Multilayer foam sheet and adhesive tape

Abstract

To provide a foam sheet excellent in both step followability and reworkability, and an adhesive tape using the foam sheet.SOLUTION: A multilayer foam sheet of the present invention includes: a substrate layer formed of a foam layer; and a surface layer formed of a foam layer or a resin film, which is laminated on at least one surface of the substrate layer directly or via another layer. The multilayer foam sheet has a 25% compressive strength of 125 kPa or less and a tensile breaking strength of 10-700 N / 10 mm. The adhesive tape of the present invention includes the multilayer foam sheet of the present invention and an adhesive material provided on at least one surface of the multilayer foam sheet.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 have been used inside electronic devices for their cushioning and waterproofing properties. For example, when the electronic device is a display, a foam sheet is placed between the front panel and the housing where the front panel is fitted. If the foam sheet cannot conform to the unevenness of the housing, which is necessary for wiring routing, the front panel will not be parallel, which may result in unevenness on the display screen. Furthermore, the foam sheet may not provide sufficient waterproofing or dustproofing. Furthermore, with the increasing emphasis on display design in recent years, more expensive housings have been used. For this reason, foam sheets are required to enable display repair without destroying the housing.

[0005] Therefore, an object of the present invention is to provide a foam sheet that is excellent in both step-conforming ability and reworkability, 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 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 tensile breaking strength, and have completed the present invention. That is, the present invention provides the following [1] to

[13] .

[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 25% compressive strength of 125 kPa or less and a tensile breaking strength of 10 to 700 N / 10 mm. [2] The multilayer foam sheet according to [1] above, wherein the surface layers are laminated on both sides of the base layer. [3] Density: 0.07 to 0.22 g / cm3 The multilayer foam sheet according to [1] or [2] above, [4] The multilayer foam sheet according to any one of the above [1] to [3], 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. [5] The multilayer foam sheet according to any one of the above [1] to [4], which has a thickness of 30 to 2000 μm. [6] The multilayer foam sheet according to any one of the above [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 the above [1] to [6], which has a closed cell rate of 90% or more. [8] The multilayer foam sheet according to any one of the above [1] to [7], which has a gel fraction of 30 to 80%. [9] The multilayer foam sheet according to any one of the above [1] to [8], wherein the base layer contains at least one resin selected from the group consisting of olefin-based thermoplastic resins and thermoplastic elastomers.

[10] An adhesive tape comprising the multilayer foam sheet according to any one of the above [1] to [9] and an adhesive material provided on at least one surface of the multilayer foam sheet.

[11] The adhesive tape according to

[10] above, which is used on the back or frame of a display.

[12] The adhesive tape according to

[10] or

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

[13] When the adhesive is attached to an adherend having a step, the area of ​​the gap between the adhesive and the adherend is 0 to 3.0 cm 2 The adhesive tape according to the above

[11] or

[12] , [Effects of the Invention]

[0008] According to the present invention, it is possible to provide a foam sheet having excellent step-conforming properties and reworkability, and a pressure-sensitive adhesive tape using the foam sheet. DETAILED DESCRIPTION OF THE INVENTION

[0009] 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 25% compressive strength of 125 kPa or less and a tensile breaking strength of 10 N / 10 mm to 700 N / 10 mm. This provides the multilayer foam sheet with excellent conformability and reworkability. From the viewpoint of further improving conformability, the surface layer is preferably a foam layer.

[0010] (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 multilayer foam sheet will have poor conformability to uneven surfaces. 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.

[0011] (Tensile breaking strength) The tensile breaking strength of the multilayer foam sheet of the present invention is 10 to 700 N / 10 mm. If the tensile breaking strength is less than 10 N / 10 mm, the multilayer foam sheet may be damaged during rework, resulting in poor reworkability. If the tensile breaking strength is greater than 700 N / 10 mm, the flexibility of the multilayer foam sheet decreases, resulting in poor conformability to uneven surfaces. From this perspective, the tensile breaking strength of the multilayer foam sheet is preferably 15 to 500 N / 10 mm, and more preferably 20 to 80 N / 10 mm. 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.

[0012] (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 3 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).

[0013] (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 step-conforming ability of the foam sheet is improved, and by setting it to be equal to or less than these upper limits, the reworkability of the foam sheet is improved.

[0014] 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, even more preferably 1.4 to 6, and from the viewpoint of dustproofness and waterproofness, even more preferably 1.4 to 3. 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 the ability to conform to unevenness is improved, and when it is equal to or less than these upper limits, the reworkability of the foam sheet is improved. The expansion ratio can be measured by the method described in the examples.

[0015] 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.

[0016] From the viewpoint of obtaining a multilayer foam sheet having excellent step-conforming ability and reworkability, 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 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 both step-conforming ability and reworkability.

[0017] (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 step-conforming properties and reworkability. From the viewpoint of improving step-following properties and reworkability, the thickness of the base layer and the surface layer are preferably as follows.

[0018] 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.

[0019] 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.

[0020] In order to provide a multilayer foam sheet with excellent conformability to unevenness and reworkability, it is preferable that the thicknesses of both the base layer and the surface layer be 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.

[0021] (gel fraction) The multilayer foam sheet of the present invention preferably has a gel fraction of 30 to 80% by mass. When the foam sheet has a gel fraction of 30 to 80% by mass, the foam sheet is more likely to have good step-conforming ability and reworkability. 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.

[0022] (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 tensile strength at break to the desired range described above, and the reworkability of the foam sheet 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.

[0023] (closed bubble rate) 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 conformability to unevenness and reworkability. From the above perspectives, 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.

[0024] 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.

[0025] Specific examples of olefin-based thermoplastic resins include polyethylene resins, polypropylene resins, and ethylene-vinyl acetate copolymers, with polyethylene resins being preferred among these. 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.

[0026] (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.

[0027] 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.

[0028] 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.

[0029] 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.

[0030] 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.

[0031] 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.

[0032] 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.

[0033] 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.

[0034] 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.

[0035] 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.

[0036] 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."

[0037] [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.

[0038] 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.

[0039] 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.

[0040] 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.

[0041] 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.

[0042] <Surface layer> The surface layer is laminated on at least one surface of the base layer. That is, the surface layer may be laminated on one surface of the base layer or on both surfaces of the base layer. In particular, from the viewpoint of improving reworkability, it is preferable that the surface layer be laminated on both surfaces of the base layer. 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.

[0043] 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.

[0044] 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. The use of olefin-based thermoplastic resins can further improve the reworkability of the foam sheet. 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.

[0045] 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 3More 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.

[0046] 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.

[0047] 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.

[0048] 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.

[0049] (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.

[0050] <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.

[0051] 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.

[0052] 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.

[0053] 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.

[0054] 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.

[0055] (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).

[0056] (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.

[0057] 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.

[0058] 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.

[0059] 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.

[0060] 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.

[0061] [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 have the adhesive material provided on both surfaces of the multilayer foam sheet or on one surface, but is preferably provided on both surfaces.

[0062] 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.

[0063] [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 and the adhesive tape comprising the same of the present invention are excellent in both step-conforming ability and reworkability. Therefore, the multilayer foam sheet and the adhesive tape comprising the same are preferably used in displays, particularly large displays, and specifically, displays of 20 inches or more. The display may be a liquid crystal display or an organic electroluminescence (EL) display. When the adhesive tape comprising the multilayer foam sheet is used in a display, the adhesive tape comprising the multilayer foam sheet is used on the back surface or frame of the display. Furthermore, as described above, the adhesive tape of the present invention has excellent step-conforming ability. Therefore, when the adhesive tape of the present invention is used in a display, the adhesive tape of the present invention can conform to the step of the display housing, thereby reducing the area of ​​the gap between the adhesive tape and the housing. Specifically, when the adhesive tape of the present invention is attached to a predetermined adherend sample having a step by a predetermined method, the area of ​​the gap between the step of the adherend and the adhesive tape is 0 to 3.0 cm. 2 Preferably, the thickness is 0 to 2.0 cm. 2 It is more preferable that the predetermined adherend sample, the method of applying the adhesive tape, and the method of measuring the gap area are as shown in the Examples.

[0064] When the foam layer is provided on only one surface of the base layer, it is preferable to arrange the multilayer foam sheet so that the base layer faces the step, thereby ensuring waterproofness and dustproofness between the adherend having the step and the multilayer foam sheet. [Example]

[0065] 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.

[0066] [Measurement method] The methods for measuring and evaluating each physical property are as follows.

[0067] <25% compressive strength> Measurement was carried out at a temperature of 23°C according to the method of JIS K 6767.

[0068] <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.).

[0069] <Expansion ratio, density> 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. The density was also calculated from the measured specific volumes.

[0070] <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.

[0071] <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.

[0072] <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.

[0073] <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

[0074] <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. The test pieces were collected uniformly along the thickness direction of the multilayer foam sheet. Gel fraction (mass%) = 100 × (B / A)

[0075] <Reworkability> Adhesive layers were formed on both sides of the foam sheet, and acrylic plates were attached to both sides and left to stand at 23°C for 24 hours. A cutter was then inserted between the attached acrylic plates to tear the housing, and the base material remaining on the housing was peeled off. If the adhesive layer interface broke, it was deemed to have reworkability and evaluated as "○", and if the adhesive layer interface did not break, it was deemed not to have reworkability and evaluated as "×".

[0076] <Step-following ability> A foam sheet with an adhesive layer on one side was attached to an acrylic plate with a 2 mm step on one side formed by a 2 mm x 2 mm x 2 mm cubic protrusion in the center, and the foam sheet was left to stand for 24 hours at 23°C. After that, a 50x magnified photograph was taken using a digital microscope (Keyence Corporation, product name "VHX-900"), and the area of ​​the gap (gap area) that occurred between the step and the adhesive layer was calculated.

[0077] <Overall rating> The reworkability rating is "○" and the gap area is 3.0 cm 2 The foam sheet having a gap area of ​​3.0 cm or less was evaluated as "G (Good)". 2 The foam sheets with a size larger than 100 mm were evaluated as "B (Bad)".

[0078] [Method for producing multilayer foam sheet] The multilayer foam sheets of Examples 1 and 2 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.

[0079] <Example 2> The resin contained in foamable resin composition 1 was changed to a styrene-based thermoplastic elastomer (Hybler 7311F manufactured by Kuraray Co., Ltd.). 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 resin composition 1 was changed from 550 μm to 320 μm. The extrusion thickness of foamable resin composition 2 was changed from 320 μm to 400 μm. Otherwise, a multilayer foam sheet was obtained in the same manner as in Example 1.

[0080] <Comparative Example 1> A single-layer foam sheet was obtained in the same manner as in Example 1, except that no raw materials were supplied to the second and third extruders, and the second and third extruders were not used.

[0081] <Comparative Example 2> A single-layer foam sheet was obtained in the same manner as in Comparative Example 1, except that the amount of the foaming agent in Expandable Resin Composition 1 was changed from 7.0 parts by mass to 4.5 parts by mass.

[0082] <Comparative Example 3> A single-layer foam sheet was obtained in the same manner as in Comparative Example 1, except that the amount of foaming agent in Foamable Resin Composition 1 was changed from 7.0 parts by mass to 1.7 parts by mass, and the extrusion thickness of Foamable Resin Composition 1 was changed from 550 μm to 380 μm.

[0083] <Comparative Example 4> The single-layer foam sheet obtained in Comparative Example 1 was sandwiched between 100 μm-thick PET films. The single-layer foam sheet sandwiched between the PET films was then continuously fed into a heating furnace maintained at approximately 100°C by hot air and an infrared heater, where it was heated and laminated. This resulted in a multilayer foam sheet of Comparative Example 4, in which films were laminated on the upper and lower layers.

[0084] [Table 1]

[0085] The configurations, performance, and evaluation results of the foam sheets of Examples 1 and 2 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 represent 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.

[0086] [Table 2]

[0087] The multilayer foam sheets of Examples 1 and 2 in Table 2 had a 25% compressive strength of 125 kPa or less and a tensile breaking strength of 10 to 700 N / 10 mm, and therefore had good reworkability and conformability to uneven surfaces. In contrast, the foam sheets of Comparative Examples 1 to 3 were single-layer foam sheets and were therefore unable to improve both reworkability and conformability to uneven surfaces. The foam sheet of Comparative Example 4 was a multi-layer foam sheet, but had a tensile breaking strength of more than 700 N / 10 mm, resulting in poor conformability to uneven surfaces.

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 base layer made of the foam layer has an expansion ratio of 12 to 40, the multilayer foam sheet has a 25% compressive strength of 125 kPa or less, the multilayer foam sheet has a tensile breaking strength of 10 to 700 N / 10 mm, and 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 4.0 or more.

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

3. Density is 0.07 to 0.22 g / cm ３ The multilayer foam sheet according to claim 1 or 2,

4. The multilayer foam sheet according to any one of claims 1 to 3, having a thickness of 30 to 2000 µm.

5. A multilayer foam sheet described in any one of claims 1 to 4, wherein the average bubble diameter of the base layer is 20 to 500 μm.

6. The multilayer foam sheet according to any one of claims 1 to 5, wherein the closed cell content is 90% or more.

7. The multilayer foam sheet according to any one of claims 1 to 6, which has a gel fraction of 30 to 80%.

8. The multilayer foam sheet according to any one of claims 1 to 7, wherein the base 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.

10. The adhesive tape according to claim 9, which is used for the back surface or frame of a display.

11. The adhesive tape according to claim 9 or 10, which is used for a display of 20 inches or more.

12. When the film is attached to an adherend having a step, the area of ​​the gap between the adherend and the film is 0 to 3.0 cm ２ The adhesive tape according to claim 10 or 11,

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