Thermally conductive sheet and method for manufacturing the same

Abstract

To provide a thermally conductive sheet with excellent compressibility and adhesiveness, and a method for manufacturing such a thermally conductive sheet.SOLUTION: A thermally conductive sheet comprises resin containing a crosslinkable resin, graphite particles, a cross-linking agent, and a foaming agent. The volume fraction of the graphite particles is 40 vol% or more.SELECTED DRAWING: None

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%.

[0006] Furthermore, Patent Document 2 describes a thermally conductive sheet containing at least one type of graphite particles selected from the group consisting of scale-like particles, ellipsoidal particles, and rod-like particles oriented in the thickness direction, and having a modulus of elasticity and tackiness each equal to or greater than a predetermined value, and also describes that if the content of liquid components in the thermally conductive sheet is equal to or greater than a certain level, the thermally conductive sheet has excellent adhesiveness. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 2023-151582 [Patent Document 2] International Publication No. 2019 / 159340 Summary of the Invention [Problem to be solved by the invention]

[0008] Here, in addition to compressibility, the thermally conductive sheet is also required to have excellent adhesiveness between the thermally conductive sheet and the member to which it is applied, such as a heat sink or a heat generator. However, the conventional sheets described above have room for improvement in terms of achieving both high levels of compressibility and adhesiveness.

[0009] Therefore, an object of the present invention is to provide a thermally conductive sheet having excellent compressibility and adhesiveness, and a method for producing such a thermally conductive sheet. [Means for solving the problem]

[0010] The present inventors have conducted extensive research to achieve the above object, and have newly discovered that, in the production of a thermally conductive sheet containing a resin and graphite particles, a thermally conductive sheet having excellent compressibility and adhesiveness can be provided by blending a crosslinkable resin, a crosslinking agent, and a foaming agent, and have completed the present invention.

[0011] That is, the present invention aims to advantageously solve the above-mentioned problems, and the present invention is a thermal conductive sheet characterized by comprising: [1] a resin containing a crosslinkable resin, graphite particles, a crosslinking agent, and a foaming agent, and the volume fraction of the graphite particles is 40 volume % or more. Such a thermally conductive sheet has excellent compressibility and adhesiveness.

[0012] [2] Here, the thermally conductive sheet of the above [1] preferably has a porosity calculated according to the following formula (1) of 15% or more and 30% or less. Porosity (%) = 100 × (1 - measured specific gravity of thermal conductive sheet ÷ theoretical specific gravity of thermal conductive sheet) (1) If the porosity is within the above range, the heat conductive sheet will have better compressibility.

[0013] [3] In the thermally conductive sheet of [1] or [2] above, the cross-linkable resin preferably contains at least one of an acrylic polymer, and a styrene-butadiene block copolymer or a hydrogenated product thereof. If the cross-linkable resin contains at least one of an acrylic polymer, and a styrene-butadiene block copolymer or a hydrogenated product thereof, the thermally conductive sheet will have even better adhesiveness.

[0014] [4] In the thermally conductive sheet of any one of the above [1] to [3], the gel fraction is preferably 85% or less. If the gel fraction in the thermal conductive sheet is equal to or less than the upper limit, this means that crosslinking of the crosslinkable resin in the thermal conductive sheet has not progressed, and when the thermal conductive sheet is applied to a target member and the two are bonded together, the adhesion between the member and the thermal conductive sheet can be improved. The gel fraction can be measured according to the method described in the examples.

[0015] [5] The present invention also provides a method for producing a thermally conductive sheet according to any one of [1] to [4] above, comprising the steps of: a pre-thermally conductive sheet forming step of pressurizing a composition containing a resin containing a cross-linkable resin, graphite particles, a cross-linking agent, and a foaming agent to form it into a sheet to obtain a pre-thermally conductive sheet; a laminate forming step of stacking a plurality of the pre-thermally conductive sheets in the thickness direction or folding or rolling the pre-thermally conductive sheet to obtain a laminate; a laminate bonding step of heating the laminate under temperature conditions such that the gel fraction in the thermally conductive sheet is 85% or less; 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 manufacturing method has excellent compressibility and adhesiveness. [Effects of the Invention]

[0016] According to the present invention, it is possible to provide a thermally conductive sheet having excellent compressibility and adhesiveness, and a method for producing such a thermally conductive sheet. DETAILED DESCRIPTION OF THE INVENTION

[0017] 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.

[0018] (thermal conductive sheet) The thermally conductive sheet of the present invention comprises a resin containing a crosslinkable resin, graphite particles, a crosslinking agent, and a foaming agent, and is characterized in that the volume fraction of the graphite particles is 40% by volume or more. The thermally conductive sheet of the present invention that satisfies these conditions has excellent compressibility and adhesiveness. That is, a thermally conductive sheet having a predetermined void and excellent compressibility can conform well to unevenness that may exist on the surface of multiple adherends when sandwiched between multiple adherends, such as heat-generating bodies and heat-dissipating bodies. Therefore, by performing the predetermined bonding process described below using such a thermally conductive sheet, multiple adherends can be bonded well.

[0019] <Resin> The thermally conductive sheet of the present invention contains a resin, which allows the heat generating element and the heat dissipating element to be well bonded to each other via the thermally 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 crosslinkable resin, and optionally contains a resin other than the crosslinkable resin (other resin).

[0020] <<Crosslinkable resin>> The resin contained in the thermal conductive sheet of the present invention contains a cross-linkable resin. The cross-linkable resin becomes a cross-linked resin when cross-linked by a cross-linking agent. The thermal conductive sheet of the present invention may contain a cross-linked resin that is unintentionally cross-linked during the manufacturing process. Even in such cases, the cross-linkable resin may account for 99% by mass or more of the total mass of the cross-linkable resin and cross-linked resin contained in the thermal conductive sheet. Furthermore, by including a cross-linkable resin as a resin, the thermal conductive sheet of the present invention can firmly bond the two at the interface with the adherends when the thermal conductive sheet is sandwiched between multiple adherends, such as a heat sink and a heat generating element, and a predetermined bonding process, as described below, is performed. The predetermined bonding process refers to a process in which the thermal conductive sheet is heated while being interposed between the heat sink and the heat generating element, and pressure is applied in the thickness direction of the thermal conductive sheet to cross-link at least a portion of the cross-linkable resin with a cross-linking agent and to foam the foaming agent to create a void structure in the thermal conductive sheet, thereby bonding the heat sink and the heat generating element via the thermal conductive sheet having a void structure. By subjecting the thermally conductive sheet of the present invention to such a predetermined bonding process, a crosslinking reaction occurs in at least a portion of the resin simultaneously with bonding, and chemical bonding may occur at the interface with at least one of the heat-generating body and the heat-dissipating body, presumably enabling strong bonding to the heat-generating body and the heat-dissipating body. Furthermore, because the thermally conductive sheet of the present invention contains a foaming agent, it has superior compressibility, even in an unfoamed state, compared to sheets that do not contain a foaming agent. Furthermore, during the predetermined bonding process, the foaming agent is expanded to create a void structure in the thermally conductive sheet, which is interposed between multiple adherends and firmly bonds them together. This results in a void structure in the thermally conductive sheet. Therefore, the thermally conductive sheet that has undergone the predetermined bonding process can have even better compressibility.

[0021] Here, the crosslinkable resin is preferably 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).

[0022] The thermally conductive sheet of the present invention may optionally contain a crosslinking accelerator.

[0023] 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.

[0024] -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 at least one of an acrylic polymer, an acrylonitrile-butadiene copolymer, and a styrene-butadiene block copolymer or a hydrogenated product thereof.

[0025] 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 -55.0°C or higher, 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 resulting thermal conductive sheet can be prevented from thermally deforming during use, thereby improving thermal conductivity. 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 adhesiveness that the resulting thermal conductive sheet can exhibit can be further improved.

[0026] The blending ratio of the crosslinkable resin that is solid at room temperature and normal pressure in the crosslinkable resin is preferably 50% by mass or more, and more preferably 70% 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, the thermal conductive sheet has excellent adhesiveness.

[0027] -Cross-linkable resin that is liquid at room temperature and pressure- The crosslinkable resin may be 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 crosslinkable resin that is liquid at room temperature and normal pressure in addition to a crosslinkable resin that is solid at room temperature and normal pressure as the crosslinkable resin, the thermal conductivity of the thermal conductive sheet can be further improved.

[0028] [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, a styrene-butadiene block copolymer, or a hydrogenated product thereof, it is preferable to use a sulfur-based crosslinking agent or a peroxide.

[0029] 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 adhesiveness of the thermal conductive sheet is improved and the orientation structure of the graphite particles within the thermal conductive sheet is well maintained.

[0030] [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.

[0031] 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, which is thought to impart appropriate strength and elasticity to the thermal conductive sheet, thereby improving the adhesiveness of the thermal conductive sheet and maintaining a good orientation structure of the graphite particles within the thermal conductive sheet.

[0032] <<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.

[0033] [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 65% ​​by volume or less, more preferably 55% by volume or less, based on the total volume of resin, graphite particles, crosslinking agent, foaming agent, 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 adhesiveness, and if it is equal to or less than the upper limit, the thermal conductive sheet will have excellent thermal conductivity.

[0034] [Crosslinkable resin content] The proportion of the crosslinkable resin contained in the resin is preferably 50% by mass or more, and more preferably 70% by mass or more. The upper limit of the proportion of the crosslinkable resin contained in the resin is not particularly limited, and may be 100% by mass. When the proportion of the crosslinkable resin contained in the resin is equal to or greater than the above lower limit, the thermal conductive sheet has excellent adhesiveness.

[0035] [Gel fraction] The thermally conductive sheet of the present invention preferably has a gel fraction of 85% or less, more preferably 80% or less. A gel fraction of less than the above upper limit indicates that crosslinking of the crosslinkable resin in the thermally conductive sheet has not progressed. When the thermally conductive sheet is applied to a target component and the two are bonded together, the adhesion between the component and the thermally conductive sheet can be improved. The lower limit of the gel fraction may be, for example, 50% or more, or 60% or more. 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 thermally 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.

[0036] <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.

[0037] 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).

[0038] 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%.

[0039] The volume fraction of graphite particles in the thermal conductive sheet must be 35% by volume or more, preferably 45% by volume or more, and preferably 70% by volume or less, and more preferably 60% by volume or less, where the total volume of the resin, graphite particles, crosslinking agent, foaming agent, 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-mentioned 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-mentioned upper limit, the flexibility of the thermal conductive sheet can be maintained.

[0040] <Foaming agent> The thermally conductive sheet of the present invention also contains a foaming agent. Various foaming agents that decompose upon heating to generate gas can be used as the foaming agent. A foaming assistant that lowers the decomposition temperature of the foaming agent and promotes its decomposition may also be used. Specific examples of the foaming agent include organic foaming agents such as azo compounds such as azodicarbonamide (ADCA) and azobisisobutyronitrile, nitroso compounds such as N,N'-dinitrosopentamethylenetetramine (DPT), and sulfonylhydrazide compounds such as p-toluenesulfonylhydrazide and 4,4'-oxybis(benzenesulfonylhydrazide) (OBSH); and gas-based foaming agents such as volatile hydrocarbon compounds such as chlorofluorocarbons, carbon dioxide, water, and pentane, and microcapsules containing these compounds. These include one or more chemical foaming agents. The blending ratio of the foaming agent is preferably 5% by mass or more, more preferably 7% by mass or more, and preferably 15% by mass or less, and more preferably 12% 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 foaming agent ratio is equal to or greater than the above lower limit, the foaming reaction proceeds smoothly, and the resulting thermal conductive sheet can have excellent compressibility. If the foaming agent ratio is equal to or less than the above upper limit, the adhesiveness of the resulting thermal conductive sheet can be improved.

[0041] 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.

[0042] [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.

[0043] 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.

[0044] 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.

[0045] <Other ingredients> The thermally conductive sheet of the present invention may optionally further contain components other than the resin, graphite particles, crosslinking agent, and foaming agent 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 production of thermally conductive sheets, 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; and antioxidants. These can be blended in the required amounts depending on the application.

[0046] (Thermal conduction sheet structure) The thermally conductive sheet of the present invention preferably has an orientation structure in which the graphite particles are oriented at an orientation angle of 60° to 90° relative to the main surface of the thermally conductive sheet. Furthermore, the orientation angle of the graphite particles relative to the main surface of the thermally conductive sheet is preferably within the range of 75° to 90°, and more preferably within the range of 78° to 90°. If the orientation angle of the graphite particles relative to the main surface of the thermally conductive sheet is within the above range, the thermal conductivity and strength of the thermally conductive sheet can be improved. The orientation structure of the graphite particles in the thermally conductive sheet can be confirmed according to the method described in JP 2023-151582 A.

[0047] (Thermal Conduction Sheet Properties) <Porosity> The thermal conductive sheet preferably has a porosity calculated according to the following formula (1) of 15% or more, preferably 18% or more, more preferably 20% or more, and preferably 30% or less, more preferably 29% or less, and even more preferably 28% or less. Porosity (%) = 100 × (1 - measured specific gravity of thermal conductive sheet ÷ theoretical specific gravity of thermal conductive sheet) (1) If the porosity is equal to or greater than the lower limit, the thermally conductive sheet has a certain degree of compressibility, resulting in excellent conformability to unevenness that may exist on the surface of an adherend (hereinafter, sometimes referred to as uneven conformability). This further enhances the adhesion between the adherend and the thermally conductive sheet when multiple adherends are bonded together using the thermally conductive sheet. Furthermore, if the porosity is equal to or less than the upper limit, the resulting thermally conductive sheet does not contain excessive voids, thereby exhibiting excellent thermal conductivity. In the method for producing a thermally conductive sheet of the present invention described below, conditions are controlled to minimize foaming of the foaming agent during the production process. However, it is believed that a very small amount of foaming agent still inevitably occurs during the production process. Due to this unavoidable foaming, the thermally conductive sheet of the present invention is believed to have a very small proportion of voids, and these voids are believed to act to increase the compressibility of the thermally conductive sheet of the present invention in its pre-foamed state. The porosity can be prevented from increasing excessively by suppressing unintended and unavoidable foaming of the foaming agent during the production of the thermally conductive sheet.

[0048] <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 9% 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.

[0049] <Thermal conductivity> The thermal conductivity of the thermal conductive sheet in the thickness direction is preferably 13 W / m K or more, and more preferably 18 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.

[0050] <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.

[0051] (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 pressed 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 laminate bonding step in which the laminate is heated at a temperature condition in which the gel fraction of the thermally conductive sheet is 85% or less; 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.

[0052] <(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.

[0053] <<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 (other components) other than the resin, graphite particles, crosslinking agent, foaming agent, and crosslinking accelerator.

[0054] [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 described above in the "Thermal Conductive Sheet" section can be used as the crosslinkable resin in the ratio described above. The blending amount of the resin in the composition corresponds to the blending range described for the "resin content" contained in the thermal conductive sheet described above. Furthermore, the blending amount of the crosslinkable resin in the resin corresponds to the blending range described for the "crosslinkable resin content" contained in the thermal conductive sheet described above. 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 ratio described above.

[0055] [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.

[0056] [Crosslinking agent] As the crosslinking agent, the crosslinking agents described above in the section "Thermal Conductive Sheet" can be used in the proportions described above.

[0057] [Crosslinking accelerator] As the crosslinking accelerator, those mentioned in the section "Thermal Conductive Sheet" can be used in the proportions mentioned above.

[0058] [Foaming agent] As the foaming agent, the foaming agents described above in the section "Thermal Conductive Sheet" can be used in the proportions described above.

[0059] [Foaming aid] As the foaming aid, the foaming aids described above in the section "Thermal Conductive Sheet" can be used in the proportions described above.

[0060] [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.

[0061] [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 a kneader; a mixer such as a Henschel mixer, a Hobart mixer, or a high-speed mixer; a twin-screw kneader; or a roll mixer. 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 an optional crosslinking accelerator and other components. The mixing time may be, for example, 5 minutes to 60 minutes. The mixing temperature is, for example, preferably 5°C or higher, more preferably 85°C or lower, and even more preferably 80°C or lower. Keeping the mixing temperature below the upper limit can improve mixing efficiency. Keeping the mixing temperature below the upper limit can effectively prevent both unintended crosslinking reactions of the crosslinking agent and unintended excessive foaming of the foaming agent.

[0062] <<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.

[0063] 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.

[0064] <<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.

[0065] <(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.

[0066] <(C) Laminate adhesion process> In the laminate bonding step, the laminate obtained in the laminate formation step (hereinafter also referred to as the first laminate) is heated under temperature conditions that result in a gel fraction of 85% or less in the thermally conductive sheet, thereby obtaining a second laminate in which the interfaces between the primary sheets are bonded to each other. Temperature conditions that result in a gel fraction of 85% or less in the thermally conductive sheet refer to temperature conditions that minimize the crosslinking reaction of the crosslinkable resin contained in the thermally conductive sheet. As described above, the gel fraction of the thermally conductive sheet obtained through the manufacturing method of the present invention is preferably 80% or less, and may be, for example, 50% or more, or 60% or more. Therefore, the temperature conditions in the laminate bonding step are also preferably temperature conditions that result in a gel fraction of the thermally conductive sheet within the above range.

[0067] Furthermore, the temperature conditions in the laminate bonding step preferably also satisfy the condition of minimizing foaming of the foaming agent contained in the primary sheets. Specifically, the temperature conditions in the laminate bonding step are preferably 100°C or lower, more preferably 95°C or lower, and even more preferably 90°C or lower. Note that there is no particular lower limit for the temperature in the laminate bonding step, but from the viewpoint of ensuring adhesion between the primary sheets, it must be 70°C or higher, and preferably 80°C or higher.

[0068] Furthermore, in the laminate bonding step, it is preferable to heat the first laminate and apply isotropic pressure (secondary pressure) to the laminate in the stacking direction or to form a second laminate, which can further strengthen the adhesion at the interface between the primary sheets and improve the orientation structure of the graphite particles in the thermal conductive sheet.

[0069] 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.

[0070] Furthermore, the heating time for the laminate is not particularly limited, but is, for example, preferably 20 minutes or more, more preferably 30 minutes or more, and preferably 2 hours or less, more preferably 1 hour or less. If the heating time is equal to or greater than the above-mentioned lower limit, the interfacial adhesion in the second laminate can be made stronger. On the other hand, if the heating time is equal to or less than the above-mentioned upper limit, both the occurrence of an unintended crosslinking reaction of the crosslinkable resin and the unintended foaming of the foaming agent can be effectively suppressed.

[0071] In the second laminate obtained through the laminate bonding step, it is presumed that most of the cross-linkable resin, for example, 90% by mass or more, exists without becoming a cross-linked resin, and most of the foaming agent, for example, 90% by mass or more, exists in an unfoamed state. Furthermore, it is presumed that the graphite particles in the second laminate are oriented in a direction substantially perpendicular to the lamination direction.

[0072] <(D) Slicing process> In the slicing step, the laminate that has undergone the laminate bonding 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.

[0073] 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).

[0074] The thermally conductive sheet thus obtained contains a cross-linkable resin and a foaming agent, and when a predetermined bonding step is carried out, for example, with the thermally conductive sheet sandwiched between multiple adherends such as heat sinks and heat generators, the two can be firmly bonded at the interface with the adherends. Therefore, the thermally conductive sheet of the present invention can be suitably used, for example, as a reactive bonding thermally conductive sheet in the manufacturing method of an electronic device according to an example described below.

[0075] (Electronic device manufacturing method) An example of a method for manufacturing an electronic device is a method for manufacturing an electronic device using the above-described thermally conductive sheet. This manufacturing method may include a bonding step of heating the thermally conductive sheet while applying pressure in a thickness direction of the thermally conductive sheet, with the thermally conductive sheet interposed between the heat sink and the heat generating element, to foam the foaming agent and crosslink at least a portion of the crosslinkable resin with the crosslinking agent, thereby bonding the heat sink and the heat generating element via the thermally conductive sheet.

[0076] <Thermal Conduction Sheet> As the thermally conductive sheet, the thermally conductive sheet of the present invention described above is used.

[0077] <Heater> The heat-generating body is an object to which a heat sink is attached via a thermally conductive sheet to dissipate heat, such as a semiconductor chip, a semiconductor package, an automotive power module, a display, or an industrial power module.

[0078] <Heat dissipator> The heat dissipator is a component that dissipates heat from a heat-generating body via a thermally conductive sheet. Examples of the heat dissipator include a heat spreader, a heat sink, and a water-cooled pipe.

[0079] <Adhesion process> In the bonding process, the thermally conductive sheet is placed between the heat sink and the heat generating element, and pressure is applied to the thermally conductive sheet in the thickness direction while the thermally conductive sheet is heated. This causes the foaming agent to foam and at least a portion of the cross-linkable resin to be cross-linked by the cross-linking agent. This cross-links at least a portion of the cross-linkable resin and the cross-linking agent to cross-link to form a cross-linked resin. It is believed that this is because chemical bonds are formed at the interface between the heat generating element and the thermally conductive sheet, and at the interface between the heat sink and the thermally conductive sheet, resulting in a strong bond between the thermally conductive sheet, the heat generating element, and the heat sink. Furthermore, the void structure formed by foaming the foaming agent can impart good compressibility to the thermally conductive sheet placed between the heat generating element and the heat sink.

[0080] Here, the pressure when pressing the thermally conductive sheet in the thickness direction in the bonding step is preferably 0.1 MPa or more, more preferably 0.2 MPa or more, even more preferably 0.6 MPa or more, preferably 2.0 MPa or less, and more preferably 1.5 MPa or less. If the pressure when pressing the thermally conductive sheet in the thickness direction is equal to or greater than the above-mentioned lower limit, the heat generating element and the heat dissipating element can be firmly bonded via the thermally conductive sheet in the bonding step, thereby improving adhesion. On the other hand, if the pressure when pressing the thermally conductive sheet in the thickness direction is equal to or less than the above-mentioned upper limit, it is possible to effectively prevent the thermally conductive sheet from being excessively crushed, which would cause the orientation structure of the graphite particles to collapse and reduce thermal conductivity.

[0081] The heating temperature in the bonding step is not particularly limited, but is preferably above 90°C, more preferably 100°C or higher, and even more preferably 155°C or higher, and is preferably 200°C or lower, and more preferably 190°C or lower.

[0082] The heating time in the bonding step is not particularly limited, but may be, for example, 1 hour or more and 8 hours or less.

[0083] Even in the thermally conductive sheet that has undergone the above-mentioned bonding process, the graphite particles are oriented at an angle of 60° to 90° relative to the main surface of the thermally conductive sheet, resulting in excellent thermal conductivity. Furthermore, as a result of the cross-linking reaction that has progressed in the bonding process, the thermally conductive sheet itself is endowed with mechanical strength, and the orientation structure of the graphite particles is also strengthened, resulting in excellent durability of thermal conductivity.

[0084] The heating in the bonding step may be performed by heat generation from a heating element, and the pressure in the bonding step may be applied by the weight of the heating element or heat dissipating element. [Example]

[0085] 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.

[0086] (Physical property measurement)

[0087] <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.

[0088] <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.

[0089] <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

[0090] <Compression ratio> The compressibility of the thermally conductive sheets manufactured in each example and comparative example was measured using a thermal resistance tester (product name: "Resin Material Thermal Resistance Measuring Device" manufactured by Hitachi Technology and Services, Ltd.) Here, the thermally conductive sheets were cut into approximately 1 cm squares as samples, and the thickness of the thermally conductive sheets was measured at a sample temperature of 50°C when a pressure of 0.9 MPa was applied in the thickness direction. The thickness of the thermal conductive sheet when pressurized 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.

[0091] <Adhesiveness> The adhesion between the thermally conductive sheet and the metal plate was measured using a peel analysis device (Kyowa Interface Science Co., Ltd., "VPA-H100"). First, a 40 mm × 40 mm × 0.3 mm thermally conductive sheet was sandwiched between copper foil of the same size on both sides, which was then sandwiched between a 50 mm × 50 mm × 10 mm copper plate and a 50 mm × 50 mm × 10 mm aluminum plate. The four corners were secured with screws to create a copper plate-copper foil-thermally conductive sheet-copper foil-aluminum plate structure. Two such structures were prepared for each example and comparative example. These structures simulated electronic devices. By adjusting the screws at the four corners, the thermally conductive sheet was heated at 180°C for 1 hour under a pressure of 0.6 N, bonding the copper foil and thermally conductive sheet. The thermally conductive sheet with copper foil on both sides was removed from the structure and cut to a length of 40 mm × 10 mm × 0.3 mm. The thermally conductive sheet was then fixed on a flat table. Next, a portion of the copper foil located on the main surface of one end of the thermal conductive sheet was peeled off. The copper foil in the peeled portion was then held and peeled off at a rate of 200 mm / min in a 90° direction in an atmosphere of 25°C. The maximum tensile strength measured when the copper foil was peeled off was taken as the peel strength (N), and the value obtained by dividing this value by the width was taken as the peel strength (unit: N / mm).

[0092] <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.

[0093] <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)

[0094] Example 1 <Preparation of Composition> As the cross-linkable resin, styrene butadiene rubber (SBR) which is solid at room temperature and normal pressure (manufactured by Zeon Corporation, trade name "Nipol (registered trademark) 1502"; glass transition temperature: -50.0°C; specific gravity: 0.94 g / cm) was used. 3 153 parts of a resin and graphite particles (product name "EC-50", volume average particle diameter: 250 μm, aspect ratio=1.5), and 372 parts (50% by volume with respect to the total volume of the resin and graphite particles) of expanded graphite particles (manufactured by Ito Graphite Industries Co., Ltd.) were prepared. These were kneaded at a temperature of 120°C for 20 minutes using a pressure kneader (manufactured by Nippon Spindle Co., Ltd.) to obtain a composition. To the obtained composition, 2.3 parts of sulfur (trade name "SULFAX (registered trademark) PMC" manufactured by Tsurumi Chemical Industry Co., Ltd.) as a crosslinking agent, 4.6 parts of zinc oxide as a vulcanization activator, 1.5 parts of stearic acid as a vulcanization activator, 3.1 parts of N-cyclohexylbenzothiazole-2-sulfenamide (CBS) (Noccela CZ-G) as a crosslinking accelerator, and 15.3 parts of a foaming agent (trade name "Neocelbon N#1000S" manufactured by Eiwa Chemical Industry Co., Ltd., 4,4-oxybis(benzenesulfonylhydrazide)) were added, and the mixture was kneaded at a temperature of 80°C for 5 minutes to obtain a composition.

[0095] <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.

[0096] <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.

[0097] <Laminated body adhesion process (second laminate formation process)> 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 90°C and a pressure of 0.8 MPa (absolute pressure) from all directions for 30 minutes to obtain a second laminate.

[0098] <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.

[0099] Example 2 The blending amounts in Example 1 were changed to 181 parts of solid SBR, and the graphite particles were changed to 519 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). Otherwise, the same procedure as in Example 1 was carried out. The results are shown in Table 1.

[0100] Example 3 The crosslinkable resin in Example 1 was replaced with acrylic rubber, which is a solid acrylic resin at room temperature and normal pressure (solid acrylic resin, manufactured by Zeon Corporation, trade name "Nipol (registered trademark) AR-14", glass transition temperature: -42.0°C, specific gravity: 1.10 g / cm). 3 The blending amount of graphite was changed to 414.3 parts, and the crosslinking agent was changed to 1.5 parts of Diak#1 (hexamethylenediamine carbamate, manufactured by Chemours), the crosslinking accelerator was changed to 6.2 parts of Noccela DT (di-o-tolylguanidine), the blending amount of foaming agent was changed to 31.0 parts, and 9.3 parts of Nocrac 224 (2,2,4-trimethyl-1,2-dihydroquinoline polymer) was added as an antioxidant. The rest of the experiment was carried out in the same manner as in Example 1. The results are shown in Table 1.

[0101] Example 4 In Example 3, the amounts of each component were changed to 285 parts crosslinkable resin, 715 parts graphite particles, 1.4 parts crosslinking agent, 5.7 parts crosslinking accelerator, 8.55 parts antioxidant, and 28.5 parts foaming agent. The rest was the same as in Example 3. The results are shown in Table 1.

[0102] Example 5 The same procedure as in Example 4 was carried out, except that the type of graphite particles was changed to expanded graphite (manufactured by Ito Graphite Industries Co., Ltd., product name "EC-300", volume average particle size: 50 μm, aspect ratio = 1.5). The results are shown in Table 1.

[0103] (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.

[0104] (Comparative Example 2) A primary sheet was produced with the same composition as in Example 1, and this was made into a laminate. When this was subjected to the laminate bonding step, the heating temperature was changed to 180°C. By setting the heating temperature in the laminate bonding step to 180°C, the crosslinking reaction of the crosslinkable resin and the foaming reaction of the foaming agent were promoted in this step. Except for this, the procedure was the same as in Example 1. The results are shown in Table 1. In the obtained thermal conductive sheet, the crosslinkable resin had mostly (99% by mass or more) become crosslinked resin, and the foaming agent had foamed and disappeared.

[0105] [Table 1]

[0106] As can be seen from Table 1, the thermally conductive sheets of Examples 1-5, which contain a resin containing a crosslinkable resin, graphite particles, a crosslinking agent, and a foaming agent, with the volume fraction of the graphite particles being 40% by volume or more, have superior compressibility compared to Comparative Example 1, which does not contain a foaming agent. Furthermore, in Comparative Example 2, in which the foaming reaction and crosslinking reaction progressed to the point where the foaming agent disappeared and most of the crosslinkable resin was converted to a crosslinked resin, the thermally conductive sheet and the metal plate did not bond, and the adhesiveness of the thermally conductive sheet could not be measured, whereas the thermally conductive sheets of Examples 1-5 have superior adhesiveness. [Industrial Applicability]

[0107] According to the present invention, a thermally conductive sheet having excellent compressibility and adhesiveness can be provided.

Claims

1. The present invention includes a resin containing a crosslinkable resin, graphite particles, a crosslinking agent, and a foaming agent, The volume fraction of the graphite particles is 40% by volume or more. Thermal conductive sheet.

2. The thermal conductive sheet according to claim 1, wherein the porosity calculated according to the following formula (1) is 15% or more and 30% or less. Porosity (%) = 100 × (1 - measured specific gravity of thermal conductive sheet ÷ theoretical specific gravity of thermal conductive sheet) (1)

3. 2. The thermally conductive sheet according to claim 1, wherein the crosslinkable resin comprises at least one of an acrylic polymer, a styrene-butadiene block copolymer, and a hydrogenated product thereof.

4. The thermally conductive sheet according to claim 1 , wherein the gel fraction is 85% or less.

5. A method for producing a thermal conductive sheet according to any one of claims 1 to 4, 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 laminate bonding step of heating the laminate under a temperature condition in which the gel fraction of the thermally conductive sheet is 85% or less; 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.

Images