JP7767920B2 - Double-sided pressure-sensitive adhesive sheet, laminate for image display device, and image display device - Google Patents

Description

The present invention relates to a double-sided pressure-sensitive adhesive sheet for use in bonding two components of an image display device, a laminate for an image display device, and an image display device.

In recent years, in order to improve the visibility of image display devices, the gap between an image display panel such as a liquid crystal display (LCD), plasma display (PDP) or electroluminescent display (ELD) and a protective panel or touch panel component placed on the front side (viewing side) of the panel has been filled with a resin such as an adhesive or glue to suppress reflection of incident light and light emitted from the displayed image at the air layer interface.

For example, Patent Document 1 discloses a method for manufacturing a laminated component for an image display device, which has a configuration in which a component for an image display device is laminated on at least one side of a transparent double-sided adhesive sheet. The method involves laminating an adhesive sheet that has been primarily crosslinked by ultraviolet light to the component for the image display device, and then irradiating the adhesive sheet with ultraviolet light through the component for the image display device to cause secondary curing.

When bonding and integrating components for image display devices with adhesives in this way, operational errors such as misalignment or the introduction of air bubbles or foreign matter can occur during the bonding process. Because it may be necessary to peel off the components of the image display device to correct these errors, adhesives used for this type of purpose are often required to be removably (reworkable). In particular, laminates of non-flexible plate-like components are not easy to separate once bonded, so adhesives with reworkability are required when bonding components for large screens that are difficult to bond, components with curved surfaces, or expensive components.

Furthermore, even when components for image display devices are completely bonded together with adhesive, if defects such as poor bonding or insufficient curing occur after bonding, it is desirable to reuse some components, such as expensive components, rather than simply discarding them.From the perspective of such reuse, there is a growing demand for removability (reworkability).

Conventionally, as an adhesive that is removably (reworkably) suitable for image display devices, for example, Patent Document 2 proposes an adhesive for optical films that uses a specific acrylic triblock copolymer, does not require chemical crosslinking, has excellent adhesive performance and durability, and can be peeled off with moderate peel strength without leaving any adhesive residue.

Patent document 3 also discloses a double-sided adhesive sheet that is configured to be removably attached to at least one of the touch panel and the display surface of a display device, and is characterized by being optically isotropic. The removability is achieved by making the adhesive strength of the adhesive layer on the display device side to the display surface of the display device smaller than the adhesive strength of the adhesive layer on the touch panel side to the bonding surface of the touch panel.

Furthermore, Patent Document 4 discloses a re-peelable adhesive sheet having an internal interface (internal peel interface) that allows peeling on a surface different from the bonding surface between the adhesive and the adherend.

Patent No. 4971529 Patent No. 5203964 Japanese Patent Application Laid-Open No. 2004-231723 International Publication No. 2010/137523

In actual manufacturing processes, components for image display devices may be peeled (separated) by cooling, but the evaluation of removability disclosed in Patent Documents 2 to 4 above was conducted at room temperature, and removability through cooling was not evaluated. Therefore, Patent Documents 2 to 4 above do not design PSA sheets from the perspective of removability through cooling.

The problem that this invention aims to solve is to provide a double-sided adhesive sheet that has excellent adhesive performance such as step absorption, adhesive strength, and lamination reliability, and that allows components for image display devices to be peeled (separated) by cooling.

As a result of extensive research into resolving the above-mentioned problems, the inventors discovered that the above-mentioned problems can be resolved by setting the ratio of the tensile storage modulus (E') to the shear storage modulus (G'), the peak temperature of the loss tangent obtained by shear mode dynamic viscoelasticity measurement, the peak temperature of the loss tangent obtained by tensile mode dynamic viscoelasticity measurement, and the 180° peel adhesive strength to specified values in a double-sided pressure-sensitive adhesive sheet.

That is, the gist of the present invention is the following [1] to [13].
[1] A double-sided pressure-sensitive adhesive sheet for use in bonding two components for an image display device, comprising:
the ratio (E'/G') of the tensile storage modulus (E') to the shear storage modulus (G') is 5.0 or more;
The peak temperature (T1) of the loss tangent (Tan δ) obtained by dynamic viscoelasticity measurement in a shear mode at a frequency of 1 Hz is −10° C. or lower,
The peak temperature (T2) of the loss tangent (Tan δ) obtained by dynamic viscoelasticity measurement in a tensile mode at a frequency of 1 Hz is −10° C. or higher (however, if two or more peak temperatures are observed, the highest temperature is taken as the peak temperature (T2)).
A double-sided pressure-sensitive adhesive sheet having a 180° peel adhesive strength of 5 N/20 mm or more at 23°C with respect to the respective adherend surfaces of the two constituent members for an image display device.
[2] The double-sided pressure-sensitive adhesive sheet according to [1], wherein the difference between the peak temperature (T2) of the loss tangent (Tan δ) obtained by dynamic viscoelasticity measurement in a tensile mode at a frequency of 1 Hz and the peak temperature (T1) of the loss tangent (Tan δ) obtained by dynamic viscoelasticity measurement in a shear mode at a frequency of 1 Hz is 5 to 50°C.
[3] The double-sided pressure-sensitive adhesive sheet according to [1] or [2], wherein the two components for an image display device are formed from different materials.
[4] The double-sided pressure-sensitive adhesive sheet according to any one of [1] to [3], wherein one of the two components for an image display device is glass, and the other is any one or a combination of two or more selected from the group consisting of a touch sensor, an image display panel, a surface protection panel, a polarizing film, and a retardation film.
[5] The double-sided pressure-sensitive adhesive sheet according to [4], wherein the glass is a cover glass having a curved surface.
[6] The double-sided pressure-sensitive adhesive sheet according to any one of [1] to [5], which is configured so that the two components for an image display device can be separated by a cooling operation.
[7] The double-sided pressure-sensitive adhesive sheet according to [6], wherein the cooling step is cooling to −50° C. or lower.
[8] The double-sided pressure-sensitive adhesive sheet according to any one of [1] to [7], having a thickness of 50 to 1000 μm.
[9] The double-sided PSA sheet has at least three layers, the outermost layer and the innermost layer being (meth)acrylic PSA layers,
The double-sided pressure-sensitive adhesive sheet according to any one of [1] to [8], wherein the ratio of the total thickness of the outermost layer and the innermost layer to the total thickness is 5 to 70%.
[10] The double-sided pressure-sensitive adhesive sheet according to any one of [1] to [9], which is photocurable.
[11] A laminate for an image display device, in which two components for an image display device are bonded together via the double-sided pressure-sensitive adhesive sheet according to any one of [1] to [10].
[12] A laminate for an image display device, in which two components for an image display device are bonded together via a pressure-sensitive adhesive layer formed by curing the double-sided pressure-sensitive adhesive sheet according to any one of [1] to [10].
[13] An image display device constructed using the laminate for an image display device according to [12].

The double-sided pressure-sensitive adhesive sheet of the present invention has excellent adhesive performance, including level difference absorption, adhesive strength, and lamination reliability, as well as excellent reworkability, allowing components for image display devices to be easily peeled off by cooling. Therefore, it is suitable for use in laminating components for image display devices, and is particularly suitable for laminating components for large screens that are difficult to laminate, components with curved surfaces, and expensive components.

1A and 1B are diagrams for explaining the evaluation test methods for ITO corrosion resistance reliability and Cu corrosion resistance reliability performed in the examples, in which (A) is a top view of the ITO pattern on an ITO glass substrate or a top view of the copper pattern on a copper glass substrate for evaluating corrosion resistance reliability, (B) is a top view showing a state in which an adhesive sheet is covered on an ITO glass substrate for evaluating ITO corrosion resistance reliability or a top view showing a state in which an adhesive sheet is covered on a copper glass substrate for evaluating Cu corrosion resistance reliability, (C) is a cross-sectional view of a sample for evaluating ITO corrosion resistance reliability, and (D) is a cross-sectional view of a sample for evaluating Cu corrosion resistance reliability.

An example of an embodiment of the present invention will be described in detail below, but the present invention is not limited to the following embodiment.
In this specification, "(meth)acrylic" encompasses "acrylic" and "methacrylic", and "(meth)acrylate" encompasses "acrylate" and "methacrylate".

The double-sided pressure-sensitive adhesive sheet of the present invention is typically composed of multiple (meth)acrylic pressure-sensitive adhesive layers and is used to bond two components of an image display device. Furthermore, from the perspective of improving bonding reliability, the double-sided pressure-sensitive adhesive sheet of the present invention preferably has photocurability, meaning that it cures when irradiated with active energy rays such as ultraviolet light.

Furthermore, the peak temperature (T1) of the loss tangent (Tan δ) obtained by dynamic viscoelasticity measurement of the double-sided pressure-sensitive adhesive sheet of the present invention in shear mode at a frequency of 1 Hz is -10°C or lower, preferably -100 to -15°C, and particularly preferably -50 to -20°C. Having the peak temperature (T1) of the loss tangent (Tan δ) in the above range enables excellent level difference absorption and lamination reliability. The peak temperature (T1) of the loss tangent obtained by dynamic viscoelasticity measurement in shear mode can be obtained by the following method.

[Dynamic viscoelasticity measurement in shear mode]
The sample to be measured is laminated to a thickness of 0.6 to 0.8 mm and punched out into a circle with a diameter of 8 mm to serve as the measurement sample. The dynamic viscoelasticity spectrum of this measurement sample is measured in shear mode using a rheometer (TA Instruments, "Discovery HR2") under the following measurement conditions. From the dynamic viscoelasticity spectrum data obtained by the measurement, the temperature at which the loss tangent (Tan δ) reaches its maximum value, i.e., the peak temperature (T1), is read.
As mentioned above, the values measured in shear mode for dynamic viscoelasticity are those obtained when the sample thickness is 0.6 to 0.8 mm. This is because, to accurately measure dynamic viscoelasticity in shear mode, it is necessary to avoid fluctuations in the measurement results due to the influence of the measurement jig caused by insufficient sample thickness. Therefore, to measure dynamic viscoelasticity in shear mode, it is necessary to adjust the sample to a certain thickness range before measurement. By adjusting the sample thickness within the above range beforehand and measuring the dynamic viscoelasticity in shear mode, the dynamic viscoelasticity spectrum of the sample can be accurately determined without being influenced by the measurement jig.
Furthermore, when the sample is photocurable, the sample used may be either before or after curing.
[Measurement conditions]
Adhesive jig: Φ8mm parallel plate Distortion: 0.1%
Frequency: 1 Hz
Temperature: -120~200℃
Temperature increase rate: 5°C/min

Furthermore, the peak temperature (T2) of the loss tangent (Tan δ) obtained by dynamic viscoelasticity measurement of the double-sided pressure-sensitive adhesive sheet of the present invention in a tensile mode at a frequency of 1 Hz is −10° C. or higher, preferably −5 to 20° C., and particularly preferably 0 to 15° C. Having the peak temperature (T2) of the loss tangent (Tan δ) in the above range allows for excellent removability (reworkability) and cutting processability.
The peak temperature (T2) of the loss tangent (Tan δ) obtained by dynamic viscoelastic measurement in a tensile mode can be obtained by the following method.

[Dynamic viscoelasticity measurement in tension mode]
The dynamic viscoelasticity spectrum of the sample to be measured is measured in tension mode using a dynamic viscoelasticity measuring device (ITKDVA-200, manufactured by IT Measurement & Control Co., Ltd.) at a vibration frequency of 1 Hz, a measurement temperature of -120 to 80°C, and a heating rate of 3°C/min. From the dynamic viscoelasticity spectrum data obtained by the measurement, the temperature at which the loss tangent (Tan δ) reaches its maximum value, i.e., the peak temperature (T2), is read. However, if two or more peak temperatures are observed, the highest temperature is taken as the peak temperature (T2).
Furthermore, when the sample is photocurable, the sample used may be either before or after curing, but is preferably after curing.

Furthermore, in the present invention, the difference between the peak temperature (T2) of the loss tangent (Tan δ) obtained by dynamic viscoelasticity measurement in a tension mode at a frequency of 1 Hz and the peak temperature (T1) of the loss tangent (Tan δ) obtained by dynamic viscoelasticity measurement in a shear mode at a frequency of 1 Hz is preferably 5 to 50°C, more preferably 10 to 40°C, and particularly preferably 15 to 30°C. By achieving this range, an excellent balance of performance characteristics such as level difference absorption, lamination reliability, impact resistance, removability (reworkability), and cutting processability can be achieved.

The double-sided pressure-sensitive adhesive sheet of the present invention has a 180° peel adhesive strength at 23°C to the adherend surfaces of two components for an image display device to be bonded, of 5 N/20 mm or more, preferably 6 N/20 mm or more, and particularly preferably 7 N/20 mm or more. The upper limit of the 180° peel adhesive strength is typically 50 N/20 mm. The above 180° peel adhesive strength is measured by the following method.

[Method for measuring 180° peel adhesive strength]
A 100 μm thick polyethylene terephthalate (PET) film (Diafoil T100, manufactured by Mitsubishi Chemical Corporation) was bonded to one side of a double-sided pressure-sensitive adhesive sheet, and the other side was roll-pressured against a soda lime glass plate to form a bonded product. The bonded product was then autoclaved (60°C, gauge pressure 0.2 MPa, 20 minutes) for finish bonding. The PET film side was then irradiated with 365 nm ultraviolet light using a high-pressure mercury lamp to achieve an integrated light intensity of 3000 mJ/ ^cm² , and then aged for 12 hours in an environment of 23°C and 50% RH to form a sample for measuring 180° peel adhesive strength. The peel strength (N/cm) measured when this sample was peeled at a peel angle of 180° and a peel rate of 300 mm/min in an environment of 23°C and 50% RH was defined as the 180° peel adhesive strength.

The double-sided pressure-sensitive adhesive sheet of the present invention preferably has a two-layer structure with the top and bottom layers being (meth)acrylic pressure-sensitive adhesive layers, more preferably at least three layers with the top and bottom layers being (meth)acrylic pressure-sensitive adhesive layers, and particularly preferably a three-layer structure with the top and bottom layers being (meth)acrylic pressure-sensitive adhesive layers [top layer (pressure-sensitive adhesive layer)/middle layer/bottom layer (pressure-sensitive adhesive layer)]. A multi-layer structure can provide excellent reworkability, step absorption, and lamination reliability.

Furthermore, in the present invention, from the viewpoints of reworkability, level difference absorbency, adhesive strength, and lamination reliability, it is preferable to have at least two layers with different glass transition temperatures, and it is particularly preferable to have an outermost layer and an innermost layer (hereinafter referred to as "low Tg layer") formed from a (meth)acrylic pressure-sensitive adhesive layer with a glass transition temperature of -10°C or lower, and an intermediate layer (hereinafter referred to as "high Tg layer") formed from a resin composition containing a (meth)acrylic resin with a glass transition temperature higher than -10°C. Unless otherwise specified, in this specification, the glass transition temperature is defined as the peak temperature of the loss tangent (Tan δ) obtained by dynamic viscoelasticity measurement in shear mode at a frequency of 1 Hz, and is measured by the method described above.

The glass transition temperature of the low Tg layer is usually -10°C or lower, preferably -100 to -15°C, and particularly preferably -50 to -20°C.
The glass transition temperature of the high Tg layer is usually higher than -10°C, preferably -5 to 20°C, and particularly preferably 0 to 15°C.

Furthermore, the difference in glass transition temperature between the high Tg layer and the low Tg layer (high Tg layer - low Tg layer) is typically 5°C or higher, preferably 10°C or higher, and particularly preferably 20°C or higher. The upper limit is typically 50°C. By keeping the glass transition temperature difference within the above range, reworkability, level difference absorbability, adhesive strength, and lamination reliability tend to be excellent. Note that when a double-sided pressure-sensitive adhesive sheet has multiple high Tg layers and low Tg layers, the difference in glass transition temperature between the high Tg layer and the low Tg layer is the difference in glass transition temperature between the high Tg layer with the highest glass transition temperature and the low Tg layer with the lowest glass transition temperature.

As described above, the preferred layer configuration of the double-sided pressure-sensitive adhesive sheet of the present invention is at least three layers, with the outermost and innermost layers being (meth)acrylic pressure-sensitive adhesive layers, and particularly preferably a three-layer configuration [(pressure-sensitive adhesive layer)/intermediate layer/innermost layer (pressure-sensitive adhesive layer)], with the outermost and innermost layers being (meth)acrylic pressure-sensitive adhesive layers. In this case, the outermost and innermost layers of the double-sided pressure-sensitive adhesive sheet (the surfaces to be attached to the components for an image display device) are preferably low-Tg layers. The intermediate layer sandwiched between the outermost and innermost layers is preferably a high-Tg layer. Furthermore, the low-Tg layers used in the outermost and innermost layers may have different glass transition temperatures, but it is preferable that the outermost and innermost layers have the same glass transition temperature. It is particularly preferable that the outermost and innermost layers are (meth)acrylic pressure-sensitive adhesive layers formed from the same resin composition.

In other words, one of the most preferred layer configurations for the double-sided pressure-sensitive adhesive sheet of the present invention is a three-layer configuration of a low Tg layer/high Tg layer/low Tg layer formed from two types of resin compositions. This layer configuration can provide excellent reworkability, step absorption, adhesive strength, and lamination reliability.

The shear storage modulus (G') of the double-sided PSA sheet is preferably 5 x ¹⁰ to 5 x ¹⁰ Pa, more preferably 1 x ¹⁰ to 1 x ¹⁰ Pa, and particularly preferably 2 x ¹⁰ to 1 x ¹⁰ Pa. When the double-sided PSA sheet has a shear storage modulus (G') within this range, it is possible to impart appropriate processability while maintaining level difference absorbency. The shear storage modulus (G') is the shear storage modulus at 25°C, and can be obtained by reading the shear storage modulus (G') at 25°C from the dynamic viscoelasticity spectrum data obtained by the dynamic viscoelasticity measurement in the shear mode described above.

Furthermore, when the double-sided pressure-sensitive adhesive sheet has at least three layers, the outermost and innermost layers of which are (meth)acrylic pressure-sensitive adhesive layers, it is preferable from the standpoint of reworkability that the shear storage modulus (G') of the middle layer be greater than the shear storage modulus (G') of the outermost and innermost layers.

The tensile storage modulus (E') of the double-sided PSA sheet is preferably 2.5 x ¹⁰ to 5 x ¹⁰ Pa, more preferably 5 x ¹⁰ to 5 x ¹⁰ Pa, and particularly preferably 1 x ¹⁰ to 5 x ¹⁰ Pa. Having a tensile storage modulus (E') in this range enables excellent handling properties and cutting processability. The tensile storage modulus (E') is the tensile storage modulus at 25°C, and can be obtained by reading the tensile storage modulus (E') at 25°C from the dynamic viscoelasticity spectrum data obtained by the dynamic viscoelasticity measurement in the tension mode described above.

As described above, the double-sided PSA sheet of the present invention preferably has at least two layers with different glass transition temperatures, and whether the double-sided PSA sheet has at least two layers with different glass transition temperatures can be determined by the ratio of the tensile storage modulus (E') obtained by tensile viscoelasticity measurement to the shear storage modulus (G') obtained by shear viscoelasticity measurement. Typically, when the PSA sheet is an isotropic elastic body, the relationship E' = 3G' holds between the tensile storage modulus (E') and the shear storage modulus (G').
However, when there are at least two layers with different glass transition temperatures, that is, when the material is an anisotropic elastic body, this relationship does not hold (E'≠3G').

The ratio (E'/G') of the tensile storage modulus (E') to the shear storage modulus (G') of the double-sided PSA sheet of the present invention is 5.0 or more, preferably 6.0 or more, more preferably 7.0 or more, and even more preferably 8.0 or more. Also, it is preferably 100 or less, more preferably 50 or less, and even more preferably 30 or less.
In other words, when the E'/G' ratio of a double-sided pressure-sensitive adhesive sheet is 5.0 or more, it can be determined that the sheet "has at least two layers with different glass transition temperatures" or "has layers with a glass transition temperature gradient in the thickness direction," and has excellent reworkability, making it easy to peel at the interface with the adherend surface of at least one of the components for an image display device by cooling.

The thickness of the double-sided pressure-sensitive adhesive sheet of the present invention is preferably 50 to 1000 μm, more preferably 60 to 500 μm, and particularly preferably 75 to 300 μm. If the sheet is too thin, its level difference absorbency tends to decrease, while if it is too thick, it tends to be difficult to achieve reworkability.

Furthermore, when the double-sided PSA sheet has at least three layers, the outermost and innermost layers of which are (meth)acrylic PSA layers, the ratio of the total thickness of the outermost and innermost layers to the total thickness is preferably 5 to 70%, more preferably 10 to 60%, and particularly preferably 20 to 45%. By keeping the thicknesses of the outermost and innermost layers within the above ranges, excellent reworkability, level difference absorbability, and lamination reliability can be achieved.

The double-sided adhesive sheet of the present invention is used to bond two components of image display devices, specifically, to bond components of image display devices such as LCDs, PDPs or ELs in personal computers, mobile terminals (PDAs), game consoles, televisions (TVs), car navigation systems, touch panels, pen tablets, etc.

In addition, it is preferable that the two components for the image display device are formed from different materials in terms of reworkability.

More specifically, as for the components for the image display device, it is preferable that one of the two components for the image display device is glass and the other is film, and it is particularly preferable that the glass is tempered glass and the film is a laminate consisting of one of the group consisting of a touch sensor, an image display panel, a surface protection panel, a polarizing film, and a retardation film, or a combination of two or more types.

In recent years, image display panels such as liquid crystal displays (LCDs), plasma displays (PDPs), and electroluminescent displays (ELDs) have increasingly used curved cover glass for design reasons. However, such cover glass is expensive and is a material that should be reused. However, the double-sided pressure-sensitive adhesive sheet of the present invention has excellent reworkability and can therefore be effectively used on curved cover glass.

The substrate of the above film is preferably a transparent resin, such as polyester resin, polyolefin resin, (meth)acrylic resin, polyurethane resin, polyethersulfone resin, polycarbonate resin, polysulfone resin, polyether resin, polyetherketone resin, (meth)acrylonitrile resin, or cycloolefin resin. Of these, polyester resin is preferred, with polyethylene terephthalate (PET) being particularly preferred. These may be used alone or in combination of two or more.

By laminating components for an image display device to both adhesive surfaces of the double-sided pressure-sensitive adhesive sheet of the present invention, a laminate for an image display device is obtained in which two components for an image display device are laminated via the double-sided pressure-sensitive adhesive sheet. Furthermore, such a laminate for an image display device is typically irradiated with active energy rays to photo-cure the pressure-sensitive adhesive sheet, thereby obtaining a laminate for an image display device in which two components for an image display device are laminated via the adhesive layer formed by curing the double-sided pressure-sensitive adhesive sheet.

The double-sided pressure-sensitive adhesive sheet of the present invention is preferably configured so that two components for an image display device can be peeled (separated) by cooling. In particular, it is preferably configured so that the cooling operation enables peeling (separation) at the interface with the adherend surface of at least one of the components for an image display device. Note that "peelable (separable)" means that the components for an image display device bonded via the double-sided pressure-sensitive adhesive sheet can be easily peeled (separated) without destroying the components, particularly at the interface between the double-sided pressure-sensitive adhesive sheet and the component for an image display device. The cooling operation is carried out by the following method.

[Cooling operation]
By cooling the laminate for an image display device, the constituent members for the image display device can be separated, particularly by peeling occurring at the interface with the adherend surface of at least one of the constituent members for the image display device.
In some cases, the laminate for an image display device may be cooled before irradiation with active energy rays and then peeled off without irradiating the active energy rays. In this case, too, the components for the image display device can be separated, particularly by peeling occurring at the interface with the adherend surface of at least one of the components for the image display device.
By the above method, the components for an image display device can be easily recycled.

The cooling temperature of the laminate for the image display device is preferably -50°C or lower, more preferably -80°C or lower, and even more preferably -100°C or lower. There is no particular lower limit to the cooling temperature, but it is usually around -300°C, preferably around -200°C.

Furthermore, when cooling the above-mentioned laminate for an image display device, it is preferable to place it so that one of the image display components of the laminate for an image display device is in contact with an SUS cooling plate that has been cooled to the above-mentioned cooling temperature, and it is particularly preferable to place it so that the side of the component for an image display device with a lower linear expansion coefficient is in contact with the SUS cooling plate.

The time required for the above peeling to occur is usually within 600 seconds, particularly preferably within 300 seconds, more preferably within 180 seconds, particularly preferably within 120 seconds, even more preferably within 60 seconds, and especially preferably within 30 seconds.

The principle by which the double-sided pressure-sensitive adhesive sheet of the present invention enables components (adherends) for image display devices to be peeled off by cooling is believed to be as follows: When a double-sided pressure-sensitive adhesive sheet bonded to two adherends is cooled, the adherends shrink as the temperature drops, generating a stress equal to the elastic modulus of the adherends multiplied by the shrinkage strain. This strain or stress becomes the peel force that separates the double-sided pressure-sensitive adhesive sheet from the adherends at their interfaces. Therefore, it is preferable that the two adherends in contact with the double-sided pressure-sensitive adhesive sheet be made of materials with different linear expansion coefficients, and it is even more preferable that the two adherends be made of different materials.

The key to providing reworkability to the double-sided pressure-sensitive adhesive sheet of the present invention is that it is difficult to relax, or becomes difficult to relax upon cooling, in response to the strain or stress (difference) exerted by the adherend. Specifically, this can be achieved by increasing the glass transition temperature (Tg) of the double-sided pressure-sensitive adhesive sheet. However, simply increasing the glass transition temperature (Tg) reduces adhesive performance such as level difference absorption, adhesive strength, and lamination reliability. For this reason, in the present invention, the ratio (E'/G') of the tensile storage modulus (E') to the shear storage modulus (G') of the double-sided PSA sheet is set to 5.0 or higher, and further, the peak temperature (T1) of the loss tangent (Tan δ) obtained by dynamic viscoelasticity measurement in shear mode at a frequency of 1 Hz of the double-sided PSA sheet is set to -10°C or lower, and further, the peak temperature (T2) of the loss tangent (Tan δ) obtained by dynamic viscoelasticity measurement in tensile mode at a frequency of 1 Hz is set to -10°C or higher (however, if two or more peak temperatures are observed, the highest temperature is taken as the peak temperature (T2)).

The double-sided pressure-sensitive adhesive sheet of the present invention having the above-mentioned properties preferably has at least two (meth)acrylic pressure-sensitive adhesive layers, and the (meth)acrylic pressure-sensitive adhesive layers are formed from a resin composition containing a (meth)acrylic copolymer.
Furthermore, when the protective film has at least three layers, the outermost and innermost layers of which are (meth)acrylic pressure-sensitive adhesive layers, the intermediate layer is also formed from a resin composition containing a (meth)acrylic copolymer.
The resin compositions forming the (meth)acrylic pressure-sensitive adhesive layer and the intermediate layer will be described below.

The above resin composition contains a (meth)acrylic copolymer as the main component, and may further contain a crosslinking agent (B), a photoinitiator (C), a silane coupling agent (D), a corrosion inhibitor (E), and other additives.

The above-mentioned "main component" means that the resin composition contains 50% by mass or more, preferably 70% by mass or more, and more preferably 80% by mass or more of (meth)acrylic copolymer based on the entire resin composition.

(Meth)acrylic copolymer
The (meth)acrylic copolymer may be, for example, a copolymer of an alkyl(meth)acrylate monomer having an alkyl group with 4 to 18 carbon atoms and a monomer component copolymerizable therewith.

Examples of alkyl (meth)acrylate monomers having an alkyl group of 4 to 18 carbon atoms include linear alkyl (meth)acrylate monomers such as n-butyl (meth)acrylate, pentyl (meth)acrylate, hexyl (meth)acrylate, heptyl (meth)acrylate, n-octyl (meth)acrylate, nonyl (meth)acrylate, decyl (meth)acrylate, undecyl (meth)acrylate, lauryl (meth)acrylate, tridecyl (meth)acrylate, tetradecyl (meth)acrylate, cetyl (meth)acrylate, and stearyl (meth)acrylate; isobutyl (meth)acrylate, sec-butyl (meth)acrylate, t-butyl (meth)acrylate; and isopentyl (meth)acrylate. Examples of suitable acrylates include branched alkyl (meth)acrylate monomers such as butyl (meth)acrylate, neopentyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, isooctyl (meth)acrylate, isononyl (meth)acrylate, isodecyl (meth)acrylate, and isostearyl (meth)acrylate, and alicyclic (meth)acrylate monomers such as cyclohexyl (meth)acrylate, t-butylcyclohexyl (meth)acrylate, 3,5,5-trimethylcyclohexane (meth)acrylate, dicyclopentanyl (meth)acrylate, dicyclopentenyl (meth)acrylate, dicyclopentenyloxyethyl (meth)acrylate, and isobornyl (meth)acrylate. These may be used alone or in combination of two or more.

Examples of monomer components copolymerizable with the alkyl (meth)acrylate monomers having 4 to 18 carbon atoms in the alkyl group include hydroxyl group-containing (meth)acrylate monomers, nitrogen atom-containing (meth)acrylate monomers, carboxy group-containing (meth)acrylate monomers, epoxy group-containing (meth)acrylate monomers, vinyl monomers, alkyl (meth)acrylate monomers having 1 to 3 carbon atoms in the alkyl group, and other copolymerizable monomers.

Examples of the hydroxyl group-containing (meth)acrylate monomer include 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 2-hydroxy-1-methylethyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, 2-hydroxybutyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, glycerin mono(meth)acrylate, polyethylene glycol mono(meth)acrylate, polypropylene glycol mono(meth)acrylate, polyethylene glycol polypropylene glycol mono(meth)acrylate, polyethylene glycol polybutylene glycol mono(meth)acrylate, and hydroxyphenyl (meth)acrylate. These may be used alone or in combination of two or more.

Examples of the nitrogen atom-containing (meth)acrylate monomer include aminoalkyl (meth)acrylates such as aminomethyl (meth)acrylate, aminoethyl (meth)acrylate, aminopropyl (meth)acrylate, and aminoisopropyl (meth)acrylate; amino group-containing (meth)acrylate monomers such as N-alkylaminoalkyl (meth)acrylate, N,N-dimethylaminoethyl (meth)acrylate, and N,N-dimethylaminopropyl (meth)acrylate; and amide group-containing (meth)acrylate monomers such as (meth)acrylamide, N,N-dimethyl (meth)acrylamide, N-butyl (meth)acrylamide, N-methylol (meth)acrylamide, N-methylolpropane (meth)acrylamide, N-methoxymethyl (meth)acrylamide, N-butoxymethyl (meth)acrylamide, diacetone (meth)acrylamide, maleic acid amide, and maleimide. These may be used alone or in combination of two or more.

Furthermore, examples of the carboxyl group-containing (meth)acrylate monomer include (meth)acrylic acid and (meth)acrylic acid dimer. These may be used alone or in combination of two or more.

Examples of the epoxy group-containing (meth)acrylate monomer include glycidyl (meth)acrylate, methyl glycidyl (meth)acrylate, 3,4-epoxycyclohexylmethyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate glycidyl ether, etc. These may be used alone or in combination of two or more.

Examples of the vinyl monomer include (meth)acrylic acid alkyl esters in which the alkyl group has 1 to 12 carbon atoms; functional monomers having functional groups such as hydroxyl groups, amide groups, and alkoxyl alkyl groups in the molecule; polyalkylene glycol di(meth)acrylates; vinyl ester monomers such as vinyl acetate, vinyl propionate, and vinyl laurate; and aromatic vinyl monomers such as styrene, chlorostyrene, chloromethylstyrene, α-methylstyrene, and other substituted styrenes. These may be used alone or in combination of two or more.

Examples of alkyl (meth)acrylate monomers in which the alkyl group has 1 to 3 carbon atoms include methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, i-propyl (meth)acrylate, etc. These may be used alone or in combination of two or more.

Examples of the other copolymerizable monomers include acid anhydride group-containing monomers such as maleic anhydride and itaconic anhydride, heterocyclic basic monomers such as vinylpyrrolidone, vinylpyridine, and vinylcarbazole, and macromonomers. These may be used alone or in combination of two or more.

In the present invention, a (meth)acrylic copolymer obtained by copolymerizing the above-mentioned various monomer components can be used so that the double-sided pressure-sensitive adhesive sheet has a specified loss tangent peak temperature and specific physical properties when made into a double-sided pressure-sensitive adhesive sheet, and the copolymerization method can be any conventional method known in the art, such as solution radical polymerization, suspension polymerization, bulk polymerization, emulsion polymerization, etc.

In particular, when the double-sided pressure-sensitive adhesive sheet of the present invention has at least three layers, the outermost and innermost layers of which are (meth)acrylic pressure-sensitive adhesive layers, it is preferable that the outermost and innermost (meth)acrylic pressure-sensitive adhesive layers be formed from a resin composition containing a (meth)acrylic copolymer (A) having a glass transition temperature of -10°C or lower, and more preferably, the (meth)acrylic copolymer contained in the resin composition consists solely of (meth)acrylic copolymer (A).

[(Meth)acrylic copolymer (A)]
The (meth)acrylic copolymer (A) having a glass transition temperature of −10° C. or less preferably contains substantially no structural units derived from a carboxy group-containing (meth)acrylate monomer, and contains, as monomer components constituting the (meth)acrylic copolymer (A), at least one polar group-containing (meth)acrylate monomer (a2) selected from the group consisting of hydroxy group-containing (meth)acrylate monomers and nitrogen atom-containing (meth)acrylate monomers, and a (meth)acrylate monomer (a1) other than the (a2), which has a glass transition temperature (Tg) of less than −30° C. when formed into a homopolymer from the monomer components.

The phrase "substantially free of structural units derived from carboxyl group-containing (meth)acrylate monomers" as used above not only refers to the complete absence of structural units, but also includes the presence of less than 0.5% by mass, preferably less than 0.1% by mass, of carboxyl group-containing (meth)acrylate monomers in the (meth)acrylic copolymer (A).

(Meth)acrylate monomer (a1) having a glass transition temperature (Tg) of less than -30°C (preferably less than -40°C, and particularly preferably less than -50°C) when formed into a homopolymer from the above monomer components includes the aforementioned alkyl (meth)acrylate monomers having an alkyl group of 4 to 18 carbon atoms, which have a glass transition temperature of less than -30°C. Specific examples include linear alkyl (meth)acrylate monomers such as n-propyl (meth)acrylate, n-butyl (meth)acrylate, n-heptyl acrylate, n-hexyl acrylate, n-octyl acrylate, nonyl acrylate, lauryl methacrylate, and stearyl methacrylate; and branched alkyl (meth)acrylate monomers such as 2-ethylhexyl acrylate, isononyl acrylate, and isodecyl acrylate. These may be used alone or in combination of two or more. Among these, branched alkyl (meth)acrylates are preferred, with 2-ethylhexyl acrylate being particularly preferred.

Examples of the polar group-containing (meth)acrylate monomer (a2) include the aforementioned hydroxyl group-containing (meth)acrylate monomers and nitrogen atom-containing (meth)acrylate monomers, of which hydroxyl group-containing (meth)acrylate monomers are preferred, with 2-hydroxyethyl acrylate being particularly preferred.

In addition, monomers other than the above-mentioned monomers (a1) and (a2) can be used as copolymerization components of the (meth)acrylic copolymer (A). The various monomers mentioned above can be used as monomers other than the above-mentioned monomers (a1) and (a2). Among these, it is preferable to use an alkyl(meth)acrylate monomer having an alkyl group containing 1 to 3 carbon atoms, and it is particularly preferable to use methyl(meth)acrylate.

The glass transition temperature of the (meth)acrylic copolymer (A) obtained by copolymerizing these is preferably -10°C or lower, more preferably -100 to -15°C, and particularly preferably -50 to -20°C, from the standpoint of level difference absorbency and lamination reliability.

The mass average molecular weight of the above (meth)acrylic copolymer (A) is preferably 50,000 to 1,500,000, more preferably 100,000 to 700,000, and particularly preferably 150,000 to 600,000.

In this specification, the mass average molecular weight is measured by the following method.
A measurement sample is prepared by dissolving 4 mg of the (meth)acrylic copolymer in 12 mL of THF, and a molecular weight distribution curve is measured under the following conditions using a gel permeation chromatography (GPC) analyzer (HLC-8320GPC, manufactured by Tosoh Corporation) to determine the mass average molecular weight (Mm).
・Guard column: TSKguardcolumnHXL
Separation column: TSKgel GMHXL (4 columns)
・Temperature: 40℃
・Injection volume: 100μL
・Polystyrene equivalent ・Solvent: THF
・Flow rate: 1.0mL/min

The hydroxyl value of the above (meth)acrylic copolymer (A) is typically 20 to 150 mgKOH/g, preferably 30 to 100 mgKOH/g, and more preferably 40 to 80 mgKOH/g.

Furthermore, when the double-sided pressure-sensitive adhesive sheet of the present invention has at least three layers, the outermost layer and the innermost layer of which are (meth)acrylic pressure-sensitive adhesive layers, it is preferable that the intermediate layer (the layer sandwiched between the outermost layer and the innermost layer) be formed from a resin composition containing a (meth)acrylic copolymer (A') having a glass transition temperature higher than -10°C, and more preferably, the (meth)acrylic copolymer contained in the resin composition consists solely of (meth)acrylic copolymer (A').

[(Meth)acrylic copolymer (A')]
The (meth)acrylic copolymer (A') having a glass transition temperature higher than -10°C preferably contains, as monomer components constituting the (meth)acrylic copolymer (A'), at least one polar group-containing (meth)acrylate monomer (a2) selected from the group consisting of hydroxyl group-containing (meth)acrylate monomers and nitrogen atom-containing (meth)acrylate monomers, and an alkyl (meth)acrylate monomer (a3) in which the alkyl group has 1 to 18 carbon atoms.

Examples of the polar group-containing (meth)acrylate monomer (a2) include the aforementioned hydroxyl group-containing (meth)acrylate monomer and nitrogen atom-containing (meth)acrylate monomer, of which nitrogen atom-containing (meth)acrylate is preferred, amide group-containing (meth)acrylate is more preferred, and (meth)acrylamide is particularly preferred.

Examples of the alkyl (meth)acrylate monomer (a3) having 1 to 18 carbon atoms in the alkyl group include the aforementioned alkyl (meth)acrylate monomer having 4 to 18 carbon atoms in the alkyl group and alkyl (meth)acrylate monomer having 1 to 3 carbon atoms in the alkyl group, of which methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, isopropyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, t-butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, lauryl (meth)acrylate, tridecyl (meth)acrylate, and isobornyl (meth)acrylate are particularly preferred. Furthermore, from the viewpoint of versatility of the monomer and making the Tg of the (meth)acrylic copolymer (A') -10°C or higher, methyl (meth)acrylate, ethyl methacrylate, t-butyl (meth)acrylate, isobutyl methacrylate, and isobornyl (meth)acrylate are more preferred.

In addition, as the copolymerization component of the (meth)acrylate copolymer (A'), a monomer other than the above-mentioned monomers (a2) and (a3) can be used. As the monomer other than the above-mentioned monomers (a2) and (a3), the various monomers described above can be used.

The glass transition temperature of the (meth)acrylic copolymer (A') obtained by copolymerizing these is preferably higher than -10°C, more preferably -5 to 20°C, and particularly preferably 0 to 15°C, from the viewpoint of reworkability.

The mass average molecular weight of the (meth)acrylic copolymer (A') is preferably 50,000 to 1,000,000, more preferably 70,000 to 700,000, and even more preferably 100,000 to 500,000.

[Crosslinking agent (B)]
The resin compositions forming each layer may contain a crosslinking agent (B) in addition to the (meth)acrylic copolymer. In particular, it is preferable to incorporate the crosslinking agent (B) into the resin composition forming the intermediate layer of the double-sided PSA sheet.

The crosslinking agent (B) is preferably one having at least a double bond crosslink. Examples include crosslinking agents having at least one crosslinkable functional group selected from (meth)acryloyl groups, epoxy groups, isocyanate groups, carboxy groups, hydroxy groups, carbodiimide groups, oxazoline groups, aziridine groups, vinyl groups, amino groups, imino groups, and amide groups. One or more crosslinking agents may be used in combination. Crosslinking agent (B) also includes those in which the crosslinking agent (B) is chemically bonded to the (meth)acrylic copolymer.

Among these, crosslinking agents containing (meth)acryloyl groups are preferred, and polyfunctional (meth)acrylates are particularly preferred in terms of cutting processability and lamination reliability. Here, "polyfunctional" refers to those having two or more crosslinkable functional groups. However, if necessary, three or more, or four or more crosslinkable functional groups may be present. Furthermore, the above crosslinkable functional groups may be protected with deprotectable protecting groups.

Examples of the polyfunctional (meth)acrylates include 1,4-butanediol di(meth)acrylate, glycerin di(meth)acrylate, neopentyl glycol di(meth)acrylate, glycerin glycidyl ether di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, 1,9-nonanediol di(meth)acrylate, tricyclodecane dimethacrylate, tricyclodecane dimethanol di(meth)acrylate, bisphenol A polyethoxy di(meth)acrylate, bisphenol A polypropoxy di(meth)acrylate, and bisphenol F poly. Ethoxy di(meth)acrylate, ethylene glycol di(meth)acrylate, neopentyl glycol di(meth)acrylate, trimethylolpropane trioxyethyl (meth)acrylate, ε-caprolactone-modified tris(2-hydroxyethyl)isocyanurate tri(meth)acrylate, pentaerythritol tri(meth)acrylate, propoxylated pentaerythritol tri(meth)acrylate, ethoxylated pentaerythritol tri(meth)acrylate, pentaerythritol tetra(meth)acrylate, propoxylated pentaerythritol tetra(meth)acrylate acrylate, ethoxylated pentaerythritol tetra(meth)acrylate, dipentaerythritol hexa(meth)acrylate, polyethylene glycol di(meth)acrylate, polypropylene glycol di(meth)acrylate, tris(acryloxyethyl)isocyanurate, pentaerythritol tetra(meth)acrylate, dipentaerythritol hexa(meth)acrylate, dipentaerythritol penta(meth)acrylate, tripentaerythritol hexa(meth)acrylate, tripentaerythritol penta(meth)acrylate, hydroxypivalic acid ne Examples of suitable UV-curable polyfunctional (meth)acrylic monomers include di(meth)acrylate of dipentyl glycol di(meth)acrylate, di(meth)acrylate of ε-caprolactone adduct of hydroxypivalic acid neopen glycol, trimethylolpropane tri(meth)acrylate, trimethylolpropane polyethoxy tri(meth)acrylate, and ditrimethylolpropane tetra(meth)acrylate, as well as polyfunctional (meth)acrylic oligomers such as polyester (meth)acrylate, epoxy (meth)acrylate, urethane (meth)acrylate, and polyether (meth)acrylate. These may be used alone or in combination of two or more. Among these, propoxylated pentaerythritol tri(meth)acrylate is preferred.

The content of the crosslinking agent (B) is typically 0.5 to 50 parts by mass, preferably 1 to 40 parts by mass, and particularly preferably 5 to 30 parts by mass, per 100 parts by mass of the (meth)acrylic copolymer. A content within the above range is preferred because it facilitates cutting processability and lamination reliability.

[Photopolymerization initiator (C)]
The resin composition preferably contains a photopolymerization initiator (C). As the photopolymerization initiator (C), currently known initiators can be used as appropriate, and among them, photopolymerization initiators that are sensitive to ultraviolet light having a wavelength of 380 nm or less are preferred from the viewpoint of ease of control of the crosslinking reaction.

Photopolymerization initiators (C) are broadly classified into two types based on the radical generation mechanism: cleavage-type photopolymerization initiators, which can generate radicals by cleaving and decomposing the single bond of the photopolymerization initiator itself, and hydrogen abstraction-type photopolymerization initiators, which form an exciplex between the photoexcited photopolymerization initiator and the hydrogen donor in the system and transfer the hydrogen from the hydrogen donor.

The cleavage-type photopolymerization initiator is preferably used because it decomposes into a different compound when it generates radicals upon irradiation with light, and once excited, it no longer functions as a reaction initiator. Therefore, it does not remain as an active species in the cured product such as an adhesive after the crosslinking reaction is completed, and there is no possibility of causing unexpected photodegradation of the cured product.
On the other hand, hydrogen abstraction photopolymerization initiators do not produce decomposition products, unlike cleavage photopolymerization initiators, during the radical generation reaction upon irradiation with active energy rays such as ultraviolet rays. Therefore, they are less likely to become volatile components after the reaction is completed, and are therefore useful in that they can reduce damage to the adherend.

Examples of the cleavage-type photopolymerization initiator include 2,2-dimethoxy-1,2-diphenylethan-1-one, 1-hydroxycyclohexyl phenyl ketone, 2-hydroxy-2-methyl-1-phenyl-propan-1-one, 1-(4-(2-hydroxyethoxy)phenyl)-2-hydroxy-2-methyl-1-propan-1-one, 2-hydroxy-1-[4-{4-(2-hydroxy-2-methyl-propionyl)benzyl}phenyl]-2-methyl-propan-1-one, oligo(2-hydroxy-2-methyl-1-(4-(1-methylvinyl)phenyl)propan-1-one, methyl phenylglyoxylate, 2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)butan-1-one, 2-methyl-1-[4-(methylthio)phenyl]-2-morpholinopropan-1-one, 2-(dimethylamino)-2-[(4-methylphenyl)methyl]-1-[4-(4-morpholinyl)phenyl]-1-butanone, bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide, 2,4,6-trimethylbenzoyldiphenylphosphine oxide, and derivatives thereof. Among these, oligo(2-hydroxy-2-methyl-1-(4-(1-methylvinyl)phenyl)propanone) is preferred.

Examples of the hydrogen abstraction photopolymerization initiator include benzophenone, 4-methylbenzophenone, 2,4,6-trimethylbenzophenone, 4-phenylbenzophenone, 3,3'-dimethyl-4-methoxybenzophenone, methyl 2-benzoylbenzoate, methyl benzoylformate, bis(2-phenyl-2-oxoacetic acid)oxybisethylene, 4-(1,3-acryloyl-1,4,7,10,13-pentaoxotridecyl)benzophenone, thioxanthone, 2-chlorothioxanthone, 3-methylthioxanthone, 2,4-dimethylthioxanthone, 2-methylanthraquinone, 2-ethylanthraquinone, 2-tert-butylanthraquinone, 2-aminoanthraquinone, and derivatives thereof. Of these, 4-methylbenzophenone and 2,4,6-trimethylbenzophenone are preferred.

The photopolymerization initiator (C) is not limited to the substances listed above. Furthermore, the photopolymerization initiator (C) may be either a cleavage-type photopolymerization initiator or a hydrogen abstraction-type photopolymerization initiator, or a combination of both.

The content of the photopolymerization initiator (C) is not particularly limited, but is usually 0.1 to 10 parts by mass, preferably 0.2 to 5 parts by mass, and particularly preferably 0.3 to 3 parts by mass, relative to 100 parts by mass of the (meth)acrylic copolymer.
By setting the content of the photopolymerization initiator (C) within the above range, it is possible to obtain an appropriate reaction sensitivity to active energy rays.

[Silane coupling agent (D)]
The resin composition preferably contains a silane coupling agent (D) to enhance adhesion to components for image display devices, particularly glass. In particular, the silane coupling agent (D) is preferably contained in a resin composition that forms a (meth)acrylic pressure-sensitive adhesive layer that contacts the double-sided pressure-sensitive adhesive sheet with the components for image display devices.

Examples of the silane coupling agent (D) include compounds having unsaturated groups such as vinyl groups, acryloxy groups, and methacryloxy groups, amino groups, epoxy groups, etc., as well as hydrolyzable functional groups such as alkoxy groups.

Examples of the silane coupling agent (D) include N-(β-aminoethyl)-γ-aminopropyltrimethoxysilane, N-(β-aminoethyl)-γ-aminopropylmethyldimethoxysilane, γ-aminopropyltriethoxysilane, γ-glycidoxypropyltrimethoxysilane, and γ-methacryloxypropyltrimethoxysilane. These may be used alone or in combination of two or more. Of these, γ-glycidoxypropyltrimethoxysilane is preferred because of its good adhesion to components for image display devices and its low discoloration, such as yellowing.

The content of the above silane coupling agent (D) is preferably 0.01 to 5 parts by mass, particularly preferably 0.2 to 3 parts by mass, per 100 parts by mass of the (meth)acrylic copolymer.

In addition, similar to the silane coupling agent (D), coupling agents such as organic titanate compounds can also be effectively used.

[Metal corrosion inhibitor (E)]
The resin composition also preferably contains a metal corrosion inhibitor (E). In particular, the metal corrosion inhibitor (E) is preferably contained in a resin composition that forms a (meth)acrylic pressure-sensitive adhesive layer that contacts the double-sided pressure-sensitive adhesive sheet with a component for an image display device.

Examples of the metal corrosion inhibitor (E) include benzotriazole compounds, benzimidazole compounds, benzothiazole compounds, and other triazole derivatives.

Preferably, the metal corrosion inhibitor is one or more selected from benzotriazole compounds, 1,2,3-triazole, and 1,2,4-triazole. Among these, triazole derivatives such as 1,2,3-triazole and 1,2,4-triazole are preferred, with 1,2,3-triazole being particularly preferred, as they offer excellent reliability as double-sided PSA sheets in addition to their metal corrosion prevention properties.

From the viewpoint of preventing bleed-out of the metal corrosion inhibitor and metal corrosion prevention effect, the content of the above-mentioned metal corrosion inhibitor (E) is preferably 0.01 to 5 parts by mass, more preferably 0.03 to 1 part by mass, and particularly preferably 0.05 to 0.5 parts by mass per 100 parts by mass of the (meth)acrylic copolymer.

[Other additives]
The resin composition may contain other additives in addition to the above components. Examples of the other additives include various additives such as light stabilizers, ultraviolet absorbers, metal deactivators, antioxidants, antistatic agents, moisture absorbers, foaming agents, antifoaming agents, inorganic particles, viscosity modifiers, tackifier resins, photosensitizers, and fluorescent agents, as well as reaction catalysts (such as tertiary amine compounds, quaternary ammonium compounds, and tin laurate compounds). These may be used alone or in combination of two or more.
In addition, other known components that are usually blended in resin compositions that form pressure-sensitive adhesives may also be appropriately contained.

The above resin composition is obtained by mixing predetermined amounts of a (meth)acrylic copolymer, and, if necessary, a crosslinking agent (B), a photopolymerization initiator (C), a silane coupling agent (D), a metal corrosion inhibitor (E), and other additives. There are no particular restrictions on the mixing method, nor on the order in which the components are mixed. A heat treatment step may also be included in the production of the resin composition. In this case, it is desirable to mix the components of the resin composition in advance and then perform the heat treatment. In the above mixing, a masterbatch made by concentrating the various mixed components may also be used.

As mentioned above, the mixing method is not particularly limited, and for example, a universal mixer, planetary mixer, Banbury mixer, kneader, gate mixer, pressure kneader, three-roll mill, two-roll mill, etc. can be used. When mixing the components of the resin composition, a solvent may be used as needed. The resin composition can also be used as a solvent-free system. Using a solvent-free system has the advantage of eliminating residual solvent, improving heat resistance and light resistance.

[Method for manufacturing double-sided pressure-sensitive adhesive sheet]
A method for producing a double-sided pressure-sensitive adhesive sheet of the present invention is described below, but is not limited to this method. The double-sided pressure-sensitive adhesive sheet of the present invention preferably has at least two (meth)acrylic pressure-sensitive adhesive layers, and more preferably has at least three layers, with the outermost and innermost layers being (meth)acrylic pressure-sensitive adhesive layers. The double-sided pressure-sensitive adhesive sheet of the present invention is preferably produced as a double-sided pressure-sensitive adhesive sheet with a release film, typically by the following steps, which has a configuration in which a double-sided pressure-sensitive adhesive sheet and a release film are laminated together. Note that the pre-curing step described below may be omitted.

First, a (meth)acrylic pressure-sensitive adhesive layer (or intermediate layer) with a release film is prepared. The (meth)acrylic pressure-sensitive adhesive layer (or intermediate layer) with a release film can be obtained by applying a hot-melt resin composition to a release film, sandwiching the resulting film between other release films, and then heating the resulting film. These (meth)acrylic pressure-sensitive adhesive layers (or intermediate layers) with release films are prepared according to the number of layers required for the double-sided pressure-sensitive adhesive sheet.

Examples of the release film include films made of polyester resin, polyolefin resin, polycarbonate resin, polystyrene resin, acrylic resin, triacetyl cellulose resin, fluororesin, etc. Furthermore, these films coated with silicone resin for release treatment, and release paper, etc., can also be used. Of these, polyester resin and polyolefin resin are preferred, and release-treated polyester resin and polyolefin resin are particularly preferred. Furthermore, it is also preferable to use release films with different peel strengths or release films with different thicknesses on both sides of the (meth)acrylic pressure-sensitive adhesive layer.

Next, the release film is peeled off from the obtained (meth)acrylic pressure-sensitive adhesive layer (or intermediate layer) with the release film, and a (meth)acrylic pressure-sensitive adhesive layer (or intermediate layer) is laminated, thereby obtaining the double-sided pressure-sensitive adhesive sheet of the present invention laminated with a release film.

Furthermore, in the case of a double-sided PSA sheet having at least three layers, in which the outermost and innermost layers are (meth)acrylic PSA layers, for example, a double-sided PSA sheet having three layers: outermost layer (adhesive layer)/intermediate layer/innermost layer (adhesive layer), the PSA sheet can be produced by a method in which the above-mentioned resin composition is applied to a release film to form the outermost layer, a resin composition for forming an intermediate layer is applied to the outermost layer to form an intermediate layer, and then an outermost layer is formed on the intermediate layer; a method in which the resin composition is applied to a release film to form the outermost layer and intermediate layer in the same manner as above, and then the coated surfaces are bonded together; or a method in which the outermost layer and intermediate layer are simultaneously formed by multi-layer coating or co-extrusion molding of the resin composition.

In addition to the above method, a multi-layer double-sided pressure-sensitive adhesive sheet can also be produced by, for example, coating a resin composition onto a release film to form a (meth)acrylic pressure-sensitive adhesive layer, and then coating another resin composition onto the formed (meth)acrylic pressure-sensitive adhesive layer to form a resin layer, and repeating this process.

The obtained double-sided PSA sheet is preferably pre-cured by crosslinking with active energy rays so that it retains latent active energy ray reactivity, in other words, so that it retains active energy ray reactivity. When pre-curing, each layer can be crosslinked with active energy rays by irradiating them through the release film. In this case, the degree of active energy ray crosslinking can be adjusted by controlling the amount of active energy ray irradiation. However, as mentioned above, it is also possible to adjust the degree of active energy ray crosslinking by irradiating ultraviolet rays through the release film, thereby partially blocking the active energy rays.

The double-sided pressure-sensitive adhesive sheet obtained in this manner is an optically transparent pressure-sensitive adhesive sheet. Here, "optically transparent" means that the total light transmittance is 80% or more, preferably 85% or more, and more preferably 90% or more.

The double-sided pressure-sensitive adhesive sheet of the present invention is typically distributed with the (meth)acrylic pressure-sensitive adhesive layers on both sides sandwiched between release films. When using the double-sided pressure-sensitive adhesive sheet, the release films are peeled off from the (meth)acrylic pressure-sensitive adhesive layer, and the (meth)acrylic pressure-sensitive adhesive layer is then attached to an image display component.

(Explanation of terms, etc.)
In general, a "sheet" is defined in JIS as a thin, flat product whose thickness is small relative to its length and width, and a "film" is generally a thin, flat product whose thickness is extremely small compared to its length and width and whose maximum thickness is arbitrarily limited, and is usually supplied in the form of a roll (Japanese Industrial Standard JIS K6900). However, the boundary between a sheet and a film is unclear, and there is no need to distinguish between the two in the present invention. Therefore, in the present invention, when the term "film" is used, it is intended to include "sheet," and when the term "sheet" is used, it is intended to include "film."
Furthermore, when the term "panel" is used, such as an image display panel or a protective panel, it encompasses a plate, a sheet, and a film.

In this specification, when it is written "x to y" (x and y are arbitrary numbers), unless otherwise specified, it means "x or more and y or less", and also includes the meanings "preferably larger than x" and "preferably smaller than y".
Furthermore, when it is stated that the amount is "x or more" (x is any number), unless otherwise specified, it includes the meaning of "x or more" as well as "preferably greater than x", and when it is stated that the amount is "y or less" (y is any number), it includes the meaning of "y or less" as well as "preferably less than y" unless otherwise specified.

The present invention will be explained in more detail below with reference to examples and comparative examples. However, the present invention is not limited to these examples.

In the examples, the hydroxyl value, mass average molecular weight, and glass transition temperature of the (meth)acrylic copolymer are measured by the following methods.

<Hydroxyl value>
The hydroxyl value of the (meth)acrylic copolymer was measured by neutralization titration.
2 g of sample was placed in an Erlenmeyer flask, and 10 mL of a 1:13 acetic anhydride:pyridine mixed solution was added using a volumetric pipette. Then, 10 mL of toluene was added. An air cooling tube was attached to the top of the Erlenmeyer flask, and the mixture was heated at 95°C for 90 minutes. After heating, 10 mL of toluene and 10 mL of pure water were added, and the mixture was allowed to cool with stirring until it reached room temperature (23°C). A few drops of phenolphthalein solution were then added, and titration was performed with 0.1 mol/L potassium hydroxide solution. A blank test was also performed in the same manner as above, without placing a sample in the Erlenmeyer flask. The hydroxyl value was calculated using the following formula (1):
[Calculation formula]
Hydroxyl value = 5.611 × (amount of potassium hydroxide solution used in blank test (mL) − amount of potassium hydroxide solution used in titration (mL)) × f / amount of sample collected (g) + acid value (1)
f: Factor of 0.1 mol/L potassium hydroxide solution

[Acid value]
The acid value was measured by the following method. Y g of a (meth)acrylic copolymer was placed in a beaker and dissolved in a mixed solvent of toluene:methanol = 7:3. After dissolution, an appropriate amount of phenolphthalein was added, and while stirring with a stirrer, titration was performed with a 0.1 mol/L potassium hydroxide solution. The amount of potassium hydroxide solution at the time when the solution turned light pink was read as X mL, which was the endpoint, and the acid value was calculated based on the following formula (2).
[Calculation formula]
Acid value (mgKOH/g)=X×(f×M×56.1)/Y...(2)
f: Factor of potassium hydroxide solution M: Molar concentration (mol/L)
X: Amount of potassium hydroxide solution (mL)
Y: sample amount (g)
When the acid value was low, a 0.01 mol/L KOH solution was used to improve accuracy.

<Mass average molecular weight>
The mass average molecular weight of the (meth)acrylic copolymer was measured using a gel permeation chromatography (GPC) analyzer (HLC-8320GPC, manufactured by Tosoh Corporation). Specifically, 4 mg of the (meth)acrylic copolymer was dissolved in 12 mL of THF to prepare a measurement sample, and the molecular weight distribution curve was measured under the following conditions to determine the mass average molecular weight (Mw).
・Guard column: TSKguardcolumnHXL
Separation column: TSKgel GMHXL (4 columns)
・Temperature: 40℃
・Injection volume: 100μL
・Polystyrene equivalent ・Solvent: THF
・Flow rate: 1.0mL/min

<Glass transition temperature (Tg)>
The glass transition temperature (Tg) of the (meth)acrylic copolymer was measured using a rheometer ("DiscoveryHR2" manufactured by TA Instruments). Specifically, for a (meth)acrylic copolymer having a thickness of 0.6 to 0.8 mm, a dynamic viscoelasticity spectrum was measured in a temperature range of -120 to 200°C under the following conditions: jig: Φ8 mm parallel plates, strain: 0.1%, frequency: 1 Hz, temperature: -120 to 200°C, and heating rate: 5°C/min. From the obtained data, the temperature at which the loss tangent (Tan δ) reached its maximum value was read, thereby determining the glass transition temperature (Tg).

Example 1
Resin composition 1 was prepared by uniformly melt-kneading 1 kg of a copolymer (A'-1) [mass average molecular weight 290,000, Tg 4°C] consisting of 2-ethylhexyl acrylate/methyl acrylate/methyl methacrylate/isobornyl methacrylate/acrylamide as a (meth)acrylic copolymer, 100 g of propoxylated pentaerythritol triacrylate (B-1) as a crosslinking agent (B), and 5 g of a mixture (C-1) of 2,4,6-trimethylbenzophenone and 4-methylbenzophenone as a photopolymerization initiator (C).

1 kg of a copolymer (A-1) consisting of 2-ethylhexyl acrylate/2-hydroxyethyl acrylate/methyl acrylate [mass average molecular weight 440,000, Tg -25°C, hydroxyl value 67 mgKOH/g] as a (meth)acrylic copolymer, 10 g of (C-1) as a photopolymerization initiator (C), and 1 g of 3-glycidoxypropyltrimethoxysilane (D-1) as a silane coupling agent (D) were uniformly melt-kneaded to produce resin composition 2.

The resin composition 1 was sandwiched between two release-treated polyethylene terephthalate films (Diafoil MRF (thickness 75 μm) manufactured by Mitsubishi Chemical Corporation/Diafoil MRT (thickness 38 μm) manufactured by Mitsubishi Chemical Corporation), i.e., two release films, and shaped into a sheet at a temperature of 80° C. to a thickness of 50 μm, to produce an intermediate layer sheet (1).
The above resin composition 2 was sandwiched between two release-treated polyethylene terephthalate films (Diafoil MRF (thickness 75 μm) manufactured by Mitsubishi Chemical Corporation / Diafoil MRT (thickness 38 μm) manufactured by Mitsubishi Chemical Corporation), i.e., two release films, and shaped into a sheet at a temperature of 80°C to a thickness of 25 μm, to produce two adhesive sheets (2) for the front and back layers.

The intermediate layer sheet (1), from which the release films on both sides had been peeled off, was attached to the adhesive surface of the front and back layer adhesive sheet (2), from which the release film on one side had been peeled off, to produce a laminate having a layer structure of (2)/(1)/(2).
The adhesive sheet (2) for the front and back layers was pre-cured by irradiating it with light from a high-pressure mercury lamp through the release film remaining on the surface thereof so that the integrated light intensity at a wavelength of 365 nm was 2000 mJ/ ^cm2 , thereby producing the double-sided adhesive sheet (pre-cured product) of Example 1.
In the double-sided pressure-sensitive adhesive sheet of Example 1, the shear storage modulus (G') of the front and back layers at a temperature of 25°C was lower than the shear storage modulus (G') of the intermediate layer, and both layers had room to be photocured by light irradiation.

Example 2
Resin composition 3 was prepared by uniformly melt-kneading 1 kg of (A-1) as a (meth)acrylic copolymer, 10 g of (C-1) as a photopolymerization initiator (C), and 1 g of 1,2,3-triazole (E-1) as a metal corrosion inhibitor (E).
The metal corrosion inhibitor (E-1) had an absorption coefficient of 0.3 mL/(g·cm) at 365 nm and a water solubility of more than 1000 g/L at 25° C.

The resin composition 1 prepared in Example 1 was sandwiched between two release-treated polyethylene terephthalate films (Diafoil MRF (thickness 75 μm) manufactured by Mitsubishi Chemical Corporation/Diafoil MRT (thickness 38 μm) manufactured by Mitsubishi Chemical Corporation), i.e., two release films, and shaped into a sheet at a temperature of 80° C. so as to have a thickness of 67 μm, to prepare an intermediate layer sheet (1).
The resin composition 3 was sandwiched between two release-treated polyethylene terephthalate films (Diafoil MRF (thickness 75 μm) manufactured by Mitsubishi Chemical Corporation/Diafoil MRT (thickness 38 μm) manufactured by Mitsubishi Chemical Corporation), i.e., two release films, and shaped into a sheet at a temperature of 80°C to a thickness of 17 μm, to produce two adhesive sheets (3) for the front and back layers.

The intermediate layer sheet (1), from which the release films on both sides had been peeled off, was attached to the adhesive surface of the front and back layer adhesive sheet (3), from which the release film on one side had been peeled off, to produce a laminate having a layer structure of (3)/(1)/(3).
The adhesive sheet (3) for the front and back layers was pre-cured by irradiating it with light from a high-pressure mercury lamp through the release film remaining on the surface thereof so that the integrated light intensity at a wavelength of 365 nm was 1000 mJ/ ^cm2 , thereby producing a double-sided adhesive sheet (pre-cured product) of Example 2.
In addition, in the double-sided pressure-sensitive adhesive sheet of Example 2, the shear storage modulus (G') of the front and back layers at a temperature of 25°C was lower than the shear storage modulus (G') of the intermediate layer, and both layers had room to be photocured by light irradiation.

Example 3
Resin composition 4 was prepared by uniformly melt-kneading 1 kg of (A'-1) as a (meth)acrylic copolymer, 100 g of (B-1) as a crosslinking agent (B), 4 g of (C-1) as a photopolymerization initiator (C), and 4 g of oligo(2-hydroxy-2-methyl-1-(4-(1-methylvinyl)phenyl)propanone) (C-2).

The resin composition 4 was sandwiched between two release-treated polyethylene terephthalate films (Diafoil MRF (thickness 75 μm) manufactured by Mitsubishi Chemical Corporation/Diafoil MRT (thickness 38 μm) manufactured by Mitsubishi Chemical Corporation), i.e., two release films, and shaped into a sheet at a temperature of 80° C. to a thickness of 75 μm, to produce an intermediate layer sheet (4).
The resin composition 2 prepared in Example 1 was sandwiched between two release-treated polyethylene terephthalate films (Diafoil MRF (thickness 75 μm) manufactured by Mitsubishi Chemical Corporation/Diafoil MRT (thickness 38 μm) manufactured by Mitsubishi Chemical Corporation), i.e., two release films, and shaped into a sheet at a temperature of 80°C to a thickness of 13 μm, to prepare two pressure-sensitive adhesive sheets (2) for the front and back layers.

The intermediate layer sheet (4) from which the release films on both sides had been peeled off was attached to the adhesive surface of the front and back layer adhesive sheet (2) from which the release film on one side had been peeled off, to produce a laminate having a layer structure of (2)/(4)/(2).
The adhesive sheet (2) for the front and back layers was pre-cured by irradiating it with light from a high-pressure mercury lamp through the release film remaining on the surface thereof so that the integrated light intensity at a wavelength of 365 nm was 1000 mJ/ ^cm2 , thereby producing the double-sided adhesive sheet (pre-cured product) of Example 3.
In addition, in the double-sided pressure-sensitive adhesive sheet of Example 3, the shear storage modulus (G') of the front and back layers at a temperature of 25°C was lower than the shear storage modulus (G') of the intermediate layer, and there was still room for both layers to be photocured by light irradiation.

<Comparative Example 1>
Resin composition 5 was prepared by uniformly melt-kneading 1 kg of (A-1) as a (meth)acrylic copolymer, 200 g of (B-1) as a crosslinking agent (B), and 8 g of (C-1) as a photopolymerization initiator (C).

1 kg of (A-1) as a (meth)acrylic copolymer and 10 g of (C-1) as a photopolymerization initiator (C) were uniformly melted and kneaded to prepare resin composition 6.

The resin composition 5 was sandwiched between two release-treated polyethylene terephthalate films (Diafoil MRF (thickness 75 μm) manufactured by Mitsubishi Chemical Corporation/Diafoil MRT (thickness 38 μm) manufactured by Mitsubishi Chemical Corporation), i.e., two release films, and shaped into a sheet at a temperature of 80° C. so as to have a thickness of 67 μm, to produce an intermediate layer sheet (5).
The above resin composition 6 was sandwiched between two polyethylene terephthalate films (Diafoil MRF (thickness 75 μm) manufactured by Mitsubishi Chemical Corporation/Diafoil MRT (thickness 38 μm) manufactured by Mitsubishi Chemical Corporation) with release films, i.e., two release films, and formed into a sheet at a temperature of 80°C to a thickness of 17 μm, to produce two pressure-sensitive adhesive sheets (6) for the front and back layers.

The intermediate layer sheet (5), from which the release films on both sides had been peeled off, was attached to the adhesive surface of the surface layer adhesive sheet (6), from which the release film on one side had been peeled off, to produce a laminate having a layer structure of (6)/(5)/(6).
The adhesive sheet for the front and back layers (6) was pre-cured by irradiating it with light from a high-pressure mercury lamp through the release film remaining on the surface thereof so that the integrated light intensity at a wavelength of 365 nm was 2000 mJ/ ^cm2 , thereby producing a double-sided adhesive sheet (pre-cured product) of Comparative Example 1.
In addition, in the double-sided pressure-sensitive adhesive sheet of Comparative Example 1, the shear storage modulus (G') of the front and back layers at a temperature of 25°C was lower than the shear storage modulus (G') of the intermediate layer, and both layers had room to be photocured by light irradiation.

<Comparative Example 2>
Resin composition 7 was prepared by uniformly melt-kneading 1 kg of (A'-1) as a (meth)acrylic copolymer, 100 g of (B-1) as a crosslinking agent (B), 5 g of (C-1) as a photopolymerization initiator (C), and 1 g of (D-1) as an additive (D).

The above resin composition 7 was sandwiched between two release-treated polyethylene terephthalate films (Diafoil MRF (thickness 75 μm) manufactured by Mitsubishi Chemical Corporation/Diafoil MRT (thickness 38 μm) manufactured by Mitsubishi Chemical Corporation), i.e., two release films, and shaped into a sheet at a temperature of 80°C so as to have a thickness of 100 μm. Light was then irradiated from one of the release films using a high-pressure mercury lamp so that the integrated light intensity at a wavelength of 365 nm was 1000 mJ/ ^cm2 , thereby pre-curing the double-sided PSA sheet (pre-cured product) of Comparative Example 2.
It should be noted that the (meth)acrylic pressure-sensitive adhesive layer in the double-sided pressure-sensitive adhesive sheet of Comparative Example 2 still had room to be photocured by light irradiation.

<Comparative Example 3>
Resin composition 8 was prepared by uniformly melt-kneading 1 kg of copolymer 1 (mass average molecular weight 540,000, Tg 1°C, hydroxyl value 62 mgKOH/g) consisting of methyl acrylate/ethyl acrylate/2-ethylhexyl acrylate/2-hydroxyethyl acrylate as a (meth)acrylic copolymer, 100 g of polypropylene glycol diacrylate (B-2, molecular weight 536) as a crosslinking agent (B), 10 g of (C-1) as a photopolymerization initiator (C), and 1 g of (D-1) as an additive (D).

The above resin composition 8 was sandwiched between two release-treated polyethylene terephthalate films (Diafoil MRF (thickness 75 μm) manufactured by Mitsubishi Chemical Corporation/Diafoil MRT (thickness 38 μm) manufactured by Mitsubishi Chemical Corporation), i.e., two release films, and shaped into a sheet at a temperature of 80°C so as to have a thickness of 100 μm. Light was then irradiated from one of the release films using a high-pressure mercury lamp so that the integrated light intensity at a wavelength of 365 nm was 1000 mJ/ ^cm2 , thereby pre-curing the double-sided PSA sheet (pre-cured product) of Comparative Example 3.
The (meth)acrylic pressure-sensitive adhesive layer in the double-sided pressure-sensitive adhesive sheet of Comparative Example 3 still had room to be photocured by light irradiation.

<Comparative Example 4>
1 kg of (A-1) as a (meth)acrylic copolymer, 180 g of (B-3) as a crosslinking agent (B), and 10 g of (C-1) as a photopolymerization initiator (C) were uniformly melt-kneaded to prepare resin composition 9.

1 kg of (A-1) as a (meth)acrylic copolymer and 12 g of (C-1) as a photopolymerization initiator (C) were uniformly melted and kneaded to prepare resin composition 10.

The resin composition 9 was sandwiched between two release-treated polyethylene terephthalate films (Diafoil MRF (thickness 75 μm) manufactured by Mitsubishi Chemical Corporation/Diafoil MRT (thickness 38 μm) manufactured by Mitsubishi Chemical Corporation), i.e., two release films, and shaped into a sheet at a temperature of 80° C. to a thickness of 67 μm, to produce an intermediate layer sheet (9).
The resin composition 10 was sandwiched between two polyethylene terephthalate films (Diafoil MRF (thickness 75 μm) manufactured by Mitsubishi Chemical Corporation/Diafoil MRT (thickness 38 μm) manufactured by Mitsubishi Chemical Corporation) with release films, i.e., two release films, and formed into a sheet at a temperature of 80°C to a thickness of 17 μm, to produce two adhesive sheets (10) for the front and back layers.

The intermediate layer sheet (9) from which the release films on both sides had been peeled off was attached to the adhesive surface of the front and back layer adhesive sheet (10) from which the release film on one side had been peeled off, to produce a laminate having a layer structure of (10)/(9)/(10).
The adhesive sheet for the front and back layers (10) was pre-cured by irradiating it with light from a high-pressure mercury lamp through the release film remaining on the surface thereof so that the integrated light intensity at a wavelength of 365 nm was 2000 mJ/ ^cm2 , thereby producing a double-sided adhesive sheet (pre-cured product) of Comparative Example 4.
In addition, in the double-sided adhesive sheet of Comparative Example 4, the shear storage modulus (G') of the front and back layers at a temperature of 25°C was lower than the shear storage modulus (G') of the intermediate layer, and there was still room for both layers to be photocured by light irradiation.

[Physical property evaluation]
The physical properties of the double-sided PSA sheets of Examples 1 to 3 and Comparative Examples 1 to 4 obtained above were measured as follows. Furthermore, for the physical properties of each layer alone, a sheet for each layer was prepared and then pre-cured under the same conditions, and the sheet was used for measurement. The physical properties of the double-sided PSA sheets of Examples 1 to 3 and Comparative Examples 1 to 4, and each layer alone, are shown in Table 1 below.

<Peak Temperature (T1) of Shear Storage Modulus (G') and Loss Tangent (Tan δ)>
For each of the double-sided PSA sheets (pre-cured products) of Examples 1 to 3 and Comparative Examples 1 to 4, i.e., the double-sided PSA sheets before curing (hereinafter collectively referred to as "pre-cured double-sided PSA sheets"), the two release films were peeled off, the PSA sheets were laminated to a thickness of 0.6 to 0.8 mm, and then punched out into circles with a diameter of 8 mm to prepare measurement samples.
Using a rheometer ("DiscoveryHR2" manufactured by TA Instruments), a dynamic viscoelasticity spectrum was measured in shear mode in a temperature range of -120 to 200°C under the following conditions: adhesive jig: Φ8 mm parallel plates, strain: 0.1%, frequency: 1 Hz, temperature: -120 to 200°C, heating rate: 5°C/min, and from the obtained data, the shear storage modulus (G') of the pre-cured double-sided PSA sheet at a temperature of 25°C was determined.
Further, the temperature at which the loss tangent (Tan δ) of the obtained dynamic viscoelastic spectrum data reached its maximum value, that is, the peak temperature (T1), was read.

Next, the uncured double-sided pressure-sensitive adhesive sheet was photocured by irradiating it with light from a high-pressure mercury lamp so that the cumulative light intensity at a wavelength of 365 nm was 3000 mJ/cm ² , thereby preparing a cured double-sided pressure-sensitive adhesive sheet.
For each of the cured double-sided PSA sheets (hereinafter collectively referred to as "cured double-sided PSA sheets"), the two release films were peeled off in the same manner as above, and the PSA sheets were laminated to a thickness of 0.6 to 0.8 mm, and then punched out into circles with a diameter of 8 mm to prepare measurement samples. The dynamic viscoelasticity spectrum of this measurement sample in shear mode was measured under the same measurement conditions as for the pre-cure double-sided PSA sheets, and the storage modulus (G') of the cured double-sided PSA sheet at a temperature of 25°C was determined from the obtained data.
Further, the temperature at which the loss tangent (Tan δ) of the obtained dynamic viscoelastic spectrum data reached its maximum value, that is, the peak temperature (T1), was read.

<Tensile storage modulus (E') and loss tangent (Tan δ) peak temperature (T2)>
For each of the double-sided PSA sheets (pre-cured products) of Examples 1 to 3 and Comparative Examples 1 to 4, i.e., the double-sided PSA sheets before curing, the pre-cured double-sided PSA sheets were photocured by irradiating them with light from a high-pressure mercury lamp so that the integrated light intensity at a wavelength of 365 nm was ^3,000 mJ/cm2. Samples were then cut to dimensions of 4 mm width x 15 mm length, and dynamic viscoelasticity spectra were measured in tension mode using a dynamic viscoelasticity measuring device (IT Measurement & Control Co., Ltd., itkDVA-200) at a tension mode oscillation frequency of 1 Hz, a measurement temperature of -120 to 80°C, and a heating rate of 3°C/min. The tensile storage modulus (E') of the cured double-sided PSA sheets at a temperature of 25°C was determined from the obtained data.
Further, the temperature at which the loss tangent (Tan δ) of the obtained dynamic viscoelastic spectrum data reached its maximum value, that is, the peak temperature (T2), was read.

The double-sided PSA sheets of Examples 1 to 3 and Comparative Examples 1 to 4 obtained above were evaluated as follows. The evaluation results are shown in Table 1 below.

<180° peel adhesive strength>
For each of the double-sided PSA sheets (pre-cured products) of Examples 1 to 3 and Comparative Examples 1 to 4, one release film was peeled off, and a 100 μm PET film (manufactured by Mitsubishi Chemical Corporation, "Diafoil T100") was laminated as a backing film. This was cut to a length of 150 mm and a width of 20 mm, and the remaining release film was peeled off, and the exposed adhesive surface was roll-pressured onto a soda-lime glass plate to form a laminated product. The laminated product was autoclaved (60°C, gauge pressure 0.2 MPa, 20 minutes) to finish bonding, and the PET film surface was irradiated with light using a high-pressure mercury lamp so that the integrated light intensity at 365 nm was 3000 mJ/ ^cm2. After aging for 12 hours at a temperature of 23°C and 50% RH, a sample for measuring 180° peel adhesive strength was prepared.
The peel strength (N/20 mm) of each sample was measured at a peel angle of 180° and a peel rate of 300 mm/min in an environment of 23° C. and 50% RH. Samples that peeled easily at a peel strength of less than 5 N/20 mm were rated as “× (poor),” and samples that peeled at 5 N/20 mm or more were rated as “◯ (good).”

<Reworkability>
The double-sided PSA sheets (pre-cured products) of Examples 1 to 3 and Comparative Examples 1 to 4 were cut to dimensions of 52 mm x 80 mm using a Thomson punch with the release film still laminated. The release film on one side was peeled off, and the exposed adhesive surface was attached to a soda-lime glass plate (54 mm x 82 mm, 0.6 mm thick) using a hand roll.
Next, the release film on the adhesive side of the adhesive polarizing plate ("VLC2" manufactured by Sanritz Co., Ltd.) was peeled off, and a polyethylene terephthalate film ("Diafoil T100" manufactured by Mitsubishi Chemical Corporation, 54 mm × 82 mm, 38 μm thick) was attached to the exposed adhesive surface using a hand roll. Furthermore, the other release film of the double-sided adhesive sheet attached to the soda lime glass plate was peeled off, and the protective film on the polarizing plate side was attached to the peeled polarizing plate using a hand roll.
Next, the sheet was autoclaved (60°C, 0.2 MPa gauge pressure, 20 minutes) for finish bonding, and then photocured by irradiating the sheet with light from a high-pressure mercury lamp from the soda-lime glass plate side so that the integrated light intensity at 365 nm was 3000 mJ/ ^cm2 (post-cured product). After that, the sheet was cured at room temperature (23°C) for 12 hours to prepare a sample for evaluating reworkability (soda-lime glass plate/double-sided PSA sheet/adhesive-attached polarizing plate/PET film).
These were placed on a SUS cooling plate (surface temperature -120°C) so that the soda-lime glass plate was in contact with the cooling plate (with the soda-lime glass plate facing downwards), and the time until the soda-lime glass plate and the polarizing plate separated was measured. Five samples for the reworkability evaluation were prepared for each double-sided PSA sheet, and measurements were taken five times for each sheet. The average time until separation was evaluated as follows.
Those that could be separated within 30 seconds were rated as "◎ (very good)", those that could not be peeled off in 30 seconds but could be separated within 120 seconds were rated as "◯ (good)", and those that could not be separated after 120 seconds or more and were not suitable for practical use were rated as "× (poor)".

<Bump absorption>
Each of the double-sided PSA sheets (pre-cured products) of Examples 1 to 3 and Comparative Examples 1 to 4 was cut into a size of 52 mm x 80 mm using a Thomson punch with the release film still laminated thereon.
The release film on one side was peeled off, and the exposed adhesive surface was pressed and bonded using a vacuum press (temperature 25°C, press pressure 0.1 MPa) to the printed surface of a soda lime glass plate (82 mm × 54 mm × thickness 0.5 mm) that had been printed with a thickness of 15 μm or 25 μm around a 5 mm periphery, so that the four sides of the double-sided adhesive sheet overlapped the printing steps.
The remaining release film was then peeled off, and a soda lime glass plate (82 mm x 54 mm x 0.5 mm thick) without printed steps was press-laminated, followed by autoclaving (60°C, gauge pressure 0.2 MPa, 20 minutes) for finish lamination, to produce a laminate (soda lime glass plate with printed steps/double-sided PSA sheet/soda lime glass plate).

The produced laminate was visually observed and evaluated as follows.
Those in which the double-sided pressure-sensitive adhesive sheet did not conform near the print step, leaving air bubbles, and were therefore unsuitable for print step lamination were rated as "x (usual)", those in which there were no air bubbles at the 15 μm print step and the sheet was able to be laminated with a good appearance but air bubbles remained at the 25 μm print step were rated as "○ (good)", and those in which there were no air bubbles at both the 15 μm and 25 μm print step and the sheet was able to be laminated with a good appearance were rated as "◎ (very good)".

<Lamination reliability>
The double-sided PSA sheets (pre-cured products) of Examples 1 to 3 and Comparative Examples 1 to 4 were cut to dimensions of 52 mm x 80 mm using a Thomson punch with the release film still laminated. The release film on one side was peeled off, and the exposed adhesive surface was bonded to soda-lime glass plate 1 (54 mm x 82 mm, 0.6 mm thick) using hand rolling. Next, the release film on the adhesive side of a pressure-sensitive adhesive polarizing plate (manufactured by Sanritz, "VLC2") was peeled off, and the exposed adhesive surface was bonded to soda-lime glass plate 2 (54 mm x 82 mm, 0.6 mm thick) using hand rolling. Furthermore, the other release film of each double-sided PSA sheet bonded to soda-lime glass plate 1 was peeled off, and the sheet was bonded to a polarizing plate from which the protective film on the polarizing plate side had been peeled off using hand rolling.
Next, the sheet was autoclaved (60°C, 0.2 MPa gauge pressure, 20 minutes) for finish bonding, and then photocured by irradiating the sheet with light from a high-pressure mercury lamp from the soda-lime glass plate 1 side so that the integrated light intensity of 365 nm was 3000 mJ/ ^cm2 (post-cured product). This was then cured at room temperature (23°C) for 12 hours to produce a sample for evaluating heat foaming resistance (soda-lime glass plate 1/double-sided PSA sheet/adhesive-attached polarizing plate/soda-lime glass plate 2).

These were left to stand in an environment at 85° C. for 300 hours, and the presence or absence of foaming was visually observed.
Those in which bubbles were generated at the interface between the adhesive polarizing plate and the double-sided adhesive sheet were rated as "x (poor)", and those in which no bubbles were generated and the appearance was good were rated as "◯ (good)".

<ITO corrosion resistance>
As shown in FIG. 1A , a substrate for evaluating ITO corrosion resistance was prepared by depositing a 200 Å ITO film on a 60 mm × 45 mm, 0.7 mm thick glass plate under the patterning conditions of a line width of 70 μm/line spacing of 30 μm (patterning accuracy of ±8 μm), a line length of 46 cm, an electrode size of 5 mm × 5 mm, and a distance between electrodes of 50 mm.
For each of the double-sided PSA sheets (pre-cured products) of Examples 1 to 3 and Comparative Examples 1 to 4, the release film on one side was peeled off, and the sheet was roll-bonded to a PET film (manufactured by Mitsubishi Chemical Corporation, "Diafoil T100", thickness 50 μm), and then cut to a width of 52 mm.
Thereafter, the release film on the other side was peeled off, and the adhesive sheet was attached to the substrate with a roll so that it was positioned between the electrodes, and then autoclaved (temperature 60°C, gauge pressure 0.2 MPa, 20 minutes) to finish the attachment.
The film was then photocured by irradiating it with light from a high-pressure mercury lamp from the PET film side at a wavelength of 365 nm to an integrated light intensity of 2000 mJ/ ^cm² (cured product). This was then left to cure at room temperature for 12 hours to prepare a sample for evaluating ITO corrosion resistance (see (C) in Figure 1).

This sample was subjected to an environmental test in a humid and hot environment at 65°C and 90% RH for 500 hours, and the increase in resistance between the electrodes was confirmed (see Figure 1 (B)). In the environmental test, resistance increases of more than 4% were rated as "× (poor)," resistance increases of more than 3% but kept to 4% or less were rated as "○ (good)," and resistance increases of 3% or less were rated as "◎ (very good)."

<Cu corrosion resistance>
As shown in FIG. 1A , a substrate for evaluating Cu corrosion resistance was prepared by depositing ITO of 1300 Å and copper (Cu) of 3000 Å in this order on a glass substrate measuring 60 mm × 45 mm and 0.7 mm in thickness under patterning conditions of line width 70 μm/line spacing 30 μm (patterning accuracy ±8 μm), line length 46 cm, electrode size 5 mm × 5 mm, and distance between electrodes 50 mm.
For each of the double-sided PSA sheets (pre-cured products) of Examples 1 to 3 and Comparative Examples 1 to 4, the release film on one side was peeled off, and the sheet was roll-bonded to a PET film (manufactured by Mitsubishi Chemical Corporation, "Diafoil T100", thickness 50 μm), and then cut to a width of 52 mm.
Thereafter, the release film on the other side was peeled off, and the adhesive sheet was attached to the substrate with a roll so that it was positioned between the electrodes, and then autoclaved (temperature 60°C, gauge pressure 0.2 MPa, 20 minutes) to finish the attachment.
Thereafter, the composition was photocured by irradiating it with light from a high-pressure mercury lamp from the PET film side so that the integrated light intensity at a wavelength of 365 nm was 3000 mJ/cm ² (fully cured product).
Thereafter, the specimen was cured at room temperature (23° C.) for 12 hours to prepare a specimen for evaluating Cu corrosion resistance (see FIG. 1(D)).

This sample was subjected to an environmental test in a humid and hot environment at 65°C and 90% RH for 500 hours, and the increase in resistance between the electrodes was confirmed (see Figure 1 (B)). In the environmental test, resistance increases of more than 3% were rated as "× (poor)," resistance increases of more than 1% but kept to 3% or less were rated as "○ (good)," and resistance increases of 1% or less were rated as "◎ (very good)."

The double-sided PSA sheets of Examples 1 to 3 had a ratio of tensile storage modulus (E') to shear storage modulus (G') of 5.0 or higher, a peak temperature (T1) of the loss tangent (Tan δ) obtained by dynamic viscoelasticity measurement in shear mode at a frequency of 1 Hz of -10°C or lower, a peak temperature (T2) of the loss tangent (Tan δ) obtained by dynamic viscoelasticity measurement in tensile mode at a frequency of 1 Hz of -10°C or higher, and an adhesive strength of 5 N/20 mm or higher. Therefore, they possessed excellent reworkability while also combining step absorption, reliability, and metal corrosion resistance. Among these, the transparent double-sided PSA sheet laminates of Examples 2 and 3 were particularly excellent in reworkability and metal corrosion resistance, due to the selection of the composition of the surface layer and intermediate layer and the layer thickness ratio, etc.

In contrast, the double-sided adhesive sheet of Comparative Example 1 had a ratio of tensile storage modulus (E') to shear storage modulus (G') of 4.9, and therefore had poor reworkability.

Furthermore, in the double-sided pressure-sensitive adhesive sheets of Comparative Examples 2 and 3, the (meth)acrylic pressure-sensitive adhesive layer was a single layer, and the peak temperature (T1) of the loss tangent (Tan δ) obtained by dynamic viscoelasticity measurement in a shear mode at a frequency of 1 Hz was higher than -10°C, or the adhesive strength was 5.0 N/20 mm or less. As a result, the double-sided pressure-sensitive adhesive sheets were hard and had poor step absorbability, and air bubbles were generated at the edge of the bonding surface in a reliability test, resulting in poor bonding reliability.
The double-sided PSA sheet of Comparative Example 4 had a peak temperature (T2) of the loss tangent (Tan δ) obtained by dynamic viscoelasticity measurement in a tensile mode at a frequency of 1 Hz that was lower than -10°C, and therefore had poor reworkability.

The above examples show specific embodiments of the present invention, but these examples are merely illustrative and should not be construed as limiting. Various modifications that are obvious to those skilled in the art are intended to be within the scope of the present invention.

The double-sided pressure-sensitive adhesive sheet of the present invention has excellent adhesive properties such as step absorbency, adhesive strength, and lamination reliability, and can easily peel (separate) components for image display devices by cooling, making it suitable for use in laminating components for image display devices.
The laminate for an image display device and the image display device of the present invention can be easily peeled (separated) into constituent members for the image display device, and are excellent in reworkability.

Claims (12)

A double-sided pressure-sensitive adhesive sheet for use in bonding two components for an image display device,
the double-sided PSA sheet has at least three layers, the outermost layer and the innermost layer of which are (meth)acrylic PSA layers,
The ratio of the total thickness of the outermost layer and the innermost layer to the total thickness is 5 to 70%;
the ratio (E'/G') of the tensile storage modulus (E') to the shear storage modulus (G') is 5.0 or more;
The peak temperature (T1) of the loss tangent (Tan δ) obtained by dynamic viscoelasticity measurement in a shear mode at a frequency of 1 Hz is −10° C. or lower,
The peak temperature (T2) of the loss tangent (Tan δ) obtained by dynamic viscoelasticity measurement in a tensile mode at a frequency of 1 Hz is −10° C. or higher (however, if two or more peak temperatures are observed, the highest temperature is taken as the peak temperature (T2)).
A double-sided pressure-sensitive adhesive sheet having a 180° peel adhesive strength of 5 N/20 mm or more at 23°C with respect to the respective adherend surfaces of the two constituent members for an image display device. The double-sided pressure-sensitive adhesive sheet of claim 1, wherein the difference between the peak temperature (T2) of the loss tangent (Tan δ) obtained by dynamic viscoelasticity measurement in a tension mode at a frequency of 1 Hz and the peak temperature (T1) of the loss tangent (Tan δ) obtained by dynamic viscoelasticity measurement in a shear mode at a frequency of 1 Hz is 5 to 50°C. The double-sided pressure-sensitive adhesive sheet of claim 1 or 2, wherein the two components for the image display device are formed from different materials. The double-sided pressure-sensitive adhesive sheet according to any one of claims 1 to 3, wherein one of the two components for an image display device is glass, and the other is made of one or a combination of two or more of the following: a touch sensor, an image display panel, a surface protection panel, a polarizing film, and a retardation film. The double-sided pressure-sensitive adhesive sheet of claim 4, wherein the glass is a cover glass having a curved surface. The double-sided pressure-sensitive adhesive sheet according to any one of claims 1 to 5, which is configured so that the two components for an image display device can be separated by a cooling operation. The double-sided pressure-sensitive adhesive sheet according to claim 6, wherein the cooling step is performed to a temperature of -50°C or below. The double-sided pressure-sensitive adhesive sheet according to any one of claims 1 to 7, having a thickness of 50 to 1000 μm. The double-sided pressure-sensitive adhesive sheet according to any one of claims 1 to 8 , which is photocurable. A laminate for an image display device, comprising two components for an image display device bonded together via the double-sided pressure-sensitive adhesive sheet according to any one of claims 1 to 9 . A laminate for an image display device, comprising two components for an image display device bonded together via a pressure-sensitive adhesive layer formed by curing the double-sided pressure-sensitive adhesive sheet according to any one of claims 1 to 9 . An image display device constructed using the laminate for an image display device according to claim 11 .