Thermally conductive sheet and method for manufacturing the same

Abstract

To provide a thermally conductive sheet with excellent compressibility and restorability.SOLUTION: A thermally conductive sheet comprises resin and plate-shaped boron nitride particles. The thermally conductive sheet includes multiple voids and contains a crosslinked resin. In the thermally conductive sheet, the plate-shaped boron nitride 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 plate-shaped boron nitride particles occupying the volume of the thermally conductive sheet is 50% or more, and the porosity calculated according to the following formula (1) is 10% 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] To prevent malfunctions 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.When using a heat sink, in order to efficiently transfer heat from the heat-generating body to the heat sink, a sheet-like member with high thermal conductivity (thermal conduction sheet) is placed between the heat-generating body and the heat sink, and a predetermined pressure is applied to this thermal conduction sheet to bring the heat-generating body and the heat sink into close contact.

[0004] When used, a single thermally conductive sheet is often attached to multiple heat sources at different heights, and therefore is required to have excellent compressibility. For example, Patent Document 1 discloses a technology for providing a thermally conductive sheet by setting the volume fraction of graphite particles with an aspect ratio greater than 1 to 50% or more, orienting the long axes of the graphite particles in the thickness direction, and further providing a structure in which the long axes of the graphite particles do not have an inflection point in a predetermined region within the thermally conductive sheet when the thermally conductive sheet is compressed in the thickness direction, thereby achieving excellent thermal conductivity and compressibility of the resulting thermally conductive sheet. Furthermore, Patent Document 2, for example, discloses a technology for preparing a laminate in which graphite heat-dissipating sheet layers and flexible adhesive layers formed by blending expandable particles into an adhesive and foaming them are alternately stacked, and then cutting the laminate to a predetermined thickness in the stacking direction to obtain a sheet having a thermal conductivity of 4 W / m·K or more and a density of 0.2 g / cm. 3 ~1.0g / cm 3Furthermore, Patent Document 3 discloses a thermally conductive sheet for electronic devices that includes a foam sheet and an adhesive material, in which the resin that constitutes the main component of the foam sheet is crosslinked, and the sheet has a 50% compressive strength of 1000 kPa or less and a thermal resistance at 50% compression of 5.0°C / W or less. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2023-151582 [Patent Document 2] International Publication No. 2020 / 202908 [Patent Document 2] Japanese Patent Application Laid-Open No. 2017-79264 Summary of the Invention [Problem to be solved by the invention]

[0006] 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.

[0007] However, the conventional sheets described above have room for improvement in terms of achieving both high levels of compressibility and restorability.

[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 plate-like boron nitride particles has excellent compressibility and restorability when the thermally conductive sheet contains a plurality of voids, the resin contains a crosslinked resin, the plate-like boron nitride particles are oriented at a predetermined angle within the sheet, the volume fraction of the plate-like boron nitride particles in the thermally conductive sheet is a certain level or more, and the porosity calculated by a predetermined formula is 20% or more, thereby completing the present invention.

[0010] That is, the present invention aims to advantageously solve the above-mentioned problems, and provides: [1] a thermally conductive sheet comprising a resin and plate-shaped boron nitride particles, wherein the thermally conductive sheet contains a plurality of voids, the resin contains a crosslinked resin, the plate-shaped boron nitride particles are oriented at an orientation angle of 60° or more and 90° or less with respect to the main surface of the thermally conductive sheet, the volume fraction of the plate-shaped boron nitride particles in the volume of the thermally conductive sheet is 50% by volume or more, and the void ratio calculated according to the following formula (1) is 10% 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. In this specification, the orientation angle of the plate-like boron nitride particles can be measured by the method described in the examples.

[0011] [2] Here, in the thermal conductive sheet of [1] above, it is preferable that, among the plurality of voids in the thermal conductive sheet, the length of the major axis of the voids, when the outline of the void is approximated as an ellipse, is 20% or more of the thickness of the thermal conductive sheet, and the major axis of the voids is oriented at an angle of 60° or more and 90° or less with respect to the main surface of the thermal conductive sheet. Such a heat conductive sheet has even better compressibility. The length of the major axis of the voids can be measured by the method described in the Examples.

[0012] [3] In the thermally conductive sheet of the above [1] or [2], the aspect ratio of the voids is preferably 8 or more. Such a thermally conductive sheet has even better compressibility. The aspect ratio of the voids can be measured by the method described in the Examples.

[0013] [4] Furthermore, any of the thermally conductive sheets [1] to [3] above may further contain a liquid material.

[0014] [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 crosslinkable resin, plate-shaped boron nitride 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]

[0015] 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]

[0016] [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

[0017] Hereinafter, embodiments of the present invention will be described in detail. Because the thermally conductive sheet of the present invention has thermal conductivity, it can be sandwiched between a heat-generating body and a heat-dissipating body. For example, the thermally conductive sheet of the present invention can be used to form an electronic device by interposing it between a heat-generating body such as a semiconductor, electronic display, or general electronic component and a heat-dissipating body such as a heat sink, heat sink plate, or heat dissipating fin to bond them together. In addition, 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 contains a resin and plate-like boron nitride particles, and also contains a plurality of voids. The resin contained in the thermally conductive sheet contains a crosslinked resin, and the plate-like boron nitride particles are oriented at an orientation angle of 60° or more and 90° or less with respect to the main surface of the thermally conductive sheet. Furthermore, in the thermally conductive sheet, the volume fraction of the plate-like boron nitride particles in the volume of the thermally conductive sheet is 50% by volume or more. The thermally conductive sheet of the present invention is characterized by having a porosity of 10% or more, 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 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.

[0019] <Plate-shaped boron nitride particles> In this specification, plate-like boron nitride refers to boron nitride particles with an aspect ratio (major axis / minor axis) of 1.4 or more. The aspect ratio of the plate-shaped boron nitride particles is preferably 10.0 or less, and more preferably 5.0 or less. It is presumed that an aspect ratio of 10.0 or less makes it easier for the plate-shaped boron nitride particles to be well oriented in the thickness direction in the thermal conductive sheet, thereby improving the thermal conductivity of the thermal conductive sheet in the thickness direction. Furthermore, flake-shaped boron nitride can be suitably used as the plate-shaped boron nitride. There is no particular lower limit to the aspect ratio, but it is preferably 1.1 or more. In the present invention, the "aspect ratio" can be determined by observing boron nitride particles with an SEM (scanning electron microscope), measuring the maximum diameter (major diameter) and the particle diameter (minor diameter) in the direction perpendicular to the maximum diameter for 50 random boron nitride particles, and calculating the average ratio of the major diameter to the minor diameter (major diameter / minor diameter). In the above, for example, when the plate-like boron nitride particles are scaly, the "major diameter" refers to the length in the direction of the major axis of the main surface of the scaly shape, and the "minor diameter" refers to the length in the direction perpendicular to the major axis of the main surface.

[0020] The volume average particle diameter of the plate-shaped boron nitride particles is preferably 10 μm or more, preferably 20 μm or more, more preferably 30 μm or more, and preferably 60 μm or less, more preferably 50 μm or less. It is presumed that if the volume average particle diameter of the plate-shaped boron nitride particles is above the above-mentioned lower limit, a good heat transfer path of the plate-shaped boron nitride particles can be formed in the heat conductive sheet, thereby improving the thermal conductivity of the heat conductive sheet. On the other hand, if the volume average particle diameter of the plate-shaped boron nitride particles is below the above-mentioned upper limit, the thickness precision of the heat conductive sheet can be ensured to be sufficiently high. Furthermore, if the volume average particle diameter of the plate-shaped boron nitride particles is within the above-mentioned specified range, the thermal conductivity of the heat conductive sheet can be further improved. In the present invention, the "volume average particle size" can be measured in accordance with JIS Z8825, and represents the particle size at which the cumulative volume calculated from the smallest diameter side becomes 50% in the particle size distribution (volume basis) measured by a laser diffraction method. It is also known that there is almost no discrepancy between the particle size of the plate-like boron nitride particles at the material stage and the particle size at the stage when the particles are contained in the thermal conductive sheet after the manufacturing process. Therefore, when the particle size of the plate-like boron nitride particles at the material stage is within the above range, it is presumed that the particle size of the plate-like boron nitride particles in the thermal conductive sheet will also be within the above range.

[0021] <<Content of plate-shaped boron nitride particles>> The content of the plate-like boron nitride particles in the thermally conductive sheet must be 50% by volume or more, preferably 80% by volume or less, and more preferably 78% by volume or less, based on the volume of the thermally conductive sheet. If the content of the plate-like boron nitride particles in the thermally conductive sheet is equal to or greater than the above-mentioned lower limit, the thermal resistance of the thermally conductive sheet can be reduced, and the thermal conductivity can be further improved. If the content of the plate-like boron nitride particles in the thermally conductive sheet is equal to or less than the above-mentioned upper limit, the compressibility can be further increased. If the content of the plate-like boron nitride particles in the thermally conductive sheet is equal to or less than the above-mentioned upper limit, separation of the thermally conductive sheet into strips can be effectively prevented, and the restorability of the thermally conductive sheet can be improved, particularly when the thermally conductive sheet has a structure in which strips are joined in parallel.

[0022] <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 plate-like boron nitride particles together. The resin contained in the thermal conductive sheet of the present invention contains a crosslinked resin.

[0023] <<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 the parallel-joined strips are crosslinked to each other, resulting in excellent restorability.

[0024] Here, the crosslinked 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).

[0025] The crosslinked resin is formed by crosslinking the crosslinkable resin with the crosslinking agent. In the crosslinking reaction, a crosslinking accelerator may be used.

[0026] [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 resin that is solid at room temperature and normal pressure can be used. Note that the crosslinkable resin preferably does not contain an epoxy group, in other words, is a resin other than an epoxy resin, in order to avoid excessive hardness and loss of elasticity.

[0027] - A resin that is solid at room temperature and pressure - Examples of resins that are solid at room temperature and 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 their hydrogenated products; acrylonitrile-butadiene copolymers (nitrile rubbers); styrene-butadiene block copolymers or their hydrogenated products; styrene-isoprene block copolymers or their hydrogenated products; silicone resins such as polyimide silicone resins; and fluororesins such as vinylidene fluoride-based fluororesins, tetrafluoroethylene-propylene-based fluororesins, and tetrafluoroethylene-purovinyl ether-based fluororesins. These may be used alone or in combination. Among these, it is preferable that the thermally conductive sheet of the present invention contain 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.

[0028] The crosslinkable resin preferably has a glass transition temperature of -50.0°C or higher, more preferably -35.0°C or higher, even more preferably -32.0°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 crosslinkable resin is above the above lower limit, the recovery of the resulting thermal conductive sheet can be further improved, resulting in further improved thermal conductivity. If the glass transition temperature of the resin used as the crosslinkable resin is below the above upper limit, the resulting thermal conductive sheet can be imparted with appropriate flexibility, resulting in further improved thermal conductivity.

[0029] [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 resins. 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 in any ratio. Among these, when the crosslinkable resin is an acrylic polymer, it is preferable to use an amine-based crosslinking agent such as hexamethylenediamine carbamate. When the crosslinkable resin is at least one of an acrylonitrile-butadiene copolymer and a styrene-butadiene copolymer, a sulfur-based crosslinking agent is preferred.

[0030] The amount of crosslinking agent 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 5.0 parts by mass or less, and more preferably 2.5 parts by mass or less, relative to 100 parts by mass of the crosslinkable resin. If the amount of crosslinking agent is within the above range, the crosslinked resin 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 recovery of the thermal conductive sheet and maintaining a good orientation structure of the plate-like boron nitride particles within the thermal conductive sheet.

[0031] [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 examples thereof include guanidine-based crosslinking accelerators and sulfenamide-based crosslinking accelerators. These may be used alone or in combination of two or more in any ratio. Among these, guanidine-based crosslinking accelerators are preferred.

[0032] 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. If the amount of crosslinking accelerator blended is within the above range, the crosslinked resin 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 restorability of the thermal conductive sheet and maintaining a good orientation structure of the plate-like boron nitride particles within the thermal conductive sheet.

[0033] [Cross-linked resin content] The proportion of the cross-linked resin contained in the thermal conductive sheet must be 50% by volume or less, preferably 48% by volume or less, more preferably 20% by volume or more, more preferably 25% by volume or more, and even more preferably 30% by volume or more, based on the volume of the thermal conductive sheet. If the proportion of the cross-linked resin based on the volume of the thermal conductive sheet is equal to or greater than the above lower limit, the thermal conductive sheet will have even better thermal conductivity. The amount of the cross-linking resin contained in the thermally conductive sheet is usually the same as the amount of the cross-linking resin used in producing the thermally conductive sheet.

[0034] [Other resins] The thermal conductive sheet may contain other resins in addition to the cross-linkable resin. The other resins other than the cross-linkable resin are not particularly limited, but include resins that do not undergo cross-linking reaction with the cross-linking agent and cross-linking accelerator that contribute to cross-linking of the above-mentioned cross-linkable resin.

[0035] <<Liquids>> The thermally conductive sheet of this specification may optionally contain a liquid material. By including a liquid material in the thermally conductive sheet, the sheet becomes more flexible and can further increase compressibility. In this specification, a liquid material refers to a substance that is liquid and fluid at room temperature and pressure, has a molecular weight of less than 10,000, and has a viscosity of 200 mPa·s to 100 Pa·s at 25°C. For example, liquid resins and plasticizers that satisfy the above conditions can be suitably used as the liquid material. It is preferable that the liquid material is not crosslinked by the same crosslinking system as the crosslinking resin described above. If the liquid material is not crosslinked by the same crosslinking system as the crosslinking resin, flexibility is imparted to the entire sheet even after the crosslinkable resin is crosslinked in the production of the thermal conductive sheet, resulting in a thermal conductive sheet with excellent compressibility.

[0036] [Liquid resin] The liquid resin is not particularly limited as long as it satisfies the above conditions. For example, liquid nitrile butadiene rubber or liquid acrylic resin can be used. These may be used alone or in combination of two or more types in any ratio. The proportion of the liquid resin in the volume of the thermally conductive sheet is preferably 5% by volume or more, more preferably 10% by volume or more, and preferably 25% by volume or less, and more preferably 15% by volume or less. If the volume proportion of the liquid resin in the volume of the thermally conductive sheet is above the above lower limit, flexibility is imparted to the entire sheet, resulting in a thermally conductive sheet with excellent compressibility. If the volume proportion of the liquid resin in the volume of the thermally conductive sheet is below the above upper limit, a thermally conductive sheet with even better recovery properties can be obtained. Note that, compared to when a plasticizer is used as the liquid, using a liquid resin as the liquid can impart strength to the entire thermally conductive sheet.

[0037] [Plasticizer] Examples of the plasticizer that can be used include an adipic acid ether ester compound such as di(butoxyethoxyethyl) adipate, and a fatty acid ester such as sebacate. The volume ratio of the plasticizer to the volume of the thermal conductive sheet is preferably 1% by volume or more, more preferably 5% by volume or more, and even more preferably 10% by volume or more, and is preferably 25% by volume or less, and more preferably 15% by volume or less. When the volume ratio of the plasticizer to the volume of the thermal conductive sheet is equal to or greater than the lower limit, flexibility is imparted to the entire sheet, resulting in a thermal conductive sheet with excellent compressibility.When the volume ratio of the plasticizer to the volume of the thermal conductive sheet is equal to or less than the upper limit, a thermal conductive sheet with excellent restorability can be obtained. In addition, when a plasticizer is used as the liquid material, the entire thermal conductive sheet can be made more flexible than when a liquid resin is used as the liquid material.

[0038] The liquid resin and plasticizer may be used alone or in combination. When only one of them is used, the preferred content is as described above. When both are used in combination, the proportion of the liquid material in the volume of the thermal conductive sheet (i.e., the total content of the liquid resin and plasticizer) is preferably 5% by volume or more, more preferably 10% by volume or more, and is preferably 25% by volume or less, and more preferably 15% by volume or less.

[0039] <Other ingredients> The thermally conductive sheet of the present invention may optionally further contain components other than the various components 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 a thermally conductive sheet, and examples thereof include fibrous carbon materials; flame retardants such as red phosphorus-based flame retardants and phosphate ester-based flame retardants; plasticizers such as fatty acid ester-based plasticizers; 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 agents described in the example above foam during the production of the thermally conductive sheet, and most of them are removed from the thermally conductive sheet, but a small amount of residue may remain in the thermally conductive sheet.

[0040] <Thermal Conduction Sheet Structure> In the thermally conductive sheet of the present invention, it is preferable that at least some of the above-mentioned multiple voids 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 multiple voids whose major axis length is 20% or more of the thickness of the thermally conductive sheet is 8 or more. Furthermore, it is preferable that the major axes of multiple voids whose major axis length is 20% or more of the thickness of the thermally conductive sheet are oriented at an angle of 60° to 90° with respect to the main surface of the thermally conductive sheet. It is presumed that multiple 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.

[0041] 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 8 or more, more preferably 9 or more, and even more preferably 10 or more, and is preferably 50 or less, more preferably 40 or less, and even more preferably 35 or less. If the aspect ratio of the specified voids is equal to or greater than the lower limit, sufficient compressibility can be exhibited. Furthermore, if the aspect ratio of the specified voids is equal to or less than the upper limit, there will be no excessively large voids, and therefore excellent thermal conductivity can be exhibited.

[0042] 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 main surface of the thermally conductive sheet. Such uniform dispersion of multiple voids within the main surface 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 originating 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 under pressure 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.

[0043] FIG. 1 shows a schematic view of a thickness cross section of an example of a thermally conductive sheet according to the present invention that satisfies the above-described 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 contains a resin 10 and plate-like boron nitride particles (shape not shown). The plate-like boron nitride particles are represented by their major axes 11 (broken lines), and their shapes are omitted for clarity. Furthermore, the thermally conductive sheet 1 contains a plurality of voids 12, which are shown as ellipses. As described above, the length of the major axis LA of at least some of the plurality of voids 12 is 20% or more of the distance between the first main surface A and the second main surface B of the thermally conductive sheet 1, i.e., the thickness of the thermally conductive sheet 1. Furthermore, it is preferable that the long axis LA, whose length is 20% or more of the thickness of the thermal conductive sheet 1, is oriented at an angle of 60° to 90° relative to the first main surface A and the second main surface B of the thermal conductive sheet 1 (90° in FIG. 1). In the embodiment shown in FIG. 1, all of the illustrated multiple voids 12 have long axes LA whose length is 20% or more of the thickness of the thermal conductive sheet 1, and all of these LAs are oriented at an angle of 90° relative to the first main surface A and the second main surface B of the thermal conductive sheet 1. Although not shown, the thermal conductive sheet 1 may contain unintentional voids generated during kneading. Such unintentional voids may often be spherical voids with very small diameters.

[0044] Here, the angle formed by the major axes of the plurality of voids, whose major axis 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°. If the major axes of the plurality of voids, whose major axis 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 plate-shaped boron nitride 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°. If the orientation angle of the plate-shaped boron nitride 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.

[0045] (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 10% or more, preferably 15% or more, more preferably 20% or more, and preferably 40% or less, more preferably 35% or less, and even more preferably 30% or less. If the porosity is equal to or greater than the lower limit, sufficient compressibility can be exhibited. On the other hand, if the porosity is equal to or less than the upper limit, there will be no excessive voids, and therefore excellent thermal conductivity can be exhibited.

[0046] <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 (2) is preferably 30% or more, more preferably 40% or more, and is preferably 80% or less, more preferably 75% or less. Compression rate (%)=100×[1-(T 0.9 / T0)][%]···(2) 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 and manufacturing method of the thermally conductive sheet. The compression ratio is a value that can be measured for a 1 mm thick thermally conductive sheet using the method described in the Examples.

[0047] <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 (3) is preferably 50% or more, and more preferably 77% or more. Recovery rate (%) = (thickness of thermal conductive sheet after recovery) ÷ (thickness of thermal conductive sheet before pressure application) (3) 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 and manufacturing method of the thermal conductive sheet. The restoration rate is a value that can be measured for a 1 mm thick thermal conductive sheet using the method described in the examples.

[0048] <Thermal resistance> The thermal resistance of the thermal conductive sheet of the present invention is preferably 1.80°C / W or less, more preferably 1.60°C / W or less, and even more preferably 1.00°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 thermal conductive sheet. The thermal resistance is a value that can be measured for a 1 mm thick thermal conductive sheet by the method described in the examples.

[0049] <Thermal conductivity> The thermal conductivity of the thermal conductive sheet in the thickness direction is preferably 12 W / m K or more, more preferably 15 W / m K or more, and even 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 thermal conductive sheet in the thickness direction can be adjusted by the types and proportions of materials and components contained in the thermal conductive sheet, as well as the manufacturing method and manufacturing conditions of the thermal conductive sheet, etc. The thermal conductivity is a value that can be measured for a 1 mm thick thermal conductive sheet using the method described in the examples.

[0050] <Thermal Conduction Sheet Thickness> The thickness of the thermally conductive sheet of the present invention is not particularly limited, but is preferably 100 μm or more, more preferably 300 μm or more, and more preferably 500 μm or more, and is preferably 5000 μm or less, more preferably 3000 μm or less, and even more preferably 1000 μm or less. If the thickness of the thermally conductive sheet is equal to or greater than the above lower limit, the compressibility of the thermally conductive sheet can be further increased. 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 increased.

[0051] From the viewpoint of resilience, the Asker C hardness of the thermally conductive sheet is preferably 64 or more, more preferably 71 or more, and from the viewpoint of compressibility, it is preferably 82 or less, more preferably 75 or less. The Asker C hardness of the thermally conductive sheet can be measured by the method described in the examples.

[0052] (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, plate-shaped boron nitride 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.

[0053] <(A) Pre-heat conductive sheet forming process> In the pre-thermal conductive sheet molding step, a composition containing a resin containing a crosslinkable resin, plate-like boron nitride particles, a crosslinking agent, and a foaming agent is pressed into a sheet to obtain a pre-thermal conductive sheet.

[0054] <<Composition>> The composition includes a resin containing a crosslinkable resin, plate-like boron nitride 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, plate-like boron nitride particles, crosslinking agent, foaming agent, and crosslinking accelerator.

[0055] [resin] Here, the resin contained in the composition contains a crosslinkable resin and, optionally, contains a resin other than the crosslinkable resin. As the crosslinkable resin, for example, the crosslinkable resin that can be used to form the crosslinked 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 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 "crosslinked resin content" contained in the thermal conductive sheet described above. As the resin other than the crosslinkable resin, for example, other resins that can be contained in the resin described above in the "Thermal Conductive Sheet" section can be used.

[0056] [Plate-shaped boron nitride particles] As the plate-like boron nitride particles, for example, the plate-like boron nitride particles described above in the section "Thermal Conduction Sheet" can be used in the proportions described above.

[0057] [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.

[0058] [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 thermal 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 thermal conductive sheet can be increased.

[0059] 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.

[0060] [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.

[0061] 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.

[0062] 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.

[0063] [Crosslinking accelerator] As the crosslinking accelerator, those mentioned in the section "Thermal Conductive Sheet" can be used in the proportions mentioned above.

[0064] [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.

[0065] [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 boron nitride particles, a crosslinking agent, a foaming agent, and optionally, a crosslinking accelerator 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.

[0066] <<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.

[0067] 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.

[0068] <<Pre-heat conductive sheet>> Furthermore, in the pre-thermal conductive sheet formed by pressing the composition into a sheet, the plate-shaped boron nitride particles are primarily oriented in the in-plane direction, which is thought to improve the thermal conductivity of the pre-thermal conductive sheet in particular in the in-plane direction.

[0069] <(B) Laminate formation process> In the laminate formation process, multiple pre-thermally conductive sheets obtained in the pre-thermally conductive sheet molding process are stacked in the thickness direction, or the pre-thermally conductive sheets are folded or wound to obtain a laminate in which multiple pre-thermally conductive sheets containing a resin, plate-shaped boron nitride particles, a crosslinking agent, and a foaming agent are formed in the thickness direction. Here, the formation of the laminate by folding the pre-thermally conductive sheets is not particularly limited and can be performed by folding the pre-thermally conductive sheets at a constant width using a folding machine. Furthermore, the formation of the laminate by winding the pre-thermally conductive sheets is not particularly limited and can be performed by winding the pre-thermally conductive sheets around an axis parallel to the short or long direction of the pre-thermally conductive sheets. Furthermore, the formation of the laminate by stacking the pre-thermally conductive sheets is not particularly limited and can be performed using a lamination device.

[0070] It is presumed that in the laminate obtained by stacking, folding or rolling the pre-heat conductive sheet, the plate-like boron nitride particles are oriented in a direction substantially perpendicular to the stacking direction.

[0071] <(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.

[0072] 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 0.1 MPa or more and 0.9 MPa or less.

[0073] 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. A heating temperature above the lower limit ensures good crosslinking and foaming reactions, thereby improving the resilience of the resulting thermally conductive sheet and generating voids within the thermally conductive sheet, thereby enhancing the compressibility of the thermally conductive sheet. A heating temperature below the upper limit prevents deterioration of the resin constituting the thermally conductive sheet. The expansion ratio in the crosslinking reaction step is preferably 1.05 times or more, more preferably 1.10 times or more, and is preferably 1.50 times or less, and more preferably 1.30 times or less. An expansion ratio above the lower limit further enhances the compressibility of the thermally conductive sheet. An expansion ratio below the upper limit further enhances the resilience of the thermally conductive sheet.

[0074] 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.

[0075] It is presumed that in the second laminate obtained through the crosslinking reaction process, the crosslinkable resin is crosslinked to form a crosslinked resin, and the plate-like boron nitride particles are oriented in a direction approximately perpendicular to the lamination direction.

[0076] <(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.

[0077] From the viewpoint of increasing the thermal conductivity of the thermally 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 thermally conductive sheet obtained through this slicing process may have a structure in which strips containing resin and plate-shaped boron nitride particles are bonded in parallel in one direction approximately perpendicular to the thickness direction of the thermally conductive sheet (a direction at an angle of approximately 90° with respect to the thickness direction).

[0078] The thermally conductive sheet thus obtained contains a plurality of voids due to foaming of the foaming agent, and a crosslinked resin is formed, so that the thermally conductive sheet has excellent compressibility and restorability. [Example]

[0079] 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 each example and each comparative example, various attributes were measured or evaluated by the following methods.

[0080] (Measurement and evaluation of the physical properties of thermal conductive sheets) <Orientation angle of plate-shaped boron nitride> The orientation angle of the plate-like boron nitride in the thermal conductive sheet obtained in each example and comparative example was determined by observing a regular octagonal cross section of the thermal conductive sheet using a scanning electron microscope (SEM, Hitachi High-Technologies Corporation, "SU-3500") at a magnification that captured the entire sheet from top to bottom. The magnification was 100x. If the filler cannot be discerned due to resolution limitations, the magnification may be increased. In this case, evaluation was performed at three locations in the thickness direction of the sheet: the top, middle, and bottom. The largest value among the three locations was taken as the orientation angle of the plate-like boron nitride in the sheet. Specifically, 50 lines were drawn along the major axis of the plate-like boron nitride in the cross section, and the average angle of the major axis relative to the surface of the thermal 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 plate-like boron nitride in the thermal conductive sheet.

[0081] <Void orientation angle and void aspect ratio> The voids in the cross section where the orientation angle of the plate-shaped boron nitride was the maximum, determined when measuring the orientation angle of the plate-shaped boron nitride, were 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, first, the outline of the voids contained in the above-mentioned "cross section with the maximum angle" of the thermal conductive sheet was approximated as an ellipse, 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 evaluated. 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.

[0082] <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.

[0083] <Thermal conductivity> [Thermal diffusivity α in the thickness direction] The thermal diffusivity of the thermal conductive sheet was measured using a thermal diffusivity / thermal conductivity measuring device (ai-Phase Mobile 1u, manufactured by i-Phase Corporation) in accordance with the provisions of ISO22007-3. [Constant pressure specific heat Cp] Using a differential scanning calorimeter (manufactured by Rigaku, product name "DSC8230"), the specific heat was measured at 25°C under conditions of a temperature increase of 10°C / min. [Specific gravity ρ (density)] Measurement was carried out using an automatic hydrometer (manufactured by Toyo Seiki Co., Ltd., trade name "DENSIMETER-H"). Then, each measurement value is calculated using the following formula (I): λ=α×Cp×ρ (I) The thermal conductivity λ (W / m K) of the thermal conductive sheet in the thickness direction at 25°C was calculated by substituting the above equation.

[0084] <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.

[0085] <Asker C hardness> The Asker C hardness was measured in accordance with the Asker C method of the Society of Rubber Industry, Japan (SRIS) using a hardness tester (manufactured by Kobunshi Keiki Co., Ltd., product name "ASKER CL-150LJ") at a temperature of 25°C. Specifically, a laminate (50 mm long x 50 mm wide x approximately 7.00 mm thick) made by stacking seven of the obtained thermally conductive sheets to a thickness of approximately 7 mm was left to stand in a thermostatic chamber maintained at a temperature of 25°C for 48 hours or more to prepare a test specimen. Next, the hardness tester was placed so that the needle tip was 2 cm from the top surface of the test specimen, and a damper was lowered to allow the top surface of the test specimen to collide with the damper. The Asker C hardness of the test specimen 60 seconds after the collision was measured twice using a hardness tester (manufactured by Kobunshi Keiki Co., Ltd., product name "ASKER CL-150LJ"), and the average of the measurement results was used.

[0086] <Thickness, thermal resistance and compressibility> The thickness, thermal resistance, and compressibility of the thermally conductive sheets manufactured in each example and comparative example were measured using a thermal resistance tester (Hitachi Technology & Services, Ltd., product name: "Resin Material Thermal Resistance Measuring Device"). A thermally conductive sheet cut into a roughly 1 cm square was used as a sample, and the thickness (T0) of the thermally conductive sheet before pressure was measured. Then, at a sample temperature of 50°C, the thermal resistance and thickness of the thermally conductive sheet were measured under pressures of 0.1 MPa and 0.9 MPa applied in the thickness direction. The thickness of the sample before pressure was T0, and the thickness of the sample after pressure of 0.9 MPa was T 0.9 Let T0 and T 0.9 The compression ratio (%) was calculated according to the following formula (2). Compression rate (%)=100×[1-(T 0.9 / T0)][%]···(2) Furthermore, after the measurement, the load was removed and 3 minutes had elapsed, 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 (3). Recovery rate (%) = (thickness of thermal conductive sheet after recovery) ÷ (thickness of thermal conductive sheet before pressure application) (3)

[0087] Example 1 <Preparation of Composition> The thermoplastic resin was acrylic rubber, which is a solid acrylic resin at room temperature and normal pressure (solid acrylic resin, manufactured by Zeon Corporation, product name "Nipol (registered trademark) AR-12", glass transition temperature: -30.3°C, specific gravity: 1.10 g / cm). 3 180 parts of di(butoxyethoxyethyl) adipate (adipic acid ether ester plasticizer, trade name "Adekacizer RS-107", manufactured by ADEKA Corporation) as a liquid plasticizer, and 450 parts of hexagonal boron nitride (h-BN) (manufactured by Dandong Chemical Engineering, trade name "HSPD50", aspect ratio: 1.5, specific gravity: 2.23, volume average particle size: 50 μm), which is plate-like (flaky) boron nitride, were kneaded using a pressure kneader (manufactured by Nippon Spindle) at a temperature of 120°C for 20 minutes to obtain a composition. To the resulting composition, 2.7 parts of Nocrac 224 (2,2,4-trimethyl-1,2-dihydroquinoline polymer) as an antioxidant, 0.90 parts of Diak#1 (hexamethylenediamine carbamate, manufactured by Chemours) as a crosslinking agent, 3.6 parts of Noccela DT (di-o-tolylguanidine) as a crosslinking accelerator, and 18 parts of a foaming agent (manufactured by Eiwa Chemical Industry Co., Ltd., trade name "Neocelbon N#1000S", 4,4-oxybis(benzenesulfonylhydrazide; median diameter 14 μm; dp=160°C)) were added, and the mixture was kneaded at a temperature of 80°C for 5 minutes to obtain a composition.

[0088] <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.

[0089] <First laminate formation 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.

[0090] <Crosslinking reaction step (second laminate formation step)> The resulting first laminate was wrapped in release PET (polyethylene terephthalate) and sealed with tape, then vacuum-packaged in a PET retort pouch. This was then placed in an autoclave (Hanida Iron Works, small autoclave "DANDELION") for isostatic pressing, and heated and pressurized for 30 minutes at 180°C with a pressure of 0.8 MPa (absolute pressure) applied from all directions to promote crosslinking and foaming reactions, yielding a second laminate.

[0091] <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 primary sheet), yielding a secondary sheet (thermally conductive sheet) measuring 50 mm long x 50 mm wide x approximately 1.00 mm (0.924 mm) thick. The thermally conductive sheet was composed of strips joined in parallel in a direction perpendicular to the thickness direction of the thermally conductive sheet (at an angle of 90° to the thickness direction). The width of the strips in this approximately perpendicular direction was approximately the same as the thickness of the pre-thermally conductive sheet. The obtained thermally conductive sheet was subjected to various measurements and evaluations as described above. The results are shown in Table 1.

[0092] Examples 2 to 6 The composition of the thermally conductive sheet was changed in various ways as shown in Table 1. Except for this, the same procedure as in Example 1 was carried out. The results are shown in Table 1.

[0093] Example 7 As a liquid material, liquid nitrile butadiene rubber (hereinafter also referred to as "liquid NBR") (Nipol 1312, manufactured by Nippon Zeon, specific gravity: 1 g / cm) was used instead of the plasticizer.3 ) was blended in 50 parts, and other components were also changed as shown in Table 1. Other than this, the same procedure as in Example 1 was carried out. The results are shown in Table 1.

[0094] Example 8 No liquid material was added, and other components were also changed as shown in Table 1. Other than this, the same procedure as in Example 1 was carried out. The results are shown in Table 1.

[0095] (Comparative Example 1) No foaming agent was added, and other components were also changed as shown in Table 1. Other than these points, the same procedure as in Example 1 was carried out. The results are shown in Table 1. In this comparative example, a thermally conductive sheet was produced that did not contain any intentionally created voids.

[0096] [Table 1]

[0097] Table 1 shows that the thermal conductive sheets of Examples 1 to 8, which are thermal conductive sheets containing a crosslinked resin and plate-shaped boron nitride particles including plate-shaped boron nitride particles, have multiple voids within the sheet, the plate-shaped boron nitride particles are oriented at a predetermined orientation angle within the sheet, and further have a volume fraction of the plate-shaped boron nitride particles of 50 volume % or more relative to the volume of the thermal conductive sheet and a void ratio calculated according to a predetermined formula of 10% or more, have excellent compressibility and restorability. On the other hand, it is clear that the heat conductive sheet according to Comparative Example 1, which does not include the predetermined voids, is significantly inferior in terms of compressibility. [Industrial Applicability]

[0098] According to the present invention, it is possible to provide a thermally conductive sheet that is excellent in compressibility and restorability.

Claims

1. A thermally conductive sheet comprising a resin and plate-like boron nitride particles, the thermally conductive sheet includes a plurality of voids; the resin comprises a crosslinked resin, the plate-like boron nitride 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 plate-like boron nitride particles in the volume of the thermal conductive sheet is 50 volume % or more; A thermally conductive sheet having a porosity of 10% 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. 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 a void whose outline is approximated as an ellipse and whose length is 20% or more of the thickness of the thermal conduction sheet is oriented at an angle of 60° or more and 90° or less with respect to the main surface of the thermal conduction sheet.

3. The thermally conductive sheet according to claim 1 , wherein the voids have an aspect ratio of 8 or more.

4. The thermally conductive sheet according to claim 1 , further comprising a liquid material.

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, plate-like boron nitride 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.

Images