Thermally conductive sheet and method for producing thermally conductive sheet

Abstract

To provide a heat conductive sheet excellent in heat conductivity, compressibility and restorability.SOLUTION: The heat conductive sheet contains a resin and plate-like boron nitride particles. When the resin contains a crosslinked resin, the plate-like boron nitride particles are oriented at a predetermined angle in the sheet, and the volume fraction of the plate-like boron nitride particles in the thermally conductive sheet is a certain value or more, such a thermally conductive sheet satisfies at least one of (a) setting the Asker C hardness within a predetermined range, or (b) using a liquid resin and a solid resin in combination as the resin and further setting the ratio of the solid resin to the total mass of the liquid resin and the solid resin to a predetermined ratio 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] 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 3] Japanese Patent Application Laid-Open No. 2017-79264 Summary of the Invention [Problem to be solved by the invention]

[0006] Here, in addition to high thermal conductivity and 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 high levels of thermal conductivity, compressibility, and restorability.

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

[0009] The present inventors have conducted extensive research to achieve the above-mentioned object, and have newly discovered that in a thermally conductive sheet comprising a resin and plate-shaped boron nitride particles, when the resin contains a crosslinked resin, the plate-shaped boron nitride particles are oriented at a predetermined angle within the sheet, and the volume fraction of the plate-shaped boron nitride particles in the thermally conductive sheet is equal to or greater than a certain level, high levels of thermal conductivity, compressibility, and restorability can be simultaneously achieved by satisfying at least one of the following conditions: (a) the Asker C hardness is within a predetermined range, or (b) a liquid resin and a solid resin are used in combination as the resin, and further, the ratio of the solid resin relative to the total mass of the liquid resin and the solid resin is equal to or greater than a predetermined ratio, and thus have completed the present invention.

[0010] That is, the present invention aims to advantageously solve the above-mentioned problems, and the thermal conductive sheet of the present invention is characterized in that it is [1] a thermal conductive sheet comprising a resin and plate-shaped boron nitride particles, wherein 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 thermal conductive sheet, the volume fraction of the plate-shaped boron nitride particles in the volume of the thermal conductive sheet is 50 volume% or more, and further satisfies at least one of the following conditions (a) and (b): (a) Asker C hardness is 40 or more and 70 or less. (b) The resin contains a liquid resin and a solid resin, and the proportion of the solid resin is 30.0% by mass or more, with the total mass of the liquid resin and the solid resin being 100% by mass. Such a thermally conductive sheet has excellent thermal conductivity, compressibility, and recovery. In this specification, the Asker C hardness and the orientation angle of the plate-like boron nitride particles of the thermally conductive sheet can be measured by the method described in the Examples.

[0011] [2] Here, it is preferable that the thermally conductive sheet of the above [1] contains a plurality of voids and has a void ratio calculated according to the following formula (1) of 10% or more. Porosity (%) = 100 × (1 - measured specific gravity of thermal conductive sheet ÷ theoretical specific gravity of thermal conductive sheet) (1) Such a heat conductive sheet has even better compressibility.

[0012] [3] Here, in the thermal conductive sheet of [2] 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 and orientation angle of the major axis of the voids can be measured by the method described in the Examples.

[0013] [4] In the thermally conductive sheet of the above [2] or [3], 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.

[0014] [5] Furthermore, any of the thermally conductive sheets [1] to [4] above may further contain a plasticizer.

[0015] [6] The present invention also provides a method for producing a thermally conductive sheet, comprising: a pre-thermally conductive sheet forming step in which a composition 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; a laminate forming step in which multiple pre-thermally conductive sheets are stacked in the thickness direction or folded or rolled to obtain a laminate; a crosslinking reaction step in which the laminate is heated under pressure to undergo crosslinking and foaming reactions to obtain a crosslinked laminate; and a slicing step in which the crosslinked laminate is sliced ​​at an angle of 45° or less relative to the stacking direction to obtain a thermally conductive sheet. In the crosslinking reaction step, the foaming reaction is carried out in a state in which both end faces of the laminate in the stacking direction can be fixed with jigs, thereby controlling the expansion ratio of the laminate. The thermally conductive sheet obtained by this production method has excellent thermal conductivity, compressibility, and recovery properties.

[0016] [7] In the method for producing a thermally conductive sheet according to [6] above, the composition used in the pre-thermal conductive sheet forming step preferably contains plate-like boron nitride particles in a volume fraction of 50% by volume or more. The thermally conductive sheet obtained by this method has even better thermal conductivity.

[0017] [8] In the method for producing a thermally conductive sheet according to [7] or [8] above, the composition used in the pre-thermal conductive sheet molding step preferably contains a liquid resin and a solid resin, and the proportion of the solid resin is 30.0% by mass or more, where the total mass of the liquid resin and the solid resin is 100% by mass. The thermally conductive sheet obtained by this method has even better recovery.

[0018] [9] Furthermore, in the method for producing a thermally conductive sheet according to any one of the above [6] to [8], it is preferable that the thermally conductive sheet obtained through the slicing step has an Asker C hardness of 40 or more and 70 or less. The thermally conductive sheet obtained by such a production method has even better compressibility. [Effects of the Invention]

[0019] According to the present invention, it is possible to provide a thermally conductive sheet having excellent thermal conductivity, compressibility, and restorability, and a method for manufacturing the thermally conductive sheet. [Brief explanation of the drawings]

[0020] [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. [Figure 2] FIG. 10 is a schematic diagram illustrating a state in which a laminate is placed in a mold according to an example. DETAILED DESCRIPTION OF THE INVENTION

[0021] 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.

[0022] (thermal conductive sheet) The thermally conductive sheet of the present invention comprises a resin and plate-like boron nitride particles. 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 satisfying at least one of the following conditions (a) and (b): (a) Asker C hardness is 40 or more and 70 or less. (b) It contains a liquid resin and a solid resin, and the ratio of the solid resin is 30.0% by mass or more, with the total mass of the liquid resin and solid resin being 100% by mass. The thermally conductive sheet of the present invention that satisfies these conditions exhibits excellent thermal conductivity, presumably due to the oriented structure of the plate-like boron nitride particles. It also exhibits excellent resilience, presumably due to the cross-linked structure of the resin within the sheet. Furthermore, it exhibits excellent compressibility, presumably due to its moderate hardness (corresponding to the above-mentioned condition (a)), or due to the inclusion of a liquid resin and a certain proportion of solid resin to achieve both flexibility and strength (corresponding to the above-mentioned condition (b)).

[0023] <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.

[0024] 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.

[0025] <<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.

[0026] <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. Furthermore, it is preferable that the resin contains at least one of a liquid resin and a solid resin. A liquid resin refers to a resin that is liquid and has fluidity at room temperature and normal pressure. A solid resin refers to 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).

[0027] <<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 includes a crosslinked resin as a resin, which crosslinks the parallel-bonded strips, resulting in excellent resilience. The crosslinked resin may be obtained by crosslinking both a liquid resin and a solid resin with a crosslinking agent, or by crosslinking at least one of the liquid resin and the solid resin with a crosslinking agent, as crosslinkable resins capable of undergoing a crosslinking reaction when used in combination with a corresponding crosslinking agent. Among these, it is preferable for the crosslinked resin to contain a crosslinked solid resin. To avoid excessive hardness and loss of elasticity, the crosslinkable resin preferably does not contain epoxy groups, in other words, is a resin other than an epoxy resin. A crosslinking accelerator can be used in the crosslinking reaction.

[0028] [Solid resin] Examples of solid 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-purovinyl ether-based fluororesins. These may be used alone or in combination of two or more. Among these, it is preferable that the thermal conductive sheet of the present invention contains a crosslinked resin, which is a crosslinked product formed using at least one solid resin selected from the group consisting of an acrylic polymer, an acrylonitrile-butadiene copolymer, and a styrene-butadiene copolymer.

[0029] The solid 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 solid resin is above the lower limit, the recovery of the resulting thermally conductive sheet can be further improved, resulting in further improved thermal conductivity. If the glass transition temperature of the solid resin is below the upper limit, the resulting thermally conductive sheet can be imparted with appropriate flexibility, resulting in further improved thermal conductivity.

[0030] [Liquid resin] The liquid resin is not particularly limited, but 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 kinds in any ratio.

[0031] [Crosslinking agent] The crosslinking agent is not particularly limited as long as it can undergo a crosslinking reaction with the solid or liquid resin used as the crosslinkable resin. Typical crosslinking agents include sulfur-based crosslinking agents, peroxides, and amine-based crosslinking agents, which crosslink unsaturated bonds contained in the resin. These agents 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 can be used.

[0032] 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.

[0033] [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.

[0034] 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.

[0035] [Solid resin content in total mass of solid and liquid resins] As stated in the above condition (b), the solid resin content in the total mass of the solid resin and liquid resin is preferably 30.0 mass% or more, more preferably 40.0 mass% or more, even more preferably 50.0 mass% or more, and preferably 90.0 mass% or less, and more preferably 80.0 mass% or less, where the total mass of the solid resin and liquid resin is 100 mass%. If the solid resin content in the total mass of the solid resin and liquid resin is equal to or greater than the above lower limit, the restorability of the thermal conductive sheet can be further improved. Furthermore, if the solid resin content in the total mass of the solid resin and liquid resin is equal to or less than the above upper limit, flexibility is imparted to the thermal conductive sheet, resulting in a thermal conductive sheet with excellent compressibility.

[0036] [Ratio of liquid resin to total mass of solid and liquid resins] The content of the liquid resin in the total mass of the solid resin and liquid resin is preferably 10.0% by mass or more, more preferably 20.0% by mass or more, and preferably 70.0% by mass or less, more preferably 60.0% by mass or less, and even more preferably 50.0% by mass or less, where the total mass of the solid resin and liquid resin is 100% by mass. If the content of the liquid resin in the total mass of the solid resin and liquid resin is equal to or greater than the above-mentioned lower limit, flexibility is imparted to the thermal conductive sheet, resulting in a thermal conductive sheet with excellent compressibility. Furthermore, if the content of the liquid resin in the total mass of the solid resin and liquid resin is equal to or less than the above-mentioned upper limit, the restorability of the thermal conductive sheet can be further improved.

[0037] [Ratio of cross-linked resin to the total resin] The content of the crosslinked resin in the entire resin is not particularly limited, but is preferably 50% by mass or more, and may be 100% by mass. If the content of the crosslinked resin is equal to or greater than the lower limit, the restorability of the thermal conductive sheet can be improved. On the other hand, if the content of the crosslinked resin is equal to or less than the upper limit, the flexibility of the thermal conductive sheet can be improved.

[0038] [Resin content in thermal conductive sheet] The proportion of resin contained in the thermal conductive sheet must be 50% by volume or less, preferably 48% by volume or less, preferably 15% by volume or more, more preferably 20% by volume or more, and even more preferably 25% by volume or more, based on the volume of the thermal conductive sheet. If the proportion of resin based on the volume of the thermal conductive sheet is equal to or greater than the above-mentioned lower limit, the thermal conductive sheet will have even better thermal conductivity and compressibility. If the proportion of resin based on the volume of the thermal conductive sheet is equal to or less than the above-mentioned upper limit, the thermal conductivity of the thermal conductive sheet can be further improved. The amount of resin contained in the thermally conductive sheet usually corresponds to the amount of solid resin and liquid resin used in producing the thermally conductive sheet.

[0039] [Other resins] The thermally conductive sheet may contain other resins in addition to the various resins described above.

[0040] <<Plasticizers>> The thermally conductive sheet of the present specification may optionally contain a plasticizer. It is preferable that the plasticizer does not crosslink in the same crosslinking system as the crosslinking resin described above. If the plasticizer does not crosslink in 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 thermally conductive sheet, resulting in a thermally conductive sheet with excellent compressibility.

[0041] Examples of plasticizers that can be used include adipic acid ether ester compounds such as di(butoxyethoxyethyl) adipate and fatty acid esters such as sebacate esters. When the thermally conductive sheet contains a plasticizer, the volume ratio of the plasticizer to the volume of the thermally conductive sheet is preferably 1% by volume or more, more preferably 5% by volume or more, even more preferably 10% by volume or more, and preferably 25% by volume or less, and more preferably 20% by volume or less. If the volume ratio of the plasticizer to the volume of the thermally conductive sheet is equal to or greater than the above-mentioned lower limit, flexibility is imparted to the entire sheet, resulting in a thermally conductive sheet with excellent compressibility. If the volume ratio of the plasticizer to the volume of the thermally conductive sheet is equal to or less than the above-mentioned upper limit, a thermally conductive sheet with excellent resilience can be obtained.

[0042] <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.

[0043] <Thermal Conduction Sheet Structure> The thermally conductive sheet of the present invention preferably includes a plurality of voids. Furthermore, it is preferable that the length of the major axis of at least some of the voids, when the outline is approximated as an ellipse, is 20% or more of the thickness of the thermally conductive sheet. Furthermore, it is preferable that the average aspect ratio calculated for the plurality of voids whose major axis length is 20% or more of the thickness of the thermally conductive sheet is 8 or more. Furthermore, it is preferable that the major axes of the plurality of voids whose major axis length is 20% or more of the thickness of the thermally conductive sheet are oriented at an angle of 60° or more and 90° or less with respect to the main surface of the thermally conductive sheet. It is presumed that the plurality of voids that satisfy this specific shape and are arranged in this specific arrangement contribute to improving the compressibility and resilience of the thermally conductive sheet in a balanced manner.

[0044] 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.

[0045] 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.

[0046] 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.

[0047] 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.

[0048] (Thermal Conduction Sheet Properties) <Asker C hardness> As defined in the above condition (a), the Asker C hardness of the thermally conductive sheet is preferably 40 or more and 70 or less. Furthermore, the Asker C hardness of the thermally conductive sheet is more preferably 45 or more, even more preferably 50 or more, even more preferably 55 or more, more preferably 68 or less, and even more preferably 63 or less. If the Asker C hardness of the thermally conductive sheet is equal to or greater than the above lower limit, the restorability can be further improved. Furthermore, if the Asker C hardness of the thermally conductive sheet is equal to or less than the above upper limit, the compressibility can be further improved. The Asker C hardness of the thermally conductive sheet can be controlled depending on the amount of crosslinking agent blended, etc. The Asker C hardness of the thermally conductive sheet can be measured by the method described in the Examples.

[0049] <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 is preferably 10% or more, more preferably 15% 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 are no excessive voids, and therefore excellent thermal conductivity can be exhibited.

[0050] <Compression ratio> The thermal conductive sheet of the present invention has a thickness of T when pressed at 0.9 MPa in the thickness direction. 0.9 The thickness of the thermally conductive sheet before pressure is T0, and the compressibility of the thermally conductive sheet calculated by the following formula (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.

[0051] <Recovery rate> Furthermore, when the thermally conductive sheet of the present invention is pressed in the thickness direction at 0.9 MPa, and then the load is removed and the sheet is restored after 3 minutes, the restoration rate calculated based on the following formula (3) is preferably 40% or more, more preferably 50% or more, and even more preferably 60% 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.

[0052] <Thermal resistance> The thermal conductive sheet of the present invention preferably has a thermal resistance of 1.10°C / W or less, more preferably 1.00°C / W or less, and even more preferably 0.90°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.

[0053] <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.

[0054] <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.

[0055] (Method of manufacturing thermal conductive sheets) The thermally conductive sheet of the present invention can be efficiently produced by a manufacturing method including the following steps: (A) a pre-thermally conductive sheet forming step in which a composition containing a 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 to obtain a laminate; (C) a crosslinking reaction step in which the laminate is heated under pressure to undergo a crosslinking and foaming reaction to obtain a crosslinked laminate; and (D) a slicing step in which the crosslinked laminate is sliced ​​at an angle of 45° or less relative to the stacking direction to obtain a thermally conductive sheet. In the crosslinking reaction step (C), the foaming reaction is carried out in a state in which both end faces of the laminate in the stacking direction can be fixed with a jig, thereby controlling the expansion ratio of the laminate. This method strengthens the adhesion between the strips derived from the pre-thermally conductive sheets that make up the laminate, while also enabling the balance between voids and adhesion between the strips to be controlled. The thermally conductive sheet of the present invention may optionally further include steps other than the above steps (A) to (D).

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

[0057] <<Composition>> The composition includes a resin containing a crosslinkable resin, 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.

[0058] [resin] Here, the resin contained in the composition contains a crosslinkable resin. Furthermore, the resin may contain a non-crosslinkable resin in addition to the crosslinkable resin. Furthermore, the resin may optionally contain other resins. For example, the various resins described above in the "Thermal Conductive Sheet" section can be used as the resin. The blending amounts of the various resins in the total resin blended in the composition correspond to the various suitable ratios described above in the "Thermal Conductive Sheet" section. Furthermore, the blending amounts of the resins in the composition correspond to the blending ranges described above in the "Ratio of Resin Contained in the Thermal Conductive Sheet" section. In the above explanation, the ratio of resin contained in the thermal conductive sheet is described as "50% by volume or less, based on the volume of the thermal conductive sheet." However, it is preferable that this volume ratio be satisfied when the total volume, which is 100% by volume, is the total volume of the resin and plate-like boron nitride particles contained in the thermal conductive sheet, plus the volume of any plasticizers contained therein.

[0059] [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 Conductive Sheet" can be used in the above-mentioned proportions. Specifically, it is preferable that the above-mentioned volume proportions are satisfied, with the total volume of the resin and plate-like boron nitride particles contained in the thermal conductive sheet, plus the volume of any plasticizer contained therein, being taken as 100 volume %.

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

[0061] [Foaming agent] Various blowing agents that decompose upon heating to generate gas can be used as the blowing agent. A blowing assistant that lowers the decomposition temperature of the blowing agent and promotes its decomposition may 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. Among these, the use of OBSH alone is preferred. The blending ratio of the foaming agent is preferably 2% 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 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 upper limit, the strength and resilience of the resulting thermal conductive sheet can be increased.

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

[0063] [Foaming aid] As described above, various foaming aids can be used that lower the decomposition temperature of the foaming agent to be combined and promote its decomposition. For example, a urea (H2NCONH2)-based foaming aid can be used as a foaming aid to be combined with ADCA. The blending ratio of the foaming aid can be set arbitrarily depending on the type of foaming agent to be combined. Of course, the foaming aid does not have to be blended.

[0064] [Crosslinking accelerator] As the crosslinking accelerator, those mentioned in the section "Thermal Conductive Sheet" can be used in the proportions mentioned above.

[0065] [Plasticizer] The plasticizer may be any of those described above in the "Thermal Conductive Sheet" section, in the proportions described above. Specifically, it is preferable to use the plasticizer in the proportions described above, with the total volume of the resin, plate-like boron nitride particles, and plasticizer contained in the thermal conductive sheet being 100% by volume.

[0066] [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.

[0067] [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.

[0068] <<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.

[0069] 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.

[0070] <<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.

[0071] <(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.

[0072] 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.

[0073] <(C) Crosslinking Reaction Step> In the crosslinking reaction step, the laminate obtained in the laminate formation step is preferably heated and pressurized to form a crosslinked laminate. Furthermore, in the crosslinking reaction step, the foaming reaction is controlled by allowing both end faces of the laminate in the stacking direction to be fixed with a jig. By heating the laminate while applying pressure to allow both end faces in the stacking direction to be fixed with a jig and allowing the foaming reaction to proceed, once the end faces of the expanded laminate come into contact with the jig, expansion in the stacking direction is restricted while the foaming reaction and crosslinking reaction proceed. This strengthens the adhesion between the interfaces of the pre-heat conductive sheets stacked together, thereby effectively improving the integrity of the manufactured heat conductive sheet. The formation of a crosslinked resin itself can also improve the restorability of the heat conductive sheet. To increase the foaming ratio, the foaming agent can be preferentially foamed at a temperature lower than the crosslinking temperature of the resin, and then the temperature can be raised to the crosslinking temperature. Furthermore, the foaming agent undergoes a foaming reaction during the crosslinking reaction, generating voids, thereby improving the compressibility of the heat conductive sheet. In this process, "allowing the foaming reaction to proceed in a state in which both end faces of the laminate in the stacking direction can be fixed with jigs" means that the laminate is placed on one surface of a pair of jigs, both of which are plate-shaped, and the laminate is foamed, so that one end face of the laminate, which expands in the stacking direction as the foaming reaction progresses, comes into contact with the other jig at a certain point, and from that point onwards, the foaming reaction is allowed to proceed while the expansion of the laminate in the stacking direction is restricted.

[0074] Here, when pressing the laminate in the crosslinking reaction step, it is preferable to perform isostatic pressing using an autoclave, etc. The pressure of the isostatic pressing is preferably in the range of 0.1 MPa or more and 0.9 MPa or less.

[0075] The jig used to fixate both end faces of the laminate in the stacking direction is not particularly limited, but may be, for example, a plate-shaped jig such as a metal plate. By restricting expansion in the stacking direction using the plate-shaped jig, the entire laminate can be well integrated. As a result, the sheet strength of the resulting thermally conductive sheet can be uniformly increased. As an example, FIG. 2 shows a schematic configuration of a mold having the above-mentioned jig and a laminate placed within the mold. The mold 20 has a structure in which a height adjustment member 22 and a support 23 are interposed between an upper metal plate 21 and a lower metal plate 24 serving as jigs. The height adjustment member 22 has a height-adjustable structure, such as a bolt and nut structure. Although not shown, wax paper or the like may be placed adjacent to both end faces of the laminate 30 placed on the lower metal plate 24 of the mold 20 in the stacking direction. The presence of an anti-sticking member such as wax paper can prevent the laminate 30 from adhering to the upper metal plate 21 and the lower metal plate 24 serving as jigs. The illustrated embodiment shows the laminate 30 placed in the mold 20. As shown, before heating and pressurizing the laminate, i.e., before the foaming and cross-linking reactions begin, the laminate 30 placed on the lower metal plate 24 is not in contact with the upper metal plate 21. As the foaming and cross-linking reactions of the laminate 30 progress in this process, the laminate 30 gradually expands in the stacking direction, and then comes into contact with the upper metal plate 21. Once the laminate 30 contacts the upper metal plate 21, its expansion in the stacking direction is restricted. This controls the expansion ratio of the laminate 30. The cross-linking and foaming reactions proceed under this restriction until the end of this process, which is believed to improve the bonding strength between the pre-heat-conductive sheets in the stacking direction. The expansion ratio of the laminate 30 can be controlled by adjusting the distance between the upper metal plate 21 and the lower metal plate 24 using the height adjustment member 22. The expansion ratio of the laminate is calculated by dividing the height of the laminate after expansion by the height of the laminate before expansion.

[0076] The heating temperature of the laminate is not particularly limited, but is preferably above 140°C, more preferably above 150°C, even more preferably above 170°C, and preferably below 210°C, and more preferably below 190°C. When the heating temperature is above the lower limit, the crosslinking and foaming reactions 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. When the heating temperature is below the upper limit, deterioration of the resin constituting the thermal conductive sheet can be suppressed. Here, the expansion ratio in the crosslinking reaction step is preferably above 105%, more preferably above 110%, and preferably below 150%, and more preferably below 130%. When the expansion ratio is above the lower limit, the compressibility of the thermal conductive sheet can be further increased. When the expansion ratio is below the upper limit, the restorability of the thermal conductive sheet can be further increased.

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

[0078] It is presumed that in the crosslinked 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.

[0079] <(D) Slicing process> In the slicing step, the crosslinked 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 crosslinked laminate. The method for slicing the crosslinked 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 crosslinked laminate is not particularly limited, and a slicing member having a smooth plate surface with a slit and a blade portion protruding from the slit portion (for example, a plane or slicer with a sharp blade) can be used.

[0080] From the viewpoint of improving the thermal conductivity of the thermally conductive sheet, the angle at which the crosslinked laminate is sliced ​​is preferably 30° or less relative to the stacking direction, more preferably 15° or less relative to the stacking direction, and preferably approximately 0° relative 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-like 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° relative to the thickness direction).

[0081] The thermally conductive sheet obtained in this manner has excellent thermal conductivity because the plate-shaped boron nitride particles are oriented in a direction approximately perpendicular to the thickness direction of the thermally conductive sheet, has excellent resilience because the thermally conductive sheet contains a cross-linked resin, and further has excellent compressibility because the thermally conductive sheet has moderate hardness or contains a liquid resin. [Example]

[0082] The present invention will be specifically described below based on examples, but the present invention is not limited to these examples. In the following description, "%" and "parts" representing amounts are based on mass unless otherwise specified. In each example and each comparative example, various attributes were measured or evaluated by the following methods.

[0083] (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.

[0084] <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 as the object 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 and comparative examples, 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 boron nitride 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.

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

[0086] <Thermal conductivity> [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.

[0087] <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.

[0088] <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.

[0089] <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 passed, and the thickness of the restored thermal conductive sheet was measured with a film thickness meter, and the restoration rate was calculated based on the following formula (3). Recovery rate (%) = (thickness of thermal conductive sheet after recovery) ÷ (thickness of thermal conductive sheet before pressure application) (3)

[0090] Example 1 <Preparation of Composition> The resin used was 34 parts of a liquid nitrile butadiene rubber (manufactured by Nippon Zeon, trade name "Nipole 1312", specific gravity: 1.0, crosslinked with the crosslinking agent Percumyl D40 to form a crosslinked resin) that is liquid at room temperature and normal pressure, 80 parts of a solid nitrile butadiene rubber (manufactured by Nippon Zeon, trade name "Nipole 3350", specific gravity: 1.0, crosslinked with the crosslinking agent Percumyl D40 to form a crosslinked resin) that is solid at room temperature and normal pressure, 40 parts of di(butoxyethoxyethyl) adipate (adipic acid ether ester-based plasticizer, trade name "Adekacizer RS-107", manufactured by ADEKA Corporation) as a plasticizer, and hexagonal boron nitride (h-BN) (manufactured by Dandong Chemical Co., Ltd.), which is plate-like (scale-like) boron nitride, as boron nitride particles. 520 parts of a cellulose ester copolymer (manufactured by Nippon Spindle Engineering, trade name "HSPD50," aspect ratio: 1.5, specific gravity: 2.23, volume average particle diameter: 50 μm) was kneaded for 20 minutes at 120°C using a pressure kneader (manufactured by Nippon Spindle). 1.7 parts of Nocrac 224 (2,2,4-trimethyl-1,2-dihydroquinoline polymer) as an antioxidant, 3.4 parts of dicumyl peroxide (product name "Percumyl D40") as a crosslinking agent, and 5.7 parts of a foaming agent (manufactured by Eiwa Chemical Industry Co., Ltd., trade name "Neocelbon 1000S," 4,4-oxybis(benzenesulfonylhydrazide; median diameter: 14 μm; dp = 160°C)) were added to the resulting composition, and the mixture was kneaded for 5 minutes at 80°C to obtain a composition.

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

[0092] <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 stack of about 50 mm height.

[0093] <Crosslinking reaction step> The resulting laminate was placed in a mold, as described above with reference to Figure 2, capable of fixing both end faces of the laminate in the stacking direction with jigs (metal plates). The mold was then placed in an autoclave (a small autoclave called "DANDELION" manufactured by Hanyuda Iron Works, Ltd.). The autoclave was heated and pressurized for 30 minutes at 180°C under an applied pressure of 0.8 MPa (absolute pressure) from all directions, allowing the crosslinking and foaming reactions to proceed, yielding a crosslinked laminate. The original height of the laminate was 5 cm, but the distance between the metal plates was set to 6.2 cm to allow the crosslinking and foaming reactions to proceed. After cooling, the height of the crosslinked laminate was measured. The height was found to be 6 cm due to slight shrinkage of the laminate upon cooling. Therefore, the expansion ratio was calculated based on a 50 mm height of the pre-foamed laminate as 120% (60 mm / 50 mm x 100) at 6 cm.

[0094] <Slicing process> Next, while pressing the laminated surface of the crosslinked 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 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 (a direction 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-thermal conductive sheet. The obtained thermally conductive sheet was subjected to various measurements and evaluations as described above. The results are shown in Table 1.

[0095] Example 2 Except for changing the amount of crosslinking agent to 1.7 parts in the <Preparation of composition> step, various operations and measurements were carried out in the same manner as in Example 1. The results are shown in Table 1.

[0096] Example 3 Except for changing the resin formulation to 80 parts liquid resin and 34 parts solid resin in the <Preparation of Composition> step, various operations and measurements were carried out in the same manner as in Example 1. The results are shown in Table 1.

[0097] Example 4 In the <Preparation of Composition> step, the amount of nitrile butadiene rubber (manufactured by Zeon Corporation, trade name "Nipole 1312", specific gravity: 1.0, does not crosslink with Diak#1 used as a crosslinking agent) which is liquid at room temperature and normal pressure was changed to 70 parts, and the solid resin was changed from that used in Example 1 to an 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", specific gravity: 1.10 g / cm 3 The crosslinking agent Diak#1 crosslinks the resin to form a crosslinked resin. The amount of boron nitride particles was changed to 470 parts, the amount of foaming agent was changed to 4.0 parts, the crosslinking agent was changed to 0.4 parts of Diak#1 (hexamethylenediamine carbamate, manufactured by Chemours), and 1.6 parts of Noccela DT (di-o-tolylguanidine) was added as a crosslinking accelerator. Except for these points, various operations and measurements were carried out in the same manner as in Example 1. The results are shown in Table 1.

[0098] (Comparative Example 1) Except for changing the resin blending amount to 114 parts of liquid resin and not blending any solid resin in the <Composition Preparation> step, various operations and measurements were carried out in the same manner as in Example 1. The results are shown in Table 1.

[0099] (Comparative Example 2) In the <Preparation of Composition> step, no liquid resin was blended, and instead, a solid resin was used instead of the one used in Example 1, using an acrylic rubber (solid acrylic resin, manufactured by Zeon Corporation, trade name "Nipol (registered trademark) AR-14", specific gravity: 1.10 g / cm) which is a solid acrylic resin at room temperature and normal pressure. 3 The blending amount of the foaming agent (manufactured by Eiwa Chemical Industry Co., Ltd., trade name "Neocelvon N#1000S", 4,4-oxybis(benzenesulfonylhydrazide; median diameter 14 μm; dp=160°C) was changed to 18 parts, the crosslinking agent was changed to 0.9 parts Diak#1 (hexamethylenediamine carbamate, manufactured by Chemours), and 3.6 parts Noccela DT (di-o-tolylguanidine) was blended as a crosslinking accelerator. Except for these points, various operations and measurements were carried out in the same manner as in Example 1. The results are shown in Table 1.

[0100] [Table 1]

[0101] Table 1 shows that in thermally conductive sheets containing resin and plate-shaped boron nitride particles, when the resin contains a crosslinked resin, the plate-shaped boron nitride particles are oriented at a predetermined angle within the sheet, and the volume fraction of the plate-shaped boron nitride particles in the thermally conductive sheet is a certain amount or more, the thermally conductive sheets of Examples 1 to 4, which satisfy at least one of the following: (a) the Asker C hardness is within a predetermined range, or (b) a liquid resin and a solid resin are used in combination as the resin, and further, the proportion of the solid resin in the total mass of the liquid resin and the solid resin is a predetermined proportion or more, have excellent thermal conductivity, compressibility, and recovery. On the other hand, the thermal conductive sheet of Comparative Example 1, which did not contain a crosslinked resin, was significantly inferior in terms of recovery. Also, the thermal conductive sheet of Comparative Example 2, which did not contain a liquid resin and had an Asker C hardness of over 70, was inferior in thermal conductivity and compressibility. [Industrial Applicability]

[0102] According to the present invention, it is possible to provide a thermally conductive sheet that is excellent in thermal conductivity, compressibility, and restorability. [Explanation of symbols]

[0103] 1. Thermal conductive sheet 10 Resin 11 Long axis of plate-shaped boron nitride particles 12 void LA long axis A First main surface of the thermal conductive sheet 20 Formwork 21 Upper metal plate 22 Height adjustment member 23 Pillar 24 Lower metal plate 30 laminate

Claims

1. A thermally conductive sheet comprising a resin and plate-like boron nitride particles, 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-shaped boron nitride particles in the volume of the thermal conductive sheet is 50 volume % or more, and further, A thermally conductive sheet that satisfies at least one of the following conditions (a) and (b): (a) Asker C hardness is 40 or more and 70 or less. (b) The resin contains a liquid resin and a solid resin, and the proportion of the solid resin is 30.0% by mass or more, with the total mass of the liquid resin and the solid resin being 100% by mass.

2. containing a plurality of voids, The thermal conductive sheet according to claim 1, wherein the porosity 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)

3. 3. The thermal conduction sheet according to claim 2, 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 orientation angle of 60° or more and 90° or less with respect to the main surface of the thermal conduction sheet.

4. The thermally conductive sheet according to claim 2 , wherein the voids have an aspect ratio of 8 or more.

5. The thermally conductive sheet according to any one of claims 1 to 4, further comprising a plasticizer.

6. 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 under pressure to carry out a crosslinking and foaming reaction to obtain a crosslinked laminate; a slicing step of slicing the crosslinked laminate at an angle of 45° or less with respect to the lamination direction to obtain a thermally conductive sheet, In the crosslinking reaction step, the foaming reaction is allowed to proceed in a state in which both end faces of the laminate in the stacking direction can be fixed with jigs, thereby controlling the foaming ratio of the laminate. A manufacturing method for a thermal conductive sheet.

7. The method for producing a thermal conductive sheet according to claim 6 , wherein the composition used in the thermal conductive sheet pre-molding step has a volume fraction of plate-like boron nitride particles of 50% by volume or more.

8. 7. The method for manufacturing a thermal conductive sheet according to claim 6, wherein the composition used in the pre-thermal conductive sheet molding step contains a liquid resin and a solid resin, and the ratio of the solid resin is 30.0 mass% or more, with the total mass of the liquid resin and the solid resin being 100 mass%.

9. 9. The method for producing a thermal conductive sheet according to claim 6, wherein the thermal conductive sheet obtained through the slicing step has an Asker C hardness of 40 or more and 70 or less.

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