Fiber sheet, method for producing fiber sheet, polymer electrolyte membrane provided with fiber sheet, fuel cell, and water electrolysis hydrogen generator

Abstract

The purpose of the present invention is to realize a fiber sheet with which a polyazole-based resin contained in constituent fibers can be prevented from being excessively oxidized, and which can be used for various industrial applications due to excellent elongation.　This fiber sheet has an O / C ratio measured by ESCA analysis of less than 0.10, preventing the polyazole-based resin contained in the constituent fibers from being excessively oxidized, and has an average breaking elongation (y) per unit weight of greater than 6.79%, making the fiber sheet easy to stretch and have excellent elongation. Therefore, the fiber sheet according to the present invention can be used in various industrial applications.

Description

Fiber sheet and its manufacturing method, polymer electrolyte membrane and fuel cell equipped with said fiber sheet, and water electrolysis hydrogen generator

[0001] The present invention relates to a fiber sheet containing fibers containing a polyazole resin as constituent fibers that can support a polymer electrolyte to form a polymer electrolyte membrane, a method for producing the fiber sheet, a polymer electrolyte membrane including the fiber sheet, and a fuel cell and an electrolytic hydrogen generation device including the same.

[0002] Polyazole resins such as polybenzimidazole are known to have excellent heat resistance. Therefore, fiber sheets (hereinafter sometimes abbreviated as fiber sheets) containing fibers containing polyazole resins as constituent fibers are used in various industrial applications, as exemplified in JP 2014-234581 A (Patent Document 1). Specifically, they are used as supports for membranes that can be used in various industrial applications, such as gas separation membranes, liquid separation membranes such as water treatment membranes, medical materials, ion exchange membranes, dialysis membranes, redox flow batteries, fuel cells, and polymer electrolyte membranes used in water electrolysis hydrogen generators. They are also used in various industrial applications, such as separators for electrochemical elements such as capacitors and primary / secondary batteries, prepregs, gas filters, and liquid filters.

[0003] Furthermore, as disclosed in Japanese Patent No. 6924742 (Patent Document 2), it is known that the mechanical strength of a fiber sheet can be improved by heat treating the fiber sheet at a temperature ranging from 50°C lower than the glass transition temperature of the polyazole resin contained in the constituent fibers of the fiber sheet to 20°C higher than the glass transition temperature. As a specific example, Patent Document 2 discloses that a nonwoven fabric made of polybenzimidazole having a glass transition temperature of 427°C and a large weight-average molecular weight (weight-average molecular weight of 180,000 or more) was heat-treated at 400°C.

[0004] JP 2014-234581 A Japanese Patent No. 6924742 A

[0005] The present applicant has found that, when using a fiber sheet for various industrial applications, the fiber sheet must be excellent not only in mechanical strength but also in extensibility. As an example, the applicant has found that, in order to realize an extensible polymer electrolyte membrane, the fiber sheet that serves as a support for the polymer electrolyte membrane must also be extensible. Therefore, in order to confirm the extensibility of a fiber sheet according to the prior art such as that disclosed in Patent Document 2, the applicant has calculated the average value of the breaking elongation in the direction parallel to the main surface of the fiber sheet in which the tensile strength is highest and the breaking elongation in the direction perpendicular to the direction in which the tensile strength is highest (hereinafter, sometimes abbreviated as the average breaking elongation) as the basis weight (unit: g / m) of the fiber sheet. ２ ) (unitless). This gave the average value of the breaking elongation per unit area weight of the fiber sheet.

[0006] As a result of the investigation, when a fiber sheet according to the prior art disclosed in Patent Document 2 was heat-treated within the above-mentioned range (corresponding to the fiber sheet prepared in Comparative Example 2 described below), the average breaking elongation per basis weight was high. In other words, the fiber sheet was easily stretched and had excellent elongation. However, the heat treatment resulted in an excessive increase in the O / C ratio of the fiber sheet measured by ESCA analysis. Specifically, as in Comparative Example 2 described below, the O / C ratio was as high as 0.10. A high O / C ratio means that the amount of oxygen is high relative to the amount of carbon constituting the polyazole-based resin contained in the constituent fibers of the fiber sheet. In other words, the increase in the O / C ratio is thought to indicate that the polyazole-based resin contained in the constituent fibers of the fiber sheet has been oxidized.

[0007] When a functional material such as a polymer electrolyte was applied to a fiber sheet, fiber sheets with an excessively high O / C ratio exhibited coating unevenness. The following reasons for this coating unevenness were considered: Fiber sheets with an excessively high O / C ratio contained excessively oxidized polyazole-based resin formed by heat treatment. The portions of the fiber sheet containing the excessively oxidized polyazole-based resin were areas where the affinity with the functional material had changed. In other words, fiber sheets with an excessively high O / C ratio had portions where the affinity with the functional material had changed. Therefore, the excessively high O / C ratio of the fiber sheet was considered to be the cause of coating unevenness when the functional material was applied to the fiber sheet. For example, as is clear from Comparative Example 2 described below, the O / C ratio of the fiber sheet according to the prior art disclosed in Patent Document 2 was as high as 0.10, and composite films prepared using the fiber sheet exhibited coating unevenness.

[0008] In addition, when a polymer electrolyte membrane is prepared by supporting a polymer electrolyte with a fiber sheet having an excessively high O / C ratio, there is a risk that the polymer electrolyte may be unintentionally modified by the excessively oxidized polyazole resin, making it unusable for various industrial applications.

[0009] Therefore, in order to prevent the O / C ratio from increasing excessively, the applicant attempted to heat-treat a fiber sheet according to the prior art, such as that disclosed in Patent Document 2, at a temperature 50°C lower than the glass transition temperature of the polyazole-based resin (corresponding to the fiber sheet prepared in Comparative Example 1 described below). However, although the excessive increase in the O / C ratio was prevented, the average breaking elongation per unit weight of the fiber sheet was low. Specifically, as shown in the Comparative Example described below, the average breaking elongation per unit weight was 6.79% or less, resulting in poor elongation and stretchability.

[0010] When such a fiber sheet is subjected to a secondary processing step, such as a step of preparing a composite film by coating the fiber sheet with a dispersion or solution of the above-mentioned functional material and drying it, the fiber sheet exhibits poor elongation. Therefore, the fiber sheet is unable to adequately withstand external forces received during handling, transport, or secondary processing, and is prone to wrinkling. For example, as is clear from Comparative Example 1 described below, when a fiber sheet having an average breaking elongation per basis weight of 6.79% or less is subjected to a secondary processing step, wrinkling occurs in the fiber sheet. Furthermore, a fiber sheet prone to wrinkling cannot be used for various industrial applications.

[0011] The present invention aims to provide a fiber sheet that can be used for various industrial applications by preventing excessive oxidation of the polyazole resin contained in the constituent fibers and by providing excellent elongation.

[0012] The present invention relates to: "(1) A fiber sheet including, as constituent fibers, fibers containing a polyazole resin, wherein the fiber sheet has an O / C ratio measured by ESCA analysis of less than 0.10, and an average value y of breaking elongation per basis weight calculated by the following formula is greater than 6.79%. The fiber sheet has: y = (a / c + b / c) / 2, where y is the average value of breaking elongation per basis weight (unit: %), a is the breaking elongation of the fiber sheet in the direction having the highest tensile strength among all directions parallel to the main surface of the fiber sheet (unit: %), b is the breaking elongation of the fiber sheet in the direction perpendicular to the direction having the highest tensile strength among all directions parallel to the main surface of the fiber sheet (unit: %), c is the basis weight of the fiber sheet (unit: g / m ２) value (unitless). (2) The fiber sheet according to (1), in which the average value z of the breaking elongation calculated by the following formula is greater than 19.00%. where z = (a + b) / 2, z: average value of the breaking elongation (unit: %), a: breaking elongation of the fiber sheet in a direction parallel to the main surface of the fiber sheet in which the tensile strength is highest (unit: %), and b: breaking elongation of the fiber sheet in a direction parallel to the main surface of the fiber sheet, perpendicular to the direction in which the tensile strength is highest (unit: %). (3) The fiber sheet according to (1), in which the weight-average molecular weight of the polyazole resin is less than 180,000. (4) A polymer electrolyte membrane configured such that the fiber sheet according to any one of (1) to (3) supports a polymer electrolyte. (5) A fuel cell comprising the polymer electrolyte membrane according to (4). (6) A water electrolysis hydrogen generation device comprising the polymer electrolyte membrane according to (5). (7) A method for producing a fiber sheet, comprising: (Step 1) preparing a fabric whose constituent fibers contain a polyazole resin having a weight-average molecular weight of less than 180,000; (Step 2) heat-treating the fabric at a temperature H (unit: °C) defined by the following formula; and (Step 3) cooling the fabric after the heat treatment: PTg - 127°C < H < PTg - 27°C, where PTg represents the glass transition temperature (unit: °C) of the polyazole resin.

[0013] The fiber sheet of the present invention has an O / C ratio of less than 0.10 as measured by ESCA analysis. Therefore, excessive oxidation of the polyazole resin contained in the constituent fibers is prevented. Furthermore, the average breaking elongation (y) per unit area is greater than 6.79%. Therefore, the fiber sheet is easily stretchable and has excellent elongation. For these reasons, the fiber sheet of the present invention can be used in a variety of industrial applications.

[0014] In addition, in a preferred embodiment of the present invention, the average breaking elongation (z) is greater than 19.00%, making the fiber sheet highly extensible. Therefore, the fiber sheet of the present invention can be used in a variety of industrial applications.

[0015] In particular, if the polyazole resin contained in the constituent fibers of the fiber sheet has a weight-average molecular weight of less than 180,000, the fiber sheet can be used for the various industrial applications described above.

[0016] Furthermore, by supporting a polymer electrolyte with the fiber sheet of the present invention, a polymer electrolyte membrane that is resistant to unintended denaturation and has excellent elongation can be realized, and a fuel cell or a water electrolysis hydrogen generation device equipped with the polymer electrolyte membrane can be realized.

[0017] The method for producing a fiber sheet according to the present invention includes a step of using a fabric whose constituent fibers contain a polyazole resin having a weight-average molecular weight of less than 180,000, and heat-treating the fabric at a temperature higher than 127° C. lower than the glass transition temperature of the polyazole resin and lower than 27° C. lower than the glass transition temperature. The method for producing a fiber sheet according to the present invention makes it easy to produce fiber sheets that can be used in the various industrial applications described above.

[0018] In the present invention, various configurations can be appropriately selected, for example, the following configurations. Note that, unless otherwise specified, the various measurements described in the present invention are performed under atmospheric pressure. Furthermore, measurements are performed under a temperature condition of 25°C. Unless otherwise specified, the various measurement results described in the present invention are measured to a value one digit smaller than the desired value, and the value is calculated by rounding off the value. As a specific example, when the desired value is expressed to one decimal place, the value is measured to two decimal places, and the obtained value is rounded to one decimal place to calculate the value to one decimal place, and this value is used as the desired value. The upper and lower limit values ​​exemplified in the present invention can be combined in any combination.

[0019] The polyazole-based resin contained in the constituent fibers of the fiber sheet of the present invention refers to an organic resin having an azole structure in the polymer chain. Here, the azole structure refers to a compound containing a five-membered heterocyclic ring structure containing one or more nitrogen atoms within the ring. The five-membered heterocyclic ring may contain atoms such as oxygen and sulfur in addition to nitrogen. Examples of polyazole-based resins of the present invention include polyimidazole, polybenzoxazole, polybenzothiazole, polybenzimidazole, polybenzopyrazole, polybenzopyrrole, and polybenzofurazan. Among these, polybenzimidazole (hereinafter sometimes abbreviated as PBI) is preferred due to its excellent heat resistance.

[0020] A typical PBI is represented by the following formula (1):

[0021] In the formula (1), R １ is a tetravalent group having at least one aromatic ring, and is, for example, a benzene ring, or two benzene rings connected by a direct bond or by —O—, —CO—, or —SO ２ Preferred examples include tetravalent groups containing aromatic rings linked by - or the like, and specifically preferred examples include the following chemical structures.

[0022] In addition, in formula (1), R ２ is a divalent group having at least one aromatic ring, and may be substituted with a substituent such as a hydroxyl group, an alkyl group, or a sulfonic acid group; an aromatic ring such as a benzene ring or a naphthalene ring; a nitrogen-containing heterocycle such as a pyridine ring; a group in which two benzene rings are directly bonded; -O-, -CO-, -SO ２ Examples of the divalent group include a divalent group containing a benzene ring or the like connected by - or the like, and specifically, for example, the following chemical structures are preferred.

[0023] Among the polyazole resins used in the present invention, PBI can be synthesized from aromatic tetraamine and aromatic dicarboxylic acid monomers, as shown in the following reaction formula. Aromatic tetraamines containing one or two benzene rings are desirable. Examples of preferred aromatic tetraamines include 3,3'-diaminobenzidine and 1,2,4,5-benzenetetraamine. Examples of preferred aromatic dicarboxylic acids include isophthalic acid, bisbenzoic acid, and 4,4'-oxybisbenzoic acid. Furthermore, by using sulfonated aromatic dicarboxylic acids, for example, when a composite membrane supporting a polymer electrolyte is formed, the amount of sulfonic acid groups in the composite membrane, in which the polymer electrolyte is supported by a fiber sheet, can be further increased. Examples of sulfonated aromatic dicarboxylic acids include 5-sulfoisophthalic acid and 4,8-disulfonyl-2,6-naphthalenedicarboxylic acid.

[0024] Furthermore, among the polyazole-based resins used in the present invention, PBI may be synthesized from an aromatic compound having two amino groups and one carboxyl group in one molecule, as shown in the following reaction formula: In particular, the use of PBI that does not contain oxygen atoms in its molecular structure tends to prevent the polyazole resin contained in the constituent fibers of the fiber sheet from being oxidized, which is preferable because it allows for the realization of a fiber sheet that can be used in a variety of industrial applications.

[0025] The weight-average molecular weight of the polyazole-based resin according to the present invention can be appropriately selected so that the polyazole-based resin contained in the constituent fibers is prevented from being excessively oxidized, and a fiber sheet having a large average breaking elongation per basis weight and excellent elongation can be realized. In particular, it is preferable to use a polyazole-based resin having a weight-average molecular weight of less than 180,000, as this makes it easy to realize the above-mentioned fiber sheet. The weight-average molecular weight of the polyazole-based resin is more preferably 150,000 or less, and most preferably 120,000 or less. The lower limit of the weight-average molecular weight of the polyazole-based resin can also be appropriately selected, but is preferably 50,000, and most preferably 90,000.

[0026] The weight-average molecular weight referred to here is a value measured by gel permeation chromatography (GPC). Specifically, the weight-average molecular weight is calculated in terms of polystyrene using commercially available standard polystyrene at a measurement temperature of 40°C, a flow rate of 1.0 mL / min, a concentration of 1.0 g / L, and an injection volume of 40 μL using a refractive index detector (RI) as a detector and THF as a solvent.

[0027] The glass transition temperature of the polyazole resin according to the present invention (hereinafter, the glass transition temperature may be abbreviated as Tg, and the glass transition temperature of the polyazole resin may be abbreviated as PTg) is appropriately adjusted. In order to prevent excessive oxidation of the polyazole resin contained in the constituent fibers and to easily produce a fiber sheet having a large average value of breaking elongation per basis weight and excellent elongation, it is preferable to use a polyazole resin having a Tg in the range of 300°C to 500°C, and more preferably a polyazole resin having a Tg in the range of 400°C to 450°C.

[0028] The glass transition temperature referred to here means the temperature at the intersection of the tangent to the baseline of the DTA curve measured by subjecting the resin to measurement to a differential thermal analyzer (DTA) and the tangent to the steep drop in the endothermic region due to the glass transition.

[0029] The fiber sheet according to the present invention includes fibers containing a polyazole-based resin as constituent fibers. The constituent fibers may include organic resins other than polyazole-based resins. Examples of organic resins other than polyazole-based resins include polyolefin-based resins (polyethylene, polypropylene, polymethylpentene, polyolefin-based resins in which a portion of hydrocarbons is substituted with a cyano group or a halogen such as fluorine or chlorine, etc.), styrene-based resins, polyether-based resins (polyethylene glycol, polypropylene glycol, polyether ether ketone, polyacetal, modified polyphenylene ether, aromatic polyether ketone, etc.), phenol-based resins, melamine-based resins, urea-based resins, epoxy-based resins, polyester-based resins (polyethylene terephthalate, polytrimethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, polybutylene naphthalate, polycarbonate, polyarylate, polylactic acid, wholly aromatic polyester resins, unsaturated polyester resins, etc.), polyisoprene-based resins, and the like. Examples of suitable resins include known resins such as amide resins, polyamideimide resins, polyamide resins (e.g., aromatic polyamide resins such as aramid resins, aromatic polyetheramide resins, nylon resins, etc.), urethane resins, fluorine-containing resins (e.g., polytetrafluoroethylene, polyvinylidene fluoride, perfluorosulfonic acid resins, etc.), polysaccharides (e.g., starch, cellulose resins, pullulan, alginic acid, hyaluronic acid), proteins (e.g., gelatin, collagen, etc.), vinyl alcohol resins (e.g., polyvinyl alcohol, polyvinyl acetate, etc.), polycaprolactone, polyglycolic acid, polyvinylpyrrolidone, and acrylic resins (e.g., polyacrylonitrile resins copolymerized with acrylic acid esters or methacrylic acid esters, modacrylic resins copolymerized with acrylonitrile and vinyl chloride or vinylidene chloride, etc.). Furthermore, the above-mentioned resins may contain sulfonic acid groups, phosphonic acid groups, or carboxylic acid groups in their side chains. The other resins may be of multiple types, and may be either linear or branched polymers, or may be block or random copolymers. The resins may have any three-dimensional structure or may have any crystallinity.

[0030] However, a fiber sheet with a high content of polyazole-based resin can be used in a wider variety of industrial applications, and therefore the percentage by mass of polyazole-based resin in the mass of resin contained in the fibers constituting the fiber sheet is preferably 70 mass % or more, more preferably 90 mass % or more, and most preferably 100 mass %.

[0031] The fiber sheet may also contain fibers that do not contain a polyazole-based resin as its constituent fibers. Fibers composed of the above-mentioned organic resins (organic resins other than polyazole-based resins) can be used as the fibers that do not contain a polyazole-based resin. However, a fiber sheet containing a high proportion of fibers containing a polyazole-based resin among its constituent fibers can realize a fiber sheet that can be used for a wider variety of industrial applications. Therefore, it is preferable that the constituent fibers of the fiber sheet are only fibers containing a polyazole-based resin.

[0032] The fiber length and average fiber diameter of the fibers constituting the fiber sheet are adjusted as appropriate. The constituent fibers can be short fibers cut to a specific length or continuous fibers (fibers not cut to a specific length prepared using a direct spinning method such as electrospinning). The average fiber length of the short fibers can be 15 to 120 mm, 20 to 90 mm, or 30 to 60 mm. The "fiber length" refers to the fiber length measured in accordance with Method C (direct method) specified in 8.4.1c) of JIS L1015:2021 "Test Methods for Staple Chemical Fibers."

[0033] The average fiber diameter of the fibers constituting the fiber sheet can be adjusted as needed, but is preferably 50 μm or less, more preferably 5 μm or less, even more preferably 3 μm or less, and most preferably 1 μm or less. A fiber sheet with a small fiber diameter can be realized that is usable for a wider variety of industrial applications. The lower limit of the average fiber diameter can be adjusted as needed, but can be 10 nm, 100 nm, 300 nm, or 500 nm. The term "average fiber diameter" refers to the arithmetic mean value of the fiber diameters of 50 fibers measured based on an optical microscope photograph or electron microscope photograph of the cross section of the fibers constituting the fiber sheet. When the cross-sectional shape of the fiber is non-circular, the diameter is considered to be the diameter of a circle having the same area as the cross-sectional area.

[0034] The fiber sheet referred to in the present invention refers to a flat or thin sheet made of fibers derived from fabrics such as nonwoven fabrics, woven fabrics, or knitted fabrics. In particular, when the constituent fibers are randomly dispersed, it is easy to realize a fiber sheet that can be used for various industrial applications. Therefore, the fiber sheet is preferably made of nonwoven fabric.

[0035] The nonwoven fabric may be, for example, a dry nonwoven fabric, a wet nonwoven fabric, an electrospun nonwoven fabric, a meltblown nonwoven fabric, or a spunbond nonwoven fabric obtained by using an electrospinning method. Alternatively, a laminated nonwoven fabric obtained by laminating multiple sheets of the same type of nonwoven fabric or multiple types of nonwoven fabric can be used. Known manufacturing apparatuses capable of implementing electrospinning and manufacturing methods using the same are disclosed, for example, in JP-A-2003-73964, JP-A-2004-238749, and JP-A-2005-194675.

[0036] If desired, the polyazole resin contained in the fiber constituting the fiber sheet of the present invention may be doped with an acid, such as phytic acid, phosphoric acid, sulfonic acid, or carboxylic acid.

[0037] Acid doping can be performed by immersing the fabric or fiber sheet used to prepare the fiber sheet in an acid solution with a concentration of, for example, 1 to 95% by mass, preferably 10 to 85% by mass. The immersion time in the acid solution varies depending on the type and content of polyazole resin contained in the fiber sheet, the immersion temperature, the required acid content, etc. Therefore, although not particularly limited, the immersion time is preferably 1 second to 72 hours, and the temperature is preferably 20 to 90°C. After acid doping, the fiber sheet is preferably dried in a vacuum or hot air oven.

[0038] The physical properties of the fiber sheet of the present invention, such as the basis weight, thickness, porosity, and tensile strength, can be adjusted as appropriate.

[0039] The weight of the fiber sheet is 0.1 to 20 g / m ２ and 0.5 to 10 g / m ２ and 1 to 5 g / m ２ The "weight per unit area" is the weight per square meter of the main surface, which is the widest surface of the fiber sheet. ２ This refers to the mass converted into per unit mass.

[0040] The thickness of the fiber sheet can be 10 mm or less, 5 mm or less, 3 mm or less, 1 mm or less, 0.5 mm or less, 0.3 mm or less, or 0.1 mm or less. On the other hand, a thickness of 1 μm or more is practical. The "thickness" is measured using a high-precision digital length measuring machine (Mitutoyo Corporation, Litematic (registered trademark), VL-50S, contact area of ​​the terminal used for measurement: 28.77 mm ２ ) when a load of 0.01 N is applied in a direction perpendicular to the main surface of the object to be measured.

[0041] The porosity of the fiber sheet is preferably 70% or more, more preferably 80% or more, and even more preferably 90% or more. A high porosity makes it easier to fill the polymer electrolyte that can constitute the polymer electrolyte membrane, for example, when a composite membrane supporting a polymer electrolyte is formed. Therefore, cavities are less likely to form between the polymer electrolyte and the constituent fibers, and the amount of polymer electrolyte that can be filled can be increased. As a result, the ionic conductivity of the polymer electrolyte membrane formed by supporting the polymer electrolyte on the fiber sheet can be increased. On the other hand, if the porosity is too high, the surface of the fiber sheet is prone to fluffing, making it difficult to handle. Therefore, the porosity is preferably less than 100% and 95% or less.

[0042] This "porosity (P)" (unit: %) is the value obtained from the following formula: P = 100 - (Fr1 + Fr2 + ... + Frn) where Frn represents the packing ratio (unit: %) of n components constituting the fiber sheet, and is the value obtained from the following formula: Frn = {(M x Prn) / (T x SGn)} x 100 where M is the basis weight of the fiber sheet (unit: g / cm ２ ), T is the thickness of the fiber sheet (unit: cm), Prn is the abundance mass ratio of the n component constituting the fiber sheet, and SGn is the specific gravity of the n component (unit: g / cm ３ ) respectively.

[0043] The tensile strength of the fiber sheet of the present invention can be adjusted as needed. To achieve a fiber sheet with high tensile strength that can be used for various industrial applications, the tensile strength of the fiber sheet in both the warp and weft directions is preferably 0.5 N / 50 mm or more, more preferably 0.7 N / 50 mm or more, and most preferably 0.9 N / 50 mm or more. The tensile strength of the fiber sheet in the warp direction can be determined by subjecting a total of three samples taken from the fiber sheet so that the warp and length directions are parallel to each other to the method described in (Step 2) of the above-mentioned (Method for measuring the average breaking elongation per basis weight), and calculating the average of the obtained maximum tensile loads (unit: N / 50 mm). The tensile strength in the transverse direction of the fiber sheet can be determined by subjecting a total of three samples taken from the fiber sheet so that the transverse direction and the longitudinal direction are parallel to each other to the method described in (Step 2) of the above-mentioned (Method for measuring the average value of breaking elongation per unit weight), and calculating the average value of the obtained maximum tensile loads (unit: N / 50 mm).

[0044] Furthermore, it is preferable that the difference in tensile strength between the longitudinal and transverse directions of the fiber sheet is small. When a polymer electrolyte membrane in which a polymer electrolyte is supported by a fiber sheet with such a small difference is used in a fuel cell or an electrolytic hydrogen generation device, the physical properties of the polymer electrolyte membrane in the longitudinal and transverse directions during use become similar. Furthermore, the difference in swelling and shrinkage between the longitudinal and transverse directions of the polymer electrolyte membrane during use becomes small. As a result, unintended degradation of the performance of the fuel cell or electrolytic hydrogen generation device can be prevented, which is preferable. The difference is preferably 0.60 N / 50 mm or less, more preferably 0.50 N / 50 mm or less, more preferably 0.40 N / 50 mm or less, more preferably less than 0.40 N / 50 mm, and more preferably 0.30 N / 50 mm or less.

[0045] The fiber sheet according to the present invention is characterized in that the O / C ratio measured by ESCA analysis is less than 0.10. The O / C ratio can be determined using the following ESCA analysis.

[0046] (ESCA Analysis) (Step 1) The fiber sheet is subjected to an X-ray photoelectron spectrometer (JPS-9010MK, JEOL (registered trademark)) and a full element scan is performed. (Step 2) After the full element scan, a narrow scan is performed on the detected elements and elements expected to be present to obtain the measured values ​​of "C: carbon atoms present on the surface of the constituent fibers of the fiber sheet (unit: atomic %)" and "O: oxygen atoms present on the surface of the constituent fibers of the fiber sheet (unit: atomic %)." (Step 3) The O / C is calculated from the obtained measured values, and this calculated value is the O / C ratio measured by ESCA analysis. Note that a lower O / C ratio indicates a lower amount of oxygen relative to the amount of carbon constituting the polyazole-based resin contained in the constituent fibers of the fiber sheet. Note that a fiber sheet composed of a polyazole-based resin that does not have oxygen atoms in its structure has a theoretical O / C ratio of 0.00.

[0047] As will be apparent from the examples described below, when a fiber sheet is heated at a high temperature (in other words, when the polyazole-based resin contained in the constituent fibers of the fiber sheet is heated at a high temperature), the polyazole-based resin contained in the constituent fibers of the fiber sheet is oxidized, and the O / C ratio of the fiber sheet excessively increases to 0.10. In contrast, the fiber sheet of the present invention has a low O / C ratio of less than 0.10, preventing oxidation of the polyazole-based resin contained in the constituent fibers of the fiber sheet.

[0048] Therefore, the fiber sheet according to the present invention is a fiber sheet that can be used for various industrial applications because the polyazole resin contained in the constituent fibers is prevented from being excessively oxidized. To realize a fiber sheet that can be used for various industrial applications, the O / C ratio is preferably 0.90 or less, more preferably 0.80 or less, even more preferably 0.70 or less, and most preferably 0.60 or less.

[0049] Furthermore, the fiber sheet according to the present invention is characterized in that the average breaking elongation per unit area weight is greater than 6.79%. The average breaking elongation per unit area weight can be determined using the following measurement.

[0050] (Method for Measuring the Average Breaking Elongation per Basis Weight) (Step 1) Six rectangular samples (length: 200 mm, width: 50 mm) are taken from the fiber sheet so that the length directions are at 30° angles relative to the direction parallel to the main surface of the fiber sheet. (Step 2) Each of the taken samples is subjected to a tensile tester (UCT-500, manufactured by Orientec Co., Ltd.) in accordance with JIS P8113:2006 "Paper and paperboard - Test methods for tensile properties - Part 2: Constant rate of extension method." The maximum tensile load (tensile strength, unit: N / 50 mm) measured in the length direction until breakage of each sample is determined. The measurement is performed under the conditions of a grip distance of 100 mm and a pulling speed of 50 mm / min. (Step 3) After performing the above (Step 2), the sample with the highest maximum tensile load (tensile strength, unit: N / 50 mm) measured until breakage is selected. Then, a total of three rectangular samples (length: 200 mm, width: 50 mm) are taken from the fiber sheet so that the length direction of the selected sample (hereinafter referred to as the longitudinal direction) is parallel to the length direction of the sample to be taken. (Step 4) Each newly taken sample is subjected to a tensile tester (UCT-500, manufactured by Orientec Co., Ltd.) in accordance with JIS P8113:2006 "Paper and paperboard -- Test methods for tensile properties -- Part 2: Constant rate of extension method." The breaking elongation in the length direction of each sample is measured. The measurement is performed under conditions of a grip distance of 100 mm and a pulling speed of 50 mm / min. The breaking elongation is a value calculated using the following formula: Elongation (%) = 100 × (D - 100) / 100 (where D means the elongation at break (mm)). The average value of the longitudinal breaking elongation of the three samples measured as described above is calculated. The calculated value is the breaking elongation (a, unit: %) of the fiber sheet in the direction (warp direction) with the highest tensile strength among the directions parallel to the main surface of the fiber sheet. (Step 5) A total of three rectangular samples (length: 200 mm, width: 50 mm) are taken from the fiber sheet such that the longitudinal direction of the sample to be taken is parallel to the transverse direction, which is perpendicular to the warp direction determined in (Step 2) above, among the directions parallel to the main surface of the fiber sheet. (Step 6) The breaking elongation in the longitudinal direction of each newly taken sample is measured in the same manner as (Step 4) above.The average value of the breaking elongation in the length direction of the three samples measured as described above is calculated. The calculated value is the breaking elongation (b, unit: %) of the fiber sheet in the direction (transverse direction) perpendicular to the direction (warp direction) in which the tensile strength is highest among the directions parallel to the main surface of the fiber sheet. (Step 7) The values ​​of a and b calculated as described above are used to calculate the basis weight (g / m) of the fiber sheet, respectively, as follows: ２ The average value (y, unit: %) of the values ​​obtained by dividing the value of a / c by the value of b / c (unitless) is calculated. The calculated average value y is the average value of the breaking elongation per unit area weight of the fiber sheet. The higher the average value, the easier the fiber sheet is to stretch and the better its elongation. y = (a / c + b / c) / 2

[0051] To realize a fiber sheet that can be used for various industrial applications, the average breaking elongation per basis weight of the fiber sheet is greater than 6.79%, preferably 6.9% or more, more preferably 7% or more, more preferably 8% or more, even more preferably 9% or more, and most preferably 10% or more. The upper limit can be adjusted as appropriate, but is preferably 50%.

[0052] Furthermore, it is preferable that the difference between the values ​​of a and b determined by the above-mentioned method (method for measuring the average breaking elongation per unit weight) is small. When a polymer electrolyte membrane in which a polymer electrolyte is supported by a fiber sheet with a small difference is used in a fuel cell or an electrolytic hydrogen generation device, the physical properties of the polymer electrolyte membrane in the longitudinal and transverse directions during use become close. Furthermore, the difference in swelling and shrinkage between the longitudinal and transverse directions of the polymer electrolyte membrane during use becomes small. As a result, unintended degradation of the performance of the fuel cell or electrolytic hydrogen generation device can be prevented, which is preferable. The difference between the values ​​of a and b is preferably 0.80% or less, preferably less than 0.80%, and preferably 0.70% or less.

[0053] Additionally, the fiber sheet according to the present invention preferably has an average breaking elongation greater than 19.00%. Such a fiber sheet is more easily stretchable. The average breaking elongation is calculated by adding the values ​​a and b obtained by the above-mentioned method (method for measuring the average breaking elongation per unit area weight) to the following formula (z, unit: %). Note that a higher average breaking elongation means that the fiber sheet is more easily stretchable and has excellent elongation. z = (a + b) / 2

[0054] To realize a fiber sheet that can be used for various industrial applications, the average breaking elongation of the fiber sheet is preferably greater than 19.00%, more preferably 19.50% or more, more preferably 20.00% or more, and most preferably 20.50% or more. The upper limit can be adjusted as appropriate, but is preferably 50% or less.

[0055] Next, an example of a method for producing a fiber sheet according to the present invention will be described. The fiber sheet according to the present invention can be easily produced by a production method including: (Step 1) preparing a fabric whose constituent fibers contain a polyazole resin having a weight-average molecular weight of less than 180,000; (Step 2) heat-treating the fabric at a temperature H (unit: °C) defined by the following formula: PTg - 127°C < H < PTg - 27°C (where PTg represents the glass transition temperature (unit: °C) of the polyazole resin); and (Step 3) cooling the fabric after the heat treatment.

[0056] Through research, the present applicant has found that by using a fabric whose constituent fibers contain a polyazole-based resin with a weight-average molecular weight of less than 180,000 and heat-treating the fabric at a temperature greater than 127°C lower than the glass transition temperature of the polyazole-based resin and less than 27°C lower than the glass transition temperature, excessive oxidation of the polyazole-based resin in the constituent fibers can be prevented, thereby improving elongation. Specifically, a fabric whose constituent fibers contain polybenzimidazole with a glass transition temperature of 427°C is preferably heat-treated at a temperature greater than 300°C and less than 400°C, more preferably at a temperature in the range of 350 to 380°C. This results in an O / C ratio measured by ESCA analysis of less than 0.10, preventing excessive oxidation of the polyazole-based resin in the constituent fibers. The inventors have also found that a fiber sheet having an average breaking elongation (y) per unit area weight of greater than 6.79%, which is easy to stretch and has excellent elongation, can be realized. In addition, the inventors have found that a fiber sheet having an average breaking elongation (z) per unit area weight of greater than 19.00%, which is easy to stretch and has excellent elongation, can be realized.

[0057] The O / C ratio measured by ESCA analysis of the fabric prepared in step 1 is preferably 0.06 or less. By using a fabric with a low O / C ratio, it is easy to realize a fiber sheet with a low O / C ratio after heat treatment.

[0058] In addition, to prevent unintended reactions such as oxidation reactions from occurring during the heat treatment in (Step 2), it is preferable to heat-treat the fabric in an atmosphere of argon gas or nitrogen gas. The heating device used for the heat treatment can also be selected appropriately, and examples include a method of heating or heating and pressurizing using a heating roll device, a method of heating using a heater such as an oven dryer, far-infrared heater, dry heat dryer, or hot air dryer, and a method of heating by irradiating with infrared rays under no pressure. The heating time is also selected appropriately so as to produce a fiber sheet with the desired physical properties.

[0059] The fabric after the heat treatment in (step 3) can be cooled by leaving it to cool or by placing it in a refrigerator.

[0060] By going through the above steps, the fiber sheet according to the present invention can be easily produced.

[0061] By applying a polymer electrolyte to the fiber sheet of the present invention and subjecting it to heat treatment, a polymer electrolyte membrane can be prepared in which the fiber sheet supports the polymer electrolyte.

[0062] The type of polymer electrolyte can be appropriately selected so as to ensure excellent proton conductivity of the polymer electrolyte membrane. For example, perfluorosulfonic acid polymers such as Nafion (registered trademark) and Aquivion (registered trademark), sulfonated polyarylene ether (SPAE), sulfonated polyethersulfone (SPES), sulfonated polyimide (SPI), sulfonated PBI (SPBI), sulfonated polyphenylene (SPP), sulfonated polyphenylene oxide (SPPO), sulfonated polyphenylene sulfide (SPPS), sulfonated polystyrene and its copolymer (SPSt), polyvinyl sulfonic acid and its copolymer (PVS), etc. can be used.

[0063] The thickness of the polymer electrolyte membrane according to the present invention is adjusted so as to realize, for example, a fuel cell with high ionic conductivity and high power generation efficiency, and, for example, so as to enable miniaturization of an electrolytic hydrogen generation device by employing the membrane as a diaphragm. The membrane thickness is preferably 50 μm or less, more preferably 45 μm or less, even more preferably 40 μm or less, even more preferably 35 μm or less, and even more preferably 30 μm or less. The polymer electrolyte membrane according to the present invention is reinforced with the fiber sheet according to the present invention. Therefore, even a thin membrane of 30 μm or less has excellent handleability. On the other hand, the lower limit of the membrane thickness of the polymer electrolyte membrane according to the present invention is preferably 1 μm, so as to have excellent mechanical strength and easy handling.

[0064] The content of the polymer electrolyte in the polymer electrolyte membrane of the present invention is preferably from 50 to 99% by mass, more preferably from 60 to 95% by mass, and even more preferably from 72 to 90% by mass.

[0065] As described above, by supporting a polymer electrolyte with the fiber sheet of the present invention, a polymer electrolyte membrane that is resistant to unintended denaturation and has excellent elongation can be realized.Furthermore, a fuel cell or a water electrolysis hydrogen generation device equipped with the polymer electrolyte membrane can be realized.

[0066] The present invention will be described in more detail below with reference to examples, but these examples are not intended to limit the scope of the present invention. The performance of the prepared fiber sheets was evaluated as follows.

[0067] (Evaluation of Coating Unevenness) A solution prepared by dissolving sulfonated polyethersulfone in N,N-dimethylacetamide was applied to one main surface of a fiber sheet. The N,N-dimethylacetamide was then removed by a drying process. This resulted in the preparation of a composite membrane in which a membrane made of sulfonated polyethersulfone was supported by a fiber sheet. The main surface of the composite membrane on which the solution was applied was then visually inspected. Whether coating unevenness (i.e., areas with a high and low amount of sulfonated polyethersulfone present) was then determined. Based on the results of the evaluation, fiber sheets constituting composite membranes with coating unevenness were marked with "Yes" in the "Presence of Coating Unevenness" column in the table. On the other hand, fiber sheets constituting composite membranes without coating unevenness were marked with "No" in the "Presence of Coating Unevenness" column in the table. It should be noted that the coating unevenness is believed to be caused by an excessively high O / C ratio of the fiber sheet, as described above.

[0068] (Evaluation of the Presence or Absence of Wrinkles) The composite membranes prepared in the above (Evaluation of the Presence or Absence of Coating Unevenness) were visually inspected for the presence or absence of wrinkles on the main surface of the membrane portion made of sulfonated polyethersulfone. As a result of the inspection, the fiber sheets constituting the composite membranes in which wrinkles were generated were marked with "Present" in the "Presence or Absence of Wrinkles" column in the table. On the other hand, the fiber sheets constituting the composite membranes in which wrinkles were not generated were marked with "Absence" in the "Presence or Absence of Wrinkles" column in the table. It is believed that the wrinkles were generated due to poor elongation of the fiber sheets in the process of preparing the composite membrane by coating the fiber sheets with the sulfonated polyethersulfone solution and drying them, which was carried out in (Evaluation of the Presence or Absence of Coating Unevenness).

[0069] From the results of the above evaluations, it was found that fiber sheets from which composite films without coating unevenness and without wrinkles could be prepared (fiber sheets with "none" in all evaluation results in the table) can be used for various industrial applications.

[0070] (Comparative Examples 1 and 2) PBI (weight average molecular weight: 180,000, glass transition temperature: 427°C) was prepared as a polyazole resin. The PBI was then dissolved in dimethylacetamide to prepare a spinning solution with a polymer concentration of 20% by mass. Using the prepared spinning solution, a nonwoven fabric was then produced by electrospinning under the following spinning conditions. The prepared nonwoven fabric was subjected to ESCA analysis, and the O / C ratio measured was 0.06.

[0071] <Spinning conditions> Discharge rate 1 mL / hr, distance between nozzle (needle) and collector (substrate) 6 cm, temperature 25° C., humidity 30% RH, needle: inner diameter 0.33 mm, voltage 15 kV.

[0072] The prepared nonwoven fabric was subjected to a heat treatment in an air atmosphere for 0.5 hours using a heat roller device with an adjusted heating temperature. The heating temperature was 350°C in Comparative Example 1 and 400°C in Comparative Example 2. The nonwoven fabric was then removed from the heat roller device and allowed to cool. Each heat-treated nonwoven fabric was prepared in this manner. The physical properties of the nonwoven fabrics are summarized in Table 1.

[0073]

[0074] The comparison between Comparative Example 1 and Comparative Example 2 revealed the following: Comparative Example 2, in which a nonwoven fabric containing polyazole-based resin-containing fibers as constituent fibers was heat-treated at 400°C (PTg -27°C), had a high average breaking elongation per basis weight of 8.57% (also an average breaking elongation of 24.00%). However, the O / C ratio excessively increased from 0.06 to 0.10 (an O / C ratio increase of 0.04). As a result, coating unevenness occurred in the composite film. Comparative Example 1, in which a nonwoven fabric containing polyazole-based resin-containing fibers as constituent fibers was heated at 350°C (a temperature below PTg -27°C), had a low O / C ratio of 0.06. However, the average breaking elongation per basis weight was low at 6.79% (also an average breaking elongation of 19.00%). As a result, wrinkles occurred in the composite film. From the above results, it was found that the fiber sheets according to the prior art did not satisfy both the requirements of preventing excessive oxidation of the polyazole resin contained in the constituent fibers and of having excellent elongation, and therefore were not fiber sheets usable for various industrial applications.

[0075] (Comparative Example 3) A nonwoven fabric was prepared in the same manner as in Comparative Example 1, except that PBI (weight average molecular weight: 98,000, glass transition temperature: 427°C) was prepared and used as the polyazole resin and that no heat treatment was performed. Various physical properties thereof are summarized in Table 2. Note that, since the produced nonwoven fabric was not subjected to heat treatment, a "-" is entered in the "Heating temperature (°C)" column in the table.

[0076] (Comparative Examples 4 to 6, Examples 1 and 2, Examples 5 to 9) Heat-treated nonwoven fabrics were prepared in the same manner as in Comparative Example 1, except that the nonwoven fabric prepared in Comparative Example 3 was used and the heating temperature was changed to the temperature shown in Table 2. The various physical properties of the nonwoven fabrics are also summarized in Table 2.

[0077]

[0078] The fiber sheet of Comparative Example 3, which was not heat-treated, the fiber sheets of Comparative Examples 4 and 5, which were heat-treated at a low temperature, and the fiber sheet of Comparative Example 6, which was heat-treated at a high temperature, had poor elongation, with an average breaking elongation per basis weight of 6.55% or less (and an average breaking elongation of 19.00% or less). As a result, composite films prepared using these fiber sheets were wrinkled. Therefore, they could not be used for various industrial applications.

[0079] In contrast, the fiber sheets of Examples 1 and 2 and Examples 5 to 9 according to the present invention had excellent elongation, with an average breaking elongation per basis weight greater than 6.79% (and an average breaking elongation greater than 19.00%), and therefore no wrinkles were generated in the composite films prepared using these fiber sheets.

[0080] Furthermore, the fiber sheet of Comparative Example 6, which was heat-treated at a high temperature, had an O / C ratio that excessively increased to 0.10. Therefore, composite films prepared using this fiber sheet had coating unevenness, making them unsuitable for various industrial applications. In contrast, the fiber sheets of Examples 1 and 2 and Examples 5 to 9 according to the present invention had O / C ratios of less than 0.10. In other words, excessive increases in the O / C ratio were prevented. Therefore, composite films prepared using this fiber sheet did not have coating unevenness.

[0081] Example 3 A nonwoven fabric was prepared in the same manner as in Comparative Example 1, except that PBI (weight average molecular weight: 120,000, glass transition temperature: 427°C) was prepared and used as the polyazole resin. The prepared nonwoven fabric was subjected to ESCA analysis, and the O / C ratio measured was 0.06. The prepared nonwoven fabric was then subjected to the same heat treatment as in Comparative Example 1 to prepare a heat-treated nonwoven fabric. The various physical properties thereof are summarized in Table 3. For ease of understanding, Table 3 lists the results of Example 1 and Comparative Example 1 together.

[0082]

[0083] The fiber sheet prepared in Example 3 had excellent elongation, with an average breaking elongation per unit area greater than 6.79% (and an average breaking elongation greater than 19.00%). The O / C ratio was less than 0.10, preventing excessive increases in the O / C ratio. Therefore, the composite film prepared using the fiber sheet prepared in Example 3 had no wrinkles or coating irregularities.

[0084] (Example 4) A nonwoven fabric was prepared in the same manner as in Example 2, except that the basis weight was reduced. The prepared nonwoven fabric was subjected to ESCA analysis, and the O / C ratio measured was 0.06. The prepared nonwoven fabric was then subjected to the same heat treatment as in Example 2 to prepare a heat-treated nonwoven fabric. The various physical properties of the nonwoven fabric are summarized in Table 4.

[0085]

[0086] From the above results, it was found that the fiber sheet of the present invention prevented excessive oxidation of the polyazole resin contained in the constituent fibers and also had excellent elongation, thereby satisfying both of these properties and making it suitable for a variety of industrial applications.

[0087] Furthermore, the results of comparing the manufacturing processes of Example 1 with those of Example 3 and Comparative Example 1 showed that by using a fabric containing, as its constituent fibers, fibers containing a polyazole-based resin with a weight-average molecular weight of less than 180,000, and the results of comparing the manufacturing processes of Comparative Examples 3 to 6 with those of Examples 1 and 2 and Examples 5 to 9 showed that by using a fabric containing, as its constituent fibers, fibers containing a polyazole-based resin and heat-treating the fabric within a temperature range from 127°C lower than the glass transition temperature of the polyazole-based resin to 27°C lower than the glass transition temperature, excessive oxidation of the polyazole-based resin contained in the constituent fibers of the fiber sheet could be prevented. Furthermore, elongation was improved, allowing the production of a fiber sheet satisfying both performance requirements. Therefore, it was revealed that the manufacturing method of the present invention can easily produce a fiber sheet usable for a variety of industrial applications.

[0088] In particular, the comparison between Examples 6 and 7 revealed that by using a fabric containing polyazole resin-containing fibers as constituent fibers and heating the fabric in the temperature range of (PTg - 127°C < H < PTg - 50°C), a fiber sheet could be produced in which: - the difference in breaking elongation of the fiber sheet in the warp and weft directions, and - the difference in tensile strength of the fiber sheet in the warp and weft directions were small. In other words, a fiber sheet with similar physical properties in the warp and weft directions could be produced. Therefore, the fiber sheet had small differences in swelling and shrinkage in the warp and weft directions of the polymer electrolyte membrane during use. As a result, it became clear that a fiber sheet could be easily produced that could prevent unintended degradation of the performance of fuel cells and electrolytic hydrogen generators.

[0089] The fiber sheet of the present invention can be used in a variety of industrial applications, including as a support for membranes usable in various industrial applications, such as liquid separation membranes (e.g., gas separation membranes, water treatment membranes), medical materials (e.g., structural materials for medical devices such as medical sealants), ion exchange membranes, dialysis membranes, redox flow batteries, fuel cells, and polymer electrolyte membranes used in water electrolysis hydrogen generators, etc. It can also be used in a variety of industrial applications, such as separators for electrochemical elements (e.g., capacitors, primary / secondary batteries), prepregs, gas filters, and liquid filters.

Claims

1. A fiber sheet whose constituent fibers contain polyazole resin, wherein the O / C ratio measured by ESCA analysis is less than 0.10, and the average breaking elongation per unit weight, y, calculated by the following formula, is greater than 6.79%. Note: y = (a / c + b / c) / 2 y: average breaking elongation per unit weight (unit: %) a: breaking elongation of the fiber sheet in the direction with the highest tensile strength among all directions parallel to the main surface of the fiber sheet (unit: %) b: breaking elongation of the fiber sheet in the direction perpendicular to the direction with the highest tensile strength among all directions parallel to the main surface of the fiber sheet (unit: %) c: basis weight of the fiber sheet (unit: g / m ２ ) value (unitless).

2. The fiber sheet according to claim 1, wherein the average value z of the breaking elongation calculated by the following formula is greater than 19.00%, where z = (a + b) / 2, z: average value of breaking elongation (unit: %), a: breaking elongation of the fiber sheet in the direction having the highest tensile strength among directions parallel to the main surface of the fiber sheet (unit: %), and b: breaking elongation of the fiber sheet in the direction perpendicular to the direction having the highest tensile strength among directions parallel to the main surface of the fiber sheet (unit: %).

3. The fiber sheet according to claim 1, wherein the weight average molecular weight of the polyazole resin is less than 180,000.

4. A polymer electrolyte membrane comprising the fiber sheet according to any one of claims 1 to 3 supporting a polymer electrolyte.

5. A fuel cell comprising the polymer electrolyte membrane according to claim 4.

6. A water electrolysis hydrogen generating device comprising the polymer electrolyte membrane according to claim 5.

7. A method for producing a fiber sheet, comprising: (Step 1) preparing a fabric whose constituent fibers contain a polyazole resin having a weight-average molecular weight of less than 180,000; (Step 2) heat-treating the fabric at a temperature H (unit: °C) defined by the following formula; and (Step 3) cooling the fabric after the heat treatment: PTg - 127°C < H < PTg - 27°C, where PTg represents the glass transition temperature (unit: °C) of the polyazole resin.

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