Wet-laid nonwoven fabric, electrolyte reinforcing film, and method for producing wet-laid nonwoven fabric

Abstract

The present invention provides a wet-laid nonwoven fabric having a small thickness, high strength, high air permeability, and high uniformity with respect to pore sizes.　This wet-laid nonwoven fabric contains drawn polyarylene sulfide fibers having an average fiber diameter of 0.3 μm to 3.0 μm inclusive, has an average pore diameter of 0.5 μm to 50.0 μm inclusive, and has a weight per square meter of 1.0 g / m2 or more and less than 8.0 g / m2.

Description

Wet-laid nonwoven fabric, electrolyte-reinforced membrane, and method for producing wet-laid nonwoven fabric

[0001] The present invention relates to a wetlaid nonwoven fabric, an electrolyte-reinforced membrane, and a method for producing the wetlaid nonwoven fabric.

[0002] Polyarylene sulfide has high heat resistance, chemical resistance, electrical insulation, and flame retardancy, as well as excellent mechanical properties and moldability, making it widely used as a metal replacement material and a material that can withstand extreme environments. Taking advantage of these properties, polyarylene sulfide fibers are also used in applications such as bag filters, papermaking canvases, electrical insulating paper, battery separators, and various diaphragms. In particular, research is being conducted to combine the heat resistance and chemical resistance of polyarylene sulfide with the ion permeability and gas separation properties of nonwoven fabric materials for use as reinforcement materials for electrolyte membranes, which are attracting attention as a clean energy source.

[0003] To improve the efficiency of electrolyte membranes, it is necessary to reduce their thickness while maintaining their mechanical strength. To achieve this, an electrolyte-reinforced membrane structure has been proposed, in which an electrolyte membrane is formed on an electrolyte membrane reinforcing material. This electrolyte membrane reinforcing material is required to be thin, have high air permeability, have high mechanical strength, and have a highly uniform pore size in the wet-laid nonwoven fabric, so as not to reduce the ion permeability of the electrolyte-reinforced membrane.

[0004] To address this issue, for example, a wet-laid nonwoven fabric for use as a reinforcing material for solid polymer electrolytes, mainly composed of polyphenylene sulfide fibers, a type of polyarylene sulfide, has been proposed (Patent Document 1). It is described that unstretched polyphenylene sulfide fibers plasticize at a lower temperature than stretched polyphenylene sulfide fibers, and therefore act as adhesive fibers, resulting in a wet-laid nonwoven fabric with excellent mechanical strength.

[0005] In addition, a wetlaid nonwoven fabric has been proposed that is excellent in gas barrier properties and ion permeability by being composed of thermoplastic fibers having an average single fiber fineness of 0.1 to 10 dtex and a specific crimp number (Patent Document 2).The patent document also describes that the thermoplastic fibers include polyphenylene sulfide fibers (PPS).

[0006] On the other hand, polyphenylene sulfide fiber has a problem of low water retention and easy breakage during the papermaking process. Therefore, a polyphenylene sulfide wetlaid nonwoven fabric has been proposed in which aramid fiber, which has higher water retention than polyphenylene sulfide fiber, is mixed (Patent Document 3). It is described that the inclusion of fibrillated aramid fiber improves the specific surface area of ​​the fiber, increasing the number of adhesion points between the fibers, resulting in a wetlaid nonwoven fabric with high paper strength.

[0007] JP 2003-77494 A JP 2016-89197 A JP 2020-76174 A

[0008] The technology disclosed in Patent Document 1 can produce polyphenylene sulfide wetlaid nonwoven fabrics with excellent mechanical strength, but has the problem that the skeletal fibers constituting the wetlaid nonwoven fabric are substantially thick, making it impossible to produce a thin wetlaid nonwoven fabric. For example, in the examples, a wetlaid nonwoven fabric made of stretched PPS fibers with a fineness of 1.1 dtex (i.e., a fiber diameter of 10 μm for polyphenylene sulfide fibers) and a wetlaid nonwoven fabric with a fineness of 2.2 dtex (i.e., a fiber diameter of 14 μm for polyphenylene sulfide fibers) cannot produce a wetlaid nonwoven fabric with a thickness of 14 μm or less. In addition, when a wetlaid nonwoven fabric with a low basis weight is produced using fibers with a thick fiber diameter, there is the problem that the strength is extremely reduced.

[0009] Furthermore, the technology disclosed in Patent Document 2 uses thick thermoplastic fibers with a fineness of 0.1 dtex or more (i.e., polyphenylene sulfide fibers with a fiber diameter of 3.03 μm or more), and the basis weight of the nonwoven fabric is large, resulting in low breathability of the nonwoven fabric. Therefore, when the nonwoven fabric is used as an electrolyte-reinforced membrane, power generation efficiency may decrease. On the other hand, reducing the thickness of the nonwoven fabric improves breathability, but reduces the number of constituent fibers of the wet-laid nonwoven fabric, which may result in reduced strength.

[0010] In Patent Document 3, a wetlaid nonwoven fabric with high wet paper strength is obtained by mixing fibrillated aramid fibers with a polyphenylene sulfide wetlaid nonwoven fabric. However, the wetlaid nonwoven fabric has low air permeability due to the inclusion of fibrillated fibers, and the aramid fibers have low acid resistance, which reduces the chemical resistance of the wetlaid nonwoven fabric. This reduces the strength of the wetlaid nonwoven fabric, making it unsuitable as a reinforcing material for an electrolyte membrane.

[0011] An object of the present invention is to overcome the problems of the prior art and to provide a wetlaid nonwoven fabric that is thin, has high tensile strength and air permeability, and is highly uniform.

[0012] As a result of further investigations, the present inventors have found that in order to reduce the thickness of a wetlaid nonwoven fabric containing polyarylene sulfide fibers and improve its breathability and tensile strength, it is effective to make the wetlaid nonwoven fabric low in basis weight and to use finer fibers. On the other hand, when ultrafine fibers are used, dispersibility is reduced, such as aggregation occurring between fibers in a fiber dispersion during the production of the wetlaid nonwoven fabric. In particular, in a low basis weight wetlaid nonwoven fabric, the wetlaid nonwoven fabric produced using the dispersion is prone to unevenness, making it difficult to obtain a wetlaid nonwoven fabric with a highly uniform pore size.

[0013] Therefore, the present inventors conducted extensive research to solve the above-mentioned problems, and as a result, they found that by setting the average fiber diameter of the polyarylene sulfide fiber within a specific range, it is possible to obtain a wetlaid nonwoven fabric having a high degree of breathability and tensile strength and a high degree of uniformity in pore size, even when the fabric weight is low, and thus completed the present invention.

[0014] The present invention aims to solve the above problems and adopts the following means: (1) A stretched polyarylene sulfide fiber having an average fiber diameter of 0.3 μm or more and 3.0 μm or less, an average pore size of 0.5 μm or more and 50.0 μm or less, and a basis weight of 1.0 g / m ２ 8.0g / m or more ２(2) The wetlaid nonwoven fabric according to (1), comprising unstretched polyarylene sulfide fibers having an average fiber diameter of 0.8 μm or more and 6.0 μm or less. (3) The wetlaid nonwoven fabric according to (1) or (2), wherein the aspect ratio (average fiber length / average fiber diameter) of the stretched polyarylene sulfide fibers is 800 or more and 2,000 or less, and the aspect ratio (average fiber length / average fiber diameter) of the unstretched polyarylene sulfide fibers is 400 or more and 1,800 or less. (4) The wetlaid nonwoven fabric according to any one of (1) to (3), wherein, when Dp (nm) is the pore diameter showing the maximum peak of the pore size distribution in a pore size distribution curve, 50% or more of all pores have a diameter of 0.9 Dp or more and 1.1 Dp or less. (5) The wetlaid nonwoven fabric according to any one of (1) to (4), which has a shrinkage rate of 8.0% or less after being left to stand in an atmosphere at a temperature of 160° C. for 24 hours. (6) The wetlaid nonwoven fabric according to any one of (2) to (5), wherein the average fiber diameter of the unstretched polyarylene sulfide fibers is smaller than the average fiber diameter of the stretched polyarylene sulfide fibers. (7) An electrolyte reinforced membrane comprising the wetlaid nonwoven fabric according to any one of (1) to (6). (8) A method for producing a wetlaid nonwoven fabric according to any one of (2) to (7), which includes a step of dispersing the stretched polyarylene sulfide fibers and the unstretched polyarylene sulfide fibers in an aqueous medium to obtain a dispersion, wherein the fiber concentration in the dispersion is 1.0 ppm by mass or more and 80.0 ppm by mass or less. (9) The method for producing a wetlaid nonwoven fabric according to (8), wherein the dispersion contains a dispersant, and the concentration of the dispersant relative to the dispersion is 0.0010% by mass or more and 8.0% by mass or less.

[0015] According to the present invention, a wetlaid nonwoven fabric can be obtained which is thin, has high strength and breathability, and has highly uniform pore diameters.

[0016] The wetlaid nonwoven fabric of the present invention satisfies the following requirements.

[0017] The fabric contains stretched polyarylene sulfide fibers having an average fiber diameter of 0.3 μm or more and 3.0 μm or less, an average pore size of 0.5 μm or more and 50.0 μm or less, and a basis weight of 1.0 g / m ２ 8.0g / m or more ２ is less than.

[0018] The present invention will be described in detail below, but the present invention is not limited to the scope of the following description as long as it does not deviate from the spirit of the invention.

[0019] <Polyarylene sulfide fiber> The polyarylene sulfide used in the polyarylene sulfide fiber is a homopolymer or copolymer having a repeating unit of -(Ar-S)-, where Ar includes structural units represented by the following formulas (1) to (12).

[0020]

[0021]

[0022]

[0023]

[0024]

[0025]

[0026]

[0027]

[0028]

[0029]

[0030]

[0031]

[0032] (In formulas (1) to (10), R １ , R ２ are each a substituent selected from a hydrogen atom, an alkyl group, an alkoxy group, and a halogen group, and R １ and R ２ may be the same or different.) Representative examples of polyarylene sulfides include polyphenylene sulfide (PPS), polyphenylene sulfide sulfone, polyphenylene sulfide ketone, etc. As the repeating unit of polyarylene sulfide, a p-arylene sulfide unit having Ar of the above formula (1) is preferred.

[0033] Furthermore, by using a resin containing, as the main structural unit of the polymer, preferably 60 mol % or more, more preferably 70 mol % or more, and even more preferably 80 mol % or more of p-phenylene units represented by the following structural formula (13), the resin has excellent heat resistance and exhibits excellent strength when processed into fibers and wet-laid nonwoven fabrics.

[0034]

[0035] Furthermore, copolymerized units may be contained within the range that does not impair the effects of the present invention, but the copolymerized units are preferably contained in an amount of 20 mol % or less relative to the polyarylene sulfide polymer.

[0036] The polyarylene sulfide fiber may contain various additives, such as inorganic substances such as titanium oxide, silica, barium oxide, and calcium carbonate, colorants such as carbon black, dyes, and pigments, flame retardants, fluorescent brighteners, antioxidants, and ultraviolet absorbers, within the range that does not impair the effects of the present invention.

[0037] The polyarylene sulfide fiber may be a single-component fiber or a composite fiber in which two or more types of resins are composited. When the polyarylene sulfide fiber is a composite fiber, the composite form is not particularly limited as long as it does not impair the effects of the present invention, and can be appropriately selected from a core-sheath type, a sea-island type, a side-by-side type, an eccentric core-sheath type, a blend type, etc. The polyarylene sulfide contained in the polyarylene sulfide fiber preferably accounts for 70 mass% or more of the total mass of the polymers in the fiber.

[0038] The cross-sectional shape of the polyarylene sulfide fiber is not limited in any way, and may be any irregular cross-sectional shape, such as a round cross section, a multi-lobal cross section such as a Y-shaped cross section or a triangular cross section, a flat cross section, an S-shaped cross section, a cross cross section, or a hollow cross section.

[0039] <Stretched Polyarylene Sulfide Fibers> The wetlaid nonwoven fabric of the present invention contains stretched polyarylene sulfide fibers having an average fiber diameter of 0.3 μm or more and 3.0 μm or less. Here, "stretched polyarylene sulfide fibers" refers to fibers having an orientation parameter of 3.0 or more for the polyarylene sulfide molecular chains. The orientation parameter is a value indicating the degree of orientation of the polyarylene sulfide molecular chains, with a larger orientation parameter indicating a higher degree of orientation of the molecular chains. Here, the orientation parameter is measured using a laser Raman device. The orientation parameter of stretched polyarylene sulfide fibers is preferably 3.0 or more, more preferably 5.0 or more, and even more preferably 7.0 or more. By setting the orientation parameter within the above range, fibers with high heat resistance and excellent tensile strength are obtained. The orientation parameter of stretched polyarylene sulfide fibers is preferably 40 or less, more preferably 30 or less, and even more preferably 25 or less. By setting the orientation parameter within the above range, fibers with high elongation are obtained.

[0040] The orientation parameter in the present invention refers to a parameter obtained by the method described in the Examples. The orientation parameter of the drawn polyarylene sulfide fiber can be adjusted to the above range by setting the draw ratio in the fiber drawing step to 2.5 times or more, but other methods may be used as long as they do not impede the object of the present invention.

[0041] The average fiber diameter of the stretched polyarylene sulfide fiber is 0.3 μm or more and 3.0 μm or less. If the average fiber diameter of the stretched polyarylene sulfide fiber is greater than 3.0 μm, the number of fibers constituting the same basis weight decreases, resulting in a decrease in the tensile strength of the wetlaid nonwoven fabric and an increase in the thickness of the wetlaid nonwoven fabric. Furthermore, if the average fiber diameter is large, the pore size variation increases and the uniformity of the pore size decreases. The average fiber diameter of the stretched polyarylene sulfide fiber is preferably 2.8 μm or less, more preferably 2.7 μm or less. On the other hand, if the average fiber diameter is less than 0.3 μm, the number of fibers constituting the same basis weight increases, resulting in a structure in which the fibers are densely packed, resulting in a decrease in the breathability of the resulting wetlaid nonwoven fabric. Furthermore, the aspect ratio also increases, resulting in a decrease in dispersibility and a decrease in the uniformity of the pore size of the wetlaid nonwoven fabric. The average fiber diameter of the drawn polyarylene sulfide fibers is preferably 0.5 μm or more, more preferably 1.0 μm. The average fiber diameter (μm) of the fibers mentioned here can be determined by the method described in the Examples.

[0042] The average fiber diameter of the drawn polyarylene sulfide fiber can be adjusted to the above range by adjusting the spinning output and the draw ratio, but other methods may also be used as long as they do not impede the object of the present invention.

[0043] The aspect ratio of the stretched polyarylene sulfide fiber (average fiber length / average fiber diameter) is preferably 800 or more and 2000 or less. By setting the aspect ratio of the stretched polyarylene sulfide fiber to 2000 or less, stretched polyarylene sulfide fiber with excellent dispersibility can be obtained, and a wetlaid nonwoven fabric with more uniform pore diameter can be obtained. The aspect ratio of the stretched polyarylene sulfide fiber is more preferably 1800 or less, and even more preferably 1500 or less. On the other hand, by setting the aspect ratio of the stretched polyarylene sulfide fiber to 800 or more, not only is the fiber less likely to fall off during the processing step, but the entanglement between the fibers further improves the tensile strength of the wetlaid nonwoven fabric. The aspect ratio of the stretched polyarylene sulfide fiber is more preferably 900 or more, and even more preferably 1000 or more.

[0044] The average fiber length can be measured in accordance with "8.4.1. Average fiber length, direct method (method C)" in JIS L1015:2010 "Testing methods for chemical fiber staples." The aspect ratio is the value obtained by dividing the average fiber diameter of the fibers by the average fiber length.

[0045] <Unstretched Polyarylene Sulfide Fiber> The wetlaid nonwoven fabric of the present invention preferably contains unstretched polyarylene sulfide fiber of 0.8 μm or more and 6.0 μm or less. Here, "unstretched polyarylene sulfide fiber" refers to a fiber whose molecular chains are not oriented and whose orientation parameter is less than 3.0. The orientation parameter of the unstretched polyarylene sulfide fiber is preferably less than 3.0, more preferably 2.5 or less, and even more preferably 2.0 or less. By setting the orientation parameter within the above range, the fiber becomes one having many amorphous portions and having high adhesive ability when used as a binder fiber.

[0046] The orientation parameter of the undrawn polyarylene sulfide fiber can be adjusted to the above range by setting the spinning speed to 1500 m / min or less, but other methods may be used as long as they do not impede the object of the present invention.

[0047] The average fiber diameter of the unstretched polyarylene sulfide fiber is 0.8 μm or more and 6.0 μm or less. If the average fiber diameter of the unstretched polyarylene sulfide fiber is greater than 6.0 μm, the number of constituent fibers per unit area is reduced, and the number of bonding points is reduced, resulting in a decrease in the tensile strength of the wetlaid nonwoven fabric and an increase in the thickness of the wetlaid nonwoven fabric. Furthermore, if the average fiber diameter is large, the pore size variation increases and the uniformity of the pore size decreases. The average fiber diameter of the unstretched polyarylene sulfide fiber is preferably 4.0 μm or less, more preferably 2.5 μm or less. On the other hand, if the average fiber diameter of the unstretched polyarylene sulfide fiber is less than 0.8 μm, the number of constituent fibers per unit area is increased, resulting in a structure in which the fibers are densely packed, resulting in a decrease in the breathability of the resulting wetlaid nonwoven fabric. Furthermore, the aspect ratio is also increased, resulting in a decrease in dispersibility and a decrease in the uniformity of the pore size of the wetlaid nonwoven fabric. The average fiber diameter of the unstretched polyarylene sulfide fibers is preferably 0.9 μm or more, and more preferably 1.0 μm or more.

[0048] The average fiber diameter of the undrawn polyarylene sulfide fibers can be adjusted to the above range by adjusting the spinning output and draw ratio, but other methods may also be used as long as they do not impede the object of the present invention.

[0049] The aspect ratio of the unstretched polyarylene sulfide fiber (average fiber length / average fiber diameter) is preferably 400 or more and 1800 or less. By setting the aspect ratio of the unstretched polyarylene sulfide fiber to more preferably 1600 or less, even more preferably 1400 or less, and most preferably 1300 or less, it is possible to obtain unstretched polyarylene sulfide fiber with excellent dispersibility, which allows the binder adhesion points to be uniform and a wetlaid nonwoven fabric with high tensile strength to be obtained. Furthermore, by setting the aspect ratio to preferably 500 or more, even more preferably 700 or more, the fibers are less likely to fall off during the processing step and the fibers are more likely to entangle with each other, resulting in a wetlaid nonwoven fabric with excellent tensile strength.

[0050] <Wetlaid Nonwoven Fabric> The wetlaid nonwoven fabric of the present invention preferably contains stretched polyarylene sulfide fibers and unstretched polyarylene sulfide fibers. The stretched polyarylene sulfide fibers function as the main fibers, and the unstretched polyarylene sulfide fibers function as the binder fibers. The unstretched polyarylene sulfide fibers have a mostly amorphous structure and melt when heated, thereby fulfilling their role as a binder. The stretched polyarylene sulfide fibers are stretched in the fiber production process, resulting in high single fiber strength and excellent dimensional stability. In the present invention, the combined use of stretched polyarylene sulfide fibers and unstretched polyarylene sulfide fibers makes it possible to provide a wetlaid nonwoven fabric with excellent chemical resistance and heat resistance. Furthermore, by forming a wetlaid nonwoven fabric, a nonwoven fabric with a low basis weight and highly uniform pore size can be obtained.

[0051] The wetlaid nonwoven fabric of the present invention preferably contains 20% by mass or more and 80% by mass or less of stretched polyarylene sulfide fibers relative to the total mass of the wetlaid nonwoven fabric. More preferably, it contains 30% by mass or more and 70% by mass or less of stretched polyarylene sulfide fibers. When the stretched polyarylene sulfide fibers, which are the main fibers of the wetlaid nonwoven fabric, are contained in an amount of 20% by mass or more, the wetlaid nonwoven fabric has higher tensile strength. On the other hand, when the stretched polyarylene sulfide fibers are contained in an amount of 80% by mass or less, the proportion of binder fibers can be increased, increasing the number of bonding points between the main fibers and resulting in a wetlaid nonwoven fabric with higher tensile strength.

[0052] Furthermore, the wetlaid nonwoven fabric preferably contains 20% by mass or more and 80% by mass or less of unstretched polyarylene sulfide fiber relative to the total mass of the wetlaid nonwoven fabric. It is even more preferably 30% by mass or more and 70% by mass or less. When the unstretched polyarylene sulfide fiber, which is the binder fiber of the wetlaid nonwoven fabric, is 20% by mass or more, the number of bonding points increases, resulting in a wetlaid nonwoven fabric with higher tensile strength. On the other hand, when the unstretched polyarylene sulfide fiber is 80% by mass or less, the proportion of the main fiber component can be increased, resulting in a wetlaid nonwoven fabric with higher tensile strength.

[0053] Other fibers may be mixed in as long as the effects of the present invention are not impaired, but the total amount of stretched polyarylene sulfide fibers and unstretched polyarylene sulfide fibers is preferably 80 mass% or more of the total mass of the wetlaid nonwoven fabric.

[0054] The mass ratio of the stretched polyarylene sulfide fiber to the unstretched polyarylene sulfide fiber is preferably (stretched polyarylene sulfide fiber):(unstretched polyarylene sulfide fiber)=10:90 to 90:10, and more preferably (stretched polyarylene sulfide fiber):(unstretched polyarylene sulfide fiber)=20:80 to 80:20. By using the stretched polyarylene sulfide fiber to the unstretched polyarylene sulfide fiber in the above ratio, the number of bonding points to the main fiber becomes appropriate, and a wetlaid nonwoven fabric with higher tensile strength can be obtained.

[0055] The average fiber diameter of the unstretched polyarylene sulfide fibers constituting the wetlaid nonwoven fabric of the present invention is preferably smaller than the average fiber diameter of the stretched polyarylene sulfide fibers. The wetlaid nonwoven fabric of the present invention generates high stress when pulled because the main fibers (i.e., stretched polyarylene sulfide fibers) are bonded together via binder fibers (i.e., unstretched polyarylene sulfide fibers). If the binder fibers are thinner than the main fibers, the bonded joints are multipointed and uniform, dispersing stress during pulling and making the fabric less susceptible to plastic deformation, resulting in higher strength of the wetlaid nonwoven fabric. The ratio of the average fiber diameter of the unstretched polyarylene sulfide fibers to the average fiber diameter of the stretched polyarylene sulfide fibers (average fiber diameter of unstretched polyarylene sulfide fibers / average fiber diameter of stretched polyarylene sulfide fibers) is preferably 0.05 or more and 0.90 or less, more preferably 0.10 or more and 0.80 or less. By setting the thickness within the above range, the bonded portions are multi-pointed and uniform in structure, stress is dispersed when pulled, plastic deformation is less likely to occur, and the tensile strength of the wetlaid nonwoven fabric is increased.

[0056] The wetlaid nonwoven fabric of the present invention has through-holes. The through-holes are formed by the gaps between fibers. It is important that the average pore diameter is 0.5 μm or more and 50.0 μm or less. In conventional wetlaid nonwoven fabrics, simply reducing the diameter of the constituent fibers to obtain a low basis weight has the problem of not being able to obtain a wetlaid nonwoven fabric with highly uniform pore diameters and reducing tensile strength. The inventors have discovered that these problems can be solved by appropriately adjusting the average pore diameter. If the average pore diameter is less than 0.5 μm, the nonwoven fabric becomes film-like, making it difficult to maintain the porosity characteristic of nonwoven fabrics, and reducing breathability. The average pore diameter is preferably 1.0 μm or more, more preferably 3.0 μm or more. On the other hand, if the average pore diameter is too large, greater than 50.0 μm, the wetlaid nonwoven fabric tends to have poor formation, lose pore diameter uniformity, and reduce tensile strength. The average pore diameter is preferably 30.0 μm or less, more preferably 20.0 μm or less.

[0057] The average pore size referred to here refers to a value calculated by the bubble point method. For example, the Perm-Porometer (manufactured by PMI), an automatic porous material pore measurement system, can be used for the bubble point method. In measurements using this Perm-Porometer, a wetlaid nonwoven sheet is immersed in a liquid with a known surface tension, and a gas pressure is supplied from above the sheet while increasing, and the average pore size is measured from the relationship between this pressure and the surface tension of the liquid on the surface of the wetlaid nonwoven sheet.

[0058] The average pore size of the polyarylene sulfide wetlaid nonwoven fabric can be adjusted to the above range by adjusting the basis weight and the average fiber diameter of the constituent fibers.

[0059] The basis weight of the wetlaid nonwoven fabric of the present invention is 1.0 g / m ２ 8.0g / m or more ２ The basis weight is less than 8.0 g / m ２ If the weight is more than this, the wetlaid nonwoven fabric will be thick, and in addition, the pore size will be extremely small due to the excessive number of constituent fibers, which tends to reduce the air permeability and may reduce the ion conductivity of the electrolyte reinforced membrane. ２ less than 5.0 g / m ２less than 3.0 g / m ２ On the other hand, the basis weight is less than 1.0 g / m ２ If the weight is less than 1.5 g / m, the number of fibers constituting the nonwoven fabric is too small, resulting in a decrease in tensile strength. In addition, the pore size distribution varies, resulting in a decrease in the uniformity of the pore size of the nonwoven fabric. ２ More preferably, 2.0 g / m ２ That's all.

[0060] The basis weight of the wetlaid nonwoven fabric can be adjusted to the above range by adjusting the amount of fiber added to the fiber dispersion for papermaking.

[0061] The thickness of the wetlaid nonwoven fabric of the present invention is preferably 15.0 μm or less. By setting the thickness to preferably 12.0 μm or less, more preferably 10.0 μm or less, and even more preferably 8.0 μm or less, the ion permeability of an electrolyte-reinforced membrane using the wetlaid nonwoven fabric may be improved, which is preferable. Furthermore, when the wetlaid nonwoven fabric is installed in a device as an electrolyte membrane reinforcing material, the volume occupied by the wetlaid nonwoven fabric is reduced, thereby enabling the device to be made smaller and lighter.

[0062] The thickness of the wetlaid nonwoven fabric can be adjusted to the above range by adjusting the basis weight, the average fiber diameter of the constituent fibers, and the calendering conditions.

[0063] In the wetlaid nonwoven fabric of the present invention, pores having a diameter of 0.9 Dp or more and 1.1 Dp or less preferably account for 50% or more of the total pores, where Dp (nm) is the pore diameter showing the maximum peak of the pore diameter distribution. Here, the pore diameter distribution curve is determined from the pressure change of the air flow rate (in the range of 0 kPa to 3000 kPa) in the dry and wet states. More preferably, pores having a diameter of 0.9 Dp or more and 1.1 Dp or less account for 55% or more, and even more preferably 60% or more of the total pores. A higher proportion of pores having a diameter of 0.9 Dp or more and 1.1 Dp or less indicates the presence of pores with uniform diameters without unevenness, and the tensile strength and air permeability of the wetlaid nonwoven fabric are less uneven. The higher this value, the more desirable it is.

[0064] The proportion of pores having a Dp of 0.9 to 1.1 can be adjusted to fall within the above range by adjusting the aspect ratio of the fibers, the concentration of the papermaking fiber dispersion, the dispersant concentration, and the thickener concentration.

[0065] The pore size distribution curve of the wetlaid nonwoven fabric of the present invention can be measured using a Perm-Porometer according to the bubble point method (ASTM F316-86).

[0066] The shrinkage rate of the wetlaid nonwoven fabric of the present invention after being kept at a temperature of 160°C for 24 hours and cooled to room temperature is preferably 8.0% or less. A shrinkage rate of 7.0% or less is more preferable, and a shrinkage rate of 6.0% or less is even more preferable. A lower shrinkage rate indicates higher dimensional stability against heat, and when used as a reinforcing material for an electrolyte membrane operated at high temperatures for long periods of time, deterioration or tearing of the membrane due to shrinkage is less likely to occur. Here, a negative shrinkage rate and expansion are also undesirable, as this can cause deterioration or tearing of the membrane. Therefore, a shrinkage rate of 0% or more is preferable.

[0067] To achieve the shrinkage rate within the above range, it is effective to subject the wet-laid nonwoven fabric to thermal crystallization. Thermal crystallization can be achieved by the method described below, but other methods may also be used as long as they do not impede the object of the present invention.

[0068] The wetlaid nonwoven fabric of the present invention preferably has a tensile strength of 0.50 N / 15 mm or more. A tensile strength of more preferably 0.55 N / 15 mm or more, even more preferably 0.60 N / 15 mm or more, and most preferably 0.80 N / 15 mm or more makes it less likely to break during the manufacturing process. Furthermore, the wetlaid nonwoven fabric is less likely to break even when tensile stress is applied during subsequent processes or during use. The tensile strength can be improved because the stretched polyarylene sulfide fibers, which have high fiber strength, are firmly bonded by the fused unstretched polyarylene sulfide fiber parts. That is, the presence of the stretched polyarylene sulfide fibers and the fused unstretched polyarylene sulfide fiber parts makes it less likely for the fibers to come loose, and the stretched polyarylene sulfide fibers can bear the stress, resulting in a wetlaid nonwoven fabric with high tensile strength. The tensile strength of the wetlaid nonwoven fabric of the present invention refers to a value measured by the method described in the Examples.

[0069] The air permeability of the wetlaid nonwoven fabric of the present invention is 3.0 cm ３ / (cm ２ s) or more. More preferably, it is 10.0 cm ３ / (cm ２ s) or more, particularly preferably 15.0 cm ３ / (cm ２ The air permeability is also necessary to ensure the properties of the nonwoven fabric, and if the air permeability is high, ion conduction is less likely to be inhibited when the nonwoven fabric is used as a reinforcing material for an electrolyte membrane, and the ion conductivity tends to be improved. On the other hand, if the air permeability of a wet-laid nonwoven fabric is too high, the dimensional stability decreases, so the air permeability is set to 300 cm ３ / (cm ２ It is preferable that the value is equal to or less than s.

[0070] <Electrolyte Reinforced Membrane> The wetlaid nonwoven fabric of the present invention can be suitably used as an electrolyte membrane reinforcing material. In this case, the wetlaid nonwoven fabric may be used as is, or may be subjected to a hydrophilization treatment within a range that does not impair the effects of the present invention. By subjecting the wetlaid nonwoven fabric to a hydrophilization treatment, the permeability of the electrolyte membrane is improved. Since the wetlaid nonwoven fabric has excellent tensile strength and air permeability even when it is thin and has a low basis weight, it serves as an electrolyte membrane reinforcing material that has excellent ion permeation efficiency and is resistant to tearing during packaging and use.

[0071] The electrolyte-reinforced membrane of the present invention can be used by integrating the wetlaid nonwoven fabric of the present invention with an electrolyte membrane, and can be suitably used in fuel cells. Examples of electrolyte membranes include fluorine-based electrolytes and hydrocarbon-based electrolytes. Examples of fluorine-based electrolytes include perfluorosulfonic acid polymers, polytrifluorostyrene sulfonic acid polymers, and perfluorocarbon phosphonic acid polymers. In particular, cation exchange membranes made of perfluorosulfonic acid containing sulfonic acid groups, such as "Nafion" (registered trademark, DuPont), are preferably used because of their excellent basic properties.

[0072] <Method for producing wetlaid nonwoven fabric> Hereinafter, a method for producing a wetlaid nonwoven fabric of the present invention will be described.

[0073] [Method for producing drawn polyarylene sulfide fiber] Various polyarylene sulfide fibers can be used, but it is preferable to obtain the drawn polyarylene sulfide fiber by disposing polyarylene sulfide in the island component fibers and removing the easily soluble sea component polymer of the islands-in-sea type composite fiber having an islands-in-sea type composite cross section.

[0074] In this production method, first, the polyarylene sulfide and the easily soluble polymer are melted separately, and then the melts are passed through polymer piping and metered by a known metering device such as a gear pump. After passing through a filter to remove foreign matter from the polymer, the melts are allowed to flow into a spinneret. The individual polymers flowing into the spinneret are shaped into an arbitrary composite form within the spinneret, merged, and extruded from the spinneret holes as islands-in-the-sea composite fibers.

[0075] The cross-sectional area of ​​the island component fibers in the islands-in-sea type composite fiber can be adjusted by adjusting the polymer discharge rate. For example, by forming the island component polyarylene sulfide into a round shape with a diameter of 2.0 μm, it is possible to produce a round cross-section fiber with an average fiber diameter of 2.0 μm.

[0076] The discharged undrawn fiber is taken up by a roller (godet roller) rotating at a constant speed. The take-up speed is preferably 300 m / min or more to improve linear uniformity and productivity, and 1500 m / min or less to prevent the orientation of molecular chains.

[0077] The undrawn fiber thus obtained is subjected to a drawing step. Drawing is performed by running the fiber between a heated first roller or a heating device provided between the first roller and the second roller. The drawing conditions are determined by the mechanical properties of the undrawn fiber obtained, and the draw ratio is determined by the ratio of the peripheral speeds of the first roller and the second roller.

[0078] The temperature of the heated first roller or heating device in the drawing step is preferably 80° C. or higher and 130° C. or lower. A temperature of 80° C. or higher fixes the drawing point, enabling stable drawing, while a temperature of 130° C. or lower can suppress yarn breakage and improve processability. Furthermore, from the viewpoint of fixing the drawing point, the temperature of the second roller is preferably the temperature of the heated first roller or heating device + 20° C. or lower.

[0079] Furthermore, after passing through the second roller, the drawn fibers may be heated and heat-set by a heated third roller or a heating device provided between the second roller and the third roller. The heat-setting temperature when heat-setting is preferably 150 to 240°C. By setting the heat-setting temperature to 150°C or higher, the shrinkage rate can be reduced by thermal crystallization. This prevents the width of the nonwoven fabric product from becoming uneven during the wetlaid nonwoven fabric production process, and also allows for the production of a wetlaid nonwoven fabric with excellent dimensional stability.

[0080] The resulting fibers are then cut with a cutter to a length of 0.3 mm to 10 mm to obtain cut fibers.

[0081] [Method for Producing Undrawn Polyarylene Sulfide Fibers] Cut fibers of undrawn fibers can be obtained in the same manner as in the method for producing drawn polyarylene sulfide fibers, except that drawing and heat setting are not carried out.

[0082] [Method for manufacturing a wetlaid nonwoven fabric] After dissolving the sea component from the two types of cut fibers obtained by the above method with sodium hydroxide, each is dispersed in an aqueous medium. Here, aqueous medium refers to a liquid primarily composed of water. These dispersions are mixed in a desired ratio to form a papermaking dispersion. The fiber concentration in the dispersion is preferably 1.0 ppm by mass or more and 80.0 ppm by mass or less. By setting the fiber concentration to 1.0 ppm by mass or more, more preferably 1.5 ppm by mass or more, production efficiency can be improved and the load on the dehydration process can be reduced. On the other hand, by setting the fiber concentration to 80.0 ppm by mass or less, more preferably 60.0 ppm by mass or less, and even more preferably 10.0 ppm by mass or less, the dispersion state can be improved, a uniform wetlaid nonwoven fabric can be obtained, and the tensile strength of the wetlaid nonwoven fabric can be increased.

[0083] The dispersion preferably contains a dispersant, and the concentration of the dispersant in the dispersion is preferably 0.0010% by mass or more and 8.0% by mass or less. By setting the dispersant concentration to 0.0010% by mass or more, more preferably 0.0075% by mass or more, and even more preferably 0.010% by mass or more, the fibers are uniformly dispersed in the dispersion, the pore size uniformity of the resulting wetlaid nonwoven fabric is improved, and the wetlaid nonwoven fabric has higher tensile strength. Furthermore, by setting the dispersant concentration to 8.0% by mass or less, more preferably 6.0% by mass or less, more preferably 3.0% by mass or less, and even more preferably 0.80% by mass or less, the fibers are uniformly dispersed in the dispersion, the pore size uniformity of the resulting wetlaid nonwoven fabric is improved, and the wetlaid nonwoven fabric has higher tensile strength.

[0084] As the dispersant, a surfactant or the like can be used, and examples of the surfactant include cationic, anionic, and nonionic surfactants.

[0085] The papermaking fiber dispersion may contain a water-soluble polymer as a thickener, or an antifoaming agent to suppress the generation of bubbles.

[0086] The papermaking dispersion prepared as described above is made into paper using a cylinder, fourdrinier, or inclined wire papermaking machine or a hand-made papermaking machine, and the paper is dried using a Yankee dryer, rotary dryer, or the like to form a dry web. This is then subjected to a heating and pressurizing treatment to obtain a wetlaid nonwoven fabric. In the present invention, the simultaneous application of heat and pressure is referred to as the heating and pressurizing treatment, and is distinguished from treatments such as drying, which involve only heating without pressurizing. A dry web refers to a wetlaid nonwoven fabric that has not been subjected to this heating and pressurizing treatment.

[0087] In a preferred method for producing the wetlaid nonwoven fabric of the present invention, unstretched polyarylene sulfide fibers are preferably produced by heat and pressure treatment or the like to improve interfiber adhesion. In this case, it is preferable to suppress crystallization during the dry web production process as much as possible. To achieve this, the drying temperature in the papermaking process is preferably 80 to 150°C, and more preferably 90 to 140°C. It is also preferable to suppress crystallization by shortening the drying process time. Insufficient drying not only reduces the paper strength of the dry web, but also causes rapid thermal shrinkage during the subsequent heat and pressure treatment, making the web more susceptible to paper breakage. Conversely, excessive drying promotes crystallization of the dry web, making it difficult for the unstretched polyarylene sulfide fibers to undergo plastic deformation during the subsequent heat and pressure treatment. As a result, the mechanical properties of the resulting wetlaid nonwoven fabric are reduced.

[0088] In the production of the wetlaid nonwoven fabric of the present invention, a dry web containing a mixture of two types of cut fibers is subjected to a heat and pressure treatment. By performing the heat and pressure treatment, the unstretched polyarylene sulfide fibers are melted and softened, as described above, and polyarylene sulfide fused portions can be formed. Any means for applying heat and pressure can be used, including, for example, a hot press using a flat plate or a hot calender. Of these, a hot calender, which allows continuous processing, is preferred. Examples of rolls that can be used for the hot calender include a metal-metal roll, a metal-paper roll, and a metal-rubber roll.

[0089] The temperature conditions for the heating and pressure treatment are preferably a temperature equal to or higher than the glass transition temperature of the unstretched polyarylene sulfide fiber and equal to or lower than the melting point of the polyarylene sulfide fiber. To achieve this, the heating temperature is preferably set to 150 to 240°C. By setting the heating temperature to preferably 150°C or higher, the unstretched polyarylene sulfide fiber melts and softens, allowing the formation of fused polyarylene sulfide portions, resulting in a wetlaid nonwoven fabric with excellent mechanical properties. By setting the heating temperature to preferably 240°C or lower, the polyarylene sulfide fiber is completely fused, preventing damage to the fiber shape.

[0090] When calendering is used as the heat and pressure treatment, the pressure is preferably 98 to 7000 N / cm. By setting the pressure to 98 N / cm or more, polyarylene sulfide fused portions can be formed, resulting in a wetlaid nonwoven fabric with excellent mechanical properties. On the other hand, by setting the pressure to 7000 N / cm or less, tearing of the wetlaid nonwoven fabric during the heat and pressure treatment step can be prevented, allowing for stable treatment. The process speed is preferably 1 to 30 m / min. By setting the speed to preferably 1 m / min or more, more preferably 2 m / min, good working efficiency can be achieved. On the other hand, by setting the speed to preferably 30 m / min or less, more preferably 20 m / min or less, heat can be conducted to the fibers inside the wetlaid nonwoven fabric, allowing for heat fusion of the binder fibers.

[0091] The wetlaid nonwoven fabric of the present invention will be described in more detail below with reference to examples. However, the present invention is not limited to these examples. Various modifications and alterations are possible within the scope of the present invention. The respective property values ​​in the examples were determined by the following methods.

[0092] [Measurement and Evaluation Methods] (1) Average Fiber Diameter Measurements were carried out as follows using a scanning electron microscope "S-5500" manufactured by Hitachi High-Technologies Corporation as the scanning electron microscope and "WinROOF2015" manufactured by Mitani Shoji Co., Ltd. as the image analysis software. Step 1: An image of the cross section of the fiber was taken with the scanning electron microscope. The observation magnification was as follows: when the fiber diameter of the fiber was 5 μm or more and 14 μm or less, the observation magnification was 1000 times, when the fiber diameter was 1 μm or more and less than 5 μm, the observation magnification was 3000 times, and when the fiber diameter was less than 1 μm, the observation magnification was 5000 times. Step 2: Using the taken image and image analysis software, the area Af (μm ２ Step 3: This is measured for 100 randomly selected fibers, and a simple number average is calculated to calculate the average fiber diameter (μm), which is then rounded off to one decimal place.

[0093] (2) Average Fiber Length Measurement was carried out based on "8.4.1. Average Fiber Length Direct Method (Method C)" of JIS L1015:2010 "Test Methods for Chemical Fiber Staples," and the average value of 200 fibers was calculated and rounded to one decimal place.

[0094] (3) Basis weight Based on JIS L1913:2010 "General nonwoven fabric test method", three 10 cm x 10 cm sample pieces are taken in accordance with the mass per unit area, and the mass (g) of each piece is measured under standard conditions. The average value is calculated as 1 m ２ Mass per unit (g / m ２ ) is expressed as

[0095] (4) Thickness According to "9.3.1 Measurement of Thickness" of JIS P8118:2014 "Paper and Paperboard - Test Methods for Thickness, Density and Specific Volume", 20 test pieces of 10 cm x 10 cm were taken, and the thickness of each of the 20 test pieces was measured using a micrometer (manufactured by Mitutoyo Corporation), and the arithmetic mean value was taken as the thickness (μm).

[0096] (5) Tensile Strength Tensile strength was measured in accordance with "6.3.1 Standard Time" in "6.3 Tensile Strength and Elongation (ISO Method)" of JIS L1913:2010 "Test Methods for Chemical Fiber Staples." Using a Tensilon ("UTM-III-100" manufactured by Orientec Co., Ltd.), the maximum point load was measured under the conditions of a sample width of 15 mm, an initial length of 20 mm, and a tensile speed of 20 mm / min. Five measurements were made per level, and the arithmetic average value was calculated and used as the tensile strength (N / 15 mm).

[0097] (6) Average pore size Pore size was calculated using a porous material automatic pore measurement system Perm-Porometer (manufactured by PMI) according to the bubble point method (based on ASTM F-316-86). The measurement sample diameter was 25 mm, and the pore size distribution was measured using Galwick (surface tension: 16 mN / m) as a measurement liquid with a known surface tension. The average flow diameter obtained by automatic calculation was used as the average pore size, and the value was rounded to one decimal place.

[0098] (7) Pore size distribution Using a porous material automatic pore measurement system Perm-Porometer (manufactured by PMI), a wet nonwoven sheet was immersed in a liquid with a known surface tension, and gas pressure was supplied from above the sheet while increasing. Measurements were then performed in the dry-up and wet-up states based on the relationship between this pressure and the liquid surface tension on the surface of the wet nonwoven sheet, and a pore size distribution curve was obtained. In the obtained pore size distribution curve, the proportion of pores with a diameter of 0.9 Dp to 1.1 Dp was calculated, where Dp (nm) represents the maximum peak in the pore size distribution, out of all pores detected in the range of 0 kPa to 3000 kPa.

[0099] (8) Air permeability According to "6.8.1 Frazier type method" of "6.8 Air permeability (JIS method)" of JIS L1913:2010 "Testing methods for synthetic fiber staples", three 15 cm x 15 cm sample pieces were taken, and the air permeability of the three test pieces was measured at a test pressure of 125 Pa using an air permeability tester ("FX3300" manufactured by TEXTEST AG), and the arithmetic average value was calculated as the air permeability (cm ３ / (cm ２ ・s))

[0100] (9) Shrinkage A 15 cm × 15 cm wet-laid nonwoven fabric in a free state was kept in a dryer at 160°C for 24 hours, and the shrinkage (%) was calculated from the dimensional change before and after heat treatment using the following formula: Shrinkage = (1 - area of ​​wet-laid nonwoven fabric after heat treatment / area of ​​wet-laid nonwoven fabric before heat treatment) × 100.

[0101] (10) Orientation Parameter Measurement was carried out using a laser Raman device (manufactured by Photon Design Co., Ltd.) as follows. First, a spectrum was obtained through a polarizer arranged in a polarization direction parallel to the polarization direction of the laser light. Next, the sample was rotated, and spectra were obtained in polarization directions parallel and perpendicular to the polarization direction of the laser light. Then, the orientation parameter was calculated using the following formula: (Orientation Parameter) = (I １０８０ / I ７４０ ) (parallel) / (I １０８０ / I ７４０ ) (vertical) I １０８０ / I ７４０ (Parallel): 1080 cm in the Raman spectrum measured in a polarization direction parallel to the fiber axis direction －１ The Raman band around 740 cm －１ Divided by the intensity of the Raman band near I １０８０ / I ７４０ (Perpendicular): 1080 cm in a Raman spectrum measured in a polarization direction perpendicular to the fiber axis direction －１ The Raman band around 740 cm －１ Divided by the intensity of the nearby Raman band.

[0102] [Production Example 1] <Production of drawn polyarylene sulfide fiber> Polyphenylene sulfide consisting only of p-phenylene sulfide units was used as island components, and polyethylene terephthalate copolymerized with 5.0 mol % of 5-sodium sulfoisophthalic acid was used as a sea component. A sea / island component composite ratio of 40 / 60 was melt-extruded using an islands-in-sea composite spinneret (number of islands: 1000) in which the island components had round shapes. The extruded yarn was cooled and solidified, and then an oil was applied thereto. The yarn was then taken up at a spinning speed of 1000 m / min, to obtain an undrawn fiber having an islands-in-sea composite cross section (output rate per hole: 2.6 g / min).

[0103] The resulting undrawn fiber was heated and drawn six times around a first roller and a second roller heated to 90°C and 100°C, respectively. The fiber was then heated and drawn six times around a third roller heated to 180°C, and heat-set. The draw ratio was 3.85, and the fiber was taken up after the third roller by an unheated roller at a peripheral speed of 400 m / min to obtain a drawn polyphenylene sulfide fiber.

[0104] The resulting drawn polyphenylene sulfide fibers were then cut with a cutter to obtain cut fibers. The orientation parameter of the resulting drawn polyphenylene sulfide fibers was measured and found to be 11.

[0105] [Production Example 2] <Production of Undrawn Polyarylene Sulfide Fiber> After obtaining an undrawn fiber by the method described in <Production of Drawn Polyarylene Sulfide Fiber>, the obtained undrawn fiber was cut with a cutter to obtain a cut fiber. The orientation parameter of the obtained undrawn polyphenylene sulfide fiber was measured and found to be 2.5.

[0106] Example 1: Stretched polyphenylene sulfide fibers and unstretched polyphenylene sulfide fibers having the average fiber diameters and aspect ratios shown in Table 1 were subjected to a sea component elution treatment using a 3% by mass aqueous sodium hydroxide solution (bath ratio 1 / 100) heated to 90°C and 65°C, respectively, to obtain a fiber dispersion. The resulting fiber dispersion was then weighed out to achieve the basis weight and blending ratio shown in Table 1, and the fibers were dispersed in the dispersion to obtain a fiber concentration of 2.0 ppm by mass, thereby obtaining a fiber dispersion for papermaking. A dispersant (Y-258 (nonionic surfactant) manufactured by Yoshimura Oil Chemical Co., Ltd.) was added to the papermaking fiber dispersion to a dispersant concentration of 0.020% by mass, and a thickener (Meipam (registered trademark) manufactured by Meisei Chemical Industry Co., Ltd.) was added to the dispersion to a dispersant concentration of 0.00030% by mass, and the mixture was thoroughly stirred. This papermaking fiber dispersion was used to make paper using a square sheet machine (250 mm square) manufactured by Kumagai Riki Kogyo Co., Ltd., and the paper was dried in a rotary dryer with a roller temperature set to 120°C to obtain a dry web, which was then subjected to thermal calendering at an iron roll surface temperature of 220°C, a linear pressure of 490 N / cm, and a roll rotation speed of 3 m / min to obtain a wetlaid nonwoven fabric, the physical properties of which were evaluated. The evaluation results are shown in Table 1.

[0107] Examples 2 to 22, Comparative Examples 1 to 8 Wetlaid nonwoven fabrics were obtained in the same manner as in Example 1, except that various conditions were changed as shown in Tables 1 to 3.

[0108] As can be seen from the table, the wetlaid nonwoven fabrics of Examples 1 to 22 were thin but had high tensile strength and air permeability, and had highly uniform pore size. In contrast, the wetlaid nonwoven fabrics of Comparative Examples 1 to 8 were poor in any one of the properties of thickness, strength, air permeability, or uniformity of pore size.

[0109]

[0110]

[0111]

Claims

1. A stretched polyarylene sulfide fiber having an average fiber diameter of 0.3 μm or more and 3.0 μm or less, an average pore size of 0.5 μm or more and 50.0 μm or less, and a basis weight of 1.0 g / m ２ 8.0g / m or more ２ Wet nonwoven fabric.

2. The wetlaid nonwoven fabric according to claim 1, which contains unstretched polyarylene sulfide fibers having an average fiber diameter of 0.8 μm or more and 6.0 μm or less.

3. The wetlaid nonwoven fabric according to claim 1 or 2, wherein the aspect ratio (average fiber length / average fiber diameter) of the stretched polyarylene sulfide fibers is 800 or more and 2,000 or less, and the aspect ratio (average fiber length / average fiber diameter) of the unstretched polyarylene sulfide fibers is 400 or more and 1,800 or less.

4. The wetlaid nonwoven fabric according to claim 1 or 2, in which, when the pore size showing the maximum peak of the pore size distribution in a pore size distribution curve is defined as Dp (nm), pores having a size of 0.9 Dp or more and 1.1 Dp or less account for 50% or more of all pores.

5. The wetlaid nonwoven fabric according to claim 1 or 2, which has a shrinkage rate of 8.0% or less after being left to stand in an atmosphere at a temperature of 160° C. for 24 hours.

6. The wetlaid nonwoven fabric according to claim 2, wherein the average fiber diameter of the unstretched polyarylene sulfide fibers is smaller than the average fiber diameter of the stretched polyarylene sulfide fibers.

7. An electrolyte-reinforced membrane comprising the wet-laid nonwoven fabric according to claim 1 or 2.

8. A method for producing a wetlaid nonwoven fabric as described in claim 2, comprising a step of dispersing the stretched polyarylene sulfide fibers and the unstretched polyarylene sulfide fibers in an aqueous medium to obtain a dispersion, wherein the fiber concentration in the dispersion is 1.0 ppm by mass or more and 80.0 ppm by mass or less.

9. The method for producing a wetlaid nonwoven fabric according to claim 8, wherein the dispersion liquid contains a dispersant, and the concentration of the dispersant in the dispersion liquid is 0.0010% by mass or more and 8.0% by mass or less.

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