JP7199708B2 - CORE-SHELL TYPE THERMAL CONDUCTIVE BEADS, METHOD FOR MANUFACTURING SAME, RESIN COMPOSITION AND MOLDED PRODUCT - Google Patents

Description

TECHNICAL FIELD The present invention relates to a core-shell type thermally conductive bead, a method for producing the same, a resin composition and a molded article.

Organic polymer materials such as resins have the characteristic of having a smaller specific gravity than inorganic materials such as metals and ceramics. Taking advantage of these characteristics, in various fields such as electronic equipment and transportation equipment, techniques have been proposed to replace inorganic materials with organic polymer materials for the purpose of weight reduction.

Efficiently dissipate heat generated from heat generating elements such as electronic parts mounted in personal computers, mobile terminals, automobiles, LED lighting, etc., and motors mounted in electric vehicles, hybrid vehicles, etc. is desired. However, organic polymer-based materials have a problem of lower thermal conductivity than inorganic materials. Attempts have therefore been made to obtain a lightweight and highly thermally conductive composition by adding a highly thermally conductive filler to an organic polymer material.

Fillers with high thermal conductivity include electrically conductive fillers such as graphite and metals, and electrically insulating fillers such as ceramics. Of these, electrically conductive fillers tend to be more thermally conductive than electrically insulating fillers. This is because electrically conductive fillers can conduct heat via lattice vibrations and conduction electrons, whereas electrically insulating fillers are more susceptible to conduction electrons than electrically conductive fillers. This is probably because the contribution to heat conduction is small.

As the filler, fine particles having so-called shape anisotropy, such as flaky or rod-shaped particles, are sometimes used as the filler. Fine particles having shape anisotropy form a heat path in the composition by contacting ends of the long diameter direction, that is, the direction having the largest outer dimension, in the composition, and the heat of the composition increases. Conductivity can be improved.

As a filler of this type, for example, Patent Document 1 describes a graphitized carbon powder obtained by pulverizing a carbonaceous foam obtained by heat-treating and carbonizing and foaming pitch after graphitization.

JP 2013-112915 A

When the resin composition containing the graphitized carbon powder of Patent Document 1 is molded by a method such as injection molding or transfer molding, the flow of the composition during molding causes the graphitized carbon powder to expand in the longitudinal direction, i.e., the outer dimension becomes maximum. It is easy to orient so that the direction to become is along the flow of the composition. Therefore, a heat path along the flow direction of the composition is more likely to be formed inside the molded article obtained by injection molding or the like. As a result of the above, the thermal conductivity of the molded body is no longer isotropic, and there is a possibility that desired thermal conductivity characteristics cannot be obtained.

In addition, when using an electrically conductive filler, it is expected that the thermal conductivity of the molded product will be higher than that of an electrically insulating filler, but the volume resistivity tends to be low, and in some cases the molding may be difficult. There is also the possibility that the body will become electrically conductive. However, organic polymer materials used in electronic parts, motors, and the like are sometimes required to have not only high thermal conductivity but also electrical insulation. From the standpoint of ensuring the electrical insulation of the molded article, the current situation is that electrically insulating fillers are used in such applications.

The present invention has been made in view of this background, and has a core-shell type heat conduction that can produce a molded body that has electrical insulation, excellent thermal conductivity, and isotropic thermal conductivity. The object of the present invention is to provide a thermally conductive bead, a method for producing the same, a resin composition comprising the core-shell type thermally conductive bead, and a molded article.

One reference embodiment of the present invention is a core portion containing a substance having electrical conductivity,
a shell layer covering the core portion and exposed to the surface,
Core-shell thermally conductive beads having a spherical shape,
The shell layer is
a resin portion made of a cured product of a curable polymer and having electrical insulation;
a filler that has higher thermal conductivity than the resin portion, has shape anisotropy and electrical insulation, and is held by the resin portion;
In the core-shell thermally conductive bead, the filler is oriented so that the long axis direction of the filler is along the surface of the core portion.

One aspect of the present invention is a method for producing a core-shell type thermally conductive bead according to the aspect described above,
and a spherical uncured shell layer that covers the surface of the core and contains the filler and the uncured curable polymer that are oriented along the surface of the core. a bead making step to make uncured beads exhibiting
a curing step of curing the curable polymer in the uncured shell layer ;
The bead production step includes:
a mixing step of creating a mixture of the filler and the curable polymer;
and a coating step of forming the uncured shell layer on the surface of the core by stirring the core and the mixture in a rotation-revolution mixer. It lies in the manufacturing method of the beads.

The core-shell type thermally conductive bead (hereinafter sometimes abbreviated as “thermally conductive bead” as appropriate) includes a core portion containing an electrically conductive substance and an electrically insulating shell covering the core portion. and a layer. Therefore, an electrically conductive material having high thermal conductivity can be used for the core portion while ensuring the electrical insulation of the thermally conductive beads by the shell layer.

In addition, the filler in the shell layer is oriented so that the long axis direction is along the surface of the core portion. By orienting the filler in the shell layer in this way, it is possible to easily form a heat path in which the fillers are connected to each other in the shell layer, thereby improving the thermal conductivity of the shell layer. Furthermore, by bringing the thermally conductive beads into contact with each other, heat can be efficiently transmitted to the adjacent thermally conductive beads via the shell layer. As a result, the shell layer can enhance the thermal conductivity of the thermally conductive beads while ensuring electrical insulation.

Also, the filler of the shell layer is held in the resin portion made of a cured product of a curable polymer. Therefore, the orientation state of the filler can be maintained even when an external force such as a shearing force is applied.

Furthermore, since the thermally conductive beads are spherical, when the resin composition containing the thermally conductive beads is molded by a molding method such as injection molding that involves the flow of the resin composition, heat conduction in the resin composition does not occur. The beads can be isotropically dispersed. Therefore, a heat path including the core portion and the shell layer is likely to be isotropically formed in the molding containing the thermally conductive beads.

As a result, the thermally conductive beads have electrical insulation, excellent thermal conductivity, and can produce a molded article having isotropic thermal conductivity.

Moreover, according to the manufacturing method of the above aspect, the thermally conductive bead provided with the shell layer can be easily manufactured.

1 is a cross-sectional view showing a main part of a thermally conductive bead in Example 1. FIG. FIG. 2 is an explanatory diagram showing a method of calculating the orientation angle θ of the filler. 3 is an example of an optical microscope image of the thermally conductive beads of Production Example 1. FIG. FIG. 4 is an explanatory view showing the essential parts of a rotation-revolution mixer used for producing thermally conductive beads in Example 1. FIG. 5 is an electron microscope image of a cross section of the thermally conductive beads of Example 1. FIG. FIG. 6 is a partial cross-sectional view showing a main part of a molded body containing thermally conductive beads in Example 2. FIG.

A. Core-Shell Thermally Conductive Bead The thermally conductive bead has a core and a shell layer covering the core.

<Core part>
The core portion contains an electrically conductive substance such as a metal such as gold, silver, or copper, or a conductive carbon material such as graphite, carbon fiber, carbon black, or carbon nanotube. That is, the core portion may be composed only of an electrically conductive material, or may include an electrically conductive material and a non-electrically conductive material.

The form of the substance having electrical conductivity in the core portion is not particularly limited. For example, the core may be a single particle made of the electrically conductive material described above. Further, the core portion may be a secondary particle formed by aggregating a plurality of electrically conductive primary particles. Also, the core portion may be a composite particle containing a plurality of electrically conductive particles and a binder that holds these particles. When the core portion is the composite particle described above, the binder may further contain additives such as flame retardants, stabilizers, plasticizers, surfactants, and antistatic agents.

The core portion preferably contains graphite or recycled crushed carbon fibers. These materials have high thermal conductivity among electrically conductive materials and are inexpensive. Therefore, in this case, the thermal conductivity of the thermally conductive beads can be further improved, and the cost can be more easily reduced.

Although the shape of the core portion is not particularly limited, it is preferably spherical from the viewpoint of making the thickness of the shell layer uniform. Also, the core preferably has a higher thermal conductivity than the filler in the shell layer. In this case, the thermal conductivity of the thermally conductive beads can be further improved.

The core portion preferably has electrical conductivity. In addition, the core part "has electrical conductivity" means that the volume resistivity of the core part is 10 ^-4 Ω·m or less.

The volume resistivity of the core tends to decrease as the volume ratio of the electrically conductive substance in the core increases. By increasing the volume ratio of the electrically conductive substance in the core portion, the thermal conductivity of the core portion can be further improved. Therefore, by increasing the volume ratio of the electrically conductive substance to the extent that the core has electrical conductivity, the thermal conductivity of the core can be further improved, and the thermal conductivity of the thermally conductive beads can be improved. It is possible to further improve the performance.

<Shell layer>
The surface of the core is covered with an electrically insulating shell layer. The thickness of the shell layer is preferably 1 μm or more and 20 μm or less. In this case, the shell layer can be evenly formed on the surface of the core portion, and electrical insulation can be imparted more reliably. Furthermore, by setting the thickness of the shell layer within the specific range, it is possible to reduce the amount of filler used while ensuring electrical insulation. This makes it possible to more easily reduce the material cost of the thermally conductive beads.

The shell layer includes a resin portion made of a cured product of a curable polymer and a filler retained within the resin portion.

·Resin part The curable polymer used in the resin part has fluidity in an uncured state, and has properties of becoming a solid having electrical insulation properties after curing. Here, the expression that the resin portion "has electrical insulation" means that the resin portion has a volume resistivity of 10 ¹⁰ Ω·m or more. The curable polymer may be a thermosetting polymer that is cured by heating, a photocurable polymer that is cured by light irradiation, or a moisture-curable polymer that is cured by moisture. Specific examples of curable polymers that can be used include epoxy resins, phenol resins, melamine resins, urea resins, unsaturated polyester resins, alkyd resins, oxazine resins, bismaleimide resins, and triazine resins. As the curable polymer, only one kind of these polymers may be used, or two or more kinds thereof may be used in combination.

-Filler The filler in the shell layer has electrical insulation and shape anisotropy, and has a higher thermal conductivity than the resin portion. Here, the filler "has electrical insulation" means that the filler has a volume resistivity of 10 ¹⁰ Ω·m or more.

In addition, the above-mentioned "having shape anisotropy" means that the minor diameter S _f [μm] of the filler, that is, the major diameter L _f [μm], that is, the value of the ratio L _f /S _f of the largest outer dimension value is 3 or more. Fillers having shape anisotropy include, for example, flake-like or scale-like shapes whose length or width is sufficiently wider than thickness, and rod-like or fiber-like shapes whose length is sufficiently long compared to thickness. A shape called a shape is included.

The value of L _f /S _f is preferably 1000 or less. When the value of L _f /S _f is excessively large, that is, when the major axis of the filler is excessively long relative to the minor axis, the fillers tend to get entangled with each other during the production process of the thermally conductive beads. It can be difficult to mold the cured beads into a spherical shape. By setting the value of L _f /S _f to 1000 or less, such a problem can be avoided and spherical heat conductive beads can be easily produced.

From the viewpoint of making spherical thermally conductive beads more easily, the major axis L _f of the filler is preferably less than 1 mm.

The ratio D _b /L _f of the major diameter L _f [μm] of the filler to the diameter D _b [μm] of the thermally conductive beads is preferably 10 or more and 1000 or less, and 20 or more and 500 or less. It is more preferable to have In this case, the filler in the shell layer can be more easily oriented in a desired direction in the process of producing the thermally conductive beads. Also, in this case, the fillers in the shell layer can be brought into contact with each other more easily, and more heat paths can be formed in the shell layer. As a result, the thermal conductivity of the thermally conductive beads can be further improved.

It is preferable that the filler be made of a material having electrical insulation and higher thermal conductivity than the resin portion. Examples of fillers include ceramic materials such as hexagonal boron nitride, aluminum oxide, aluminum nitride, silicon nitride, magnesium oxide, and silicon carbide, and highly crystalline materials such as PBO (polyparaphenylenebenzoxazole) fibers and cellulose nanofibers. One or more substances selected from organic fibers and the like can be employed.

In addition, the filler may be composed of a thermally isotropic substance, that is, a substance that can conduct heat equally in any direction, or a thermally anisotropic substance, that is, It may be composed of a material having a property that heat is easily conducted in a specific direction. When the filler is composed of a substance having thermal anisotropy, it is preferable that the longitudinal direction of the filler coincides with the direction in which heat is easily conducted. In this case, heat can be transferred more efficiently within the shell layer. As a result, the thermal conductivity of the molded article containing the thermally conductive beads can be further improved.

As the filler, hexagonal boron nitride can be particularly suitably used. Hexagonal boron nitride usually has a scaly or flaky shape, and has the property of being easily conductive of heat in the longitudinal direction, that is, in the direction perpendicular to the thickness direction. Therefore, by orienting the hexagonal boron nitride in the specific direction in the shell layer, the thermal anisotropy of the hexagonal boron nitride is effectively utilized to further improve the thermal conductivity of the shell layer. be able to. As a result, the thermal conductivity of the thermally conductive beads can be further improved.

The content of the filler in the shell layer is preferably 30% by volume or more and 95% by volume or less. If the filler content in the shell layer is too low, it may become difficult to form the shell layer. Moreover, in this case, it becomes difficult to form a heat path through the filler in the shell layer, which may lead to a decrease in thermal conductivity. On the other hand, if the content of the filler in the shell layer is excessively high, the resin portion becomes insufficient, so voids are likely to be formed between the fillers, which may lead to a decrease in strength and thermal conductivity. be. By setting the content of the filler within the specific range, these problems can be easily avoided and the thermal conductivity of the thermally conductive beads can be sufficiently enhanced.

Orientation state of filler The filler in the shell layer is oriented so that the long axis direction is along the surface of the core portion. For example, when the filler is in the form of flakes, the filler present in the shell layer is oriented such that its thickness direction is along the radial direction of the thermally conductive beads and its plate surface faces the surface of the shell layer. are doing. Further, for example, when the filler is rod-shaped, the filler present in the shell layer extends in the circumferential direction of the thermally conductive beads.

The degree of orientation of the filler in the shell layer can be evaluated based on the average value and standard deviation of orientation angles calculated by the following method. First, the thermally conductive bead is cut along a plane passing through its center of gravity to expose a cross section containing the center of gravity. This cross section is observed under a microscope to obtain a microscope image including the filler placed closest to the surface of the thermally conductive bead. Next, on the microscopic image, draw a straight line passing through the center of gravity of the thermally conductive bead and the center of gravity of the filler placed closest to the surface of the thermally conductive bead, and draw this straight line and the surface of the thermally conductive bead Determine the point of intersection with A tangent to the contour of the thermally conductive bead passing through this intersection is then determined. The orientation angle is defined as the angle between the tangent to the outline of the thermally conductive bead determined as described above and the straight line extending in the direction parallel to the major axis of the filler. In determining the orientation angle, the straight line extending parallel to the major axis of the filler is tilted counterclockwise with respect to the tangential line of the contour of the thermally conductive bead, and the straight line is tilted clockwise. Sometimes it is tilted. For the sake of convenience, it is assumed that the value of the orientation angle takes a positive value in the former case and takes a negative value in the latter case.

The orientation angles of a plurality of fillers can be calculated by the above method, and the arithmetic mean of these orientation angles can be used as the average orientation angle. Moreover, the standard deviation of the orientation angles can be calculated by statistically processing the orientation angles calculated for a plurality of fillers. In calculating the average value and standard deviation of the orientation angles, the more fillers used for the measurement of the orientation angles, the more accurate values can be calculated. The number of fillers used when calculating the average value and standard deviation of the orientation angles may be, for example, 100 or more.

Specifically, the average value of the orientation angles of the thermally conductive beads should be −10° or more and +10° or less.

The average orientation angle of the filler in the shell layer is preferably −8° or more and +8° or less, more preferably −5° or more and +5° or less. Also, the standard deviation of the orientation angles is preferably 25° or less, more preferably 15° or less. In this case, a heat path along the surface of the thermally conductive beads can be more easily formed in the shell layer. As a result, it is possible to further improve the thermal conductivity of the thermally conductive beads and the molded article provided therewith.

• Other Components The shell layer may contain additives such as flame retardants, stabilizers, plasticizers, surfactants, and antistatic agents within a range that does not impair the effects described above.

<Shape of thermally conductive beads>
The thermally conductive beads are spherical. Here, the above-mentioned "spherical shape" is a concept including a perfect sphere, an elliptical sphere, and a shape obtained by imparting unevenness to the surface of these shapes. Whether or not the thermally conductive beads are spherical can be determined by the average value of the oblateness calculated by the following method.

In calculating the oblateness, first, thermally conductive beads are placed on a flat plate and an optical microscope image is obtained. Two points are set on the outline of the thermally conductive bead in this optical microscope image so that the distance between them is the longest, and the length between the two points is measured. The length between these two points, that is, the value of the longest outer dimension measured based on the optical microscope image is defined as the major axis _Lb of the thermally conductive bead. Next, based on the optical microscope image, the outer dimension in the direction perpendicular to the major axis _Lb is measured, and this value is defined as the minor axis _Sb of the thermally conductive bead.

The flatness of each thermally conductive bead is a value calculated by the following formula (1) based on the value of the major axis L _b and the minor axis S _b obtained by the method described above.
Oblateness = (L _b - S _b )/L _b (1)

The flattening of a plurality of thermally conductive beads can be calculated by the above method, and the arithmetic mean of the flattening can be used as the average flattening. In calculating the average flattening ratio, the more the number of beads used for measuring the flattening ratio, the more accurate the calculation. The number of thermally conductive beads used for calculating the average flattening value may be, for example, 100 or more.

The oblateness takes a value of 0 or more and less than 1, and the closer the value is to 0, the closer the shape of the thermally conductive bead is to a true sphere. The average flatness of the thermally conductive beads is 0.4 or less.

The average flattening ratio is preferably less than 0.2, more preferably 0.1 or less. In this case, since the thermally conductive beads have a shape that is closer to a true sphere, uneven placement of the thermally conductive beads in the compact can be suppressed. As a result, a molded article having isotropic thermal conductivity can be obtained more easily.

In addition, the size of unevenness on the surface of the thermally conductive bead can be evaluated based on the average value of surface unevenness calculated by the following method. First, a thermally conductive bead is placed on a flat plate and an optical microscope image is obtained. Then, the length of the contour of the thermally conductive bead in the optical microscope image is measured, and this value is defined as the peripheral length P [μm]. Also, the area of the thermally conductive beads in the optical microscope image is measured, and this value is defined as the projected area S [μm ² ].

The degree of surface unevenness is a value calculated by the following formula (2) based on the peripheral length P of the thermally conductive beads and the projected area S of the thermally conductive beads.
Surface unevenness=P ² /4πS (2)

The surface unevenness is calculated for a plurality of thermally conductive beads by the above method, and the arithmetic mean of the surface unevenness can be used as the average value of the surface unevenness. When calculating the average value of the degree of surface unevenness, the more the number of beads used for measuring the degree of surface unevenness, the more accurate the value can be calculated. The number of thermally conductive beads used for calculating the average value of surface unevenness may be, for example, 100 or more.

The degree of surface unevenness takes a value of 1 or more, and the closer the value is to 1, the smaller the unevenness of the surface of the thermally conductive bead. The average value of surface unevenness is preferably less than 1.5, more preferably 1.2 or less. In this case, since the surfaces of the thermally conductive beads become smoother, the thermally conductive beads are more likely to come into contact with each other in the compact. As a result, the thermal conductivity of the molded body can be further improved.

The size of the thermally conductive beads is not particularly limited, but can be appropriately set within a range of, for example, a diameter of 10 μm or more and 3 mm or less. Specifically, the value of the diameter D _b of the thermally conductive bead is a sphere-equivalent diameter calculated by the following formula (3) based on the projected area S of the thermally conductive bead, that is, a true sphere having the same volume. diameter is used.
D _b =(4S/π) ^1/2 (3)

B. Method for producing thermally conductive beads The method for producing thermally conductive beads comprises:
and a spherical uncured shell layer that covers the surface of the core and contains the filler and the uncured curable polymer that are oriented along the surface of the core. a bead making step to make uncured beads exhibiting
and a curing step of curing the curable polymer in the uncured shell layer.

The bead making step includes a mixing step of making a mixture of the filler and the curable polymer;
and a coating step of forming the uncured shell layer on the surface of the core by stirring the core and the mixture in a rotation-revolution mixer.

In the mixing step, first, the filler and the uncured curable polymer are mixed using a kneader, a stirrer, or the like. The uncured curable polymer is liquid. In addition, the uncured curable polymer includes, for example, a base material that forms the skeleton of the polymer after curing, a curing agent that bonds and cures the base materials, and a curing accelerator that accelerates the curing reaction.

In mixing the filler and the uncured curable polymer, the components can be mixed in any order. For example, the main agent and the like may be mixed in a predetermined ratio to prepare a curable polymer, and then the curable polymer and the filler may be mixed, or the main agent and the filler may be mixed in advance, and then the curing agent and the like may be added. may be mixed further.

The viscosity of the curable polymer can also be adjusted by diluting the uncured curable polymer with an organic solvent before or during mixing with the filler. In this case, the organic solvent may be removed by volatilization or other means after the completion of mixing with the filler until the uncured beads are cured.

In the coating step, the core portion and the mixture are stirred in a rotation-revolution mixer. Here, the rotation-revolution type mixer is configured so that the container containing the core portion and the mixture is rotated around the revolution axis, and the container itself rotates around the rotation axis tilted with respect to the revolution axis. It means a mixer.

When the core part and the mixture are stirred in the rotation-revolution mixer, the core part in the container can be rolled by rotation while centrifugal force is applied to the core part in the container by revolution. Thereby, the mixture can be uniformly adhered to the surface of the core portion. Further, by rolling the core portion as described above, the filler in the mixture can be oriented so that the surface of the core portion and the long axis direction are parallel to each other, and the entire particle can be formed into a spherical shape. As a result, uncured beads can be produced.

In the curing step, the curable polymer in the uncured beads obtained in the bead making step is cured. As a method for curing the uncured beads, an appropriate method can be adopted according to the type of curable polymer. For example, if the curable polymer is a thermoset polymer, the uncured beads can be heated for a predetermined time and temperature.

C. Resin Composition A resin composition can be obtained by mixing the curable beads and a resin matrix made of a resin. The resin matrix may be composed of any resin. For example, the resin matrix may be thermosetting resins such as epoxy resins, phenolic resins, melamine resins, urea resins, unsaturated polyester resins, alkyd resins, oxazine resins, bismaleimide resins, triazine resins, polyamides, Thermoplastic resins such as polyester, polycarbonate, acrylic polymer, methacrylic polymer, polyimide, liquid crystal polymer, polyolefin, polystyrene, and polyphenylene ether may also be used. Moreover, as the resin matrix, one of a thermosetting resin and a thermoplastic resin may be used, or two or more of them may be used in combination.

Furthermore, the resin matrix may contain additives such as fillers, flame retardants, stabilizers, plasticizers, surfactants and antistatic agents. The filler in the resin matrix may have shape anisotropy or may have an isotropic shape. Even if the filler in the resin matrix has shape anisotropy, the filler in the resin matrix after molding can be prevented from being oriented in a specific direction by mixing it with the thermally conductive beads. . As a result, it is possible to easily obtain a molded article having isotropic heat conduction properties. From the viewpoint of ensuring the electrical insulation of the molded body, it is preferable that the filler in the resin matrix has electrical insulation.

The resin composition may contain one type of thermally conductive beads, or may contain a plurality of types of thermally conductive beads with different fillers and/or curable polymers. Also, the thermally conductive beads in the resin composition may have a single diameter, or may have various diameters. From the viewpoint of further improving the filling property of the thermally conductive beads in the compact, it is preferable to use a plurality of types of thermally conductive beads having different particle size distributions together as the thermally conductive beads.

The content of the thermally conductive beads in the resin composition can be appropriately set within a range of, for example, 10% by volume or more and 95% by volume or more. If the content of the thermally conductive beads is too low, it becomes difficult to form heat paths in the resin composition. As a result, there is a possibility that the thermal conductivity of the molded body will be lowered. Also, if the content of the thermally conductive beads is excessively high, the resin matrix may become insufficient, making it difficult to mold the resin composition into a desired shape. These problems can be easily avoided by setting the content of the thermally conductive beads within the specific range.

The content of the thermally conductive beads in the resin composition is preferably 30% by volume or more and 95% by volume or less. In this case, the thermally conductive beads are more likely to come into contact with each other, so that heat paths are more likely to be formed in the compact. Therefore, in this case, the thermal conductivity of the molded body can be made higher.

D. Molded Article By molding the resin composition, a molded article having isotropic thermal conductivity can be obtained. The molding method of the resin composition is not particularly limited, and can be selected from known molding methods such as compression molding, vacuum compression molding, transfer molding, injection molding, extrusion molding, and cast molding, and the type of resin matrix and desired molding. It can be appropriately selected according to the shape of the body and the like.

As described above, a heat path including the shell layer and the core portion of the thermally conductive beads is formed inside the molded body formed by molding the resin composition. Therefore, the molded body has isotropic thermal conductivity, that is, the property that heat is transmitted to the same degree in any direction. More specifically, the molded body has a thermal conductivity λ _A [W/m·K] in the direction in which the thermal conductivity is maximized and a thermal conductivity λ _B [W /m·K], the value of thermal conductivity anisotropy λ _A /λ _B is 1.5 or less.

The thermal conductivity λ _B is preferably 7 W/m·K or more. In this case, the thermal conductivity of the entire molded body can be further improved. Molded bodies having such thermal conductivity are suitable for applications such as heat dissipating members, for example.

Moreover, the volume resistivity of the compact is 1×10 ¹⁴ Ω·m or more . A molded article having such properties is suitable for applications requiring high thermal conductivity and electrical insulation.

(Example 1)
An embodiment of the thermally conductive beads and a method for producing the same will be described with reference to FIGS. 1 to 5. FIG. As shown in FIG. 1, the core-shell type thermally conductive bead 1 of this example includes a core portion 2 containing an electrically conductive substance, and a shell layer 3 covering the core portion 2 and exposed to the surface. It has a spherical shape as shown in FIG. As shown in FIG. 1, the shell layer 3 includes a resin portion 31 made of a cured product of a curable polymer and having electrical insulation, and a resin portion 31 having a higher thermal conductivity than the resin portion 31 and having shape anisotropy and electrical insulation. and a filler 32 held in the resin portion 31 . In addition, the filling material 32 in the shell layer 3 is oriented such that its long axis direction is along the surface 21 of the core portion 2 .

Although not shown in the drawing, the core portion 2 of this example is a composite particle in which a plurality of primary particles made of flaky graphite are held by a binder made of a cured bisphenol A epoxy resin. Moreover, the core part 2 of this example has electrical conductivity.

The resin portion 31 in the shell layer 3 of this example is a cured product of bisphenol A type epoxy resin. The filler 32 in the shell layer 3 is hexagonal boron nitride (GP grade manufactured by Denka Co., Ltd.) having an average major axis of 3.44 μm and an average minor axis of 0.33 μm.

The thermally conductive bead 1 of this example includes the above-described core portion 2, the filler 32 that covers the surface 21 of the core portion 2 and is oriented so that the major axis direction is along the surface 21 of the core portion 2, and the uncured cured and a curing step of curing the curable polymer in the uncured bead 100 . A method for producing the thermally conductive beads 1 of this example will be described in detail below.

- Manufacturing method of the core part 2 As the primary particles, "UP-20" (average major axis: 3.37 µm, average minor axis: 0.28 µm) manufactured by Nippon Graphite Co., Ltd. was used. A bisphenol A type epoxy resin was used as a binder. Specifically, the binder includes a main agent having a bisphenol A skeleton ("jER (registered trademark) 827" manufactured by Mitsubishi Chemical Corporation) and 4-methylcyclohexane-1,2-dicarboxylic anhydride (Tokyo Kasei Kogyo Co., Ltd.) and 2-ethyl-4-methylimidazole (Tokyo Kasei Kogyo Co., Ltd.) as a curing accelerator.

An uncured curable polymer was prepared by mixing 108 parts by mass of a curing agent and 1 part by mass of a curing accelerator with 100 parts by mass of the main agent. Next, 15 parts by mass of primary particles were added to 5 parts by mass of this curable polymer and mixed to obtain a clay-like core mixture. The obtained core mixture was formed into granules using a pelletizer to prepare a core portion precursor.

Next, after putting the core portion precursor into the container 41 of the rotation/revolution type mixer 4 ("ARE-310" manufactured by Thinky Co., Ltd.) described later, the revolution speed of the container 41 was set to 2000 rpm, and the rotation speed was set to 800 rpm. Stirring was performed for 2 minutes. As a result, the core portion precursor was molded into a spherical shape to obtain an uncured core portion.

Thereafter, the uncured core portion is heated at a temperature of 80° C. for 1 hour, then at a temperature of 120° C. for 1 hour, and then at a temperature of 150° C. for 3 hours. The curable polymer was cured. As described above, the core portion 2 including the primary particles made of flaky graphite and the binder made of the cured bisphenol A type epoxy resin and holding the primary particles was obtained.

Bead Making Step Next, a bead making step was performed to form an uncured shell layer on the surface 21 of the core portion 2 obtained by the method described above. The bead production process of this example includes a mixing process of producing a shell mixture, which is a mixture of the filler 32 and the curable polymer, and agitating the core part 2 and the shell mixture in the rotation-revolution mixer 4 shown in FIG. and a coating step of forming an uncured shell layer on the surface 21 of the core portion 2 .

In the mixing step, an uncured curable polymer was prepared in the same manner as for the core portion 2 . Next, 15 parts by mass of filler 32 was added to 5 parts by mass of this curable polymer and mixed to obtain a clay-like shell mixture.

In the coating step, 10 parts by mass of the core part 2 and 1 part by mass of the shell mixture were placed in the container 41 of the rotation-revolution mixer 4 (“ARE-310” manufactured by Thinky Co., Ltd.) shown in FIG. After that, the container 41 was stirred for 2 minutes by setting the revolution speed to 2000 rpm and the rotation speed to 800 rpm. After the stirring is completed, 1 part by mass of the shell mixture is added to the container 41 with respect to 10 parts by mass of the core part 2, and the container 41 is stirred for 2 minutes with the revolution speed set to 2000 rpm and the rotation speed set to 800 rpm. was repeated four times.

The container 41 of the rotation/revolution type mixer 4 used in this example has a cylindrical shape as shown in FIG. Further, the rotation axis 42 of the container 41 is arranged so as to be inclined at 45° with respect to the revolution axis 43 . The container 41 is configured to revolve around the revolution axis 43 and rotate about the rotation axis 42 (that is, the central axis of the container 41).

The core part 2 in the container 41 rolls by rotation while being subjected to centrifugal force due to the revolution of the container 41 . As a result, the shell mixture can be evenly adhered to the surface 21 of the core portion 2, and the uncured beads 100 can be molded into a spherical shape. Furthermore, by rolling the core portion 2, the filler 32 adhering to the core portion 2 can be oriented so that the surface of the core portion 2 and the long axis direction are parallel to each other, and the entire particle can be formed into a spherical shape. As a result, an uncured shell layer is formed on the surface 21 of the core portion 2, and an uncured bead 100 can be obtained.

In the curing step, the uncured beads 100 were heated to cure the curable polymer. Specifically, in the curing step of this example, the uncured bead 100 is formed with a temperature history of holding at a temperature of 80° C. for 1 hour, then holding at a temperature of 120° C. for 1 hour, and then holding at a temperature of 150° C. for 3 hours. heated. As described above, a core-shell type thermally conductive bead 1 was obtained in which 5 parts by mass of the shell layer 3 was attached to the surface 21 with respect to the 10 parts by mass of the core part 2 . Table 1 shows the mass ratio and volume ratio between the filler 32 and the resin portion 31 in the shell layer 3 of the thermally conductive bead 1 of this example.

Next, the shape of the thermally conductive beads 1, the thickness of the shell layer 3, and the orientation state of the filler 32 in the shell layer 3 were evaluated by the following methods.

- Shape of Thermally Conductive Bead 1 An optical microscope image was obtained by observing the thermally conductive bead 1 with an optical microscope. Using image analysis software (“WinROOF2015” manufactured by Mitani Shoji Co., Ltd.), the major axis L _b [μm], the minor axis S _b [μm], and the perimeter P of the thermally conductive beads 1 in the optical microscope image shown in FIG. [μm] and projected area S [μm ² ] were calculated. Then, for each thermally conductive bead 1, the values of flatness, surface unevenness and diameter _Db were calculated based on the following formulas (1) to (3).
Oblateness = (L _b - S _b )/L _b (1)
Surface unevenness=P ² /4πS (2)
D _b =(4S/π) ^1/2 (3)

After measuring the values of flatness, surface unevenness and diameter D _b for 100 or more thermally conductive beads 1, statistical processing was performed to calculate the average value and standard deviation. Table 1 shows the average values and standard deviations of the flatness, surface unevenness, and diameter D _b of the thermally conductive beads 1 of this example.

- Thickness of Shell Layer 3 After cutting the thermally conductive bead 1, the cross section was polished to expose the cross section passing through the center of gravity. A region including the surface of the thermally conductive bead 1 in this cross section was observed with an optical microscope, and the thickness of the shell layer 3 was measured at 10 randomly selected locations. The average value of these was used as the thickness of the shell layer 3 . The thickness of the shell layer 3 of this example was as shown in Table 1.

Orientation state of filler 32 After cutting the thermally conductive beads 1, the cross section was polished to expose the cross section passing through the center of gravity. This cross section was observed with a scanning electron microscope, and the electron microscope image shown in FIG. 5 was obtained. Although not shown in the figure, observation of the shell layer 3 at a higher magnification revealed that the thickness direction of the filler 32 in the shell layer 3 was along the radial direction of the thermally conductive beads 1 and The major axis direction of the material 32 was oriented along the surface 11 of the thermally conductive bead 1 .

Next, using image analysis software ("WinROOF2015" manufactured by Mitani Shoji Co., Ltd.), the orientation angle θ of the filler 32 arranged closest to the surface 11 of the thermally conductive bead 1 by the following method (see FIG. 2) was calculated. First, on an electron microscope image, draw a straight line L1 passing through the center of gravity W _b of the thermally conductive bead 1 and the center of gravity W _f of the filler 32 located closest to the surface 11 of the thermally conductive bead 1, An intersection point 12 between this straight line L1 and the surface 11 of the thermally conductive bead 1 was determined. Then, a tangent line L2 of the contour of the thermally conductive bead 1 passing through this intersection point 12 was determined. The angle formed by the tangent line L2 determined as described above and the straight line L3 extending in the direction parallel to the major axis of the filler 32 was defined as the orientation angle θ. For the sake of convenience, part of the description of the filler 32 is omitted in FIG.

After measuring the value of the orientation angle θ for 100 or more fillers 32, statistical processing was performed to calculate the average value and standard deviation of the orientation angle θ. Table 1 shows the average value and standard deviation of the orientation angle θ in the thermally conductive beads 1 of this example.

As shown in Table 1, in the thermally conductive bead 1 of this example, the surface 21 of the core portion 2 is covered with the shell layer 3 . Since the filler 32 and the resin portion 31 constituting the core portion 2 have electrical insulation properties, by covering the core portion 2 with the shell layer 3, the electrical insulation property of the thermally conductive beads 1 is ensured. be able to.

Moreover, the filler 32 in the shell layer 3 is oriented in the specific direction. As a result, a heat path in which the fillers 32 are connected to each other can be easily formed in the shell layer 3 , and heat can be efficiently transmitted along the shell layer 3 . Furthermore, by bringing the thermally conductive beads 1 into contact with each other, heat can be efficiently transmitted to adjacent thermally conductive beads 1 via the shell layer 3 .

Also, the filler 32 of the thermally conductive beads 1 is held in the resin portion 31 made of a cured product of a curable polymer. Therefore, when an external force such as a shearing force is applied, the collapse of the thermally conductive beads 1 can be suppressed, and the oriented state of the filler 32 in the shell layer 3 can be maintained.

Furthermore, since the thermally conductive beads 1 are spherical, when the resin composition containing the thermally conductive beads 1 is molded by a molding method such as injection molding that involves the flow of the resin composition, heat is generated in the resin composition. Conductive beads 1 can be isotropically dispersed. Therefore, a heat path including the shell layer 3 and the core portion 2 is likely to be isotropically formed in the molding including the thermally conductive beads 1 .

As a result, according to the thermally conductive beads 1 of this example, it is possible to produce a molded body having electrical insulation, excellent thermal conductivity, and isotropic thermal conductivity.

(Example 2)
In this example, an example of a molded body 5 containing thermally conductive beads 1 will be described with reference to FIG. Note that, of the reference numerals used in the examples after this example, the same reference numerals as those used in the previous examples denote the same components and the like as those in the previous examples unless otherwise specified.

As shown in FIG. 6 , the molded body 5 of this example includes a resin matrix 51 and thermally conductive beads 1 dispersed in the resin matrix 51 . Specifically, in this example, two types of molded bodies 5 (specimens T1 and T2) were produced as follows.

・ Specimen T1
The resin matrix 51 of the specimen T1 contains a cured product 511 of bisphenol A type epoxy resin and a filler 512 dispersed in the cured product 511 . Specifically, the cured product 511 in the resin matrix 51 of the specimen T1 is a main agent having a bisphenol A skeleton (“jER (registered trademark) 827” manufactured by Mitsubishi Chemical Corporation, 4-methylcyclohexane-1 as a curing agent, It is a bisphenol A type epoxy resin obtained by curing a mixture of 2-dicarboxylic anhydride (manufactured by Tokyo Chemical Industry Co., Ltd.) and 2-ethyl-4-methylimidazole (manufactured by Tokyo Chemical Industry Co., Ltd.) as a curing accelerator. Further, the filler 512 in the specimen T1 is specifically hexagonal boron nitride (GP grade manufactured by Denka Co., Ltd.).

In preparing the test sample T1, first, 108 parts by mass of a curing agent and 1 part by mass of a curing accelerator were mixed with 100 parts by mass of a main agent to prepare an uncured curable polymer. Next, 15 parts by mass of filler 512 was added to 5 parts by mass of this curable polymer and mixed to obtain an uncured resin matrix.

Next, 3 parts by mass of the thermally conductive beads 1 of Example 1 were mixed with 1 part by mass of the uncured resin matrix to prepare a resin composition. This resin composition was placed in a cylindrical mold having an inner diameter of φ40 mm, and compression molding was performed. In compression molding, after heating under pressure of 37 MPa and temperature of 80° C. for 1 hour, the temperature was raised to 120° C. while maintaining the pressure, and the temperature of 120° C. was maintained for 1 hour. After that, the molded body 5 removed from the mold was heated at a temperature of 150° C. for 3 hours to sufficiently harden the resin matrix 51 . As a result, a disk-shaped compact 5 having a thickness of 1.2 mm and a diameter of 40 mm was obtained. This molded body 5 was designated as test body T1.

Table 2 shows the mass ratio of the core portion, the shell layer, and the resin matrix in the specimen T1. Moreover, the mass ratio of the hexagonal boron nitride and the graphite contained in the specimen T1 was hexagonal boron nitride:graphite=50:50.

・ Specimen T2
A compact 5 was produced under the same conditions as those for the test sample T1, except that 3 parts by mass of the thermally conductive beads 1 of Example 1 were mixed with 3 parts by mass of the uncured resin matrix. This molded body 5 was used as a test body T2. Table 2 shows the mass ratio of the core portion, the shell layer, and the resin matrix in the specimen T2. Moreover, the mass ratio of the hexagonal boron nitride and the graphite contained in the specimen T2 was hexagonal boron nitride:graphite=33:67.

In this example, molded bodies (specimens T3 and T4) produced without using the thermally conductive beads 1 were prepared for comparison with the specimens T1 and T2. A specific method for producing the specimens T3 and T4 is as follows.

・ Specimen T3
Instead of the thermally conductive beads 1 of Example 1, the core part 2 prepared by the same method as in Example 1 was used, and 1 part by mass of the core part 2 was mixed with 1 part by mass of the uncured resin matrix. Except for this, a compact was produced under the same conditions as those for the test sample T1. This compact was designated as test sample T3. That is, the electrically conductive core portions 2 are dispersed in the resin matrix 51 of the specimen T3. Further, the electrically insulating shell layer 3 is not provided on the surface 21 of the core portion 2 .

Table 2 shows the mass ratio of the core portion, the shell layer, and the resin matrix in the specimen T3. Moreover, the mass ratio of the hexagonal boron nitride and the graphite contained in the specimen T3 was hexagonal boron nitride:graphite=50:50.

・ Specimen T4
Instead of the thermally conductive beads 1 of Example 1, the core part 2 prepared by the same method as in Example 1 was used, and 1 part by mass of the core part 2 was mixed with 2 parts by mass of the uncured resin matrix. Except for this, a compact was produced under the same conditions as those for the test sample T2. This compact was designated as test sample T4.

Table 2 shows the mass ratio of the core portion, the shell layer, and the resin matrix in the specimen T4. Moreover, the mass ratio of the hexagonal boron nitride and the graphite contained in the specimen T4 was hexagonal boron nitride:graphite=33:67.

The thermal conductivity of the specimens T1 to T4 obtained above was evaluated by the following method.

・Method for measuring thermal conductivity and thermal conductivity anisotropy Using a thermophysical property measuring device ("Thermo Wave Analyzer TA35" manufactured by Bethel Co., Ltd.), the thickness direction of the specimen and the direction perpendicular to the thickness direction (that is, , direction along the circular surface of the ^specimen ) ^were measured. Also, the constant pressure specific heat Cp [J·K ⁻¹ ·kg ⁻¹ ] of the specimen was measured using a differential scanning calorimeter (“DSC7000” manufactured by Hitachi High-Tech Science Co., Ltd.). Furthermore, the density ρ [kg·m ^-3 ] of the specimen was measured by the Archimedes method. All of these measurements were performed in an environment of 25±2°C.

By substituting the values of the thermal diffusivity λ, the constant pressure specific heat Cp and the density ρ obtained above into the following formula (4), the thermal conductivity value α [W m ^-1・K ⁻¹ ] can be calculated. Table 2 shows the thermal conductivity in the thickness direction and the thermal conductivity in the direction perpendicular to the thickness direction of the specimens T1 to T4. In the manufacturing method of this example, one of the thickness direction of the specimen and the direction perpendicular to the thickness direction is the direction in which the thermal conductivity is maximized, and the other is the direction in which the thermal conductivity is minimized. estimated to be
α=λ×Cp×ρ (4)

As shown in Table 2, the volume resistivity of specimens T1 and T2 is 1×10 ¹⁴ Ω·m or more, and it can be understood that these specimens have electrical insulation. In addition, the thermal conductivity λ _{A [W / m K] in the direction in which the thermal conductivity of these specimens is the maximum, and the thermal conductivity λ B [} W / m K] in the direction in which the thermal conductivity is the minimum ], the thermal conductivity anisotropy λ _A /λ _B is 1.5 or less. From these results, it can be understood that these specimens have isotropic thermal conductivity.

It is presumed that the arrangement of the filler 32 of the thermally conductive beads 1 is maintained inside the test bodies T1 and T2 even after molding, as shown in an example in FIG. It is considered that the heat path including the shell layers 3 is formed isotropically by the shell layers 3 of the thermally conductive beads 1 coming into contact with each other.

On the other hand, although the specimens T3 and T4 had thermal conductivity equal to or higher than that of the specimens T1 and T2, they could not ensure electrical insulation.

Specific aspects of the thermally conductive beads 1, the resin composition, and the molded article 5 according to the present invention are not limited to the aspects described in Examples 1 and 2, which impairs the gist of the present invention. The configuration can be changed as appropriate within a range that is not necessary.

For example, in Example 1, an example of using composite particles of graphite and epoxy resin as the core portion 2 was shown, but the core portion 2 is, for example, spherical metal particles, spherical graphite particles, etc., which have electrical conductivity. Particles formed by forming a substance into a spherical shape may also be used.

In addition, in Example 2, an example of the resin composition and molded body 5 using the resin matrix 51 having the same composition as the thermally conductive beads 1 was shown, but the composition of the resin matrix 51 was different from that of the thermally conductive beads. It can be different from 1.

REFERENCE SIGNS LIST 1 thermally conductive bead 2 core portion 21 surface 3 shell layer 31 resin portion 32 filler

Claims (9)

a core portion containing a substance having electrical conductivity;
a shell layer covering the core portion and exposed to the surface,
It has a spherical shape,
The shell layer is
a resin portion made of a cured product of a curable polymer and having electrical insulation;
a filler that has higher thermal conductivity than the resin portion, has shape anisotropy and electrical insulation, and is held by the resin portion;
A method for producing a core-shell type thermally conductive bead, wherein the filler is oriented so that the major axis direction of the filler is along the surface of the core portion ,
and a spherical uncured shell layer that covers the surface of the core and contains the filler and the uncured curable polymer that are oriented along the surface of the core. a bead making step to make uncured beads exhibiting
a curing step of curing the curable polymer in the uncured shell layer ;
The bead production step includes:
a mixing step of creating a mixture of the filler and the curable polymer;
and a coating step of forming the uncured shell layer on the surface of the core by stirring the core and the mixture in a rotation-revolution mixer. Bead manufacturing method. a core portion containing a substance having electrical conductivity;
a shell layer covering the core portion and exposed to the surface,
Core-shell thermally conductive beads having a spherical shape,
The core portion is a composite particle containing a plurality of electrically conductive particles and a binder that holds these particles,
The shell layer is
a resin portion made of a cured product of a curable polymer and having electrical insulation;
a filler that has higher thermal conductivity than the resin portion, has shape anisotropy and electrical insulation, and is held by the resin portion;
The filler is oriented such that the longitudinal direction of the filler is along the surface of the core portion,
The core-shell type, wherein the ratio D _b /L _f of the major axis L _f [μm] of the filler and the diameter D _b [μm] of the core-shell thermally conductive bead is 10 or more and 1000 or less. thermally conductive beads. 3. The core-shell type thermally conductive bead according to claim 2 , wherein the shell layer has an average thickness of 1 μm or more and 20 μm or less. 4. The core-shell type thermally conductive bead according to claim 2 , wherein the content of said filler in said shell layer is 30% by volume or more and 95% by volume or less. The core-shell type thermally conductive bead according to any one of claims 2 to 4 , wherein the filler is composed of hexagonal boron nitride. a resin matrix made of resin;
and the core-shell type thermally conductive beads according to any one of claims 2 to 5 dispersed in the resin matrix. 7. The resin composition according to claim 6 , wherein the content of said core-shell type thermally conductive beads is 30% by volume or more and 95% by volume or less. a resin matrix made of resin;
core-shell type thermally conductive beads dispersed in the resin matrix, and
Thermal conductivity value λ _A [W/m K] in the direction of maximum thermal conductivity and λ _B [W/m K] in the direction of minimum thermal conductivity The value of the thermal conductivity anisotropy λ _A /λ _B , which is the ratio, is 1.5 or less ,
A volume resistivity of 1×10 ¹⁴ Ω·m or more,
The core-shell type thermally conductive bead comprises a core portion containing a substance having electrical conductivity;
a shell layer covering the core portion and exposed to the surface,
It has a spherical shape,
The shell layer is
a resin portion made of a cured product of a curable polymer and having electrical insulation;
a filler that has higher thermal conductivity than the resin portion, has shape anisotropy and electrical insulation, and is held by the resin portion;
The molded article , wherein the filler is oriented such that the longitudinal direction of the filler is along the surface of the core portion . 9. The molded article according to claim 8 , wherein the thermal conductivity value λ _B [W/m·K] in the direction in which the thermal conductivity is minimized is 7 W/m·K or more.

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