JP2021088685A - Impact absorption sheet - Google Patents

Abstract

To provide an impact absorption sheet having good water resistance even when thickness is thin.SOLUTION: The impact absorption sheet contains a foam resin layer having a thickness of 200 μm or less. An X value obtained by sticking an adhesive tape to both surfaces of the foam resin layer to obtain a laminate, subjecting it to heavy water swelling treatment of leaving it in heavy water at 80°C for 24 hours, and calculating it from the following Expression (1) based on a measurement value obtained by measuring it at 30°C by pulse NMR by Solid Echo method before and after the heavy water swelling treatment is 0.6 or more. Expression (1): X value=(hard component amount before heavy water swelling treatment/middle component amount before heavy water swelling treatment)×(relaxation time of soft component before heavy water swelling treatment/relaxation time of soft component after heavy water swelling treatment).SELECTED DRAWING: None

Description

The present invention relates to a shock absorbing sheet, for example, a thin shock absorbing sheet used in an electronic device or the like.

In display devices used in various electronic devices such as personal computers, mobile phones, and electronic paper, the space between the glass plate and the display unit that make up the surface of the device, the housing body 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 restrictions, and accordingly, the shock absorber is also required to be in the form of a thin sheet.

As a thin shock absorbing material, a polyolefin resin foam and an acrylic resin foam are widely known, and various improvements have been made to improve the shock absorbing performance. For example, Patent Document 1 discloses that in a polyolefin-based resin foam, the impact absorption performance is improved by making the shape of bubbles constant and controlling the flexibility. Further, Patent Document 2 discloses that the distribution of bubbles in the thickness direction is made uniform to improve the shock absorption performance.

Japanese Unexamined Patent Publication No. 2014-214205 International Publication No. 2019/06602

In recent years, mobile electronic devices such as mobile phones have become more sophisticated, and for example, waterproofness and water resistance may be required. In order to improve waterproofness and water resistance, the shock absorber is also required to play a role as a sealing material to prevent the ingress of water. However, the conventional shock absorbing material has insufficient water resistance and often cannot sufficiently fulfill the function as a sealing material for preventing the ingress of water.

Therefore, an object of the present invention is to provide a shock absorbing sheet having good water resistance even if it is thin.

As a result of diligent studies, the present inventors have obtained an X value calculated based on the measured values obtained by measuring each of the foamed resin layers before the heavy water swelling treatment and after the heavy water swelling treatment by pulse NMR. As a result, it was found that the shock absorbing sheet containing the foamed resin layer has good water resistance even if the thickness is thin, and the following invention has been completed. The gist of the present invention is the following [1] to [12].
[1] A shock absorbing sheet containing a foamed resin layer having a thickness of 200 μm or less.
Adhesive tapes are attached to both sides of the foamed resin layer to form a laminate, which is subjected to heavy water swelling treatment by allowing it to stand in heavy water for 24 hours at 80 ° C., and measured by pulse NMR at 30 ° C. at 30 ° C. before and after the heavy water swelling treatment. A shock absorbing sheet having an X value of 0.6 or more calculated from the following formula (1) based on the measured values obtained.
X value = (amount of hard component before heavy water swelling treatment / amount of middle component before heavy water swelling treatment) x (relaxation time of soft component before heavy water swelling treatment / relaxation time of soft component after heavy water swelling treatment) (1)
[2] Adhesive tapes are attached to both sides of the foamed resin layer to form a laminate, which is subjected to heavy water swelling treatment by allowing it to stand in heavy water at 80 ° C. for 24 hours, and pulse NMR at 30 ° C. by the Solid Echo method before and after the heavy water swelling treatment. The shock absorbing sheet according to the above [1], wherein the middle component change rate calculated from the following formula (2) is 10% or less based on the measured value obtained by the above.
Middle component change rate = (Middle component amount before heavy water swelling treatment-Middle component amount after heavy water swelling treatment) / Middle component amount before heavy water swelling treatment x 100 (2)
[3] The following water swelling test is carried out using the foamed resin layer as a sample, and the water absorption rate represented by the weight increase rate of the sample after the water swelling test as compared with that before the water swelling test is 20% or less [1]. Or the shock absorbing sheet according to [2].
<Water swelling test>
Adhesive tapes are attached to both sides of the foamed resin layer to form a laminate, and the laminate is immersed in a sample tube filled with water and allowed to stand at 80 ° C. for 24 hours.
[4] The shock absorbing sheet according to any one of the above [1] to [3], wherein the density of the foamed resin layer is 0.3 g / cm ³ or more and 0.9 g / cm ^{3 or less.}
[5] The shock absorbing sheet according to any one of the above [1] to [4], wherein the foamed resin layer is an acrylic foamed resin layer.
[6] The shock absorbing sheet according to any one of the above [1] to [5], wherein the glass transition temperature is 0 ° C. or higher and 25 ° C. or lower.
[7] The shock absorbing sheet according to any one of the above [1] to [6], wherein the 7-time average shock absorbing rate is 30% or more.
[8] The shock absorbing sheet according to any one of the above [1] to [7] used in electronic devices.
[9] The shock absorbing sheet according to any one of the above [1] to [8], which is arranged on the back side of the display device.
[10] An adhesive tape comprising the shock absorbing sheet according to any one of the above [1] to [9] and an adhesive material provided on at least one surface of the shock absorbing sheet.
[11] A display device including the shock absorbing sheet according to any one of the above [1] to [9] or the adhesive tape according to the above [10].
[12] A shock absorbing sheet containing a foamed resin layer having a thickness of 200 μm or less.
A shock absorbing sheet in which the following water swelling test is carried out using the foamed resin layer as a sample, and the water absorption rate represented by the weight increase rate of the sample after the water swelling test as compared with that before the water swelling test is 20% or less.
<Water swelling test>
Adhesive tapes are attached to both sides of the foamed resin layer to form a laminate, and the laminate is immersed in a sample tube filled with water and allowed to stand at 80 ° C. for 24 hours.

According to the present invention, it is possible to provide a shock absorbing sheet having a thin thickness and good water resistance.

Hereinafter, the present invention will be described in more detail using embodiments.
[Shock absorption sheet]
The shock absorbing sheet according to one aspect of the present invention is a shock absorbing sheet including a foamed resin layer having a thickness of 200 μm or less. In one aspect of the present invention, adhesive tapes are attached to both sides of the foamed resin layer to form a laminate, which is subjected to heavy water swelling treatment in which it is allowed to stand in heavy water at 80 ° C. for 24 hours, and pulses are performed before and after the heavy water swelling treatment by a method described later. Based on the measured value obtained by measuring by NMR, the X value is calculated from the following formula (1), and the calculated X value is 0.6 or more. The pulse NMR was measured by the Solid Echo method at a temperature of 30 ° C.
X value = (amount of hard component before heavy water swelling treatment / amount of middle component before heavy water swelling treatment) x (relaxation time of soft component before heavy water swelling treatment / relaxation time of soft component after heavy water swelling treatment) (1)

In general, the foamed resin layer is measured by pulse NMR of the Solid Echo method to obtain a 1H spin-spin relaxation free induction decay curve. The obtained free induction decay curve can be waveform-separated into three curves derived from the three components of the hard component, the middle component, and the soft component in ascending order of relaxation time. That is, the actually measured free induction decay curve is a superposition of the free induction decay curves derived from the three components of the hard component, the middle component, and the soft component. A method for separating and analyzing three components using such pulse NMR is known, and examples of the literature include JP-A-2018-2983.

The hard component is a component having a short relaxation time in pulse NMR measurement, and means a component having low molecular motility. On the other hand, the soft component is a component having a long relaxation time in pulse NMR measurement, and means a component having high molecular motility. The middle component has a relaxation time in the pulse NMR measurement between the hard component and the soft component, so that the molecular motility is also between the hard component and the soft component. The amount of the component is a ratio to the total amount of the hard component, the middle component, and the soft component.

In the present invention, when the X value is 0.6 or more, the water absorption rate when the foamed resin layer is immersed in water becomes low, and the water resistance of the foamed resin layer becomes good. When the water resistance of the foamed resin layer becomes good, for example, by arranging the shock absorbing sheet of the present invention between the parts of the electronic device, it is possible to prevent water from entering between the parts and improve the waterproof property of the electronic device. Can be done.
The principle that the water resistance becomes good when the X value is 0.6 or more is not clear, but it is estimated as follows. As the X value increases, as is clear from the above formula (1), the ratio of the hard component to the middle component before the heavy water swelling treatment increases, and as a result, the foamed resin layer has less molecular motion and more hard components. It is presumed that water makes it difficult to swell and improves water resistance. Further, a large X value means that the relaxation time of the soft component is prevented from being lengthened due to the swelling of water, and it is presumed that the water resistance of the foamed resin layer is also improved by this. On the other hand, if the ratio of the hard component to the middle component before immersion becomes small or the relaxation time of the soft component becomes long by immersing in water, the X value becomes less than 0.6 and the water resistance of the foamed resin layer becomes water resistant. Is insufficient. From the viewpoint of improving water resistance, the higher the X value, the better, preferably 0.7 or more, more preferably 0.8 or more, and further preferably 0.85 or more.

The X value is preferably 1.2 or less. By setting the X value to 1.2 or less, it is possible to prevent the proportion of hard components from becoming larger than necessary, thereby ensuring a certain degree of flexibility in the foamed resin layer and improving the shock absorption of the shock absorbing sheet. Can be done well. From the viewpoint of further improving the shock absorption, the X value is more preferably 1.0 or less, and further preferably 0.95 or less.

The shock absorbing sheet in another aspect of the present invention is a shock absorbing sheet containing a foamed resin layer having a thickness of 200 μm or less, and the following water swelling test is carried out using the foamed resin layer as a sample, as compared with the case before the water swelling test. The water absorption rate represented by the weight increase rate (%) of the sample after the water swelling test is 20% or less. The weight increase rate (%) of the foamed resin layer is (BA) / A ×, where A is the weight of the foamed resin layer before the water swelling test and B is the weight of the foamed resin layer after the water swelling test. It is represented by 100.
<Water swelling test>
Adhesive tapes are attached to both sides of the foamed resin layer to form a laminate, and the laminate is immersed in a sample tube filled with water and allowed to stand at 80 ° C. for 24 hours.

Since the shock absorbing sheet has a water absorption rate of 20% or less, the water resistance becomes good, so that the waterproof property of the electronic device can be ensured by using it in the electronic device or the like. The water absorption rate of the shock absorbing sheet is preferably 15% or more, more preferably 10% or less, still more preferably 9% or less, from the viewpoint of improving the water resistance of the shock absorbing sheet. Further, the water absorption rate of the shock absorbing sheet should be as low as possible, and the lower limit thereof is 0%, but preferably 1% or more. By setting it to 1% or more, it becomes easy to improve the shock absorption of the shock absorbing sheet.
In the present invention, in each aspect, it is preferable that the X value is within the predetermined range or the water absorption rate is within the predetermined range as described above, but the X value is within the above-mentioned predetermined range and the water absorption rate is within the above-mentioned predetermined range. It is preferable to be inside. Hereinafter, in the present invention, the details of the shock absorbing sheet on each side surface will be described in more detail.

(Middle component change rate)
In the present invention, based on the measured values obtained by pulse NMR at 30 ° C. by the Solid Echo method using the samples composed of the foamed resin layers before the heavy water swelling treatment and after the heavy water swelling treatment, the following The rate of change in the middle component calculated from the formula (2) is preferably 10% or less.
Middle component change rate = (Middle component amount before heavy water swelling treatment-Middle component amount after heavy water swelling treatment) / Middle component amount before heavy water swelling treatment x 100 (2)

When the rate of change of the middle component is 10% or less, the water resistance can be improved and the shock absorption can be maintained well. The principle is not clear, but if the reduction rate is 10% or less, a certain amount of a component having a certain hardness and flexibility can be secured even after water swelling, and thereby water resistance even after being immersed in water for a certain period of time. It is considered that both properties and shock absorption can be improved. The rate of change of the middle component is more preferably 7% or less from the viewpoint of water resistance and shock absorption.
The rate of change in the middle component may be a negative value, that is, less than 0% as long as it is 10% or less. A negative value indicates that the middle component is increased by the heavy water swelling treatment, and the absolute value indicates the amount of increase. The rate of change in the middle component is preferably -15% or more, more preferably -12% or more, still more preferably -7% or more, in order to prevent a large change in the physical properties of the foamed resin layer.
In the present invention, the rate of change of the middle component and the above-mentioned X value can be within the above range by appropriately adjusting the composition of the resin component constituting the foamed resin layer, the monomer component, the additive and the like.

In each of the above water swelling tests and bals NMR measurements, the reason why the adhesive tapes were attached to both sides of the foamed resin layer was that the foamed resin layer was placed between electronic devices in terms of actual specifications, so the state at the time of use This is to reproduce it. The adhesive tape used in the water swelling test and the bals NMR measurement may be the one specified in the examples.
When the shock absorbing sheet is made of a single foamed resin layer, the shock absorbing sheet itself may be used as a sample for pulse NMR measurement and water absorption measurement. On the other hand, when the shock absorbing sheet has a layer other than the foamed resin layer such as a skin layer, the layer other than the foamed resin layer is removed by a known means that the foam does not deform, and then the foamed resin layer single layer is removed. It is advisable to measure each measured value as a sample. As a method for removing the layer other than the foamed resin layer, physical peeling, cooling peeling, and slicing are preferable.

(Glass transition temperature (Tg))
In the present invention, the shock absorbing sheet containing the foamed resin layer preferably has a glass transition temperature (Tg) of 0 ° C. or higher and 25 ° C. or lower. When the glass transition temperature (Tg) is within the above range, the shock absorption performance of the shock absorbing sheet can be improved. From the viewpoint of improving the shock absorption performance, the glass transition temperature (Tg) is more preferably 5 ° C. or higher and 24 ° C. or lower, and further preferably 9 ° C. or higher and 19 ° C. or lower.
The glass transition temperature (Tg) in the present invention can be adjusted by appropriately changing the resin component constituting the foamed resin layer, as will be described later. For example, in the case of an acrylic foamed resin layer, the glass transition temperature (Tg) can be adjusted by appropriately selecting the component of the monomer component (a) as described later. The glass transition temperature (Tg) of the shock absorbing sheet can be measured by the method described in Examples described later.

(Thickness of foamed resin layer)
In the present invention, the thickness of the foamed resin layer is 200 μm or less. When the thickness of the foamed resin layer is larger than 200 μm, the shock absorbing sheet also becomes thicker accordingly, and it becomes difficult to achieve thinness and miniaturization of electronic devices and the like. From the viewpoint of thinning and miniaturization, the thickness of the foamed resin layer is preferably 180 μm or less, more preferably 150 μm or less, and even more preferably 120 μm or less. The thickness of the foamed resin layer is preferably 20 μm or more, more preferably 50 μm or more, from the viewpoint of improving shock absorption and shock absorption performance of electronic devices incorporating a shock absorption sheet and preventing panel damage. It is preferable, and 70 μm or more is more preferable.

(density)
The density of the foamed resin layer is preferably 0.3 g / cm ³ or more and 0.9 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 ^{0.85g / cm 3, 0.6g / cm} 3 or more 0.8 g / cm It is more preferably ^{3 or less.}

(Shock absorption rate)
The shock absorbing sheet of the present invention preferably has a 7-time average shock absorbing rate of 30% or more. When the 7-time average shock absorption rate is set to 30% or more, good shock absorption can be maintained even if a shock is repeatedly applied to the shock absorption sheet. The 7-time average shock absorption rate is obtained by measuring the shock absorption rate 7 times in a row by the method described in Examples described later and averaging the measured values.
The 7-time average shock absorption rate is preferably 33% or more, more preferably 36% or more, from the viewpoint of further improving the shock absorption when repeated shocks are applied.

(Bubble)
The foamed resin layer of the present invention contains hollow particles, and bubbles may be formed by the space inside the hollow particles. Further, the bubbles in the foamed resin layer may be formed by other means, and are preferably 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 resin foam layer may have closed cells, may have open cells, or may have both closed cells and open cells.

(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. Sufficient shock absorption can be obtained by setting the average particle size of the hollow particles to 10 μm or more and 150 μm or less.
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 preferably 0.2 or more and 0.85 or less. preferable. When the average particle size / thickness is in the above range and the viscosity as described later is within a predetermined range, some of the hollow particles are lifted and bubbles are unevenly distributed when the foamed resin layer is formed. Can be prevented.
The density of the hollow particles 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 preferred. By setting the density of the hollow particles to 0.01 g / cm ³ or more and 0.4 g / cm ³ or less, it is possible to prevent floating and uniformly disperse when forming the foamed resin layer.

(Average bubble diameter)
The foamed resin layer of the present invention contains bubbles as described above, but the average bubble diameter is preferably 10 μm or more and 200 μm or less. When the average bubble diameter is not more than the above upper limit value, it becomes easy to achieve both water resistance and shock absorption performance. Further, when the value is not less than the above upper limit value and more than the lower limit value, both water resistance and shock absorption performance can be easily improved. From the viewpoint of water resistance and shock absorption, the average bubble diameter is more preferably 20 μm or more and 160 μm or less, and further preferably 50 μm or more and 120 μm or less.
The average bubble diameter is obtained by taking a cross-sectional image of a plane perpendicular to the thickness direction at the center of the foamed resin layer in the thickness direction and measuring the average value of the diameters of all bubbles reflected in the cross-sectional image. You can ask. The detailed measurement method is as described in Examples.

(Other layers)
The shock absorbing sheet of the present invention is preferably composed of a single foamed resin layer, but a layer other than the foamed resin layer may be provided as long as the effects of the present invention are 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-foaming 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 other resins. The resin forming the skin layer may be 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 resin and the thermoplastic elastomer 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 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, 1 μm or more and 50 μm or less, preferably 1 μm or more and 20 μm or less.

The thickness of the shock absorbing sheet may be larger than 200 μm, but is preferably 200 μm or less from the viewpoint of miniaturization and thinning. Further, from the viewpoint of shock absorption, waterproofness, miniaturization and thinning, it is preferably 20 μm or more and 180 μm or less, and more preferably 50 μm or more and 150 μm or less.
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, it is easy to improve the shock absorbing performance.

Examples of the resin constituting the foamed resin layer in the shock absorbing sheet of the present invention include acrylic resins and thermoplastic elastomers.
Among these, it is preferable that the resin constituting the foamed resin layer is an acrylic resin, and the foamed resin layer is an acrylic foamed resin layer. By using an acrylic resin as the resin constituting the foamed resin layer, it becomes easy to adjust the above-mentioned X value, middle component change rate, and water absorption rate within the above-mentioned predetermined ranges.
In the acrylic foamed resin layer, the acrylic resin is the main component of the resin component in the foamed resin layer, and the content thereof is preferably 70% by mass or more with respect to the total amount of the resin component of the foamed resin layer. It is more preferably 90% by mass or more, and most preferably 100% by mass.

Hereinafter, a mode in which the resin constituting the foamed resin layer is an acrylic resin will be described more specifically.
(Acrylic resin)
The acrylic resin is an acrylic polymer obtained by polymerizing the monomer component (a) containing the acrylic monomer component. The monomer component (a) is a monomer component having one addition-polymerizable double bond, and is preferably a monomer component having one vinyl group. The monomer component (a) is not particularly limited, but preferably contains an alkyl (meth) acrylate as the acrylic monomer component.
In addition, in this specification, (meth) acrylate means acrylate or methacrylate, and the same applies to other similar terms.

Examples of the alkyl (meth) acrylate include an alkyl (meth) acrylate having a linear or branched alkyl group, and the number of carbon atoms of the alkyl group is, for example, 1 to 18, preferably 1 to 14, more preferably. 1 to 10, more preferably 1 to 8. By using an alkyl (meth) acrylate having 1 to 8 carbon atoms in the alkyl group, it becomes easy to adjust the glass transition temperature (Tg) of the foamed resin layer within a desired range. Further, by using an alkyl (meth) acrylate, it becomes easy to adjust the X value and the rate of change of the middle component within a desired range.
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 is preferably contained in an amount of 50% by mass or more, more preferably 60% by mass or more and 100% by mass or less, in the monomer component (a) constituting the acrylic polymer. It is more preferably contained in an amount of 75% by mass or more and 100% by mass or less, and further preferably contained in an amount of 90% by mass or more and 100% by mass or less. By using an alkyl (meth) acrylate at least these lower limit values, it becomes easy to impart the mechanical strength required for the resin foam layer. Moreover, the monomer component (a) may be all alkyl (meth) acrylate.

Alkyl (meth) acrylates can be used alone or in combination of two or more. When two or more kinds are used in combination, it may be appropriately used so that the glass transition temperature (Tg) of the foamed resin layer is within the above-mentioned desired range.
Further, the alkyl (meth) acrylate is a first component (A1) which is an alkyl (meth) acrylate having an alkyl group having 1 to 3 carbon atoms and an alkyl (meth) acrylate having an alkyl group having 4 or more carbon atoms. It is preferable to use the second component (A2) in combination. By using the first component (A1) and the second component (A2) together, it becomes easier to adjust the X value and the rate of change of the middle component within a desired range, and it becomes easier to improve both water resistance and shock absorption. ..
Here, the first component (A1) is preferably an alkyl (meth) acrylate having an alkyl group having 1 or 2 carbon atoms, and more preferably an alkyl group having 1 or 2 carbon atoms, from the viewpoint of improving shock absorption. 1 or 2 alkyl acrylates, more preferably methyl acrylates.
Further, as the second component (A2), the number of carbon atoms of the alkyl group is preferably 4 to 8 from the viewpoint of making it easy to adjust the X value and the rate of change of the middle component within a desired range and improving the water resistance. It is an alkyl (meth) acrylate, more preferably an alkyl acrylate having an alkyl group having 4 to 8 carbon atoms. Further, it is more preferable to include an alkyl acrylate having an alkyl group having 4 carbon atoms such as n-butyl acrylate from the viewpoint of improving shock absorption. Further, from the viewpoint of increasing the X value and reducing the rate of change of the middle component to improve the water resistance, it is more preferable to contain an alkyl acrylate having an alkyl group having 8 carbon atoms such as 2-ethylhexyl acrylate. ..

As described above, when the first component (A1) and the second component (A2) are used in combination, the mass ratio (A2 / A1) of the second component to the first component (A1) is 5/95 or more and 70/30. The following is preferable, 10/90 or more and 50/50 or less is more preferable, and 20/80 or more and 40/60 or less is further preferable.

Further, the monomer component (a) may contain a monomer component other than the alkyl (meth) acrylate in addition to the above-mentioned alkyl (meth) acrylate. Examples of the monomer and the component other than the alkyl (meth) acrylate include a carboxyl group-containing monomer or an anhydride thereof, a hydroxyl group-containing (meth) acrylic monomer, a nitrogen-containing vinyl monomer, and a styrene-based monomer.

Examples of the carboxyl group-containing monomer include carboxylic acids containing an addition-polymerizable double bond such as (meth) acrylic acid, crotonic acid, silicic acid, itaconic acid, maleic acid, fumaric acid, and citraconic acid.
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 the nitrogen-containing vinyl monomer include (meth) acrylamide, N-methyl (meth) acrylamide, N, N-dimethyl (meth) acrylamide, N-ethyl (meth) acrylamide, N, N-diethyl (meth) acrylamide, and N-. Propyl (meth) acrylamide, N, N-dipropyl (meth) acrylamide, N-isopropyl (meth) acrylamide, N, N-diisopropyl (meth) acrylamide, N-butyl (meth) acrylamide, N, N-dibutyl (meth) Acrylamides such as acrylamide, amino group-containing (meth) acrylic monomers such as aminoethyl (meth) acrylate and t-butylaminoethyl (meth) acrylate, (meth) acrylonitrile, N-vinylpyrrolidone, N-vinylcaprolactam, N- Examples thereof include vinyl laurilolactam, (meth) acryloylmorpholine, and dimethylaminomethyl (meth) acrylate.
Examples of the styrene-based monomer include styrene, α-methylstyrene, o-methylstyrene, p-methylstyrene and the like.
The monomer components other than the alkyl (meth) acrylate may be used alone or in combination of two or more.

As the monomer component other than the alkyl (meth) acrylate, one or more selected from styrene-based monomers, acrylamides, and carboxyl group-containing monomers are preferable among the above-mentioned monomers. The monomer component other than the alkyl (meth) acrylate is preferably contained in an amount of 50% by mass or less, and 0% by mass or more and 40% by mass or less, in the monomer component (a) constituting the acrylic polymer. Is more preferably 0% by mass or more and 25% by mass or less, more preferably 0% by mass or more and 10% by mass or less, and it may not be contained.

The acrylic polymer is preferably crosslinked with a crosslinking agent and has a crosslinked structure. Since the acrylic polymer has a crosslinked structure, the amount of the hard component of the foamed resin layer increases, and it is prevented that the relaxation time of the soft component becomes long due to immersion in water, and the above-mentioned X value is increased. It will be easier. In addition, the rate of change in the middle component can be reduced.
When the acrylic polymer is crosslinked by a cross-linking agent, for example, when the monomer component (a) is polymerized, a cross-linking agent is further added to copolymerize the monomer component (a) and the cross-linking agent. The acrylic polymer may be crosslinked with a crosslinking agent. Further, the acrylic polymer may be crosslinked with the crosslinking agent by adding a crosslinking agent to the partially polymerized monomer component (a) and further polymerizing the monomer component (a).

Examples of the cross-linking agent include those having two or more addition-polymerizable double bonds, preferably those having two or more vinyl groups, and more preferably polyfunctional having two or more (meth) acryloyl groups. Examples include (meth) acrylate. Such a cross-linking agent is incorporated into a main chain composed of the monomer component (a), and the main chains are cross-linked to form a network.
Specific cross-linking agents 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) isocia. Nurate triacrylate, ε-caprolactone-modified tris (2-acryloxyethyl) isocyanurate, caprolactone-modified ethoxylated isocyanuric acid triacrylate, ethosylated trimethylol propanetriacrylate, proxyed trimethylol propanetriacrylate, proxyed glyceryl triacrylate, Examples thereof include neopentyl glycol adipate diacrylate, polyurethane acrylate, epoxy acrylate, polyester acrylate, and liquid hydrogenated 1,2-polybutadiene diacrylate. The cross-linking agent may be used alone or in combination of two or more.

The blending amount of the cross-linking agent is preferably 0.5 parts by mass or more and 10 parts by mass or less with respect to 100 parts by mass of the monomer component (a). When the content is 0.5 parts by mass or more, the crosslinked structure can be appropriately formed, and the X value and the rate of change of the middle component can be easily adjusted within a desired range. Further, when the amount is 10 parts by mass or less, the flexibility of the foamed resin layer can be ensured, and the shock absorption can be easily improved. From these viewpoints, the blending amount of the cross-linking agent is more preferably 1 part by mass or more and 8 parts by mass or less, and further preferably 2 parts by mass or more and 6 parts by mass or less.

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). To. In the following description, when the monomer component (a) is described as 100 parts by mass, the content of the monomer component (a) and the content of the structural unit derived from the monomer component (a) are used. Means the total amount of.
The above-mentioned cross-linking agent may be copolymerized with the monomer component (a) to perform the above-mentioned partial polymerization or complete polymerization, or the acrylic resin composition may be obtained without copolymerizing with the acrylic polymer. It may be blended.
Among the above, the foamed resin layer is preferably formed from an acrylic resin composition containing an acrylic polymer obtained by copolymerizing the monomer component (a) with a cross-linking agent.

When the bubbles in the foamed resin layer are formed by the hollow particles, the acrylic resin composition may contain 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 mass by mass with respect to 100 parts by mass of the monomer component (a). Parts or more and 5 parts by mass or less, preferably 1 part by mass or more and 3 parts by mass or less.

In addition to the above, the acrylic resin composition includes a polymerization initiator, a surfactant, a metal damage inhibitor, an antistatic agent, a stabilizer, a nucleating agent, a cross-linking aid, a pigment, a dye, a halogen-based, and a phosphorus-based resin. And other flame retardants such as, and other additives such as fillers may be contained within a range that does not impair the object of the present invention.

(Manufacturing method of shock absorbing sheet)
Hereinafter, the method for manufacturing the shock absorbing sheet will be described in more detail.
In the present invention, the foamed resin layer is preferably produced using, for example, a resin emulsion as a raw material. By using the resin emulsion as a raw material, the bubble stability at the time of producing the foamed resin layer is improved. Further, in the present invention, even when an emulsion is used, the water resistance is improved by using a cross-linking agent as described above or by using a reactive surfactant as described later.
The resin emulsion is an aqueous dispersion of various resins. The foamed resin layer is preferably composed of an acrylic foamed resin layer as described above, and therefore, the resin emulsion is preferably an acrylic emulsion. In the acrylic emulsion, the volume 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 is preferably 5 μm or less. Further, in order to improve the stability of the foam, the average particle size is preferably 500 nm or less, more preferably 300 nm or less. The 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).

When the foamed resin layer is used as a raw material, the foamed resin layer is formed from an emulsion composition containing an emulsion, and the emulsion composition is preferably an acrylic resin composition as described above.
The acrylic resin composition constituting the emulsion composition is, for example, a single amount in water or a mixture of water and a polar solvent in the presence of a polymerization initiator, a surfactant and the like, which are blended as needed. It can be produced by subjecting the body component (a) or the monomer component (a) and a cross-linking agent to emulsion polymerization, suspension polymerization, dispersion polymerization or the like.

As the surfactant, it is preferable to use a reactive surfactant. Reactive surfactants are surfactants that have an addition-polymerizable double bond in the molecule. The reactive surfactant may be an anion, a nonion, or a cation. Specific examples of the reactive surfactant include sodium alkylallyl sulfosuccinate, sodium methacryloyloxypolyoxypropylene sulfate, polyoxyethylene-propenylalkylphenyl ether, polyoxyethylene-propenyl alkylphenyl ether sulfate, propenyl group or allyl. Examples thereof include an ammonium salt of a group-added polyoxyethylene alkyl phenyl ether sulfonic acid.
Commercially available reactive surfactants include the Eleminor series such as "Ereminol JS-20" and "Ereminol RS-3000" manufactured by Sanyo Chemical Industries, Ltd., and "Aqualon HS" manufactured by Daiichi Kogyo Seiyaku Co., Ltd. / BC series ”,“ Aqualon KH series ”,“ Aqualon AR series ”,“ Adecaria Soap SR, ER, SE, NE, PP series ”manufactured by ADEKA CORPORATION.
The reactive surfactant is incorporated into the polymer by reacting with the monomer component (a), the cross-linking agent, etc. at the time of polymerization. Therefore, it is possible to prevent the water resistance of the polymer from being hindered by the surfactant, improve the water resistance of the foamed resin layer, and facilitate the adjustment of the above-mentioned X value and water absorption rate within a predetermined range. Further, as the surfactant, in addition to the reactive surfactant, a normal surfactant having no addition-polymerizable double bond in the molecule may be used.

As the polymerization initiator, a known radical initiator may be used. As the radical initiator, a water-soluble peroxide such as ammonium peroxodisulfate or potassium peroxodisulfate may be used, or a water-insoluble initiator may be used. Examples of the water-insoluble initiator include azobisisobutyronitrile, benzoyl peroxide and the like. The water-insoluble initiator may be added by dissolving it in a solvent.

As described above, the emulsion composition contains water as a dispersion medium, and may contain a polar solvent in addition to water. The solid content of the emulsion composition is, for example, 30% by mass or more and 70% by mass or less, more preferably 35% by mass or more and 60% by mass or less. Examples of the polar solvent used with water include methyl alcohol, ethyl alcohol, isopropyl alcohol and the like.

Bubbles are formed in the emulsion composition described above by mixing a gas, and a foamed resin layer can be produced by layering 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 bubbles are formed is then applied onto an appropriate support such as a release film or a base material to form a coating layer, and the layer is heated and dried to form a foamed resin layer. Obtainable. Here, the heating temperature is not particularly limited, but is preferably 45 to 155 ° C, more preferably 50 to 150 ° C.
The coating method used when applying the emulsion composition is not particularly limited, and a usual 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.

The viscosity of the emulsion composition in which bubbles are formed is preferably adjusted to 500 mPa · s or more and 50,000 mPa · s or less, and more preferably 800 mPa · s or more and 45,000 mPa · s or less. By adjusting the viscosity within the above range, it is possible to prevent the mixed bubbles from floating and facilitate the uniform arrangement of the bubbles in the thickness direction. 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. 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.

When the bubbles are formed of hollow particles, the hollow particles may be added to the emulsion composition instead of mixing the gas. Further, when bubbles are formed by both the hollow particles and the gas mixed in the resin composition, it is preferable to further add the hollow particles to the emulsion composition in which the gas is mixed by the mechanical floss method.

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.

Further, although the method of producing the foamed resin layer using the resin emulsion as a raw material has been described above, the foamed resin layer may be produced by using a resin emulsion other than the resin emulsion.
For example, the foamed resin layer is coated with an acrylic resin composition containing at least one of a monomer component (a) and an acrylic polymer obtained by partially polymerizing the monomer component (a). After that, it can be obtained by further polymerizing the monomer component (a) by irradiating it with an active energy ray such as ultraviolet rays and curing the acrylic resin composition. At this time, the acrylic resin composition may be appropriately blended with a cross-linking agent or the like as described above.

[How to use the shock absorbing sheet]
Hereinafter, a method of using the shock absorbing sheet of the present invention described above will be described.
The shock absorbing sheet of the present invention is used for, for example, various electronic devices, preferably portable electronic devices such as notebook personal computers, mobile phones, electronic papers, digital cameras, and video cameras. More specifically, it is used as a shock absorbing sheet for a display device (display) provided in these electronic devices. Examples of the display device include an organic EL display device, a liquid crystal display device, and the like, but an organic EL display device is preferable. Further, the display device, particularly the organic EL display device, is preferably a flexible display.

When used in a display device, the shock absorbing sheet may be 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 present invention has high shock absorbing performance even if it is thin, it is possible to appropriately prevent damage to the display device while making the electronic device thinner. In addition, 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 flexible display. Becomes possible.

Further, the shock absorbing sheet of the present invention is also used as a sealing material, and by arranging it between two parts, it is possible to prevent water from entering the inside of an electronic device from between the two parts. Therefore, for example, as described above, by arranging the display device on the back side, it is possible to prevent water from entering the back surface of the display device. Therefore, for example, it is possible to provide an electronic device having high waterproofness even if there is a gap in an edge portion of the electronic device.

Further, the shock absorbing sheet may be used as an adhesive tape by providing an adhesive material on one side or both sides. By using an adhesive tape, the shock absorbing sheet can be adhered to parts such as a housing of an electronic device via an adhesive material. 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 material may be one having pressure-sensitive adhesiveness and at least provided with a pressure-sensitive adhesive layer, and may be composed of a single pressure-sensitive adhesive layer laminated on the surface of the shock-absorbing sheet, or the shock-absorbing sheet. Although it may be a double-sided pressure-sensitive adhesive sheet attached to the surface of the above, 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 the shock absorbing sheet and 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 to 200 μm, more preferably 7 to 150 μm. 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.

The present invention will be described in more detail by way of examples, but the present invention is not limited to these examples.
The measurement method and evaluation method of each physical property are as follows.

<Water absorption rate>
A single-sided adhesive tape provided with an adhesive layer on one surface of the base material was attached to both sides of the foamed resin layer and cut into a size of 10 mm × 40 mm to obtain a laminated sheet. The weight of the obtained laminate sample was measured and used as the weight before the water swelling test. The laminated sample was placed in a sample tube (trade name: Screw Tube No. 7, manufactured by Maruem Co., Ltd.), ion-exchanged water was added thereto, and the sample was completely submerged in water. Then, after allowing to stand at 80 ° C. for 24 hours, the laminate sample was taken out from the sample tube, and the moisture adhering to the surface was removed from the laminate sample without extruding the moisture in the foamed resin layer. The weight of the laminate sample was measured and used as the weight after the water swelling test.
Assuming that the weight of the foamed resin layer before the water swelling test is "A" and the weight of the foamed resin layer after the water swelling test is "B", the weight increase rate represented by (BA) / A × 100 is obtained. It was calculated and used as the water absorption rate.
[Single-sided adhesive tape]
The single-sided adhesive tape used in the water swelling test and the pulse NMR measurement described later was prepared as follows.
52 parts by mass of ethyl acetate was placed in a reactor equipped with a thermometer, a stirrer, and a cooling tube to replace with nitrogen, and then the reactor was heated to start reflux. Thirty minutes after the ethyl acetate boiled, 0.08 parts by mass of azobisisobutyronitrile was added as a polymerization initiator. The monomer mixture was added dropwise and reacted evenly and gradually over 1 hour and 30 minutes. As the monomer mixture, 65 parts by mass of butyl acrylate (BA), 31.9 parts by mass of 2-ethylhexyl acrylate (2EHA), 3 parts by mass of acrylic acid (AAc), and 2-hydroxyethyl acrylate (2HEA). ) 0.1 parts by mass was used. 30 minutes after the completion of the dropping, 0.1 part by mass of azobisisobutyronitrile was added, the polymerization reaction was further carried out for 5 hours, and ethyl acetate was added to the reactor to cool the mixture while diluting. An acrylic copolymer-containing solution of / Mn = 10 was obtained.
Ethyl acetate was added to 100 parts by mass of the non-volatile content of the obtained acrylic copolymer-containing solution and stirred, and a total of 35 parts by mass of the tackifier resin and 4 parts by mass of the cross-linking agent were added and stirred. A 30% by mass pressure-sensitive adhesive composition was obtained. As the tackifier resin, 10 parts by mass of a modified rosin ester having a softening point of 100 ° C., 15 parts by mass of a polymerized rosin ester having a softening point of 135 ° C., and 10 parts by mass of a terpene phenol resin having a softening point of 130 ° C. were used. As the cross-linking agent, an isocyanate-based cross-linking agent (trade name "Coronate L45" manufactured by Nippon Polyurethane Industry Co., Ltd.) was used. Next, a release paper was prepared, the pressure-sensitive adhesive composition was applied to the release-treated surface of the release paper, and the pressure-sensitive adhesive layer was dried at 100 ° C. for 5 minutes to form a pressure-sensitive adhesive layer having a thickness of 10 μm. This pressure-sensitive adhesive layer was attached to a PET film having a thickness of 12 μm to obtain a single-sided pressure-sensitive adhesive tape. The softening point is a softening point measured by the JIS K2207 ring-and-ball method.

<Pulse NMR measurement>
Similar to the above water swelling test, a single-sided adhesive tape was attached to both sides of the foamed resin layer and cut into 8 mm × 20 mm to obtain a laminated body sample.
The laminate sample was introduced into a glass sample tube having a diameter of 10 mm (manufactured by Bruker, product number 1824511, 10 mm in diameter and length 180 mm, flat bottom). The sample tube was placed in a pulse NMR apparatus (“the minispec mq20” manufactured by BRUKER) and measured at 30 ° C. using the Solid Echo method. The obtained 1H spin-spin relaxation free induction decay curve was waveform-separated into three curves derived from the three components of the hard component, the middle component, and the soft component. Waveform separation was performed by fitting using both Gaussian type and exponential type. The ratio and relaxation time of each component were determined from the curves derived from the three components obtained in each measurement. In addition, the same kind of measurement was performed twice, and the ratio and relaxation time of each component were calculated as the average value to obtain the amount of each component before the heavy water swelling treatment and the relaxation time. It was submerged in water and allowed to stand at 80 ° C. for 24 hours. Then, only heavy water was extracted from the sample tube and held at 30 ° C. for 10 minutes, and the Solid Echo method was measured. The obtained 1H spin-spin relaxation free induction decay curve was waveform-separated into three curves derived from the three components of the hard component, the middle component, and the soft component. Waveform separation was performed by fitting using both Gaussian type and exponential type. The ratio and relaxation time of each component were determined from the curves derived from the three components obtained in each measurement. The same kind of measurement was performed twice, and the ratio and relaxation time of each component were obtained as the average value, and the amount and relaxation time of each component after the heavy water swelling treatment were used to obtain the X value and the rate of change of the middle component.
Using the analysis software "TD-NMRA (Version 4.3 Rev 0.8)" manufactured by BRUKER, the hard component was fitted in the Gaussian type, and the middle component and the soft component were fitted in the exponential type according to the product manual. In the analysis, fitting was performed using points up to 0.6 msec on the transition curve, and the amount of each component and the relaxation time were determined.

The following formula was used for fitting.
Y = A1 * exp (-1 / w1 * (t / T2A) ^ w1) + B1 * exp (-1 / w2 * (t / T2B) ^ w2) + C1 * exp (-1 / w3 * (t /) T2C) ^ w3)
Here, w1 to w3 are Weibull coefficients, w1 takes a value of 2, and w2 and w3 take a value of 1. A1 is the amount of the hard component, B1 is the middle component, C1 is the amount of the soft component, T2A is the hard component, T2B is the middle component, and T2C is the relaxation time of the soft component. t is the time.
The hard component, middle component, and soft component are components defined in ascending order of relaxation time in pulse NMR measurement, and the value of each relaxation time is not particularly limited, but the normal relaxation time is determined by the hard component. The range is less than 0.02 ms, the middle component is 0.02 ms or more and less than 0.1 ms, and the soft component is 0.1 ms or more.
<Solid Echo method>
Scans: 256times
Recycle Day: 1sec
Acquisition scale: 1ms

<Thickness>
The thickness was measured with a dial gauge.
<Apparent density>
The density of the foamed resin layer is a value of the apparent density measured in accordance with JIS K6767.

<Glass transition temperature (Tg)>
Measuring device: Using DVA-200 (manufactured by IT Measurement Control Co., Ltd.), shear mode: 1 Hz, strain amount: 1.0%, temperature range: -100 ° C to 100 ° C, temperature rise rate: 1 ° C / min The loss coefficient (tan δ) was measured under the conditions of. As the measurement sample, a shock absorbing sheet was laminated so as to have a thickness of about 1 mm and cut out to a size of 10 mm × 5 mm. The temperature when tan δ reached the maximum peak value was determined, and that temperature was defined as the glass transition temperature (Tg).

<Average cell diameter>
The shock absorbing sheet was attached to a PET film having a thickness of 50 μm, cut out to a size of 3 mm in width and 15 mm in length, and three-dimensional measurement was performed by an X-ray CT apparatus. The X-ray CT apparatus is not particularly limited, but in this embodiment, TDM1000H-II (2K) manufactured by Yamato Scientific Co., Ltd. was used. The resolution is about 1.5 μm / pixel. When cutting out the shock absorbing sheet, the MD direction is the length direction and the TD direction is the width direction, but when the MD direction is unknown, any one direction is set to the length direction. .. Here, the MD direction is an abbreviation for Machine Direction, and coincides with the flow direction of the resin at the time of coating or the like. The TD direction is an abbreviation for Transverse Direction, and is a direction orthogonal to the MD direction.
Next, a reference plane of the PET film and the boundary surface of the impact-absorbing sheet was counted total number S _T of the cross-sectional image that exists in a direction perpendicular (thickness direction) to the surface. The cross-sectional image existing in the thickness direction was from the image of the boundary surface (reference surface) between the PET film and the shock absorbing sheet to the last image of the shock absorbing sheet on the opposite surface.
Thereafter, to 0.5S _T th cross-sectional image, performs binarization processing using an image processing software "Avizo9.2.0" (FEI Inc.), separating the gap portion and the resin portion, the gap portion Was used as a bubble, and the diameter of each bubble was measured. The diameter of each bubble is an average value of the length along the length direction and the length along the width direction in the cross-sectional image. The diameter was measured for all the bubbles reflected in the cross-sectional image having a width of 2.2 mm and a length of 2.2 mm, and the average value was taken as the average of the bubble diameters.

<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-time average shock absorption rate (%) = (L _0a −L _1a ) / L _0a × 100

The monomer component (a) used in each Example and Comparative Example is as follows.
2EHA: 2-Ethylhexyl Acrylate BA: n-Butyl Acrylate EA: Ethyl Acrylate MA: Methyl Acrylate St: Styrene

The components used in Examples and Comparative Examples other than the monomer component (a) are as follows.
Crosslinking agent: Polyfunctional (meth) acrylate hollow particles (trade name "Expansel 920DE80d30", manufactured by Nippon Phillite Co., Ltd.), average particle size: 80 μm

[Example 1]
The monomer component (a) and the cross-linking agent are emulsified and polymerized in water in the presence of a polymerization initiator and a reactive surfactant having an addition-polymerizable double bond in the molecule in the amounts shown in Table 1, and solidified. An emulsion composition containing an aqueous dispersion of an acrylic polymer (average particle size 120 nm) having a volume of 50% by mass was obtained. The emulsion composition was stirred using a stirrer (TESCOM1200) under room temperature conditions at the second speed for 1.0 minute, and air was mixed by the mechanical floss method to form bubbles. The emulsion composition in which bubbles were formed was applied onto a release paper and then heated at 100 ° C. for 5 minutes to dry to obtain a shock absorbing sheet composed of a foamed resin layer. Table 1 shows the evaluation results of the shock absorbing sheet.

[Example 2]
The same procedure as in Example 1 was carried out except that the stirring time when air was mixed by the mechanical floss method was changed to 1.5 minutes. Table 1 shows the evaluation results of the shock absorbing sheet.

[Example 3]
The same procedure as in Example 1 was carried out except that the stirring time when air was mixed by the mechanical floss method was changed to 1.25 minutes. Table 1 shows the evaluation results of the shock absorbing sheet.

[Example 4]
25 parts by mass of butyl acrylate, 75 parts by mass of methyl acrylate, and 0.5 parts by mass of photoinitiator are mixed, and the mixture is partially polymerized by polymerization using ultraviolet rays to cure the syrup-like viscosity at 2000 mPa · s. A sex acrylic resin material was obtained. An acrylic resin composition was prepared by adding 3 parts by mass of a cross-linking agent and 2 parts by mass of hollow particles A to the present resin material and mixing them. The obtained acrylic resin composition was applied onto a release paper and irradiated with ultraviolet rays under the conditions of ^{illuminance: 4 mW / cm 2} and light intensity: 720 mJ / cm ^{2 to obtain a shock absorbing sheet.} Table 1 shows the evaluation results of the shock absorbing sheet.

[Example 5]
The same procedure as in Example 1 was carried out except that the composition of the monomer component (a) was changed as shown in Table 1 and the stirring time when air was mixed by the mechanical floss method was changed to 1.0 minute. Table 1 shows the evaluation results of the shock absorbing sheet.

[Comparative Example 1]
The same as in Example 1 except that the emulsion composition was prepared in the same manner as in Example 1 except that the cross-linking agent was not blended, and the stirring time when air was mixed by the mechanical floss method was changed to 1.5 minutes. It was carried out in. Table 1 shows the evaluation results of the shock absorbing sheet.

[Comparative Example 2]
The same procedure as in Comparative Example 1 was carried out except that the composition of the monomer component (a) was changed as shown in Table 1 and the stirring time when air was mixed by the mechanical floss method was changed to 1.25 minutes. Table 1 shows the evaluation results of the shock absorbing sheet.

[Comparative Example 3]
The same procedure as in Comparative Example 1 was carried out except that the composition of the monomer component (a) was changed as shown in Table 1 and the stirring time when air was mixed by the mechanical floss method was changed to 1.0 minute. Table 1 shows the evaluation results of the shock absorbing sheet.

[Comparative Example 4]
The emulsion composition prepared in the same manner as in Example 1 except that the composition of the monomer component (a) was changed as shown in Table 1 was applied on a release paper and then heated at 100 ° C. for 1 hour. And dried to obtain a shock absorbing sheet. Table 1 shows the evaluation results of the shock absorbing sheet.

[Comparative Example 5]
The blending amount of the cross-linking agent was changed as shown in Table 1, and after emulsion polymerization was carried out in the same manner as in Example 1, an emulsion composition was prepared by further adding 2 parts by mass of hollow particles. The emulsion composition containing the hollow particles was applied onto a release paper and then heated at 100 ° C. for 5 minutes to dry to obtain a shock absorbing sheet composed of a foamed resin layer. Table 1 shows the evaluation results of the shock absorbing sheet.

※なお、比較例１では、発泡樹脂層の吸水量が多かったため、粘着テープの剥離が発生し、Ｘ値、ミドル成分変化率を測定できなかった。

* In Comparative Example 1, since the amount of water absorbed by the foamed resin layer was large, the adhesive tape was peeled off, and the X value and the rate of change in the middle component could not be measured.

As shown in Table 1, the shock absorbing sheet of each example had a low water absorption rate and a high water resistance when the X value was 0.6 or more. On the other hand, the shock absorbing sheet of each comparative example had a low X value, a high water absorption rate, and poor water resistance.

Claims (12)

A shock absorbing sheet containing a foamed resin layer having a thickness of 200 μm or less.
Adhesive tapes are attached to both sides of the foamed resin layer to form a laminate, which is subjected to heavy water swelling treatment by allowing it to stand in heavy water for 24 hours at 80 ° C., and measured by pulse NMR at 30 ° C. at 30 ° C. before and after the heavy water swelling treatment. A shock absorbing sheet having an X value of 0.6 or more calculated from the following formula (1) based on the measured values obtained.
X value = (amount of hard component before heavy water swelling treatment / amount of middle component before heavy water swelling treatment) x (relaxation time of soft component before heavy water swelling treatment / relaxation time of soft component after heavy water swelling treatment) (1) Adhesive tapes are attached to both sides of the foamed resin layer to form a laminate, which is subjected to heavy water swelling treatment by allowing it to stand in heavy water for 24 hours at 80 ° C., and measured by pulse NMR at 30 ° C. at 30 ° C. before and after the heavy water swelling treatment. The shock absorbing sheet according to claim 1, wherein the middle component change rate calculated from the following formula (2) is 10% or less based on the measured values obtained.
Middle component change rate = (Middle component amount before heavy water swelling treatment-Middle component amount after heavy water swelling treatment) / Middle component amount before heavy water swelling treatment x 100 (2) The present invention according to claim 1 or 2, wherein the following water swelling test is carried out using the foamed resin layer as a sample, and the water absorption rate represented by the weight increase rate of the sample after the water swelling test is 20% or less as compared with that before the water swelling test. Shock absorbing sheet.
<Water swelling test>
Adhesive tapes are attached to both sides of the foamed resin layer to form a laminate, and the laminate is immersed in a sample tube filled with water and allowed to stand at 80 ° C. for 24 hours. 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 0.9 g / cm ^{3 or less.} The shock absorbing sheet according to any one of claims 1 to 4, wherein the foamed resin layer is an acrylic foamed resin layer. The shock absorbing sheet according to any one of claims 1 to 5, wherein the glass transition temperature is 0 ° C. or higher and 25 ° C. or lower. The shock absorbing sheet according to any one of claims 1 to 6, wherein the 7-time average shock absorbing rate is 30% or more. The shock absorbing sheet according to any one of claims 1 to 7, which is used in an electronic device. The shock absorbing sheet according to any one of claims 1 to 8, 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 9 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 9 or the adhesive tape according to claim 10. A shock absorbing sheet containing a foamed resin layer having a thickness of 200 μm or less.
A shock absorbing sheet in which the following water swelling test is carried out using the foamed resin layer as a sample, and the water absorption rate represented by the weight increase rate of the sample after the water swelling test as compared with that before the water swelling test is 20% or less.
<Water swelling test>
Adhesive tapes are attached to both sides of the foamed resin layer to form a laminate, and the laminate is immersed in a sample tube filled with water and allowed to stand at 80 ° C. for 24 hours.