Polymer electrolyte fuel cells (or solid polymer fuel cells) have a basic component called a membrane electrode assembly (MEA). The MEA is obtainable by laminating an electrode membrane (a catalyst layer or an electrode catalyst membrane) on either side of a polymer electrolyte membrane and putting the resulting laminate between a fuel gas supply layer and an air supply layer; the electrode membrane contains, as a main component, a platinum group metal catalyst supported on a carbon powder, the polymer electrolyte membrane is an ion exchange membrane, and the fuel gas supply layer and the air supply layer are an electroconductive porous membrane. For the MEA, both electrolyte membrane and electrode membrane contain an ion exchange polymer. The electrolyte membrane and the electrode membrane are usually formed by a casting method and/or a coating method, or other methods. A method of laminating the electrolyte membrane and the electrode membrane usually comprises allowing an electrolyte membrane formed on a support to closely contact with an electrode membrane formed on a support, thermocompression bonding these membranes at about 130 to 150° C. (about 150 to 200° C. according to a material used) and a pressure of about 1 to 10 MPa, and then separating the supports from these membranes. Thus, as the support, a release film is used. The release film requires a moderate (or reasonable) releasability (peelability) from the electrolyte membrane and the electrode membrane and a moderate (or reasonable) adhesion to the electrolyte membrane and the electrode membrane. The ion exchange polymer contained in the electrolyte membrane and the electrode membrane has a unique structure that has a main chain being a highly releasable fluoropolymer and a side chain containing a poorly releasable sulfonic acid group. This makes it difficult to predict the releasability behavior and to balance the releasability and the adhesion. The release film is practically laminated on a substrate film having high mechanical characteristics in order to achieve improved handling or improved production. Unfortunately, it is difficult to improve the adhesion of the release film to a commonly-used substrate film having no reactive groups or other groups. The release film for the production of a fuel cell further requires heat resistance for reasons of production process. The release film, which is produced by a roll-to-roll processing in terms of efficient production, also requires flexibility. As the release film, a fluorine-containing film is widely used. The fluorine-containing film has excellent heat resistance, releasability, and stain resistance, while the film is expensive and less combustible in waste incineration after use, and easily generates a poisonous gas. The film, which has a low elastic modulus, is difficult to produce by a roll-to-roll processing. In place of the fluorine-containing film, a release film containing a cyclic olefin polymer is also reported.
Japanese Patent Application Laid-Open Publication No. 2010-234570 (JP-2010-234570A, Patent Document 1) discloses a release film made of a cycloolefin copolymer and also describes a release film formed by coating a solution of a cycloolefin copolymer on a substrate film such as a poly(ethylene terephthalate) film. In working examples of the document, a solution containing a copolymer of ethylene and norbornene is cast on a poly(ethylene terephthalate) film using a flow-casting apparatus to form a release film having a thickness of 0.5 μm.
This release film has an excellent releasability from an electrolyte membrane or an electrode membrane containing an ion exchange polymer, while the release film has a low adhesion to a poly(ethylene terephthalate) film and is easily separated from the substrate film. This reduces the handleability of the release film or the production of the MEA. Further, the releasability of the release film from the electrolyte membrane or the electrode membrane is insufficient for a high-temperature production process of the MEA.
Japanese Patent Application Laid-Open Publication No. 2009-102558 (JP-2009-102558A, Patent Document 2) discloses a laminated film stacked on a plastic substrate. The laminated film has a first layer composed of a chlorine-containing polymer, and a second layer laminated on the first layer and composed of a cyclic olefin polymer. Each of the first layer and the second layer is formed by coating. This document describes that the laminated film is utilizable for an industrial material such as an industrial release film, a packaging film for food, pharmaceutical, chemical, or other products, and an optical member such as a polarization plate for liquid crystal. The document also describes a copolymer of a cyclic olefin and a chain olefin as a preferred cyclic olefin polymer.
However, this document is silent on a fuel cell. Further, the complicated structure of the laminated film would reduce fuel cell production or film handleability if the film is used as a release film for producing a fuel cell. This document fails to disclose the reason why a copolymer of a cyclic olefin and a chain olefin is preferred among cyclic olefin polymers.
Japanese Patent Application Laid-Open Publication No. 2000-95957 (JP-2000-95957A, Patent Document 3) discloses a reversible thermotropic molding material having a first component and a second component; the first component consists of a transparent plastic material, and the second component consists of non-liquid crystal plastic material which is thermodynamically immiscible with the first component and is different in temperature dependency of refraction index from the first component. This document describes that an example of the first component includes a cycloolefin and that an example of a monomer constituting the second component includes vinylidene chloride. In working examples of this document, a molding material having a copolyamide in combination with a terpolymer containing a styrenic monomer or other monomers is prepared. The document also discloses that the applications of the molding material include reversible thermotropic plastic molding materials such as shielding systems for regulating the amount of light and temperature, for example, the glazing of buildings, greenhouses, cars, solar collecting systems and the like.
This document is also silent on a fuel cell. Further, this document fails to describe the meanings of combination of a cyclic olefin polymer and a chlorine-containing polymer.