Thermally conductive sheet and method for manufacturing the same

Abstract

To provide a thermally conductive sheet with excellent compressibility and restorability, and a method for manufacturing such a thermally conductive sheet.SOLUTION: A thermally conductive sheet comprises resin and graphite particles. The thermally conductive sheet includes multiple voids. The resin contains a cross-linked resin. The graphite particles are oriented at an orientation angle of 60° to 90° relative to the major surface of the thermally conductive sheet. The volume fraction of the graphite particles in the thermally conductive sheet is 35 vol% or more, and the porosity calculated according to the following formula (1) is 20% or more. Porosity (%)=100×(1 - measured specific gravity of thermally conductive sheet / theoretical specific gravity of thermally conductive sheet) (1)SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a thermally conductive sheet and a method for manufacturing the thermally conductive sheet. [Background technology]

[0002] In recent years, the amount of heat generated by electronic components such as power semiconductors (e.g., IGBT modules) and integrated circuit (IC) chips has increased as their performance has improved. As a result, electronic devices that use these components need to take measures to prevent malfunctions caused by temperature increases in the electronic components.

[0003] As a countermeasure against functional failures caused by temperature rise in electronic components, a common method is to promote heat dissipation by attaching a heat sink, heat sink plate, heat dissipation fin, or other heat sink made of metal to the heat generating body of the electronic component, etc. When using a heat sink, a sheet-like member with high thermal conductivity (hereinafter also referred to as a "thermal conduction sheet") is used between the heat generating body and the heat sink to efficiently transfer heat from the heat generating body to the heat sink.

[0004] Thermally conductive sheets are required to have high flexibility as well as high thermal conductivity to promote heat dissipation. In recent years, various methods for manufacturing thermally conductive sheets have been proposed to further improve the properties of thermally conductive sheets.

[0005] For example, Patent Document 1 discloses a thermally conductive sheet containing a resin and graphite particles, the thermally conductive sheet having a first main surface and a second main surface separated by a thickness, the graphite particles having an aspect ratio of more than 1.0, the long axes of the graphite particles being oriented in the thickness direction of the thermally conductive sheet, the volume fraction of the graphite particles in the thermally conductive sheet being 50% by volume or more, and after the thermally conductive sheet is compressed by 30% in the thickness direction from the first main surface side, the long axes of the graphite particles do not have an inflection point in a region located between the first main surface and the second main surface in a cross-sectional view in the thickness direction of the thermally conductive sheet, the region being 30% or more and 100% or less of the thickness of the thermally conductive sheet compressed by 30%. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2023-151582 Summary of the Invention [Problem to be solved by the invention]

[0007] Here, in addition to compressibility, the thermal conductive sheet is also required to have excellent recovery properties after pressure in the thickness direction is removed, in order to further improve its ability to follow the expansion and contraction of the component to which the thermal conductive sheet is applied.

[0008] Therefore, an object of the present invention is to provide a thermally conductive sheet that is excellent in compressibility and restorability, and a method for manufacturing such a thermally conductive sheet. [Means for solving the problem]

[0009] The present inventors have conducted extensive research to achieve the above object, and have newly discovered that a thermally conductive sheet containing a resin and graphite particles has excellent compressibility and restorability when the thermally conductive sheet contains a plurality of voids, the resin contains a crosslinked resin, the graphite particles are oriented at a predetermined angle within the sheet, the volume fraction of the graphite particles in the thermally conductive sheet is a certain level or higher, and the porosity calculated by a predetermined formula is 20% or higher, thereby completing the present invention.

[0010] That is, the present invention aims to advantageously solve the above-mentioned problems, and the present invention is a thermally conductive sheet comprising: [1] a resin and graphite particles, wherein the thermally conductive sheet contains a plurality of voids, the resin contains a crosslinked resin, the graphite particles are oriented at an orientation angle of 60° or more and 90° or less with respect to a main surface of the thermally conductive sheet, the volume fraction of the graphite particles in the thermally conductive sheet is 35% by volume or more, and the porosity calculated according to the following formula (1) is 20% or more. Porosity (%) = 100 × (1 - measured specific gravity of thermal conductive sheet ÷ theoretical specific gravity of thermal conductive sheet) (1) Such a thermally conductive sheet has excellent compressibility and restorability.

[0011] [2] Here, the thermally conductive sheet of [1] above preferably has a porosity of 20% or more and 35% or less. If the porosity is within the above range, the thermally conductive sheet can achieve both sufficient compressibility and excellent thermal conductivity.

[0012] [3] In the thermally conductive sheet of [1] or [2] above, it is preferable that, among the plurality of voids in the thermally conductive sheet, when the outline of the voids is approximated as an ellipse, the length of the major axis of the voids is 20% or more of the thickness of the thermally conductive sheet, the major axis of the voids is oriented at an angle of 60° to 90° with respect to the main surface of the thermally conductive sheet, and the voids have an average aspect ratio of 15 or more. If the thermally conductive sheet has voids that satisfy the above conditions, the thermally conductive sheet has even better compressibility.

[0013] [4] In the thermally conductive sheet of any one of the above [1] to [3], the graphite particles preferably have a volume average particle diameter of 40 μm or more. When the graphite particles have a volume average particle diameter of 40 μm or more, the thermally conductive sheet has excellent thermal conductivity.

[0014] [5] In the thermally conductive sheet of any one of [1] to [4] above, the crosslinked resin preferably includes a crosslinked product of a crosslinkable resin that is solid at room temperature and normal pressure, and a crosslinked product of a crosslinkable resin that is liquid at room temperature and normal pressure. If the thermally conductive sheet includes a crosslinked product of a crosslinkable resin that is solid at room temperature and normal pressure, and a crosslinked product of a crosslinkable resin that is liquid at room temperature and normal pressure, the thermally conductive sheet will have even better thermal conductivity.

[0015] [6] In the thermally conductive sheet of any one of the above [1] to [5], the gel fraction is preferably 80% or more. When the gel fraction is equal to or greater than the above lower limit, the thermally conductive sheet has even better recovery properties. The gel fraction can be measured by the method described in the examples of the present specification.

[0016] [7] The present invention also provides a method for producing a thermally conductive sheet according to any one of [1] to [6] above, comprising the steps of: a pre-thermally conductive sheet forming step of pressurizing a composition containing a resin containing a crosslinkable resin, graphite particles, a crosslinking agent, and a foaming agent to form a sheet to obtain a pre-thermally conductive sheet; a laminate forming step of stacking a plurality of pre-thermally conductive sheets in the thickness direction or folding or rolling the pre-thermally conductive sheets to obtain a laminate; a crosslinking reaction step of heating the laminate while applying pressure to cause a crosslinking and foaming reaction; and a slicing step of slicing the laminate at an angle of 45° or less to the stacking direction to obtain a thermally conductive sheet. The thermally conductive sheet obtained by this production method has excellent compressibility and recovery properties. [Effects of the Invention]

[0017] According to the present invention, it is possible to provide a thermally conductive sheet having excellent compressibility and restorability, and a method for manufacturing such a thermally conductive sheet. [Brief explanation of the drawings]

[0018] [Figure 1] 1 is a schematic diagram of a thickness direction cross section of an example of a thermally conductive sheet of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0019] Hereinafter, embodiments of the present invention will be described in detail. The thermally conductive sheet of the present invention can be used by being sandwiched between a heat generating body and a heat dissipating body. That is, the thermally conductive sheet of the present invention can function as a heat dissipating member and can constitute a heat dissipating device together with a heat dissipating body such as a heat sink, a heat dissipating plate, or a heat dissipating fin. The thermally conductive sheet of the present invention can be produced using the method for producing a thermally conductive sheet of the present invention.

[0020] (thermal conductive sheet) The thermal conductive sheet of the present invention includes a resin and graphite particles, and also includes a plurality of voids. The resin included in the thermal conductive sheet contains a crosslinked resin, and the graphite particles are oriented at an angle of 60° to 90° relative to the main surface of the thermal conductive sheet. The thermal conductive sheet is further characterized in that the volume fraction of the graphite particles in the thermal conductive sheet is 35% by volume or more, and the porosity calculated according to the following formula (1) is 20% or more. Porosity (%) = 100 × (1 - measured specific gravity of thermal conductive sheet ÷ theoretical specific gravity of thermal conductive sheet) (1) The thermally conductive sheet of the present invention that satisfies these conditions has excellent compressibility due to the presence of a certain proportion of voids, and also has excellent restorability due to the cross-linked structure of the resin in the sheet.

[0021] <Resin> The thermal conductive sheet of the present invention contains a resin, which allows the heat generating element and the heat dissipating element to be well adhered to each other via the thermal conductive sheet. In this specification, rubber and elastomer are included in the term "resin." The resin that can be contained in the thermally conductive sheet of the present invention constitutes a matrix resin and also functions as a binder that binds the graphite particles together. The resin contained in the thermal conductive sheet of the present invention contains a crosslinked resin, and optionally contains a resin other than the crosslinked resin (another resin).

[0022] <<Crosslinked resin>> The resin contained in the thermal conductive sheet of the present invention contains a crosslinked resin. The crosslinked resin is a resin crosslinked by a crosslinking agent. The thermal conductive sheet of the present invention contains a crosslinked resin as the resin, and thus has appropriate strength and elasticity, resulting in excellent recovery.

[0023] Here, the crosslinked resin preferably contains a resin that is solid at room temperature and normal pressure. In this specification, "room temperature" refers to 23° C., and "normal pressure" refers to 1 atm (absolute pressure).

[0024] The crosslinked resin is formed by crosslinking a crosslinkable resin with a crosslinking agent. A crosslinking accelerator can be used in the crosslinking reaction. The crosslinking accelerator can be, but is not limited to, a peroxide crosslinking agent such as dicumyl peroxide or sulfur.

[0025] [Crosslinkable resin] The crosslinkable resin is not particularly limited, and although it depends on the types of crosslinking agent and crosslinking accelerator, for example, a crosslinkable resin that is solid at room temperature and normal pressure can be used. Note that, in order to avoid a crosslinkable resin that is solid at room temperature and normal pressure becoming excessively hard and losing elasticity, it is preferable that the crosslinkable resin does not contain an epoxy group, in other words, be a resin other than an epoxy resin.

[0026] -Cross-linkable resin that is solid at room temperature and pressure- Examples of crosslinkable resins that are solid at room temperature and normal pressure include acrylic polymers such as poly(2-ethylhexyl acrylate), copolymers of acrylic acid and 2-ethylhexyl acrylate, polymethacrylic acid or its esters, and polyacrylic acid or its esters; styrene-acrylonitrile copolymers; acrylonitrile-butadiene-styrene copolymers (ABS resins); styrene-butadiene copolymers or hydrogenated products thereof; acrylonitrile-butadiene copolymers (nitrile rubbers); styrene-butadiene block copolymers or hydrogenated products thereof; styrene-isoprene block copolymers or hydrogenated products thereof; silicone resins such as polyimide silicone resins; and fluororesins such as vinylidene fluoride-based fluororesins, tetrafluoroethylene-propylene-based fluororesins, and tetrafluoroethylene-purple vinyl ether-based fluororesins. These may be used alone or in combination of two or more. Among these, it is preferable that the thermally conductive sheet of the present invention contains a crosslinked resin, which is a crosslinked product formed using at least one of an acrylic polymer, an acrylonitrile-butadiene copolymer, and a styrene-butadiene copolymer.

[0027] The crosslinkable resin that is solid at room temperature and normal pressure preferably has a glass transition temperature of -60.0°C or higher, more preferably -50.0°C or higher, and preferably -8.0°C or lower, more preferably -15.0°C or lower, and even more preferably -20.0°C or lower. If the glass transition temperature of the resin used as the solid crosslinkable resin at room temperature and normal pressure is equal to or higher than the above-mentioned lower limit, the restorability of the resulting thermal conductive sheet can be further improved. Furthermore, if the glass transition temperature of the resin used as the solid crosslinkable resin at room temperature and normal pressure is equal to or lower than the above-mentioned upper limit, the resulting thermal conductive sheet can be imparted with appropriate flexibility.

[0028] The blending ratio of the crosslinkable resin that is solid at room temperature and normal pressure in the crosslinkable resin is preferably 25% by mass or more, and more preferably 30% by mass or more. The upper limit of the blending ratio of the crosslinkable resin that is solid at room temperature and normal pressure in the crosslinkable resin is not particularly limited, and may be 100% by mass. When the blending ratio of the crosslinkable resin that is solid at room temperature and normal pressure in the crosslinkable resin is equal to or greater than the above lower limit, such a thermal conductive sheet has excellent restorability.

[0029] -Cross-linkable resin that is liquid at room temperature and pressure- The crosslinkable resin preferably includes a crosslinkable resin that is a liquid at room temperature and normal pressure in addition to a crosslinkable resin that is a solid at room temperature and normal pressure. Examples of the crosslinkable resin that is a liquid at room temperature and normal pressure include thermoplastic resins that are liquid at room temperature and normal pressure, such as acrylic resin, epoxy resin, silicone resin, fluororesin, acrylonitrile-butadiene copolymer, and polybutene. By including a cross-linkable resin that is liquid at room temperature and normal pressure in addition to a cross-linkable resin that is solid at room temperature and normal pressure, the thermal conductivity of the thermal conductive sheet can be further improved. Furthermore, for the purpose of imparting flexibility, the recovery of the thermal conductive sheet can be improved compared to when a plasticizer is added instead of a cross-linkable resin that is liquid at room temperature and normal pressure.

[0030] [Crosslinking agent] The crosslinking agent is not particularly limited as long as it can undergo a crosslinking reaction with the above-mentioned crosslinkable resin, and is not limited as long as it is used as a crosslinking agent for the resin. Typical crosslinking agents include sulfur-based crosslinking agents, peroxides, and amine-based crosslinking agents that crosslink unsaturated bonds contained in the resin. These may be used alone or in combination of two or more types in any ratio. Among these, when the crosslinkable resin is an acrylic polymer, it is preferable to use an amine-based crosslinking agent, and when the crosslinkable resin is at least one of an acrylonitrile-butadiene copolymer and a styrene-butadiene copolymer, it is preferable to use a sulfur-based crosslinking agent or a peroxide.

[0031] The amount of crosslinking agent blended is preferably 0.1 parts by mass or more, more preferably 0.2 parts by mass or more, and even more preferably 0.3 parts by mass or more, and is preferably 15.0 parts by mass or less, and more preferably 12.0 parts by mass or less, relative to 100 parts by mass of the crosslinkable resin that is solid at room temperature and normal pressure. If the amount of crosslinking agent blended is within the above range, the crosslinked resin formed is sufficiently crosslinked by the crosslinking agent, and it is thought that by imparting appropriate strength and elasticity to the thermal conductive sheet, the recovery of the thermal conductive sheet is improved and the orientation structure of the graphite particles within the thermal conductive sheet is well maintained.

[0032] [Crosslinking accelerator] The crosslinking accelerator that can be used in the crosslinking reaction between the crosslinkable resin and the crosslinking agent is not particularly limited, and sulfenamide crosslinking accelerators and the like can be used. These may be used alone or in combination of two or more in any ratio. Among them, sulfenamide crosslinking accelerators are preferred.

[0033] The amount of crosslinking accelerator blended is preferably 0.5 parts by mass or more, more preferably 0.7 parts by mass or more, and even more preferably 1.0 part by mass or more, and is preferably 5.0 parts by mass or less, and more preferably 4.0 parts by mass or less, relative to 100 parts by mass of the crosslinkable resin that is solid at room temperature and normal pressure. If the amount of crosslinking accelerator blended is within the above range, the crosslinked resin that is formed is sufficiently crosslinked by the crosslinking agent, and it is thought that by imparting appropriate strength and elasticity to the thermal conductive sheet, the recovery of the thermal conductive sheet is improved and the orientation structure of the graphite particles within the thermal conductive sheet is well maintained.

[0034] <<Other resins>> Resins other than the crosslinkable resin are not particularly limited, and examples include resins that do not undergo crosslinking reactions with the crosslinking agents and crosslinking accelerators that contribute to the crosslinking of the crosslinkable resins described above. Examples of such resins include liquid resins that are liquid at room temperature and normal pressure. Examples of liquid resins that can be used include thermoplastic resins that are liquid at room temperature and normal pressure. Examples of other resins include acrylic resins, epoxy resins, silicone resins, fluororesins, acrylonitrile-butadiene copolymers, and polybutene, which are thermoplastic resins that are liquid at room temperature and normal pressure and are compounds different from the crosslinkable resins described above.

[0035] [Resin content] The proportion of resin contained in the thermal conductive sheet is preferably 30% by volume or more, more preferably 40% by volume or more, and preferably 55% by volume or less, and more preferably 65% ​​by volume or less, based on the total volume of resin, graphite particles, plasticizer, and other components. If the proportion of resin contained in the thermal conductive sheet is equal to or greater than the lower limit, the thermal conductive sheet will have excellent restoring properties, and if it is equal to or less than the upper limit, the thermal conductive sheet will have excellent thermal conductivity.

[0036] [Cross-linked resin content] The proportion of the crosslinked resin contained in the resin is preferably 25% by mass or more, and more preferably 30% by mass or more. The upper limit of the proportion of the crosslinked resin contained in the resin is not particularly limited, and may be 100% by mass. When the proportion of the crosslinked resin contained in the resin is equal to or greater than the above lower limit, the thermal conductive sheet has excellent restorability. The amount of cross-linkable resin contained in the thermally conductive sheet is usually the same as the amount of cross-linkable resin used in producing the thermally conductive sheet.

[0037] [Gel fraction] The thermally conductive sheet of the present invention preferably has a gel fraction of 80% or more, more preferably 90% or more, and even more preferably 95% or more. The upper limit of the gel fraction is not particularly limited, but it may be 100%. If the gel fraction is equal to or greater than the above lower limit, the thermally conductive sheet will have even better recovery properties. In this specification, the gel fraction can be calculated according to the following formula (2) based on the mass (Y) of the dried product obtained after immersing a predetermined amount (X) of the thermal conductive sheet in a solvent such as methyl ethyl ketone to elute the non-crosslinked resin. Gel fraction (%) = (Y) / (X) × 100 (2) The gel fraction can be controlled depending on the type and amount of resin used in the production of the thermal conductive sheet, the production method, etc.

[0038] <Graphite particles> The graphite particles are not particularly limited as long as they have an aspect ratio of greater than 1.0, and examples thereof include artificial graphite and natural graphite. Artificial graphite includes carbon black and pyrolytic graphite. Natural graphite includes flake graphite such as expanded graphite and spherical graphite, as well as flake graphite. These may be used alone or in combination of two or more. Among these, the graphite particles are preferably scaly graphite. If the graphite particles are scaly graphite, the graphite particles are well oriented in the heat conductive sheet, thereby improving the thermal conductivity of the heat conductive sheet. If the graphite particles are scaly graphite, the "major axis" refers to the length in the direction of the major axis of the main surface of the scale shape, and the "minor axis" refers to the length in the direction perpendicular to the major axis of the main surface. The scaly graphite is preferably expanded graphite. If the scaly graphite is expanded graphite, the thermal conductivity of the heat conductive sheet can be further improved.

[0039] The aspect ratio (major axis / minor axis) of the graphite particles is preferably 1.2 or more, preferably 1.3 or more, preferably 1.4 or more, and preferably 10 or less, more preferably 5 or less, and even more preferably 3 or less. When the aspect ratio of the graphite particles is within the above range, the graphite particles are well oriented in the heat conductive sheet, thereby improving the thermal conductivity of the heat conductive sheet. In this specification, the aspect ratio of graphite particles can be calculated by observing the graphite particles with a scanning electron microscope (SEM), measuring the maximum diameter (major diameter) and the particle diameter (minor diameter) in the direction perpendicular to the maximum diameter for 50 random graphite particles, and averaging the ratio of the major diameter to the minor diameter (major diameter / minor diameter).

[0040] The average particle size of the graphite particles is preferably 40 μm or more, more preferably 70 μm or more, and preferably 500 μm or less, and more preferably 300 μm or less. If the average particle size of the graphite particles is within the above range, the thermal conductivity of the thermal conductive sheet can be further improved. In this specification, the volume average particle size of the graphite particles can be measured by a laser diffraction scattering method in accordance with JIS Z8825, and refers to the particle size at which the cumulative volume calculated from the smallest diameter side in the measured particle size distribution (volume basis) is 50%.

[0041] The volume fraction of graphite particles in the thermal conductive sheet must be 35% by volume or more, preferably 45% by volume or more, preferably 70% by volume or less, and more preferably 60% by volume or less, where the total volume of the resin, graphite particles, optional plasticizer, and other components is 100% by volume. If the volume fraction of graphite particles in the thermal conductive sheet is equal to or greater than the above lower limit, the thermal conductivity of the thermal conductive sheet can be improved. If the volume fraction of graphite particles in the thermal conductive sheet is equal to or less than the above upper limit, the flexibility of the thermal conductive sheet can be maintained.

[0042] <Plasticizer> The thermally conductive sheet of the present invention may optionally contain a plasticizer such as a fatty acid ester-based plasticizer. The plasticizer is not particularly limited, and examples thereof include phthalate esters, dioctyl adipate, diisononyl adipate, di-normal alkyl adipate, dialkyl adipate, dioctyl azelate, dialkyl sebacate, dioctyl sebacate, trialkyl cetate, epoxidized unsaturated fatty acid esters, epoxidized unsaturated fatty acid esters, trimellitic acid esters, and polyether esters. The volume fraction of the plasticizer in the thermally conductive sheet is preferably 30% by volume or less, where the total volume of the resin, graphite particles, plasticizer, and any other components is 100% by volume. The lower limit of the volume fraction of the plasticizer is not particularly limited and may be 0% by volume. If the volume fraction of the plasticizer in the thermally conductive sheet is equal to or less than the upper limit, the thermal conductivity of the thermally conductive sheet can be further improved.

[0043] <Other ingredients> The thermally conductive sheet of the present invention may optionally further contain components other than the resin, graphite particles, and plasticizer described above (hereinafter, sometimes referred to as "other components"). The other components are not particularly limited as long as they are components that can be used in the manufacture of a thermally conductive sheet, and examples include fibrous carbon materials; flame retardants such as red phosphorus-based flame retardants and phosphate ester-based flame retardants; toughness improvers such as urethane acrylates; moisture absorbents such as calcium oxide and magnesium oxide; adhesion improvers such as silane coupling agents, titanium coupling agents, and acid anhydrides; wettability improvers such as nonionic surfactants and fluorine-based surfactants; ion trapping agents such as inorganic ion exchangers; antioxidants; and foaming agents such as sulfonyl hydrazide compounds. These may be blended in the required amount depending on the application. Note that the foaming agent described in the example above foams during the manufacture of the thermally conductive sheet, and most of it is removed from the thermally conductive sheet, but a small amount of residue may remain in the thermally conductive sheet.

[0044] (Thermal conduction sheet structure) In the thermally conductive sheet of the present invention, it is preferable that at least some of the plurality of voids described above have a major axis length of 20% or more of the thickness of the thermally conductive sheet when the outline is approximated as an ellipse. Furthermore, it is preferable that the average aspect ratio calculated for the plurality of voids whose major axis length is 20% or more of the thickness of the thermally conductive sheet is 15 or more. Furthermore, it is preferable that the major axes of the plurality of voids whose major axis length is 20% or more of the thickness of the thermally conductive sheet are oriented at an angle of 60° or more and 90° or less with respect to the main surface of the thermally conductive sheet. It is presumed that the plurality of voids that satisfy this specific shape and are arranged in this specific arrangement contribute to improving the compressibility and resilience of the thermally conductive sheet in a balanced manner.

[0045] As described above, the aspect ratio of voids in the thermal conductive sheet whose major axis length is 20% or more of the thickness of the thermal conductive sheet is preferably 15 or more, more preferably 17 or more, even more preferably 19 or more, and preferably 50 or less, more preferably 40 or less, and even more preferably 35 or less. If the aspect ratio of a given void is equal to or greater than the above-mentioned lower limit, the thermal conductive sheet can exhibit sufficient compressibility. Furthermore, if the aspect ratio of a given void is equal to or less than the above-mentioned upper limit, the thermal conductive sheet does not have excessively large voids, and can therefore exhibit excellent thermal conductivity.

[0046] Furthermore, the thermally conductive sheet of the present invention preferably has a structure in which the above-mentioned multiple voids are uniformly dispersed within the thermally conductive sheet. Such uniform dispersion of multiple voids within the thermally conductive sheet can further improve the compressibility and resilience of the thermally conductive sheet in a balanced manner. It is also believed that at least some of the multiple voids have a so-called open-cell structure, in which voids derived from multiple foaming agent particles are interconnected. It is believed that such an open-cell structure can be efficiently produced by carrying out the "crosslinking reaction step in which the laminate is heated while being pressurized to cause a crosslinking and foaming reaction" in the manufacturing method of the present invention described below. It is also believed that such an open-cell structure can form voids whose major axes are oriented at a predetermined angle relative to the main surface of the thermally conductive sheet.

[0047] FIG. 1 shows a schematic cross section of an example of a thermally conductive sheet according to the present invention, which satisfies the specific structure. FIG. 1 partially illustrates a cross section of the thermally conductive sheet 1 cut perpendicular to the main surface, i.e., in the thickness direction. The thermally conductive sheet 1 includes a resin 10 and graphite particles (the shape of which is not shown). The graphite particles are represented by their major axes 11 (broken lines), and their shapes are omitted for clarity. Furthermore, the thermally conductive sheet 1 includes a plurality of voids 12, which are shown as ellipses. As described above, at least some of the voids 12 have a major axis LA that is 20% or more of the distance between the first and second main surfaces A and B of the thermally conductive sheet 1, i.e., the thickness of the thermally conductive sheet 1. Furthermore, the major axis LA, whose length is 20% or more of the thickness of the thermally conductive sheet 1, is preferably oriented at an angle of 60° to 90° relative to the first and second main surfaces A and B of the thermally conductive sheet 1 (90° in FIG. 1 ). In the embodiment shown in Fig. 1, all of the illustrated voids 12 have a long axis LA whose length is 20% or more of the thickness of the thermally conductive sheet 1, and all of these LAs are oriented at an orientation angle of 90° with respect to the first principal surface A and the second principal surface B of the thermally conductive sheet 1. Although not shown, the thermally conductive sheet 1 may contain unintentional voids that were generated during kneading. Such unintentional voids may often be spherical voids with very small diameters.

[0048] Here, the angle formed by the major axes of the plurality of voids, whose length is 20% or more of the thickness of the thermal conductive sheet, with respect to the main surface of the thermal conductive sheet 1 is preferably 60° to 90°, more preferably 75° to 90°, and even more preferably 78° to 90°. When the major axes of the plurality of voids, whose length is 20% or more of the thickness of the thermal conductive sheet, are oriented within the above-mentioned angle range, the compressibility and resilience of the thermal conductive sheet can be improved in a more balanced manner. Furthermore, the orientation angle of the graphite particles with respect to the main surface of the thermal conductive sheet must be 60° to 90°, preferably 75° to 90°, and more preferably 78° to 90°. When the orientation angle of the graphite particles with respect to the main surface of the thermal conductive sheet is within the above-mentioned range, the thermal conductivity and strength of the thermal conductive sheet can be improved.

[0049] (Thermal Conduction Sheet Properties) <Porosity> The porosity of the thermally conductive sheet is a value calculated according to the following formula (1). Porosity (%) = 100 × (1 - measured specific gravity of thermal conductive sheet ÷ theoretical specific gravity of thermal conductive sheet) (1) The porosity must be 20% or more, preferably 22% or more, more preferably 23% or more, and preferably 35% or less, more preferably 33% or less, and even more preferably 32% or less. If the porosity is equal to or greater than the above-mentioned lower limit, the resulting thermally conductive sheet can exhibit sufficient compressibility. If the porosity is equal to or less than the above-mentioned upper limit, the resulting thermally conductive sheet does not have excessive voids, and can therefore exhibit excellent thermal conductivity.

[0050] <Compression ratio> The thermal conductive sheet of the present invention has a thickness of T when pressed at 0.9 MPa in the thickness direction. 0.9 The thickness of the thermally conductive sheet before pressure is T0, and the compressibility of the thermally conductive sheet calculated by the following formula (3) is preferably 8% or more, more preferably 10% or more, and is preferably 35% or less, more preferably 30% or less. Compression rate (%)=100×[1-(T 0.9 / T0)][%]···(3) If the compression ratio is equal to or greater than the lower limit, the thermally conductive sheet has excellent adhesion to the heat generating element and the heat dissipating element and excellent conformability to uneven surfaces. If the compression ratio is equal to or less than the upper limit, the thermally conductive sheet has adequate strength and excellent durability. The compression ratio can be controlled depending on the composition, manufacturing method, etc. of the thermally conductive sheet.

[0051] <Recovery rate> Furthermore, when the thermally conductive sheet of the present invention is pressed in the thickness direction at 0.9 MPa, and then the load is removed and the sheet is restored after 3 minutes, the restoration rate calculated based on the following formula (4) is preferably 70% or more, more preferably 80% or more, and even more preferably 90% or more. Recovery rate (%) = (thickness of thermal conductive sheet after recovery) ÷ (thickness of thermal conductive sheet before pressure application) (4) If the restoration rate is equal to or greater than the lower limit, the sheet can better conform to the component. The upper limit of the restoration rate is not particularly limited and may be, for example, 100%. The restoration rate can be controlled depending on the composition, manufacturing method, etc. of the thermal conductive sheet.

[0052] <Thermal resistance> The thermal conductive sheet of the present invention preferably has a thermal resistance of 0.85°C / W or less, and more preferably 0.45°C / W or less, when a pressure of 0.1 MPa is applied in the thickness direction at a sample temperature of 50°C. If the thermal resistance is equal to or less than the above upper limit, the thermal conductivity of the thermal conductive sheet can be increased. There is no particular lower limit for the thermal resistance, but it can usually be 0.10°C / W or more. The thermal resistance value can be controlled depending on the composition, manufacturing method, etc. of the thermally conductive sheet.

[0053] <Thermal conductivity> The thermal conductivity of the thermal conductive sheet in the thickness direction is preferably 15 W / m K or more, and more preferably 20 W / m K or more. There is no particular upper limit to the thermal conductivity of the thermal conductive sheet in the thickness direction, but it is, for example, 65 W / m K or less. The thermal conductivity of the thermally conductive sheet in the thickness direction can be adjusted by the types and proportions of materials and components contained in the thermally conductive sheet, as well as the manufacturing method and manufacturing conditions of the thermally conductive sheet.

[0054] <Thermal Conduction Sheet Thickness> The thickness of the thermally conductive sheet of the present invention is not particularly limited, but is preferably 50 μm or more, more preferably 100 μm or more, and more preferably 200 μm or more, and is preferably 2000 μm or less, more preferably 1000 μm or less, and even more preferably 500 μm or less. If the thickness of the thermally conductive sheet is equal to or greater than the above lower limit, the thermally conductive sheet does not become excessively thin, thereby improving the strength and handleability of the thermally conductive sheet. If the thickness of the thermally conductive sheet is equal to or less than the above upper limit, the thermal conductivity of the thermally conductive sheet in the thickness direction can be improved.

[0055] (Method of manufacturing thermal conductive sheets) The thermally conductive sheet of the present invention can be efficiently produced by a manufacturing method including the following steps: (A) a pre-thermally conductive sheet forming step in which a composition containing a resin containing a crosslinkable resin, graphite particles, a crosslinking agent, and a foaming agent is pressurized and molded into a sheet to obtain a pre-thermally conductive sheet; (B) a laminate forming step in which multiple pre-thermally conductive sheets are stacked in the thickness direction or folded or rolled up to obtain a laminate; (C) a crosslinking reaction step in which the laminate is heated under pressure to cause a crosslinking and foaming reaction; and (D) a slicing step in which the laminate is sliced ​​at an angle of 45° or less relative to the stacking direction to obtain a thermally conductive sheet. The thermally conductive sheet of the present invention may optionally include further steps other than (A) to (D) above.

[0056] <(A) Pre-heat conductive sheet forming process> In the pre-thermal conductive sheet molding step, a composition containing a resin containing a crosslinkable resin, graphite particles, a crosslinking agent, and a foaming agent is pressed into a sheet to obtain a pre-thermal conductive sheet.

[0057] <<Composition>> The composition includes a resin containing a crosslinkable resin, graphite particles, a crosslinking agent, and a foaming agent. The composition may further include a crosslinking accelerator. Furthermore, the composition may further include components (plasticizers and other components) other than the resin, graphite particles, crosslinking agent, foaming agent, and crosslinking accelerator.

[0058] [resin] Here, the resin contained in the composition contains a crosslinkable resin and, optionally, contains a resin other than the crosslinkable resin. For example, the crosslinkable resin that can be used to form the crosslinkable resin described above in the "Thermal Conductive Sheet" section can be used in the proportions described above. The blending amount of the resin in the composition corresponds to the blending range described above for the "resin content" contained in the thermal conductive sheet. Furthermore, the blending amount of the crosslinkable resin in the resin corresponds to the blending range described above for the "crosslinkable resin content" contained in the thermal conductive sheet. For example, the resin other than the crosslinkable resin can be another resin (e.g., a liquid resin) that can be contained in the resin described above in the "Thermal Conductive Sheet" section, and can be used in the proportions described above.

[0059] [Graphite particles] As the graphite particles, for example, the graphite particles described above in the section "Thermal Conduction Sheet" can be used in the proportions described above.

[0060] [Crosslinking agent] As the crosslinking agent, the crosslinking agents that can be used to form the crosslinked resin described above in the section "Thermal Conductive Sheet" can be used in the proportions described above.

[0061] [Foaming agent] Various blowing agents that decompose upon heating to generate gas can be used. A blowing assistant that lowers the decomposition temperature of the blowing agent and promotes its decomposition can also be used in combination. Specific examples of the blowing agent include organic blowing agents such as azo compounds (e.g., azodicarbonamide (ADCA) and azobisisobutyronitrile), nitroso compounds (e.g., N,N'-dinitrosopentamethylenetetramine (DPT)), and sulfonylhydrazide compounds (e.g., p-toluenesulfonylhydrazide and 4,4'-oxybis(benzenesulfonylhydrazide) (OBSH)); and gas-based blowing agents such as volatile hydrocarbon compounds (e.g., chlorofluorocarbons, carbon dioxide, water, and pentane), and microcapsules containing these compounds. These blowing agents include one or more of the following chemical blowing agents: The blending ratio of the foaming agent is preferably 5% by mass or more and 15% by mass or less relative to the resin contained in the composition. If the foaming agent ratio is equal to or greater than the above lower limit, the foaming reaction proceeds smoothly, and the resulting thermally conductive sheet can have excellent compressibility. If the foaming agent ratio is equal to or less than the above upper limit, the strength and recovery of the resulting thermally conductive sheet can be increased. Furthermore, if the foaming agent ratio is equal to or greater than the above lower limit, the compressibility can be increased.

[0062] The foaming temperature depends on the type of foaming agent, but is preferably 80° C. or higher and 220° C. or lower, and more preferably 150° C. or higher and 200° C. or lower.

[0063] [Foaming aid] As described above, various foaming aids can be used that lower the decomposition temperature of the foaming agent to be combined with it and promote its decomposition. For example, a foaming aid that can be combined with ADCA is a urea (H2NCONH2)-based foaming aid.

[0064] The blending ratio of the foaming aid can be set arbitrarily depending on the type of foaming agent to be combined, but is preferably 1% by mass or more and 15% by mass or less relative to the resin contained in the composition.

[0065] As the foaming agent, it is particularly preferred to use ADCA in combination with a urea-based foaming aid or to use OBSH alone, and it is particularly preferred to use OBSH alone.

[0066] [Crosslinking accelerator] As the crosslinking accelerator, those mentioned in the section "Thermal Conductive Sheet" can be used in the proportions mentioned above.

[0067] [Plasticizer] As the plasticizer, the plasticizers described above in the section "Thermal Conductive Sheet" can be used in the proportions described above.

[0068] [Other ingredients] As other components that can be contained in the composition, other components that can be contained in the thermally conductive sheet described above in the section "Thermal Conductive Sheet" can be used.

[0069] [Preparation of Composition] The composition is not particularly limited and can be prepared by mixing the above-mentioned components. The mixing of the above-mentioned components can be carried out using known mixing devices, such as kneaders; mixers such as Henschel mixers, Hobart mixers, and high-speed mixers; twin-screw kneaders; and roll mixers. The mixing may also be carried out in the presence of a solvent such as ethyl acetate. The resin may be dissolved or dispersed in a solvent in advance to form a resin solution, which may then be mixed with graphite particles, a crosslinking agent, a foaming agent, and optionally a crosslinking accelerator, a plasticizer, and other components. The mixing time may be, for example, 5 minutes to 60 minutes. The mixing temperature may be, for example, 5°C to 150°C.

[0070] <<Molding of the composition>> The composition prepared as described above can be optionally degassed and crushed, and then pressed to form into a sheet. The sheet-shaped composition thus pressure-molded can be used as a pre-heat conductive sheet. If a solvent is used during mixing, it is preferable to remove the solvent before forming into a sheet. For example, if degassing is performed using vacuum degassing, the solvent can be removed simultaneously during degassing.

[0071] Here, the composition can be formed into a sheet using any known forming method, such as press molding, rolling, or extrusion, as long as the forming method involves applying pressure. Among these, the composition is preferably formed into a sheet by rolling (primary processing), and more preferably by a predetermined roll forming method in which the composition is passed between a first roll and a second roll having a faster peripheral speed than the first roll. The peripheral speed ratio of the second roll to the first roll ("peripheral speed of the second roll" / "peripheral speed of the first roll") is preferably 1.03 / 1 or more and 2 / 1 or less. The distance between the first roll and the second roll can be, for example, 1 mm or more and 3 mm or less.

[0072] <<Pre-heat conductive sheet>> In the pre-heat conductive sheet obtained by pressing the composition into a sheet, the graphite particles are presumably oriented mainly in the in-plane direction, thereby improving the thermal conductivity of the pre-heat conductive sheet in the in-plane direction in particular.

[0073] <(B) Laminate formation process> In the laminate formation process, multiple pre-thermal conductive sheets obtained in the pre-thermal conductive sheet molding process are stacked in the thickness direction, or the pre-thermal conductive sheets are folded or rolled to obtain a laminate in which multiple pre-thermal conductive sheets containing resin, graphite particles, a crosslinking agent, and a foaming agent are formed in the thickness direction. Here, the formation of the laminate by folding the pre-thermal conductive sheets is not particularly limited and can be performed by folding the pre-thermal conductive sheets at a constant width using a folding machine. Furthermore, the formation of the laminate by rolling the pre-thermal conductive sheets is not particularly limited and can be performed by rolling the pre-thermal conductive sheets around an axis parallel to the short or long direction of the pre-thermal conductive sheets. Furthermore, the formation of the laminate by stacking the pre-thermal conductive sheets is not particularly limited and can be performed using a lamination device.

[0074] <(C) Crosslinking Reaction Step> In the crosslinking reaction step, the laminate obtained in the laminate formation step (hereinafter sometimes referred to as the first laminate) is preferably heated and isotropically pressed (secondary pressurization) in the stacking direction or against the laminate to form a second laminate. By heating the laminate and promoting the crosslinking reaction, the crosslinkable resin undergoes a crosslinking reaction to form a crosslinked resin, thereby improving the restorability of the produced thermal conductive sheet. Furthermore, simultaneously with the crosslinking reaction, the foaming agent undergoes a foaming reaction to generate voids, thereby improving the compressibility of the thermal conductive sheet.

[0075] The pressure applied to the laminate in the stacking direction can be 0.05 MPa or more and 0.90 MPa or less. Alternatively, isostatic pressing can be performed using an apparatus such as an autoclave. The pressure of the isostatic pressing is preferably in the range of 0.3 MPa or more and 0.9 MPa or less.

[0076] The heating temperature of the laminate is not particularly limited, but is preferably above 140°C, more preferably above 150°C, even more preferably above 170°C, and preferably below 210°C, more preferably below 190°C. If the heating temperature is above the lower limit, the crosslinking and foaming reactions proceed smoothly, improving the restorability of the produced thermally conductive sheet and generating voids within the thermally conductive sheet, thereby increasing the compressibility of the thermally conductive sheet. If the heating temperature is below the upper limit, deterioration of the resin that constitutes the thermally conductive sheet can be suppressed.

[0077] Furthermore, the heating time for the laminate is not particularly limited, but can be, for example, 30 minutes to 6 hours. If the heating time is equal to or greater than the above-mentioned lower limit, the crosslinking reaction and foaming reaction proceed smoothly, improving the restorability of the produced thermal conductive sheet and generating voids within the thermal conductive sheet, thereby increasing the compressibility of the thermal conductive sheet. On the other hand, if the heating time is equal to or less than the above-mentioned upper limit, the crosslinking reaction is prevented from proceeding excessively, and the flexibility of the thermal conductive sheet can be maintained well, resulting in further increasing the compressibility of the thermal conductive sheet.

[0078] It is presumed that in a laminate obtained by stacking, folding or rolling the pre-heat conductive sheet, the graphite particles are oriented in a direction substantially perpendicular to the stacking direction.

[0079] <(D) Slicing process> In the slicing step, the laminate that has undergone the crosslinking reaction and foaming reaction in the crosslinking reaction step is sliced ​​at an angle of 45° or less relative to the lamination direction to obtain a thermally conductive sheet consisting of slices of the laminate. The method for slicing the laminate is not particularly limited, and examples thereof include a multi-blade method, a laser processing method, a water jet method, and a knife processing method. The cutting tool used to slice the laminate is not particularly limited, and a slicing member having a smooth plate surface with a slit and a blade protruding from the slit (for example, a plane or slicer with a sharp blade) can be used.

[0080] From the viewpoint of increasing the thermal conductivity of the thermal conductive sheet, the angle at which the laminate is sliced ​​is preferably 30° or less with respect to the stacking direction, more preferably 15° or less with respect to the stacking direction, and preferably approximately 0° with respect to the stacking direction (i.e., in the direction along the stacking direction).The thermal conductive sheet obtained through such a slicing process may have a structure in which strips containing resin and graphite particles are bonded in parallel in one direction approximately perpendicular to the thickness direction of the thermal conductive sheet (a direction at an angle of approximately 90° with respect to the thickness direction).

[0081] The thermally conductive sheet obtained in this manner has a plurality of voids formed by the foaming of the foaming agent, and a crosslinked resin is formed, so that the thermally conductive sheet has excellent compressibility and restorability. [Example]

[0082] The present invention will be specifically described below based on examples, but the present invention is not limited to these examples. In the following description, "%" and "parts" representing amounts are based on mass unless otherwise specified. In addition, when calculating volume fractions, etc., the volume of each blended component was calculated by dividing the mass of each blended component by its theoretical specific gravity. Various measurements and evaluations in the examples and comparative examples were carried out according to the following methods.

[0083] (Physical property measurement) <Orientation angle of the long axis of graphite particles, orientation angle of voids, aspect ratio of voids> The cross-sections of the thermally conductive sheets obtained in the examples and comparative examples were observed by cutting the thermally conductive sheet into a regular octagon and observing the cross-section with a scanning electron microscope (SEM, Hitachi High-Technologies Corporation's "SU-3500") at a magnification that captured the entire sheet from top to bottom. The magnification was 180x. Thirty lines were drawn along the long axes of the graphite particles in this cross-section, and the average angle of the long axes relative to the main surfaces (first and second main surfaces) of the thermally conductive sheet was calculated. If the angle was 90° or greater, a supplementary angle was used. This was performed on eight surfaces, and the largest value among the eight surfaces was taken as the orientation angle of the long axes of the graphite particles in the thermally conductive sheet. The voids in the cross section with the maximum angle were also evaluated. Specifically, the average aspect ratio of the voids, the orientation angle of the voids, and the length of the major axis of the voids, which could be observed at the same magnification as above, were evaluated. To calculate the average aspect ratio of the voids, the outlines of the voids contained in the "cross section with the maximum angle" of the thermal conductive sheet were first approximated as ellipses, and the major and minor axes were set for each ellipse. Then, voids whose major axis length was 20% or more of the thickness of the thermal conductive sheet were judged to be voids caused by the foaming agent and were subject to evaluation. Voids whose length was less than 20% of the thickness of the thermal conductive sheet were judged to be unintentional voids generated during kneading. Furthermore, the aspect ratio of the voids was calculated by dividing the length of the major axis of the voids determined to be the subject of evaluation above by the length of the minor axis. Ten voids with the largest aspect ratios were selected in descending order, and the average aspect value was calculated. For all Examples 1-6, ten voids were selected, and the average aspect ratio was calculated. The angle of the major axis of the selected voids was then calculated in the same manner as for the graphite particles. When evaluating the void orientation angle and void aspect ratio of a thermally conductive sheet having less than 10 voids in its cross section, the target is a thermally conductive sheet having five or more voids in its cross section, and these five voids are selected as the measurement targets and evaluated in the same manner as above.

[0084] <Volume average particle size and aspect ratio of graphite particles in the thermal conductive sheet> 1 g of the thermally conductive sheet obtained in each of the examples and comparative examples was placed in methyl ethyl ketone as a solvent to dissolve the resin components of the thermally conductive sheet, thereby obtaining a suspension in which the graphite particles contained in the thermally conductive sheet were separated and dispersed. Next, the particle size of the graphite particles contained in the suspension was measured using a laser diffraction / scattering particle size distribution analyzer (manufactured by Horiba, Ltd., model "LA960"). A particle size distribution curve was then created, with the obtained particle size on the horizontal axis and the volumetric particle frequency on the vertical axis. Furthermore, the particle size (D50) at which the cumulative volume calculated from the smallest diameter side reached 50% was determined in the particle size distribution curve, and this was used as the volume-average particle size of the graphite particles. Furthermore, the graphite particles separated as described above were observed with an SEM to measure the aspect ratio, and it was confirmed that the aspect ratio of the graphite particles used in the examples and comparative examples was greater than 1.0.

[0085] <Thermal Conduction Sheet Thickness> The thickness of the thermally conductive sheets obtained in the examples and comparative examples was measured using a thickness gauge (manufactured by Mitutoyo Corporation, product name "Digimatic Indicator ID-C112XBS"). Ten measurements were taken, and the arithmetic mean of the obtained thickness measurement data was taken as the thickness of the thermally conductive sheet in question.

[0086] <Thermal conductivity> The thermal conductivity of the thermally conductive sheets obtained in the examples and comparative examples was measured as follows. Within the main surface of the thermal conductive sheet, the thermal diffusivity α (m 2 / s), specific heat at constant pressure Cp (J / g K) and specific gravity ρ (g / m 3 ) was measured by the following method. [Thermal diffusivity α(m 2 / s)] The thermal diffusivity was measured using a thermophysical property measuring device (manufactured by Bethel Corporation, product name "Thermowave Analyzer TA35"). [Specific heat at constant pressure Cp (J / g K)] The specific heat was measured using a differential scanning calorimeter (manufactured by Rigaku, product name "DSC8230") under the condition of a temperature increase of 10°C / min. [Specific gravity ρ(g / m 3 )] Specific gravity (density) (g / m) was measured using an automatic hydrometer (manufactured by Toyo Seiki Co., Ltd., product name "DENSIMETER-H"). 3 ) was measured. Then, the obtained measurement values ​​are used to calculate the following formula (5): λ=α×Cp×ρ (5) The thermal conductivity λ (W / m K) of the thermal conductive sheet was calculated by substituting

[0087] <Thermal resistance, compression rate and recovery rate> The thermal resistance and thickness of the thermally conductive sheets manufactured in each example and comparative example when pressure was applied in the thickness direction were measured using a thermal resistance tester (Hitachi Technology and Services Co., Ltd., product name "Resin Material Thermal Resistance Measuring Device"). Using samples of thermally conductive sheets cut into approximately 1 cm squares, the thermal resistance (°C / W) and thickness of the thermally conductive sheets were measured when pressures of 0.1 MPa and 0.9 MPa were applied at a sample temperature of 50°C. The smaller the thermal resistance value, the better the thermal conductivity of the thermally conductive sheet, indicating, for example, excellent heat dissipation properties when interposed between a heat generating body and a heat sink. In addition, the thickness of the thermal conductive sheet when pressed in the thickness direction at 0.9 MPa is T 0.9 and the thickness of the thermal conductive sheet before pressure is T0, the following formula (3): Compression rate (%)=100×[1-(T 0.9 / T0)][%]···(3) The compressibility (%) of the thermal conductive sheet was calculated by the following formula. Furthermore, after the measurement, the load was removed and 3 minutes had passed, and the thickness of the restored thermal conductive sheet was measured with a film thickness meter, and the restoration rate was calculated based on the following formula (4). Recovery rate (%) = (thickness of thermal conductive sheet after recovery) ÷ (thickness of thermal conductive sheet before pressure application) (4)

[0088] <Theoretical specific gravity and porosity> For the thermally conductive sheets manufactured in each of the examples and comparative examples, the theoretical specific gravity was calculated from the specific gravity of the raw materials and their blending amounts. From the calculated theoretical specific gravity and the measured specific gravity, the following formula (1): Porosity (%) = 100 × (1 - measured specific gravity of thermal conductive sheet ÷ theoretical specific gravity of thermal conductive sheet) (1) The porosity (%) was calculated by the following formula.

[0089] <Gel fraction> A predetermined amount (X) (approximately 500 mg) of thermally conductive sheet was weighed out and immersed in 100 ml of methyl ethyl ketone at room temperature for 3 days. The insoluble matter was then filtered through a 200-mesh wire screen, air-dried at room temperature for 15 hours, dried at 100°C for 2 hours, and cooled at room temperature. The mass (Y) of the sample was then measured. The gel fraction was calculated by substituting X and Y into the following formula (2). Gel fraction (%) = (Y) / (X) × 100 (2)

[0090] Example 1 <Preparation of Composition> As a cross-linkable resin, nitrile rubber (NBR) which is solid at room temperature and normal pressure (manufactured by Zeon Corporation, product name "Nipol (registered trademark) 3350"; glass transition temperature: -38.0°C; specific gravity: 1 g / cm) was used. 372.2 parts of a resin containing 1,000 ppm or more of ethylenediaminetetraacetic acid (E2), 168.5 parts of nitrile rubber (NBR) (manufactured by Nippon Zeon Co., Ltd., product name "Nipol (registered trademark) 1312") that is liquid at room temperature and normal pressure were prepared, and 536.9 parts (50% by volume with respect to the total volume of the resin and graphite particles) of expanded graphite (manufactured by Ito Graphite Industries Co., Ltd., product name "EC-50", volume average particle diameter: 250 μm, aspect ratio = 1.5) were prepared as graphite particles. These were kneaded using a pressure kneader (manufactured by Nippon Spindle Co., Ltd.) at a temperature of 120°C for 20 minutes to obtain a composition. To the obtained composition, 3.6 parts of Nocrac 224 as an antioxidant, 7.2 parts of dicumyl peroxide (product name: Percumyl D40) as a crosslinking agent, and 24.1 parts of a foaming agent (manufactured by Eiwa Chemical Industry Co., Ltd., product name "Neocelbon N#1000S", 4,4-oxybis(benzenesulfonylhydrazide)) were added, and the mixture was kneaded at a temperature of 80°C for 5 minutes to obtain a composition.

[0091] <Pre-heat conductive sheet forming process> Next, 500 g of the obtained composition was rolled into a sheet using a first roll and a second roll under the following conditions: a gap between the first roll and the second roll of 1 mm, a roll temperature of 25°C, a sheet discharge speed (peripheral speed of the first roll) of 2 m / min, and a peripheral speed ratio of the second roll to the first roll (second roll / first roll): 1.15 / 1. The rolling process was repeated while maintaining the same sheet conveyance direction. A total of 10 rolling processes were performed to obtain a pre-heat-conductive sheet with a thickness of 1 mm.

[0092] <Laminated body forming process (first laminate forming process)> Next, the obtained pre-heat conductive sheet was cut into a size of 50 mm length x 50 mm width, and 50 sheets were stacked in the thickness direction to obtain a first stack having a height of about 50 mm.

[0093] <Crosslinking reaction step (second laminate formation step)> The obtained first laminate was wrapped in release PET (polyethylene terephthalate) and sealed with tape, then vacuum-packaged in a PET retort pouch. This was then heated and pressurized in an autoclave (Hanida Iron Works, small autoclave "DANDELION") at a temperature of 180°C and a pressure of 0.8 MPa (absolute pressure) from all directions for 30 minutes to allow the crosslinking and foaming reaction to proceed, yielding a second laminate.

[0094] <Slicing process> Next, while pressing the laminated surface of the secondary-pressurized laminate with a pressure of 0.3 MPa, a woodworking slicer (Marunaka Iron Works Co., Ltd., product name "Super Mecha S Super Finishing Planer") was used to slice the laminate at an angle of 0° to the lamination direction (in other words, in the normal direction to the main surface of the laminated pre-thermal conductive sheet), yielding a secondary sheet (thermal conductive sheet) measuring 50 mm long x 50 mm wide x 0.3 mm thick. The thermal conductive sheet is composed of strips joined in parallel in a direction perpendicular to the thickness direction of the thermal conductive sheet (a direction at an angle of 90° to the thickness direction). The width of the strips in this approximately perpendicular direction is approximately the same as the thickness of the pre-thermal conductive sheet. The obtained thermally conductive sheet was subjected to various measurements and evaluations as described above. The results are shown in Table 1.

[0095] Example 2 The blending amounts in Example 1 were changed to 93.8 parts solid NBR, 218.8 parts liquid NBR, 382.8 parts graphite particles, 9.4 parts crosslinking agent, 4.7 parts primary antioxidant, and 31.3 parts foaming agent. The rest was the same as in Example 1. The results are shown in Table 1.

[0096] Example 3 The graphite particles in Example 1 were changed to 481.6 parts of expanded graphite (manufactured by Ito Graphite Industries Co., Ltd., product name "EC-300", volume average particle size: 50 μm, aspect ratio = 1.5), and the amounts of other components were changed to 64.7 parts of solid NBR, 151.0 parts of liquid NBR, 6.5 parts of crosslinking agent, 3.2 parts of primary antioxidant, and 21.6 parts of foaming agent. The rest of the experiment was carried out in the same manner as in Example 1. The results are shown in Table 1.

[0097] Example 4 The crosslinkable resin in Example 1 was replaced with styrene-butadiene rubber (SBR) (manufactured by Zeon Corporation, trade name "Nipol (registered trademark) 1502", glass transition temperature: -50.0°C, specific gravity: 0.94 g / cm) which is solid at room temperature and normal pressure. 3 The amount of the additive was changed to 152.7 parts. Furthermore, 4.6 parts of zinc oxide as a vulcanization activator, 1.5 parts of stearic acid as a vulcanization activator, 2.3 parts of sulfur (manufactured by Tsurumi Chemical Industry Co., Ltd., product name "SULFAX (registered trademark) PMC") as a sulfur-based crosslinking agent, and 3.1 parts of N-cyclohexylbenzothiazole-2-sulfenamide (CBS) (Noccela CZ-G) as a crosslinking accelerator were blended. The amounts of the other components were changed to 372.3 parts of graphite particles, 2.3 parts of primary antioxidant, and 15.3 parts of foaming agent. The rest of the experiment was carried out in the same manner as in Example 1. The results are shown in Table 1.

[0098] Example 5 The blending amounts in Example 4 were changed to 100.0 parts of solid SBR, 500.0 parts of graphite particles, 3.0 parts of zinc oxide, 1.0 part of stearic acid, 1.5 parts of sulfur, 2.0 parts of CBS, 3.0 parts of a primary antioxidant, and 20.0 parts of a foaming agent. Liquid polybutene (manufactured by Nippon Soda Co., Ltd., trade name "NISSO PB B-3000", specific gravity: 0.88 g / cm) was used as a liquid crosslinkable resin at room temperature and normal pressure. 3 The rest of the procedure was the same as in Example 4. The results are shown in Table 1.

[0099] Example 6 In Example 1, liquid NBR was not blended as a crosslinkable resin, and instead, a plasticizer (manufactured by ADEKA Corporation, trade name "ADEKA Cizer RS-107", specific gravity: 1.02 g / cm 3 The amounts of other components were changed to 150.0 parts of solid NBR, 500.0 parts of graphite particles, 4.5 parts of crosslinking agent, 2.3 parts of primary antioxidant, and 15.0 parts of foaming agent. The rest of the procedure was the same as in Example 1. The results are shown in Table 1.

[0100] (Comparative Example 1) The same procedure as in Example 1 was carried out except that no foaming agent was added. The results are shown in Table 1.

[0101] (Comparative Example 2) The procedure was the same as in Example 1, except that the crosslinking agent, primary antioxidant, and foaming agent were not added. The results are shown in Table 1.

[0102] [Table 1]

[0103] Table 1 shows that Examples 1-6, in which the resin contains a crosslinked resin, the graphite particles are oriented at an angle of 60° or more and 90° or less with respect to the main surface of the thermal conductive sheet, the volume fraction of the graphite particles in the thermal conductive sheet is 35% by volume or more, and the porosity is 20% or more, are superior in compressibility compared to Comparative Example 1, in which the porosity is less than 20%. Furthermore, compared to Comparative Example 2, in which the resin does not contain a crosslinked resin and the porosity is less than 20%, Examples 1-6 are superior in restorability. In addition, in Comparative Examples 1 and 2, there were no voids whose major axis length was 20% or more of the thickness of the thermal conductive sheet, which would be evaluated as voids caused by a foaming agent, so the orientation angle and aspect ratio of the voids could not be measured. [Industrial Applicability]

[0104] According to the present invention, a thermally conductive sheet having excellent compressibility and restorability can be provided. [Explanation of symbols]

[0105] 1. Thermal conductive sheet 10 Resin 11 Long axis of graphite particle 12 void A First principal surface B Second principal surface

Claims

1. A thermally conductive sheet comprising a resin and graphite particles, the thermally conductive sheet includes a plurality of voids; the resin comprises a crosslinked resin, the graphite particles are oriented at an orientation angle of 60° or more and 90° or less with respect to the main surface of the thermal conductive sheet, the volume fraction of the graphite particles in the thermal conductive sheet is 35% by volume or more; A thermally conductive sheet having a porosity of 20% or more as calculated according to the following formula (1): Porosity (%) = 100 × (1 - measured specific gravity of thermal conductive sheet ÷ theoretical specific gravity of thermal conductive sheet) (1)

2. The thermal conductive sheet according to claim 1 , wherein the porosity is 20% or more and 35% or less.

3. 2. The thermal conduction sheet according to claim 1, wherein, among the plurality of voids in the thermal conduction sheet, the length of the major axis of the voids, when the outline of the voids is approximated as an ellipse, is 20% or more of the thickness of the thermal conduction sheet, the major axis of the voids is oriented at an orientation angle of 60° or more and 90° or less with respect to the main surface of the thermal conduction sheet, and has an average aspect ratio of 15 or more.

4. The thermal conductive sheet according to claim 1 , wherein the graphite particles have a volume average particle size of 40 μm or more.

5. The thermal conductive sheet according to claim 1 , wherein the crosslinked resin comprises a crosslinked product of a crosslinkable resin that is solid at room temperature and normal pressure, and a crosslinked product of a crosslinkable resin that is liquid at room temperature and normal pressure.

6. The thermally conductive sheet according to claim 1 , wherein the gel fraction is 80% or more.

7. A method for producing a thermal conductive sheet according to any one of claims 1 to 6, a pre-thermal conductive sheet forming step of pressurizing a composition containing a resin containing a crosslinkable resin, graphite particles, a crosslinking agent, and a foaming agent into a sheet to obtain a pre-thermal conductive sheet; a laminate forming step of stacking a plurality of the pre-thermal conductive sheets in the thickness direction or folding or rolling the pre-thermal conductive sheet to obtain a laminate; a crosslinking reaction step of heating the laminate while applying pressure to the laminate to cause a crosslinking and foaming reaction; and a slicing step of slicing the laminate at an angle of 45° or less with respect to the lamination direction to obtain a thermally conductive sheet.

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