foam

Abstract

To provide a foam having excellent impact resistance even with a narrow bonding area.SOLUTION: A foam has a temperature range of glass transition points (Tg) from -60 to 15°C, a peak top intensity (maximum value) of loss tangent (tanδ) of 0.25 or more, an average cell diameter of 140 μm or less, and an elastic modulus of 2.0×103 Pa or more at 23°C.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to foams. [Background technology]

[0002] In portable electronic devices such as notebook personal computers, mobile phones, smartphones, and tablets, a cushioning material is sometimes placed on the back side of the display device to prevent damage or malfunction. High flexibility is required for the cushioning material, and foam sheets have been widely used. Furthermore, the cushioning material may be used as a foam tape for adhering the housing and the display panel by providing an adhesive layer or the like on the foam sheet and placing it, for example, in a frame shape, between the housing and the display panel.

[0003] It is known that the foam used for foam tape adjusts the peak top temperature range of loss tangent (tan δ), which is the ratio of storage modulus to loss modulus, and its peak top strength. For example, Patent Document 1 discloses that the peak top temperature of tan δ is set to a range of -60 to 30°C, and the peak top strength is set to 0.2 or more.

[0004] In recent years, as electronic devices have become more sophisticated, their housings have also become larger. Furthermore, the risk of dropping electronic devices, such as smartphones and tablets, which are carried around while in use, is increasing, as is the risk of damage between the display panel and the housing. Therefore, methods of fixing the panel and housing that are less likely to cause damage are being sought, and for example, foam sheets used in foam tape are required to have high impact resistance. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] International Publication No. 2019 / 187388 Summary of the Invention [Problem to be solved by the invention]

[0006] As the screen size of electronic devices such as smartphones increases and their designs improve, the width of the adhesive area between the display panel and the housing becomes narrower. For example, if the screen size increases while the external dimensions of the housing remain the same, or if the side of the housing is curved, the width of the foam tape used as a cushioning material will become narrower. However, narrowing the width of the foam tape reduces the number of cells per width, resulting in a problem of reduced impact resistance. Furthermore, the impact of the drop described above is a high-speed impact, and conventional foams often do not have sufficient high-speed impact resistance. Therefore, an object of the present invention is to provide a foam that has good impact resistance, such as high-speed impact resistance, even when the width is narrowed. [Means for solving the problem]

[0007] As a result of intensive research, the present inventors have found that a loss tangent (tanδ) peak value is 0.25 or more, at least one loss tangent (tanδ) peak value is present in the temperature range of -60 to 15°C, the average cell diameter is 140 μm or less, and the elastic modulus at 23°C is 2.0 × 10 3 The present invention was completed based on the discovery that the above problems can be solved by a foam having a compressive strength of 100 Pa or more. That is, the present invention provides the following [1] to [8].

[0008] [1] The peak value of the loss tangent (tanδ) is 0.25 or more, at least one peak value of the loss tangent (tanδ) exists in the temperature range of -60 to 15 ° C, the average bubble diameter is 140 μm or less, and the storage modulus at 23 ° C is 2.0 × 10 3 Pa or more, foam. [2] The foam according to [1], wherein the heat generation peak starts at 200°C or higher when the calorific value at each temperature is measured using a differential scanning calorimeter (DSC). [3] The foam according to [1] or [2], having a 25% compressive strength of 800 kPa or less. [4] Density: 0.1 to 0.7 g / cm 3 The foam according to any one of [1] to [3], wherein [5] The foam according to any one of [1] to [4], which has a thickness of 0.03 to 2.0 mm. [6] The foam according to any one of [1] to [5], which has a closed cell ratio of 80% or more. [7] The foam according to any one of [1] to [6], which has an average cell diameter of 20 to 100 μm. [8] A pressure-sensitive adhesive tape comprising the foam according to any one of [1] to [7] and a pressure-sensitive adhesive layer on at least one surface of the foam sheet. [Effects of the Invention]

[0009] According to the present invention, it is possible to provide a foam that has good impact resistance, such as high-speed impact resistance, even when the width is narrowed. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a conceptual diagram showing a smartphone screen, a housing, and foam tape. [Figure 2] FIG. 1 is a schematic diagram showing the results of measuring the amount of heat at each temperature using a differential scanning calorimeter (DSC). DETAILED DESCRIPTION OF THE INVENTION

[0011] The foam of the present invention will be described in more detail below. [Foam] The foam of the present invention has a peak value of loss tangent (tan δ) of 0.25 or more, at least one peak value of loss tangent (tan δ) exists in the temperature range of −60 to 15° C., an average cell diameter of 140 μm or less, and an elastic modulus at 23° C. of 2.0×10 3 Pa or more.

[0012] <Loss tangent> The foam of the present invention has a peak value of loss tangent (tan δ) of 0.25 or more. Tan δ is the ratio (G" / G') of storage modulus (G') to loss modulus (G"). It indicates how much energy a material absorbs (converts to heat) when deforming. If the peak value of tan δ is less than 0.25, the impact resistance of the foam decreases, and the high-speed impact resistance also decreases. Furthermore, the impact absorption may become insufficient. From the viewpoint of improving impact resistance, particularly high-speed impact resistance, the peak value of tan δ of the foam is preferably 0.40 or more, and more preferably 0.50 or more. Note that a larger value of the loss tangent (tan δ) of the foam is preferable from the viewpoint of improving impact resistance, and there is no upper limit, but in practice the peak value of the loss tangent (tan δ) is, for example, 5 or less, and may be 3 or less. It should be noted that a plurality of peak values ​​of the loss tangent (tan δ) may be confirmed, and in this case, it is sufficient that at least one of the peak values ​​is within the above range.

[0013] The foam of the present invention has at least one peak loss tangent (tan δ) in the temperature range of -60 to 15°C. If the peak loss tangent (tan δ) is not observed within the above temperature range, the foam will not be able to fully exhibit its impact resistance, particularly its high-speed impact resistance. From the viewpoint of more effectively improving the impact resistance, particularly the high-speed impact resistance, of the foam, it is preferable that at least one peak be present in the temperature range of -55 to 0°C, and more preferably at least one peak be present in the temperature range of -50 to -10°C. The peak value of the loss tangent (tanδ) may exist in a temperature range other than the above-mentioned specific temperature range, but preferably exists only in the above-mentioned specific temperature range.Furthermore, it is sufficient that at least one peak value of the loss tangent (tanδ) is within the above-mentioned specific temperature range, but it is preferable that the maximum value of the peak is within the above-mentioned temperature range. In this specification, the temperature (peak top temperature) at which the loss tangent (tan δ) shows a peak value is also referred to as the glass transition temperature (Tg). The peak value of the loss tangent (tan δ) can be adjusted by the type and amount of the resin constituting the foam, the manufacturing conditions of the foam, and the like. The peak value of the loss tangent (tan δ) is measured by dynamic viscoelasticity measurement at a measurement frequency of 10 Hz, and the details are measured by the method described in the examples.

[0014] <Average bubble diameter> The foam of the present invention has an average cell diameter of 140 μm or less. If the average cell diameter exceeds 140 μm, narrowing the width of the foam makes it difficult to maintain a certain number of cells in the width direction, making it difficult to improve impact resistance. From the viewpoint of more effectively improving the impact resistance of the foam, the average cell diameter is preferably 120 μm or less, more preferably 100 μm or less, even more preferably 95 μm or less, and particularly preferably 85 μm or less. The average cell diameter is not particularly limited in terms of the lower limit, but from the viewpoint of ensuring a certain level of flexibility and elasticity of the foam, it is preferably 20 μm or more, more preferably 30 μm or more, even more preferably 40 μm or more, and even more preferably 45 μm or more. The average cell diameter in the present invention is the larger of the average cell diameter in the machine direction (MD) and the average cell diameter in the direction perpendicular to the MD (TD). The average cell diameter was measured by the method described in the examples.

[0015] <Storage modulus> The foam of the present invention has a storage modulus of 2.0×10 at 23° C. 3 Pa or more. Storage modulus is 2.0×10 3 If the storage modulus is less than 1.0×10 Pa, the impact resistance cannot be sufficiently improved. In addition, the impact absorption properties may be insufficient. From the viewpoint of more effectively improving the impact resistance, the storage modulus is 1.0×10 4 Pa or more is more preferable, and 1.0 × 10 5 It is more preferable that the viscosity is 100 Pa or more. There is no particular upper limit for the storage modulus, but it is usually 1.0 × 10 12 Pa or less, and from the viewpoint of flexibility, it is 1.0 × 1010 It is preferably below Pa. The storage elastic modulus is a value measured by the method described in the examples.

[0016] <Peak start point of heat generation in DSC measurement> When measuring the heat quantity at each temperature using a differential scanning calorimeter (DSC) (hereinafter sometimes referred to as "DSC measurement"), it is preferable that the peak start point of heat generation is 200°C or higher. The above heat generation peak is a peak derived from the foaming agent. As described later, the foam is foamed by a foaming agent in a preferred embodiment. However, even after foaming, the foaming agent residue remains in the foam. In DSC measurement, as described above, the heat generation peak derived from the foaming agent appears. That the peak start point of heat generation is 200°C or higher indicates that the foam start point of the foamable sheet before foaming is 200°C or higher. And when the foam start point is 200°C or higher, the bubbles in the foam tend to be uniform and fine. That is, when the peak start point of the above heat generation is 200°C or higher, the average bubble diameter in the foam becomes small as described above, and the impact resistance of the foam is more effectively improved. From these viewpoints, the peak start point of the above heat generation is more preferably 210°C or higher, and even more preferably 215°C or higher. The peak start point of heat generation can be adjusted, for example, by appropriately selecting the type of foaming agent and the type of decomposition temperature regulator described later. The peak start point of heat generation in DSC measurement is not particularly limited, but from the viewpoint of imparting appropriate foaming properties, it is preferably 300°C or lower, more preferably 270°C or lower, and even more preferably 250°C or lower.

[0017] <Closed cell ratio> The foam of the present invention preferably has a closed cell ratio of 80% or more. When the closed cell ratio is 80% or more, it becomes easier to improve the impact resistance. From these viewpoints, the closed cell ratio of the foam is more preferably 90% or more. The higher the closed cell ratio, the better, as long as it is 100% or less. The closed cell ratio is a value measured by the method described in the examples.

[0018] <Apparent density> The apparent density of the foam of the present invention is 0.05 to 0.75 g / cm 3 It is preferable that the density is 0.10 to 0.70 g / cm 3 More preferably, it is 0.12 to 0.60 g / cm 3 More preferably, it is 0.18 to 0.55 g / cm 3 It is even more preferred that: When the apparent density is within the above range, the foam tends to have good flexibility, cushioning properties, etc. Furthermore, the foam tends to have a certain level of mechanical strength, which makes it easy to improve impact resistance, etc.

[0019] <Thickness> The foam of the present invention is preferably a foam sheet in the form of a sheet. The thickness of the foam of the present invention is preferably 0.03 to 2.0 mm. A thickness of 0.03 mm or more facilitates ensuring the cushioning properties of the foam, and also improves impact resistance. A thickness of 2.0 mm or less allows for a thinner design, making it suitable for use in thin electronic devices such as smartphones and tablets. Furthermore, flexibility of the foam is also more easily ensured. From these viewpoints, the thickness of the foam is more preferably 0.1 to 1.0 mm, and further preferably 0.15 to 0.7 mm. The thickness is a value measured by a dial gauge.

[0020] <25% compressive strength> The foam of the present invention preferably has a 25% compressive strength of 800 kPa or less. A 25% compressive strength of 800 kPa or less improves flexibility, resulting in excellent conformability, and when used as a tape substrate or the like, improves adhesive strength during bonding. From the above viewpoints, the 25% compressive strength is more preferably 750 kPa or less, even more preferably 600 kPa or less, and particularly preferably 470 kPa or less. There is no particular restriction on the lower limit, but it is usually about 10 kPa, and preferably 20 kPa or more. The 25% compressive strength is a value measured at a temperature of 23°C using a measurement method in accordance with JIS K 6767.

[0021] <Degree of cross-linking (gel fraction)> The foam of the present invention may be crosslinked. If it is crosslinked, the degree of crosslinking, expressed as gel fraction, is preferably 30 to 80% by mass. By setting the degree of crosslinking within this range, the foam can easily have finer cell diameters and have better impact resistance while maintaining a certain level of flexibility and cushioning properties. From the above viewpoints, the gel fraction is more preferably 40 to 75% by mass, and even more preferably 50 to 70% by mass. The gel fraction is a value measured by the method described in the examples.

[0022] The foam of the present invention preferably contains at least an elastomer as a resin. The use of an elastomer makes it easier to ensure flexibility, impact resistance, etc. while improving foamability, etc. From these viewpoints, the foam sheet of the present invention preferably contains an elastomer (A) and a polyolefin resin (B).

[0023] <Elastomer> Examples of the elastomer include thermoplastic elastomers, ethylene-α-olefin copolymer rubbers, etc. Examples of the thermoplastic elastomer include olefin thermoplastic elastomers, styrene thermoplastic elastomers, vinyl chloride thermoplastic elastomers, polyurethane thermoplastic elastomers, polyester thermoplastic elastomers, polyamide thermoplastic elastomers, etc. As the elastomer, 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 preferred, styrene-based thermoplastic elastomers and ethylene-α-olefin-based copolymer rubbers are more preferred, and styrene-based thermoplastic elastomers are even more preferred.

[0024] (olefin-based thermoplastic elastomer) Olefin-based thermoplastic elastomers (TPO) generally have polyolefins such as polyethylene and polypropylene as hard segments and rubber components such as butyl rubber, halobutyl rubber, EPDM (ethylene-propylene-diene rubber), EPM (ethylene-propylene rubber), NBR (acrylonitrile-butadiene rubber), and natural rubber as soft segments. Any of the blend, dynamic crosslink, and polymerization types of olefin-based thermoplastic elastomers (TPO) can be used. Specific examples of suitable rubber components include the above-mentioned EPM and EPDM, with EPDM being particularly preferred. EPDM includes ethylene-propylene-5-ethylidene-2-norbornene copolymer rubber and ethylene-propylene-dicyclopentadiene copolymer rubber, with ethylene-propylene-dicyclopentadiene copolymer rubber being preferred.

[0025] Further, olefin-based thermoplastic elastomers include block copolymer types. Block copolymer types include those having a crystalline block and a soft segment block, and more specifically, crystalline olefin block-ethylene-butylene copolymer-crystalline olefin block copolymer (CEBC). In CEBC, the crystalline olefin block is preferably a crystalline ethylene block, and commercially available CEBCs include "DYNARON 6200P" manufactured by JSR Corporation.

[0026] (styrene-based thermoplastic elastomer) Examples of styrene-based thermoplastic elastomers include block copolymers having a styrene polymer or copolymer block and a conjugated diene compound polymer or copolymer block, such as isoprene and butadiene. The styrene-based thermoplastic elastomer used in the present invention may or may not be hydrogenated, but is preferably hydrogenated. When hydrogenated, the hydrogenation can be carried out by a known method.

[0027] The styrene-based thermoplastic elastomer is typically a block copolymer, such as styrene-isoprene block copolymer (SI), styrene-isoprene-styrene block copolymer (SIS), styrene-butadiene block copolymer (SB), styrene-butadiene-styrene block copolymer (SBS), styrene-ethylene / butylene-styrene block copolymer (SEBS), styrene-ethylene / propylene-styrene block copolymer (SEPS), styrene-ethylene / ethylene / propylene-styrene block copolymer (SEEPS), styrene-ethylene / butylene block copolymer (SEB), styrene-ethylene / propylene block copolymer (SEP), styrene-ethylene / butylene-crystalline olefin block copolymer (SEBC), etc. As the styrene-based thermoplastic elastomer, block copolymers are preferred, and among them, SIS, SEBS, SEPS, SEEPS, and SEBC are more preferred, with SEEPS and SEBS being even more preferred.

[0028] An example of SIS is manufactured by Kuraray Co., Ltd. under the trade name "Hybler (registered trademark) 5125" (styrene content 20% by mass, Tg = -13°C), and an example of a commercially available SEPS is manufactured by Kuraray Co., Ltd. under the trade name "Hybler (registered trademark) 7125F" (styrene content 20% by mass, Tg = -15°C). An example of a commercially available SEEPS is manufactured by Kuraray Co., Ltd. under the trade name "Hybler (registered trademark) 7311F" (styrene content 12% by mass, Tg = -32°C). Commercially available SEBS products include the Tuftec (registered trademark) series manufactured by Asahi Kasei Corporation and MD6951 (styrene content 34.5% by mass, Tg=9° C.) manufactured by Kraton.

[0029] The styrene-based thermoplastic elastomer according to the present invention preferably has a maximum peak temperature of tan δ measured by dynamic viscoelasticity measurement of -60 to 25°C. When the maximum peak temperature of tan δ is relatively low, heat loss in a high-speed deformation region such as impact fracture increases, and the fracture strength of the foamed sheet tends to improve. From the above viewpoints, the maximum peak temperature of tan δ of the styrene-based thermoplastic elastomer is more preferably -35 to 10°C, and even more preferably -35 to 0°C. In this specification, the "maximum peak temperature of tan δ" refers to a value measured using a dynamic viscoelasticity measuring device in a tensile mode at a temperature rise rate of 10°C / min and a frequency of 10 Hz. Examples of dynamic viscoelasticity measuring devices that can be used for the measurement include the "Rheovibron DDV-III" manufactured by Orientec Co., Ltd.

[0030] The styrene-based thermoplastic elastomer contains structural units derived from styrene, which allows the foam sheet to have good impact resistance. The styrene content in the styrene-based thermoplastic elastomer is preferably 5 to 50% by mass. By setting the styrene content within this range, excellent impact resistance can be obtained. Furthermore, by setting the styrene content to the above upper limit or less, compatibility with the polyolefin resin (B), which will be described in detail later, is improved, and 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, and even more preferably 7 to 30% by mass.

[0031] The number average molecular weight of the styrene-based thermoplastic elastomer is not particularly limited, but from the viewpoints of breaking strength and processability, it is preferably 30,000 to 800,000, more preferably 120,000 to 180,000. The number average molecular weight is a polystyrene-equivalent value measured by gel permeation chromatography (GPC).

[0032] (Ethylene-α-olefin copolymer rubber) The α-olefin used in the ethylene-α-olefin copolymer rubber may be one or more α-olefins having 3 to 15 carbon atoms, preferably 3 to 10 carbon atoms, such as propylene, 1-butene, 2-methylpropylene, 3-methyl-1-butene, 1-pentene, 1-hexene, 4-methyl-1-pentene, 1-octene, etc. Among these, propylene and 1-butene are preferred, and 1-butene is more preferred. The ethylene-α-olefin copolymer rubber used here is an amorphous or low-crystalline rubber-like substance in which two or more olefin monomers are copolymerized substantially randomly.

[0033] The ethylene-α-olefin copolymer rubber may have other monomer units in addition to the ethylene units and α-olefin units. Examples of monomers that form the other monomer units include conjugated dienes having 4 to 8 carbon atoms, such as 1,3-butadiene, 2-methyl-1,3-butadiene (isoprene), 1,3-pentadiene, and 2,3-dimethyl-1,3-butadiene; non-conjugated dienes having 5 to 15 carbon atoms, such as dicyclopentadiene, 5-ethylidene-2-norbornene, 1,4-hexadiene, 1,5-dicyclooctadiene, 7-methyl-1,6-octadiene, and 5-vinyl-2-norbornene; vinyl ester compounds, such as vinyl acetate; unsaturated carboxylic acid esters, such as methyl acrylate, ethyl acrylate, butyl acrylate, methyl methacrylate, and ethyl methacrylate; and unsaturated carboxylic acids, such as acrylic acid and methacrylic acid. These monomers may be used alone or in combination of two or more. Among these, non-conjugated dienes having 5 to 15 carbon atoms are preferred, and from the viewpoint of availability, 5-ethylidene-2-norbornene, 1,4-hexadiene, and dicyclopentadiene (DCPD) are more preferred.

[0034] The ethylene unit content of the ethylene-α-olefin copolymer rubber is usually 30 to 85 mass%, preferably 40 to 80 mass%, more preferably 45 to 75 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 mass%, preferably 15 to 50 mass%, and the content of other monomer units such as non-conjugated dienes is usually 0 to 20 mass%, preferably 1 to 10 mass%.

[0035] The ethylene-α-olefin copolymer rubber is preferably a terpolymer such as EPDM (ethylene-propylene-diene rubber) or EBDM (ethylene-butene-1-diene rubber). Examples of the ethylene-α-olefin copolymer include "EBT K-9330" manufactured by Mitsui Chemicals, Inc.

[0036] <Polyolefin resin (B)> The polyolefin resin (B) is a thermoplastic resin, and specific examples thereof include polyethylene resin, polypropylene resin, polybutene resin, ethylene-vinyl acetate copolymer, etc., and among these, polyethylene resin is preferred. As the polyethylene resin, low density polyethylene (LDPE) is preferred, and linear low density polyethylene (LLDPE) is more preferred. Furthermore, examples of the polyethylene resin include polyethylene resins polymerized with a polymerization catalyst such as a Ziegler-Natta catalyst, a metallocene catalyst, or a chromium oxide compound, and polyethylene resins polymerized with a metallocene catalyst are preferably used.

[0037] (Metallocene catalyst) Examples of metallocene catalysts include compounds such as bis(cyclopentadienyl) metal complexes, which have a structure in which a transition metal is sandwiched between π-electron unsaturated compounds. More specifically, examples include compounds in which one or more cyclopentadienyl rings or analogs thereof exist as ligands on a tetravalent transition metal such as titanium, zirconium, nickel, palladium, hafnium, or platinum. Such metallocene catalysts have uniform properties of active sites, and each active site has the same activity. Polymers synthesized using metallocene catalysts have high uniformity in molecular weight, molecular weight distribution, composition, composition distribution, etc., so when a sheet containing a polymer synthesized using a metallocene catalyst is crosslinked, the crosslinking proceeds uniformly. A uniformly crosslinked sheet is foamed uniformly, making it easier to stabilize its physical properties. In addition, since it can be stretched uniformly, the thickness of the foam can be made uniform.

[0038] Examples of the ligand include a cyclopentadienyl ring and an indenyl ring. These cyclic compounds may be substituted with a hydrocarbon group, a substituted hydrocarbon group, or a hydrocarbon-substituted metalloid group. Examples of hydrocarbon groups 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, various decyl groups, various cetyl groups, and a phenyl group. Note that "various" refers to various isomers including n-, sec-, tert-, and iso-. Alternatively, a cyclic compound may be polymerized as an oligomer and used as the ligand. Furthermore, in addition to the π-electron unsaturated compounds, monovalent anionic ligands such as chlorine and bromine, or divalent anionic chelate ligands, hydrocarbons, alkoxides, arylamides, aryloxides, amides, phosphides, arylphosphides, etc. may also be used.

[0039] Examples of metallocene catalysts containing a tetravalent transition metal or a ligand include cyclopentadienyltitanium tris(dimethylamide), methylcyclopentadienyltitanium tris(dimethylamide), bis(cyclopentadienyl)titanium dichloride, and dimethylsilyltetramethylcyclopentadienyl-t-butylamide zirconium dichloride. Metallocene catalysts, when combined with a specific cocatalyst (promoter), function as a catalyst during the polymerization of various olefins. Specific examples of the cocatalyst include methylaluminoxane (MAO) and boron-based compounds. The ratio of the cocatalyst to the metallocene catalyst is preferably 100,000 to 1,000,000 moles, and more preferably 50 to 5,000 moles.

[0040] Moreover, the polyethylene resin is preferably a linear low-density polyethylene. The linear low-density polyethylene is more preferably a linear low-density polyethylene obtained by copolymerizing ethylene (for example, 75% by mass or more, preferably 90% by mass or more, based on the total amount of monomers) with a small amount of an α-olefin as needed. Specific examples of the α-olefin include propylene, 1-butene, 1-pentene, 4-methyl-1-pentene, 1-hexene, 1-heptene, and 1-octene. Of these, α-olefins having 4 to 10 carbon atoms are preferred. The density of the polyethylene resin, for example, the linear low-density polyethylene described above, is 0.870 to 0.925 g / cm from the viewpoint of flexibility. 3 is preferable, and 0.890 to 0.925 g / cm 3 More preferably, 0.910 to 0.925 g / cm 3 As the polyethylene resin, a plurality of polyethylene resins may be used, and a polyethylene resin having a density outside the above range may be added.

[0041] The ethylene-vinyl acetate copolymer used as the polyolefin resin (B) is, for example, an ethylene-vinyl acetate copolymer containing 50% by mass or more of ethylene. Examples of polypropylene resins include homopolypropylene and propylene-α-olefin copolymers containing 50% by mass or more of propylene. These may be used alone or in combination of two or more. Specific examples of α-olefins constituting the propylene-α-olefin copolymers include ethylene, 1-butene, 1-pentene, 4-methyl-1-pentene, 1-hexene, 1-heptene, and 1-octene. Of these, α-olefins having 6 to 12 carbon atoms are preferred. Examples of polybutene resins include homopolymers of butene-1 and copolymers with ethylene or propylene.

[0042] <Mass ratio of elastomer (A) to polyolefin resin (B)> The mass ratio of the elastomer (A) to the polyolefin resin (B) is preferably 100:0 to 30:70. Within this range, a foam exhibiting the effects of the present invention can be easily produced. From the viewpoint of obtaining a foam with even greater effects, the mass ratio of the (A) component to the (B) component is more preferably within the range of 95:5 to 35:65, even more preferably 90:10 to 40:60, and particularly preferably 85:15 to 45:55.

[0043] <Additives> The foam of the present invention is preferably obtained by foaming a foamable composition containing a resin and a foaming agent. As the resin, for example, as described above, it is preferable to contain at least an elastomer (A), and more preferably an elastomer (A) and a polyolefin resin (B). As the foaming agent, a thermal decomposition type foaming agent is preferable. The thermal decomposition type blowing agent may be an organic blowing agent or an inorganic blowing agent. Examples of the organic blowing agent include azo compounds such as azodicarbonamide, azodicarboxylic acid metal salts (e.g., barium azodicarboxylate), and azobisisobutyronitrile; nitroso compounds such as N,N'-dinitrosopentamethylenetetramine; hydrazine derivatives such as hydrazodicarbonamide, 4,4'-oxybis(benzenesulfonylhydrazide), and toluenesulfonylhydrazide; and semicarbazide compounds such as toluenesulfonylsemicarbazide. Examples of inorganic foaming agents include ammonium carbonate, sodium carbonate, ammonium hydrogen carbonate, sodium hydrogen carbonate, ammonium nitrite, sodium borohydride, and anhydrous monosodium citrate. Among these, from the viewpoint of obtaining fine bubbles, and from the viewpoints of economy and safety, azo compounds are preferred, and azodicarbonamide is more preferred. The thermal decomposition type foaming agents may be used alone or in combination of two or more.

[0044] The amount of foaming agent in the foamable 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 even more preferably 3 parts by mass or more and 10 parts by mass or less, relative to 100 parts by mass of the resin. By using a foaming agent amount of 1 part by mass or more, the foamable sheet is foamed appropriately, making it possible to impart appropriate flexibility and impact absorption to the foam. Furthermore, by using a foaming agent amount of 20 parts by mass or less, the foam is prevented from foaming more than necessary, making it possible to improve the mechanical strength of the foam.

[0045] The foamable composition preferably contains a decomposition temperature regulator. The decomposition temperature regulator is incorporated to have a regulating function, such as lowering the decomposition temperature of the thermally decomposable foaming agent or accelerating the decomposition rate. Examples of the decomposition temperature regulator include zinc compounds and nitrogen atom-containing compounds. Nitrogen atom-containing compounds are preferred from the viewpoint of improving impact resistance by increasing the foaming initiation temperature to a certain temperature or higher, thereby forming fine and uniform bubbles. Examples of the zinc compound include zinc oxide and zinc stearate. Examples of the nitrogen atom-containing compound include urea and amino group-containing compounds. Among these, amino group-containing compounds are preferred from the viewpoint of forming fine and uniform bubbles and improving impact resistance. As the amino group-containing compound, aromatic amino group-containing compounds are preferred, and 3-(N-salicyloyl)amino-1,2,4-triazole is particularly preferred. The content of the decomposition temperature regulator is preferably 0.01 to 5 parts by mass, more preferably 0.05 to 3 parts by mass, and even more preferably 0.1 to 1 part by mass, relative to 100 parts by mass of the resin.

[0046] The foamable composition may contain an antioxidant. Examples of the antioxidant include phenol-based antioxidants such as 2,6-di-t-butyl-p-cresol, sulfur-based antioxidants, phosphorus-based antioxidants, and amine-based antioxidants. The antioxidant is blended in an amount of, for example, 0.01 to 5 parts by mass per 100 parts by mass of the resin. In addition to the above, the foamable composition may contain additives that are generally used in foams, such as heat stabilizers, colorants, flame retardants, antistatic agents, and fillers.

[0047] In the foam, the elastomer (A) and the polyolefin resin (B) are the main components, and the total content of the (A) component and the (B) component is, for example, 70 mass % or more, preferably 80 mass % or more, and more preferably 90 mass % or more, based on the total amount of the foam.

[0048] <Method of manufacturing foam> The foam 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 thermally decomposable foaming agent to foam the thermally decomposable foaming agent. Preferably, the foamable sheet is crosslinked and the crosslinked foam is heated to foam. More specifically, the method for producing a foam preferably includes the following steps (1) to (3). Step (1): A step of forming a foamable sheet from a foamable composition containing at least a resin and a thermally decomposable foaming agent. Step (2): A step of irradiating the foamable sheet with ionizing radiation to crosslink the foamable sheet. Step (3): Heating the crosslinked expandable sheet to expand the thermally decomposable foaming agent, thereby obtaining a foam.

[0049] In step (1), the method for forming the foamable sheet is not particularly limited, but may be, for example, by feeding the resin and additives to an extruder, melt-kneading them, and extruding the foamable composition from the extruder into a sheet. Alternatively, the foamable sheet may be formed by pressing the foamable composition. The forming temperature of the foamable sheet (i.e., the temperature during extrusion or pressing) is preferably 50°C or higher and 250°C or lower, more preferably 80°C or higher and 180°C or lower.

[0050] In step (2), the foamable sheet is crosslinked by irradiating it with ionizing radiation such as electron beams, α-rays, β-rays, γ-rays, etc. The dose of the ionizing radiation may be adjusted so that the degree of crosslinking of the resulting foam falls within the desired range, and is preferably 1 to 12 Mrad, more preferably 1.5 to 10 Mrad.

[0051] In step (3), the heating temperature when the foamable sheet is heated to foam the thermally decomposable foaming agent may be equal to or higher than the foaming temperature of the thermally decomposable foaming agent, and is preferably 200 to 300° C., more preferably 220 to 280° C. In step (3), the foamable sheet is foamed to form bubbles, thereby forming a foam.

[0052] In the present production method, the foam may be thinned by rolling, stretching, or the like.

[0053] However, the present production method is not limited to the above, and a foam may be obtained by a method other than the above. For example, instead of irradiating with ionizing radiation, crosslinking may be performed by a method in which an organic peroxide is previously blended into a foamable sheet, and the foamable sheet is heated to decompose the organic peroxide. If crosslinking is not required, step (2) may be omitted, and in that case, in step (3), the uncrosslinked expandable sheet may be heated to expand it.

[0054] [Adhesive tape] The foam of the present invention may be used in an adhesive tape using a foam sheet as a substrate. The adhesive tape (also called foam tape) comprises, for example, a foam and an adhesive material provided on at least one surface of the foam. The adhesive tape can be adhered to other members such as a support member via the adhesive material. The adhesive tape may have adhesive materials provided on both surfaces of the foam or on one surface. The adhesive material may be any material that includes at least a pressure-sensitive adhesive layer, and may be a single pressure-sensitive adhesive layer laminated on the surface of a foam, or a double-sided pressure-sensitive adhesive sheet attached to the surface of a foam, but is preferably a single pressure-sensitive adhesive layer. The double-sided pressure-sensitive adhesive sheet includes a substrate and pressure-sensitive adhesive layers provided on both sides of the substrate. The double-sided pressure-sensitive adhesive sheet is used to adhere one pressure-sensitive adhesive layer to a foam, and the other pressure-sensitive adhesive layer to another member.

[0055] The adhesive constituting the adhesive layer is not particularly limited, and for example, an acrylic adhesive, a urethane adhesive, a rubber adhesive, a silicone adhesive, etc. Furthermore, a release sheet such as release paper may be further attached onto 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.

[0056] [Foam applications] The use of the foam is not particularly limited, but it is preferably used in electronic devices. Examples of electronic devices include mobile phones such as smartphones, game devices, electronic organizers, tablet devices, and notebook personal computers. The foam can be used as a buffer material inside the electronic device, and is preferably used as a cushioning material for display devices. It may also be used as a sealing material to fill gaps between components inside the electronic device.

[0057] The foam may be used in any shape, but is preferably narrow, such as an elongated rectangle, a frame shape such as a square frame, an L-shape, a U-shape, etc. The width of these is, for example, 5 mm or less, preferably 3 mm or less, more preferably 1 mm or less, and is, for example, 0.1 mm or more.

[0058] The foam sheet used as a cushioning material for a display device may be placed, for example, on the back side of a display panel installed in various electronic devices to cushion impacts acting on the display panel. In this case, the foam may be placed on a support member placed on the back side of the display panel. The support member may be, for example, a part of the housing of the various electronic devices. The foam used in the electronic device may be provided with an adhesive material as described above, and may be attached to a display panel, a support member, or the like by the adhesive material.

[0059] An example in which a frame-shaped foam as a cushioning material for a display device is used as a substrate for an adhesive tape and applied to a smartphone is shown in Fig. 1. As shown in Fig. 1, a square-frame-shaped adhesive tape 2 is placed, for example, between a display panel 1 of the smartphone and a square-frame-shaped support member of a housing 3, thereby bonding the display panel 1 to the housing 3 via the adhesive tape 2. However, Fig. 1 is just one example of a cushioning material for a display device, and the present invention is not particularly limited to this configuration. [Example]

[0060] The present invention will be described below with reference to examples, but the present invention is not limited to these examples in any way. The methods for measuring and evaluating each physical property are as follows.

[0061] (1) Apparent density and expansion ratio The apparent density of the foam was measured in accordance with JIS K 7222, and the reciprocal thereof was taken as the expansion ratio.

[0062] (2) Average bubble diameter The foam was cut in the thickness direction along both the MD and TD, and a 200x magnified photograph was taken using a digital microscope (Keyence Corporation, product name "VHX-900"). In the enlarged photograph, the cell diameters in MD and TD were measured for all bubbles present in a 2 mm-long cut surface in each of the MD and TD, and this procedure was repeated five times. The average values ​​of the cell diameters in MD and TD for all bubbles were taken as the average cell diameters in MD and TD. The larger of the average cell diameter in MD and the average cell diameter in TD was taken as the average cell diameter.

[0063] (3) Glass transition temperature (Tg), peak value of loss tangent (tanδ), and storage modulus The Tg, peak tan δ, and storage modulus were determined under the following measurement conditions using a tensile storage modulus measuring device, product name "DVA-200 / L2," manufactured by AT Measurement & Control Co., Ltd. In Table 1, the temperature at which the tan δ value was maximum was recorded as Tg. (Measurement conditions) Gauge length: 2.5cm Sample width: 0.5cm Sample thickness: foam thickness Deformation mode: tension Static / dynamic stress ratio: 1.5 Set distortion: 1.0% Set temperature rise rate: 10℃ / min Measurement frequency 10Hz Temperature range: -150℃~100℃

[0064] (4) 25% compressive strength Measurement was carried out by a method conforming to JIS K 6767. The foam sheets were laminated until the thickness reached 10 mm or more, and then the 25% compressive strength was measured.

[0065] (5) Closed cell ratio A flat square test piece with a side length of 5 cm was cut out from the foam. The thickness of the test piece was measured to calculate the apparent volume V1 of the test piece, and the weight W1 of the test piece was also measured. Next, the volume V2 occupied by the cells was calculated using the following formula. The density of the test piece was defined as ρ (g / cm 3 ) Volume occupied by the bubble V2=V1-W1 / ρ Next, the test piece was submerged in distilled water at 23°C to a depth of 500 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 of the water, and the water adhering to the surface of the test piece was removed. The weight W2 of the test piece was measured, and the closed cell ratio F1 was calculated according to the following formula. Closed cell rate F1 (%) = 100 - 100 x ((W2 - W1) / V2)

[0066] (6) Onset of the exothermic peak in DSC measurement Measurements were carried out according to the following steps 1 to 3. 1. Sample Preparation Test pieces of 5 mg to 20 mg were cut out from the foam and then placed in a dedicated sample container.

[0067] 2. Measurement preparation and measurement The test piece prepared in 1 above was set together with the sample container in a predetermined measuring device (Hitachi High-Tech Science, product name "DSC7020"). After that, the measuring device was operated to set the baseline to zero, and measurement was started. The measurement conditions were as follows. <Measurement temperature> Initial temperature: 30℃ Final temperature: 300℃ Heating rate: 10℃ / min

[0068] 3. Analysis of the foam initiation point The analysis results are generally as shown in Figure 2. In Figure 2, the temperature a at the point where the tangent line L1 of the flat baseline before foaming intersects with the tangent line L2 of the rising part of the decomposition peak is defined as the onset of the exothermic peak (the onset of foaming).

[0069] (7) Gel fraction A test piece of approximately 100 mg was taken from the foam, and the mass A (mg) of the test piece was precisely weighed. Next, this test piece was immersed in 30 cm of xylene at 115 °C. 3 After immersion for 24 hours, the insoluble matter on the wire mesh was filtered through a 200-mesh mesh, dried in vacuum, and the mass B (mg) of the insoluble matter was precisely weighed. The crosslinking rate (mass%) was calculated from the obtained value using the following formula: Gel fraction (mass%) = 100 × (B / A)

[0070] [evaluation] (8) Impact resistance test (tumbling test) Two test pieces measuring 0.15 cm long x 7 cm wide were cut out from the foams produced in each Example and Comparative Example to prepare test pieces (thickness: shown in the table). The two test pieces were placed 5 cm apart, sandwiched between two acrylic plates (9.5 cm long x 7 cm wide) with a 1.5 cm vertical offset, and fixed with an adhesive to prepare test pieces for tumbling. The above-mentioned tumbling test specimen was placed in a commercially available 18L can (length of top and bottom plates: approximately 24 cm, height: approximately 35 cm), and rotated 750 times, resulting in 1,500 consecutive drops. The test specimen was then removed and visually observed for breakage between the foam layers. The evaluation criteria were as follows: ◯: No breakage was observed between the foam layers. ×: Destruction was observed between the layers of the foam.

[0071] [material] The materials used in the examples and comparative examples are as follows. <Elastomer (A)> Hydrogenated styrene-isoprene-butadiene block copolymer (manufactured by Kuraray Co., Ltd., trade name "Hybra (registered trademark) 7311F", styrene content 12% by mass) Ethylene / butene / non-conjugated diene terpolymer (EBDM) (Mitsui Chemicals, Inc., product name "K-9330M") Styrene-ethylene / butylene-styrene block copolymer (SEBS) (Kraton, product name "MD6951")

[0072] <Polyolefin resin (B)> Metallocene-catalyzed ethylene / α-olefin copolymer (LLDPE): (Dow Chemical Company, product name "2036P")

[0073] <Thermal decomposition type foaming agent> Azodicarbonamide

[0074] <Phenol-based antioxidant> 2,6-di-t-butyl-p-cresol

[0075] <Decomposition temperature regulator> 3-(N-salicyloyl)amino-1,2,4-triazole (manufactured by ADEKA Corporation, product name "CDA-1") Zinc stearate (manufactured by Dainichi Chemical Industry Co., Ltd., product name "Daiwax ZF")

[0076] [Examples and Comparative Examples] Example 1 The foam raw materials were prepared as follows: 80 parts by mass of elastomer (A), 20 parts by mass of polyolefin resin (B), 2.5 parts by mass of a thermally decomposable foaming agent, 0.4 parts by mass of a decomposition temperature regulator, and 0.5 parts by mass of a phenolic antioxidant. These materials were melt-kneaded and then pressed to obtain a foamable sheet with a thickness of 0.22 mm. Both surfaces of the resulting foamable sheet were crosslinked by irradiating them with 11 Mrad of electron beam at an acceleration voltage of 540 keV. The sheet was then heated to 250°C to foam the foamable sheet, resulting in a density of 0.56 g / cm. 3 A foam with a thickness of 0.10 mm was obtained. The foam thus obtained was subjected to measurement and evaluation of the physical properties after molding as described above.

[0077] <Examples 2 to 8, Comparative Examples 1 to 3> A foam was obtained in the same manner as in Example 1, except that the composition of each material, the thickness of the foamable sheet, and the irradiation dose of the electron beam were changed as shown in Table 1. Then, the physical properties of the foam after molding were measured and evaluated.

[0078] [Table 1]

[0079] The foams of the examples all satisfied the requirements of the present invention in terms of the peak value of loss tangent (tanδ), glass transition temperature (Tg), storage modulus at 23°C, and average cell diameter, and therefore showed good results in the impact resistance test and were excellent in impact resistance. On the other hand, the foams of the comparative examples did not satisfy the requirements of the present invention in at least one of the peak value of loss tangent (tanδ), glass transition temperature (Tg), storage modulus at 23°C, and average cell diameter, and therefore had inferior impact resistance compared to the foams of the examples. [Explanation of symbols]

[0080] 1 screen 2 foam tapes 3. Housing L1: A straight line indicating a flat baseline before foaming Linear line at the rising edge of the L2 decomposition peak a Temperature when lines L1 and L2 intersect

Claims

1. The peak value of the loss tangent (tan δ) is 0.25 or more, at least one peak value of the loss tangent (tan δ) exists in the temperature range of −60 to 15° C., the average bubble diameter is 140 μm or less, and the storage modulus at 23° C. is 2.0×10 ３ Pa or more, and the closed cell rate is 80% or more, A foam comprising an elastomer and a polyolefin resin.

2. 2. The foam according to claim 1, wherein the heat generation peak starts at 200°C or higher when the heat quantity at each temperature is measured using a differential scanning calorimeter (DSC).

3. A pressure-sensitive adhesive tape comprising the foam according to claim 1 or 2 and a pressure-sensitive adhesive layer on at least one surface of the foam.

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