JP2020076029A - Thermoconductive sheet - Google Patents

Abstract

To provide a thermoconductive sheet that prevents outgasing and has excellent handleability.SOLUTION: A thermoconductive sheet has graphite with an amount of oil absorption of 250 ml/100 g or less, and a resin. In the thermoconductive sheet, the graphite is oriented in the thickness direction, and the graphite has a volume fraction of 40 vol.% or more.SELECTED DRAWING: None

Description

  The present invention relates to a heat conductive sheet.

  2. Description of the Related Art In recent years, electronic components such as power semiconductors and integrated circuit (IC) chips have increased the amount of heat generation as their performance has increased. As a result, in an electronic device using an electronic component, it is necessary to take measures for a functional failure due to a temperature rise of the electronic component.

  As a measure against functional failure due to temperature rise of electronic parts, generally, a method of accelerating heat dissipation by attaching a heat sink such as a metal heat sink, heat sink, or heat dissipation fin to a heat generator such as an electronic part is adopted. ing. When using the heat radiator, a sheet-shaped member having heat conductivity (that is, a heat conductive sheet) is interposed between the heat generator and the heat radiator to bring the heat generator and the heat radiator into close contact with each other. Is commonly done.

  And patent document 1 discloses a heat conductive pressure-sensitive adhesive sheet having high heat conductivity and high strength. More specifically, in Patent Document 1, a heat-conductive pressure-sensitive adhesive sheet obtained by molding a composition containing a resin containing a (meth) acrylic acid ester polymer and expanded graphite into a sheet shape is provided. It is disclosed.

JP, 2011-26531, A

Here, in recent years, along with the expansion of applications of the heat conductive sheet, it is required to further enhance various attributes of the heat conductive sheet. More specifically, the heat conductive sheet is required to be less likely to generate outgas and to have an appropriate hardness and excellent handleability.
However, the above-mentioned conventional heat conductive sheet has room for improvement in that it is difficult to generate outgas and exhibits excellent handling properties.

  Therefore, it is an object of the present invention to provide a heat conductive sheet that does not easily generate outgas and is excellent in handleability.

  The present inventors have earnestly studied to achieve the above object. Then, the present inventors have newly found that the amount of outgas such as acid gas generated is reduced by adopting graphite having a relatively small oil absorption amount as graphite to be blended in the heat conductive sheet. When the amount of outgas generated from the heat conductive sheet is small, when the heat conductive sheet is mounted on an electronic device or the like, it is possible to suppress adverse effects on the electronic device or the like due to the outgas generated due to the use of the electronic device. .. However, as a result of the studies conducted by the present inventors, in the formation of the heat conductive sheet, when the graphite having a relatively small oil absorption is adopted, the heat conductive sheet obtained particularly when the graphite content is small. It has become clear that the hardness of the is easily lowered. Here, the hardness of the heat conductive sheet is excessively low, that is, when the heat conductive sheet is too soft, the heat conductive sheet is easily broken, and it becomes difficult to carry the heat conductive sheet. There is a possibility that the problem of. The handleability of the heat conductive sheet may be deteriorated even if the heat conductive sheet becomes sticky. Therefore, when forming a heat conductive sheet using graphite having an oil absorption amount equal to or less than a predetermined value, the present inventors set the volume fraction of graphite in the heat conductive sheet to 40% by volume or more to generate outgas. It was newly found that it is possible to provide a heat conductive sheet that is difficult to handle and is excellent in handleability, and completed the present invention.

  That is, the present invention is intended to advantageously solve the above problems, the thermal conductive sheet of the present invention is a thermal conductive sheet containing graphite and resin, the oil absorption of the graphite is 250ml / It is 100 g or less, the graphite is oriented in the thickness direction of the heat conductive sheet, and the volume fraction of the graphite in the heat conductive sheet is 40 vol% or more. If the oil absorption amount of graphite that is oriented in the thickness direction in the heat conductive sheet and is contained at a ratio such that the volume fraction is 40% by volume or more is 250 ml / 100 g or less, the heat conductive sheet is Outgassing is difficult to occur and handling is excellent. The oil absorption of graphite and the volume fraction of graphite in the heat conductive sheet can be measured by the methods described in Examples. Further, in the present specification, the “sheet” means an object having a shape in which main surfaces including a front surface and a back surface are arranged with a distance of thickness therebetween.

  In the heat conductive sheet of the present invention, it is preferable that the graphite has a volume average particle size of 40 μm or more and 260 μm or less. If the volume average particle diameter of graphite contained in the heat conductive sheet is at least the above lower limit, the thermal conductivity can be further enhanced, and if the volume average particle size is at most the above upper limit, the resulting heat conductive sheet will be sticky. Can be suppressed, and the handleability of the seat can be further enhanced. The volume average particle diameter of graphite can be measured by the method described in Examples.

  Further, in the heat conductive sheet of the present invention, it is preferable that the graphite is artificial graphite. If the heat conductive sheet contains artificial graphite as graphite, it is more difficult to generate outgas.

Further, in the heat conductive sheet of the present invention, it is preferable that the BET specific surface area of the graphite is 1 m ² / g or more and 30 m ² / g or less. When the BET specific surface area of graphite contained in the heat conductive sheet is 1 m ² / g or more and 30 m ² / g or less, the tensile strength of the heat conductive sheet can be increased. The BET specific surface area of graphite can be measured by the method described in Examples.

  Further, in the heat conductive sheet of the present invention, it is preferable that the resin is a thermoplastic fluororesin. If the resin contained in the heat conductive sheet is a thermoplastic fluororesin, the durability of the heat conductive sheet can be improved.

  ADVANTAGE OF THE INVENTION According to this invention, the heat conductive sheet which is hard to generate | occur | produce an outgas and is excellent in handleability can be provided.

Hereinafter, embodiments of the present invention will be described in detail.
Since the heat conductive sheet of the present invention has heat conductivity, it can be used by being sandwiched between a heat generating body and a heat radiating body. That is, the heat conductive sheet of the present invention can constitute a heat dissipation device together with a heat dissipation member such as a heat sink, a heat dissipation plate, and a heat dissipation fin as a heat dissipation member.

(Heat conduction sheet)
The heat conductive sheet of the present invention is a heat conductive sheet containing graphite and resin. In the heat conductive sheet of the present invention, the oil absorption of graphite is 250 ml / 100 g or less, the graphite is oriented in the thickness direction of the heat conductive sheet, and the volume fraction of graphite in the heat conductive sheet is 40. It is characterized by being at least volume%. The heat conductive sheet of the present invention may optionally contain other components than the above-mentioned predetermined contained components. The heat conductive sheet of the present invention contains, as a graphite material, graphite having an oil absorption of 250 ml / 100 g or less, so that outgas generation is small. For this reason, when the heat conductive sheet of the present invention is mounted on an electronic device or the like, it is difficult for the electronic device to be affected by outgas. Furthermore, when an acid gas such as H ₂ SO ₄ gas is generated as an outgas from the heated heat conductive sheet with the heat conductive sheet attached to the adherend, the surroundings of the adherend and the heat conductive sheet. There is a risk of corroding other parts and the like arranged in the. As described above, in the heat conductive sheet, it is important in various aspects that the outgas generation is small. Therefore, as a result of various studies by the present inventors, it was revealed that when graphite having an oil absorption amount of 250 ml / 100 g or less was used as the graphite material, the generation of outgas could be effectively suppressed. However, as a result of further study by the present inventors, graphite having an oil absorption of 250 ml / 100 g or less is used as a heat conductive sheet if the content ratio in the heat conductive sheet is small when used for forming the heat conductive sheet. It was revealed that sufficient hardness could not be imparted, and the resulting heat conductive sheet lacked in hardness and was inferior in handleability. Therefore, the present inventors set the volume fraction of graphite having an oil absorption of 250 ml / 100 g or less in the heat conductive sheet to 40% by volume or more, thereby suppressing the generation of outgas from the heat conductive sheet, We succeeded in securing appropriate handleability by giving appropriate hardness to the conductive sheet.

<Graphite>
In the heat conductive sheet of the present invention, graphite is oriented in the thickness direction of the sheet and forms a heat conduction network in the thickness direction of the sheet. Here, whether or not the graphite is in the state of being “oriented in the thickness direction of the sheet” can be confirmed by observing the cross section of the heat conductive sheet with a scanning electron microscope (SEM). .. If the graphite is oriented in the thickness direction of the heat conductive sheet, such a sheet exhibits heat conductivity at least in the thickness direction.

<< Oil absorption of graphite >>
The graphite contained in the heat conductive sheet of the present invention needs to have an oil absorption of 250 ml / 100 g or less. When the oil absorption of graphite contained in the heat conductive sheet is 250 ml / 100 g or less, the heat conductive sheet is not only difficult to generate outgas, but also can be given appropriate flexibility to the sheet. As a result, the adhesion between the heat conductive sheet and the adherends (for example, the heat generating body and the heat radiating body) sandwiching the heat conductive sheet can be enhanced, and the interface between the adherend and the heat conductive sheet can be increased. The thermal resistance (i.e., interfacial resistance) at 1 can be reduced. In the case where a graphite sheet having an oil absorption amount of more than 250 ml / 100 g and a graphite sheet having an oil absorption amount of 250 ml / 100 g or less are used to form a heat conductive sheet having the same volume fraction value in the heat conductive sheet, The hardness of the heat conductive sheet formed by using graphite having an amount of more than 250 ml / 100 g is higher than the hardness of the heat conductive sheet formed by using graphite having an oil absorption amount of 250 ml / 100 g or less. Further, the difference becomes remarkable when the volume fraction of graphite in the heat conductive sheet is increased. Therefore, the heat conductive sheet according to the present invention can prevent the hardness of the heat conductive sheet from becoming excessively high even when the volume fraction of graphite is high. Therefore, according to the present invention, it is possible to provide a heat conductive sheet having appropriate flexibility and hardness. The oil absorption of graphite can be measured according to a predetermined method for graphite as a material, as described in the examples. The value of the oil absorption amount for a certain graphite may be constant without substantially changing in various steps for manufacturing the heat conductive sheet.

  Hereinafter, the graphite having an oil absorption of 250 ml / 100 g or less contained in the heat conductive sheet of the present invention is also referred to as “graphite A”. Further, the oil absorption of graphite A is preferably 50 ml / 100 g or more, more preferably 80 ml / 100 g or more, and more preferably 100 ml / 100 g or more, from the viewpoint of further enhancing the handleability of the heat conductive sheet. More preferable. When the oil absorption of the graphite A is 50 ml / 100 g or more, the affinity between the graphite A and the resin in the heat conductive sheet is appropriately high, and it is possible to prevent the hardness of the heat conductive sheet from becoming excessively low. Because you can.

<< Volume of Graphite A >>
The volume fraction of graphite (graphite A) having an oil absorption of 250 ml / 100 g or less in the heat conductive sheet needs to be 40% by volume or more, preferably 80% by volume or less, and 60% by volume or less. More preferably. When the volume fraction of graphite A in the heat conductive sheet is 40% by volume or more, the heat conductive sheet with a sufficiently small amount of outgas has an appropriate hardness and is excellent in handleability. Further, when the volume fraction of graphite A in the heat conductive sheet is equal to or less than the above upper limit value, it is possible to prevent the hardness of the heat conductive sheet from becoming excessively high and the interface resistance from increasing.

<< Type of Graphite A >>
The graphite (graphite A) having an oil absorption amount of 250 ml / 100 g or less is not particularly limited, and artificial graphite and natural graphite such as flake graphite, exfoliated graphite, acid-treated graphite, expandable graphite, and expanded graphite are used. It may be graphite. The oil absorption is not particularly limited as long as the above conditions are satisfied, and a mixture of these may be used as the graphite A. Above all, it is preferable that the graphite A contained in the heat conductive sheet is made of artificial graphite from the viewpoints of further improving the handling property and durability of the heat conductive sheet, the homogenization of the heat conductive sheet, and the like. The artificial graphite can be obtained, for example, by heat-treating coke derived from petroleum or coal at a high temperature (for example, 1000 ° C. or higher).

<< Volume Average Particle Diameter of Graphite A >>
The volume average particle size of graphite (graphite A) having an oil absorption of 250 ml / 100 g or less is preferably 40 μm or more, more preferably 50 μm or more, preferably 260 μm or less, and more preferably 250 μm or less. When the volume average particle diameter of the graphite A contained in the heat conductive sheet is not less than the lower limit value described above, the handleability of the heat conductive sheet is improved by increasing the hardness of the heat conductive sheet, and the heat conductivity of the obtained heat conductive sheet is increased. The rate can be increased. Further, when the volume average particle diameter of the graphite A is not more than the above upper limit value, it is possible to suppress stickiness of the obtained heat conductive sheet and further improve the handleability of the sheet. Furthermore, if the volume average particle diameter of the graphite A is not more than the above upper limit value, the interface resistance can be reduced without excessively deteriorating the smoothness of the surface of the heat conductive sheet. The volume average particle diameter of graphite can be measured according to a predetermined method for graphite as a material, as described in Examples. The value of the volume average particle diameter of the graphite A having an oil absorption of 250 ml / 100 g or less does not substantially change in various steps for producing the heat conductive sheet and may be constant. This is because graphite A having an oil absorption of 250 ml / 100 g or less has a high compressive strength, and even when it is subjected to a series of steps for producing a heat conductive sheet and stirred and compressed. It is assumed that this is because the particle size does not substantially change.

<< BET specific surface area of graphite A >>
BET specific surface area of the graphite A is preferably at least ^{1 m} 2 / g, preferably ^{30 m} 2 / g or less, more preferably ^{15m 2 / g, 5m 2 /} g or less is more preferable. When the BET specific surface area of graphite A is within the above range, the tensile strength of the heat conductive sheet can be increased. More specifically, if the BET specific surface area of the graphite A is within the above range, the resin and the graphite A interact appropriately to increase the affinity and act to increase the ductility of the heat conductive sheet. The tensile strength can be increased. The BET specific surface area of graphite A can be measured according to a predetermined method for graphite as a material, as described in the examples. The value of the BET specific surface area of a certain graphite A does not substantially change in various steps for producing a heat conductive sheet and may be constant.

<< Aspect ratio of graphite A >>
The aspect ratio of the graphite A is preferably 10 or less, more preferably 6 or less, usually 1 or more, and preferably 1.5 or more. If the aspect ratio of the graphite A is equal to or less than the above upper limit value, the graphite A does not exert the reinforcing effect, so that the softness of the heat conductive sheet can be maintained. Further, when the aspect ratio of the graphite A is not less than the lower limit value described above, the thermal conductivity of the heat conductive sheet can be increased, and the tensile strength and hardness of the heat conductive sheet can be appropriately increased. The "aspect ratio" is a value obtained by dividing the length of the major axis of the particulate matter by the length of the minor axis orthogonal to the major axis, and specifically, measured by the method described in Examples. can do. The aspect ratio of graphite A can be measured according to a predetermined method for graphite A as a material, as described in the examples. The value of the aspect ratio of graphite A having an oil absorption of 250 ml / 100 g or less does not substantially change in various steps for producing the heat conductive sheet and may be constant. This is because graphite A having an oil absorption of 250 ml / 100 g or less has a high compressive strength, and even when it is subjected to a series of steps for producing a heat conductive sheet and stirred and compressed. It is assumed that this is because the shape does not substantially change.

<Resin>
The resin contained in the heat conductive sheet of the present invention functions as a matrix resin for the heat conductive sheet and also as a binder for binding graphite and the like in the heat conductive sheet. More specifically, the resin contained in the heat conductive sheet may be a thermoplastic resin. By including a thermoplastic resin in the heat conductive sheet, the flexibility of the heat conductive sheet is improved in a high temperature environment during use (when heat is dissipated), and the heat generating element and the heat radiating element are satisfactorily provided through the heat conductive sheet. Can be closely attached. Further, a thermosetting resin can be used in combination with the heat conductive sheet provided that the properties and effects of the heat conductive sheet of the present invention are not lost. In this specification, rubber and elastomer are included in the "resin".

  Examples of the thermoplastic resin include "a thermoplastic resin which is liquid at normal temperature and normal pressure" and "a thermoplastic resin which is solid at normal temperature and normal pressure". These may be used alone or in combination of two or more at an arbitrary ratio. The interface resistance between the adherend and the heat conductive sheet can be lowered, and the thermal conductivity of the heat conductive sheet can be further improved, so that at least "a thermoplastic resin which is liquid under normal temperature and normal pressure" is used. It is preferable to use. Furthermore, in addition to the effect of reducing the interfacial resistance, from the viewpoint of imparting hardness to the heat conductive sheet to enhance handling properties, in addition to "thermoplastic resin which is liquid under normal temperature and normal pressure", "solid heat under normal temperature and normal pressure" It is preferable to use "plastic resin". In the present specification, “normal temperature” refers to 23 ° C., and “normal pressure” refers to 1 atm (absolute pressure).

  When a liquid / solid thermoplastic resin is used together at room temperature and normal pressure, the content ratio of the liquid thermoplastic resin at room temperature and normal pressure is the same as that of the liquid thermoplastic resin at room temperature and normal pressure and the solid content at normal temperature and normal pressure. The total content of the thermoplastic resin is preferably 40% by mass or more, more preferably 60% by mass or more, preferably 90% by mass or less, and more preferably 75% by mass or less. . This is because if the content ratio of the liquid thermoplastic resin at room temperature and normal pressure is within the above range, the thermal conductivity of the thermal conductive sheet can be increased by increasing the flexibility of the thermal conductive sheet.

<< thermoplastic resin that is liquid under normal temperature and pressure >>
Examples of the thermoplastic resin that is liquid at room temperature and normal pressure include acrylic resin, epoxy resin, silicone resin, and fluororesin. These may be used alone or in combination of two or more at an arbitrary ratio.
Among these, from the viewpoint of improving the flame retardancy, heat resistance, oil resistance, and chemical resistance of the heat conductive sheet, and from the viewpoint of enhancing the durability of the heat conductive sheet, a liquid thermoplastic fluororesin at room temperature and normal pressure is used. preferable.

[Thermoplastic fluororesin that is liquid at normal temperature and pressure]
The thermoplastic fluororesin that is liquid at room temperature and atmospheric pressure is not particularly limited as long as it is a liquid thermoplastic fluororesin at room temperature and atmospheric pressure. Examples of the thermoplastic fluororesin that is liquid under normal temperature and pressure include, for example, vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-hexafluoropentene-tetrafluoroethylene terpolymer, perfluoropropene oxide polymer, And tetrafluoroethylene-propylene-vinylidene fluoride copolymer. These may be used alone or in combination of two or more at an arbitrary ratio.
Examples of commercially available thermoplastic fluororesins that are liquid at room temperature and normal pressure include Viton (registered trademark) LM manufactured by DuPont Co., Ltd., Daier (registered trademark) G-101 manufactured by Daikin Industries, Ltd., and 3M. Examples include Dyneon FC2210 manufactured by Co., Ltd. and SIFEL series manufactured by Shin-Etsu Chemical Co., Ltd.

<< thermoplastic resin solid at room temperature and pressure >>
Examples of the solid thermoplastic resin at room temperature and normal pressure include poly (2-ethylhexyl acrylate), copolymers of acrylic acid and 2-ethylhexyl acrylate, polymethacrylic acid or its ester, polyacrylic acid or its ester. Acrylic resin such as; Silicone resin; Fluorine resin; Polyethylene; Polypropylene; Ethylene-propylene copolymer; Polymethylpentene; Polyvinyl chloride; Polyvinylidene chloride; Polyvinyl acetate; Ethylene-vinyl acetate copolymer; Polyvinyl alcohol; Polyacetal Polyethylene terephthalate; Polybutylene terephthalate; Polyethylene naphthalate; Polystyrene; Polyacrylonitrile; Styrene-acrylonitrile copolymer; Acrylonitrile-butadiene-styrene copolymer (ABS resin); Styrene-butadiene block copolymer or hydrogenated product thereof; Styrene-isoprene block copolymer or hydrogenated product thereof; polyphenylene ether; modified polyphenylene ether; aliphatic polyamides; aromatic polyamides; polyamideimide; polycarbonate; polyphenylene sulfide; polysulfone; polyethersulfone; polyethernitrile; poly Ether ketone; polyketone; polyurethane; liquid crystal polymer; ionomer; and the like. These may be used individually by 1 type, and may be used together 2 or more types by arbitrary ratios.
Among these, from the viewpoint of improving flame retardancy, heat resistance, oil resistance, chemical resistance, etc. of the heat conductive sheet, and from the viewpoint of enhancing the durability of the heat conductive sheet, a solid thermoplastic fluororesin under normal temperature and normal pressure. Is preferred.

[Thermoplastic fluororesin that is solid under normal temperature and pressure]
The thermoplastic fluororesin that is solid under normal temperature and normal pressure is not particularly limited as long as it is a solid thermoplastic fluororesin under normal temperature and normal pressure. As the solid thermoplastic fluororesin at room temperature and atmospheric pressure, for example, vinylidene fluoride-based fluororesin, tetrafluoroethylene-propylene-based fluororesin, tetrafluoroethylene-purple orovinyl ether-based fluororesin, etc. are polymerized with a fluorine-containing monomer. The resulting elastomer may be mentioned. More specifically, polytetrafluoroethylene, tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer, tetrafluoroethylene-hexafluoropropylene copolymer, tetrafluoroethylene-ethylene copolymer, polyvinylidene fluoride, polychloro. Trifluoroethylene, ethylene-chlorofluoroethylene copolymer, tetrafluoroethylene-perfluorodioxole copolymer, polyvinyl fluoride, tetrafluoroethylene-propylene copolymer, vinylidene fluoride-hexafluoropropylene copolymer, Vinylidene fluoride-tetrafluoroethylene-hexafluoropropylene copolymer, acrylic modified product of polytetrafluoroethylene, ester modified product of polytetrafluoroethylene, epoxy modified product of polytetrafluoroethylene and silane modified product of polytetrafluoroethylene , And so on. These may be used alone or in combination of two or more at an arbitrary ratio.
Among these, from the viewpoint of processability, polytetrafluoroethylene, tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer, vinylidene fluoride-tetrafluoroethylene-hexafluoropropylene copolymer, acrylic modified product of polytetrafluoroethylene. , Are preferred.

  Examples of commercially available thermoplastic fluororesin that is solid at room temperature and atmospheric pressure include Daiel (registered trademark) G-912, G-700 series, and Daiel G-550 series / G- manufactured by Daikin Industries, Ltd. 600 series, DAI-EL G-310; KYNAR (registered trademark) series, KYNAR FLEX (registered trademark) series manufactured by ALKEMA; Dyneon FC2211, FPO3600ULV manufactured by 3M, and the like.

<< thermosetting resin >>
Thermosetting resins that can be optionally used in the heat conductive sheet are, for example, natural rubber; butadiene rubber; isoprene rubber; nitrile rubber; hydrogenated, provided that the properties and effects of the heat conductive sheet of the present invention are not lost. Nitrile rubber; chloroprene rubber; ethylene propylene rubber; chlorinated polyethylene; chlorosulfonated polyethylene; butyl rubber; halogenated butyl rubber; polyisobutylene rubber; epoxy resin; polyimide resin; bismaleimide resin; benzocyclobutene resin; phenol resin; unsaturated polyester Diallyl phthalate resin; polyimide silicone resin; polyurethane; thermosetting polyphenylene ether; thermosetting modified polyphenylene ether; and the like. These may be used alone or in combination of two or more at an arbitrary ratio.

<Other ingredients>
The heat conductive sheet of the present invention may contain components other than the above-mentioned various contained components, provided that the properties and effects of the heat conductive sheet of the present invention are not lost. Such other components are not particularly limited, and include fibrous carbon materials such as carbon nanotubes; inorganic nitride materials; flame retardants such as red phosphorus flame retardants and phosphoric acid ester flame retardants; fatty acid ester plasticizers. Plasticizers such as urethane acrylates; toughness improving agents such as urethane acrylates; moisture absorbing agents such as calcium oxide and magnesium oxide; silane coupling agents, titanium coupling agents, and adhesion improvers such as acid anhydrides; nonionic surfactants and And a wetting improver such as a fluorinated surfactant; and an ion trapping agent such as an inorganic ion exchanger.

<Tensile strength of heat conductive sheet>
The tensile strength of the heat conductive sheet is preferably 0.5 MPa or more, and usually 1.5 MPa or less. When the tensile strength of the heat conductive sheet is 0.5 MPa or more, it is possible to prevent the heat conductive sheet from being easily broken when the heat conductive sheet is sandwiched between adherends (for example, a heat generating body and a heat radiating body). be able to.

<Thermal conductivity of the heat conductive sheet>
Moreover, as described above, in the heat conductive sheet of the present invention, the graphite A is oriented in the thickness direction of the heat conductive sheet. Therefore, the heat conductive sheet of the present invention exhibits heat conductivity at least in the thickness direction of the heat conductive sheet. The thermal conductivity in the thickness direction of the heat conductive sheet can be measured according to the method described in the examples. Here, the thermal conductivity in the thickness direction of the heat conductive sheet is preferably 1.5 W / m · K or more. If the thermal conductivity of the heat conductive sheet in the thickness direction is 1.5 W / m · K or more, it can be sufficiently mounted as a heat dissipation member.

<Specific gravity and thickness of heat conductive sheet>
The specific gravity of the heat conductive sheet is not particularly limited, and can take various values depending on the type of resin mixed in the heat conductive sheet, the amount of graphite mixed, and the like. For example, in the case of using a thermoplastic fluororesin as the resin has a specific gravity of the resulting thermally conductive sheet may be 1.20 g / cm ³ or more 2.00 g / cm ³ or less. If the specific gravity of the heat conductive sheet is within such a range, it can be suitably used for various purposes. The specific gravity of the heat conductive sheet can be measured by the method described in Examples. The thickness of the heat conductive sheet is not particularly limited and may be 50 μm or more and 500 μm or less. If the thickness of the heat-conducting sheet is less than or equal to the above upper limit, when it becomes necessary to interpose the heat-conducting sheet between the heat-generating body and the heat-dissipating body, heat generated between the heat-generating body and the heat-dissipating body via the heat-conducting sheet. It can be moved easily. When the thickness of the heat conductive sheet is at least the above lower limit, the strength of the heat conductive sheet can be increased.

<Method of forming heat conductive sheet>
The heat conductive sheet of the present invention is prepared by a heat conductive sheet forming method including, for example, (i) pre-heat conductive sheet forming step, (ii) laminate forming step, (iii) slicing step, and the like, which will be described in detail below. To be done.

<< (i) Pre-heat conductive sheet forming step >>
In the pre-heat conductive sheet forming step, a composition containing a resin and graphite A having an oil absorption of 250 ml / 100 g or less and further containing any other component is pressed to form a sheet to obtain a pre-heat conductive sheet. ..

〔Composition〕
Here, the composition can be prepared by mixing the resin, graphite A, and any other component. As the resin, graphite A, and any other component, the above-mentioned various components can be preferably used. In other words, it is preferable that the graphite A used in the pre-heat conductive sheet forming step is artificial graphite having a volume average particle diameter of 40 μm or more and 260 μm or less and a BET specific surface area of 1 m ² / g or more and 30 m ² / g or less. .. Further, the resin used in this step is preferably a thermoplastic fluororesin.

  The mixing of the above-mentioned components is not particularly limited, and can be performed using a known mixing device such as a kneader, roll, or mixer. Further, the mixing may be performed in the presence of a solvent such as an organic solvent. The mixing time can be, for example, 5 minutes or more and 60 minutes or less. Further, the mixing temperature can be set to, for example, 5 ° C or higher and 150 ° C or lower.

  The composition prepared as described above may be defoamed and crushed, and then pressed (primary pressure) to be molded into a sheet. Then, in the pre-heat conductive sheet formed by pressing the composition into a sheet shape, it is presumed that the graphite is mainly arranged in the in-plane direction, and particularly the thermal conductivity in the in-plane direction is improved. The thickness of the pre-heat conductive sheet is not particularly limited and can be, for example, 0.05 mm or more and 2 mm or less.

The Asker C hardness of the pre-heat conductive sheet measured by the method described in the examples (hereinafter, simply referred to as “Asker C hardness of pre-heat conductive sheet”) is preferably 20 or more, and 25 or more. Is more preferable, 75 or less is preferable, 70 or less is more preferable, and 60 or less is further preferable. When the Asker C hardness of the pre-heat conductive sheet is at least the above lower limit value, the obtained heat conductive sheet is not excessively soft and is excellent in handleability. Further, when the Asker C hardness of the pre-heat conductive sheet is not more than the above upper limit value, the heat conductive sheet has appropriate flexibility, and the adhesion between the heat conductive sheet and the adherend is improved. Therefore, the interface resistance can be reduced.
The hardness of the pre-heat conductive sheet is increased when graphite having a high oil absorption amount is used as a material and when a large amount of graphite is used.

<< (ii) Laminated body forming step >>
In the laminate forming step, a plurality of pre-heat conductive sheets obtained in the pre-heat conductive sheet forming step are laminated in the thickness direction, or the pre-heat conductive sheet is folded or wound to obtain a laminate. Here, it is preferable that the laminated body in which the pre-heat conductive sheets are laminated is further hot pressed (secondary pressure) in the laminating direction. The conditions of the secondary pressurization are not particularly limited, and the pressure in the stacking direction is 0.05 MPa or more and 0.5 MPa or less, and the temperature can be 80 ° C. or more and 170 ° C. or less for 10 seconds to 30 minutes. Then, in the laminated body obtained by laminating, folding or winding the pre-heat conductive sheets, it is presumed that graphite is arranged in a direction substantially orthogonal to the laminating direction. Therefore, in such a laminate, it is presumed that the thermal conductivity in the direction substantially orthogonal to the laminating direction is higher than the thermal conductivity in the laminating direction.

<< (iii) Slicing process >>
In the slicing step, the laminated body obtained in the laminated body forming step is sliced at an angle of 45 ° or less with respect to the laminating direction to obtain a heat conductive sheet made of sliced pieces of the laminated body. Here, the method for slicing the laminate is not particularly limited, and examples thereof include a multi-blade method, a laser processing method, a water jet method, and a knife processing method. Among them, the knife processing method is preferable because it is easy to make the thickness of the heat conductive sheet uniform. The cutting tool for slicing the laminate is not particularly limited, and has, for example, a fixture such as a metal plate for pressing and fixing the laminate in the stacking direction, and a double-edged cutting blade. It is possible to use a slicer that includes a slicing member and that slices the laminated body by moving a cutting blade while pressing the laminated body with a fixture.

  From the viewpoint of enhancing the thermal conductivity of the heat conductive sheet, the angle at which the laminate is sliced is preferably about 0 ° with respect to the laminating direction (that is, the direction along the laminating direction). The heat conductive sheet made of sliced pieces of the laminate thus obtained exhibits heat conductivity in the thickness direction. More specifically, in the heat conductive sheet obtained through each of the above steps, the strip containing resin and graphite is in a direction substantially perpendicular to the thickness direction of the heat conductive sheet (the angle with respect to the thickness direction is approximately 90). (Direction of °) has a structure in which they are joined in parallel. In such a heat conductive sheet, graphite or the like is oriented in the thickness direction of the heat conductive sheet in each strip, so that the heat conductivity is excellent.

  Hereinafter, the present invention will be specifically described 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 Examples and Comparative Examples, the oil absorption of graphite, the volume average particle diameter of graphite, the specific surface area of graphite, the aspect ratio of graphite, the volume fraction of graphite in the heat conductive sheet, the Asker C hardness of the heat conductive sheet, and the heat conductive sheet. The tensile strength, the specific gravity of the heat conductive sheet, the thermal conductivity in the thickness direction of the heat conductive sheet, and the amount of outgas generated were measured or evaluated as follows.

<Oil absorption of graphite>
The oil absorption of each type of graphite used in the examples and comparative examples was measured according to JIS K 5101-13-1: 2004.
<Volume average particle diameter of graphite>
The volume average particle diameter of each type of graphite used in Examples and Comparative Examples is measured by a laser diffraction method using a laser diffraction / scattering particle size distribution measuring device (manufactured by Horiba, Ltd., model “LA-960”). In the obtained particle diameter distribution, the particle diameter (D50) was calculated when the cumulative volume calculated from the small diameter side was 50%. The volume average particle diameter of graphite was the same as the value when measured for graphite as a material and the value for graphite in the state of being contained in the heat conductive sheet.
<Specific surface area of graphite>
The various graphites used in Examples and Comparative Examples were subjected to vacuum deaeration treatment at 120 ° C. for 8 hours, and then an automatic specific surface area / pore distribution measuring device “BELSORP-miniII” (manufactured by Microtrac Bell) was used. Nitrogen adsorption isotherm was measured by the constant volume method gas adsorption method under the condition of liquid nitrogen temperature (−196 ° C.). A BET plot is prepared from the obtained adsorption isotherm, and two points having the highest correlation coefficient within the range of relative pressure of 0.05 to relative pressure of 0.30 are selected, and the BET specific surface area (m ² / g ) Was asked.
<Aspect ratio of graphite>
Aspect ratios of various autographites used in Examples and Comparative Examples were calculated as follows. Using SEM, the maximum diameter (major axis) and the particle diameter (minor axis) in a direction orthogonal to the maximum diameter are measured for 50 randomly selected graphites, and the ratio of major axis and minor axis (major axis / minor axis) is measured. The number average value of (diameter) was calculated. Note that the aspect ratio of graphite was the same as the value when measured for graphite as a material and the value for graphite in the state of being contained in the heat conductive sheet.

<Volume fraction of graphite in heat conductive sheet>
The volume fraction of graphite in the heat conductive sheet was calculated using the addition amount and specific gravity of graphite, and the addition amount and specific gravity of each resin.

<Asker C hardness of pre-heat conductive sheet>
Based on the Asker C method of the Japan Rubber Association Standard (SRIS), using a hardness meter (manufactured by Kobunshi Keiki Co., Ltd., trade name "ASKER CL-150LJ", at a temperature of 25 ° C, a pre-heat conductive sheet Asker C hardness was measured.
First, each of the pre-heat conductive sheets obtained in Examples and Comparative Examples was cut into a size of width 25 mm × length 50 mm × thickness 0.5 mm, and 24 sheets were stacked to obtain a test piece. The obtained test piece was allowed to stand in a thermostatic chamber kept at a temperature of 25 ° C. for 48 hours or more to obtain a pre-heat conductive sheet laminate as a test piece. Next, the height of the damper was adjusted so that the pointer was 95 to 98, and the pre-heat conductive sheet laminate was made to collide with the damper. The Asker C hardness of the pre-heat conductive sheet laminate 60 seconds after the collision was measured twice using a hardness meter (manufactured by Kobunshi Keiki Co., Ltd., trade name "ASKER CL-150LJ"), and the average value of the measurement results was measured. Was calculated. The smaller Asker C hardness indicates that the pre-heat conductive sheet is more flexible and flexible, and the larger Asker C hardness indicates that the pre-heat conductive sheet is harder. If the Asker C hardness of the pre-heat conductive sheet is appropriately high, the resulting heat conductive sheet will be excellent in handleability.

<Tensile strength of heat conductive sheet>
The heat conductive sheets obtained in Examples and Comparative Examples were punched out in a size of 20 mm × 50 mm to obtain test pieces. At the time of punching, the longitudinal direction of the test piece was made to coincide with the following X direction. About the obtained test piece, using a small bench tester ("FGS-500TV" manufactured by Nidec Shinpo Co., Ltd., FGP-50 is used as a digital force gauge), a pulling speed was set to 20 mm / min in the X direction. The tensile test was performed. The distance between chucks was 30 mm. The sheet strength (N / mm) of the heat conductive sheet was calculated by dividing the maximum strength (N) in the tensile test by the thickness (mm) of the test body. The X direction was the direction perpendicular to the stacking direction of the laminate of the pre-heat conductive sheets on the main surface of the heat conductive sheet. The X direction may be a direction generally corresponding to the direction in which the thermal conductivity is highest in the main surface of the heat conductive sheet.

<Specific gravity of heat conduction sheet>
The specific gravity (density) (g / m ³ ) of the heat conductive sheets obtained in Examples and Comparative Examples was measured using an automatic densitometer (trade name “DENSIMETER-H” manufactured by Toyo Seiki Co., Ltd.).

<Thermal conductivity in the thickness direction of the heat conductive sheet>
The thermal diffusivity α (m ² / s) and the constant pressure specific heat Cp (J / g · K) of the heat conductive sheets obtained in Examples and Comparative Examples were measured by the following methods. Further, the value of specific gravity of each sheet measured according to the above: ρ (g / m ³ ) was also used.
[Thermal diffusivity α (m ² / s)]
The thermal diffusivity was measured using a thermophysical property measuring device (product name “Thermo Wave Analyzer TA35” manufactured by Bethel Co., Ltd.).
[Constant pressure specific heat Cp (J / gK)]
Using a differential scanning calorimeter (manufactured by Rigaku, product name "DSC8230"), specific heat was measured under a temperature rising condition of 10 ° C / min.
Then, each measured value is represented by the following formula (I):
λ = α × Cp × ρ (I)
And the thermal conductivity λ (W / m · K) in the thickness direction of the heat conductive sheet was determined.

<Outgassing amount>
From the heat conductive sheets obtained in Examples and Comparative Examples, 1.5 g each was sampled to obtain a measurement sample. For each measurement sample, air (standard atmosphere according to JIS W 0201: 1990) was heated at 175 ° C. for 2 hours while supplying air at a flow rate of 100 mL / min, and the outgas released from the heat conductive sheet had a concentration of 0.01. % Hydrogen peroxide solution (20 ml) to collect the solution. Further resulting captured solution, ion chromatography (Dionex Corp., DX-320) was analyzed using, SO ₄ ^2- perform quantification of the components, SO unit mass (g) per sheet ₄ ^2- The component amounts were calculated and evaluated according to the following criteria. In addition, IonPac AS4A-SC (size is 4 mm in diameter × 250 mm in length) was used as a column, and the injection amount of the collection liquid was 0.5 ml.
A: The amount of SO ₄ ^2- component is 3 μg / g or less. B: The amount of SO ₄ ^2- component is more than 3 μg / g.

(Example 1)
<Pre-heat conductive sheet molding process>
-Preparation of composition 70 parts of a thermoplastic fluororesin (Daikin Industry Co., Ltd., trade name "Dayer G-101") which is liquid at room temperature and atmospheric pressure as a resin, and a thermoplastic fluororesin which is solid at room temperature and atmospheric pressure ( 30 parts manufactured by 3M Japan Co., Ltd., trade name "Dynion FC2211", and artificial graphite as graphite (graphite A) having an oil absorption of 250 ml / 100 g or less (manufactured by Oriental Sangyo Co., Ltd., trade name "AT-No. 5"). 90 parts of oil absorption: 79 ml / 100 g, volume average particle diameter: 50 μm, aspect ratio: 5) were stirred and mixed at a temperature of 150 ° C. for 20 minutes using a pressure kneader (manufactured by Nippon Spindle). Next, the obtained mixture was put into a crusher (trade name “Wonder Crush Mill D3V-10” manufactured by Osaka Chemical Co., Ltd.) and crushed for 10 seconds to obtain a composition.
-Molding of pre-heat conductive sheet Next, 50 g of the obtained composition is sandwiched by a sandblasted PET film (protective film) having a thickness of 50 μm, a roll gap of 550 μm, a roll temperature of 50 ° C., a roll linear pressure of 50 kg / cm, Roll forming (primary pressure) was performed under a roll speed of 1 m / min to obtain a pre-heat conductive sheet having a thickness of 0.5 mm.
<Laminate formation process>
Subsequently, the obtained pre-heat conductive sheet was cut into a length of 50 mm, a width of 50 mm and a thickness of 0.5 mm, and 110 sheets were laminated in the thickness direction of the pre-heat conductive sheet to obtain a laminate having a height of about 55 mm. Further, by pressing (secondary pressing) in the stacking direction at a temperature of 80 ° C. and a pressure of 0.2 MPa for 1 minute, a stack having a height of about 50 mm was obtained.
<Slicing process>
Then, the entire upper surface of the obtained laminated body was pressed with a metal plate, leaving a necessary length for slicing, and a pressure of 0.1 MPa was applied in the laminating direction (that is, from above) to fix the laminated body. .. The side surface and back surface of the laminate were not fixed. At this time, the temperature of the laminated body was 25 ° C.
Next, a cutting blade (double-edged blade, blade angle 2θ: 20 °, maximum blade thickness: 3.5 mm, material: super steel, Rockwell hardness: 91) was applied to the press portion of a servo press machine (manufactured by Electric Discharge Precision Machining Laboratory). .5, silicon processing of the blade surface: none, total length: 200 mm was attached, and the stacking direction of the stack body (in other words, the main surface of the stacked pre-heat conductive sheet) under the conditions of a slicing speed of 200 mm / sec and a slice width of 100 μm. (In the direction corresponding to the normal line of 1) to obtain a heat conductive sheet having a length of 50 mm, a width of 50 mm and a thickness of 0.10 mm. Such a heat conductive sheet had a structure in which strips containing resin and graphite (pre-heat conductive sheets) were joined in parallel. Further, in the heat conductive sheet obtained according to this example, it was confirmed by SEM observation that graphite was oriented in the thickness direction of the heat conductive sheet.
And about the obtained heat conductive sheet, the above-mentioned measurement and evaluation were performed according to the above-mentioned method. The results are shown in Table 1.

(Example 2)
Each step, measurement, and evaluation were performed in the same manner as in Example 1 except that the amount of artificial graphite added was 180 parts. The results are shown in Table 1.

(Example 3)
Artificial graphite to be added as graphite A is made into artificial graphite (“4010S” manufactured by Oriental Sangyo Co., Ltd., oil absorption: 248 ml / 100 g, volume average particle diameter: 250 μm, aspect ratio: 2) satisfying various attributes shown in Table 1. changed. Except for this point, each step, measurement, and evaluation were performed in the same manner as in Example 1. The results are shown in Table 1.

(Example 4)
Each step, measurement, and evaluation were performed in the same manner as in Example 3 except that the amount of artificial graphite added was 160 parts. The results are shown in Table 1.

(Example 5)
Artificial graphite to be added as graphite A, artificial graphite satisfying various attributes shown in Table 1 (manufactured by Toyo Tanso Co., Ltd., trade name “TEG-200”, oil absorption: 80 ml / 100 g, volume average particle diameter: 70 μm, aspect ratio Changed to 1). Other than that was performed like Example 1.

(Example 6)
Each step, measurement, and evaluation were performed in the same manner as in Example 5 except that the amount of artificial graphite added was 130 parts. The results are shown in Table 1.

(Example 7)
Each step, measurement, and evaluation were performed in the same manner as in Example 5 except that the amount of artificial graphite added was 180 parts. The results are shown in Table 1.

(Comparative Example 1)
Using 100 parts of an acrylic resin prepared as described below as a resin, instead of graphite A, expanded graphite (manufactured by Ito Graphite Industry Co., Ltd., trade name "EC-50", oil absorption: 310 ml / 100 g, volume average) Particle diameter: 250 μm, aspect ratio: 2) 355 parts by mass were used. Except for these points, each step, measurement, and evaluation were performed in the same manner as in Example 1. The results are shown in Table 1.
<Preparation of acrylic resin>
100 parts of a monomer mixture consisting of 94 parts of 2-ethylhexyl acrylate and 6 parts of acrylic acid, 0.03 part of 2,2′-azobisisobutyronitrile, and 700 parts of ethyl acetate were placed in a reactor. After being uniformly dissolved and the atmosphere in the vessel was replaced with nitrogen, a polymerization reaction was carried out at 80 ° C. for 6 hours. The polymerization conversion rate was 97%. Then, the obtained polymer was dried under reduced pressure and ethyl acetate was evaporated to obtain a viscous solid acrylic resin. The weight average molecular weight (Mw) of the acrylic resin was 270000, and the molecular weight distribution (weight average molecular weight (Mw) / number average molecular weight (Mn)) was 3.1. The weight average molecular weight (Mw) and the number average molecular weight (Mn) were calculated in terms of standard polystyrene by gel permeation chromatography using tetrahydrofuran as an eluent.

(Comparative example 2)
As the resin, the solid thermoplastic fluororesin at room temperature and normal pressure was not compounded, but the compounding amount of the liquid thermoplastic fluororesin at room temperature and normal pressure was 100 parts. Further, instead of the graphite A, 50 parts of expanded graphite (trade name “EC-300” manufactured by Ito Graphite Industry Co., Ltd., oil absorption: 303 ml / 100 g, volume average particle diameter: 50 μm, aspect ratio: 2) was blended. did. Except for these points, each step, measurement, and evaluation were performed in the same manner as in Example 1. The results are shown in Table 1.

(Comparative example 3)
Instead of graphite A, 90 parts of expanded graphite (Ito Graphite Industry Co., Ltd., trade name “EC-300”, oil absorption: 303 ml / 100 g, volume average particle diameter: 50 μm, aspect ratio: 2) was blended. In the same manner as in Example 1, each step, measurement, and evaluation were performed. The results are shown in Table 1.

(Comparative example 4)
Example 1 except that 30 parts of expanded graphite (trade name “EC-300” manufactured by Ito Graphite Industry Co., Ltd., oil absorption: 303 ml / 100 g, volume average particle diameter: 50 μm) was mixed in place of graphite A Each step, measurement, and evaluation were performed in the same manner as in. The results are shown in Table 1.

(Comparative example 5)
In preparing the composition, each step, measurement, and measurement were performed in the same manner as in Example 1 except that the blending amount of artificial graphite was decreased so that the volume fraction of graphite in the heat conductive sheet was 28% by volume. An evaluation was carried out. The results are shown in Table 1.

(Comparative example 6)
Instead of graphite A, 15 parts of expanded graphite (trade name “EC-300” manufactured by Ito Graphite Industry Co., Ltd., oil absorption: 303 ml / 100 g, volume average particle diameter: 50 μm, aspect ratio: 2) was blended. In the same manner as in Example 1, each step, measurement, and evaluation were performed. The results are shown in Table 1.

From Table 1, it can be seen that the heat conductive sheets according to Examples 1 to 7 containing 40% by volume or more of graphite having an oil absorption of 250 ml / 100 g or less had little outgas and also had handleability. From the results of measuring the thermal conductivity of the heat conductive sheets according to Examples 1 to 7, it can be seen that these sheets exhibited thermal conductivity in the thickness direction.
On the other hand, the heat conductive sheets according to Comparative Examples 1 to 4 and 6 in which the graphite having an oil absorption amount of more than 250 ml / 100 g is mixed, have a large amount of outgas even if they have handleability, and outgas is hardly generated and handled. It can be seen that the heat conductive sheet did not have excellent properties. Also, it can be seen that the heat conductive sheet according to Comparative Example 5 in which the content ratio of the graphite satisfying the predetermined oil absorption amount is less than 40% by volume was inferior in handleability although outgas was not generated.

  ADVANTAGE OF THE INVENTION According to this invention, the heat conductive sheet which is hard to generate | occur | produce an outgas and is excellent in handleability can be provided.

Claims (5)

A heat conductive sheet containing graphite and resin,
The oil absorption of the graphite is 250 ml / 100 g or less,
The graphite is oriented in the thickness direction of the heat conductive sheet, and,
The volume fraction of the graphite in the heat conductive sheet is 40% by volume or more,
Thermal conductive sheet.   The heat conductive sheet according to claim 1, wherein the graphite has a volume average particle size of 40 μm or more and 260 μm or less.   The heat conductive sheet according to claim 1, wherein the graphite is artificial graphite. The heat conductive sheet according to claim 1, wherein the BET specific surface area of the graphite is 1 m ² / g or more and 30 m ² / g or less.   The heat conductive sheet according to claim 1, wherein the resin is a thermoplastic fluororesin.