JP7697273B2 - Manufacturing method of laminate, manufacturing method of sheet, and sheet - Google Patents

Description

The present invention relates to a method for producing a laminate having a layer containing a sintered product of particles of a specific tetrafluoroethylene-based polymer and an inorganic filler having an aspect ratio of more than 1, a method for producing a sheet containing the sintered product and the inorganic filler, and a sheet containing the sintered product and the inorganic filler.

Tetrafluoroethylene-based polymers have excellent physical properties such as electrical insulation, water and oil repellency, chemical resistance, and heat resistance, and are widely used in electronic equipment parts, automobile parts, etc. In particular, tetrafluoroethylene-based polymers are used as various electronic components in the form of coatings, laminates, sheets, films, etc., taking advantage of their low dielectric properties and low dielectric loss tangent.

On the other hand, electronic components are becoming more dense, more integrated, and more powerful, and measures to prevent heat generation from these electronic components are becoming increasingly important. In particular, power semiconductors such as LEDs generate a lot of heat, so improving their heat dissipation performance is particularly important.
Patent Documents 1 and 2 disclose a sheet obtained by injection molding a powder composition containing a tetrafluoroethylene-based polymer and a boron nitride filler, and propose an attempt to improve the thermal conductivity of the sheet.

International Publication No. 2020/045260 International Publication No. 2021/010320

However, as electrical equipment becomes more sophisticated, its power consumption increases, and the demand for heat dissipation from electronic components is becoming higher and higher. The sheets disclosed in Patent Documents 1 and 2 still do not have sufficient heat dissipation properties.

The inventors aimed to develop a sheet containing a heat-fusible tetrafluoroethylene-based polymer and an inorganic filler, which has high thermal conductivity, and investigated a manufacturing method that would result in a highly oriented inorganic filler, which led to the completion of the present invention.

The present invention provides a laminate having a layer with high thermal conductivity by improving the orientation of the inorganic filler in a layer containing a heat-fusible tetrafluoroethylene-based polymer and an inorganic filler, a manufacturing method for producing a sheet comprising said layer, and a sheet with high thermal conductivity.

[1]
A method for producing a laminate having a substrate and a layer containing the fired product and the inorganic filler formed on the surface of the substrate, the method comprising: forming a layer containing a powder of a tetrafluoroethylene-based polymer containing particles of a heat-fusible tetrafluoroethylene-based polymer and an inorganic filler having an aspect ratio of more than 1 on a surface of the substrate; and forming a layer containing a fired product of the powder and the inorganic filler, the layer and the substrate being pressed together during any one of the steps.
[2]
The method according to the above [1], wherein the powder of the tetrafluoroethylene-based polymer contains particles of a heat-fusible tetrafluoroethylene-based polymer and particles of a non-heat-fusible tetrafluoroethylene-based polymer.
[3]
The method according to [1] or [2], wherein the heat-meltable tetrafluoroethylene polymer is a tetrafluoroethylene polymer having a carbonyl group-containing group or a hydroxyl group-containing group and a melting temperature of 200° C. or higher and 320° C. or lower.
[4]
The manufacturing method according to any one of [1] to [3], wherein the mass ratio of the heat-fusible tetrafluoroethylene-based polymer to the inorganic filler in the layer is such that the mass ratio of the heat-fusible tetrafluoroethylene-based polymer to the inorganic filler is 1, and the mass ratio of the inorganic filler is 0.2 to 2.
[5]
The method according to any one of [1] to [4], wherein the average particle size of the particles of the heat-fusible tetrafluoroethylene polymer is smaller than the average particle size of the inorganic filler.
[6]
The method according to any one of [1] to [5], wherein the content of the inorganic filler in the layer containing the particles of the heat-fusible tetrafluoroethylene polymer and the inorganic filler is 60 mass % or less.
[7]
The method according to any one of [1] to [6], wherein the inorganic filler contains at least one of boron nitride and silicon oxide.
[8]
The method for producing a substrate according to any one of [1] to [7], wherein the layer containing the powder and the inorganic filler further contains a silane coupling agent.
[9]
The manufacturing method according to any one of [1] to [8], further comprising pressing the layer containing the powder and the inorganic filler against the substrate.
[10]
The manufacturing method according to any one of [1] to [9], wherein the pressing pressure is 0.2 MPa or more and 10 MPa or less.
[11]
The method according to any one of [1] to [10], wherein the pressing temperature is equal to or lower than the melting temperature of the heat-fusible tetrafluoroethylene polymer.
[12]
The manufacturing method according to any one of [1] to [11], comprising: coating a surface of a substrate with a liquid composition containing the tetrafluoroethylene-based polymer powder, the inorganic filler, and a liquid dispersion medium; and removing at least a portion of the liquid dispersion medium to form a layer containing the powder and the inorganic filler.
[13]
The method according to [12], wherein the liquid composition further contains a silane coupling agent.
[14]
A method for producing a sheet containing the fired product and the inorganic filler, comprising the steps of: forming a layer containing a powder of a tetrafluoroethylene-based polymer containing particles of a heat-fusible tetrafluoroethylene-based polymer and an inorganic filler having an aspect ratio of more than 1 on a surface of a substrate; forming a layer containing a fired product of the powder and the inorganic filler; pressing the layer and the substrate together to obtain a laminate having the substrate and the layer containing the fired product and the inorganic filler formed on the surface of the substrate; and removing the substrate.
[15]
A sheet comprising a fired product of tetrafluoroethylene-based polymer powder containing particles of a heat-fusible tetrafluoroethylene-based polymer and an inorganic filler having an aspect ratio of more than 1, the sheet having a thickness of 50 μm or more and a thermal conductivity of more than 5 W/m·K.

According to the present invention, it is possible to improve the orientation of the inorganic filler in a layer containing a tetrafluoroethylene-based polymer and an inorganic filler, and to produce a laminate having a layer with high thermal conductivity, and a sheet comprising said layer. Furthermore, according to the present invention, a sheet containing a tetrafluoroethylene-based polymer and an inorganic filler and having high thermal conductivity is provided.

The following terms have the following meanings.
The term "thermally meltable tetrafluoroethylene polymer" refers to a polymer containing units (hereinafter also referred to as TFE units) based on tetrafluoroethylene (hereinafter also referred to as TFE), and having melt fluidity at a temperature that is 20° C. or more higher than the melting temperature of the polymer under a load of 49 N, at which the melt flow rate is 1 to 1000 g/10 min.
The term "non-thermoplastic tetrafluoroethylene polymer" refers to a polymer containing TFE units, which is a non-melt-flowable polymer at which there is no temperature at which the melt flow rate exceeds 1 g/10 min under a load of 49 N.
The "glass transition point (Tg) of a polymer" is a value measured by analyzing a polymer using a dynamic mechanical analysis (DMA) method.
The "melting temperature (melting point) of a polymer" is the temperature corresponding to the maximum of the melting peak as measured by differential scanning calorimetry (DSC).
"D50" is the average particle diameter of the particles or inorganic filler, and is the volume-based cumulative 50% diameter of the particles or inorganic filler obtained by a laser diffraction/scattering method. That is, the particle size distribution of the particles or inorganic filler is measured by a laser diffraction/scattering method, a cumulative curve is obtained with the total volume of the group of particles or fillers being 100%, and the "D50" is the particle diameter at the point on the cumulative curve where the cumulative volume is 50%.
"D90" is the cumulative volume particle size of a particle or an inorganic filler, and is the volume-based cumulative 90% diameter of a particle that is determined in the same manner as "D50".
The "viscosity" is a value measured for the liquid composition using a Brookfield viscometer at room temperature (25° C.) and a rotation speed of 30 rpm. The measurement is repeated three times, and the average value of the three measured values is used.
The "thixotropy ratio" is a value calculated by dividing the viscosity of a liquid composition measured at a rotation speed of 30 rpm by the viscosity of a liquid composition measured at a rotation speed of 60 rpm.
The term "unit based on a monomer" refers to an atomic group based on the monomer formed by polymerization of the monomer. The unit may be a unit formed directly by a polymerization reaction, or may be a unit in which a part of the unit is converted into a different structure by treating the polymer. Hereinafter, a unit based on monomer a is also referred to simply as a "monomer a unit."

The manufacturing method of the present invention (hereinafter also referred to as the present method) is a method of forming a layer containing a powder (hereinafter also referred to as the present powder) of a tetrafluoroethylene-based polymer (hereinafter also referred to as the "F polymer") containing particles (hereinafter also referred to as molten particles) of a heat-fusible tetrafluoroethylene-based polymer (hereinafter also referred to as the "molten F polymer") and an inorganic filler (hereinafter also referred to as the present inorganic filler) having an aspect ratio of more than 1 on the surface of a substrate, forming a layer containing a fired product of the present powder and the inorganic filler, and pressing the layer and the substrate together during any of the steps of the formation of the layer containing the fired product and the inorganic filler formed on the surface of the substrate, thereby obtaining a laminate (hereinafter also referred to as the present laminate).

The present powder is an aggregate of particles including molten particles, and may consist of only molten particles, or may contain molten particles and other particles. The other particles are preferably particles of a non-thermofusible tetrafluoroethylene-based polymer (hereinafter also referred to as "non-molten F polymer") (hereinafter also referred to as "non-molten particles"), and preferably contain both molten particles and non-molten particles.
In the latter case, the inorganic filler is more easily supported in the layer due to the progress of fibrillation of the non-thermofusible F polymer when the layer and the substrate are pressed together, and the strength of the layer is more likely to be improved.
In this case, the content of the non-molten particles in the total amount of the present powder is preferably 50 mass % or more.
The non-meltable F polymer is preferably non-thermofusible polytetrafluoroethylene.
The D50 of the non-fused particles is preferably from 0.1 to 1 μm.
When the present powder is an aggregate of molten particles and non-molten particles, the content of the molten particles and non-molten particles in the present powder is preferably such that the total mass of the content of the molten particles and non-molten particles is 100 mass%, with the mass of the present powder being 100 mass%.
When the present powder is an aggregate of molten particles and non-molten particles, the content of the molten particles in the present powder is preferably from 5 to 80 mass%, and more preferably from 10 to 60 mass%, with the total mass of the contents of the molten particles and non-molten particles being 100 mass%.

The melting temperature of the molten F polymer is preferably 200°C or higher, more preferably 250°C or higher, and even more preferably 280°C or higher. From the viewpoint of moldability, a melting temperature of 320°C or lower is preferable.

The glass transition temperature of the molten F polymer is preferably from 30 to 150°C, more preferably from 75 to 125°C.
As the molten F polymer, the polymer (hereinafter also referred to as PFA) that contains TFE unit and the unit (hereinafter also referred to as PAVE) that is based on perfluoro(alkyl vinyl ether) or the copolymer (hereinafter also referred to as FEP) that contains TFE unit and the unit that is based on hexafluoropropylene are preferred, and PFA is particularly preferred.These polymers may further contain the unit that is based on other comonomers.

As PAVE, CF2=CFOCF3, CF2=CFOCF2CF3 or CF2=CFOCF2CF2CF3 (hereinafter also referred to as PPVE) is preferred, and PPVE is more preferred.
The molten F polymer preferably has a polar functional group. The molten F polymer having a polar functional group is likely to further improve the adhesiveness to a substrate described later and the reliability of the laminate, such as the peel strength and water resistance.

The polar functional group may be contained in a monomer unit in the molten F polymer, or may be contained in a terminal group of the main chain of the molten F polymer. Examples of the latter embodiment include a molten F polymer having a polar functional group as an end group derived from a polymerization initiator, a chain transfer agent, etc., and a molten F polymer having a polar functional group obtained by subjecting a molten F polymer to plasma treatment or ionizing radiation treatment.
The polar functional group is preferably a hydroxyl group-containing group or a carbonyl group-containing group, and particularly preferably a carbonyl group-containing group.

The hydroxyl-containing group is preferably a group containing an alcoholic hydroxyl group, more preferably -CF2CH2OH or -C(CF3)2OH.
The carbonyl group-containing group is a group containing a carbonyl group (>C(O)), and is preferably a carboxyl group, an alkoxycarbonyl group, an amide group, an isocyanate group, a carbamate group (-OC(O)NH2), an acid anhydride residue (-C(O)OC(O)-), an imide residue (-C(O)NHC(O)- etc.) or a carbonate group (-OC(O)O-), and particularly preferably an acid anhydride residue.

Suitable embodiments of the molten F polymer include polymer (1) containing TFE units and PAVE units and having a polar functional group, or polymer (2) containing TFE units and PAVE units, containing 2.0 to 5.0 mol% of PAVE units based on the total monomer units, and having no polar functional group, with polymer (1) being preferred. These polymers form microspherulites in the product, which tends to improve the properties of the resulting product.

The polymer (1) is preferably a polymer containing TFE units, PAVE units, and a monomer having a hydroxyl group-containing group or a carbonyl group-containing group. The polymer (1) preferably contains 90 to 99 mol% of TFE units, 0.5 to 9.97 mol% of PAVE units, and 0.01 to 3 mol% of units based on the monomers, based on all units.
The monomer is preferably itaconic anhydride, citraconic anhydride, or 5-norbornene-2,3-dicarboxylic anhydride (also called himic anhydride; hereinafter, also referred to as "NAH").
Specific examples of the polymer (1) include the polymers described in WO 2018/16644.

The polymer (2) is composed only of TFE units and PAVE units, and preferably contains 95.0 to 98.0 mol % of TFE units and 2.0 to 5.0 mol % of PAVE units based on the total monomer units.
The content of PAVE units in the polymer (2) is preferably 2.1 mol % or more, more preferably 2.2 mol % or more, based on the total monomer units.
In addition, the polymer (2) having no polar functional groups means that the number of polar functional groups that the polymer has per 1×106 carbon atoms constituting the polymer main chain is less than 500. The number of polar functional groups is preferably 100 or less, more preferably less than 50. The lower limit of the number of polar functional groups is usually 0.

Polymer (2) may be produced using a polymerization initiator or a chain transfer agent that does not generate a polar functional group as the end group of the polymer chain, or may be produced by fluorinating a molten F polymer having a polar functional group. Fluorination methods include a method using fluorine gas (see JP 2019-194314 A, etc.).

When the molten F polymer has a carbonyl group-containing group or a hydroxyl group-containing group, the number of carbonyl group-containing groups or hydroxyl group-containing groups in the molten F polymer is preferably 10 to 5000, more preferably 50 to 4000, and even more preferably 100 to 2000 per 1×106 carbon atoms in the main chain. In this case, the molten F polymer is likely to interact with the inorganic filler, and the inorganic filler is likely to have excellent dispersibility in the resulting layer. The number of carbonyl group-containing groups or hydroxyl group-containing groups in the molten F polymer can be quantified from the composition of the polymer. The carbonyl group-containing groups can be quantified by the method described in International Publication WO 2020/145133.

The molten particles are particles containing a molten F polymer, and the amount of the molten F polymer in the molten particles is preferably 80% by mass or more, and more preferably 100% by mass.
The average particle size D50 of the molten particles is preferably 50 μm or less, more preferably 20 μm or less, and even more preferably 8 μm or less. The D50 of the molten particles is preferably 0.1 μm or more, more preferably 0.3 μm or more, and even more preferably 1 μm or more. The D90 of the molten particles is preferably less than 100 μm, and more preferably 90 μm or less. If the D50 and D90 of the molten particles are within the above ranges, the surface area of the molten particles is increased, and the dispersibility of the molten particles is more likely to be improved.

The molten particles may contain other resins or inorganic materials different from the molten F polymer.
Specific examples of other resins include aromatic polymers such as aromatic polyimides, aromatic maleimides, styrene elastomers, and aromatic polyamic acids.
A specific example of the inorganic material is silica.

The inorganic filler has an aspect ratio of more than 1, and may be granular, needle-like, fibrous, or plate-like in shape. Specific shapes of the inorganic filler include spherical, scale-like, layered, leaf-like, apricot kernel-like, columnar, cockscomb-like, equiaxial, leaf-like, micaceous, block-like, flat, wedge-like, rosette-like, mesh-like, and prismatic shapes.
In addition to the above shapes, the inorganic filler may have various shapes such as a plate shape, a hollow shape, a honeycomb shape, and the like.
The aspect ratio of the present inorganic filler is preferably at least 5, and more preferably at least 10. The aspect ratio of the present inorganic filler is preferably at most 1,000.

Examples of the inorganic filler include aluminum oxide, magnesium oxide, boron nitride, aluminum nitride, silicon nitride, silicon oxide, and silicon carbide. Among these, from the viewpoint of thermal conductivity, alumina, boron nitride, and silicon oxide are preferred, boron nitride and silicon oxide are more preferred, and boron nitride, in particular hexagonal boron nitride, is even more preferred.

The inorganic filler is preferably surface-treated with a silane coupling agent. In this case, the inorganic filler and the molten particles are easily interacted with each other, and the inorganic filler is easily dispersed in the resulting layer. In addition, the binding force between the powder and the inorganic filler is improved, and the powder or inorganic particles are easily prevented from falling off in the powder layer.
The silane coupling agent is a compound having a hydrolyzable silyl group and an organic group.
The silane coupling agent may be partially reacted and may form a polysiloxane skeleton.

The hydrolyzable silyl group is preferably a monoalkoxysilyl group, a dialkoxysilyl group, or a trialkoxysilyl group, and more preferably a trialkoxysilyl group. The hydrolyzable silyl group may be hydrolyzed.
The organic group includes vinyl group, epoxy group, styryl group, acryloyloxy group, methacryloyloxy group, amino group, isocyanate group, mercapto group, benzotriazole group, and acid anhydride group, and epoxy group, benzotriazole group, phenyl group, and ureido group are preferable, and epoxy group is more preferable. The silane coupling agent may have a plurality of different types of organic groups, or may have a plurality of the same type of organic groups.
The silane coupling agent is preferably a compound having a trialkoxysilyl group and a benzotriazole group or an epoxy group, and more preferably a compound having a trialkoxysilyl group and an epoxy group.

Examples of the silane coupling agent include a compound having a benzotriazole group and a trimethoxysilyl group at both ends of the main chain, a compound having three epoxy groups in the main chain and a plurality of triethoxysilyl groups in the side chain, a compound having a siloxane structure in the main chain and amino groups at both ends of the main chain, a compound having a butadiene structure in the main chain and one acid anhydride group and one trimethoxysilyl group in the side chain, and a compound having an alkoxysiloxane structure in the main chain and a plurality of epoxy groups in the side chain.
Specific examples of the silane coupling agent include N-phenyl-3-aminopropyltrimethoxysilane, 3-glycidoxypropyltrimethoxysilane, 3-aminopropyltrimethoxysilane, 3-aminopropyltriethoxysilane, p-styryltrimethoxysilane, 3-trimethoxysilylpropylsuccinic anhydride, N-2-(aminomethyl)-8-aminooctyltrimethoxysilane, 3-glycidoxypropylmethyldimethoxysilane, 3-glycidoxypropylmethyldiethoxysilane, 3-glycidoxypropyltriethoxysilane, and 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane.

Specific examples of silane coupling agents include "KBM-573", "KBM-403", "KBM-903", "KBE-903", "KBM-1403", "X-12-967C", "X-12-1214A", "X-12-984S", "X-12-1271A", "KBP-90", "KBM-6803", "X-12-1287A", "KBM-402", "KBE-402", "KBE-403", "KR-516", "KBM-303", "KBM-4803", "KBM-3063", and "KBM-13" (all manufactured by Shin-Etsu Chemical Co., Ltd.).

A method for surface-treating the inorganic filler with a silane coupling agent includes mixing a solution containing the silane coupling agent with the inorganic filler and drying the mixture. In the mixing process, the mixture of the solution and the inorganic filler may be heated or hydrated to promote the reaction of the silane coupling agent. The reaction of the silane coupling agent may also be accelerated by a reaction catalyst. Furthermore, after drying, the inorganic filler that has been surface-treated with the silane coupling agent may be crushed or classified.

The D50 of the inorganic filler is preferably 0.1 to 50 μm, and it is more preferable to use inorganic fillers with different D50s. When inorganic fillers with different D50s are used, a mixture of coarse powder with a D50 of 10 to 50 μm and fine powder with a D50 of 0.5 to 4 μm is preferable. By using a mixture of coarse powder and fine powder as the inorganic filler, the fine powder can be filled between the coarse powders, thereby increasing the filling rate of the inorganic filler in the obtained layer. When the inorganic filler is formed from coarse powder and fine powder, the blending ratio of the coarse powder is preferably 70% or more, and more preferably 75% or more. If the coarse powder ratio is within this range, the inorganic filler in the obtained layer tends to be densely packed.

The average particle size D50 of the fused particles is preferably smaller than the D50 of the present inorganic filler, and the D50 of the fused particles is preferably 0.1 or more and 0.6 or less than the D50 of the present inorganic filler.
When inorganic fillers having different D50 are used, specifically when the coarse powder and the fine powder are used, it is preferable that the D50 of the molten particles is smaller than the D50 of the fine powder.

In this method, a layer containing the present powder and the present inorganic filler is formed on the surface of a substrate.
The substrate may be a metal substrate or a resin substrate.
The metal substrate is preferably a metal foil. Examples of metals constituting the metal foil include copper, copper alloys, stainless steel, nickel, nickel alloys, aluminum, aluminum alloys, titanium, and titanium alloys.
The metal foil is preferably a copper foil, more preferably a rolled copper foil with no distinction between the front and back sides or an electrolytic copper foil with a distinction between the front and back sides, and even more preferably a rolled copper foil.
The resin substrate is more preferably a polyimide film.
The surface of the substrate may be treated with a silane coupling agent or the like.
The ten-point average roughness of the surface of the substrate is preferably 0.01 to 0.05 μm.

Methods for forming a layer containing the present powder and the present inorganic filler (hereinafter also referred to as "powder layer") on the surface of the substrate include, for example, a method of extruding a powder composition containing the present powder and the present inorganic filler onto the substrate, a method of co-extruding the substrate and a powder composition containing the present powder and the present inorganic filler, a method of preparing a sheet containing the present powder and the present inorganic filler and bonding the substrate and the obtained sheet by adhesion or pressure bonding, a method of coating the surface of the substrate with a liquid composition containing the present powder, the present inorganic filler, and a liquid dispersion medium and removing at least a part of the liquid dispersion medium by heating, etc.

When a powder composition containing the present powder and the present inorganic filler is extrusion molded, the powder composition may further contain a thermoplastic resin, a thermosetting resin, or a semi-curable thermoplastic resin. The powder composition may be a pellet containing the present powder, the present inorganic filler, and these resins.
Examples of such resins include polyolefins such as polyethylene, polypropylene, ABS resin, polymethyl methacrylate, polystyrene, ethylene-vinyl acetate copolymer, modified polyolefin, etc.; polyesters such as tetrafluoroethylene polymer, polyethylene terephthalate, polybutylene terephthalate, polyester elastomer, etc.; polyamides such as nylon 6, nylon 66, nylon 610, nylon 612, nylon 12, aromatic amorphous nylon, methoxymethylated polyamide resin, polyamide elastomer, etc.; engineering plastics such as polycarbonate, polyacetal, polyarylate, polysulfone, etc.; polyurethane, polyurethane elastomer, or polymer blends and polymer alloys thereof.

When a liquid composition containing the present powder, the present inorganic filler, and a liquid dispersion medium is coated on the surface of a substrate, and at least a part of the liquid dispersion medium is removed by heating to form a powder layer, the liquid dispersion medium may be water or a non-aqueous dispersion medium. The liquid dispersion medium may be an aprotic dispersion medium or a protic dispersion medium. The liquid state means a state in which the viscosity at 25° C. is 10 mPa·s or less.
The liquid dispersion medium is preferably at least one selected from the group consisting of water, amides, ketones, esters, and glycols.Specific examples of the liquid dispersion medium include water, N-methyl-2-pyrrolidone, γ-butyrolactone, methyl ethyl ketone, cyclohexanone, cyclopentanone, ethylene glycol, propylene glycol, trimethylene glycol, diethylene glycol, dipropylene glycol, triethylene glycol, and tripropylene glycol.
The liquid dispersion medium may be one type or two or more types. When two or more types are used, it is preferable that the different liquid dispersion media are compatible with each other.

When coating this dispersion, the following coating methods can be used: spray, roll coating, spin coating, gravure coating, microgravure coating, gravure offset, knife coating, kiss coating, bar coating, die coating, fountain-meyer bar, and slot die coating.

When the liquid dispersion medium is used, it may further contain a nonionic surfactant from the viewpoint of further improving the dispersion stability and handling properties of the liquid composition.
The surfactant is preferably an acetylene-based surfactant, a silicone-based surfactant or a fluorine-based surfactant, and more preferably a silicone-based surfactant.

Specific examples of such surfactants include the "Ftergent" series (manufactured by Neos Co., Ltd., Ftergent is a registered trademark), the "Surflon" series (manufactured by AGC Seimi Chemical Co., Ltd., Surflon is a registered trademark), the "Megafac" series (manufactured by DIC Corporation, Megafac is a registered trademark), the "Unidyne" series (manufactured by Daikin Industries, Ltd., Unidyne is a registered trademark), "BYK-347", "BYK-349", "BYK-378", "BYK-3450", "BYK-3451", "BYK-3455", "BYK-3456" (manufactured by BYK Japan KK), "KF-6011", and "KF-6043" (manufactured by Shin-Etsu Chemical Co., Ltd.).
When a surfactant is contained, the content of the surfactant in the liquid composition is preferably 1% by mass or more and 15% by mass or less, in which case the affinity between the components increases, and the dispersion stability and handleability of the liquid composition are likely to be further improved.

The liquid composition may include an aromatic polymer.
As the aromatic polymer, aromatic polyimide, aromatic polyamideimide, precursor of aromatic polyamideimide, aromatic maleimide, aromatic elastomer (styrene elastomer, etc.), aromatic polyamic acid or polyphenylene ether is preferred, and aromatic polyimide, aromatic polyamideimide, precursor of aromatic polyamideimide or aromatic polyamic acid is more preferred. The aromatic polyimide may be thermoplastic or thermosetting. Thermoplastic polyimide means polyimide in which imidization is completed and no further imidization reaction occurs.

Specific examples of aromatic polyimides include the "Neoprim (registered trademark)" series (manufactured by Mitsubishi Gas Chemical Company, Inc.), the "Spiccellia (registered trademark)" series (manufactured by Somar), the "Q-PILON (registered trademark)" series (manufactured by PI Technical Research Institute, Inc.), the "WINGO" series (manufactured by Wingo Technology Co., Ltd.), the "Tomide (registered trademark)" series (manufactured by T&K Toka Corporation), the "KPI-MX" series (manufactured by Kawamura Sangyo Co., Ltd.), the "Upia (registered trademark)-AT" series (manufactured by Ube Industries, Ltd.), "HPC-1000" and "HPC-2100D" (all manufactured by Showa Denko Materials Co., Ltd.).

The liquid composition preferably contains a silane coupling agent. In this case, the binding strength between the powder and the inorganic filler is improved, and the powder or inorganic particles in the powder layer are more likely to be prevented from falling off. The silane coupling agent may be mixed into the liquid composition and contained in the liquid composition, or may be contained in the liquid composition as a surface treatment agent for the inorganic filler.

As the silane coupling agent contained in the liquid composition, a compound having a trialkoxysilyl group and a phenyl group, a vinyl group, an epoxy group, an acryloyloxy group, a methacryloyloxy group, an amino group, a ureido group, a mercapto group or an isocyanate group is preferred, and a compound having a trialkoxysilyl group and an amino group is more preferred.

The thermal decomposition temperature of the silane coupling agent contained in the liquid composition is preferably 200°C or higher, more preferably 300°C or higher. The upper limit of the thermal decomposition temperature is preferably 400°C. In this case, the heat resistance of the resulting laminate is more likely to be improved. The thermal decomposition temperature of the silane coupling agent is the temperature at which the mass of the silane coupling agent becomes 50% of the mass at the start of heating when the silane coupling agent is heated from 50°C to 400°C at a rate of 5°C/min in a nitrogen atmosphere.

Specific examples of the silane coupling agent contained in the liquid composition include methyltrimethoxysilane, methyltriethoxysilane, n-propyltrimethoxysilane, n-propyltriethoxysilane, hexyltrimethoxysilane, hexyltriethoxysilane, phenyltrimethoxysilane, vinyltrimethoxysilane, vinyltriethoxysilane, 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane, 3-glycidoxypropyltrimethoxysilane, 3-glycidoxypropyltriethoxysilane, p-styryltrimethoxysilane, 3-methacryloxypropyltrimethoxysilane, 3-methacryloxypropyltriethoxysilane, 3-acryloxypropyltrimethoxysilane, and n-phenyl-3-aminopropyltrimethoxysilane.
The above-mentioned silane coupling agent, which may be used as a surface treatment agent for the present inorganic filler, may be mixed into the liquid composition for use.

When a liquid composition containing a silane coupling agent is obtained by mixing the silane coupling agent, the liquid composition is preferably obtained by adding the silane coupling agent to the inorganic filler, further adding the powder, and performing a mixing process. When adding the silane coupling agent, the inorganic filler may be dispersed in a liquid dispersion medium, or the silane coupling agent may be diluted with the liquid dispersion medium. In addition, in the mixing process, the liquid composition containing the silane coupling agent, the inorganic filler, and the powder may be heated or hydrated to promote the reaction of the silane coupling agent. The reaction of the silane coupling agent may also be accelerated by a reaction catalyst.

When the liquid composition contains a surfactant, an aromatic polyimide, or a silane coupling agent, the content of each of these is preferably independently 1 to 10% by mass relative to the content of the powder.

The content of the liquid dispersion medium in the liquid composition is preferably 30% by mass or more, and more preferably 90% by mass or less, and more preferably 80% by mass or less.
The solid content in the liquid composition is preferably 20% by mass or more, and more preferably 30% by mass or more, based on the total mass of the liquid composition being 100%. From the viewpoint of dispersibility of the liquid composition, the solid content is preferably 60% by mass or less, and more preferably 50% by mass or less.
In addition, the liquid dispersion medium is preferably degassed in order to prevent a decrease in uniformity of the component distribution in the powder layer and to prevent voids.

The viscosity of the liquid composition is preferably 10 mPa·s or more, more preferably 100 mPa·s or more. The viscosity of the liquid composition is preferably 10,000 mPa·s or less, more preferably 1,000 mPa·s or less. In this case, the liquid composition has excellent coatability.
The thixotropy ratio of the liquid composition is preferably equal to or greater than 1. The thixotropy ratio of the liquid composition is preferably equal to or less than 3, and more preferably equal to or less than 2. In this case, the liquid composition not only has excellent coatability, but also has excellent homogeneity.

After the liquid composition is coated on the substrate, at least a portion of the liquid dispersion medium is removed by heating to form a powder layer. Heating may be continued until at least a portion of the liquid dispersion medium is removed, and the liquid dispersion medium may be removed to such an extent that the powder and the inorganic filler maintain a layered shape. It is preferable that 80% by mass or more of the liquid dispersion medium contained in the liquid composition is removed. The liquid dispersion medium may be completely removed.

The temperature for removing the liquid dispersion medium is preferably a temperature below the melting temperature of the molten F polymer and below the boiling point of the liquid dispersion medium, and more preferably a temperature below 100°C lower than the melting temperature of the molten F polymer and 10°C to 100°C lower than the boiling point of the liquid dispersion medium. For example, when using a molten F polymer with a melting temperature of 300°C and N-methyl-2-pyrrolidone with a boiling point of about 200°C, the temperature for removing the liquid dispersion medium is preferably 150°C or lower, more preferably 100 to 120°C. From the viewpoint of forming a powder layer with excellent smoothness, it is preferable to blow air onto the surface of the powder layer to be formed when removing the liquid dispersion medium.

The mass ratio of the molten F polymer to the inorganic filler in the powder layer obtained by the above method is preferably 0.2 or more, more preferably 0.5 or more, based on the mass of the molten F polymer being 1. The mass of the inorganic filler is preferably 2 or less, more preferably 1.5 or less.
The content of the inorganic filler in the powder layer is preferably 60% by mass or less, and more preferably 50% by mass or less, based on the total mass of the powder layer being 100% by mass. When the powder layer is formed using the liquid composition, the total mass of the powder layer is the mass when 80% by mass or more of the liquid dispersion medium has been removed.
The mass ratio of the molten F polymer to the inorganic filler in the powder layer and the content of the inorganic filler can be set within the above range by appropriately setting the mass ratio and content of the powder and the inorganic filler in the composition or the liquid composition.

The obtained powder layer may contain a third component in addition to the present powder and the present inorganic filler. Examples of the third component include a silane coupling agent, an ultraviolet absorber, and a resin other than the tetrafluoroethylene-based polymer. The powder layer preferably contains a silane coupling agent or a resin other than the tetrafluoroethylene-based polymer, and more preferably contains a silane coupling agent and a resin other than the tetrafluoroethylene-based polymer. In this case, the binding force between the present powder and the present inorganic filler is improved, and the peeling of the present powder or the present inorganic particles in the powder layer is easily suppressed.
The silane coupling agent may be contained in the powder layer as a surface treatment agent for the inorganic filler. When the powder layer is formed using the liquid composition, the powder layer containing the silane coupling agent may be obtained by forming the powder layer from the liquid composition mixed with the silane coupling agent.

The ultraviolet absorber is preferably an ultraviolet absorber having a phenolic hydroxyl group and a nitrogen-containing heterocyclic structure, more preferably an ultraviolet absorber having a hydroxybenzophenone structure or an ultraviolet absorber having a phenolic hydroxyl group and a triazine structure or a benzotriazole structure, the latter preferably having a hydroxyphenyltriazine structure or a structure in which various substituents are substituted on the hydroxyphenyl structure.
Other resins different from the F polymer include the aromatic polymers mentioned above.

In addition to the third component, the powder layer may further contain additives such as a thixotropic agent, a viscosity regulator, an antifoaming agent, a dehydrating agent, a plasticizer, a weathering agent, an antioxidant, a heat stabilizer, a lubricant, an antistatic agent, a whitening agent, a colorant, a conductive agent, a release agent, a surface treatment agent, a flame retardant, and various fillers.

The obtained powder layer is further fired to form a fired product of the present powder (hereinafter also referred to as the "fired product") and a layer containing the present inorganic filler (hereinafter also referred to as the "fired product layer"). The present powder may be completely fired or partially fired.
The baking temperature may be any temperature equal to or higher than the melting point of the molten F polymer, and may be, for example, in the range of 300 to 400°C.

When the powder layer is formed on the surface of the substrate by the melt extrusion method, the co-extrusion method, or the method of bonding with the substrate, a fired layer can be formed by firing the substrate and the powder layer within the firing temperature range.
When the powder layer is formed on the surface of a substrate by coating the substrate with the liquid composition and removing at least a part of the liquid dispersion medium by heating, a fired layer can be formed by subsequently firing the substrate within the firing temperature range. The liquid dispersion medium may be completely removed before firing.

The fired layer may contain, for example, the surfactant, the third component, the additives, and residues thereof.
Similarly to the powder layer, the mass ratio of the molten F polymer to the inorganic filler in the fired layer is preferably 0.2 or more, more preferably 0.5 or more, relative to the mass of the molten F polymer being 1. The mass ratio of the inorganic filler is preferably 2 or less, more preferably 1.5 or less.
The content of the inorganic filler in the fired layer is preferably 60% by mass or less, and more preferably 50% by mass or less, with the total mass of the fired layer being 100% by mass.

In this method, the layer and the substrate are pressed together during the process of forming either the powder layer or the fired layer.
When the powder layer is formed on the surface of the substrate by the melt extrusion method, the co-extrusion method, or the method of bonding with the substrate, pressing is preferably performed at any of the following stages: (1) the stage of forming the powder layer on the surface of the substrate, (2) the stage after the powder layer is formed and before firing, (3) the stage of firing, and (4) the stage after the fired layer is formed and before the fired layer is cooled. In the above stage (2), the powder layer may be cooled once, or may be fired as it is without cooling.
The pressing is preferably carried out in the above-mentioned step (1) or (2), more preferably in the step (2). The pressing may be carried out in a plurality of the above-mentioned steps.

When the powder layer is formed on the surface of the substrate by a method of coating the surface of the substrate with the liquid composition and removing at least a part of the liquid dispersion medium by heating, pressing is preferably performed at any of the following stages: (11) immediately after the liquid dispersion medium is partially removed by heating and the powder layer is formed, (21) after the powder layer is formed and before the powder layer is further heated to form a fired layer, (31) when the powder layer is fired to form a fired layer, and (41) after firing and before the fired layer is cooled. In the above (11), it is preferable that 80% by mass or more of the liquid dispersion medium contained in the liquid composition is removed. In addition, in the above (21), the powder layer may be cooled once after the powder layer is formed, or the powder layer may be fired without cooling after the powder layer is formed. Pressing is more preferably performed at the above (11) or (21) stages, and even more preferably at the (21) stage. Pressing may be performed at a plurality of the above stages.

The pressing is preferably carried out in an atmosphere of atmospheric pressure or reduced pressure, and more preferably in an atmosphere of atmospheric pressure.
The pressure of the pressing is preferably 0.2 MPa or more, more preferably 0.5 MPa or more, and is preferably 10 MPa or less, more preferably 5 MPa or less.

The pressing temperature is preferably higher than the glass transition temperature of the molten F polymer, and more preferably 30°C or more higher than the glass transition temperature. The pressing temperature is preferably lower than the melting temperature of the molten F polymer, and more preferably 100°C or more lower than the melting temperature. The pressing temperature is preferably higher than the glass transition temperature of the molten F polymer and 100°C or more lower than the melting temperature of the molten F polymer.

The pressing method may be a method of passing the powder layer or the fired layer and the substrate between a pair of heated rolls during any process from the formation of the powder layer to the formation of the fired layer, a method of blowing hot air onto the powder layer or the fired layer and the substrate while passing them between a pair of rolls, or a method of pressing the powder layer or the fired layer and the substrate with a hot plate.
By the above pressing, the ratio of the thickness of the powder layer or the fired layer after pressing to the thickness of the powder layer or the fired layer before pressing is preferably 0.5 to 0.9. For example, the ratio of the thickness of the powder layer after pressing to the thickness of the powder layer before pressing is preferably 0.5 to 0.9, or the ratio of the thickness of the fired layer after pressing to the thickness of the fired layer before pressing is preferably 0.5 to 0.9.

In order to suppress adhesion of the powder layer or fired layer to the roll or hot plate during pressing, a release film may be placed between the surface of the powder layer or fired layer and the roll or hot plate, or the surface of the roll or hot plate may be surface-treated with a release agent.
When the powder layer or the calcined layer is pressed by passing it between a pair of rolls, it is preferable that the release film contacts the powder layer or the calcined layer only on the pressure surfaces of the rolls and is peeled off when the powder layer or the calcined layer leaves the rolls.
The release film preferably has a thickness of 50 to 150 μm.
The release film may be a polyimide film, and specific examples include "Apical NPI" (manufactured by Kaneka Corporation), "Kapton EN" (Toray DuPont Co., Ltd.), and "Upilex S" (Ube Industries, Ltd.).

In the present invention, the process from the formation of the powder layer to the formation of the fired layer can be considered to be in a state in which the inorganic filler is fluidly dispersed in the molten F polymer. It is believed that pressing the layer in this state exerts the effect of orienting the long axis direction of the inorganic filler in the horizontal direction of the substrate surface. In particular, the powder layer can be considered to be in a state in which the inorganic filler is dispersed in the voids formed by the packing of the powder. It is believed that pressing the powder layer in such a state crushes the voids and makes it easier for the long axis direction of the inorganic filler to be oriented in the horizontal direction of the substrate. As a result, it is believed that this method has produced a dense laminate containing the inorganic filler whose long axis direction is oriented and dispersed in the molten F polymer in the horizontal direction of the substrate.

By this method, it is possible to obtain the present laminate comprising the fired layer in which the present inorganic filler is highly oriented and the substrate.
The thickness of the fired layer is preferably 50 μm or more, more preferably 75 μm or more. The upper limit of the thickness is preferably 500 μm, more preferably 250 μm.

A sheet containing the present fired product and the present inorganic filler can be produced by removing the base material from the present laminate obtained by the present method. Methods for removing the base material include peeling and etching.
In the sheet thus obtained, the voids between the fired products are crushed, the voids in the layers are reduced, and the inorganic filler is oriented in-plane, resulting in excellent thermal conductivity.

The sheet of the present invention (hereinafter also referred to as "the sheet") comprises a sintered product of tetrafluoroethylene-based polymer powder containing particles of a heat-fusible tetrafluoroethylene-based polymer and an inorganic filler having an aspect ratio of more than 1, and has a thickness of 50 μm or more and a thermal conductivity of more than 5 W/m·K.
The heat-fusible tetrafluoroethylene polymer, the powder containing it, the tetrafluoroethylene polymer, the powder containing it and the fired product are the same as the molten F polymer, the molten particles, the F polymer, the present powder and the fired product.
The inorganic filler having an aspect ratio of more than 1 is the same as the present inorganic filler described above.

The thickness of the sheet is preferably 50 μm or more, more preferably 75 μm or more. The upper limit of the thickness is preferably 500 μm, more preferably 250 μm.
The thermal conductivity of the present sheet measured by the method described below is preferably more than 5 W/m·K, more preferably 8 W/m·K or more. The upper limit of the thermal conductivity of the present sheet is not particularly limited, and is 100 W/m·K.

Because the present laminate and sheet have excellent thermal conductivity, they can be suitably used as thermally conductive and insulating sheets used for heat dissipation in electronic components, and as metal base substrates and circuit boards having a thermally conductive and insulating layer.
In particular, since the inorganic filler in the present laminate and sheet is highly oriented, the direction of heat dissipation can be controlled.
For example, when a plurality of sheets of the present sheet are laminated and then the laminate is sliced in the thickness direction to form an insulating sheet with the cut end surface being flat, the insulating sheet has the present inorganic filler oriented in the thickness direction of the insulating sheet. Such a sheet has high thermal conductivity in the thickness direction, and is therefore useful when heat dissipation in the thickness direction is required.

The laminate and sheet of the present invention are useful as electronic substrate materials such as flexible printed wiring boards and rigid printed wiring boards, protective films and heat dissipation substrates, particularly heat dissipation substrates for automobiles.
Specifically, the laminate and the sheet can also be suitably used as a heat dissipation member for a wireless communication device. For example, in a wireless communication device having a base material made of resin or the like, an antenna pattern formed on the base material, and an RFIC package which is a power supply circuit connected to the antenna pattern, by arranging the laminate or the sheet around the RFIC package, the heat dissipation of the RFIC package can be promoted and the temperature rise can be effectively suppressed. In this case, the laminate or the sheet is preferably arranged on the surface of the RFIC package opposite to the surface on which the antenna pattern is provided, from the viewpoint of not interfering with the radiation of the antenna. Examples of such wireless communication devices include the wireless communication devices described in International Publication No. 2020/008691 and International Publication No. 2020/031419.
The laminate and sheet of the present invention can also be suitably used as a heat dissipation substrate for mounting power devices.
When using the present laminate or the present sheet as these components, the present laminate or the present sheet may be directly attached to the target substrate, or the present laminate or the present sheet may be attached to the target substrate via an adhesive layer such as a silicone-based adhesive layer.

Although the present method, the sheet manufacturing method in which the substrate is removed from the laminate obtained by the present method, and the present sheet have been described above, the present invention is not limited to the configurations of the above-mentioned embodiments.
For example, the method for producing a sheet in which the substrate is removed from the laminate obtained by this method and the method may have any other step added to the configuration of the above embodiment, or may be replaced with any other step that produces a similar effect. Also, the sheet in the configuration of the above embodiment may have any other step added to it, or may be replaced with any other step that produces a similar function.

The present invention will be described in detail below with reference to examples, but the present invention is not limited to these.
1. Preparation of each ingredient [Powder]
Powder 1: Molten particles (D50: 2.1 μm) made of a polymer containing 97.9 mol%, 0.1 mol%, and 2.0 mol% of TFE units, NAH units, and PPVE units, in that order, having 1,000 carbonyl group-containing groups per 1×106 carbon atoms in the main chain, and having a melting temperature of 300° C.
Powder 2: Non-melting particles made of non-thermofusible polytetrafluoroethylene (D50: 0.3 μm)
[Inorganic filler]
Filler 1: Scaly boron nitride filler (aspect ratio: greater than 1, D50: 14.6 μm)
[Liquid Compound]
NMP: N-methyl-2-pyrrolidone

2. Examples of Dispersion Preparation [Example 1]
The varnish of thermoplastic aromatic polyimide (PI1), NMP and aminosilane coupling agent were put into the pot and mixed. Furthermore, the powder mixture of Powder 1 and Filler 1 was put into the pot and mixed to prepare a composition. This composition was kneaded in a planetary mixer and then taken out to obtain a kneaded powder 1 containing 50 parts by weight of Powder 1, 40 parts by weight of Filler 1, 6 parts by weight of PI1, 1 part by weight of aminosilane coupling agent and 30 parts by weight of NMP. The kneaded powder 1 was lumpy and clay-like.
NMP was added to the kneaded powder 1 in several portions, and the mixture was stirred at 2000 rpm with a planetary mixer while being defoamed. Furthermore, NMP was added in several portions, and 80 parts by mass of NMP in total was added to the kneaded powder 1 to prepare a dispersion, and dispersion 1 was obtained. The viscosity of dispersion 1 was 300 mPa·s.

[Example 2]
In the preparation of the composition of Example 1, Powder 2 was further used to obtain a kneaded powder 2 containing 25 parts by mass of Powder 1, 25 parts by mass of Powder 2, 40 parts by mass of Filler 1, 6 parts by mass of PI1, 1 part by mass of aminosilane coupling agent, and 30 parts by mass of NMP. NMP was added to the kneaded powder 2 in several batches, and the mixture was stirred with a planetary mixer at 2000 rpm while being degassed. Furthermore, NMP was stirred in several batches, and a total of 80 parts by mass of NMP was added to the kneaded powder 2 to prepare a dispersion, and Dispersion 2 was obtained.

3. Example of laminate production [Laminate 1]
Dispersion Liquid 1 was applied to the surface of a long copper foil having a thickness of 18 μm using a bar coater to form a wet film. Next, the copper foil on which the wet film was formed was passed through a drying oven at 110° C. for 5 minutes to dry, and a powder layer 1 containing Powder 1 and Filler 1, which was a dry film, was formed on the surface of the substrate.
This substrate was passed between a pair of rolls heated to 120° C. and adjusted so as to press the powder layer 1 at 0.5 MPa, thereby pressing the powder layer 1 .
Thereafter, the substrate was further heated in a nitrogen oven at 380° C. for 3 minutes, thereby producing Laminate 1 having copper foil and, on its surface, a polymer layer having a thickness of 200 μm as a molded product, which contained the molten and fired product of Powder 1, Filler 1, and PI 1.
[Laminate 2]
The substrate on which the powder layer 1 was formed was heated at 380°C without being passed between a pair of rolls, to obtain a laminate 2 having a copper foil and, on its surface, a polymer layer having a thickness of 200 μm as a molded product, which contained a molten and fired product of the powder 1, a filler 1 and PI 1.
[Laminate 3]
A laminate 3 was produced in the same manner as in the production of the laminate 1, except that the dispersion 1 was changed to the dispersion 2.

4. Evaluation 4-1. Evaluation of Linear Expansion Coefficient of Sheets For each laminate, the copper foil of the laminate was removed by etching with an aqueous ferric chloride solution to prepare a sheet that was a single polymer layer. A square test piece measuring 180 mm square was cut out from the prepared sheet, and the linear expansion coefficient of the test piece was measured in the range of 25°C to 260°C according to the measurement method specified in JIS C 6471:1995. As a result, the linear expansion coefficients of the sheets obtained from Laminate 1 and Laminate 3 were 50 ppm/°C or less, and the linear expansion coefficient of the sheet obtained from Laminate 2 was more than 75 ppm/°C.

4-2. Evaluation of thermal conductivity of sheets For each laminate, the copper foil of the laminate was removed by etching with an aqueous solution of ferric chloride to prepare a sheet that was a single polymer layer. A test piece measuring 10 mm x 10 mm was cut from the center of the prepared sheet, and the thermal conductivity (W/m·K) in the in-plane direction was measured. As a result, the thermal conductivity of the sheets obtained from laminate 1 and laminate 3 was 10 W/m·K, and the thermal conductivity of the sheet obtained from laminate 2 was 5 W/m·K or less.

Furthermore, the sheet obtained from Laminate 3 had better bending properties and sheet strength than the sheet obtained from Laminate 1.

As is clear from the above results, the cross section of the sheet produced by this method is dense and void-free, and the inorganic filler is highly oriented, so the sheet obtained by this method has excellent thermal conductivity.

Claims (13)

A method for producing a laminate having a substrate and a layer containing the fired product and the inorganic filler formed on the surface of the substrate, the method comprising the steps of: forming a layer containing a powder of a tetrafluoroethylene-based polymer containing particles of a heat-fusible tetrafluoroethylene-based polymer and an inorganic filler which is aluminum oxide, magnesium oxide, boron nitride, aluminum nitride , silicon nitride, silicon oxide or silicon carbide and has an aspect ratio of more than 1 on the surface of the substrate; and forming a layer containing a fired product of the powder and the inorganic filler , the layer containing the inorganic filler and the substrate being pressed at a temperature equal to or lower than the melting temperature of the heat-fusible tetrafluoroethylene-based polymer. The manufacturing method according to claim 1, wherein the tetrafluoroethylene-based polymer powder contains particles of a heat-fusible tetrafluoroethylene-based polymer and particles of a non-heat-fusible tetrafluoroethylene-based polymer. The method according to claim 1 or 2, wherein the heat-meltable tetrafluoroethylene polymer is a tetrafluoroethylene polymer having a carbonyl group-containing group or a hydroxyl group-containing group and a melting temperature of 200°C or higher and 320°C or lower. The method according to any one of claims 1 to 3, wherein the mass ratio of the heat-fusible tetrafluoroethylene polymer to the inorganic filler in the layer is such that the mass of the heat-fusible tetrafluoroethylene polymer is 1 and the mass of the inorganic filler is 0.2 to 2. The method according to any one of claims 1 to 4, wherein the average particle size of the particles of the heat-fusible tetrafluoroethylene-based polymer is smaller than the average particle size of the inorganic filler. The manufacturing method according to any one of claims 1 to 5, wherein the content of the inorganic filler in the layer containing the particles of the heat-fusible tetrafluoroethylene-based polymer and the inorganic filler is 60 mass% or less. The method according to any one of claims 1 to 6, wherein the inorganic filler contains at least one of boron nitride and silicon oxide. The manufacturing method according to any one of claims 1 to 7, wherein the layer containing the powder and the inorganic filler further contains a silane coupling agent. The manufacturing method according to any one of claims 1 to 8, in which the layer containing the powder and the inorganic filler is pressed against the substrate. The manufacturing method according to any one of claims 1 to 9, wherein the pressure of the pressing is 0.2 MPa or more and 10 MPa or less. The method according to any one of claims 1 to 10, further comprising: coating a surface of a substrate with a liquid composition containing the tetrafluoroethylene-based polymer powder, the inorganic filler, and a liquid dispersion medium; and removing at least a portion of the liquid dispersion medium to form a layer containing the powder and the inorganic filler. The method according to claim 11 , wherein the liquid composition further comprises a silane coupling agent. A method for producing a sheet containing the fired product and the inorganic filler, comprising the steps of: forming a layer containing a powder of a tetrafluoroethylene-based polymer containing particles of a heat-fusible tetrafluoroethylene-based polymer and an inorganic filler which is aluminum oxide, magnesium oxide, boron nitride, aluminum nitride, silicon nitride, silicon oxide or silicon carbide having an aspect ratio of more than 1 on a surface of a substrate; and forming a fired product of the powder and a layer containing the inorganic filler; pressing the layer containing the inorganic filler and the substrate at a temperature equal to or lower than the melting temperature of the heat-fusible tetrafluoroethylene-based polymer to obtain a laminate having the substrate and the fired product and the layer containing the inorganic filler formed on the surface of the substrate; and removing the substrate.