JP2018172681A - Thermally conductive foam sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a thermally conductive foam sheet having high thermal conductivity and a good surface state.SOLUTION: A thermally conductive foam sheet contains an elastomer resin (A), a thermally conductive filler (B), and an olefinic low-molecular-weight resin (C) with a weight average molecular weight of 900-50,000.SELECTED DRAWING: None

Description

  The present invention relates to a thermally conductive foam sheet, for example, relates to a thermally conductive foam sheet used inside an electronic device for radiating heat to the outside.

In the vicinity of display devices used in various electronic devices such as personal computers, mobile phones, and electronic papers, and other electronic components, shock absorbers for absorbing shock and vibration, to fill gaps to be dustproof, waterproof, etc. The sealing material is used. In addition, in electronic devices, integrated electronic components generate heat and may cause failure. Therefore, shock absorbers and seal materials are required to have heat dissipation performance to suppress heat generation of electronic devices. is there.
Conventionally, as a foam having both heat dissipation and shock absorption or sealing properties, for example, as disclosed in Patent Document 1, thermal conductivity obtained by foaming an elastomer containing a thermally conductive filler such as aluminum oxide is used. Foam is known.

JP 2013-231166 A

  However, in order to improve heat dissipation, when a thermally conductive filler is blended and foamed in the presence of the thermally conductive filler, abnormal bubbles are generated on the surface of the foam, and the foam is poor in surface condition. The tendency to become a body was confirmed.

  The present invention has been made in view of the above circumstances, and an object thereof is to provide a thermally conductive foam sheet having a high thermal conductivity and a good surface state.

As a result of intensive studies, the present inventors can solve the above-mentioned problems by including a olefin-based low molecular weight resin having a specific weight average molecular weight in a heat conductive foam containing an elastomer resin and a heat conductive filler. The following invention was completed.
That is, the present invention provides the following [1] to [8].
[1] A thermally conductive foam sheet containing an elastomer resin (A), a thermally conductive filler (B), and an olefinic low molecular weight resin (C) having a weight average molecular weight of 900 to 50,000.
[2] The thermally conductive foam sheet according to [1], including 100 to 500 parts by mass of the thermally conductive filler (B) with respect to 100 parts by mass of the elastomer resin (A).
[3] The thermally conductive foam sheet according to [1] or [2], wherein the thermally conductive filler (B) contains a plate-like filler (b1).
[4] The thermally conductive foam sheet according to [3], wherein the thermally conductive filler (B) further contains a spherical filler (b2).
[5] The thermally conductive foam sheet according to any one of [1] to [4], wherein the olefinic low molecular weight resin (C) is polypropylene.
[6] The thermally conductive foam sheet according to any one of [1] to [5], wherein the elastomer resin (A) is ethylene propylene diene rubber.
[7] The thermally conductive foam sheet according to any one of [1] to [6], wherein the elastomer resin (A) is a mixture of solid ethylene propylene diene rubber and liquid ethylene propylene diene rubber. .
[8] The thermally conductive foam sheet according to any one of [1] to [7], wherein the apparent density is 0.5 to 2.0 g / cm ³ .

  According to the present invention, it is possible to provide a thermally conductive foam sheet having a high thermal conductivity and a good surface state.

The thermally conductive foam sheet of the present invention contains an elastomer resin (A), a thermally conductive filler (B), and an olefinic low molecular weight resin (C) having a weight average molecular weight of 900 to 50,000. The reason why the surface state of the thermally conductive foam sheet of the present invention is good is not clear, but the olefinic low molecular weight resin (C) present in the foam sheet functions as an internal lubricant, and an elastomer resin (A ) And the thermally conductive filler (B) are improved to suppress the generation of abnormal bubbles during foaming.
Hereinafter, the present invention will be described in detail.

[Elastomer resin (A)]
The thermally conductive foam sheet of the present invention contains an elastomer resin (A). The elastomer resin (A) is not particularly limited, but acrylonitrile butadiene rubber, ethylene propylene diene rubber, ethylene propylene rubber, natural rubber, polybutadiene rubber, polyisoprene rubber, styrene-butadiene block copolymer, hydrogenated styrene-butadiene block. Examples of the copolymer include a hydrogenated styrene-butadiene-styrene block copolymer, a hydrogenated styrene-isoprene block copolymer, and a hydrogenated styrene-isoprene-styrene block copolymer. Among these, acrylonitrile butadiene rubber and ethylene propylene diene rubber are preferable, and ethylene propylene diene rubber is more preferable in order to ensure high flexibility even when a relatively large amount of the heat conductive filler (B) is blended. These elastomer resins (A) may be used alone or in combination of two or more.

The elastomer resin (A) may be a liquid elastomer that becomes liquid at room temperature (23 ° C.) and normal pressure (1 atm), or may be a solid or a mixture thereof. Although it may be present, it is preferred to use a mixture. From the viewpoint of ensuring high flexibility and a favorable surface condition, the mixture is preferably a mixture of the same kind of solid elastomer and liquid elastomer, and solid ethylene propylene diene rubber and liquid It is more preferable to use a mixture of ethylene propylene diene rubber.
From the viewpoint of improving the surface state of the thermally conductive foam sheet, it is preferable that the content of the solid elastomer is larger than the content of the liquid elastomer. The content of the solid elastomer is preferably 40 to 70% by mass and the liquid elastomer is preferably 30 to 60% by mass based on the total amount of the elastomer resin. The solid elastomer content is preferably 50 to 65 mass%, and the liquid elastomer content is more preferably 35 to 50 mass%. It is more preferable that the content of the solid elastomer is 55 to 65% by mass and the content of the liquid elastomer is 35 to 45% by mass.

The Mooney viscosity (ML _{1 + 4} , 100 ° C.) of the solid elastomer is 10 to 100, preferably 10 to 50, and more preferably 15 to 30. Here, Mooney viscosity (ML _{1 + 4} , 100 ° C.) is a value measured according to JIS K6300-1. By setting the Mooney viscosity of the solid elastomer within these ranges, it becomes easy to improve foamability, mechanical strength, and the like.
The liquid elastomer has a viscosity at 23 ° C. of 1 to 1000 Pa · s, preferably 5 to 500 Pa · s, and more preferably 5 to 100 Pa · s. Here, the viscosity at 23 ° C. is a value measured at a rotational speed of 1 rpm with a B-type rotational viscometer. By setting the viscosity of the liquid elastomer within these ranges, the kneadability when kneading the thermally conductive filler into the elastomer resin is improved.

[Thermal conductive filler (B)]
The heat conductive foam sheet of the present invention contains a heat conductive filler (B). The content of the thermally conductive filler (B) is preferably 100 to 500 parts by mass, more preferably 150 to 450 parts by mass, and still more preferably 200 to 100 parts by mass with respect to 100 parts by mass of the elastomer resin (A). 400 parts by mass. When the content of the heat conductive filler (B) is 100 parts by mass or more, the heat conductivity of the heat conductive foam sheet is improved, the heat dissipation is improved, and the content of the heat conductive filler (B) is high. When it is 500 parts by mass or less, the foamability is improved.

From the viewpoint of improving thermal conductivity, the heat conductive filler (B) preferably contains at least one of a plate filler (b1) and a spherical filler (b2), and contains at least a plate filler (b1). It is more preferable that both the plate-like filler (b1) and the spherical filler (b2) are contained. Here, the plate-like filler (b1) is a filler in the form of flakes and flakes, and the major axis of each filler is sufficiently larger than the thickness. For example, the ratio of the thickness to the major axis is 2 or more, Preferably it is 3 or more.
The spherical filler (b2) has a filler shape close to a sphere and a sphere, and the ratio of the major axis to the minor axis of each filler is close to 1 or 1, and the ratio is, for example, 0.6 to 1.7. , Preferably 0.8 to 1.5.
By using the plate-like filler (b1), it is easy to increase the thermal conductivity in the thickness direction of the thermally conductive foam sheet, and it is easy to improve heat dissipation. Moreover, such improvement in heat dissipation is further enhanced by using the plate filler (b1) and the spherical filler (b2) in combination.
This is presumably because the plate-like filler (b1) is oriented in the thickness direction of the thermally conductive foam sheet by the spherical filler (b2) and bubbles formed by foaming.

In the case where the plate-like filler (b1) and the spherical filler (b2) are used in combination, the content of the plate-like filler (b1) in the thermally conductive filler is spherical filler from the viewpoint of improving thermal conductivity and foamability. The content is preferably less than the content of (b2). From the same viewpoint, the content of the plate-like filler (b1) is 5 to 60% by mass and the content of the spherical filler (b2) is 40 to 95% by mass based on the total mass of the thermally conductive filler. It is preferable that More preferably, the content of the plate-like filler (b1) is 10 to 40% by mass and the content of the spherical filler (b2) is 60 to 90% by mass based on the total mass of the thermally conductive filler. is there. More preferably, the content of the plate-like filler (b1) is 15 to 35% by mass and the content of the spherical filler (b2) is 65 to 85% by mass based on the total mass of the thermally conductive filler. is there.
In the thermally conductive filler (B), the total content of the plate filler (b1) and the spherical filler (b2) is preferably 80% by mass or more, more preferably 95% by mass or more, and 100% by mass. More preferably.

  As described above, the thermal conductivity is improved by including the plate-like filler (b1) in the thermally conductive foam sheet. However, generally, when the plate-like filler (b1) is included, the foamability is deteriorated. The surface state of the thermally conductive foam sheet tends to deteriorate. However, as in the present invention, by containing the olefinic low molecular weight resin (C) in the thermally conductive foam sheet, deterioration of the surface state caused by the plate filler (b1) can be suppressed, and the thermal conductivity and A heat conductive foam sheet having a good surface condition can be obtained.

For example, the plate filler (b1) and the spherical filler (b2) preferably have a thermal conductivity of 30 W / m · K or more, but the plate filler (b1) has a thermal conductivity of the spherical filler (b2). It is preferable that the thermal conductivity is higher. By increasing the thermal conductivity of the plate-like filler, the thermal conductivity in the thickness direction of the thermally conductive foam sheet can be easily increased, and the heat dissipation can be easily improved.
Further, the thermal conductivity of the plate filler (b1) is more preferably 30 to 400 W / m · K, and further preferably 40 to 350 W / m · K. On the other hand, the thermal conductivity of the spherical filler (C) is more preferably 30 to 150 W / m · K, and further preferably 40 to 100 W / m · K.
In addition, the heat conductivity of a filler can be measured by the advance wirecraft thermal conductivity measuring device TC-1000 made from Advance Riko Co., Ltd.

Each of the plate filler (b1) and the spherical filler (b2) is selected from, for example, silica, aluminum oxide, magnesium oxide, boron nitride, talc, aluminum nitride, carbon-based filler, and conductive metal particles such as copper and aluminum. At least one kind. In addition, as a carbon-type filler, a graphite, a graphene, etc. are mentioned, However, A graphene is preferable. As the plate filler (b1) and the spherical filler (b2), fillers made of the same type of material may be used, or fillers made of different types of materials may be used.
Among the above, the plate filler (b1) is more preferably boron nitride, and the spherical filler (b2) is more preferably aluminum oxide or magnesium oxide, and the plate filler (b1) is boron nitride. The spherical filler (b2) is particularly preferably magnesium oxide.
In addition, a plate-like filler (b1) and a spherical filler (b2) may each be used individually by 1 type, and may use 2 or more types together.

The average particle size of the spherical filler (b2) is preferably larger than the average particle size of the plate filler (b1). It is considered that the average particle size of the spherical filler (b2) is larger than the average particle size of the plate filler (b1), so that it becomes easier to orient the plate filler (b1) in the thickness direction. The heat dissipation of the body sheet is improved. The average particle diameter of the plate filler (b1) is, for example, 5 to 100 μm, preferably 10 to 50 μm, and more preferably 15 to 40 μm. On the other hand, the average particle diameter of the spherical filler (b2) is, for example, 10 to 150 μm, preferably 20 to 100 μm, more preferably greater than 40 μm and 80 μm or less.
The plate-like filler (b1) has a relatively small average particle size, thereby improving the foamability and improving the smoothness of the surface of the thermally conductive foam sheet. Moreover, various performances, such as thermal conductivity and a softness | flexibility, can be improved with sufficient balance by making the average particle diameter of a plate-like filler (b1) and a spherical filler (b2) into the said range.
In the present invention, the total amount of the elastomer resin (A) and the heat conductive filler (B) is preferably 70% by mass or more, and more preferably 90% by mass or more in the heat conductive foam sheet. And more preferably 95% by mass or more.

[Olefinic low molecular weight resin (C)]
The heat conductive foam sheet of the present invention contains an olefinic low molecular weight resin (C) having a weight average molecular weight of 900 to 50,000. By containing the olefinic low molecular weight resin (C), the surface state of the thermally conductive foam sheet is improved. This is because the olefinic low molecular weight resin (C) used in the present invention has a relatively low molecular weight, improves the affinity between the elastomer resin (A) and the heat conductive filler (B), and causes abnormal foaming during foaming. It is thought to suppress.
The weight average molecular weight of the olefinic low molecular weight resin (C) is preferably 900 to 30,000, more preferably 1,000 to 10,000, and preferably 2,000 to 8,000. Further preferred. By setting the weight average molecular weight within the above range, in addition to improving the surface state of the thermally conductive foam sheet, it becomes easy to obtain a foam sheet having a low apparent density, and the thermal conductivity and flexibility are high. In addition, it is possible to obtain a thermally conductive foam sheet having a good surface condition and excellent physical property balance. The weight average molecular weight can be measured by gel permeation chromatography (GPC), and a polystyrene equivalent value is used.

The type of the polyolefin-based low molecular weight resin (C) is not particularly limited as long as it is a polyolefin, and examples thereof include polyethylene, polypropylene, polybutene, etc. Among them, the surface state of the heat conductive foam sheet is good. From the viewpoint of, polypropylene is preferable. Polypropylene can be selected from homopolypropylene, random polypropylene, block polypropylene, and the like.
The polyolefin-based low molecular weight resin (C) may be polymerized with any polymerization catalyst such as a Ziegler-Natta compound, a metallocene compound, or a chromium oxide compound, but is polymerized using a metallocene compound as a polymerization catalyst. It is preferable. Polyolefin-based low molecular weight resin (C) polymerized using a metallocene compound has uniform molecular weight, molecular weight distribution, composition, etc., and improves the affinity between elastomer resin (A) and thermally conductive filler (B). Thus, it is considered that the effect of reducing abnormal foaming and improving the surface condition is high.

  Although content in particular of polyolefin system low molecular weight resin (C) is not restrict | limited, Preferably it is 0.1-20 mass parts with respect to 100 mass parts of elastomer resins (A), More preferably, it is 0.5-15. It is a mass part, More preferably, it is 1-10 mass parts. By setting it as 0.1 mass part or more, the surface state of a heat conductive foam sheet can be made more favorable, and mechanical strength can be made favorable by setting it as 20 mass parts or less.

[Other ingredients]
The thermally conductive foam sheet may contain components other than the components (A) to (C). Examples of such components include various additives usually used for foams. Examples of additives include foaming aids, lubricants, shrinkage inhibitors, cell nucleating agents, crystal nucleating agents, colorants (pigments, dyes, etc.), UV absorbers, antioxidants, anti-aging agents, and fillers other than the above fillers. Agents, reinforcing agents, flame retardants, flame retardant aids, antistatic agents, surfactants, vulcanizing agents, surface treatment agents, softening agents and the like. Such additives can be used alone or in combination of two or more. The amount of these additives can be selected as appropriate as long as the effects of the present invention are not impaired, and the amount used for ordinary foams can be employed.
Among the additives mentioned above, it is preferable to use an antioxidant in the heat conductive foam sheet. Examples of the antioxidant include phenolic antioxidants, sulfur antioxidants, phosphorus antioxidants, amine antioxidants, etc. Among them, phenolic antioxidants are preferable. An antioxidant may be used independently and 2 or more types may be used together.
0.03-10 mass parts is preferable with respect to 100 mass parts of elastomer resins (A), and, as for content of antioxidant, 0.05-5 mass parts is more preferable.

[Foaming agent]
The thermally conductive foam sheet of the present invention is foamed with a foaming agent, and in addition to the components (A) to (C) and other additives contained as necessary, the foaming agent is usually used. Is obtained by foaming a foamable composition containing a foaming agent. Moreover, it is preferable to foam the crosslinked foamable composition. A thermal decomposition type foaming agent is preferable as the foaming agent. Specific examples of the pyrolytic foaming agent include organic or inorganic chemical foaming agents having a decomposition temperature of about 140 ° C to 270 ° C.
Organic foaming agents include azodicarbonamide, azodicarboxylic acid metal salts (such as barium azodicarboxylate), azo compounds such as azobisisobutyronitrile, nitroso compounds such as N, N′-dinitrosopentamethylenetetramine, Examples thereof include hydrazine derivatives such as hydrazodicarbonamide, 4,4′-oxybis (benzenesulfonylhydrazide) and toluenesulfonylhydrazide, and semicarbazide compounds such as toluenesulfonyl semicarbazide.
Examples of the inorganic foaming agent include ammonium acid, sodium carbonate, ammonium hydrogen carbonate, sodium hydrogen carbonate, ammonium nitrite, sodium borohydride, anhydrous monosodium citrate, and the like.
Among these, azo compounds and nitroso compounds are preferable from the viewpoint of obtaining fine bubbles, and from the viewpoints of economy and safety, and azodicarbonamide, azobisisobutyronitrile, N, N′-dinitrosopentamethylene. Tetramine is more preferred, and azodicarbonamide is particularly preferred. These pyrolytic foaming agents can be used alone or in combination of two or more.
As for the compounding quantity of a thermal decomposition type foaming agent, 1-30 mass parts is preferable with respect to 100 mass parts of elastomer resins (A). By setting it as such a compounding quantity, it can foam appropriately, without a bubble bursting. Moreover, when the compounding quantity of a thermal decomposition type foaming agent is increased, an apparent density will become low and it is possible to improve a softness | flexibility. Therefore, as for the compounding quantity of a thermal decomposition type foaming agent, 5-25 mass parts is more preferable, and 10-25 mass parts is further more preferable.

(The thickness of the heat conductive foam sheet)
The thickness of the thermally conductive foam sheet is preferably 0.05 to 1 mm. By setting the thickness of the thermally conductive foam sheet to 0.05 mm or more, the mechanical strength of the thermally conductive foam sheet is increased, and tearing is prevented. Moreover, it becomes easy to arrange | position inside a small electronic device because it shall be 1 mm or less. Moreover, 0.05-0.8 mm is more preferable and, as for the thickness of a heat conductive foam sheet, 0.1-0.5 mm is further more preferable.

(Apparent density)
The thermal conductive foam sheet preferably has an apparent density of 0.5 to 2.0 g / cm ³ . By making the apparent density within such a range, it becomes easy to improve the flexibility and thermal conductivity of the thermally conductive foam sheet. The apparent density is more preferably 0.6 to 1.2 g / cm ³ and further preferably 0.65 to 0.77 g / cm ³ from the viewpoint of improving the flexibility and thermal conductivity in a balanced manner. .

(Thermal conductivity)
The thermal conductivity of the thermally conductive foam sheet is preferably 0.70 to 3.0 W / m · K in the state of the sheet-like foamable composition before foaming. Good heat dissipation can be obtained when the thermal conductivity is 0.70 W / m · K or more.
From the viewpoint of heat dissipation of the thermally conductive foam sheet and ease of production, the thermal conductivity before foaming is more preferably 0.85 to 2.8 W / m · K, and 1.2 to 2.6 W / m · K is more preferable. In addition, thermal conductivity can be measured according to the method as described in the Example mentioned later.
Further, the thermal conductivity of the thermally conductive foam sheet after foaming is preferably 0.6 to 2.6 W / m · K, more preferably 0.8 to 2.5 W / m · K. preferable.

<The manufacturing method of a heat conductive foam sheet>
As a manufacturing method of the heat conductive foam sheet of this invention, the foamable composition which mix | blended the heat conductive filler (B) and the olefin type low molecular weight resin (C) with the elastomer (A) is made to foam. The method of obtaining a heat conductive foam sheet is mentioned. Hereinafter, this production method will be described in more detail.

  In this production method, the foamable composition contains, for example, an elastomer (A), a heat conductive filler (B), and an olefinic low molecular weight resin (C), and, if necessary, a foaming agent and other optional components. What is necessary is just to shape | mold into a sheet form by supplying to an extruder, melt-kneading, and extruding from an extruder. Alternatively, the elastomer (A), the heat conductive filler (B), and the olefinic low molecular weight resin (C), and further, if necessary, a foaming agent and other optional components are kneaded using a calendar, conveyor belt casting, or the like. However, the foamable composition may be formed into a sheet by continuously conveying it. In addition, the foamable composition is obtained by pressing the elastomer (A), the thermally conductive filler (B), the olefinic low molecular weight resin (C), and, if necessary, a kneaded foaming agent and other optional components. It is good also as a sheet form.

The method for foaming the foamable composition is not particularly limited, but as described above, foaming is preferably performed with a foaming agent such as a pyrolytic foaming agent. When foaming with a pyrolytic foaming agent, the foamable composition may be heated at a temperature higher than the decomposition temperature of the pyrolytic foaming agent. The heating temperature is, for example, 200 to 400 ° C, preferably 220 to 300 ° C. Moreover, it is good to perform a foaming process with respect to the foamable composition shape | molded in the sheet form as mentioned above.
The method for decomposing and foaming the pyrolytic foaming agent is not particularly limited. For example, the foamable composition is heated with hot air, the infrared heating method, the salt bath heating method, or the oil bath heating method. These may be used, and these may be used in combination.

In this production method, it is preferable to crosslink the foamable composition before foaming. The crosslinking treatment may be performed after the foamable composition is formed into a sheet shape. Examples of the method for crosslinking the foamable composition include, for example, a method of irradiating the foamable composition with ionizing radiation such as electron beam, α-ray, β-ray, and γ-ray, and an organic peroxide previously applied to the foamable composition. In addition, a method in which a sulfur-based compound such as sulfur is blended, the foamable composition is heated to decompose the organic peroxide, or vulcanized with a sulfur compound is included. Good. In these, the method of irradiating ionizing radiation is preferable, and it is more preferable to use an electron beam especially. The irradiation amount of ionizing radiation is, for example, 0.5 to 15 Mrad, preferably 1 to 5 Mrad.
Further, the thermally conductive foam sheet may be stretched after foaming or while foaming. In addition, the manufacturing method of a heat conductive foam sheet is not limited to the said method, You may manufacture by another method.

<How to use thermally conductive foam sheet>
Although the heat conductive foam sheet of this invention is not specifically limited, It is preferable to use it for an electronic device use. The electronic device is preferably a mobile electronic device such as a mobile phone such as a smartphone, a tablet terminal, electronic paper, a notebook PC, a video camera, or a digital camera.
The thermally conductive foam sheet is disposed in the vicinity of the heat source inside the electronic device, and can be used as a heat dissipation material that diffuses or dissipates heat generated from the heat source. Further, it can also be suitably used as an impact absorbing material for absorbing impacts and vibrations, and as a sealing material for filling a gap to prevent dust and waterproofing. Moreover, since a heat conductive foam sheet is a thin material, it can be appropriately arranged in a narrow space.

  A heat conductive foam sheet is good to arrange | position in the space between various electronic components and a heat sink, for example inside an electric equipment. The heat conductive foam sheet arranged in this way can release heat generated in various electronic components to the heat sink. The heat source is an electronic component that generates heat when driven or used, and specifically includes a display device such as a CPU, a battery, a power amplifier, a liquid crystal panel, and an organic EL panel. In addition, examples of the heat sink include metal members such as iron and stainless steel, materials having high thermal conductivity such as graphite, or composites and laminates thereof, and may constitute a housing of an electronic device.

<Adhesive tape>
Moreover, you may use for the adhesive tape which uses the heat conductive foam sheet of this invention as a base material. The pressure-sensitive adhesive tape includes, for example, a heat conductive foam sheet and a pressure-sensitive adhesive layer provided on at least one surface of the heat conductive foam sheet, and both surfaces provided with a pressure-sensitive adhesive layer on both sides. An adhesive tape may be used.
The thickness of the pressure-sensitive adhesive layer constituting the pressure-sensitive adhesive tape is preferably 5 to 200 μm. The thickness of the pressure-sensitive adhesive layer is more preferably 7 to 150 μm, still more preferably 10 to 100 μm. The thickness of the structure fixed using the adhesive tape can be made thin as the thickness of an adhesive layer is the range of 5-200 micrometers.
There is no restriction | limiting in particular as an adhesive used for an adhesive layer, For example, an acrylic adhesive, a urethane type adhesive, a rubber-type adhesive etc. can be used. Further, a release sheet such as a release paper may be further bonded on the pressure-sensitive adhesive layer.

  Examples The present invention will be described in more detail with reference to examples, but the present invention is not limited to these examples.

The measurement method and evaluation method of each physical property are as follows.
[Weight average molecular weight]
The weight average molecular weight can be measured by gel permeation chromatography (GPC), and a polystyrene equivalent value is used. Specifically, ethylene propylene diene rubber was dissolved in orthodichlorobenzene using a Prominence system manufactured by Shimadzu Corporation and measured using a STANDARD SM-105 column.
[Mooney viscosity and viscosity at 23 ° C.]
The Mooney viscosity (ML _{1 + 4} , 100 ° C.) of the elastomer is a value measured according to JIS K6300-1.
The viscosity at 23 ° C. is a value measured with a B-type rotational viscometer at a rotational speed of 1 rpm.
[Average particle size of thermally conductive filler]
Using a laser diffraction / scattering particle size distribution measuring device (HELOS / BFM, manufactured by Sympatec GmbH), the particle size distribution was measured by a conventional method, and the average value of the average particle size when measured five times was defined as the average particle size. .
[Apparent density]
It measured based on JISK7222.
[Thermal conductivity]
The long sheet-like cross-linked foamable composition or foam sheet in each Example and Comparative Example was cut into 2 cm squares, and a sample was prepared by stacking until the thickness became 1 cm or more. About the sample, the thermal conductivity was measured at 23 degreeC by the hot disk method using "TPS-1500" by Kyoto Electronics Industry Co., Ltd.
[Surface condition]
The surface was confirmed by visual inspection, and A was given when the surface was good, with few irregularities, and the surface was uniform [Comprehensive judgment].
A sample having a good surface state and a low density was judged as A, a surface state was good, but a sample having a slightly high density was judged as B, and a sample having a poor surface state was judged as C.

[Example 1]
57 parts by mass of solid ethylene propylene diene rubber, 40 parts by mass of liquid ethylene propylene diene rubber, 3 parts by mass of olefinic low molecular weight resin (C), 17.5 parts by mass of pyrolytic foaming agent, and spherical filler (B2) 255 parts by mass, 85 parts by mass of a plate-like filler (b1), and 3 parts by mass of a phenolic antioxidant were melt-kneaded. The melt kneading was performed at 130 ° C. with a plast mill to obtain a foamable composition. The amount of the thermally conductive filler with respect to 100 parts by mass of the elastomer resin in the foamable composition is about 351 parts by mass. Subsequently, the foamable composition was pressed into a long sheet having a thickness of 0.3 mm.
Next, the foamable composition was crosslinked by irradiating an electron beam with an acceleration voltage of 500 kV for 1.5 Mrad on both surfaces of the long sheet-like foamable composition.
Next, the long sheet-like foamable composition was continuously fed into a foaming furnace maintained at 250 ° C. with hot air and an infrared heater and heated to foam to obtain a thermally conductive foam sheet. This heat conductive foam sheet had an apparent density of 0.75 g / cm ³ and a thickness of 0.3 mm.
The surface state of this thermally conductive foam sheet was evaluated. The thermal conductivity was measured before and after foaming. The results are shown in Table 1.
The details of the raw materials used in Example 1 are as follows.
Solid ethylene propylene diene rubber: manufactured by Mitsui Chemicals, trade name “EMB-EPT4021”, Mooney viscosity 24 (ML _{1 + 4} , 100 ° C.)
Liquid ethylene propylene diene rubber: manufactured by Mitsui Chemicals, trade name “PX-68”, viscosity at 23 ° C. 10 Pa · S
-Olefin-based low molecular weight resin (C): Polypropylene low molecular weight resin having a weight average molecular weight of 6,400 (manufactured by Clariant Japan Co., Ltd., trade name "LicocenePP1302")
Spherical filler (b2): Magnesium oxide (RF-70C-SC, manufactured by Ube Materials Co., Ltd., average particle size 70 μm, thermal conductivity: 50 W / m · K)
Plate filler (b1): Boron nitride (PTX-25S, manufactured by Momentive, average particle size 5 μm, thermal conductivity: 60 W / m · K)
・ Pyrolytic foaming agent: Azodicarbonamide

[Examples 2 to 4]
In Example 1, the heat conductive foam sheet was obtained like Example 1 except having changed the kind of polypropylene low molecular weight resin which is an olefin type low molecular weight resin (C) as Table 1. FIG. The results are shown in Table 1.

[Comparative Example 1]
60 parts by mass of solid ethylene propylene diene rubber, 40 parts by mass of liquid ethylene propylene diene rubber, 17.5 parts by mass of pyrolytic foaming agent, 255 parts by mass of spherical filler (b2), and platy filler (b1 ) 85 parts by mass and 3 parts by mass of a phenolic antioxidant were melt-kneaded. The melt kneading was performed at 130 ° C. with a plast mill, and the obtained foamable composition was pressed into a long sheet having a thickness of 0.3 mm.
The same solid ethylene propylene diene rubber, liquid ethylene propylene diene rubber, pyrolytic foaming agent, spherical filler (b2) and plate-like filler (b1) as in Example 1 were used.
Next, the foamable composition was crosslinked by irradiating an electron beam with an acceleration voltage of 500 kV for 1.5 Mrad on both surfaces of the long sheet-like foamable composition.
Next, the long sheet-like foamable composition was continuously fed into a foaming furnace maintained at 250 ° C. with hot air and an infrared heater and heated to foam to obtain a thermally conductive foam sheet. This heat conductive foam sheet had an apparent density of 0.74 g / cm ³ and a thickness of 0.3 mm.
The surface state of this thermally conductive foam sheet was evaluated. The thermal conductivity was measured in the state before foaming. The results are shown in Table 1.

  In the above Examples 1 to 4, the elastomer resin (A), the thermally conductive filler (B), and the olefinic low molecular weight resin (C) having a weight average molecular weight of 900 to 50,000 are added to the thermally conductive foam sheet. It was found that the inclusion had a high thermal conductivity and the surface state after foaming was good. In particular, when the olefin-based low molecular weight resin (C) having a weight average molecular weight of 1000 to 10,000 is used, it can be seen that a thermally conductive foam sheet having a better surface state and a lower density can be obtained. It was. On the other hand, in Comparative Example 1 in which the olefinic low molecular weight resin (C) was not used, the surface state was not good.

Claims (8)

  A thermally conductive foam sheet comprising an elastomer resin (A), a thermally conductive filler (B), and an olefinic low molecular weight resin (C) having a weight average molecular weight of 900 to 50,000.   The thermally conductive foam sheet according to claim 1, comprising 100 to 500 parts by mass of the thermally conductive filler (B) with respect to 100 parts by mass of the elastomer resin (A).   The thermally conductive foam sheet according to claim 1 or 2, wherein the thermally conductive filler (B) contains a plate-like filler (b1).   The thermally conductive foam sheet according to claim 3, wherein the thermally conductive filler (B) further contains a spherical filler (b2).   The heat conductive foam sheet in any one of Claims 1-4 whose said olefin type low molecular weight resin (C) is a polypropylene.   The heat conductive foam sheet in any one of Claims 1-5 whose elastomer resin (A) is ethylene propylene diene rubber.   The thermally conductive foam sheet according to any one of claims 1 to 6, wherein the elastomer resin (A) is a mixture of solid ethylene propylene diene rubber and liquid ethylene propylene diene rubber. The thermally conductive foam sheet according to any one of claims 1 to 7, wherein the apparent density is 0.5 to 2.0 g / cm ³ .