JP7770972B2 - thermal conductive materials - Google Patents

Description

The present invention relates to a thermally conductive material.

Crystalline carbon materials such as graphite and carbon nanotubes have high thermal conductivity and are therefore sometimes used as thermal conductive materials (see, for example, Patent Document 1). However, due to the high thermal resistance between the resin and the carbon, the thermal conductivity is often not as high as expected. Furthermore, because these carbon materials have low affinity with resin, increasing the amount added to improve thermal conductivity can cause problems with kneading, molding, strength, etc.

Graphene sheets are two-dimensional single-layer sheets in which carbon atoms are arranged in a honeycomb lattice pattern, and are also the building blocks of graphite, fullerenes, carbon nanotubes, and other materials. Flake-like carbon (in this specification, "flake-like carbon" includes graphene sheets) is made up of graphene sheets stacked to a thickness of approximately 100 nm or less. Due to its unique physical properties, it has attracted attention as a new material that can be used in a variety of applications.

However, while nanocarbon materials such as flaky carbon have extremely high thermal conductivity, the finer their structure, the more likely they are to aggregate, making it difficult to fully realize their potential and preventing sufficient improvement in thermal conductivity and heat dissipation.

Nanocarbon materials also have usability issues, such as being difficult to create into a single film or to turn into a paint. When combined with resins, there are many cases where they do not disperse due to incompatibility (they cannot be applied as a paint or do not have sufficient strength as a sheet), and their thermal conductivity drops significantly.

Furthermore, it is preferable that both the coating and the sheet are insulating.

WO 2014/080743

In light of the above circumstances, the object of the present invention is to provide a thermally conductive material with excellent thermal conductivity.

As a result of extensive research into solving the above problems, the inventors discovered that the above problems could be solved by using a specific organic compound, thin carbon flakes having a specific thickness, and specific nanofibers. Based on this finding, the inventors conducted further research and completed the present invention.

That is, the present invention provides the following thermally conductive material.
Item 1.
flaky carbon having a thickness of 1 to 100 nm;
an organic compound having a hydrophilic group and a carbon-philic hydrophobic group;
A thermally conductive material comprising cellulose or a cellulose derivative and nanofibers having a fiber diameter of 1 to 500 nm.
Item 2.
Item 2. The thermally conductive material according to Item 1, wherein the hydrophilic group is at least one selected from the group consisting of the following general formulas (1) to (4):
[The above formula (1) represents an alcoholic hydroxyl group or a phenolic hydroxyl group. In formula (2), R represents a divalent organic group, and both oxygen atoms represent ether bonds. In formula (3), _X1 represents a hydrogen atom, an alkali metal, _NH4 , or an organic ammonium. In formula (4), _X2 represents a hydrogen atom, an alkali metal, _NH4 , an organic ammonium, or an alkyl group.]
Item 3.
Item 3. The thermally conductive material according to Item 1 or 2, wherein the hydrophilic group is a phenolic hydroxyl group or a polyoxyethylene group.
Item 4.
Item 4. The thermal conductive material according to any one of items 1 to 3, wherein the carbon affinity hydrophobic group is at least one selected from the group consisting of an alkyl group, an alkenyl group, a cycloalkyl group, an aryl group, and a polyoxyalkylene group having 3 or more carbon atoms.
Item 5.
Item 5. The thermally conductive material according to any one of Items 1 to 4, wherein the carbon-affinity hydrophobic group is an aryl group having two or more aromatic rings.
Item 6.
Item 6. The thermally conductive material according to Items 1 to 5, wherein the nanofibers are cellulose nanofibers having a cellulose purity of 85% or more.
Section 7.
Item 7. The thermal conductive material according to any one of items 1 to 6, wherein the nanofibers comprise at least one nanofiber selected from the group consisting of methyl cellulose, carboxymethyl cellulose salt, hydroxyethyl cellulose, hydroxypropyl cellulose, hydroxyethyl methyl cellulose, and hydroxypropyl methyl cellulose.
Section 8.
Item 8. A paint containing the thermally conductive material according to any one of Items 1 to 7, or a coating film formed from the paint.
Item 9.
Item 8. A sheet comprising the thermally conductive material according to any one of Items 1 to 7.
Item 10.
Item 8. A method for producing a thermally conductive material according to any one of Items 1 to 7, comprising the step of mixing a flaky carbon dispersion or a wet solid of the flaky carbon dispersion, the flaky carbon dispersion containing 1 to 100 parts by mass of an organic compound having the hydrophilic group and the carbon-affinity hydrophobic group per 100 parts by mass of the flaky carbon, with a nanofiber dispersion or a wet solid of the nanofiber dispersion.

The thermally conductive material of the present invention, as described above, has excellent thermal conductivity.

In this specification, the term "containing" encompasses the concepts of "comprise," "consist essentially of," and "consist only of." Furthermore, in this specification, when a numerical range is indicated as "A to B," it means A or more and B or less.

(1. Thermal Conductive Materials)
The thermally conductive material of the present invention contains flaky carbon having a thickness of 1 to 100 nm, an organic compound having a hydrophilic group and a carbon-affinitive hydrophobic group, and nanofibers made of cellulose or a cellulose derivative and having a fiber diameter of 1 to 500 nm.

(1.1. Flake-like carbon)
Thinner flaky carbon is preferable because it has better thermal conductivity and heat dissipation. Specifically, it is 1 to 100 nm, and preferably 1 to 20 nm. If the thickness of the flaky carbon exceeds 100 nm, it is not possible to obtain a thermally conductive material with sufficient thermal conductivity and heat dissipation.

Furthermore, the content of flaky carbon with a thickness of 1 to 10 nm is preferably 80% or more, and more preferably 90% or more, with the total number of flaky carbon being 100%. In other words, although flaky carbon with a thickness of more than 10 nm may be included, it is preferable that the thickness of most of the flaky carbon be 10 nm or less. The thickness of the flaky carbon is measured by observation using a transmission electron microscope (TEM).

The flaky carbon is preferably flaky carbon having a layered structure with 300 or fewer layers of stacked graphene (i.e., 1 to 300 layers), and more preferably flaky carbon having a layered structure with 1 to 60 layers of stacked graphene. Similarly, the content of flaky carbon having 1 to 30 stacked layers is preferably 80% or more, and more preferably 90% or more, of the total number of flaky carbon being 100%. In other words, thicker flaky carbon may be included, but the thickness of the majority of flaky carbon is preferably 30 layers or less. The stacked flaky carbon is calculated from the thickness measured by observation with a transmission electron microscope (TEM).

Flake carbon typically has a planar shape with many convex and concave angles, making it difficult to define its dimensions in general terms other than thickness. In this specification, the size of a piece of flake carbon is defined as the distance between the furthest convex angles on that piece of flake carbon.

The size of such flaky carbon is preferably 20 nm or more, more preferably 100 nm or more, and even more preferably 200 nm or more. By using flaky carbon of such a size, thermal conductivity and heat dissipation can be further improved. It is known that larger flaky carbons have better thermal conductivity and heat dissipation properties, so the upper limit of the size is not limited, but is usually 100 μm. The size of the flaky carbon is measured by observation using a transmission electron microscope (TEM).

In the thermal conductive material of the present invention, the content of flaky carbon is not particularly limited, but from the viewpoint of thermal conductivity and heat dissipation, it is preferably 50 to 99.5% by mass, and more preferably 60 to 99.2% by mass, where the total amount of the thermal conductive material of the present invention is 100% by mass.

(1.2. Organic Compounds Having Hydrophilic Groups and Carbon-Affinity Hydrophobic Groups)
In the thermal conductive material of the present invention, an organic compound having a hydrophilic group and a carbon-affinitive hydrophobic group is used. If this organic compound is not used, the flaky carbon maintaining the graphene structure will aggregate and will not be able to be maintained in a uniformly dispersed state. The organic compound can also function as a dispersant to uniformly disperse the flaky carbon.

The organic compound having such a hydrophilic group and a carbon-affinitive hydrophobic group is not particularly limited, and a wide variety of organic compounds (especially water-soluble compounds) that can function as a dispersant for flaky carbon can be used.

The carbon-affinity hydrophobic group in the organic compound is not particularly limited, and examples include alkyl groups, alkenyl groups, cycloalkyl groups, aryl groups, and polyoxyalkylene groups having 3 or more carbon atoms. Organic compounds having a hydrophilic group and a carbon-affinity hydrophobic group can contain one or more types of such hydrophobic groups. Furthermore, when multiple hydrophobic groups are present, the compounds may contain multiple of the same hydrophobic groups, or multiple different hydrophobic groups.

The alkyl group may be a chain alkyl group or a branched alkyl group. However, from the viewpoint of affinity with carbon, a chain alkyl group is preferable. Furthermore, taking into consideration affinity with carbon, the number of carbon atoms in the alkyl group is preferably 6 or more, more preferably 8 to 28, and even more preferably 10 to 22. Examples of such alkyl groups include hexyl, octyl, decyl, undecyl, dodecyl (or lauryl), tridecyl, tetradecyl (or myristyl), pentadecyl, hexadecyl (or cetyl), octadecyl, and icosyl groups.

This alkyl group may or may not have a substituent. Examples of such substituents include a cycloalkyl group, an aryl group, and an aralkyl group. Examples of cycloalkyl groups and aryl groups are described below.

As an aralkyl group as a substituent of an alkyl group, an aralkyl group having 7 to 14 carbon atoms and an aryl group (described below) and an alkyl group having 1 to 6 carbon atoms is preferred, specifically, a benzyl group, a phenethyl group, etc. are preferred.

The substituents are not limited to those mentioned above, and may also have groups derived from the fluorene structure (such as fluorenyl groups). In particular, when water solubility is important, phenyl groups and the like are preferred as substituents, and when compatibility with flaky carbon is important, naphthyl groups, fluorenyl groups, and the like are preferred as substituents.

Taking into consideration the affinity with carbon and the water solubility of the compound, the alkenyl group preferably has 4 or more carbon atoms, more preferably 6 to 100, and even more preferably 8 to 30. Examples of such alkenyl groups include oleyl and linoleyl groups.

This alkenyl group may or may not have a substituent. Examples of such substituents include alkyl groups, cycloalkyl groups, aryl groups, and aralkyl groups. Examples of aralkyl groups include those listed above, and examples of cycloalkyl groups and aryl groups include those listed below.

As the alkyl group as a substituent on the alkenyl group, an alkyl group having 1 to 6 carbon atoms is preferred, and specifically preferred are methyl, ethyl, propyl, butyl, and tert-butyl groups.

The substituents are not limited to those mentioned above, and may also have groups derived from the fluorene structure (such as fluorenyl groups). In particular, when water solubility is important, phenyl groups and the like are preferred as substituents, and when compatibility with flaky carbon is important, naphthyl groups, fluorenyl groups, and the like are preferred as substituents.

Preferred cycloalkyl groups are those having 5 to 10 carbon atoms (preferably 5 to 8, and particularly 5 to 6), and specific examples include cyclopentyl and cyclohexyl groups.

This cycloalkyl group may or may not have a substituent. Such substituents include alkyl groups, aryl groups, and aralkyl groups.

As the alkyl group as a substituent of the cycloalkyl group, an alkyl group having 1 to 6 carbon atoms is preferred, and specifically preferred are methyl, ethyl, propyl, butyl, tert-butyl, etc.

Aryl groups and aralkyl groups as substituents on cycloalkyl groups include those exemplified above.

The substituents are not limited to those mentioned above, and may also have groups derived from the fluorene structure (such as fluorenyl groups). In particular, when water solubility is important, phenyl groups and the like are preferred as substituents, and when compatibility with flaky carbon is important, naphthyl groups, fluorenyl groups, and the like are preferred as substituents.

The aryl group is preferably an aryl group having 6 to 18 carbon atoms (particularly 6 to 14 carbon atoms), and can be any of a monocyclic aryl group, a fused ring aryl group, and a polycyclic aryl group, such as a phenyl group, a naphthyl group, an anthracenyl group, a phenanthrenyl group, a biphenyl group, a terphenyl group, a fluorenyl group, a pyrenyl group, and a triphenylenyl group. From the standpoint of affinity with carbon, aryl groups having two or more aromatic rings (fused ring aryl groups and polycyclic aryl groups) are preferred.

This aryl group may or may not have a substituent. Examples of such substituents include alkyl groups, cycloalkyl groups, and aralkyl groups.

As the alkyl group as a substituent on the aryl group, an alkyl group having 1 to 6 carbon atoms is preferred, and specifically preferred are methyl, ethyl, propyl, butyl, and tert-butyl groups.

Cycloalkyl groups and aralkyl groups as substituents on aryl groups include those exemplified above.

The substituents are not limited to those listed above, and may also include groups derived from the fluorene structure (such as a fluorenyl group).

Polyoxyethylene groups are generally hydrophilic, but polyoxyalkylene groups with three or more carbon atoms, such as polyoxypropylene and polyoxybutylene groups, become more hydrophobic as the degree of polymerization increases, and they function as hydrophobic groups. Polyoxypropylene groups with a degree of polymerization of four or more and polyoxybutylene groups with a degree of polymerization of three or more are particularly preferred. For example, when polyoxyethylene-polyoxypropylene or polyoxyethylene-polyoxybutylene is used as an organic compound having a hydrophilic group and a carbon-philic hydrophobic group, the polyoxypropylene group and polyoxybutylene group can also function as hydrophobic groups.

This polyoxyalkylene group having 3 or more carbon atoms may or may not have a substituent. Examples of such substituents include alkyl groups, cycloalkyl groups, aralkyl groups, and aryl groups.

As the alkyl group as a substituent on a polyoxyalkylene group having 3 or more carbon atoms, an alkyl group having 1 to 6 carbon atoms is preferred, and specifically preferred are methyl, ethyl, propyl, butyl, tert-butyl, etc.

Examples of cycloalkyl groups, aralkyl groups, and aryl groups that can be used as substituents on polyoxyalkylene groups having 3 or more carbon atoms include those exemplified above.

The substituents are not limited to those mentioned above, and may also have groups derived from the fluorene structure (such as fluorenyl groups). In particular, when water solubility is important, phenyl groups and the like are preferred as substituents, and when compatibility with flaky carbon is important, naphthyl groups, fluorenyl groups, and the like are preferred as substituents.

From the viewpoint of affinity with carbon, such hydrophobic groups are preferably aryl groups and polyoxyalkylene groups having 3 or more carbon atoms, more preferably aryl groups, and even more preferably aryl groups having two or more aromatic rings (fused aryl groups and polycyclic aryl groups). Specific examples include naphthyl groups, anthracenyl groups, phenanthrenyl groups, biphenyl groups, terphenyl groups, fluorenyl groups, pyrenyl groups, triphenylenyl groups, polyoxypropylene groups with a degree of polymerization of 4 or more, and polyoxybutylene groups with a degree of polymerization of 3 or more.

The hydrophilic group possessed by the organic compound having a hydrophilic group and a carbon-affinity hydrophobic group is not particularly limited as long as it can increase the solubility of the organic compound having the hydrophilic group and the carbon-affinity hydrophobic group in water. However, taking into consideration the water solubility of the organic compound having the hydrophilic group and the carbon-affinity hydrophobic group, the dispersibility of the flaky carbon, the thermal conductivity, and heat dissipation properties, etc., it is preferable that the hydrophilic group be one of the following general formulas (1) to (4).

[The above formula (1) represents an alcoholic hydroxyl group or a phenolic hydroxyl group. In formula (2), R represents a divalent organic group, and both oxygen atoms are ether bonds. In formula (3), _X1 represents a hydrogen atom, an alkali metal, _NH4 , or an organic ammonium. In formula (4), _X2 represents a hydrogen atom, an alkali metal, _NH4 , an organic ammonium, or an alkyl group.]

An organic compound having a hydrophilic group and a carbon-philic hydrophobic group can contain one or more types of such hydrophilic groups. Furthermore, when multiple hydrophilic groups are used, multiple types of the same hydrophilic group may be used, multiple types of hydrophilic groups represented by the same general formula may be used, or multiple types of hydrophilic groups represented by different general formulas may be used.

In general formula (1), -OH can be either an alcoholic hydroxyl group or a phenolic hydroxyl group. However, from the standpoint of the water solubility of organic compounds having hydrophilic groups and carbon-affinity hydrophobic groups, the dispersibility of flaky carbon, thermal conductivity, heat dissipation, etc., alcoholic hydroxyl groups are preferred. However, when a phenolic hydroxyl group is included (especially when multiple phenolic hydroxyl groups are included), it inevitably also contains a benzene ring, which has excellent hydrophobicity, and is therefore preferred because it provides excellent overall water solubility of organic compounds having hydrophilic groups and carbon-affinity hydrophobic groups, the dispersibility of flaky carbon, thermal conductivity, heat dissipation, etc.

In general formula (2), the divalent organic group represented by R is not particularly limited, but a divalent hydrocarbon group is preferred. Examples of divalent hydrocarbon groups include aliphatic hydrocarbon groups (such as alkylene groups (or alkylidene groups), cycloalkylene groups, alkylene (or alkylidene)-cycloalkylene groups, and bi- or tricycloalkylene groups), and aromatic hydrocarbon groups (such as arylene groups and alkylene (or alkylidene)-arylene groups).

In general formula (2), the alkylene group (or alkylidene group) represented by the group R is preferably an alkylene group, more preferably a _C1-8 alkylene group, even more preferably a _C1-4 alkylene group, particularly preferably a _C2-4 alkylene group, and most preferably a _C2-3 alkylene group. Specific examples include a methylene group, an ethylene group, an ethylidene group, a trimethylene group, a propylene group, a propylidene group, a tetramethylene group, an ethylethylene group, a butan-2-ylidene group, a 1,2-dimethylethylene group, a pentamethylene group, and a pentane-2,3-diyl group.

In general formula (2), the cycloalkylene group represented by group R is preferably a C _5-10 cycloalkylene group, more preferably a C _5-8 cycloalkylene group. Specific examples include a cyclopentylene group, a cyclohexylene group, a methylcyclohexylene group, and a cycloheptylene group.

In general formula (2), the alkylene (or alkylidene)-cycloalkylene group represented by the group R is preferably an alkylene-cycloalkylene group, more preferably a _C1-6 alkylene- _C5-10 cycloalkylene group, and even more preferably a _C1-4 alkylene- _C5-8 cycloalkylene group. Specific examples include a methylene-cyclohexylene group, an ethylene-cyclohexylene group, an ethylene-methylcyclohexylene group, and an ethylidene-cyclohexylene group.

In general formula (2), specific examples of the bicycloalkylene or tricycloalkylene group represented by the group R include norbornane-diyl groups.

In general formula (2), the arylene group represented by group R is preferably a C _6-10 arylene group, specifically exemplified by a phenylene group and a naphthalenediyl group.

In general formula (2), the alkylene (or alkylidene)-arylene group represented by the group R is preferably an alkylene-arylene group, more preferably a _C1-6 alkylene- _C6-20 arylene group, still more preferably a _C1-4 alkylene- _C6-10 arylene group, and particularly preferably a _C1-2 alkylene-phenylene group. Specific examples include a methylene-phenylene group, an ethylene-phenylene group, an ethylene-methylphenylene group, and an ethylidenephenylene group.

Of these, divalent aliphatic hydrocarbon groups, particularly alkylene groups (for example, C _1-4 alkylene groups such as methylene and ethylene groups) are preferred.

Note that the alkylene (or alkylidene)-cycloalkylene group and alkylene (alkylidene)-arylene group refer to a group represented by -Ra-Rb- (where Ra represents an alkylene group or alkylidene group bonded to different oxygen atoms in general formula (2), and Rb represents a cycloalkylene group or arylene group).

The hydrophilic group represented by general formula (2) is not particularly limited, and examples thereof include -OC ₂ H ₄ O-, -OC ₃ H ₆ O-, and -OCH ₂ O-. Groups having a plurality of these groups (preferably 3 to 100 groups) can also be preferably used, and examples of such groups include polyoxymethylene groups, polyoxyethylene groups, and polyoxypropylene groups. In particular, when the hydrophilic group represented by general formula (2) has a structure in which three or more hydrophilic groups have been polymerized, the more carbon atoms in R (for example, three or more carbon atoms), the lower the hydrophilicity and the higher the hydrophobicity. Therefore, -OC ₂ H ₄ O- and -OCH ₂ O-, which can maintain hydrophilicity even with an increased degree of polymerization, are preferred.

In the general formula (3), the alkali metal represented by X ₁ is not particularly limited, and examples thereof include sodium, potassium, and lithium.

In the general formula (3), the organic ammonium represented by _X1 is preferably a quaternary ammonium, and tetramethylammonium, tetraethylammonium, tetrapropylammonium, tetrabutylammonium, etc. can be suitably used.

The hydrophilic group represented by general formula (3) is not particularly limited, and examples thereof include —SO ₃ ⁻ H ⁺ , —SO ₃ ⁻ Na ⁺ , —SO ₃ ⁻ K ⁺ , —SO ₃ ⁻ Li ⁺ , —SO ₃ ⁻ NH ₄ ⁺ , —SO ₃ ⁻ N(CH ₃ ) ₄ ⁺ , —SO ₃ ⁻ N(C ₂ H ₅ ) ₄ ⁺ , —SO ₃ ⁻ N(C ₃ H ₇ ) ₄ ⁺ , and —SO ₃ ⁻ N(C ₄ H ₉ ) ₄ ⁺ .

In the general formula (4), examples of the alkali metal and organic ammonium represented by _X2 include those exemplified above.

In general formula (4), the alkyl group represented by _X2 may be a chain alkyl group or a branched alkyl group. However, from the viewpoint of affinity with carbon, a chain alkyl group is preferable. Furthermore, from the viewpoint of affinity with carbon, the number of carbon atoms in the alkyl group is preferably 1 to 2.

The hydrophilic group represented by the general formula (4) is not particularly limited, and examples thereof include -COOH, -COONa, -COOK, -COOLi, -COONH4, -COON( _{CH3 ) 4} , -COON( _C2H5 ) ₄ , -COON( _C3H7 ) ₄₊ ^, _and -COON ⁽ _{C4H9 ) 4+} .

Among these hydrophilic groups, the hydrophilic group represented by general formula (2) is preferred, taking into consideration the water solubility of organic compounds having hydrophilic groups and carbon-affinitive hydrophobic groups, stability independent of pH, dispersibility of flaky carbon, thermal conductivity, and heat dissipation properties.

However, when a compound has multiple identical hydrophilic groups represented by general formula (2), i.e., a polymerized structure, the hydrophilicity of the water-soluble compound increases as the degree of polymerization increases for compounds with two or fewer carbon atoms, but the hydrophobicity of compounds with three or more carbon atoms may increase as the degree of polymerization increases.

In the organic compound having a hydrophilic group and a carbon-affinity hydrophobic group used in the present invention, the number of carbon atoms in the constituent parts other than the hydrophilic group (such as the hydrophobic group) is preferably 6 or more, and more preferably 8 to 18, from the viewpoints of the water solubility of the organic compound having a hydrophilic group and a carbon-affinity hydrophobic group, the dispersibility of the flaky carbon, thermal conductivity, heat dissipation, etc.

Furthermore, in the present invention, when a nonionic material (such as a nonionic surfactant) is used as the organic compound having a hydrophilic group and a carbon-affinity hydrophobic group, its HLB value is preferably 12 or higher, and more preferably 13 to 19, from the viewpoints of the water solubility of the organic compound having the hydrophilic group and the carbon-affinity hydrophobic group, the dispersibility of the flaky carbon, thermal conductivity, heat dissipation, etc. Furthermore, when the hydrophobic group is the same (when the affinity with the flaky carbon is the same), the higher the HLB value, the better.

There are no particular limitations on the organic compounds having a hydrophilic group and a carbon-affinity hydrophobic group that satisfy the above conditions. For example, aromatic water-soluble compounds or non-aromatic water-soluble compounds may be used, with aromatic water-soluble compounds being preferred. Examples of organic compounds having a hydrophilic group and a carbon-affinity hydrophobic group include polyoxyethylene lauryl ether, polyoxyethylene decyl ether, polyoxypropylene decyl ether, polyoxyethylene lauryl ether, polyoxyethylene naphthyl ether, polyoxypropylene lauryl ether, polyoxypropylene naphthyl ether, polyoxyethylene myristyl ether, polyoxypropylene myristyl ether, polyoxyethylene cetyl ether, polyoxypropylene cetyl ether, polyoxyethylene octylphenyl ether, polyoxypropylene octylphenyl ether, and polyoxyethylene undecylphenyl ether. ether, polyoxypropylene undecyl phenyl ether, polyoxyethylene tridecyl phenyl ether, polyoxypropylene tridecyl phenyl ether, polyoxyethylene pentadecyl phenyl ether, polyoxypropylene pentadecyl phenyl ether, polyoxyethylene polyoxypropylene glycol, polyoxypropylene polyglyceryl ether, sodium cholate, potassium cholate, sodium dodecyl sulfonate, potassium dodecyl sulfonate, sodium dilauroyl glutamate lysine, potassium dilauroyl glutamate lysine, decaglycerin laurate, n-decyl alcohol, etc.

Examples of organic compounds having such hydrophilic groups and carbon-affinity hydrophobic groups include Emulgen 103, Emulgen 104P, Emulgen 105, Emulgen 106, Emulgen 108, Emulgen 109P, Emulgen 120, Emulgen 123P, Emulgen 130K, Emulgen 147, Emulgen 150, Emulgen 210P, and Emulgen 220 (all polyoxyethylene alkyl ethers manufactured by Kao Corporation), Triton X-100, Triton X-114, Triton X-305, and Triton X-405 (polyoxyethylene octylphenyl ethers manufactured by Dow Chemical Company), and Noigen EN and Noigen EN-10 (polyoxyethylene naphthyl ethers manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd.).

The content of the organic compound having a hydrophilic group and a carbon-affinity hydrophobic group in the thermally conductive material of the present invention is not particularly limited. However, in order to impart sufficient thermal conductivity and heat dissipation properties to the thermally conductive material, the content of the organic compound having a hydrophilic group and a carbon-affinity hydrophobic group is preferably 0.5 to 50 mass%, and more preferably 0.8 to 40 mass%, of the total amount of the thermally conductive material taken as 100 mass%.

In addition, to provide the thermal conductive material with sufficient thermal conductivity and heat dissipation properties, the content of the organic compound having a hydrophilic group and a carbon-affinitive hydrophobic group is preferably 1 to 100 parts by mass, and more preferably 2 to 80 parts by mass, per 100 parts by mass of flaky carbon.

When the content of the organic compound having a hydrophilic group and a carbon-affinity hydrophobic group is low, the thermally conductive material of the present invention has a configuration in which the surface of the flaky carbon is coated with an organic compound having a hydrophilic group and a carbon-affinity hydrophobic group. On the other hand, when the content of the organic compound having a hydrophilic group and a carbon-affinity hydrophobic group is high, the thermally conductive material of the present invention has a configuration in which the flaky carbon is dispersed in the organic compound having a hydrophilic group and a carbon-affinity hydrophobic group. In either case, the presence of the organic compound having a hydrophilic group and a carbon-affinity hydrophobic group around the flaky carbon suppresses aggregation of the flaky carbon, resulting in a material with excellent thermal conductivity and heat dissipation properties.

(1.3. Nanofibers)
The thermally conductive material of the present invention includes nanofibers, and the nanofibers used are cellulose-based nanofibers such as cellulose or cellulose derivatives.

Using cellulose-based nanofibers together with flake carbon has an advantage over resin-based thermal conductive materials in that the flake carbon is adsorbed onto the hydrophobic groups of the cellulose, allowing for easy and uniform compounding.

In other words, by using nanofibers and highly dispersible flaky carbon, the flaky carbon is adsorbed evenly over a large surface area, heat is transferred by the flaky carbon along the nanofibers, and the nanofibers themselves also assist in thermal conduction. Furthermore, by creating a state in which the flaky carbon is adsorbed intermittently, it is possible to create a state in which electricity does not flow.

Conversely, if nanofibers are not used, the flaky carbon will be densely adsorbed onto a small surface area, resulting in conductivity. Furthermore, if highly dispersible flaky carbon is not used, the nanofibers and carbon cannot be uniformly combined, and the unevenly distributed carbon will become conductive.

When the strength of the thermally conductive material is important, it is preferable to use cellulose nanofibers as cellulose-based nanofibers. If a thermally conductive material with even higher thermal conductivity and strength is required, materials with high cellulose purity, such as dissolving pulp (from which hemicellulose and lignin have been purified), cotton, and cotton linter pulp, can be used.

When uniformity of the thermally conductive material is important, it is preferable to use a cellulose derivative obtained by hydrophilizing cellulose. Examples include methyl cellulose, carboxymethyl cellulose salt, hydroxyethyl cellulose, hydroxypropyl cellulose, hydroxyethyl methyl cellulose, and hydroxypropyl methyl cellulose.

To improve the strength of the thermal conductive material, it is preferable to use carboxymethyl cellulose salts (salts of Na, K, NH4, etc.). On the other hand, to improve affinity with flaky carbon, it is preferable to use hydroxypropyl cellulose or hydroxypropyl methyl cellulose. Furthermore, if fluidity is a priority, it is preferable to use the same cellulose (derivative) with a low molecular weight (degree of polymerization) of the cellulose portion, while if strength is a priority, it is preferable to use a high molecular weight.

(1.4. Other Ingredients)
The thermally conductive material of the present invention may contain other components in addition to the flaky carbon and the organic compound having a hydrophilic group and a carbon-affinic hydrophobic group. Examples of such other components include carbon fiber (particularly, carbon nanofibers having a fiber diameter of 500 nm or less), activated carbon, carbon black (acetylene black, oil furnace black, etc.; particularly, ketjen black, which has high conductivity and a large specific surface area), glassy carbon, carbon microcoils, fullerene, biomass-based carbon materials (materials made from bagasse, sorghum, wood chips, sawdust, bamboo, bark, rice straw, rice husks, coffee grounds, used tea leaves, soybean pulp refuse, rice bran, pulp waste, etc.; carbon fiber produced from lignin, etc.), cellulose nanofibers, boron nitride, molybdenum compounds (molybdenum disulfide, organic molybdenum, etc.), tungsten disulfide, fluororesins (polytetrafluoroethylene (PTFE), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), etc.), melamine cyanurate, phthalocyanine, lead oxide, calcium fluoride, and layered minerals (mica, talc, etc.).

However, from the standpoint of making it easier to disperse in the resin and further improving the uniformity and adhesion of the coating film when applied, it is preferable that the content of other components is low. Taking the total amount of the thermal conductive material of the present invention as 100% by mass, it is preferably 0.01 to 10% by mass, and more preferably 0.02 to 5% by mass.

The shape of the thermally conductive material of the present invention is not particularly limited, and examples include a coating film, sheet, block, fiber, etc.

As described above, the thermally conductive material of the present invention is a material that not only has excellent thermal conductivity but also excellent heat dissipation properties. Therefore, the thermally conductive material of the present invention not only has excellent thermal conductivity, but is also able to dissipate heat more quickly after being heated, thereby cooling the temperature. Therefore, the thermally conductive material of the present invention can also function as a thermally conductive heat dissipation material.

The thermally conductive material of the present invention has excellent thermal conductivity and heat dissipation properties, and can be used in applications such as thermally conductive grease for electronic materials, thermally conductive paint for electronic materials, thermally conductive rubber for electronic materials, thermally conductive paint for LEDs, paint for heat sinks, and coatings for various heat exchangers. Furthermore, by adding cellulose or a cellulose derivative, it is possible to produce a free-standing film.

(2. Method for manufacturing thermally conductive material)
The present invention includes an invention relating to a method for producing a thermally conductive material, which includes a step of mixing a flaky carbon dispersion containing 1 to 100 parts by mass of an organic compound having a hydrophilic group and a carbon-affinity hydrophobic group per 100 parts by mass of flaky carbon with a nanofiber dispersion.

(2.1. Method for producing flaky carbon dispersion liquid)
In a dispersion (flaky carbon dispersion) containing flaky carbon, an organic compound having a hydrophilic group and a carbon-affinity hydrophobic group, and a solvent, the above-mentioned explanations can be used for the flaky carbon and the organic compound having a hydrophilic group and a carbon-affinity hydrophobic group. In addition, the flaky carbon dispersion can contain other components as needed.

When preparing the flaky carbon dispersion, it is preferable to use water as the main solvent, from the viewpoints of the dispersibility of the flaky carbon, the thermal conductivity and heat dissipation properties of the resulting thermal conductive material, etc.

There are no particular restrictions on the water content in the solvent used, but from the perspective of the dispersibility of the flaky carbon and the thermal conductivity and heat dissipation properties of the resulting thermal conductive material, the water content is preferably 70% by mass or more (70 to 100% by mass), and more preferably 75 to 100% by mass, of the total amount of solvent taken as 100% by mass.

In the present invention, only water may be used as the solvent, and organic solvents are not necessarily required. However, to further improve the solubility of organic compounds having hydrophilic groups and carbon-affinity hydrophobic groups in water, organic solvents such as alcohols such as methanol, ethanol, 2-propanol, and tert-butyl alcohol; glycols such as ethylene glycol; glycerin; and 2-methoxyethanol may be used.

The content of organic solvent in the solvent used is preferably 30% by mass or less (0 to 30% by mass), and more preferably 5 to 25% by mass, based on 100% by mass of the total amount of solvent, taking into consideration the solubility of the organic compound having a hydrophilic group and a carbon-affinity hydrophobic group, and the thermal conductivity and heat dissipation properties of the resulting thermal conductive material.

In the above-mentioned flaky carbon dispersion, the content of flaky carbon is not particularly limited. From the viewpoint of facilitating the composition of the thermal conductive material of the present invention, the content is preferably 20% by mass or less, more preferably 0.0001 to 15% by mass, and even more preferably 0.001 to 10% by mass, based on 100% by mass of the total amount of the flaky carbon dispersion.

Furthermore, the solvent content is not particularly limited, but from the perspective of facilitating the composition of the thermal conductive material of the present invention, it is preferably 40 to 99.9998 mass%, more preferably 63 to 99.998 mass%, and even more preferably 85 to 99.98 mass%, of the total amount of the flaky carbon dispersion liquid being 100 mass%.

The method for producing a flaky carbon dispersion is not particularly limited, and can be obtained, for example, by adding flaky carbon and an organic compound having a hydrophilic group and a carbon-affinity hydrophobic group to a solvent. More specifically, flaky carbon can be added to a dispersion of an organic compound having a hydrophilic group and a carbon-affinity hydrophobic group, or an organic compound having a hydrophilic group and a carbon-affinity hydrophobic group can be added to a dispersion of flaky carbon. Furthermore, flaky carbon and an organic compound having a hydrophilic group and a carbon-affinity hydrophobic group can be added simultaneously to a solvent.

However, in order to further improve the dispersibility of the flaky carbon, making it less likely to agglomerate, and further enhance the thermal conductivity and heat dissipation properties of the resulting thermal conductive material of the present invention, it is preferable to place a composition containing a carbonaceous material with a layered structure (a material for producing flaky carbon, as described below) and an organic compound having hydrophilic groups and carbon-affinitive hydrophobic groups between a rotating turntable and a plate placed approximately parallel to the turntable, and then shear the carbonaceous material in the composition while adjusting the shortest distance between the turntable and the plate to 200 μm or less (grinding method).

Shearing treatment results in the atomization of the carbonaceous material having a layered structure. While this may result in the graphene structure not being maintained depending on the conditions, it can efficiently flaking the carbonaceous material having a layered structure and reduce processing time. When performing this shearing treatment, the rotating disk and the disk are arranged approximately parallel to each other, but they do not need to be strictly parallel. Specifically, the angle between the axis perpendicular to the rotating disk and the axis perpendicular to the disk is preferably 10° or less, more preferably 5° or less. It is most preferable that the axis perpendicular to the rotating disk and the axis perpendicular to the disk are strictly parallel. The minimum distance between the two surfaces when performing this shearing treatment is not particularly limited as long as it is sufficient to flaking the carbonaceous material having a layered structure; however, it is preferably 200 μm or less, more preferably 1 to 50 μm, and even more preferably 2 to 30 μm. Although the rotating disk and the disk are arranged approximately parallel to each other, the distance between the rotating disk and the disk may vary depending on the location. In this case, the shortest distance between the turntables refers to the shortest distance between the turntables. Furthermore, it is not necessary to separate the turntables from the platen in advance; the material to be processed may be sandwiched between the turntables and the platen. Alternatively, the turntables and the platen may be brought into contact with each other, and the gap between the turntables and the platen may widen as a result of the layered carbonaceous material being sandwiched between them. Such shearing treatment can be performed using any device that has a mechanism for rotating a plate-shaped object, such as a millstone, a vibration mixer, a spin coater, or a grinder.

There are no particular restrictions on the size of the turntable that can be used in this process, but a size of 5 to 500 mm is preferred, and 10 to 200 mm is more preferred. There are also no particular restrictions on the rotation speed of the turntable when performing the shearing process, but it is preferably set within a range that can sufficiently flake the carbonaceous material having a layered structure; for example, 1,000 to 10,000 ppm is preferred, and 2,000 to 5,000 ppm is more preferred.

By performing this type of shearing treatment, the plate and the carbonaceous material having a layered structure, and the carbonaceous material having a layered structure and the carbonaceous material having a layered structure can be brought into contact with each other, and shear can be applied to the carbonaceous material having a layered structure in a direction parallel to the graphene layers of the carbonaceous material having a layered structure.

By reducing the shortest distance between the rotating discs during shearing and increasing the rotation speed of the rotating discs, it is possible to make the conditions stronger, allowing the layered carbonaceous material to be flaked more efficiently and reducing the processing time. This shearing operation can be performed one or more times, preferably three or more times.

There are no particular restrictions on the temperature at which the shearing treatment is carried out, as long as it is a temperature that can sufficiently exfoliate the carbonaceous material having a layered structure, and it can be 0°C or higher, preferably 0 to 100°C, and particularly 20 to 95°C. The temperature at which the shearing treatment is carried out is preferably one that provides high solubility for organic compounds having hydrophilic groups and hydrophobic groups that have a high affinity for carbon. If higher temperatures increase solubility, higher temperatures are preferred. If a water-soluble compound with a cloud point is used, it is preferable to maintain the temperature below the cloud point.

Before carrying out the shearing treatment, in order to bring the carbonaceous material having a layered structure into intimate contact with the organic compound having hydrophilic groups and hydrophobic groups that have a high affinity for carbon, the materials may be stirred in advance using a stirring device, ultrasonic dispersion device, or the like before preparing the composition, allowing the organic compound having hydrophilic groups and hydrophobic groups that have a high affinity for carbon to blend with the surface of the carbonaceous material having a layered structure.

In the present invention, when graphite oxide is used as the carbonaceous material having a layered structure, it exists as a flaky carbon oxide in the dispersion after the shearing treatment. Therefore, when graphite oxide is used as the carbonaceous material having a layered structure, it is preferable to perform a reduction treatment as a post-treatment. Various methods, such as chemical reduction and electrochemical reduction, can be used for the reduction treatment, but chemical reduction is preferred. Among these, chemical reduction using a reducing agent such as hydrazine or sodium borohydride is preferred. The amount of reducing agent is preferably 0.01 to 10 parts by mass, more preferably 0.1 to 5 parts by mass, and even more preferably 0.5 to 3 parts by mass, per part by mass of flaky carbon oxide. Furthermore, heating during reduction facilitates reduction. The heating temperature is preferably 40 to 200°C, more preferably 50 to 150°C, and even more preferably 60 to 120°C. The reduction time is preferably 10 minutes to 64 hours, more preferably 30 minutes to 48 hours, and even more preferably 1 to 24 hours. However, it is preferable to do so to the extent that the graphene structure is not excessively destroyed.

The above-described manufacturing method allows the flaky carbon to be obtained as the above-described flaky carbon dispersion. This manufacturing method contains an organic compound having a hydrophilic group and a hydrophobic group with a high affinity for carbon, and therefore the flaky carbon dispersion also contains an organic compound having a hydrophilic group and a hydrophobic group with a high affinity for carbon. This organic compound having a hydrophilic group and a hydrophobic group with a high affinity for carbon can adsorb to the surface of the flaky carbon and isolate and disperse the flaky carbon at a high concentration in the solvent, thereby functioning as a dispersant in the flaky carbon dispersion. Furthermore, the organic compound having a hydrophilic group and a hydrophobic group with a high affinity for carbon can be commercially available, and is superior to conventional products in both cost and dispersibility. Furthermore, this organic compound having a hydrophilic group and a hydrophobic group with a high affinity for carbon remains on the surface of the flaky carbon, thereby exhibiting sufficient thermal conductivity and heat dissipation.

Furthermore, in conventional methods involving oxidation and reduction, it was impossible to form a flaky carbon dispersion on a plastic substrate because the plastic substrate was hydrolyzed during the reduction treatment and the flaky carbon aggregated when the reduction treatment was performed, making it impossible to exist as a dispersion. However, in the present invention, by incorporating an organic compound having the above-mentioned hydrophilic group and a hydrophobic group that has a high affinity for carbon and performing a specific treatment, it is possible to form a flaky carbon dispersion on a plastic substrate such as polyethylene terephthalate (PET) without the substrate being hydrolyzed.

Flake-like carbon dispersions can also be produced by applying a pressure of 30 MPa or more to a composition containing a carbonaceous material with a layered structure and an organic compound having hydrophilic groups and carbon-affinitive hydrophobic groups (high-pressure dispersion method).

When using the high-pressure dispersion method, as described above, it is preferable to apply a pressure of 30 MPa or more to a composition containing a carbonaceous material having a layered structure and an organic compound having a hydrophilic group and a hydrophobic group that has a high affinity for carbon.

Pressure treatment causes the carbonaceous material having a layered structure to become finer. While this may result in the graphene structure not being maintained depending on the conditions, it can efficiently flaking the carbonaceous material having a layered structure and reduce processing time. There are no particular restrictions on the pressure level used when performing this type of pressure treatment, as long as it is sufficient to flaking the carbonaceous material having a layered structure. However, a pressure of 30 MPa or higher is preferred, with 50 to 400 MPa being more preferred, and 100 to 300 MPa being even more preferred. This type of pressure treatment can be performed using a high-pressure dispersion device, a supercritical water production device, or the like. High-pressure dispersion devices can disperse the material by applying mechanical pressure, and supercritical water production devices can increase the pressure of the system by heating the water.

Such pressure can, for example,
(i) causing two or more of the carbonaceous material dispersions to collide with each other;
(ii) causing the carbonaceous material dispersion to collide with a metal or ceramic material (a high-hardness material such as silicon carbide or alumina);
(iii) A treatment such as passing the carbonaceous material dispersion through a space having a cross-sectional area of 1 ^cm2 or less may be carried out.

According to (i) and (ii) above, it is possible to apply stronger pressure conditions, which allows the carbonaceous material having a layered structure to be flaked more efficiently and the processing time to be reduced. Furthermore, according to (iii) above, the carbonaceous material having a layered structure can be flaked more appropriately while better maintaining the graphene structure. This pressure operation can be performed one or more times, preferably ten or more times.

There are no particular restrictions on the pressurizing temperature, as long as it is a temperature that can sufficiently exfoliate the carbonaceous material having a layered structure. In the cases of (i) and (ii) above, it can be 0 to 100°C, particularly 20 to 95°C. In the case of (iii) above, if pressure is applied mechanically, 0 to 100°C is preferred, and if pressure is generated by the supercritical state of water, 373 to 700°C is preferred, and 380 to 450°C is more preferred.

When carrying out the pressure treatment, it is preferable to carry out ultrasonic dispersion treatment as a preliminary treatment (pretreatment) to atomize the carbonaceous material having a layered structure. This can have the effect of preventing clogging, etc.

There are no particular restrictions on the output power used in ultrasonic dispersion treatment, but from the perspective of flaking carbonaceous materials having a layered structure, it is preferable to use a power that is more powerful than the power typically used in ultrasonic dispersion treatment (approximately 40 to 50 W). Specifically, the output power of the ultrasonic dispersion treatment is preferably 100 W or more, more preferably 300 to 20,000 W, and even more preferably 400 to 18,000 W.

There are no particular restrictions on the ultrasonic dispersion temperature, as long as it is a temperature that can sufficiently exfoliate the carbonaceous material having a layered structure, and it can be 0 to 80°C, particularly 10 to 70°C. There are no particular restrictions on the ultrasonic dispersion time, as long as it is a time that can sufficiently exfoliate the carbonaceous material having a layered structure, and it can be 1 to 600 minutes, particularly 3 to 120 minutes.

In addition, as a pre- or post-treatment of these processes, dispersion treatment using other dispersion devices such as conventional mechanical stirring, dispersion treatment using an emulsifier, or dispersion treatment using a bead mill may be used in combination.

In the present invention, when graphite oxide is used as the carbonaceous material having a layered structure, it exists as a flaky carbon oxide in the dispersion that has been subjected to the above-mentioned pressure treatment. Therefore, when graphite oxide is used as the carbonaceous material having a layered structure, it is preferable to perform a reduction treatment as a post-treatment. Various methods, such as chemical reduction and electrochemical reduction, can be used for the reduction treatment, but chemical reduction is preferred. Among these, chemical reduction using a reducing agent such as hydrazine or sodium borohydride is preferred. The amount of reducing agent is preferably 0.01 to 10 parts by mass, more preferably 0.1 to 5 parts by mass, and even more preferably 0.5 to 3 parts by mass, per part by mass of flaky carbon oxide. Furthermore, heating during reduction facilitates reduction. The heating temperature is preferably 40 to 200°C, more preferably 50 to 150°C, and even more preferably 60 to 120°C. The reduction time is preferably 10 minutes to 64 hours, more preferably 30 minutes to 48 hours, and even more preferably 1 to 24 hours. However, it is preferable to do so to the extent that the graphene structure is not excessively destroyed.

In addition, from the viewpoint of the thermal conductivity and heat dissipation properties of the resulting product, the grinding method is most preferable.

Conventionally, when producing flaky carbon using a wet method, a water dispersion containing flaky carbon oxide and an aqueous solvent is subjected to a reduction treatment. However, this method makes it difficult to maintain the graphene structure, and the resulting flaky carbon aggregates severely, making it difficult to obtain a flaky carbon water dispersion. There are also safety issues.

When high-pressure treatment is performed, it is possible to obtain a water dispersion of flaky carbon, but the resulting flaky carbon is easily destroyed, production tends to take a long time, and clumps that have not been peeled off may remain.

In contrast, in the present invention, by using an organic compound having a hydrophilic group and a carbon-affinity hydrophobic group, it is possible to obtain flaky carbon that maintains its graphene structure without agglomeration and in a uniformly dispersed state (flaky carbon dispersion liquid).The resulting flaky carbon is also less likely to be destroyed, can be obtained in a short period of time, and is less likely to leave behind any clumps that have not been peeled off.In this case, the organic compound having a hydrophilic group and a carbon-affinity hydrophobic group can also function as a dispersant to uniformly disperse the flaky carbon.

Similarly, the content of the organic compound having a hydrophilic group and a carbon-affinitive hydrophobic group is not particularly limited. However, because it is easier to formulate the thermal conductive material of the present invention, the content is preferably 0.00001 to 99.9 mass%, more preferably 0.0001 to 50 mass%, and even more preferably 0.001 to 30 mass%, assuming the total amount of flaky carbon dispersion to be 100 mass%.

Similarly, the content of the organic compound having a hydrophilic group and a carbon-affinity hydrophobic group in the flaky carbon dispersion is not particularly limited, but from the viewpoint of facilitating the composition of the thermal conductive material of the present invention, it is preferably 1 to 100 parts by mass, and more preferably 2 to 80 parts by mass, per 100 parts by mass of flaky carbon.

Note that the lower the content of organic compounds having hydrophilic groups and carbon-affinity hydrophobic groups, the higher the content of carbonaceous material with a layered structure, which tends to improve thermal conductivity and heat dissipation, and makes processing easier and less expensive. On the other hand, the higher the content of organic compounds having hydrophilic groups and carbon-affinity hydrophobic groups, the more likely it is that exfoliation (delamination) will occur, and flaky carbon tends to be obtained more efficiently, but higher viscosity may also result in lower exfoliation efficiency.

For this reason, it is preferable to appropriately set the content of the organic compound having a hydrophilic group and a carbon-affinity hydrophobic group from the perspective of balancing thermal conductivity, heat dissipation, cost, exfoliation efficiency, etc. When a carbonaceous material dispersion is used in this manufacturing method, it is preferable to set the content of the organic compound having a hydrophilic group and a carbon-affinity hydrophobic group in the carbonaceous material dispersion within the above range.

Furthermore, with the shearing method, the direction of force application is parallel to the surface direction of the carbonaceous material having a layered structure, and processing is carried out in a narrow space. This results in less breakage and allows for larger-sized flake carbon (e.g., flake carbon with a size of 1 μm or more) to be obtained compared to conventional manufacturing methods using high-speed stirring, ultrasonic treatment, high-pressure treatment, etc., and the peeling is efficient, allowing processing to be carried out in a short time (few passes), while also leaving fewer thick lumps that were not peeled.

The carbonaceous material having the layered structure described above is not particularly limited, and examples include natural graphite, artificial graphite, expanded graphite, amorphous graphite, and graphite oxide. Graphite oxide can be graphite oxidized with one or more oxidizing agents, such as sulfuric acid, nitric acid, potassium permanganate, and hydrogen peroxide. For example, when obtaining graphite oxide using the Hummers process, graphite is immersed in concentrated sulfuric acid, potassium permanganate is added to oxidize the graphite, and the reaction mixture is quenched with dilute sulfuric acid and/or hydrogen peroxide. The reaction mixture is then washed with distilled water, and oxygen atoms are bonded to the carbon atoms, introducing oxygen atoms between the layers to obtain graphite oxide.

In particular, when attempting to obtain highly pure flaky carbon that does not contain heteroatoms such as oxygen, it is preferable to use graphite as the raw material, and it is even more preferable to use natural graphite and expanded graphite. When using expanded graphite, it is preferable to use expanded graphite that has less oxidation of the graphene structure. Furthermore, when using expanded graphite, it may be heat-treated at approximately 300 to 1000°C for 10 seconds to 5 hours before use. This makes it possible to obtain expanded graphite that has been appropriately expanded.

Graphite oxide may also be used when ease of production is important. The use of graphite oxide allows solvent molecules to easily intercalate between layers, facilitating exfoliation only in the layer direction, improving exfoliation efficiency and dispersibility, and enabling shorter processing times. However, when graphite oxide is used, a subsequent reduction treatment is required, and from the perspective of better maintaining the graphene structure, conductivity, and strength, other materials (natural graphite, artificial graphite, expanded graphite, amorphous graphite) are preferred.

On the other hand, amorphous graphite can also be used to further improve dispersibility. However, from the standpoint of crystallinity, purity, and structural maintenance, other materials (natural graphite, artificial graphite, expanded graphite, graphite oxide) are preferred.

In addition, if the crystallinity, strength, and structural integrity of the resulting flaky carbon are important, artificial graphite can also be used.

In the present invention, when shearing is performed between a rotating turntable and a plate placed approximately parallel to it, with the shortest distance between the two surfaces maintained at 200 μm or less, the content of the carbonaceous material having a layered structure in the system is not particularly limited. However, it is preferably 20% by mass or less, more preferably 0.0001 to 15% by mass, and even more preferably 0.001 to 10% by mass, based on 100% by mass of the total amount of the composition used to produce the flaky carbon dispersion. Note that a thinner content of the carbonaceous material having a layered structure is more likely to cause exfoliation (delamination), resulting in more efficient production of flaky carbon and fewer treatment cycles. It also tends to maintain an appropriate viscosity and facilitate shearing and other processes. On the other hand, a higher content of the carbonaceous material having a layered structure results in better productivity. Therefore, it is preferable to appropriately set the content of the carbonaceous material having a layered structure from the perspective of balancing exfoliation efficiency, viscosity, productivity, and other factors. When using a carbonaceous material dispersion, it is preferable to keep the content of the carbonaceous material having a layered structure in the flaky carbon dispersion within the above range.

When producing the above-mentioned flaky carbon dispersion, as described above, it is preferable to subject the carbonaceous material having a layered structure to a specific treatment in the presence of an organic compound having hydrophilic groups and carbon-affinity hydrophobic groups. However, from the standpoint of the exfoliation efficiency of the carbonaceous material having a layered structure, the thermal conductivity and heat dissipation properties of the resulting flaky carbon, etc., it is preferable to subject a carbonaceous material dispersion containing the carbonaceous material having a layered structure and an organic compound having hydrophilic groups and carbon-affinity hydrophobic groups to a specific treatment.

The solvents described above can be used. In this case, the solvents used to prepare the carbonaceous material dispersion (carbonaceous material dispersion or carbonaceous material coating) can be used as well.

In the present invention, when a specific process is performed using a carbonaceous material dispersion liquid containing a solvent, the total amount of solvent in the carbonaceous material dispersion liquid is not particularly limited. However, from the viewpoints of the exfoliation efficiency of the carbonaceous material having a layered structure, the solubility of organic compounds having hydrophilic groups and carbon-affinic hydrophobic groups, and the like, the total amount of the carbonaceous material dispersion liquid is preferably 40 to 99.9998 mass%, more preferably 63 to 99.998 mass%, and even more preferably 85 to 99.98 mass%, taking the total amount of the carbonaceous material dispersion liquid as 100 mass%.

In the present invention, when a specific process is performed using a carbonaceous material dispersion using a solvent, the carbonaceous material dispersion may be prepared by adding a carbonaceous material having a layered structure to a dispersion of an organic compound having hydrophilic groups and carbon-affinity hydrophobic groups, or by adding an organic compound having hydrophilic groups and carbon-affinity hydrophobic groups to a dispersion of a carbonaceous material having a layered structure. Alternatively, the carbonaceous material having a layered structure and the organic compound having hydrophilic groups and carbon-affinity hydrophobic groups may be simultaneously added to the solvent.

In the present invention, other components may be included in a composition (e.g., a carbonaceous material dispersion, etc.) containing a carbonaceous material having a layered structure and an organic compound having a hydrophilic group and a hydrophobic group that has a high affinity for carbon. This allows these other components to be included in the final flaky carbon dispersion or thermally conductive material. The above-mentioned components can be used as such other components, and they may be used within a range that does not impair the effects of the present invention. However, from the perspective of easily dispersing these other components in a resin and easily obtaining a thermally conductive material that further improves the uniformity and adhesion of the coating film when applied, it is preferable that the content of these other components is low. Taking the total amount of the carbonaceous material dispersion as 100% by mass, 0.00001 to 5% by mass is preferred, and 0.0001 to 2% by mass is more preferred.

(2.2. Method for producing nanofiber dispersion)
When producing the nanofiber dispersion, nanofibers made of the above-mentioned cellulose or cellulose derivative and having a fiber diameter of 1 to 500 nm are used.

The dispersion medium for the nanofiber dispersion can be water, methanol, ethanol, 1-propanol, 2-propanol, ethylene glycol, propylene glycol, glycerin, or a mixture thereof. Among these, water is preferred because it has the highest polarity and affinity with nanofibers, does not impair the dispersibility of the flaky carbon having adsorbed organic matter with hydrophilic groups on its surface as described in 1.1, and is easily removed by heating or reducing pressure.

The content of nanofibers in the nanofiber dispersion is preferably 0.1% by mass or more, more preferably 0.5% by mass or more, and even more preferably 1.0% by mass or more, because this reduces the amount of solvent that is distilled off after compounding with the flaky carbon and allows for greater flexibility in blending with the flaky carbon.

On the other hand, in order to ensure the fluidity of the nanofiber dispersion and facilitate mixing with the flaky carbon dispersion, the content of nanofibers in the nanofiber dispersion is preferably 10% by mass or less, more preferably 7% by mass or less, and even more preferably 5% by mass or less.

However, if the solvent ratio of the flaky carbon dispersion to be mixed is high, the fluidity of the nanofiber dispersion does not necessarily need to be high, and a wet solid may be used. In this case, the nanofiber content may be 10% by mass or more and 50% by mass or less, and preferably 10% by mass or more and 30% by mass or less.

(3.3. Step of mixing flaky carbon dispersion and nanofiber dispersion)
A uniform composite can be prepared by mixing an aqueous dispersion of flaky carbon with an aqueous dispersion of nanofibers of cellulose or a cellulose derivative, but it is also possible to use a wet solid prepared by filtration or centrifugation as one of the components.

Both can also be made into wet solids and mixed under high shear.

A dispersion or wet solid thereof in which one or both solvents are replaced with an organic solvent may also be used, allowing a water-insoluble resin to be added.

The above describes embodiments of the present invention, but the present invention is not limited to these examples and can, of course, be embodied in various forms without departing from the spirit of the present invention.

The following provides a more detailed explanation of embodiments of the present invention based on examples, but the present invention is not limited to these examples.

Example 1
500 g of natural graphite (manufactured by Ito Graphite Industries Co., Ltd.), 50 g of polyoxyethylene naphthyl ether (HLB value 17), and 10,000 g of water were mixed and processed once in a ceramic grinder. The resulting dispersion had a solids content of 5.5 wt%, of which the carbon content was 5.0 wt%. Similarly, 300 g of cotton linter pulp and 9,700 g of water were mixed and processed five times in a grinder to produce a 3 wt% cellulose nanofiber dispersion. When 10 g of the resulting dispersion (carbon 5 wt%) was mixed with 16.7 g of a CNF dispersion (cellulose 3 wt%), carbon was adsorbed onto the CNF surface. The liquid was filtered and dried at 200°C under a pressure of 20 MPa, resulting in a 50/50 carbon/cellulose sheet. Thermal conductivity was measured and found to be high at 67 W/m·K.

Example 2
A sheet was produced in the same manner as in Example 1, except that the carbon/cellulose ratio was 5/95. When the thermal conductivity was measured, a high thermal conductivity of 6 W/m·K was obtained. Furthermore, when the surface resistance was measured, it was greater than 10 ⁹ Ω/□, indicating insulation.

Example 3
A sheet was produced in the same manner as in Example 1, except that the carbon/cellulose ratio was 10/90. When the thermal conductivity was measured, a high thermal conductivity of 10 W/m·K was obtained. Furthermore, when the surface resistance was measured, it was greater than 10 ⁹ Ω/□, indicating insulation.

Example 4
A sheet was produced in the same manner as in Example 1, except that the carbon/cellulose ratio was 30/70. The thermal conductivity was measured and found to be high at 31 W/m·K. The surface resistance was also measured and found to be 15 Ω/□.

(Comparative Example 1)
500 g of natural graphite (manufactured by Ito Graphite Industries Co., Ltd.), 50 g of tannic acid, and 10,000 g of water were mixed and processed once in a ceramic grinder. The resulting solution was concentrated to obtain a powder containing 90.9% by mass of flaky carbon. 0.55 g of this powder (containing 0.50 g of flaky carbon), 0.23 g of 1,6-hexanediol diglycidyl ether, and 0.22 g of the curing agent hexahydrophthalic anhydride (total: 0.45 g) were mixed in a mortar and a planetary mill and press-molded at 150 °C to obtain a sheet containing 50% by mass of flaky carbon. The thermal conductivity was measured to be 19 W/m·K, lower than that of Example 1.

(Comparative Example 2)
A test was conducted in the same manner as in Example 1, except that 0.23 g of 1,6-hexanediol diglycidyl ether and 0.22 g of the curing agent hexahydrophthalic anhydride (total 0.45 g) were used instead of 16.7 g of the CNF dispersion (cellulose 3% by mass), to obtain a sheet containing 50% by mass of carbon. The thermal conductivity was measured and found to be 30 W/m K, which was lower than that of Example 1.

(Comparative Example 3)
33.3 g of the CNF dispersion (cellulose 3% by mass) was filtered, and a sheet was prepared in the same manner as in Example 1. Approximately 5% by mass of carbon relative to the cellulose was supported by carbon spray. The thermal conductivity was measured to be 2 W/m·K, which was lower than that of Example 2. The surface resistance was also measured to be 9×10 ⁴ Ω/□, which was conductive.

(Comparative Example 4)
500 g of natural graphite (manufactured by Ito Graphite Industries Co., Ltd.), 50 g of tannic acid, and 10,000 g of water were mixed and processed once in a ceramic grinder. The resulting solution was concentrated to obtain a powder containing 90.9% by mass of flaky carbon. 0.055 g of this powder (containing 0.050 g of flaky carbon), 0.548 g of bisphenol A diglycidyl ether, and 0.397 g of curing agent hexahydrophthalic anhydride (total 0.945 g) were mixed in a mortar and a planetary mill and press-molded at 150°C to obtain a sheet containing 5% by mass of flaky carbon. The thermal conductivity was measured to be 1.2 W/m·K, lower than that of Example 2. The surface resistivity was also measured to be 1.3 × 10 ⁵ Ω/□, demonstrating electrical conductivity.

(Comparative Example 5)
An experiment was conducted in the same manner as in Comparative Example 4, except that 0.11 g of powder containing 90.9 mass% flaky carbon (containing 0.10 g of flaky carbon), 0.516 g of bisphenol A diglycidyl ether, and 0.374 g of curing agent hexahydrophthalic anhydride were mixed to obtain a sheet containing 10 mass% flaky carbon. The thermal conductivity was measured to be 1.3 W/m·K, which was lower than that of Example 3. The surface resistance was also measured to be 1.2 × 10 ³ Ω/□, indicating electrical conductivity.

(Comparative Example 6)
An experiment was conducted in the same manner as in Comparative Example 4, except that 0.33 g of powder containing 90.9 mass% flaky carbon (containing 0.30 g of flaky carbon) was mixed with 0.388 g of bisphenol A diglycidyl ether and 0.282 g of a curing agent, hexahydrophthalic anhydride, to obtain a sheet containing 30 mass% flaky carbon. The thermal conductivity was measured and found to be 3.2 W/m K, which was lower than that of Example 4.

Comparisons between Example 1 and Comparative Examples 1 and 2, between Example 2 and Comparative Examples 3 and 4, between Example 3 and Comparative Example 5, and between Example 4 and Comparative Example 6 show that a combination of flaky carbon and cellulose nanofibers can produce a material with high thermal conductivity even with the same carbon content. Furthermore, comparisons between Example 2 and Comparative Example 4, and between Example 3 and Comparative Example 5 show that when the carbon content is low, it is easier to obtain a material with both insulation and high thermal conductivity even with the same carbon ratio.

Claims (6)

flaky carbon having a thickness of 1 to 100 nm;
Polyoxyethylene naphthyl ether ,
Nanofibers made of cellulose or a cellulose derivative and having a fiber diameter of 1 to 500 nm are contained,
A thermally conductive material in which the mass ratio of the flaky carbon to the nanofibers is 5/95 to 50/50 . The thermally conductive material according to claim 1 , wherein the nanofibers are cellulose nanofibers having a cellulose purity of 85% or more. 3. The thermally conductive material according to claim 1 , wherein the nanofibers are at least one type of nanofibers selected from the group consisting of methyl cellulose, carboxymethyl cellulose salts, hydroxyethyl cellulose, hydroxypropyl cellulose, hydroxyethyl methyl cellulose, and hydroxypropyl methyl cellulose. A paint containing the thermally conductive material according to any one of claims 1 to 3 , or a coating film formed from the paint. A sheet comprising the thermally conductive material according to any one of claims 1 to 3 . 4. The method for producing a thermally conductive material according to claim 1, comprising a step of mixing a flaky carbon dispersion or a wet solid of the flaky carbon dispersion, the flaky carbon dispersion containing 1 to 100 parts by mass of the polyoxyethylene naphthyl ether per 100 parts by mass of the flaky carbon, with a nanofiber dispersion or a wet solid of the nanofiber dispersion.