Insulating heat transfer sheet

Abstract

The present invention provides an insulating heat transfer sheet which is characterized by being a cured product of a thermally conductive silicone resin composition that contains the components (A) to (D) described below, the insulating heat transfer sheet being characterized in that the peak intensity ratio (degree of orientation) in a thickness direction as detected by X-ray diffraction method of the insulating heat transfer sheet is [2θ = 41 to 43°] / [2θ = 25 to 27°] = 1 or more: (A) an organopolysiloxane which has two or more alkenyl groups in each molecule, the alkenyl groups being each bonded to a silicon atom, (B) an organohydrogenpolysiloxane which contains a component (B1) and a component (B2), and has two or more hydrosilyl groups in each molecule, (C) a hydrosilylation catalyst, (D) a flat boron nitride which has an average particle diameter of 30 to 100 μm and an aspect ratio of 20 to 100 as measured by a laser diffraction method. Consequently, the present invention provides an insulating heat transfer sheet which has high sheet strength while having excellent thermal conductivity in the thickness direction.

Description

Insulating and thermally conductive sheet

[0001] The present invention relates to an insulating and thermally conductive sheet.

[0002] 2. Description of the Related Art In recent years, as electronic devices have become more sophisticated, semiconductor elements have become increasingly dense and highly packaged.

[0003] In particular, the computerization of automobiles has been remarkable, and the number of electronic components installed has increased, leading to an increased demand for heat dissipation materials. Heat dissipation materials are available in sheet and grease types, with the sheet type being preferred due to its ease of implementation. Furthermore, with autonomous driving as a backdrop, further improvements in thermal conductivity are required for such heat dissipation materials. Generally, to achieve high thermal conductivity, it is necessary to increase the filling rate of the filler dispersed within the polymer, which tends to increase the specific gravity, but this poses a problem when promoting weight reduction to improve the fuel efficiency of gasoline vehicles and the electric power efficiency of electric vehicles.

[0004] For example, Patent Document 1 proposes a method for producing a thermally conductive resin sheet, which includes at least a curing step of extruding a thermally conductive composition containing a polymer, an anisotropic filler, and a bulking agent using an extruder, and curing the extrusion to obtain a cured product, and a cutting step of cutting the cured product to a predetermined thickness in a direction perpendicular to the extrusion direction using an ultrasonic cutter.

[0005] Furthermore, for example, Patent Document 2 also proposes a method for manufacturing a thermally conductive sheet, in which a magnetic field is applied to a thermally conductive composition containing a polymer, an anisotropic filler, and a bulking agent to orient the anisotropic thermally conductive filler, form a bulk compact, and slice the compact to a desired sheet thickness.

[0006] Furthermore, Patent Documents 3 and 4 propose a method for manufacturing a thermally conductive sheet in which a primary sheet is produced by filling a resin with an anisotropic filler and aligning it in the plane direction, and then these sheets are laminated to form a molded body, which is then sliced ​​perpendicular to the alignment direction. This method makes it relatively easy to obtain a large-area thermally conductive sheet, and is a manufacturing method that is excellent for mass production.

[0007] However, when stacking primary sheets to create a molded product, interlayer adhesion between the primary sheets presents a major challenge. In other words, if the adhesive strength of the primary sheets is insufficient, the strength of the thermally conductive sheet obtained by slicing them significantly decreases, making it difficult to withstand use as an insulating thermally conductive sheet. Furthermore, when adhesive is used between the primary sheets, the expansion coefficients of the respective materials affect the material, leading to embrittlement due to temperature changes, raising major concerns about long-term reliability.

[0008] JP 2012-023335 A JP 2019-186555 A JP 2017-126614 A International Publication No. 2021 / 090929

[0009] The present invention has been made in view of the above circumstances, and has as its object to provide an insulating thermally conductive sheet that has excellent thermal conductivity in the thickness direction and high sheet strength.

[0010] In order to solve the above problems, the present invention provides an insulating thermally conductive sheet, which is a cured product of a thermally conductive silicone resin composition containing the following components (A) to (D), wherein the insulating thermally conductive sheet has a peak intensity ratio (degree of orientation) in the thickness direction as detected by X-ray diffraction of the insulating thermally conductive sheet, of [2θ=41 to 43°] / [2θ=25 to 27°]=1 or more: (A) 100 parts by mass of an organopolysiloxane having two or more alkenyl groups bonded to silicon atoms per molecule; (B) An organohydrogenpolysiloxane containing the following components (B1) and (B2), and having two or more hydrosilyl groups per molecule: the amount of component (B) added is such that the number of moles of hydrosilyl groups derived from component (B) is 1.3 to 2.0 per mole of alkenyl groups derived from component (A), and the ratio of the number of moles of hydrosilyl groups in component (B1) to component (B2) is 1:1 to 1:5. (B1) An organohydrogenpolysiloxane represented by the following formula (1): (m is an integer of 4 to 20, n is an integer of 10 to 40, and the bonding order of the siloxane units bounded by m and n may be block or random). (B2) An organohydrogenpolysiloxane having hydrosilyl groups only at both ends, represented by the following formula (2): (p is an integer of 40 to 200) (C) a hydrosilylation catalyst: an amount equivalent to 100 to 3,000 ppm by mass of platinum group metal element relative to component (A), and (D) 200 to 800 parts by mass of flat boron nitride having an average particle size of 30 to 100 μm as measured by laser diffraction and an aspect ratio of 20 to 100.

[0011] The insulating thermally conductive sheet of the present invention has excellent thermal conductivity in the thickness direction and high sheet strength.

[0012] The insulating thermally conductive sheet of the present invention preferably has a thermal conductivity in the thickness direction of 17 to 30 W / m·K.

[0013] Such an insulating heat conductive sheet has excellent heat conductivity in the thickness direction.

[0014] Furthermore, the insulating thermally conductive sheet of the present invention preferably has a Shore A hardness of 5 to 60.

[0015] Such an insulating and heat-conductive sheet is unlikely to deform or lose its orientation, and the contact thermal resistance between the insulating and heat-conductive sheet and the heat-generating body and the cooling body is not increased.

[0016] The insulating thermally conductive sheet of the present invention preferably has a breakdown voltage of 5 kV / mm or more.

[0017] Such an insulating heat conductive sheet has excellent insulating properties.

[0018] Furthermore, the insulating and thermally conductive sheet of the present invention preferably has a specific gravity of 1.2 to 2.0.

[0019] Such an insulating heat-conductive sheet is preferable for reducing the weight of the heat dissipation member.

[0020] As described above, the insulating thermally conductive sheet of the present invention has excellent thermal conductivity in the thickness direction and high sheet strength. The insulating thermally conductive sheet also has excellent mechanical strength, and there is no risk of the sheet being damaged during use.

[0021] An example of a mold used in the filling process. A cross-sectional structure of the mold in Figure 1. An example of a mold used in the molding process.

[0022] As described above, there has been a demand for the development of an insulating thermally conductive sheet that has excellent thermal conductivity in the thickness direction and high sheet strength.

[0023] As a result of extensive research into the above-mentioned problems, the inventors discovered that by highly orienting flat boron nitride and controlling the crosslinking structure of the silicone resin, it is possible to obtain an insulating thermally conductive sheet that has excellent thermal conductivity in the thickness direction and high sheet strength, and thus completed the present invention.

[0024] That is, the present invention provides an insulating thermally conductive sheet that is a cured product of a thermally conductive silicone resin composition containing the following components (A) to (D), wherein the insulating thermally conductive sheet has a peak intensity ratio (degree of orientation) in the thickness direction as detected by X-ray diffraction of the insulating thermally conductive sheet, such that [2θ=41 to 43°] / [2θ=25 to 27°]=1 or more: (A) 100 parts by mass of an organopolysiloxane having two or more alkenyl groups bonded to silicon atoms per molecule; (B) An organohydrogenpolysiloxane containing the following components (B1) and (B2), and having two or more hydrosilyl groups per molecule: the amount of component (B) added is such that the number of moles of hydrosilyl groups derived from component (B) is 1.3 to 2.0 per mole of alkenyl groups derived from component (A), and the ratio of the number of moles of hydrosilyl groups in component (B1) to component (B2) is 1:1 to 1:5. (B1) An organohydrogenpolysiloxane represented by the following formula (1): (m is an integer of 4 to 20, n is an integer of 10 to 40, and the bonding order of the siloxane units bounded by m and n may be block or random). (B2) An organohydrogenpolysiloxane having hydrosilyl groups only at both ends, represented by the following formula (2): (p is an integer of 40 to 200) (C) a hydrosilylation catalyst: an amount equivalent to 100 to 3,000 ppm by mass of platinum group metal element relative to component (A), and (D) 200 to 800 parts by mass of flat boron nitride having an average particle size of 30 to 100 μm as measured by laser diffraction and an aspect ratio of 20 to 100.

[0025] The present invention will be described in detail below, but the present invention is not limited thereto.

[0026] [Insulating Thermally Conductive Sheet] The insulating thermally conductive sheet of the present invention is obtained by molding a thermally conductive silicone resin composition containing flat boron nitride (D) in addition to the above components (A) to (C) using the method described below.

[0027] [Thermal Conductive Silicone Resin Composition] The thermally conductive silicone resin composition of the present invention is an addition-curable organopolysiloxane composition containing the following components (A) to (C) and (D) flat boron nitride: (A) an organopolysiloxane having two or more alkenyl groups per molecule, (B) an organohydrogenopolysiloxane, and (C) a hydrosilylation catalyst.

[0028] [(A) Organopolysiloxane having two or more alkenyl groups per molecule] Component (A) is an organopolysiloxane having two or more alkenyl groups bonded to silicon atoms per molecule, and serves as the main component of the thermally conductive silicone resin composition of the present invention. Usually, the main chain portion is basically composed of a repeating diorganosiloxane unit. Note that the molecular structure may contain a branched structure or may be cyclic, but linear diorganopolysiloxanes are preferred from the viewpoint of physical properties such as the mechanical strength of the cured product.

[0029] Examples of the alkenyl group include those typically having about 2 to 8 carbon atoms, such as vinyl, allyl, propenyl, isopropenyl, butenyl, hexenyl, and cyclohexenyl groups, and among these, lower alkenyl groups having 2 to 4 carbon atoms, such as vinyl and allyl, are preferred, with vinyl being particularly preferred. Preferably, two or more alkenyl groups are present in the molecule, and in order to improve the flexibility of the resulting cured product, it is more preferred that they are present bonded only to silicon atoms at the molecular chain terminals.

[0030] The functional group other than the alkenyl group is a monovalent hydrocarbon group, and particularly, those having 1 to 10 carbon atoms, particularly 1 to 6 carbon atoms, are exemplified. Examples include alkyl groups such as methyl, ethyl, propyl, isopropyl, butyl, isobutyl, tert-butyl, pentyl, neopentyl, hexyl, heptyl, octyl, nonyl, and decyl; cycloalkyl groups such as cyclopentyl, cyclohexyl, and cycloheptyl; aryl groups such as phenyl, tolyl, xylyl, and naphthyl; and aralkyl groups such as benzyl, phenylethyl, phenylpropyl, and methylbenzyl. Preferred are alkyl groups having 1 to 3 carbon atoms, such as methyl, ethyl, and propyl, and phenyl groups. Furthermore, the functional groups other than the alkenyl group bonded to the silicon atom do not necessarily have to be the same.

[0031] The kinematic viscosity of this organopolysiloxane at 23°C is 10 to 100,000 mm ２ / s is preferred, and more preferably 100 to 100,000 mm ２ / s. The kinematic viscosity is in the range of 10 mm ２ / s or more, the storage stability of the resulting resin composition is good, and ２ If the viscosity is 1 / s or less, the extensibility of the resulting resin composition is good.

[0032] In this specification, the kinematic viscosity is a value measured at 25°C using a Cannon-Fenske viscometer described in JIS Z 8803:2011.

[0033] The organopolysiloxane of component (A) may be used alone or in combination of two or more different organopolysiloxanes having different kinematic viscosities.

[0034] [(B) Organohydrogenpolysiloxane] Component (B) is an organohydrogenpolysiloxane, characterized in that it is a mixture of component (B1) of the following formula (1) and component (B2) of the following formula (2): (B1) (m is an integer of 4 to 20, n is an integer of 10 to 40, and the bonding order of the siloxane units bounded by m and n may be block or random.) (B2) (p is an integer from 40 to 200)

[0035] The organohydrogenpolysiloxane of component (B1) is an organohydrogenpolysiloxane having 4 to 20 hydrogen atoms (hydrosilyl groups) directly bonded to silicon atoms per molecule, and functions as a crosslinking agent for component (A). That is, the hydrosilyl groups in component (B1) add to the alkenyl groups in component (A) via a hydrosilylation reaction promoted by the hydrosilylation catalyst of component (C), described below, thereby forming a three-dimensional network structure.

[0036] On the other hand, the organohydrogenpolysiloxane having hydrosilyl groups at both ends of component (B2) only extends the molecular chain and does not interfere with the free movement of the molecules, even when added to the alkenyl groups in component (A).

[0037] The molar ratio of hydrosilyl groups in component (B1) to component (B2) is 1:1 to 1:5, preferably 1:1 to 1:4. If the molar ratio of hydrosilyl groups in component (B1) to component (B2) is greater than 1:5, the proportion of the three-dimensional network structure in the composition decreases, and sufficient sheet strength cannot be obtained. On the other hand, if the molar ratio is less than 1:1, the sheet becomes rigid and loses flexibility, which is undesirable.

[0038] The amount of component (B) added is an amount such that the number of hydrosilyl groups derived from component (B) is 1.3 to 2.0 moles per mole of alkenyl groups derived from component (A) (i.e., an amount such that the number of moles of hydrosilyl groups is 1.3 to 2.0 times the number of moles of alkenyl groups derived from component (A)). If the amount of hydrosilyl groups derived from component (B) is less than 1.3 moles per mole of alkenyl groups derived from component (A), the proportion of crosslinked structures decreases, resulting in insufficient curing. On the other hand, if the amount exceeds 2.0 moles, the amount of hydrosilyl groups becomes excessive, which may cause foaming on the sheet due to dehydrogenation.

[0039] [(C) Hydrosilylation Catalyst] The hydrosilylation catalyst of component (C) is a catalyst for promoting the addition reaction between the alkenyl group derived from component (A) and the hydrosilyl group derived from component (B), and examples thereof include catalysts well known for use in hydrosilylation reactions. Specific examples thereof include platinum group metals such as platinum (including platinum black), rhodium, and palladium; ２ PtCl ４ ・nH ２ O, H ２ PtCl ６ ・nH ２ O, NaHPtCl ６ ・nH ２ O, KHPtCl ６ ・nH ２ O, Na ２ PtCl ６ ・nH ２ O.K. ２ PtCl ４ ・nH ２ O, PtCl ４ ・nH ２ O, PtCl ２ , Na ２ HPtCl ４ ・nH ２ O (wherein n is an integer of 0 to 6, preferably 0 or 6), chloroplatinic acid and chloroplatinate salts, alcohol-modified chloroplatinic acid (see U.S. Pat. No. 3,220,972), complexes of chloroplatinic acid with olefins (see U.S. Pat. Nos. 3,159,601, 3,159,662 and 3,775,452), platinum black, platinum group metals such as palladium supported on a support such as alumina, silica or carbon, rhodium-olefin complexes, chlorotris(triphenylphosphine)rhodium (Wilkinson's catalyst), complexes of platinum chloride, chloroplatinic acid or chloroplatinate salts with vinyl group-containing siloxanes, particularly vinyl group-containing cyclic siloxanes.

[0040] The content of component (C) is 100 to 3,000 ppm, preferably 100 to 2,000 ppm, calculated as the mass of platinum group metal element relative to component (A). If the content of component (C) is less than 100 ppm, sufficient catalytic activity cannot be obtained, and if it exceeds 3,000 ppm, it is an excessive amount as a catalyst and is not preferable from the standpoint of cost.

[0041] [(D) Flat Boron Nitride] The flat boron nitride of component (D) is a particle that is oriented in the thickness direction of the sheet and promotes thermal conduction. From the viewpoint of thermal conduction and electrical insulation in the thickness direction of the heat dissipation sheet, hexagonal boron nitride is particularly preferred. Furthermore, the shape is flat, and scale-like, plate-like, or flake-like is particularly preferred.

[0042] The aspect ratio of the flat boron nitride is 20 to 100, and preferably 50 to 100. If the aspect ratio is less than 20, the orientation important for increasing thermal conductivity cannot be ensured, and if it exceeds 100, the boron nitride itself becomes susceptible to damage due to stirring shear during filling into silicone resin or pumping, which is undesirable.

[0043] The aspect ratio is the longest diameter of a particle divided by the thickness of the particle, i.e., longest diameter / thickness. When a particle is spherical, the aspect ratio is 1, and as the particle becomes more flat, the aspect ratio increases.

[0044] The aspect ratio can be obtained by measuring the major axis and thickness of a particle at a magnification of 1,500 times using a scanning electron microscope and calculating the ratio of major axis to thickness.

[0045] The average particle size of the flat boron nitride is 30 to 100 μm, preferably 40 to 90 μm, and more preferably 50 to 80 μm.

[0046] In order to efficiently form heat conduction paths in the thickness direction of the insulating heat conductive sheet, it is necessary to reduce the number of contact points between fillers. If the average particle size of the flat boron nitride is less than 30 μm, the number of contact points between the flat boron nitride particles increases, resulting in insufficient thermal conductivity. If the average particle size exceeds 100 μm, the flexibility of the insulating heat conductive sheet is impaired, making it prone to cracking. The flat boron nitride may have a single average particle size, or multiple types with different average particle sizes may be mixed.

[0047] The average particle size is the median diameter (the particle size at which, when a powder is divided into two at a certain particle size, the particles larger than that particle size and the particles smaller than that particle size are equal in amount; generally also referred to as D50) measured by a laser diffraction method using a laser diffraction / scattering particle size distribution measuring device.

[0048] The content of the flat boron nitride is 200 to 800 parts by mass, and preferably 300 to 600 parts by mass, per 100 parts by mass of component (A). If the content of flat boron nitride is less than 200 parts by mass, the insulating thermally conductive sheet will not have sufficient thermal conductivity in the thickness direction, and if it exceeds 800 parts by mass, the insulating thermally conductive sheet will become brittle and will have insufficient strength.

[0049] In addition to the flaky boron nitride, insulating inorganic particles such as alumina, magnesia, aluminum hydroxide, aluminum nitride, silicon nitride, silica, etc. can be used in combination. The viscosity of the composition can be adjusted by using other insulating inorganic particles in combination, which is a preferred embodiment in terms of processability.

[0050] [(E) Dilution Solvent] The thermally conductive silicone resin composition can be blended with a dilution solvent to reduce the viscosity of the composition during kneading. Examples of solvents that can be used include non-polar solvents such as normal hexane, cyclohexane, benzene, toluene, and xylene; alcohols such as methanol, ethanol, and isopropanol; ketones such as acetone and methyl ethyl ketone; and ethers such as diethyl ether and tetrahydrofuran. If the solvent remains in the composition, it can cause foaming during molding or reduce the thermal conductivity of the molded product, so it is preferable to use a solvent with a boiling point of 150°C or less that can be easily removed in a heating step before the molding step.

[0051] [Other Components] The thermally conductive silicone resin composition may further contain, as needed, components such as a thixotropy-imparting agent, a dispersant, a curing agent, a curing accelerator, a retarder, a slight tackifier, a plasticizer, a flame retardant, an antioxidant, a stabilizer, and a colorant.

[0052] The thermally conductive silicone resin composition may also contain a filler other than component (D). Specific examples include silica and titanium oxide. The amount of filler to be added is not particularly limited, but is preferably 0 to 60 parts by mass, and more preferably 0 to 40 parts by mass, per 100 parts by mass of component (A).

[0053] [Method for Producing Thermally Conductive Silicone Resin Composition] The thermally conductive silicone resin composition of the present invention can be produced by mixing the above-mentioned thermosetting silicone resin, the above-mentioned flat boron nitride, and, if necessary, the above-mentioned other components using a mixer or the like.

[0054] [Method for producing insulating thermally conductive sheet] The method for producing the insulating thermally conductive sheet of the present invention includes the steps of molding and curing the thermally conductive silicone resin composition, and examples thereof include the following methods.

[0055] One example of a method for producing an insulating thermally conductive sheet of the present invention includes a filling step in which a thermally conductive silicone resin composition is filled into a mold; a vibration step in which the mold is ultrasonically vibrated to degas and align the flat boron nitride particles; a heating step in which string-shaped semi-cured silicone composition is obtained from the thermally conductive silicone resin composition by heating; a molding step in which the string-shaped semi-cured silicone composition is arranged in the longitudinal direction and heated and pressurized to obtain a thermally conductive silicone molded body; and a slicing step in which the thermally conductive silicone molded body is sliced ​​to obtain an insulating thermally conductive sheet of the desired thickness.

[0056] By using the above method, it is possible to obtain a flexible insulating heat conductive sheet that has excellent thermal conductivity in the thickness direction by highly orienting the flat boron nitride.

[0057] Each of the steps will be described in detail below.

[0058] Filling Step (Mold Used in Filling Step) The mold used in the filling step has a width of 5.0 mm or less, a depth of the width length + 1.0 mm or more, and a length of 10.0 mm or more, and has one or more grooves whose bottom shape is not particularly limited, but examples thereof include wedge-shaped, semicircular, wavy, and flat grooves.

[0059] (Filling Method) In the filling step, a specified amount of the thermally conductive silicone resin composition may be filled into the mold groove, or a large amount of the thermally conductive silicone resin composition may be filled and then the unnecessary portion may be scraped off with a squeegee or the like.

[0060] Adding a solvent to the thermally conductive silicone resin composition is preferable because it reduces the viscosity of the composition and improves filling properties. Furthermore, when the flat boron nitride particles are degassed and allowed to settle in the subsequent vibration step, the long axis direction of the flat boron nitride particles tends to align with the long axis direction of the string-like semi-cured silicone composition.

[0061] Vibration Step The vibration step is performed by ultrasonically vibrating the mold to degas and align the flat boron nitride particles. In order to obtain a string-shaped semi-cured silicone composition in the next heating step, the mold is ultrasonically vibrated to degas and precipitate and orient the flat boron nitride particles, aligning them in the longitudinal direction of the string-shaped semi-cured silicone composition.

[0062] Heating Step The heating step is a step of heating the thermally conductive silicone resin composition to obtain the string-shaped semi-cured silicone composition. If the thermally conductive silicone resin composition contains a solvent, the heating step also includes a step of volatilizing the solvent by heating. The heating temperature in the heating step is preferably 40 to 100°C, more preferably 60 to 80°C. The heating time is preferably 1 to 60 minutes, more preferably 5 to 30 minutes.

[0063] Molding process (mold used in the molding process) The mold is a concave mold with inner dimensions of 10.0 mm or more in width, 10.0 mm or more in length, and 10.0 mm or more in depth, and is characterized by having a lower mold with a flat bottom shape and an upper mold whose width and length are 0.01 to 0.2 mm shorter than the inner dimensions of the lower mold. There are no particular restrictions on the height of the upper mold, but it must be high enough to be able to pressurize the upper mold.

[0064] (Filling Method) The string-shaped semi-cured silicone composition obtained in the heating step is filled into the lower mold section in the following manner (i) to (ii): (i) The pieces are aligned in the mold so that their longitudinal direction is parallel to the X axis, and then stacked in the Z axis direction and filled. (ii) The pieces are aligned in the mold so that their longitudinal direction is parallel to the Y axis, and then stacked in the Z axis direction and filled.

[0065] In this way, a thermally conductive silicone molded body in which flat boron nitride particles are oriented can be obtained more preferably and efficiently.

[0066] The number of times of stacking in the Z-axis direction is preferably 2 or more, and more preferably 5 to 3,000.

[0067] (Molding Method) The molding step is a step in which the lower mold is pressed in the Z-axis direction by the upper mold and heated to completely cure the string-like semi-cured silicone composition, thereby obtaining a thermally conductive silicone molded body.

[0068] As a method of applying pressure, it is preferable to gradually increase pressure from atmospheric pressure until the desired pressure is reached.

[0069] The pressure applied to the mold is selected appropriately depending on the desired shape and hardness of the thermally conductive silicone molded article, but is preferably 0.1 to 20.0 MPa, and more preferably 0.5 to 5.0 MPa.

[0070] The temperature at which the mold is pressed is preferably −10 to 40° C., and more preferably 0 to 30° C. The curing temperature of the string-shaped semi-cured silicone composition is preferably 50 to 200° C., and more preferably 100 to 150° C. The curing time is preferably 1 minute to 24 hours, and more preferably 5 minutes to 1 hour.

[0071] Slicing Process The slicing process is a process in which the thermally conductive silicone molded body obtained in the molding process is sliced ​​to obtain an insulating thermally conductive sheet of the desired thickness. Specifically, the process involves the following steps (i) and (ii): (i) The molded bodies are aligned in a mold so that their longitudinal direction is parallel to the X-axis, and then stacked in the Z-axis direction. The filled molded body is then sliced ​​parallel to the Y-Z plane. (ii) The molded bodies are aligned in a mold so that their longitudinal direction is parallel to the Y-axis, and then stacked in the Z-axis direction. The filled molded body is then sliced ​​parallel to the X-Z plane to obtain an insulating thermally conductive sheet in which the longitudinal direction of the flat boron nitride particles is oriented in the thickness direction of the sheet. By slicing the thermally conductive silicone molded body to the desired thickness, an insulating thermally conductive sheet in which the longitudinal direction of the flat boron nitride particles is oriented in the thickness direction of the sheet can be obtained.

[0072] [Degree of Orientation] The degree of orientation of the insulating thermally conductive sheet of the present invention is at least 1. If the degree of orientation is less than 1, the thermal conduction path in the thickness direction by the flat boron nitride particles deteriorates, which is not preferable.

[0073] [Thermal Conductivity] The thermal conductivity of the insulating thermally conductive sheet of the present invention in the thickness direction is preferably 17 to 30 W / m·K. If the thermal conductivity in the thickness direction is in the range of 17 to 30 W / m·K, smooth heat transfer between the heating element and the cooling element is possible via the insulating thermally conductive sheet.

[0074] [Hardness] From the viewpoint of sheet strength and heat dissipation, the hardness of the insulating thermally conductive sheet of the present invention is preferably a Shore A hardness of 5 to 60, more preferably 10 to 50. If the Shore A hardness is in the range of 5 to 60, the insulating thermally conductive sheet will not deform and there is no possibility of losing its orientation, and the contact thermal resistance between the insulating thermally conductive sheet and the heating element or cooling element will not increase.

[0075] [Breakdown Voltage] The insulating thermally conductive sheet of the present invention preferably has a breakdown voltage of 5 kV / mm or more. When the breakdown voltage is 5 kV / mm or more, the insulating properties are excellent.

[0076] [Specific Gravity] The specific gravity of the insulating thermally conductive sheet of the present invention is preferably 1.2 to 2.0. A specific gravity in the range of 1.2 to 2.0 is preferable for reducing the weight of the heat dissipation member.

[0077] EXAMPLES The present invention will be specifically explained below using examples and comparative examples, but the present invention is not limited to these.

[0078] The kinematic viscosity is a value measured at 25°C using a Cannon-Fenske viscometer according to JIS Z 8803:2011, the aspect ratio is determined by SEM measurement, and the average particle size is a cumulative average particle size (median diameter) on a volume basis measured using a Microtrac MT3300EXII particle size analyzer manufactured by Microtrac Bell Corporation.

[0079] (Preparation of Thermally Conductive Silicone Resin Composition) Components (A) to (C) used in the following examples are shown below. Component (A): Alkenyl group-containing organopolysiloxane represented by the following formula (3): (A1) In the above formula, q is a kinematic viscosity at 25°C of 600 mm ２ (A2) In the above formula, q is the number at which the kinematic viscosity at 25°C is 100,000 mm ２ / s.

[0080] Component (B): Organohydrogenpolysiloxanes represented by the following formulas (4) to (9) Component (B1-a) (The siloxane units in the parentheses are arranged in a random order.) Component (B1-b) (The siloxane units in the parentheses are arranged in a random order.) Component (B1-c) (The siloxane units in the parentheses are arranged in a random order.) Component (B2-d) (B2-e) Component (B2-f) Component

[0081] Component (C): 5% chloroplatinic acid 2-ethylhexanol solution as an addition reaction catalyst

[0082] Component (D): Flat boron nitride HSPD50 (aspect ratio 75:1, average particle size 50 μm, manufactured by Dandong Chemical Engineering Institute Co.)

[0083] Component (E): Toluene as a dilution solvent during kneading

[0084] The above components (A) to (D) and component (E) were added in the specified amounts shown in Examples 1 to 5 and Comparative Examples 1 to 9 in Table 1 below, and kneaded for 60 minutes in a planetary mixer to obtain a thermally conductive silicone composition.

[0085] (Production of Electrically Insulating and Thermally Conductive Sheet) Hereinafter, the mold used in the examples and comparative examples will be described with reference to the drawings.

[0086] (Filling Step) Figure 1 shows a mold 1 used in the filling step of the present invention. Figure 2 shows a cross section of a mold 2 used in the filling step. A mold used in the filling step having a groove with a width of 1.5 mm, a length of 50.0 mm, and a depth of 2.5 mm and a semicircular bottom shape, a lower mold used in the molding step with inner dimensions of a width of 33.0 mm (X-axis), a length of 52.0 mm (Y-axis), and a depth of 50.0 mm (Z-axis), and an upper mold used in the molding step with a width of 32.9 mm, a length of 51.9 mm, and a height of 40.0 mm were prepared. The thermally conductive silicone resin composition prepared by the above method was filled so that it overflowed from the groove, and the overflowing portion was scraped off with a squeegee.

[0087] (Vibration Step) The mold filled with the above thermally conductive silicone composition was vibrated for 30 minutes using a Sharp Corporation tabletop ultrasonic cleaner UT-106 at a water temperature of 25°C, a frequency of 37 kHz, and an output of 100%, and then dried for 2 minutes in an 80°C environment, yielding a string-like semi-cured silicone composition.

[0088] (Molding step) The mold 3 used in the molding step is shown in Figure 3. The string-shaped semi-cured silicone composition described above was filled into the bottom of the mold shown in Figure 3 so that it was lined up in the Y axis direction, forming 21 rows in the X axis direction and 40 columns in the Z axis direction, and then heated at 120°C for 30 minutes while being pressurized in the Z axis direction at 1.0 MPa using a hydraulic press, to obtain a molded product.

[0089] (Slicing Step) The molded body was sliced ​​parallel to the XZ plane with a cutter knife to obtain an electrically insulating and heat-conductive sheet.

[0090] (Evaluation of Electrically Insulating and Thermally Conductive Sheets) The electrically insulating and thermally conductive sheets obtained in Examples 1 to 5 and Comparative Examples 1 to 8 were evaluated as follows. In Comparative Example 9, the sheet was directly poured into a 60 mm x 60 mm x 2 mm mold without undergoing the vibration and molding steps described above, and then pressed at 1.0 MPa using a hydraulic press and heated at 120°C for 10 minutes to produce a molded body, which was then evaluated in the same manner. The results are shown in Table 1.

[0091] (Degree of Orientation) An insulating and heat-conductive sheet sliced ​​to a thickness of 2 mm was punched out into a circle with a diameter of 12.7 mm, and this was used as a test piece. Measurements were carried out using a BRUKER JAPAN tabletop powder X-ray diffractometer D2 PHASER 2nd Generation under the following conditions: scan range: 20-90°, step size: 0.024°, sample rotation: 10 rpm, voltage: 30 KV, current: 10 mA. The sum of the counts at 2θ = 41 to 43° (100 plane) and 25 to 27° (002 plane) was calculated. The degree of orientation was defined by the following formula: Degree of orientation = sum of counts at [2θ = 41 to 43°] / sum of counts at [2θ = 25 to 27°]

[0092] (Thermal Conductivity) The insulating and thermally conductive sheet was sliced ​​to a thickness of 2 mm and punched out into a circle having a diameter of 12.7 mm. This was used as a test piece, and the thermal conductivity was measured using a laser flash method (LFA 447 Nanoflash, manufactured by Netzsch GmbH) in accordance with JIS R 1611:2010.

[0093] (Hardness) The insulating and thermally conductive sheet was sliced ​​to a thickness of 12 mm and measured at 25° C. using a Shore A hardness tester.

[0094] (Breakdown Voltage) An insulating heat conductive sheet sliced ​​to a thickness of 1 mm was immersed in oil, and the breakdown voltage was measured by a method in accordance with JIS C 2110-1:2016.

[0095] (Specific Gravity) The specific gravity of the insulating and thermally conductive sheet sliced ​​to a thickness of 2 mm was measured at 25° C. by the underwater displacement method using an automatic specific gravity measuring device (SGM-6, manufactured by Mettler) in accordance with JIS K 6249:2003.

[0096] (Compression test) The flat surface of an insulating thermally conductive sheet sliced ​​to a thickness of 1 mm was compressed at a rate of 0.5 mm / min using a cylindrical compression terminal with a diameter of 15 mm attached to a precision autograph (AG-Xplus, manufactured by Shimadzu Corporation) until the stress reached a maximum of 1.0 MPa, and the pressure was then maintained for 5 minutes. The pressure was then released, and the insulating thermally conductive sheet was visually inspected for chips or cracks.

[0097]

[0098] Comparing Examples 1 to 4 with Comparative Examples 1 to 3, and Example 5 with Comparative Example 4, even though the molar ratio of hydrosilyl groups in Component B to alkenyl groups in Component A was the same, Examples 1 to 3 and Example 4, which contained appropriate amounts of Component (B1) and Component (B2), showed no cracks or chips after the compression test, whereas Comparative Examples 1 to 3 and 4, which did not contain both components appropriately, showed cracks after the compression test. These results demonstrate that an insulating thermally conductive sheet with excellent mechanical strength can be obtained by using an appropriate blend of Component B1 and Component B2.

[0099] On the other hand, it was found that if the molar ratio of the hydrosilyl groups in component B to the alkenyl groups in component A is not appropriate, as in Comparative Examples 5 and 6, an insulating heat conductive sheet with sufficient hardness cannot be obtained, and there is a concern that the insulating heat conductive sheet may be damaged during use.

[0100] Furthermore, the results of Comparative Examples 7 and 8 showed that the strength and thermal conductivity of the insulating thermally conductive sheet are also affected by the content of flat boron nitride (component (D)). In addition, the results of Example 1 and Comparative Example 9 showed that when the degree of orientation is less than 1, thermal conduction paths parallel to the thickness direction of the sheet are not formed, and sufficient thermal conductivity in the thickness direction cannot be obtained.

[0101] As described above, the present invention can provide an insulating thermally conductive sheet that has excellent thermal conductivity in the thickness direction and high sheet strength.

[0102] The present invention is not limited to the above-described embodiments. The above-described embodiments are merely examples, and anything that has substantially the same configuration as the technical idea described in the claims of the present invention and that exhibits similar effects is included within the technical scope of the present invention.

Claims

1. An insulating thermally conductive sheet, which is a cured product of a thermally conductive silicone resin composition containing the following components (A) to (D), wherein the insulating thermally conductive sheet has a peak intensity ratio (degree of orientation) in the thickness direction as detected by X-ray diffraction method of [2θ=41-43°] / [2θ=25-27°]=1 or more: (A) 100 parts by mass of an organopolysiloxane having two or more alkenyl groups bonded to silicon atoms per molecule; (B) an organohydrogenpolysiloxane containing the following components (B1) and (B2) and having two or more hydrosilyl groups per molecule: the amount of component (B) added is such that the number of moles of hydrosilyl groups derived from component (B) is 1.3 to 2.0 per mole of alkenyl groups derived from component (A), and the ratio of the number of moles of hydrosilyl groups of component (B1) to component (B2) is 1:1 to 1:

5. (B1) An organohydrogenpolysiloxane represented by the following formula (1): (m is an integer of 4 to 20, n is an integer of 10 to 40, and the bonding order of the siloxane units bounded by m and n may be block or random.) (B2) An organohydrogenpolysiloxane having hydrosilyl groups only at both ends, as represented by the following formula (2): (p is an integer of 40 to 200); (C) a hydrosilylation catalyst: an amount equivalent to 100 to 3,000 ppm by mass of platinum group metal element relative to the component (A); and (D) 200 to 800 parts by mass of flat boron nitride having an average particle size of 30 to 100 μm as measured by laser diffraction methods and an aspect ratio of 20 to 100.

2. The insulating heat conductive sheet according to claim 1, characterized in that the thermal conductivity of said insulating heat conductive sheet in the thickness direction is 17 to 30 W / m·K.

3. The insulating and heat conductive sheet according to claim 1, characterized in that the Shore A hardness of said insulating and heat conductive sheet is 5 to 60.

4. The insulating heat conductive sheet according to claim 1, characterized in that the insulating heat conductive sheet has a dielectric breakdown voltage of 5 kV / mm or more.

5. The insulating and heat conductive sheet according to claim 1, characterized in that the specific gravity of said insulating and heat conductive sheet is 1.2 to 2.0.