Release film for use in manufacturing membrane electrode assembly

Abstract

A release film 2 is for use in manufacturing a membrane electrode assembly. The release film 2 has a base material layer 4 and a release layer 6 laminated on the base material layer 4. The main material of the base material layer 4 is a polyester. The release layer 6 contains an atactic polystyrene. Preferably, the average roughness Sa of the back-side surface of the base material layer 4 is 20 nm or less. Preferably, the thickness of the release layer 6 is 0.1-5 μm. Preferably, the water contact angle on the surface of the release layer 6 is 92° or less.

Description

Release film used in the manufacture of membrane electrode assemblies

[0001] The present specification discloses a release film used in the manufacturing process of a membrane electrode assembly, which is a component of a polymer electrolyte fuel cell. The present specification further discloses a method for manufacturing the membrane electrode assembly.

[0002] A polymer electrolyte fuel cell has a membrane electrode assembly. The membrane electrode assembly includes a solid polymer electrolyte membrane and two catalyst layers. Each catalyst layer is bonded to the surface of the electrolyte membrane. Both the electrolyte membrane and the catalyst layers are ion-exchange resin layers.

[0003] In the manufacture of a membrane electrode assembly, an electrolyte membrane is formed on a first release film, and a catalyst layer is formed on a second release film. This catalyst layer is brought into contact with the electrolyte membrane. The electrolyte membrane and catalyst layer are heated and pressurized, thereby bonding the catalyst layer to the electrolyte membrane. The first release film is then peeled off from the electrolyte membrane. The first release film is required to have appropriate adhesion to the electrolyte membrane and appropriate releasability. The second release film is peeled off from the catalyst layer. The second release film is required to have appropriate adhesion to the catalyst layer and appropriate releasability.

[0004] Japanese Patent Application Laid-Open No. 2014-154273 discloses a release film having a substrate layer and a release layer. The substrate layer is made of polyethylene terephthalate. The surface of this substrate layer is smooth. Therefore, this release film can contribute to a long life of the membrane electrode assembly.

[0005] Japanese Patent Application Laid-Open No. 2014-175116 discloses a release film having a release layer made of syndiotactic polystyrene, which has excellent releasability from an ion-exchange resin layer.

[0006] JP 2014-154273 A JP 2014-175116 A

[0007] A laminate is obtained by bonding the back side of an ion exchange resin layer to the front side of a release film disclosed in JP 2014-154273 A. This laminate is then wound onto a reel. Upon winding, the front side of the ion exchange resin layer comes into contact with the back side of a release film on a different rotation. Because the base layer is smooth, the front side of the ion exchange resin layer can adhere to the back side of the release film. Due to this adhesion, when the laminate is unwound from the reel, the ion exchange resin layer is pulled by the release film on a different rotation. This can result in unintended peeling of the back side of the ion exchange resin layer from the front side of the release film on the same rotation. This phenomenon is called blocking.

[0008] The release film disclosed in JP 2014-154273 A has insufficient wettability on the front side. In particular, when an aqueous or water-rich coating liquid is used for the ion exchange resin layer, there is a strong demand for improved wettability.

[0009] The release film disclosed in JP 2014-175116 A has high solvent resistance because the release layer is made of syndiotactic polystyrene. This release film presents difficulties when applying a coating solution for the ion exchange resin layer. If the release layer is made of low-molecular-weight syndiotactic polystyrene, coating is possible. However, the use of this polystyrene can cause defects in the release film, such as curling and fisheyes.

[0010] The release film disclosed in JP 2014-175116 A has insufficient adhesion to the ion-exchange resin layer, and therefore, unintended peeling of the ion-exchange resin layer from the release film may occur during the manufacturing process of the membrane electrode assembly.

[0011] The present applicant intends to provide a release film that is less susceptible to unintended peeling of the ion exchange resin layer, has excellent wettability, and can further contribute to the productivity of membrane electrode assemblies.

[0012] The release film disclosed herein can be used in the production of a membrane electrode assembly. The release film has a substrate layer and a release layer laminated on the substrate layer. The substrate layer is primarily made of polyester. The release layer contains atactic polystyrene.

[0013] This release film can prevent unintended peeling of the ion-exchange resin layer, has excellent wettability, and can contribute to the productivity of membrane electrode assemblies.

[0014] Fig. 1 is a cross-sectional view showing a part of a release film according to one embodiment. Fig. 2 is a flow chart showing a method for manufacturing a membrane electrode assembly using the release film of Fig. 1. Fig. 3 is a schematic view showing an example of a process of the manufacturing method of Fig. 2.

[0015] Preferred embodiments will be described below with reference to the drawings as appropriate. Each configuration and combination thereof in each embodiment is an example. Modifications such as omission or substitution of configurations in the embodiments are possible, and other configurations can also be added to the embodiments, without departing from the spirit of the present disclosure. The scope of the disclosure of this specification should not be interpreted as being limited by the embodiments. Each aspect disclosed in this specification can be combined with any other feature disclosed in this specification.

[0016] [Release Film] Figure 1 shows a release film 2. This release film 2 has a base layer 4 and a release layer 6. The release layer 6 is laminated on the base layer 4. The base layer 4 has a back side 8 and a front side 10. The release layer 6 has a back side 12 and a front side 14. The release film 2 may have another layer located between the base layer 4 and the release layer 6.

[0017] [Substrate Layer] A typical material for the substrate layer 4 is polyester. As described below, the release film 2 is used in the manufacturing process of a membrane electrode assembly. In this manufacturing process, the temperature of the release film 2 rises. Furthermore, tension is applied to this release film 2 in the manufacturing process. The substrate layer 4 made of polyester is unlikely to stretch even when tension is applied in a high-temperature environment. This substrate layer 4 can suppress the stretching of the release film 2 in the manufacturing process of the membrane electrode assembly. The release film 2 that is unlikely to stretch can suppress unintended peeling of the electrolyte membrane or catalyst layer, which will be described below, in the manufacturing process of the membrane electrode assembly.

[0018] The base layer 4 may contain a combination of polyester and other resins. In this case, the ratio of polyester to the total amount of resins is 50% by mass or more. In other words, the main material of the base layer 4 is polyester. The ratio of polyester is more preferably 70% by mass or more, and particularly preferably 80% by mass or more. This ratio may be 100% by mass. The material of the base layer 4 may be a composition containing a resin and an additive.

[0019] Preferred polyesters include polyethylene terephthalate, polytrimethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, and polybutylene naphthalate, with polyethylene terephthalate and polyethylene naphthalate being more preferred, and polyethylene terephthalate being particularly preferred.

[0020] A preferred base material layer 4 from the viewpoint of suppressing elongation of the release film 2 is a stretched film. A uniaxially stretched film and a biaxially stretched film can be used as the base material layer 4. From the viewpoint of strength, a biaxially stretched film is preferred. The longitudinal stretching ratio and the transverse stretching ratio of the biaxially stretched film are each preferably 1.5 or more, more preferably 2.5 or more, and particularly preferably 3.0 or more.

[0021] The average roughness Sa of the substrate layer 4 is preferably 20 nm or less. In other words, the surface of the substrate layer 4 has excellent smoothness. As described below, the release layer 6 is thin. Therefore, the surface state of this release layer 6 can reflect the surface state of the substrate layer 4. A substrate layer 4 with excellent surface smoothness can achieve a release layer 6 with excellent surface smoothness. As described below, this release layer 6 can contribute to a long life of the membrane electrode assembly. From the viewpoint of a long life of the membrane electrode assembly, the average roughness Sa of the substrate layer 4 is more preferably 15 nm or less, and particularly preferably 13 nm or less. The lower limit of the average roughness Sa that can be achieved is 1 nm.

[0022] As shown in FIG. 1 , the front surface 10 of the base material layer 4 is covered with the release layer 6. Therefore, the average roughness Sa is measured on the back surface 8. A suitable device for the measurement is a non-contact surface shape measurement system ("VertScan 2.0", Ryoka Systems Co., Ltd.). The measurement conditions are as follows: Mode: WAVE Objective lens magnification: 5x 0.5x Tube lens

[0023] In FIG. 1 , an arrow T1 indicates the thickness of the base material layer 4. The thickness T1 is preferably 1 μm or more and 300 μm or less. A base material layer 4 having a thickness T1 of 1 μm or more can suppress elongation of the release film 2 during the manufacturing process of the membrane electrode assembly. From this viewpoint, the thickness T1 is more preferably 10 μm or more, and particularly preferably 20 μm or more. A base material layer 4 having a thickness T1 of 300 μm or less does not inhibit the productivity of the membrane electrode assembly. From this viewpoint, the thickness T1 is more preferably 200 μm or less, and particularly preferably 100 μm or less.

[0024] [Release Layer] The release layer 6 is a resin film. A suitable resin for the release layer 6 is atactic polystyrene. As will be described later, in the manufacturing process of a membrane electrode assembly, an electrolyte membrane is formed on the release film 2. The release film 2 must have adhesion to this electrolyte membrane. In the manufacturing process of a membrane electrode assembly, a catalyst layer is also formed on the release film 2. The release film 2 must have adhesion to the catalyst layer. The release layer 6 is in direct contact with the electrolyte membrane or the catalyst layer. The release layer 6, whose material is atactic polystyrene, can contribute to adhesion to the electrolyte membrane and the catalyst layer.

[0025] Atactic polystyrene is amorphous. Atactic polystyrene resin can be obtained by radical polymerization of aromatic vinyl monomers. Examples of aromatic vinyl monomers include styrene; alkyl-substituted styrenes such as o-methylstyrene, m-methylstyrene, p-methylstyrene, vinylxylene, p-ethylstyrene, p-isopropylstyrene, butylstyrene, p-t-butylstyrene, and 2,4-dimethylstyrene; alkoxy-substituted styrenes such as methoxystyrene and ethoxystyrene; halogen-substituted styrenes such as o-chlorostyrene, m-chlorostyrene, p-chlorostyrene, o-bromostyrene, and o-fluorostyrene; α-alkyl-substituted styrenes; aryl-substituted styrenes; and vinylnaphthalene. Atactic polystyrene may be obtained from two or more monomers. The release layer 6 may contain two or more types of atactic polystyrene.

[0026] From the viewpoint of heat resistance of the release layer 6, the weight average molecular weight of the atactic polystyrene is preferably 10,000 or more, more preferably 100,000 or more, and particularly preferably 150,000 or more. From the viewpoint of ease of production, the weight average molecular weight is preferably 1,000,000 or less, more preferably 500,000 or less, and particularly preferably 350,000 or less. The weight average molecular weight is measured by gel permeation chromatography in terms of polystyrene.

[0027] The material of the release layer 6 may be a composition containing atactic polystyrene and additives. The material of the release layer 6 may be a composition containing atactic polystyrene and other resins. The material of the release layer 6 may be a composition containing atactic polystyrene, other resins, and additives. In either composition, the ratio of atactic polystyrene to the total amount of resin is preferably 50% by mass or more. In other words, it is preferable that the main material of the release layer 6 is atactic polystyrene. The ratio of atactic polystyrene is more preferably 70% by mass or more, and particularly preferably 80% by mass or more. This ratio may be 100% by mass.

[0028] Examples of additives that can be contained in the composition of the release layer 6 include fillers, lubricants such as waxes, fatty acid esters, and fatty acid amides, antistatic agents, stabilizers such as antioxidants, heat stabilizers, and light stabilizers, flame retardants, viscosity modifiers, thickeners, and antifoaming agents. The composition may also contain organic or inorganic particles.

[0029] The water contact angle of the surface of the release layer 6 is preferably 92° or less. A release film 2 with a water contact angle of 92° or less has excellent adhesion to the ion exchange resin layer. From this perspective, the water contact angle is more preferably 90° or less, and particularly preferably 89° or less. As shown in FIG. 1 , the back surface 12 of the release layer 6 overlaps with the base layer 4. Therefore, the water contact angle is measured at the front surface 14. Atactic polystyrene can contribute to the small water contact angle.

[0030] In FIG. 1 , an arrow T2 indicates the thickness of the release layer 6. The thickness T2 is preferably 0.1 μm or more and 20 μm or less. A release layer 6 having a thickness T2 of 0.1 μm or more can be sufficiently adhered to the base material layer 4. From this viewpoint, the thickness T2 is more preferably 0.3 μm or more, and particularly preferably 0.5 μm or more. A release layer 6 having a thickness T2 of 20 μm or less does not inhibit the productivity of the membrane electrode assembly. From this viewpoint, the thickness T2 is more preferably 10 μm or less, and particularly preferably 5 μm or less.

[0031] [Thickness Ratio] The ratio (T2 / T1) of the thickness T2 of the release layer 6 to the thickness T1 of the base layer 4 is preferably 5 / 1 or more and 1 / 10 or less, more preferably 3 / 1 or more and 1 / 5 or less, and particularly preferably 2 / 1 or more and 1 / 3 or less.

[0032] [Method for producing release film] Examples of methods for laminating the release layer 6 onto the base layer 4 include coating, coextrusion, extrusion lamination, pressure bonding, and adhesive methods. From the viewpoint of smoothness of the release layer 6, coating is preferred. In the coating method, a solution or dispersion containing atactic polystyrene is coated onto the base layer 4. Examples of solvents for the solution and dispersion include aromatic hydrocarbons such as benzene, toluene, and xylene; alicyclic hydrocarbons such as cyclohexane, cyclohexanone, and cyclohexene; halogenated hydrocarbons such as dichloromethane and dichloroethane; cyclic amides such as N-methyl-2-pyrrolidone; and cyclic ethers such as dioxane. Two or more solvents may be used in combination.

[0033] Preferred coating methods include a roll coater method, an air knife coater method, a blade coater method, a rod coater method, a reverse coater method, a bar coater method, a comma coater method, a die coater method, a gravure coater method, a screen coater method, a spray method, and a spinner method. From the viewpoint of versatility, the blade coater method and the bar coater method are preferred.

[0034] A coating film is obtained by the coating method. The coating film is dried to obtain the release layer 6. The release layer 6 is in close contact with the substrate layer 4.

[0035] A release layer 6 may be formed on a surface different from that of the base layer 4 by a coating method or the like. This release layer 6 is laminated to the base layer 4 via an adhesive layer. The adhesive layer may be formed from an adhesive or a pressure-sensitive adhesive. Examples of adhesives include urethane-based adhesives, acrylic-based adhesives, polyester-based adhesives, and polyamide-based adhesives. Examples of pressure-sensitive adhesives include rubber-based adhesives, acrylic-based adhesives, olefin-based adhesives, and silicone-based adhesives. The thickness of the adhesive layer is preferably 1 μm or more and 40 μm or less.

[0036] [Method for Manufacturing a Membrane Electrode Assembly] An example of a method for manufacturing a membrane electrode assembly is shown in Figure 2. This manufacturing method includes forming an electrolyte membrane (STEP 1), forming a front catalyst layer (STEP 2), bonding (STEP 3), peeling (STEP 4), forming a back catalyst layer (STEP 5), bonding (STEP 6), and peeling (STEP 7).

[0037] In the formation of the electrolyte membrane (STEP 1), first, the resin of this electrolyte membrane is dissolved in a solvent to obtain a first coating liquid. Preferred examples of the solvent include water; alcohols such as methanol, ethanol, isopropanol, and 1-butanol; ketones such as acetone and methyl ethyl ketone; ethers such as dioxane and tetrahydrofuran; and sulfoxides such as dimethyl sulfoxide.

[0038] This first coating liquid is coated onto the release layer 6 of the release film 2 (first release film) shown in Figure 1. Preferred coating methods include a roll coater method, an air knife coater method, a blade coater method, a rod coater method, a reverse coater method, a bar coater method, a comma coater method, a die coater method, a gravure coater method, a screen coater method, a spray method, and a spinner method. A coating film is obtained by coating.

[0039] This coating film is heated. The heating temperature is preferably 80°C or higher and 200°C or lower, and particularly preferably 100°C or higher and 150°C or lower. The coating film dries by heating, and the electrolyte membrane 16 shown in FIG. 3 is obtained. Drying may be performed at room temperature. This electrolyte membrane 16 is in close contact with the first release film 2a. The back side 18 of the electrolyte membrane 16 abuts against the first release film 2a. The front side 20 of the electrolyte membrane 16 is exposed.

[0040] The first release film 2a and the electrolyte membrane 16 constitute a first laminate 22a. This first laminate 22a is wound around a reel. A first roll is formed by the winding. This first roll has multiple turns of the first laminate 22a. In this first roll, different turns of the first release film 2a abut against the front side surface 20 of the electrolyte membrane 16. The first roll is stored for a predetermined period of time.

[0041] In forming the front catalyst layer (STEP 2), first, the resin of this catalyst layer is dissolved in a solvent to obtain a second coating liquid. Preferred solvents are the same as those described above for the first coating liquid. This second coating liquid is coated onto the release layer 6 of the second release film 2b (see FIG. 1). In this embodiment, the specifications of the second release film 2b are the same as those of the first release film 2a. Preferred coating methods are the same as those described above for the first coating liquid. A coating film is obtained by coating.

[0042] This coating film is heated. The heating temperature is preferably 50°C or higher and 150°C or lower, and particularly preferably 60°C or higher and 120°C or lower. The coating film dries by heating, and the front catalyst layer 24 shown in FIG. 3 is obtained. Drying may be performed at room temperature. This front catalyst layer 24 is in close contact with the second release film 2b. The front side surface 26 of the front catalyst layer 24 abuts against the second release film 2b. The back side surface 28 of the front catalyst layer 24 is exposed.

[0043] The second release film 2b and the front catalyst layer 24 constitute a second laminate 22b. This second laminate 22b is wound onto another reel. A second roll is formed by the winding. This second roll has multiple turns of the second laminate 22b. In this second roll, the second release film 2b with a different turn abuts against the back surface 28 of the front catalyst layer 24. The second roll is stored for a predetermined period of time.

[0044] In the bonding step (STEP 3), the first laminate 22a is unwound from the first roll. Because the first release film 2a has excellent adhesion to the electrolyte membrane 16, blocking can be suppressed during unwinding even if the surface of the first release film 2a is smooth. In other words, the electrolyte membrane 16 does not adhere to the first release film 2a on a different rotation. In the bonding step (STEP 3), the second laminate 22b is further unwound from the second roll. Because the second release film 2b has excellent adhesion to the front catalyst layer 24, blocking can be suppressed during unwinding even if the surface of the second release film 2b is smooth. In other words, the front catalyst layer 24 does not adhere to the second release film 2b on a different rotation.

[0045] The back surface 28 of the front catalyst layer 24 of this second laminate 22b is mated with the front surface 20 of the electrolyte membrane 16 of the first laminate 22a. This results in a laminate including the first release film 2a, the electrolyte membrane 16, the front catalyst layer 24, and the second release film 2b. This laminate is also referred to as an intermediate laminate.

[0046] This intermediate laminate is heated and pressurized. The heating temperature is preferably 80°C or higher and 200°C or lower, more preferably 100°C or higher and 180°C or lower, and particularly preferably 110°C or higher and 150°C or lower. The pressing pressure is preferably 0.1 MPa or higher and 20 MPa or lower, more preferably 0.5 MPa or higher and 15 MPa or lower, and particularly preferably 1 MPa or higher and 10 MPa or lower. Heating and pressurization firmly adhere the front catalyst layer 24 to the electrolyte membrane 16. As described above, each release film 2 has a flexible and non-stretchable substrate layer 4. Furthermore, this release film 2 includes a release layer 6 that has adequate adhesion to the ion-exchange resin layer. Therefore, unintended peeling of the first release film 2a from the electrolyte membrane 16 can be suppressed during the heating and pressurization process. Furthermore, unintended peeling of the second release film 2b from the front catalyst layer 24 can be suppressed.

[0047] In peeling (STEP 4), the first release film 2a is peeled off from the electrolyte membrane 16. The peeling exposes the back side surface 18 of the electrolyte membrane 16. As described above, the surface of the release layer 6 is smooth. Therefore, the back side surface 18 of the electrolyte membrane 16 can be made smooth.

[0048] In forming the backside catalyst layer (STEP 5), the resin of this catalyst layer is first dissolved in a solvent to obtain a third coating liquid. Preferred solvents are the same as those described above for the first coating liquid. This third coating liquid is coated onto the release layer 6 of the third release film 2c (see FIG. 1). In this embodiment, the specifications of the third release film 2c are the same as those of the first release film 2a. The preferred coating method is the same as the coating method described above for the first coating liquid. A coating film is obtained by coating.

[0049] This coating film is heated. The preferred heating temperature is the same as that described above for the second coating liquid. The coating film dries by heating, resulting in the back-side catalyst layer 30 shown in FIG. 3. Drying may be performed at room temperature. This back-side catalyst layer 30 is in close contact with the third release film 2c. The back side surface 32 of the back-side catalyst layer 30 abuts against the third release film 2c. The front side surface 34 of the back-side catalyst layer 30 is exposed.

[0050] The third release film 2c and the back-side catalyst layer 30 constitute a third laminate 22c. This third laminate 22c is further wound onto another reel. By winding, a third roll is formed. This third roll has multiple turns of the third laminate 22c. In this third roll, the front surface 34 of the back-side catalyst layer 30 abuts against a third release film 2c of a different turn. The third roll is stored for a predetermined period of time.

[0051] In the bonding step (STEP 6), the third laminate 22c is unwound from the third roll. Because the third release film 2c has excellent adhesion to the catalyst layer, blocking during unwinding can be suppressed even if the surface of the third release film 2c is smooth. In other words, the back-side catalyst layer 30 does not adhere to the third release film 2c on a different circumference.

[0052] The front surface 34 of the back-side catalyst layer 30 of this third laminate 22c is mated with the back surface 18 of the electrolyte membrane 16. This results in a laminate including the third release film 2c, the back-side catalyst layer 30, the electrolyte membrane 16, the front-side catalyst layer 24, and the second release film 2b. This laminate is heated and pressurized. The heating temperature is preferably 80°C or higher and 200°C or lower, more preferably 100°C or higher and 180°C or lower, and particularly preferably 110°C or higher and 150°C or lower. The pressurizing pressure is preferably 0.1 MPa or higher and 20 MPa or lower, more preferably 0.5 MPa or higher and 15 MPa or lower, and particularly preferably 1 MPa or higher and 10 MPa or lower. Heating and pressurization firmly adhere the back-side catalyst layer 30 to the electrolyte membrane 16. As described above, each release film 2 has a flexible and non-stretchable substrate layer 4. Furthermore, this release film 2 includes a release layer 6 that has adequate adhesion to the ion-exchange resin layer. Therefore, in the heating and pressurizing process, unintended peeling of the second release film 2b from the front-side catalyst layer 24 can be suppressed. Furthermore, unintended peeling of the third release film 2c from the back-side catalyst layer 30 can be suppressed.

[0053] In peeling (STEP 7), the second release film 2b is peeled off from the front-side catalyst layer 24, and the third release film 2c is peeled off from the back-side catalyst layer 30. This peeling process results in a laminate consisting of the back-side catalyst layer 30, the electrolyte membrane 16, and the front-side catalyst layer 24. A gas supply layer and other layers are further laminated on this laminate to obtain a membrane electrode assembly. As described above, the back surface 18 of the electrolyte membrane 16 is smooth. Therefore, the back-side catalyst layer 30 is firmly bonded to the electrolyte membrane 16. This bonding contributes to the long life of the membrane electrode assembly.

[0054] The specifications of the second release film 2b may be different from those of the first release film 2a. The specifications of the third release film 2c may be different from those of the first release film 2a.

[0055] The effects of the examples will be explained below, but the scope of the present disclosure should not be construed as being limited based on the description of these examples.

[0056] Example 1 A coating solution was obtained by dissolving 10% by mass of atactic polystyrene ("G100C", Toyo Styrene Co., Ltd.) in 90% by mass of toluene. On the other hand, a polyester film with an easy-adhesion layer ("Cosmoshine (registered trademark) A4100", Toyobo Co., Ltd.) was prepared as a substrate layer. The polyester film had a thickness of 50 μm and a surface roughness Sa of 6 nm. The coating solution was applied to the surface of the easy-adhesion layer of the polyester film using a wire bar #10 to obtain a coating film. This coating film was held at a temperature of 100° C. for 1 minute and dried to obtain the release film of Example 1.

[0057] [Example 2] A release film of Example 2 was obtained in the same manner as in Example 1, except that a polyester film (Lumirror (registered trademark) T60, Toray Industries, Inc.) was used as the base layer. The polyester film had a thickness of 75 μm and a surface roughness Sa of 13 nm.

[0058] [Example 3] A release film of Example 3 was obtained in the same manner as in Example 1, except that 10% by mass of atactic polystyrene ("G200C", Toyo Styrene Co., Ltd.) was dissolved in 90% by mass of toluene to obtain a coating liquid.

[0059] [Example 4] A release film of Example 4 was obtained in the same manner as in Example 1, except that 10% by mass of atactic polystyrene ("G320C", Toyo Styrene Co., Ltd.) was dissolved in 90% by mass of toluene to obtain a coating liquid.

[0060] [Example 5] A release film of Example 5 was obtained in the same manner as in Example 1, except that 10% by mass of atactic polystyrene ("H350", Toyo Styrene Co., Ltd.) was dissolved in 90% by mass of toluene to obtain a coating liquid.

[0061] [Example 6] A release film of Example 6 was obtained in the same manner as in Example 1, except that 10% by mass of atactic polystyrene ("H485", Toyo Styrene Co., Ltd.) was dissolved in 90% by mass of toluene to obtain a coating liquid.

[0062] Comparative Example 1 A release film of Comparative Example 1 was obtained in the same manner as in Example 1, except that 10% by mass of a cyclic olefin resin ("TOPAS-6015", Polyplastics Co., Ltd.) was dissolved in 90% by mass of toluene to obtain a coating liquid.

[0063] Comparative Example 2 A release film of Comparative Example 2 was obtained in the same manner as in Example 1, except that 10% by mass of a cyclic olefin resin ("ZEONEX 480R", Zeon Corporation) was dissolved in 90% by mass of toluene to obtain a coating liquid.

[0064] Comparative Example 3 A release film of Comparative Example 3 was obtained in the same manner as in Example 1, except that 10% by mass of a cyclic olefin resin ("ARTONF3500", JSR Corporation) was dissolved in 90% by mass of toluene to obtain a coating liquid.

[0065] Comparative Example 4 A release film of Comparative Example 4 was obtained in the same manner as in Example 1, except that a coating liquid was obtained by dissolving 10% by mass of a polyarylate resin ("UNIFINER M-2040", Unitika Ltd.) in a mixed liquid of 70% by mass of toluene and 20% by mass of methyl ethyl ketone.

[0066] Comparative Example 5 A release film of Comparative Example 5 was obtained in the same manner as in Comparative Example 1, except that a polyester film (Lumirror (registered trademark) T60, Toray Industries, Inc.) was used as the base layer. This polyester film had a thickness of 75 μm and a surface roughness Sa of 12 nm.

[0067] [Comparative Example 6] A release film of Comparative Example 6 was obtained in the same manner as in Comparative Example 1, except that a polyester film (Diafoil (registered trademark) T100, Mitsubishi Chemical Corporation) was used as the base layer. This polyester film had a thickness of 75 μm and a surface roughness Sa of 36 nm.

[0068] [Comparative Example 7] A commercially available resin film ("HN-200", Kurabo Industries, Ltd.) was prepared as a release film for Comparative Example 7. The material of this resin film was syndiotactic polystyrene. The thickness of this resin film was 35 μm, and the surface roughness Sa was 72 nm.

[0069] [Blocking] An ion exchange resin solution (Nafion (registered trademark) 20 dispersion Solution DE2020CS type, DuPont) was prepared as a coating liquid. This coating liquid was applied onto the release layer of a release film to obtain a coating film. This coating film was maintained at a temperature of 100°C for 3 minutes to obtain a laminate of a release film and an ion exchange resin layer. The thickness of the ion exchange resin layer was 10 µm. Two test pieces were cut out from this laminate. Each test piece had a size of 10 cm x 15 cm. The release film of one test piece was brought into contact with the ion exchange resin layer of the other test piece. A roller with a load of 2 kgf was passed back and forth over these test pieces twice to apply pressure. These test pieces were maintained in an environment of 23°C and 65% Rh for 24 hours. One test piece was peeled from the other test piece in 0.5 to 1.0 seconds. The condition of one test piece was visually observed and ranked based on the following criteria: A: The ion exchange resin layer was significantly peeled off from the release film. B: The ion exchange resin layer was slightly peeled off from the release film. C: The ion exchange resin layer was not peeled off at all from the release film. The results are shown in Tables 1 and 2 below.

[0070] [Peel Force] An ion exchange resin solution (Nafion (registered trademark) DS2020CS, DuPont) was prepared as a coating liquid. The solids content of this ion exchange resin solution was 21% by mass ± 1% by mass. This coating liquid was applied to a release film using a Mayer bar to obtain a coating film. This coating film was held at a temperature of 100°C for 3 minutes and dried. This coating film was further subjected to a heat treatment by holding at a temperature of 160°C for 30 minutes to obtain a laminate of a release film and an ion exchange resin layer. The thickness of the ion exchange resin layer was 4 μm. A test piece was cut out from this laminate. The ion exchange resin layer was peeled from the release film in this test piece, and the peel force was measured. The conditions were as follows: Apparatus: Autograph "AGS-X", Shimadzu Corporation Test piece width: 25 mm Angle: 180° Speed: 600 mm / min The results are shown in Tables 1 and 2 below. In the release films of Comparative Examples 3 and 4, the ion exchange resin layer was not peeled off.

[0071] [Handling] Test pieces identical to those used to measure peel strength were prepared. These test pieces were subjected to a heat treatment. The surfaces of these test pieces were visually observed and ranked based on the following criteria: A: Creases were observed. B: Creases were slightly observed. C: Creases were not observed. The results are shown in Tables 1 and 2 below.

[0072]

[0073]

[0074] As is clear from Tables 1 and 2, the release films of the examples are excellent in various performances. The superiority of this release film is clear from these evaluation results.

[0075] Disclosed Items Each of the following items discloses a preferred embodiment.

[0076] [Item 1] A release film used in the production of a membrane electrode assembly, comprising a base layer and a release layer laminated on the base layer, wherein the base layer is mainly made of polyester, and the release layer contains atactic polystyrene.

[0077] [Item 2] The release film according to item 1, wherein the surface of the release layer has a water contact angle of 92° or less.

[0078] [Item 3] The release film according to item 1 or 2, wherein the thickness of the release layer is 0.1 μm or more and 5 μm or less.

[0079] [Item 4] The release film according to any one of items 1 to 3, wherein the average roughness Sa of the back surface of the base material layer is 20 nm or less.

[0080] [Item 5] A method for producing a membrane electrode assembly, comprising: (1) a step of forming an electrolyte membrane on a surface of a first release film, the first release film including a base layer and a release layer laminated on the base layer, the base layer being mainly made of polyester and the release layer including atactic polystyrene, to obtain a first laminate; (2) a step of winding up the first laminate to obtain a roll; (3) a step of forming a catalyst layer on the surface of a second release film to obtain a second laminate; (4) a step of unwinding the first laminate from the roll and bonding the catalyst layer to the electrolyte membrane to obtain an intermediate laminate; and (5) a step of heating and pressurizing the intermediate laminate.

[0081] [Item 6] In the step (1), the electrolyte membrane is formed on the surface of the first release film, which has an average roughness Sa of the back side of 20 nm or less and a release layer thickness of 0.1 μm or more and 5 μm or less. The manufacturing method according to item 5.

[0082] [Item 7] A laminate comprising: (1) a release film having a base layer and a release layer laminated on the base layer, the base layer being mainly made of polyester and the release layer containing atactic polystyrene; and (2) an electrolyte membrane laminated on the release film.

[0083] The release film described above is suitable for producing various molded articles made of ion exchange resins.

[0084] DESCRIPTION OF THE REFERENCE NUMERALS 2...Release film 2a...First release film 2b...Second release film 2c...Third release film 4...Substrate layer 6...Release layer 8...Rear side of substrate layer 10...Front side of substrate layer 12...Rear side of release film 14...Front side of release film 16...Electrolyte membrane 18...Rear side of electrolyte membrane 20...Front side of electrolyte membrane 22a...First laminate 22b...Second laminate 22c...Third laminate 24...Front catalyst layer 26...Front side of front catalyst layer 28...Rear side of front catalyst layer 30...Rear catalyst layer 32...Rear side of rear catalyst layer 34...Front side of rear catalyst layer

Claims

1. A release film for use in the manufacture of a membrane electrode assembly, comprising a base layer and a release layer laminated on the base layer, the base layer being mainly made of polyester, and the release layer containing atactic polystyrene.

2. The release film according to claim 1, wherein the surface of said release layer has a water contact angle of 92° or less.

3. The release film according to claim 1 or 2, wherein the thickness of the release layer is from 0.1 μm to 5 μm.

4. The release film according to claim 1 or 2, wherein the average roughness Sa of the back surface of the base layer is 20 nm or less.

5. A method for manufacturing a membrane electrode assembly, comprising: (1) a step of forming an electrolyte membrane on the surface of a first release film, the first release film having a base layer and a release layer laminated on the base layer, the main material of the base layer being polyester and the release layer containing atactic polystyrene, to obtain a first laminate; (2) a step of winding up the first laminate to obtain a roll; (3) a step of forming a catalyst layer on the surface of a second release film to obtain a second laminate; (4) a step of unwinding the first laminate from the roll, bonding the catalyst layer to the electrolyte membrane, and obtaining an intermediate laminate; and (5) a step of heating and pressurizing the intermediate laminate.

6. The manufacturing method described in claim 5, wherein in the step (1), the electrolyte membrane is formed on the surface of the first release film, the surface of which has an average roughness Sa of 20 nm or less and a release layer having a thickness of 0.1 μm or more and 5 μm or less.

7. A laminate comprising: (1) a base layer and a release layer laminated on the base layer, the base layer being mainly made of polyester and the release layer containing atactic polystyrene; and (2) an electrolyte membrane laminated on the release film.

Images