Fluorinated Ionomers available in Hardcover
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- Elsevier Science
Extensive coverage has been given to perfluorinated ionomers because of the practical importance of this group of polymers. Commercial products such as Nafion®, Aciplex® (Asahi Chemical) and Flemion® (Asahi Glass) are fluorinated ionomers have been discussed in detail. Whether you need information about use of fluorinated ionomers in fuel cells, batteries, chlor-alkali cells, sensors, fabrication techniques, or commercial products you will find it in this valuable handbook.
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By Walther Grot
William AndrewCopyright © 2011 Elsevier Inc.
All right reserved.
Fluorinated ionomers, particularly the perfluorinated ionomers developed in the 1960s, have revolutionized the chlor-alkali industry. In this process, the use of hazardous materials such as mercury and asbestos has been eliminated; and the economics, particularly in regard to reduced energy consumption, has substantially improved. This application has now matured to such an extent that the complete replacement of the two older technologies is only a question of time.
More recently, a new application has emerged in the field of fuel cells. This development is still in flux and is the subject of considerable research in both industry and government institutions. It appears that the full potential of this application is yet to be realized.
The combination of hydrophilic and hydrophobic groups in the same polymer molecule of polymeric fluorinated ionomers results in unique properties and morphologies. This process has attracted the attention of industry, researchers, and theoreticians. However, many questions regarding the inner workings of this material remain still unanswered.
Both partially fluorinated and perfluorinated polymers, containing sufficient ionic groups to dominate the transport properties of the polymer, have been described in this book. Ionic groups may include sulfonic and carboxylic groups as well as sulfonamides and sulfonimides. Due to their importance in the synthesis and fabrication of these ionomers, precursor polymers, containing sulfonyl fluoride or carboxylic ester groups are also discussed. However, it should be emphasized that these precursor polymers are not ionomers, and that they have properties which are quite different from those of the corresponding ionomers.
The synthesis of a perfluorinated ionomer containing phosphonic acid groups has been discussed in detail in. Perfluorinated ionomers containing sulfonyl imide functional groups have also received some attention.
Within this broad scope, perfluorinated ionomers containing sulfonic or carboxylic functional groups have been covered most extensively due to their many commercial uses. Within this narrower group, the emphasis has been placed on Nafion®, which has been available for about ten years longer than any of the other competitive materials in its class. DuPont has made both information and samples of Nafion® and its precursor polymer readily available to research groups and commercial users, which has resulted in extensive coverage of Nafion® in the literature.
More recently, Solvay-Solexis and Minnesota Mining and Manufacturing (3M) have published information about their perfluorinated ionomers, based on monomers of molecular weight 280 and 380 respectively. These polymers differ from Nafion® in that they lack the second ether linkage in the side chain (see Chapter 3). While this structure offers the promise of inherently superior properties it remains to be seen whether the control of important polymerization parameters, such as molecular weight (MW) and its distribution, will allow the realization of this promise. Another consideration is the cost of the manufacturing of the new monomers.
1.2 Physical Shapes
Most fluorinated ionomers are sold as flat sheets and films, such as extruded or solution cast films, or as composite membranes containing fabric reinforcement, which is added to one or more layers of the ionomer. Extruded capillary tubing is also available. Smaller quantities are sold in the form of pellets for applications such as catalysts or for conversion to liquid compositions.
In Chapter 3, monomer synthesis, copolymerization, fabrication, including lamination to a reinforcement, and finishing are described in enough detail to allow the manufacture of these products on a laboratory scale. In Chapter 9, test procedures are provided to determine the properties of both the precursor form as well as the final ionomer. Most important among these properties are equivalent weight (EW) and melt flow (MF). Chapter 3 (Section 3.3) then gives the procedures to adjust these parameters during the copolymerization. A laboratory chlor-alkali cell is described in a way that will allow testing in its most important application, particularly in direct comparison with commercially available membrane types.
The end-use properties as well as the morphology and structure of these products are discussed in Chapter 4.
Chapter 5 deals with the commercial applications. This includes the discussion of chlor-alkali electrolysis, which is the production of chlorine and sodium hydroxide through the electrolysis of brine. This process is still by far the largest and most important application for these ionomers. The major manufacturers have introduced new and improved membranes, known as high performance membranes, which will be covered in Chapter 7. Chapter 8 will discuss how the replacement of the two older chlor-alkali technology by membrane technology is continuing.
In Chapter 6 a section on redox batteries has been added. These are rechargeable batteries, in which the energy storage occurs in liquid electrolytes outside of the cell. Large quantities of energy can then be stored by providing large storage tanks for the electrolytes. This type of large scale energy storage can be used in conjunction with renewable energy sources, such as wind and solar energy, which may not be available at the time of need.
The first fluorinated ionomer was discovered in the early 1960s at the DuPont Experimental Station near Wilmington, Delaware. This perfluorinated ionomer later became known as Nafion®. At that time, an exploratory chemistry group within DuPont's Plastics Department, headed by Frank Gresham, was pursuing a newly discovered synthetic route to prepare perfluorinated vinyl ethers. This route allowed the conversion of almost any perfluorinated acyl fluoride to the corresponding vinyl ether. This conversion was achieved through the addition of hexafluoropropylene epoxide followed by dehalocarbonylation. These perfluoro vinyl ethers were promising monomers for the production of melt-fabricable copolymers of tetrafluoroethylene. Several important monomers, including perfluoro methyl-, ethyl-, and propyl-vinyl ether, became commercially available as result of this work.
One of the vinyl ethers synthesized by this method was based on the reaction product of sulfur trioxide and tetrafluoroethylene. It offered the opportunity to introduce ionic groups into a perfluorinated polymer. Initially, the motivation for this work was simply curiosity to study polymers with a broad range of compositions; but the presence of ionic groups made this polymer different from anything known before. It was shown that the reactive groups of the precursor polymer allowed vulcanization through the use of curing agents such as magnesium oxide. It was hoped, that a terpolymer of TFE (or HFP) with perfluoro methyl vinyl ether and the sulfonyl fluoride containing monomer would yield a perfluorinated elastomer of improved properties.
Another early expectation was that ionic cross-linking would result in improved mechanical properties, particularly resistance to creep, of fluoropolymers. However, none of these early approaches led to any useful products. Instead, the presence of ionic groups had an adverse effect on most of the useful properties of perfluorinated polymers such as unsurpassed dielectric properties, exceptionally low coefficient of friction, and non-stick and hydrophobic behavior. At this point, the identification of commercial uses for this polymer required thinking "outside the box".
The use of Nafion® as a separator membrane in a chlor-alkali cell was demonstrated by Grot in 1964. In 1966, Grot and Selman approached General Electric (GE) regarding the use of this polymer in fuel cells. These two applications weave like supporting threads throughout the entire development of perfluorinated ionomers. While one application was in the limelight, research on the other was taking place in the background in preparation for its gaining the ascendancy.
While initial experiments indicated that a Nafion® membrane could be used in a chlor-alkali cell, improvements in strength and in hydroxide ion rejection were necessary to meet the needs of the industrial chlor-alkali market. In addition the electrolysis cells developed for the two incumbent technologies, asbestos and mercury, were not suitable for membrane operation. As a result, the chlor-alkali industry saw little incentive in abandoning the existing technologies, which had been optimized during many decades of development, in favor of a new one, which needed significant improvement in terms of membrane performance and cell design.
Fortunately, at that time there was a critical need for high-performance fuel cell membranes in connection with the space program. The use of Nafion® in this application proved an immediate success. The sales from this program supported a small production project and allowed improvements in monomer synthesis, polymerization processes, and fabrication techniques. At the price of several thousands of dollars per square meter it attracted the attention of the management of a department involved in the sale of low-cost products such as polyethylene and Mylar® films.
In the meantime, work on the chlor-alkali application continued, facilitated by the easily available starting materials and fabricated shapes. The problem of poor mechanical strength, particularly poor resistance to tear propagation, was solved by the introduction of a reinforcing fabric made of polytetrafluoroethylene. The use of a thin barrier layer, made of a highly selective version of a perfluorinated ionomer, proved to be a powerful and versatile approach to improved hydroxide ion rejection [4–7]. Two other developments overseas had a major effect on the eventual success of Nafion® in this application:
In Europe, the introduction of titanium-based anodes, dimensional-stable anodes or DSAs, by Beer and de Nora provided long-term stability of the anode combined with lower cell voltage. The use of these DSAs was synergistic with the use of membrane technology.
In 1968 in Japan, the government concluded that the mysterious disease that had plagued a local population around Minamata Bay for over a decade was caused by the ingestion of fish and shellfish contaminated with methyl mercury. This disease, aptly named Minamata Disease, had caused 46 fatalities in 1956 and was responsible for several thousand cases of serious illness. The source of the mercury was traced back to the Chisso acetaldehyde plant. This plant used mercuric sulfate as a catalyst and then discharged the waste into the streams feeding into Minamata Bay. While it appears that the chlor-alkali industry was not involved in this environmental disaster, the Japanese government ordered the phase-out of the use of mercury in the production of chlorine and caustic soda. As a result, the Japanese industry, with substantial government support, launched a crash program to adopt the newly emerging membrane technology for the manufacture of chlorine and caustic soda.
Asahi Glass's discovery of a barrier layer containing carboxylic acid groups, which provides particularly effective rejection of hydroxyl ions was an important aspect of this work. The improvement in performance was so significant that DuPont developed its own version of a carboxylic barrier layer. Asahi Chemical was the third company to introduce a similar type of membrane. Since 1980, these three companies have offered comparable membranes for chlor-alkali applications. All of these membranes consist of a main layer of a sulfonic polymer with an imbedded fabric reinforcement, which is coated on one surface with a thin barrier layer of carboxylic polymer. The sulfonic polymers used contain fairly long linkages, made up of five or six carbons in addition to two ether groups, between the sulfonic acid group and the polymer backbone.
Excerpted from Fluorinated Ionomers by Walther Grot Copyright © 2011 by Elsevier Inc.. Excerpted by permission of William Andrew. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
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Table of Contents
1Introduction 1.1Polymers 1.2Physical Shapes 1.3References 2History 2.1References 3Manufacture 3.1Introduction 3.2Perfluorinated Ionomers 3.3Polymerization 3.4Fabrication 3.5Hydrolysis and Acid Exchange 3.6Finishing and Testing 3.7Liquid Compositions 3.8Fluorinated Ionomers with Phosphonic or Sulfonyl Imide Functional Groups 3.9Partially Fluorinated Ionomers 3.10Composite Materials of Ionomers and Inorganic Oxides 3.11Composite Materials of Ionomers and a Porous Matrix 3.12Remanufactured Membranes 3.13References 4Properties 4.1Properties of the Precursor Polymers 4.2Properties of the Ionic Forms 4.3Morphology 4.4Transport Properties 4.5Optical Properties 4.6Thermal Properties 4.7Stability 4.8References 5Applications 5.1Electrolysis 5.2Sensors and Actuators 5.3Dialysis 5.4Gas and Vapor Diffusion 5.5Protective Clothing 5.6Catalysis 5.7References 6Fuel Cells and Batteries 6.1Introduction 6.2Operating Parameters 6.3Ionomer Stability 6.4Direct Methanol Fuel Cells (DMFCs) 6.5Manufacture of MEAs 6.6Rechargeable Flow Through Batteries 6.7References 6.8Further Reading 7Commercial Membrane Types 7.1Unreinforced Perfluorinated Sulfonic Acid Films 7.2Reinforced Perfluorinated Membranes 8Economic Aspects 8.1Chlor-Alkali Cells 8.2Fuel Cells 8.3References 9Experimental Methods 9.1Infrared Spectra 9.2Hydrolysis, Surface Hydrolysis and Staining 9.3Other Reactions of the Precursor Polymer 9.4Ion Exchange Equilibrium 9.5Determination of EW by Titration or Infrared Analysis 9.6Determining Melt Flow 9.7Distinguishing the Precursor Polymer from Various Ionic Forms 9.8Fenton’s Test for Oxidative Stability 9.9Examination of a Membrane 9.10Determining the Permselectivity 9.11Measuring Pervaporation Rates 9.12Simple Electrolytic Cells 9.13References 10Heat Sealing and Repair 10.1Reference 11Handling and Storage 11.1Handling the Film 11.2Pretreatment 11.3Installation 11.4Sealing and Gasketing 12Toxicology, Safety and Disposal 12.1Toxicology 12.2Safety 12.3Disposal 12.4References Appendix AA Chromic Acid Regeneration System Appendix BLaboratory Chlor-alkali Cell Appendix CSolution Cast Nafion Film Appendix DPlastic-Based Bipolar Plates Suppliers and Resources Glossary and Web Sites Index