JP7723474B2 - Multilayer foam sheet and adhesive tape - Google Patents

Description

The present invention relates to a multilayer foam sheet and an adhesive tape comprising the foam sheet.

Foam sheets are widely used as sealing materials or shock-absorbing materials in electronic devices such as mobile phones, cameras, displays, game devices, electronic organizers, and personal computers. Foam sheets are also sometimes used inside electronic devices, for example, by applying an adhesive to at least one surface to form adhesive tape. 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).

JP 2014-28925 A

In recent years, foam sheets have been used inside electronic devices for their cushioning and waterproofing properties. As televisions with large displays are increasingly being hung on the wall or diagonally, 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.

The present invention was made in consideration of the above circumstances, and aims to provide a foam sheet that has excellent flexibility and mechanical strength in the shear direction, and an adhesive tape that uses such a foam sheet.

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 including 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].

[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, wherein the multilayer foam sheet has 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 the above [1], wherein the surface layer is laminated on both sides of the base layer.
[3] The multilayer foam sheet according to [1] or [2] above, having a tensile breaking strength of 10 N/10 mm or more.
[4] The multilayer foam sheet according to any one of the above [1] to [3], having a density of 0.07 to 0.22 g/cm ³ .
[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 [1] to [5] above, having a thickness of 30 to 2000 μm.
[7] The multilayer foam sheet according to any one of [1] to [6] above, wherein the base layer has an average cell diameter of 20 to 500 μm.
[8] The multilayer foam sheet according to any one of [1] to [7] above, having a closed cell ratio of 90% or more.
[9] The multilayer foam sheet according to any one of [1] to [8] above, having a gel fraction of 30 to 80%.
[10] The multilayer foam sheet according to any one of [1] to [9] above, wherein the base layer contains at least one resin selected from the group consisting of an olefin-based thermoplastic resin and a thermoplastic elastomer.
[11] An adhesive tape comprising the multilayer foam sheet according to any one of [1] to [10] above and an 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 a display of 20 inches or more.

The present invention provides a multilayer foam sheet with excellent flexibility and mechanical strength in the shear direction, and an adhesive tape using the multilayer foam sheet.

FIG. 1 is an explanatory diagram illustrating a method for measuring elastic modulus by low shear. FIG. 1 is an explanatory diagram illustrating a method for a shear retention test.

The present invention will be described in detail below using embodiments.
[Foam sheet]
The multilayer foam sheet of the present invention is 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, and has a low 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.

(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 in consideration of 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.

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 ("Quickbond 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 the wooden board 12 and one end side 13a of the 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 was fixed, and the other end 13b of the wooden board 13 was 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 was taken as the elastic modulus under slow shear stress in the present invention.

(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.

(Tensile Breaking Strength)
The tensile breaking strength of the multilayer foam sheet of the present invention is preferably 10 N/10 mm or more. A tensile breaking strength of 10 N/10 mm or more tends to improve the mechanical strength of the multilayer foam sheet in the shear direction. 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.

(density)
The density of the multilayer foam sheet of the present invention is preferably 0.07 to 0.22 g/ ^cm3 , more preferably 0.10 to 0.21 g/ ^cm3 , and even more preferably 0.13 to 0.20 g/ ^cm3 . When 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).

(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.

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.

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

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.

(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 facilitates 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.

The thickness of the substrate layer in the multilayer foam sheet of the present invention is not particularly limited, but is preferably 20 to 1,400 μm, more preferably 80 to 1,200 μm, and even more preferably 170 to 1,000 μm.

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 the substrate, it is preferable that the thickness of at least one, and preferably both, of the surface layers be within the above range.

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.

(Gel fraction)
The gel fraction of the multilayer foam sheet of the present invention is preferably 30 to 80% by mass. When the foam sheet has a gel fraction of 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.

(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 bubble 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.

(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.

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, but 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 makes it easier to ensure the flexibility and mechanical strength of the base layer while improving foamability, etc. The resin used in the base layer may be used alone or in combination of two or more.

Specific examples of olefin-based thermoplastic resins include polyethylene resin, polypropylene resin, ethylene-vinyl acetate copolymer, etc., and among these, polyethylene resin is 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.

(Metallocene catalyst)
Examples of metallocene catalysts include compounds such as bis(cyclopentadienyl) metal complexes having a structure in which a transition metal is sandwiched between π-electron unsaturated compounds. More specifically, examples include compounds in which one or more cyclopentadienyl rings or analogs thereof are present 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.

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.

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 cocatalysts 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.

Furthermore, 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.
From the viewpoint of flexibility, the density of the polyethylene resin, for example, the linear low-density polyethylene described above, is preferably 0.870 to 0.925 g/cm ³ , more preferably 0.890 to 0.925 g/cm ³ , and even more preferably 0.910 to 0.925 g/cm ^3. As the polyethylene resin, a plurality of polyethylene resins can be used, and a polyethylene resin having a density outside the above-mentioned range can also be added.

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 the 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 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.

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.

Furthermore, 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 examples of such CEBC include "DYNARON 6200P" manufactured by JSR Corporation.

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.

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

Commercially available styrene-based thermoplastic elastomers include "DYNARON 8600P" (styrene content: 15% by mass), "DYNARON 4600P" (styrene content: 20% by mass), and "DYNARON 1321P" (styrene content: 10% by mass), all manufactured by JSR Corporation, and "HYBRAR 7311" (styrene content: 10% by mass), all manufactured by Kuraray Co., Ltd.

[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, and examples of the organic blowing agent include azo compounds such as azodicarbonamide, metal azodicarboxylate (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.

The amount of foaming agent in the foamable resin composition used to form 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 resin. By using a foaming agent 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 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, etc., of the foam layer.

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. 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 resin to adjust the surface condition of the foam layer.

The foamable resin composition may contain an antioxidant. Examples of antioxidants 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 typically blended in an amount of 0.01 to 5 parts by mass per 100 parts by mass of resin.

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.

<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.

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, the resin may be used alone or in combination of two or more kinds. When the surface layer is 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 viewpoint of productivity and from the viewpoint of easily making the physical properties of both layers the same or similar, it is preferable that the resins are the same.

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.

Examples of olefin-based thermoplastic resins include polyethylene resins, polypropylene resins, etc. Also usable are ethylene-vinyl acetate copolymers, ethylene-ethyl acrylate copolymers, etc.
Examples of polyethylene resins include low-density polyethylene (density: less than 0.930 g/cm ³ ), medium-density polyethylene (density: 0.930 g/cm ³ or more and less than 0.942 g/cm ³ ), and high-density polyethylene (density: 0.942 g/cm ³ or more).
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.

The ethylene-vinyl acetate copolymer used in the surface layer may be, 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.

In the surface layer, the resin is the main component, and the content of the resin 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 amount of each layer. In addition to the resin, the surface layer may contain additives such as an antioxidant, a heat stabilizer, a colorant, a flame retardant, an antistatic agent, and a filler.
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.

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 can be formed by blending a resin that can be used in the surface layer with a foaming agent, as described for the base layer, and foaming the resin. The amount of foaming agent in the foamable resin composition used to form 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 is easy to adjust the expansion ratio of the surface layer to the desired range described above.

(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, flexibility, etc. 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 1/2 or less of the thickness of the surface layer.

<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 multilayer foam sheet is made of only one type of resin.

More specifically, production method 1 includes the following steps I to 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 obtaining a foam sheet by foaming the layer made of the foamable resin composition of the multilayer laminate sheet.

Each step will be described below.
(Step (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. Also, the resin for forming the base layer, a foaming agent such as a thermal decomposition type 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 joined together 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, thereby obtaining a multilayer laminate sheet having a three-layer structure.

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 thermal decomposition type 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 co-extrusion molding, either a feed block method or a multi-manifold method may be used, but the feed block method is preferred.

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.

(Step (II))
In step (II), the multilayer laminate sheet obtained in step (I) is subjected to a foaming treatment to foam the layer comprising the foamable resin composition. The layer comprising the foamable resin composition may be treated so that the foaming agent foams, but when the foaming agent is a thermally decomposable foaming agent, the multilayer laminate sheet is heated to foam it. The heating temperature may be any temperature at or above 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 description has been given of 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 into the resin composition for forming the surface layer, and foamed in step (II).

(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.

In the second manufacturing method, a foam can be obtained by melt-kneading the resin for forming the base layer, the thermally decomposable foaming agent, and any additives that are added as needed 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). There are no particular limitations on the method for melt-kneading the foamable resin composition and molding it into a sheet, but it is preferable to use an extruder.

The resulting foamable resin composition sheet is preferably further crosslinked before foaming, as described below. One crosslinking method involves blending an organic peroxide into the foamable resin composition sheet in advance and heating the sheet to crosslink it, but it is preferable 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.

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. If 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 manufacturing method above. The foamable resin composition sheet may also be stretched as appropriate during or after foaming.

A separately prepared resin film or foam for forming the surface layer can then be superimposed on one or both sides of the foam layer that forms the base layer and bonded to obtain a multilayer foam sheet. Specifically, this can be achieved by applying heat and pressure using a press or similar device to achieve thermocompression bonding. Alternatively, a pressure sensitive adhesive or adhesive can be applied to the bonding surface between the foam layer and the surface layer, or double-sided adhesive tape can be applied to bond them together using the pressure sensitive adhesive, adhesive, double-sided adhesive tape, etc.

[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.

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.

[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 members.
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 EL display.

The present invention will be explained in more detail using examples, but the present invention is not limited to these examples in any way.

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

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

<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.).

<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 ^cm3 of xylene at 120°C and left for 24 hours, and then filtered through a 200-mesh wire net. The insoluble matter on the wire net was collected and vacuum-dried, and the mass B (mg) of the insoluble matter was precisely weighed. From the obtained value, the gel fraction (mass%) was calculated using the following formula.
Gel fraction (mass%)=100×(B/A)

<Closed cell ratio>
Measurement was carried out according to the method of ASTM D2856 (1998).
Specifically, the measurement was 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 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 air bubbles was calculated based on the following formula: The density of the matrix resin constituting the test piece was taken as ρ (g/cm ³ ).
Volume occupied by bubbles V ₂ = V ₁ - W ₁ /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 ratio _F1 and closed cell ratio _F2 were calculated according to the following formula.
Open cell ratio F ₁ (%) = 100 × (W ₂ - _{W 1} ) / V ₂
Closed cell ratio F ₂ (%) = 100 - F ₁

<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.

<Average bubble diameter>
The multilayer foam sheet was cut in the thickness direction along each of the MD and TD, and a 200x magnified photograph was taken using a digital microscope (Keyence Corporation, product name "VHX-900"). In the substrate layer portion of the enlarged photograph, the MD and TD cell diameters were measured for all bubbles present in a 2 mm long cut surface in each of the MD and TD, and this operation was repeated five times. The average values of the cell diameters in each of the 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.

<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.

<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.

<Elastic modulus at low shear>
The measurement method is explained with reference to Fig. 1. The multilayer foam sheet of the present invention is cut into 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 ("Quickbond 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 the wooden board 12 and one end side 13a of the 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 was fixed, and the other end 13b of the wooden board 13 was 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 was taken as the elastic modulus under slow shear stress in the present invention.

<Shear retention test>
The multilayer foam sheet was cut into a test piece of 25 mm x 25 mm, and an 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 release film with a thickness of 75 μm was prepared, and an adhesive solution was applied to the release-treated surface of this release film and dried at 110°C for 5 minutes to form an adhesive layer with a thickness of 0.05 mm. This adhesive layer was bonded to the surface of a multilayer foam sheet. Next, in the same manner, the same adhesive layer as above was bonded to the opposite surface of the multilayer foam sheet. After that, the product was 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, one release film was peeled off, and the tape was 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 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 sides 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, preparing a holding strength test sample. The prepared holding strength test sample was left at 23°C and 50% RH for 24 hours, then transferred to a thermostatic chamber at 60°C and 90% RH, and the other end 15b of the glass plate 15 was fixed so that it 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 (peeling time) was measured. A rating of ◯ was given when the elapsed time from hanging the weights until the sample peeled and fell was 300 hours or more, △ was given when it was 200 hours or more but less than 300 hours, and × was given when it was less than 200 hours.

[Method for producing a multilayer foam sheet]
The multi-layer foam sheets of Examples 1 to 7 and Comparative Examples 1 to 4 were produced as follows.
Example 1
A metallocene-catalyzed linear low-density polyethylene resin (metallocene LLDPE, manufactured by Japan Polyethylene Corporation, product name "Kernel KF283") was prepared as the resin for forming the base layer, which is the foam layer. Azodicarbonamide was prepared as the thermal decomposition type foaming agent. Zinc oxide (manufactured by Sakai Chemical Industry Co., Ltd., product name "OW-212F") was prepared as the decomposition temperature regulator. 2,6-di-t-butyl-p-cresol, a phenolic antioxidant, was also prepared as the antioxidant.
100 parts by mass of a 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 prepare foamable resin composition 1 for a base layer.
The resin, foaming agent, decomposition temperature regulator, and antioxidant for the surface layer were prepared in the same manner as for the base layer. 100 parts by mass of a linear low-density polyethylene resin (metallocene LLDPE), 1.5 parts by mass of a thermal decomposition type foaming agent, 1 part by mass of a decomposition temperature regulator, and 0.5 parts by mass of an antioxidant were supplied to a second and a third extruder and melt-kneaded at 130°C to prepare a 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 a base layer and layers made of the foamable resin composition 2 for surface layers laminated on both sides of the layer.
Next, the multilayer laminate sheet was crosslinked by irradiating it with an electron beam at an acceleration voltage of 500 kV for 4.0 Mrad. 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 step, thereby obtaining a multilayer foam sheet of Example 1 in which foam layers having different stretch ratios were laminated.

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.

Example 3
A multilayer foam sheet was obtained in the same manner as in Example 1, except that the amount of the 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 the 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.

Example 4
A multilayer foam sheet was obtained in the same manner as in Example 3, except that the amount of the 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.

Example 5
A multi-layer 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 the 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.

Example 6
A multi-layer 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").

Example 7
A multi-layer 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.).

<Comparative Example 1>
A multilayer foam sheet was obtained in the same manner as in Example 1, except that the amount of the 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.

<Comparative Example 2>
A multilayer foam sheet was obtained in the same manner as in Example 2, except that the amount of the 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.

<Comparative Example 3>
A multilayer foam sheet was obtained in the same manner as in Example 1, except that the amount of the 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 the 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.

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

The structure, 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.

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 strengths exceeding 125 kPa, indicating poor flexibility.

11 Test piece 12 Wooden plate 13 Wooden plate 14 SUS plate 15 Glass plate 16 Weight

Claims (9)

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%. The multilayer foam sheet according to claim 1, wherein the surface layer is laminated on both sides of the base layer. The multilayer foam sheet according to claim 1 or 2, having a tensile breaking strength of 10 N/10 mm or more. The multilayer foam sheet according to any one of claims 1 to 3, having a density of 0.07 to 0.22 g/ ^cm3 . The multilayer foam sheet according to any one of claims 1 to 4, having a thickness of 30 to 2000 μm. 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. The multilayer foam sheet according to any one of claims 1 to 6, having a closed cell rate of 90% or more. The multilayer foam sheet according to any one of claims 1 to 7, wherein the substrate layer contains at least one resin selected from the group consisting of olefin-based thermoplastic resins and thermoplastic elastomers. An adhesive tape comprising the multilayer foam sheet according to any one of claims 1 to 8 and an adhesive material provided on at least one surface of the multilayer foam sheet.