Foam sheet and electronic device using same

Abstract

To provide a thin foam sheet having high surface impact absorption performance, and an electronic device which is resistant to surface impact using the foam sheet.SOLUTION: A foam sheet is obtained by foaming and curing a resin composition, where thickness of the foam sheet is 0.05-1.0 mm, and the foam sheet has compression restoration time that is time until thickness of the foam sheet is restored to thickness of 90% of thickness before compressive load application after the compressive load has been applied to the foam sheet with a weight of a diameter of 10 mm and a mass of 500 g at 25°C for 30 seconds and then the weight has been removed of 6-85 seconds.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to a foam sheet and an electronic device using the same. [Background technology]

[0002] In recent years, as electronic and electrical devices become more precise, smaller, and more mobile, it has become important to prevent damage to the parts installed inside the devices. In particular, in the display devices of electronic and electrical devices, foams made of various materials are sandwiched as cushioning materials between the glass plate on the surface and the image display material to absorb shocks and vibrations and prevent damage. Furthermore, as the miniaturization progresses, the space for installing the above-mentioned cushioning materials is limited, making it impossible to give the cushioning materials a sufficient thickness, and so there is a demand for thin but sufficient shock absorption performance. In particular, organic EL displays use frit glass as a sealing material and do not have a backlight unit, so they are considered to be vulnerable to shocks, and the above-mentioned cushioning materials are required to have high shock resistance.

[0003] Patent documents 1 and 2 propose a foam sheet having sufficient shock absorbing properties as a cushioning material, and in particular, for electronic or electrical devices having a display unit, they propose using a foam sheet on the back of the display unit as a damage-preventing cushioning material.

[0004] Patent Document 1 proposes a foam sheet having excellent resistance to repeated impacts as an impact absorbing material. Patent Document 1 discloses that a highly elastic foam (high recovery) is useful against repeated impacts, in which a load is applied to the foam sheet, the load is removed after 5 seconds, and the time from the removal of the load until the foam sheet fully recovers to its initial thickness (recovery time) is 0.1 seconds or less. In addition, in the examples, it is disclosed that the foam sheet has excellent effects on pooling resistance in liquid crystal.

[0005] Patent Document 2 proposes a foam sheet as a shock absorbing material that has excellent resistance to repeated shocks and can highly suppress the occurrence of display unevenness in the display section caused by touch operations on a touch panel. Patent Document 2 discloses that a highly elastic foam (high recovery) with a thickness recovery rate of 90% or more is useful against repeated shocks and can highly suppress the occurrence of display unevenness (pooling), when a load is applied to the foam sheet and the ratio of the thickness of the foam sheet before it is compressed (initial thickness) to the thickness 0.5 seconds after the compression state is released is defined as the thickness recovery rate. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] JP 2019-218557 A [Patent Document 2] JP 2018-021170 A DISCLOSURE OF THEINVENTION [Problem to be solved by the invention]

[0007] In recent years, highly elastic foam sheets as disclosed in Patent Documents 1 and 2 have been used as protective materials for the thin display parts of various display devices. However, although these shock absorbing materials are excellent in resistance to repeated impacts and in suppressing display unevenness that occurs in impacted parts of liquid crystal, there have been occasional cases where they are unable to absorb the impact and break. As a result of research by the inventors, it was found that, although evaluation using the impact loading method using steel balls as disclosed in Patent Documents 1 and 2 is capable of evaluating localized impacts, there is a risk that it is ineffective against impacts (surface impacts) that are applied over a wide area of ​​the display panel, and there is a risk that the protection of the components inside the device will be insufficient.

[0008] Therefore, an object of the present invention is to provide a thin foamed sheet having high surface impact absorbing performance, and an electronic device using the foamed sheet that is resistant to surface impact. [Means for solving the problem]

[0009] The present inventors have conducted extensive research and found that the above problems can be solved by a foamed sheet having specific properties, and have thus completed the present invention.

[0010] The present invention (1) is A foamed sheet obtained by foaming and curing a resin composition, The foamed sheet has a thickness of 0.05 to 1.0 mm. The compression recovery time of the foamed sheet is 6 to 85 seconds; The compression and recovery time is measured by subjecting the foamed sheet to a compression test at 25° C. using a 10 mm diameter foam sheet having a mass of 500 g. The foamed sheet is characterized in that a compressive load is applied to the foamed sheet after a weight is placed on the foamed sheet for 30 seconds, and the time from when the weight is removed until the foamed sheet restores to 90% of the thickness before the compressive load is applied. The present invention (2) is The apparent density of the foamed sheet is 0.10 to 0.70 g / cm 3 The foamed sheet according to the above invention (1), The present invention (3) is The foamed sheet according to the above invention (1) or (2) is characterized in that the resin composition contains an acrylic emulsion. The present invention (4) is The foamed sheet according to the third aspect of the present invention is characterized in that the resin composition contains an EVA-acrylic emulsion. The present invention (5) is The foamed sheet according to any one of the above inventions (1) to (4), wherein the resin composition contains a urethane-based emulsion. The present invention (6) is The foamed sheet according to any one of the above inventions (1) to (5), wherein the foamed sheet has a 25% compressive stress of 0.005 to 0.300 MPa. The present invention (7) is The electronic device includes the foamed sheet according to any one of the above aspects (1) to (6). Effect of the Invention

[0011] According to the present invention, it is possible to provide a thin foamed sheet having high surface impact absorbing performance, and an impact-resistant electronic device using the foamed sheet. [Brief description of the drawings]

[0012] [Figure 1] FIG. 2 is a schematic diagram showing a drop type impact absorption test machine used in the evaluation of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] The following items will be explained in order below.

[0014] 1. Foam sheet The foamed sheet of the present invention is a foamed sheet obtained by foaming and curing a resin composition. The thickness of the foamed sheet is 0.05 to 1.0 mm, and the compression recovery time of the foamed sheet is 6 to 85 seconds. Here, the compression recovery time is the time from when a weight having a diameter of 10 mm and a mass of 500 g is applied to the foamed sheet at 25°C for 30 seconds to apply a compressive load, and then when the weight is removed, until the foamed sheet recovers to 90% of the thickness before the compressive load is applied.

[0015] The compression restoration time of a foam sheet is a value representing the viscoelastic properties of the foam sheet. Here, the foam sheet is mainly configured in such a manner that bubbles are contained in the cured resin. Therefore, the compression restoration time of a foam sheet is considered to be a characteristic value that changes due to the combination and action of the viscoelastic properties of the resin and the elastic properties of the bubbles. The compression restoration time of a foam sheet is considered to be shorter when the elastic effect of the foam sheet is relatively large (the viscous effect is relatively small), that is, when the elastic effect of the resin and the bubbles is relatively strong (the viscous effect of the resin is relatively weak), it is considered to be shorter. Conversely, when the viscous effect of the resin is relatively high, the compression restoration time of a foam sheet is longer. Therefore, it is presumed that the compression recovery time of a foamed sheet is a characteristic value that depends on many variables such as the resin material and the structure of the bubbles. That is, the following description of the foamed sheet is an example for setting the compression recovery time of the foamed sheet in a predetermined range, and is not limited thereto. The foamed sheet of the present invention will be described in detail below.

[0016] The foamed sheet of the present invention has a thickness of 0.05 to 1.0 mm, and may be either a foamed sheet having a predetermined thickness obtained by extrusion molding, or a foamed sheet having a predetermined thickness obtained by splitting after molding.

[0017] In this specification, the term "thickness of foam sheet" refers to the thickness of the "foam formed into a sheet" and does not include the thicknesses of other layers including the pressure-sensitive adhesive layer, the substrate, the release liner, etc.

[0018] The foamed sheet of the present invention may have a deteriorated layer, called a skin layer, on the surface thereof.

[0019] The type of cells contained in the foamed sheet of the present invention is not particularly limited as long as it does not impair the effects of the present invention, and may be any of closed cells, open cells, and semi-open cells. Here, the term "semi-open cell structure" refers to a structure in which cells have small pores, unlike closed cells, and in which the pores between adjacent cells are smaller than those in an open cell structure. Specifically, the semi-open cell structure has an air permeability of 2 to 80 ml / (cm) according to the Frazier type method of JIS L1096 A method. 2 / s). Closed cells have strong elastic properties compared to foam sheets because gas is trapped inside the cells, and foam sheets containing more closed cells are more elastic. This means that the compression recovery time of foam sheets is shorter. Conversely, open cells have relatively weak elastic properties compared to foam sheets because the gas inside the cells can move freely. This means that the compression recovery time of foam sheets is longer. Semi-open cells have an effect that is intermediate between closed cells and open cells.

[0020] The apparent density of the foamed sheet of the present invention is not particularly limited as long as the effects of the present invention are not inhibited. For example, it can be 0.10 to 0.70 g / cm 3 can be used. The apparent density is measured in accordance with JIS K7222:2005 "Foamed Plastics and Rubbers - Method for Determining Apparent Density". When the apparent density of the foamed sheet is small, since many bubbles are contained in the foamed sheet, the elastic characteristics of the bubbles are more strongly affected. Conversely, when the apparent density of the foamed sheet is large, the viscoelastic characteristics of the resin are relatively more strongly affected. By adjusting these elastic characteristics of the bubbles and the viscoelastic characteristics of the resin, the compression recovery time of the foamed sheet can be adjusted.

[0021] The foamed sheet according to the present invention can be formed on a base material. By doing so, it becomes possible to give strength to the foamed sheet. The method of forming the foamed sheet on the base material is not particularly limited, and examples include a method of directly coating a foam on the base material and a method of providing an adhesive layer and bonding them. Further, a laminate in which the base material, the adhesive layer, and the release liner are arranged in this order may be formed in advance, and the foam may be integrally molded on the surface of the base material on the side opposite to the side where the adhesive layer is present.

[0022] A release agent layer may be provided on the surface of the base material on the foamed sheet forming side. By doing so, the base material can be used as a release liner.

[0023] Furthermore, a release agent layer may be provided on the surface of the base material on the side opposite to the foamed sheet forming side. By doing so, the foamed sheet can be made into a roll body wound in a roll shape, saving space during transportation and storage and protecting the foamed sheet from damage.

[0024] In addition, the foamed sheet according to the present invention can be provided with an adhesive layer on one side or both sides of its surface. By providing the adhesive layer, it becomes easy to fix electronic and electrical components using the foamed sheet.

[0025] The foam sheet having a pressure-sensitive adhesive layer on one or both surfaces may further have a release liner on the surface of the pressure-sensitive adhesive layer, which can prevent damage to the pressure-sensitive adhesive layer during transportation or before use.

[0026] Here, the foam sheet according to the present invention may be laminated with a foam other than the foam according to the present invention, a substrate, a pressure-sensitive adhesive layer, and other known layers in a desired number and in a desired order depending on the application, so long as it contains the foam according to the present invention. Furthermore, although an embodiment in which one layer of the foam according to the present invention is provided as the foam is preferred, the foam sheet may have a plurality of foams according to the present invention, such as substrate / foam according to the present invention / pressure-sensitive adhesive layer / foam according to the present invention / pressure-sensitive adhesive layer, etc.

[0027] 2. Foam sheet raw material (resin composition) The foamed sheet of the present invention is produced by foaming and curing a resin composition. The resin composition is not particularly limited as long as it can produce the foamed sheet of the present invention, and includes, for example, a resin component, a foaming agent, a dispersion medium, a crosslinking agent, and other additives. The effects of the present invention are not particularly limited by the material of the foamed sheet, but are exhibited by the physical properties of the foamed sheet. These will be described in detail.

[0028] 2-1. Resin components The resin component according to the present invention is not particularly limited as long as it does not impair the effects of the present invention, and examples thereof include (meth)acrylic resins; polyurethane resins such as polyether and polyester; polystyrene resins; polyolefin resins such as polyethylene, polypropylene, and polyethylene-polypropylene copolymers; vinyl acetate copolymer (EVA) resins; polyester resins such as polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polyethylene naphthalate (PEN), polybutylene naphthalate (PBN), and polytrimethylene terephthalate (PTT); resol and novolac phenolic resins; epoxy resins; silicone resins; polyvinyl chloride resins; urea resins; polyimide resins; ethylene propylene diene rubber (EPDM); and the like can be used, and copolymers obtained by copolymerizing the constituent monomers of these resins can be used. Furthermore, emulsions of these resins and copolymers obtained by copolymerizing the constituent monomers of these resins can be used. These can be used alone or in combination. In addition, it is preferable to use emulsions of these resins and copolymers obtained by copolymerizing the constituent monomers of these resins, since it is easy to adjust the density and bubble (cell) diameter by the gas mixing method. Furthermore, it is more preferable to use at least an acrylic emulsion among the emulsions. Furthermore, the use of a urethane emulsion can further impart material strength, and is particularly preferable when the adherend is adhesive glass or the like, and the obtained urethane resin foam has the advantage of being excellent in flexibility and low in compression residual strain. The following describes acrylic emulsions and urethane emulsions that are preferable as the resin component according to the present invention.

[0029] Acrylic emulsion The acrylic emulsion (aqueous dispersion of acrylic resin) can be produced by, for example, copolymerizing a mixture of (meth)acrylic acid ester monomers as essential polymerizable monomer components and, if necessary, other polymerizable monomers copolymerizable with these monomers in the presence of a polymerization initiator and, if necessary, an emulsifier and a dispersion stabilizer. Two or more kinds of acrylic emulsions may be used in combination.

[0030] Examples of polymerizable monomers that can be used in the preparation of the acrylic emulsion include (meth)acrylic acid ester monomers such as methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, butyl (meth)acrylate, hexyl (meth)acrylate, heptyl (meth)acrylate, octyl (meth)acrylate, octadecyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, cyclohexyl (meth)acrylate, nonyl (meth)acrylate, dodecyl (meth)acrylate, stearyl (meth)acrylate, isobornyl (meth)acrylate, dicyclopentanyl (meth)acrylate, phenyl (meth)acrylate, and benzyl (meth)acrylate; acrylic acid, methacrylic acid, β-carboxyethyl (meth)acrylate, 2-(meth)acryloylpropionic acid, crotonic acid, itaconic acid, maleic acid, fumaric acid, isopropyl alcohol, ethyl ... Examples of the monomers include unsaturated bond-containing monomers having a carboxyl group, such as taconic acid half ester, maleic acid half ester, maleic anhydride, and itaconic anhydride; glycidyl group-containing polymerizable monomers, such as glycidyl (meth)acrylate and allyl glycidyl ether; hydroxyl group-containing polymerizable monomers, such as 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, polyethylene glycol mono(meth)acrylate, and glycerol mono(meth)acrylate; ethylene glycol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, neopentyl glycol di(meth)acrylate, trimethylolpropane tri(meth)acrylate, polyethylene glycol di(meth)acrylate, polypropylene glycol di(meth)acrylate, diallyl phthalate, divinylbenzene, and allyl (meth)acrylate. For the acrylic emulsion, other resin-based polymerizable monomers such as styrene-based polymerizable monomers (for example, resin components contained in styrene-acrylic emulsions) or copolymers obtained by copolymerizing vinyl acetate-based polymerizable monomers, ethylene, and acrylic-based polymerizable monomers with acrylic polymerizable monomers (for example, resin components of EVA-acrylic emulsions) can be used. Also, copolymers obtained by copolymerizing acrylic polymerizable monomers with other resin-based polymers (or copolymers) can be used.These can be selected in consideration of the viscoelastic properties of the resin contained in the foamed sheet, and the composition ratio when these are copolymerized can also be selected in consideration of the viscoelastic properties. By selecting these, the compression recovery time of the foamed sheet can be set within a predetermined range.

[0031] As a copolymer of an acrylic polymerizable monomer with another resin polymerizable monomer, a copolymer with a styrene polymerizable monomer, which is a preferred embodiment, will be described below. Examples of copolymers of acrylic polymerizable monomers and styrene polymerizable monomers include copolymer resins such as styrene-maleic anhydride, styrene-(meth)acrylic acid, styrene-methacrylic acid-methyl methacrylate, and styrene-acrylonitrile, and ternary copolymer resins such as acrylonitrile-butadiene-styrene, etc. More specifically, examples of styrene polymerizable monomers include α-methylstyrene, ethylstyrene, isopropylstyrene, dimethylstyrene, paramethylstyrene, t-butylstyrene, chlorostyrene, and bromostyrene. Examples of monomers that form copolymers with such styrene-based monomers include esters of acrylic acid and methacrylic acid, such as methyl acrylate, ethyl acrylate, butyl acrylate, methyl methacrylate, ethyl methacrylate, butyl methacrylate, and cetyl methacrylate, as well as acrylonitrile, dimethyl fumarate, ethyl fumarate, vinyl toluene, vinyl xylene, butadiene, and maleic anhydride.

[0032] As a copolymer of an acrylic polymerizable monomer and another resin polymer (or copolymer), a copolymer of a vinyl acetate polymerizable monomer, ethylene, and an acrylic polymerizable monomer (which becomes a resin component contained in an EVA-acrylic emulsion) will be described below. Examples of the vinyl acetate polymerizable monomer include those having functional groups such as a carboxyl group, an epoxy group, a sulfonic acid group, a hydroxyl group, a methylol group, an alkoxy acid group, etc. These can be used alone or in combination. Examples of such vinyl acetate polymerizable monomers and monomers that form copolymers with ethylene include esters of acrylic acid and methacrylic acid, such as methyl acrylate, ethyl acrylate, butyl acrylate, methyl methacrylate, ethyl methacrylate, butyl methacrylate, and cetyl methacrylate, as well as acrylonitrile, dimethyl fumarate, ethyl fumarate, vinyl toluene, vinyl xylene, butadiene, and maleic anhydride.

[0033] When an emulsifier is used in preparing the acrylic emulsion, a known emulsifier may be used.

[0034] · Urethane emulsion Examples of methods for preparing a urethane emulsion (aqueous dispersion of a urethane resin) include the following methods (I) to (III). (I) A method of obtaining a urethane emulsion by mixing an organic solvent solution or organic solvent dispersion of a urethane resin having a hydrophilic group obtained by reacting an active hydrogen-containing compound, a compound having a hydrophilic group, and a polyisocyanate, with an aqueous solution containing a neutralizing agent, as necessary. (II) A method in which a urethane emulsion is obtained by mixing an isocyanate-terminated urethane prepolymer having a hydrophilic group obtained by reacting an active hydrogen-containing compound, a compound having a hydrophilic group, and a polyisocyanate with an aqueous solution containing a neutralizing agent, or by adding a neutralizing agent to the prepolymer in advance, mixing with water to disperse the prepolymer in water, and then reacting the prepolymer with a polyamine. (III) A method in which a urethane emulsion is obtained by mixing a terminal isocyanate group-containing urethane prepolymer having a hydrophilic group obtained by reacting an active hydrogen-containing compound, a compound having a hydrophilic group, and a polyisocyanate with an aqueous solution containing a neutralizing agent and a polyamine, or by adding a neutralizing agent to the prepolymer in advance and then adding and mixing an aqueous solution containing a polyamine.

[0035] Examples of the polyisocyanate used in the preparation of the urethane resin include 2,4-tolylene diisocyanate, 2,6-tolylene diisocyanate, m-phenylene diisocyanate, p-phenylene diisocyanate, 4,4'-diphenylmethane diisocyanate, 2,4'-diphenylmethane diisocyanate, 2,2'-diphenylmethane diisocyanate, 3,3'-dimethyl-4,4'-biphenylene diisocyanate, 3,3'-dimethoxy-4,4'-biphenylene diisocyanate, 3,3'-dichloro-4,4'-biphenylene diisocyanate, 1,5-naphthalene diisocyanate, 1,5-tetramethylphenyl ... Examples of the isocyanate include tetrahydronaphthalene diisocyanate, tetramethylene diisocyanate, 1,6-hexamethylene diisocyanate, dodecamethylene diisocyanate, trimethylhexamethylene diisocyanate, 1,3-cyclohexylene diisocyanate, 1,4-cyclohexylene diisocyanate, xylylene diisocyanate, tetramethylxylylene diisocyanate, hydrogenated xylylene diisocyanate, lysine diisocyanate, isophorone diisocyanate, 4,4'-dicyclohexylmethane diisocyanate, and 3,3'-dimethyl-4,4'-dicyclohexylmethane diisocyanate. In addition, a polyisocyanate having a valence of 3 or more may be used in combination within a range that does not impair the effects of the invention. These can be selected in consideration of the viscoelastic properties of the resin contained in the foamed sheet. This selection can adjust the compression recovery time of the foamed sheet.

[0036] Examples of the compound having a hydrophilic group include polyester polyol, polyether polyol, polycarbonate polyol, polyacetal polyol, polyacrylate polyol, polyesteramide polyol, polythioether polyol, polybutadiene-based polyolefin polyol, etc. These high molecular weight compounds may be used in combination of two or more kinds. The polyester polyol may be a known one. These may be selected in consideration of the viscoelastic properties of the resin contained in the foamed sheet. The compression restoration time of the foamed sheet can be adjusted by this selection.

[0037] In the above methods (I) to (III), an emulsifier may be further used within a range that does not impair the effects of the invention. Examples of such emulsifiers include nonionic emulsifiers such as polyoxyethylene nonylphenyl ether, polyoxyethylene lauryl ether, polyoxyethylene styrenated phenyl ether, and polyoxyethylene sorbitol tetraoleate; anionic emulsifiers such as fatty acid salts such as sodium oleate, alkyl sulfate ester salts, alkylbenzene sulfonates, alkyl sulfosuccinates, naphthalene sulfonates, sodium alkanesulfonates, and sodium alkyldiphenyl ether sulfonates; and nonionic and anionic emulsifiers such as polyoxyethylene alkyl sulfates and polyoxyethylene alkylphenyl sulfates.

[0038] ·Dispersion medium In this embodiment, the dispersion medium of the resin emulsion is essentially water, but may be a mixture of water and a water-soluble solvent. The water-soluble solvent may be, for example, alcohols such as methyl alcohol, ethyl alcohol, isopropyl alcohol, ethyl carbitol, ethyl cellosolve, butyl cellosolve, etc., polar solvents such as N-methylpyrrolidone, etc., and one or a mixture of two or more of these may be used.

[0039] Among the resin components used in the present invention, the physical properties of the resin emulsion, which is a preferred embodiment, will be described below.

[0040] ·Viscosity (mPa·s) The viscosity can be measured, for example, by a Brookfield viscometer (25° C.).

[0041] The viscosity of the resin emulsion can be, for example, 100 to 15,000 mPa·s, and preferably 2000 to 15,000 mPa·s. When the viscosity is within this range, if it is 100 or more, the bubble retention during molding is sufficient, and finer cells can be molded. In this way, the bubble (cell) size and bubble shape of the foamed sheet can be adjusted depending on the viscosity of the resin component, and the compression recovery time of the foamed sheet is also affected by these factors. When the viscosity is within this range, it is easy to keep the compression recovery time of the foamed sheet within a predetermined range.

[0042] Glass transition temperature (℃): Tg E Glass transition temperature (Tg) of resin emulsion E is not particularly limited, and may be the glass transition temperature Tg S It is sufficient that the temperature can be adjusted to within a predetermined range, for example, −60 to −20° C. Glass transition temperature Tg E The glass transition temperature can be determined as the temperature showing the peak value of tan δ measured using a dynamic viscoelasticity device (Anton Paar: Model MCR302) according to a procedure conforming to JIS-K7198 under conditions of -80°C to 150°C, heating at a rate of 5°C / min and a frequency of 1 Hz.

[0043] 2-2. Foaming agents (anionic surfactants) The anionic surfactant (foaming anionic surfactant) functions as a foaming agent for the resin composition.

[0044] Specific examples of anionic surfactants include sodium laurate, sodium myristate, sodium stearate, ammonium stearate, sodium oleate, potassium oleate soap, potassium castor oil soap, potassium coconut oil soap, sodium lauroyl sarcosine, sodium myristoyl sarcosine, sodium oleyl sarcosine, sodium cocoyl sarcosine, sodium coconut alcohol sulfate, sodium polyoxyethylene lauryl ether sulfate, sodium alkyl sulfosuccinate, sodium lauryl sulfoacetate, sodium alkylbenzene sulfonate, and sodium α-olefin sulfonate, with sodium alkyl sulfosuccinate being particularly preferred.

[0045] Here, the anionic surfactant used in this embodiment preferably has an HLB of 10 or more, and more preferably 20 or more, so that the resin composition can be easily dispersed when made into a resin-based emulsion.

[0046] 2-3. Amphoteric surfactants In the foam according to the present embodiment, the bubbles are made fine and uniform by using an amphoteric surfactant in addition to an anionic surfactant. That is, by adding an amphoteric surfactant, the bubble (cell) size and bubble density of the foam sheet can be adjusted, and the compression recovery time of the foam sheet is affected. In addition, when an amphoteric surfactant is used, the bubble density and distribution in the foam sheet are easily made uniform, so that a foam sheet having uniform surface impact resistance in the sheet plane can be obtained.

[0047] In particular, when an anionic surfactant and an amphoteric surfactant are used in combination, the charges of the hydrophilic groups of the anionic surfactant molecules repel each other, and while the anionic surfactant molecules are kept at a certain distance from each other, the electrically neutral amphoteric surfactant enters between the anionic surfactant molecules, thereby making the bubbles more stable and reducing the size of the bubbles. Therefore, by using an anionic surfactant and an amphoteric surfactant in combination, the bubble (cell) diameter and bubble density of the foamed sheet can be further adjusted, and the compression recovery time of the foamed sheet is affected.

[0048] The amphoteric surfactant that can be used in the present invention is not particularly limited, and may be an amino acid type, a betaine type, an amine oxide type, or the like amphoteric surfactant. A betaine type amphoteric surfactant is preferable because it has a higher effect as described above. Furthermore, from the viewpoint of ease of incorporation between the molecules of an anionic surfactant, C10-12 is preferable.

[0049] Examples of the amino acid type amphoteric surfactant include N-alkyl or alkenyl amino acids or salts thereof. N-alkyl or alkenyl amino acids have a structure in which an alkyl or alkenyl group is bonded to a nitrogen atom, and one or two groups represented by "-R-COOH" (wherein R represents a divalent hydrocarbon group, preferably an alkylene group, and particularly preferably has 1 to 2 carbon atoms) are bonded to the nitrogen atom. In a compound with one "-R-COOH" bonded, a hydrogen atom is further bonded to the nitrogen atom. A compound with one "-R-COOH" is called a mono-form, and a compound with two "-R-COOH"s is called a di-form. Either of these mono-forms or di-forms can be used as the amphoteric surfactant of the present invention. In the N-alkyl or alkenyl amino acid, the alkyl or alkenyl group may be linear or branched. Specific examples of amino acid type amphoteric surfactants include sodium lauryldiaminoethylglycine, sodium trimethylglycine, sodium cocoyl taurate, sodium methyl cocoyl taurate, sodium lauroyl glutamate, potassium lauroyl glutamate, and lauroyl methyl-β-alanine.

[0050] Examples of betaine-type amphoteric surfactants include alkyl betaines, imidazolinium betaines, carbobetaines, amido carbobetaines, amido betaines, alkyl amido betaines, sulfobetaines, amido sulfobetaines, phosphobetaines, etc. Specific examples of betaine-type amphoteric surfactants include lauryl betaine, stearyl betaine, lauryl dimethylamino acetic acid betaine, stearyl dimethylamino acetic acid betaine, lauric acid amido propyl dimethylamino acetic acid betaine, isostearic acid amido ethyl dimethylamino acetic acid betaine, isostearic acid amido propyl dimethylamino acetic acid betaine, isostearic acid amido ethyl diethylamino acetic acid betaine, isostearic acid amido propyl diethylamino acetic acid betaine, isostearic acid amido ethyl dimethylamino hydroxysulfobetaine, isostearic acid amido propyl dimethylamino hydroxysulfobetaine, isostearic acid amido ethyl diethylamino hydroxysulfobetaine, isostearic acid amido propyl diethylamino hydroxysulfobetaine, isostearic acid amido ethyl diethylamino hydroxysulfobetaine, isostearic acid amido propyl diethylamino hydroxysulfobetaine, N-lauryl-N,N -dimethylammonium-N-propyl sulfobetaine, N-lauryl-N,N-dimethylammonium-N-(2-hydroxypropyl) sulfobetaine, N-lauryl-N,N-dimethyl-N-(2-hydroxy-1-sulfopropyl)ammonium sulfobetaine, lauryl hydroxysulfobetaine, dodecylaminomethyl dimethyl sulfopropyl betaine, octadecylaminomethyl dimethyl sulfopropyl betaine, 2-alkyl-N-carboxymethyl-N-hydroxyethyl imidazolinium betaine (2-lauryl-N-carboxymethyl-N-hydroxyethyl imidazolinium betaine 2-stearyl-N-carboxymethyl-N-hydroxyethyl imidazolinium betaine, etc.), coconut oil fatty acid amidopropyl betaine, coconut oil fatty acid amidopropyl hydroxysultaine, etc.

[0051] Examples of the amine oxide type amphoteric surfactant include lauryl dimethylamine-N-oxide and oleyl dimethylamine-N-oxide.

[0052] Among the above-mentioned amphoteric surfactants, for the method for producing a foamed sheet according to the present invention, it is preferable to use a betaine-type amphoteric surfactant. Among the betaine-types, alkyl betaine, imidazolinium betaine, and carbobetaine are particularly preferable. Examples of the alkyl betaine that can be used in the present invention include stearyl betaine and lauryl betaine. Examples of the imidazolinium betaine include 2-alkyl-N-carboxymethyl-N-hydroxyethylimidazolinium betaine.

[0053] 2-4. Crosslinking agent (hardening agent) By using a crosslinking agent (hardening agent), it becomes possible to improve the strength of the foamed sheet according to the present embodiment, and at the same time, the viscoelastic properties of the resin in the foamed sheet can be adjusted more elastically. As a result, it becomes possible to adjust the compression recovery time of the foamed sheet.

[0054] Such a crosslinking agent is not particularly limited, and an appropriate amount may be added according to the use and the like. As crosslinking methods using a crosslinking agent, for example, there are physical crosslinking, ionic crosslinking, and chemical crosslinking, and the crosslinking method can be selected according to the type of the water-dispersible resin. As the crosslinking agent, known crosslinking agents can be used, and epoxy-based crosslinking agents, melamine-based crosslinking agents, isocyanate-based crosslinking agents, carbodiimide-based crosslinking agents, oxazoline-based crosslinking agents, etc. can be used in appropriate amounts according to the type and amount of functional groups contained in the resin formulation system used. Epoxy-based crosslinking agents and isocyanate-based crosslinking agents are preferable in order to improve the adhesive strength, tack strength, and interlayer peeling strength. Isocyanate-based and epoxy-based crosslinking agents can prevent material breakage of the adherend and the porous foam by increasing the material strength. Among them, aliphatic isocyanate is more preferable. These crosslinking agents may be used in combination of two or more.

[0055] 2-5. Other additives The resin composition according to the present invention may contain other additives, which are known additives added to foams. As other additives, it is preferable to use a surfactant for dispersing water-dispersible resins in order to promote the dispersion of the resin in the emulsion. The surfactant for dispersing water-dispersible resins is a surfactant for dispersing water-dispersible resins, and unlike anionic surfactants, it does not have to have an effect as a foaming agent. Such surfactants may be appropriately selected according to the water-dispersible resin to be selected.

[0056] 3. Manufacturing method of foam sheet 3-1. Composition of raw materials The composition of the resin composition will be described below in the case where a resin emulsion is used. In the present specification, the components constituting the "solid content" of the resin emulsion are the components excluding the dispersion medium from the entire resin emulsion. The resin emulsion has a solid content concentration of 30 to 80% by mass, preferably 40 to 70% by mass, and more preferably 50 to 60% by mass, when the entire resin emulsion is taken as 100% by mass. By setting the solid content concentration in such a range, it is possible to obtain an effect that a stable foamed sheet can be formed. For example, when an acrylic emulsion and a urethane emulsion are used in combination, the acrylic emulsion can be 60 to 95 parts by mass in terms of solid content, and the urethane emulsion can be 5 to 40 parts by mass in terms of solid content, and it is more preferable that the acrylic emulsion contains an EVA-acrylic emulsion in a solid content of 1 to 38 parts by mass. By setting the blending amount in such a manner, it is possible to obtain a foamed sheet with better surface impact performance.

[0057] The amount of the anionic surfactant is preferably 1.0 to 10 parts by weight, more preferably 3 to 10 parts by weight, based on the total amount of the resin emulsion in the resin composition (the total solid content is 100 parts by weight). By setting the amount in this range, it is possible to obtain the effect that suitable foaming is easily achieved and a fine cell structure can be formed.

[0058] The amount of the amphoteric surfactant is preferably 0.5 to 10 parts by weight, more preferably 1 to 5 parts by weight, based on the total amount of the resin emulsion in the resin composition (the total of the solid content and the non-solid content being 100 parts by weight). By setting the amount in this range, it is possible to obtain the effect of easily achieving appropriate foaming and forming a fine cell structure.

[0059] The amount of the crosslinking agent (curing agent) in the resin composition is preferably 0.5 to 10 parts by weight, more preferably 1 to 5 parts by weight, based on the total amount of the resin emulsion (the total of the solid content and non-solid content being 100 parts by weight). By setting the amount in this range, a foam with small compression set can be formed.

[0060] 3-2. Manufacturing of foam sheets The method for producing a foamed sheet according to the present invention includes a raw material preparation step, and a foaming / curing step (for example, a step of foaming a resin composition containing at least an emulsion and a foaming agent, for example, by using a mechanical froth method to form a foamable composition, and curing the foamable composition). The resin composition may further contain a crosslinking agent, and in the step, energy may be applied to crosslink the resin constituting the emulsion via the crosslinking agent, thereby curing the foamable composition. Each step will be described in detail below.

[0061] 3-2-1. Raw material preparation process In the raw material preparation step, the raw materials described above are mixed to prepare a resin composition, which is a raw material mixture for the foam. The mixing method is not particularly limited, but may be, for example, mixing the components while stirring them in a container such as a mixing tank.

[0062] 3-2-2. Foaming and curing process In the foaming and curing process, a predetermined foaming gas is added to the resin composition obtained in the raw material preparation process, and they are thoroughly mixed to produce a state in which a large number of bubbles exist in the resin composition (foamable composition). This foaming and curing process is usually carried out by thoroughly mixing the liquid porous foam raw material mixture obtained in the raw material preparation process with the foaming gas using a mixing device such as a mixing head.

[0063] 3-2-3. Foaming gas The foaming gas mixed into the resin composition in the stirring and foaming process forms bubbles (cells) in the foam, and the amount of the foaming gas mixed in determines the expansion ratio and density of the resulting foam. In other words, it contributes to the compression recovery time of the foam sheet. In order to adjust the density of the porous foam, the weight of the raw material of the porous foam required is calculated from the desired density of the porous foam and the volume of the raw material of the porous foam (for example, the internal volume of the mold into which the raw material of the porous foam is injected), and the amount of the foaming gas is determined so that the desired volume is obtained for this weight. In addition, air is mainly used as the type of foaming gas, but other inert gases such as nitrogen, carbon dioxide, helium, and argon can also be used.

[0064] 3-2-4. Foaming method and conditions As the foaming method according to the present invention, it is preferable to use a mechanical froth (mechanical foaming) method. The mechanical froth method is a method in which the resin composition is foamed by mixing air in the atmosphere into the resin composition by stirring the resin composition with a stirring blade or the like. As the stirring device, a stirring device generally used in the mechanical froth method can be used without any particular limitation, and for example, a homogenizer, a dissolver, a mechanical froth foaming machine, etc. can be used. According to this mechanical froth method, a foamed sheet having a density suitable for various applications can be obtained by adjusting the mixing ratio of the resin composition and the air. Although it is possible to use other foaming methods in combination, when a foaming method using a chemical foaming agent is used in combination, the ratio of closed bubbles increases, and it is possible to increase the density. By increasing the density, it is also possible to adjust the compression recovery time of the foam sheet.

[0065] The mixing time of the resin composition and the air is not particularly limited, but is usually 1 to 10 minutes, preferably 2 to 6 minutes. The mixing temperature is also not particularly limited, but is usually room temperature. The stirring speed in the above mixing is preferably 200 rpm or more (more preferably 500 rpm or more) in order to make the bubbles fine, and is preferably 2000 rpm or less (more preferably 800 rpm or less) in order to smoothly discharge the foamable composition from the foaming machine.

[0066] 3-2-5. Forming foam sheets The foamable composition (foamed emulsion composition) foamed as described above is formed into a foamed sheet having a desired thickness by known means such as a doctor knife or a doctor roll.

[0067] 3-2-6. Hardening A known method can be used to harden the foamed sheet. The foamed body according to the present embodiment can be self-crosslinked, but the foamed sheet can also be hardened by applying energy to crosslink the resin constituting the emulsion via a crosslinking agent. The process of applying energy is not particularly limited, but may be, for example, a heating process (thermal crosslinking).

[0068] In the heating step, the dispersion medium in the molded foamable composition is evaporated. The drying method is not particularly limited, but for example, hot air drying may be used. The drying temperature and drying time are also not particularly limited, but for example, it may be about 80°C for about 1 to 3 minutes.

[0069] In addition, in this heating step, the dispersion medium evaporates from the foamable composition, and the path through which the vapor escapes is connected from the inside to the outside of the foamed sheet, so that open or semi-open cells can be formed. If the foaming gas mixed in the stirring and foaming step remains as it is, the foamed sheet obtained will have closed cells, whereas if the mixed foaming gas is connected when the vapor escapes in this step, the foamed sheet obtained will have open or semi-open cells. That is, in the present invention, some of the bubbles in the foamed sheet are open, and the remaining bubbles are closed. The compression recovery time of the foamed sheet can be adjusted by such a bubble structure.

[0070] When a crosslinking agent is added, the heating step advances and completes the crosslinking (curing) reaction of the raw materials. Specifically, the raw materials are crosslinked with each other by the above-mentioned crosslinking agent, and a hardened foamed sheet is formed. The heating means at this time is not particularly limited as long as it can sufficiently heat the raw materials and crosslink (cur) the raw materials, and for example, a tunnel-type heating furnace or the like can be used. In addition, the heating temperature and heating time may be any temperature and time that can crosslink (cur) the raw materials, and may be, for example, 80 to 150°C (particularly, about 120°C is preferable) for about 1 to 3 minutes.

[0071] 4. Characteristics of foam sheets Compression and decompression time The compression recovery time of the foamed sheet is 6 to 85 seconds, preferably 10 to 55 seconds, and more preferably 15 to 50 seconds. When the compression recovery time of the foamed sheet is within this range, it is possible to obtain a foamed sheet with better surface impact performance. The compression recovery time of the foamed sheet can be adjusted to a predetermined value by adjusting the viscoelastic properties of the resin contained in the foamed sheet and the elastic properties of the bubbles contained in the foamed sheet. The compression recovery time is measured by applying a compressive load of a 500g weight with a diameter of 10mm to a foamed sheet for 30 seconds in an environment of 25°C, and then measuring the time from when the weight is removed until the foamed sheet recovers to 90% of the thickness before the compression load was applied. The measurement is performed by taking images of the foamed sheet during the compression recovery process using a digital microscope, and using its length measuring function, measuring the time until the foamed sheet recovers to 90% of the thickness before the compression load was applied.

[0072] Glass transition temperature of foam: Tg S Glass transition temperature Tg of foam sheet S is not particularly limited as long as it does not impair the effects of the present invention, but is preferably at least lower than the ambient air temperature in order to set the compression recovery time of the foamed sheet within a predetermined range. Sの The upper limit can be set to -60 to 0°C. S When the compression rate is within the above range, the compression recovery time of the foamed sheet can be easily adjusted. Glass transition temperature Tg S The glass transition temperature is determined by measuring the peak value of tan δ at a temperature range of -80°C to 150°C at a rate of 5°C / min and a frequency of 1 Hz using a dynamic viscoelasticity device (Anton Paar: Model MCR302).

[0073] Loss tangent of foam sheet (tanδ) The loss tangent (tan δ) of the foamed sheet is not particularly limited as long as it does not impair the effects of the present invention, but it is preferable that the foamed sheet is more viscous in order to set the compression recovery time of the foamed sheet within a predetermined range, and for example, the lower limit of the loss tangent tan δ of the foamed sheet in an environment of 25° C. is 0.3 to 0.8. In the case of the loss tangent (tan δ) of the foamed sheet, it is easy to adjust the compression recovery time of the foamed sheet within a predetermined range.

[0074] The 25% compressive stress of the foamed sheet at 25°C is not particularly limited as long as it does not impair the effects of the present invention, and is, for example, 0.005 to 0.300 MPa, preferably 0.010 to 0.200 MPa, and more preferably 0.030 to 0.150 MPa. When the 25% compressive stress of the foamed sheet is within this range, a foamed sheet having better surface impact resistance can be provided. The 25% compressive stress is measured by the method described in JIS K6254-2:2016 "Vulcanized rubber and thermoplastic rubber - Determination of stress-strain characteristics".

[0075] 3. Uses and usage of foam sheets The foamed sheet according to the present invention is thin and has high surface impact absorption performance, and therefore can be used as an impact absorbing sheet for protecting precision electronic and electrical components inside electronic devices, etc. It is particularly useful for protecting thin, wide-area display components such as liquid crystal panels, organic EL panels, and touch panels. Furthermore, the foamed sheet according to the present invention can be attached in advance to the electronic or electrical parts to be protected, thereby improving workability when assembling the electronic or electrical parts. EXAMPLES

[0076] Next, the present invention will be described in more detail with reference to examples, but the present invention is not limited to these examples. In addition, unless otherwise specified, "%" representing the content means % by mass.

[0077] <Preparation of foam sheet> First, in each of the Examples and Comparative Examples, the following materials were used as the raw materials for the foamed sheet. Acrylic emulsion 1: Acrylonitrile, acrylic acid alkyl ester, itaconic acid copolymer, pH 9, solid content 54%, viscosity 12,000 mPa·s, Tg-20℃ Acrylic emulsion 2: Ethylene vinyl acetate-acrylic-special ester copolymer, pH 8, solid content 60%, viscosity 5,000 mPa·s, Tg-50℃ Urethane emulsion 1: Polyether, polycarbonate, pH 8, solid content 60%, viscosity 100mPa·s, Tg-40℃ Anionic surfactant 1: Ammonium stearate, 30% solids Anionic surfactant 2: Sodium alkylsulfosuccinate, solids content 35% Betaine-based amphoteric surfactant: Alkyl betaine, solid content 30% Crosslinking agent: Hydrophobic HDI isocyanurate (functional group number 3.5) Solid content 100%

[0078] The raw materials described in each of the examples and comparative examples were blended to prepare the resin compositions of each of the examples and comparative examples.

[0079] The resin compositions of each Example and Comparative Example were foamed by mechanical froth method (foaming condition 100-1000 rpm) by adding air or inert gas such as nitrogen gas, cast on a PET release liner, and then heat-treated (in an oven or drying furnace) to obtain foamed sheets of each Example and Comparative Example. The density of the foamed sheets of each Example and Comparative Example was adjusted by changing the injection amount of inert gas such as air or nitrogen gas, the rotation speed of the mixer, and the drying conditions.

[0080] <Evaluation test> The foamed sheets obtained in each of the Examples and Comparative Examples were subjected to the following measurements and evaluations. The results are shown in Table 1.

[0081] Thickness The thickness was measured by a thickness gauge.

[0082] ·density It was measured by calculating the weight per unit volume.

[0083] 25% compressive stress at 25℃ The 25% compressive stress at 25°C was determined by the method described in JIS K6254-2:2016 "Vulcanized rubber and thermoplastic rubber - Determination of stress-strain characteristics."

[0084] Compression and decompression time The compression recovery time was measured by applying a compressive load of a weight of 10 mm in diameter and 500 g in mass to the foamed sheet of each Example and Comparative Example for 30 seconds under an environment of 25° C., and then measuring the time from when the weight was removed until the foamed sheet recovered to 90% of the thickness before the compressive load was applied. The measurement was performed by taking images of the foamed sheet during the recovery process using a digital microscope (VHX-7000 manufactured by Keyence Corporation) and using its length measurement function. The results are shown in Table 1.

[0085] Surface impact absorption rate The surface impact absorption rate was calculated by the following formula 1 after performing a surface impact absorption test using a drop type impact absorption tester (see Figure 1). The measurement was performed by placing a sample cut to a size of φ50 mm on the sample stage, and further placing a 5 mm thick acrylic plate at the position where the impactor of the sample would come into contact. The measurement conditions were an air temperature of 23°C, an impactor weight of 35.76 g, and the impactor was dropped from a height of 100 mm. Surface impact absorption rate (%) = {(f a0 -f a1 ) / f a0}×100 (Formula 1) In the above formula (1), f a0 is the impact load when the shock absorption test is performed without placing the sample on the sample stage and only the acrylic plate is placed, and f a1 is the impact load when an impact absorption test was performed by placing a sample on the sample stage and further placing a 5 mm thick acrylic plate at the position where the sample would come into contact with the impactor. Measurements were taken at drop heights of 100 mm, 200 mm, and 300 mm. The impact load was measured using a sensor installed on the sample stage. The areal impact absorption rate was evaluated according to the following criteria. The results are shown in Table 1. ○: Surface impact absorption rate is 30% or more ×: Surface impact absorption rate is less than 30%

[0086] ·comprehensive evaluation A comprehensive evaluation was conducted based on the following criteria, and the results are shown in Table 1. ○: All surface impact absorption rates are ○ ×: At least one × in the evaluation of surface impact absorption rate

[0087] [Table 1] [Explanation of symbols]

[0088] 1. Drop-type impact absorption test machine 10 Impactor (steel ball) 11 Acrylic board 12 Samples 13 Sample stage (SUS) 14 Sensors

Claims

1. A foamed sheet obtained by foaming and curing a resin composition, The resin composition includes an acrylic emulsion and a urethane emulsion, The foamed sheet has a thickness of 0.05 to 1.0 mm. The foamed sheet has a compression recovery time of 6 to 85 seconds, which is a time required for the foamed sheet to recover to 90% of the thickness before the compression load is applied after a compressive load of a weight having a diameter of 10 mm and a mass of 500 g is applied to the foamed sheet for 30 seconds at 25°C and the weight is removed.

2. The apparent density of the foamed sheet is 0.10 to 0.70 g / cm ３ The foamed sheet according to claim 1 ,

3. 2. The foam sheet according to claim 1, wherein the acrylic emulsion comprises an EVA-acrylic emulsion.

4. The foamed sheet according to any one of claims 1 to 3, wherein the foamed sheet has a 25% compressive stress at 25°C of 0.005 to 0.300 MPa.

5. An electronic device comprising the foam sheet according to any one of claims 1 to 4.

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