JP2021055043A - Conductive foam sheet - Google Patents

Abstract

To provide a foamed composite sheet having high flexibility and excellent conductivity, and a method of producing the same.SOLUTION: A conductive foam sheet has a polyolefin resin and at least one carbon filler, the conductive foam sheet having a 25% compression strength of 1 MPa or less and a surface resistivity of 2000 Ω/sq or less.SELECTED DRAWING: None

Description

The present invention relates to a conductive foam sheet.

In electronic devices, due to the influence of electrical (electromagnetic) noise of internal noise whose noise source is inside the electronic device and external noise whose noise source is outside the electronic device, parts mounted on the electronic device may be damaged or malfunction. It may occur. For example, in communication electronic devices such as smartphones and base stations, electrical noise may interfere with the built-in wireless system and communication may be hindered, resulting in deterioration of electronic device performance and malfunction. Further, in vehicle electronic devices such as an engine control unit (ECU) and an advanced driver assistance system (ADAS), if a vehicle malfunctions due to electrical noise, a serious accident may occur. ..
Therefore, in order to suppress the influence of electrical noise on the electronic device, measures may be taken to impart conductivity to the members of the electronic device. For example, it has been proposed to add a conductive additive to a foam in which a vinyl or vinylide monomer and an aliphatic α-olefin are distributed to impart conductivity (see, for example, Patent Document 1).

Special Table 2001-520291

The present inventors have paid attention to the gasket material and the cushioning material provided for sealing the gap between the housing of the electronic device and the substrate and absorbing the impact and vibration, and as the gasket material and the cushioning material. We are studying measures to reduce the effects of electrical noise by adding conductivity to the foam sheet. However, although the conventional foamed sheet has flexibility, it is difficult to impart sufficient conductivity, and it is difficult to sufficiently reduce the influence of electrical noise. Further, if the conductivity of the foam sheet is insufficient, the electromagnetic wave shield case or the like may be charged, and the charged electromagnetic wave shield case itself generates electromagnetic waves, so that it cannot serve as an electromagnetic wave shield case. There was a case that it ended up.
The present invention has been made in view of the above problems, and an object of the present invention is to provide a conductive foamed sheet having high flexibility and excellent conductivity.

As a result of diligent studies, the present inventors have found that the above problems can be solved by using a conductive foamed sheet containing a polyolefin resin and at least one type of carbon filler, and have completed the following invention. ..
That is, the present invention provides the following [1] to [13].
[1] A conductive foamed sheet containing at least one resin of a polyolefin resin and a thermoplastic elastomer and at least one carbon filler, having a 25% compressive strength of 1 MPa or less and a surface resistivity of 2000 Ω. A conductive foamed sheet having a value of / sq or less.
[2] The conductive foamed sheet according to [1], which contains a polyolefin-based resin.
[3] The conductive foamed sheet according to [1] or [2], wherein the carbon filler contains scaly graphite.
[4] The conductive foamed sheet according to [3], wherein the scaly graphite has an average aspect ratio of 0.001 to 0.200.
[5] The conductive foamed sheet according to any one of [1] to [4], wherein the carbon filler contains carbon black.
[6] The conductive foamed sheet according to [5], wherein the carbon black has a primary particle size of 5 nm or more and 100 nm or less.
[7] The conductive foamed sheet according to any one of [1] to [6], wherein the polyolefin-based resin contains a polyethylene-based resin.
[8] The conductive foamed sheet according to any one of [1] to [7], which has a thickness of 0.1 mm or more and 5.0 mm or less.
[9] The conductive foamed sheet according to any one of [1] to [8], wherein the foaming ratio is 1.2 times or more and 20 times or less.
[10] The conductive foamed sheet according to any one of [1] to [9], wherein the closed cell ratio is 70% or more.
[11] The conductive foam sheet according to any one of [1] to [10], wherein the electromagnetic wave shielding property at a frequency of 5 GHz by the coaxial method is 7 dB or more.
[12] The conductive foamed sheet according to any one of [1] to [11], which is crosslinked.
[13] The conductive foamed sheet according to [12], wherein the gel fraction is 10% by mass or more and 60% by mass or less.

According to the present invention, it is possible to provide a conductive foamed sheet having high flexibility and excellent conductivity.

Hereinafter, the present invention will be described in more detail using embodiments.
[Conductive foam sheet]
The conductive foamed sheet in the present invention is a conductive foamed sheet containing at least one resin of a polyolefin resin and a thermoplastic elastomer and at least one carbon filler, and has a 25% compressive strength of 1 MPa or less. , The surface resistance is 2000Ω / sq or less.

(25% compression strength)
If the 25% compressive strength of the conductive foamed sheet is more than 1 MPa, the flexibility becomes insufficient, and the sealing property and the shock absorption property are lowered. The 25% compressive strength of the conductive foamed sheet is preferably 500 kPa or less, more preferably 460 kPa or less, further preferably 430 kPa or less, and further preferably 400 kPa or less, from the viewpoint of having sufficient flexibility. Is particularly preferred.
The 25% compressive strength of the conductive foamed sheet is preferably 50 kPa or more, more preferably 100 kPa or more, and even more preferably 150 kPa or more, from the viewpoint of improving the mechanical strength.
The 25% compressive strength in the present invention can be measured according to the method described in Examples described later.

(Surface resistivity)
When the surface resistivity of the conductive foamed sheet exceeds 2000 Ω / sq, the conductivity of the conductive foamed sheet is low, and the shielding property against electrical noise becomes unsuitable. The surface resistivity of the conductive foamed sheet is preferably 2000 Ω / sq or less, and more preferably 1500 Ω / sq or less, from the viewpoint of improving conductivity and improving shielding property against electrical noise and antistatic property. It is more preferably 1000 Ω / sq or less, further preferably 500 Ω / sq or less, further preferably 300 Ω / sq or less, further preferably 250 Ω / sq or less, and even more preferably 200 Ω. It is even more preferably / sq or less, even more preferably 68Ω / sq or less, even more preferably 65Ω / sq or less, and particularly preferably 63Ω / sq or less. The lower the surface resistivity of the conductive foamed sheet, the better the conductivity. However, for example, it is 0.1 Ω / sq or more, and practically 1 Ω / sq or more.
The surface resistivity in the present invention can be measured by the four-probe method in accordance with JIS K7194: 1994, and can be measured by the method described in Examples.

(Thickness)
The thickness of the conductive foamed sheet is preferably 0.1 mm or more and 5.0 mm or less, more preferably 0.1 mm or more and 4.0 mm or less, and further preferably 0.1 mm or more and 3.0 mm or less. It is preferable, and it is particularly preferable that it is 0.5 mm or more and 3.0 mm or less. When the thickness of the conductive foamed sheet is at least the above lower limit value, it becomes easy to secure electromagnetic wave shielding property, sealing property, shock absorbing property and the like. Further, when the thickness of the conductive foamed sheet is not more than the above upper limit value, the molded product can be made thinner, and it becomes easy to secure the flexibility.

(Effervescence magnification)
The foaming ratio of the conductive foamed sheet is preferably 1.2 times or more and 20 times or less, more preferably 1.5 times or more and 10 times or less, and preferably 1.8 times or more and 5 times or less. It is more preferable, and it is particularly preferable that it is 2.0 times or more and 5 times or less. When the foaming ratio of the conductive foamed sheet is within the above range, the conductive foamed sheet is appropriately foamed to improve the flexibility, and the sealing property, shock absorption and the like are likely to be improved. In addition, a certain level of mechanical strength is imparted to the conductive foamed sheet, and impact resistance and the like are also improved.
The foaming ratio in the present invention can be measured according to the method described in Examples described later.

(Closed cell ratio)
The conductive foamed sheet preferably has a closed cell ratio of 70% or more from the viewpoint of obtaining sufficient sealing properties. From the above viewpoint, the closed cell ratio of the conductive foamed sheet is preferably 80 to 100%, more preferably 90 to 100%.
The closed cell ratio in the present invention can be measured according to the method described in Examples described later.

(Electromagnetic wave shielding property)
The electromagnetic wave shielding property of the conductive foam sheet at a frequency of 5 GHz by the coaxial method is preferably 7 dB or more, more preferably 10 dB or more, further preferably 13 dB or more, and particularly preferably 16 dB or more. preferable. When the electromagnetic wave shielding property of the conductive foam sheet at a frequency of 5 GHz by the coaxial method is at least the above lower limit value, electrical noise can be blocked and the influence of electrical noise can be suppressed.
The electromagnetic wave shielding property in the present invention can be measured by the coaxial method, and can be measured by the method described in Examples.

(Tensile strength)
The tensile strength of the conductive foamed sheet is not particularly limited, but from the viewpoint of increasing the mechanical strength of the conductive foamed sheet, the tensile strength in MD is 1.5 to 7.0 MPa, and the tensile strength in TD is 1. The tensile strength in MD is preferably 2.0 to 5.0 MPa, and the tensile strength in TD is more preferably 1.2 to 4.0 MPa.
The tensile strength in MD and TD refers to the tensile strength at break when the conductive foamed sheet is stretched to MD and TD at 23 ° C., and is measured in accordance with JIS K6767: 1999.
Further, the tensile elongation is measured in both the MD and TD directions, and the one having the higher tensile elongation is adopted. When it is unclear which direction is the MD or TD direction, the tensile elongation in the direction in which the tensile elongation is maximized may be measured. In the present invention, the MD direction is an abbreviation for Machine Direction, and coincides with the flow direction of the resin at the time of coating or the like described later. Further, in the present invention, the TD direction is an abbreviation for Transverse Direction, and is a direction orthogonal to the MD direction.

(Elongation at break)
The elongation at break of the conductive foamed sheet is preferably 20% or more, more preferably 30% or more, and further preferably 40% or more. When the breaking elongation of the conductive foamed sheet is within the above range, it is possible to prevent the sheet from breaking while maintaining the shape stability.
In the present specification, the breaking elongation of the conductive foamed sheet means the elongation when the conductive foamed sheet is pulled until it breaks and the tensile force is maximized. The elongation at break is measured according to the method described in JIS K6767: 1999.
As the breaking elongation in the present invention, the breaking elongation in both the MD and TD directions is measured, and the breaking elongation in the MD and TD directions is averaged. Further, in the highly flexible conductive sheet, when it is unclear which direction is the MD direction or the TD direction, the breaking elongation in the direction in which the breaking elongation is maximized and the direction in which the breaking elongation is maximized. The tensile elongation in the direction orthogonal to is measured, and the average of them is adopted.

(Gel fraction)
The conductive foamed sheet is preferably a crosslinked body. The gel content indicating the degree of cross-linking of the conductive foamed sheet is preferably 10% by mass or more and 60% by mass or less, more preferably 10% by mass or more and 50% by mass or less, and 12% by mass or more and 47% by mass or less. It is more preferably 15% by mass or more, and particularly preferably 45% by mass or less. When the gel fraction of the conductive foamed sheet is at least the above lower limit value, sufficient crosslinks are formed, and by foaming the crosslinks, the moldability is improved. Further, when the gel fraction of the conductive foamed sheet is not more than the above upper limit value, flexibility can be ensured, and sealing property, shock absorption and the like can be improved.
The gel fraction in the present invention can be measured according to the method described in Examples described later.

The conductive foamed sheet of the present invention contains at least one resin of a polyolefin resin and a thermoplastic elastomer, and preferably contains a polyolefin resin.
(Polyolefin resin)
Examples of the polyolefin-based resin include polyethylene-based resins, polypropylene-based resins, and mixtures thereof, and among these, polyethylene-based resins are preferable.

<Polyethylene resin>
Examples of the polyethylene-based resin include an ethylene homopolymer, an ethylene-α-olefin copolymer, and an ethylene-vinyl acetate copolymer. The ethylene-α-olefin copolymer is a polyethylene-based copolymer obtained by copolymerizing ethylene with α-olefin, which contains ethylene as a main component (usually 70% by mass or more, preferably 90% by mass or more of all the monomers). It is a resin. Specific examples of the α-olefin constituting the polyethylene-based resin include propylene, 1-butene, 1-pentene, 4-methyl-1-pentene, 1-hexene, 1-hexene, 1-octene and the like. ..
Further, examples of the ethylene-vinyl acetate copolymer usually include a copolymer in which ethylene is used in an amount of 50% by mass or more of all the monomers.
The polyethylene-based resin may be any of linear low-density polyethylene, low-density polyethylene, medium-density polyethylene and high-density polyethylene, but linear low-density polyethylene and low-density polyethylene are preferable. The density of the linear low density polyethylene is preferably 0.870~0.930g / cm ^3, more preferably 0.875~0.925g / cm ^3, more preferably 0.880~0.921g / cm ³ .. As the polyethylene resin, a plurality of polyethylene resins may be used, or a polyethylene resin other than the above-mentioned density range may be added.

The polyethylene-based resin preferably contains a polyethylene-based resin polymerized with a polymerization catalyst of a metallocene compound, and particularly preferably contains a linear low-density polyethylene polymerized with a polymerization catalyst of a metallocene compound. By using the polyethylene-based resin obtained by the polymerization catalyst of the metallocene compound, it becomes easy to obtain a conductive foamed sheet having high flexibility, high sealing property and shock absorption.

《Metallocene compound》
Suitable metallocene compounds include compounds such as bis (cyclopentadienyl) metal complexes having a structure in which a transition metal is sandwiched between π-electron unsaturated compounds. More specifically, one or more cyclopentadienyl rings or their analogs are present as ligands in tetravalent transition metals such as titanium, zirconium, nickel, palladium, hafnium, and platinum. Examples of the compound.
In such a metallocene compound, the properties of active sites are uniform, and each active site has the same activity. Therefore, the polymer synthesized using the metallocene compound has high uniformity in molecular weight, molecular weight distribution, composition, composition distribution, etc. Therefore, when a sheet containing the polymer synthesized using the metallocene compound is crosslinked, the polymer is crosslinked. Crosslinking proceeds uniformly. Since the uniformly crosslinked sheet is easily stretched uniformly, it is easy to make the thickness of the resin conductive foamed sheet uniform, and it is easy to maintain high performance even if the thickness is reduced.

Examples of the ligand include a cyclopentadienyl ring, an indenyl ring and the like. These cyclic compounds may be substituted with hydrocarbon groups, substituted hydrocarbon groups or hydrocarbon-substituted metalloid groups. Examples of the hydrocarbon group include methyl group, ethyl group, various propyl group, various butyl group, various amyl group, various hexyl group, 2-ethylhexyl group, various heptyl group, various octyl group, various nonyl group and various decyl group. , Various cetyl groups, phenyl groups and the like. In addition, "various" means various isomers including n-, sec-, tert-, and iso-.
Further, a product obtained by polymerizing a cyclic compound as an oligomer may be used as a ligand.
Furthermore, in addition to π-electron unsaturated compounds, monovalent anion ligands such as chlorine and bromine, divalent anion chelate ligands, hydrocarbons, alkoxides, arylamides, aryloxides, amides, arylamides, phosphides, and aryls. You may use phosphide or the like.

Examples of metallocene compounds containing tetravalent transition metals and ligands include cyclopentadienyl titanium tris (dimethylamide), methylcyclopentadienyl titanium tris (dimethylamide), bis (cyclopentadienyl) titanium dichloride, and dimethyl. Examples thereof include silyltetramethylcyclopentadienyl-t-butylamide zirconium dichloride.
The metallocene compound exerts an action as a catalyst in the polymerization of various olefins by combining with a specific co-catalyst (co-catalyst). Specific examples of the co-catalyst include methylaluminoxane (MAO) and boron-based compounds. The ratio of the cocatalyst used to the metallocene compound is preferably 10 to 1 million mol times, more preferably 50 to 5,000 mol times.

<Polypropylene resin>
The polypropylene-based resin is obtained, for example, by using a propylene homopolymer or propylene as a main component (usually 70% by mass or more, preferably 90% by mass or more of all the monomers) and copolymerizing propylene and α-olefin. Examples thereof include propylene-α-olefin copolymers and the like. One of these may be used alone, or two or more thereof may be used in combination.
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-hexene, and 1-. Octene and the like can be mentioned.

<Other resin components>
As the resin component constituting the conductive foamed sheet, only the polyolefin-based resin may be contained, but a resin component other than the polyolefin-based resin may be contained as long as the object of the present invention is not impaired. Examples of the resin component other than the polyolefin-based resin include various elastomers. The content of the resin component other than the polyolefin resin is preferably 30% by mass or less, more preferably 20% by mass or less, still more preferably 10% by mass or less, based on the total amount of the resin component. ..

《Elastomer》
The elastomer is not particularly limited, but an olefin rubber is preferable. The olefin-based rubber is an amorphous or low-crystalline rubber-like substance in which two or more types of olefin-based monomers are copolymerized substantially randomly, and an ethylene-α-olefin-based copolymer rubber is preferable. Here, as the α-olefin used in the ethylene-α-olefin copolymer rubber, the number of carbon atoms of propylene, 1-butene, 2-methylpropylene, 3-methyl-1-butene, 1-hexene and the like is 3 Examples thereof include one or more olefins of about 10 to 10, and propylene is preferable among these.
Further, the olefin-based rubber may contain a repeating unit composed of a monomer other than the olefin, and the monomer includes 5 carbon atoms such as ethylidene norbornene, 1,4-hexadiene, and dicyclopentadiene. Diene compounds typified by about 15 non-conjugated diene compounds can be mentioned.
The olefin rubber may be used alone or in combination of two or more. The olefin-based rubber may be a liquid rubber that becomes liquid at room temperature (23 ° C.) or a solid rubber that becomes solid at room temperature.
Specific examples of the preferable olefin-based rubber include ethylene-propylene rubber (EPM) and ethylene-propylene-diene rubber (EPDM), and EPDM is more preferable.
Examples of the elastomer include acrylonitrile-butadiene rubber, natural rubber, polybutadiene rubber, polyisoprene rubber, styrene-butadiene block copolymer, hydrogenated styrene-butadiene block copolymer, and hydrogenated styrene, in addition to the above-mentioned olefin rubber. -Butadiene-styrene block copolymer, hydrogenated styrene-isoprene block copolymer, hydrogenated styrene-isoprene-styrene block copolymer and the like may also be used.

(Thermoplastic elastomer)
The conductive foamed sheet of the present invention may contain a thermoplastic elastomer in place of the polyolefin-based resin or together with the polyolefin-based resin. A thermoplastic elastomer is a material that flows when heated and can be molded in the same manner as ordinary thermoplastics, and exhibits rubber elasticity at room temperature. The thermoplastic elastomer is not particularly limited, but for example, a styrene-based thermoplastic elastomer, an olefin-based thermoplastic elastomer, a vinyl chloride-based thermoplastic elastomer, a urethane-based thermoplastic elastomer, a polyester-based thermoplastic elastomer, a polyamide-based thermoplastic elastomer, and an ionomer. Examples thereof include thermoplastic elastomers and liquid crystal thermoplastic elastomers. Among these, styrene-based thermoplastic elastomers are preferable from the viewpoint of further improving the flexibility of the conductive foamed sheet.

Examples of the styrene-based thermoplastic elastomer include styrene-ethylene-propylene copolymer (SEP), styrene-ethylene-propylene-styrene copolymer (SEPS), and styrene-ethylene-ethylene-propylene-styrene copolymer (SEEPS). , Styrene-ethylene-butylene-styrene copolymer (SEBS), styrene-butadiene block copolymer (SB), styrene-butadiene-styrene block copolymer (SBS), hydrogenated styrene-butadiene block copolymer (HSB) ), Hydrogenated styrene-butadiene-styrene block copolymer (HSBS), hydrogenated styrene-isoprene block copolymer (HSI), hydrogenated styrene-isoprene-styrene block copolymer (HSIS) and the like. These styrene-based thermoplastic elastomers may be used alone or in combination of two or more. Among these, styrene-ethylene-propylene copolymer (SEP), styrene-ethylene-propylene-styrene copolymer (SEPS), and styrene, from the viewpoint of further improving the flexibility and conductivity of the conductive foamed sheet. -Ethethylene-ethylene-propylene-styrene copolymer (SEEPS) and styrene-ethylene-butylene-styrene copolymer (SEBS) are preferred.

<Other resin components>
The conductive foamed sheet of the present invention may contain only a thermoplastic elastomer as a resin component, but may contain a resin component other than the thermoplastic elastomer. For example, the above-mentioned polyolefin resin may be further contained from the viewpoint of further improving the flexibility and conductivity of the conductive foamed sheet. In this case, the content of the polyolefin resin is preferably 25 to 75 parts by mass, preferably 25 to 60 parts by mass, based on 100 parts by mass of the total content of the polyolefin resin and the thermoplastic elastomer. More preferably, 30 to 55 parts by mass is further preferable.
The total content of the thermoplastic elastomer content and the olefin resin content is preferably 70% by mass or more, more preferably 80% by mass or more, and 90% by mass, based on the total amount of the resin components. It is more preferably% or more.
The conductive foamed sheet of the present invention may contain various elastomers other than the thermoplastic elastomer as a resin component other than the thermoplastic elastomer.
The content of the resin component other than the thermoplastic elastomer and the polyolefin-based resin is preferably 50% by mass or less, more preferably 30% by mass or less, and 10% by mass or less, based on the total amount of the resin component. Is even more preferable.

At this time, the elastomer used is not particularly limited as long as it is other than the thermoplastic elastomer, but among the above-mentioned olefin-based rubbers, those other than the thermoplastic elastomer are preferable.
As the elastomer, in addition to the above-mentioned olefin rubbers other than the thermoplastic elastomer, for example, acrylonitrile butadiene rubber, natural rubber, polybutadiene rubber, polyisoprene rubber and the like may be used.

(Carbon filler)
The conductive foamed sheet of the present invention contains at least one carbon filler. Examples of the carbon filler include graphite and carbon black. Examples of the shape of graphite include scaly, needle-like, fibrous, spherical, flake-like, agglomerate-like, and porous-like shapes.
The conductive foamed sheet preferably contains two or more types of carbon fillers having different functions. For example, it is preferable to contain a carbon filler that enhances the conductivity of the conductive foamed sheet, and it is preferable that the scaly graphite enhances the conductivity of the conductive foamed sheet due to the characteristics of the shape of graphite. Further, it is preferable to contain a carbon filler that forms a conductive path in the conductive foam sheet, and it is preferable to contain carbon black that forms a conductive path by chaining the carbon fillers to form a structure.

The content of the carbon filler is preferably 7% by mass or more and 70% by mass or less, more preferably 10% by mass or more and 60% by mass or less, and 15% by mass or more and 55% by mass or less, based on the total amount of the conductive foamed sheet. It is more preferably 20% by mass or less, and even more preferably 20% by mass or more and 50% by mass or less. When the content of the carbon filler is within the above range, the conductivity of the conductive foamed sheet can be improved, and the electromagnetic wave shielding property can be improved.

<Scale graphite>
The scaly graphite is not particularly limited as long as it is a carbon filler having a scaly shape. The scale-like graphite has a large aspect ratio of particles, exhibits high conductivity in the major axis direction, and can improve the conductivity of the conductive foamed sheet. Here, the aspect ratio means the thickness / major axis. In the present specification, the aspect ratio is an average aspect ratio. Specifically, 50 arbitrarily selected particles are observed with an electron microscope or an optical microscope, and the average value of the thickness / major axis of each particle is calculated. It is required by doing. The aspect ratio of the scaly graphite is preferably 0.001 to 0.200, more preferably 0.005 to 0.150, and even more preferably 0.010 to 0.100.

The average particle size (major diameter) of the scaly graphite is preferably 10 μm or more and 500 μm or less, more preferably 15 μm or more and 400 μm or less, and further preferably 20 μm or more and 300 μm or less. When the average particle size (major axis) of the scaly graphite is not more than the above upper limit value, the dispersibility of the scaly graphite in the effervescent composition can be improved. When the average particle size (major axis) of the scaly graphite is at least the above lower limit value, the contact probability between the scaly graphites increases, and the conductivity can be improved. Here, for the average particle size, for example, scaly graphite is sufficiently dispersed in a THF solution, and the particle size is measured using a laser diffraction type particle size distribution meter (manufactured by Shimadzu Corporation, product name "SALD-2200"). It is an average value.

The content of scaly graphite is preferably 10% by mass or more and 50% by mass or less, more preferably 15% by mass or more and 45% by mass or less, and 20% by mass or more and 40% by mass or less, based on the total amount of the conductive foamed sheet. It is more preferably less than or equal to%. When the content of scaly graphite is within the above range, the contact probability between scaly graphites can be ensured and the conductivity can be improved.

<Carbon black>
The shape of carbon black is not particularly limited. The carbon black includes, for example, a hollow carbon filler such as Ketjen black having a hollow shape having a hollow inside.
In carbon black, a conductive path is formed by chaining carbon blacks with each other, and the conductivity of the conductive foamed sheet can be improved. In order for the carbon blacks to be linked well, the amount of dibutyl phthalate (DBP) oil absorbed by the carbon blacks is preferably 100 mL / 100 g or more, more preferably 200 mL / 100 g or more, and 300 mL / 100 g or more. It is more preferable to have. When the DBP oil absorption amount of the carbon black is at least the above lower limit value, the bonding between the carbon blacks becomes good, and a wide range of conductive paths can be formed. Here, the DBP oil absorption amount of carbon black can be measured by using, for example, an absorption amount measuring device (manufactured by Asahi Soken Co., Ltd., trade name "S500") in accordance with JIS K 6217-4: 2008.

The primary particle size of carbon black is preferably 5 nm or more and 100 nm or less, more preferably 7 nm or more and 80 nm or less, and further preferably 10 nm or more and 60 nm or less. When the primary particle size of the carbon black is within the above range, the dispersibility is good, and the bonding between the carbon blacks is good, so that a wide range of conductive paths can be formed. Here, the primary particle diameter of carbon black can be obtained by measuring the diameters of 100 particles with a transmission electron microscope and calculating the average value thereof.

The content of carbon black is preferably 0.01% by mass or more and 20% by mass or less, more preferably 0.1% by mass or more and 15% by mass or less, based on the total amount of the conductive foamed sheet. It is more preferably 5% by mass or more and 10% by mass or less. When the content of carbon black is within the above range, the contact probability between carbon blacks can be ensured and the conductivity can be improved.

The conductive foamed sheet of the present invention is formed by foaming a foaming composition containing a foaming agent in addition to the above-mentioned resin component and carbon filler, and preferably the foamable composition is crosslinked and foamed. It was done.

<< Foaming agent >>
As the foaming agent, a thermal decomposition type foaming agent can be used, and for example, a chemical foaming agent such as an organic foaming agent and an inorganic foaming agent having a decomposition temperature of about 160 to 270 ° C. can be used.
Examples of the organic foaming agent include azodicarboxylic amides, azodicarboxylic acid metal salts (Azodicarboxylic acid barium, etc.), azo compounds such as azobisisobutyronitrile, and nitroso compounds such as N, N'-dinitrosopentamethylenetetramine. Examples thereof include hydrazine derivatives such as hydrazodicarboxylic amide, 4,4'-oxybis (benzenesulfonyl hydrazide) and toluenesulfonylhydrazide, and semicarbazide compounds such as toluenesulfonyl semicarbazide.
Examples of the inorganic foaming agent include ammonium carbonate, sodium carbonate, ammonium hydrogencarbonate, sodium hydrogencarbonate, ammonium nitrite, sodium borohydride, monosoda anhydrous citrate and the like.
As the foaming agent, azo compounds and nitroso compounds are preferable from the viewpoint of obtaining fine bubbles, and from the viewpoint of economy and safety, azodicarbonamide, azobisisobutyronitrile, N, N'-dinitrosopentamethylene. Tetramine is more preferred, and azodicarbonamide is particularly preferred. These foaming agents can be used alone or in combination of two or more.
The amount of the foaming agent added is preferably 1 to 30 parts by mass, more preferably 2 to 20 parts by mass, and even more preferably 3 to 10 parts by mass with respect to 100 parts by mass of the resin component. When the amount of the foaming agent added is within the above range, bubbles of the foam can be appropriately foamed without bursting.

<< Other additives >>
The effervescent composition of the present invention contains additives such as antioxidants, effervescent aids, cross-linking aids, flame retardants, metal damage inhibitors, antistatic agents, stabilizers, fillers, pigments, etc., as required. It may be contained.

Examples of the antioxidant include phenol-based antioxidants, sulfur-based antioxidants, phosphorus-based antioxidants, amine-based antioxidants, etc. Among these, phenol-based antioxidants and sulfur-based antioxidants. Is preferable, and it is more preferable to use a phenol-based antioxidant and a sulfur-based antioxidant in combination.
Examples of the phenolic antioxidant include 2,6-di-tert-butyl-p-cresol, n-octadecyl-3- (3,5-di-tert-butyl-4-hydroxyphenyl) propionate, and 2-tert-. Butyl-6- (3-tert-butyl-2-hydroxy-5-methylbenzyl) -4-methylphenylacrylate, tetrakis [methylene-3- (3,5-di-tert-butyl-4-hydroxyphenyl) propionate ] Methyl and the like can be mentioned. These phenolic antioxidants may be used alone or in combination of two or more.
Examples of the sulfur-based antioxidant include dilauryl thiodipropionate, dilaurylthiodipropionate, dimyristylthiodipropionate, distearylthiodipropionate, and pentaerythrityl tetrakis (3-laurylthiopropionate). And so on. These sulfur-based antioxidants may be used alone or in combination of two or more.
The content of the antioxidant is preferably 0.1 to 10 parts by mass, more preferably 0.2 to 5 parts by mass, and even more preferably 0.3 to 3 parts by mass with respect to 100 parts by mass of the resin component.

[Manufacturing method of conductive foam sheet]
The conductive foamed sheet heat-foams a composition containing, for example, at least one resin of an olefin resin and a thermoplastic elastomer, other resin components to be blended as necessary, at least one carbon filler and an additive. It can be manufactured by allowing it to be produced. Specifically, it is industrially advantageous to manufacture by a method including the following steps (1) and (2).
Step (1): Each component of the composition containing a pyrolysis foaming agent as a foaming agent is supplied to a kneading device, melted and kneaded at a temperature lower than the decomposition temperature of the pyrolysis foaming agent, and then foamed in a sheet shape. Step of making a sex composition Step (2): A step of heating the sheet-shaped foamable composition obtained in step (1) to a temperature higher than the decomposition temperature of the foaming agent to foam it to obtain a foam.

When carbon black such as Ketjen black is blended in the above step (1), it may be difficult to secure dispersibility in the foamable composition. Therefore, in order to improve dispersibility, it is preferable to use a carbon black masterbatch in which the base resin is at least one resin of a polyolefin resin and a thermoplastic elastomer, preferably a polyolefin resin, and more preferably a polyethylene resin. ..
The content of carbon black in the carbon black masterbatch is 1.0% by mass or more and 12.0% by mass or less with respect to the total amount of the carbon black masterbatch from the viewpoint of improving the dispersibility in the foamable composition. It is more preferably 2.0% by mass or more and 10.0% by mass or less, and further preferably 3.0% by mass or more and 8.0% by mass or less.

After the step (1), it is preferable to include a step of irradiating the effervescent composition obtained in the step (1) with ionizing radiation to crosslink the effervescent composition. In the step of cross-linking, the effervescent composition is irradiated with ionizing radiation so that the gel fraction of the cross-linked effervescent composition is 10% by mass or more and 60% by mass or less. Examples of the ionizing radiation used in the step of cross-linking include α-rays, β-rays, γ-rays, electron beams and the like, but electron beams are preferable. The irradiation amount of the ionizing radiation may be as long as a desired degree of cross-linking can be obtained, but is preferably 0.1 to 10 Mrad, more preferably 0.3 to 8 Mrad, and even more preferably 0.5 to 6 Mrad. Since the irradiation amount of ionizing radiation is affected by the blending of at least one resin of olefin resin and thermoplastic elastomer and additives, the irradiation amount is usually adjusted while measuring the degree of cross-linking.

Examples of the kneading device in the above step (1) include an extruder such as a single-screw extruder and a twin-screw extruder, a general-purpose kneading device such as a Banbury mixer and a roll, and the extruder is preferable.
In the step (2), the temperature at which the effervescent composition is heated and foamed depends on the decomposition temperature of the thermally decomposable foaming agent used as the foaming agent, but is usually 140 to 300 ° C., preferably 150 to 260 ° C.

[How to use conductive foam sheet]
The conductive foam sheet of the present invention is used, for example, inside an electronic device. Electronic devices include mobile devices such as mobile phones, tablet terminals, electronic papers, notebook PCs, video cameras and digital cameras, and vehicle-based electronic devices such as engine control units (ECU) and advanced driver assistance systems (ADAS). In addition, wearable terminal devices and the like used in aspects such as clocks and eyeglasses are preferable. The conductive foamed sheet is arranged in the vicinity of an electronic component that generates electrical noise or an insulator that is easily charged, and is used as a conductive sheet that suppresses electrical noise and charging. As described above, the conductive foamed sheet of the present invention can be suitably used as a sheet for parts inside an electronic device that is a source of electrical noise or charging. Since the conductive foam sheet has high flexibility and can be made thin, it can be appropriately placed even in a narrow space.

Further, the conductive foam sheet is attached to a specific component inside the electronic device or provided around the component, and is used as a shock absorber for preventing the component from being impacted. It is also used as a sealing material that fills the gaps inside the electronic device and prevents dust, moisture, etc. from entering the inside of the electronic device. Further, the conductive foam sheet is arranged in the vicinity of the heat source inside the electronic device, and is used as a heat radiating sheet that diffuses or dissipates the heat generated from the heat source.
That is, the conductive foam sheet can be used as a shock absorbing material, a sealing material, and a heat radiating sheet while being used as a conductive sheet.

When the conductive foam sheet is used as a conductive sheet, for example, it is arranged between an electric noise source (including an electromagnetic noise source) and a conductive heat sink, and together with the conductive heat sink, noise from the electric noise source is emitted. It constitutes a noise attenuation mechanism to suppress. The electrical noise source is included in electronic components in mobile wireless communication devices such as smartphones such as SoC (System on a chip), memory, and power supply modules, and printed circuit boards used when mounting other electronic components. There are various noises and the like derived from the wiring.
Further, for example, it is arranged between an insulator such as plastic which is easily charged and a conductive heat sink, and together with the conductive heat sink, constitutes a mechanism for suppressing static electricity.

The present invention will be described in more detail by way of examples, but the present invention is not limited to these examples.
The method for measuring and evaluating each physical property in the present invention is as follows.

(Effervescence magnification)
^{The specific volume (unit: cm 3} / g) of the foamable resin sheet before foaming and the conductive foamed sheet obtained in Examples and Comparative Examples was measured, and the specific volume of the conductive foamed sheet / the foamable resin before foaming was measured. It was calculated by the specific volume of the sheet.

(Thickness)
The thickness of the foam sheet obtained in Examples and Comparative Examples was measured with a dial gauge.

(Surface resistivity)
The surface resistivity of the conductive foamed sheet obtained in Examples and Comparative Examples was measured by a four-probe method based on JIS K7194: 1994 using the product name "Loresta-GP MCP-T600" manufactured by Mitsubishi Chemical Corporation. It was measured. The measurement was performed at five randomly selected points in the central portion of the conductive foamed sheet, and the average value was taken as the surface resistivity of the conductive foamed sheet.

(Electromagnetic wave shielding property)
The electromagnetic wave shielding property of the conductive foam sheets obtained in Examples and Comparative Examples was measured by the coaxial method using the following measuring equipment at a measurement frequency of 5 GHz.
-Coaxial tube jig: Made by Keycom Co., Ltd., product name "GPC-7"
-Network analyzer: Product name "E5071C-4K5" manufactured by Agilent Technologies, Inc.

(25% compression strength)
The 25% compressive hardness of the conductive foam sheets obtained in Examples and Comparative Examples was measured according to JIS K6767: 1999.

(Tensile strength)
The tensile strength of the conductive foamed sheets obtained in Examples and Comparative Examples was measured according to JIS K6767: 1999.

(Elongation at break)
The elongation at break of the conductive foamed sheets obtained in Examples and Comparative Examples was measured according to JIS K6767: 1999.

(Closed cell ratio)
From the conductive foam sheets obtained in Examples and Comparative Examples, test pieces of each foam layer having a square shape with a side of 5 cm and a constant thickness are cut out. Then, by measuring the thickness of the test piece to calculate the apparent volume V ₁ of the test piece, measuring the weight W ₁ of the specimen.
Next, the volume V ₂ occupied by the bubbles is calculated based on the following formula. The density of the resin constituting the test piece is ρg / cm ³ .
Volume occupied by bubbles V ₂ = V ₁ −W ₁ / ρ
Subsequently, the test piece is submerged in distilled water at 23 ° C. to a depth of 100 mm from the water surface, and a pressure of 15 kPa is applied to the test piece for 3 minutes. After that, the test piece is taken out from the water, the water adhering to the surface of the test piece is removed, the weight W _{2 of the} test piece is measured, and the closed cell ratio F ₁ is calculated based on the following formula.
Closed cell ratio F ₁ (%) = 100-100 x (W _2- W ₁ ) / V ₂

(Gel fraction)
The gel fractions of the conductive foamed sheets obtained in Examples and Comparative Examples were measured as follows.
The conductive foam sheet was weighed (A [g]). This was immersed in xylene at 120 ° C. for 24 hours, the insoluble matter was filtered through a 200 mesh wire mesh, the residue on the wire mesh was vacuum dried, and the weight of the dried residue was measured (B [g]).
Gel fraction (mass%) = (B / A) x 100

(Conductivity)
The conductivity of the conductive foam sheets obtained in Examples and Comparative Examples was evaluated based on the following evaluation criteria.
(Evaluation criteria)
A: The surface resistivity is 100Ω / sq or less.
B: The surface resistivity is more than 100Ω / sq and less than 2,000Ω / sq.
C: The surface resistivity is over 2,000 Ω / sq.

(Effervescent)
The foamability of the conductive foam sheets obtained in Examples and Comparative Examples was evaluated based on the following evaluation criteria.
(Evaluation criteria)
A: Abnormal foaming, unevenness, and surface roughness did not occur on the surface of the foam, and foaming could be performed satisfactorily.
B: Abnormal foaming, unevenness, and surface roughness were slightly generated on the surface of the foam, and foaming could be performed almost satisfactorily.
C: Abnormal foaming, unevenness, and surface roughness occurred on the surface of the foam, and foaming could not be performed satisfactorily.

(Flexibility)
The flexibility of the conductive foamed sheets obtained in Examples and Comparative Examples was evaluated based on the following evaluation criteria.
(Evaluation criteria)
A: The 25% compression strength is 400 kPa or less.
B: The 25% compression strength is more than 400 kPa and less than 1 MPa.
C: 25% compression strength is over 1 MPa.

[Example 1]
(Making a foam sheet)
As the olefin resin, a polyethylene resin (manufactured by Dow Chemical Co., Ltd., product name "PL1850G") obtained by a polymerization catalyst of a metallocene compound was prepared. As a carbon filler, the base resin is a polyethylene resin and the carbon black is Ketjen Black. Carbon Black Masterbatch (manufactured by Lion Specialty Chemicals Co., Ltd., product name "FD-9002K", ratio of carbon black 3.0 ~ 10.0% by mass) was prepared. Further, as the carbon filler, scaly graphite (manufactured by Nippon Carbon Filler Industry Co., Ltd., product name "F # 3", average particle size 50 μm, aspect ratio 0.075) was prepared. As a foaming agent, azodicarbonamide (manufactured by Otsuka Chemical Co., Ltd., trade name "SO-G3"), which is a thermal decomposition type foaming agent, was prepared. As antioxidants, a phenolic antioxidant (manufactured by BASF, trade name "Irganox 1010") and dilauryl thiodipropionate (manufactured by Ouchi Shinko Kagaku Kogyo Co., Ltd., trade name "Nocrack 400") were prepared.
Next, 50 parts by mass of the polyethylene-based resin, 50 parts by mass of carbon black masterbatch, 50 parts by mass of scaly graphite, 3 parts by mass of azodicarboxylic amide, and 0.4 parts by mass of the phenol-based antioxidant prepared above were prepared. , 0.2 parts by mass of dilauryl thiodipropionate was melt-kneaded with a plastic mill at 140 ° C. to obtain an effervescent composition. Then, the obtained foamable composition was molded by a press machine to form a long sheet having a thickness of 0.5 mm.
Next, the foam material was crosslinked by irradiating both sides of the long sheet-shaped foamable composition with an electron beam having an acceleration voltage of 800 kV for 2.5 Mrad.
Then, this foam material was continuously sent into a foaming furnace held at 250 ° C. by hot air and an infrared heater and heated to foam to obtain a foamed sheet.
Table 1 shows the physical characteristics and evaluation results of this foam sheet.

[Example 2]
Examples except that scaly graphite (manufactured by Nippon Carbon Filler Industry Co., Ltd., product name "UP-35N", average particle size 30 μm, aspect ratio 0.044) was used instead of the scaly graphite used in Example 1. A foam sheet was obtained in the same manner as in 1.
Table 1 shows the physical characteristics and evaluation results of this foam sheet.

[Example 3]
In Example 1, a foamed sheet was obtained in the same manner as in Example 1 except that the number of parts of scaly graphite was changed.
Table 1 shows the physical characteristics and evaluation results of this foam sheet.

[Example 4]
In Example 2, a foamed sheet was obtained in the same manner as in Example 2 except that the number of parts of scaly graphite was changed.

[Example 5]
In Example 1, a foamed sheet was obtained in the same manner as in Example 1 except that the number of parts of scaly graphite was changed.

[Example 6]
In Example 1, a thermoplastic elastomer (styrene thermoplastic elastomer, manufactured by Kuraray Co., Ltd., product name "Septon 2002", styrene-ethylene / propylene-styrene block copolymer (SEPS)) is used instead of the olefin resin. A foamed sheet was obtained in the same manner as in Example 1.
Table 1 shows the physical characteristics and evaluation results of this foam sheet.

[Comparative Example 1]
As the olefin resin, a polyethylene resin (manufactured by Dow Chemical Co., Ltd., product name "PL1850G") obtained by a polymerization catalyst of a metallocene compound was prepared. As a carbon filler, the base resin is a polyethylene resin and the carbon black is Ketjen Black. Carbon Black Masterbatch (manufactured by Lion Specialty Chemicals Co., Ltd., product name "FD-9002K", ratio of carbon black 3.0 ~ 10.0% by mass) was prepared. As a foaming agent, azodicarbonamide (manufactured by Otsuka Chemical Co., Ltd., trade name "SO-G3"), which is a thermal decomposition type foaming agent, was prepared. As antioxidants, a phenolic antioxidant (manufactured by BASF, trade name "Irganox 1010") and dilauryl thiodipropionate (manufactured by Ouchi Shinko Kagaku Kogyo Co., Ltd., trade name "Nocrack 400") were prepared.
Next, 50 parts by mass of the polyethylene-based resin, 50 parts by mass of the carbon black masterbatch, 3 parts by mass of azodicarboxylic amide, 0.4 parts by mass of the phenol-based antioxidant, and 0 dilauryl thiodipropionate prepared above. .2 parts by mass were melt-kneaded with a plastic mill at 140 ° C. to obtain an effervescent composition. Then, the obtained foamable composition was molded by a press machine to form a long sheet having a thickness of 0.5 mm.
Next, the foam material was crosslinked by irradiating both sides of the long sheet-shaped foamable composition with an electron beam having an acceleration voltage of 800 kV for 2.5 Mrad.
Then, this foam material was continuously sent into a foaming furnace held at 250 ° C. by hot air and an infrared heater and heated to foam to obtain a foamed sheet.
Table 1 shows the physical characteristics and evaluation results of this foam sheet.

[Comparative Example 2]
As a carbon filler, the base resin is a polyethylene resin and the carbon black is Ketjen Black. Carbon Black Masterbatch (manufactured by Lion Specialty Chemicals Co., Ltd., product name "FD-9002K", ratio of carbon black 3.0 ~ 10.0% by mass) was prepared. As a foaming agent, azodicarbonamide (manufactured by Otsuka Chemical Co., Ltd., trade name "SO-G3"), which is a thermal decomposition type foaming agent, was prepared. As antioxidants, a phenolic antioxidant (manufactured by BASF, trade name "Irganox 1010") and dilauryl thiodipropionate (manufactured by Ouchi Shinko Kagaku Kogyo Co., Ltd., trade name "Nocrack 400") were prepared.
Next, 100 parts by mass of the carbon black masterbatch prepared above, 3 parts by mass of azodicarbonamide, 0.4 parts by mass of the phenolic antioxidant, and 0.2 parts by mass of dilauryl thiodipropionate were added to the plastomill. The mixture was melt-kneaded at 140 ° C. to obtain an effervescent composition. Then, the obtained foamable composition was molded by a press machine to form a long sheet having a thickness of 0.5 mm.
Next, the foam material was crosslinked by irradiating both sides of the long sheet-shaped foamable composition with an electron beam having an acceleration voltage of 800 kV for 2.5 Mrad.
Then, this foam material was continuously sent into a foaming furnace held at 250 ° C. by hot air and an infrared heater and heated to foam to obtain a foamed sheet.
Table 1 shows the physical characteristics and evaluation results of this foam sheet.

[Comparative Example 3]
In Example 1, a foamed sheet was obtained only with a polyethylene-based resin without blending a carbon black masterbatch or a carbon filler.

In Examples 1 to 6, the good results of 25% compression strength and surface resistivity made it possible for the foamed sheet to have particularly excellent conductivity while maintaining flexibility.
On the other hand, in Comparative Examples 1 and 3, although the foamed sheet had flexibility, the surface resistivity result was poor, so that the conductivity was not sufficient. Further, in Comparative Example 2, although the foamed sheet had excellent conductivity, the flexibility of the foamed sheet was insufficient.

Claims (13)

A conductive foamed sheet containing at least one resin of a polyolefin resin and a thermoplastic elastomer and at least one carbon filler.
The 25% compression strength is 1 MPa or less,
A conductive foam sheet having a surface resistivity of 2000 Ω / sq or less. The conductive foamed sheet according to claim 1, which contains a polyolefin-based resin. The conductive foamed sheet according to claim 1 or 2, wherein the carbon filler contains scaly graphite. The conductive foamed sheet according to claim 3, wherein the scaly graphite has an average aspect ratio of 0.001 to 0.200. The conductive foamed sheet according to any one of claims 1 to 4, wherein the carbon filler contains carbon black. The conductive foamed sheet according to claim 5, wherein the primary particle size of the carbon black is 5 nm or more and 100 nm or less. The conductive foamed sheet according to any one of claims 1 to 6, wherein the polyolefin-based resin contains a polyethylene-based resin. The conductive foamed sheet according to any one of claims 1 to 7, wherein the thickness is 0.1 mm or more and 5.0 mm or less. The conductive foamed sheet according to any one of claims 1 to 8, wherein the foaming ratio is 1.2 times or more and 20 times or less. The conductive foamed sheet according to any one of claims 1 to 9, wherein the closed cell ratio is 70% or more. The conductive foam sheet according to any one of claims 1 to 10, wherein the electromagnetic wave shielding property at a frequency of 5 GHz by the coaxial method is 7 dB or more. The conductive foamed sheet according to any one of claims 1 to 11, which is crosslinked. The conductive foamed sheet according to claim 12, wherein the gel fraction is 10% by mass or more and 60% by mass or less.