JP7000310B2 - Shock absorbing sheet - Google Patents

Description

The present invention relates to a thin shock absorbing sheet, and more particularly to a shock absorbing sheet used in a display device of a portable electronic device.

In display devices used in various electronic devices such as personal computers, mobile phones, and electronic paper, the space between the glass plate and the display unit that make up the surface of the device, the housing body and the display unit to which the display unit is attached, etc. A shock absorbing material for absorbing shock and vibration is provided between them. On the other hand, electronic devices provided with display devices, particularly portable electronic devices, are required to be thin due to space restrictions, and accordingly, the shock absorbing material is also required to be in the form of a thin sheet.
It is widely known that such a thin shock absorber is formed of a foam made of a polyolefin resin typified by polyethylene, and the foam made of these polyolefin resins has a constant shape of bubbles. It is considered that the impact absorption performance is improved by controlling the flexibility by controlling the one (see, for example, Patent Document 1).

Japanese Unexamined Patent Publication No. 2014-214205

However, when the impact absorbing material is a thin foam, it may not be possible to sufficiently improve the impact absorbing performance simply by controlling the shape of the bubbles to a constant shape and controlling the flexibility. For example, the glass constituting the surface of the display device may be damaged when a relatively large impact force of about several tens to 100 MPa is locally applied, but the flexibility of the foam sheet made of a polyolefin resin is controlled. However, it is difficult to sufficiently alleviate such impact force.

The present invention has been made in view of the above problems, and the subject of the present invention is that it has excellent impact absorption performance even when it is thin, and in particular, it absorbs a relatively large impact force applied locally. It is to provide a shock absorbing sheet capable of improving performance.

As a result of diligent studies to solve the above problems, the present inventors have found that the above problems can be solved by adjusting the storage elastic modulus, tan δ, and density of the impact absorbing sheet, and completed the present invention. .. That is, the present invention provides the following (1) to (11).
(1) Impact absorbing sheet having a storage elastic modulus of 1.0 × 10 ⁵ Pa or more and 2.5 × 10 ⁷ Pa or less at 23 ° C, a tan δ of 0.3 or more at 23 ° C, and a density of 600 kg / m ³ or more. ..
(2) The shock absorbing sheet according to (1) above, which has a thickness of 0.02 mm or more and 0.9 mm or less.
(3) The impact absorbing sheet according to (1) or (2) above, which contains a solid resin (A) and a liquid rubber (B).
(4) The shock absorbing sheet according to (3) above, wherein the resin (A) is a styrene-based thermoplastic elastomer.
(5) The shock absorbing sheet according to (3) or (4) above, further containing the process oil (C).
(6) The above (3) to (5), wherein the total content of the liquid rubber (B) and the process oil (C) is 10 parts by mass or more and 130 parts by mass or less with respect to 100 parts by mass of the resin (A). The shock absorbing sheet according to any one item.
(7) The shock absorbing sheet according to any one of (1) to (6) above, which has a density of 1200 kg / m ³ or less.
(8) The shock absorbing sheet according to any one of (1) to (7) above, wherein the shock absorbing rate is 50% or more.
(9) The shock absorbing sheet according to any one of (1) to (8) above, which is a foam.
(10) The shock absorbing sheet according to any one of (1) to (9) above, which is used for electronic devices.
(11) The shock absorbing sheet according to any one of (1) to (10) above, which is arranged on the back side of the display device.

According to the present invention, it is possible to provide a shock absorbing sheet having excellent shock absorbing performance, for example, even if it is thin, which improves the absorbing performance against a relatively large impact force applied locally.

Hereinafter, the present invention will be described in more detail using embodiments.
[Shock absorption sheet]
The impact absorbing sheet of the present invention has a storage elastic modulus of 1.0 × 10 ⁵ Pa or more and 2.5 × 10 ⁷ Pa or less, a tan δ of 0.3 or more, and a density of 600 kg / m ³ or more. In the present specification, the storage elastic modulus and tan δ mean the storage elastic modulus at 23 ° C. and tan δ at 23 ° C., respectively, unless otherwise specified.
The impact absorbing sheet of the present invention can improve the impact absorbing performance even if the sheet itself is thin and a relatively large impact force is locally applied to the resin sheet. Although the principle is not clear, when the storage elastic modulus is 2.5 × 10 ⁷ Pa or less and the tan δ is 0.3 or more, the point load is easily converted into the surface load and the energy loss becomes large. The density is as high as 600 kg / m ³ or more and the storage elastic modulus is 1.0 × 10 ⁵ Pa or more, so that the shock wave easily propagates along the surface direction of the resin sheet, so that sufficient shock absorption is sufficient. It is presumed to exhibit performance.

If the storage elastic modulus of the shock absorbing sheet is out of the above range, the shock absorbing performance cannot be sufficiently improved. The storage elastic modulus of the shock absorbing sheet is preferably 2.0 × 10 ⁵ Pa or more and 2.0 × 10 ⁷ Pa or less, and more preferably 4.0 × 10 ⁵ Pa or more from the viewpoint of improving the shock absorbing performance. It is 0 × 10 ⁷ Pa or less.
Further, when tan δ is less than 0.30, the energy loss cannot be increased and the shock absorption performance cannot be sufficiently improved. From the viewpoint of increasing the energy loss and further improving the shock absorption performance, the tan δ is preferably 0.33 or more, and more preferably 0.36 or more. The upper limit of tan δ is not particularly limited, but is preferably 1.5 or less, more preferably 1.2 or less, still more preferably 0.9 or less, in order to facilitate the storage elastic modulus in the above range.
Furthermore, if the density of the shock absorbing sheet is less than 600 kg / m ³ , when a shock is applied to the shock absorbing sheet, the shock cannot be absorbed by the shock absorbing sheet and propagates to the member on the back side. It may occur. The density of the shock absorbing sheet is preferably 650 kg / m ³ or more, more preferably 700 kg / m ³ or more, and further preferably more than 700 kg / m ³ from the viewpoint of further improving the shock absorbing performance. .. The upper limit of the density of the shock absorbing sheet is not particularly limited, but is usually 1200 kg / m ³ or less due to the characteristics of the resin used for the shock absorbing sheet.

The peak temperature of tan δ is preferably less than 15 ° C., more preferably −60 ° C. or higher and lower than 15 ° C., and even more preferably −50 ° C. or higher and lower than 15 ° C. from the viewpoint of improving the shock absorption performance. As will be described later, the peak temperature can be within the above range by using a styrene-based thermoplastic elastomer or the like.

The thickness of the shock absorbing sheet is preferably 0.02 mm or more and 0.9 mm or less. By setting the thickness of the shock absorbing sheet to 0.02 mm or more, it is possible to prevent so-called bottoming from occurring when a shock is applied to the shock absorbing sheet. Further, by making it 0.9 mm or less, it becomes possible to contribute to thinning and miniaturization of electronic devices. Further, in the present invention, even a thin impact absorbing sheet having a thickness of 0.9 mm or less can be provided with sufficient impact resistance. From these viewpoints, the thickness of the shock absorbing sheet is more preferably 0.02 mm or more and 0.5 mm or less, and further preferably 0.05 mm or more and 0.2 mm or less.

The shock absorbing sheet may be a non-foaming material or a foaming material as long as the density is within the above range. Further, the foam may have closed cells, may have open cells, or may have both closed cells and open cells, but mainly has closed cells. It is preferable that it is a thing. Specifically, the closed cell ratio of the shock absorbing sheet is preferably 60% or more and 100% or less, more preferably 70% or more and 100% or less, and further preferably 80% or more and 100% or less. The closed cell ratio can be obtained, for example, in accordance with JIS K7138 (2006). Further, the shock absorbing sheet of the present invention has a relatively high density as described above, and therefore, even if it is a foam, the foaming ratio is low and the foaming is slightly foamed.

The shock absorbing sheet may be a crosslinked body or a non-crosslinked body. However, when the shock absorbing sheet is a foam, it is preferably a crosslinked body. When the shock absorbing sheet is a crosslinked body, the gel fraction indicating the degree of crosslinking of the shock absorbing sheet is preferably 10% by mass or more and 70% by mass or less, and more preferably 20% by mass or more and 60% by mass or less. The gel fraction is measured by the following method.
A test piece (mass A (mg) of the test piece) is cut out from the shock absorbing sheet so that the mass becomes about 50 mg, the test piece is immersed in 30 cm ³ of xylene at 105 ° C., left for 24 hours, and then 200 mesh is used. The insoluble matter on the wire mesh is collected by filtering with the wire mesh of the above, vacuum dried, and the mass B (mg) of the insoluble matter is precisely weighed. From the obtained value, it is calculated by the following formula.
Degree of cross-linking (% by mass) = (B / A) x 100

The shock absorbing sheet of the present invention preferably has a shock absorbing rate of 50% or more. The impact absorption rate is measured by the method described in Examples described later. By setting the impact absorption rate to 50% or more, it becomes possible to increase the impact absorption against local impact, and for example, it becomes easy to prevent breakage of glass and display defects in a flexible display.

The shock absorbing sheet is formed by adding various additives described later to the resin component as needed. It is preferable that the resin component constituting the shock absorbing sheet contains an elastomer. The impact absorbing sheet is not limited as long as it can obtain the above various characteristics, but it is preferable to use the resin (A) and the liquid rubber (B) shown below as the resin component, and the resin (A) is the elastomer (A1). ) Is more preferable.

[Resin (A)]
As the resin (A) such as the elastomer (A1), a solid resin (A) that does not show fluidity under the conditions of 23 ° C. and 1 atm (1.01 × 10 ^-1 MPa) is used. As the resin (A), for example, a resin having a storage elastic modulus higher than that of the impact absorbing sheet is used. Specifically, the resin (A) such as the elastomer (A1) preferably has a storage elastic modulus of 5.0 × 10 ⁵ Pa or more and 1.0 × 10 ⁸ Pa or less, and more preferably storage elasticity. Those with a rate of 1.0 × 10 ⁶ Pa or more and 5.0 × 10 ⁷ Pa or less are used. The storage elastic modulus was measured when the resin (A) was made into a sheet having a thickness of 150 μm, and the details of the measuring method are as shown in Examples described later.
The MFR (230 ° C., 21.2N) of the resin (A) is preferably 2 g / 10 min or more and 30 g / 10 min or less, more preferably 4 g / 10 min or more and 20 g / 10 min or less, and 6 g / 10 min or more and 15 g. It is more preferably / 10 min or less. The MFR was measured in accordance with JIS K 7210.
The content of the resin (A) is preferably 30% by mass or more and 90% by mass or less, more preferably 35% by mass or more and 80% by mass or less, still more preferably 40% by mass or more and 70% by mass or less, based on the total amount of the shock absorbing sheet. ..
As the resin (A), styrene-based thermoplastic elastomers, olefin-based elastomers, vinyl chloride-based elastomers, urethane-based elastomers, polyvinyl butyral resins, polyvinyl alcohol resins and the like can be used, but styrene-based thermoplastic elastomers are particularly preferable.

(Styrene-based thermoplastic elastomer)
Examples of the styrene-based thermoplastic elastomer include a styrene-conjugated diene copolymer and a hydrogenated product thereof. The styrene-based thermoplastic elastomer may be, for example, a random copolymer or a block copolymer in which each block constitutes either a hard segment or a soft segment, but the block copolymer may be used. preferable. The hard segment or the soft segment may be composed of two or more components randomly copolymerized.
Examples of the conjugated diene include isoprene and butadiene. Further, in the styrene-conjugated diene copolymer, the styrene content is preferably 5% by mass or more and 30% by mass or less, and more preferably 5% by mass or more and 20% by mass or less.

The styrene-conjugated diene copolymer has a polystyrene block as a hard segment (X) at the molecular end, and a small amount of styrene is randomly copolymerized with the conjugated diene polymer block as a soft segment (Y) or the conjugated diene. Examples include those having a styrene-conjugated diene random copolymer block. The small amount of styrene is, for example, 20 mol% or less of the conjugated diene in the soft segment. As the structures that can be adopted by these, for example, the following can be exemplified.
(XY) m, (XY) nX, (XY) pZ However, in the formula, m and n indicate an integer of 1 or more, p indicates an integer of 2 or more and 6 or less, and X. Is a hard segment, Y is a soft segment, and Z is a coupling agent residue.
Among these, the styrene-conjugated diene copolymer preferably has an XYX structure.

As a preferable specific example of the styrene-conjugated diene copolymer, the styrene content in the copolymer is 5% by mass or more and 20% by mass or less, and the styrene content constituting the hard segment (X) is the same weight. Examples thereof include those having an amount of 3% by mass or more and 15% by mass or less with respect to the entire coalescence. Here, when styrene is copolymerized in the soft segment, the styrene content in the segment is preferably 10% by mass or less with respect to the entire copolymer. Further, such a styrene-conjugated diene copolymer is preferably a hydrogenated agent. Further, the conjugated diene is preferably butadiene. Therefore, such a styrene-conjugated diene copolymer may be a styrene-ethylene-butylene-styrene block copolymer (SEBS).
Specific examples of the hydrogenated styrene-conjugated diene elastomer described above include the trade names "Dynaron 1320P" and "Dynaron 1321P" manufactured by JSR Corporation.

Further, another preferable specific example of the styrene-conjugated diene copolymer is a styrene-isoprene-styrene polymer (SIS) in which the blocks at both ends are made of polystyrene and the intermediate block is made of isoprene.
The above-mentioned resin (A) may be used alone or in combination of two or more.

[Liquid rubber (B)]
The shock absorbing sheet preferably contains a liquid rubber (B) in addition to the resin (A), and more preferably a styrene-based thermoplastic elastomer and the liquid rubber (B) are used in combination. The liquid rubber (B) has fluidity under the conditions of 20 ° C. and 1 atm (1.01 × 10 ^-1 MPa) and becomes liquid. By mixing the liquid rubber (B) with the resin (A) and using it, it is possible to sufficiently improve the tan δ of the impact absorbing sheet and also reduce the storage elastic modulus. The principle is not clear, but it is possible that the component (A) and the component (B) are pseudo-double network gels.
The liquid rubber (B) has a viscosity at 38 ° C. of preferably 5 Pa · s or more and 1500 Pa · s or less, and more preferably 10 Pa · s or more and 1300 Pa · s or less. In the present specification, the viscosity is a value measured at a rotation speed of 100 rpm with a B-type rotational viscometer.

Examples of the liquid rubber (B) include liquid acrylonitrile-butadiene rubber, liquid acrylonitrile-butadiene rubber, liquid carboxylated acrylonitrile-butadiene rubber, liquid acrylonitrile-butadiene-isoprene rubber, and liquid acrylonitrile-isoprene. Liquid acrylonitrile rubber such as rubber and liquid acrylonitrile rubber such as liquid ternary copolymer of acrylonitrile, butadiene and functional monomer having anti-aging function, liquid chloroprene rubber, liquid isoprene rubber, liquid butyl rubber, liquid butadiene rubber, liquid ethylene-propylene -Diene rubber, liquid ethylene-propylene rubber, liquid natural rubber and the like. These may be used individually by 1 type and may be used in combination of 2 or more type.
As the liquid rubber (B), when the resin (A) is a styrene-based thermoplastic elastomer, it is preferable to use the liquid isoprene rubber from the viewpoints of compatibility, storage elastic modulus, tan δ and the like.

[Procell oil (C)]
The shock absorbing sheet may contain procel oil (C) in addition to the above resin (A) and liquid rubber (B). By mixing the components (A) and (B) with the procel oil (C), the storage elastic modulus of the impact absorbing sheet can be set to an appropriate value, and the moldability and the like can be improved. Examples of the procel oil (C) include mineral oils such as paraffin oil and vegetable oils, and among these, paraffin oil is preferable. When procel oil is used, for example, an oil-expanded elastomer obtained by preliminarily adding procel oil to a resin (A) such as an elastomer (A1) may be used.
The procel oil (C) preferably has a viscosity at 40 ° C. of 0.01 Pa · s or more and 1.0 Pa · s or less, and more preferably 0.02 Pa · s or more and 0.5 Pa · s or less.
The process oil (C) may be used alone or in combination of two or more.

In the shock absorbing sheet, the total content of the liquid rubber (B) and the procel oil (C) is preferably 10 parts by mass or more and 130 parts by mass or less with respect to 100 parts by mass of the resin (A) described above. It is more preferably 40 parts by mass or more and 120 parts by mass or less, and further preferably 70 parts by mass or more and 110 parts by mass or less. By setting the total content within these ranges, it becomes easy to adjust the storage elastic modulus and the value of tan δ within the above ranges. When the contents of the components (B) and (C) are within the above range, the shock absorbing sheet may or may not contain the component (C).
When the shock absorbing sheet contains process oil, the ratio (C / B) of the content of procell oil (C) to the content of liquid rubber (B) is preferably 0.1 or more and 1.5 or less. It is more preferably 0.3 or more and 1.3 or less, and further preferably 0.5 or more and 1.1 or less.

[Antioxidant]
The shock absorbing sheet preferably contains an antioxidant. Examples of the antioxidant include a phenol-based antioxidant, a sulfur-based antioxidant, a phosphorus-based antioxidant, an amine-based antioxidant, and the like.
For example, examples of the phenolic antioxidant include 2,6-di-tert-butyl-p-cresol, n-octadecyl-3- (3,5-di-tert-butyl-4-hydroxyphenyl) propionate, 2-. tert-Butyl-6- (3-tert-butyl-2-hydroxy-5-methylbenzyl) -4-methylphenylacrylate, tetrakis [methylene-3- (3,5-di-tert-butyl-4-hydroxyphenyl) ) Propionate] methane and the like.
Examples of the sulfur-based antioxidant include dilaurylthiodipropionate, dimyristylthiodipropionate, disstearylthiodipropionate, pentaerythrityltetrakis (3-laurylthiopropionate) and the like. ..
The antioxidant may be used alone or in combination of two or more.
The antioxidant is preferably 0.1 part by mass or more and 2.0 part by mass or less with respect to 100 parts by mass in total of the above-mentioned resin (A), liquid rubber (B), and procel oil (C). , 0.2 parts by mass or more and 1.5 parts by mass or less is more preferable.

[Other additives]
In addition to antioxidants, shock absorbing sheets include metal damage inhibitors, antistatic agents, stabilizers, nucleating agents, cross-linking agents, cross-linking aids, pigments, halogen-based and phosphorus-based flame retardants, and fillers. Other additives such as the above may be contained in a range that does not impair the object of the present invention.

[Manufacturing method of shock absorbing sheet]
Hereinafter, a method for manufacturing a shock absorbing sheet will be described. As described above, the shock absorbing sheet of the present invention is a mixture of a resin (A), a liquid rubber (B), a process oil (C) to be blended as necessary, an antioxidant, and other additives. Then, it can be obtained by molding into a sheet.
Specifically, each component can be supplied to an extruder such as a single-screw extruder or a twin-screw extruder, melted and kneaded, and extruded by extrusion molding or the like to obtain a sheet-shaped resin composition. Although it is possible, the sheet-shaped resin composition can be used as a shock absorbing sheet.

In addition, the resin composition may be crosslinked and / or foamed as appropriate. For foaming of the resin composition, for example, a pyrolyzable foaming agent is mixed with each of the above components in the resin composition, and the sheet-shaped resin composition is heated above the decomposition temperature of the pyrolyzable foaming agent to heat the resin composition. It is preferable to foam the sheet-shaped resin composition by thermally decomposing the decomposition-type foaming agent.
As the thermal decomposition type foaming agent, one having a decomposition temperature higher than the melting temperature of the resin can be used. For example, an organic or inorganic chemical foaming agent having a decomposition temperature of 160 ° C. or higher and 270 ° C. or lower can be used.
Examples of the organic foaming agent include azodicarbonamides, azodicarboxylic acid metal salts (azodicarboxylic acid barium and the like), azo compounds such as azobisisobutyronitrile, and nitroso compounds such as N, N'-dinitrosopentamethylenetetramine. Examples thereof include hydrazodicarbonamides, 4,4'-oxybis (benzenesulfonyl hydrazide), hydrazine derivatives such as toluenesulfonyl hydrazide, and semicarbazide compounds such as toluenesulfonyl semicarbazide.
Examples of the inorganic foaming agent include ammonium acid, sodium carbonate, ammonium hydrogencarbonate, sodium hydrogencarbonate, ammonium nitrite, sodium boron hydride, anhydrous monosoda citrate and the like.
Among these, azo compounds and nitroso compounds are preferable from the viewpoint of obtaining fine bubbles, and from the viewpoint of economy and safety, azodicarbonamide, azobisisobutyronitrile, N, N'-dinitrosopentamethylene. Tetramine is more preferred, and azodicarbonamide is even more preferred.
These pyrolyzable foaming agents can be used alone or in combination of two or more.
Even when the shock absorbing sheet foams, it becomes slightly foamed as described above. Therefore, it is preferable that the amount of the pyrolytic foaming agent compounded is small. Specifically, the blending amount of the pyrolyzable foaming agent in the resin composition is preferably 0.1 part by mass or more and 5 parts by mass or less with respect to 100 parts by mass in total of the above components (A) to (C), and is 0. .3 parts by mass or more and 3.0 parts by mass or less are more preferable.

Further, as a material for lowering the decomposition temperature of the thermally decomposable foaming agent or accelerating or adjusting the decomposition rate, for example, a decomposition temperature adjusting agent such as zinc oxide, zinc stearate, or urea may be blended into the resin composition. good. The decomposition temperature control agent is used, for example, in an amount of 0.01 part by mass or more and 5 parts by mass or less with respect to 100 parts by mass of the above components (A) to (C) in order to adjust the surface condition of the heating equipment and the foam. can do.

Further, the foaming may be performed by a method other than the above-mentioned thermal decomposition type foaming agent, and may be performed by, for example, physical foaming. In this case, instead of using the above-mentioned pyrolysis foaming agent, it is preferable to impregnate the resin composition with a gas for forming bubbles. As the bubble forming gas, it is preferable to use a high-pressure inert gas. Examples of the inert gas include carbon dioxide, nitrogen gas, air, butane gas and the like, and carbon dioxide is preferable. These gases may be used alone or in combination of two or more. The inert gas is preferably impregnated in the resin composition in a supercritical state or a subcritical state.

Further, the foaming in the present invention is not limited to the above-mentioned chemical foaming and physical foaming, and includes those performed by other means for containing air bubbles in the shock absorbing sheet. For example, hollow particles may be contained in the shock absorbing sheet.
Examples of the hollow particles include inorganic hollow particles and organic hollow particles. Examples of the inorganic hollow particles include alumina bubbles, glass microballoons, shirasu balloons, fly ash balloons, canamite, obsidian pearlite, diamond balloons, and inorganic microcapsules. On the other hand, the organic hollow particles include phenol resin microballoons, polyvinylidene chloride resin hollow particles such as Saran Microsphere (trade name: manufactured by Dow), epoxy resin hollow particles such as ecosphere, and crecus fairy. Other examples include hollow carbon particles such as carbon spheres.

Further, the cross-linking of the resin composition may be performed when the shock absorbing sheet is a non-foaming material or a foaming material, but it is preferably performed when the foaming material is used. Cross-linking of the resin composition may be performed on the resin composition molded into a sheet shape, but in the case of a foam, it may be performed on the sheet-shaped resin composition before foaming.
Crosslinking is preferably carried out by irradiating the resin composition formed into a sheet with ionizing radiation. Examples of the ionizing radiation include α-rays, β-rays, γ-rays, electron beams and the like, but electron beams are more preferable. The irradiation amount of ionizing radiation is preferably 0.5 Mrad or more and 10 Mrad or less, and more preferably 1 Mrad or more and 8 Mrad or less.

The cross-linking may be carried out by a method in which a cross-linking agent such as an organic peroxide is mixed in advance with the resin composition and the resin composition is heated to decompose the organic peroxide. Examples of such an organic peroxide include 1,1-bis (t-butylperoxy) 3,3,5-trimethylcyclohexane, 1,1-bis (t-butylperoxy) cyclohexane and the like. ..

Further, the impact absorbing sheet may be appropriately stretched. When the shock absorbing sheet is a foam, the stretching may be performed while foaming the resin composition, or may be performed after foaming the resin composition. When the foam is stretched after the resin sheet is foamed to obtain a foam, the foam may be continuously stretched while maintaining the molten state at the time of foaming without cooling the foam. After cooling the foam, the foam may be heated again to be in a melted or softened state, and then the foam may be stretched. Stretching may be performed in one or both of the MD direction and the TD direction.
Further, even when the impact absorbing sheet is a non-foaming material, stretching may be performed on the sheet-shaped resin composition, but in the case of cross-linking, it may be performed after cross-linking.

[How to use the shock absorbing sheet]
The shock absorbing sheet of the present invention is used for, for example, various electronic devices, preferably portable electronic devices such as notebook personal computers, mobile phones, electronic papers, digital cameras, and video cameras. More specifically, it is used as a shock absorbing sheet for a display device provided in these electronic devices. Examples of the display device include an organic EL display device, a liquid crystal display device, and the like, but an organic EL display device is preferable.
Further, the display device, particularly the organic EL display device, is preferably a flexible display. The organic EL display device is flexible by forming an organic EL element having both electrodes, a light emitting layer formed between the two electrodes, and a sealing material for sealing the light emitting layer on a film substrate. It can be a display.

When used in a display device, the shock absorbing sheet is arranged on the back side of various display devices to absorb the impact applied to the display device. More specifically, the shock absorbing sheet is placed, for example, on the housing of an electronic device and is arranged between the housing and the display device. Further, the shock absorbing sheet is usually compressed and arranged between a component constituting an electronic device such as a housing and a display device.
Since the shock absorbing sheet of the present invention has high shock absorbing performance even if it is thin, it is possible to appropriately prevent damage to the display device while making the electronic device thinner. In addition, the shock absorbing sheet can absorb the shock appropriately even when a relatively large impact is applied locally, so that the glass may be damaged or display defects that occur in flexible displays may occur. It will be possible to prevent it appropriately.

If necessary, the shock absorbing sheet may be used by laminating a resin sheet on one surface and both sides thereof as appropriate. Examples of the resin used for the resin sheet include polyolefin resins such as polyethylene and polypropylene, and thermoplastic resins such as polyethylene terephthalate resin. Each resin sheet is preferably thinner than the shock absorbing sheet, and has a thickness of, for example, 10 μm or more and 300 μm or less, preferably 10 μm or more and 200 μm or less. The resin sheet may be adhered to the impact absorbing sheet by thermocompression bonding, or may be adhered to the impact absorbing sheet by using an adhesive or the like.

Further, the shock absorbing sheet may be used as an adhesive tape by providing an adhesive material on one side or both sides. By using an adhesive tape, the shock absorbing sheet can be adhered to parts such as a housing of an electronic device via an adhesive material.
The adhesive material may be at least provided with an adhesive layer, may be composed of a single adhesive layer laminated on the surface of the shock absorbing sheet, or may be double-sided adhesive attached to the surface of the shock absorbing sheet. It may be a sheet, but it is preferably a single adhesive layer. The double-sided pressure-sensitive adhesive sheet includes a base material and a pressure-sensitive adhesive layer provided on both sides of the base material. The double-sided pressure-sensitive adhesive sheet is used to bond one pressure-sensitive adhesive layer to a shock-absorbing sheet and to bond the other pressure-sensitive adhesive layer to another component.
The pressure-sensitive adhesive constituting the pressure-sensitive adhesive layer is not particularly limited, and for example, an acrylic pressure-sensitive adhesive, a urethane-based pressure-sensitive adhesive, a rubber-based pressure-sensitive adhesive, or the like can be used. The thickness of the adhesive material is preferably 5 μm or more and 200 μm or less, and more preferably 7 μm or more and 150 μm or less. Further, a release sheet such as a release paper may be further attached on the adhesive material to protect the adhesive layer with the release paper before use.

The present invention will be described in more detail by way of examples, but the present invention is not limited to these examples.

[Evaluation methods]
In the present invention, each physical property and performance of the shock absorbing sheet are evaluated by the following methods.
<Storage modulus and tanδ>
Measuring device: Using DVA-200 (manufactured by IT Measurement Control Co., Ltd.), tensile mode: 10 Hz, strain amount: 0.1%, temperature range: -100 ° C or higher and 100 ° C or lower, temperature rise rate: 10 ° C / Under the condition of min, the storage elastic modulus and tan δ at 23 ° C. were determined. The sample size was 40 mm in length (however, the distance between grips was 25 mm) and 5 mm in width.
<Thickness>
The thickness was measured with a dial gauge.
<Density>
The density of the shock absorbing sheet is a value of the apparent density measured according to JIS K6767.
<Impact absorption test>
A shock absorbing sheet (50 mm square) was placed in the center of an acrylic plate (100 mm square, thickness 10 mm), and an acceleration sensor was attached to the surface opposite to the surface of the acrylic plate on which the shock absorbing sheet was placed. In the acrylic plate, the four corners are fixed to the pedestal with bolts having a length of 35 mm, and the upper surface of the acrylic plate is held so as to be at a position 25 mm from the pedestal surface.
An iron ball of 13.8 g (diameter 15 mm) was dropped from a height of 100 mm with respect to the center position of the shock absorbing sheet, and the acceleration when colliding with the shock absorbing sheet was measured. Further, the same iron ball drop and acceleration measurement were repeated 19 times without replacing the shock absorbing sheet, and the average value of the accelerations for all 20 times was taken as the acceleration (L1). Further, the average value of the accelerations for all 20 times of the same iron ball drop and acceleration measurement performed without placing the shock absorbing sheet on the acrylic plate is defined as the acceleration (L0), and the obtained accelerations (L1) and accelerations (L0) are taken. ), The impact absorption rate was calculated by the following formula.
Impact absorption rate (%) = (L0-L1) / L0 × 100

The components used in the examples and comparative examples are as follows.
Styrene-based thermoplastic elastomer (TPS1): Trade name "Dynaron 1321P", manufactured by JSR Corporation, MFR (230 ° C, 21.2N) 10g / 10min, storage elastic modulus at 23 ° C 1.7 × 10 ⁶ Pa, 23 ° C Tan δ = 0.11, styrene content 10% by mass
Liquid isoprene rubber (IR): Product name "Claplen LIR-290", manufactured by Kuraray Co., Ltd., viscosity at 38 ° C: 1200 Pa · s
Process oil (PO): Product name "Diana Process Oil PW-32", manufactured by Idemitsu Kosan Co., Ltd., viscosity at 40 ° C: 0.026 Pa · s
Polyethylene-based thermoplastic elastomer (TPS2): trade name "Arneston JS20N", styrene-based thermoplastic elastomer (TPS3) manufactured by Kuraray Plastics Co., Ltd .: trade name "Arneston CJ103", polyethylene-based resin manufactured by Kuraray Plastics Co., Ltd. PE): Product name "Affinity PL1850G", Polyvinyl butyral resin (PVB) manufactured by The Dow Chemical Company: Product name "Eslek Clear Film", Antioxidant (phenolic antioxidant) manufactured by Sekisui Chemical Co., Ltd .: Product name "Adecastab AO-60", heat-decomposable foaming agent (azodicarboxylic amide) manufactured by ADEKA Co., Ltd .: Product name "Vinihole AC # 3", manufactured by Eiwa Kasei Co., Ltd.

[Example 1]
50 parts by mass of styrene-based thermoplastic elastomer (TPS1) as resin (A), 50 parts by mass of liquid isoprene rubber (IR) as liquid rubber (B), and 1 part by mass of antioxidant are kneaded at 160 ° C. The resin composition formed into a sheet having a thickness of 0.15 mm was used as a shock absorbing sheet.

[Example 2]
50 parts by mass of styrene-based thermoplastic elastomer (TPS1) as resin (A), 25 parts by mass of liquid isoprene rubber (IR) as liquid rubber (B), and paraffin oil (PO) 25 as process oil (C). A mass part and 1 part by mass of the antioxidant were kneaded at 160 ° C. to form a resin composition formed into a sheet having a thickness of 0.15 mm as a shock absorbing sheet.

[Example 3]
The same procedure as in Example 1 was carried out except that both sides of the sheet-shaped resin composition were crosslinked by irradiating both sides with an electron beam having an acceleration voltage of 800 kV by 4.0 rad.
[Example 4]
The same procedure as in Example 2 was carried out except that both sides of the sheet-shaped resin composition were crosslinked by irradiating both sides with an electron beam having an acceleration voltage of 800 kV by 4.0 rad.

[Example 5]
In addition to the above components (A) and (B) and the antioxidant, the sheet-like form is the same as in Example 3 except that 1 part by mass of azodicarbonamide (ADCA) as a pyrolytic foaming agent is added. The resin composition was molded and then crosslinked. Then, the crosslinked sheet-shaped resin composition is passed through a heating furnace at 250 ° C. to be foamed and stretched in the MD and TD directions to obtain a crosslinked foam having a thickness of 0.15 mm, and the crosslinked foam is obtained. Was used as a shock absorbing sheet.
[Example 6]
Same as Example 4 except that 1 part by mass of azodicarbonamide (ADCA) as a pyrolytic foaming agent is added in addition to the above components (A), (B) and (C) and an antioxidant. , A sheet-shaped resin composition was formed, and then crosslinked. Then, the crosslinked sheet-shaped resin composition is passed through a heating furnace at 250 ° C. to be foamed and stretched in the MD and TD directions to obtain a crosslinked foam having a thickness of 0.15 mm, and the crosslinked foam is obtained. Was used as a shock absorbing sheet.
[Example 7]
The procedure was carried out in the same manner as in Example 5 except that 0.8 parts by mass of azodicarbonamide (ADCA) as a pyrolysis foaming agent was added.

[Comparative Examples 1 to 7, 9]
A resin sheet obtained by kneading each component so as to have the composition shown in Table 1 and forming a sheet having a thickness of 0.15 mm was used as a shock absorbing sheet.
[Comparative Example 8]
The same procedure as in Example 5 was carried out except that the blending amount was changed as shown in Table 1.
[Comparative Examples 10 to 12]
Each component was kneaded so as to have the composition shown in Table 1, and both sides of a resin sheet formed into a sheet having a thickness of 0.15 mm were crosslinked by irradiating both sides with an electron beam having an acceleration voltage of 800 kV by 4.0 Mrad. The crosslinked resin sheet was used as a shock absorbing sheet.
[Comparative Example 13]
The procedure was carried out in the same manner as in Example 5 except that the blending amount of azodicarbonamide (ADCA) as a pyrolysis foaming agent was 2 parts by mass.

As is clear from the above examples, the impact absorption rate could be as high as 50% or more by setting the storage elastic modulus, tan δ, and density within the predetermined ranges. On the other hand, as shown in each comparative example, when any one of the storage elastic modulus, tan δ, and density is not within the predetermined range, the impact absorption rate is less than 50%, and the impact absorption performance is not sufficient. rice field.

Claims (8)

It contains a styrene-based thermoplastic elastomer that is a solid resin (A), has a storage elastic modulus of 1.0 × 10 ⁵ Pa or more and 2.5 × 10 ⁷ Pa or less at 23 ° C, and has a tan δ of 0.3 or more at 23 ° C. A shock absorbing sheet that has a density of more than 700 kg / m ³ and a thickness of 0.02 mm or more and 0.9 mm or less, and is a foam. The shock absorbing sheet according to claim 1, which comprises liquid rubber (B). The shock absorbing sheet according to claim 1 or 2 , further comprising the process oil (C). The shock absorbing sheet according to claim 2 or 3 , wherein the total content of the liquid rubber (B) and the process oil (C) is 10 parts by mass or more and 130 parts by mass or less with respect to 100 parts by mass of the resin (A). The shock absorbing sheet according to any one of claims 1 to 4 , wherein the density is 1200 kg / m ³ or less. The shock absorbing sheet according to any one of claims 1 to 5 , wherein the shock absorbing rate is 50% or more. The shock absorbing sheet according to any one of claims 1 to 6 used for electronic devices. The shock absorbing sheet according to any one of claims 1 to 7 , which is arranged on the back side of the display device.