JPWO2018131619A1 - Shock absorbing sheet - Google Patents

Abstract

The impact-absorbing sheet according to the present invention has a storage elastic modulus at 23 ° C. of 1.0 × 10 5 Pa or more and 2.5 × 10 7 Pa or less, a tan δ at 23 ° C. of 0.3 or more, and a density of 600 kg / m 3 or more. ADVANTAGE OF THE INVENTION According to this invention, it can provide the impact-absorbing sheet which has the outstanding impact-absorbing performance, for example, even if it is thin, makes the absorption performance with respect to the comparatively big impact force added locally locally.

Description

  The present invention relates to a thin impact absorbing sheet, and more particularly to an impact 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, between the glass plate constituting the surface of the device and the display unit, and the case body and display unit to which the display unit is attached In the middle, an impact absorbing material for absorbing shock and vibration is provided. On the other hand, electronic devices including a display device, particularly portable electronic devices, are required to be thin due to space limitations, and accordingly, the shock absorber is also required to be a thin sheet.
As such a thin impact absorbing material, it is widely known that it is formed of a foam made of a polyolefin resin typified by polyethylene. In the foam made of a polyolefin resin, the shape of bubbles is constant. It is considered that the impact absorption performance is improved by controlling the flexibility by controlling the material (see, for example, Patent Document 1).

JP 2014-214205 A

  However, when the shock absorbing material is a thin foam, the shock absorbing performance may not be sufficiently improved by simply controlling the shape of the bubbles to be constant 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 one hundred MPa is locally applied, but the flexibility of the foam sheet made of polyolefin resin is controlled. Even so, it is difficult to sufficiently mitigate such impact force.

  The present invention has been made in view of the above problems, and an object of the present invention is to have excellent shock absorption performance even when it is thin, and particularly, to absorb relatively large impact force applied locally. An object of the present invention is to provide an impact absorbing sheet capable of improving the performance.

As a result of intensive 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 shock absorbing sheet, and completed the present invention. . That is, the present invention provides the following (1) to (11).
(1) An impact-absorbing sheet having a storage elastic modulus at 23 ° C. of 1.0 × 10 ⁵ Pa to 2.5 × 10 ⁷ Pa, tan δ at 23 ° C. of 0.3 or more, and a density of 600 kg / m ³ or more. .
(2) The impact-absorbing sheet according to (1), wherein the thickness is 0.02 mm or more and 0.9 mm or less.
(3) The impact-absorbing sheet according to (1) or (2), which comprises a solid resin (A) and a liquid rubber (B).
(4) The impact-absorbing sheet according to (3), wherein the resin (A) is a styrene-based thermoplastic elastomer.
(5) The impact-absorbing sheet according to (3) or (4), further including process oil (C).
(6) 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 impact-absorbing sheet according to any one of the above.
(7) The shock absorbing sheet according to any one of (1) to (6), wherein the density is 1200 kg / m ³ or less.
(8) The impact absorbing sheet according to any one of (1) to (7), wherein the impact absorbing rate is 50% or more.
(9) The impact-absorbing sheet according to any one of (1) to (8), which is a foam.
(10) The impact-absorbing sheet according to any one of (1) to (9), which is used for an electronic device.
(11) The impact absorbing sheet according to any one of (1) to (10), which is disposed on the back side of the display device.

  ADVANTAGE OF THE INVENTION According to this invention, it can provide the impact-absorbing sheet which has the outstanding impact-absorbing performance, for example, even if it is thin, makes the absorption performance with respect to the comparatively big impact force added locally locally.

Hereinafter, the present invention will be described in more detail using embodiments.
[Shock absorbing sheet]
The impact-absorbing sheet of the present invention has a storage elastic modulus of 1.0 × 10 ⁵ Pa to 2.5 × 10 ⁷ Pa, tan δ of 0.3 or more, and a density of 600 kg / m ³ or more. In this 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 is thin and the impact-absorbing performance can be improved even when 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 tan δ is 0.3 or more, the point load is easily converted into the surface load and the energy loss is increased. And since the density becomes 600 kg / m ³ or more and the storage elastic modulus becomes 1.0 × 10 ⁵ Pa or more, the shock wave easily propagates along the surface direction of the resin sheet. It is estimated that performance will be exhibited.

If the storage elastic modulus of the shock absorbing sheet is outside the above range, the shock absorbing performance cannot be sufficiently improved. The storage elastic modulus of the impact absorbing sheet is preferably 2.0 × 10 ⁵ Pa or more and 2.0 × 10 ⁷ Pa or less, more preferably 4.0 × 10 ⁵ Pa or more, from the viewpoint of improving impact absorbing performance. 0 × 10 ⁷ Pa or less.
On the other hand, 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 impact absorption performance, tan δ is preferably 0.33 or more, and more preferably 0.36 or more. Further, the upper limit of tan δ is not particularly limited, but is preferably 1.5 or less, more preferably 1.2 or less, and still more preferably 0.9 or less, in order to make the storage elastic modulus easily in the above range.
Furthermore, the density of the impact-absorbing sheet, when an impact is applied to the impact-absorbing sheet when less than 600 kg / m ^3, a so-called bottoming propagating on the back side of the member to not be absorbed its impact in impact-absorbing sheet May occur. The density of the impact absorbing sheet is preferably 650 kg / m ³ or more, more preferably 700 kg / m ³ or more, and even more preferably 700 kg / m ³ from the viewpoint of further improving the impact 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 because of 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 from −60 ° C. to less than 15 ° C., and even more preferably from −50 ° C. to less than 15 ° C. from the viewpoint of improving the impact 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 impact absorbing sheet is preferably 0.02 mm or more and 0.9 mm or less. By setting the thickness of the impact absorbing sheet to 0.02 mm or more, it is possible to prevent so-called bottoming from occurring when an impact is applied to the impact absorbing sheet. Moreover, it becomes possible to contribute to thickness reduction and size reduction of an electronic device by setting it as 0.9 mm or less. In the present invention, even a shock absorbing sheet that is as thin as 0.9 mm or less can provide sufficient impact resistance. From these viewpoints, the thickness of the impact 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 impact-absorbing sheet may be non-foamed or foamed as long as the density falls 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. 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 still more preferably 80% or more and 100% or less. The closed cell ratio can be determined based on, for example, JIS K7138 (2006). Further, as described above, the impact-absorbing sheet of the present invention has a relatively high density. Therefore, even a foamed body has a low foaming ratio and becomes finely foamed.

The impact-absorbing sheet may be a crosslinked body or a non-crosslinked body. However, when the impact absorbing sheet is a foam, it is preferably a crosslinked body. When the impact-absorbing sheet is a crosslinked body, the gel fraction indicating the degree of crosslinking of the impact-absorbing sheet is preferably 10% by mass to 70% by mass, and more preferably 20% by mass to 60% by mass. The gel fraction is measured by the following method.
A test piece (test piece mass A (mg)) was cut out from the shock-absorbing sheet so as to have a mass of about 50 mg, and the test piece was immersed in 30 cm ³ of xylene at 105 ° C. and allowed to stand for 24 hours, and then 200 mesh. The undissolved portion on the wire mesh is collected by filtration through a wire mesh, vacuum dried, and the mass B (mg) of the insoluble portion is precisely weighed. From the obtained value, it is calculated by the following formula.
Crosslinking degree (mass%) = (B / A) × 100

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

  The impact absorbing sheet is obtained by adding various additives, which will be described later, to the resin component as necessary. The resin component constituting the shock absorbing sheet preferably contains an elastomer. Although it will not be limited if the said various characteristics are acquired, it is preferable that an impact-absorbing sheet uses the resin (A) and liquid rubber (B) which are shown below as a resin component, and resin (A) is an elastomer (A1). ) Is more preferable.

[Resin (A)]
As the resin (A) such as the elastomer (A1), a solid resin that does not exhibit fluidity under conditions of 23 ° C. and 1 atm (1.01 × 10 ⁻¹ MPa) is used. As the resin (A), for example, one having a storage elastic modulus higher than the storage elastic modulus of the shock absorbing sheet is used. Specifically, the elastomer (A1) a resin, such as (A) is preferably the storage modulus those 1.0 × 10 ⁸ Pa or less 5.0 × 10 ⁵ Pa or more is used, more preferably a storage elastic Those having a rate of 1.0 × 10 ⁶ Pa to 5.0 × 10 ⁷ Pa are used. The storage elastic modulus was measured when the resin (A) was made into a sheet having a thickness of 150 μm, and details of the measurement method are as shown in the examples described later.
The resin (A) has an MFR (230 ° C., 21.2 N) of preferably 2 g / 10 min to 30 g / 10 min, more preferably 4 g / 10 min to 20 g / 10 min, and more preferably 6 g / 10 min to 15 g. / 10 min or less is more preferable. The MFR is 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, and further 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 thermoplastic elastomer, olefin elastomer, vinyl chloride elastomer, urethane elastomer, polyvinyl butyral resin, polyvinyl alcohol resin and the like can be used, and among them, styrene thermoplastic elastomer is preferable.

(Styrenic thermoplastic elastomer)
Examples of the styrenic thermoplastic elastomer include a styrene-conjugated diene copolymer and a hydrogenated product thereof. The styrenic 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. preferable. In addition, the hard segment or the soft segment may be configured by randomly copolymerizing two or more components.
Examples of conjugated dienes include isoprene and butadiene. 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 the hard segment (X) at the molecular end, and a small amount of styrene is randomly copolymerized with the conjugated diene polymer block or conjugated diene as the soft segment (Y). And those having a styrene-conjugated diene random copolymer block. The said small amount of styrene is a case where it is 20 mol% or less of the conjugated diene in a soft segment, for example. Examples of structures that can be adopted include the following.
(XY) m, (XY) nX, (XY) p-Z, wherein m and n are integers of 1 or more, p is an integer of 2 to 6, and X Represents a hard segment, Y represents a soft segment, and Z represents a coupling agent residue.
Among these, as the styrene-conjugated diene copolymer, those having a structure of XYX are preferable.

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 co-polymerized. What is 3 mass% or more and 15 mass% or less with respect to the whole coalescence is mentioned. Here, when styrene is copolymerized in the soft segment, the styrene content in the segment is preferably 10% by mass or less based on the entire copolymer. Such a styrene-conjugated diene copolymer is preferably a hydrogenated product. 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 trade names “Dynalon 1320P” and “Dynalon 1321P” manufactured by JSR Corporation.

Another preferred specific example of the styrene-conjugated diene copolymer includes 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 resin (A) described above may be used alone or in combination of two or more.

[Liquid rubber (B)]
The impact-absorbing sheet preferably contains liquid rubber (B) in addition to the resin (A), and it is more preferable to use a styrene-based thermoplastic elastomer and liquid rubber (B) 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 and using the liquid rubber (B) in the resin (A), it becomes possible to sufficiently improve the tan δ of the shock absorbing sheet and to lower the storage elastic modulus. The principle is not clear, but it may be because the (A) component and the (B) component are pseudo double network gels.
The liquid rubber (B) has a viscosity at 38 ° C. of preferably 5 Pa · s to 1500 Pa · s, more preferably 10 Pa · s to 1300 Pa · s. In the present specification, the viscosity is a value measured with a B-type rotational viscometer at a rotational speed of 100 rpm.

Examples of the liquid rubber (B) include liquid acrylonitrile-butadiene rubber, liquid hydrogenated acrylonitrile-butadiene rubber, liquid carboxylated acrylonitrile-butadiene rubber, liquid acrylonitrile-butadiene-isoprene rubber, liquid acrylonitrile-isoprene. Liquid acrylonitrile rubbers such as rubber and liquid terpolymers of acrylonitrile, butadiene and functional monomers having anti-aging functions, liquid chloroprene rubber, liquid isoprene rubber, liquid butyl rubber, liquid butadiene rubber, liquid ethylene-propylene -Diene rubber, liquid ethylene-propylene rubber, liquid natural rubber, etc. are mentioned. 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 liquid isoprene rubber from the viewpoint of compatibility, storage elastic modulus, tan δ, and the like.

[Procell oil (C)]
The impact absorbing sheet may contain a process oil (C) in addition to the resin (A) and the liquid rubber (B). By mixing and using the process oil (C) with the components (A) and (B), the storage modulus of the impact-absorbing sheet can be set to a suitable value and the moldability and the like can be improved. Examples of the procell oil (C) include mineral oil such as paraffin oil, vegetable oil, and the like. Among these, paraffin oil is preferable. When using a process oil, for example, an oil-extended elastomer obtained by adding a process oil to a resin (A) such as an elastomer (A1) in advance may be used.
The process oil (C) preferably has a viscosity at 40 ° C. of 0.01 Pa · s to 1.0 Pa · s, more preferably 0.02 Pa · s to 0.5 Pa · s.
Process oil (C) may be used individually by 1 type, and may use 2 or more types together.

In the impact absorbing sheet, the total content of the liquid rubber (B) and the process 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. 40 parts by mass or more and 120 parts by mass or less are more preferable, and 70 parts by mass or more and 110 parts by mass or less are more preferable. 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-described ranges. In addition, when content of (B) and (C) component exists in the said range, an impact-absorbing sheet may contain (C) component and does not need to contain it.
Further, when the impact absorbing sheet contains process oil, the ratio (C / B) of the content of the process oil (C) to the content of the liquid rubber (B) is preferably 0.1 or more and 1.5 or less, 0.3 or more and 1.3 or less are more preferable, and 0.5 or more and 1.1 or less are still more preferable.

[Antioxidant]
The shock absorbing sheet preferably contains an antioxidant. Examples of the antioxidant include phenol-based antioxidants, sulfur-based antioxidants, phosphorus-based antioxidants, and amine-based antioxidants.
For example, phenolic antioxidants 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-methylphenyl acrylate, tetrakis [methylene-3- (3,5-di-tert-butyl-4-hydroxyphenyl) ) Propionate] methane and the like.
Examples of the sulfur-based antioxidant include dilauryl thiodipropionate, dimyristyl thiodipropionate, distearyl thiodipropionate, pentaerythrityl tetrakis (3-lauryl thiopropionate), and the like. .
An antioxidant may be used individually by 1 type and may use 2 or more types together.
The antioxidant is preferably 0.1 parts by mass or more and 2.0 parts by mass or less with respect to 100 parts by mass in total of the contents of the resin (A), the liquid rubber (B), and the process oil (C) described above. 0.2 parts by mass or more and 1.5 parts by mass or less are more preferable.

[Other additives]
In addition to the antioxidants, the impact absorbing sheet includes metal damage inhibitors, antistatic agents, stabilizers, nucleating agents, crosslinking agents, crosslinking aids, pigments, halogen-based, phosphorus-based flame retardants, and fillers. Other additives such as these may be contained within a range not impairing the object of the present invention.

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

In addition, the resin composition may appropriately perform either one or both of crosslinking and foaming. The foaming of the resin composition is performed by, for example, mixing a thermal decomposable foaming agent together with the above components in the resin composition, heating the sheet-shaped resin composition to a temperature higher than the decomposition temperature of the pyrolyzable foaming agent, It is preferable to foam the sheet-shaped resin composition by thermally decomposing the decomposable foaming agent.
As the thermally decomposable 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.
Organic foaming agents include azodicarbonamide, azodicarboxylic acid metal salts (such as barium azodicarboxylate), azo compounds such as azobisisobutyronitrile, nitroso compounds such as N, N′-dinitrosopentamethylenetetramine, Examples thereof include hydrazine derivatives such as hydrazodicarbonamide, 4,4′-oxybis (benzenesulfonylhydrazide) and toluenesulfonylhydrazide, and semicarbazide compounds such as toluenesulfonyl semicarbazide.
Examples of the inorganic foaming agent include ammonium acid, sodium carbonate, ammonium hydrogen carbonate, sodium hydrogen carbonate, ammonium nitrite, sodium borohydride, anhydrous monosodium citrate, and the like.
Among these, azo compounds and nitroso compounds are preferable from the viewpoint of obtaining fine bubbles, and from the viewpoints of economy and safety, and azodicarbonamide, azobisisobutyronitrile, N, N′-dinitrosopentamethylene. Tetramine is more preferred, and azodicarbonamide is still more preferred.
These pyrolytic foaming agents can be used alone or in combination of two or more.
Even when the shock absorbing sheet is foamed, it is slightly foamed as described above. For this reason, it is preferable that the amount of the pyrolytic foaming agent is small. Specifically, the blending amount of the pyrolytic foaming agent in the resin composition is preferably 0.1 parts 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). More preferably, it is 3 parts by mass or more and 3.0 parts by mass or less.

  Further, as a means for lowering the decomposition temperature of the pyrolytic foaming agent or increasing the decomposition rate, for example, a decomposition temperature adjusting agent such as zinc oxide, zinc stearate, urea or the like may be added to the resin composition. Good. In order to adjust the surface condition of heating equipment and foam, the decomposition temperature regulator is used in an amount of about 0.01 to 5 parts by mass with respect to 100 parts by mass in total of the above components (A) to (C), for example. can do.

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

Further, the foaming in the present invention is not limited to the above-described chemical foaming and physical foaming, but includes foaming performed by other means for containing bubbles in the impact 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, kanamite, obsidian perlite, diamond balloons, and inorganic microcapsules. On the other hand, as the organic hollow particles, hollow particles made of polyvinylidene chloride resin such as phenol resin microballoons, Saran microspheres (trade name: manufactured by Dow), hollow particles made of epoxy resins such as ecospheres, clecas spheres, Carbon hollow particles, such as other carbon spheres, etc. are mentioned.

The crosslinking of the resin composition may be performed when the impact absorbing sheet is a non-foamed body or a foamed body, but is preferably performed when the impact absorbing sheet is a foamed body. Crosslinking of the resin composition may be performed on the resin composition formed 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 performed by irradiating the resin composition formed into a sheet shape with ionizing radiation. Examples of ionizing radiation include α-rays, β-rays, γ-rays, and electron beams, and electron beams are more preferable. The irradiation dose of ionizing radiation is preferably 0.5 Mrad to 10 Mrad, and more preferably 1 Mrad to 8 Mrad.

  The crosslinking may be performed by a method in which a crosslinking agent such as an organic peroxide is previously blended in the resin composition and the organic peroxide is decomposed by heating the resin composition. Examples of such organic peroxides 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 impact absorbing sheet is a foam, the stretching may be performed while foaming the resin composition, or may be performed after foaming the resin composition. In addition, when the foam is stretched after foaming the resin sheet, the foam may be stretched while maintaining the molten state during foaming without cooling the foam. After cooling the foam, the foam may be stretched again by heating it to a molten or softened state. Stretching may be performed in one or both of the MD direction and the TD direction.
In addition, even when the impact absorbing sheet is a non-foamed body, the stretching is preferably performed on the sheet-like resin composition, but when it is crosslinked, it may be performed after crosslinking.

[How to use shock absorbing sheet]
The shock absorbing sheet of the present invention is used for various electronic devices, preferably portable electronic devices such as notebook personal computers, mobile phones, electronic paper, digital cameras, video cameras and the like. More specifically, it is used as an impact absorbing sheet for a display device provided in these electronic devices. Examples of the display device include an organic EL display device and a liquid crystal display device, and an organic EL display device is preferable.
Further, it is preferable that the display device, particularly the organic EL display device, be a flexible display. An organic EL display device is flexible by forming an organic EL element including both electrodes, a light emitting layer formed between the 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 impact absorbing sheet is disposed on the back side of various display devices and absorbs impacts applied to the display device. More specifically, the shock absorbing sheet is placed on, for example, a housing of an electronic device and is disposed between the housing and the display device. In addition, the shock absorbing sheet is usually compressed and arranged between a component constituting an electronic device such as a housing and the display device.
Since the impact absorbing sheet of the present invention has high impact absorbing performance even if it is thin, it is possible to appropriately prevent damage to the display device while reducing the thickness of the electronic device. In addition, even when a relatively large impact is applied locally, the impact-absorbing sheet can absorb the impact appropriately, so that display defects such as glass breakage and flexible displays can be avoided. It becomes possible to prevent appropriately.

  If necessary, the shock absorbing sheet may be used by appropriately laminating a resin sheet on one side and both sides thereof. 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 impact absorbing sheet, and has a thickness of, for example, 10 μm to 300 μm, preferably 10 μm to 200 μm. The resin sheet may be bonded to the shock absorbing sheet by thermocompression bonding, or may be bonded to the shock absorbing sheet using an adhesive or the like.

Further, the impact absorbing sheet may be used as an adhesive tape by providing an adhesive material on one side or both sides. By making the impact absorbing sheet an adhesive tape, it becomes possible to adhere to a component such as a casing of an electronic device via an adhesive material.
The pressure-sensitive adhesive material only needs to have at least a pressure-sensitive adhesive layer, may be composed of a single pressure-sensitive adhesive layer laminated on the surface of the shock-absorbing sheet, or double-sided pressure-sensitive adhesive stuck to the surface of the shock-absorbing sheet. Although it may be a sheet, it is preferably a pressure-sensitive adhesive layer alone. In addition, a double-sided adhesive sheet is provided with a base material and the adhesive layer provided in both surfaces of the base material. The double-sided pressure-sensitive adhesive sheet is used for bonding one pressure-sensitive adhesive layer to the shock absorbing sheet and bonding the other pressure-sensitive adhesive layer to other components.
There is no restriction | limiting in particular as an adhesive which comprises an adhesive layer, For example, an acrylic adhesive, a urethane type adhesive, a rubber-type adhesive etc. 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 bonded onto the adhesive material, and the adhesive layer may be protected by the release paper before use.

  Examples The present invention will be described in more detail with reference to 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 elastic modulus and tan δ>
Measuring apparatus: DVA-200 (manufactured by IT Measurement Control Co., Ltd.), tensile mode: 10 Hz, strain amount: 0.1%, temperature range: −100 ° C. to 100 ° C., heating rate: 10 ° C. / The storage elastic modulus and tan δ at 23 ° C. were determined under the condition of min. The sample size was 40 mm in length (however, the distance between grips was 25 mm) and 5 mm in width.
<Thickness>
Thickness was measured with a dial gauge.
<Density>
The density of the shock absorbing sheet is an apparent density value measured according to JIS K6767.
<Shock absorption test>
A shock absorbing sheet (50 mm square) was placed on 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. The acrylic plate has four corners fixed to a pedestal with bolts having a length of 35 mm, and the upper surface of the acrylic plate is held 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. Moreover, the same iron ball dropping and acceleration measurement were repeated 19 times without replacing the shock absorbing sheet, and the average value of accelerations for all 20 times was defined as acceleration (L1). Moreover, the average value of the acceleration of all 20 times when the same iron ball dropping and acceleration measurement were performed without placing the shock absorbing sheet on the acrylic plate was defined as acceleration (L0), and the obtained acceleration (L1) and acceleration (L0) ), The impact absorption rate was calculated by the following equation.
Impact absorption rate (%) = (L0−L1) / L0 × 100

Each component used in Examples and Comparative Examples is as follows.
Styrenic thermoplastic elastomer (TPS1): trade name “Dynalon 1321P”, manufactured by JSR Corporation, MFR (230 ° C., 21.2 N) 10 g / 10 min, storage elastic modulus at 23 ° C. 1.7 × 10 ⁶ Pa, 23 ° C. Tan δ = 0.11 and styrene content 10% by mass
Liquid isoprene rubber (IR): trade name “Kuraprene LIR-290”, manufactured by Kuraray Co., Ltd., viscosity at 38 ° C .: 1200 Pa · s
Process oil (PO): trade name “Diana Process Oil PW-32”, manufactured by Idemitsu Kosan Co., Ltd., viscosity at 40 ° C .: 0.026 Pa · s
Styrenic thermoplastic elastomer (TPS2): Trade name “Arneston JS20N”, Kuraray Plastics Co., Ltd. Styrenic thermoplastic elastomer (TPS3): Trade name “Arneston CJ103”, Kuraray Plastics Co., Ltd. polyethylene resin ( PE): Trade name “Affinity PL1850G”, Polyvinyl butyral resin (PVB) manufactured by The Dow Chemical Company: Trade name “S-Rec Clear Film”, Sekisui Chemical Co., Ltd. antioxidant (phenolic antioxidant): Product name “ADK STAB AO-60”, ADEKA thermal decomposition type foaming agent (azodicarbonamide): Product name “VINYHALL AC # 3”, manufactured by Eiwa Kasei Co., Ltd.

[Example 1]
50 parts by mass of styrene 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. Thus, the resin composition molded into a sheet having a thickness of 0.15 mm was used as an impact 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 resin composition obtained by kneading a mass part and 1 part by mass of an antioxidant at 160 ° C. into a sheet having a thickness of 0.15 mm was used as an impact absorbing sheet.

[Example 3]
The same operation as in Example 1 was carried out except that both surfaces of the sheet-shaped resin composition were crosslinked by irradiation with an electron beam of 800 kV at an acceleration voltage of 4.0 Mrad.
[Example 4]
The same operation as in Example 2 was performed except that both surfaces of the sheet-shaped resin composition were crosslinked by irradiation with an electron beam of 800 kV at an acceleration voltage of 4.0 Mrad.

[Example 5]
In addition to the components (A) and (B) and the antioxidant, in the same manner as in Example 3, except that 1 part by mass of azodicarbonamide (ADCA) as a pyrolytic foaming agent was added. The resin composition was molded and then crosslinked. Then, the cross-linked 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 cross-linked foam having a thickness of 0.15 mm. Was used as an impact absorbing sheet.
[Example 6]
In the same manner as in Example 4 except that 1 part by mass of azodicarbonamide (ADCA) as a pyrolytic foaming agent was added in addition to the components (A), (B) and (C) and the antioxidant. Next, a sheet-like resin composition was molded and then crosslinked. Then, the cross-linked 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 cross-linked foam having a thickness of 0.15 mm. Was used as an impact absorbing sheet.
[Example 7]
The same procedure as in Example 5 was performed except that 0.8 part by mass of azodicarbonamide (ADCA) as a pyrolytic foaming agent was added.

[Comparative Examples 1-7, 9]
Each component was kneaded so as to have the composition described in Table 1, and a resin sheet formed into a sheet having a thickness of 0.15 mm was used as an impact absorbing sheet.
[Comparative Example 8]
The same procedure as in Example 5 was performed except that the blending amount was changed as described in Table 1.
[Comparative Examples 10-12]
Each component was kneaded so as to have the composition shown in Table 1, and both surfaces of a resin sheet formed into a sheet having a thickness of 0.15 mm were crosslinked by irradiation with an electron beam with an acceleration voltage of 800 kV for 4.0 Mrad. The cross-linked resin sheet was used as an impact absorbing sheet.
[Comparative Example 13]
The same procedure as in Example 5 was performed except that the amount of azodicarbonamide (ADCA) as the pyrolytic foaming agent was 2 parts by mass.

  As is clear from the above examples, the impact absorption rate can be increased to 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 shock absorption rate is less than 50%, and the shock absorption performance is not sufficient. It was.

Claims (11)

An impact-absorbing sheet having a storage elastic modulus at 23 ° C. of 1.0 × 10 ⁵ Pa to 2.5 × 10 ⁷ Pa, tan δ at 23 ° C. of 0.3 or more, and a density of 600 kg / m ³ or more.   The impact-absorbing sheet according to claim 1, wherein the thickness is 0.02 mm or more and 0.9 mm or less.   The impact-absorbing sheet according to claim 1 or 2, comprising a solid resin (A) and a liquid rubber (B).   The impact-absorbing sheet according to claim 3, wherein the resin (A) is a styrene-based thermoplastic elastomer.   The impact absorbing sheet according to claim 3 or 4, further comprising process oil (C).   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). Shock absorbing sheet. The impact-absorbing sheet according to any one of claims 1 to 6, wherein the density is 1200 kg / m ³ or less.   The impact absorbing sheet according to any one of claims 1 to 7, wherein the impact absorbing rate is 50% or more.   It is a foam, The impact-absorbing sheet of any one of Claims 1-8.   The impact-absorbing sheet according to any one of claims 1 to 9, which is used in an electronic device. The impact-absorbing sheet according to any one of claims 1 to 10, which is disposed on the back side of the display device.