Nanodiamond aqueous dispersion

Abstract

Provided is a nanodiamond aqueous dispersion that achieves high dispersion of nanodiamond particles in water. This nanodiamond aqueous dispersion includes water and nanodiamond particles that are dispersed in the water. The nanodiamond aqueous dispersion contains no more than 1 part by mass of heavy metals per 100 parts by mass of the nanodiamond particles, and the horizontal projection area of the nanodiamond particles as calculated from a low-temperature electron microscope image satisfies at least one condition selected from among (a)–(d). (a) The percentage 100 nm2 or greater is no more than 20%. (b) The percentage 80 nm2 or greater is no more than 24%. (c) The percentage 50 nm2 or greater is no more than 30%. (d) The percentage 30 nm2 or greater is no more than 39%.

Description

[Technical Field]

[0001] This disclosure relates to a nanodiamond aqueous dispersion comprising water and nanodiamond particles dispersed in water. This application claims priority to Japanese Patent Application No. 2021-135080, filed in Japan on August 20, 2021, the contents of which are incorporated herein by reference. [Background technology]

[0002] Nano-sized micromaterials, such as nanocarbon materials, are known to possess novel properties that cannot be exhibited in their bulk state. For example, among nanocarbon materials, graphite (a layered structure of graphene) is a two-dimensional material with a large specific surface area that exhibits excellent conductivity and sensitivity to small molecules, and has been used in a variety of electrode materials. However, nanocarbon materials generally have a large proportion of surface atoms, so the sum of van der Waals forces that can act between the surface atoms of adjacent particles is large, and they are known to be prone to aggregation. For this reason, it has been extremely difficult to stably disperse nanodiamond particles in dispersion media such as water or organic solvents.

[0003] In recent years, methods have been reported for introducing modifying groups containing polyglycerin chains onto the surface of nanodiamond particles with the aim of suppressing aggregation of nanodiamonds and improving their dispersibility in water (Patent Documents 1 and 2). [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2010-248023 [Patent Document 2] Japanese Patent Publication No. 2012-82103 [Overview of the project] [Problems that the invention aims to solve]

[0005] The DLS method is known for evaluating the dispersibility of nanodiamond aqueous dispersions. When conventional nanodiamond aqueous dispersions were evaluated using the DLS method, results sometimes indicated that the dispersed nanodiamond particles were small in size and exhibited high dispersibility. However, even in such nanodiamond aqueous dispersions, clustering and chaining of nanodiamond particles were sometimes observed. Therefore, there is a need for nanodiamond aqueous dispersions with even higher dispersion of nanodiamond particles than conventional ones.

[0006] Therefore, the object of this disclosure is to provide a nanodiamond aqueous dispersion that can contain nanodiamond particles in a highly dispersed state in water. [Means for solving the problem]

[0007] In other words, this disclosure relates to a nanodiamond aqueous dispersion comprising water and nanodiamond particles dispersed in water, The heavy metal content is 1 part by mass or less per 100 parts by mass of nanodiamond particles. The present invention provides a nanodiamond aqueous dispersion in which the horizontal projected area of ​​nanodiamond particles, calculated from analytical images obtained by cryo-electron microscopy, satisfies at least one condition selected from the group consisting of (a) to (d) below. (a) 100nm 2 The above percentage must be 20% or less. (b) 80nm 2 The percentage above must be 24% or less. (c) 50nm 2 The above percentage must be 30% or less. (d) 30nm 2 The percentage above is 39% or less.

[0008] Preferably, the aforementioned nanodiamond aqueous dispersion further contains a metal salt.

[0009] The aforementioned metal salt is preferably a light metal salt.

[0010] The aforementioned metal salt is preferably an alkali metal salt.

[0011] The nanodiamond particles preferably have surface-modifying groups that include hydrophilic polymer chains.

[0012] The hydrophilic polymer chain is preferably a polyglycerin chain.

[0013] The zeta potential of the nanodiamond particles is preferably -12 to +20 mV.

[0014] Preferably, the zeta potential of the nanodiamond particles is greater than 0mV and less than or equal to +20mV. [Effects of the Invention]

[0015] The nanodiamond aqueous dispersion of this disclosure allows nanodiamond particles to exist stably in water in a highly dispersed state. Therefore, even when the dispersibility of the nanodiamond aqueous dispersion of this disclosure is evaluated by cryo-electron microscopy, nanodiamond particles with high dispersibility, without clustering or chaining, can be stably present in water. [Brief explanation of the drawing]

[0016] [Figure 1] This figure shows (a) a TEM image and (b) a binarized image of nanodiamond particles in the nanodiamond aqueous dispersion of Example 1, obtained by cryo-electron microscopy. [Figure 2] This figure shows the distribution of the horizontal projected area (particle size distribution) of nanodiamond particles in the nanodiamond aqueous dispersion of Example 1, as measured by cryo-electron microscopy. [Figure 3] This figure shows (a) a TEM image and (b) a binarized image of nanodiamond particles in the nanodiamond aqueous dispersion of Comparative Example 1, obtained by cryo-electron microscopy. [Figure 4]This figure shows the distribution of the horizontal projected area (particle size distribution) of nanodiamond particles in the nanodiamond aqueous dispersion of Comparative Example 1, as measured by cryo-electron microscopy. [Figure 5] This figure shows (a) a TEM image and (b) a binarized image of nanodiamond particles in the nanodiamond aqueous dispersion of Comparative Example 2, obtained by cryo-electron microscopy. [Figure 6] This figure shows the distribution of the horizontal projected area (particle size distribution) of nanodiamond particles in the nanodiamond aqueous dispersion of Comparative Example 2, as measured by cryo-electron microscopy. [Modes for carrying out the invention]

[0017] [Nanodiamond aqueous dispersion] This disclosure relates to a nanodiamond aqueous dispersion comprising water and nanodiamond particles dispersed in water, The heavy metal content is 1 part by mass or less per 100 parts by mass of nanodiamond particles. The nanodiamond aqueous dispersion is such that the horizontal projected area of ​​the nanodiamond particles, calculated from the analysis image by cryo-electron microscopy, satisfies at least one condition selected from the group consisting of (a) to (d) below. (a) 100nm 2 The above percentage must be 20% or less. (b) 80nm 2 The percentage above must be 24% or less. (c) 50nm 2 The above percentage must be 30% or less. (d) 30nm 2 The percentage above is 39% or less.

[0018] The nanodiamond aqueous dispersion of this disclosure can stably contain highly dispersible nanodiamond particles in water, such that clusters and chains of nanodiamond particles do not exist in the water, by ensuring that the horizontal projected area of ​​the nanodiamond particles calculated from analytical images by cryo-electron microscopy satisfies at least one selected from the group consisting of (a) to (d) above.

[0019] From the perspective of imparting high dispersibility to the nanodiamond particles, the nanodiamond aqueous dispersion of the present disclosure preferably satisfies at least two conditions selected from the group consisting of the above (a) to (d) (for example, (a) and (b), (a) and (c), (a) and (d), (b) and (c), (b) and (d), (c) and (d)), more preferably satisfies at least three conditions selected from the group consisting of the above (a) to (d) (for example, (a), (b), and (c); (a), (b), and (d); (b), (c), and (d)), and even more preferably satisfies all the conditions of the above (a) to (d).

[0020] Regarding the above (a), if the proportion of nanodiamond particles with a horizontal projected area of 100 nm 2 or more is not particularly limited as long as it is 20% or less. For example, it is preferably 15% or less, more preferably 12% or less, even more preferably 10% or less, still more preferably 5% or less, and particularly preferably 2% or less. Regarding the above (b), if the proportion of nanodiamond particles with a horizontal projected area of 80 nm 2 or more is not particularly limited as long as it is 24% or less. For example, it is preferably 16% or less, more preferably 12% or less, even more preferably 8% or less, and particularly preferably 4% or less. Regarding the above (c), if the proportion of nanodiamond particles with a horizontal projected area of 50 nm 2 or more is not particularly limited as long as it is 30% or less. For example, it is preferably 20% or less, more preferably 15% or less, even more preferably 10% or less, and particularly preferably 7% or less. Regarding the above (d), if the proportion of nanodiamond particles with a horizontal projected area of 30 nm 2 or more is not particularly limited as long as it is 39% or less. For example, it is preferably 35% or less, more preferably 30% or less, even more preferably 25% or less, and particularly preferably 20% or less. When the above (a) to (d) are within the above ranges, there is a tendency to stably have nanodiamond particles with high dispersibility present in water.

[0021] The content of nanodiamond particles in the nanodiamond aqueous dispersion of this disclosure is not particularly limited, but is preferably 0.1 ppm to 10% by mass, more preferably 0.5 ppm to 5% by mass, and even more preferably 1.0 ppm to 3% by mass. The nanodiamond particle content can be calculated from the absorbance at 350 nm. If the nanodiamond particle content is low (for example, 2000 ppm or less by mass), the content can also be determined by detecting compounds that surface-modify the nanodiamond particles using inductively coupled plasma emission spectroscopy (ICP emission spectroscopy) and basing the calculation on the detected amount.

[0022] The nanodiamond aqueous dispersion of this disclosure contains nanodiamond particles in a highly dispersed state. The median diameter (particle size D50) of the nanodiamond particles in the nanodiamond aqueous dispersion of this disclosure is, for example, 100 nm or less, preferably 60 nm or less, more preferably 50 nm or less, and even more preferably 30 nm or less. The lower limit of the median diameter of the nanodiamond particles is, for example, 5 nm.

[0023] In the nanodiamond aqueous dispersion of this disclosure, the zeta potential of the nanodiamond particles is preferably, for example, -15 to +20 mV, more preferably -12 to +20 mV, even more preferably -10 to +15 mV, and particularly preferably -5 to +10 mV. Having the zeta potential of the nanodiamond particles within this range tends to allow highly dispersible nanodiamond particles to exist stably in water. In particular, when the zeta potential of the nanodiamond particles is positive, more specifically, when it is greater than 0 mV and less than or equal to +20 mV, the nanodiamond particles tend to exist in a highly dispersed state in the nanodiamond aqueous dispersion of this disclosure. Conventionally, it was thought that a high zeta potential in nanodiamond aqueous dispersions led to high dispersion of nanodiamond particles. However, surprisingly, it has been found that even when the zeta potential is close to 0, the nanodiamond particles do not aggregate and exhibit high dispersibility. This is thought to be due to surface modification of the nanodiamond particles. The zeta potential of nanodiamond particles can be measured, for example, by laser Doppler electrophoresis.

[0024] (Nanodiamond particles) In the nanodiamond aqueous dispersion of the present disclosure, the nanodiamond particles preferably include primary nanodiamond particles. Furthermore, it may also include secondary particles formed by the aggregation (adhesion) of multiple primary particles.

[0025] As the nanodiamond particles, for example, detonation-produced nanodiamonds (i.e., nanodiamonds produced by the detonation method) or high-temperature, high-pressure-produced nanodiamonds (i.e., nanodiamonds produced by the high-temperature, high-pressure-produced method) can be used. Among these, detonation-produced nanodiamonds are preferred because they have superior dispersibility in the dispersion medium, that is, the particle size of the primary particles is an order of magnitude nanometers.

[0026] The aforementioned detonation-processed nanodiamonds include air-cooled detonation-processed nanodiamonds (i.e., nanodiamonds produced by the air-cooled detonation method) and water-cooled detonation-processed nanodiamonds (i.e., nanodiamonds produced by the water-cooled detonation method). Among these, air-cooled detonation-processed nanodiamonds are preferred over water-cooled detonation-processed nanodiamonds because their primary particles are smaller.

[0027] The nanodiamond particles may be unmodified nanodiamond particles or nanodiamond particles equipped with surface-modifying groups (sometimes referred to as "surface-modified nanodiamonds (particles)"). When the nanodiamond particles are surface-modified nanodiamond particles, it is preferable that they are equipped with surface-modifying groups containing hydrophilic polymer chains from the viewpoint of improving the dispersibility of the nanodiamond particles in water. In addition, the nanodiamond particles may have one or more functional groups other than surface-modifying groups containing hydrophilic polymer chains. Examples of such functional groups include amino groups, hydroxyl groups, and carboxyl groups.

[0028] Examples of the hydrophilic polymer chain include polymer chains containing structural units derived from monomers having hydrophilic groups, such as polyether chains (polyethylene oxide, polypropylene oxide, and copolymers thereof) and polyglycerin chains (C3H6O(CH2CH(OH)CH2O)nH). Among these, polyglycerin chains are preferred from the viewpoint of dispersion stability in water. In other words, the hydrophilic polymer chain is preferably a polyglycerin chain. The nanodiamond particles of this disclosure may contain only one type of hydrophilic polymer chain as a surface modifying group, or they may contain two or more types.

[0029] The polyglycerin chain is preferably a polyglycerin chain represented by the following formula (1). -(X 1 C3H5)-(OC3H5)p-(X 2 R 1 )q (1) [In equation (1), p represents an integer greater than or equal to 1, and q represents an integer satisfying q = p + 2. X 1 This indicates a divalent group, [X 1 X in C3H5] 1 The bonding hand extending to the left from this point binds to the nanodiamond particles. [X 2 R 1 ] indicates the end of the polyglycerol chain, X 2 R indicates a single bond or a divalent group. 1 [This represents a hydrogen atom or a monovalent organic group.]

[0030] In the above equation (1) [X 1 C3H5] is [-X 1 It is represented as [-CH2-C(-)H-CH2-]. The above [X 1 X in C3H5] 1 The two carbon atoms are bonded to the nanodiamond particles, and the two carbon atoms are each either the oxygen or the Xyzite in [OC3H5]. 2 R 1 ] X inside 2 It joins to the aforementioned [X 1 X in C3H5] 1 This indicates a divalent group.

[0031] X 1 The divalent group in this is, for example, an amino group (-NR a -), amide group (-NR a- C(=O)-), ether bond (-O-), ester bond (-OC(=O)-), phosphinic acid group (-PH(=O)O-), phosphonic acid group (-P(-OH)(=O)O-), phosphate ester (-OP(=O)(OH)-O-), sulfide bond (-S-), carbonyl group (-C(=O)-), urethane bond (-R a NC(=O)-O-), imide bond(-C(=O)-NR aExamples include -C(=O)-, thiocarbonyl group (-C(=S)-), siloxane bond (-Si-O-), sulfate ester group (-OS(=O)2-O-), sulfonyl group (-S(=O)2-O-), sulfone group (-S(=O)2-), sulfoxide (-S(=O)-), and groups in which two or more of these are bonded. In the case of asymmetric divalent groups, the orientation of the divalent group relative to the nanodiamond particle side and the R side is not particularly limited. a This represents a hydrogen atom or a monovalent organic group. Among the divalent groups, -NR a -, -O-, -C(=O)O-, -NR a- C(=O)-, -PH(=O)O-, and -S- are preferred, and more preferably -NR a -, -O-, -NR a- It is C(=O)- or -C(=O)O-.

[0032] In formula (1) above, the [OC3H5] with p attached is a structure derived from glycerin represented as [-O-CH2-C(-)H-CH2-], and [X 1 It forms a polyglycerin chain with [C3H5]. p represents the repeating unit of [OC3H5] and is an integer of 1 or more, preferably 3 to 2000, more preferably 5 to 500, and even more preferably 10 to 200. p may be the same or different in the group containing the plurality of polyglycerin chains.

[0033] In the above formula (1), [X 2 R 1 ] indicates the end of the polyglycerin chain, and [X 1 It binds to the C in [C3H5] or the C in [OC3H5]. 1 This represents a hydrogen atom or a monovalent organic group.

[0034] The aforementioned [X 2 R 1 ] X inside 2 This indicates a single bond or a divalent group. The divalent group is as described above [X 1 X in C3H5] 1 The divalent group is given as an example and explained. Also, X 2 In Ra and R 1 It may also be bonded to form a ring. 2 Among them, -NR a -, -NR a C(=O)-, -O-, -C(=O)O-, -OC(=O)-, -PH(=O)O-, -OP(=O)(OH)-O-, -S-, -OS(=O)2-O-, -OS(=O)2- are preferred, and more preferably -NR a -or -O-. The above [X 1 X in C3H5] 1 and the aforementioned [X 2 R 1 ] X inside 2 These may be the same or different. The aforementioned multiple [X 2 R 1 ] may be the same or different. Also, the above [X 2 R 1 ] X inside 2 In the group containing the multiple polyglycerin chains, q may be the same or different. q is an integer greater than or equal to 3, and its value depends on the value of p, satisfying q = p + 2. q may be the same or different in the group containing the multiple polyglycerin chains.

[0035] R 1 Examples of the monovalent organic group in R include substituted or unsubstituted hydrocarbon groups (monovalent hydrocarbon groups, particularly monovalent aliphatic or aromatic hydrocarbon groups), substituted or unsubstituted heterocyclic groups (monovalent heterocyclic groups), and groups in which two or more of the monovalent hydrocarbon groups and / or the monovalent heterocyclic groups are bonded. The bonded groups may be directly bonded or bonded via linking groups. Examples of the linking groups include amino groups, ether bonds, ester bonds, phosphinic acid groups, sulfide bonds, carbonyl groups, organic group-substituted amide groups, organic group-substituted urethane bonds, organic group-substituted imide bonds, thiocarbonyl groups, siloxane bonds, and groups in which two or more of these are bonded. 1 If is a monovalent organic group, then the multiple R in formula (1) 1 They may be the same or they may be different.

[0036] R a The monovalent organic group in is the R 1 Examples of monovalent organic groups in this context include those described and illustrated. Among these, examples include substituted or unsubstituted hydrocarbon groups (monovalent hydrocarbon groups), substituted or unsubstituted heterocyclic groups (monovalent heterocyclic groups), and groups formed by the bonding of two or more of these groups. Furthermore, the monovalent organic group may have an ionic form. The bonded groups may be directly bonded or bonded via a linking group. The hydrocarbon group in the substituted or unsubstituted hydrocarbon group is preferably an alkyl group, more preferably an alkyl group having 1 to 18 carbon atoms, even more preferably an alkyl group having 1 to 6 carbon atoms, and particularly preferably an ethyl group, a propyl group, a butyl group, or a hexyl group.

[0037] [X 2 R 1 Specific examples of [X] include OH, NH2, CH3, alkoxy groups, acyl groups, mono or dialkylamino groups, mono or dialkenylamino groups, alkylamide groups, alkenylamide groups, quaternary ammonium-substituted alkoxy groups, chlorine-substituted alkoxy groups, polyalkylene oxide groups, cyclic imide groups, carboxyl-substituted alkylamino groups, carboxyl-substituted alkyloxy groups, etc. Two or more of the above [X] 2 R 1 ] is R 1 A ring may be formed via the hydroxyl group. Among these, OH, cyclic imide groups, and carboxyl-substituted alkylamino groups are preferred from the viewpoint of superior water dispersibility of surface-modified nanodiamonds.

[0038] The number-average degree of polymerization of glycerin in the polyglycerin chain is 3 to 2000, more preferably 5 to 500, and even more preferably 10 to 200. A higher number-average degree of polymerization allows sufficient repulsive force between nanodiamonds to act, further improving the dispersibility of nanodiamond particles. A number-average degree of polymerization of 5000 or less suppresses entanglement between polyglycerin chains among nanodiamonds, further improving the dispersibility of nanodiamond particles in water. The number-average degree of polymerization is defined by the number of glycidol units constituting the polyglycerin chain in the group bonded to surface functional group 1 of the raw material nanodiamond. The number of surface functional groups of the raw material nanodiamond can be determined by measuring the elemental analysis value of the raw material nanodiamond, measuring the acid value, or a combination of both.

[0039] When the nanodiamond particles have surface-modifying groups, the mass ratio of nanodiamond particles to surface-modifying groups [nanodiamond particles / surface-modifying groups] is not particularly limited, but is preferably 0.5 to 1.0, and more preferably 0.6 to 0.8. When the mass ratio is 0.5 or higher (particularly 0.6 or higher), the properties as a nanodiamond material are less likely to be impaired. When the mass ratio is 1.0 or lower (particularly 0.8 or lower), the degree of modification of the surface-modifying groups becomes sufficient, resulting in superior dispersibility in water. The mass ratio is determined based on the weight loss rate measured by thermogravimetric analysis, with the weight loss being used as the mass of the polyglycerin chain-containing surface-modifying groups.

[0040] If the nanodiamond particles have surface-modifying groups including polyglycerin chains, the nanodiamond particles can be obtained by directly ring-opening polymerization of glycidol onto the nanodiamond particles. Nanodiamond particles inherently have carboxyl groups and hydroxyl groups generated during the manufacturing process, and by reacting these functional groups with glycidol, the surface of the nanodiamonds can be modified with polyglycerin chains.

[0041] The reaction between nanodiamond particles and glycidol (ring-opening polymerization) can be carried out, for example, by adding glycidol and a catalyst to nanodiamond particles under an inert gas atmosphere and heating to 50-100°C. Both acidic and basic catalysts can be used. Examples of acidic catalysts include trifluoroboron etherate, acetic acid, and phosphoric acid. Examples of basic catalysts include triethylamine, pyridine, dimethylaminopyridine, and triphenylphosphine.

[0042] For information on the ring-opening polymerization conditions of glycidol, refer to SRSandler et al.'s J.Polym.Sci.,Polym.Chem.Ed., Vol.4,1253 (1966), EJ Vanderberg's J.Polym.Sci.,Polym.Chem.Ed., vol.23,915 (1985), and GR Newcome et al.'s Dendritic Macromolecules: Concepts, Syntheses, Perspectives, VCH, Weinheim (1996), as appropriate.

[0043] Surface-modified nanodiamonds can also be obtained by ring-opening polymerization of glycidol onto nanodiamond particles to which a functional group containing active hydrogen has been introduced on the surface. The functional group containing active hydrogen is not particularly limited, but examples include amino groups, hydroxyl groups, carboxyl groups, mercapto groups (thiol groups), and phosphinic acid groups. For methods of introducing the functional group containing active hydrogen into nanodiamond particles, refer to Japanese Patent Publication No. 2012-82103, Japanese Patent Publication No. 2010-248023, etc. Ring-opening polymerization of glycidol onto nanodiamond particles to which a functional group containing active hydrogen has been introduced on the surface can be carried out in the same manner as the ring-opening polymerization of glycidol onto nanodiamond particles described above.

[0044] (Metal salts) The nanodiamond aqueous dispersion of this disclosure is not particularly limited as long as it contains water and nanodiamond particles dispersed in water, but may further contain a metal salt (e.g., a light metal salt and / or a heavy metal salt). The metal salt may be used alone or in a mixture of two or more types.

[0045] The metal elements constituting the metal salt are not particularly limited, but include, for example, light metal elements such as aluminum, magnesium, beryllium, alkali metals, and alkaline earth metals; and heavy metal elements such as iron, lead, gold, silver, copper, chromium, zinc, manganese, nickel, molybdenum, and tin. Specifically, the metal salts include sulfides, hydroxides, halides (fluorides, chlorides, bromides, iodides, etc.), sulfates, and organic acid salts (acetates, etc.) containing the above-mentioned metal elements. Among these, light metal salts are preferred, alkali metal salts and alkaline earth metal salts are more preferred, and alkali metal salts are even more preferred.

[0046] Examples of the alkali metal salts include sodium chloride, potassium chloride, sodium nitrate, potassium nitrate, and sodium acetate, with sodium chloride being particularly preferred from the viewpoint of dispersion stability in water and ease of handling.

[0047] The concentration of the metal salt is not particularly limited, but for example, 1.0 × 10 -6 Preferably, it is ~2.0M, and more preferably 5.0×10 -5 ~1.0M, more preferably 1.0×10 -5 ~5.0×10 -1 M is the concentration of the metal salt. When the concentration of the metal salt is within the above range, the dispersion stability in water is improved, and the dispersibility of the nanodiamond particles tends to improve to the extent that clusters and chains are not observed even when the dispersibility is evaluated by cryo-electron microscopy. However, the heavy metal content in the nanodiamond aqueous dispersion of this disclosure is 1 part by mass or less per 100 parts by mass of nanodiamond particles, preferably 0.1 parts by mass or less, and more preferably 0.01 parts by mass or less.

[0048] (Other ingredients) The nanodiamond aqueous dispersion of this disclosure may further contain components other than water, nanodiamond particles, and light metal salts. Examples of such components (hereinafter sometimes referred to as "other components") include dispersion media other than water, surfactants, thickeners, coupling agents, dispersants, rust inhibitors, corrosion inhibitors, freezing point depressants, defoaming agents, wear-resistant additives, preservatives, and colorants.

[0049] Examples of dispersion media other than water include organic solvents and ionic liquids. One type of dispersion medium may be used, or two or more types may be used. Examples of organic solvents include aliphatic hydrocarbons (especially linear saturated aliphatic hydrocarbons) such as hexane, heptane, and octane; aromatic hydrocarbons such as benzene, toluene, and xylene; alicyclic hydrocarbons such as cyclohexane and methylcyclohexane; aprotic polar solvents such as dimethylformamide (DMF), dimethylacetamide, N-methylpyrrolidone, and dimethyl sulfoxide; alcohols such as methanol; halogenated hydrocarbons such as chloroform, dichloromethane, dichloroethane, carbon tetrachloride, chlorobenzene, and trifluoromethylbenzene; linear or cyclic ethers such as diethyl ether, diisopropyl ether, dimethoxyethane, tetrahydrofuran (THF), and dioxane; esters such as ethyl acetate and butyl acetate; linear ketones such as methyl ethyl ketone (MEK) and methyl isobutyl ketone; and nitriles such as acetonitrile.

[0050] In the nanodiamond aqueous dispersion of this disclosure, the water content relative to 100% by mass of the dispersion medium is not particularly limited, but is preferably 90% by mass or more, more preferably 95% by mass or more, even more preferably 99% by mass or more, and particularly preferably 99.9% by mass or more.

[0051] DLS (Deep Loss Spectroscopy) and cryo-electron microscopy are known methods for evaluating the dispersibility of nanodiamond aqueous dispersions. When conventional nanodiamond aqueous dispersions were evaluated using DLS, the results sometimes indicated that the dispersed nanodiamond particles were small in size and exhibited high dispersibility. However, when the dispersibility of conventional nanodiamond aqueous dispersions was evaluated using cryo-electron microscopy, clustering and chain formation of nanodiamond particles were sometimes observed. In other words, evaluation using cryo-electron microscopy suggests that while conventional nanodiamond aqueous dispersions appeared to have high dispersibility using DLS, they may not actually possess the desired high dispersibility.

[0052] Although the reason why the evaluation of nanodiamond particle dispersibility differs between DLS and cryo-electron microscopy is unclear, cryo-electron microscopy can be said to have high reliability from the perspective of evaluating the dispersibility of nanodiamond particles, and therefore, the water dispersibility of nanodiamond particles in conventional nanodiamond aqueous dispersions is insufficient. The nanodiamond aqueous dispersion of this disclosure does not exhibit clustering or chaining of nanodiamond particles even when evaluated by cryo-electron microscopy, and it is possible to have nanodiamond particles exist in a highly dispersed state in water.

[0053] The nanodiamond aqueous dispersion of this disclosure can be preferably used as an additive to composite materials, for example, to impart properties of fine nanodiamond particles (e.g., mechanical strength, high refractive index, thermal conductivity, insulating properties, antioxidant properties, crystallization promoting effect, dendrite suppression effect, etc.) to resins (e.g., thermal or photocurable resins, thermoplastic resins, etc.). Furthermore, compositions obtained by adding the nanodiamond aqueous dispersion of this disclosure to a resin can be preferably used as materials for functional hybrid materials, thermal functional materials (heat resistance, heat storage, thermal conductivity, heat insulation, etc.), photonics (organic EL elements, LEDs, liquid crystal displays, optical discs, etc.), bio- and biocompatible materials, coating materials, film materials (hard coat films for touch panels and various displays, heat shielding films, etc.), sheet materials, screen materials (transmissive transparent screens, etc.), filler materials (heat dissipation fillers, mechanical property improvement fillers, etc.), heat-resistant plastic substrate materials (substrates for flexible displays, etc.), lithium-ion batteries, and the like. Furthermore, the nanodiamond aqueous dispersion of this disclosure can also be used in other applications, such as medical applications, and as a friction reducer or lubricant for sliding parts of mechanical components (e.g., automobiles and aircraft).

[0054] Each embodiment disclosed herein can be combined with any other features disclosed herein. Each configuration and combination thereof in each embodiment is an example, and additions, omissions, substitutions, and other modifications are permitted as appropriate, without departing from the spirit of this disclosure. Furthermore, each invention relating to this disclosure is not limited by the embodiments or the following examples, but is limited solely by the claims. [Examples]

[0055] An embodiment of this disclosure will be described in more detail below based on examples.

[0056] [Example 1] A nanodiamond aqueous dispersion was manufactured through the following process.

[0057] (Nanodiamond fabrication) First, the process of producing nanodiamonds using the detonation method was carried out. In this process, the molded explosives, with electric detonators attached, were first placed inside a pressure-resistant container for detonation, and the container was sealed. The container was made of iron and had a volume of 15 m³. 3 The following was done. 0.50 kg of a mixture of TNT and RDX was used as the explosive. The mass ratio of TNT to RDX in this explosive (TNT / RDX) was 50 / 50. Next, an electric detonator was detonated, causing the explosive to detonate inside the container (nanodiamond generation by detonation). Then, the container and its interior were allowed to cool down by leaving it at room temperature for 24 hours. After this cooling, the crude nanodiamond product (containing aggregates of nanodiamond particles and soot generated by the above detonation method) adhering to the inner wall of the container was scraped off with a spatula, and the crude nanodiamond product was recovered.

[0058] The crude nanodiamond product obtained by performing the above-described generation process multiple times was then subjected to an acid treatment process. Specifically, a slurry obtained by adding 6 L of 10% hydrochloric acid to 200 g of the crude nanodiamond product was heat-treated for 1 hour under reflux conditions at atmospheric pressure. The heating temperature in this acid treatment was 85-100°C. Next, after cooling, the solid components (including nanodiamond aggregates and soot) were washed with water by decantation. The washing of the solid components by decantation was repeated until the pH of the precipitate liquid went from a low pH to 2.

[0059] Next, an oxidation treatment process was carried out. Specifically, 6 L of 98% by mass sulfuric acid and 1 L of 69% by mass nitric acid were added to the precipitate (containing nanodiamond adsorbents) obtained after decantation following the acid treatment to form a slurry. This slurry was then heat-treated under reflux at atmospheric pressure for 48 hours. The heating temperature during this oxidation treatment was 140-160°C. After cooling, the solid components (containing nanodiamond adsorbents) were washed with water by decantation. Initially, the supernatant was colored, and the washing of the solid components by decantation was repeated until the supernatant was visibly clear.

[0060] Next, the precipitate (liquid containing nanodiamond aggregates) obtained after the aforementioned washing process was subjected to a drying process to obtain a dried powder (nanodiamond aggregates). For the drying process, evaporation to dryness using an evaporator was employed.

[0061] Next, 4.5 g of the dried powder (nanodiamond aggregate) obtained through the above drying process was placed in the furnace tube of a gas atmosphere furnace (product name "Gas Atmosphere Tube Furnace KTF045N1", manufactured by Koyo Thermo Systems Co., Ltd.). Nitrogen gas was continuously flowed through the furnace tube at a flow rate of 1 L / min for 30 minutes. Then, the flowing gas was switched from nitrogen to a mixed gas of oxygen and nitrogen, and this mixed gas was continuously flowed through the furnace tube at a flow rate of 1 L / min. The oxygen concentration in the mixed gas was 4 volume percent. After switching to the mixed gas, the furnace temperature was raised to the heating set temperature of 400°C. The heating rate was set at 10°C / min until 380°C, which is 20°C lower than the heating set temperature, and then at 1°C / min from 380°C to 400°C. Then, while maintaining the temperature condition inside the furnace at 400°C, the nanodiamond powder inside the furnace was subjected to oxygen oxidation treatment. The treatment time was 3 hours.

[0062] Next, the hydrogenation process was carried out using the gas atmosphere furnace described above. Specifically, nitrogen gas was continuously flowed through the gas atmosphere furnace, which contained the nanodiamond powder that had undergone the oxygen oxidation process, at a flow rate of 1 L / min for 30 minutes. Then, the flowing gas was switched from nitrogen to a mixture of hydrogen and nitrogen, and this mixture was continuously flowed through the furnace tube at a flow rate of 1 L / min. The hydrogen concentration in the mixed gas was 2 volume percent. After switching to the mixed gas, the furnace was heated to the set temperature of 600°C. The heating rate was 10°C / min. Then, while maintaining the temperature inside the furnace at 600°C, the nanodiamond powder inside the furnace was subjected to hydrogenation treatment. The treatment time was 5 hours. In this way, nanodiamond powder that had undergone hydrogenation treatment was obtained.

[0063] Next, a crushing process was carried out. Specifically, 0.9 g of nanodiamond powder that had undergone the hydrogenation process described above and 29.1 ml of pure water were first added to a 50 ml sample bottle and mixed to obtain approximately 30 ml of slurry. After adjusting the pH to 4 using 1 N hydrochloric acid, the slurry was subjected to ultrasonic treatment. For the ultrasonic treatment, an ultrasonic irradiator (product name "Ultrasonic Cleaner AS-3", manufactured by AS ONE) was used to irradiate the slurry with ultrasound for 2 hours. After this, bead milling was performed using a bead milling device (product name "Parallel Four-Cylinder Sand Grinder LSG-4U-2L", manufactured by AIMEX Corporation). Specifically, 30 ml of the ultrasonically irradiated slurry and 30 μm diameter zirconia beads were placed into a 100 ml milling container (Vessel, manufactured by AIMEX Corporation), sealed, and the device was driven to perform bead milling. In this bead milling process, the amount of zirconia beads added is, for example, 33% of the volume of the milling vessel, the rotation speed of the milling vessel is 2570 rpm, and the milling time is 2 hours.

[0064] Next, the slurry that had undergone the crushing process described above was subjected to centrifugation using a centrifuge (classification operation). The centrifugal force used in this centrifugation was 20,000 × g, and the centrifugation time was 10 minutes. Next, 10 ml of the supernatant of the nanodiamond-containing solution after the centrifugation was collected. In this way, a nanodiamond aqueous dispersion in which nanodiamonds were dispersed in pure water was obtained. The solid content concentration of this nanodiamond aqueous dispersion was 2.1% by mass, and the pH was 5.40. The median diameter (particle size D50) of the nanodiamond aqueous dispersion obtained as described above (the "nanodiamond aqueous dispersion" in Comparative Example 2 described later) was 5.0 nm.

[0065] Next, the nanodiamond aqueous dispersion obtained through the crushing process described above was dried using an evaporator to obtain a black dried powder. The obtained dried powder (100 mg) was added to 12 mL of glycidol in a glass reactor and dissolved by sonication in an ultrasonic cleaner (product name "BRANSON2510", manufactured by Marshall Scientific) at room temperature for 2 hours. This was reacted at 140°C for 20 hours with stirring under a nitrogen atmosphere. After cooling the reaction mixture, 120 mL of methanol was added, sonication was performed, and then the mixture was centrifuged at 50400 × g for 2 hours to obtain a precipitate. 120 mL of methanol was added to this precipitate, and the washing-centrifugation process was repeated 5 times. Finally, the precipitate was subjected to pure water dialysis using a dialysis membrane (Spectra / Prodialysis membrane, MWCO: 12-14 kDa), the residual methanol was replaced with water, and freeze-dried to obtain a gray powder of polyglycerin-modified nanodiamond particles (PG-ND particles). TG-DTA thermal analysis revealed that the ratio of nanodiamond particles to surface-modifying groups was 1:1.39.

[0066] To a 100 ml PG-ND particle aqueous dispersion, 1 ml of 1 M NaCl aqueous solution was added to achieve a final NaCl concentration of 0.01 M, and the mixture was sonicated for 10 minutes to obtain the desired nanodiamond aqueous dispersion. The nanodiamond particle content was 1% by mass. The nanodiamond particle content was calculated by drying the nanodiamond aqueous dispersion, recovering the solid content, and analyzing the solid content using TG-DTA. The zeta potential of the nanodiamond aqueous dispersion was 4.89 mV. The horizontal projected area of ​​this nanodiamond aqueous dispersion was calculated using cryo-electron microscopy (Cryo-TEM). Specifically, the nanodiamond aqueous dispersion was frozen with liquid nitrogen, and a TEM image was obtained using an electron microscope (Krios G4, Thermo Fisher Scientific) (Figure 1(a)). The obtained TEM images were analyzed using ImageJ (particle size analysis software), and the horizontal projection area (Figure 2) was calculated from the binarized image (Figure 1(b)). The results are summarized in Table 1.

[0067] [Comparative Example 1] For the nanodiamond aqueous dispersion prepared in the same manner as in Example 1, except that a 1M NaCl aqueous solution was not used, a TEM image (Figure 3(a)) was obtained by cryo-electron microscopy, and the horizontal projected area (Figure 4) was calculated from the binarized image (Figure 3(b)). The results are summarized in Table 1.

[0068] [Comparative Example 2] A nanodiamond aqueous dispersion (without polyglycerin modification) described in Example 1 was prepared, and a TEM image (Figure 5(a)) was obtained by cryo-electron microscopy. The horizontal projected area (Figure 6) was calculated from the binarized image (Figure 5(b)). The results are summarized in Table 1.

[0069] [Table 1]

[0070] <Particle size D50> The median diameter (particle size D50) of nanodiamond particles in the nanodiamond aqueous dispersions of the examples and comparative examples obtained as described above was measured from the particle size distribution of nanodiamonds obtained by dynamic light scattering. Specifically, the particle size distribution of nanodiamonds was measured by dynamic light scattering (non-contact backscattering) using a Malvern instrument (product name "Zetasizer Nano ZS"). As a result, the median diameter (particle size D50) of the nanodiamond particles in Example 1 was 26.95 nm.

[0071] In summary, the structure of this disclosure and its variations are described below. [1] A nanodiamond aqueous dispersion comprising water and nanodiamond particles dispersed in water, The heavy metal content is 1 part by mass or less per 100 parts by mass of nanodiamond particles. A nanodiamond aqueous dispersion in which the horizontal projected area of ​​nanodiamond particles calculated from analytical images obtained by cryo-electron microscopy satisfies at least one condition selected from the group consisting of (a) to (d) below. (a) 100nm 2 The above percentage must be 20% or less. (b) 80nm 2 The percentage above must be 24% or less. (c) 50nm 2 The above percentage must be 30% or less. (d) 30nm 2 The percentage above is 39% or less. [2] The nanodiamond aqueous dispersion described in [1] that satisfies the conditions of (a) and (b) above; the conditions of (a) and (c); the conditions of (a) and (d); the conditions of (b) and (c); the conditions of (a), (b), and (c); the conditions of (a), (b), and (d); the conditions of (b), (c), and (d); or the conditions of (a), (b), (c), and (d). [3] In (a) above, the horizontal projection area is 100 nm 2The proportion of the above-mentioned nanodiamond particles is 15% or less, 12% or less, 10% or less, 5% or less, or 2% or less, and the nanodiamond aqueous dispersion described in [1] or [2]. [4] In the above (b), the horizontal projected area is 80 nm 2 The proportion of the above-mentioned nanodiamond particles is 16% or less, 12% or less, 8% or less, or 4% or less, and the nanodiamond aqueous dispersion described in any one of [1] to [3]. [5] In the above (c), the horizontal projected area is 50 nm 2 The proportion of the above-mentioned nanodiamond particles is 20% or less, 15% or less, 10% or less, or 7% or less, and the nanodiamond aqueous dispersion described in any one of [1] to [4]. [6] In the above (d), the horizontal projected area is 30 nm 2 The proportion of the above-mentioned nanodiamond particles is 35% or less, 30% or less, 25% or less, or 20% or less, and the nanodiamond aqueous dispersion described in any one of [1] to [5]. [7] The content of the nanodiamond particles is 0.1 mass ppm to 10 mass, 0.5 mass ppm to 5 mass%, or 1.0 mass ppm to 3 mass%, and the nanodiamond aqueous dispersion described in any one of [1] to [6]. [8] The median diameter (particle size D50) of the above-mentioned nanodiamond particles is 100 nm or less, 60 nm or less, 50 nm or less, or 30 nm or less, or the lower limit thereof is 5 nm, and the nanodiamond aqueous dispersion described in any one of [1] to [7]. [9] The zeta potential of the above-mentioned nanodiamond particles is -15 to +20 mV, -12 to +20 mV, -10 to +15 mV, or -5 to +10 mV, and the nanodiamond aqueous dispersion described in any one of [1] to [8].

[10] The zeta potential of the above-mentioned nanodiamond particles exceeds 0 mV and is 20 mV or less, and the nanodiamond aqueous dispersion described in any one of [1] to [9].

[11] The above-mentioned nanodiamond particles include primary particles of nanodiamond particles, or the above-mentioned primary particles include secondary particles aggregated (agglomerated) by a plurality of them, and the nanodiamond aqueous dispersion described in any one of [1] to

[10] .

[12] The nanodiamond aqueous dispersion according to any one of [1] to

[11] , wherein the nanodiamond particles are detonation method nanodiamonds.

[13] The nanodiamond aqueous dispersion according to

[12] , wherein the detonation method nanodiamond is an air-cooled detonation method nanodiamond or a water-cooled detonation method nanodiamond.

[14] The nanodiamond aqueous dispersion according to any one of [1] to

[13] , wherein the nanodiamond particles are unmodified nanodiamond particles or nanodiamond particles having a surface modification group (surface-modified nanodiamonds).

[15] The nanodiamond aqueous dispersion according to any one of [1] to

[14] , wherein the nanodiamond particles have a surface modification group containing a hydrophilic polymer chain.

[16] The nanodiamond aqueous dispersion according to

[15] , wherein the hydrophilic polymer chain is a polyether chain (such as polyethylene oxide, polypropylene oxide, or a copolymer thereof) or a polyglycerol chain (such as C3H6O(CH2CH(OH)CH2O)n-H).

[17] The nanodiamond aqueous dispersion according to

[16] , wherein the polyglycerol chain is a polyglycerol chain represented by the following formula (1). -(X 1 C3H5)-(OC3H5)p-(X 2 R 1 )q (1) [In formula (1), p represents an integer of 1 or more, and q represents an integer satisfying q = p + 2. X 1 represents a divalent group, and the bond extending from X 1 in [X 1 C3H5] to the left is bonded to the nanodiamond particles. [X 2 R 1 represents the end of the polyglycerol chain, X 2 represents a single bond or a divalent group, and R 1 represents a hydrogen atom or a monovalent organic group.]

[18] The nanodiamond aqueous dispersion according to

[16] or

[17] , wherein the number average degree of polymerization of glycerol in the polyglycerol chain is 3 to 2000, 5 to 500, or 10 to 200.

[19] If the nanodiamond particles have a surface modifying group, the nanodiamond aqueous dispersion according to any one of [1] to

[18] , wherein the mass ratio of nanodiamond particles to surface modifying groups [nanodiamond particles / surface modifying group] is 0.5 to 1.0 or 0.6 to 0.8.

[20] The nanodiamond aqueous dispersion according to any one of [1] to

[19] , wherein the nanodiamond particles have a surface modifying group including a polyglycerin chain, and the nanodiamond particles can be obtained by directly ring-opening polymerization of glycidol onto the nanodiamond particles.

[21] Furthermore, a nanodiamond aqueous dispersion according to any one of [1] to

[20] , which contains a metal salt.

[22] The nanodiamond aqueous dispersion according to

[21] , wherein the metal elements constituting the metal salt are at least one light metal element selected from aluminum, magnesium, beryllium, alkali metals, and alkaline earth metals, or at least one heavy metal element selected from iron, lead, gold, silver, copper, chromium, zinc, manganese, nickel, molybdenum, and tin.

[23] The nanodiamond aqueous dispersion according to

[22] , wherein the metal salt is at least one selected from sulfides, hydroxides, halides (such as fluorides, chlorides, bromides, and iodides), sulfates, and organic acid salts (such as acetates) containing the metal element.

[24] A nanodiamond aqueous dispersion according to any one of

[21] to

[23] , wherein the metal salt is a light metal salt.

[25] The nanodiamond aqueous dispersion according to any one of

[21] to

[24] , wherein the metal salt is an alkali metal salt or an alkaline earth metal salt.

[26] The nanodiamond aqueous dispersion according to

[25] , wherein the alkali metal salt is sodium chloride, potassium chloride, sodium nitrate, potassium nitrate, or sodium acetate.

[27] The concentration of the above metal salt is 1.0 × 10 -6 ~2.0M, 5.0×10 -5 ~1.0M, or 1.0×10 -5 ~5.0×10 -1A nanodiamond aqueous dispersion as described in any one of

[21] to

[26] , which is M.

[28] The nanodiamond aqueous dispersion according to any one of [1] to

[27] , wherein the heavy metal content in the nanodiamond aqueous dispersion is 0.1 parts by mass or less, or 0.01 parts by mass or less, per 100 parts by mass of nanodiamond particles.

[29] The nanodiamond aqueous dispersion according to any one of [1] to

[28] , wherein the water content per 100% by mass of the dispersion medium is 90% by mass or more, 95% by mass or more, 99% by mass or more, or 99.9% by mass or more. [Industrial applicability]

[0072] The nanodiamond aqueous dispersion of this disclosure allows nanodiamond particles to exist stably in water in a highly dispersed state. Therefore, even when the dispersibility of the nanodiamond aqueous dispersion of this disclosure is evaluated by cryo-electron microscopy, nanodiamond particles with high dispersibility, without clustering or chaining, can be stably present in water.

Claims

1. A nanodiamond aqueous dispersion containing water and nanodiamond particles dispersed in water, The heavy metal content is 1 part by mass or less per 100 parts by mass of nanodiamond particles. A nanodiamond aqueous dispersion in which the horizontal projected area of ​​nanodiamond particles calculated from analytical images obtained by cryo-electron microscopy satisfies at least one condition selected from the group consisting of (a) to (d) below. (a) 100 nm ２ The above percentage must be 20% or less. (b) 80 nm ２ The percentage above must be 24% or less. (c) 50 nm ２ The above percentage must be 30% or less. (d) 30 nm ２ The above percentage must be 39% or less.

2. Furthermore, the nanodiamond aqueous dispersion according to claim 1, further comprising a metal salt.

3. The nanodiamond aqueous dispersion according to claim 2, wherein the metal salt is a light metal salt.

4. The nanodiamond aqueous dispersion according to claim 2 or 3, wherein the metal salt is an alkali metal salt.

5. The nanodiamond aqueous dispersion according to claim 1 or 2, wherein the nanodiamond particles are provided with a surface-modifying group containing a hydrophilic polymer chain.

6. The nanodiamond aqueous dispersion according to claim 5, wherein the hydrophilic polymer chain is a polyglycerin chain.

7. The nanodiamond aqueous dispersion according to claim 1 or 2, wherein the zeta potential of the nanodiamond particles is -12 to +20 mV.

8. The nanodiamond aqueous dispersion according to claim 1 or 2, wherein the zeta potential of the nanodiamond particles is greater than 0 mV and less than or equal to +20 mV.

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