JP6859494B1 - Shock absorbing sheet, adhesive tape and display device - Google Patents

Abstract

Provided is a shock absorbing sheet having good bending resistance even in a low temperature environment while maintaining good shock absorbing performance. A shock absorbing sheet containing a foamed resin layer, at least one having a loss coefficient (tan δ) peak between 25 and 25 ° C. and having a tensile elongation at −35 ° C. of 6% or more. Absorption sheet.

Description

The present invention relates to a shock absorbing sheet, and an adhesive tape and a display device having the shock absorbing sheet.

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 form the surface of the device, the housing body to which the display unit is attached, the display unit, etc. A shock absorbing material for absorbing shock and vibration is provided between them. Electronic devices equipped with a display device, particularly portable electronic devices, are required to be thin due to space constraints, and the shock absorbing material is often made into a sheet (shock absorbing sheet) accordingly. In addition, the diversification of electronic devices is progressing. For example, in display devices, flexible displays such as Foldable and Rollable are being developed, and shock absorption used in electronic devices is being developed. The sheet may be required to have bending resistance.

It is widely known that the shock absorbing sheet is formed of a polyolefin-based foam made of a polyolefin-based resin typified by polyethylene. Further, as a shock absorbing material, for example, as disclosed in Patent Document 1, an ultraviolet curable foam formed by foaming a urethane acrylate oligomer and curing it with ultraviolet rays is also known.

Japanese Unexamined Patent Publication No. 2008-156544

In recent years, it is necessary to assume that electronic devices, especially portable electronic devices such as mobile phones, are used in various environments, and shock absorbing sheets used in electronic devices have various performances, for example, in a low temperature environment. May be required to maintain. However, it is difficult for the foam used in the conventional shock absorbing sheet to improve the bending resistance in a low temperature environment while maintaining high shock absorbing property.

Therefore, the first object of the present invention is to provide a shock absorbing sheet having good bending resistance even in a low temperature environment while maintaining good shock absorbing performance.
A second object of the present invention is to provide a shock absorbing sheet having good bending resistance even in a low temperature environment.

As a result of diligent studies to solve the first problem, the present inventors have at least one peak of loss coefficient (tan δ) between 25 and 25 ° C., and tensile elongation at −35 ° C. It has been found that the first problem can be solved by setting the coefficient to a predetermined value or more, and the following first invention has been completed. Alternatively, the first problem is solved by ensuring that the shock absorption rate at 23 ° C. is 25% or more and the shock absorbing sheet bent 4,000 times in the dynamic bending resistance test is not scratched by 1 mm or more. He found what he could do and completed the following first invention.
In addition, as a result of diligent studies to solve the second problem, the present inventors have formed a foamed resin layer with an acrylic polymer, and used a predetermined amount of urethane (meth) acrylate in the acrylic polymer. By doing so, it was found that the above-mentioned second problem could be solved, and the following second invention was completed.

The present invention has been made in view of the above problems, and the following [1] to [28] are the gist of the present invention. The present invention provides, for example, the first and second inventions in view of the first and second problems, respectively, and the first invention of the present invention is described in the following [1] to [1] to [ 13] is the gist. The second invention of the present invention has the following gist of [14] to [28].

[1] A shock absorbing sheet containing a foamed resin layer, at least one having a loss coefficient (tan δ) peak between 25 and 25 ° C., and having a tensile elongation at −35 ° C. of 6% or more. There is a shock absorbing sheet.
[2] The shock absorbing sheet according to the above [1], which has a thickness of 200 μm or less.
[3] The shock absorbing sheet according to the above [1] or [2], wherein the resin constituting the foamed resin layer contains an acrylic polymer.
[4] The shock absorbing sheet according to any one of [1] to [3] above, wherein the density of the foamed resin layer is 0.3 g / cm ³ or more and 0.8 g / cm ^{3 or less.}
[5] The shock absorbing sheet according to any one of the above [1] to [4], wherein the 25% compression strength at 23 ° C. is 200 kPa or more and 700 kPa or less.
[6] The shock absorbing sheet according to any one of the above [1] to [5], wherein the shock absorbing rate at 23 ° C. is 25% or more.
[7] The shock absorbing sheet according to any one of the above [1] to [6], wherein the foamed resin layer contains hollow particles.
[8] The shock absorbing sheet according to any one of the above [1] to [7], wherein the foamed resin layer has bubbles made of gas mixed inside.
[9] The shock absorbing sheet according to any one of the above [1] to [8] used in electronic devices.
[10] The shock absorbing sheet according to any one of the above [1] to [9], which is arranged on the back side of the display device.
[11] An adhesive tape comprising the shock absorbing sheet according to any one of the above [1] to [10] and an adhesive material provided on at least one surface of the shock absorbing sheet.
[12] A display device comprising the shock absorbing sheet according to any one of [1] to [10] above or the adhesive tape according to [11].
[13] A shock absorbing sheet containing a foamed resin layer having a shock absorbing rate of 25% or more at 23 ° C., and the shock absorbing sheet bent 4,000 times in the following dynamic bending resistance test has a scratch of 1 mm or more. A shock absorbing sheet that does not occur.
[Dynamic flexion resistance test]
In an environment of −35 ° C., the pair of pedestals on which the shock absorbing sheet is erected at a distance between the pedestals of 30 mm are brought close to each other so that the distance between the pedestals is 6 mm to bend the shock absorbing sheet, and then the pair again. The reciprocating motion of moving away the gantry to the original distance between the gantry is repeated 4,000 times in a cycle of 46 times per minute.

[14] A shock absorbing sheet containing a foamed resin layer, wherein the foamed resin layer contains an acrylic polymer obtained by polymerizing a monomer component (A) containing urethane (meth) acrylate (A1).
A shock absorbing sheet containing 5% by mass or more of the urethane (meth) acrylate (A1) with respect to the monomer component (A).
[15] The shock absorbing sheet according to the above [14], wherein the urethane (meth) acrylate (A1) has a weight average molecular weight of 5,000 or more and 45,000 or less.
[16] The shock absorbing sheet according to the above [14] or [15], wherein the urethane (meth) acrylate (A1) contains a structural unit derived from propylene oxide.
[17] The shock absorbing sheet according to any one of the above [14] to [16], which has a thickness of 200 μm or less.
[18] The shock absorbing sheet according to any one of [14] to [17] above, wherein the density of the foamed resin layer is 0.3 g / cm ³ or more and 0.8 g / cm ^{3 or less.}
[19] The shock absorbing sheet according to any one of [14] to [18] above, wherein the 25% compression strength at 23 ° C. is 200 kPa or more and 700 kPa or less.
[20] The shock absorbing sheet according to any one of the above [14] to [19], wherein the shock absorbing rate at 23 ° C. is 20% or more.
[21] The shock absorbing sheet according to any one of [14] to [20] above, wherein at least one has a peak loss coefficient (tan δ) between 25 and 25 ° C.
[22] The shock absorbing sheet according to any one of the above [14] to [21], wherein the shock absorbing sheet bent 4000 times in the following dynamic bending resistance test is not scratched by 1 mm or more.
[Dynamic flexion resistance test]
In an environment of −35 ° C., the pair of pedestals on which the shock absorbing sheet is erected at a distance between pedestals of 30 mm are brought close to each other so that the distance between the pedestals is 6 mm to bend the shock absorbing sheet, and then the pair again. The reciprocating motion of moving the pedestal away from the pedestal to the original distance between the pedestals is repeated 4000 times in a cycle of 46 times per minute.
[23] The shock absorbing sheet according to any one of [14] to [22] above, wherein the foamed resin layer contains hollow particles.
[24] The shock absorbing sheet according to any one of [14] to [23] above, wherein the foamed resin layer has bubbles made of gas mixed inside.
[25] The shock absorbing sheet according to any one of the above [14] to [24] used in an electronic device.
[26] The shock absorbing sheet according to any one of the above [14] to [25], which is arranged on the back side of the display device.
[27] An adhesive tape comprising the shock absorbing sheet according to any one of the above [14] to [26] and an adhesive material provided on at least one surface of the shock absorbing sheet.
[28] A display device comprising the shock absorbing sheet according to any one of the above [14] to [26] or the adhesive tape according to the above [27].

According to the first invention of the present invention, it is possible to provide a shock absorbing sheet having good bending resistance even in a low temperature environment while maintaining good shock absorbing performance.
Further, according to the second invention of the present invention, it is possible to provide a shock absorbing sheet having good bending resistance even in a low temperature environment.

Hereinafter, the present invention will be described in more detail using embodiments.

(First invention)
[Shock absorption sheet]
The shock absorbing sheet according to the first aspect of the first aspect of the present invention is a shock absorbing sheet containing a foamed resin layer, which has a tensile elongation of 6% or more at −35 ° C. and a tensile elongation of 25 to 25 ° C. At least one of them is characterized by having a peak of the loss coefficient (tan δ).
The shock absorbing sheet according to the second embodiment of the first invention of the present invention is a shock absorbing sheet containing a foamed resin layer, has a shock absorbing rate of 25% or more at 23 ° C., and is 4 in a dynamic bending resistance test. It is characterized in that the shock absorbing sheet bent 000 times is not scratched by 1 mm or more.
Since the shock absorbing sheet of the first invention is in the first form or the second form, it has good bending resistance even in a low temperature environment while maintaining good shock absorbing performance. Further, the shock absorbing sheet according to the second embodiment of the first invention of the present invention is not required for the display device of the electronic device because it does not cause scratches in the dynamic bending resistance test showing the bending resistance in a low temperature environment. Stress is suppressed. Further, the shock absorbing sheet according to the second embodiment of the first invention of the present invention does not break in the dynamic bending resistance test showing the bending resistance in a low temperature environment, so that the shock absorbing property at the bending point is not lost.
Hereinafter, the present invention will be described in more detail.

(Loss coefficient (tan δ))
The shock absorbing sheet according to the first embodiment of the first invention has at least one peak of a loss coefficient (tan δ) between 25 and 25 ° C. If there is no peak of the loss coefficient (tan δ) within this temperature range, the impact resistance performance of the impact absorbing sheet becomes insufficient. From the viewpoint of further improving the impact resistance performance, it is preferable to have at least one peak of the loss coefficient (tan δ) between -20 to 23 ° C., more preferably between -18 and 20 ° C. The peak within the above temperature range is preferably the maximum peak when the loss coefficient (tan δ) is measured by the measurement method described later.
The loss coefficient (tan δ) may be measured using a dynamic viscoelasticity measuring device in a temperature range of −100 ° C. to 100 ° C.
The loss coefficient (tan δ) in the present invention can be adjusted, for example, by appropriately changing the resin component constituting the foamed resin layer, as will be described later. For example, in the case of an acrylic polymer, it can be adjusted according to the type and content of the monomer component.

(Tensile elongation at -35 ° C)
The shock absorbing sheet according to the first aspect of the present invention has a tensile elongation of 6% or more at −35 ° C. If the tensile elongation of the shock absorbing sheet at −35 ° C. is less than 6%, the bending resistance in a low temperature environment becomes insufficient. When the tensile elongation of the shock absorbing sheet is high in a low temperature environment, the bending resistance in a low temperature environment becomes good. From the viewpoint of bending resistance in a low temperature environment, the tensile elongation at −35 ° C. is preferably 6.5% or more, more preferably 7.0% or more, still more preferably 7.5% or more.
The upper limit of the tensile elongation at −35 ° C. is not particularly limited, but is, for example, 600% or less, preferably 500% or less, and 25% or less from the viewpoint of ensuring a certain mechanical strength and high impact absorption performance. It is preferably 20% or less, more preferably 15% or less.
The tensile elongation at −35 ° C. in the present invention can be adjusted, for example, by appropriately changing the resin component constituting the foamed resin layer, as will be described later. For example, in the case of an acrylic polymer, it can be adjusted according to the type and content of the monomer component.
The tensile elongation means the elongation when the impact absorbing sheet is pulled until it breaks and the tensile force is maximized (also referred to as "maximum tensile strength elongation"). The tensile elongation is measured according to the method described in JIS K6767, and the measurement is performed in an environment of −35 ° C. It can also be adjusted by the density and the like.
Further, the tensile elongation is measured in both the MD and TD directions, and the one having the higher tensile elongation is adopted. When it is unclear which direction is the MD or TD direction, the tensile elongation in the direction in which the tensile elongation is maximized may be measured. The MD direction is an abbreviation for Machine Direction, and coincides with the resin flow direction at the time of coating or the like described later. The TD direction is an abbreviation for Transverse Direction, and is a direction orthogonal to the MD direction.

(Shock absorption rate at 23 ° C)
From the viewpoint of improving the shock absorption, the shock absorbing sheet according to the first aspect of the first invention preferably has a shock absorbing rate of 25% or more, more preferably 28% or more at 23 ° C. , 32% or more is more preferable.
The shock absorbing sheet according to the second embodiment of the first invention has a shock absorbing rate of 25% or more at 23 ° C. The shock absorbing sheet according to the second aspect of the first invention has insufficient shock absorbing property when the shock absorbing rate at 23 ° C. is less than 25%. From the viewpoint of improving the shock absorption, the shock absorbing sheet according to the second aspect of the first invention preferably has a shock absorbing rate of 28% or more, more preferably 30% or more at 23 ° C. , 32% or more is more preferable.
The shock absorption rate at 23 ° C. is measured by the method described in Examples described later. By setting the shock absorption rate at 23 ° C. to 25% or more, it is possible to improve the shock absorption performance, particularly the shock absorption to a local shock. The shock absorption rate can be measured by the measuring method shown in Examples described later.

(Bending resistance in low temperature environment)
The shock absorbing sheet according to the second embodiment of the first invention does not cause a scratch of 1 mm or more when bent 4,000 times in the following dynamic bending resistance test. When the shock absorbing sheet is bent 4,000 times in the dynamic bending resistance test, if a scratch of 1 mm or more is generated at the center of the bending, the bending resistance in a low temperature environment becomes insufficient. From the viewpoint of improving the bending resistance in a low temperature environment, the shock absorbing sheet preferably does not have a scratch of 0.8 mm or more, and does not have a scratch of 0.6 mm or more after the dynamic bending resistance test. It is more preferable that scratches of 0.4 mm or more do not occur, and it is particularly preferable that scratches do not occur (0 mm).
The scratches generated in the dynamic bending resistance test are those that penetrate in the thickness direction of the shock absorbing sheet at the center of the bending. If there was a scratch other than the central part, it was measured again.
If the sample is scratched by 10 mm or more, record the number of breaks.
<Dynamic flexion resistance test>
In an environment of −35 ° C., the shock absorbing sheet is bent by bringing the pair of pedestals on which the shock absorbing sheet is erected at a distance of 30 mm so that the distance between the pedestals is 6 mm, and then the pair of pedestals are again used. The reciprocating motion of moving the gantry away from the gantry to the original distance between the gantry is repeated 4,000 times at a cycle of 46 times per minute.
The dynamic bending resistance test can be performed by a commercially available durability tester as described in the examples.

(Tensile strength at -35 ° C)
The shock absorbing sheet of the first invention preferably has a tensile strength at −35 ° C. of 4 MPa or more and 55 MPa or less. When it is 4 MPa or more, it becomes easy to secure good mechanical strength and shock absorption of the shock absorbing sheet even in a low temperature environment. Further, when it is 55 MPa or less, a certain degree of flexibility can be ensured in a low temperature environment, and bending resistance in a low temperature environment tends to be improved. From these viewpoints, the tensile strength of the shock absorbing sheet at −35 ° C. is more preferably 5 MPa or more, further preferably 6 MPa or more, further preferably 20 MPa or less, further preferably 17 MPa or less, still more preferably 15 MPa or less.
The tensile strength means the value of the maximum tensile force when the shock absorbing sheet is pulled until it breaks. The tensile strength was measured according to the method described in JIS K6767, and the measurement is performed in an environment of −35 ° C.
Further, as the tensile strength, the one having a high tensile elongation is adopted by measuring both the MD and TD directions. When it is unclear which direction is the MD or TD direction, the tensile strength in the direction in which the tensile elongation is maximized may be measured.

(Thickness of shock absorbing sheet)
The thickness of the shock absorbing sheet is preferably 200 μm or less, more preferably 20 μm or more and 180 μm or less, and further preferably 50 μm or more and 150 μm or less. By reducing the thickness of the shock absorbing sheet to 200 μm or less, it becomes possible to contribute to the thinning and miniaturization of electronic devices, and it becomes easy to prevent the bending resistance from being lowered. Further, by setting the thickness of the shock absorbing sheet to 20 μm or more, it is possible to prevent so-called bottoming from occurring when a shock is applied to the shock absorbing sheet, and the shock absorbing performance tends to be improved.
The thickness of the foamed resin layer preferably occupies 70% or more, more preferably 90% or more, and even more preferably 100% of the total thickness of the shock absorbing sheet. Since most of the shock absorbing sheet is formed of the foamed resin layer, the shock absorbing performance and the bending resistance in a low temperature environment can be improved.

(25% compression strength at 23 ° C)
The shock absorbing sheet of the first invention preferably has a 25% compression strength at 23 ° C. of 200 kPa or more and 700 kPa or less. When the 25% compression strength is within the above range, the shock absorbing sheet has appropriate flexibility, and it is easy to improve both the shock absorbing performance and the bending resistance. From these viewpoints, the 25% compression strength is more preferably 240 kPa or more and 600 kPa or less, and further preferably 280 kPa or more and 500 kPa or less.

[Foam resin layer]
Examples of the resin constituting the foamed resin layer in the shock absorbing sheet of the first invention include acrylic polymers, thermoplastic elastomers, and polyolefin resins. The resin constituting the foamed resin layer preferably contains an acrylic polymer. The acrylic polymer is preferably a polymer obtained by polymerizing the monomer component (A) containing urethane (meth) acrylate (A1). By using urethane (meth) acrylate (A1) as the acrylic polymer, while maintaining high shock absorption performance, the elongation at break in a low temperature environment becomes high and the bending resistance becomes good.
Further, as the resin constituting the foamed resin layer in the shock absorbing sheet of the first invention, a thermoplastic elastomer and a polyolefin resin may be used in combination. By foaming a resin containing these to obtain a foamed resin layer as described later, the shock absorbing sheet has high breaking elongation in a low temperature environment and good bending resistance while maintaining high shock absorbing performance. become.

(Acrylic polymer)
The urethane (meth) acrylate (A1) used in the acrylic polymer is preferably contained in an amount of 5% by mass or more and 80% by mass or less with respect to the monomer component (A). When it is 5% by mass or more, the effect of using the urethane (meth) acrylate (A1) can be sufficiently exhibited, and the elongation at break in a low temperature environment becomes high. On the other hand, when it is 80% by mass or less, the performance originally possessed by the acrylic polymer is exhibited, and the shock absorption performance is improved.
From these viewpoints, the urethane (meth) acrylate (A1) is more preferably contained in an amount of 10% by mass or more and 60% by mass or less, still more preferably 14% by mass or more and 40% by mass or less, based on the monomer component (A). ..

<Urethane (meth) acrylate (A1)>
The urethane (meth) acrylate (A1) is a compound having at least a urethane bond and a (meth) acryloyl group, and preferably has a polyalkylene oxide skeleton. The urethane (meth) acrylate (A1) has a polyalkylene oxide skeleton, which imparts flexibility, and the elongation at break in a low temperature environment tends to be high, so that the bending resistance tends to be good.
Urethane (meth) acrylate (A1) may be monofunctional having one (meth) acryloyl group in one molecule, or may have two or more (meth) acryloyl groups in one molecule. It may be functional, but it is preferably monofunctional. The monofunctionality makes it difficult for the flexibility of the shock absorbing sheet to be impaired, whereby the bending resistance in a low temperature environment can be improved while improving the shock absorbing performance. Further, as long as the urethane (meth) acrylate (A1) contains a monofunctional, it is preferable to use both the monofunctional and the polyfunctional in combination, and the polyfunctional at that time is more preferably bifunctional. When monofunctional and polyfunctional are used in combination, the content (parts by mass) of the monofunctional urethane (meth) acrylate (A1) may be higher than the content of the polyfunctional urethane (meth) acrylate (A1). preferable.
The (meth) acryloyl group means that it is at least one of a methacryloyl group and an acryloyl group, and other similar terms have the same meaning.

The polyalkylene oxide skeleton contained in the urethane (meth) acrylate (A1) is a skeleton obtained by polymerizing an oxylan compound such as ethylene oxide, propylene oxide, butylene oxide, or tetrahydrofuran, and among these, propylene is preferable. Use oxide. That is, the urethane (meth) acrylate (A1) preferably contains a structural unit derived from propylene oxide. Since the acrylic polymer contains a structural unit derived from propylene oxide, the elongation at break in a low temperature environment tends to be high, so that the bending resistance tends to be good. Propylene oxide may be used alone or in combination with other oxylan compounds such as ethylene oxide.

More specifically, the urethane (meth) acrylate (A1) reacts with an alcohol (a1) having a polyalkylene oxide skeleton with a compound (a2) having an isocyanate group and a (meth) acryloyl group. It is preferable to obtain it in. In the urethane (meth) acrylate (A1) thus obtained, a urethane bond is formed by reacting the isocyanate group of the compound (a2) with the hydroxyl group contained in the alcohol (a1).
Examples of the alcohol (a1) include polyalkylene glycols such as polypropylene glycol, polyethylene glycol, and polytetramethylene glycol, and the polyalkylene glycol may have one end sealed with an alkyl ether group or the like.
Examples of the compound (a2) include (meth) acryloyloxyalkyl isocyanates such as 2- (meth) acryloyloxyethyl isocyanate, 4- (meth) acryloyloxybutyl isocyanate, and 6- (meth) acryloyloxyhexyl isocyanate. Of these, 2- (meth) acryloyloxyethyl isocyanate is more preferred.
The urethane (meth) acrylate (A1) may be used alone or in combination of two or more.

The weight average molecular weight (MW) of urethane (meth) acrylate (A1) is preferably 4,000 or more and 50,000 or less. By setting the weight average molecular weight (MW) within the above range, sufficient flexibility can be obtained, and it becomes easy to improve the bending resistance in a low temperature environment.
The weight average molecular weight (MW) of the urethane (meth) acrylate (A1) is more preferably 5,000 or more and 45,000 or less, and further preferably 10,000 or more and 40,000 or less.
The weight average molecular weight (MW) is a standard polystyrene-equivalent weight average molecular weight measured by gel permeation chromatography (GPC).
Specifically, the weight average molecular weight can be measured under the following measurement conditions. The diluted solution obtained by diluting urethane (meth) acrylate 50 times with tetrahydrofuran (THF) was filtered through a filter (material: polytetrafluoroethylene, pore diameter: 0.2 μm). The obtained filtrate was supplied to a gel permeation chromatograph (2690 Separations Model manufactured by Waters), and GPC measurement was performed under the conditions of a sample flow rate of 1 ml / min and a column temperature of 40 ° C. to measure the polystyrene-equivalent molecular weight of the polymer. Then, the weight average molecular weight (Mw) and the molecular weight distribution (Mw / Mn) were determined. As the column, two "GPC KF-806L" manufactured by shodex were connected in series and used.

The acrylic polymer is preferably a copolymer of urethane (meth) acrylate (A1) and a monomer component other than the component (A1). The acrylic polymer is preferably a copolymer of urethane (meth) acrylate (A1) and alkyl (meth) acrylate (A2), or urethane (meth) acrylate (A1) and alkyl (meth) acrylate (A2). ) And other monomers (A3) other than these (A1) and (A2) components.

<Alkyl (meth) acrylate (A2)>
Examples of the alkyl (meth) acrylate include an alkyl (meth) acrylate having a linear or branched alkyl group, and the alkyl group has, for example, 1 to 18 carbon atoms. Alkyl (meth) acrylate is a monofunctional monomer having one (meth) acryloyl group.
Examples of the alkyl (meth) acrylate include methyl (meth) acrylate, ethyl (meth) acrylate, n-propyl (meth) acrylate, isopropyl (meth) acrylate, n-butyl (meth) acrylate, and isobutyl (meth) acrylate. sec-butyl (meth) acrylate, t-butyl (meth) acrylate, n-pentyl (meth) acrylate, n-hexyl (meth) acrylate, n-heptyl (meth) acrylate, 2-ethylhexyl (meth) acrylate, n- Octyl (meth) acrylate, isooctyl (meth) acrylate, n-nonyl (meth) acrylate, n-decyl (meth) acrylate, n-undecyl (meth) acrylate, n-dodecyl (meth) acrylate, n-tridecyl (meth) Examples thereof include acrylate and n-tetradecyl (meth) acrylate. The alkyl (meth) acrylate can be used alone or in combination of two or more.
The alkyl (meth) acrylate preferably has an alkyl group having 1 to 14 carbon atoms, more preferably 1 to 10 carbon atoms, and further preferably 1 to 8 carbon atoms, and the peak temperature of the loss coefficient (tan δ) is within the above-mentioned predetermined range. From the viewpoint of facilitating adjustment to the above, the most preferable is an alkyl acrylate having an alkyl group having 1 to 8 carbon atoms.

Adjusting the amount of alkyl (meth) acrylate (A2) used within a certain range makes it easier to improve the shock absorption performance. From such a viewpoint, the alkyl (meth) acrylate (A2) is preferably 20% by mass or more and 94% by mass or less, more preferably 30% by mass or more and 89% by mass or less, based on the monomer component (A). More preferably, it is contained in an amount of 50% by mass or more and 84% by mass or less.

<Other monomers (A3)>
The other monomer (A3) used in the acrylic polymer may be a monomer other than the components (A1) and (A2), and may be a monofunctional monomer or a polyfunctional monomer. Polyfunctional monomers are preferred. Further, as the other monomer (A3), a monofunctional monomer and a polyfunctional monomer may be used in combination. The other monomer (A3) may be a compound capable of polymerizing with the components (A1) or (A1) and (A2), but preferably has an unsaturated double bond, and more preferably has a (meth) acryloyl group. preferable.
Examples of the monofunctional monomer include a carboxyl group-containing monomer or an anhydride thereof, a hydroxyl group-containing (meth) acrylic monomer, a nitrogen-containing monomer, and a styrene-based monomer.

Examples of the carboxyl group-containing monomer include carboxylic acids having an unsaturated double bond such as (meth) acrylic acid, crotonic acid, silicic acid, itaconic acid, maleic acid, fumaric acid, and citraconic acid. Of these, (meth) acrylic acid is preferable.
Examples of the hydroxyl group-containing (meth) acrylic monomer include 2-hydroxyethyl (meth) acrylate, 2-hydroxypropyl (meth) acrylate, 4-hydroxybutyl (meth) acrylate, caprolactone-modified (meth) acrylate, and polyoxyethylene (meth). ) Acrylate and a hydroxyl group-containing (meth) acrylate such as polyoxypropylene (meth) acrylate can be mentioned.
Examples of nitrogen-containing monomers include (meth) acrylamide, N-methyl (meth) acrylamide, N, N-dimethyl (meth) acrylamide, N-ethyl (meth) acrylamide, N, N-diethyl (meth) acrylamide, and N-propyl. (Meta) acrylamide, N, N-dipropyl (meth) acrylamide, N-isopropyl (meth) acrylamide, N, N-diisopropyl (meth) acrylamide, N-butyl (meth) acrylamide, N, N-dibutyl (meth) acrylamide Acrylamides such as acrylamide, aminoethyl (meth) acrylate, amino group-containing (meth) acrylic monomer such as t-butylaminoethyl (meth) acrylate, (meth) acrylonitrile, N-vinylpyrrolidone, N-vinylcaprolactam, N-vinyl. Laurilolactam, (meth) acryloylmorpholine, dimethylaminomethyl (meth) acrylate and the like can be mentioned.
Examples of the styrene-based monomer include styrene, α-methylstyrene, o-methylstyrene, p-methylstyrene and the like.
In addition, the monofunctional monomer as the monomer (A3) may be used alone or in combination of two or more.
The monofunctional monomer as the other monomer (A3) is contained, for example, 0% by mass or more and 20% by mass or less with respect to the monomer component (A). By setting the content to 20% by mass or less, it becomes easy to maintain good shock absorption performance and bending resistance in a low temperature environment. From these viewpoints, the monofunctional monomer as the other monomer (A3) is preferably contained in an amount of 0% by mass or more and 8% by mass or less, more preferably 0% by mass or more and 4% by mass or less, based on the monomer component (A). ..

In addition, the polyfunctional monomer as the other monomer (A3) is incorporated into the main chain of the acrylic polymer, crosslinks the main chains to form a network, and has a function as a so-called cross-linking agent. The polyfunctional monomer as the other monomer (A3) is preferably a polyfunctional (meth) acrylate having two or more (meth) acryloyl groups. The polyfunctional monomer may be bifunctional or higher, but is preferably 2 to 4 functional, more preferably 2 or trifunctional.

Specific polyfunctional monomers include hexanediol di (meth) acrylate, polyethylene glycol di (meth) acrylate, polypropylene glycol di (meth) acrylate, ethicylated bisphenol A di (meth) acrylate, and tris (2-hydroxyethyl). Isocyanurate tri (meth) acrylate, ε-caprolactone-modified tris (2- (meth) acryloxyethyl) isocyanurate, caprolactone-modified ethoxylated isocyanurate tri (meth) acrylate, ethosylated trimethyl propantri (meth) acrylate, proxy Examples thereof include trimethylol propanetri (meth) acrylate, proxilated glyceryl tri (meth) acrylate, neopentyl glycol adipate di (meth) acrylate, and liquid hydride 1,2-polybutadienedi (meth) acrylate.
In addition, the polyfunctional monomer as the monomer (A3) may be used alone or in combination of two or more.

The polyfunctional monomer as the other monomer (A3) in the acrylic polymer is contained, for example, preferably 0.5% by mass or more and 20% by mass or less with respect to the monomer component (A). When the content of the polyfunctional monomer is within the above range, a network can be formed appropriately, and the shock absorption performance and the bending resistance in a low temperature environment can be easily improved. The content of the polyfunctional monomer as the other monomer (A3) is more preferably 1% by mass or more and 15% by mass or less, and further preferably 2% by mass or more and 10% by mass or less.

The resin component in the foamed resin layer is preferably an acrylic polymer, but may contain other resin components as long as the effects of the present invention are not impaired. The acrylic polymer is the main component of the resin component in the foamed resin layer, and its content is preferably 70% by mass or more, more preferably 90% by mass or more, most preferably, with respect to the total amount of the resin component in the foamed resin layer. It is preferably 100% by mass.

The foamed resin layer is formed from an acrylic resin composition containing at least one of the above-mentioned monomer component (A) and an acrylic polymer obtained by partially or completely polymerizing the monomer component (A). In the following description, when the term “100 parts by mass” of the monomer component (A) is used, it means the total amount of the content of the monomer component (A) and the content of the structural unit derived from the monomer component (A). ..
Therefore, when the bubbles of the foamed resin layer are formed by the hollow particles, it is preferable that the acrylic resin composition contains the hollow particles. The content of the hollow particles in the acrylic resin composition may be adjusted so that the apparent density can be adjusted within a desired range, and is, for example, 0.05 to 5% by mass with respect to 100 parts by mass of the monomer component (A). Parts, preferably 0.1 to 3 parts by mass.

When the above-mentioned monomer component (A) is polymerized by photopolymerization, a photopolymerization initiator may be added to the acrylic resin composition. In the acrylic resin composition, when bubbles are formed from hollow particles, it is preferable to form a resin foam layer by photopolymerization. Therefore, when bubbles are formed from hollow particles, it is preferable to include a photopolymerization initiator in the acrylic resin composition.
The photopolymerization initiator is not particularly limited, but for example, a ketal-based photopolymerization initiator, an α-hydroxyketone-based photopolymerization initiator, an α-aminoketone-based photopolymerization initiator, an acylphosphine oxide-based photopolymerization initiator, and a benzoin. Ether-based photopolymerization initiator, acetophenone-based photopolymerization initiator, alkylphenone-based photopolymerization initiator, aromatic sulfonyl chloride-based photopolymerization initiator, photoactive oxime-based photopolymerization initiator, benzoin-based photopolymerization initiator, benzyl-based Examples thereof include a photopolymerization initiator, a benzophenone-based photopolymerization initiator, and a thioxanthone-based photopolymerization initiator. The polymerization initiator can be used alone or in combination of two or more.
The amount of these photopolymerization initiators used is not particularly limited, but is preferably 0.03 parts by mass or more and 3 parts by mass or less, and 0.05 parts by mass or more and 1.5 parts by mass with respect to 100 parts by mass of the monomer component (A). It is more preferably less than or equal to parts by mass.

In addition to the above, the acrylic resin composition contains flame retardants such as surfactants, metal damage inhibitors, antistatic agents, stabilizers, nucleating agents, cross-linking aids, pigments, dyes, halogen-based and phosphorus-based agents. , And other additives such as fillers may be contained as long as the object of the present invention is not impaired.

(Thermoplastic elastomer)
Examples of the thermoplastic elastomer contained as the resin constituting the foamed resin layer in the shock absorbing sheet of the first invention include olefin-based thermoplastic elastomers, styrene-based thermoplastic elastomers, vinyl chloride-based thermoplastic elastomers, polyurethane-based thermoplastic elastomers, and polyesters. Examples thereof include based thermoplastic elastomers and polyamide-based thermoplastic elastomers. As the thermoplastic elastomer, these components may be used alone or in combination of two or more.
Among these, olefin-based thermoplastic elastomers, styrene-based thermoplastic elastomers, and ethylene-α-olefin-based copolymer rubbers are preferable, styrene-based thermoplastic elastomers and ethylene-α-olefin-based copolymer rubbers are more preferable, and styrene-based ones. Thermoplastic elastomers are more preferred.

Olefin-based thermoplastic elastomers (TPOs) generally have polyolefins such as polyethylene and polypropylene as hard segments, and butyl rubber, halobutyl rubber, EPDM (ethylene-propylene-diene rubber), EPM (ethylene-propylene rubber), NBR ( Rubber components such as acrylonitrile-butadiene rubber) and natural rubber are used as soft segments. As the olefin-based thermoplastic elastomer (TPO), any of a blend type, a dynamic cross-linking type, and a polymerization type can be used.
Preferable specific examples of the rubber component include the above-mentioned EPM and EPDM, and EPDM is particularly preferable. Examples of EPDM include ethylene-propylene-5-ethylidene-2-norbornene copolymer rubber and ethylene-propylene-dicyclopentadiene copolymer rubber. Among these, ethylene-propylene-dicyclopentadiene copolymer rubber is used. preferable.

Further, as the olefin-based thermoplastic elastomer, a block copolymer type can also be mentioned. Examples of the block copolymer type include those having a crystalline block and a soft segment block, and more specifically, a crystalline olefin block-ethylene / butylene copolymer-crystalline olefin block copolymer (CEBC) is exemplified. Will be done. In CEBC, the crystalline olefin block is preferably a crystalline ethylene block, and examples of such a commercially available product of CEBC include "DYNARON 6200P" manufactured by JSR Corporation.

Examples of the styrene-based thermoplastic elastomer include a block copolymer having a polymer or copolymer block of styrene and a polymer or copolymer block of a conjugated diene compound. Examples of the conjugated diene compound include isoprene and butadiene.
The styrene-based thermoplastic elastomer used in the present invention may or may not be hydrogenated, but hydrogenation is preferable. When hydrogenating, hydrogenation can be performed by a known method.

The styrene-based thermoplastic elastomer is usually a block copolymer, and is a styrene-isoprene block copolymer (SI), a styrene-isoprene-styrene block copolymer (SIS), or a styrene-butadiene block copolymer (SB). , Styrene-butadiene-styrene block copolymer (SBS), styrene-ethylene / butylene-styrene block copolymer (SEBS), styrene-ethylene / propylene-styrene block copolymer (SEPS), styrene-ethylene / ethylene / Styrene-styrene block copolymer (SEEPS), styrene-ethylene / butylene block copolymer (SEB), styrene-ethylene / propylene block copolymer (SEP), styrene-ethylene / butylene-crystalline olefin block copolymer (SEBC) and the like. Further, the styrene-based thermoplastic elastomer may be hydrogenated styrene-butadiene rubber (HSBR).
As the above-mentioned styrene-based thermoplastic elastomer, block copolymers are preferable, and SIS, SEBS, SEPS, SEEPS, and SEBC are more preferable, and SEEPS and SEBS are particularly preferable from the viewpoint of shock absorption performance and bending resistance in a low temperature environment. More preferred.

Examples of SIS include Kuraray Co., Ltd., trade name "Hybler (registered trademark) 5125" (styrene content 20% by mass, Tg = -13 ° C.), and SEPS's commercial product includes SEPS Co., Ltd. Examples thereof include Kuraray's trade name "Hybler (registered trademark) 7125F" (styrene content 20% by mass, Tg = -15 ° C.). Examples of commercially available SEEPS products include Kuraray Co., Ltd., trade name "Hybler (registered trademark) 7311F" (styrene content 12% by mass, Tg = −32 ° C.).
Examples of commercially available SEBS products include the Tough Tech (registered trademark) series manufactured by Asahi Kasei Corporation. Further, as a commercially available product of HSBR, a trade name "DYNARON 1321P" manufactured by JSR Corporation can be mentioned.

The styrene-based thermoplastic elastomer has a styrene-derived structural unit, which makes it possible to improve the impact resistance of the foamed sheet. The styrene content in the styrene-based thermoplastic elastomer is preferably 5 to 50% by mass. By setting the styrene content in the above range, the crosslinkability and foamability are improved, and excellent impact resistance can be obtained. From these viewpoints, the styrene content in the styrene-based thermoplastic elastomer is more preferably 7 to 40% by mass, further preferably 7 to 30% by mass.

The number average molecular weight of the styrene-based thermoplastic elastomer is not particularly limited, but is preferably 30,000 to 800,000, more preferably 120,000 to 180,000 from the viewpoint of breaking strength and processability.
The number average molecular weight is a potistyrene-equivalent value measured by gel permeation chromatography (GPC).

(Polyolefin resin)
The polyolefin resin contained as the resin constituting the foamed resin layer in the shock absorbing sheet of the first invention is a thermoplastic resin, and specific examples thereof include polyethylene resin, polypropylene resin, polybutene resin, ethylene-vinyl acetate copolymer and the like. Among these, polyethylene resin is preferable.
Further, as the polyethylene resin, low density polyethylene (LDPE) is preferable, and linear low density polyethylene (LLDPE) is more preferable.
Further, examples of the polyethylene resin include polyethylene resins polymerized with a polymerization catalyst such as a Cheegler-Natta catalyst, a metallocene catalyst, and a chromium oxide compound, and a polyethylene resin polymerized with a metallocene catalyst is preferably used.

Examples of the metallocene catalyst include compounds such as a bis (cyclopentadienyl) metal complex having a structure in which a transition metal is sandwiched between π-electron unsaturated compounds. More specifically, one or more cyclopentadienyl rings or their analogs are present as ligands in tetravalent transition metals such as titanium, zirconium, nickel, palladium, hafnium, and platinum. Can be mentioned.
In such a metallocene catalyst, the properties of active sites are uniform, and each active site has the same activity. A polymer synthesized using a metallocene catalyst has high uniformity in molecular weight, molecular weight distribution, composition, composition distribution, etc. Therefore, when a sheet containing a polymer synthesized using a metallocene catalyst is crosslinked, the cross-linking is uniform. Proceed to. Since the uniformly crosslinked sheet is uniformly foamed, it becomes easy to stabilize the physical properties. Moreover, since it can be stretched uniformly, the thickness of the foam can be made uniform.

Examples of the ligand include a cyclopentadienyl ring, an indenyl ring and the like. These cyclic compounds may be substituted with hydrocarbon groups, substituted hydrocarbon groups or hydrocarbon-substituted metalloid groups. Examples of the hydrocarbon group include methyl group, ethyl group, various propyl group, various butyl group, various amyl group, various hexyl group, 2-ethylhexyl group, various heptyl group, various octyl group, various nonyl group and various decyl group. , Various cetyl groups, phenyl groups and the like. In addition, "various" means various isomers including n-, sec-, tert-, and iso-.
Further, a product obtained by polymerizing a cyclic compound as an oligomer may be used as a ligand.
Furthermore, in addition to π-electron unsaturated compounds, monovalent anion ligands such as chlorine and bromine, divalent anion chelate ligands, hydrocarbons, alkoxides, arylamides, aryloxides, amides, phosphides, arylphosphides, etc. May be used.

Examples of metallocene catalysts containing tetravalent transition metals and ligands include cyclopentadienyl titanium tris (dimethylamide), methylcyclopentadienyl titanium tris (dimethylamide), bis (cyclopentadienyl) titanium dichloride, and dimethyl. Examples thereof include silyltetramethylcyclopentadienyl-t-butylamide zirconium dichloride.
The metallocene catalyst exerts an action as a catalyst in the polymerization of various olefins by combining with a specific co-catalyst (co-catalyst). Specific examples of the co-catalyst include methylaluminoxane (MAO) and boron-based compounds. The ratio of the co-catalyst used to the metallocene catalyst is preferably 10 to 1 million mol times, more preferably 50 to 5,000 mol times.

Further, as the polyethylene resin, linear low-density polyethylene is preferable, and linear low-density polyethylene polymerized with a metallocene catalyst is more preferable. The linear low-density polyethylene is obtained by copolymerizing ethylene (for example, 75% by mass or more, preferably 90% by mass or more) with respect to the total amount of monomers and, if necessary, a small amount of α-olefin. Chain low density polyethylene is more preferred. Specific examples of the α-olefin include propylene, 1-butene, 1-pentene, 4-methyl-1-pentene, 1-hexene, 1-heptene, 1-octene and the like. Of these, α-olefins having 4 to 10 carbon atoms are preferable.
Polyethylene resin, for example the density of the above-mentioned linear low density polyethylene, from the viewpoint of flexibility, preferably 0.870~0.925g / cm ^3, more preferably 0.890~0.925g / cm ^3, 0 .910-0.925 g / cm ³ is even more preferred. As the polyethylene resin, a plurality of polyethylene resins may be used, or a polyethylene resin other than the above-mentioned density range may be added.

Examples of the ethylene-vinyl acetate copolymer used as the polyolefin resin include an ethylene-vinyl acetate copolymer containing 50% by mass or more of ethylene.
Examples of the polypropylene resin include homopolypropylene and a propylene-α-olefin copolymer containing 50% by mass or more of propylene. One of these may be used alone, or two or more thereof may be used in combination. Specific examples of the α-olefin constituting the propylene-α-olefin copolymer include ethylene, 1-butene, 1-pentene, 4-methyl-1-pentene, 1-hexene, 1-hexene, and 1-. Examples thereof include octene, and among these, α-olefins having 6 to 12 carbon atoms are preferable.
Examples of the polybutene resin include a homopolymer of butene-1 and a copolymer with ethylene or propylene.

The polyolefin resin may be used in combination with the thermoplastic elastomer as described above. The mass ratio of the polyolefin resin to the thermoplastic elastomer in the foamed resin layer (polyolefin resin / thermoplastic elastomer) is, for example, 5/95 to 70/30, preferably 10/90 to 55/45. At this time, the resin component in the foamed resin layer may contain other resin components as long as the effects of the present invention are not impaired. However, the polyolefin resin and the thermoplastic elastomer are the main components of the resin component in the foamed resin layer, and the total content thereof is preferably 70% by mass or more, more preferably 90% by mass or more, based on the total amount of the resin component in the foamed resin layer. By mass or more, most preferably 100% by mass.

(Bubble)
The foamed resin layer of the first invention preferably has air bubbles. The bubbles in the foamed resin layer can be formed by means for containing hollow particles. It is preferable that the foamed resin layer contains hollow particles and bubbles are formed by the space inside the hollow particles.
The hollow particles contained in the foamed resin layer are not particularly limited, and may be hollow inorganic microspheres, hollow organic microspheres, or a hollow organic-inorganic composite. It may be a microsphere. Examples of the hollow inorganic microspheres include a hollow glass balloon such as a hollow glass balloon, a hollow silica balloon, a hollow balloon made of a metal compound such as a hollow alumina balloon, and a porcelain hollow balloon such as a hollow ceramic balloon. Can be mentioned. Examples of the hollow organic microspheres include hollow acrylic balloons, hollow vinylidene chloride balloons, phenol balloons, and resin-made hollow balloons such as epoxy balloons.

The average particle size of the hollow particles is not particularly limited as long as it is not more than the thickness of the foamed resin layer, but is preferably 10 μm or more and 150 μm or less, more preferably 20 μm or more and 130 μm or less, and 30 μm or more and 100 μm or less. Is even more preferable. When the average particle size of the hollow particles is within the above range, sufficient shock absorption can be obtained.
The average particle size of the hollow particles can be measured by, for example, a laser diffraction method or a low-angle laser light scattering method.

The ratio of the average particle size of the hollow particles to the thickness of the foamed resin layer (average particle size / thickness) is preferably 0.1 or more and 0.9 or less, and 0.2 or more and 0.85 or less. Is preferable. The foamed resin layer is formed by setting the ratio of the average particle size of the hollow particles to the thickness of the foamed resin layer (average particle size / thickness) within the above range and setting the viscosity as described later within a predetermined range. It is possible to prevent a part of the hollow particles from being lifted during the process and the final porosity distribution from becoming non-uniform.

The density of the hollow particles in the foamed resin layer is not particularly limited ^{, but is preferably 0.01 g / cm 3} or more and 0.4 g / cm ³ or less, and 0.02 g / cm ³ or more and 0.3 g / cm ³ or less. More preferably. When the density of the hollow particles in the foamed resin layer is within the above range, it is possible to prevent the hollow particles from floating when forming the foamed resin layer and to uniformly disperse the hollow particles.

Further, the bubbles in the foamed resin layer may be formed by means other than the hollow particles, and may be formed by, for example, a gas mixed in the resin composition. In this case, the bubbles are voids directly formed in the resin composition constituting the foamed resin layer, and the inner surface of the bubbles is made of the resin composition. That is, the bubbles in the foamed resin layer are bubbles whose inner wall does not have a shell structure. The gas mixed in the foamed resin layer may be a gas generated from a foaming agent or the like blended in the resin composition constituting the foamed resin layer, but is mixed from the outside of the resin composition by a mechanical floss method or the like described later. It is preferably a gas.

The density of the foamed resin layer ^{is preferably 0.3 g / cm 3} or more and 0.8 g / cm ³ or less. When the density is within the above range, when an impact is applied to the impact absorbing sheet, the impact can be sufficiently absorbed by the foamed resin layer. The density of the foamed resin layer, from the viewpoint of further improving the impact absorption performance, more preferably at most 0.45 g / cm ³ or more ^{0.8g / cm 3, 0.6g / cm} 3 or more 0.78 g / cm It is more preferably ^{3 or less.}
The density of the foamed resin layer means the apparent density.

The resin foam layer may have closed cells, may have open cells, or may have both closed cells and open cells, but mainly has closed cells. 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 further preferably 80% or more and 100% or less. The closed cell ratio can be obtained, for example, in accordance with JIS K7138 (2006).

The thickness of the foamed resin layer may be any thickness as long as it can exhibit appropriate shock absorption performance and bending performance, preferably 200 μm or less, more preferably 20 μm or more and 180 μm or less, and 50 μm or more and 150 μm or less. More preferred.

When the resin constituting the foamed resin layer in the shock absorbing sheet of the first invention contains the thermoplastic elastomer and the polyolefin resin as described above, the foamed resin layer is preferably obtained by foaming with a foaming agent. That is, the foamed resin layer may be obtained by foaming a foamable composition containing a resin containing a thermoplastic elastomer and a polyolefin resin and a foaming agent. As the foaming agent, a thermally decomposable foaming agent is preferable.
As the pyrolytic foaming agent, an organic foaming agent and an inorganic foaming agent can be used. Examples of the organic foaming agent include azodicarboxylic amides, azodicarboxylic acid metal salts (such as barium azodicarboxylic acid), azo compounds such as azobisisobutyronitrile, nitroso compounds such as N, N'-dinitrosopentamethylenetetramine, and hydrazine. Examples thereof include zodicarboxylic amides, hydrazine derivatives such as 4,4'-oxybis (benzenesulfonyl hydrazide) and toluenesulfonyl hydrazide, and semicarbazide compounds such as toluenesulfonyl semicarbazide.
Examples of the inorganic foaming agent include ammonium carbonate, sodium carbonate, ammonium hydrogencarbonate, sodium hydrogencarbonate, ammonium nitrite, sodium borohydride, monosoda anhydrous citrate and the like.
Among these, an azo compound is preferable, and an azodicarbonamide is more preferable, from the viewpoint of obtaining fine bubbles, and from the viewpoint of economy and safety.
One type of pyrolysis foaming agent may be used alone, or two or more types may be used in combination.

The blending amount of the foaming agent in the effervescent composition is preferably 1 part by mass or more and 20 parts by mass or less, more preferably 1.5 parts by mass or more and 15 parts by mass or less, and 2 parts by mass or more and 10 parts by mass with respect to 100 parts by mass of the resin. More preferably, it is by mass or less. By setting the blending amount of the foaming agent to the above lower limit value or more, the foamable sheet is appropriately foamed, and it is possible to impart appropriate flexibility and shock absorption to the foam sheet. Further, by setting the blending amount of the foaming agent to the above upper limit value or less, it is possible to prevent the foamed resin layer from foaming more than necessary, and to improve the mechanical strength of the shock absorbing sheet.

The effervescent composition containing the above-mentioned foaming agent may contain a decomposition temperature adjusting agent. The decomposition temperature adjusting agent is compounded as having an adjusting function such as lowering the decomposition temperature of the thermally decomposing foaming agent and accelerating the decomposition rate, and specific compounds include zinc oxide, zinc stearate, and urea. And so on. The decomposition temperature adjusting agent is blended in an amount of 0.01 to 5 parts by mass with respect to 100 parts by mass of the resin, for example, in order to adjust the surface condition of the foam sheet.

In addition, an antioxidant may be blended in the foaming composition containing the above-mentioned foaming agent. Examples of the antioxidant include phenolic antioxidants such as 2,6-di-t-butyl-p-cresol, sulfur-based antioxidants, phosphorus-based antioxidants, amine-based antioxidants and the like. For example, the antioxidant is blended in an amount of 0.01 to 5 parts by mass with respect to 100 parts by mass of the resin.
In addition to these, the effervescent composition containing a foaming agent may contain additives generally used for foams such as heat stabilizers, colorants, flame retardants, antistatic agents, and fillers. ..

[Shock absorption sheet]
The shock absorbing sheet of the first invention is preferably composed of the above-mentioned foamed resin layer alone, but a layer other than the foamed resin layer may be provided as long as the effect of the present invention is not impaired. By providing a layer other than the foamed resin layer, it is possible to impart light-shielding properties and improve processability and handleability. For example, the skin layer may be provided on one side or both sides of the foamed resin layer. The skin layer is, for example, a non-foamed resin layer composed of various resins.

The skin layer may be formed of the same type of resin as the resin constituting the foamed resin layer, or may be formed of any other resin. The resin forming the skin layer may be a thermoplastic elastomer, a polyolefin resin, a polyester resin, a urethane resin, a polyimide, a polyethylene naphthalate, or the like, but may be a rubber resin, in addition to the acrylic polymer described above. Further, the skin layer may be a metal foil, a non-woven fabric, or the like.
The skin layer may have a thickness that does not interfere with the function of the foamed resin layer, and the thickness of each skin layer may be less than the thickness of the foamed resin layer. The thickness of each skin layer is, for example, preferably 1 μm or more and 50 μm or less, and more preferably 1 μm or more and 20 μm or less.

[Manufacturing method of shock absorbing sheet (1)]
Hereinafter, a method for manufacturing the shock absorbing sheet will be described in detail. In the following, first, a method for producing a foamed resin layer when bubbles are formed from hollow particles will be described.
The foamed resin layer may be formed from a resin composition containing a resin component or a precursor of a resin component that becomes a resin component by curing, hollow particles, and other additives to be blended as needed. Good. Hereinafter, a case where the resin component is an acrylic polymer will be described in detail as an example.
In the formation of the foamed resin layer, first, the monomer component (A) or a component obtained by polymerizing at least a part of the monomer component (A), hollow particles, and other additives to be blended as needed are contained. An acrylic resin composition may be prepared.
Next, the acrylic resin composition may be formed into a film, and the foamed resin layer may be formed by polymerizing the above-mentioned monomer component (A) or a component obtained by polymerizing at least a part of the monomer component (A). Specifically, the foamed resin layer is preferably a component obtained by polymerizing at least a part of the monomer component (A) or the monomer component (A) on an appropriate support such as a release paper, a release film, or a base material. It is preferable to apply an acrylic resin composition containing hollow particles, a photopolymerization initiator and the like to form a coating layer, and to cure the layer with active energy rays. A release paper, a release film, or the like may be further laminated on the formed coating layer before irradiating the active energy rays.
The separators such as the release paper and the release film used for forming the acrylic foamed resin layer may be appropriately peeled off before using the shock absorbing sheet after production.

Here, the monomer component (A) contained in the acrylic resin composition is preferably partially polymerized. The monomer component (A) generally has a very low viscosity. Therefore, by using a partially polymerized (partially polymerized) acrylic resin composition, the shock absorbing sheet of the first invention can be produced more efficiently.
The acrylic resin composition in which the monomer component (A) is partially polymerized can be produced, for example, as follows. First, it is active against a composition that does not contain hollow particles, contains a monomer component (A) excluding a polyfunctional monomer as another component (A3), and, if necessary, contains a photopolymerization initiator or the like. Partial polymerization is carried out by polymerization using energy rays to prepare a so-called syrup-like curable acrylic resin material. The viscosity at this time is preferably adjusted to 200 mPa · s or more and 5,000 mPa · s or less, and more preferably 300 mPa · s or more and 4,000 mPa · s or less. By adjusting the viscosity within the above range, it is possible to prevent the hollow particles from floating and to make the porosity in the thickness direction uniform. The viscosity is the viscosity measured under the conditions of a measurement temperature of 23 ° C. and 100 rpm in the viscosity measurement with a B-type viscometer.
Next, hollow particles and, if necessary, a polyfunctional monomer as another component (A3) were added to the curable acrylic resin material, and the mixture was stirred and mixed to disperse the hollow particles in the curable acrylic resin material. A based resin composition can be prepared.

The coating method used when applying the acrylic resin composition is not particularly limited, and a normal method can be adopted. Examples of such a coating method include a slot die method, a reverse gravure coating method, a microgravure method, a dip method, a spin coating method, a brush coating method, a roll coating method, and a flexographic printing method.
Examples of active energy rays include ionizing radiation such as α-rays, β-rays, γ-rays, neutron rays, and electron beams, and ultraviolet rays. Among these, ultraviolet rays are particularly preferably used. The irradiation energy of the active energy ray and the irradiation time thereof are not particularly limited as long as the monomer component (A) can be appropriately polymerized, but the illuminance is, for example, 3.0 to 4.5 mW / cm ² , preferably 3.5 to 4. It is .5 mW / cm ² , and the amount of light is, for example, 150 to 1,000 mJ / cm ² , preferably 300 to 900 mJ / cm ² .

Next, an example of a method for producing the foamed resin layer when bubbles are formed by the gas mixed in the acrylic resin composition will be described. When the bubbles are formed by the gas mixed in the acrylic resin composition, the foamed resin layer may be produced from, for example, an acrylic emulsion as a raw material. Acrylic emulsions are aqueous dispersions of acrylic polymers. In the acrylic emulsion, the average particle size of the acrylic polymer must be smaller than the sheet thickness, preferably 100 μm or less. Further, in order to stabilize the captured bubbles, the average particle size of the acrylic polymer is more preferably 5 μm or less. Further, in order to improve the stability of the foam, the average particle size of the acrylic polymer is further preferably 1 μm or less, further preferably 500 nm or less, still more preferably 300 nm or less. The average particle size of the resin can be measured as a volume average particle size measured by a particle size distribution measuring device (Nanotrac 150 manufactured by Microtrac).

The acrylic emulsion can be obtained, for example, by subjecting the monomer component (A) to emulsion polymerization, suspension polymerization, dispersion polymerization, etc. in the presence of a polymerization initiator, an emulsifier, a dispersion stabilizer, etc., which are blended as necessary. be able to. The foamed resin layer can be produced by a method described later using an emulsion composition (acrylic resin composition) containing an emulsion such as an acrylic emulsion as a raw material.
The emulsion composition contains water as a dispersion medium. In addition to water, polar solvents such as methyl alcohol, ethyl alcohol, and isopropyl alcohol may be contained. Further, the emulsion composition may contain a foaming agent made of a surfactant or the like, a cross-linking agent or the like, if necessary. The solid content of the emulsion composition is, for example, preferably 30% by mass or more and 70% by mass or less, and more preferably 35% by mass or more and 60% by mass or less.

A foamed resin layer can be produced by forming bubbles in the above-mentioned emulsion composition by mixing a gas and forming a film of the emulsion composition in which the bubbles are formed.
It is preferable to mix the gas into the emulsion composition by the mechanical floss method. Specifically, it is a method in which the emulsion composition is stirred with a stirring blade or the like and air or gas in the atmosphere is mixed, and the supply may be a continuous type or a batch type. As the gas, nitrogen, air, carbon dioxide, argon or the like can be used. The amount of gas mixed may be appropriately adjusted so that the obtained foamed resin layer has the above-mentioned density. Specifically, it is advisable to adjust the stirring time and the mixing ratio with air or gas.
The emulsion composition in which the bubbles are formed is then applied onto an appropriate support such as a release paper, a release film, or a base material to form a coating layer, and the layer is heated and dried to cause foaming. A resin layer can be obtained. Here, the heating temperature is not particularly limited, but is preferably 45 to 155 ° C, more preferably 50 to 150 ° C.

The viscosity of the emulsion composition in which the bubbles are formed is preferably adjusted to 1,000 mPa · s or more and 50,000 mPa · s or less, and is adjusted to 2,000 mPa · s or more and 45,000 mPa · s or less. Is more preferable. By adjusting the viscosity within the above range, it is possible to prevent the mixed air bubbles from rising and make the porosity in the thickness direction uniform. The viscosity of the emulsion composition can be adjusted by the stirring time when the gas is mixed, the amount of the gas mixed, the solid content of the emulsion composition, and the like. Specifically, it is possible to increase the viscosity by extending the stirring time, increasing the mixing amount, increasing the solid content, and the like.

When a skin layer is provided in addition to the foamed resin layer in the shock absorbing sheet, the skin layer is formed by applying a resin material for forming the skin layer to the foamed resin layer and drying it if necessary. It is good to do it. Further, the skin layer may be formed by laminating the resin layer on one side or both sides of the foamed resin layer.

[Manufacturing method of shock absorbing sheet (2)]
Hereinafter, a method for manufacturing the shock absorbing sheet will be described in detail. Hereinafter, a method for producing the foamed resin layer in the case where bubbles are formed by the gas generated from the foaming agent or the like blended in the resin composition constituting the foamed resin layer will be described.

The foamed resin layer of the shock absorbing sheet of the first invention is not particularly limited, but the foamable sheet made of a foamable composition containing at least a resin and a pyrolytic foaming agent is heated to foam the pyrolytic foaming agent. It can be manufactured by letting it. Further, preferably, the foamable sheet is crosslinked, and the crosslinked foamable sheet is heated to foam.
More specifically, the method for producing the foam sheet preferably includes the following steps (1) to (3).
Step (1): A step of forming an effervescent sheet composed of an effervescent composition containing at least a resin and a pyrolytic foaming agent Step (2): Irradiating the effervescent sheet with ionizing radiation to crosslink the effervescent sheet. Step Step (3): A step of heating a crosslinked foamable sheet to foam a pyrolyzable foaming agent to obtain a foam sheet.

In the step (1), the method for forming the foamable sheet is not particularly limited, but for example, the resin and the additive are supplied to the extruder, melt-kneaded, and the foamable composition is extruded into a sheet from the extruder. It may be molded by this. Further, the foamable sheet may be molded by pressing the foamable composition or the like. The molding temperature of the foamable sheet (that is, the temperature at the time of extrusion or the temperature at the time of pressing) is preferably 50 ° C. or higher and 250 ° C. or lower, and more preferably 80 ° C. or higher and 180 ° C. or lower.

As a method for cross-linking the effervescent composition in the step (2), a method of irradiating the effervescent sheet with ionizing radiation such as electron beam, α ray, β ray, and γ ray is used. The irradiation amount of the ionizing radiation may be adjusted so that the degree of cross-linking of the obtained foam sheet is within the above-mentioned desired range, but it is preferably 1 to 12 Mrad, and preferably 1.5 to 10 Mrad. More preferred.

In the step (3), the heating temperature at which the foamable composition is heated to foam the pyrolysis foaming agent may be equal to or higher than the foaming temperature of the pyrolysis foaming agent, but is preferably 200 to 300 ° C. More preferably, it is 220 to 280 ° C. In step (3), the effervescent composition is foamed to form bubbles to form a foam.

Further, in the present production method, the foam sheet may be thinned by a method such as rolling or stretching.

However, the present production method is not limited to the above, and a foam sheet may be obtained by a method other than the above. For example, instead of irradiating with ionizing radiation, an organic peroxide may be blended in advance in the effervescent composition, and the effervescent sheet may be heated to decompose the organic peroxide, or the like for cross-linking. ..
If cross-linking is not necessary, step (2) may be omitted. In that case, in step (3), the uncrosslinked foamable sheet may be heated to foam.

[How to use the shock absorbing sheet]
Hereinafter, a method of using the shock absorbing sheet of the first invention described above will be described.
The shock absorbing sheet of the first 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.

When used in a display device, the shock absorbing sheet is arranged on the back side of various display devices to absorb the shock acting on the display device. The back surface of the display device is a surface opposite to the surface on which the image of the display device is displayed.
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 first 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. Further, since the shock absorbing sheet can appropriately absorb the shock even when a relatively large shock is locally applied, it is necessary to appropriately prevent display defects caused by the display device. Becomes possible.

Further, the display device is preferably a flexible display. Examples of the flexible display include a foldable display and a rollable display. Since portable electronic devices are used over a wide temperature range, shock absorbing sheets can be folded in low temperature environments when used in flexible displays. Since the shock absorbing sheet of the first invention has good bending resistance in a low temperature environment in addition to shock absorbing properties, it can be used for a long period of time without being damaged even when used for a flexible display. ..

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 each component such as a housing of an electronic device via an adhesive material. In this case, the adhesive tape includes a shock absorbing sheet and an adhesive material provided on at least one surface of the shock absorbing sheet.
The pressure-sensitive adhesive is a member having pressure-sensitive adhesiveness, and may be a member having at least a pressure-sensitive adhesive layer, and may be composed of a single pressure-sensitive adhesive layer laminated on the surface of a shock-absorbing sheet, or may be a shock-sensitive material. It may be a double-sided pressure-sensitive adhesive sheet attached to the surface of the absorption sheet, but it is preferably a single pressure-sensitive adhesive layer. The double-sided pressure-sensitive adhesive sheet includes a base material and pressure-sensitive adhesive layers 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 part.
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 paper pattern may be further attached on the adhesive material to protect the pressure-sensitive adhesive layer with the paper pattern before use.

(Second invention)
[Foam resin layer]
The shock absorbing sheet of the second invention of the present invention contains a foamed resin layer, and the foamed resin layer contains an acrylic polymer. The acrylic polymer is a polymer obtained by polymerizing a monomer component (A) containing urethane (meth) acrylate (A1). By using urethane (meth) acrylate (A1) as the acrylic polymer, the bending resistance in a low temperature environment is improved.

(Acrylic polymer)
The urethane (meth) acrylate (A1) used in the acrylic polymer is contained in an amount of 5% by mass or more based on the monomer component (A). If it is less than 5% by mass, the effect of using the urethane (meth) acrylate (A1) cannot be sufficiently exhibited, and the bending resistance in a low temperature environment is not improved.
Further, the urethane (meth) acrylate (A1) is preferably contained in an amount of, for example, 95% by mass or less with respect to the monomer component (A) in order to contain another monomer component. Further, in the present invention, when the compounding composition is set to a certain range, it is possible to provide a shock absorbing sheet having good bending resistance even in a low temperature environment while maintaining good shock absorbing performance. From such a viewpoint, the urethane (meth) acrylate (A1) is contained in an amount of preferably 8% by mass or more and 60% by mass or less, more preferably 10% by mass or more and 50% by mass or less, based on the monomer component (A). More preferably, it is 15% by mass or more and 40% by mass or less.

<Urethane (meth) acrylate (A1)>
The urethane (meth) acrylate (A1) is a compound having at least a urethane bond and a (meth) acryloyl group, and preferably has a polyalkylene oxide skeleton. Urethane (meth) acrylate (A1) has a polyalkylene oxide skeleton, which imparts flexibility and tends to have good bending resistance in a low temperature environment.
Urethane (meth) acrylate (A1) may be monofunctional having one (meth) acryloyl group in one molecule, or may have two or more (meth) acryloyl groups in one molecule. It may be functional, but it is preferably monofunctional. The monofunctionality makes it difficult for the flexibility of the shock absorbing sheet to be impaired, whereby the bending resistance in a low temperature environment can be improved. Further, as long as the urethane (meth) acrylate (A1) contains a monofunctional, it is preferable to use both the monofunctional and the polyfunctional in combination, and the polyfunctional at that time is more preferably bifunctional. When monofunctional and polyfunctional are used in combination, the content (parts by mass) of the monofunctional urethane (meth) acrylate (A1) may be higher than the content of the polyfunctional urethane (meth) acrylate (A1). preferable.
The urethane (meth) acrylate (A1) is the same as that described in the first invention described above, and the description thereof will be omitted.

The acrylic polymer is preferably a copolymer of urethane (meth) acrylate (A1) and a monomer component other than the component (A1). The acrylic polymer is preferably a copolymer of urethane (meth) acrylate (A1) and alkyl (meth) acrylate (A2), or urethane (meth) acrylate (A1) and alkyl (meth) acrylate (A2). ) And other monomers (A3) other than these (A1) and (A2) components.

<Alkyl (meth) acrylate (A2)>
When the amount of the alkyl (meth) acrylate (A2) used is adjusted within a certain range, the peak temperature of tan δ is adjusted within a desired range as described later, and it becomes easy to improve the shock absorption performance. From such a viewpoint, the alkyl (meth) acrylate (A2) is preferably contained in an amount of, for example, 4% by mass or more and 94% by mass with respect to the monomer component (A). From the viewpoint of facilitating improvement of shock absorption performance while improving bending resistance in a low temperature environment, alkyl (meth) acrylate (A2) is preferably 30% by mass or more and 89% by mass with respect to the monomer component (A). Hereinafter, it is more preferably contained in an amount of 45% by mass or more and 84% by mass or less, and further preferably 50% by mass or more and 78% by mass or less.
The alkyl (meth) acrylate (A2) is the same as that described in the first invention described above, and the description thereof will be omitted.

<Other monomers (A3)>
The monofunctional monomer as the other monomer (A3) is contained, for example, 0% by mass or more and 20% by mass or less with respect to the monomer component (A). By setting the content to 20% by mass or less, it becomes easy to maintain good shock absorption performance and bending resistance in a low temperature environment. From these viewpoints, the monofunctional monomer as the other monomer (A3) is preferably contained in an amount of 0% by mass or more and 8% by mass or less, more preferably 0% by mass or more and 4% by mass or less, based on the monomer component (A). ..

The other monomer (A3) used in the acrylic polymer may be a monomer other than the components (A1) and (A2), and may be a monofunctional monomer or a polyfunctional monomer. Polyfunctional monomers are preferred. Further, as the other monomer (A3), a monofunctional monomer and a polyfunctional monomer may be used in combination.
The polyfunctional monomer as the other monomer (A3) in the acrylic polymer is contained, for example, 0.5% by mass or more and 20% by mass or less with respect to the monomer component (A). When the content of the polyfunctional monomer is within the above range, a network can be formed appropriately, and the shock absorption performance and the bending resistance in a low temperature environment can be easily improved. The content of the polyfunctional monomer as the other monomer (A3) is preferably 1% by mass or more and 15% by mass or less, and more preferably 2% by mass or more and 10% by mass or less.
The other monomer (A3) is the same as that described in the first invention described above, and the description thereof will be omitted.

The resin component in the foamed resin layer is preferably composed of an acrylic polymer, and may contain other resin components as long as the effects of the present invention are not impaired. The acrylic polymer is the main component of the resin component in the foamed resin layer, and its content is preferably 70% by mass or more, more preferably 90% by mass or more, most preferably, with respect to the total amount of the resin component in the foamed resin layer. It is preferably 100% by mass.

(Bubble)
The foamed resin layer of the second invention has bubbles. It is preferable that the foamed resin layer contains hollow particles and bubbles are formed by the space inside the hollow particles. Further, the bubbles in the foamed resin layer may be formed by means other than the hollow particles, and may be formed by, for example, a gas mixed in the resin composition.
The bubbles and the hollow particles forming the bubbles are the same as those described in the first invention described above, and the description thereof will be omitted.

The foamed resin layer is formed from an acrylic resin composition containing at least one of the above-mentioned monomer component (A) and an acrylic polymer obtained by partially or completely polymerizing the monomer component (A). The acrylic resin composition is the same as that described in the first invention described above, and the description thereof will be omitted.

The density of the foamed resin layer is preferably 0.3 g / cm ³ or more and 0.8 g / cm ³ or less. When the density is within the above range, when an impact is applied to the shock absorbing sheet, the impact can be sufficiently absorbed by the shock absorbing sheet. The density of impact-absorbing sheet, from the viewpoint of further improving the impact absorption performance, more preferably at most 0.45 g / cm ³ or more ^{0.8g / cm 3, 0.6g / cm} 3 or more 0.78 g / cm It is more preferably ^{3 or less.} The density of the foamed resin layer means the apparent density.

The resin foam layer may have closed cells, may have open cells, or may have both closed cells and open cells, but mainly has closed cells. 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 further preferably 80% or more and 100% or less. The closed cell ratio can be obtained, for example, in accordance with JIS K7138 (2006).

The thickness of the foamed resin layer may be as long as it can exhibit appropriate shock absorption performance and bending performance even if the shock absorbing sheet is thin, preferably 200 μm or less, more preferably 20 μm or more and 180 μm or less, and 50 μm. It is more preferably 150 μm or less.

[Shock absorption sheet]
The shock absorbing sheet of the second invention is preferably composed of the above-mentioned foamed resin layer alone, but a layer other than the foamed resin layer may be provided as long as the effect of the present invention is not impaired. By providing a layer other than the foamed resin layer, it is possible to impart light-shielding properties and improve processability and handleability. For example, the skin layer may be provided on one side or both sides of the foamed resin layer. The skin layer is, for example, a non-foamed resin layer composed of various resins.
The skin layer is the same as that described in the first invention described above, and the description thereof will be omitted.

The thickness of the shock absorbing sheet is, for example, 200 μm or less, preferably 20 μm or more and 180 μm or less, and more preferably 50 μm or more and 150 μm or less. By setting the thickness to 200 μm or less, it becomes possible to contribute to making the electronic device thinner and smaller, and it becomes easier to prevent the bending resistance from being lowered. Further, by setting the thickness of the shock absorbing sheet to 20 μm or more, it is possible to prevent so-called bottoming from occurring when a shock is applied to the shock absorbing sheet, and the shock absorbing performance tends to be improved.
The thickness of the foamed resin layer preferably occupies 70% or more, more preferably 90% or more, and even more preferably 100% of the total thickness of the shock absorbing sheet. Since most of the shock absorbing sheet is formed of a foamed resin layer, the shock absorbing performance and bending resistance in a low temperature environment tend to be improved.

The shock absorbing sheet preferably has at least one peak loss coefficient (tan δ) between 25-25 ° C. When the loss coefficient (tan δ) has a peak within this temperature range, the impact resistance performance of the impact absorbing sheet becomes good. From the viewpoint of further improving the impact resistance performance, the acrylic polymer more preferably has a peak loss coefficient (tan δ) between -20 to 23 ° C., more preferably between -18 and 20 ° C. Has. The peak within the above temperature range is preferably the maximum peak when the loss coefficient is measured by the measurement method described later.
The loss coefficient may be measured using a dynamic viscoelasticity measuring device in a temperature range of −100 ° C. to 100 ° C.

The shock absorbing sheet preferably has a 25% compression strength at 23 ° C. of 200 kPa or more and 700 kPa or less. When the 25% compression strength is within the above range, the shock absorbing sheet has appropriate flexibility, and it is easy to improve both the shock absorbing performance and the bending resistance. From these viewpoints, the 25% compression strength is more preferably 240 kPa or more and 600 kPa or less, and further preferably 280 kPa or more and 500 kPa or less.

In the second invention, as described above, by using the specific acrylic polymer, the bending resistance in a low temperature environment is improved. Specifically, in the following dynamic bending resistance test, it is preferable that the central portion of the bending does not break even when it is bent 2,000 times, and more preferably it does not break even when it is bent 4,000 times. .. More preferably, when it is bent 4,000 times, a scratch of 1 mm or more does not occur at the center of the bending. The scratches generated in the dynamic bending resistance test are those that penetrate in the thickness direction of the shock absorbing sheet at the center of the bending. If there was a scratch other than the central part, it was measured again. If the sample is scratched by 10 mm or more, the number of breaks is recorded assuming that the center is broken.
[Dynamic flexion resistance test]
In an environment of −35 ° C., a pair of pedestals on which shock absorbing sheets are erected at a distance between pedestals of 30 mm are brought close to each other so that the distance between pedestals is 6 mm to bend the shock absorbing sheets, and then the pair of pedestals are placed again. The reciprocating motion of moving away from the original gantry distance is repeated 4,000 times at a cycle of 46 times per minute.
The dynamic bending resistance test can be performed by a commercially available durability tester as described in the examples.

Further, the shock absorbing sheet of the second invention preferably has a shock absorbing rate at 23 ° C. of 20% or more, more preferably 25% or more, and further preferably 28% or more. The shock absorption rate is measured by the method described in Examples described later. By setting the shock absorption rate to 20% or more, the shock absorption performance, particularly the shock absorption against a local shock can be improved. The shock absorption rate can be measured by the measuring method shown in Examples described later.

The shock absorbing sheet of the second invention preferably has a tensile elongation of 7% or more at −35 ° C. When the tensile elongation of the shock absorbing sheet is high in a low temperature environment, the bending resistance in a low temperature environment becomes good. From the viewpoint of bending resistance in a low temperature environment, the tensile elongation at −35 ° C. is more preferably 7.5% or more, further preferably 8.0% or more.
The upper limit of the tensile elongation at −35 ° C. is not particularly limited, but is preferably 20%, more preferably 15% from the viewpoint of ensuring a certain mechanical strength and high impact absorption performance.

The shock absorbing sheet of the second invention preferably has a tensile strength at −35 ° C. of 2 MPa or more and 20 MPa or less. When it is 2 MPa or more, it becomes easy to secure good mechanical strength and shock absorption of the shock absorbing sheet even in a low temperature environment. Further, when it is set to 20 MPa or less, a certain degree of flexibility can be ensured in a low temperature environment, and bending resistance in a low temperature environment tends to be improved. From these viewpoints, the tensile strength of the shock absorbing sheet at −35 ° C. is more preferably 3 MPa or more and 17 MPa or less, further preferably 4 MPa or more and 15 MPa or less.

The tensile elongation means the elongation when the impact absorbing sheet is pulled until it breaks and the tensile force is maximized (also referred to as "tensile strength maximum elongation"), and the tensile strength is similarly when pulled. Means the maximum tensile strength value. The tensile elongation and tensile strength were measured according to the method described in JIS K6767, but the measurement is performed in an environment of −35 ° C.
Further, the tensile elongation and the tensile strength are measured in both the MD and TD directions, and the tensile elongation and the tensile strength in the direction in which the tensile elongation is high are adopted. When it is unclear which direction is the MD or TD direction, the tensile elongation and the tensile strength in the direction in which the tensile elongation is maximized may be measured. The MD direction is an abbreviation for Machine Direction, and coincides with the resin flow direction at the time of coating or the like described later. The TD direction is an abbreviation for Transverse Direction, and is a direction orthogonal to the MD direction.

[Manufacturing method of shock absorbing sheet]
Hereinafter, a method for manufacturing the shock absorbing sheet will be described in detail. In the following, first, a method for producing a foamed resin layer when bubbles are formed from hollow particles will be described.
In the formation of the foamed resin layer, first, the monomer component (A) or a component obtained by polymerizing at least a part of the monomer component (A), hollow particles, and other additives to be blended as needed are contained. An acrylic resin composition may be prepared.
Next, the acrylic resin composition may be formed into a film, and the foamed resin layer may be formed by polymerizing the above-mentioned monomer component (A) or a component obtained by polymerizing at least a part of the monomer component (A). Specifically, the foamed resin layer is preferably a component obtained by polymerizing at least a part of the monomer component (A) or the monomer component (A) on an appropriate support such as a release paper, a release film, or a base material. It is preferable to apply an acrylic resin composition containing hollow particles, a photopolymerization initiator and the like to form a coating layer, and to cure the layer with active energy rays. A release paper, a release film, or the like may be further laminated on the formed coating layer before irradiating the active energy rays.
Separator such as release paper and release film used for forming the acrylic foamed resin layer may be appropriately peeled off before using the shock absorbing sheet after production.
The shock absorbing sheet manufacturing method is substantially the same as that of the shock absorbing sheet manufacturing method (No. 1) described in the first invention described above, and the description thereof will be omitted.

[How to use the shock absorbing sheet]
Hereinafter, a method of using the shock absorbing sheet of the second invention described above will be described.
The shock absorbing sheet of the second 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.
The method of using the shock absorbing sheet is substantially the same as that described in the first invention described above, and the description thereof will be omitted.

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

(First invention)
[Evaluation method]
The physical characteristics and performance of the shock absorbing sheet were evaluated by the following methods.
<Thickness>
The thickness was measured with a dial gauge.
<Density>
The density of the foamed resin layer is a value of the apparent density measured according to JIS K6767.
<Tensile strength and maximum tensile strength extension>
The measurement was carried out in an environment of −35 ° C. according to the method described in JIS K6767.
<Compression strength>
The measurement was carried out at 23 ° C. according to the method described in JIS K6767.

<Loss coefficient (tan δ)>
Measuring device: Using a dynamic viscoelasticity measuring device (product name "DVA-200", manufactured by IT Measurement Control Co., Ltd.), shear mode, 1 Hz, strain amount: 0.1%, temperature range: -100 ° C ~ The loss coefficient (tan δ) was measured under the conditions of 100 ° C. and a heating rate of 1 ° C./min.
As the measurement sample, a shock absorbing sheet laminated to a thickness of 1 mm and cut out to a size of 10 mm × 5 mm was used.
In the measurement sample, if there was at least one peak of the loss coefficient (tan δ) between 25 and 25 ° C., it was evaluated as “A”, and if there was no peak, it was evaluated as “B”.
If there is a peak of the loss coefficient (tan δ) between 25 and 25 ° C., it indicates the peak temperature.

<Shock 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. The four corners of the acrylic plate 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 6 times without replacing the shock absorbing sheet, and the average value of the accelerations for all 7 times was taken as the acceleration (L _1a ). Further, the average value of the accelerations of all seven times in which the same iron ball was dropped and the acceleration was measured without placing the shock absorbing sheet on the acrylic plate was taken as the acceleration (L _0a ), and the obtained acceleration (L _1a ) and the acceleration (L 1a). From L _0a ), the 7-time average shock absorption rate was calculated by the following formula. The test was conducted under the conditions of a temperature of 23 ° C. and a humidity of 50 RH%.
7 times average shock absorption rate (%) = (L _0a −L _1a ) / L _0a × 100
Based on the results of the 7-time average shock absorption rate, evaluation was performed according to the following evaluation criteria.
A: The average shock absorption rate for 7 times was 25% or more.
B: The average shock absorption rate for 7 times was less than 25%.

<Dynamic flexion resistance test>
The bending resistance of the shock absorbing sheet was evaluated by a high temperature and high humidity environment durability tester (“CL09-typeD01-FSC90” manufactured by Yuasa System Co., Ltd.) using a sample obtained by cutting the shock absorbing sheet into 50 mm × 10 mm. The high-temperature and high-humidity durability tester was provided with a pair of pedestals, and samples were erected between the pedestals. The distance between the gantry was 30 mm. At this time, the sample was erected so that its longitudinal direction coincided with the opposite direction of the gantry. In addition, both ends of the sample were attached to the upper surface of each frame with double-sided tape (trade name "Nystack", manufactured by Nichiban Co., Ltd.).

When erection, the samples were placed on the gantry by 7 mm on each side and pasted together so as not to extend from the original length or to hang down too much between the gantry. A slide mechanism is attached to the lower part of each gantry, and the gantry can reciprocate along the directions in which they face each other. In the high temperature and high humidity environment durability tester (manufactured by Yuasa System Co., Ltd.), the samples were bonded at room temperature to prevent the samples from being deformed by wind or unnecessary stress, and the oven was set to −35 ° C. After confirming that the temperature inside the furnace had reached −35 ° C. and letting it stand for 30 minutes, the reciprocating motion was started.
After that, the pedestals bend the samples by bringing them closer to each other so that the distance between the pedestals is 6 mm, and then reciprocate the pedestals again to make the distance between the pedestals 30 mm at a cycle of 46 times per minute. Repeated 4,000 times. After bending 4,000 times, the maximum length (mm) of the maximum wound was measured.
If the sample breaks before bending 4,000 times, the number of breaking points at which the sample breaks is shown.

The components used in Examples 1-1 to 1-8 and Comparative Examples 1-1 to 1-7 are as follows.
(1) Methyl acrylate: MA (manufactured by Nippon Shokubai Co., Ltd.)
(2) 2-Ethylhexyl acrylate: 2EHA (manufactured by Nippon Shokubai Co., Ltd.)
(3) n-Butyl acrylate: BA (manufactured by Nippon Shokubai Co., Ltd.)
(4) Acrylic acid (5) Urethane acrylate: Urethane acrylate obtained by reacting one end of polypropylene glycol with acryloyloxyethyl isocyanate, monofunctional (however, containing a small amount of bifunctionality), weight average molecular weight: 15,000 , Product name "2.0 PEM-X264" (manufactured by Asahi Glass Co., Ltd.)
(6) Bifunctional (meth) acrylate: trade name "NK ester APG-400", manufactured by Shin Nakamura Chemical Industry Co., Ltd. (7) Trifunctional (meth) acrylate: trade name "NK ester A-9300-3CL", Shin Nakamura Chemical Industry Co., Ltd. (8) Photopolymerization initiator (trade name "Irgacure184", manufactured by BASF Japan Co., Ltd.)
(9) Hollow particles (trade name "Expansel 920DE80d30", manufactured by Nippon Philite Co., Ltd.), average particle size: 80 μm
(10) Thermoplastic elastomer (1): Product name "DYNARON 1321P", HSBR, manufactured by JSR Corporation (11) Thermoplastic elastomer (2): Product name "Hybler (registered trademark) 7311F", manufactured by Kuraray Co., Ltd., water Styrene-isoprene-butadiene block copolymer, styrene content 12% by mass
(12) Polyolefin resin: trade name "Kernel KF283", linear low-density polyethylene resin obtained by polymerization catalyst of metallocene compound, density: 0.921 g / cm ³ , manufactured by Nippon Polyethylene Co., Ltd. (14) Pyrolysis type Foaming agent: Azodicarboxylic amide (15) Decomposition temperature adjuster: Trade name "OW-212F", Zinc oxide, manufactured by Sakai Chemical Industry Co., Ltd. (16) Phenolic antioxidant: 2,6-di-t-butyl- p-cresol

[Example 1-1]
Methyl acrylate, 2-ethylhexyl acrylate, urethane acrylate, and photopolymerization initiator are mixed according to the formulation shown in Table 1, and the mixture is partially polymerized by polymerization using ultraviolet rays to increase the viscosity to 2,000 mPa · s. A prepared syrup-like curable acrylic resin material was obtained. Bifunctional (meth) acrylate, trifunctional (meth) acrylate, and hollow particles were mixed with the present resin material according to the formulation shown in Table 1 to prepare a curable acrylic resin composition. This was applied on the release paper at 23 ° C., and the release paper was further superposed on the obtained coating layer. Then, it was irradiated with ultraviolet rays to prepare a shock absorbing sheet having a thickness of 104 μm. The ultraviolet rays were irradiated under the conditions of an illuminance of 4 mW / cm ² and a light intensity of 720 mJ / cm ^2.
The peak of the loss coefficient (tan δ) of the shock absorbing sheet was 2.3 ° C.

[Examples 1-2 to 1-6]
A shock absorbing sheet was prepared in the same manner as in Example 1-1 except that the blending amount of each component was changed as shown in Table 1.

[Example 1-7]
Thermoplastic elastomer (1) 90.0 parts by mass, polyolefin resin 10.0 parts by mass, pyrolysis foaming agent 2.3 parts by mass, decomposition temperature adjuster 1 part by mass, phenol-based antioxidant 0.5 parts by mass A mass portion was prepared as a raw material. These materials were melt-kneaded and then pressed to obtain a foamable resin sheet having a thickness of 0.3 mm. Both sides of the obtained foamable resin sheet were irradiated with an electron beam of 4.5 Mrad at an acceleration voltage of 500 keV to crosslink the foamable sheet. Next, the crosslinked foamable sheet was heated to 250 ° C. to foam the foamable resin sheet ^{to prepare a shock absorbing sheet having a density of 0.4 g / cm 3} and a thickness of 199 μm.
The peak of the loss coefficient (tan δ) of the shock absorbing sheet was −24.2 ° C.

[Example 1-8]
Thermoplastic elastomer (2) 50.0 parts by mass, polyolefin resin 50.0 parts by mass, pyrolysis type foaming agent 9.0 parts by mass, decomposition temperature adjuster 1 part by mass, phenol-based antioxidant 0.5 A mass portion was prepared as a raw material. These materials were melt-kneaded and then pressed to obtain a foamable resin sheet having a thickness of 0.3 mm. Both sides of the obtained foamable resin sheet were irradiated with an electron beam of 6.0 Mrad at an acceleration voltage of 500 keV to crosslink the foamable sheet. Next, the crosslinked foamable sheet was heated to 250 ° C. to foam the foamable resin sheet ^{to prepare a shock absorbing sheet having a density of 0.56 g / cm 3} and a thickness of 198 μm.
The peak loss coefficient (tan δ) of the shock absorbing sheet was -22.1 ° C.

[Comparative Examples 1-1 to 1-7]
A shock absorbing sheet was prepared in the same manner as in Example 1-1 except that the blending amount of each component was changed as shown in Table 1.

As is clear from Table 1 above, in each example, at least one peak of the loss coefficient (tan δ) was formed between 25 and 25 ° C., and the tensile elongation at −35 ° C. was equal to or higher than a predetermined value. As a result, both impact resistance and bending resistance in a low temperature environment were improved.
Further, in each example, the impact absorption rate at 23 ° C. is 25% or more, and the impact absorption sheet bent 4,000 times in the dynamic bending resistance test is not scratched by 1 mm or more. Both bending resistance in a low temperature environment were improved.

(Second invention)
[Evaluation method]
The physical properties and performance of the shock absorbing sheet are the same as in the first invention, such as thickness, density, tensile strength and maximum tensile strength elongation, compressive strength, loss coefficient (tan δ), shock absorption test, and dynamic bending. It was evaluated in a resistance test.
In the shock absorption test, 20% or more was designated as "A" and less than 20% was designated as "B".
In the dynamic bending resistance test, after bending 4,000 times, those having no scratches of 1 mm or more were designated as "A", and those having scratches of 1 mm or more were designated as "B". If the sample breaks before bending 4,000 times, the number of breaking points at which the sample breaks is shown.

As each component used in Examples 2-1 to 2-8 and Comparative Examples 2-1 to 2-3, the same components as those used in the Examples and Comparative Examples of the first invention were used.

[Examples 2-1 to 2-8, Comparative Examples 2-1 to 2-3]
Monomer components other than the bifunctional (meth) acrylate and trifunctional (meth) acrylate shown in Table 2 and the photopolymerization initiator are mixed according to the formulation shown in Table 1, and the mixture is partially polymerized by polymerization using ultraviolet rays. By doing so, a syrup-like curable acrylic resin material having a viscosity adjusted to 2,000 mPa · s was obtained. Bifunctional (meth) acrylate, trifunctional (meth) acrylate, and hollow particles were mixed with the present resin material according to the formulation shown in Table 2 to prepare an acrylic resin composition. This was applied on the release paper at 23 ° C., and the release paper was further superposed on the obtained coating layer. Then, it was irradiated with ultraviolet rays to prepare a shock absorbing sheet having a thickness of 103 μm. The ultraviolet rays were irradiated under the conditions of an illuminance of 4 mW / cm ² and a light intensity of 720 mJ / cm ^2.

As is clear from Table 2 above, in each example, an acrylic polymer is used as the resin constituting the foamed resin layer, and a predetermined amount of urethane (meth) acrylate is contained in the constituent monomer. Bending resistance in a low temperature environment has improved.

Claims (22)

A shock absorbing sheet containing a foamed resin layer.
At least one has a loss factor (tan δ) peak between 25-25 ° C.
A shock absorbing sheet having a tensile elongation of 6% or more at −35 ° C. The shock absorbing sheet according to claim 1, which has a thickness of 200 μm or less. The shock absorbing sheet according to claim 1 or 2, wherein the resin constituting the foamed resin layer contains an acrylic polymer. The shock absorbing sheet according to any one of claims 1 to 3, wherein the density of the foamed resin layer is 0.3 g / cm ³ or more and 0.8 g / cm ^{3 or less.} The shock absorbing sheet according to any one of claims 1 to 4, wherein the 25% compressive strength at 23 ° C. is 200 kPa or more and 700 kPa or less. The shock absorbing sheet according to any one of claims 1 to 5, wherein the shock absorbing rate at 23 ° C. is 25% or more. The shock absorbing sheet according to any one of claims 1 to 6, wherein the foamed resin layer contains hollow particles. The shock absorbing sheet according to any one of claims 1 to 7, wherein the foamed resin layer has bubbles made of gas mixed inside. The shock absorbing sheet according to any one of claims 1 to 8, which is used in an electronic device. The shock absorbing sheet according to any one of claims 1 to 9, which is arranged on the back side of the display device. An adhesive tape comprising the shock absorbing sheet according to any one of claims 1 to 10 and an adhesive material provided on at least one surface of the shock absorbing sheet. A display device comprising the shock absorbing sheet according to any one of claims 1 to 10 or the adhesive tape according to claim 11. A shock absorbing sheet containing a foamed resin layer.
The shock absorption rate at 23 ° C. is 25% or more.
A shock absorbing sheet that does not cause scratches of 1 mm or more on the shock absorbing sheet that has been bent 4,000 times in the following dynamic bending resistance test.
[Dynamic flexion resistance test]
In an environment of −35 ° C., the pair of pedestals on which the shock absorbing sheet is erected at a distance between the pedestals of 30 mm are brought close to each other so that the distance between the pedestals is 6 mm to bend the shock absorbing sheet, and then the pair again. The reciprocating motion of moving away the gantry to the original distance between the gantry is repeated 4,000 times in a cycle of 46 times per minute. A shock absorbing sheet containing a foamed resin layer.
The foamed resin layer contains an acrylic polymer obtained by polymerizing a monomer component (A) containing urethane (meth) acrylate (A1).
The urethane (meth) acrylate (A1) is contained in an amount of 5% by mass or more based on the monomer component (A).
A shock absorbing sheet having at least one loss factor (tan δ) peak between 25-25 ° C. The shock absorbing sheet according to claim 14, wherein the shock absorbing rate at 23 ° C. is 20% or more. The shock absorbing sheet according to claim 14 or 15 , wherein the foamed resin layer has bubbles made of a gas mixed therein. A shock absorbing sheet containing a foamed resin layer.
The foamed resin layer contains an acrylic polymer obtained by polymerizing a monomer component (A) containing urethane (meth) acrylate (A1).
The urethane (meth) acrylate (A1) is contained in an amount of 5% by mass or more based on the monomer component (A).
The 25% compression strength at 23 ° C. is 200 kPa or more and 700 kPa or less.
A shock absorbing sheet having a shock absorbing rate of 20% or more at 23 ° C. A shock absorbing sheet containing a foamed resin layer.
The foamed resin layer contains an acrylic polymer obtained by polymerizing a monomer component (A) containing urethane (meth) acrylate (A1).
The urethane (meth) acrylate (A1) is contained in an amount of 5% by mass or more based on the monomer component (A).
The shock absorption rate at 23 ° C. is 20% or more,
A shock absorbing sheet that does not cause scratches of 1 mm or more on the shock absorbing sheet that has been bent 4,000 times in the following dynamic bending resistance test.
[Dynamic flexion resistance test]
In an environment of −35 ° C., the pair of pedestals on which the shock absorbing sheet is erected at a distance between pedestals of 30 mm are brought close to each other so that the distance between the pedestals is 6 mm to bend the shock absorbing sheet, and then the pair again. The reciprocating motion of moving the pedestal away from the pedestal to the original distance between the pedestals is repeated 4,000 times at a cycle of 46 times per minute. A shock absorbing sheet containing a foamed resin layer.
The foamed resin layer contains an acrylic polymer obtained by polymerizing a monomer component (A) containing urethane (meth) acrylate (A1).
The urethane (meth) acrylate (A1) is contained in an amount of 5% by mass or more based on the monomer component (A).
The foamed resin layer contains hollow particles and
A shock absorbing sheet having a shock absorbing rate of 20% or more at 23 ° C. A shock absorbing sheet containing a foamed resin layer.
The foamed resin layer contains an acrylic polymer obtained by polymerizing a monomer component (A) containing urethane (meth) acrylate (A1).
The urethane (meth) acrylate (A1) is contained in an amount of 5% by mass or more based on the monomer component (A).
The 25% compression strength at 23 ° C. is 200 kPa or more and 700 kPa or less.
A shock absorbing sheet in which the foamed resin layer has bubbles made of gas mixed inside. A shock absorbing sheet containing a foamed resin layer.
The foamed resin layer contains an acrylic polymer obtained by polymerizing a monomer component (A) containing urethane (meth) acrylate (A1).
The urethane (meth) acrylate (A1) is contained in an amount of 5% by mass or more based on the monomer component (A).
The foamed resin layer has bubbles made of gas mixed inside, and has
A shock absorbing sheet that does not cause scratches of 1 mm or more on the shock absorbing sheet that has been bent 4,000 times in the following dynamic bending resistance test.
[Dynamic flexion resistance test]
In an environment of −35 ° C., the pair of pedestals on which the shock absorbing sheet is erected at a distance between pedestals of 30 mm are brought close to each other so that the distance between the pedestals is 6 mm to bend the shock absorbing sheet, and then the pair again. The reciprocating motion of moving the pedestal away from the pedestal to the original distance between the pedestals is repeated 4,000 times at a cycle of 46 times per minute. A shock absorbing sheet containing a foamed resin layer.
The foamed resin layer contains an acrylic polymer obtained by polymerizing a monomer component (A) containing urethane (meth) acrylate (A1).
The urethane (meth) acrylate (A1) is contained in an amount of 5% by mass or more based on the monomer component (A).
The foamed resin layer contains hollow particles and
A shock absorbing sheet in which the foamed resin layer has bubbles made of gas mixed inside.