Multilayer foam sheet and adhesive tape

Abstract

To provide a multilayer foam sheet that has excellent flexibility and mechanical strength in the shear direction.SOLUTION: This multilayer foam sheet is equipped with a substrate layer comprising a foam layer, and a surface layer comprising a foam layer or a resin film layered on at least one surface of the substrate layer either directly or via another layer. The elasticity resulting from low-speed shearing is 50 MPa or more, and the 25% compressive strength is 125 kPa or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a multilayer foam sheet and an adhesive tape comprising the foam sheet. [Background technology]

[0002] Foam sheets are widely used as sealing materials or shock-absorbing materials in electronic devices such as mobile phones, cameras, displays, game devices, electronic notebooks, and personal computers. Foam sheets are also sometimes used inside electronic devices, for example, by applying an adhesive to at least one surface thereof to form adhesive tapes. A known foam sheet used in these applications is a crosslinked polyolefin resin foam sheet obtained by foaming and crosslinking an expandable polyolefin resin sheet containing a thermally decomposable foaming agent (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2014-28925 Summary of the Invention [Problem to be solved by the invention]

[0004] In recent years, foam sheets used inside electronic devices have been used for their cushioning and water-repellent properties. As televisions with large displays are increasingly being used for wall-mounting or diagonal mounting, there is a demand for foam sheets that can withstand forces perpendicular to the thickness of the sheet (shear direction). In order to increase the mechanical strength in the shear direction, it is conceivable to reduce the expansion ratio of the foam. However, in this case, although the mechanical strength is increased, the flexibility is reduced, which makes the foam more susceptible to defects when used in electronic devices. Therefore, a foam sheet that is excellent in both flexibility and mechanical strength in the shear direction is desired.

[0005] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a foam sheet that is excellent in both flexibility and mechanical strength in the shear direction, and a pressure-sensitive adhesive tape using the foam sheet. [Means for solving the problem]

[0006] The present inventors have conducted extensive research to solve the above-mentioned problems, and as a result have found that the above-mentioned problems can be solved by a multilayer foam sheet comprising a base layer made of a foam layer and a surface layer made of a foam layer or a resin film laminated on at least one surface of the base layer directly or via another layer, the multilayer foam sheet having a predetermined 25% compressive strength and low-speed shear elastic modulus, and have completed the present invention. That is, the present invention provides the following [1] to

[12] .

[0007] [1] A multilayer foam sheet comprising a base layer made of a foam layer and a surface layer made of a foam layer or a resin film laminated on at least one surface of the base layer, the multilayer foam sheet having a low-speed shear elastic modulus of 50 MPa or more and a 25% compressive strength of 125 kPa or less. [2] The multilayer foam sheet according to [1] above, wherein the surface layers are laminated on both sides of the base layer. [3] The multilayer foam sheet according to [1] or [2] above, which has a tensile breaking strength of 10 N / 10 mm or more. [4] Density: 0.07 to 0.22 g / cm 3 The multilayer foam sheet according to any one of the above [1] to [3], wherein [5] The multilayer foam sheet according to any one of the above [1] to [4], wherein the ratio of the thickness of the base layer to the thickness of the surface layer (thickness of the base layer / thickness of the surface layer) is 2.0 or more. [6] The multilayer foam sheet according to any one of the above [1] to [5], which has a thickness of 30 to 2000 μm. [7] The multilayer foam sheet according to any one of the above [1] to [6], wherein the base layer has an average cell diameter of 20 to 500 μm. [8] The multilayer foam sheet according to any one of the above [1] to [7], which has a closed cell rate of 90% or more. [9] The multilayer foam sheet according to any one of the above [1] to [8], which has a gel fraction of 30 to 80%.

[10] The multilayer foam sheet according to any one of the above [1] to [9], wherein the base layer contains at least one resin selected from the group consisting of olefin-based thermoplastic resins and thermoplastic elastomers.

[11] A pressure-sensitive adhesive tape comprising the multilayer foam sheet according to any one of the above [1] to

[10] and a pressure-sensitive adhesive material provided on at least one surface of the multilayer foam sheet.

[12] The adhesive tape according to

[11] above, which is used for displays of 20 inches or more. [Effects of the Invention]

[0008] According to the present invention, it is possible to provide a multilayer foam sheet having excellent flexibility and mechanical strength in the shear direction, and a pressure-sensitive adhesive tape using the multilayer foam sheet. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is an explanatory diagram illustrating a method for measuring elastic modulus by low shear. [Figure 2] FIG. 1 is an explanatory diagram illustrating a method for a shear retention test. DETAILED DESCRIPTION OF THE INVENTION

[0010] The present invention will be described in detail below using embodiments. [Foam sheet] The multilayer foam sheet of the present invention comprises a base layer made of a foam layer and a surface layer made of a foam layer or a resin film laminated on at least one surface of the base layer, and has a low-speed shear modulus of elasticity of 50 MPa or more and a 25% compressive strength of 125 kPa or less, thereby providing the multilayer foam sheet with excellent flexibility and mechanical strength in the shear direction.

[0011] (Low shear modulus) The multilayer foam sheet of the present invention has a low shear modulus of elasticity of 50 MPa or more. If the low shear modulus is less than 50 MPa, the multilayer foam sheet will have reduced mechanical strength in the shear direction. Here, the shear direction means the direction perpendicular to the thickness direction of the sheet. The low-shear modulus of the multilayer foam sheet is preferably 60 MPa or more, more preferably 80 MPa or more, and even more preferably 100 MPa or more, from the viewpoint of further improving the mechanical strength in the shear direction. From the viewpoint of increasing the mechanical strength in the shear direction of the multilayer foam sheet, a high low-shear modulus is preferable, but considering the balance with flexibility, the low-shear modulus is preferably 300 MPa or less, more preferably 200 MPa or less. The low-shear modulus is obtained by a tensile test at a tensile speed of 100 mm / min, based on the measurement method described below. The low-shear elastic modulus can be adjusted to a desired value by adjusting the thickness of the base layer and surface layer constituting the multilayer foam sheet, the expansion ratio, the cell diameter, etc.

[0012] The method for measuring the elastic modulus by low shear will be explained below with reference to FIG. The multilayer foam sheet of the present invention is cut to a size of 5 mm x 30 mm to prepare a test piece 11. Separately, wooden boards 12 and 13 are prepared. These two wooden boards each have a size of 30 mm x 98 mm and a thickness of 5 mm. An adhesive ("Quick Bond F9 Chemical Reaction Adhesive" manufactured by Kansai Polymer Research Institute Co., Ltd.) is applied to both sides of the test piece 11, and wooden boards 12 and 13 are laminated and fixed to both sides of the test piece 11. At this time, as shown in Figure 1, one end side 12a of wooden board 12 and one end side 13a of wooden board 13 are placed on both sides of the test piece 11, and the other end sides 12b and 13b of each wooden board are laminated and fixed so that they are separated from each other. Next, a 5 kg weight is placed on one end 12a of the wooden board 12 for 10 seconds to apply pressure, and then the board is left at 23° C. for 24 hours to prepare a sample for measuring the elastic modulus under low-speed shear. The other end 12b of the wooden board 12 of the measurement sample prepared as described above is fixed, and the other end 13b of the wooden board 13 is pulled in the shear direction (the direction of the arrow in Figure 1) at 100 mm / min at 23°C using a tensile tester, and the initial slope of the obtained stress-strain curve is the elastic modulus under low-speed shear in this invention.

[0013] (25% compressive strength) The 25% compression strength of the multilayer foam sheet of the present invention is 125 kPa or less. If the 25% compression strength of the multilayer foam sheet is greater than 125 kPa, the flexibility will be insufficient and the function as a foam sheet will be impaired. From this perspective, the 25% compression strength of the multilayer foam sheet is preferably 110 kPa or less, more preferably 100 kPa or less, and even more preferably 90 kPa or less. The lower limit of the range of the 25% compression strength of the foam sheet of the present invention is not particularly limited, but is, for example, 20 kPa. The 25% compression strength of the multilayer foam sheet can be measured by the method described in the Examples below.

[0014] (Tensile breaking strength) The tensile breaking strength of the multilayer foam sheet of the present invention is preferably 10 N / 10 mm or more. When the tensile breaking strength is 10 N / 10 mm or more, the mechanical strength of the multilayer foam sheet in the shear direction is likely to be improved. From this perspective, the tensile breaking strength of the multilayer foam sheet is more preferably 15 N / 10 mm or more, and even more preferably 20 N / 10 mm or more. The tensile breaking strength of the multilayer foam sheet is the tensile breaking strength in the machine direction (MD), and can be measured by the method described in the Examples below.

[0015] (density) The density of the multilayer foam sheet of the present invention is preferably 0.07 to 0.22 g / cm 3 and more preferably 0.10 to 0.21 g / cm 3 and more preferably 0.13 to 0.20 g / cm 3When the density of the multilayer foam sheet is within this range, the 25% compressive strength can be easily adjusted to a desired value. Here, density refers to apparent density, and is a value measured in accordance with JIS K 7222 (2005).

[0016] (Expansion ratio) In the multilayer foam sheet of the present invention, the expansion ratio of the base layer consisting of the foam layer is preferably 3 to 40, more preferably 5 to 30, and even more preferably 7 to 25. By setting the expansion ratio of the base layer to be equal to or greater than these lower limits, the flexibility of the foam sheet is improved, and by setting it to be equal to or less than these upper limits, the mechanical strength of the foam sheet in the shear direction is increased.

[0017] When the surface layer is a foam layer, the expansion ratio of the foam layer is preferably 1.2 to 10, more preferably 1.3 to 8, and even more preferably 1.4 to 6. When the expansion ratio of the surface foam layer is equal to or greater than these lower limits, the flexibility of the foam sheet is improved, and when it is equal to or less than these upper limits, the mechanical strength of the foam sheet in the shear direction is increased. The expansion ratio can be measured by the method described in the examples.

[0018] The expansion ratio of the multilayer foam sheet of the present invention (expansion ratio of the entire sheet) is preferably 2-20, more preferably 3-15, and even more preferably 4-10.

[0019] From the viewpoint of obtaining a multilayer foam sheet having excellent flexibility and mechanical strength in the shear direction, it is preferable to adjust the expansion ratio of the base layer within the above range and also adjust the expansion ratio of the foam layer of the surface layer within the above range. Furthermore, it is preferable that the expansion ratio of the foam layer of the surface layer is lower than that of the base layer. In the case of a multilayer foam sheet having foam layers on both sides of a base layer, it is preferable that the expansion ratio of the foam layer on at least one side is lower than that of the base layer, and it is more preferable that the expansion ratios of the foam layers on both sides are both lower than that of the base layer. By adjusting the expansion ratio in this way, it becomes easier to obtain a multilayer foam sheet that is excellent in flexibility and mechanical strength in the shear direction.

[0020] (Thickness) The thickness of the multilayer foam sheet of the present invention is not particularly limited, but is preferably 30 to 2000 μm, more preferably 100 to 1700 μm, and even more preferably 200 to 1300 μm. A thickness within this range makes it easy to use in thin electronic devices, and makes it easier to ensure appropriate flexibility and mechanical strength in the shear direction. Furthermore, from the viewpoint of improving flexibility and mechanical strength in the shear direction, the thickness of the base layer and the surface layer are preferably as follows.

[0021] The thickness of the substrate layer in the multilayer foam sheet of the present invention is not particularly limited, but is preferably 20 to 1400 μm, more preferably 80 to 1200 μm, and even more preferably 170 to 1000 μm.

[0022] The thickness of the surface layer in the multilayer foam sheet of the present invention is not particularly limited, but is preferably 5 to 300 μm, more preferably 10 to 250 μm, and even more preferably 15 to 150 μm. In the case of a multilayer foam sheet having surface layers on both sides of a substrate, it is preferred that the thickness of at least one, preferably both, of the surface layers is within the above range.

[0023] In order to obtain a multilayer foam sheet with excellent flexibility and mechanical strength in the shear direction, it is preferable that the thicknesses of both the base layer and the surface layer are within the above ranges. From the same viewpoint, the ratio of the thickness of the base layer to the thickness of the surface layer (thickness of base layer / thickness of surface layer) is preferably 2.0 or more, more preferably 3.0 or more, even more preferably 4.0 or more, still more preferably 5.0 or more, and preferably 30 or less. In the case of a multilayer foam sheet having surface layers on both sides of a base layer, the thickness of the surface layer when determining the thickness ratio is the average value of the thicknesses of the surface layers on both sides.

[0024] (gel fraction) The gel fraction of the multilayer foam sheet of the present invention is preferably 30 to 80% by mass. When the gel fraction of the foam sheet is 30 to 80% by mass, the flexibility and mechanical strength in the shear direction of the foam sheet are likely to be improved. From this perspective, the gel fraction of the multilayer foam sheet is more preferably 32 to 70% by mass, and even more preferably 33 to 65% by mass. The gel fraction of the multilayer foam sheet can be measured by the method described in the Examples.

[0025] (average bubble diameter) The average cell diameter of the base layer of the multilayer foam sheet of the present invention is preferably 20 to 500 μm. When the average cell diameter is within this range, it becomes easier to adjust the low-speed shear elastic modulus to the desired range, and the mechanical strength of the foam sheet in the shear direction is improved. From the above viewpoints, the average cell diameter is more preferably 30 to 400 μm, and even more preferably 40 to 300 μm. 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 can be measured by the method described in the Examples.

[0026] (closed cell ratio) The multilayer foam sheet of the present invention preferably has a closed cell ratio of 90% or more. A closed cell ratio of 90% or more can improve mechanical strength and flexibility in the shear direction. From the above viewpoints, the closed cell ratio of the foam sheet is more preferably 92% or more. The higher the closed cell ratio, the better, and it is sufficient if it is 100% or less. The closed cell ratio can be measured by the method described in the examples.

[0027] The substrate and surface layer made of a foam layer will be described in more detail below. <Base layer (foam layer)> The base layer in the multilayer foam sheet of the present invention comprises a foam layer. The type of resin constituting the base layer is not particularly limited, and examples include olefin-based thermoplastic resins, acrylic resins, polyurethane-based resins, and thermoplastic elastomers. These may be used alone or in combination of two or more. The base layer preferably contains at least one resin selected from the group consisting of olefin-based thermoplastic resins and thermoplastic elastomers, and more preferably contains an olefin-based thermoplastic resin. The use of an olefin-based thermoplastic resin or a thermoplastic elastomer improves foamability and other properties while ensuring the flexibility and mechanical strength of the base layer. The resin used in the base layer may be used alone or in combination of two or more.

[0028] Specific examples of olefin-based thermoplastic resins include polyethylene resins, polypropylene resins, and ethylene-vinyl acetate copolymers, and among these, polyethylene resins are preferred. Examples of the polyethylene resin include polyethylene resins polymerized with a polymerization catalyst such as a Ziegler-Natta compound, a metallocene catalyst, or a chromium oxide compound, and preferably polyethylene resins polymerized with a metallocene catalyst are used.

[0029] (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.

[0030] 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, arylamides, phosphides, arylphosphides, and the like may also be used.

[0031] 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.

[0032] 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.

[0033] The ethylene-vinyl acetate copolymer used as the olefin-based thermoplastic resin 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.

[0034] 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, etc. As the thermoplastic elastomer, one of these components may be used alone, or two or more of them may be used in combination. Of these, olefin-based thermoplastic elastomers and styrene-based thermoplastic elastomers are preferred, with styrene-based thermoplastic elastomers being more preferred.

[0035] Examples of olefin-based thermoplastic elastomers include blend-type, dynamically crosslinked-type, and polymerization-type elastomers. More specifically, examples include thermoplastic elastomers that use a thermoplastic crystalline polyolefin such as polypropylene or polyethylene for the hard segment and a fully vulcanized or partially vulcanized rubber for the soft segment. Examples of the thermoplastic crystalline polyolefin include a homopolymer of an α-olefin having 1 to 4 carbon atoms or a copolymer of two or more α-olefins, with polyethylene or polypropylene being preferred. Examples of the soft segment component include butyl rubber, halobutyl rubber, EPDM, EPM, acrylonitrile / butadiene rubber, NBR, and natural rubber, with EPDM being preferred.

[0036] 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.

[0037] 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. When hydrogenated, the hydrogenation can be carried out by a known method.

[0038] Styrene-based thermoplastic elastomers are usually block copolymers, and examples thereof include styrene-isoprene block copolymers, styrene-isoprene-styrene block copolymers, styrene-butadiene block copolymers, styrene-butadiene-styrene block copolymers, styrene-ethylene / butylene-styrene block copolymers (SEBS), styrene-ethylene / propylene-styrene block copolymers (SEPS), styrene-ethylene / butylene block copolymers (SEB), styrene-ethylene / propylene block copolymers (SEP), and styrene-ethylene / butylene-crystalline olefin block copolymers (SEBC). As the styrene-based thermoplastic elastomer, a block copolymer is preferred, and among them, SEBS and SEBC are more preferred.

[0039] Commercially available styrene-based thermoplastic elastomers include those manufactured by JSR Corporation under the trade name "DYNARON 8600P" (styrene content: 15% by mass), "DYNARON 4600P" (styrene content: 20% by mass), and "DYNARON 1321P" (styrene content: 10% by mass), as well as those manufactured by Kuraray Co., Ltd. under the trade name "HYBRAR 7311."

[0040] [Additives] The base layer made of a foam layer used in the present invention is preferably obtained by foaming a foamable resin composition containing the above-mentioned resin and a foaming agent, preferably a thermal decomposition type foaming agent. 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.

[0041] The amount of foaming agent in the foamable resin composition for forming the base layer is preferably 1 to 20 parts by mass, more preferably 3 to 15 parts by mass, and even more preferably 5 to 10 parts by mass, per 100 parts by mass of the resin. By using a foaming agent in an amount of 1 part by mass or more, it is possible to impart a certain level of flexibility to the foam sheet. Furthermore, by using a foaming agent in an amount of 20 parts by mass or less, it is possible to prevent the foam layer from expanding more than necessary, thereby improving the mechanical strength of the foam layer.

[0042] The foamable resin composition may contain a decomposition temperature regulator. The decomposition temperature regulator is added to lower the decomposition temperature of the thermally decomposable foaming agent or to accelerate or adjust the decomposition rate, and specific examples of such compounds include zinc oxide, zinc stearate, and urea. The decomposition temperature regulator is added in an amount of, for example, 0.01 to 5 parts by mass per 100 parts by mass of the resin in order to adjust the surface condition of the foam layer.

[0043] The foamable resin 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.

[0044] In the base layer, the resin is the main component, and the resin content is, for example, 70 mass % or more, preferably 80 mass % or more, and more preferably 90 mass % or more, based on the total mass of the foam layer. In addition to the above, the foamable resin composition may contain additives generally used in foams, such as a heat stabilizer, a colorant, a flame retardant, an antistatic agent, and a filler, as necessary.

[0045] <Surface layer> The surface layer is laminated on at least one surface of the base material layer. That is, the surface layer may be laminated on one surface of the base material layer or on both surfaces of the base material layer. In particular, it is preferable that the surface layer be laminated on both surfaces of the base material layer from the viewpoint of improving reworkability and preventing a significant change in mechanical strength in the shear direction depending on the arrangement when the base material layer is arranged inside an electronic device. The surface layer is a layer made of a foam layer or a resin film. The resin film is a non-foamed resin layer. First, the case where the surface layer is a resin film will be described.

[0046] The type of resin constituting the resin film in the surface layer is not particularly limited. Examples of resins constituting the resin film include, similar to the resin in the base layer, olefin-based thermoplastic resins, acrylic resins, polyurethane-based resins, and thermoplastic elastomers. In addition to these, silicone-based resins, vinyl chloride-based resins, styrene-based resins, polyester-based resins, polyamide-based resins, ionomer-based resins, and the like may also be used. In the surface layer, one type of resin may be used alone, or two or more types may be used in combination. When surface layers are laminated on both sides of the foam layer, the resins used in the two surface layers may be the same or different, but from the viewpoints of productivity and making it easier to make the physical properties of both layers the same or similar, it is preferable that the resins be the same.

[0047] The resin constituting the surface layer is preferably a thermoplastic resin, and the use of a thermoplastic resin makes it possible to easily bond the surface layer to the base layer. Among the resins mentioned above, olefin-based thermoplastic resins are preferred for use in the surface layers. Use of olefin-based thermoplastic resins facilitates increasing the mechanical strength of the foam sheet in the shear direction. Furthermore, when an olefin-based thermoplastic resin or a thermoplastic elastomer is used for the base layer, the adhesion of the surface layer to the foam layer can be particularly improved.

[0048] Examples of olefin-based thermoplastic resins include polyethylene resins and polypropylene resins. Also usable are ethylene-vinyl acetate copolymers and ethylene-ethyl acrylate copolymers. Examples of polyethylene resins include low-density polyethylene (density: 0.930 g / cm 3 less than 0.930 g / cm 3 More than 0.942g / cm 3 less than 0.942 g / cm 3 (The above are some examples.) Here, the low-density polyethylene is preferably linear low-density polyethylene. Details of the linear low-density polyethylene are as described in the description of the base layer, so they will not be described here. In addition, details of the polypropylene resin are as described in the description of the base layer, so they will not be described here.

[0049] The ethylene-vinyl acetate copolymer used in the surface layer is, for example, an ethylene-vinyl acetate copolymer containing 50% by mass or more of structural units derived from ethylene. Furthermore, the ethylene-ethyl acrylate copolymer may be an ethylene-ethyl acrylate copolymer containing 50% by mass or more of structural units derived from ethylene. Among the above, ethylene resin, particularly linear low-density polyethylene, is preferred from the viewpoints of adhesion to the foam layer, bending stress, and tensile strength. From the same viewpoints, ethylene-vinyl acetate copolymer is also preferred.

[0050] The resin is the main component of the surface layer, and the resin content is, for example, 70% by mass or more, preferably 80% by mass or more, and more preferably 90% by mass or more, based on the total mass of each layer. In addition to the resin, the surface layer may contain additives such as antioxidants, heat stabilizers, colorants, flame retardants, antistatic agents, and fillers. The surface layer may be crosslinked. As described later in Production Method 1, the surface layer may be crosslinked in conjunction with the crosslinking of the base layer, and therefore is preferably crosslinked by ionizing radiation.

[0051] The surface layer is preferably a layer made of a foam, as described above. When the surface layer is a foam layer, for example, a foamable resin composition obtained by blending a foaming agent, etc., as described for the base layer, with a resin that can be used in the surface layer may be foamed. The amount of foaming agent in the foamable resin composition for forming the surface layer is preferably 0.1 to 10 parts by mass, more preferably 0.5 to 5 parts by mass, and even more preferably 1 to 3 parts by mass, per 100 parts by mass of resin. If the amount of foaming agent is within this range, it becomes easier to adjust the expansion ratio of the surface layer to the desired range.

[0052] (Adhesive layer) In the present invention, the substrate layer and the surface layer may be laminated directly or via another layer. Examples of such another layer include an adhesive layer. Known adhesives, pressure-sensitive adhesives, etc. may be used as the adhesive layer. Alternatively, the adhesive layer may be a double-sided pressure-sensitive adhesive tape having pressure-sensitive adhesive layers on both sides of the substrate. The adhesive layer may have a thickness that does not significantly affect the physical properties of the multilayer foam sheet, such as mechanical strength and flexibility. Therefore, the thickness of the adhesive layer that bonds the base layer and the surface layer is preferably thinner than that of the surface layer, and more preferably not more than half the thickness of the surface layer.

[0053] <Method of manufacturing foam sheet> (Manufacturing method 1) The multilayer foam sheet of the present invention is not particularly limited, and can be produced, for example, by a method in which a plurality of layers made of a resin composition are laminated to obtain a multilayer laminate sheet, and then the multilayer laminate sheet is foamed (hereinafter also referred to as "production method 1"). In this specification, the term "resin composition" conceptually includes a case in which the composition is made of only one type of resin.

[0054] More specifically, production method 1 includes the following steps I and II. (I) A step of obtaining a multilayer laminate sheet having a layer made of a foamable resin composition and a surface layer formed on one or both sides of the layer. (II) A step of foaming the layer of the foamable resin composition of the multilayer laminate sheet to obtain a foam sheet.

[0055] Each step will be described below. (Process (I)) The method for obtaining the multilayer laminate sheet in step (I) is not particularly limited, but is preferably carried out by co-extrusion molding. Specifically, when surface layers are formed on both sides of a layer made of a foamable resin composition, the resin for forming the surface layer and other additives blended as necessary are supplied to a first and a third extruder, respectively, and melt-kneaded to obtain a resin composition for forming the surface layer.Furthermore, the resin for forming the base layer, a foaming agent such as a thermally decomposable foaming agent, and other additives blended as necessary are supplied to a second extruder, and melt-kneaded to obtain a foamable resin composition for forming the base layer. Next, the resin compositions supplied from the first to third extruders are merged so that the composition supplied from the second extruder forms the middle layer, and the resulting mixture is extruded into a sheet using a T-die or the like to obtain a multilayer laminate sheet with a three-layer structure.

[0056] When a surface layer is formed on one side of a layer made of a foamable resin composition, the resin for forming the surface layer and other additives blended as necessary are supplied to a first extruder and melt-kneaded to obtain a resin composition for forming the surface layer, and the resin for forming the base layer, a foaming agent such as a thermally decomposable foaming agent, and other additives blended as necessary are supplied to a second extruder and melt-kneaded to obtain a foamable resin composition for forming the base layer. Next, the resin composition supplied from the first extruder is joined with the composition supplied from the second extruder and extruded into a sheet using a T-die or the like to obtain a multilayer laminate sheet having a two-layer structure. In coextrusion molding, either the feedblock method or the multi-manifold method may be used, but the feedblock method is preferred.

[0057] In step (I), it is preferable to further crosslink the multilayer laminate sheet obtained above. As a crosslinking method, an organic peroxide is previously blended, and the multilayer laminate sheet obtained in step (I) is heated to crosslink, but it is preferable to crosslink the multilayer laminate sheet by irradiating it with ionizing radiation. Examples of ionizing radiation include electron beams and β rays, and electron beams are preferred. The dose of ionizing radiation is preferably 1 to 10 Mrad, more preferably 1.5 to 5 Mrad.

[0058] (Step (II)) In step (II), the multilayer laminate sheet obtained in step (I) is foamed to foam the layer comprising the foamable resin composition. The layer comprising the foamable resin composition may be foamed by treating it so that the foaming agent foams. When the foaming agent is a thermally decomposable foaming agent, the multilayer laminate sheet is heated to foam it. The heating temperature may be equal to or higher than the temperature at which the thermally decomposable foaming agent decomposes, and is, for example, about 150 to 320°C. The method for heating the multilayer laminate sheet is not particularly limited, and examples thereof include a method for heating the multilayer laminate sheet with hot air, a method for heating with infrared rays, a method for heating in a salt bath, a method for heating in an oil bath, and the like, and these may be used in combination. The multilayer laminate sheet may be appropriately stretched during or after foaming. The above explanation has been given for an example in which the surface layer is a resin film (non-foamed body). However, if the surface layer is a foamed body, a foaming agent such as a thermally decomposable foaming agent may be blended as an additive in the resin composition for forming the surface layer, and foamed in step (II).

[0059] (Second manufacturing method) The foam sheet of the present invention can also be produced by another method, specifically, a method in which a foam layer constituting the base layer is produced in advance, and a resin film or foam layer constituting the surface layer is superimposed on one or both sides of the foam layer and bonded (also referred to as a "second production method") can be mentioned.

[0060] In the second production method, a foam can be obtained by melt-kneading a resin for forming the base layer, a thermally decomposable foaming agent, and additives blended as necessary to obtain a foamable resin composition for forming the base layer, and then molding the foamable resin composition into a sheet (foamable resin composition sheet). The method for melt-kneading the foamable resin composition and molding it into a sheet is not particularly limited, but it is preferably carried out using an extruder.

[0061] The obtained foamable resin composition sheet is preferably further crosslinked before foaming, which will be described later. A crosslinking method may involve blending an organic peroxide in advance and heating the foamable resin composition sheet to crosslink it, but it is preferred to crosslink the foamable resin composition sheet by irradiating it with ionizing radiation. The type and dose of ionizing radiation are as described in the first production method above.

[0062] Next, the foamable resin composition sheet may be foamed. The layer made of the foamable resin composition may be treated so that the foaming agent foams. When the foaming agent is a thermally decomposable foaming agent, the multilayer laminate sheet is heated to foam it. The heating temperature and heating method are as described in the first production method above. The foamable resin composition sheet may be appropriately stretched during or after foaming.

[0063] Then, a separately prepared resin film or foam for forming the surface layer is superimposed on one or both sides of the foam layer constituting the base layer and bonded to obtain a multilayer foam sheet. Specifically, the layers may be thermocompression bonded by heating and pressurizing using a press or the like. Alternatively, the foam layer and the surface layer may be bonded together by applying a pressure-sensitive adhesive, adhesive, or the like to the bonding surface between them, or by applying a double-sided adhesive tape.

[0064] [Adhesive tape] The multilayer foam sheet of the present invention may be used in an adhesive tape using the multilayer foam sheet as a substrate. The adhesive tape comprises, for example, a multilayer foam sheet and an adhesive material provided on at least one surface of the multilayer foam sheet. The adhesive tape can be adhered to other members via the adhesive material. The adhesive tape may comprise a multilayer foam sheet having adhesive materials provided on both surfaces thereof or on one surface thereof.

[0065] 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 the multilayer foam sheet, or a double-sided pressure-sensitive adhesive sheet attached to the surface of the multilayer foam sheet, 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 the multilayer foam sheet and the other pressure-sensitive adhesive layer to another member. The adhesive constituting the adhesive layer is not particularly limited, and for example, an acrylic adhesive, a urethane adhesive, a rubber adhesive, etc. A release sheet such as release paper may be further attached onto the adhesive material. The thickness of the adhesive material is preferably 5 to 200 μm, more preferably 7 to 150 μm, and even more preferably 10 to 100 μm.

[0066] [Application] The multilayer foam sheet of the present invention and the pressure-sensitive adhesive tape comprising the multilayer foam sheet are not particularly limited, but are preferably used, for example, inside electronic devices, and can be used, for example, as an impact absorbing material, a sealing material, etc. by being placed between two components. Examples of electronic devices include mobile phones, cameras, displays, game devices, electronic organizers, and personal computers. As described above, the multilayer foam sheet of the present invention and the pressure-sensitive adhesive tape comprising the same have high flexibility and excellent mechanical strength in the shear direction. Therefore, the multilayer foam sheet and the pressure-sensitive adhesive tape comprising the same are preferably used in large displays, particularly displays of 20 inches or more. The display may be a liquid crystal display or an organic electroluminescence display. [Example]

[0067] The present invention will be explained in more detail by way of examples, but the present invention is not limited to these examples in any way.

[0068] [Measurement method] The methods for measuring and evaluating each physical property are as follows.

[0069] <25% compressive strength> Measurement was carried out at a temperature of 23°C according to the method of JIS K 6767.

[0070] <Tensile breaking strength> The multilayer foam sheets produced in each example and comparative example were cut into a dumbbell No. 1 shape as specified in JIS K6251 4.1. These were used as samples and subjected to measurements by pulling in the MD direction at a measurement temperature of 23°C and a speed of 500 mm / min using a tensile tester (product name: Tensilon RTF235, manufactured by A&D Co., Ltd.).

[0071] <Gel fraction> A test piece of approximately 100 mg was taken from the multilayer foam sheet, and the mass A (mg) of the test piece was precisely weighed. Next, this test piece was immersed in 30 cm of xylene at 120°C. 3 After immersion for 24 hours, the insoluble matter on the mesh was filtered through a 200-mesh wire netting, vacuum-dried, and the mass B (mg) of the insoluble matter was precisely weighed. The gel fraction (mass%) was calculated from the obtained value using the following formula: Gel fraction (mass%) = 100 × (B / A)

[0072] <Closed bubble rate> Measurement was carried out according to the method of ASTM D2856 (1998). Specifically, the measurements were carried out as follows. First, a flat square test piece with a side length of 5 cm was cut out from the multilayer foam sheet. The thickness of the test piece was then 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 bubbles was calculated based on the following formula: The density of the matrix resin constituting the test piece was ρ (g / cm 3 ) was decided. 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 100 mm from the water surface, and a pressure of 15 kPa was applied to the test piece for 3 minutes. After that, the pressure was released in the water and the test piece was left to stand for 1 minute, and then the test piece was taken out of the water and the water adhering to the surface of the test piece was removed, and the weight W2 of the test piece was measured, and the open cell fraction F1 and closed cell fraction F2 were calculated according to the following formula. Open cell rate F1 (%) = 100 × (W2 - W1) / V2 Closed bubble rate F2 (%) = 100 - F1

[0073] <Expansion ratio> The specific volumes (unit: cc / g) of the multilayer foam sheet, the base layer, and the surface layer constituting the multilayer foam sheet before and after foaming were measured, and the expansion ratio was calculated by dividing the specific volume after foaming by the specific volume before foaming.

[0074] <Average bubble diameter> The multilayer foam sheet 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 base layer portion of the enlarged photograph, the MD and TD cell diameters of all bubbles present in a 2 mm-long cut surface in each of the MD and TD were measured, and this procedure was repeated five times. The average values of the cell diameters in the MD and TD for all bubbles were taken as the average cell diameters in the MD and TD. The larger of the average cell diameter in the MD and the average cell diameter in the TD was taken as the average cell diameter.

[0075] <Thickness of base layer and surface layer> The cross section of the foam sheet was photographed using a digital microscope (manufactured by Keyence Corporation, product name VHX-900), and the thicknesses of the base layer and the surface layer were measured from the photographed image.

[0076] <Thickness of multi-layer foam sheet> The total thickness of the base layer and the surface layer was defined as the thickness of the multilayer foam sheet.

[0077] <Elastic modulus due to low shear> The measurement method is explained with reference to Figure 1. The multilayer foam sheet of the present invention is cut to a size of 5 mm x 30 mm to prepare a test piece 11. Separately, wooden boards 12 and 13 are prepared. These two wooden boards each have a size of 30 mm x 98 mm and a thickness of 5 mm. An adhesive ("Quick Bond F9 Chemical Reaction Adhesive" manufactured by Kansai Polymer Research Institute Co., Ltd.) is applied to both sides of the test piece 11, and wooden boards 12 and 13 are laminated and fixed to both sides of the test piece 11. At this time, as shown in Figure 1, one end side 12a of wooden board 12 and one end side 13a of wooden board 13 are placed on both sides of the test piece 11, and the other end sides 12b and 13b of each wooden board are laminated and fixed so that they are separated from each other. Next, a 5 kg weight is placed on one end 12a of the wooden board 12 for 10 seconds to apply pressure, and then the board is left at 23° C. for 24 hours to prepare a sample for measuring the elastic modulus under low-speed shear. The other end 12b of the wooden board 12 of the measurement sample prepared as described above is fixed, and the other end 13b of the wooden board 13 is pulled in the shear direction (the direction of the arrow in Figure 1) at 100 mm / min at 23°C using a tensile tester, and the initial slope of the obtained stress-strain curve is the elastic modulus under low-speed shear in this invention.

[0078] <Shear retention test> The multilayer foam sheet was cut into a 25 mm x 25 mm test piece, and adhesive was applied to both sides to prepare an adhesive tape. The adhesive tape is manufactured by the following method. To 100 parts by weight of an acrylic adhesive ("1882S" manufactured by Soken Chemical & Engineering Co., Ltd., solid content 16.5%), 0.037 parts by weight of an isocyanate crosslinking agent ("Coronate L45" manufactured by Nippon Polyurethane Co., Ltd., solid content 45%) and 0.119 parts by weight of an epoxy curing agent ("E-5XM" manufactured by Soken Chemical & Engineering Co., Ltd., solid content 5%) were added and stirred to obtain an adhesive solution. A 75 μm-thick release film was prepared, and an adhesive solution was applied to the release-treated surface of this release film. This was then dried at 110°C for 5 minutes to form an adhesive layer with a thickness of 0.05 mm. This adhesive layer was then bonded to the surface of a multilayer foam sheet. Next, the same adhesive layer as above was bonded to the opposite surface of the multilayer foam sheet in the same manner. The resulting adhesive tape was then cured by heating at 40°C for 48 hours. This resulted in an adhesive tape covered with a release film. The obtained adhesive tape was used as a measurement sample, and a SUS plate 14 (thickness 1 mm×30 mm×50 mm, unpolished, with through holes) and a glass plate 15 (thickness 3 mm×50 mm×75 mm) were prepared. The SUS plate 14 and the glass plate 15 were washed with ethanol and then thoroughly dried. The adhesive tape was cut to a size of 25 mm x 25 mm, and one release film was peeled off. The tape was then attached to the SUS plate 14 while being careful not to trap air bubbles. The other release film was peeled off, and the tape was then attached to the glass plate 15 while being careful not to trap air bubbles. As shown in FIG. 2, the adhesive tape 11 was laminated so that one end 14a of the SUS plate 14 and one end 15a of the glass plate 15 were positioned on both sides of the adhesive tape 11, and the other end 14b and 15b of the two plates were laminated and fixed so that they were separated from each other. In the case of a multilayer foam sheet having a surface layer on only one side of the base layer, the surface on the surface layer side can be attached to the glass plate 15, and the side of the base layer not having the surface layer can be attached so that it is in contact with the SUS plate 14. Next, a 5 kg weight was placed on the sample for 10 seconds to press the sample together, creating a holding strength test sample. The resulting holding strength test sample was then left at 23°C and 50% RH for 24 hours, then transferred to a thermostatic chamber at 60°C and 90% RH, where the other end 15b of the glass plate 15 was positioned upward. 1.6 kg and 0.8 kg weights 16 were attached to the through holes in the SUS14 plate, and the time until the weights fell (peel time) was measured. A rating of ◯ was given for a time elapsed from the time the weights were hung until the sample peeled and fell, a rating of △ was given for a time elapsed from 200 hours to less than 300 hours, and an X was given for a time elapsed from 200 hours to less than 200 hours.

[0079] [Method for producing multilayer foam sheet] The multilayer foam sheets of Examples 1 to 7 and Comparative Examples 1 to 4 were produced as follows. Example 1 The resin used to form the base layer (the foam layer) was a metallocene-catalyzed linear low-density polyethylene resin (metallocene LLDPE, manufactured by Japan Polyethylene Co., Ltd., product name "Kernel KF283"), and the thermal decomposition foaming agent was azodicarbonamide. Furthermore, zinc oxide (manufactured by Sakai Chemical Industry Co., Ltd., product name "OW-212F") was used as a decomposition temperature regulator, and the phenolic antioxidant 2,6-di-t-butyl-p-cresol was used as an antioxidant. 100 parts by mass of linear low-density polyethylene resin (metallocene LLDPE), 7.0 parts by mass of a thermally decomposable foaming agent, 1 part by mass of a decomposition temperature regulator, and 0.5 parts by mass of an antioxidant were supplied to a first extruder and melt-kneaded at 130°C to produce foamable resin composition 1 for the base layer. The resin, foaming agent, decomposition temperature regulator, and antioxidant for the surface layer were the same as those for the base layer. 100 parts by mass of linear low-density polyethylene resin (metallocene LLDPE), 1.5 parts by mass of a thermally decomposable foaming agent, 1 part by mass of a decomposition temperature regulator, and 0.5 parts by mass of an antioxidant were fed to a second and third extruder and melt-kneaded at 130°C to prepare foamable resin composition 2 for the surface layer. The foamable resin composition 1 was co-extruded from the first extruder at a thickness of 550 μm, and the foamable resin composition 2 was co-extruded from the second and third extruders at a thickness of 320 μm each, to obtain an unfoamed multilayer laminate sheet having a layer made of the foamable resin composition 1 for the base layer and layers made of the foamable resin composition 2 for the surface layer laminated on both sides of the layer. Next, the multilayer laminate sheet was crosslinked by irradiating it with 4.0 Mrad of an electron beam at an acceleration voltage of 500 kV. Thereafter, the crosslinked multilayer laminate sheet was continuously fed into a foaming furnace maintained at 250°C by hot air and an infrared heater, where it was heated and foamed, and stretched to a target thickness in a stretching process, thereby obtaining a multilayer foam sheet of Example 1 in which foam layers with different stretch ratios were laminated.

[0080] <Example 2> A multilayer foam sheet was obtained in the same manner as in Example 1, except that the amount of foaming agent in foamable resin composition 2 was changed from 1.5 parts by mass to 2.2 parts by mass, the extrusion thickness of foamable resin composition 2 was changed from 320 μm to 400 μm, and the extrusion thickness of foamable resin composition 1 was changed from 550 μm to 320 μm.

[0081] Example 3 A multilayer foam sheet was obtained in the same manner as in Example 1, except that the amount of foaming agent in foamable resin composition 2 was changed from 1.5 parts by mass to 2.8 parts by mass, the extrusion thickness of foamable resin composition 2 was changed from 320 μm to 200 μm, the amount of foaming agent in foamable composition 1 was changed from 7.0 parts by mass to 5.0 parts by mass, and the extrusion thickness of foamable resin composition 1 was changed from 550 μm to 350 μm.

[0082] Example 4 A multilayer foam sheet was obtained in the same manner as in Example 3, except that the amount of foaming agent in foamable resin composition 2 in Example 3 was changed from 2.8 parts by mass to 2.0 parts by mass, the extrusion thickness of foamable resin composition 2 was changed from 200 μm to 80 μm, the extrusion thickness of foamable composition 1 was changed from 350 μm to 240 μm, and the electron beam irradiation dose was changed from 4.0 Mrad to 5.0 Mrad.

[0083] <Example 5> A multilayer foam sheet was obtained in the same manner as in Example 1, except that the extrusion thickness of foamable resin composition 2 in Example 1 was changed from 320 μm to 500 μm, the amount of foaming agent in foamable resin composition 1 was changed from 7.0 parts by mass to 6.5 parts by mass, and the extrusion thickness of foamable composition 1 was changed from 550 μm to 450 μm.

[0084] Example 6 A multilayer foam sheet was obtained in the same manner as in Example 5, except that the resin contained in the foamable resin composition 1 and the foamable resin composition 2 was changed to a polypropylene resin (manufactured by Prime Polymer Co., Ltd., product name "E-333GV").

[0085] Example 7 A multilayer foam sheet was obtained in the same manner as in Example 5, except that the resin contained in the foamable resin composition 1 was changed to a styrene-based thermoplastic elastomer (Hybler 7311F manufactured by Kuraray Co., Ltd.).

[0086] <Comparative Example 1> A multilayer foam sheet was obtained in the same manner as in Example 1, except that the amount of foaming agent in foamable resin composition 2 was changed from 1.5 parts by mass to 4.0 parts by mass, the extrusion thickness of foamable resin composition 2 was changed from 320 μm to 100 μm, and the extrusion thickness of foamable resin composition 1 was changed from 550 μm to 500 μm.

[0087] <Comparative Example 2> A multilayer foam sheet was obtained in the same manner as in Example 2, except that the amount of foaming agent in foamable resin composition 2 was changed from 2.2 parts by mass to 0.8 parts by mass, the extrusion thickness of foamable resin composition 2 was changed from 400 μm to 700 μm, and the extrusion thickness of foamable resin composition 1 was changed from 320 μm to 250 μm.

[0088] <Comparative Example 3> A multilayer foam sheet was obtained in the same manner as in Example 1, except that the amount of foaming agent in foamable resin composition 2 was changed from 1.5 parts by mass to 8 parts by mass, the extrusion thickness of foamable resin composition 2 was changed from 320 μm to 90 μm, the amount of foaming agent in foamable composition 1 was changed from 7.0 parts by mass to 3.0 parts by mass, and the extrusion thickness of foamable composition 1 was changed from 550 μm to 790 μm.

[0089] <Comparative Example 4> A multilayer foam sheet was obtained in the same manner as in Comparative Example 2, except that the extrusion thickness of Expandable Resin Composition 2 was changed from 700 μm to 900 μm and the extrusion thickness of Expandable Resin Composition 1 was changed from 250 μm to 190 μm.

[0090] [Table 1]

[0091] The configurations, performance, and evaluation results of the multilayer foam sheets of Examples 1 to 7 and Comparative Examples 1 to 4 are shown in Table 2. Note that the thickness and expansion ratio of the surface layer in the table refer to the thickness and expansion ratio of one of the surface layers provided on both sides of the substrate, but the surface layers on both sides have the same thickness and expansion ratio.

[0092] [Table 2]

[0093] The multilayer foam sheets of Examples 1 to 7 in Table 2 had a 25% compressive strength of 125 kPa or less and good results in the shear retention test. These results demonstrate that the multilayer foam sheets of each Example were excellent in flexibility and mechanical strength in the shear direction. In contrast, the multilayer foam sheets of Comparative Examples 1 and 3 showed poor results in the shear retention test, indicating poor mechanical strength in the shear direction. The multilayer foam sheets of Comparative Examples 2 and 4 also showed 25% compressive strength exceeding 125 kPa, indicating poor flexibility. [Explanation of symbols]

[0094] 11 Test specimen 12 Wooden board 13 Wooden board 14 SUS board 15 Glass Plate 16 weights

Claims

1. A multilayer foam sheet comprising a base layer made of a foam layer and a surface layer made of a foam layer laminated on at least one surface of the base layer, wherein the multilayer foam sheet has a low-speed shear elastic modulus of 50 MPa or more, a 25% compressive strength of 125 kPa or less, a ratio of the thickness of the base layer to the thickness of the surface layer (thickness of the base layer / thickness of the surface layer) of 4.0 or more, and a gel fraction of 38 to 80%.

2. The multilayer foam sheet according to claim 1 , wherein the surface layers are laminated on both sides of the base layer.

3. 3. The multilayer foam sheet according to claim 1, wherein the multilayer foam sheet has a tensile breaking strength of 10 N / 10 mm or more.

4. Density is 0.07 to 0.22 g / cm ３ The multilayer foam sheet according to any one of claims 1 to 3, wherein

5. The multilayer foam sheet according to any one of claims 1 to 4, having a thickness of 30 to 2000 µm.

6. The multilayer foam sheet according to any one of claims 1 to 5, wherein the base layer has an average cell diameter of 20 to 500 µm.

7. The multilayer foam sheet according to any one of claims 1 to 6, wherein the closed cell content is 90% or more.

8. The multilayer foam sheet according to any one of claims 1 to 7, wherein the substrate layer comprises at least one resin selected from the group consisting of an olefin-based thermoplastic resin and a thermoplastic elastomer.

9. 9. An adhesive tape comprising the multilayer foam sheet according to claim 1 and an adhesive material provided on at least one surface of the multilayer foam sheet.

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