Date of application: December 20, 1991.) (21) Tsou, Y.-M.; Kimble, M. C.; White, R. E. Hydrogen Diffusion, Solubility and Water Uptake in Dow’s Short-Side-Chain
Perfluorocarbon Membranes. J. Electrochem. Soc. 1992, 139 (7), 1913-1917. (22) Ezzell, B. R.; Carl, W. P.; Mod, W. A. (The Dow Chemical Company). Preparation of Vinyl Ethers, U.S. Patent 4,358,412, November 9, 1982. (23) Apostolo, M.; Arcella, V. (Ausimont S.p. A.). European Patent No. 1 172 382 A2, 2002. (Date of application: June 19, 2001.) (24) Arcella, V., et al. (Ausimont S.p. A.). European Patent No. 1 167 400 A1, 2002. (Date of application: June 19, 2001.) (25) Maccone, P.; Zompatori, A. (Ausimont S.p.A.). Preparation of Sulphonic Fluorinated Polymer Solutions, U.S. Patent No. 6,197,903, March 6, 2001. (26) Martin, C. R.; Rhoades, T. A.; Ferguson, J. A. Dissolution of Perfluorinated Ion Containing Polymers. Anal. Chem. 1982, 54, 1639-1641. (27) Bahar, B.; Hobson, A. R.; Kolde, J. A.; Zuckerbrod, D. (W. L. Gore & Associates). Ultra-thin Integral Composite Membrane, U.S. Patent No. 5,547,551, August 20, 1996.
Received for review January 18, 2005 Accepted May 23, 2005 IE058008A
