JP7682003B2 - Foam sheet and electronic device using same - Google Patents

Description

The present invention relates to a foam sheet and an electronic device using the same.

In recent years, as electronic and electrical devices have 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 between the glass plate on the surface and the image display member as cushioning material to absorb shocks and vibrations and prevent damage. Furthermore, as the devices become smaller, the space available for installing the cushioning material is limited, making it impossible to provide the cushioning material with a sufficient thickness, and so there is a demand for the cushioning material to have sufficient shock absorption performance even if it is thin. In particular, organic EL displays use frit glass as a sealant and do not have a backlight unit, making them vulnerable to shocks, and therefore the cushioning material must have high shock resistance.

Patent documents 1 and 2 propose a foam sheet with sufficient shock absorbing properties as a cushioning material, and in particular, for electronic and electrical devices with a display unit, they propose using a foam sheet on the back of the display unit as a damage prevention cushioning material.

Patent Document 1 proposes a foam sheet as an impact absorbing material that has excellent resistance to repeated impacts. Patent Document 1 discloses that a highly elastic foam (high recovery) is useful against repeated impacts when 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, the examples disclose that the foam sheet also has excellent resistance to pooling in liquid crystals.

Patent Document 2 proposes a foam sheet as an impact absorbing material that has excellent resistance to repeated impacts and can highly suppress the occurrence of display unevenness on the display unit due to 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 impacts 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.

JP 2019-218557 A JP 2018-021170 A

In recent years, highly elastic foam sheets such as those 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 terms of resistance to repeated impacts and suppression of display unevenness that occurs in impacted parts of the liquid crystal, there have been occasional cases where they are unable to fully absorb the impact and break. As a result of research by the inventors, it was found that while the evaluation using the impact loading method using steel balls disclosed in Patent Documents 1 and 2 makes it possible to evaluate 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 may be insufficient.

Therefore, the present invention aims to provide a thin foam sheet with high surface impact absorption performance, and an electronic device using said foam sheet that is resistant to surface impact.

The inventors conducted extensive research and discovered that the above problems could be solved by using a foamed sheet with specific properties, leading to the completion of the present invention. That is, the present invention is as follows.

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 recovery time is a time period during which a compressive load is applied to the foamed sheet at 25°C by placing a weight having a diameter of 10 mm and a mass of 500 g for 30 seconds, and then the weight is removed until the foamed sheet recovers to 90% of the thickness before the compressive load is applied.
The present invention (2) is
The foamed sheet of the above-mentioned invention (1) is characterized in that the foamed sheet has an apparent density of 0.10 to 0.70 g/ ^cm3 .
The present invention (3) is
The foamed sheet according to the above invention (1) or (2), wherein the resin composition contains an acrylic emulsion.
The present invention (4) is
The foamed sheet according to the above-mentioned invention (3), characterized in that the above-mentioned 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
An electronic device comprising the foamed sheet according to any one of (1) to (6).

The present invention provides a thin foam sheet with high surface impact absorption performance, and an impact-resistant electronic device using the foam sheet.

FIG. 2 is a schematic diagram showing a drop type impact absorption test machine used in the evaluation of the present invention.

The following items will be explained in order below.

1. Foamed Sheet The foamed sheet of the present invention 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, and a compression recovery time of 6 to 85 seconds. Here, the compression recovery time is the time required to recover the thickness of the foamed sheet to 90% of the thickness before the compression load is applied after a weight having a diameter of 10 mm and a mass of 500 g is removed from the foamed sheet at 25° C. to apply a compressive load to the foamed sheet for 30 seconds.

The compression restoration time of the foam sheet is a value representing the viscoelastic properties of the foam sheet. Here, the foam sheet is mainly configured in a form in which air bubbles are contained in the cured resin. Therefore, the compression restoration time of the 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 air bubbles. The compression restoration time of the 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 air 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 the 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.

The foam sheet of the present invention has a thickness of 0.05 to 1.0 mm, and may be either a foam sheet that has been extruded to a desired thickness, or a foam sheet that has been cut to a desired thickness after molding.

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

The foam sheet of the present invention may have a modified layer, called a skin layer, on its surface.

The bubbles contained in the foamed sheet of the present invention are not particularly limited as long as they do not impair the effects of the present invention, and may be any of closed, open, and semi-open cells. Here, the semi-open cell structure means a structure in which the bubbles have small pores, unlike closed cells, and the pores between adjacent cells are smaller than those of an open cell structure, specifically, a structure having an air permeability of 2 to 80 ml/(cm ² /s) according to the Frazier type method of JIS L1096 A method. Since closed cells have gas trapped in them, they have strong elastic properties relative to foam sheets, and a foam sheet containing more closed cells becomes more elastic. Therefore, the compression recovery time of the foam sheet is short. Conversely, since open cells allow the gas in the cells to move freely, they have relatively weak elastic properties relative to foam sheets. Therefore, the compression recovery time of the foam sheet is long. Semi-open cells show an intermediate effect between closed cells and open cells.

The apparent density of the foamed sheet of the present invention is not particularly limited as long as the effect of the present invention is not impaired, and can be, for example, 0.10 to 0.70 g/ ^cm3 . The apparent density is measured according to JIS K7222:2005 "Foamed plastics and rubber - Determination of apparent density". When the apparent density of the foamed sheet is low, many bubbles are contained in the foamed sheet, so that the elastic properties of the bubbles have a stronger effect. Conversely, when the apparent density of the foamed sheet is high, the viscoelastic properties of the resin have a stronger effect. By adjusting the elastic properties of the bubbles and the viscoelastic properties of the resin, the compression restoration time of the foamed sheet can be adjusted.

The foam sheet according to the present invention can be formed on a substrate. In this way, it is possible to provide the foam sheet with strength. There are no particular limitations on the method for forming the foam sheet on the substrate, and examples include a method in which a foam is directly applied to the substrate, and a method in which an adhesive layer is provided and then bonded. Alternatively, a laminate in which the substrate, adhesive layer, and release liner are arranged in this order may be formed in advance, and a foam may be integrally molded on the surface of the substrate opposite to the side where the adhesive layer is present.

A release agent layer may be provided on the surface of the substrate on the side where the foam sheet is formed. In this way, the substrate can be used as a release liner.

Furthermore, a release agent layer may be provided on the surface of the substrate opposite the side on which the foam sheet is formed. In this way, the foam sheet can be wound into a roll, which saves space during transportation and storage and protects the foam sheet from damage.

The foamed sheet of the present invention can also have an adhesive layer on one or both sides of its surface. By providing an adhesive layer, it becomes easier to fix electronic/electrical components using the foamed sheet.

The foam sheet having an adhesive layer on one or both surfaces can further have a release liner on the surface of the adhesive layer. By providing a release liner, damage to the adhesive layer during transportation or before use can be prevented.

Here, as long as the foam sheet of the present invention contains the foam of the present invention, it may be laminated with foams other than the foam of the present invention, a substrate, an adhesive layer, and other known layers in a desired number and in a desired order depending on the application. Furthermore, although an embodiment in which a single layer of the foam of the present invention is provided as the foam is preferred, the foam sheet may have multiple foams of the present invention, such as substrate/foam of the present invention/adhesive layer/foam of the present invention/adhesive layer, etc.

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.

2-1. Resin Component 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 copolymers in which the constituent monomers of these resins are copolymerized can be used. Furthermore, emulsions of these resins and copolymers in which the constituent monomers of these resins are copolymerized 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, because it is easy to adjust the density and cell diameter by the gas mixing method. Furthermore, it is more preferable to use at least an acrylic emulsion among the emulsions. In addition, 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.

Acrylic emulsions Acrylic emulsions (water dispersions of acrylic resins) can be produced by copolymerizing, for example, a (meth)acrylic acid ester monomer as an essential polymerizable monomer component, and, if necessary, a mixture of 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 types of acrylic emulsions may be used in combination.

Polymerizable monomers that can be used in the preparation of the above 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 monomer 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. In addition, for the acrylic emulsion, other resin-based polymerizable monomers such as styrene-based polymerizable monomers (for example, the resin component contained in a styrene-acrylic emulsion) or copolymers obtained by copolymerizing vinyl acetate-based polymerizable monomers, ethylene, and acrylic-based polymerizable monomers (for example, the resin component of an EVA-acrylic emulsion) can be used. In addition, copolymers obtained by copolymerizing acrylic-based 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 foam sheet, and the composition ratio when copolymerizing these can also be selected in consideration of the viscoelastic properties. By selecting these, the compression recovery time of the foam sheet can be set within a predetermined range.

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, styrene-acrylonitrile, and ternary copolymer resins such as acrylonitrile-butadiene-styrene. 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.

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.

When using an emulsifier to prepare the acrylic emulsion, a known emulsifier may be used.

Urethane-Based Emulsion Examples of a method for preparing a urethane-based emulsion (aqueous dispersion of a urethane resin) include the following methods (I) to (III).
(I) A method of obtaining a urethane emulsion by mixing, as necessary, an aqueous solution containing a neutralizing agent with 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.
(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.

The polyisocyanates 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 as long as it 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 makes it possible to adjust the compression recovery time of the foamed sheet.

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. As the polyester polyol, a known one may be used. These can be selected in consideration of the viscoelastic properties of the resin contained in the foam sheet. The compression restoration time of the foam sheet can be adjusted by this selection.

In the above methods (I) to (III), an emulsifier may be further used within the scope of not impairing 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.

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. Examples of the water-soluble solvent include alcohols such as methyl alcohol, ethyl alcohol, isopropyl alcohol, ethyl carbitol, ethyl cellosolve, and butyl cellosolve, and polar solvents such as N-methylpyrrolidone, and a mixture of one or more of these may be used.

Among the resin components used in the present invention, the physical properties of the resin emulsion, which is a preferred embodiment, are described below.

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

The viscosity of the resin emulsion can be, for example, 100 to 15,000 mPa·s, and 2000 to 15,000 mPa·s is preferable. If the viscosity is within this range, a viscosity of 100 or more will provide sufficient bubble retention during molding, allowing finer cells to be molded. In this way, the bubble (cell) size and bubble shape of the foam sheet can be adjusted by the viscosity of the resin component, and these factors also affect the compression and recovery time of the foam sheet. If the viscosity is within this range, it will be easy to keep the compression and recovery time of the foam sheet within the specified range.

Glass transition temperature (°C): Tg _E
The glass transition temperature Tg _E of the resin emulsion is not particularly limited as long as it can adjust the glass transition temperature Tg _S of the foamed sheet described later to a predetermined range, and can be, for example, −60 to −20° C.
The glass transition temperature Tg _E can be determined as the temperature showing the peak value of tan δ measured using a dynamic viscoelasticity device (manufactured by Anton Paar: model MCR302) in accordance with JIS-K7198 under conditions of heating from −80° C. to 150° C. at a rate of 5° C./min and a frequency of 1 Hz.

2-2. Foaming agent (anionic surfactant)
The anionic surfactant (foaming anionic surfactant) functions as a foaming agent for the resin composition.

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.

Here, the anionic surfactant used in this embodiment preferably has an HLB of 10 or more, and more preferably 20 or more, in order to facilitate dispersion when the resin composition is made into a resin-based emulsion.

2-3. Amphoteric Surfactant 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.

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 penetrates between the anionic surfactant molecules, making the bubbles more stable and reducing the size of the bubbles. Therefore, by using an anionic surfactant in combination with an amphoteric surfactant, the bubble (cell) diameter and bubble density of the foam sheet can be further adjusted, and the compression recovery time of the foam sheet is affected.

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 other amphoteric surfactant. Betaine type amphoteric surfactants are preferred because they have a higher effect than the above-mentioned one. Furthermore, C10-12 surfactants are preferred because they can easily enter between the molecules of anionic surfactants.

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 is 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. Compounds with one "-R-COOH" are called mono-compounds, and compounds with two "-R-COOH" are called di-compounds. Either of these mono-compounds or di-compounds can be used as the amphoteric surfactant of the present invention. In N-alkyl or alkenyl amino acids, the alkyl or alkenyl group may be linear or branched. Specifically, 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.

Examples of betaine-type amphoteric surfactants include alkyl betaines, imidazolinium betaines, carbobetaines, amido carbobetaines, amido betaines, alkyl amido betaines, sulfobetaines, amido sulfobetaines, and phosphobetaines. Specifically, examples of betaine-type amphoteric surfactants include lauryl betaine, stearyl betaine, lauryl dimethylaminoacetate betaine, stearyl dimethylaminoacetate betaine, lauric acid amidopropyl dimethylaminoacetate betaine, isostearic acid amidoethyl dimethylaminoacetate betaine, isostearic acid amidopropyl dimethylaminoacetate betaine, isostearic acid amidoethyl diethylaminoacetate betaine, isostearic acid amidopropyl diethylaminoacetate betaine, isostearic acid amidoethyl dimethylaminohydroxysulfobetaine, isostearic acid amidopropyl dimethylaminohydroxysulfobetaine, isostearic acid amidoethyl diethylaminohydroxysulfobetaine, isostearic acid amidopropyl diethylaminohydroxysulfobetaine, 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.

Examples of amine oxide type amphoteric surfactants include lauryl dimethylamine N-oxide and oleyl dimethylamine N-oxide.

Of the above-mentioned amphoteric surfactants, it is preferable to use a betaine-type amphoteric surfactant in the manufacturing method of the foamed sheet according to the present invention, and among the betaine-type surfactants, alkyl betaines, imidazolinium betaines, and carbobetaines are particularly preferable. Examples of alkyl betaines that can be used in the present invention include stearyl betaine and lauryl betaine, and examples of imidazolinium betaines include 2-alkyl-N-carboxymethyl-N-hydroxyethyl imidazolinium betaine.

2-4. Crosslinking agent (curing agent)
By using a crosslinking agent (curing agent), it is possible to improve the strength of the foamed sheet according to the present embodiment and at the same time, it is possible to more elastically adjust the viscoelastic properties of the resin in the foamed sheet, thereby making it possible to adjust the compression recovery time of the foamed sheet.

Such crosslinking agents are not particularly limited, and may be added in the required amount depending on the application. Examples of crosslinking methods using crosslinking agents include physical crosslinking, ionic crosslinking, and chemical crosslinking, and the crosslinking method can be selected depending on the type of 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 depending on the type and amount of functional groups contained in the resin blend used. In order to improve the adhesive strength, tack strength, and interlayer peel strength, epoxy-based crosslinking agents and isocyanate-based crosslinking agents are preferred. Isocyanate-based and epoxy-based crosslinking agents can prevent material destruction of the adherend and the porous foam by increasing the material strength. Among them, aliphatic isocyanates are more preferred. Two or more of these crosslinking agents may be used in combination.

2-5. Other Additives The resin composition according to the present invention may contain other additives that are known to be 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 the effect of a foaming agent. Such surfactants may be appropriately selected depending on the water-dispersible resin to be selected.

3. Method for producing foamed 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 of forming a stable foamed sheet. For example, when an acrylic emulsion and a urethane emulsion are used in combination, the acrylic emulsion may be 60 to 95 parts by mass in terms of solid content, and the urethane emulsion may 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 superior surface impact performance.

The amount of anionic surfactant in the resin composition is preferably 1.0 to 10 parts by weight, and more preferably 3 to 10 parts by weight, based on the total amount of the resin emulsion (the total solid content being 100 parts by weight). By using an amount in this range, it is possible to obtain the effect of easily achieving appropriate foaming and forming a fine cell structure.

The amount of amphoteric surfactant in the resin composition is preferably 0.5 to 10 parts by weight, and 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 using an amount in this range, it is easy to achieve appropriate foaming and it is possible to form a fine cell structure.

The amount of crosslinking agent (curing agent) in the resin composition is preferably 0.5 to 10 parts by weight, and 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 using an amount in this range, a foam with small residual compression set can be formed.

3-2. Manufacturing of Foam Sheet The manufacturing method of the foam 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.

In the raw material preparation step, the raw materials described above are mixed to prepare a resin composition, which is a mixture of raw materials for the foam. The mixing method is not particularly limited, but may be, for example, mixing the components while stirring in a container such as a mixing tank.

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 these 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.

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. That is, 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 at 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.

3-2-4. Foaming method and foaming conditions As the foaming method according to the present invention, it is preferable to use the 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, it is possible to obtain a foamed sheet with a density suitable for various applications by adjusting the mixing ratio of the resin composition and the air. Although it is possible to use other foaming methods in combination, it is possible to increase the density by using a foaming method using a chemical foaming agent in combination, as the ratio of closed bubbles increases. By increasing the density, it is also possible to adjust the compression recovery time of the foam sheet.

The time for mixing the resin composition with air is not particularly limited, but is usually 1 to 10 minutes, and 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.

3-2-5. Formation of Foam Sheet The foamable composition (foamed emulsion composition) foamed as described above is formed into a foam sheet having a desired thickness by a known means such as a doctor knife or a doctor roll.

3-2-6. Curing The foam sheet can be cured by a known method. The foam according to the present embodiment can be self-crosslinked, but the foam sheet can also be cured by applying energy to crosslink the resin constituting the emulsion via a crosslinking agent. The step of applying energy is not particularly limited, but may be, for example, a heating step (thermal crosslinking).

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 can be used. The drying temperature and drying time are also not particularly limited, but for example, it can be about 80°C for about 1 to 3 minutes.

In addition, in this heating step, the dispersion medium evaporates from the foamable composition, and the path through which this steam escapes is connected from the inside to the outside of the foam sheet, forming open or semi-open cells. If the foaming gas mixed in during the stirring and foaming step remains as it is, the resulting foam sheet will have closed cells, and if the mixed foaming gas is connected when the steam escapes in this step, the resulting foam sheet will have open or semi-open cells. That is, in the present invention, some of the bubbles in the foam sheet are open, and the remaining bubbles are closed. The compression recovery time of the foam sheet can also be adjusted depending on the bubble structure.

When a crosslinking agent is added, the heating step advances and completes the crosslinking (hardening) reaction of the raw materials. Specifically, the raw materials are crosslinked with each other by the above-mentioned crosslinking agent, and a hardened foam sheet is formed. The heating means used in this step is not particularly limited as long as it can sufficiently heat the raw materials to crosslink (harden) them, but for example, a tunnel-type heating furnace can be used. The heating temperature and heating time may also be such that they can crosslink (harden) the raw materials, for example, at 80 to 150°C (especially, about 120°C is preferable) for about 1 to 3 minutes.

4. Characteristics of Foam Sheet: Compression Recovery Time The compression recovery time of the foam 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 foam sheet is within this range, it is possible to obtain a foam sheet with better surface impact performance. The compression recovery time of the foam sheet can be adjusted to a predetermined value by adjusting the viscoelastic properties of the resin contained in the foam sheet and the elastic properties of the bubbles contained in the foam sheet.
The compression recovery time is measured by applying a compressive load to a foamed sheet by placing a weight having a diameter of 10 mm and a mass of 500 g 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 is applied. For the measurement, an image of the foamed sheet during the compression recovery process is taken with a digital microscope, and the length measuring function is used to measure the time until the foamed sheet recovers to 90% of the thickness before the compression load is applied.

Glass transition temperature of foam: Tg _S
The glass transition temperature Tg _S 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 glass transition temperature Tg _{S is at least lower than the ambient air temperature in order to set the compression and recovery time of the foamed sheet within a predetermined range. That is, the upper limit of the glass transition temperature Tg S} of the foamed sheet can be set to -60 to 0°C. When the glass transition temperature Tg _S is within this range, the compression and recovery time of the foamed sheet can be easily adjusted.
The glass transition temperature Tg _S is measured using a dynamic viscoelasticity device (manufactured by Anton Paar: model MCR302) in the temperature range of -80°C to 150°C, with a temperature rise of 5°C/min and a frequency of 1 Hz. The glass transition temperature is determined as the temperature showing the peak value of tan δ.

Loss tangent (tan δ) of foam sheet
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.

- The 25% compressive stress of the foam sheet at 25°C is not particularly limited as long as it does not impair the effects of the present invention, but 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 foam sheet is within this range, a foam sheet with 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."

3. Uses and Methods of Using the Foam Sheet The foam 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. It is particularly useful for protecting thin, wide-area display components such as liquid crystal panels, organic EL panels, and touch panels.
In addition, the foamed sheet according to the present invention can be attached in advance to the electronic/electrical parts to be protected, thereby improving workability when assembling the electronic/electrical parts.

Next, the present invention will be described in more detail with reference to examples, but the present invention is not limited to these examples. Unless otherwise specified, "%" indicating the content means % by mass.

<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°C
Acrylic emulsion 2: ethylene vinyl acetate-acrylic-special ester copolymer, pH 8, solid content 60%, viscosity 5,000 mPa·s, Tg-50°C
Urethane emulsion 1: Polyether, polycarbonate type, pH 8, solid content 60%, viscosity 100 mPa·s, Tg -40°C
Anionic surfactant 1: Ammonium stearate, solid content 30%
Anionic surfactant 2: Sodium alkylsulfosuccinate, solid content 35%
Betaine-based amphoteric surfactant: Alkyl betaine, solid content 30%
Crosslinking agent: Hydrophobic HDI isocyanurate (functional group number 3.5) Solid content 100%

The raw materials described in each example and comparative example were mixed to produce the resin composition of each example and comparative example.

The resin compositions obtained in each Example and Comparative Example were mixed with an inert gas such as air or nitrogen gas, foamed by the mechanical froth method (foaming conditions: 100-1000 rpm), cast onto a PET release liner, and then heated (in an oven or drying furnace) to obtain foamed sheets for each Example and Comparative Example. The density of the foamed sheets for each Example and Comparative Example was adjusted by changing the amount of inert gas such as air or nitrogen gas injected, the rotation speed of the mixer, and the drying conditions.

<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.

Thickness The thickness was measured using a thickness gauge.

Density: Measured by calculating the weight per unit volume.

・25% compressive stress at 25 ° C. 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”.

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

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 processed to a size of φ50 mm on a sample stage, and further placing an acrylic plate with a thickness of 5 mm at the position where the impactor of the sample would come into contact. The measurement conditions were: air temperature 23°C, impactor weight 35.76 g, and impactor 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 impact absorption test was performed without placing the sample on the sample stage and only the acrylic plate, and f _a1 is the impact load when the impact absorption test was performed with the sample placed on the sample stage and a 5 mm thick acrylic plate placed at the position where the impactor of the sample contacts. The drop heights were 100 mm, 200 mm, and 300 mm. The impact load was measured by a sensor placed on the sample stage. The surface 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%.

Comprehensive evaluation was performed based on the following criteria. The results are shown in Table 1.
○: All surface impact absorption ratings are ○. ×: At least one surface impact absorption rating is ×.

1 Drop-type impact absorption tester 10 Impactor (steel ball)
11 Acrylic plate 12 Sample 13 Sample stand (SUS)
14 Sensor

Claims (5)

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 foamed sheet according to claim 1, wherein the apparent density of the foamed sheet is 0.10 to 0.70 g/ ^cm3 . 2. The foam sheet according to claim 1 , wherein the acrylic emulsion comprises an EVA-acrylic emulsion. 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. An electronic device comprising the foamed sheet according to any one of claims 1 to 4 .

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