JP2022059536A - Foam sheet - Google Patents

Abstract

To provide a foam sheet having cushioning, vibration-proof, and low-dielectric properties.SOLUTION: A foam sheet has at least one glass transition temperature (Tg) of 0-40°C, a loss tangent (tanδ) peak value of 0.30 or more at the glass transition temperature (Tg), a 25% compression strength of 1000 kPa or less, and a relative dielectric constant of 2 or less.SELECTED DRAWING: None

Description

The present invention relates to a foam sheet, for example, a foam sheet used as a cushioning material for a display.

In portable electronic devices such as notebook personal computers, mobile phones, smartphones, and tablets, a cushioning material may be arranged on the back side of the display device in order to prevent damage or failure. High flexibility is required for the cushion material, and a foam sheet has been widely used conventionally.
The foam sheet may be used as an adhesive tape inside an electronic device, for example, by applying an adhesive to at least one surface. Conventionally, as a foam sheet used in these applications, a crosslinked polyolefin resin foam sheet obtained by foaming and cross-linking a foamable polyolefin resin sheet containing a pyrolytic foaming agent is known (for example, a patent). See Document 1).

Japanese Unexamined Patent Publication No. 2014-28925

In recent years, as electronic devices have become more sophisticated, the acoustic functions of portable electronic devices such as smartphones have improved. As a result, there is a problem that the vibration in the portable electronic device becomes large, and this problem becomes remarkable especially when a glass or polycarbonate material is used for the back cover for wireless power supply. Specifically, a foam sheet is used as a cushioning material in order to impart a cushioning property between the battery and the back glass, and the foam sheet is required to have an anti-vibration effect.
In addition, 5G smartphones are being promoted, and foam sheets are also required to have low dielectric properties in order to suppress communication delays.
Therefore, it is an object of the present invention to provide a foam sheet having both anti-vibration property and low dielectric property in addition to the cushioning property conventionally required for a foam material.

As a result of diligent studies, the present inventor solves the above problems by setting the glass transition temperature (Tg), loss tangent (tanδ), and compressive strength within a certain range and the relative permittivity within a certain value. We found what we could do and completed the present invention. That is, the present invention provides the following [1] to [16].

[1] At least one glass transition temperature (Tg1) is 0 to 40 ° C., the peak value of loss tangent (tanδ) at the glass transition temperature (Tg1) is 0.30 or more, and the 25% compressive strength is 1000 kPa. A foam sheet having a specific dielectric constant of 2 or less and having a specific dielectric constant of 2 or less.
[2] The foam sheet according to the above [1], which has a 25% compressive strength of 800 kPa or less.
[3] The foam sheet according to the above [1] or [2], which has a thickness of 0.03 to 2 mm.
[4] The foam sheet according to any one of the above [1] to [3], which has a breaking point strength of 5 N / 10 mm or more at 23 ° C.
[5] The foam sheet according to any one of the above [1] to [4], wherein the closed cell ratio is 80% or more.
[6] The foam sheet according to any one of the above [1] to [5], which has an average bubble diameter of 20 to 400 μm.
[7] The foam sheet according to any one of the above [1] to [6], which has a gel fraction of 30 to 80% by mass.
[8] The foam sheet according to any one of the above [1] to [7], which has an apparent density of 0.05 to 0.70 g / cm ³ .
[9] The foam sheet according to any one of the above [1] to [8], wherein the moisture permeability (WVTR) is 400 g / m ² · day or less.
[10] The foam sheet according to any one of the above [1] to [9], wherein the glass transition temperature (Tg2) is −40 ° C. or lower.
[11] The foam sheet according to any one of the above [1] to [10], which has a breaking point elongation at 23 ° C. of 200% or more.
[12] The foam sheet according to any one of the above [1] to [11], wherein the resin constituting the foam sheet contains a polyolefin resin.
[13] The foam sheet according to the above [12], wherein the resin constituting the foam sheet further contains an elastomer.
[14] The foam sheet according to the above [13], wherein the mass ratio of the elastomer to the polyolefin resin is 90:10 to 15:85.
[15] An adhesive tape comprising the foam sheet according to any one of the above [1] to [14] and an adhesive material provided on at least one surface of the foam sheet.
[16] The roll made of the foam sheet according to any one of the above [1] to [14].

According to the present invention, it is possible to provide a foam sheet having both anti-vibration property and low dielectric property in addition to cushioning property.

Hereinafter, the foam sheet of the present invention will be described in more detail.
[Foam sheet]
The foam sheet of the present invention has at least one glass transition temperature (hereinafter, may be referred to as “Tg”) of 0 to 40 ° C., and has a peak loss tangent (tan δ) at the glass transition temperature (Tg1). It is characterized in that the value is 0.30 or more, the 25% compression strength is 1000 kPa or less, and the relative dielectric constant is 2 or less.

[Glass transition temperature and loss tangent]
In the foam sheet of the present invention, at least one glass transition temperature (Tg1) is in the range of 0 to 40 ° C., and the peak value of the loss tangent (tan δ) at the Tg1 is 0.30 or more. When Tg1 and tanδ are in the above ranges, the foam sheet of the present invention exhibits excellent anti-vibration properties. In the present invention, the temperature at the peak top of the loss tangent (tan δ) obtained by viscoelasticity measurement was defined as Tg. Further, in the present specification, Tg in the range of 0 to 40 ° C. may be described as Tg1.
tan δ is the ratio (G'' / G') of the storage shear modulus (G') and the loss shear modulus (G''), and how much energy the material absorbs when the material is deformed (heat). It becomes an index showing whether it changes to). The foam sheet of the present invention has a glass transition temperature (Tg1) in the range of 0 to 40 ° C., which is close to room temperature, and the peak value of tan δ at the glass transition temperature is 0.30 or more. The energy loss of the glass can be improved, and as a result, an excellent anti-vibration effect is exhibited. From the above viewpoint, the peak value of tan δ is preferably 0.35 or more, more preferably 0.37 or more, further preferably 0.39 or more, and preferably 0.40 or more. Especially preferable.
The glass transition temperature (Tg) and the loss tangent (tan δ) are values measured by the method described in the examples.
Here, when two or more Tg points are observed, it is preferable that the Tg at at least one point is in the range of 0 to 40 ° C., and the peak value of tan δ at that Tg is within the above range.

Further, the foam sheet of the present invention preferably has a glass transition temperature of −40 ° C. or lower. By having Tg at −40 ° C. or lower, flexibility can be ensured even in cold regions, and cracking of the foam sheet can be prevented. In this specification, Tg at −40 ° C. or lower may be referred to as Tg2. Tg2 is more preferably −60 ° C. or lower, still more preferably −80 ° C. or lower.
The lower limit of Tg2 is not particularly limited, but is preferably −150 ° C. or higher, more preferably −140 ° C. or higher, and even more preferably −130 ° C. or higher.

[Compressive strength]
The foam sheet of the present invention has a 25% compressive strength of 1000 kPa or less. If the compressive strength exceeds 1000 kPa, the flexibility is insufficient and the internal members of the portable electronic device may be damaged. Further, due to insufficient flexibility, the followability is inferior, and when used as a tape base material, for example, the adhesive force at the time of adhesion becomes insufficient. From the above viewpoint, the 25% compressive strength is more preferably 900 kPa or less, further preferably 800 kPa or less, and particularly preferably 600 kPa or less.
The lower limit of the 25% compressive strength is not particularly limited, but is usually about 10 kPa, preferably 20 kPa or more.
The 25% compressive strength is a value measured at a measurement temperature of 23 ° C. by a measurement method based on JIS K6767.

[Relative permittivity]
The foam sheet of the present invention has a relative permittivity of 2 or less. If the relative permittivity exceeds 2, communication delay may occur, and especially when used for 5G-compatible electronic devices, the problem of communication delay is likely to occur. Further, if the relative permittivity exceeds 2, it may cause an operation error of the electronic device. From the above viewpoint, the relative permittivity is preferably 1 to 1.80, more preferably 1 to 1.70, and even more preferably 1 to 1.55.
The relative permittivity is a value measured by the method described in Examples.
Further, the relative permittivity can be appropriately adjusted depending on the type of resin constituting the foam sheet, the apparent density, and the like.

[Thickness]
The thickness of the foam sheet of the present invention is preferably 0.03 to 2 mm. When the thickness is 0.03 mm or more, it becomes easy to secure the cushioning property of the foam sheet. Further, when the thickness is 2 mm or less, the thickness can be reduced, and it can be suitably used for thin electronic devices such as smartphones and tablets. Furthermore, it becomes easy to secure the flexibility of the foam sheet.
From these viewpoints, the thickness of the foam sheet is more preferably 0.1 to 1.8 mm, further preferably 0.15 to 1.6 mm, and 0.15 to 0.7 mm. Is even more preferable.
The thickness was measured with a dial gauge.

[Breaking point strength]
The breaking point strength of the foam sheet of the present invention at 23 ° C. is preferably 5N / 10 mm or more. When the breaking point strength is 5 N / 10 mm or more, good impact resistance can be obtained. From the above viewpoint, the breaking point strength is more preferably 5.5 N / 10 mm or more, further preferably 6 N / 10 mm or more, and further preferably 7 N / 10 mm or more.
The upper limit of the breaking point strength is not particularly limited, but is usually about 50 N / 10 mm, preferably 40 N / 10 mm or less.
The breaking point strength is a value measured by the method described in the examples.

[Break point elongation]
The elongation at break point of the foam sheet of the present invention at 23 ° C. is preferably 200% or more. When the elongation at break is 200% or more, good impact resistance can be obtained. From the above viewpoint, the elongation at break point is more preferably 300% or more, further preferably 400% or more.
The upper limit of the elongation at break is not particularly limited, but is usually about 800%, preferably 600% or less.
The elongation at break is a value measured by the method described in Examples.

[Closed cell ratio]
The foam sheet of the present invention preferably has a closed cell ratio of 80% or more. When the closed cell ratio is 80% or more, the cushioning property and the impact resistance are good, and it becomes easy to maintain the original elasticity of the foam sheet even after heating or cooling. There is also an advantage that the rate of change in compressive strength tends to be low.
From the above viewpoint, the closed cell ratio of the foam sheet is more preferably 90% or more. The higher the closed cell ratio, the better, and it may be 100% or less.
The closed cell ratio was measured by the method described in Examples.

[Average bubble diameter]
The foam sheet of the present invention preferably has an average cell diameter of 20 to 400 μm. When the average bubble diameter is in the above range, the impact resistance is good. From the above viewpoint, the average bubble diameter is more preferably 50 to 350 μm, further preferably 70 to 300 μm, and even more preferably 70 to 220 μm. The average bubble diameter in the present invention is mechanical. It is the larger value of the average value of the bubble diameter in the direction (MD: Machine Direction) and the average value of the bubble diameter in the direction perpendicular to the MD (TD: Transverse Direction).
The average bubble diameter was measured by the method described in Examples.

[Crosslinkability (gel fraction)]
The foam sheet of the present invention is preferably crosslinked, and the degree of crosslinking represented by the gel fraction is preferably 30 to 80% by mass. By setting the degree of cross-linking within the above range, the foam sheet tends to have good impact resistance while ensuring a certain degree of flexibility and cushioning property. From the above viewpoint, the gel fraction is more preferably 35 to 70% by mass, further preferably 38 to 65% by mass.
The method for measuring the gel fraction is a value measured by the method described in Examples.

[Apparent density]
The apparent density of the foam sheet of the present invention is preferably 0.05 g / cm ³ to 0.70 g / cm ³ , more preferably 0.06 g / cm ³ to 0.65 g / cm ³ . It is more preferably 0.07 g / cm ³ to 0.60 g / cm ³ , and particularly preferably 0.10 g / cm ³ to 0.45 g / cm ³ .
When the apparent density is within the above range, it becomes easy to improve the flexibility, cushioning property, relative permittivity and the like of the foam sheet. In addition, a certain level of mechanical strength is imparted to the foam sheet, and it becomes easy to improve impact resistance and handleability.
The apparent density is a value measured in accordance with JIS K7222 (2005).

[Humidity permeability]
The foam sheet of the present invention preferably has a water vapor transmission rate (WVTR: Water Vapor Transmission Rate) of 400 g / m ² · day or less. When the moisture permeability is 400 g / m ² · day or less, it is possible to prevent the intrusion of moisture into the inside of an electronic device or the like. From the above viewpoint, the moisture permeability is preferably 200 g / m ² · day or less, more preferably 100 g / m ² · day or less, and particularly preferably 80 g / m ² · day or less.
The lower limit is not particularly limited, but is usually about 5 g / m ² · day.
The moisture permeability is a value measured by the method described in the examples.

[Constituent resin]
The resin constituting the foam sheet of the present invention preferably contains a polyolefin-based resin. By containing the polyolefin-based resin, strength can be imparted to the foam sheet. Further, the constituent resin preferably contains an elastomer, and the constituent resin preferably contains an elastomer (A) and a polyolefin resin (B). By using an elastomer and a polyolefin-based resin, it is possible to improve flexibility, impact resistance, relative permittivity, etc. while improving foamability and the like.

The elastomer according to the present invention preferably has a maximum peak temperature of tan δ of 0 to 40 ° C. as measured by dynamic viscoelasticity. As described above, when the maximum peak temperature of tan δ is close to normal temperature, the sound absorption characteristics are improved and the vibration isolation property is likely to be improved. From the above viewpoint, the maximum peak temperature of tan δ of the elastomer is more preferably 5 to 35 ° C, further preferably 10 to 30 ° C.
In the present specification, the "maximum peak temperature of tan δ" refers to a value measured by a dynamic viscoelasticity measuring device in a tensile mode, a temperature rising rate of 10 ° C./min, and a frequency of 10 Hz. Examples of the dynamic viscoelasticity measuring device that can be used for the measurement include "Leovibron DDV-III" manufactured by Orientec Co., Ltd.

(Elastomer (A))
Examples of the elastomer (A) include thermoplastic elastomers, ethylene-α-olefin copolymer rubbers, and amorphous 4-methyl-1pentene copolymers. Examples of the thermoplastic elastomer include olefin-based thermoplastic elastomers, styrene-based thermoplastic elastomers, vinyl chloride-based thermoplastic elastomers, polyurethane-based thermoplastic elastomers, polyester-based thermoplastic elastomers, polyamide-based thermoplastic elastomers, and the like. As the elastomer (A), these components may be used alone or in combination of two or more.
Among these, olefin-based thermoplastic elastomers, styrene-based thermoplastic elastomers, and ethylene-α-olefin-based copolymer rubbers are preferable, styrene-based thermoplastic elastomers, ethylene-α-olefin-based copolymer rubbers are more preferable, and styrene-based rubbers are more preferable. Thermoplastic elastomers are more preferred.

<Olefin-based thermoplastic elastomer>
Olefin-based thermoplastic elastomers (TPOs) generally have polyolefins such as polyethylene and polypropylene as hard segments, and butyl rubber, halobutyl rubber, EPDM (ethylene-propylene-diene rubber), EPM (ethylene-propylene rubber), NBR ( Acrylonitrile-polyolefin rubber), natural rubber and other rubber components are used as soft segments. As the olefin-based thermoplastic elastomer (TPO), any of a blend type, a dynamic cross-linking type, and a polymerization type can be used.
Preferable specific examples of the rubber component include the above-mentioned EPM and EPDM, and EPDM is particularly preferable. Examples of EPDM include ethylene-propylene-5-ethylidene-2-norbornene copolymer rubber and ethylene-propylene-dicyclopentadiene copolymer rubber. Among these, ethylene-propylene-dicyclopentadiene copolymer rubber is used. preferable.

Further, examples of the olefin-based thermoplastic elastomer include a block copolymer type. Examples of the block copolymer type include those having a crystalline block and a soft segment block, and more specifically, a crystalline olefin block-ethylene / butylene copolymer-crystalline olefin block copolymer (CEBC) is exemplified. Will be done. In CEBC, the crystalline olefin block is preferably a crystalline ethylene block, and examples of such a commercially available product of CEBC include "DYNARON 6200P" manufactured by JSR Corporation.

<Styrene-based thermoplastic elastomer>
Examples of the styrene-based thermoplastic elastomer include block copolymers having a polymer or copolymer block of styrene and a polymer or copolymer block of a conjugated diene compound. Examples of the conjugated diene compound include isoprene and butadiene.
The styrene-based thermoplastic elastomer used in the present invention may or may not be hydrogenated, but hydrogenation is preferable. When hydrogenating, hydrogenation can be performed by a known method.

The styrene-based thermoplastic elastomer is usually a block copolymer, and is a styrene-isoprene block copolymer (SI), a styrene-isoprene-styrene block copolymer (SIS), or a styrene-butadiene block copolymer (SB). , Steryl-butadiene-styrene block copolymer (SBS), styrene-ethylene / butylene-styrene block copolymer (SEBS), styrene-ethylene / propylene-styrene block copolymer (SEPS), styrene-ethylene / ethylene / Propropylene-styrene block copolymer (SEEPS), styrene-ethylene / butylene block copolymer (SEB), styrene-ethylene / propylene block copolymer (SEP), styrene-ethylene / butylene-crystalline olefin block copolymer (SEBC) and the like.
As the above-mentioned styrene-based thermoplastic elastomer, block copolymers are preferable, among them, SIS, SEBS, SEPS, SEEPS and SEBC are more preferable, SEEPS and SEBS are further preferable, and SEBS is particularly preferable.
As a commercial product of SEBS, Tough Tech (registered trademark) series manufactured by Asahi Kasei Corporation, S.A. O. E. (Registered trademark) series can be mentioned.

The styrene-based thermoplastic elastomer has a structural unit derived from styrene, which makes it possible to improve the impact resistance of the foamed sheet. The styrene content in the styrene-based thermoplastic elastomer is preferably 5 to 50% by mass. By setting the styrene content in the above range, excellent impact resistance can be obtained. Further, when the value is not more than the above upper limit, the compatibility with the polyolefin resin (B) described in detail later becomes good, and the crosslinkability and foamability tend to be good. From these viewpoints, the styrene content in the styrene-based thermoplastic elastomer is more preferably 7 to 40% by mass, further preferably 7 to 30% by mass.

The number average molecular weight of the styrene-based thermoplastic elastomer is not particularly limited, but is preferably 30,000 to 800,000, more preferably 120,000 to 180,000 from the viewpoint of fracture strength and processability.
The number average molecular weight is a potistyrene equivalent value measured by gel permeation chromatography (GPC).

<Ethylene-α-olefin copolymer rubber>
The α-olefins used in the ethylene-α-olefin copolymer rubber include propylene, 1-butene, 2-methylpropylene, 3-methyl-1-butene, 1-pentene, 1-hexene and 4-methyl-. Examples thereof include one or more α-olefins having 3 to 15 carbon atoms, preferably 3 to 10 carbon atoms such as 1-pentene and 1-octene. Among these, propylene and 1-butene are preferable, and 1-butene is more preferable.
The ethylene-α-olefin-based copolymer rubber used here is an amorphous or low-crystalline rubber-like substance obtained by substantially randomly copolymerizing two or more kinds of olefin-based monomers.

The ethylene-α-olefin copolymer rubber may have other monomer units in addition to the ethylene unit and the α-olefin unit.
Examples of the monomer forming the other monomer unit include 1,3-butadiene, 2-methyl-1,3-butadiene (isoprene), 1,3-pentadiene, 2,3-dimethyl-1,3-butadiene and the like. Conjugate diene with 4 to 8 carbon atoms, dicyclopentadiene, 5-ethylidene-2-norbornene, 1,4-hexadiene, 1,5-dicyclooctadiene, 7-methyl-1,6-octadien, 5-vinyl- Non-conjugated diene having 5 to 15 carbon atoms such as 2-norbornene, vinyl ester compound such as vinyl acetate, unsaturated carboxylic acid ester such as methyl acrylate, ethyl acrylate, butyl acrylate, methyl methacrylate, ethyl methacrylate, etc. Examples thereof include unsaturated carboxylic acids such as acrylic acid and methacrylic acid. These monomers can be used alone or in combination of two or more. Among these, non-conjugated diene having 5 to 15 carbon atoms is preferable, and 5-ethylidene-2-norbornene, 1,4-hexadiene, and dicyclopentadiene (DCPD) are more preferable from the viewpoint of availability.

The ethylene unit content of the ethylene-α-olefin copolymer rubber is usually 30 to 85% by mass, preferably 40 to 80% by mass, and more preferably 45 to 75% by mass. The content of α-olefin units having 3 to 15 carbon atoms, preferably 3 to 10 carbon atoms such as propylene is usually 10 to 60% by mass, preferably 15 to 50% by mass. The content of other monomeric units such as non-conjugated diene is usually 0 to 20% by mass, preferably 1 to 10% by mass.

As the ethylene-α-olefin copolymer rubber, a ternary copolymer such as EPDM (ethylene-propylene-diene rubber) or EBDM (ethylene-butene-1-diene rubber) is preferable. Examples of the ethylene-α-olefin copolymer include "EBT K-9330" manufactured by Mitsui Chemicals, Inc.

<Amorphous 4-methyl-1pentene copolymer>
Examples of the amorphous 4-methyl-1pentene copolymer include copolymers of 4-methyl-1pentene and α-olefins other than 4-methyl-1pentene. Examples of the α-olefin include α-olefins having 2 to 20 carbon atoms, preferably ethylene, propylene, 1-butene, 1-pentene, 1-hexene, 1-octene, 1-decene, and more preferably ethylene. , Prochine, etc. Of these, 4-methyl-1pentene / propylene copolymer is preferable.
Examples of the amorphous 4-methyl-1pentene copolymer include "EP1001" manufactured by Mitsui Chemicals, Inc.

(Polyolefin resin (B))
The polyolefin resin is a thermoplastic resin, and specific examples thereof include polyethylene resin, polypropylene resin, polybutene resin, ethylene-vinyl acetate copolymer, and the like, and polyethylene resin is preferable among them. By using polyethylene resin, it becomes easy to lower the relative permittivity. In addition, it becomes easy to lower the moisture permeability.
Further, as the polyethylene resin, low density polyethylene (LDPE) is preferable, and linear low density polyethylene (LLDPE) is more preferable. Therefore, it is particularly preferable to use a styrene-based thermoplastic elastomer as the elastomer (A) and LLDPE as the polyolefin resin (B).
Further, examples of the polyethylene resin include polyethylene resins polymerized with a polymerization catalyst such as a Ziegler-Natta catalyst, a metallocene catalyst, and a chromium oxide compound, and a polyethylene resin polymerized with a metallocene catalyst is preferably used.

(Metallocene catalyst)
Examples of the metallocene catalyst include compounds such as a bis (cyclopentadienyl) metal complex 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. Can be mentioned.
In such a metallocene catalyst, the properties of active sites are uniform, and each active site has the same activity. Since the polymer synthesized using the metallocene catalyst has high uniformity in molecular weight, molecular weight distribution, composition, composition distribution, etc., when a sheet containing the polymer synthesized using the metallocene catalyst is crosslinked, the cross-linking is uniform. Proceed to. Since the uniformly crosslinked sheet is uniformly foamed, it becomes easy to stabilize the physical properties. Moreover, since it can be uniformly stretched, the thickness of the foam can be made uniform.

Examples of the ligand include a cyclopentadienyl ring, an indenyl ring and the like. These cyclic compounds may be substituted with a hydrocarbon group, a substituted hydrocarbon group or a hydrocarbon-substituted metalloid group. Examples of the hydrocarbon group include a methyl group, an ethyl group, various propyl groups, various butyl groups, various amyl groups, various hexyl groups, 2-ethylhexyl groups, various heptyl groups, various octyl groups, various nonyl groups and various decyl groups. , Various cetyl groups, phenyl groups and the like. In addition, "various" means various isomers including n-, sec-, tert-, and iso-.
Further, a cyclic compound polymerized 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, phosphides, arylphosphides, etc. May be used.

Examples of metallocene catalysts containing tetravalent transition metals and ligands include cyclopentadienyl titaniumtris (dimethylamide), methylcyclopentadienyl titaniumtris (dimethylamide), bis (cyclopentadienyl) titanium dichloride, and dimethyl. Examples thereof include silyltetramethylcyclopentadienyl-t-butylamide zirconium dichloride.
The metallocene catalyst 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), boron-based compounds and the like. The ratio of the cocatalyst used to the metallocene catalyst is preferably 100 to 1 million mol times, more preferably 50 to 5,000 mol times.

Further, as the polyethylene resin, linear low-density polyethylene is preferable. The linear low-density polyethylene is obtained by copolymerizing ethylene (for example, 75% by mass or more, preferably 90% by mass or more with respect to the total amount of monomers) and, if necessary, a small amount of α-olefin. Chain low density polyethylene is more preferred. Specific examples of the α-olefin include propylene, 1-butene, 1-pentene, 4-methyl-1-pentene, 1-hexene, 1-heptene, 1-octene and the like. Of these, α-olefins having 4 to 10 carbon atoms are preferable.
The density of the polyethylene resin, for example, the above-mentioned linear low-density polyethylene is preferably 0.870 to 0.925 g / cm ³ , more preferably 0.890 to 0.925 g / cm ³ , and 0. .910 to 0.925 g / cm ³ is more preferable. As the polyethylene resin, a plurality of polyethylene resins may be used, or polyethylene resins other than the above-mentioned density range may be added.

Examples of the ethylene-vinyl acetate copolymer used as the polyolefin resin include an ethylene-vinyl acetate copolymer containing 50% by mass or more of ethylene.
Examples of the polypropylene resin include homopolypropylene and a propylene-α-olefin copolymer containing 50% by mass or more of propylene. 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, and 1-. Examples thereof include octene, and among these, α-olefins having 6 to 12 carbon atoms are preferable.
Examples of the polybutene resin include a homopolymer of butene-1 and a copolymer with ethylene or propylene.

[Mass ratio of elastomer (A) and polyolefin resin (B)]
The mass ratio of the elastomer (A) to the polyolefin resin (B) is preferably 90:10 to 15:85. Within this range, it is possible to easily manufacture a foam sheet that exhibits the effects of the present invention. From the viewpoint of obtaining a more effective foam sheet, the mass ratio of the component (A) to the component (B) is more preferably in the range of 80:20 to 20:80, and 80:20 to 30. : 70 is more preferable.

[Additive]
The foam sheet of the present invention is preferably obtained by foaming an effervescent composition containing the above resin and a foaming agent. As the foaming agent, a pyrolytic foaming agent is preferable.
As the pyrolytic foaming agent, an organic foaming agent and an inorganic foaming agent can be used. Examples of the organic foaming agent include azodicarbonamide, azodicarboxylic acid metal salt (azodicarboxylic acid barium, etc.), azo compounds such as azobisisobutyronitrile, nitroso compounds such as N, N'-dinitrosopentamethylenetetramine, and hydra. Examples thereof include hydrazine derivatives such as zodicarbonamide, 4,4'-oxybis (benzenesulfonyl hydrazide) and toluenesulfonyl hydrazide, 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 boron hydride, anhydrous monosoda citrate and the like.
Among these, azo compounds are preferable, and azodicarbonamides are more preferable, from the viewpoint of obtaining fine bubbles, economy, and safety.
One type of pyrolysis foaming agent may be used alone, or two or more types may be used in combination.

The blending amount of the foaming agent in the effervescent composition is preferably 1 part by mass or more and 20 parts by mass or less, more preferably 1.5 parts by mass or more and 15 parts by mass or less, and 3 parts by mass or more and 10 parts by mass with respect to 100 parts by mass of the resin. It is more preferably parts by mass or less. By setting the blending amount of the foaming agent to 1 part by mass or more, the foamable sheet is appropriately foamed, and it becomes possible to impart appropriate flexibility and shock absorption to the foam sheet. Further, by setting the blending amount of the foaming agent to 20 parts by mass or less, it is possible to prevent the foam sheet from foaming more than necessary, and it is possible to improve the mechanical strength of the foam sheet.

The effervescent composition may contain a decomposition temperature adjusting agent. The decomposition temperature adjusting agent is compounded as having a controlling function such as lowering the decomposition temperature of the thermally decomposing foaming agent and accelerating the decomposition rate, and specific compounds include zinc oxide, zinc stearate, and urea. And so on. The decomposition temperature adjusting agent is blended in an amount of 0.01 to 5 parts by mass with respect to 100 parts by mass of the resin, for example, in order to adjust the surface condition of the foam sheet.

The effervescent composition may contain an antioxidant. Examples of the antioxidant include phenolic antioxidants such as 2,6-di-t-butyl-p-cresol, sulfur-based antioxidants, phosphorus-based antioxidants, amine-based antioxidants and the like. For example, the antioxidant is blended in an amount of 0.01 to 5 parts by mass with respect to 100 parts by mass of the resin.
In addition to these, the effervescent composition may contain additives generally used for foams such as heat stabilizers, colorants, flame retardants, antistatic agents, and fillers.

In the foam sheet, the elastomer (A) and the polyolefin resin (B) are the main components, and the total content of the components (A) and (B) is, for example, 70% by mass or more based on the total amount of the foam sheet. It is preferably 80% by mass or more, more preferably 90% by mass or more.

[Manufacturing method of foam sheet]
The foam sheet of the present invention is not particularly limited, but can be produced by heating a foamable sheet made of a foamable composition containing at least a resin and a pyrolysis foaming agent to foam the pyrolysis foaming agent. Further, preferably, the effervescent sheet is crosslinked, and the crosslinked effervescent sheet is heated to foam.
More specifically, the method for producing the foam sheet preferably includes the following steps (1) to (3).
Step (1): Step of forming an effervescent sheet made of an effervescent composition containing at least a resin and a heat-decomposable foaming agent Step (2): Irradiating the effervescent sheet with ionizing radiation to crosslink the effervescent sheet. Step Step (3): A step of heating a crosslinked foamable sheet to foam a heat-decomposable foaming agent to obtain a foam sheet.

In the step (1), the method for forming the foamable sheet is not particularly limited, but for example, the resin and the additive are supplied to the extruder, melt-kneaded, and the foamable composition is extruded into a sheet from the extruder. It may be molded by this. Further, the effervescent sheet may be formed by pressing an effervescent composition or the like. The molding temperature of the foamable sheet (that is, the temperature at the time of extrusion or the temperature at the time of pressing) is preferably 50 ° C. or higher and 250 ° C. or lower, and more preferably 80 ° C. or higher and 180 ° C. or lower.

As a method for cross-linking the effervescent composition in the step (2), a method of irradiating the effervescent sheet with ionizing radiation such as electron beam, α ray, β ray, and γ ray is used. The irradiation amount of the ionizing radiation may be adjusted so that the degree of cross-linking of the obtained foam sheet is within the above-mentioned desired range, but is preferably 1 to 12 Mrad, and preferably 1.5 to 10 Mrad. More preferred.

In the step (3), the heating temperature at which the foamable composition is heated to foam the pyrolyzable foaming agent may be equal to or higher than the foaming temperature of the pyrolyzable foaming agent, but is preferably 200 to 300 ° C. More preferably, it is 220 to 280 ° C. In the step (3), the effervescent composition is foamed to form bubbles to form a foam.

Further, in the present production method, the foam sheet may be thinned by a method such as rolling or stretching.

However, the present production method is not limited to the above, and a foam sheet may be obtained by a method other than the above. For example, instead of irradiating with ionizing radiation, the effervescent composition may be preliminarily blended with an organic peroxide, and the effervescent sheet may be heated to decompose the organic peroxide for cross-linking. ..
Further, if cross-linking is not necessary, the step (2) may be omitted. In that case, in the step (3), the uncrosslinked foamable sheet may be heated and foamed.

[Adhesive tape]
The foam sheet of the present invention may be used for an adhesive tape using the foam sheet as a base material. The adhesive tape includes, for example, a foam sheet and an adhesive material provided on at least one surface of the foam sheet. The adhesive tape can be adhered to another member such as a support member via the adhesive material. The adhesive tape may be a foam sheet provided with an adhesive material on both sides, or may be provided with an adhesive material on one side.
Further, the adhesive material may be at least one having an adhesive layer, may be a single adhesive layer laminated on the surface of the foam sheet, or may be a double-sided adhesive sheet attached to the surface of the foam sheet. However, it is preferable that the pressure-sensitive adhesive layer is a single substance. The double-sided pressure-sensitive adhesive sheet includes a base material and a pressure-sensitive adhesive layer provided on both sides of the base material. The double-sided pressure-sensitive adhesive sheet is used for adhering one pressure-sensitive adhesive layer to a foam sheet and adhering the other pressure-sensitive adhesive layer to another member.

The pressure-sensitive adhesive constituting the pressure-sensitive adhesive layer is not particularly limited, and for example, an acrylic pressure-sensitive adhesive, a urethane-based pressure-sensitive adhesive, a rubber-based pressure-sensitive adhesive, a silicone-based pressure-sensitive adhesive, and the like can be used. Further, a release sheet such as a release paper may be further bonded on the adhesive material.
The thickness of the pressure-sensitive adhesive layer is preferably 5 to 200 μm, more preferably 7 to 150 μm, and even more preferably 10 to 100 μm.

[Foam sheet roll]
The foam sheet of the present invention can be a roll. The roll makes it easy to store and convenient for transportation. When used, it can be unwound from the roll and used.

[Use of foam sheet]
The use of the foam sheet is not particularly limited, but it is preferably used as a cushioning material for a display device. Specifically, it may be arranged on the back side of a display panel provided in various electronic devices and used so as to cushion an impact applied to the display panel. In this case, the foam sheet may be arranged on a support member arranged on the back side of the display panel. The support member constitutes, for example, a part of a housing or the like of various electronic devices.
Further, it is preferable to use it as a cushioning material between the battery of a portable electronic device such as a smartphone and the back cover material. Vibration generated by the back cover material can be effectively suppressed.
The foam sheet may be provided with an adhesive material as described above, and may be attached to a display panel, a support member, a back cover material, or the like by the adhesive material.
Further, since the foam sheet of the present invention has a peak of tan δ near room temperature, it can exert an anti-vibration effect, and is particularly effective for electronic devices such as smartphones whose back cover material is glass or the like. .. An acoustic member such as a speaker is built in an electronic device, and the back cover material made of glass or the like is easily vibrated by the acoustic member. However, the foam sheet of the present invention has excellent vibration isolation in a low frequency band. Since it has an effect, vibration can be effectively prevented.
Further, since the foam sheet of the present invention has a low relative permittivity, it does not cause a communication delay when used in a portable electronic device such as a smartphone. In particular, when used in high-speed communication equipment such as the 5th generation mobile communication system (5G), the effect is further exhibited.

Hereinafter, the present invention will be described with reference to examples, but the present invention is not limited to these examples.
The method for measuring each physical property and the method for evaluating the foam sheet are as follows.

[Physical characteristics after molding]
(1) Glass transition temperature (Tg) and loss tangent (tanδ)
Tg and tan δ were determined under the following measurement conditions using a tensile storage elastic modulus measuring device manufactured by IT Measurement Control Co., Ltd. and having a trade name of “DVA-200 / L2”.
(Measurement condition)
Length between marked lines: 2.5 cm
Sample width: 0.5 cm
Sample thickness: Foam sheet thickness Deformation mode: Tension static / dynamic stress ratio: 1.5
Setting distortion: 1.0%
Set temperature rise rate: 10 ° C / min
Measurement frequency 10Hz
Temperature range: -150 ° C to 100 ° C

(2) Breaking point strength and breaking point elongation The foamed sheets produced in each Example and Comparative Example were cut into a dumbbell-shaped No. 1 shape specified in JIS K6251 4.1. Using this as a sample, tensile was performed in the MD direction at a measurement temperature of 23 ° C. and a speed of 500 mm / min using a tensile tester (product name: Tencilon RTF235, manufactured by A & D Co., Ltd.) at break. Tensile strength (breaking point strength, unit: N / 10 mm) and tensile elongation (breaking point elongation, unit:%) were measured.

(3) Degree of cross-linking (gel fraction)
About 100 mg of the test piece was collected from the foam sheet, and the mass A (mg) of the test piece was precisely weighed. Next, this test piece was immersed in 30 cm ³ of xylene at 120 ° C. and left for 24 hours, then filtered through a 200 mesh wire mesh to collect the insoluble matter on the wire mesh, vacuum dried, and the mass of the insoluble matter. B (mg) was precisely weighed. From the obtained values, the degree of cross-linking (mass%) was calculated by the following formula.
Gel fraction (% by mass) = 100 x (B / A)

(4) Closed cell ratio A planar square-shaped test piece having a side of 5 cm was cut out from the foam sheet. Then, the thickness of the test piece is measured to calculate the apparent volume V1 of the test piece, and the mass W1 of the test piece is measured. Next, the volume V2 occupied by the bubbles was calculated based on the following formula. The density of the test piece is ρ (g / cm ³ ).
Volume occupied by bubbles V2 = V1-W1 / ρ
Subsequently, the test piece was submerged in distilled water at 23 ° C. to a depth of 100 mm from the water surface, and a pressure of 15 kPa was applied to the test piece for 3 minutes. After that, the test piece was taken out from the water, the water adhering to the surface of the test piece was removed, the mass W2 of the test piece was measured, and the closed cell ratio F1 was calculated based on the following formula.
Closed cell ratio F1 (%) = 100-100 × (W2-W1) / V2

(5) Average bubble diameter A foam sheet is cut in the thickness direction along each of MD and TD, and a 200-fold magnified photograph is taken using a digital microscope (manufactured by KEYENCE CORPORATION, product name "VHX-900"). I took a picture. In the magnified photograph taken, the bubble diameter of MD and the bubble diameter of TD were measured for all the bubbles existing on the cut surface having a length of 2 mm in each of MD and TD, and the operation was repeated 5 times. Then, the average value of the bubble diameters of MD and TD of all the bubbles was taken as the average bubble diameter of MD and TD.
In addition, MD means Machine direction, and is the direction which coincides with the extrusion direction and the like. TD means Transverse direction, which is orthogonal to MD and parallel to the foam sheet.

(6) 25% Compressive Strength The measurement was performed at a measurement temperature of 23 ° C. by a measurement method based on JIS K6767.
(7) Apparent density and foaming ratio The apparent density of the foam was measured in accordance with JIS K7222, and the reciprocal of the measured was taken as the foaming ratio.
(8) Thickness Measured with a dial gauge.

(9) WVTR
At a temperature of 40 ° C., the foam sheets produced in each Example and Comparative Example were cut into pieces of about 10 cm × 10 cm and installed in the measuring unit of a water vapor transmittance measuring device (MOCON, PERMATRAN-W 1/50), and then 40. The water vapor permeability was measured under the conditions of ° C. and 90% RH.
The evaluation criteria are as follows.
〇: Moisture permeability (WVTR) is 400 g / m ² · day or less ×: Moisture permeability (WVTR) is over 400 g / m ² · day

(10) Relative permittivity Obtained by measuring 33 points and 1 cycle at room temperature in the air with an LCR (impedance) analyzer "PSM3750" manufactured by Iwasaki Communication Equipment Co., Ltd., dividing frequencies from 100 MHz to 10 MHz on a log scale. By reading the waveform, the relative permittivity with a frequency of 1 MHz was obtained.

[evaluation]
(11) Anti-vibration evaluation (frequency vs loss tangent (tanδ))
A master curve was created under the following measurement conditions using a tensile storage elastic modulus measuring device manufactured by IT Measurement Control Co., Ltd. under the trade name "DVA-200 / L2", and tan δ at each frequency (Hz) was obtained. The evaluation was made based on the height of the tanδ peak in the low frequency range (10 to 1000 Hz) considered to be effective for vibration. If the tanδ peak height was 0.3 or more, it was considered good (〇), and if it was less than 0.3, it was considered defective (×).
(Measurement condition)
Length between marked lines: 2.5 cm
Sample width: 0.5 cm
Sample thickness: Foam sheet thickness Deformation mode: Tension static / dynamic stress ratio: 1.5
Setting distortion: 1.0%
Set temperature rise rate: 10 ° C / min
Measurement frequency: Measurement was performed at 8 frequency points (0.32, 0.63, 1.25, 2.5, 5, 10, 20, 40 Hz) in the following temperature range every 5 ° C.
Temperature range: -70 ° C to 25 ° C

(12) Radio wave inhibition evaluation The propagation delay time was obtained from the relative permittivity based on the following equation. If the propagation delay time was less than 4.5 ns / m, it was regarded as good (◯), and if it was 4.5 ns / m or more, it was regarded as poor (×).
In the following equation, ε is the relative permittivity, τ is the propagation delay time (ns / m), К is the wavelength shortening rate, and C is the speed of light (3 × ¹⁰⁸ m / s).

The materials used in the examples and comparative examples are as follows.
-Elastomer resin (a): S. O. E. (Registered Trademark) S1609, Hydrogenated Styrene-based Thermoplastic Elastomer (SEBS)
Elastomer resin (b): EP1001 (manufactured by Mitsui Chemicals, Inc., 4-methyl-1pentene / propylene copolymer)
Elastomer resin (c): Clarity (registered trademark) LA3320 (manufactured by Kuraray Co., Ltd., acrylic thermoplastic elastomer)
Polyolefin resin (a): Kernel (registered trademark) KF283 (ethylene / α-olefin copolymer (LLDPE) polymerized with a metallocene catalyst manufactured by Nippon Polyethylene Co., Ltd.)
Polyolefin resin (b): PP-E-333GV (manufactured by Prime Polymer Co., Ltd., density: 0.9 g / cm ³ , melt flow rate: 2.4 g / 10 minutes)
-Pyrolytic foaming agent: Azodicarbonamide-Decomposition temperature adjuster: Zinc oxide, manufactured by Sakai Chemical Industry Co., Ltd., trade name "OW-212F"
・ Phenolic antioxidant: 2,6-di-t-butyl-p-cresol ・ Cross-linking agent A: 1,9-nonanediol dimethacrylate ・ Cross-linking agent B: trimethylolpropane trimethacrylate

Example 1
40 parts by mass of elastomer resin (a), 60 parts by mass of polyolefin resin (a), 5 parts by mass of pyrolysis foaming agent, 1 part by mass of decomposition temperature adjuster, and 0.5 part by mass of phenolic antioxidant. Prepared as a raw material. These materials were melt-kneaded and then pressed to obtain a foamable resin sheet having a thickness of 0.38 mm. Both sides of the obtained foamable resin sheet were irradiated with an electron beam for 5 mad at an acceleration voltage of 500 keV to crosslink the foamable resin sheet. Next, the crosslinked foamable resin sheet was foamed by heating to 250 ° C. to obtain a foam sheet having an apparent density of 0.24 g / cm ³ and a thickness of 0.60 mm. After that, it was stretched to obtain a foam sheet having an apparent density of 0.24 g / cm ³ and a thickness of 0.2 mm.
The results of evaluation by the above method are shown in Table 1.

Example 2
A foam sheet was obtained in the same manner as in Example 1 except that the pyrolyzable foaming agent was changed to 2.5 parts by mass and the thickness of the foamable resin sheet was 0.36 mm in Example 1. The apparent density of the foam sheet was 0.48 g / cm ³ , and the thickness was 0.45 mm. After that, it was stretched to obtain a foam sheet having an apparent density of 0.48 g / cm ³ and a thickness of 0.15 mm. The evaluation results are shown in Table 1.

Example 3
Same as Example 1 except that the pyrolyzable foaming agent was changed to 6.5 parts by mass, the thickness of the foamable resin sheet was 0.33 mm, and the irradiation dose of the electron beam was 4Mrad. And obtained a foam sheet. The apparent density of the foam sheet was 0.16 g / cm ³ , and the thickness was 0.60 mm. After that, it was stretched to obtain a foam sheet having an apparent density of 0.16 g / cm ³ and a thickness of 0.2 mm. The evaluation results are shown in Table 1.

Example 4
Same as Example 1 except that the pyrolyzable foaming agent was changed to 7 parts by mass, the thickness of the foamable resin sheet was 0.58 mm, and the irradiation dose of the electron beam was 5.5 Mrad in Example 1. And obtained a foam sheet. The apparent density of the foam sheet was 0.10 g / cm ³ , and the thickness was 1.25 mm. After that, it was stretched to obtain a foam sheet having an apparent density of 0.10 g / cm ³ and a thickness of 0.5 mm. The evaluation results are shown in Table 1.

Example 5
In Example 1, the same as in Example 1 except that the pyrolysis foaming agent was changed to 9 parts by mass, the thickness of the foamable resin sheet was 0.61 mm, and the irradiation dose of the electron beam was 6 Mrad. , A foam sheet was obtained. The apparent density of the foam sheet was 0.06 g / cm ³ , and the thickness was 1.5 mm. The evaluation results are shown in Table 1.

Example 6
In Example 1, the same as in Example 1 except that the blending amount of the elastomer resin (a) was 30 parts by mass, the blending amount of the polyolefin resin (a) was 70 parts by mass, and the irradiation dose of the electron beam was 4Mrad. A foam sheet having an apparent density of 0.24 g / cm ³ and a thickness of 0.2 mm after foaming was obtained. The evaluation results are shown in Table 1.

Example 7
In Example 1, the same as in Example 1 except that the blending amount of the elastomer resin (a) was 50 parts by mass, the blending amount of the polyolefin resin (a) was 50 parts by mass, and the irradiation dose of the electron beam was 4Mrad. A foam sheet having an apparent density of 0.24 g / cm ³ and a thickness of 0.2 mm after foaming was obtained. The evaluation results are shown in Table 1.

Example 8
In Example 1, the apparent density after foaming is 0 in the same manner as in Example 1 except that the blending amount of the elastomer resin (a) is 70 parts by mass and the blending amount of the polyolefin resin (a) is 30 parts by mass. A foam sheet having a thickness of .23 g / cm ³ and a thickness of 0.2 mm was obtained. The evaluation results are shown in Table 1.

Example 9
In Example 6, the elastomer resin (b) was used instead of the elastomer resin (a), and the apparent density after foaming was 0. A foam sheet having a thickness of 25 g / cm ³ and a thickness of 0.2 mm was obtained. The evaluation results are shown in Table 1.

Example 10
In Example 7, the apparent density after foaming is 0.25 g / cm ³ , and the thickness is 0.2 mm, as in Example 7, except that the elastomer resin (b) is used instead of the elastomer resin (a). Foam sheet was obtained. The evaluation results are shown in Table 1.

Example 11
In Example 7, the apparent density after foaming is the same as in Example 7, except that the polyolefin resin (b) is used instead of the polyolefin resin (a) and the irradiation dose of the electron beam is 2.5 Mrad. A foam sheet having a thickness of 0.24 g / cm ³ and a thickness of 0.2 mm was obtained. The evaluation results are shown in Table 1.

Comparative Example 1
In Example 1, 100 parts by mass of the polyolefin resin (a) was used without using the elastomer resin, and the irradiation dose of the electron beam was set to 4 Mrad. A foam sheet having a thickness of 25 g / cm ³ and a thickness of 0.2 mm was obtained. The evaluation results are shown in Table 1. In addition, Tg1 was not observed under the measurement conditions of the present application.

Comparative Example 2
In Example 1, the apparent density after foaming is the same as in Example 1 except that the blending amount of the elastomer resin (a) is 10 parts by mass and the blending amount of the polyolefin resin (a) is 90 parts by mass. A foam sheet having a thickness of 0.25 g / cm ³ and a thickness of 0.2 mm was obtained. The evaluation results are shown in Table 1.

Comparative Example 3
100 parts by mass of elastomer resin (c), 5 parts by mass of pyrolysis type foaming agent, 1 part by mass of decomposition temperature adjuster, 0.5 part by mass of phenolic antioxidant, and 1.8 parts by mass of cross-linking agent A. , 1.2 parts by mass of the cross-linking agent B was prepared as a raw material. These materials were melt-kneaded and then pressed to obtain a foam resin sheet having a thickness of 0.38 mm. The foamable resin sheet was crosslinked by irradiating both sides of the obtained foam resin sheet with an electron beam at an acceleration voltage of 800 keV for 2.5 mad. Next, the crosslinked foamable resin sheet was foamed by heating to 250 ° C. to obtain a foam sheet having a density of 0.27 g / cm ³ and a thickness of 0.60 mm. After that, it was stretched to obtain a foam sheet having an apparent density of 0.27 g / cm ³ and a thickness of 0.2 mm. The evaluation results are shown in Table 1.

As described above, all of the foam sheets of the examples show good results in the vibration isolation evaluation, and when used in a portable electronic device such as a smartphone, vibration on the back side can be suppressed. This effect is particularly effective when a material such as glass or polycarbonate is used for the backing material.
On the other hand, it was found that the foam sheet of the comparative example did not have a peak of tan δ near room temperature in the vibration isolation evaluation, so that the vibration isolation effect could not be obtained.
In addition, the foam sheet of the present invention has a short delay time and shows a good effect on radio wave obstruction.

Claims (16)

At least one glass transition temperature (Tg1) is 0 to 40 ° C., the peak value of the loss dielectric (tanδ) at the glass transition temperature (Tg1) is 0.30 or more, and the 25% compressive strength is 1000 kPa or less. A foam sheet having a relative dielectric constant of 2 or less. The foam sheet according to claim 1, wherein the 25% compressive strength is 800 kPa or less. The foam sheet according to claim 1 or 2, wherein the thickness is 0.03 to 2 mm. The foam sheet according to any one of claims 1 to 3, wherein the breaking point strength at 23 ° C. is 5 N / 10 mm or more. The foam sheet according to any one of claims 1 to 4, wherein the closed cell ratio is 80% or more. The foam sheet according to any one of claims 1 to 5, wherein the average cell diameter is 20 to 400 μm. The foam sheet according to any one of claims 1 to 6, wherein the gel fraction is 30 to 80% by mass. The foam sheet according to any one of claims 1 to 7, wherein the apparent density is 0.05 to 0.70 g / cm ³ . The foam sheet according to any one of claims 1 to 8, wherein the moisture permeability (WVTR) is 400 g / m ² · day or less. The foam sheet according to any one of claims 1 to 9, wherein the glass transition temperature (Tg2) is −40 ° C. or lower. The foam sheet according to any one of claims 1 to 10, wherein the elongation at break at 23 ° C. is 200% or more. The foam sheet according to any one of claims 1 to 11, wherein the resin constituting the foam sheet contains a polyolefin-based resin. The foam sheet according to claim 12, wherein the resin constituting the foam sheet further contains an elastomer. The foam sheet according to claim 13, wherein the mass ratio of the elastomer to the polyolefin resin is 90:10 to 15:85. An adhesive tape comprising the foam sheet according to any one of claims 1 to 14 and an adhesive material provided on at least one surface of the foam sheet. A roll made of the foam sheet according to any one of claims 1 to 14.