Wire harness

Abstract

To provide a thermally conductive filler that can exhibit high thermal conductivity while maintaining a low specific gravity, a thermally conductive composite material and a wire harness containing the thermally conductive filler, and a method for manufacturing the thermally conductive filler.SOLUTION: A thermally conductive filler 10 comprises a substrate particle 11 and a coating layer 12 that coats the substrate particle 11. The coating layer 12 includes a gel-like substance 12a, which is chemically bonded to the surface of the substrate particle 11, covering it, and a thermally conductive substance 12b dispersed within the gel-like substance 12a and having higher thermal conductivity and greater specific gravity than both the substrate particle 11 and the gel-like substance 12a. The thermally conductive filler 10 is dispersed within a matrix material 2 to form a thermally conductive composite material 1. A wire harness comprises the thermally conductive composite material 1.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present disclosure relates to a thermally conductive filler, a thermally conductive composite material, a wire harness, and a method for manufacturing a thermally conductive filler.

Background Art

[0002] In an insulating member constituting an electric and electronic component, a thermally conductive filler may be added to an organic polymer material for the purpose of enhancing heat dissipation and suppressing the influence of heat generation due to energization or the like. The thermally conductive filler is often composed of an inorganic compound having high thermal conductivity, such as alumina, aluminum nitride, boron nitride, or the like.

[0003] In recent years, in various electric and electronic components including automotive electronics, large current and integration have been progressing, and the amount of heat generated during energization tends to increase. As a means of suppressing the influence of such heat generation, for example, in the case of an automotive wire harness, the wire is flattened to increase the surface area of the wire, or the wire is efficiently brought into contact with a high thermal conductivity exterior material, etc., and the heat dissipation is improved by improving the shape and structure of the member. On the other hand, increasing the thermal conductivity of the material itself that constitutes the insulating member of electric and electronic components, such as wire coating and wire exterior material, is also important for improving heat dissipation.

[0004] If a large amount of filler is mixed into an organic polymer material or the like, the thermal conductivity of the material can be increased. However, if a large amount of filler made of an inorganic compound is mixed into an organic polymer material, the specific gravity of the material will increase, making it difficult to reduce the weight of electric and electronic components. From the perspective of weight reduction of the entire product, such as an automobile, weight reduction is important in electric and electronic components mounted on the product. Therefore, weight reduction is also desired in materials containing a thermally conductive filler. As a method for that, attempts have been made to keep the amount of filler added small.

[0005] To maintain high thermal conductivity while minimizing the amount of filler added, various measures have been taken regarding the shape and particle arrangement of the filler. For example, Patent Document 1 discloses a filler having internal voids and a porosity within a predetermined range. Patent Document 2 discloses an inorganic-organic composite composition in which boron nitride particles are dispersed in a resin matrix in the form of delaminate flattened particles produced by a delamination process that delaminates secondary particles, which are laminates of primary particles. Patent Document 3 discloses a high thermal conductivity composite in which anisotropic high thermal conductivity fillers are in direct contact with each other, forming a network structure in the matrix resin. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2019-1849 [Patent Document 2] Japanese Patent Publication No. 2012-255055 [Patent Document 3] Japanese Patent Publication No. 2010-13580 [Patent Document 4] Japanese Patent Publication No. 2012-122057 [Patent Document 5] Japanese Patent Publication No. 2015-178543 [Patent Document 6] Japanese Patent Publication No. 2015-108058 [Patent Document 7] Japanese Patent Publication No. 2003-221453 [Patent Document 8] Japanese Patent Publication No. 2014-133678 [Patent Document 9] Japanese Patent Publication No. 2020-29524 [Patent Document 10] Japanese Patent Publication No. 2019-123983 [Non-patent literature]

[0007] [Non-Patent Document 1] Hideo Watanabe et al., "Improvement of polymer dispersibility of nanosilica hollow particles by surface modification treatment," Abstracts of the 40th Autumn Meeting of the Society of Chemical Engineers, Japan, K216, September 2008. [Overview of the project] [Problems that the invention aims to solve]

[0008] Inorganic compounds such as alumina, aluminum nitride, and boron nitride exhibit high thermal conductivity, but their high specific gravity makes it difficult to achieve high thermal conductivity while keeping the overall specific gravity low when they are added as fillers to organic polymer materials and other composite materials. Fillers made of oxides such as alumina tend to have a particularly high specific gravity. As described in Patent Documents 1 to 3, it is possible to reduce the amount of inorganic compounds added to some extent by devising the shape and particle arrangement of the fillers, but there are limits. If the specific gravity of the fillers themselves can be reduced by examining the constituent materials of the fillers, it may be possible to achieve a higher level of balance between lightweight and high thermal conductivity in composite materials with added fillers.

[0009] For example, if a filler can be formed by combining a material with a low specific gravity, such as hollow particles, with a material that has high thermal conductivity, it may be possible to achieve both lightness and high thermal conductivity in the filler as a whole. Materials that combine hollow particles such as glass with other materials are disclosed in Patent Documents 8-10 and Non-Patent Document 1, etc. However, it is not easy to firmly fix a layer of a substance responsible for heat conduction to the surface of inorganic compound particles such as glass, and to provide it with a thickness sufficient to exhibit thermal conductivity. If a thermally conductive substance can be bonded to the surface of hollow particles made of glass, etc., via chemical bonds, it is expected that the layer of thermally conductive substance can be firmly fixed to the hollow particles. However, the density of chemical bonds that can be directly formed on the surface of inorganic compound particles such as glass is limited, and the range in which the influence of chemical bonds extends is limited to a very thin region of the interface between the particle and the thermally conductive substance.

[0010] Therefore, the objective is to provide a thermally conductive filler that can exhibit high thermal conductivity while keeping its specific gravity low, as well as a thermally conductive composite material and wire harness containing such a thermally conductive filler, and a method for manufacturing such a thermally conductive filler. [Means for solving the problem]

[0011] The thermally conductive filler of this disclosure comprises a base particle and a coating layer covering the base particle, wherein the coating layer includes a gel-like substance bonded to the surface of the base particle via chemical bonds to cover the surface of the base particle, and a thermally conductive substance dispersed within the layer of the gel-like substance having a higher thermal conductivity and a larger specific gravity than the base particle and the gel-like substance.

[0012] The thermally conductive composite material of the present disclosure comprises the thermally conductive filler and a matrix material, wherein the thermally conductive filler is dispersed in the matrix material.

[0013] The wire harness of this disclosure includes the thermally conductive composite material.

[0014] The present disclosure provides a method for manufacturing a thermally conductive filler, comprising: a gel preparation step of preparing a gel-like substance in which the thermally conductive substance is dispersed; and a coating step of bonding the gel-like substance in which the thermally conductive substance is dispersed, prepared in the gel preparation step, to the surface of the substrate particles via chemical bonding. [Effects of the Invention]

[0015] The thermally conductive filler according to this disclosure is a thermally conductive filler that can exhibit high thermal conductivity while keeping its specific gravity low. Furthermore, the thermally conductive composite material and wire harness according to this disclosure contain such a thermally conductive filler. The method for manufacturing the thermally conductive filler according to this disclosure makes it possible to manufacture such a thermally conductive filler. [Brief explanation of the drawing]

[0016] [Figure 1] Figures 1A and 1B are schematic diagrams for explaining the configurations of the thermally conductive filler and the thermally conductive composite material according to an embodiment of the present disclosure. Figure 1A shows the thermally conductive composite material containing the thermally conductive filler, and Figure 1B expands and explains the coating layer of the thermally conductive filler. [Figure 2] Figure 2 is a side view showing a wire harness according to an embodiment of the present disclosure. [Figure 3] Figures 3A and 3B are optical micrographs of the fillers "30-PC80" and "30-PA80" produced in the examples, respectively. The left photographs are focused on the central part of the filler particles, and the right photographs are focused on the surface part of the fillers.

Mode for Carrying Out the Invention

[0017] [Description of Embodiments of the Present Disclosure] First, the embodiments of the present disclosure will be listed and described.

[0018] The thermally conductive filler according to the present disclosure has a base material particle and a coating layer covering the base material particle. The coating layer is bonded to the surface of the base material particle through a chemical bond, and includes a gel-like substance covering the surface of the base material particle and a thermally conductive substance dispersed in the layer of the gel-like substance and having a higher thermal conductivity and a larger specific gravity than the base material particle and the gel-like substance.

[0019] The above-described thermally conductive filler has a coating layer on the surface of the base particles containing a thermally conductive substance and a gel-like substance. By using a material with a lower specific gravity than the thermally conductive substance as the base particles, the overall specific gravity of the thermally conductive filler can be reduced compared to a case where the entire filler is composed of a thermally conductive substance. On the other hand, the thermally conductive substance contained in the coating layer, which has a higher thermal conductivity than the base particles, contributes to improving the thermal conductivity of the filler. The gel-like substance constituting the coating layer is a highly viscous substance with a cross-linking structure, and the thermally conductive substance can be stably held by being dispersed within this viscous material. The thermally conductive substance held in the gel-like substance forms thermal conduction paths within the coating layer of a single filler particle, between adjacent filler particles, and between the filler particles and other materials surrounding them, thereby contributing to heat conduction. In this way, even when using base particles or gel-like substances with low thermal conductivity, the thermally conductive filler particles as a whole can exhibit high thermal conductivity due to the contribution of the thermally conductive substance held in the gel-like substance. Therefore, it is possible to ensure high thermal conductivity in thermally conductive fillers while keeping the specific gravity low.

[0020] Furthermore, the gel-like substance constituting the coating layer is chemically bonded to the surface of the substrate particles, resulting in the coating layer being firmly fixed to the surface of the substrate particles. Because the gel-like substance forms a network through a cross-linking structure, combined with the effect of chemical bonding with the substrate particles, the entire coating layer is firmly fixed to the substrate particles along its thickness, and this state is stably maintained. Therefore, a layer of gel-like substance with sufficient thickness to hold the amount of thermally conductive material necessary to obtain high thermal conductivity can be firmly bonded to the surface of the substrate particles, thereby stably obtaining a thermally conductive filler with low specific gravity and high thermal conductivity.

[0021] Here, the base material particles are preferably hollow. The presence of the hollow portion effectively reduces the specific gravity of the base material particles, resulting in a significant reduction in the overall specific gravity of the thermally conductive filler. While the presence of the hollow portion lowers the thermal conductivity of the base material particles, providing a coating layer containing a thermally conductive substance on the surface ensures high thermal conductivity for the thermally conductive filler as a whole.

[0022] The aforementioned base material particles are composed of hollow glass bodies, and preferably have functional groups on their surface that can form chemical bonds with the functional groups of the gel-like substance. Hollow glass particles with controlled particle size and shape can be obtained relatively easily and inexpensively. Furthermore, since a variety of functional groups can be easily introduced at high density onto the surface of the glass particles using a silane coupling agent, it is easy to create base material particles that can stably bond the gel-like substance constituting the coating layer via chemical bonds.

[0023] The gel-like substance has carbonyl groups, the base particles have basic groups on their surface, and the gel-like substance is preferably bonded to the surface of the base particles via chemical bonds between the carbonyl groups and the basic groups. Many substances having carbonyl groups, including polyacrylic acid, polyvinylpyrrolidone, and their derivatives, are known to constitute gels and can be suitably used as gel-like substances constituting the coating layer. Since carbonyl groups can form chemical bonds with basic groups, the coating layer can be bonded to the base particles via chemical bonds by providing basic groups on the surface of the base particles.

[0024] In this case, the gel-like substance may contain an organic polymer having a carboxyl group as the carbonyl group. Many organic polymers having carboxyl groups, including polyacrylic acid, are known to form stable gels and are highly versatile, making them suitable for use as a gel-like substance constituting the coating layer of a thermally conductive filler.

[0025] The base particles may have primary amino groups on their surface as basic groups. Primary amino groups can form ionic bonds with acidic carbonyl groups, such as carboxyl groups, to form salts. Primary amino groups can also form covalent bonds with neutral carbonyl groups, such as ketone groups, to form amide or imide bonds. Therefore, by forming primary amino groups on the surface of the base particles, a gel-like substance containing carbonyl groups can be firmly bonded to the base particles.

[0026] The aforementioned thermally conductive material is preferably composed of particles having anisotropy in shape. While thermally conductive materials often exhibit high thermal conductivity due to their anisotropic shape, if used directly as a thermally conductive filler, anisotropy occurs in the heat conduction, making it difficult to effectively contribute to improving thermal conductivity in any desired direction. However, even with such highly anisotropic thermally conductive materials, dispersing and holding them in a gel-like substance reduces the likelihood of anisotropy in the orientation of the thermally conductive material, allowing for a highly uniform contribution to heat conduction in each direction. As a result, an excellent thermally conductive filler exhibiting high thermal conductivity in any desired direction can be obtained.

[0027] In this case, the thermally conductive material is preferably carbon fiber. Although carbon fiber has high thermal conductivity and an elongated shape with high anisotropy, by forming a coating layer while being held in a gel-like substance, it provides a thermally conductive filler that exhibits high thermal conductivity in any direction.

[0028] The thermally conductive filler should have a specific gravity of 1.8 or less. This ensures that the low specific gravity of the thermally conductive filler is sufficiently maintained.

[0029] The thermally conductive composite material according to this disclosure comprises the thermally conductive filler and a matrix material, wherein the thermally conductive filler is dispersed in the matrix material.

[0030] The above-mentioned thermally conductive composite material contains the thermally conductive filler according to the embodiments of the present disclosure described above. Because the thermally conductive filler is formed by a coating layer containing a gel-like substance and a thermally conductive substance on the surface of the substrate particles, it is possible to improve heat dissipation by utilizing the high thermal conductivity of the thermally conductive filler while keeping the specific gravity of the thermally conductive composite material as low.

[0031] Here, the matrix material may contain an organic polymer. Many organic polymers have low thermal conductivity, but by mixing them with the thermally conductive filler having a coating layer containing a thermally conductive substance, high heat dissipation can be ensured for the thermally conductive composite material as a whole. On the other hand, many organic polymers have a relatively low specific gravity, but by using a thermally conductive filler with a low specific gravity, the specific gravity of the thermally conductive composite material can be kept low even with the addition of the thermally conductive filler.

[0032] The thermally conductive composite material should preferably have a specific gravity of 1.4 or less. In this case, the specific gravity of the thermally conductive composite material as a whole will be kept sufficiently low.

[0033] The aforementioned thermally conductive composite material should preferably have a thermal conductivity of 0.9 W / (m·K) or higher at room temperature. In this case, the thermally conductive composite material as a whole will have sufficiently high thermal conductivity.

[0034] The wire harness relating to this disclosure includes the thermally conductive composite material.

[0035] Because the above wire harness contains the thermally conductive composite material described above, it is possible to utilize high thermal conductivity while keeping the specific gravity of the constituent components low. Therefore, high heat dissipation can be obtained while keeping the overall mass of the wire harness low. As a result, even if heat is generated due to the current flowing through the wires constituting the wire harness, the impact of that heat can be kept to a minimum while maintaining the lightweight nature of the wire harness.

[0036] A method for manufacturing a thermally conductive filler according to this disclosure comprises a gel preparation step of preparing a gel-like substance in which the thermally conductive substance is dispersed internally, and a coating step of bonding the gel-like substance in which the thermally conductive substance is dispersed, prepared in the gel preparation step, to the surface of the substrate particles via chemical bonding, thereby manufacturing the thermally conductive filler.

[0037] According to the above manufacturing method, a coating layer is formed on the surface of the base particles, in which a thermally conductive substance is dispersed in a gel-like substance, making it possible to easily manufacture a thermally conductive filler with low specific gravity and high thermal conductivity.

[0038] [Details of the embodiments of this disclosure] The following describes in detail, with reference to the drawings, a thermally conductive filler, a thermally conductive composite material, a wire harness, and a method for manufacturing the thermally conductive filler according to embodiments of the present disclosure. The thermally conductive composite material according to embodiments of the present disclosure is composed of the thermally conductive filler according to embodiments of the present disclosure. The wire harness according to embodiments of the present disclosure is composed of the thermally conductive composite material according to embodiments of the present disclosure. Furthermore, the thermally conductive filler according to embodiments of the present disclosure can be manufactured by the manufacturing method according to embodiments of the present disclosure.

[0039] In this specification, all physical properties shall be measured at room temperature in air unless otherwise specified. Furthermore, in this specification, a component being the main component of a material means that the component accounts for 50% or more by mass of the total mass of all components constituting the material. In addition, in this specification, "organic polymer" includes low-polymerization materials such as oligomers.

[0040] <Thermal conductive filler> First, a thermally conductive filler according to one embodiment of the present disclosure (hereinafter sometimes simply referred to as "filler") will be described.

[0041] (Overall structure) As shown in Figure 1A, the thermally conductive filler 10 according to one embodiment of the present disclosure has base particles 11 and a coating layer 12, and is in particulate form. The coating layer 12 covers the surface of the base particles 11.

[0042] As shown in Figure 1B, the coating layer 12 contains a gel-like substance 12a and a thermally conductive substance 12b, with the thermally conductive substance 12b dispersed within the layer of the gel-like substance 12a. The coating layer 12 is fixed to the surface of the substrate particles 11 by chemical bonding of the gel-like substance 12a to the surface of the substrate particles 11. In the gel-like substance 12a, the constituent molecules are crosslinked by bonding forces such as hydrogen bonds, forming a mesh-like network and resulting in a highly viscous state. The particles of the thermally conductive substance 12b are held within the structure of the gel-like substance 12a.

[0043] The thermally conductive material 12b has a higher thermal conductivity than the base material particles 11 and the gel-like material 12a. Furthermore, the thermally conductive material 12b has a higher specific gravity than the base material particles 11 and the gel-like material 12a. As will be explained later, the base material particles 11 are preferably composed of hollow bodies or porous bodies. However, if there are regions within the particle that are not occupied by solid material, such as the hollow portion of a hollow body or the pores of a porous body, the specific gravity and thermal conductivity of the base material particles 11 are determined for the entire particle, including these regions not occupied by solid material.

[0044] Since the filler 10 has a structure in which a coating layer 12 containing a thermally conductive material 12b is formed on the surface of the base material particles 11, the base material particles 11 occupy a large portion of the volume of the filler 10. Because the base material particles 11 have a lower specific gravity than the thermally conductive material 12b, the specific gravity of the filler as a whole is lower than if the filler were composed entirely of the thermally conductive material 12b.

[0045] On the other hand, the coating layer 12 covering the surface of the base particles 11 contains a thermally conductive material 12b with high thermal conductivity, thereby increasing the overall thermal conductivity of the filler 10. As shown in Figure 1B, the particles of the thermally conductive material 12b dispersed within the coating layer 12 come into contact with each other within the layer, ensuring thermal conductivity within the coating layer 12. Furthermore, as shown in Figure 1A, the coating layer 12 on the surface of the filler particles 10 comes into contact with the matrix material 2 surrounding the filler particles 10 and the coating layer 12 on the surface of other filler particles 10, so that the thermally conductive material 12b contained in the coating layer 12 contributes to heat conduction between the filler particles 10 and the matrix material 2, and between the filler particles 10. Since the coating layer 12 is provided only on the surface of the base particles 11, the overall volume of the filler particles 10 is maintained by the base particles 11, while the small volume of thermally conductive material 12b allows for thermal conductivity. Adjacent filler particles 10 come into contact with each other through the surface coating layer 12, thereby forming a heat conduction path.

[0046] From the viewpoint of avoiding an increase in the mass of the filler 10, the specific gravity of the filler 10 as a whole should be 1.8 or less, more preferably 1.5 or less, or 1.2 or less. On the other hand, from the viewpoint of avoiding the inability to include in the coating layer 12 the amount of thermal conductive material 12b necessary to ensure sufficient thermal conductivity by keeping the specific gravity too low, the specific gravity of the filler 10 as a whole should be 0.5 or more, more preferably 0.8 or more. The specific gravity of the filler 10 can be measured, for example, as the true density of the powdered filler 10 using a hydrometer.

[0047] (base material particles) As described above, the base material particles 11 have a specific gravity lower than the thermally conductive material 12b contained in the coating layer 12. As long as they have such a specific gravity, the specific structure and constituent materials of the base material particles 11 are not particularly limited. Examples of the structure of the base material particles 11 include solid bodies where solid material occupies the entire surface, hollow bodies having a hollow portion not occupied by solid material, and porous bodies having numerous fine pores not occupied by solid material. The embodiment shown in Figure 1A shows a case where the base material particles 11 are hollow particles having a hollow portion 11a.

[0048] When the base particles 11 are composed of solid materials, the solid material constituting the base particles 11 must have a specific gravity lower than that of the thermally conductive material 12b. Suitable examples of such solid materials include various resins, elastomers, rubbers, and other organic polymers. Since the base particles 11 need to bond the gel-like material 12a to their surface via chemical bonds, it is preferable that the organic polymer has functional groups that can form chemical bonds with the functional groups of the gel-like material 12a when the base particles 11 are made of an organic polymer. The organic polymer may have such functional groups in its main chain, or it may have functional groups introduced into its side chains through modification or other means.

[0049] If the base material particles 11 have regions not occupied by solid material, such as hollow bodies or porous bodies, the presence of these regions reduces the overall specific gravity of the base material particles 11. Therefore, compared to the case of solid bodies, even if the solid material itself has a high specific gravity (density), the overall specific gravity of the base material particles 11 can be kept lower than that of the thermally conductive material 12b. Thus, the base material particles 11 can be made of a variety of constituent materials. In particular, as shown in Figure 1A, if the base material particles 11 are hollow bodies, the hollow portion (cavity) 11a not occupied by solid material is completely surrounded by the shell 11b formed by the solid material, and is maintained as a space that is shielded from the external environment of the base material particles 11. Therefore, it is easy to form a coating layer 12 with a clear layered structure on the surface of the base material particles 11 and maintain the effect of reducing specific gravity by securing a space (hollow portion 11a) not occupied by other materials. From these viewpoints, a form in which the base material particles 11 are composed of hollow bodies is particularly preferred.

[0050] When the base particles 11 are composed of hollow or porous materials, various inorganic materials can be suitably used as constituent materials, in addition to the organic polymers mentioned above for the case of solid materials. Examples of such inorganic materials include metals, or inorganic compounds such as glass and ceramics. Various functional groups can be introduced to the surface of the inorganic material by surface treatment. The gel-like substance 12a can be bonded to the surface of the base particles 11 through chemical bonds between these functional groups and the functional groups of the gel-like substance 12a.

[0051] Glass can be cited as a preferred example of an inorganic material constituting the base particle 11 (or its shell 11b). As a material, glass has a relatively low specific gravity among various inorganic compounds and high thermal conductivity compared to organic polymers, etc. Therefore, using it as the material for the base particle 11 constituting the thermal conductive filler 10 is highly effective in reducing the specific gravity and increasing the thermal conductivity of the thermal conductive filler 10. Furthermore, techniques for producing hollow particles using glass and controlling their particle size and shape have already been established, and glass hollow particles can be obtained at a lower cost compared to other types of hollow particles. The type of glass constituting the base particle 11 is not particularly limited, and various types of glass can be used, such as soda-lime glass, silica glass, boric acid glass, borosilicate glass, soda-lime borosilicate glass, lead glass, and phosphate glass. Among these types of glass, it is preferable to use glass containing silicon atoms in its structure that can form siloxane bonds with a silane coupling agent, such as soda-lime glass, silica glass, borosilicate glass, and soda-lime borosilicate glass, so that functional groups can be introduced using a silane coupling agent, as will be described later. The material constituting the base particle 11 may be a single type, or two or more types may be used in combination by mixing or lamination.

[0052] The base particle 11 has functional groups on its surface that can form chemical bonds with the functional groups of the gel-like substance 12a. The type of functional groups that the base particle 11 has is not particularly limited; it is sufficient that the base particle 11 has functional groups on its surface that can react with the functional groups of the gel-like substance 12a, depending on the type of functional groups that the gel-like substance 12a has. The chemical bond may be an electrostatic bond (ionic bond, hydrogen bond) or a covalent bond. When an electrostatic bond is formed, it is sufficient that polar groups having the opposite polarity to the polar groups of the gel-like substance 12a are present on the surface of the base particle 11. The polar groups that the base particle 11 has may be ionic or nonionic. In many cases, the gel-like substance 12a and its raw materials are negatively charged, so it is preferable that positively charged polar groups be present on the surface of the base particle 11.

[0053] As will be explained later, many compounds that constitute gels have carbonyl groups, and if functional groups that can form chemical bonds with carbonyl groups are provided on the surface of the base particles 11, various gel-like substances 12a can be chemically bonded to the surface of the base particles 11. Basic groups can be given as functional groups that can form chemical bonds with carbonyl groups. Examples of basic groups include amino groups, amide groups, and imide groups. Among these, it is preferable that amino groups, especially primary amino groups, be present on the surface of the base particles 11. The formation of chemical bonds between these functional groups and the gel-like substances 12a will be explained later, along with examples of gel-like substances 12a.

[0054] The distribution and method of introducing functional groups are not particularly limited, as long as the base particles 11 have functional groups capable of bonding with the gel-like substance 12a, at least on their surface. In other words, even if the entire base particles 11 contain a compound having polar groups as a constituent material, the base particles 11 may be composed of a material that substantially does not contain functional groups, or contains only a very small amount of functional groups, and functional groups may be introduced only on the surface (and its vicinity) by surface treatment or the like. From the viewpoint of simplicity in introducing functional groups to the surface of the base particles 11, if the base particles 11 are composed of an organic substance, it is preferable that the organic substance constituting the base particles 11 itself has functional groups, and that these functional groups are exposed on the surface of the base particles 11. On the other hand, if the base particles 11 are composed of an inorganic substance, such as glass, it is preferable to introduce functional groups to the surface by surface treatment. If the base particles 11 are composed of glass containing silicon, or have hydroxyl groups on their surface, various functional groups can be easily introduced to the surface by using a silane coupling agent.

[0055] The specific shape and particle size of the base particles 11 are not particularly limited. However, having a highly isotropic shape, such as one that approximates a sphere, is preferable in terms of facilitating the formation of the coating layer 12 on the surface and increasing affinity with the matrix material 2. The particle size of the base particles 11 (median diameter D50; the same applies below) is preferably 1 μm or larger, and more preferably 5 μm or larger, from the viewpoint of keeping the specific gravity of the filler 10 as a whole low. On the other hand, from the viewpoint of minimizing the influence of the filler 10 on the properties of the matrix material 2 to which it is added and increasing the specific surface area, the particle size of the base particles 11 is preferably 100 μm or smaller, and more preferably 60 μm or smaller.

[0056] From the viewpoint of keeping the overall specific gravity of the filler 10 low, it is preferable that the specific gravity of the base particles 11 individually is also low. The specific specific gravity of the base particles 11 is not particularly limited as long as the base particles 11 as a whole are lower than the specific gravity of the thermally conductive material 12b, but the specific gravity (true density) of the base particles 11 as a whole, including the hollow parts 11a, etc., should be, for example, 1.0 or less, preferably 0.5 or less. There is no particular lower limit set for the specific gravity of the base particles 11, but the specific gravity of base particles 11 composed of hollow bodies of inorganic materials such as glass or solid bodies of organic polymers is generally 0.1 or more.

[0057] (covering layer) In the coating layer 12, the gel-like substance 12a is bonded to the surface of the substrate particles 11 via chemical bonds, covering the surface of the substrate particles 11. The thermally conductive substance 12b is dispersed and held within the layer of the gel-like substance 12a.

[0058] The gel-like substance 12a forms intermolecular bonds, creating a network-like cross-linked structure. The bonds forming the cross-linked structure may be physical bonds such as van der Waals forces, but stronger chemical bonds are preferable. The chemical bonds may be electrostatic bonds such as ionic bonds or hydrogen bonds, or covalent bonds, but from the viewpoint of ease of forming the coating layer 12, electrostatic bonds are preferred, and from the viewpoint of stability of the cross-linked structure, hydrogen bonds are even more preferred.

[0059] The gel-like substance 12a has functional groups capable of forming chemical bonds with the functional groups on the surface of the base particles 11, and the coating layer 12 is formed with the gel-like substance 12a bonded to the surface of the base particles 11 by the chemical bonds formed between the two functional groups. In the gel-like substance 12a, the functional groups that form bonds with the surface of the base particles 11 may be the same as the functional groups that form a crosslinking structure between molecules, or they may be contained in the molecule of the gel-like substance 12a as functional groups different from the functional groups that participate in crosslinking. Preferably, the common functional groups are involved in both intermolecular crosslinking and bonding with the base particles 11. In the gel-like substance 12a having carbonyl groups, which will be described later, the carbonyl groups may form bonds in the crosslinking structure and, if basic groups such as amino groups are present on the surface of the base particles 11, bond the gel-like substance 12a to the surface of the base particles 11 via bonding with those basic groups.

[0060] The gel-like substance 12a is bonded to the surface of the substrate particles 11 via chemical bonds, resulting in the coating layer 12 being firmly fixed to the substrate particles 11. Although the direct effect of these chemical bonds is limited to the interface between the coating layer 12 and the substrate particles 11 and its immediate vicinity, the gel-like substance 12a has a continuous structure within the layer due to cross-linking. Therefore, the chemical bonds at the interface and the cross-linking structure within the layer ensure that the entire layer of gel-like substance 12a is stably fixed to the substrate particles 11.

[0061] Because the gel-like substance 12a forms a network structure and has high viscosity, as shown in Figure 1B, when particles of the thermally conductive substance 12b are dispersed in the gel-like substance 12a, the thermally conductive substance 12b remains stably held within the layer of the gel-like substance 12a. The gel-like substance 12a often forms a gel when it contains water or other polar solvents, but even if these polar solvents are removed, the state in which the thermally conductive substance 12b is dispersed and held within the structure of the gel-like substance 12a is maintained. The particles of the thermally conductive substance 12b dispersed in the gel-like substance 12a come into contact with each other, and thermal conduction paths are formed between the particles, causing the coating layer 12 as a whole to exhibit thermal conductivity.

[0062] Since no chemical interaction is required to retain the thermally conductive material 12b within the gel-like substance 12a, the thermally conductive material 12b does not need to be compatible with the gel-like substance 12a in order to disperse and retain the thermally conductive material 12b within the layer of the gel-like substance 12a. Furthermore, since the thermally conductive material 12b is not fixed to the surface of the substrate particles 11 by direct interaction with the surface of the substrate particles 11, the type of thermally conductive material 12b is not limited by the type of substrate particles 11. In other words, various thermally conductive materials 12b can be dispersed within the layer of the gel-like substance 12a and placed on the surface of the substrate particles 11, without being limited by the constituent materials or shape of the thermally conductive material 12b, and this state can be stably maintained. In addition, even when the coating layer 12 is formed thickly from the viewpoint of improving thermal conductivity, the entire layer can be maintained in a state of being stably fixed to the surface of the substrate particles 11. Similarly, from the viewpoint of improving thermal conductivity, increasing the density of the thermally conductive substance 12b in the coating layer 12 can be done relatively easily, without being limited by the density of chemical bond formation, as long as the gel-like substance 12a can retain the thermally conductive substance 12b within its structure.

[0063] From the viewpoint of obtaining sufficiently high thermal conductivity for the filler 10 as a whole, it is preferable that the thickness of the coating layer 12 be such that the ratio of [dry volume of the mixed gel containing the gel-like substance 12a and the thermally conductive substance 12b] to [dry volume of the base material particles 11] is 10:90 or more, more preferably 20:80 or more, or 30:70 or more. On the other hand, from the viewpoint of avoiding peeling or damage to the coating layer 12 due to it being too thick, and the resulting difficulty in mixing the filler 10 into the matrix material 2, it is preferable to keep the thickness of the coating layer 12 to such an extent that the above ratio is 80:20 or less, or more preferably 70:30 or less. Here, dry volume refers to the volume in a state where liquid components such as water and other solvents contained in the gel-like substance 12a have been removed by drying or other means. The value of the thickness of the coating layer 12 is generally such that it is 1 μm or more, more preferably 5 μm or more, or 30 μm or less.

[0064] It should be noted that even if the thermal conductive substance 12b and the base particles 11 are added to the matrix material 2 independently, without using the gel-like substance 12a, a certain degree of thermal conductivity improvement can be obtained. However, in this case, the thermal conductive substance 12b will either cover a very small area of ​​the surface of the base particles 11 or be dispersed in the matrix resin independently of the base particles 11. As in the filler 10 of this embodiment, by using the gel-like substance 12a to stably hold the thermal conductive substance 12b on the surface of the base particles 11, the thermal conductive substance 12b can efficiently contribute to the formation of thermal conduction paths.

[0065] (Gel-like substance) The gel-like substance 12a constituting the coating layer 12 is not particularly limited, as long as it can form a gel state and has functional groups capable of forming chemical bonds with the functional groups on the surface of the substrate particles 11. In the filler 10, the gel-like substance 12a constituting the coating layer 12 may be in a swollen state by incorporating water or other solvents, or it may be in a state where those solvents have been removed by drying or extraction. From the viewpoint of the stability and ease of handling of the filler 10, it is preferable that the gel-like substance 12a is in a state from which the solvent has been removed.

[0066] Many gel-like substances 12a are known to be composed of polymers having carbonyl groups, and these substances can be suitably used in this embodiment as well. Due to the high polarity of carbonyl groups, molecules having carbonyl groups tend to form stable gels. Here, carbonyl groups include various functional groups having a C=O bond, and can be ketone groups, carboxyl groups, amide groups, ester groups, etc. For example, polymers having carboxyl groups, such as polyacrylic acid, polymers having amide groups, such as polyvinylpyrrolidone, or derivatives of these compounds swell with water or highly polar solvents and form a stable gel state. Polymers having ketone groups also gel when swollen with polyethylene glycol (PEG). Specific examples of polymers constituting the gel-like substance 12a include polyacrylic acid, polyacrylate salts, polyethylene acrylic acid copolymers, polyethylene acrylic acid copolymer salts, polyvinylpyrrolidone, carboxymethylcellulose, carboxymethylcellulose salts, alginic acid, alginate salts, etc.

[0067] When the molecules constituting the gel-like substance 12a have carbonyl groups, if basic groups such as amino groups are present on the surface of the base particles 11, the gel-like substance 12a can be stably bonded to the surface of the base particles 11 by bond formation between the carbonyl groups and the basic groups. For example, if the gel-like substance 12a has acidic carbonyl groups such as carboxyl groups, it can form ionic bonds with the basic groups of the base particles 11, forming a salt. On the other hand, if the gel-like substance 12a has neutral carbonyl groups such as ketone groups, it can form covalent bonds with the basic groups on the surface of the base particles 11. For example, if primary amino groups are present on the surface of the base particles 11, amide bonds or imide bonds can be formed by condensation reactions.

[0068] The specific gravity and thermal conductivity of the gel-like substance 12a are not specifically designated, but the gel-like substance 12a is often composed of organic matter and has a lower specific gravity than the thermally conductive substance 12b which is composed of inorganic matter. The specific gravity of the gel-like substance 12a should be 1.5 or less, preferably 1.2 or less, in the state of the raw material (gelling polymer) before gelation. There is no particular lower limit set for its specific gravity, but the specific gravity of organic polymers capable of forming a gel is generally 0.5 or more. The low specific gravity of the gel-like substance 12a, combined with the effect of the low specific gravity of the base particles 11, helps to keep the overall specific gravity of the filler 10 low. In addition, the gel-like substance 12a is often composed of organic matter and has a lower thermal conductivity than the thermally conductive substance 12b. However, as described above, the thermally conductive material 12b is dispersed and held within the layer of the gel-like material 12a, forming thermal conduction paths, thereby achieving high thermal conductivity for the entire coating layer 12.

[0069] (Thermal conductive material) The thermally conductive substance 12b contained in the coating layer 12 is not particularly limited in type or shape, as long as it has a higher thermal conductivity and a larger specific gravity than the base material particles 11 and the gel-like substance 12a, and is dispersible in the gel-like substance 12a. However, from the viewpoint of high thermal conductivity and dispersibility in the gel-like substance 12a, it is preferable that the thermally conductive substance 12b is composed of particles of inorganic substances such as metals, metal compounds, and carbon materials.

[0070] Specific examples of materials constituting the thermally conductive material 12b include, as metals, non-magnetic metals such as copper and aluminum, and alloys mainly composed of these elements. Furthermore, as metal compounds, examples include oxides, nitrides, carbides, oxynitrides, carbonitrides, carbon oxides, hydroxides, borides, etc., containing metallic elements (including metalloids such as B and Si), as well as metallic silicates, aluminates, titanates, etc. Suitable examples of metal oxides include aluminum oxide, silicon oxide, magnesium oxide, iron oxide, beryllium oxide, titanium oxide, zirconium oxide, etc.; suitable examples of metal nitrides include aluminum nitride, silicon nitride, boron nitride, etc.; suitable examples of metal hydroxides include aluminum hydroxide, magnesium hydroxide, etc.; suitable examples of metal carbides include silicon carbide, etc. In addition, examples of carbon materials that can constitute the thermally conductive material 12b include graphite, carbon flax, carbon fiber, carbon nanotubes, graphite, graphene, and artificial diamond. The thermal conductive material 12b contained in the coating layer 12 may be of one type or multiple types. Furthermore, when multiple types of thermal conductive material 12b are used, they may be mixed together or form a composite. The thermal conductive material 12b may be surface-treated with an organic substance or the like, but since chemical interaction is not necessary for the retention of the thermal conductive material 12b by the gel-like material 12a, surface treatment of the thermal conductive material 12b is not necessary from the viewpoint of enhancing the retention by the gel-like material 12a.

[0071] Of those listed above, metal oxides such as aluminum oxide and magnesium oxide, carbon black, and carbon fibers are particularly preferred as thermal conductive material 12b in terms of high thermal conductivity. Furthermore, it is preferable that the thermal conductive material 12b is composed of particles having anisotropy in shape, such as plate-shaped, needle-shaped, rod-shaped, or fibrous. Particles of metal compounds and carbon materials having anisotropy in shape often have high thermal conductivity in a specific direction due to their anisotropy. While the high thermal conductivity of these particles is advantageous for use as a thermal conductive material, if the particles are oriented and distributed in a specific direction due to the anisotropy of their particle shape, they will have excellent thermal conductivity in a specific direction, but low thermal conductivity in other directions, making them difficult to use as a thermal conductive material. However, in the filler 10 according to this embodiment, since the particles of the thermally conductive material 12b are dispersed and held in the gel-like material 12a, within the layer of the coating layer 12 and throughout the filler 10 as a whole, the particles of the anisotropically shaped thermally conductive material 12b are arranged in a random orientation rather than a specific orientation. Therefore, by utilizing the high thermal conductivity of the particles of the anisotropically shaped thermally conductive material 12b, it is possible to create a thermally conductive filler 10 that has excellent thermal conductivity in any direction, rather than in a specific direction. Carbon fiber can be given as a thermally conductive material 12b that has an anisotropic shape and high thermal conductivity. Carbon fiber has an elongated, highly anisotropic shape and exhibits great anisotropy in thermal conductivity, but by dispersing it in the gel-like material 12a, this great anisotropy can be eliminated, and by forming thermal conduction paths between adjacent carbon fibers, it exhibits high thermal conductivity in any direction.

[0072] The particle size of the thermally conductive material 12b is not particularly limited, but from the viewpoint of exhibiting sufficiently high thermal conductivity, it is preferable that it be 0.1 μm or larger on average, and more preferably 0.3 μm or larger. On the other hand, from the viewpoint of making it easier to disperse and retain within the structure of the gel-like material 12a in the coating layer 12, the particle size of the thermally conductive material 12b is preferable that it be 30 μm or smaller on average, and more preferably 20 μm or smaller. Furthermore, it is preferable that the particle size of the thermally conductive material 12b is smaller than the particle size of the base material particles 11, and more preferably half or less the particle size of the base material particles 11. Here, the particle size of the thermally conductive material 12b refers to the approximate diameter when the particles can be approximated as spherical, and to the major axis (length of the major axis) when the shape is highly anisotropic, such as needle-shaped or fibrous.

[0073] The amount of thermal conductive material 12b in the coating layer 12 is not particularly limited, but from the viewpoint of sufficiently increasing the thermal conductivity within the coating layer 12 and, as a result, ensuring that the filler 10 as a whole exhibits sufficiently high thermal conductivity, it is preferable that the amount of thermal conductive material 12b contained in the coating layer 12 is such that the ratio of [dry volume of gel-like substance 12a] to [dry volume of thermal conductive material 12b] is 90:10 or higher, or even 50:50 or higher, or 30:70 or higher. On the other hand, from the viewpoint of stably retaining the thermal conductive material 12b within the structure of the gel-like substance 12a, it is preferable to keep the amount of thermal conductive material 12b contained in the coating layer 12 to 1:99 or lower, or even 10:90 or lower, in the above ratio.

[0074] <Method for manufacturing thermally conductive fillers> Next, a method for manufacturing the thermal conductive filler according to one embodiment of the present disclosure, which can produce the thermal conductive filler 10 described above, will be explained. The thermal conductive filler 10 can be manufactured by carrying out a gel preparation step and a coating step.

[0075] Prior to manufacturing the thermally conductive filler 10, base particles 11 are prepared. Many solid particles of organic polymers, and hollow or porous particles of inorganic compounds such as glass are commercially available, and these particles can be used as appropriate.

[0076] If the base particles 11 do not have functional groups on their surface that can form chemical bonds with the functional groups of the gel-like substance 12a, it is necessary to surface treat the base particles 11 to introduce functional groups to their surface. For surface treatment, a compound having the desired functional group can be bonded to the surface of the base particles 11 by a chemical reaction. In this case, another compound may be interposed between the compound having the functional group and the surface of the base particles 11. When the base particles 11 are composed of a material containing silicon atoms, such as glass, or a material having hydroxyl groups on its surface, it is preferable to introduce the functional groups to the surface of the base particles 11 using a silane coupling agent.

[0077] In the gel preparation step, a mixed gel to be formed as the coating layer 12 is prepared. That is, a gel-like substance 12a is prepared with thermally conductive substance 12b dispersed inside. The specific method is not particularly limited, but it is possible to form the gel-like substance 12a by performing crosslinking and swelling on the raw materials constituting the gel, such as a gelling polymer, and simultaneously, or after forming the gel-like substance 12a, disperse the particles of thermally conductive substance 12b within the gel-like substance 12a. For example, when using a gelling polymer that swells in a polar solvent such as water to become a gel-like substance 12a as the raw material, the gelling polymer and the particles of thermally conductive substance 12b can be stirred in a solvent containing a polar solvent such as water. At this time, a small amount of metal salt aqueous solution may be added so that the gel-like substance 12a is obtained in the form of a salt. In the gel preparation process, the amounts of raw materials to be formed into the gel-like substance 12a and the amount of thermally conductive substance 12b to be added should be selected so as to obtain a desired value in terms of the ratio of [drying capacity of gel-like substance 12a (gelling polymer)]:[drying capacity of thermally conductive substance 12b].

[0078] In the coating step, the surface of the base particles 11 is coated with the mixed gel prepared in the gel preparation step, and then the gel-like substance 12a is bonded to the surface of the base particles 11 via chemical bonds. At this time, the base particles 11, which have predetermined functional groups on their surface, can be mixed and kneaded with the mixed gel prepared above. Alternatively, the base particles 11 and the mixed gel may be mixed in a solvent that does not affect the gel properties of the gel-like substance 12a or the modification of the functional groups on the surface of the base particles 11. After these mixing and kneading operations or mixing in a solvent, the mixed gel surrounds the surface of the base particles 11, and further, chemical bonds are formed between the functional groups of the gel-like substance 12a constituting the mixed gel and the functional groups present on the surface of the base particles 11. To promote the formation of chemical bonds, operations such as heating or the addition of a reactant may be performed as appropriate. The heating temperature is preferably below the boiling point of the solvent used.

[0079] The particles obtained through the coating process may be used as is as the thermally conductive filler 10, or they may be used after the liquid component has been removed as appropriate. The liquid component can be removed by heating under normal pressure or vacuum, air drying, etc. If the liquid component is present on the surface of the filler particles 10, such as when incorporated into the gel-like substance 12a, and there is a possibility that the liquid component may volatilize or flow during use of the filler 10, reducing the handling ease of the filler 10, it is preferable to remove the liquid component.

[0080] <Thermally conductive composite materials> Next, a thermally conductive composite material (hereinafter sometimes simply referred to as "composite material") according to one embodiment of the present disclosure will be described. As shown in Figure 1, the thermally conductive composite material 1 according to this embodiment includes the thermally conductive filler 10 according to the embodiment of the present disclosure described above and a matrix material 2. The filler 10 is dispersed in the matrix material 2.

[0081] The composite material 1 according to this embodiment contains a thermally conductive filler 10 having a coating layer 12 containing a thermally conductive substance 12b on the surface of the base material particles 11 described above. Due to the high thermal conductivity of the thermally conductive substance 12b, the composite material 1 as a whole exhibits high thermal conductivity and excellent heat dissipation. At the same time, due to the effect of the base material particles 11 in reducing the specific gravity of the thermally conductive filler 10, the composite material 1 as a whole has a low specific gravity.

[0082] The type of matrix material 2 is not particularly limited, but it is preferable that the matrix material 2 contains an organic polymer, and more preferably that it is mainly composed of an organic polymer. Specific examples of organic polymers constituting the matrix material 2 include various resins, thermoplastic elastomers, rubber, etc. When a resin material is used as the matrix material 2, it may be a curable resin, a thermoplastic resin, or a plastic soluble in a solvent, depending on the desired application. Examples of resins constituting the matrix material 2 include olefin resins such as polyethylene and polypropylene, halogen resins such as polyvinyl chloride, polylactic acid, polystyrene resins, polyvinyl acetate, ABS resin, AS resin, acrylic resin, methacrylic resin, polyamide resin, urethane resin, silicone resin, fluororesin, polyvinyl alcohol, polyimide, polyacetal, polycarbonate, modified polyphenylene ether (PPE), polyethylene terephthalate, polybutylene terephthalate, polyphenylene sulfide, and epoxy resin, or copolymers and polymer alloys of these resins. Matrix material 2 may contain only one type of organic polymer or multiple types. Matrix material 2 is not a gel, but rather becomes solid at room temperature after curing or other appropriate processes. In addition to organic polymers, matrix material 2 may appropriately contain additives such as flame retardants, fillers, and colorants.

[0083] The specific gravity of the matrix material 2 itself is not particularly limited, but from the viewpoint of keeping the specific gravity of the composite material 1 with the filler 10 added low, it is preferable to keep it below 1.5. There is no particular lower limit for the specific gravity of the matrix material 2, but when an organic polymer is used as the matrix material 2, its specific gravity is generally 0.8 or higher. Similarly, the thermal conductivity of the matrix material 2 itself is not particularly limited, but from the viewpoint of ensuring high thermal conductivity for the composite material 1 with the filler 10 added, it is preferable to keep it above 0.1 W / (m·K). There is no particular upper limit for the thermal conductivity of the matrix material 2, but when an organic polymer is used as the matrix material 2, its thermal conductivity is generally 0.6 W / (m·K) or lower. The specific gravity of the matrix material 2 and composite material 1 can be measured by methods such as the water displacement method. The thermal conductivity of these materials can be measured by methods such as the laser flash method and the hot-wire method.

[0084] In the composite material 1 according to this embodiment, the content of filler 10 can be appropriately determined so that the composite material 1 as a whole has the desired specific gravity and thermal conductivity. The higher the content of filler 10, the higher the thermal conductivity of the composite material 1. Therefore, the content of filler 10 can be determined with the content that provides the desired thermal conductivity as the lower limit. For example, the content of filler 10 can be determined so that the thermal conductivity of the composite material 1 is 5 times or more, and even 7 times or more, 10 times or more, or 15 times or more, than the thermal conductivity of the matrix material 2 without filler 10. Alternatively, the content of filler 10 can be determined so that the thermal conductivity of the composite material 1 is 0.9 W / (m·K) or more, and even 1.5 W / (m·K) or more, 2.0 W / (m·K) or more, or 3.0 W / (m·K) or more. While a higher thermal conductivity for composite material 1 is preferable, to avoid an increase in specific gravity due to excessive addition of filler 10, it is advisable to keep it below 50 times, or even 30 times, the thermal conductivity of matrix material 2, and below 8.0 W / (m·K), or even below 5.0 W / (m·K).

[0085] There is no specific upper limit for the content of filler 10 in composite material 1, but the content of filler 10 should be determined so that the specific gravity of composite material 1 is kept at 1.3 times or less, and even 1.2 times or less, the specific gravity of matrix material 2 without filler 10. More preferably, the specific gravity of composite material 1 should be less than or equal to the specific gravity of matrix material 2 without filler 10. Alternatively, the content of filler 10 should be determined so that the specific gravity of composite material 1 is kept at 1.8 or less, even 1.5 or less, and even 1.3 or less. Note that a lower specific gravity of composite material 1 is preferable, and there is no specific lower limit.

[0086] When the content of filler 10 is defined as the proportion of filler 10 to the entire composite material 1, the content of filler 10 should be approximately 10% by volume or more, more preferably 20% by volume or more, or 30% by volume or more, from the viewpoint of sufficiently improving the thermal conductivity of the composite material 1. Alternatively, the content of the thermal conductive substance 12b itself should be 2% by volume or more, more preferably 4% by volume or more, or 6% by volume or more. On the other hand, from the viewpoint of suppressing the increase in the specific gravity of the composite material 1 and avoiding saturation of the thermal conductivity improvement effect due to the addition of a large amount of filler 10, it should be 60% by volume or less, or 50% by volume or less. Alternatively, the content of the thermal conductive substance 12b itself should be 15% by volume or less, or more preferably 12% by volume or less.

[0087] As described above, the composite material 1 according to this embodiment achieves both high thermal conductivity and low specific gravity. Therefore, this composite material 1 can be suitably used as a material for components that require both lightness and heat dissipation. The specific applications of the composite material 1 are not particularly limited, but next, we will give a detailed example of its use as a component material for a wire harness. The composite material 1 according to this embodiment can be manufactured by mixing powdered filler 10, manufactured by the manufacturing method described above, with matrix material 2 at a predetermined mixing ratio.

[0088] <Wire Harness> Finally, a wire harness according to an embodiment of the present disclosure will be described. The wire harness according to this embodiment includes the thermally conductive composite material 1 according to the embodiment of the present disclosure described above. As shown in Figure 2, the wire harness 5 has a connector 52 including a connection terminal (not shown) at the end of an insulated wire 51, which has an insulating coating on the outer circumference of the wire conductor. In the wire harness 5, multiple insulated wires 51 may be bundled together, and in this case, tape 53 can be used as an outer covering material for bundling the insulated wires 51.

[0089] In the wire harness 5 according to this embodiment, the composite material 1 according to the embodiment of the present disclosure described above can constitute various components that require heat dissipation. Preferably, the composite material 1, in which a filler 10 is added to an organic polymer as the matrix material 2, is used as an insulating component. Examples of such insulating components include the insulating coating that constitutes the insulated wire 51, the outer covering material such as tape 53 or protective tube placed on the outside of the insulated wire 51, the adhesive used for bonding and waterproofing between components, and the connector housing that constitutes the connector 52. The composite material 1 may also be placed between a protective tube such as a corrugated tube and the insulated wire 51.

[0090] In recent years, in the automotive sector, particularly in electric and hybrid vehicles, the current flowing through wires has increased, and consequently, the amount of heat generated from these wires has also tended to increase. Furthermore, numerous wires and electrical connection components are increasingly being arranged in close proximity. In these cases, it is important that the various components constituting the wire harness 5 have high heat dissipation properties, from the standpoint of minimizing the impact of heat dissipation from wires and electrical connection components. By constructing components in the wire harness 5 that are potentially affected by heat dissipation using the composite material 1, which has high thermal conductivity, efficient heat dissipation becomes possible. In addition, in the automotive sector, reducing the weight of components is an important issue, and by using the composite material 1, which has a low specific gravity, it is possible to contribute to the weight reduction of the wire harness 5. [Examples]

[0091] Examples are shown below. The present invention is not limited to these examples. Here, a thermally conductive filler having a coating layer on the surface of substrate particles was prepared, and the state of the filler particles, as well as the specific gravity and thermal conductivity of the composite material containing the filler, were evaluated. Unless otherwise specified, the preparation and evaluation of samples were carried out in air at room temperature.

[0092] [Test Method] (1) Preparation of filler First, several fillers were prepared, each having a coating layer on the surface of the base particles. In preparing the fillers, the raw material particles were surface-treated, and in the gel preparation step, a mixed gel was prepared. Then, in the coating step, a coating layer was formed on the surface of the raw material particles.

[0093] (1-1) Preparation of base particles As base particles, hollow glass particles with amino groups introduced to the surface were prepared. Specifically, hollow soda-lime borosilicate glass particles (3M's "Glass Bubbles iM16K"; median diameter 20 μm; specific gravity 0.46) were prepared as untreated hollow glass particles (GB). Then, 5 g of GB and 100 mL of acetone were placed in a round-bottom flask and gently stirred at room temperature to suspend. While continuing to stir, 0.5 g of 3-aminopropyltriethoxysilane (AP) ((C2H5O)3Si-C3H6-NH2) was added to the suspension. After stirring at room temperature for 2 hours, a condenser was attached, 200 mL of pure water was added, and the mixture was stirred at 50°C for 24 hours. After that, it was filtered and air-dried, and then heated in an oven at 140°C for 24 hours. Through the above process, base particles (AP-GB) surface-treated with aminopropyltriethoxysilane were obtained. Aminopropyltriethoxysilane is bonded to the surface of glass hollow particles via siloxane bonds, resulting in a state where amino groups are attached to the particle surface.

[0094] (1-2) Gel preparation process The following materials were prepared as gelling polymer materials to be used as raw materials for forming gel-like substances. • PAA: Polyacrylic acid (average molecular weight approximately 1,000,000; manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) • EAA: (Ethylene-acrylic acid copolymer; acrylic acid content 15% by mass; manufactured by Sigma-Aldrich)

[0095] In addition, the following materials were prepared as thermally conductive materials. • CF: Carbon fiber (10 μm length, 150 nm diameter; manufactured by Showa Denko, vapor-phase carbon fiber VGCF) • AlO: α-alumina (0.5 μm diameter; manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) • MgO: Magnesium oxide (7-15 μm diameter; manufactured by Ube Materials Co., Ltd.)

[0096] The gelling polymer materials in the amounts listed in Table 1, along with 17 ml of tetrahydrofuran (THF) and 33 ml of pure water, were placed in a round-bottom flask. A reflux condenser was attached, and the mixture was stirred at 60°C for 12 hours to dissolve and disperse the gelling polymers. While continuing to stir, each thermally conductive substance was added in the amounts listed in Table 1. The reflux condenser was then attached again, and the mixture was stirred at 60°C for 2 hours to disperse the thermally conductive substances. In this way, a mixed gel was prepared.

[0097] (1-3) Coating process While continuing to stir the mixed gel prepared in the gel preparation step described above, the amount of base particles (untreated GB or AP-GB) listed in Table 1 were added to the mixed gel, and the reflux tube was reattached. The mixture was stirred at 60°C for 2 hours to disperse the base particles. The dispersion was then cooled to room temperature. Then, the dispersion was gradually added to 1 L of isopropanol in a separate beaker while stirring to form a suspension. The resulting suspension was filtered by suction, and the resulting filtrate was dried at 80°C for 24 hours. In this way, a thermally conductive filler was obtained.

[0098] Table 1 below summarizes the types, specific gravities, and input amounts of gelling polymer materials, thermal conductive substances, and base particles used in the preparation of each thermally conductive filler. The table also shows the dry volume ratio of [gelling polymer]:[thermal conductive substance], the dry volume ratio of [mixed gel]:[base particles], and the specific gravity of the filler particles, calculated from these input amounts and specific gravities. The dry volume ratio of [gelling polymer]:[thermal conductive substance] is calculated as the ratio of the input amounts of gelling polymer material and thermal conductive substance converted to volume. The dry volume ratio of [mixed gel]:[base particles] is calculated as the ratio of the volume of the base particles to the sum of the calculated volumes of gelling polymer material and thermal conductive substance, and both are displayed with one significant figure. The specific gravity of the filler particles is calculated from the sum of the input amounts of gelling polymer, thermal conductive substance, and base particles, and the sum of the calculated volumes. Furthermore, as will be explained later based on the micrographs in Figures 3A and 3B, it has been confirmed that almost the entire amount of the gelling polymer material and thermally conductive substance used as raw materials forms a mixed gel that coats the surface of the substrate particles. This demonstrates the validity of calculating the dry volume ratio and specific gravity of each component based on the amount of each raw material added, as described above.

[0099] [Table 1]

[0100] (2) Preparation of composite materials The fillers prepared above were dispersed in a matrix material to prepare composite materials for samples A1-A14 and B1-B7. Here, the matrix material constituting the composite material was a cured product of the following two-component epoxy resin. • Epoxy main component: Bisphenol A glycidyl ether (Mitsubishi Chemical Corporation, "jER828"; epoxy equivalent: 190 g / eq.) • Epoxy hardener: Amine type (Mitsubishi Chemical's "ST12"; Amine value: 345-385 KOH mg / g)

[0101] As shown in Table 2 below, various fillers, epoxy main agent, and epoxy curing agent were mixed in an agate mortar at room temperature in mass ratios, and degassed for 1 minute under room temperature vacuum. The mixture was then heated in a hot press molding machine at 100°C for 10 minutes to cure. From the cured body, portions in which no air bubbles were visually observed were cut out to prepare resin cured product test pieces (10 mm × 10 mm × 1 mm). For sample B1, resin cured product test pieces were prepared from epoxy resin only, without the addition of fillers.

[0102] (3) Evaluation of the state of the filler and the properties of the composite material Each filler prepared as described above was dispersed in liquid paraffin and observed under an optical microscope to evaluate the state of the filler particles.

[0103] Furthermore, the specific gravity and thermal conductivity were measured for each resin cured material test specimen prepared as a composite material as described above. Specific gravity was measured by the water displacement method. Thermal conductivity was measured using a thermal conduction device (NETZSCH "LFA447") by the laser flash method. The direction of measurement for thermal conductivity was perpendicular to the surface of the resin cured material test specimen.

[0104] [Test Results] As representative examples of the fabricated fillers, Figures 3A and 3B show optical microscope images of fillers "30-PC80" and "30-PA80," respectively. In each image, the left image is focused on the center of the particle, and the right image is focused on the surface of the particle. In both types of fillers, the base particles maintain a hollow state, as can be seen mainly in the images focused on the center of the particle. Furthermore, as can be seen mainly in the images focused on the particle surface, the somewhat blurred layered regions observed on the surface of the hollow base particles correspond to the coating layer formed from the mixed gel. In the image of "30-PC80," the numerous elongated structures observed as spiky structures in the coating layer correspond to the carbon fibers added as a thermal conductive material. In the image of "30-PA80," the numerous point-like structures observed as dark shadows in the coating layer correspond to the alumina particles added as a thermal conductive material.

[0105] Thus, microscopic images confirm that the fabricated filler has a structure in which a coating layer consisting of a mixed gel in which a thermally conductive substance is dispersed is formed on the surface of the hollow particles used as the base particles. The mixed gel coats the surface of the hollow particles in a layered manner, forming a coating layer, and the presence of gel-like material is not confirmed in areas other than the surface of the particles. In other words, almost the entire amount of the mixed gel used as a raw material coats the surface of the hollow base particles. From this, it is confirmed that a coating layer containing a thermally conductive substance can be stably formed on the surface of the base particles by the manufacturing method in which surface-treated base particles are mixed with a mixed gel, as described above.

[0106] In the image of "30-PC80" in Figure 3A, as explained above, numerous elongated materials protrude from the surface of the coating layer, and these materials correspond to the carbon fibers added as a thermal conductive material. On the surface of the substrate particles, the protrusion direction of the carbon fibers is random, and the carbon fibers are dispersed in the gel-like material, resulting in a state where there is no specific orientation.

[0107] Table 2 summarizes the composition and property measurement results for the composite materials of samples A1-A14 and B1-B7. The top row shows the mixing ratio (unit: mass%) of filler and matrix materials, and the middle row shows the amount of filler (unit: volume%) and the content of thermally conductive material (unit: volume%). Here, the content of thermally conductive material is calculated from the volume ratio of thermally conductive material in the filler and the amount of filler in the composite material. The bottom row summarizes the measurement results for specific gravity and thermal conductivity.

[0108] [Table 2]

[0109] Samples A1 to A14 were constructed by adding a filler to the matrix material by applying a coating layer, in which a thermally conductive substance was dispersed in a gel-like material, to the surface of substrate particles into which amino groups had been introduced by surface treatment. In samples A1 to A14, even though up to 50% by volume of filler was added, the specific gravity of the composite material was kept below the specific gravity of sample B1, which did not have any filler added, or an increase of 10% or less compared to the specific gravity of sample B1.

[0110] Furthermore, in samples A1 to A14, the thermal conductivity is 1.5 W / (m·K) or higher. These values ​​are more than seven times higher than the thermal conductivity of sample B1, which does not have any fillers added. From these results, it can be seen that in samples A1 to A14, in which a coating layer of thermally conductive material dispersed in a gel-like substance is provided on the surface of the base particles, the inclusion of low-density hollow base particles in the filler allows for high thermal conductivity while keeping the overall specific gravity low for the composite material with the added filler. It can be interpreted that the coating layer stably holds the thermally conductive material within the structure of the gel-like substance, and the gel-like substance forms chemical bonds with the base particles, dispersing the thermally conductive material and stably coating the base particles with a coating layer exhibiting high thermal conductivity. It is thought that the particles of the thermally conductive material held in a dispersed state in the gel-like substance come into contact with each other within the coating layer, forming continuous heat conduction paths on the surface of each filler particle. In addition, due to the volume occupied by the base material particles, the coating layers on the surfaces of adjacent fillers come into contact with each other, forming heat conduction paths between the filler particles, which is interpreted as having a high effect in improving thermal conductivity.

[0111] Here, we examine samples B2 to B5. In sample B2, surface-treated hollow glass particles themselves (AP-GB) are added to the matrix material, resulting in a lower specific gravity compared to sample B1, which does not contain any filler. However, because the filler does not contain a thermally conductive substance, the thermal conductivity has not improved compared to sample B1. The thermal conductivity of the glass itself is approximately 1.0 W / (m·K), which is higher than that of the matrix material. However, because the particles are hollow and contain air, phonon scattering occurs inside the particles, making heat conduction through the particles difficult. Thus, hollow glass particles themselves cannot be used as a thermally conductive filler.

[0112] As thermally conductive materials, sample B3 contains carbon fiber itself, and sample B4 contains alumina particles themselves, both added at a volume of 7.2%. This amount of addition is the same as that of samples A1-A5 in terms of thermally conductive material content. However, the thermal conductivity of samples B3 and B4 is 0.6 W / (m·K) or less. Sample B3 uses the same thermally conductive material as samples A1 and A4, and sample B4 uses the same thermally conductive material as samples A2 and A5, but the thermal conductivity obtained in samples B3 and B4 is less than 1 / 4 of that of samples A1, A4 and A2, A5. This is thought to be because, in samples B3 and B4, the volume occupied by the filler in the composite material is small, resulting in a small contact area between filler particles, and thus the formation of thermal conduction paths between the filler particles is not effectively achieved. In particular, the carbon fibers used in sample B3 have high anisotropy in shape, and in the press-molded resin cured test piece, the fiber axis is oriented in the in-plane direction of the test piece, which is perpendicular to the direction of thermal conductivity measurement. Therefore, it is thought that the heat conduction paths along the fiber axis, which are effective in improving the measured thermal conductivity, are not effectively formed in the thickness direction of the test piece.

[0113] In samples B5 and B6, carbon fiber and alumina, respectively, are added as thermal conductive materials, similar to samples B3 and B4, but the amount added has been increased to 30% by volume. This amount is the same as the filler content (volume percentage occupied by filler) in samples A1 to A5. In samples B5 and B6, the thermal conductivity is significantly improved compared to sample B1, which does not have any added filler. This result indicates that, compared to samples B3 and B4, the increased amount of thermal conductive material added increased the contact area between filler particles, forming effective heat conduction paths. However, because the volume occupied by the thermal conductive material itself is larger, the specific gravity of the composite material is nearly 1.4 times that of sample B1, or even greater. Sample B5 uses the same thermal conductive material as samples A1 and A4, and sample B6 uses the same material as samples A2 and A5, so these pairs are compared with each other. In samples B5 and B6, despite having more than four times the amount of thermally conductive material, the obtained thermal conductivity did not improve compared to samples A1, A4 and A2, A5. In fact, sample B5, which uses carbon fiber, showed a lower thermal conductivity. Thus, from the results for samples B3 to B6, it can be said that it is difficult to achieve both low specific gravity and high thermal conductivity when using fillers composed solely of thermally conductive materials such as carbon fiber or alumina. In particular, when using a thermally conductive material with high anisotropy, such as carbon fiber, alone, it is difficult to control the anisotropic orientation of the thermally conductive material and achieve high thermal conductivity in the desired direction.

[0114] In sample B7, the filler is composed of a thermally conductive material, base particles, and a gel-like substance, but the base particles used are hollow glass particles that have not undergone surface treatment. Sample B7 contains the same amount of carbon fiber as samples A1, A4, and B3 as the thermally conductive material, but the thermal conductivity of sample B7 is considerably lower than that of samples A1 and A4, which use a gel-like substance, and is close to that of sample B3, which has carbon fiber added alone as the thermally conductive material. In sample B7, since the base particles do not have functional groups on their surface that can form chemical bonds with the gel-like substance, it is thought that the coating layer is not firmly bonded to the surface of the base particles via chemical bonds. Correspondingly, in sample B7, it is thought that the thermally conductive material is not stably maintained in a state where it is dispersed in the gel-like substance and coating the surface of the base particles, but rather it is coating a very small area of ​​the surface of the base particles, or it is dispersed in the matrix resin independently of the base particles.

[0115] Finally, we compare samples A1 to A14 with each other. First, the sets of samples A1 to A3, and the sets of samples A4 and A5, use different types of thermally conductive materials. Comparing these samples with each other, samples A1 and A4, which use carbon fibers as the thermally conductive material, show significantly higher thermal conductivity compared to the other samples that use alumina particles or magnesium oxide particles. This corresponds to the fact that carbon fibers have higher thermal conductivity than alumina or magnesium oxide. Carbon fibers have high anisotropy in shape, and as explained above for sample B3, they show high thermal conductivity in the direction along the oriented fiber axis, but the effect of improving thermal conductivity in other directions is limited. However, as shown in the microscope image in Figure 3A, by dispersing carbon fibers in a gel-like substance, the carbon fibers are randomly dispersed within the coating layer and incorporated into the matrix resin, the effect of anisotropy in shape is reduced, and excellent thermal conductivity can be exhibited in any direction.

[0116] The types of gelling polymer materials used differ between the A1 and A4 sample set and the A2 and A5 sample set. However, when comparing the thermal conductivity of each set, the thermal conductivity does not change significantly regardless of which gelling polymer material is used. From this, it can be said that as long as the thermally conductive substance can be dispersed and held, the type of gelling substance does not have a significant effect on the thermal conductivity of the resulting filler.

[0117] In samples A1, A6-A9, the amount of carbon fiber added as a thermal conductive material to the gel-like substance in the mixed gel used as the raw material for the coating layer (dry volume ratio of [gelling polymer]:[thermal conductive material]) differs among the samples, and correspondingly, the content of the thermal conductive material differs among the samples. The content of the thermal conductive material is highest in samples A6, A7, A1, A8, and A9, and the thermal conductivity of the composite material is higher in that order. In other words, the higher the content of the thermal conductive material in the filler, the greater the effect of improving thermal conductivity. In sample A8, the dry volume ratio of [gelling polymer]:[thermal conductive material] is 30:70, but a high thermal conductivity of 2.0 W / (m·K) or higher is obtained in the region where the content of the thermal conductive material is higher than that.

[0118] In samples A1, A10-A12, the amount of mixed gel used relative to the substrate particles (dry volume ratio of [mixed gel]:[substrate particles]) differs from sample to sample, resulting in different content of thermally conductive material. The content is highest in samples A12, A11, A1, and A10, and in that order, the specific gravity of the composite material increases, as does the thermal conductivity. In other words, in the filler, the larger the proportion of the coating layer and the thicker the coating layer is formed, the greater the effect of improving thermal conductivity. In sample A1, the dry volume ratio of [mixed gel]:[substrate particles] is 30:70, but a high thermal conductivity of 2.0 W / (m·K) or more is obtained in the region where the proportion of the coating layer is larger than that.

[0119] In samples A1, A13, and A14, the amount of filler added to the composite material differs from one another. The amount of filler increases in the order of A14, A1, and A13, and in that order, the specific gravity of the composite material increases, as does the thermal conductivity. In other words, the more filler added, the greater the effect on improving thermal conductivity. Samples A1 and A14, with filler amounts exceeding 20 volume%, achieved high thermal conductivity of 2.0 W / (m·K) or higher. However, even when the filler content is increased to 50 volume% or more, as in sample A14, the effect on improving thermal conductivity becomes limited.

[0120] Although embodiments of the present disclosure have been described in detail above, the present invention is not limited in any way to the above embodiments, and various modifications are possible without departing from the spirit of the present invention. [Explanation of Symbols]

[0121] 1. (Thermally conductive) composite materials 10 (Thermally conductive) filler 11 Base material particles (hollow particles) 11a Hollow part 11b shell 12 Covering layer 12a Gel-like substance 12b Thermally conductive materials 2 Matrix Materials 5 Wire Harness 51 Insulated wire 52 connectors 53 Tapes

Claims

1. A wire harness containing a thermally conductive composite material, The aforementioned thermally conductive composite material is Thermally conductive filler, Includes a matrix material, The thermally conductive filler is dispersed in the matrix material. The aforementioned thermally conductive filler is Substrate particles and It comprises a coating layer that covers the aforementioned substrate particles, The aforementioned coating layer is A gel-like substance is bonded to the surface of the substrate particles via chemical bonds, thereby coating the surface of the substrate particles. A wire harness comprising a thermally conductive material having a higher thermal conductivity and a larger specific gravity than the base material particles and the gel-like material, dispersed within a layer of the gel-like material.

2. The wire harness according to claim 1, wherein the base material particles are hollow.

3. The wire harness according to claim 1 or claim 2, wherein the thermally conductive filler has a specific gravity of 1.8 or less.

4. The wire harness according to any one of claims 1 to 3, wherein the matrix material comprises an organic polymer.

5. The wire harness according to any one of claims 1 to 4, wherein the thermally conductive composite material has a specific gravity of 1.4 or less.

6. The wire harness according to any one of claims 1 to 5, wherein the thermally conductive composite material has a thermal conductivity of 0.9 W / (m·K) or more at room temperature.

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