JP7498603B2 - Conductive Sheet - Google Patents

Description

The present invention relates to a conductive sheet.

In recent years, there has been active development of conductive sheets as wiring materials, shielding materials, or components for electrochemical elements such as fuel cells. As disclosed in, for example, JP 2010-192361 A (Patent Document 1), JP 2011-233274 A (Patent Document 2), JP 2012-074319 A (Patent Document 3), and JP 2013-093189 A (Patent Document 4), conductive sheets in which conductive particles are present in the voids of a fiber assembly are known. In a conductive sheet having this structure, the conductive particles present in the voids of the fiber assembly are in contact with each other so as to bridge the two main surfaces of the conductive sheet, and can play the role of a current path between the two main surfaces of the conductive sheet.

And it is known that a conductive sheet having such a configuration can be suitably used as a moisture management sheet placed between the catalyst layer and the gas diffusion layer of a fuel cell, as disclosed in Patent Documents 1 to 4.

JP 2010-192361 A JP 2011-233274 A JP 2012-074319 A JP 2013-093189 A

Depending on the application, such as a moisture management sheet, a conductive sheet having the above-mentioned configuration is required to have low electrical resistance in the thickness direction (between both main surfaces) and excellent conductivity, as well as a low Gurley value and excellent breathability. However, it has been difficult to provide a conductive sheet that satisfies the above-mentioned requirements by only referring to conventional technology.

In other words, if the amount of conductive particles present in the voids of the fiber aggregate is increased in order to reduce the electrical resistance in the thickness direction of the conductive sheet and improve conductivity, the voids in the fiber aggregate will be blocked, increasing the Gurley value of the conductive sheet and decreasing its breathability.

On the other hand, if the amount of conductive particles present in the voids of the fiber aggregate is reduced in order to reduce the Gurley value in the thickness direction of the conductive sheet and improve breathability, the number of conductive particles that act as electrical paths between the two main surfaces of the conductive sheet will decrease, and the electrical resistance in the thickness direction of the conductive sheet will increase and the electrical conductivity will decrease.

As such, in a conductive sheet having the above-mentioned configuration, there is a trade-off between conductivity and breathability, making it difficult to provide a conductive sheet that is excellent in both conductivity and breathability in the thickness direction.

The present invention aims to provide a conductive sheet that has excellent conductivity and breathability in the thickness direction.

The first invention is "a conductive sheet in which conductive particles are present only in the voids of a fiber aggregate in which the intersections of constituent fibers are integrated by fiber bonding using partially heat-fused composite fibers , the conductive particles including two or more types of conductive particles having different average particle diameters , the conductive particles having the largest average particle diameter of the two or more types of conductive particles being graphite, and the conductive sheet having through holes in the thickness direction."

The second invention is "the conductive sheet according to claim 1, used as a moisture management sheet disposed between a catalyst layer and a gas diffusion layer of a fuel cell."

As a result of the applicant's continued research, it was discovered that a conductive sheet having conductive particles present in the voids of a fiber assembly, which contains two or more types of conductive particles with different average particle sizes and has through holes in the thickness direction, can provide a conductive sheet that has excellent conductivity and breathability in the thickness direction. The reason for this is not completely clear, but it is thought to be due to the following effects.

The presence of two or more types of conductive particles with different average particle diameters (conductive particles with a large average particle diameter and conductive particles with a small average particle diameter) in the voids of the fiber aggregate causes the conductive particles to be distributed unevenly, which is thought to facilitate the formation of voids between the conductive particles and between the fiber aggregate and the conductive particles. As a result, an increase in the Gurley value of the conductive sheet can be prevented, and a highly breathable conductive sheet can be provided.

In addition, the presence of conductive particles with a large average particle size reduces the number of conductive particles required to bridge the two main surfaces of the conductive sheet, and it is believed that the total resistance of the electrical resistance that occurs when electricity passes through the contact parts between the conductive particles that exist between the two main surfaces of the conductive sheet is reduced. Furthermore, conductive particles with a small average particle size are present in the gaps between the conductive particles with a large average particle size, and it is believed that the conductive particles with a small average particle size act as bypasses that allow electricity to pass through. As a result, a conductive sheet with excellent conductivity in the thickness direction can be provided.

Furthermore, the conductive sheet has through holes in the thickness direction, which further improves the breathability of the conductive sheet. Since the conductive sheet satisfying the configuration of the present invention has excellent conductivity as described above, the conductive sheet of the present invention is unlikely to lose conductivity, even though it has through holes that do not contain conductive particles and are not thought to contribute to improving conductivity.

From the above, the present invention can provide a conductive sheet that has excellent conductivity and breathability in the thickness direction.

The conductive sheet of the present invention has excellent conductivity and breathability in the thickness direction, so it can be suitably used as a moisture management sheet to be placed between the catalyst layer and gas diffusion layer of a fuel cell.

In the present invention, various configurations can be appropriately selected, such as the following configurations. Unless otherwise specified, the various measurements described in the present invention were performed under normal pressure and at 25°C (room temperature). Furthermore, unless otherwise specified, the results of the various measurements described in the present invention were measured to a value one digit smaller than the desired value, and the desired value was calculated by rounding off the value. As a specific example, when the desired value is to one decimal place, the value was measured to one decimal place, and the obtained value to one decimal place was rounded off to the nearest decimal place to calculate the value to one decimal place, and this value was used as the desired value. Furthermore, the upper and lower limits exemplified in the present invention can be combined in any combination.

The fiber aggregate referred to in the present invention is, for example, a fiber web, nonwoven fabric, or a sheet-like fabric having voids, such as a woven fabric or knitted fabric, and mainly serves as the framework of the conductive sheet. The conductive sheet of the present invention is flexible because it contains a fiber aggregate (particularly a nonwoven fabric), and has excellent shape stability and dimensional stability.

The type of fibers constituting the fiber aggregate can be selected as appropriate, and can be glass fibers, metal oxide fibers such as alumina, and/or fibers made of organic resin. The type and mixing ratio of the fibers constituting the fiber aggregate can be selected as appropriate, and the fiber aggregate can be made of only one type of fiber, or a fiber aggregate made of a mixture of multiple types of fibers. Since this makes it easier to provide a conductive sheet with excellent flexibility, it is preferable that the fibers constituting the fiber aggregate are only fibers made of organic resin.

The type of organic resin can be appropriately selected, but examples of the organic resin include polyolefin resins (polyethylene, polypropylene, polymethylpentene, polyolefin resins in which part of the hydrocarbon is replaced with a nitrile group or a halogen such as fluorine or chlorine, etc.), styrene resins, polyether resins (polyethylene glycol, polypropylene glycol, polyether ether ketone, polyacetal, modified polyphenylene ether, aromatic polyether ketone, etc.), phenol resins, melamine resins, urea resins, epoxy resins, polyester resins (polyethylene terephthalate, polytrimethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, polybutylene naphthalate, polycarbonate, polyarylate, polylactic acid, wholly aromatic polyester resins, unsaturated polyester resins, etc.), polyimide resins, polyamideimide resins, polyamide resins (e.g., aramid Resins such as aromatic polyamide resins, aromatic polyetheramide resins, nylon resins, etc.), urethane resins, epoxy resins, polysulfone resins (polysulfone, polyethersulfone, sulfonated polyethersulfone, etc.), fluorine resins (polytetrafluoroethylene, polyvinylidene fluoride, perfluorosulfonic acid resins, etc.), vinyl alcohol resins (polyvinyl alcohol, polyvinyl acetate, etc.), polycaprolactone, polyglycolic acid, polyvinylpyrrolidone, polybenzimidazole resins, acrylic resins (for example, polyacrylonitrile resins copolymerized with acrylic acid esters or methacrylic acid esters, modacrylic resins copolymerized with acrylonitrile and vinyl chloride or vinylidene chloride, etc.), etc., may be fibers containing known organic resins, and may be fibers composed of only one type of organic resin, or may be fibers composed of multiple types of organic resins by mixing, etc.

These organic resins may be either linear or branched polymers, and may be block or random copolymers. The organic resins may have any three-dimensional structure or may be crystalline or non-crystalline.

The fibers may be monofilaments, fibril fibers, or composite fibers. Composite fibers may be, for example, sheath-core, island-in-the-sea, side-by-side, orange, or bimetallic fibers.

The fiber assembly preferably includes partially heat-sealed composite fibers. Fiber adhesion using partially heat-sealed composite fibers allows the intersections between the constituent fibers of the fiber assembly to be integrated without using unnecessary components such as binders, providing a highly rigid conductive sheet. In particular, it is preferable that the composite fibers are partially heat-sealed high-strength core-sheath composite fibers or other fibers with high tensile strength, since this provides a conductive sheet with excellent penetration resistance. As such fibers, partially heat-sealed high-strength polyolefin-based core-sheath composite fibers (for example, a core component of polypropylene and a sheath component of high-density polyethylene) with a tensile strength of 3 cN/dtex or more can be used.

The cross-sectional shape of the fibers may be a substantially circular or elliptical shape, or may be an irregular cross-sectional shape. Examples of irregular cross-sectional fibers include fibers having a cross section with a hollow shape, a polygonal shape such as a triangular shape, an alphabetic shape such as a Y-shape, an irregular shape, a multi-lobed shape, a symbolic shape such as an asterisk shape, or a shape that combines multiple of these shapes.

The method for preparing the fibers can be appropriately selected, but known methods such as melt spinning, dry spinning, wet spinning, direct spinning (melt blowing, spunbonding, electrostatic spinning in which an electric field is applied to the spinning solution to spin, spinning using centrifugal force, spinning using an accompanying airflow as described in JP 2011-012372 A, neutralization spinning, which is a type of electrostatic spinning as described in JP 2005-264374 A, etc.), and methods for extracting fibers with a small fiber diameter by removing one or more resin components from composite fibers can be used.

The fibers prepared by the above-mentioned method can be subjected to, for example, a dry method or a wet method to prepare a fiber web, and the constituent fibers of the prepared fiber web can be entangled and/or integrated to prepare a nonwoven fabric. Examples of methods for entangling and/or integrating the constituent fibers include a method of entangling with a needle or a fluid flow such as a water flow or steam/gas, and a method of subjecting the fiber web to a heat treatment to bond or melt the constituent fibers together using a binder or an adhesive fiber as described above.

The heat treatment method can be appropriately selected, but examples of the method that can be used include heating or heating and pressurizing with a roll, heating in a heater such as an oven dryer, far-infrared heater, dry heat dryer, or hot air dryer, and irradiating with infrared rays without pressure.

The type of binder that can be used is appropriately selected, but examples that can be used include polyolefins (such as modified polyolefins), ethylene-vinyl alcohol copolymers, ethylene-acrylate copolymers such as ethylene-ethyl acrylate copolymers, various rubbers and their derivatives (styrene-butadiene rubber (SBR), polyethylene oxide (PEO), fluororubber, urethane rubber, ethylene-propylene-diene rubber (EPDM), etc.), cellulose derivatives (carboxymethyl cellulose (CMC), hydroxyethyl cellulose, hydroxypropyl cellulose, etc.), polyvinyl alcohol (PVA), polyvinyl butyral (PVB), polyvinylpyrrolidone (PVP), polyurethane, epoxy resins, polyvinylidene fluoride (PVdF), vinylidene fluoride-hexafluoropropylene copolymer (PVdF-HFP), acrylic resins, etc.

The average fiber diameter of the fibers constituting the fiber assembly can be appropriately selected and can be 0.1 to 20 μm, 0.5 to 15 μm, or 1 to 10 μm. The "average fiber diameter" referred to here refers to the arithmetic average value of the fiber diameters of 50 fibers measured based on an electron microscope photograph taken at 5000 times magnification of the cross section or surface of an object such as a fiber assembly. If the fiber diameter is too thin to measure, it can be measured based on an electron microscope photograph at a magnification of more than 5000 times. If the cross section of the fiber is noncircular, the diameter of a circle with the same area as the cross section is considered to be the fiber diameter.

The fiber length of the fibers that make up the fiber aggregate is selected appropriately, but they can be short or long fibers with a specific length, or continuous fibers with a length that makes it difficult to actually measure the fiber length. The "fiber length" referred to here can be measured based on 5000x electron microscope photographs of the cross section or surface of an object such as a fiber aggregate. If the fiber length is too long to measure, it can be measured based on electron microscope photographs with a magnification of less than 5000x.

When the fiber assembly is a woven or knitted fabric, the woven or knitted fabric can be prepared by weaving or knitting the fibers prepared as described above.

The fiber aggregate may be integrated by entangling the constituent fibers, or may be integrated with a binder, or may be integrated at the intersections of the fibers by melting some of the constituent fibers. A nonwoven fabric obtained by spinning and collecting the fibers using a direct spinning method is preferable because it can form a fiber aggregate without using unnecessary components such as binders. In addition to fiber webs, fiber aggregates such as nonwoven fabrics, woven fabrics, and knitted fabrics may be subjected to the above-mentioned method of entangling and/or integrating the constituent fibers.

Various physical properties such as basis weight and thickness of the fiber assembly can be appropriately selected. The basis weight can be 0.1 to 200 g/ ^m2 , 0.3 to 100 g/ ^m2 , or 0.5 to 20 g/m2. This "basis weight" refers to the basis weight obtained based on the method specified in JIS P8124 (Paper and paperboard - basis weight measurement method). The thickness can be 0.5 to 100 μm, ¹ to 70 μm, or 2 to 60 μm. This "thickness" refers to the value measured using an outside micrometer (measurable thickness: 0 to 25 mm) specified in JIS B7502.

The more voids the fiber aggregate has, the easier it is to provide a conductive sheet that has excellent conductivity and breathability in the thickness direction. Therefore, the porosity of the fiber aggregate is preferably 40% or more, more preferably 50% or more, more preferably 70% or more, and even more preferably 80% or more. On the other hand, if the fiber aggregate has too many voids, the conductive sheet may have poor strength, so the porosity is preferably 99% or less, more preferably 95% or less, and even more preferably 90% or less.

The "void ratio" refers to a value obtained by the following formula: Here, M is the basis weight of the fiber assembly (unit: g/ ^m2 ), T is the thickness of the fiber assembly (unit: μm), and d is the average density of the components constituting the fiber assembly (unit: g/ ^cm3 ).
P = [1 - M / (T x d)] x 100

The conductive sheet of the present invention contains conductive particles. The conductive particles referred to here are particles containing a component having electrical conductivity, and can be metal particles or particles composed of carbon components. The type of carbon component can be selected appropriately, and examples of the carbon component that can be used include graphite, ketjen black, acetylene black, and charcoal black. Carbon nanofibers and carbon nanotubes may also be used as the conductive particles.

The particle shape of the conductive particles can be appropriately selected and can be fibrous, flat, spherical, beaded, columnar or rod-like, and can be solid or hollow, or have a porous shape.

The mass of the conductive particles contained in the conductive sheet can be adjusted as appropriate, but can be 1 to 100 g/ ^m2 , 2 to 90 g/ ^m2 , or 3 to 80 g/ ^m2 .

The conductive sheet of the present invention contains two or more types of conductive particles with different average particle sizes. Whether or not the conductive sheet contains two or more types of conductive particles with different average particle sizes can be determined by subjecting the conductive sheet to the following determination method.

(Method of determining whether or not a conductive material contains two or more types of conductive particles with different average particle sizes)
1. Using an electron microscope, obtain an image of the cross section or surface of the measurement target (e.g., conductive sheet) magnified 100,000 times.
2. Conductive particles are selected from the particles captured in the image. Whether or not a particle is conductive can be determined using various analytical methods and means, such as energy dispersive X-ray analysis (EDX), atomic force microscope (AFM), and Raman spectroscopy. If 50 particles are randomly collected from the object to be measured and subjected to various analytical methods and means, and all 50 particles are found to be conductive, it can be determined that the object to be measured contains only conductive particles.
Then, from the selected conductive particles, conductive particles whose outlines are fully visible are selected (number of selected conductive particles: 50; if the number of conductive particles whose outlines are fully visible is less than 50, all conductive particles whose outlines are fully visible are selected), and their particle diameters are measured. If the conductive particles are not spherical, the diameter of the circumscribed circle of the conductive particles is taken as the particle diameter. If the conductive particles are fibrous, such as carbon nanofibers, with an aspect ratio of fiber length to fiber diameter of 50 or more, fibrous conductive particles whose fiber diameters are known are selected, and the fiber diameters are measured and taken as the particle diameters.
3. The average value of the measured particle diameters is set as the average particle diameter of the first type of conductive particles.
4. If there are other types of conductive particles in addition to the first type of conductive particles in the image, do not change the viewing position of the electron microscope (the center point of the image) in item 1, but change the magnification of the electron microscope to obtain an image with a magnification of 30,000 times.
5. Among the conductive particles captured in the image, select the other types of conductive particles mentioned above whose entire outlines are captured, and measure the particle diameter of each. If the conductive particles are not spherical, the diameter of the circumscribing circle of the conductive particles is taken as the particle diameter.
6. The average value of the measured particle diameters is set as the average particle diameter of the second type of conductive particles.
7. If there are other types of conductive particles present in the image in addition to the first and second types of conductive particles, do not change the viewing position of the electron microscope in item 4 (the center point of the image), but change the magnification of the electron microscope to obtain an image with a magnification of 2,000 times.
8. Among the conductive particles captured in the image, select the other types of conductive particles mentioned above whose entire outlines are captured, and measure the particle diameter of each. If the conductive particles are not spherical, the diameter of the circumscribing circle of the conductive particles is taken as the particle diameter.
9. The average value of the measured particle diameters is set as the average particle diameter of the third type of conductive particles.
10. Repeat steps 1 to 9 above at two other locations on the object to be measured (another cross section, another surface, etc.) to determine the types of conductive particles contained in the object to be measured and the average particle diameters of each.

If, as a result of the above-mentioned judgment, only the first type of conductive particles is captured in each image at all three measurement locations on the object to be measured, it is judged that the conductive sheet contains only one type of conductive particles. Also, if the first and second types of conductive particles are captured in each image at all three measurement locations on the object to be measured, it is judged that the conductive sheet contains two or more types of conductive particles. Furthermore, if the first to third types of conductive particles are captured in each image at all three measurement locations on the object to be measured, it is judged that the conductive sheet contains three or more types of conductive particles.

Note that, if the manufacturing process of the conductive sheet is known, for example, and the type of conductive particles contained in the conductive sheet is known, and the average particle diameter is listed in a catalog or specification, the listed value can be determined to be the average particle diameter of the conductive particles. Therefore, even if the manufacturing process of a conductive sheet is known, it is possible to determine whether or not it contains two or more types of conductive particles with different average particle diameters (as well as the type and average particle diameter of each conductive particle).

In order to provide a conductive sheet that has both excellent conductivity and breathability in the thickness direction, it is preferable that two or more types of conductive particles with different average particle sizes are present in the voids of the fiber aggregate.

Examples of combinations of two or more types of conductive particles with different average particle sizes that may be included in the conductive sheet of the present invention include combinations of acetylene black and graphite with a larger average particle size than acetylene black, ketjen black and acetylene black and graphite with a larger average particle size than these, carbon nanofiber and acetylene black with a larger average particle size than these, and graphite with a larger average particle size than these.

The mass ratio of two or more types of conductive particles with different average particle diameters contained in the conductive sheet can be adjusted as appropriate. As an example, when the conductive sheet contains conductive particles A and conductive particles B with a larger average particle diameter, the mass ratio of the mass of conductive particles A to the mass of conductive particles B can be 99% by mass:1% to 1% by mass:99% by mass, 95% by mass:5% to 5% by mass:95% by mass, or 10% by mass:90% to 90% by mass:10% by mass. As another example, when the conductive sheet contains the conductive particles A and B, as well as conductive particles C (e.g., carbon nanofibers) having a smaller average particle size than these, the mass ratio of the combined mass of the conductive particles A and B to the mass of the conductive particles C can be 99.9 mass%:0.1 mass% to 60 mass%:40 mass%, 99.7 mass%:0.3 mass% to 70 mass%:30 mass%, or 99.5 mass%:0.5 mass% to 80 mass%:20 mass%.

In the conductive particles A to C exemplified above, the average particle diameters of conductive particles A and conductive particles B can be appropriately selected, but for example, the average particle diameter of conductive particles A can be 0.05 to 1.0 μm, 0.1 to 0.8 μm, or 0.2 to 0.6 μm, the average particle diameter of conductive particles B can be 1 to 20 μm, 2 to 10 μm, or 3 to 9 μm, and the average particle diameter of conductive particles C can be 0.005 to 0.1 μm, 0.008 to 0.05 μm, or 0.01 to 0.03 μm.

In particular, when the conductive sheet contains conductive particles A and conductive particles B having a larger average particle diameter than conductive particles A, the average particle diameter of conductive particles A is preferably smaller than 0.4 times the average particle diameter of conductive particles B. When conductive particles A and B having such an average particle diameter relationship are provided, four conductive particles B are arranged in a square shape, and two conductive particles A can be arranged on a perpendicular line passing through the center of the square, so that a conductive sheet in which conductive particles A and conductive particles B are closely packed can be provided, which is preferable. Alternatively, it is more preferable that the average particle diameter of conductive particles A is smaller than 0.16 times the average particle diameter of conductive particles B. When conductive particles A and conductive particles B having such an average particle diameter relationship are provided, four conductive particles B are arranged with the apexes of a regular triangular pyramid as their centers, and conductive particles A can be arranged with the most contact points with conductive particles B, so that a conductive sheet in which conductive particles A and conductive particles B are more closely packed can be provided, which is preferable.

The manner in which the conductive particles are present in the voids of the fiber assembly can be adjusted as appropriate, but examples include a manner in which the conductive particles are adhered to the surface of the fibers or conductive particles by a binder, a manner in which the conductive particles are simply attached to the surface of the fibers or conductive particles, and a manner in which the fiber assembly contains fibers with a thermoplastic resin exposed on the fiber surface and the conductive particles are fixed to the surface of the fibers by thermal fusion of the thermoplastic resin. When the conductive particles are adhered to the surface of the fibers by a binder, the mass of the binder can be adjusted as appropriate. If the amount of binder is too small, the conductive particles may fall off during handling or when the particles come into contact with liquid in the usage environment of a fuel cell, etc., and the desired effect may not be obtained. On the other hand, if the amount of binder is too large, the conductivity and breathability may decrease, and a high-performance conductive sheet may not be obtained. Therefore, the mass ratio of the total mass A of the conductive particles to the total mass B of the binder can be 99% by mass:1% by mass to 90% by mass:10% by mass, can be 98% by mass:2% by mass to 92% by mass:8% by mass, and is preferably 97% by mass:3% by mass to 94% by mass:6% by mass.

The conductive sheet of the present invention has through holes in its thickness direction. Whether or not the conductive sheet has through holes in its thickness direction can be determined by subjecting the conductive sheet to the following determination method.

(Method of determining the presence or absence of a through hole)
1. Prepare a piece of opaque cardboard with a 5 cm square cut out in the center.
2. The main surface of the cardboard is placed on the main surface of the measurement object (such as a conductive sheet) and fixed in place. In the present invention, the main surface refers to the surface having the largest area, and the back surface that faces the main surface is referred to as the other main surface.
3. Place the measurement object and the cardboard in a dark room.
4. With the light-emitting part of the A3-sized LED tracing board in contact with the other main surface of the object to be measured exposed in the cut-out part of the cardboard, irradiate the object with light of 1000 lux from the other main surface to the one main surface.
5. Visually check whether or not there is one or more areas within the 5 cm square of the object to be measured that is exposed in the cut-out portion of the cardboard where light passes from one main surface of the object to the other main surface.
6. Repeat the operations in items 4 and 5 three times on other parts of the measurement object, and if there is at least one part where light passes from the other main surface of the measurement object to one main surface in any 5 cm square in all parts (total of four places), the measurement object is determined to have a through hole in the thickness direction.

The conductive sheet having through holes in the thickness direction according to the present invention can be realized by satisfying at least one of the following configurations.
The fiber assembly has a large pore size.
- The structure has a fiber assembly with a high porosity.
The content of the conductive particles (or the conductive particles and the binder) is appropriately adjusted so that the voids in the fiber aggregate are not blocked by the conductive particles (or the conductive particles and the binder).
The fiber aggregate is subjected to a hydrophilic treatment so that the voids in the fiber aggregate are less likely to be blocked by the conductive particles (or the conductive particles and the binder).
The conductive particles are supported on the surfaces of the fibers constituting the fiber assembly by a particulate binder so that the voids in the fiber assembly are less likely to be blocked by the binder.

Various physical properties such as the basis weight, thickness, Gurley value, and electrical resistance in the thickness direction of the conductive sheet can be appropriately selected. The basis weight can be 5 to 100 g/ ^m2 , 6 to 80 g/ ^m2 , or 7 to 60 g/ ^m2 . The thickness can be 10 to 300 μm, 15 to 200 μm, or 20 to 180 μm. A lower Gurley value means better breathability. Therefore, the Gurley value of the conductive sheet is preferably less than 180 (sec/100 ml), preferably 140 (sec/100 ml) or less, more preferably 100 (sec/100 ml) or less, and more preferably 60 (sec/100 ml) or less.

The Gurley value can be determined by subjecting the conductive sheet to the following measurement method.

(Method of measuring Gurley value)
A test piece is taken from the measurement object (such as a conductive sheet) and subjected to the method specified in "JIS P8117:2009 (Paper and paperboard -- Air permeability and air resistance test method (intermediate range) -- Gurley method) a) Gurley tester method," to calculate the Gurley value (seconds/100 ml).

The lower the electrical resistance in the thickness direction, the more conductive the sheet is. Therefore, the electrical resistance in the thickness direction of the conductive sheet is preferably less than 45 (Ω/cm), more preferably 15 (Ω/cm) or less, more preferably 12 (Ω/cm) or less, and even more preferably 10 (Ω/cm) or less.

The electrical resistance in the thickness direction can be determined by subjecting the conductive sheet to the following measurement method.

(Method of measuring electrical resistance in the thickness direction)
1. A circular sample with a diameter of 2.5 cm (area 4.9 cm ² ) is taken from the measurement object (conductive sheet, etc.).
2. Gold-plated metal plates (2.5 cm or more in diameter) are placed on both main surfaces of the sample to sandwich it in place. The pressure between the metal plates is adjusted to 2 MPa.
3. Calculate the resistance (unit: Ω) from the voltage (unit: V) applied between the metal plates when a current of 1 A flows between the metal plates and the current value (1 A).
4. The resistance value is divided by 4.9, and the calculated value is further divided by the thickness of the sample (unit: cm) to calculate the electrical resistance (unit: Ω/cm) in the thickness direction of the measurement object (e.g., conductive sheet).

The conductive sheet of the present invention can be used alone, but if necessary, a laminate can be prepared by laminating a substrate separately onto the conductive sheet.

The conductive sheet or laminate produced in the above manner may be subjected to a pressure treatment process such as a reliant press process to adjust the thickness, or may be subjected to various secondary processes such as punching or molding, depending on the application and mode of use.

The conductive sheet of the present invention has excellent conductivity and breathability in the thickness direction, and can therefore be suitably used as a moisture management sheet to be placed between the catalyst layer and gas diffusion layer of a fuel cell.

Next, a method for manufacturing a conductive sheet according to the present invention will be illustrated and explained. Note that explanations of the same configuration as those already explained will be omitted.

The method for producing the conductive sheet according to the present invention can be appropriately selected. As an example,
(1) preparing a fiber assembly;
(2) preparing a coating liquid in which two or more types of conductive particles having different average particle sizes are mixed in a dispersion medium;
(3) applying a coating liquid to a fiber assembly;
(4) removing the dispersion medium from the fiber assembly to which the coating liquid has been applied;
A method for manufacturing a conductive sheet comprising the steps of:

We will now explain step (2).

The type of dispersion medium is appropriately selected, but examples include water, acetone, methanol, ethanol, propanol, isopropanol, tetrahydrofuran, dimethyl sulfoxide, 1,4-dioxane, pyridine, N,N-dimethylformamide, N,N-dimethylacetamide, N-methyl-2-pyrrolidone, acetonitrile, formic acid, toluene, benzene, cyclohexane, cyclohexanone, carbon tetrachloride, methylene chloride, chloroform, trichloroethane, ethylene carbonate, diethyl carbonate, and propylene carbonate. The dispersion medium may be one type or a mixture of multiple types.

It is preferable to select a dispersion medium so that the fiber aggregate or conductive particles are unlikely to dissolve or have unintended changes in physical properties, and also so that, when binder particles are dispersed in the coating liquid, the binder particles are unlikely to dissolve or have unintended changes in physical properties. The mass (solid mass) of the conductive particles contained in the coating liquid is selected appropriately, but can be 5 to 50 mass%, 8 to 40 mass%, or 10 to 30 mass%. The coating liquid may contain binder particles as well as additives such as a surfactant or an antifoaming agent. The mass (solid mass) of the additives contained in the coating liquid is selected appropriately, but can be 0.1 to 30 mass%, 0.5 to 20 mass%, or 1 to 15 mass%.

The temperature and viscosity of the coating liquid are appropriately selected. The temperature of the coating liquid may be 5 to 40°C, 10 to 35°C, or 15 to 30°C. The viscosity of the spinning liquid may be 0.05 to 8 Pa·s, 0.1 to 6 Pa·s, or 0.2 to 5 Pa·s. The "viscosity" here refers to the value measured using a viscosity measuring device at a temperature of 25°C at a shear rate of 100 s ^-1 .

Next, step (3) will be explained.

The method of applying the coating liquid to the fiber assembly can be selected as appropriate, and examples of methods that can be used include applying the coating liquid directly or in a foamed state to one or both main surfaces of the fiber assembly using a spray or an impregnated roll, or immersing the fiber assembly in the coating liquid. The amount of coating liquid applied to the fiber assembly can be adjusted as appropriate.

Then, we will explain step (4).

The method for removing the dispersion medium from the fiber aggregate to which the coating liquid has been applied can be selected as appropriate, and one example is a method of subjecting the fiber aggregate to a heat treatment. The type of heating device can be selected as appropriate, and for example, a method using a device that heats or heats and presses the fiber aggregate with a roll, an oven dryer, a far-infrared heater, a dry heat dryer, a hot air dryer, or a device that can heat the fiber aggregate by irradiating infrared rays can be used. The heating temperature of the heating device can be selected as appropriate, and is adjusted as appropriate so that the temperature can volatilize and remove the remaining dispersion medium, and that the constituent components such as the fibers and conductive particles do not unintentionally decompose or denature.

If the fibers contain adhesive components or crosslinkable resins, the fibers may be bonded by the adhesive components or the crosslinkable resins may be crosslinked by subjecting the fibers to a heat treatment.

In addition, in this process, at least a portion of the binder (particularly the binder particles) may be softened, and the conductive particles may be adhered and supported on the surface of the fibers by the binder.

By the above-mentioned manufacturing method, it is possible to manufacture the conductive sheet according to the present invention, in which two or more types of conductive particles having different average particle sizes are present in the voids of the fiber assembly.

The conductive sheet of the present invention can be used alone, but if necessary, after the above-mentioned process, a laminate can be prepared by laminating a substrate separately onto the conductive sheet.

The conductive sheet or laminate produced in the above manner may be subjected to a pressure treatment process such as a reliant press process to adjust the thickness, or may be subjected to various secondary processes such as punching or molding, depending on the application and mode of use.

The following describes examples of the present invention, but the present invention is not limited to the following examples.

(Preparation of fiber assembly)
White water in which E-glass fibers (average fiber diameter: 7 μm, fiber length: 10 mm) were dispersed was prepared, and wet-laid paper was formed into a wet web. The wet web was then impregnated with a dispersion in which a binder mainly composed of acrylic resin and a polyvinyl acetate binder were mixed in a solid content mass ratio of 1:1, and dried in a dry heat dryer (heating temperature: 140° C.) to prepare wet nonwoven fabric A (basis weight: 11 g/m ² , thickness: 110 μm, solid content of binder: 15% by mass).

A white water containing dispersed therein a partially heat-bonded high-strength polyolefin-based core-sheath composite fiber (tensile strength: 6.5 cN/dtex, fineness: 0.8 dtex, fiber diameter: 10 μm, fiber length: 5 mm) having a core component of polypropylene (high melting point component, melting point: 168°C) and a sheath component of high density polyethylene (low melting point component, melting point: 135°C) was prepared and wet-formed into a wet web. The resulting web was then dried in a dry heat dryer (heating temperature: 140°C) to prepare a wet nonwoven fabric B (basis weight: 7 g/ ^m2 , thickness: 40 μm).

(Preparation of Coating Fluid)
Coating solution A
Acetylene black was added to pure water and mixed using a dispersion type stirring blade. Then, a dispersion of polyvinylidene fluoride particles, which is a binder component, was added and stirring was continued to prepare coating solution A (solid content concentration: 15 mass%). The mixing ratio was as shown below.
Acetylene black (average particle size: 50 nm): 97 parts by mass.
Dispersion of polyvinylidene fluoride particles (average particle size: 0.4 μm, melting point of polyvinylidene fluoride: 170° C., solid content concentration: 30% by mass): 3 parts by mass.

Coating solution B
Acetylene black and graphite were added to pure water and mixed using a dispersing type stirring blade. Then, a dispersion of polyvinylidene fluoride particles, which is a binder component, was added and stirring was continued to prepare coating solution B (solid content concentration: 15 mass%). The mixing ratio was as shown below.
Acetylene black (average particle size: 50 nm): 33 parts by mass.
Graphite (average particle size: 6 μm): 64 parts by mass,
Dispersion of polyvinylidene fluoride particles (average particle size: 0.4 μm, melting point of polyvinylidene fluoride: 170° C., solid content concentration: 30% by mass): 3 parts by mass.

Coating solution C
Acetylene black, graphite, and Ketjen black were added to pure water and mixed using a dispersing type stirring blade. A dispersion of polyvinylidene fluoride particles, which is a binder component, was then added and the mixture was stirred to prepare coating solution C (solid content concentration: 15% by mass). The mixing ratio was as follows:
Acetylene black (average particle size: 50 nm): 23 parts by mass.
Graphite (average particle size: 6 μm): 44 parts by mass,
Ketjen black (average particle size: 40 nm): 30 parts by mass,
Dispersion of polyvinylidene fluoride particles (average particle size: 0.4 μm, melting point of polyvinylidene fluoride: 170° C., solid content concentration: 30% by mass): 3 parts by mass.

Coating solution D
Acetylene black, graphite, and carbon nanofibers were added to pure water and mixed using a dispersing type stirring blade. A dispersion of polyvinylidene fluoride particles, which serves as a binder component, was then added and stirred to prepare coating solution D (solid content concentration: 15% by mass). The mixing ratio was as follows:
Acetylene black (average particle size: 50 nm): 32.5 parts by mass.
Graphite (average particle size: 6 μm): 64 parts by mass,
Carbon nanofiber (average fiber diameter (average particle diameter): 20 nm, average fiber length: 1 μm): 0.5 parts by mass,
Dispersion of polyvinylidene fluoride particles (average particle size: 0.4 μm, melting point of polyvinylidene fluoride: 170° C., solid content concentration: 30% by mass): 3 parts by mass.

(Comparative Example 1)
Using a gravure roll having oblique grooves on its surface, Coating Liquid A was applied to one main surface of the wetlaid nonwoven fabric A. Thereafter, the coating liquid was dried at 100° C. to remove the dispersion medium in the coating liquid, thereby preparing a conductive sheet.

(Comparative Examples 2 to 4)
Using a gravure roll having oblique grooves on its surface, the coating liquid A was applied to one main surface of the wetlaid nonwoven fabric B. The coating amount of the coating liquid was changed in each of Comparative Examples 2 to 4. Thereafter, the coating liquid was dried at 100° C. to remove the dispersion medium in the coating liquid, thereby preparing a conductive sheet.

The various configurations and evaluation results of each conductive sheet prepared as described above are summarized in Table 1. In the following tables, included items are marked with a "〇" and not included items are marked with a "-".

None of the conductive sheets of Comparative Example 1 and Comparative Examples 2 to 4, which are related to the prior art, were conductive sheets that had excellent conductivity and air permeability in the thickness direction (between both main surfaces). Specifically, they were not conductive sheets with a conductivity of less than 45 (Ω/cm) in the thickness direction and a Gurley value of less than 180 (sec/100 ml) like the examples described below.

Furthermore, a comparison of Comparative Examples 2 to 4 showed that although the conductivity (resistance to electrical current in the thickness direction) and breathability (Gurley value) of the conductive sheet could be changed by increasing or decreasing the mass of the conductive particles contained in the conductive sheet, there was a trade-off between the conductivity and breathability (Gurley value) of the conductive sheet.

From the above, it has become clear that it is difficult to provide a conductive sheet that has both excellent conductivity and breathability by simply increasing or decreasing the mass of the conductive particles.

Example 1
A conductive sheet was prepared in the same manner as in Comparative Example 4, except that Coating Liquid B was used.

The configurations and evaluation results of each conductive sheet prepared as described above are summarized in Table 2. For ease of understanding, the results of Comparative Example 4 are also shown in the table.

Compared to the conductive sheet of Comparative Example 4, the conductive sheet of Example 1 had excellent conductivity (low electrical resistance in the thickness direction) and excellent breathability (small Gurley value). This was thought to be because the conductive sheet of Example 1 contained two or more types of conductive particles with different average particle sizes and had through holes.

(Examples 2 to 3)
Conductive sheets were prepared in the same manner as in Example 1, except that the coating amounts of the coating solutions were changed.

The configurations and evaluation results of each conductive sheet prepared as described above are summarized in Table 3. For ease of understanding, the table also includes the results of Comparative Examples 2 and 3.

These results demonstrate that by satisfying the configuration of the present invention, it is possible to prevent an increase in breathability (Gurley value) even when the conductivity (electrical resistance in the thickness direction) of the conductive sheet is improved by increasing the mass of the conductive particles contained in the conductive sheet.

As a result, it has been found that the present invention can easily provide a conductive sheet that is more conductive (low electrical resistance in the thickness direction) and more breathable (small Gurley value) than conventional techniques.

(Examples 4 to 5)
A conductive sheet was prepared in the same manner as in Example 1, except that the coating liquid was changed to Coating Liquid C. In Examples 4 and 5, the coating amounts of the coating liquids were changed.

Example 6
A conductive sheet was prepared in the same manner as in Example 1, except that the coating liquid was changed to Coating Liquid D. In Example 6, the coating amount of the coating liquid was changed.

The configurations and evaluation results of each conductive sheet prepared as described above are summarized in Table 4.

The conductive sheets of Examples 4 to 6 were found to have excellent conductivity (low electrical resistance in the thickness direction) and excellent breathability (small Gurley value). This was thought to be because the conductive sheets of Examples 4 to 6 contained two or more types of conductive particles with different average particle sizes and had through holes.

As a result, it has been found that the present invention can provide a conductive sheet that has excellent electrical conductivity (low electrical resistance in the thickness direction) and excellent breathability (small Gurley value).

The present invention provides a conductive sheet that can be used as a wiring material, a shielding material, or a component of an electrochemical element such as a fuel cell or a lithium ion battery (primary battery or secondary battery). In particular, it can be used as an electrode for a lithium ion battery or the like, an electrode for an electric double layer capacitor or the like, or a support for these. Furthermore, since the conductive sheet according to the present invention has excellent conductivity and air permeability in the thickness direction (between both main surfaces), it can be suitably used as a moisture management sheet to be placed between the catalyst layer and the gas diffusion layer of a fuel cell.

Claims (2)

A conductive sheet in which conductive particles are present only in voids of a fiber assembly in which intersections between constituent fibers are integrated by fiber bonding using partially heat-fused composite fibers ,
The conductive particles include two or more types of conductive particles having different average particle sizes,
Among the two or more kinds of conductive particles, the conductive particles having the largest average particle size are graphite particles,
A conductive sheet having through holes in the thickness direction. 10. The conductive sheet according to claim 1, used as a moisture management sheet disposed between a catalyst layer and a gas diffusion layer of a fuel cell.