JP2019038911A - 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) dispersed in the resin (A), with an average cell diameter of 70 μm or less.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 present inventors have found that the above problem can be solved by a thermally conductive foam sheet containing a resin and a plate-like filler dispersed in the resin and having an average cell diameter of 70 μm or less. The following present invention was completed.
That is, the present invention provides the following [1] to [8].
[1] A thermally conductive foam sheet comprising a resin (A) and a plate-like filler (B) dispersed in the resin (A) and having an average cell diameter of 70 μm or less.
[2] The thermally conductive foam sheet according to the above [1], comprising a spherical filler (C) dispersed in the resin (A).
[3] The above [1] or [2], wherein the plate-like filler (B) is at least one selected from the group consisting of aluminum oxide, magnesium oxide, boron nitride, talc, aluminum nitride, graphite, and graphene. Thermally conductive foam sheet as described in 1.
[4] The heat conduction according to [2], wherein 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. Foam sheet.
[5] Any of the above [1] to [4], wherein the resin (A) contains as a main component at least one selected from the group consisting of olefin rubber, polyolefin resin, silicone resin, and acrylic resin. Thermally conductive foam sheet as described in 1.
[6] The thermally conductive foam sheet according to any one of [1] to [5], wherein the thickness is 0.05 to 2.0 mm.
[7] The thermally conductive foam sheet according to any one of [1] to [6], wherein the apparent density is 0.1 to 1.5 g / cm ³ .
[8] Any of the above [1] to [7], wherein the foamable composition containing the resin (A), the plate-like filler (B), and a thermal decomposable foaming agent having an average particle size of 15 μm or less is foamed. The manufacturing method of the heat conductive foam sheet of description.

  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 a heat conductive foam sheet having an average cell diameter of 70 μm or less.
The reason why the foam sheet of the present invention exhibits high thermal conductivity in the thickness direction is not certain, but by reducing the average cell diameter to a certain value or less, the length direction of the plate filler (B) is foamed. It is estimated that it becomes easy to orient in the thickness direction of the body sheet. 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.

[Average bubble diameter]
The foam sheet of the present invention has many bubbles, and the average cell diameter of the foam sheet is 70 μm or less. When the average cell diameter exceeds 70 μm, the thermal conductivity in the thickness direction of the foam sheet tends to decrease. From the viewpoint of improving the thermal conductivity in the thickness direction of the foam sheet, the average cell diameter of the foam sheet is preferably 60 μm or less. Further, the smaller the average cell diameter, the better the thermal conductivity in the thickness direction of the foam sheet, but from the viewpoint of ease of production, the average cell diameter is usually 10 μm or more, preferably 20 μm or more, More preferably, it is 40 μm or more.
The average cell diameter of the foam sheet can be determined as follows. First, a foam sheet is cut | disconnected in the thickness direction along MD direction and TD direction, and the cross section of MD direction and the cross section of TD direction are produced. Next, the bubble diameter of each bubble present in the cross section is observed with a digital microscope, and the average bubble diameter in the MD direction and the average bubble diameter in the TD direction are obtained. And the average value of this average bubble diameter of MD direction and the average bubble diameter of TD direction is calculated | required, and let this be an average bubble diameter of a foam sheet. In the present invention, “MD” means “Machine Direction” and means a direction coinciding with the extrusion direction of the foam sheet. “TD” means Transverse Direction, which means a direction perpendicular to MD and parallel to the foam sheet.
In the present invention, the bubble diameter of each bubble present in each cross section means the longest diameter in the cross-sectional shape of the bubble, for example, if the cross-sectional shape is a perfect circle, it represents the diameter, and the cross-sectional shape is elliptical. If present, it represents the major axis.
In addition, when the foam sheet is manufactured, for example, by simply hot-pressing the foamable composition described later, that is, when the MD direction and the TD direction are not particularly determined in manufacturing, Thus, the average cell diameter of a foam sheet can be calculated | required. First, an arbitrary cross section in the thickness direction of the foam sheet and another cross section in the thickness direction in a direction orthogonal to the cross section are prepared. And the average bubble diameter of a foam sheet can be calculated | required like the above by making each into the cross section of MD direction, and the cross section of TD direction.

[Resin (A)]
Although resin (A) which comprises the foam sheet | seat of this invention is not specifically limited, As a main component, it contains at least 1 sort (s) chosen from the group which consists of olefin rubber, polyolefin resin, silicone resin, and an acrylic resin. preferable. 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), 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 foamable composition to be described later, 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 filler (B) is dispersed. The plate filler (B) dispersed in the resin (A) is a heat conductor. Here, the plate-like filler (B) is, for example, a flaky or scaly filler, 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. As described above, preferably 3 or more.
Moreover, it is preferable that spherical resin (C) disperse | distributed in resin (A) is included in resin (A) of the foam sheet of this invention. The spherical filler (C) is preferably a heat conductive material. When both the plate-like filler (B) and the spherical filler (C) are dispersed in the resin (A), the thermal conductivity in the thickness direction of the foam sheet is easily improved. The spherical filler (C) has a spherical shape and a shape close to a spherical shape, 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-like filler (B) and the spherical filler (C) preferably have a thermal conductivity of 30 W / m · K or more, but the plate-like filler (B) and the spherical filler (C) are used in combination. It is preferable that the thermal conductivity of the plate-like filler (B) is higher than the thermal conductivity of the spherical filler (C). 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.

  The plate filler (B) is at least one selected from, for example, aluminum oxide, magnesium oxide, boron nitride, talc, aluminum nitride, graphite, and graphene.

Moreover, when using together a plate-like filler (B) and a spherical filler (C), a plate-like filler (B) and a spherical filler (C) are respectively aluminum oxide, magnesium oxide, boron nitride, talc, aluminum nitride, for example. , 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.
As the plate-like filler (B), boron nitride is more preferable among the above. Among the above, the spherical filler (C) is more preferably aluminum oxide or magnesium oxide, and it is particularly preferable to use boron nitride as the plate-like filler (B) and magnesium oxide as the spherical filler (C).
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 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, from the viewpoint of improving the thermal conductivity and flexibility of the foam sheet.
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.

From the viewpoint of increasing the thermal conductivity of the foam sheet, the plate filler (B) is preferably 10 parts by mass or more, and more preferably 30 parts by mass or more with respect to 100 parts by mass of the resin (A). More preferably, it is more preferably 60 parts by mass or more. In addition, the plate-like filler (B) is 200 with respect to 100 parts by mass of the resin (A) from the viewpoint of improving the foamability of the foamable composition described later and securing the flexibility of the foam sheet. The amount is preferably at most 100 parts by mass, more preferably at most 100 parts by mass.
In the case where the plate-like filler (B) and the spherical filler (C) are used in combination, the spherical filler (C) is 30% relative to 100 parts by mass of the resin (A) from the viewpoint of increasing the thermal conductivity of the foam sheet. The amount is preferably at least part by mass, more preferably at least 100 parts by mass. In addition, the spherical filler (C) is 500 masses with respect to 100 parts by mass of the resin (A) from the viewpoint of improving the foamability of the foamable composition described later and securing the flexibility of the foam sheet. Part or less, preferably 350 parts by weight or less.
The total amount of the plate-like filler (B) and the spherical filler (C) is based on 100 parts by mass of the resin (A) from the viewpoint of improving the balance of thermal conductivity, flexibility and foamability of the foam sheet. 40-700 mass parts is preferable, 130-550 mass parts is more preferable, 160-450 mass parts is more preferable.
In addition, the quantity of the plate-like filler (B) or the spherical filler (C) with respect to 100 parts by mass of the resin (A) in the foamable composition described later is the plate-like filler with respect to 100 parts by mass of the resin (A) in the foam sheet. It becomes substantially the same as the amount of (B) or spherical filler (C).

In the foamable composition described later, the total volume% of the plate-like filler (B) and the spherical filler (C) is from the viewpoint of improving the balance of thermal conductivity, flexibility and foamability of the foam sheet. It is preferably 20 to 70% by volume, and more preferably 30 to 60% by volume.
The volume% can be calculated from the mass of each component constituting the foamable composition. For example, the volume% can be calculated by dividing the mass of each component by the density at 23 ° C. of each component.

[Foaming agent]
The foam sheet is foamed with a foaming agent. Usually, a foamable composition further containing a foaming agent is added to the components (A) to (B), preferably the components (A) to (C). Foamed with 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 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.

The average particle size of the pyrolytic foaming agent is preferably 15 μm or less, more preferably 10 μm or less, and even more preferably 8 μm or less. By using such a pyrolytic foaming agent having a small average particle diameter, bubbles having a small diameter are likely to be generated, and as a result, the thermal conductivity of the foam sheet can be increased. In addition, by using a pyrolytic foaming agent having a small average particle diameter, foamability is improved, the apparent density of the foam sheet is lowered, and a flexible foam sheet is easily obtained. That is, when a pyrolytic foaming agent having a small average particle diameter is used, a foam sheet having an excellent balance between thermal conductivity and flexibility can be obtained. The average particle size of the pyrolytic foaming agent is usually 0.1 μm or more, preferably 1.0 μm or more.
The average particle size of the pyrolytic foaming agent can be determined by measuring with a laser diffraction / scattering particle size distribution analyzer.

[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 1.3 to 3.0 W / m · K. Good heat dissipation can be obtained when the thermal conductivity is 1.3 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 1.5 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 manufacturing method of the foam sheet of this invention, the method of foaming the foamable composition which mix | blended resin (A) and plate-like filler (B), and obtaining a foam sheet is mentioned. A preferable method for producing a foam sheet includes a method of foaming a resin (A), a plate-like filler (B), and a foamable composition containing a thermally decomposable foaming agent having an average particle size of 15 μm or less. By foaming a pyrolytic foaming agent having a small average particle diameter, a foam sheet having a small average cell diameter can be easily obtained. Hereinafter, this production method will be described in more detail.

  In this production method, the foamable composition is formed by, for example, extruding a resin (A), a plate-like filler (B), a spherical filler (C) as necessary, and a foaming agent and other optional components as necessary. What is necessary is just to shape | mold into a sheet form by supplying to a machine, melt-kneading, and extruding from an extruder. Alternatively, a resin (A), a plate-like filler (B), a spherical filler (C) as necessary, and a foaming agent and other optional components as necessary are kneaded using a calendar, conveyor belt casting, or the like. However, what is necessary is just to make a foamable composition into a sheet form by conveying continuously. Further, the foamable composition can be obtained by pressing a resin (A), a plate-like filler (B), a spherical filler (C) if necessary, and a foaming agent and other optional components as necessary. It may be a sheet. When the foamable composition is made into a sheet by these methods, the plate-like filler may be oriented in the surface direction. However, such a plate-like filler also has a small diameter in the foam sheet as described above. By doing so, it is assumed that it becomes easy to orient in the thickness direction of the sheet at the time of foaming.

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, a raw material for 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), and a spherical filler (if necessary) A thermally conductive foam sheet may be obtained by generating and curing bubbles in a curable composite material containing C) and / or other components.

<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.
[Average particle size of foaming agent]
The average particle size of the foaming agent was measured using a laser diffraction / scattering particle size distribution analyzer (HELOS / BFM, manufactured by Sympatec GmbH).
[Apparent density]
It measured based on JISK7222.
[Average bubble diameter]
Samples for measurement were prepared by cutting the thermally conductive foam sheets obtained in Examples and Comparative Examples into 50 mm squares. This was immersed in liquid nitrogen for 1 minute and then cut with a razor blade in the thickness direction along the MD and TD directions. This cross-section is taken with a digital microscope (Keyence Co., Ltd. “VHX-900”), and a 200 times magnified photograph is taken, and all the closed cells present on the cut surface of 2 mm in length in each of MD and TD directions. The bubble diameter was measured and the operation was repeated 5 times. The average value of all the bubbles is defined as the average bubble diameter in the MD direction and the TD direction, and the average value of the average bubble diameter in the MD direction and the average bubble diameter in the TD direction [(average bubble diameter in the MD direction + average bubble in the TD direction). Diameter) / 2] was defined as the average bubble diameter in the present invention.
[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, “EP21”, solid state)
Ethylene propylene diene rubber (2) (Mitsui Chemicals, "PX-68", liquid)
<Plate filler (B)>
Boron Nitride (Momentive, “PTX25S” average length: 25 μm, thermal conductivity: 60 W / m · K)
<Spherical filler (C)>
Magnesium oxide (Ube Materials Co., Ltd., “RF-70C-SC” average particle size: 70 μm, thermal conductivity: 50 W / m · K)
<Foaming agent>
Azodicarbonamide (1) (Otsuka Chemical Co., Ltd., “Unihome AZ Ultra # 1035-I” average particle size 3.5 μm)
Azodicarbonamide (2) (Otsuka Chemical Co., Ltd., “Unihome AZ Ultra # 1067-I” average particle size 6.7 μm)
Azodicarbonamide (3) (Otsuka Chemical Co., Ltd., “Unihome AZ Ultra # 1195-I” average particle size 20 μm)
Azodicarbonamide (4) (manufactured by Eiwa Chemical Industry Co., Ltd., “Binihol AC-K3-TA” average particle size 20 μm)

[Example 1]
Each component of resin (A), plate-like filler (B), spherical filler (C), foaming agent and phenolic antioxidant is mixed in parts by mass shown in Table 1, and melt-kneaded at 130 ° C. with a plastmill. The foamable composition thus obtained 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 1 shows the evaluation results.

[Example 2, 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.

It was found that the foam sheets satisfying the requirements of the present invention in Examples 1 and 2 had high thermal conductivity after foaming and good flexibility.
On the other hand, from the results of Comparative Examples 1 and 2, when the average cell diameter is outside the range of the present invention, the thermal conductivity is inferior, and the apparent density is higher and the flexibility is inferior compared with the Examples. It was.

Claims (8)

  A thermally conductive foam sheet comprising a resin (A) and a plate-like filler (B) dispersed in the resin (A) and having an average cell diameter of 70 μm or less.   The heat conductive foam sheet of Claim 1 containing the spherical filler (C) disperse | distributed in resin (A).   The thermal conductivity according to claim 1 or 2, wherein the plate-like filler (B) is at least one selected from the group consisting of aluminum oxide, magnesium oxide, boron nitride, talc, aluminum nitride, graphite, and graphene. Foam sheet.   The thermally conductive foam sheet according to claim 2, wherein 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 thermal conductivity according to any one of claims 1 to 4, 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. Foam sheet.   The heat conductive foam sheet in any one of Claims 1-5 whose thickness is 0.05-2.0 mm. The heat conductive foam sheet according to any one of claims 1 to 6, wherein the apparent density is 0.1 to 1.5 g / cm ³ . The thermal conductivity according to any one of claims 1 to 7, wherein a foamable composition containing a resin (A), a plate-like filler (B), and a pyrolytic foaming agent having an average particle size of 15 µm or less is foamed. A method for producing a foam sheet.