Method for manufacturing a sheet, method for manufacturing a laminated sheet, and sheet

Abstract

[Problem] A manufacturing method of a sheet is provided that contains polytetrafluoroethylene-containing particles, inorganic particles, and a specific tetrafluoroethylene-base polymer. Further, a method for manufacturing a laminate sheet that comprises multiple sheets obtained with the aforementioned manufacturing method, and a manufacturing method of a laminate body that comprises the obtained laminate sheet and a substrate are provided. [Solution] This sheet manufacturing method involves: mixing an agglomerate that contains polytetrafluoroethylene-containing particles and inorganic particles, and particles that contain a tetrafluoroethylene-base polymer that has an oxygen-containing polar group and a melting temperature of no more than 320C°; and forming the obtained mixture.

Description

[Technical Field]

[0001] The present invention relates to a method for manufacturing a sheet, and to a method for manufacturing a laminated sheet having a plurality of sheets obtained by the said manufacturing method. Furthermore, the present invention relates to a sheet. [Background technology]

[0002] In the field of information and communication, metal-clad laminates, which have a metal layer made of metal foil and a resin layer, are used, for example, as printed circuit boards. In recent years, particularly with the development of high-speed communication technology, there has been a demand for improved performance of metal-clad laminates for printed circuit boards.

[0003] Polytetrafluoroethylene (PTE) is expected to be used in the resin layer of metal-clad laminates for printed circuit boards due to its excellent physical properties such as heat resistance and electrical properties. In order to form a resin layer with even better physical properties, sheets molded from coagulations of polytetrafluoroethylene particles and inorganic particles have been proposed (see Patent Documents 1 and 2). [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2015-044288 [Patent Document 2] Japanese Patent Publication No. 2015-164801 [Overview of the project] [Problems that the invention aims to solve]

[0005] However, the uniform dispersion of inorganic particles in the above-mentioned sheet was insufficient, and the inorganic particles tended to fall off the sheet easily. Therefore, the resulting sheet did not adequately provide functionality through the inorganic particles. In addition, the sheet had poor adhesion, making it difficult to directly bond the sheet to the substrate at low temperatures.

[0006] The inventors have found that by creating a sheet from a mixture obtained by mixing polytetrafluoroethylene-containing particles, an aggregate containing inorganic particles, and a specific tetrafluoroethylene-based polymer, the uniform dispersion of inorganic particles in the sheet and the low-temperature adhesion to the substrate can be improved, and the shedding of inorganic particles from the sheet can be suppressed.

[0007] The present invention provides a method for producing a sheet containing polytetrafluoroethylene particles, inorganic particles, and a specific tetrafluoroethylene-based polymer. The present invention also provides a method for producing a laminated sheet of the sheet obtained by the above production method. Furthermore, the sheet is provided that contains inorganic particles with a particle size and specific surface area that are more prone to shedding, while still sufficiently suppressing shedding. [Means for solving the problem]

[0008] The present invention has the following aspects. [1] A method for producing a sheet, comprising mixing agglomerates containing polytetrafluoroethylene particles and inorganic particles with particles containing a tetrafluoroethylene-based polymer having oxygen-containing polar groups and a melting temperature of 320°C or lower, and then molding the resulting mixture. [2] The manufacturing method according to [1], wherein the aggregate is an aggregate obtained by co-aggregating from a mixture containing polytetrafluoroethylene particles, inorganic particles, and water. [3] The manufacturing method according to [1] or [2], wherein the content of the polytetrafluoroethylene-containing particles and the inorganic particles in the aggregate is such that, with the total mass of the polytetrafluoroethylene-containing particles and the inorganic particles being 100% by mass, the polytetrafluoroethylene-containing particles are 5% by mass or more and 60% by mass or less, and the inorganic particles are 40% by mass or more and 95% by mass or less. [4] The manufacturing method according to any one of [1] to [3], wherein the inorganic particles include at least one selected from the group consisting of metal oxides, silicon oxides, and nitrides. [5] The average particle diameter of the inorganic particles is 1 to 20 μm, and the specific surface area is 1 to 20 m². 2 A manufacturing method according to any one of the above [1] to [4], wherein the amount is / g. [6] The manufacturing method according to any one of [1] to [5], wherein the average particle size of the particles containing the tetrafluoroethylene polymer is 0.1 μm or more and 25 μm or less. [7] A manufacturing method according to any one of the above [1] to [6], wherein particles containing the tetrafluoroethylene polymer are mixed with the aggregate in powder form. [8] A manufacturing method according to any one of [1] to [7], comprising dispersing particles containing the tetrafluoroethylene polymer in a dispersion medium and mixing them with the aggregate. [9] The manufacturing method according to any one of [1] to [8], wherein particles containing the tetrafluoroethylene polymer are mixed in an amount of 5 to 30 parts by mass with respect to 100 parts by mass of the polytetrafluoroethylene.

[10] The manufacturing method according to any one of the above [1] to [9], wherein the molding is extrusion molding or rolling molding.

[11] The manufacturing method according to any one of [1] to

[10] , wherein the thickness of the sheet is 0.5 mm or more and 5 mm or less.

[12] A method for manufacturing a laminated sheet, comprising stacking two or more sheets obtained by the manufacturing method described in any of [1] to

[11] above.

[13] Polytetrafluoroethylene, a tetrafluoroethylene-based polymer having oxygen-containing polar groups and a melting temperature of 320°C or lower, and an average particle size of 1 to 20 μm and a specific surface area of ​​1 to 20 m². 2 A sheet containing inorganic particles at a concentration of / g, wherein the content of polytetrafluoroethylene and the inorganic particles is 5% to 60% by mass and 40% to 95% by mass, respectively, with the total mass of polytetrafluoroethylene and the inorganic particles being 100% by mass.

[14] The sheet according to the above

[13] , comprising the tetrafluoroethylene-based polymer in an amount of 5 parts by mass or more and 30 parts by mass or less per 100 parts by mass of the polytetrafluoroethylene.

[15] The sheet according to the above

[13] or

[14] , wherein the inorganic particles comprise at least one selected from the group consisting of metal oxides, silicon oxide and nitrides.

Effect of the Invention

[0009] According to the present invention, there is provided a method for producing a sheet that has high uniform dispersibility of inorganic particles, suppresses falling of inorganic particles, and is excellent in low-temperature adhesiveness. Further, the laminated sheet and laminate obtained according to the present invention sufficiently have the properties inherently possessed by polytetrafluoroethylene and inorganic particles.

Mode for Carrying Out the Invention

[0010] The following terms have the meanings given below. The "tetrafluoroethylene-based polymer" is a polymer containing units derived from tetrafluoroethylene (hereinafter also referred to as "TFE") (hereinafter also referred to as "TFE units"). The "glass transition temperature (Tg) of a polymer" is a value measured by analyzing the polymer by dynamic viscoelasticity measurement (DMA). The "melting temperature (melting point) of a polymer" is the temperature corresponding to the maximum value of a melting peak measured for the polymer by differential scanning calorimetry (DSC). "D50" is the average particle diameter of particles, and is the 50% volume-based cumulative diameter of particles determined by a laser diffraction / scattering method. That is, it is the particle diameter at the point where the cumulative volume reaches 50% on a cumulative curve obtained by measuring the particle size distribution of particles by the laser diffraction / scattering method, with the total volume of the particle group taken as 100%. "D90" is the cumulative volume particle diameter of particles, and is the 90% volume-based cumulative diameter of particles determined in the same manner as for "D50". A "monomer-based unit" refers to an atomic group based on a monomer, formed by the polymerization of the monomer. The unit may be one directly formed by the polymerization reaction, or it may be a unit in which a part of the unit is converted to a different structure by processing the polymer. Hereinafter, a unit based on monomer a will also be simply referred to as a "monomer a unit."

[0011] The present invention's method for producing a sheet (hereinafter also referred to as "this method") involves mixing an aggregate containing particles of polytetrafluoroethylene (hereinafter also referred to as "PTFE") and inorganic particles with particles containing a tetrafluoroethylene-based polymer (hereinafter also referred to as "F polymer") having oxygen-containing polar groups and a melting temperature of 320°C or lower (hereinafter also referred to as "F particles"), and then forming the resulting mixture into a sheet.

[0012] PTFE is a polymer with low surface tension and extremely low affinity to other components. Therefore, even when PTFE is mixed with inorganic particles, the dispersibility of the inorganic particles in the PTFE is low. However, by first forming aggregates of PTFE-containing particles and inorganic particles, and then mixing these aggregates with F particles, the F polymer acts as a binder or adhesive component between the PTFE and inorganic particles. This is thought to improve the uniform dispersion of the inorganic particles in the resulting sheet, suppress powder shedding of the inorganic particles, and exhibit low-temperature adhesion.

[0013] In the present invention, PTFE may be a homopolymer of TFE, or it may be a so-called modified PTFE, which is a copolymer of TFE with a trace amount of comonomers such as perfluoro(alkyl vinyl ether) (hereinafter also referred to as "PAVE"), hexafluoropropylene (hereinafter also referred to as "HFP"), or fluoroalkylethylene. The proportion of TFE units in PTFE is preferably 99.5 mol% or more, and more preferably 99.9 mol% or more, of the total units.

[0014] PTFE is preferred if its number-average molecular weight, Mn, calculated based on the following formula (1), is 200,000 or more. Mn = 2.1 × 10⁻⁶ 10 ×ΔHc-5.16 ... (1) In equation (1), Mn represents the number-average molecular weight of PTFE, and ΔHc represents the calorific value (cal / g) of PTFE as measured by differential scanning calorimetry.

[0015] The D50 of the PTFE-containing particles is preferably 0.1 μm or larger, more preferably 0.2 μm or larger. The D50 is preferably 3 μm or smaller, and more preferably 0.5 μm or smaller.

[0016] In the present invention, the shape of the inorganic particles is preferably spherical, flaky, layered, needle-shaped, or plate-shaped, more preferably spherical, flaky, or layered, and even more preferably spherical or flaky. The spherical inorganic particles are preferably nearly spherical. Nearly spherical means that when the inorganic particles are observed with a scanning electron microscope (SEM), the ratio of the minor axis to the major axis is 0.7 or greater. It is preferable that nearly spherical inorganic particles account for 95% or more of the total. The aspect ratio of the non-spherical inorganic particles is preferably 2 or greater, and preferably 5 or greater. The aspect ratio is preferably 10000 or less.

[0017] The inorganic particles may be hollow. In this case, the sheet obtained by this method (hereinafter also referred to as "this sheet") tends to have excellent electrical properties. The inorganic particles are particles containing at least one inorganic substance, and preferably contain at least one selected from the group consisting of metal oxides, silicon oxides, and nitrides. Specific examples of inorganic substances include carbon, boron nitride, aluminum nitride, beryllia, silica, wollastonite, talc, cerium oxide, aluminum oxide, magnesium oxide, zinc oxide, barium titanate, lead zirconate titanate, lead titanate, zirconium oxide, and titanium oxide. From the viewpoint of reducing the dielectric constant and dielectric loss tangent of the sheet and improving low linear expansion properties, the inorganic particles are preferably particles containing silica or boron nitride. The silica is preferably amorphous silica. The boron nitride is preferably hexagonal boron nitride. Furthermore, from the viewpoint of improving dielectric constant, inorganic particles containing titanium dioxide or barium titanate are preferred.

[0018] When inorganic particles contain silicon dioxide, it is preferable that the inorganic particles are hollow silica from the viewpoint of electrical properties. When inorganic particles contain nitrides, from the viewpoint of the electrical properties and low linear expansion of the sheet, the inorganic particles preferably contain boron nitride or aluminum nitride, and more preferably flaky boron nitride or columnar aluminum nitride. The flaky boron nitride may aggregate to form secondary particles.

[0019] The D50 of the inorganic particles is preferably 20 μm or less, more preferably 10 μm or less. The D50 is preferably 0.01 μm or more, more preferably 0.1 μm or more, even more preferably 1 μm or more, and particularly preferably 2 μm or more. The specific surface area of ​​inorganic particles ranges from 1 to 20 m². 2 / g is preferable. The D50 is between 1 and 20 μm, and the specific surface area is between 1 and 20 m². 2 Inorganic particles with a density of / g tend to form interparticle paths within the sheet, improving sheet properties such as thermal conductivity, low linear expansion, and electrical properties. However, they also tend to have low interaction with PTFE-containing particles and are more prone to being detached from the sheet. In this invention, however, this can be highly suppressed by the action of the F polymer.

[0020] The surface of the inorganic particles may be surface-treated with a silane coupling agent. In this case, the affinity between the inorganic particles and the PTFE and F polymer is improved, the inorganic particles disperse more uniformly within the sheet, and the inorganic particles are less likely to peel off the sheet. In addition, the sheet tends to have excellent electrical properties and low thermal expansion. The preferred silane coupling agent is one having a functional group such as 3-aminopropyltriethoxysilane, vinyltrimethoxysilane, 3-mercaptopropyltrimethoxysilane, 3-glycidoxypropylmethyldiethoxysilane, 3-methacryloxypropyltriethoxysilane, or 3-isocyanatetopropyltriethoxysilane.

[0021] Specific examples of silica-containing particles include the "AdmaFine" series (manufactured by Admatex), the "SFP" series (manufactured by Denka), the "E-SPHERES" series (manufactured by Taiheiyo Cement Corporation), the "Silinax" series (manufactured by Nippon Steel Mining Co., Ltd.), the "Ecocosfiyer" series (manufactured by Emerson & Cumming), and the "Hydrophobic AEROSIL" series ("RX200," etc.) (manufactured by Nippon Aerosil Co., Ltd.). A specific example of particles containing zinc oxide is the "FINEX" series (manufactured by Sakai Chemical Industry Co., Ltd.). Specific examples of particles containing titanium dioxide include the "Typeque" series (manufactured by Ishihara Sangyo Co., Ltd.) and the "JMT" series (manufactured by Teika Co., Ltd.). A specific example of particles containing talc is the "SG" series (manufactured by Nippon Talc Co., Ltd.). A specific example of particles containing steatite is the "BST" series (manufactured by Nippon Talc Co., Ltd.). Specific examples of particles containing boron nitride include the "UHP" series (manufactured by Showa Denko Corporation) and the "GP" and "HGP" grades of the "Denka Boron Nitride" series (manufactured by Denka Corporation).

[0022] One type of inorganic particle may be used, or two or more types may be used. For example, silica particles, boron nitride particles, and titanium dioxide particles may be used in combination as inorganic particles. In this case, the content of silica particles, boron nitride particles, and titanium dioxide particles in relation to the total amount of inorganic particles is preferably 10 to 60% by mass, 10 to 60% by mass, and 5 to 40% by mass, in that order.

[0023] Aggregates containing PTFE particles and inorganic particles can be obtained, for example, by the following method. First, inorganic particles are added to a dispersion of PTFE-containing particles, or the PTFE-containing particles and inorganic particles are mixed beforehand, and the mixture is dispersed in a dispersion medium to obtain a mixture containing PTFE-containing particles, inorganic particles, and a dispersion medium. Water is preferred as the dispersion medium. A commercially available aqueous dispersion of PTFE-containing particles may be used as the dispersion of PTFE-containing particles, or it may be a dispersion of PTFE-containing particles that has been further diluted with water. The mixture may contain a surfactant. The solid content concentration in the resulting mixture is, for example, 3 to 50% by mass. If the mixture contains water, the content of PTFE particles is, for example, 0.5 to 20% by mass, and the content of inorganic particles is, for example, 10 to 30% by mass.

[0024] Mixing equipment used for mixing includes agitators equipped with blades such as Henschel mixers, pressure kneaders, Banbury mixers, and planetary mixers; grinding equipment equipped with media such as ball mills, attritors, basket mills, sand mills, sand grinders, Dino mills, disper mats, SC mills, spike mills, or agitator mills; and dispersion equipment equipped with other mechanisms such as microfluidizers, nanomizers, ultimateizers, ultrasonic homogenizers, desolvers, dispersers, high-speed impellers, rotational and revolving agitators, or thin-film swirling high-speed mixers.

[0025] The mixture may contain only PTFE particles and inorganic particles, or it may contain other fluororesins other than PTFE. Examples of other fluororesins include polymers other than the F polymer described later, such as polymers containing TFE units and ethylene-based units (ETTE), polymers containing TFE units and propylene-based units (TFEP), polymers containing TFE units and perfluoro(alkyl vinyl ether) (PAVE)-based units (PAVE units) (PFA), and polymers containing TFE units and hexafluoropropylene-based units (FEP). It is preferable to use a fluororesin that is well miscible with PTFE. When the mixture contains PTFE and other fluororesins as fluororesin components, the PTFE content relative to the total fluororesin components is preferably 5% by weight or more, and more preferably 10% by weight or more.

[0026] By removing the dispersion medium from the mixture obtained above, an aggregate containing PTFE particles and inorganic particles is obtained. Methods for obtaining aggregates include, for example, freeze-drying, spray-drying, stirring and shearing a mixture to bond PTFE-containing particles and inorganic particles and remove the dispersion medium, and a method of agglomerating PTFE-containing particles and inorganic particles in a mixture and removing the dispersion medium. Among these methods, the co-aggregation method, in which PTFE-containing particles and inorganic particles are agglomerated in a mixture containing PTFE-containing particles, inorganic particles, and water, is preferred. In other words, it is preferable that the aggregates are obtained by co-aggregation from a mixture containing PTFE-containing particles, inorganic particles, and water.

[0027] One method for coagulation is to add a flocculant to the mixture. As the flocculant, for example, a solvent such as alcohol or a material with a large specific surface area such as activated carbon can be used. In addition, if the mixture contains a surfactant, another method is to raise the temperature of the mixture to deactivate the surfactant and promote coagulation.

[0028] Co-aggregates obtained by co-aggregation can be separated from the dispersion medium to obtain co-aggregates. The obtained co-aggregates may be further dried. Known methods such as filtration can be used to separate the co-aggregates from the dispersion medium. The drying method is not particularly limited.

[0029] The amount of inorganic particles in the aggregate is set appropriately according to the required physical properties of the sheet, but with the total amount of PTFE-containing particles and inorganic particles being 100% by mass, it is preferable that the amount of PTFE-containing particles is 5% by mass or more and 60% by mass or less, and the amount of inorganic particles is 40% by mass or more and 95% by mass or less, and it is more preferable that the amount of PTFE-containing particles is 20% by mass or more and 50% by mass or less, and the amount of inorganic particles is 60% by mass or more and 80% by mass or less.

[0030] Alternatively, the above mixture may be prepared to agglomerate the PTFE-containing particles and the inorganic particles, the resulting aggregates may be separated from the dispersion medium to obtain aggregates, the obtained aggregates may be mixed with the dispersion medium to form another mixture, and then agglomerated again to obtain aggregates.

[0031] In this method, the aggregates obtained above are mixed with F particles containing F polymer. The melting temperature of polymer F is 320°C or lower, preferably 200°C or higher, and more preferably 260°C or higher.

[0032] The glass transition temperature of the F polymer is preferably 50°C or higher, and more preferably 75°C or higher. The glass transition temperature of the F polymer is preferably 150°C or lower, and more preferably 125°C or lower. The fluorine content of the F polymer is preferably 70% by mass or more, and more preferably 72 to 76% by mass. The surface tension of the F polymer is preferably 16 to 26 mN / m. The surface tension of the F polymer can be measured by placing a droplet of wetting index reagent (manufactured by Wako Pure Chemical Industries, Ltd.) on a flat plate made of the F polymer. While F polymers with a high fluorine content exhibit excellent physical properties such as electrical properties, they tend to have low surface tension and poor adhesion. However, because F polymers have oxygen-containing polar groups, they readily act as a binder or adhesive component between PTFE and inorganic particles.

[0033] The F polymer is preferably ETTE, TFEP, PFA, or FEP, more preferably PFA and FEP, and even more preferably PFA. These polymers may also contain units based on other comonomers. PAVE is preferably CF2=CFOCF3, CF2=CFOCF2CF3, and CF2=CFOCF2CF2CF3 (hereinafter also referred to as "PPVE"), with PPVE being more preferred.

[0034] The oxygen-containing polar groups of the F polymer include hydroxyl group-containing groups and carbonyl group-containing groups, and from the viewpoint of improving adhesion, carbonyl group-containing groups are preferred as the oxygen-containing polar groups. The hydroxyl group-containing group is preferably a group containing an alcoholic hydroxyl group, and -CF2CH2OH and -C(CF3)2OH are more preferred. The carbonyl group-containing groups are preferably carboxyl groups, alkoxycarbonyl groups, amide groups, isocyanate groups, carbamate groups (-OC(O)NH2), acid anhydride residues (-C(O)OC(O)-), imide residues (-C(O)NHC(O)-, etc.), and carbonate groups (-OC(O)O-), with acid anhydride residues being more preferred. The number of oxygen-containing polar groups in F polymer is 1 × 10⁶ carbon atoms in the main chain. 6 The number of oxygen-containing polar groups per polymer is preferably 10 to 5000, and more preferably 100 to 3000. The number of oxygen-containing polar groups in polymer F can be quantified by the polymer composition or by the method described in International Publication No. 2020 / 145133.

[0035] The oxygen-containing polar group may be included in the monomer-based units in the F polymer, or it may be included in the terminal groups of the main chain of the F polymer, with the former being preferred. Examples of the latter include an F polymer having an oxygen-containing polar group as a terminal group derived from a polymerization initiator, a chain transfer agent, etc., and an F polymer obtained by plasma treatment or ionization treatment of the F polymer. The monomers having a carbonyl group are preferably itaconic anhydride, citraconic anhydride, and 5-norbornene-2,3-dicarboxylic acid anhydride (hereinafter also referred to as "NAH"), with NAH being more preferred. The F polymer is preferably a polymer having carbonyl group-containing groups, including TFE units and PAVE units, and more preferably a polymer containing TFE units, PAVE units, and units based on monomers having carbonyl group-containing groups, with these units present in the following proportions relative to the total number of units: 90 to 99 mol%, 0.99 to 9.97 mol%, and 0.01 to 3 mol% in that order. A specific example of such an F polymer is the polymer described in International Publication No. 2018 / 16644.

[0036] The D50 of the F particles is preferably 0.1 μm or larger, more preferably greater than 0.3 μm, and even more preferably 1 μm or larger. The D50 of the F particles is preferably 25 μm or smaller, more preferably 10 μm or smaller, and even more preferably 8 μm or smaller. The specific surface area of ​​F particles ranges from 1 to 25 m². 2 / g is preferable. One type of F particle may be used, or two or more types may be used.

[0037] F particles are particles containing F polymer, and it is preferable that they consist of F polymer. The F particles may contain resins or inorganic compounds other than the F polymer, and may form a core-shell structure with the F polymer as the core and a resin or inorganic compound other than the F polymer as the shell, or may form a core-shell structure with the F polymer as the shell and a resin or inorganic compound other than the F polymer as the core.

[0038] Examples of resins other than F polymers include aromatic polyesters, polyamide-imides, polyimides, and maleimides. Examples of inorganic compounds include the same inorganic substances that may be contained in the aforementioned inorganic particles, with silica and boron nitride being particularly preferred.

[0039] In this method, the aggregates and the F particles are mixed to obtain a mixture. Methods for mixing aggregates and F particles include mixing the aggregates and F particles together, dispersing the F particles in a dispersion medium such as water or an organic solvent to make a dispersion, and mixing the dispersion with the aggregates, dispersing the aggregates in a dispersion medium such as water or an organic solvent to make a dispersion, and mixing the dispersion with the F particle dispersion, and dispersing the aggregates in a dispersion medium such as water or an organic solvent to make a dispersion, and mixing the dispersion with the F particles. Mixing can be performed using the same mixer as described above.

[0040] Among these methods, the method of mixing the aggregates with F particles, and the method of dispersing the F particles in a dispersion medium such as water or an organic solvent and then mixing the dispersion with the aggregates are preferred from the viewpoint of uniform dispersion of inorganic particles in this sheet. When mixing aggregates with F particles, it is preferable to mix them in a powder state, which is an aggregate of dry F particles.

[0041] When mixing a dispersion of F particles with aggregates, from the viewpoint of improving the dispersion stability of the dispersion, the dispersion medium is preferably a compound selected from the group consisting of water, amides, ketones, and esters, with water being more preferable. Alternatively, alkanes such as decane and dodecane, which serve as molding aids during sheet formation as described later, are also preferred. Examples of amides include N-methyl-2-pyrrolidone, N,N-dimethylformamide, N,N-dimethylacetamide, N,N-dimethylpropanamide, 3-methoxy-N,N-dimethylpropanamide, 3-butoxy-N,N-dimethylpropanamide, N,N-diethylformamide, hexamethylphosphoric triamide, and 1,3-dimethyl-2-imidazolidinone.

[0042] Examples of ketones include acetone, methyl ethyl ketone, methyl isopropyl ketone, methyl isobutyl ketone, methyl n-pentyl ketone, methyl isopentyl ketone, 2-heptanone, cyclopentanone, cyclohexanone, and cycloheptanone. Examples of esters include methyl acetate, ethyl acetate, butyl acetate, methyl lactate, ethyl lactate, methyl pyruvate, ethyl pyruvate, methyl methoxypropionate, ethyl ethoxypropionate, ethyl 3-ethoxypropionate, γ-butyrolactone, and γ-valerolactone.

[0043] The content of the dispersion medium in the dispersion of F particles is preferably 40% by mass or more, and more preferably 60% by mass or more. The content of the liquid dispersion medium is preferably 90% by mass or less, and more preferably 80% by mass or less. The content of F particles in the dispersion of F particles is preferably 10% by mass or more, and more preferably 20% by mass or more. The content of F particles is preferably 60% by mass or less, and more preferably 40% by mass or less.

[0044] When the dispersion medium of the dispersion of F particles is water, the pH of the dispersion of F particles is preferably 5 to 10, and more preferably 8 to 10. To adjust the pH of the dispersion, a pH adjuster or pH buffer may be further included. Examples of pH adjusters include amines, ammonia, and citric acid. Examples of pH buffers include tris(hydroxymethyl)aminomethane, ethylenediaminetetraacetic acid, ammonium bicarbonate, ammonium carbonate, and ammonium acetate.

[0045] The dispersion of F particles may contain a nonionic surfactant. The nonionic surfactant is preferably a glycol-based surfactant, acetylene-based surfactant, silicone-based surfactant, or fluorine-based surfactant, with silicone-based surfactant being more preferred. One nonionic surfactant may be used, or two or more may be used. When two nonionic surfactants are used, it is preferable that the nonionic surfactants be a silicone-based surfactant and a glycol-based surfactant.

[0046] Specific examples of nonionic surfactants include the "Futergent" series (manufactured by Neos Co., Ltd.), the "Surflon" series (manufactured by AGC Seimi Chemical Co., Ltd.), the "Megafac" series (manufactured by DIC Corporation), the "Unidyne" series (manufactured by Daikin Industries, Ltd.), "BYK-347", "BYK-349", "BYK-378", "BYK-3450", "BYK-3451", "BYK-3455", "BYK-3456" (manufactured by Bic Chemie Japan Co., Ltd.), "KF-6011", "KF-6043" (manufactured by Shin-Etsu Chemical Co., Ltd.), and the "Tergitol" series (manufactured by Dow Chemical Corporation, such as "Tergitol TMN-100X"). If the dispersion contains a nonionic surfactant, the content of the nonionic surfactant in the dispersion is preferably 1 to 15% by mass.

[0047] From the viewpoint of suppressing powder shedding of the sheet, the mixing of the aggregates and F particles is preferably such that the amount of F polymer is 5 to 30 parts by mass, more preferably 10 to 30 parts by mass, and more preferably 15 to 25 parts by mass, with 100 parts by mass of PTFE contained in the aggregates.

[0048] The mixture may also contain a resin different from the PTFE and F polymer (hereinafter referred to as "different resin") or inorganic particles different from the inorganic particles contained in the aggregate (hereinafter referred to as "different inorganic particles"). The different resins may be thermosetting or thermoplastic, with thermoplastic being preferred. Examples of different resins include crystalline aromatic polyesters, polyester resins such as polyarylate resins, amide resins, imide resins, epoxy resins, maleimide resins, urethane resins, polyphenylene ether resins, polyphenylene oxide resins, polyphenylene sulfide resins, polyolefin resins, polycarbonate resins, and polyacetal resins.

[0049] Among these, aromatic polymers are more preferred, and at least one aromatic imide polymer selected from the group consisting of aromatic polyimides, aromatic polyamic acids, aromatic polyamideimides, and precursors of aromatic polyamideimides is even more preferred. In this case, the polymer layer tends to have excellent adhesion, low thermal expansion, and UV processability. Specific examples of aromatic imide polymers include the "Yupia-AT" series (manufactured by Ube Industries), the "Neoprim®" series (manufactured by Mitsubishi Gas Chemical Company), the "Spixeria®" series (manufactured by Somar), the "Q-PILON®" series (manufactured by PI Technical Research Institute), the "WINGO" series (manufactured by Wingo Technology), the "Tomide®" series (manufactured by T&K TOKA), the "KPI-MX" series (manufactured by Kawamura Industries), and "HPC-1000" and "HPC-2100D" (both manufactured by Showa Denko Materials). If the mixture contains different resins, the content of the different resins in the mixture is preferably 0.1 to 40% by mass, and more preferably 1 to 10% by mass.

[0050] Examples of different inorganic particles include those similar to the inorganic particles that may be used to obtain the aggregates. If the mixture contains inorganic particles, the content of inorganic particles in the mixture is preferably 1 to 50% by mass, and more preferably 3 to 30% by mass.

[0051] The aforementioned mixture may also contain particles containing PTFE, in addition to the PTFE contained in the aggregate. Furthermore, the mixture may, as necessary, contain additives other than the components mentioned above, such as plasticizers, weathering agents, antioxidants, heat stabilizers, lubricants, antistatic agents, whitening agents, colorants, conductive agents, mold release agents, surface treatment agents, flame retardants, and various fillers.

[0052] In this method, a sheet is formed from the mixture obtained above. One molding method is to cast the mixture. Casting makes it easier for the PTFE to fibrillate, and makes it easier to obtain a sheet in which inorganic particles do not easily peel off. Furthermore, it makes it easier to obtain a sheet in which the electrical properties and toughness are excellent. Methods for rolling a mixture include press molding, extrusion molding, and calendering. Calendering refers to a method of rolling a mixture by passing it between multiple rolls.

[0053] The mixture containing the dispersion medium may be molded after the dispersion medium has been removed. From the mixture containing the dispersion medium, an aggregate is obtained by further agglomerating F particles into an aggregate containing PTFE particles and inorganic particles, and this aggregate can be used for molding. A method for obtaining the aggregate is the same as the method for obtaining the aggregate containing PTFE particles and inorganic particles.

[0054] The mixture may be mixed with a molding aid before molding to produce a paste-like mixture, which may then be cast. It is desirable to mix with the molding aid under conditions that minimize the fiber formation of the PTFE. Specifically, it is desirable to reduce the rotation speed and shorten the mixing time, and to mix without kneading, so as not to apply shear force to the PTFE. If PTFE fiber formation occurs during the material mixing stage, the PTFE fibers formed during casting may be cut, potentially destroying the PTFE network structure and making it difficult to maintain the sheet shape. Therefore, by mixing in a way that suppresses PTFE fiber formation, the subsequent processing of this sheet with PTFE as the matrix becomes easier. For example, alkanes such as dodecane and decane can be used as molding aids. The molding aid should be added in an amount of 20 to 55% by mass relative to the total mass.

[0055] Casting of the mixture may be performed using one type of molding method, or may be performed by combining two or more types of molding methods. Further, casting may be performed by repeating one type of molding method a plurality of times. For example, a mother sheet obtained by extrusion molding of the mixture may be further calendered and cast, or a mother sheet obtained by calender molding of the mixture may be further calendered and cast. In this case, the present sheet having any thickness and excellent in toughness and uniformity is easily obtained. A plurality of rolls may be used in calender molding, and it is preferable to use four rolls in combination. Examples of arrangements of the four rolls include I-type, S-type, inverted L-type, Z-type and oblique Z-type.

[0056] Casting of the mixture may be performed while heating at a temperature lower than the melting temperature of PTFE, or may be performed without heating. When a molding aid is used during molding of the present sheet, heating may be performed after casting to remove the molding aid. As described above, the present sheet is obtained. The thickness of the present sheet is, for example, 0.5 to 5 mm. The present sheet may be heated at a temperature equal to or higher than the melting temperature of PTFE and fired.

[0057] Further, as described above, D50 is 1 to 20 µm, and the specific surface area is 1 to 20 m 2 / g inorganic particles form inter-particle paths in the sheet, and tend to improve sheet physical properties such as thermal conductivity, low linear expansion, and electrical properties. On the other hand, they have low interaction with particles containing PTFE and tend to be more easily removed from the sheet. However, by allowing the F polymer to coexist with the inorganic particles, powder falling can be more highly suppressed even for particles that are more prone to such powder falling.

[0058] Therefore, the present invention provides PTFE, an F polymer, having an average particle diameter of 1 to 20 µm, and a specific surface area of 1 to 20 m 2Further, we provide a sheet (hereinafter also referred to as "the other sheet") which contains inorganic particles at a concentration of / g, wherein the content of PTFE and the inorganic particles is 5% to 60% by mass and 40% to 95% by mass, respectively, with the total mass of PTFE and the inorganic particles being 100% by mass. The PTFE and F polymers are as described above, and the preferred form is also the same as described above. In the other sheets, the PTFE content is preferably 20% to 50% by mass, where the total mass of the PTFE and the inorganic particles is 100% by mass. Furthermore, the inorganic particle content in the other sheets is more preferably 60% to 80% by mass, where the total mass of the PTFE and the inorganic particles is 100% by mass.

[0059] The other sheets preferably contain 5 to 30 parts by mass of the F polymer per 100 parts by mass of the PTFE, more preferably 10 to 30 parts by mass, and even more preferably 15 to 25 parts by mass. Furthermore, the inorganic particles preferably include at least one selected from the group consisting of metal oxides, silicon oxides, and nitrides, as described above.

[0060] Other sheets can be manufactured, for example, by the method described above. In the manufacture of other sheets, the preferred form is the same as the preferred form of the method described above.

[0061] A laminated sheet can be obtained by stacking two or more of the sheets obtained above or other sheets. The laminated sheet is obtained by stacking two or more sheets and rolling them. The number of layers in the laminated sheet is, for example, about 2 to 10 layers. From the viewpoint of strength between layers, it is preferable to stack and roll the sheets one by one.

[0062] Alternatively, this sheet or other sheets may be folded and laminated. By folding and rolling this sheet or other sheets, the sheet strength can be improved, and the inorganic particles can be firmly fixed to the PTFE matrix. As a result, a laminated sheet with a high inorganic particle content and flexibility can be produced. Multiple of the aforementioned laminated sheets may be stacked to create a laminated sheet with an even greater number of layers.

[0063] When creating laminated sheets, it is desirable to change the rolling direction. For example, when rolling a second sheet onto a first sheet and then laminating a third sheet, it is preferable to change the rolling direction by 90 degrees from the rolling direction of the second sheet. By rolling while changing the direction in this way, the PTFE network extends both vertically and horizontally, improving the strength of the sheet and enabling firm fixation of inorganic particles to the PTFE matrix. The resulting laminated sheet may be pressure-molded. Pressure molding can reduce the pores in the laminated sheet.

[0064] When the laminated sheet obtained above is heat-pressed to a substrate, a laminate is obtained having a substrate layer and a polymer layer containing PTFE, inorganic particles, and F polymer.

[0065] Examples of substrates include metal substrates such as metal foils made of copper, nickel, aluminum, titanium, and their alloys; heat-resistant resin films such as polyimide, polyamide, polyetheramide, polyphenylene sulfide, polyallyl ether ketone, polyamide-imide, liquid crystalline polyester, and tetrafluoroethylene polymers; prepreg substrates which are precursors to fiber-reinforced resin substrates; ceramic substrates such as silicon carbide, aluminum nitride, or silicon nitride; and glass substrates. The substrate can be planar, curved, or uneven. Furthermore, the substrate may be foil-like, plate-like, film-like, or fibrous. The ten-point average roughness of the substrate surface is preferably 0.01 to 0.05 μm.

[0066] The surface of the substrate may be surface-treated with a silane coupling agent or plasma-treated. The preferred silane coupling agent is one having a functional group such as 3-aminopropyltriethoxysilane, vinyltrimethoxysilane, 3-mercaptopropyltrimethoxysilane, 3-glycidoxypropylmethyldiethoxysilane, 3-methacryloxypropyltriethoxysilane, or 3-isocyanatetopropyltriethoxysilane.

[0067] Methods of heat-pressure bonding include a method of sandwiching the substrate and the sheet between a pair of opposing hot plates and applying pressure, a method of passing the substrate and the sheet between a pair of opposing rolls, and a method of applying pressure to the substrate and the sheet with rolls on a hot plate. The temperature for heat bonding is preferably 200°C or higher, more preferably above the melting point of PTFE, and even more preferably 350°C or higher. A temperature of 400°C or lower is preferred. It is preferable to sinter the PTFE during the heating process.

[0068] The heat-compression bonding may be performed under reduced pressure. In this case, it is preferable to perform the bonding at a vacuum level of 20 kPa or less, from the viewpoint of suppressing deterioration due to oxidation of the substrate and the sheet. It is preferable to perform the heat-compression bonding using a vacuum press. During heat sealing, it is preferable to either place a release film between the surface of the sheet and the heat plate or roll, or to surface treat the surface of the heat plate or roll with a release agent, in order to suppress adhesion of the sheet to the heat plate or roll.

[0069] The thickness of the release film is preferably 50 to 150 μm. Polyimide films are used as release films, with specific examples including "Apical NPI" (manufactured by Kaneka Corporation), "Kapton EN" (Toray DuPont), and "Upirex S" (manufactured by Ube Industries).

[0070] This laminated sheet may be heat-pressed onto only one surface of the substrate, or it may be heat-pressed onto both surfaces of the substrate. In the former case, a laminate is obtained having a substrate layer and a polymer layer on one surface of the substrate layer, and in the latter case, a laminate is obtained having a substrate layer and polymer layers on both surfaces of the substrate layer. Suitable examples of laminates include a metal-clad laminate having a metal foil and a polymer layer on at least one surface of the metal foil, and a polyimide film and a multilayer film having polymer layers on both surfaces of the polyimide film. The peel strength between the polymer layer and the substrate layer is preferably 10 to 100 N / cm. Furthermore, the substrate layer may be removed from the laminate to obtain a sheet containing PTFE, inorganic particles, and F polymer.

[0071] This sheet, this laminated sheet, and laminates containing this laminated sheet are useful as antenna components, printed circuit boards, aircraft components, automobile components, sports equipment, food industry products, heat dissipation components, paints, cosmetics, etc. Specifically, this includes wire insulation materials for aircraft and other applications, enamel wire insulation materials used in motors for electric vehicles and other applications, electrical insulation tapes, insulating tapes for oil drilling, oil transport hoses, hydrogen tanks, printed circuit board materials, separation membranes such as microfiltration membranes, ultrafiltration membranes, reverse osmosis membranes, ion exchange membranes, dialysis membranes and gas separation membranes, electrode binders for lithium secondary batteries and fuel cells, copy rolls, furniture, car dashboards, covers for home appliances, sliding members, tension ropes, wear pads, wear strips, tube lamps, test sockets, wafer guides, wear parts for centrifugal pumps, chemical and water supply pumps, tools such as shovels, files, drills and saws, boilers, hoppers, pipes, ovens, baking molds, chutes, racket strings, dies, toilets, container insulation materials, power devices, transistors, thyristors, rectifiers, transformers, and power MOS It is useful in FETs, CPUs, heat sinks, metal heat sinks, blades for wind turbines, wind power generation equipment, and aircraft, casings for personal computers and displays, electronic device materials, interior and exterior parts of automobiles, sealing materials for processing machines and vacuum ovens that perform heat treatment under low oxygen conditions, plasma processing equipment, heat dissipation components in processing units such as sputtering and various dry etching equipment, and as electromagnetic shielding. Examples of sliding members include load bearings, yaw bearings, sliding shafts, valves, bearings, bushings, seals, thrust washers, wear rings, pistons, slide switches, gears, cams, belt conveyors, and food transport belts.

[0072] Although the manufacturing methods for this sheet and laminated sheet, and other versions of this sheet have been described above, the present invention is not limited to the configurations of the embodiments described above. For example, the manufacturing method of the sheet and the laminated sheet may have additional steps in the configuration of the above embodiment, or may be replaced with any other steps that produce a similar effect. Other sheets may have additional configurations in the configuration of the above embodiment, or may be replaced with any other configurations that perform a similar function. [Examples]

[0073] The present invention will be described in detail below with reference to examples, but the present invention is not limited to these examples. 1. Preparation of each component [particle] PTFE particles: PTFE particles (D50: 0.3 μm) F particle 1: Contains TFE units, NAH units, and PPVE units in the following order: 97.9 mol%, 0.1 mol%, and 2.0 mol%, respectively, with a carbonyl group in the main chain having 1 × 10¹⁶ carbon atoms. 6 Particles of F polymer 1 (melting temperature: 300℃) with 1000 particles per unit (D50: 1.7μm) F Particle 2: Particles of F polymer 2 (melting temperature: 300°C) containing 97.5 mol% and 2.5 mol% TFE units and PPVE units, respectively, and lacking oxygen-containing polar groups (D50: 2.1 μm).

[0074] [Dispersion] Dispersion 1: Aqueous dispersion containing 60% by mass of PTFE particles. [Inorganic particles] Inorganic particle 1: Scale-like boron nitride particles (D50: 7.0 μm, specific surface area: 2 m²) 2 / g) Inorganic particles 2: Spherical silica particles (D50: 2.0 μm, specific surface area: 5 m²) 2 / g)

[0075] 2. Manufacturing of sheets, laminated sheets, and laminates (Example 1) Dispersion 1 was diluted 20 times with water, and inorganic particles 1 were added to the resulting diluted dispersion so that the mass ratio of inorganic particles 1 to PTFE particles was 80:20. This yielded a mixture containing PTFE particles 1 and inorganic particles 1. Isopropyl alcohol was added to this mixture to co-aggregate the PTFE particles and inorganic particles 1 in the mixture. The resulting aggregates were filtered to separate them from the liquid components and dried at 150°C for 24 hours to obtain aggregates containing PTFE particles and inorganic particles 1.

[0076] This aggregate was mixed with F particle 1 at a ratio of 10 parts by mass per 100 parts by mass of PTFE particles, and the molding aid "Isopar M" (manufactured by ExxonMobil), such that the ratio of the total mass of PTFE particles, inorganic particles 1, and F particle 1 to the mass of the molding aid was 5:2. The mixture was then mixed in a rotary mill for 10 minutes under conditions that minimized the formation of PTFE fibers.

[0077] The obtained mixture was pre-molded and pelletized. These pellets were extruded in an extruder to obtain a base sheet with a width of 45 mm and a thickness of 2 mm. The base sheet was rolled in the MD direction (longitudinal direction) using a pair of rolling rolls to obtain a sheet 1 with a thickness of 1 mm. The two sheets 1 were laminated in a roll-to-roll process with the MD and TD directions aligned, and rolled in the TD direction to obtain a laminated sheet 1 with a thickness of 0.25 mm. There was no powder fallout during the sheet preparation, and the content of PTFE, inorganic particles 1, and F polymer 1 in each sheet was 20% by mass, 78% by mass, and 2% by mass, respectively. The sheet contains 25% by mass of PTFE and 75% by mass of inorganic particles 1, with the total mass of PTFE and inorganic particles 1 being 100% by mass, and contains 10 parts by mass of F polymer 1 per 100 parts by mass of PTFE.

[0078] A laminated sheet 1 was placed on top of an unroughened copper foil (average surface roughness at 10 points: 0.05 μm or less, thickness: 18 μm), and the two were bonded together by heat-pressing at 380°C to obtain a laminated body 1 having a polymer layer made of the laminated sheet and a copper foil.

[0079] (Example 2) In Example 1, laminated sheet 2 and laminated body 2 were obtained in the same manner except that F particle 1 was not added. During the preparation of laminated sheet 2, streaks were observed in the sheet due to powder fallout, and contamination of the equipment was observed. Furthermore, when measuring the peel strength between the copper foil and the laminated sheet in each laminate, the peel strength of laminate 1 was greater than 10 N / cm, while the peel strength of laminate 2 was less than 10 N / cm.

[0080] (Example 3) Dispersion 1 was diluted 20-fold with water, and inorganic particles 2 were added to the resulting diluted dispersion in such a mass ratio of inorganic particles 2 to PTFE particles as 60:40. This yielded a mixture containing PTFE particles and inorganic particles 2. Isopropyl alcohol was added to this mixture to co-aggregate the PTFE particles and inorganic particles 2 within the mixture. The resulting aggregates were filtered to separate them from the liquid components and dried at 150°C for 24 hours to obtain aggregates containing PTFE particles and inorganic particles 2.

[0081] This aggregate was mixed with 8 parts by mass of F particle 1 per 100 parts by mass of PTFE particles, and the molding aid "Isopar M" (manufactured by ExxonMobil), such that the ratio of the total mass of PTFE particles, inorganic particles 2, and F particle 2 to the mass of the molding aid was 5:2. The mixture was then mixed in a rotary mill for 10 minutes under conditions that minimized the formation of PTFE fibers.

[0082] The obtained mixture was pre-molded and pelletized. These pellets were extruded in an extruder to obtain a base sheet with a width of 45 mm and a thickness of 2 mm. The base sheet was rolled in the MD direction (longitudinal direction) using a pair of rolling rolls to obtain a sheet 3 with a thickness of 1 mm. Two sheets 3 were laminated in a roll-to-roll process with the MD and TD directions aligned, and rolled in the TD direction to obtain a laminated sheet 3 with a thickness of 0.25 mm. There was no powder fallout during sheet preparation. The content of PTFE, inorganic particles 2, and F polymer 1 in each sheet was 39% by mass, 59% by mass, and 2% by mass, respectively. Each sheet contains 40% by mass of PTFE and 60% by mass of inorganic particles 2, with the total mass of PTFE and inorganic particles 2 being 100% by mass, and contains 5 parts by mass of F polymer 1 per 100 parts by mass of PTFE. Laminated sheet 3 was an adhesive sheet with low relative permittivity and dielectric loss tangent, as well as low linear expansion properties, and excellent surface smoothness with suppressed powder shedding during use.

[0083] (Example 4) Except for changing F polymer 1 to F polymer 2, we attempted to form the base sheet 4 and laminated sheet 4 in the same manner as in Example 3. However, powder shedding occurred during sheet fabrication, making it impossible to form sheets with excellent surface smoothness, and the sheets also shed powder during use.

[0084] As is clear from the results above, the sheets and laminated sheets obtained by this method exhibit uniform dispersion of inorganic particles, suppression of inorganic particle shedding, and excellent low-temperature adhesion. Furthermore, the sheets, laminated sheets, and laminates obtained by this method fully possess the inherent properties of polytetrafluoroethylene and inorganic particles.

Claims

1. A method for producing a sheet, comprising mixing particles containing polytetrafluoroethylene, an aggregate containing inorganic particles, and particles containing a tetrafluoroethylene-based polymer having oxygen-containing polar groups and a melting temperature of 320°C or lower, with the particles containing the tetrafluoroethylene-based polymer being 5 to 30 parts by mass per 100 parts by mass of the polytetrafluoroethylene, and forming the resulting mixture.

2. The manufacturing method according to claim 1, wherein the aggregate is obtained by co-aggregation from a mixture containing the polytetrafluoroethylene particles, the inorganic particles, and water.

3. The manufacturing method according to claim 1, wherein the content of the polytetrafluoroethylene-containing particles and the inorganic particles in the aggregate is such that, with the total mass of the polytetrafluoroethylene-containing particles and the inorganic particles being 100% by mass, the polytetrafluoroethylene-containing particles are 5% by mass or more and 60% by mass or less, and the inorganic particles are 40% by mass or more and 95% by mass or less.

4. The manufacturing method according to claim 1, wherein the inorganic particles include at least one selected from the group consisting of metal oxides, silicon oxides, and nitrides.

5. The average particle diameter of the inorganic particles is 1 to 20 μm, and the specific surface area is 1 to 20 m². ２ The manufacturing method according to claim 1, wherein the amount is / g.

6. The manufacturing method according to claim 1, wherein the average particle size of the particles containing the tetrafluoroethylene polymer is 0.1 μm or more and 25 μm or less.

7. The manufacturing method according to claim 1, comprising mixing the particles containing the tetrafluoroethylene polymer with the aggregate in powder form.

8. The manufacturing method according to claim 1, comprising dispersing the particles containing the tetrafluoroethylene polymer in a dispersion medium and mixing them with the aggregates.

9. The manufacturing method according to claim 1, wherein the molding is performed by extrusion molding or rolling molding.

10. The manufacturing method according to claim 1, wherein the thickness of the sheet is 0.5 mm or more and 5 mm or less.

11. A method for manufacturing a laminated sheet, comprising stacking two or more sheets obtained by the manufacturing method described in any one of claims 1 to 10.

12. Polytetrafluoroethylene, a tetrafluoroethylene-based polymer having oxygen-containing polar groups and a melting temperature of 320°C or lower, and an average particle size of 1 to 20 μm and a specific surface area of ​​1 to 20 m². ２ A sheet comprising an inorganic particle of a certain weight per gram, wherein the content of the polytetrafluoroethylene and the inorganic particle is such that, with the total mass of the polytetrafluoroethylene and the inorganic particle being 100% by mass, the amounts are 5% to 60% and 40% to 95% by mass, respectively, and the tetrafluoroethylene polymer is contained in an amount of 5 to 30 parts by mass per 100 parts by mass of the polytetrafluoroethylene.

13. The sheet according to claim 12, wherein the inorganic particles include at least one selected from the group consisting of metal oxides, silicon oxides, and nitrides.