JP2019038912A - Thermally conductive foam sheet - Google Patents

Abstract

To provide a thermally conductive foam sheet having excellent flexibility and high thermal conductivity in a thickness direction.SOLUTION: A thermally conductive foam sheet has a resin (A), and a tabular filler (B) and a spherical filler (C) dispersed in the resin (A). A total of the tabular filler (B) and spherical filler (C) is 30 vol.% or more relative to the total volume of the thermally conductive foam sheet. The ratio of an average length of the tabular filler (B) to an average particle size of the spherical filler (C) ((B) average length/(C) average particle size) is 0.4-12.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 a resin containing a heat conductive filler such as aluminum oxide is used. Foam sheets are known.

JP 2013-231166 A

In recent years, there has been a need for a thermally conductive foam sheet having better heat dissipation performance than before, and in particular, a thermally conductive foam sheet having excellent thermal conductivity in the thickness direction has been desired. On the other hand, when a thermally conductive filler is blended for such a purpose, there may be a problem that the flexibility of the sheet is lowered due to deterioration of foamability or the presence of the filler. From such a viewpoint, there is a demand for a thermally conductive foam sheet that exhibits higher thermal conductivity in the thickness direction while maintaining basic performance as a foam such as flexibility.
This invention is made | formed in view of the above situation, and it aims at providing the heat conductive foam sheet which is excellent in a softness | flexibility and has high thermal conductivity of the thickness direction.

As a result of intensive studies, the inventors of the present invention have found that the resin, and the thermally conductive foam sheet containing the plate-like filler and the spherical filler dispersed in the resin, the total volume% of both fillers and the average length of the plate-like filler. When the ratio of the average particle size of the spherical filler is controlled, the inventors have found that the above problem can be solved and completed the following present invention.
That is, the present invention provides the following [1] to [6].
[1] A resin (A) and a plate-like filler (B) and a spherical filler (C) dispersed in the resin (A), and the sum of the plate-like filler (B) and the spherical filler (C) is 30% by volume or more based on the total amount of the thermally conductive foam sheet, and the ratio of the average length of the plate-like filler (B) to the average particle diameter of the spherical filler (C) (average length of (B) / (C) The heat conductive foam sheet whose average particle diameter is 0.4-12.
[2] The plate filler (B) and the spherical filler (C) are each at least one selected from the group consisting of aluminum oxide, magnesium oxide, boron nitride, talc, aluminum nitride, graphite, and graphene. The thermally conductive foam sheet according to [1].
[3] The above-mentioned [1] or [2], wherein the resin (A) contains at least one selected from the group consisting of olefin rubber, polyolefin resin, silicone resin, and acrylic resin as a main component. Thermally conductive foam sheet.
[4] The thermally conductive foam sheet according to any one of [1] to [3], wherein the resin (A) is 40 to 70% by volume based on the total amount of the thermally conductive foam sheet.
[5] The thermally conductive foam sheet according to any one of [1] to [4], wherein the thickness is 0.05 to 2.0 mm.
[6] The thermally conductive foam sheet according to any one of [1] to [5], wherein the apparent density is 0.1 to 1.5 g / cm ³ .

  According to the present invention, it is possible to provide a thermally conductive foam sheet having high thermal conductivity in the thickness direction and excellent flexibility.

The thermally conductive foam sheet of the present invention (in this specification, the thermally conductive foam sheet is also simply referred to as a foam sheet) includes a resin (A) and a plate-like filler dispersed in the resin (A) ( B) and the spherical filler (C), the total of the plate-like filler (B) and the spherical filler (C) is 30% by volume or more based on the total amount of the thermally conductive foam sheet, and the plate-like filler (B) The ratio of the average length of the spherical filler (C) to the average particle diameter (average length of (B) / average particle diameter of (C)) is 0.4-12.
The reason why the foam sheet of the present invention exhibits high thermal conductivity in the thickness direction is not clear, but is estimated as follows. The total amount of the plate-like filler (B) and the spherical filler (C), and the ratio of the average length of the plate-like filler (B) to the average particle size of the spherical filler (C) (average length of (B) / (C) It is considered that the length direction of the plate-like filler (B) is easily oriented in the thickness direction of the foam sheet by appropriately controlling the (average particle size). Since the plate-like filler (B) has a high thermal conductivity in the length direction, it is estimated that a foam sheet having a high thermal conductivity in the thickness direction can be obtained by the orientation described above.

[Resin (A)]
The resin (A) constituting the foam sheet of the present invention is not particularly limited, but preferably contains at least one selected from the group consisting of olefin rubber, polyolefin resin, silicone resin, and acrylic resin as a main component. . By using the resin (A) containing these resins as a main component, the balance between the flexibility and the thermal conductivity of the foam sheet can be improved when a heat conductive filler described later is blended.
Including the main component means that the resin component of the resin (A) has the largest content. Specifically, at least one selected from the group consisting of an olefin rubber, a polyolefin resin, a silicone resin, and an acrylic resin is preferably 50% by mass or more of the resin component contained in the resin (A), and more Preferably it is 75 mass% or more, More preferably, it is 90 mass% or more, More preferably, it is 100 mass%.

(Olefin rubber)
The olefin rubber is an amorphous or low crystalline rubber-like material in which two or more kinds of olefin monomers are copolymerized substantially randomly, and an ethylene-α-olefin copolymer rubber is preferable.
Here, the α-olefin used in the ethylene-α-olefin copolymer rubber has 3 carbon atoms such as propylene, 1-butene, 2-methylpropylene, 3-methyl-1-butene and 1-hexene. 1 type or 2 types or more of about 10 to 10 olefins may be mentioned, and among these, propylene is preferable.
The olefin rubber may contain a repeating unit composed of a monomer other than olefin, and the monomer has 5 carbon atoms such as ethylidene norbornene, 1,4-hexadiene, dicyclopentadiene and the like. The diene compound represented by about 15 non-conjugated diene compounds is mentioned.
The olefin rubbers may be used alone or in combination of two or more. The olefin rubber may be a liquid rubber that becomes liquid at room temperature (23 ° C.) or may be a solid rubber that becomes solid at room temperature. However, from the viewpoint of improving thermal conductivity, these mixtures may be used. Preferably there is.
Specific examples of preferable olefin rubbers include ethylene-propylene rubber (EPM) and ethylene-propylene-diene rubber (EPDM), and EPDM is more preferable.

(Polyolefin resin)
The polyolefin resin is a resin other than the olefin rubber described above, and is a polymer of one or more olefin monomers. The polyolefin resin is preferably a polyethylene resin, a polypropylene resin, or a mixture thereof.
The polyethylene resin may be an ethylene homopolymer, or a copolymer obtained by copolymerizing ethylene with a small amount (for example, 30% by mass or less, preferably 10% by mass or less of all monomers) of an α-olefin as required. It may be a polymer. These may be used alone or in combination of two or more.
Specific examples of the α-olefin constituting the polyethylene resin include propylene, 1-butene, 1-pentene, 1-hexene, 1-heptene, and 1-octene.
The polypropylene resin may be a propylene homopolymer or a propylene-α-olefin copolymer containing 50% by mass or more of propylene units. These may be used alone or in combination of two or more.
Specific examples of the α-olefin constituting the propylene-α-olefin copolymer include ethylene, 1-butene, 1-pentene, 4-methyl-1-pentene, 1-hexene, 1-heptene, 1- Examples include octene.

(Silicone resin)
The silicone resin is preferably a cured product of a curable silicone resin material. The silicone resin is preferably a two-component liquid type addition reaction type silicone resin. Such curable silicone resin materials include, for example, organopolysiloxanes having at least two alkenyl groups in one molecule, organohydrogenpolysiloxanes having at least two silicon-bonded hydrogen atoms in one molecule, It consists of a platinum-based catalyst.
Examples of commercially available curable silicone resin materials include two-component heat-curable liquid silicone rubber “TSE3032” manufactured by Momentive Performance Materials Japan GK.

(acrylic resin)
The acrylic resin is preferably a polymer obtained by polymerizing monomers including acrylate, methacrylate, and both. Hereinafter, one or both of acrylate and methacrylate are collectively referred to as (meth) acrylate.
The acrylic resin usually has a structural unit derived from alkyl (meth) acrylate. As the alkyl (meth) acrylate, those having an alkyl group having 12 or less carbon atoms are usually used, and those having an alkyl group having 3 to 12 carbon atoms are preferably used. Specifically, n-propyl (meth) acrylate, n-butyl (meth) acrylate, n-amyl (meth) acrylate, n-hexyl (meth) acrylate, 2-ethylhexyl (meth) acrylate, n-octyl (meta ) Acrylate, isooctyl (meth) acrylate, n-nonyl (meth) acrylate, isononyl (meth) acrylate, n-decyl (meth) acrylate, and other alkyl (meth) acrylates.
The acrylic resin preferably contains 70 to 100% by mass, more preferably 80 to 100% by mass of a structural unit derived from an alkyl (meth) acrylate having an alkyl group having 3 to 12 carbon atoms in the acrylic resin. It is particularly preferable to contain ~ 100% by mass.

In addition, as a monomer for constituting the acrylic resin, as other monomers polymerizable with alkyl (meth) acrylate, acrylic acid, methacrylic acid, itaconic acid, vinyl acetate, hydroxyalkyl (meth) acrylate, and (meth) acrylamide Etc. may be included. The amount of these monomer-derived structural units is not limited, but is preferably 30 parts by mass or less and 1 to 25 parts by mass with respect to 100 parts by mass of the structural unit derived from the alkyl (meth) acrylate. It is more preferable that
The monomer for constituting the acrylic resin may contain a slight amount of polyfunctional (meth) acrylate such as 1,6-hexanediol diacrylate. Such a monomer has an effect of giving a crosslinked structure to the acrylic polymer after curing and contributing to the cohesiveness of the foam sheet. Moreover, the acrylic resin may have a structural unit derived from a monomer copolymerizable with the monomer other than the monomer.

(Other resin components)
The resin (A) used in the foam sheet of the present invention contains other resin components other than the above-described olefin rubber, polyolefin resin, silicone resin, and acrylic resin, unless they are contrary to the object of the present invention. You may let them.
Other resin components include acrylonitrile butadiene rubber, natural rubber, polybutadiene rubber, polyisoprene rubber, styrene-butadiene block copolymer, hydrogenated styrene-butadiene block copolymer, hydrogenated styrene-butadiene-styrene block copolymer. , Hydrogenated styrene-isoprene block copolymer, hydrogenated styrene-isoprene-styrene block copolymer, and the like.

  The amount of the resin (A) in the foam sheet of the present invention is preferably 40 to 70% by volume based on the total amount of the thermally conductive foam sheet, from the viewpoint of increasing the thermal conductivity while maintaining flexibility. More preferably, it is 50-70 volume%.

[Plate-like filler (B), spherical filler (C)]
In the resin (A) of the foam sheet of the present invention, a plate-like filler (B) and a spherical filler (C) are dispersed. Both the plate-like filler (B) and the spherical filler (C) dispersed in the resin (A) are heat conductors. Here, the plate-like filler (B) is a filler in the form of flakes and scaly, 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.
In addition, the spherical filler (C) 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.

  For example, the plate filler (B) and the spherical filler (C) preferably have a thermal conductivity of 30 W / m · K or more, but the plate filler (B) has a thermal conductivity of the spherical filler (C). 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 foam sheet can be easily increased, and the heat dissipation can be easily improved.

Each of the plate filler (B) and the spherical filler (C) is at least one selected from, for example, aluminum oxide, magnesium oxide, boron nitride, talc, aluminum nitride, graphite, and graphene. As the plate-like filler (B) and the spherical filler (C), fillers made of the same kind of material may be used, or fillers made of different kinds of materials may be used.
Further, among the above, the plate filler (B) is more preferably boron nitride, and the spherical filler (C) is more preferably aluminum oxide or magnesium oxide, and the plate filler (B) is boron nitride. The spherical filler (C) is particularly preferably magnesium oxide.
In addition, a spherical filler (C) and a plate-shaped filler (B) may each be used individually by 1 type, and may use 2 or more types together.

The ratio of the average length of the plate-like filler (B) and the average particle diameter of the spherical filler (C) (average length of (B) / average particle diameter of (C)) in the foam sheet of the present invention is 0.4 to. 12. By setting the average length of (B) / average particle diameter of (C) within the above range, the thermal conductivity in the thickness direction of the foam sheet is improved. This is considered because the length direction of the plate-like filler (B) is easily oriented in the thickness direction of the foam sheet by setting the above range.
When the average particle size of (B) average length / (C) is less than 0.4, the thermal conductivity in the thickness direction of the foam sheet tends to be inferior. On the other hand, if the average length of (B) / (C) average particle diameter exceeds 12, the foamability may deteriorate and the flexibility of the foam sheet may decrease.
The average length of (B) / (C) is preferably 0.5 to 10, more preferably 1 to 5.
The average length of the plate-like filler (B) means the average value of the length (major axis) in the longitudinal direction, and the cross section in the thickness direction of the foam sheet is observed with a scanning electron microscope, and any filler It can be obtained as an average value of the lengths of 100 major axes.
Similarly, the average particle diameter of the spherical filler (C) means the average value of the spherical diameter, and the cross section in the thickness direction of the foam sheet is observed with a scanning electron microscope. It can be obtained as an average value.
The average length of the plate-like filler (B) is preferably 5 to 150 μm, more preferably 10 to 120 μm, and still more preferably 10 to 50 μm. When the average length is 5 μm or more, the thermal conductivity in the thickness direction of the foam sheet is likely to be good, and when the average length is 150 μm or less, the foamability is likely to be good.
On the other hand, the average particle diameter of the spherical filler (C) is preferably 5 to 150 μm, more preferably 5 to 100 μm, and further preferably 5 to 60 μm, from the viewpoint of improving the thermal conductivity and flexibility of the foam sheet. is there.

(volume%)
The total of the plate-like filler (B) and the spherical filler (C) is 30% by volume or more based on the total amount of the foam sheet. If it is less than 30% by volume, the thermal conductivity tends to be low. From the viewpoint of further increasing the thermal conductivity, the total of the plate filler (B) and the spherical filler (C) is preferably 32% by volume or more based on the total amount of the foam sheet. On the other hand, from the viewpoint of improving the foamability of the foamable composition described later, and from the viewpoint of ensuring good flexibility of the foam sheet, the total of the plate filler (B) and the spherical filler (C) is the foam sheet. The total amount is preferably 60% by volume or less, more preferably 50% by volume or less, and still more preferably 40% by volume or less.
From the viewpoint of increasing the thermal conductivity of the foam sheet, the plate filler (B) is preferably 10% by volume or more, more preferably 15% by volume or more, based on the total amount of the foam sheet. More preferably, it is at least volume%. Further, the plate-like filler (B) is 30% by volume or less based on the total amount of the foam sheet from the viewpoint of improving the foamability of the foamable composition to be described later and securing the flexibility of the foam sheet. It is preferable that the content is 25% by volume or less.
From the viewpoint of increasing the thermal conductivity of the foam sheet, the spherical filler (C) is preferably 5% by volume or more and more preferably 10% by volume or more based on the total amount of the foam sheet. Further, the spherical filler (C) is 30% by volume or less based on the total amount of the foam sheet from the viewpoint of improving the foamability of the foamable composition to be described later and securing the flexibility of the foam sheet. It is preferably 25% by volume or less.
Further, the ratio of the volume percent of the plate-like filler (B) to the volume percent of the spherical filler (C) (volume percent of the plate-like filler (B) / volume percent of the spherical filler) based on the total amount of the foam sheet is the foam sheet. From the viewpoint of increasing the thermal conductivity of the resin, it is preferably 0.5 to 3.0, more preferably 1.0 to 2.0.

  The plate-like filler (B), the individual volume% of the spherical filler (C), and the total volume% are calculated on the basis of the total volume of the foam sheet, but the volume of the foam sheet is, for example, It can be calculated by subtracting the volume of the additive (foaming agent) that decomposes and disappears during foaming from the total volume of the foamable composition described below. Moreover, the volume of said (A), (B), and (C) component can be calculated from the mass of each component mix | blended, for example, the mass in 23 degreeC of each component is the mass of each component. It can be calculated by dividing by.

[Foaming agent]
The foam sheet is foamed with a foaming agent, and is usually obtained by foaming a foamable composition containing a foaming agent in addition to the components (A) to (C). 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 resin (A). By setting it as such a compounding quantity, it can foam appropriately, without the bubble of a foam sheet 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.

[Other ingredients]
The foam sheet may contain components other than those described above. 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 described above, it is preferable to use an antioxidant in the above-described 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 resin (A), and, as for content of antioxidant, 0.05-5 mass parts is more preferable.

(Foam sheet thickness)
The thickness of the foam sheet is preferably 0.05 to 2.0 mm. By setting the thickness of the foam sheet to 0.05 mm or more, the mechanical strength of the 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. Further, the thickness of the foam sheet is more preferably 0.05 to 1.5 mm, further preferably 0.1 to 1.0 mm.

(Apparent density)
The foam sheet preferably has an apparent density of 0.1 to 1.5 g / cm ³ .
By making the apparent density within such a range, it becomes easy to improve the flexibility and thermal conductivity of the foam sheet. The apparent density is more preferably 0.5 to 1.2 g / cm ³ and further preferably 0.7 to 1.0 g / cm ³ from the viewpoint of improving the flexibility and thermal conductivity in a balanced manner. .

(Thermal conductivity)
The thermal conductivity of the foam sheet is preferably 1.2 to 3.0 W / m · K. Good heat dissipation can be obtained when the thermal conductivity is 1.2 W / m · K or more.
From the viewpoint of heat dissipation and ease of manufacture of the foam sheet, the thermal conductivity is more preferably 1.4 to 2.5 W / m · K, and further preferably 1.6 to 2.3 W / m · K. In addition, thermal conductivity can be measured according to the method as described in the Example mentioned later.

[Method for producing foam sheet]
As a method for producing a foam sheet of the present invention, a foam sheet is obtained by foaming a foamable composition in which at least a plate filler (B) and a spherical filler (C) are blended with a resin (A). Is mentioned. Hereinafter, this production method will be described in more detail.

  In this production method, the foamable composition supplies, for example, a resin (A), a plate-like filler (B) and a spherical filler (C), and further, if necessary, a foaming agent and other optional components to the extruder. Then, it may be melt-kneaded and formed into a sheet by extrusion from an extruder. Alternatively, the resin (A), the plate-like filler (B), and the spherical filler (C), and further, if necessary, a foaming agent and other optional components are continuously kneaded using a calendar, conveyor belt casting, or the like. What is necessary is just to make a foamable composition into a sheet form by conveying. Also, the foamable composition can be made into a sheet by pressing the resin (A), the plate-like filler (B) and the spherical filler (C), and further kneading the foaming agent and other optional components as necessary. Good.

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 acceleration voltage of ionizing radiation is, for example, 400 to 3000 kV, preferably 500 to 1500 kV, and 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 foam sheet is not limited to the said method, You may manufacture by another method.

  For example, when a silicone resin or an acrylic resin is used as the resin (A), a foam sheet may be obtained as follows. That is, the raw material of each resin (a curable silicone resin material in the case of a silicone resin, an alkyl (meth) acrylate in the case of an acrylic resin, etc.), a plate-like filler (B), a spherical filler (C), and other as required The thermally conductive foam sheet may be obtained by generating bubbles and curing the curable composite material containing the optional components to be blended. The method of generating bubbles is not particularly limited, for example, a method of forming bubbles by a mechanical floss method in which bubbles are formed in a curable composite material by stirring and mixing while injecting bubble forming gas in a stirring and mixing device, The method of foaming with a foaming agent is mentioned.

<How to use foam sheet>
Although the 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 foam sheet is preferably disposed in the vicinity of the heat source inside the electronic device and used as a heat radiating material for diffusing or radiating 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.

  A 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 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 a metal member such as iron or stainless steel, a material having high thermal conductivity such as graphite, or a composite or laminate of these materials, and may constitute a housing of an electronic device.

The present invention will be described in more detail with reference to examples, but the present invention is not limited to these examples.
In addition, the measuring method and evaluation method of each physical property in this invention are as follows.

[Average length of plate filler (B)]
The cross section in the thickness direction of the thermally conductive foam sheet produced in each of the examples and comparative examples was observed with a scanning electron microscope (model number: S-3400N, manufactured by Hitachi High-Technologies Corporation), and the major axis of 100 arbitrary fillers. The average value of the length was taken as the average length.
[Average particle diameter of spherical filler (C)]
The cross section in the thickness direction of the thermally conductive foam sheet produced in each example / comparative example was observed with a scanning electron microscope (model number: S-3400N, manufactured by Hitachi High-Technologies Corporation), and 100 arbitrary filler balls The average value of the diameter was defined as the average particle diameter.
[Apparent density]
It measured based on JISK7222.
[Thermal conductivity (before foaming)]
The long sheet-like cross-linked foamable composition 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 of the thickness direction was measured at 23 degreeC with the hot disk method using "TPS-1500" by Kyoto Electronics Industry Co., Ltd.
[Thermal conductivity (after foaming)]
Samples were prepared by cutting the foam sheets in each Example and Comparative Example into 2 cm squares and stacking them until the thickness became 1 cm or more. The sample was compressed by 50% in the thickness direction, and the thermal conductivity in the thickness direction was measured at 23 ° C. by a hot disk method using “TPS-1500” manufactured by Kyoto Electronics Industry Co., Ltd.

<Resin (A)>
Ethylene propylene diene rubber (1) (manufactured by JSR Corporation, “EP22”, solid state)
Ethylene propylene diene rubber (2) (Mitsui Chemicals, "PX-68", liquid)
<Plate filler (B)>
Boron nitride (1) (Momentive, “PT620” average length: 20 μm, thermal conductivity: 60 W / m · K)
Boron nitride (2) (Momentive, “PT350” average length: 100 μm, thermal conductivity: 60 W / m · K)
Boron nitride (3) (manufactured by Momentive, “PTX25” average length: 25 μm, thermal conductivity: 60 W / m · K)
<Spherical filler (C)>
Magnesium oxide (1) (manufactured by Ube Materials, “RF-10C-SC”, average particle size: 10 μm, thermal conductivity: 50 W / m · K)
Magnesium oxide (2) (manufactured by Ube Materials, “RF-50C-SC”, average particle size: 50 μm, thermal conductivity: 50 W / m · K)
Magnesium oxide (3) (Ube Materials Co., Ltd., “RF-70C-SC” average particle size: 70 μm, thermal conductivity: 50 W / m · K)

[Example 1]
Each component of resin (A), plate-like filler (B), spherical filler (C), azodicarbonamide and phenolic antioxidant is mixed in parts by mass shown in Table 1 showing the composition of the foamable composition. The foamable composition obtained by melting and kneading at 130 ° C. with a plast mill was pressed into a long sheet having a thickness of 0.6 mm.
Next, the foamable composition was crosslinked by irradiating an electron beam with an acceleration voltage of 800 kV for 2.1 Mrad on both surfaces of the long sheet-like foamable composition.
Next, this foamable composition was continuously sent to a foaming furnace maintained at 250 ° C. by hot air and an infrared heater, and heated to foam to obtain a thermally conductive foam sheet.
Table 2 shows the volume percentages of the resin (A), the plate-like filler (B), and the spherical filler (C) in the foam sheet. In addition, volume% was computed by making the volume which deducted the volume of the foaming agent from the foamable composition whole quantity the volume of the foam whole quantity. Moreover, volume% was computed from the mass part and density of each component. Table 2 shows the evaluation results.

[Examples 2-3, Comparative Examples 1-2]
A thermally conductive foam sheet was obtained in the same manner as in Example 1 except that the composition of the foamable composition was changed as shown in Table 1.
The volume% of each component was calculated in the same manner as in Example 1. Table 2 shows the evaluation results.

It was found that the foam sheets satisfying the requirements of the present invention in Examples 1 to 3 have high thermal conductivity and good flexibility.
On the other hand, from the results of Comparative Examples 1 and 2, when the ratio of the average length of the plate filler (B) and the average particle diameter of the spherical filler (C) is outside the range of the present invention, the thermal conductivity may be inferior. I understood.

Claims (6)

Including resin (A), plate-like filler (B) and spherical filler (C) dispersed in resin (A),
The total of the plate-like filler (B) and the spherical filler (C) is 30% by volume or more based on the total amount of the thermally conductive foam sheet, and the average length of the plate-like filler (B) and the average particle of the spherical filler (C) A thermally conductive foam sheet having a diameter ratio (average length of (B) / average particle diameter of (C)) of 0.4 to 12.   Each of the plate-like filler (B) and the spherical filler (C) is at least one selected from the group consisting of aluminum oxide, magnesium oxide, boron nitride, talc, aluminum nitride, graphite, and graphene. The thermally conductive foam sheet as described.   The thermally conductive foam sheet according to claim 1 or 2, wherein the resin (A) contains, as a main component, at least one selected from the group consisting of an olefin rubber, a polyolefin resin, a silicone resin, and an acrylic resin. .   The heat conductive foam sheet in any one of Claims 1-3 whose said resin (A) is 40-70 volume% on the basis of heat conductive foam sheet whole quantity.   The heat conductive foam sheet according to any one of claims 1 to 4, wherein the thickness is 0.05 to 2.0 mm. The heat conductive foam sheet according to any one of claims 1 to 5, wherein an apparent density is 0.1 to 1.5 g / cm ³ .