JP2015044288A - Method for producing heat-conductive sheet, and heat-conductive sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a heat-conductive sheet having further-improved heat radiability.SOLUTION: A method for producing the heat-conductive sheet comprises: a step (I) of co-flocculating polytetrafluoroethylene fine particles and heat-conductive inorganic particles in an aqueous dispersion liquid containing the polytetrafluoroethylene fine particles and the heat-conductive inorganic particles, separating an obtained co-flocculate from a liquid component, and drying the co-flocculate to prepare a powder mixture containing the polytetrafluoroethylene fine particles and the heat-conductive inorganic particles; a step (II) of using the power mixture and a molding aid to prepare a plurality of sheet-shaped moldings each containing polytetrafluoroethylene, the heat-conductive inorganic particles and the molding aid; a step (III) of superposing the plurality of sheet-shaped moldings on one another and rolling a superposed material; and a step (IV) of removing the molding aid.

Description

  The present invention relates to a method for manufacturing a heat conductive sheet and a heat conductive sheet.

  In an electronic device typified by a mobile computer and a mobile phone, “heat dissipation” has become a major issue due to heat generation of the member itself due to improved processing capability and high-density mounting accompanying downsizing. Therefore, as a heat radiating member that has high heat dissipation performance and mechanical strength, and has excellent handling properties without adverse effects when applied to electronic devices, insulation with thermally conductive inorganic particles dispersed in a fluororesin matrix A heat conductive sheet has been proposed (Patent Document 1).

  In recent years, hybrid vehicles and electric vehicles have been developed from the viewpoint of environmental friendliness. A motor is used for such a vehicle drive system, and the motor is required to have a high output. When the motor output is increased, the amount of heat generated increases. Therefore, as a means for cooling the vehicle motor, a heat radiating member using an insulating heat conductive sheet in which heat conductive inorganic particles are dispersed in a fluororesin matrix has been proposed (Patent Document 2). Thus, the insulating heat conductive sheet can also be used for cooling the vehicle motor.

JP 2010-137562 A JP2013-208159A

  In recent years, further improvement in heat dissipation has been demanded for heat dissipation members used for electronic devices and vehicles. In the heat conductive sheet described in Patent Documents 1 and 2, as one means for obtaining higher heat dissipation, the amount of heat conductive inorganic particles contained in the heat conductive sheet is increased to increase the heat conductivity. Can be considered. However, the heat conductive sheets described in Patent Documents 1 and 2 are already allowed to contain heat conductive inorganic particles up to 95% by mass. Therefore, even if it is attempted to improve the heat dissipation property of the heat conductive sheet by adding more heat conductive inorganic particles, the fluororesin matrix becomes too small to maintain the strength of the heat conductive sheet.

  Therefore, the present invention provides a heat conductive sheet capable of realizing higher heat dissipation than the conventional heat conductive sheet even if the amount of the heat conductive inorganic particles contained is similar to that of the conventional heat conductive sheet. Objective.

The present invention
(I) Aggregation obtained by co-aggregating the PTFE fine particles and the thermally conductive inorganic particles in an aqueous dispersion containing polytetrafluoroethylene (hereinafter referred to as PTFE) particles and thermally conductive inorganic particles. Preparing a mixed powder containing the PTFE fine particles and the thermally conductive inorganic particles, which is prepared by separating an object from a liquid component and drying;
(II) using the mixed powder and the molding aid, preparing a plurality of sheet-like molded bodies containing polytetrafluoroethylene, the thermally conductive inorganic particles, and the molding aid;
(III) a step of superposing and rolling a plurality of the sheet-like molded bodies,
(IV) removing the molding aid;
The manufacturing method of the heat conductive sheet containing is provided.

  This invention further provides the heat conductive sheet obtained by the manufacturing method of the heat conductive sheet of the said invention.

  In the manufacturing method of the heat conductive sheet of this invention, the mixed powder containing PTFE microparticles | fine-particles and a heat conductive inorganic particle is used for the material for preparing the sheet-like molded object containing PTFE, a heat conductive inorganic particle, and a shaping | molding adjuvant. It is used. This mixed powder is obtained by co-aggregating the PTFE fine particles and the heat conductive inorganic particles in an aqueous dispersion containing the PTFE fine particles and the heat conductive inorganic particles, and separating and drying the obtained aggregate from the liquid component. It was produced. A heat conductive sheet obtained by preparing a plurality of sheet-like molded bodies using such a mixed powder and then rolling and laminating a plurality of sheet-like molded bodies in a subsequent process, and removing the molding aid is Further, it is possible to achieve higher heat dissipation than conventional heat conductive sheets in which the amount of heat conductive inorganic particles is comparable.

It is a figure which shows the thermal-characteristic evaluation apparatus used in the Example, (a) is a front view, (b) is a side view. 2 is a SEM (Scanning Electron Microscope) photograph of a cross section of the heat conductive sheet of Example 1. FIG. 4 is a SEM photograph of a cross section of a heat conductive sheet of Example 2. 4 is a SEM photograph of a cross section of a heat conductive sheet of Example 3. 4 is a SEM photograph of a cross section of a heat conductive sheet of Comparative Example 1. 4 is a SEM photograph of a cross section of a heat conductive sheet of Comparative Example 2. 6 is a SEM photograph of a cross section of a heat conductive sheet of Comparative Example 3.

  Embodiments of the present invention will be described below. The following description does not limit the present invention.

The manufacturing method of the heat conductive sheet of this embodiment is
(I) By coaggregating the PTFE fine particles and the thermally conductive inorganic particles in an aqueous dispersion containing the PTFE fine particles and the thermally conductive inorganic particles, and separating and drying the obtained aggregate from the liquid component. Preparing a prepared mixed powder containing the PTFE fine particles and the thermally conductive inorganic particles;
(II) using the mixed powder and the molding aid, preparing a plurality of sheet-like molded bodies containing polytetrafluoroethylene, the thermally conductive inorganic particles, and the molding aid;
(III) a step of superposing and rolling a plurality of the sheet-like molded bodies,
(IV) removing the molding aid;
including.

  Moreover, the manufacturing method of the heat conductive sheet of this Embodiment may further include the process (process (V)) of pressure-molding the sheet-like material obtained by the said process (IV). In step (V), it is desirable to perform pressure molding at a temperature within the PTFE firing temperature range.

  The heat conductive sheet obtained through the above steps (I) to (IV) usually has a porous structure. Therefore, for the purpose of increasing the thermal conductivity, increasing the insulating property and / or imparting adhesiveness, etc., the impregnating material selected appropriately is impregnated in the thermal conductive sheet, and the pores contained in the thermal conductive sheet May be filled with an impregnating material. The impregnating material can be appropriately selected from known impregnating materials according to the purpose. When a heat conductive sheet impregnated with an impregnating material is manufactured, the heat conductive sheet is impregnated with the impregnating material after step (IV) (after step (V) when step (V) is performed). Step (Step (VI)) is performed.

  Hereinafter, each step will be specifically described.

  An example of the step (I) will be described.

  In step (I), a mixed powder containing PTFE fine particles and thermally conductive inorganic particles is prepared. This mixed powder is obtained by co-aggregating the PTFE fine particles and the heat conductive inorganic particles in an aqueous dispersion containing PTFE fine particles and heat conductive inorganic particles, and separating the obtained agglomerates from the liquid components and drying. It is produced by making it.

The mixed powder may be composed of only PTFE fine particles and thermally conductive inorganic particles, or may contain other fluororesin other than PTFE. Other fluororesins include, for example, a tetrafluoroethylene / perfluoroalkyl vinyl ether copolymer (hereinafter referred to as PFA), a tetrafluoroethylene / hexafluoropropylene copolymer (hereinafter referred to as FEP), and the like. It is preferable to use a molten fluororesin having good compatibility with PTFE. When such a molten fluororesin is used, the porosity can be efficiently reduced in the subsequent hot pressing step (step (V)), and thus the thermal conductivity of the thermal conductive sheet can be further improved. Become. Therefore, as the fluororesin component contained in the mixed powder, for example,
(A) PTFE only,
(B) PTFE and PFA, or
(C) PTFE and FEP,
Is preferred. When the mixed powder contains PTFE and another fluororesin as a fluororesin component, the content of PTFE with respect to the entire fluororesin component is preferably 5% by weight or more, and more preferably 10% by weight or more. .

  An example of a method for producing a mixed powder will be described. First, an aqueous dispersion containing PTFE fine particles and thermally conductive inorganic particles is prepared. Examples of the dispersion medium include water or water containing a surfactant. The solid concentration in the aqueous dispersion is, for example, 3 to 50% by mass. In the aqueous dispersion, the content of PTFE fine particles is, for example, 0.5 to 20% by mass, and the content of thermally conductive inorganic particles is, for example, 10 to 30% by mass. This aqueous dispersion is prepared, for example, by preparing an aqueous dispersion of PTFE fine particles (for example, PTFE aqueous dispersion), diluting it with water, and then adding thermally conductive inorganic particles to the obtained dispersion. Can be made. In addition, the aqueous dispersion of PTFE fine particles used here is not particularly limited, and a commercially available product may be used. The particle size of the PTFE fine particles in the aqueous dispersion is, for example, 0.2 to 0.5 μm. When preparing a mixed powder containing other fluororesin other than PTFE, an aqueous dispersion containing PTFE fine particles and other fluororesin particles is prepared. Moreover, the heat conductive inorganic particle added is as follows.

The heat conductive inorganic particles are preferably formed of an inorganic material having a heat conductivity of 1 to 200 W / mK in order to impart sufficient heat conductivity to the heat conductive sheet. Further, when it is necessary to impart high electrical insulation to the heat conductive sheet, the heat conductive inorganic particles are preferably formed of an inorganic material having an electric resistivity of 10 ¹⁰ to 10 ¹⁷ Ω · m. Since the thermal conductivity is high, boron nitride is preferably used for the thermally conductive inorganic particles in the present embodiment. Therefore, it is preferable that the thermally conductive inorganic particles in the present embodiment are substantially made of boron nitride. The “thermally conductive inorganic particles substantially composed of boron nitride” means that the thermally conductive inorganic particles contain no substances other than boron nitride or contain other substances. However, it means that it is a very small amount (for example, 10% by weight or less) that does not significantly reduce the characteristics (thermal conductivity characteristics) when using thermally conductive inorganic particles (boron nitride particles) that do not contain other substances. .

  The shape of the thermally conductive inorganic particles is not particularly limited, but in order to obtain a thermally conductive sheet having thermal conductivity anisotropy, it is preferable that the thermally conductive inorganic particles have a flat plate shape or scale shape that can be easily aligned in the in-plane direction by rolling. For the same reason, it is preferable that the thermally conductive inorganic particles themselves have thermal conductivity anisotropy. Moreover, when improving the heat conductivity of the thickness direction, you may use the heat conductive inorganic particle of the aggregation shape currently sold from each company.

  The thermally conductive inorganic particles are preferably blended so that the content is 40 to 95% by weight in the state of the thermally conductive sheet, and more preferably 60% by weight or more. By setting the blending amount of the heat conductive inorganic particles in such a range, the thermal conductivity of the sheet can be sufficiently increased, so that better heat dissipation performance can be realized.

  The heat conductive inorganic particles are supported on the PTFE matrix without falling off, and since it is sufficient that the obtained heat conductive sheet has sufficient heat conductivity, the particle diameter is not particularly limited. The thing with a particle size of 0.3-500 micrometers is desirable. However, the heat conductive inorganic particles preferably have a larger particle size in order to achieve high heat conductivity. This is because even if the content of the thermally conductive inorganic particles is the same, the larger the particle size, the smaller the number of interfaces, and the lower the thermal resistance. Here, the particle diameter is a value measured by a laser diffraction / scattering particle diameter / particle size distribution measuring device (microtrack).

  Next, the PTFE fine particles and the heat conductive inorganic particles are co-aggregated in an aqueous dispersion containing the PTFE fine particles and the heat conductive inorganic particles. The method for causing co-aggregation is not particularly limited. For example, a method of adding a flocculant to the aqueous dispersion and raising the temperature to deactivate the surfactant contained in the aqueous dispersion. Can be used. As the flocculant, for example, a solvent having a low solubility with respect to the heat conductive inorganic particles (poor solvent) and a material having a large specific surface area such as activated carbon can be used.

  By separating the agglomerate obtained by coaggregation from the liquid component and further drying, a mixed powder can be produced. The separation method of the aggregate is not particularly limited, and a known method such as filtration can be used. The drying method is not particularly limited, and a known method can be used.

  In addition, the manufacturing method of this embodiment is the step of producing the mixed powder before the step (I), that is, in the aqueous dispersion containing the PTFE fine particles and the thermally conductive inorganic particles, and the PTFE fine particles and the A step of co-aggregating the thermally conductive inorganic particles, separating the obtained aggregate from the liquid component and drying to produce a mixed powder containing the PTFE fine particles and the thermally conductive inorganic particles; Is also possible.

  Next, an example of the step (II) will be described.

  The mixed powder prepared in step (I) and the molding aid are mixed to produce a paste-like mixture. It is desirable that this mixing be performed under conditions that suppress fiber formation of PTFE as much as possible. Specifically, it is desirable to reduce the number of rotations, shorten the mixing time, and mix without kneading so as not to apply a shearing force to PTFE. If fiber formation of PTFE occurs at the stage of mixing the materials, there is a possibility that the PTFE fiber already formed is cut and the PTFE network structure is destroyed when rolling in the step (III). It may be difficult to maintain the sheet shape. Therefore, by mixing so as to suppress the fiberization of PTFE as in the present embodiment, processing of a sheet-like material using PTFE as a matrix in a later step becomes easy.

  As the molding aid, for example, saturated hydrocarbons such as dodecane and decane can be used. What is necessary is just to add a shaping | molding adjuvant so that it may become 20 to 55 weight% with respect to the total weight. A mother sheet obtained by forming such a mixture into a sheet by extrusion and rolling can be used as the sheet-like molded article of the present invention (first example of a sheet-like molded article). The thickness of the sheet-like molded body thus obtained is, for example, 0.5 to 5 mm.

  In addition, as another example of the sheet-like molded body prepared in the step (II), a laminated sheet (second example of the sheet-like molded body) obtained by rolling a plurality of the mother sheets stacked together is also given. It is done. The number of laminated sheets is not particularly limited, and can be appropriately determined in consideration of the number of constituent layers of the heat conductive sheet to be manufactured (the number of layers constituting the heat conductive sheet).

  In addition, the sheet-like molded body may contain a material other than the fluororesin component, the thermally conductive inorganic particles and the molding aid, or is formed only by the fluororesin component, the thermally conductive inorganic particles and the molding aid. May be.

  A sheet-like molded body can be prepared as described above.

  Next, an example of the step (III) will be described.

  In step (III), the plurality of sheet-like molded bodies prepared in step (II) are overlapped and rolled. Specifically, a plurality of sheet-like molded bodies prepared in step (II) are laminated, and the laminate is rolled to obtain a laminated sheet. As described above, the sheet-like molded body may be the mother sheet (sheet-like molded body of the first example) or a laminated sheet (first sheet) obtained by rolling a plurality of mother sheets. The sheet-like molded body of the example of 2) may be used. The number of the sheet-like molded bodies to be overlaid in the step (II) is not particularly limited, and can be about 2 to 10 sheets, for example. In order to achieve high strength, it is desirable to roll the sheet-like molded bodies on top of each other.

  As another example of the method of superposing a plurality of sheet-like molded bodies, a method of folding the sheet-like molded bodies can also be mentioned. The number of sheet-shaped molded bodies to be folded (the number of sheet-shaped molded bodies to be stacked) is not particularly limited, and is, for example, about 2 to 10 sheets. By folding and rolling the sheet-like molded body, the sheet strength can be improved and the thermally conductive inorganic particles can be firmly fixed to the PTFE matrix. As a result, it is possible to produce a flexible sheet with a high blending ratio of the heat conductive inorganic particles.

  In the manufacturing method of the heat conductive sheet of this Embodiment, process (II) and process (III) may be repeated alternately. A specific example in this case will be described below.

  First, a plurality of (for example, 2 to 10) mother sheets are prepared (step (II)). Next, a plurality of mother sheets are laminated, and the laminate is rolled to obtain a laminated sheet (first laminated sheet) (step (III)). A plurality of (for example, 2 to 10) first laminated sheets obtained here are further prepared, and the first laminated sheet is used as a sheet-like formed body in the step (II). Next, a plurality of (for example, 2 to 10) first laminated sheets are laminated, and the laminated product is rolled to obtain a laminated sheet (second laminated sheet) (step (III)). Furthermore, a plurality of (for example, 2 to 10) second laminated sheets obtained are prepared, and the second laminated sheet is used as a sheet-like formed body in step (II). Next, a plurality of (for example, 2 to 10) second laminated sheets are laminated, and the laminated product is rolled to obtain a laminated sheet (third laminated sheet) (step (III)). Thus, the step (II) and the step (III) can be alternately repeated until the desired number of layers of the heat conductive sheet is reached. In the example described here, the lamination sheets having the same number of laminations (first lamination sheets, second lamination sheets, etc.) are overlapped and rolled, but the lamination numbers are different from each other. It is also possible to roll the sheets by overlapping them.

  It is desirable to change the rolling direction when repeating step (III). For example, in rolling performed to obtain the second laminated sheet, the rolling direction may be changed by 90 degrees from the direction of rolling performed to obtain the first laminated sheet. By rolling while changing the direction in this way, the PTFE network extends vertically and horizontally, and the sheet strength can be improved and the heat-conductive inorganic particles can be firmly fixed to the PTFE matrix.

  When the number of constituent layers of the heat conductive sheet is represented by the total number of mother sheets included in the heat conductive sheet, the number of constituent layers can be, for example, 2 to 5000 layers. In order to increase the sheet strength, the number of layers is preferably 200 or more. Further, in order to reduce the thickness (for example, a sheet having a thickness of 1 mm or less), the number of layers is preferably 1500 layers or less. The greater the number of constituent layers, the higher the strength of the resulting sheet.

  At the beginning of rolling (the stage where the total number of mother sheets included is small), it is difficult to withstand high-strength rolling with low strength. And the solid fixation of the thermally conductive inorganic particles to the PTFE matrix becomes possible. The laminated structure (number of constituent layers) is also related to the thermal conductivity and insulating properties of the obtained sheet. Therefore, in order to obtain a sheet having sufficient thermal conductivity and insulation, the number of constituent layers is preferably 10 to 1000.

  Finally, a sheet having a thickness of about 0.1 to 3 mm is prepared, and then, as a step (IV), a heat conduction sheet can be obtained by heating to remove the molding aid.

  Next, examples of steps (V) and (VI) performed as necessary will be described.

  After removing the molding aid, the sheet-like material obtained in the step (IV) may be pressure-molded (step (V)). By including such a pressure forming step, pores can be reduced, which contributes to improvement in thermal conductivity. That is, in order to further improve the thermal conductivity of the obtained heat conductive sheet, it is desirable to reduce the porosity, for example, the porosity is preferably 40% or less. In addition, the porosity here is a value calculated | required by the measuring method performed in the below-mentioned Example. In the step (IV), it is desirable to perform pressure molding at a temperature within the PTFE firing temperature range. By performing pressure molding at such a firing temperature, the porosity can be efficiently reduced.

  When impregnating the heat conductive sheet with the impregnating material, impregnating the sheet-like material obtained by the step (IV) (or the sheet-like material obtained by the step (V) when the step (V) is carried out) The material is impregnated (step (VI)).

  In the step (VI), since it is easy to impregnate the impregnating material at a high impregnation rate in a short time, the sheet-like material obtained in the step (IV) (when the step (V) is carried out) The sheet-like material obtained in the step (V) is preferably immersed in an impregnating material and pressurized. Such an operation can be performed using a pressurized container.

  As described above, the impregnating material is not particularly limited because it can be appropriately selected depending on the purpose such as enhancing the thermal conductivity, enhancing the insulating properties and / or imparting the adhesiveness. However, for the impregnating material, for example, a material that does not have volatility and has an insulating property is preferably used. Since the impregnating material needs to be included in the pores of the heat conductive sheet, the impregnating material preferably has a viscosity of 1 to 100000 mPa · s, and more preferably 1 to 1000 mPa · s. The lower the viscosity, the easier the impregnation material can be contained in the pores.

  The electrical insulation and thermal conductivity of the impregnating material are not particularly limited, but the higher the better.

  As the impregnating material, for example, acrylic and silicone oils, adhesives such as thermosetting adhesives (for example, epoxy resins) and hot melt adhesives, grease, and resins can be used. These oils and adhesives are publicly known and are also commercially available. What is necessary is just to select the kind of impregnation material suitably according to the use of a heat conductive sheet, and also the insulation and heat dissipation which desire.

  In the manufacturing method of the present embodiment, a sheet-like molded body is produced using a mixed powder obtained by wet-mixing PTFE fine particles and thermally conductive inorganic particles in advance, and the sheet-like molded body is stacked. A heat conductive sheet is manufactured through processes such as bonding and rolling. The heat conductive sheet manufactured by this method is manufactured by a method in which a PTFE powder and a heat conductive inorganic particle are mixed with a molding aid without using a mixed powder to produce a sheet-like molded body. Compared with the sheet, it has higher heat dissipation.

  In the production method of the present embodiment, in the step (II), when a mixed powder containing PTFE fine particles and thermally conductive inorganic particles and a molding aid are mixed to produce a paste-like mixture, PTFE fibers Mixing is performed under the condition that suppresses the conversion as much as possible. Thereby, in the rolling of process (III) which follows, the change to a sheet shape and the fiberization of PTFE advance simultaneously. Therefore, in the rolling of the step (III), the heat conductive inorganic particles are exposed to the pressing of the rolling without being constrained by the PTFE fibers, and are arranged in a state substantially parallel to the sheet. Further, when scaly particles are used as the thermally conductive inorganic particles, the particles are oriented in the flow direction during rolling, and therefore the thermal conductivity in the in-plane direction becomes higher. Furthermore, the thermal conductivity in the in-plane direction can be further increased by using particles having thermal conductivity anisotropy such as boron nitride particles. By disposing the heat conductive inorganic particles in such a state, anisotropy appears in heat conduction in the obtained heat conductive sheet. That is, according to the manufacturing method of the present embodiment, it is possible to obtain a heat conductive sheet whose thermal conductivity in the in-plane direction of the sheet is higher than the thermal conductivity in the thickness direction.

  In the heat conductive sheet produced by the manufacturing method of the present embodiment, only the fluororesin component is used as a matrix, and it is preferable that other organic materials, rubber components, vulcanizing agents, and the like are not included. By using only the fluororesin component as the matrix, it is not necessary to consider the influence on the device when applied to the electronic device. In addition, the thermal conductivity in the in-plane direction is high, and it is optimal for heat diffusion and heat dissipation. Therefore, a sheet having both high heat diffusion functions can be realized. Furthermore, this heat conductive sheet has high mechanical strength, and even when heat conductive inorganic particles are blended at a high ratio, sufficient mechanical strength can be realized.

  According to the manufacturing method of the present embodiment, a heat conductive sheet having a low Young's modulus (for example, Young's modulus of 0.5 GPa or less) can be produced. That is, the heat conductive sheet obtained by the manufacturing method of the present embodiment is flexible and has high unevenness followability. Therefore, when this heat conductive sheet is installed in an electronic device or a vehicle as a heat radiating member, it can be arranged at a desired location without twisting the shape of the installation location.

  In addition, in the manufacturing method of this embodiment, since fiber formation of PTFE does not occur so much at the time of mixing materials, even if the rolling process of step (III) is repeated, the fibers of PTFE are cut and cannot keep the shape. Therefore, the sheet shape can be easily maintained. Moreover, in this Embodiment, since several sheet-like molded objects are laminated | stacked and rolled, even when a defect arises in a certain layer by rolling, the defect can be supplemented with another layer. Therefore, the problem that the sheet shape cannot be maintained does not occur. Further, when the rolling direction is changed when repeating the step (III), PTFE isotropically bound to obtain a beautiful sheet. For these reasons, according to the manufacturing method of the present embodiment, a long sheet or a continuous sheet can be obtained.

  Moreover, as described above, it is also possible to provide a heat radiating member provided with the heat conductive sheet produced by the manufacturing method of the present embodiment. The heat radiating member may be a heat radiating sheet made of a heat conductive sheet, or may be constituted by another component such as a heat conductive sheet and a metal plate.

  Next, the manufacturing method of the heat conductive sheet of this invention and a heat conductive sheet are demonstrated concretely using an Example.

Example 1
After diluting a PTFE aqueous dispersion (Asahi Glass Co., Ltd., product number “AD938E”) 20 times with water, boron nitride (BN) particles (made by Mizushima Alloy Iron Co., Ltd.) as thermally conductive inorganic particles were added to the obtained dispersion. , Product number “HP-40”) was added so that the mass ratio of boron nitride particles to PTFE fine particles was 80:20 (boron nitride particles: PTFE fine particles). As a result, an aqueous dispersion containing PTFE fine particles and boron nitride particles was obtained. Isopropyl alcohol was added to the aqueous dispersion to coaggregate the PTFE fine particles and the boron nitride particles in the aqueous dispersion. The obtained aggregate was filtered and separated from the liquid component, and dried at 150 ° C. for 24 hours to obtain a mixed powder containing PTFE fine particles and boron nitride particles.

  This mixed powder was mixed with “Isopar M” (manufactured by ExxonMobil Co., Ltd.) used as a molding aid so that the mass ratio was 5: 2 (mixed powder: molding aid). The mixture was mixed for 10 minutes in a rotary mill under the condition that the conversion was not as much as possible.

  The resulting mixture was preformed and pelletized. The pellets were extruded using an extruder to obtain a sheet-like product having a width of 45 mm and a thickness of 2 mm. This sheet material was rolled in the MD direction (extrusion direction (longitudinal direction)) with a pair of rolling rolls to obtain a sheet material having a thickness of 1 mm. This sheet material having a thickness of 1 mm was cut to prepare 16 sheet-like molded bodies having a length of 250 mm and a width of 45 mm. These 16 sheet-like molded bodies were laminated with each other in the MD direction and the TD direction (direction perpendicular to the MD direction) and rolled in the TD direction. Rolling in the TD direction was repeatedly performed until the thickness of the laminated sheet-like formed body became 0.25 mm.

  Next, the obtained sheet was heated at 150 ° C. for 1 hour to remove the molding aid. Next, 7 MPa pressure molding was performed on the sheet-like material at 380 ° C. for 1 minute, and then cooled to 170 ° C. over 5 minutes. This obtained the heat conductive sheet of Example 1 which has thickness 0.172mm.

  About the heat conductive sheet of Example 1 produced as described above, thermal resistance, effective thermal conductivity, porosity, and orientation were measured by the following methods. The measurement results are as shown in Table 1. Moreover, the SEM photograph of the cross section of a heat conductive sheet is shown in FIG.

(Example 2)
A thickness of 0.22 mm was obtained in the same manner as in Example 1 except that the mixed powder was prepared so that the mass ratio of boron nitride particles to PTFE fine particles was 85:15 (boron nitride particles: PTFE fine particles). A heat conductive sheet of Example 2 was prepared. About the obtained heat conductive sheet, thickness, thermal resistance, effective thermal conductivity, porosity, and orientation were measured by the following methods. The measurement results are as shown in Table 1. Moreover, the SEM photograph of the cross section of a heat conductive sheet is shown in FIG.

Example 3
A thickness of 0.192 mm was obtained in the same manner as in Example 1 except that the mixed powder was prepared so that the mass ratio of boron nitride particles and PTFE fine particles was 90:10 (boron nitride particles: PTFE fine particles). A heat conductive sheet of Example 3 was prepared. About the obtained heat conductive sheet, thermal resistance, effective thermal conductivity, porosity, and orientation were measured by the following methods. The measurement results are as shown in Table 1. Moreover, the SEM photograph of the cross section of a heat conductive sheet is shown in FIG.

(Comparative Example 1)
Boron nitride particles (manufactured by Mizushima Alloy Iron Co., Ltd., product number “HP-40”) as PTFE fine powder (product number “F104U” manufactured by Daikin Industries, Ltd.) as a heat conductive inorganic particle are 80 by mass ratio. It mixed so that it might become 20 (boron nitride particle | grains: PTFE fine powder). To this, “Isopar M” (manufactured by ExxonMobil Co., Ltd.) was added as a molding aid so as to be 28.5% by mass, and mixed under conditions such that PTFE fiberization did not occur as much as possible. The mixing conditions were a V-type mixer with a rotation speed of 10 rpm, a temperature of 25 ° C., and a mixing time of 5 minutes. This mixture was extruded to obtain a sheet-like molded body having a thickness of 2 mm, a width of 50 mm, and a length of 2500 mm. Except for using the sheet-like molded body thus prepared, rolling, removal of the molding aid, and pressure molding were performed in the same manner as in Example 1. This produced the heat conductive sheet of the comparative example 1 which has thickness 0.21mm. About the obtained heat conductive sheet, thermal resistance, effective thermal conductivity, porosity, and orientation were measured by the following methods. The measurement results are as shown in Table 1. Moreover, the SEM photograph of the cross section of a heat conductive sheet is shown in FIG.

(Comparative Example 2)
The same method as in Comparative Example 1 except that boron nitride particles and PTFE fine powder were mixed so that the mass ratio of boron nitride particles to PTFE fine powder was 85:15 (boron nitride particles: PTFE fine particles). A heat conductive sheet of Comparative Example 2 having a thickness of 0.2 mm was produced. About the obtained heat conductive sheet, thermal resistance, effective thermal conductivity, porosity, and orientation were measured by the following methods. The measurement results are as shown in Table 1. Moreover, the SEM photograph of the cross section of a heat conductive sheet is shown in FIG.

(Comparative Example 3)
The same method as in Comparative Example 1 except that the boron nitride particles and the PTFE fine powder were mixed so that the mass ratio of the boron nitride particles and the PTFE fine powder was 90:10 (boron nitride particles: PTFE fine particles). A heat conductive sheet of Comparative Example 3 having a thickness of 0.23 mm was produced. About the obtained heat conductive sheet, thickness, thermal resistance, effective thermal conductivity, porosity, and orientation were measured by the following methods. The measurement results are as shown in Table 1. Moreover, the SEM photograph of the cross section of a heat conductive sheet is shown in FIG.

<Measurement of thermal resistance and effective thermal conductivity>
The heat dissipation of the heat conductive sheets of Examples and Comparative Examples was evaluated by their thermal resistance. The measurement of thermal resistance was performed using the thermal characteristic evaluation apparatus 10 shown in FIGS. 1 (a) and 1 (b). The thermal property evaluation apparatus 10 has a heating element (heater block) 11 in the upper part and a radiator (cooling base plate configured to circulate cooling water) 12 in the lower part. The heat generating body 11 and the heat radiating body 12 each have a rod 13 made of brass (A5052, thermal conductivity: 108 W / m · K) formed to be a cylinder (diameter: 22.5 mm). A temperature sensor 14 of a thermometer 15 is attached to the back side of the heating element 11 and the upper and lower rods 13. Specifically, the temperature sensors 14 are attached to one place of the heating element 11 and three places at equal intervals in the vertical direction of each rod 13.

First, the sample 20 was sandwiched between the pair of rods 13 from above and below. Sample 20 was obtained by cutting out the heat conduction sheets of each of the examples and comparative examples into a square of 25 mm × 25 mm and cutting the four corners protruding from the outside of the rod 13 after setting it on the rod 13. That is, the sample 20 had an octagonal shape. The thermal measurement evaluation apparatus 10 is incorporated in a tensilon apparatus (not shown), and the tensilon apparatus applies pressure to the sample 20 in a direction in which the sample 20 is compressed in the thickness direction from the outside of the heating element 11 and the radiator 12. added. While the pressure was applied to the sample 20, the temperature of the heating element 11 was set to 120 ° C., and 15 ° C. cooling water was circulated through the radiator 12. Then, after the temperatures of the heating element 11 and the upper and lower rods 13 were stabilized, the temperature of the upper and lower rods 13 was measured by each temperature sensor 14. After the measurement, the thickness of the sample 20 was measured with a dial gauge. The heat flux passing through the sample 20 was calculated from the thermal conductivity (W / m · K) of the upper and lower rods 13 and the temperature gradient, and the temperature at the interface between the upper and lower rods 13 and the sample 20 was calculated. And using these, the thermal resistance (K / W) in the said pressure was computed using the thermal conductivity equation (Fourier's law). Furthermore, the effective thermal conductivity (W / m · K) (apparent thermal conductivity of the sample including the interface) was calculated using the calculated thermal resistance. In addition, the thermal resistance was calculated | required about the case where the pressure of 400N was applied to the sample 20. FIG.
Q = −λgradT
R = 1 / λ (L / A)
λ ′ = L / R · A
Q: Heat flow rate per unit area gradT: Temperature gradient L: Sample (heat conduction sheet) thickness A: Rod area λ: Thermal conductivity R: Thermal resistance λ ′: Effective thermal conductivity

<Measurement of porosity>
The weight and volume of the heat conductive sheet were measured, and the actual density was determined from the results. Using this measured density and true density, the porosity was determined by the following equation.
Porosity (%) = (1−actual density / true density) × 100

<Orientation degree>
The degree of orientation of transmission and reflection was measured using a two-dimensional detector-mounted X-ray diffractometer “D8 DISCOVER with GADDS” manufactured by Brucker AXS.

  From the results shown in Table 1, when the same mass ratios of boron nitride and PTFE are compared, the thermal conductive sheet of the example has a lower thermal resistance than the thermal conductive sheet of the comparative example, and the effective thermal conductivity is low. it was high. That is, the heat conductive sheet produced by the production method of the present invention had higher heat dissipation than the heat conductive sheet produced by the conventional method.

  When comparing the SEM photographs of the heat conductive sheets of Examples 1 to 3 and Comparative Examples 1 to 3, in the heat conductive sheets of Examples 1 to 3, boron nitride particles are present in a dispersed manner in the PTFE matrix. In the heat conductive sheets of Examples 1 to 3, it can be confirmed that the PTFE matrix and the boron nitride particles are present in layers in the cross section.

  Since the heat conductive sheet obtained by this invention has high heat dissipation, it can be utilized suitably as a heat radiating member for electronic devices and vehicles.

DESCRIPTION OF SYMBOLS 10 Thermal characteristic evaluation apparatus 11 Heat generating body 12 Heat radiating body 13 Rod 14 Temperature sensor 15 Thermometer 20 Sample

Claims (9)

(I) The polytetrafluoroethylene fine particles and the heat conductive inorganic particles are co-aggregated in an aqueous dispersion containing the polytetrafluoroethylene fine particles and the heat conductive inorganic particles, and the obtained aggregate is separated from the liquid component. And preparing a mixed powder containing the polytetrafluoroethylene fine particles and the thermally conductive inorganic particles, which is prepared by drying
(II) using the mixed powder and the molding aid, preparing a plurality of sheet-like molded bodies containing polytetrafluoroethylene, the thermally conductive inorganic particles, and the molding aid;
(III) a step of superposing and rolling a plurality of the sheet-like molded bodies,
(IV) removing the molding aid;
The manufacturing method of the heat conductive sheet containing this. The mixed powder prepared in the step (I) is prepared by diluting an aqueous dispersion of polytetrafluoroethylene fine particles with water, adding the thermally conductive inorganic particles to the obtained dispersion, Preparing the aqueous dispersion containing tetrafluoroethylene fine particles and the thermally conductive inorganic particles, and co-aggregating the polytetrafluoroethylene fine particles and the thermally conductive inorganic particles in the aqueous dispersion, the obtained Produced by separating agglomerates from the liquid component and drying;
The manufacturing method of the heat conductive sheet of Claim 1. The thermally conductive inorganic particles are boron nitride particles;
The manufacturing method of the heat conductive sheet of Claim 1 or 2. (V) a step of pressure-molding the sheet-like material obtained by the step (IV),
Further including
The manufacturing method of the heat conductive sheet of any one of Claims 1-3. In the step (V), pressure molding is performed at a temperature within the firing temperature range of polytetrafluoroethylene.
The manufacturing method of the heat conductive sheet of Claim 4. (VI) impregnating the sheet-like material obtained by the step (IV) with an impregnating material,
Further including
The manufacturing method of the heat conductive sheet of any one of Claims 1-5. The step (II) and the step (III) are alternately repeated.
The manufacturing method of the heat conductive sheet of any one of Claims 1-6.   The manufacturing method of the heat conductive sheet of Claim 7 which changes a rolling direction when repeating the said process (III). The heat conductive sheet containing the polytetrafluoroethylene and heat conductive inorganic particle obtained by the method of any one of Claims 1-8.

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