JP7666262B2 - Adhesive sheet - Google Patents

Description

The present invention relates to an adhesive sheet.

Adhesive sheets are used, for example, to bond components (adherends) together, and are widely used in a variety of applications. In recent years, such adhesive sheets have also been used in display devices such as liquid crystal displays (LCDs), and input devices such as touch panels. By using adhesive sheets to bond the various components of the display device or input device, such as the optical displays, various devices can be manufactured with high precision and ease.

When an adhesive sheet is used for applications such as display devices or optical displays, the adhesive sheet is required to have the ability to bond and fix components (adhesiveness) and the ability to fill printing gaps (step-following ability). Furthermore, since these applications are usually used for long periods of time, the adhesive sheet is required to have the ability to stably maintain its adhesive strength over a long period of time and to be less likely to peel off due to shrinkage of the adherend (i.e., stress relaxation properties). From this perspective, for example, Patent Document 1 proposes a technology that uses an adhesive sheet containing a (meth)acrylic acid ester copolymer, an acrylic crosslinking monomer, and a specific photopolymerization initiator to prevent the retention strength from decreasing over time and to prevent the generation of air bubbles at the interface.

JP 2009-292986 A

When the adhesive sheet contains a photopolymerization initiator, as in the technology disclosed in Patent Document 1, when ultraviolet light is irradiated onto a laminate formed by bonding members (adherends) together with the adhesive sheet, further curing (post-curing) proceeds, and the members in the laminate are more firmly bonded together. However, because the hydrogen abstraction photopolymerization initiator remains in the adhesive sheet even after ultraviolet light irradiation, if the laminate is further exposed to ultraviolet light during use, the adhesive sheet continues to cure and becomes too hard. This results in a decrease in stress relaxation properties, causing problems such as the generation of air bubbles at the interface between the adhesive sheet and the adherend.

In addition, hydrogen abstraction photoinitiators (e.g., benzophenone-based) have a main absorption wavelength in the short wavelength region of 350 nm or less, so there is a problem in that the curing properties differ when cured with a high-pressure mercury or metal halide lamp that also emits light on the short wavelength side and when cured with a UV LED lamp that only emits wavelengths around 365 nm.

The present invention has been made in view of the above, and aims to provide an adhesive sheet and a laminate including the adhesive sheet that has excellent conformability to unevenness, is prevented from becoming too hard even when exposed to ultraviolet light after curing, has excellent stress relaxation properties even after exposure to ultraviolet light, and has a small difference in curing properties between high-pressure mercury lamps and UV LED lamps.

As a result of intensive research into achieving the above object, the inventors discovered that the above object can be achieved by including a specific crosslinking agent (B) and a photopolymerization initiator in a specified amount, and thus completed the present invention.

That is, the present invention includes, for example, the subject matter described in the following sections.
Item 1
A (meth)acrylic copolymer (A),
A crosslinking agent (B) having two or more acryloyl groups;
A hydrogen abstraction type photoinitiator (C);
and a self-cleavage type photoradical initiator (D),
The crosslinking agent (B) is contained in an amount of less than 5 parts by mass relative to 100 parts by mass of the (meth)acrylic copolymer (A),
the hydrogen abstraction type photoinitiator (C) is contained in an amount of 0.3 parts by mass or less based on 100 parts by mass of the (meth)acrylic copolymer (A);
The self-cleavage type photoradical initiator (D) is contained in an amount of 0.2 parts by mass or more per 100 parts by mass of the (meth)acrylic copolymer (A),
The following formula (1)
R=S(LED)/S(HTML) (1)
(Here, S(HPML) indicates the stress value [N/mm2] when an adhesive sheet is irradiated with light from a high-pressure mercury lamp with an illuminance of 160 mW/ ^cm2 to an accumulated light amount of 3000 mJ/ ^cm2 , and then pulled at a speed of 300 mm/min until the tensile elongation reaches 500%, and S(LED) indicates the stress value [N/ ^mm2 ] when an adhesive sheet is irradiated with light from an LED lamp with an illuminance of 160 mW/ ^cm2 and a wavelength of 365 nm to an accumulated light amount of 3000 mJ/ ^cm2 , and then pulled at a speed of ³⁰⁰ mm/min until the tensile elongation reaches 500%)
The pressure-sensitive adhesive sheet has an R value of 0.8 to 1.2.
Item 2
Item 2. The pressure-sensitive adhesive sheet according to item 1, wherein the (meth)acrylic copolymer (A) has a glass transition temperature of −50° C. to −40° C. and a mass average molecular weight of 250,000 to 450,000.
Item 3
Item 3. The pressure-sensitive adhesive sheet according to item 1 or 2, wherein the (meth)acrylic copolymer (A) does not contain an acryloyl group in its structural unit.
Item 4
Item 4. The pressure-sensitive adhesive sheet according to any one of items 1 to 3, which does not contain a thermal crosslinking agent.
Item 5
Item 5. A laminate comprising the pressure-sensitive adhesive sheet according to any one of items 1 to 4.

The adhesive sheet of the present invention has excellent conformability to uneven surfaces, is prevented from becoming too hard even when exposed to ultraviolet light after curing, and has excellent stress relaxation properties even after exposure to ultraviolet light. Furthermore, the adhesive sheet of the present invention has a small difference in curing properties between high-pressure mercury lamps and UV LED lamps.

The embodiments of the present invention will be described in detail below. In this specification, the expressions "contain" and "include" include the concepts of "contain," "include," "consist essentially of," and "consist only of."

In the numerical ranges described in this specification in stages, the upper or lower limit of a certain stage of the numerical range can be arbitrarily combined with the upper or lower limit of the numerical range of another stage. In the numerical ranges described in this specification, the upper or lower limit of the numerical range may be replaced with a value shown in an example or a value that can be unambiguously derived from an example. In addition, in this specification, a numerical value connected with "~" means a numerical range that includes the numerical values before and after "~" as the upper and lower limits.

1. Pressure-sensitive adhesive sheet The pressure-sensitive adhesive sheet of the present invention is
A (meth)acrylic copolymer (A),
A crosslinking agent (B) having two or more acryloyl groups;
A hydrogen abstraction type photoinitiator (C);
and (D) a self-cleavage type photoradical initiator.

In the pressure-sensitive adhesive sheet of the present invention, the crosslinking agent (B) is contained in an amount of less than 5 parts by mass per 100 parts by mass of the (meth)acrylic copolymer (A), the hydrogen abstraction type photoinitiator (C) is contained in an amount of 0.3 parts by mass or less per 100 parts by mass of the (meth)acrylic copolymer (A), and the self-cleavage type photoradical initiator (D) is contained in an amount of 0.2 parts by mass or more per 100 parts by mass of the (meth)acrylic copolymer (A).

The pressure-sensitive adhesive sheet of the present invention is a polymerizable compound represented by the following formula (1):
R=S(LED)/S(HTML) (1)
The value of R represented by the formula is 0.8 to 1.2. Here, S(HPML) indicates the stress value [N/mm 2 ] when the adhesive sheet is irradiated with light from a high pressure mercury lamp with an illuminance of 160 mW/cm ² so that the accumulated light amount is 3000 mJ/cm ² , and then pulled at a speed of 300 mm/min until the tensile elongation is 500%. S(LED) indicates the stress value [N/mm ² ] when the adhesive sheet is irradiated with light from an LED lamp with an illuminance of 160 mW/cm ² and a wavelength of 365 nm so that the accumulated light amount is 3000 mJ/cm ² , and then pulled at a speed of 300 mm/min until the tensile elongation is ⁵⁰⁰ %.

The adhesive sheet of the present invention has excellent conformability to uneven surfaces, is prevented from becoming too hard even when exposed to ultraviolet light after curing, and has excellent stress relaxation properties even after exposure to ultraviolet light. In addition, the adhesive sheet of the present invention has a small difference in curing properties between high-pressure mercury lamps and UV LED lamps, so its performance is stable regardless of the type of lamp used for post-curing.

((Meth)acrylic copolymer)
The pressure-sensitive adhesive sheet of the present invention contains a (meth)acrylic copolymer (A) as a constituent component. The (meth)acrylic copolymer (A) is the main component of the pressure-sensitive adhesive sheet of the present invention, and is the so-called base polymer of the pressure-sensitive adhesive sheet.

The type of (meth)acrylic copolymer (A) is not particularly limited, and for example, a wide range of (meth)acrylic copolymers used to form known pressure-sensitive adhesive sheets can be used.

The (meth)acrylic copolymer (A) may have, for example, a (meth)acrylic acid alkyl ester unit. In this specification, a "unit" is a repeating structural unit (also called a monomer unit) that constitutes a polymer. The (meth)acrylic acid alkyl ester unit is derived from a (meth)acrylic acid alkyl ester. Examples of the (meth)acrylic acid alkyl ester include methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, isopropyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, t-butyl (meth)acrylate, n-pentyl (meth)acrylate, n-hexyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, n-octyl (meth)acrylate, isooctyl (meth)acrylate, n-nonyl (meth)acrylate, isononyl (meth)acrylate, n-decyl (meth)acrylate, isodecyl (meth)acrylate, n-undecyl (meth)acrylate, n-dodecyl (meth)acrylate, stearyl (meth)acrylate, methoxyethyl (meth)acrylate, ethoxyethyl (meth)acrylate, cyclohexyl (meth)acrylate, benzyl (meth)acrylate, and isobornyl (meth)acrylate. The (meth)acrylic acid alkyl ester units contained in the (meth)acrylic copolymer (A) can be one type alone, or two or more types can be used in combination.

Among the above (meth)acrylic acid alkyl esters, at least one selected from the group consisting of methyl (meth)acrylate, n-butyl (meth)acrylate, and 2-ethylhexyl (meth)acrylate is preferred because it has high adhesion.

The (meth)acrylic copolymer (A) may have other acrylic monomer units other than the (meth)acrylic acid alkyl ester units. Examples of the other acrylic monomer units include acrylic monomer units having a crosslinkable functional group, such as hydroxyl group-containing acrylic monomer units and carboxyl group-containing acrylic monomer units. The (meth)acrylic copolymer (A) may contain one type of these monomer units, or may contain two or more types.

The hydroxyl group-containing acrylic monomer unit is derived from a hydroxyl group-containing acrylic monomer. Examples of hydroxyl group-containing acrylic monomers include hydroxyalkyl (meth)acrylates such as 2-hydroxyethyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, and 2-hydroxypropyl (meth)acrylate, (meth)acrylic acids [(mono-, di-, or poly)alkylene glycols] such as mono(diethylene glycol) (meth)acrylate, and (meth)acrylic acid lactones such as monocaprolactone (meth)acrylate. Examples of carboxyl group-containing acrylic monomer units include acrylic acid and methacrylic acid.

When the (meth)acrylic copolymer (A) has acrylic monomer units other than the (meth)acrylic acid alkyl ester units, the content of the other acrylic monomer units is preferably 0.01 to 20% by mass, and more preferably 0.5 to 10% by mass, based on the total mass of the (meth)acrylic copolymer (A). In particular, it is preferable that the content of the acrylic monomer units having a crosslinkable functional group is within the above range. If the content of the other acrylic monomer units is equal to or greater than the above lower limit, the cohesive strength can be sufficiently increased, and if it is equal to or less than the above upper limit, sufficient adhesive strength can be easily ensured.

It is preferable that the (meth)acrylic copolymer (A) does not contain an acryloyl group in its structural units. This makes it easier to produce the (meth)acrylic copolymer (A).

The glass transition temperature (Tg) of the (meth)acrylic copolymer (A) is not particularly limited. For example, the glass transition temperature (Tg) of the (meth)acrylic copolymer (A) can be set to -50°C to -40°C. In this case, the step-following ability of the adhesive sheet can be more effectively improved, the cohesive force of the adhesive sheet can be further increased, and the adhesive sheet has excellent durability and adhesion. In addition, the adhesive sheet can be easily processed because its handleability can be improved. The glass transition temperature (Tg) of the (meth)acrylic copolymer (A) is preferably -49°C or higher, more preferably -48°C or higher, and preferably -41°C or lower, more preferably -42°C or lower.

The mass average molecular weight (Mw) of the (meth)acrylic copolymer (A) is not particularly limited. For example, the mass average molecular weight of the (meth)acrylic copolymer (A) can be 250,000 to 450,000. This allows a balance between step followability and durability, which are in a trade-off relationship. The mass average molecular weight (Mw) of the (meth)acrylic copolymer (A) is preferably 260,000 or more, more preferably 280,000 or more, even more preferably 300,000 or more, and is preferably 430,000 or less.

It is particularly preferable that the (meth)acrylic copolymer (A) has a glass transition temperature of -50°C to -40°C and a mass average molecular weight of 250,000 to 450,000.

The weight average molecular weight of the (meth)acrylic copolymer (A) is a value measured by gel permeation chromatography (GPC) and calculated in terms of standard polystyrene. The measurement conditions for gel permeation chromatography (GPC) are as follows.
Solvent: tetrahydrofuran Column: Shodex KF801, KF803L, KF800L, KF800D (four columns connected together, manufactured by Showa Denko K.K.)
Column temperature: 40°C
Sample concentration: 0.5% by mass
Detector: RI-2031plus (JASCO)
Pump: RI-2080plus (JASCO)
Flow rate (flow rate): 0.8ml/min
Injection volume: 10 μl
Calibration curve: Standard polystyrene Shodex standard polystyrene (manufactured by Showa Denko) A calibration curve using 10 samples with Mw=1,320 to 2,500,000 was used.

The (meth)acrylic copolymer (A) can be produced, for example, by a known method, for example, by a polymerization reaction of acrylic monomers. In this case, various polymerization methods can be used, for example, a solution polymerization method, an emulsion polymerization method, a suspension polymerization method, etc. In addition, the (meth)acrylic copolymer (A) can also be obtained from commercial products, for example, OP-9200-1, OP-9200-23, OP-9200-3, OP-9200-4, OP-9200-5, OP-9200-7, etc. manufactured by Aica Kogyo Co., Ltd.

(Crosslinking Agent (B))
The pressure-sensitive adhesive sheet of the present invention contains a crosslinking agent (B) having two or more acryloyl groups as a constituent. The crosslinking agent (B) having two or more acryloyl groups is a so-called polyfunctional monomer. The crosslinking agent (B) having two or more acryloyl groups is referred to as a unit "crosslinking agent (B)".

The type of crosslinking agent (B) is not particularly limited, and for example, a wide variety of known polymerizable monomers having two or more acryloyl groups can be used. Examples of crosslinking agents (B) include ethylene glycol di(meth)acrylate, triethylene glycol di(meth)acrylate, 1,3-butylene glycol di(meth)acrylate, 1,4-butylene glycol di(meth)acrylate, 1,9-nonanediol di(meth)acrylate, 1,6-hexanediol diacrylate, polybutylene glycol di(meth)acrylate, neopentyl glycol di(meth)acrylate, tetraethylene glycol di(meth)acrylate, tripropylene glycol di(meth)acrylate, polypropylene glycol di(meth)acrylate, diacrylate of bisphenol A diglycidyl ether, trimethylolpropane tri(meth)acrylate, pentaerythritol tri(meth)acrylate, pentaerythritol tetra(meth)acrylate, and other polyhydric alcohol (meth)acrylic acid esters, vinyl methacrylate, and the like.

The crosslinking agent (B) can be produced, for example, by a known method, or can be obtained from a commercially available product. Examples of commercially available products include ATM-4PL and A-TMM-3L manufactured by Shin-Nakamura Chemical Co., Ltd., PET-30 manufactured by Nippon Kayaku Co., Ltd., and Viscoat #230 manufactured by Osaka Organic Chemical Industry Co., Ltd.

(Hydrogen Abstraction Photoinitiator (C))
The pressure-sensitive adhesive sheet of the present invention contains a hydrogen abstraction type photoinitiator (C) as a constituent. The hydrogen abstraction type photopolymerization initiator (C) is a photopolymerization initiator in which an initiator photoexcited by irradiation with active energy rays and a hydrogen donor in the system form an exciplex, and the hydrogen of the hydrogen donor is transferred to promote polymerization. Therefore, the hydrogen abstraction type photoinitiator (C) initiates the polymerization reaction of the crosslinking agent (B) and other polymerizable components contained in the pressure-sensitive adhesive sheet of the present invention, and also acts on the (meth)acrylic copolymer (A) to form a crosslinked structure.

Here, "active energy rays" refers to electromagnetic waves or charged particle rays that have an energy quantum, and examples of such rays include ultraviolet rays, electron beams, visible light, X-rays, and ion beams. Among these, from the viewpoint of versatility, ultraviolet rays or electron beams are preferred, and ultraviolet rays are particularly preferred.

The type of hydrogen abstraction type photoinitiator (C) is not particularly limited, and examples thereof include benzophenone, benzoylbenzoic acid, methyl benzoylbenzoate, 4-phenylbenzophenone, hydroxybenzophenone, 3,3'-dimethyl-4-methoxybenzophenone, 2,4,6-trimethylbenzophenone, 4-methylbenzophenone, thioxanthone, 2-chlorothioxanthone, 2-methylthioxanthone, 2,4-dimethylthioxanthone, isopropylthioxanthone, camphorquinone, dibenzosuberone, 2-ethylanthraquinone, 3,3',4,4'-tetra(t-butylperoxycarbonyl)benzophenone, benzil, and 9,10-phenanthrenequinone. Of these, it is preferable that the hydrogen abstraction type photopolymerization initiator (C) is a benzophenone-based photopolymerization initiator. Benzophenone-based photopolymerization initiators include, for example, benzophenone, 4-methylbenzophenone, and 2,4,6-trimethylbenzophenone.

The hydrogen abstraction type photopolymerization initiator (C) can be produced by a known method, or can be obtained from a commercial product. Examples of commercial products include TZT and MBF manufactured by IGM Resins, and "SPEED CURE MBP" (e.g., "4MBP") manufactured by Lambson.

(Self-Cleavage Type Photoradical Initiator (D))
The pressure-sensitive adhesive sheet of the present invention contains a self-cleavage type photoradical initiator (D) as a constituent. The self-cleavage type photoradical initiator (D) initiates a polymerization reaction of the crosslinking agent (B) and other polymerizable components that may be contained in the pressure-sensitive adhesive sheet of the present invention when irradiated with active energy rays. The self-cleavage type photoradical initiator (D) is other than the hydrogen abstraction type photopolymerization initiator (C).

The type of self-cleavage type photoradical initiator (D) is not particularly limited, and examples thereof include acetophenone-based initiators, benzoin ether-based initiators, hydroxyalkylphenone-based initiators, thioxanthone-based initiators, amine-based initiators, and acylphosphine oxide-based initiators.

Specific examples of acetophenone initiators include diethoxyacetophenone and benzyl dimethyl ketal.Specific examples of benzoin ether initiators include benzoin and benzoin methyl ether.Specific examples of hydroxyalkylphenone initiators include 1-hydroxycyclohexylphenyl ketone.Specific examples of thioxanthone initiators include 2-isopropylthioxanthone and 2,4-dimethylthioxanthone.Specific examples of amine initiators include triethanolamine and ethyl 4-dimethylbenzoate.Specific examples of acylphosphine oxide initiators include phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide.

The self-cleavage type photoradical initiator (D) can have an absorption wavelength peak at a wavelength of, for example, 220 to 450 nm. In particular, it is preferable that the self-cleavage type photoradical initiator (D) has an absorption wavelength peak at 340 to 390 nm. When the absorption wavelength peak of the self-cleavage type photoradical initiator (D) is less than 340 nm, it is preferable to use an appropriate sensitizer.

The self-cleavage type photoradical initiator (D) can be produced by a known method or can be obtained from a commercial product. Examples of commercial products include EsacureOne, Omnirad 184, Omnirad 819, Omnirad 1173, Omnirad 2959, Omnirad 127, Omnirad 907, Omnirad 369, Omnirad 379G, Omnirad TPO, Omnirad TPO H, Irgacure OXE01, Irgacure OXE02, etc., manufactured by IGM RESINS B.V.

(Adhesive sheet)
As described above, the pressure-sensitive adhesive sheet of the present invention contains the (meth)acrylic copolymer (A), the crosslinking agent (B), the hydrogen abstraction type photoinitiator (C), and the self-cleavage type photoradical initiator (D), and therefore can be so-called post-cured. For example, by irradiating an active energy ray such as ultraviolet light to a laminate formed by bonding adherends together with the pressure-sensitive adhesive sheet, the post-curing proceeds and the adherends are more firmly bonded to each other.

In the adhesive sheet of the present invention, the content of the crosslinking agent (B) is less than 5 parts by mass, the content of the hydrogen abstraction type photoinitiator (C) is 0.3 parts by mass or less, and the content of the self-cleavage type photoradical initiator (D) is 0.2 parts by mass or more, relative to 100 parts by mass of the (meth)acrylic copolymer (A). As a result, the adhesive sheet of the present invention is prevented from becoming too hard even when exposed to ultraviolet light after curing, and has excellent stress relaxation properties even after exposure to ultraviolet light. In addition, the difference in curing properties between a high-pressure mercury lamp and a UV LED lamp can be reduced. Therefore, the adhesive sheet of the present invention has stable performance regardless of the type of lamp used during post-curing.

When the content of the crosslinking agent (B) is 5 parts by mass or more relative to 100 parts by mass of the (meth)acrylic copolymer (A), the adhesive sheet after post-curing becomes too hard, so that the stress relaxation property is insufficient, and air bubbles and the like are easily generated. When the content of the hydrogen abstraction type photoinitiator (C) exceeds 0.3 parts by mass relative to 100 parts by mass of the (meth)acrylic copolymer (A), the adhesive sheet becomes too hard when exposed to ultraviolet light after curing, so that the stress relaxation property is deteriorated, and air bubbles and the like are easily generated. Note that an adhesive sheet that does not contain the hydrogen abstraction type photoinitiator (C) is difficult to cure even by post-curing, so that the stress relaxation property is deteriorated and air bubbles and the like are easily generated. When the content of the self-cleavage type photoradical initiator (D) is less than 0.2 parts by mass relative to 100 parts by mass of the (meth)acrylic copolymer (A), the crosslinking agent (B) and the like contained in the adhesive sheet are not sufficiently cured, causing a decrease in adhesive strength, and the value of R is also small.

The content of the crosslinking agent (B) is preferably 4.5 parts by mass or less, more preferably 4 parts by mass or less, and even more preferably 3.5 parts by mass or less, per 100 parts by mass of the (meth)acrylic copolymer (A). The content of the crosslinking agent (B) is preferably 0.1 parts by mass or more, more preferably 0.5 parts by mass or more, and even more preferably 1 part by mass or more, per 100 parts by mass of the (meth)acrylic copolymer (A).

The content of the hydrogen abstraction type photoinitiator (C) is preferably 0.25 parts by mass or less, and more preferably 0.2 parts by mass or less, relative to 100 parts by mass of the (meth)acrylic copolymer (A). The content of the hydrogen abstraction type photoinitiator (C) is preferably 0.01 parts by mass or more, and more preferably 0.05 parts by mass or more, and even more preferably 0.1 parts by mass or more, relative to 100 parts by mass of the (meth)acrylic copolymer (A).

The adhesive sheet of the present invention contains a hydrogen abstraction type photoinitiator (C) in a specified amount or less, so even if the adhesive sheet is exposed to UV rays for a longer period of time after post-curing, the excessive reaction of the hydrogen abstraction type photoinitiator is suppressed and the adhesive sheet does not become too hard. As a result, the adhesive sheet of the present invention can significantly suppress the generation of air bubbles due to a decrease in stress relaxation properties.

The content of the self-cleavage type photoradical initiator (D) is preferably 0.25 parts by mass or more, and more preferably 0.3 parts by mass or more, per 100 parts by mass of the (meth)acrylic copolymer (A). The content of the self-cleavage type photoradical initiator (D) is preferably 1 part by mass or less, and more preferably 0.8 parts by mass or less, per 100 parts by mass of the (meth)acrylic copolymer (A).

The pressure-sensitive adhesive sheet of the present invention may contain other components in addition to the (meth)acrylic copolymer (A), the crosslinking agent (B), the hydrogen abstraction type photoinitiator (C) and the self-cleavage type photoradical initiator (D) to the extent that the effects of the present invention are not impaired.

For example, the pressure-sensitive adhesive sheet of the present invention may contain a monofunctional monomer. A monofunctional monomer is a monomer having one reactive double bond (particularly a polymerizable double bond) in the molecule. Examples of the monofunctional monomer include a wide range of known monofunctional monomers contained in pressure-sensitive adhesive sheets, such as (meth)acrylic ester compounds, (meth)acrylamide compounds, and other vinyl compounds.

(Meth)acrylic ester compounds include isobornyl acrylate, isostearyl acrylate, methyl methacrylate, ethyl methacrylate, butyl methacrylate, benzyl methacrylate, and (meth)acrylates having a cyclic ether structure with a 4- to 6-membered ring. (Meth)acrylamide compounds include N-acryloyloxyethylhexahydrophthalimide, acrylamide, N,N-dimethylacrylamide, N,N-diethylacrylamide, etc. Other examples include acryloylmorpholine, vinylpyrrolidone, etc.

When the pressure-sensitive adhesive sheet of the present invention contains a monofunctional monomer, the content is not particularly limited, and for example, the monofunctional monomer is preferably contained in an amount of 30 parts by mass or less, more preferably 20 parts by mass or less, and even more preferably 10 parts by mass or less, per 100 parts by mass of the (meth)acrylic copolymer (A).

The adhesive sheet of the present invention may contain a thermal crosslinking agent if necessary, but in terms of being particularly excellent in step conformability, the adhesive sheet of the present invention preferably contains a small amount of thermal crosslinking agent, and particularly preferably does not contain any thermal crosslinking agent. Therefore, the adhesive sheet of the present invention preferably contains 1 part by mass or less of thermal crosslinking agent per 100 parts by mass of (meth)acrylic copolymer (A), more preferably 0.1 parts by mass or less, and particularly preferably 0 parts by mass or less. The thermal curing agent referred to here means a material that can be cured by heat, and means, for example, known thermal crosslinking agents such as isocyanate compounds, epoxy compounds, oxazoline compounds, aziridine compounds, metal chelate compounds, and butylated melamine compounds.

The adhesive sheet of the present invention may contain additives that can be used as additives for the adhesive sheet, as necessary, such as sensitizers, plasticizers, antioxidants, metal corrosion inhibitors, tackifiers, silane coupling agents, ultraviolet absorbers, light stabilizers, etc. As described above, when the adhesive sheet contains a self-cleavage type photoradical initiator (D) with an absorption wavelength peak of less than 340 nm, it is preferable to use an appropriate sensitizer.

The pressure-sensitive adhesive sheet of the present invention may be a single-sided pressure-sensitive adhesive sheet or a double-sided pressure-sensitive adhesive sheet. The pressure-sensitive adhesive sheet of the present invention may also have a release sheet on one or both sides. Examples of the release sheet include a release laminate sheet having a release sheet substrate and a release agent layer provided on one side of the release sheet substrate, or a polyolefin film such as a polyethylene film or a polypropylene film as a low-polarity substrate.

Paper or polymer film may be used as the substrate for the release sheet in the release laminate sheet. For example, a general-purpose addition type or condensation type silicone-based release agent or a long-chain alkyl group-containing compound is used as the release agent constituting the release agent layer. In particular, an addition type silicone-based release agent having high reactivity is preferably used. Specific examples of the silicone-based release agent include BY24-4527 and SD-7220 manufactured by Toray Dow Corning Silicone Co., Ltd., and KS-3600, KS-774, and X62-2600 manufactured by Shin-Etsu Chemical Co., Ltd. In addition, it is preferable that the silicone-based release agent contains a silicone resin, which is an organosilicon compound having SiO ₂ units and (CH ₃ ) ₃ SiO _1/2 units or CH ₂ ═CH(CH ₃ ) SiO _1/2 units. Specific examples of silicone resins include BY24-843, SD-7292, SHR-1404, etc. manufactured by Toray Dow Corning Silicone Co., Ltd., and KS-3800, X92-183, etc. manufactured by Shin-Etsu Chemical Co., Ltd.

Commercially available products may be used as the peelable laminate sheet. For example, a heavy separator film made by Toyobo Co., Ltd., which is a release-treated polyethylene terephthalate film, or a light separator film made by Toyobo Co., Ltd., which is a release-treated polyethylene terephthalate film, may be used.

The pressure-sensitive adhesive sheet of the present invention preferably has a pair of release sheets with different peel strengths. If the releasability from one side differs from the other side, it becomes easier to first peel off only the release sheet with the stronger peel strength.

The method for post-curing the pressure-sensitive adhesive sheet of the present invention is not particularly limited, and any known method can be widely used. For example, the sheet can be post-cured by irradiating the laminate with active energy rays, as described below.

(Physical properties of adhesive sheet)
The pressure-sensitive adhesive sheet of the present invention is a polymerizable compound represented by the following formula (1):
R=S(LED)/S(HTML) (1)
The value of R represented by the formula is 0.8 to 1.2.

S(HPML) indicates the stress value [N/mm2] when the adhesive sheet is irradiated with light from a high pressure mercury lamp with an illuminance of 160 mW/ ^cm2 to an integrated light amount of 3000 mJ/ ^cm2 , and then pulled at a speed of 300 mm/min until the tensile elongation is 500%. S(LED) indicates the stress value [N/ ^mm2 ] when the adhesive sheet is irradiated with light from an LED lamp with an illuminance of 160 mW/ ^cm2 and a wavelength of 365 nm to an integrated light amount of 3000 mJ/ ^cm2 , and then pulled at a speed of ³⁰⁰ mm/min until the tensile elongation is 500%.

S (HPML) is measured by the following procedure. An adhesive sheet with two release sheets with different peel strengths attached to both sides is cut out to a length of 50 mm and a width (width direction) of A mm. At this time, the value of A in the width direction is determined to be the thickness (mm) of the adhesive sheet x A mm = 6 ^mm2 . Next, post-curing is performed by irradiating light using a high-pressure mercury lamp with an illuminance of 160 mW/ ^cm2 so that the accumulated light amount is 3000 mJ/ ^cm2 . Thereafter, the release sheet with the lower peel strength is peeled off, and only the adhesive sheet is rolled in the width direction on the release sheet with the higher peel strength to obtain a cylindrical sample with a circle diameter of 2.8 mm and a height of 50 mm. The regions up to 10 mm from the upper and lower ends of the cylindrical sample are sandwiched between two PET films (total of four) with a thickness of 188 μm, a length of 25 mm, and a width of 50 mm. The area sandwiched between the two PET films is then used as the chuck part of a tensile tester, and the chuck distance is fixed at 30 mm. The test piece is then pulled at a tensile speed of 300 mm/min under conditions of a measurement temperature of 23° C. and a relative humidity of 50% until the tensile elongation reaches 500%, and the stress value at this time is designated as S(HPML).

S (LED) is measured by the following procedure. An adhesive sheet with two release sheets with different peel strengths attached to both sides is cut out to a length of 50 mm and a width (width direction) of A mm. At this time, the value of A in the width direction is determined to be the thickness (mm) of the adhesive sheet x A mm = 6 ^mm2 . Next, post-curing is performed by irradiating light using an LED lamp with an illuminance of 160 mW/ ^cm2 and a wavelength of 365 nm so that the accumulated light amount is 3000 mJ/ ^cm2 . Thereafter, the release sheet with the lower peel strength is peeled off, and only the adhesive sheet is rolled in the width direction on the release sheet with the higher peel strength to obtain a cylindrical sample with a circle diameter of 2.8 mm and a height of 50 mm. The regions up to 10 mm at the top and bottom of the cylindrical sample are sandwiched between two PET films (total of four) with a thickness of 188 μm, a length of 25 mm, and a width of 50 mm. The area sandwiched between the two PET films is then used as the chuck part of the tensile tester, and the chuck distance is fixed at 30 mm. The specimen is then pulled at a tensile speed of 300 mm/min under an environment of a measurement temperature of 23° C. and a relative humidity of 50% until the tensile elongation reaches 500%, and the stress value at this time is designated as S(LED).

If the value of R in formula (1) is outside the range of 0.8 to 1.2, the degree of cure of the adhesive sheet will vary greatly depending on the type of lamp used, resulting in problems with the stability of product performance. Therefore, by keeping the value of R in formula (1) in the range of 0.8 to 1.2, whether the lamp used to irradiate the adhesive sheet is a high-pressure mercury lamp or an LED, there will be no significant variation in the performance of the adhesive sheet after post-curing, making it possible to provide a stable product.

The value of R represented by the formula (1) is preferably 0.82 or more, more preferably 0.85 or more, more preferably 0.87 or more, and preferably 1.1 or less, more preferably 1.05 or less, and even more preferably 1.0 or less. Both S(HPML) and S(LED) are preferably 0.05 N/ ^mm2 or more, more preferably 0.1 N/ ^mm2 or more. In this case, the pressure-sensitive adhesive sheet is unlikely to be insufficiently cured. In addition, both S(HPML) and S(LED) are preferably 0.4 N/mm2 or ^less , more preferably 0.3 N/ ^mm2 or less. In this case, the pressure-sensitive adhesive sheet is unlikely to be excessively cured by post-curing, so that it can have sufficient stress relaxation properties.

The adhesive sheet of the present invention exhibits excellent conformability to the steps of the adherend, even when the distance between steps on the adherend is narrow or the height or depth of the uneven shape that constitutes the steps is large. In addition, even if the adhesive sheet of the present invention is applied to the steps and post-cured with active energy rays such as ultraviolet rays, and then exposed to ultraviolet rays, it is unlikely to deteriorate and the generation of air bubbles in the steps is also suppressed.

Therefore, the pressure-sensitive adhesive sheet of the present invention is preferably an adhesive sheet for attachment to an adherend having at least one type selected from convex portions and concave portions. In particular, the pressure-sensitive adhesive sheet of the present invention is preferably an adhesive sheet for attachment to two adherends, and specifically, it is preferably for attachment to a first adherend having convex portions and a second adherend having at least one type selected from convex portions and concave portions.

The gel fraction (initial gel fraction) of the adhesive sheet of the present invention before post-curing is, for example, 5.0% or less, preferably 4.5% or less, more preferably 4.0% or less, even more preferably 3.5% or less, and particularly preferably 2.5% or less. The lower limit of the gel fraction of the adhesive sheet is not particularly limited, and may be, for example, 0%. By setting the gel fraction of the adhesive sheet before photocrosslinking within the above range, an adhesive sheet with excellent step-following properties can be obtained.

The gel fraction of the adhesive sheet is a value measured by the following method. First, about 0.1 g of the adhesive sheet (only the adhesive layer) is collected in a sample bottle, 30 ml of ethyl acetate is added, and the mixture is shaken for 24 hours. The contents of the sample bottle are then filtered through a 150 mesh stainless steel wire mesh, and the residue on the wire mesh is dried at 100° C. for 1 hour to measure the dry weight W (g). From the obtained dry weight, the following formula is calculated: gel fraction (%)=(dry mass W/collected mass of adhesive sheet)×100
The gel fraction (%) is calculated from

When the pressure-sensitive adhesive sheet of the present invention is post-cured by irradiating it with light using a high-pressure mercury lamp with an illuminance of 160 mW/ ^cm2 so that the accumulated light amount is 3000 mJ/ ^cm2 , the gel fraction at that time is, for example, preferably 5% or more, more preferably 10% or more, and even more preferably 20% or more.

Furthermore, when the pressure-sensitive adhesive sheet of the present invention is post-cured by irradiating it with light using an LED lamp having an illuminance of 160 mW/ ^cm2 and a wavelength of 365 nm so that the accumulated light amount is 3000 mJ/ ^cm2 , the gel fraction after post-curing is preferably 5% or more, more preferably 10% or more, and even more preferably 20% or more.

The thickness of the adhesive sheet of the present invention is not particularly limited and can be within an appropriate range depending on the purpose of use. For example, it is preferably 5 to 500 μm, more preferably 30 to 300 μm, and even more preferably 50 to 200 μm. By setting the thickness of the adhesive sheet within the above range, it is possible to sufficiently improve the step-following ability and durability.

2. Method for Producing the Pressure-Sensitive Adhesive Sheet The method for producing the pressure-sensitive adhesive sheet of the present invention is not particularly limited, and as long as it contains specific components, for example, any known production method can be widely adopted. For example, the pressure-sensitive adhesive sheet of the present invention can be produced using a method of coating a pressure-sensitive adhesive composition.

In this case, the components contained in the adhesive composition are the same as those contained in the adhesive sheet described above, and the content ratios thereof are also the same as those of the adhesive sheet. Therefore, the adhesive composition contains at least a (meth)acrylic copolymer (A), a crosslinking agent (B), a hydrogen abstraction type photoinitiator (C), and a self-cleavage type photoradical initiator (D), and the crosslinking agent (B) is contained in an amount of less than 5 parts by mass per 100 parts by mass of the (meth)acrylic copolymer (A), the hydrogen abstraction type photoinitiator (C) is contained in an amount of 0.3 parts by mass or less per 100 parts by mass of the (meth)acrylic copolymer (A), and the self-cleavage type photoradical initiator (D) is contained in an amount of 0.2 parts by mass or more per 100 parts by mass of the (meth)acrylic copolymer (A). The adhesive composition can be prepared, for example, by a known method.

The adhesive composition can be applied to a substrate to form a coating film, for example, to form an adhesive layer. The adhesive layer thus formed may be directly obtained as an adhesive sheet. The adhesive composition can be applied using a known coating device. Examples of coating devices include a blade coater, an air knife coater, a roll coater, a bar coater, a gravure coater, a microgravure coater, a rod blade coater, a lip coater, a die coater, and a curtain coater.

The thickness of the adhesive composition after application is not particularly limited and can be set appropriately depending on the desired thickness of the adhesive layer. If the adhesive composition contains a solvent as described below, the solvent can be removed by an appropriate method.

In order to improve the coatability of the adhesive composition, the adhesive composition may contain a solvent. Examples of the solvent include hydrocarbons such as hexane, heptane, octane, toluene, xylene, ethylbenzene, cyclohexane, and methylcyclohexane; halogenated hydrocarbons such as dichloromethane, trichloroethane, trichloroethylene, tetrachloroethylene, and dichloropropane; alcohols such as methanol, ethanol, propanol, isopropyl alcohol, butanol, isobutyl alcohol, and diacetone alcohol; ethers such as diethyl ether, diisopropyl ether, dioxane, and tetrahydrofuran; ketones such as acetone, methyl ethyl ketone, methyl isobutyl ketone, isophorone, and cyclohexanone; esters such as methyl acetate, ethyl acetate, butyl acetate, isobutyl acetate, amyl acetate, and ethyl butyrate; and polyols and derivatives thereof such as ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monomethyl ether acetate, propylene glycol monomethyl ether, propylene glycol monoethyl ether, and propylene glycol monomethyl ether acetate.

When the adhesive composition contains a solvent, the content ratio is not particularly limited and can be, for example, within the same range as that of known adhesive compositions.

When applying the adhesive composition, the substrate used is not particularly limited. For example, the adhesive composition can be applied to various substrates such as a resin substrate or a glass substrate. When the substrate is the release sheet described above, the adhesive composition can be applied onto this release laminate sheet, or the adhesive composition can be applied directly onto the member to be adhered.

When the substrate is the release sheet described above, an adhesive sheet having a release sheet on one side as described above can be obtained. In addition, by protecting the adhesive surface of an adhesive sheet having a release sheet on one side with a release sheet having the same or different release properties, an adhesive sheet having a release sheet on both sides can be obtained.

The adhesive sheet obtained as described above contains at least the (meth)acrylic copolymer (A), the crosslinking agent (B), the hydrogen abstraction type photoinitiator (C) and the self-cleavage type photoradical initiator (D), and is therefore in a so-called semi-cured state and has post-curing properties.

3. Laminate The laminate of the present invention includes the pressure-sensitive adhesive sheet. For example, the laminate has a structure in which a first adherend, a pressure-sensitive adhesive sheet, and a second adherend are laminated in this order. In the laminate, the pressure-sensitive adhesive sheet is, for example, only a pressure-sensitive adhesive layer. The method of laminating the pressure-sensitive adhesive sheet to the adherend is not particularly limited, and for example, known methods such as a roll lamination method and a vacuum lamination method can be adopted.

In the laminate of the present invention, the adhesive sheet has post-curing properties, so that, for example, irradiation with active energy rays causes a curing reaction in the adhesive sheet (adhesive layer), whereby the first adherend and the second adherend are firmly attached to the adhesive sheet. The method of irradiating active energy rays is not particularly limited, and for example, methods similar to known methods can be widely adopted. In addition, irradiation of the laminate with active energy rays can be performed from either the side of the first adherend or the side of the second adherend.

The first adherend and the second adherend may have at least one type selected from a convex portion and a concave portion. In addition, it is preferable that the first adherend and the second adherend are each optical members. Examples of optical members include each component member of an optical product such as a touch panel or an image display device.

Examples of components of a touch panel include an ITO film in which an ITO film is provided on a transparent resin film, ITO glass in which an ITO film is provided on the surface of a glass plate, a transparent conductive film in which a transparent resin film is coated with a conductive polymer, a hard coat film, and a fingerprint-resistant film. As the first and second adherends, an ITO film in which an ITO film is provided on a transparent resin film, an ITO glass in which an ITO film is provided on the surface of a glass plate, and a transparent conductive film in which a transparent resin film is coated with a conductive polymer are preferred.

Constituent members of image display devices include, for example, anti-reflection films, alignment films, polarizing films, retardation films, and brightness enhancement films used in liquid crystal display devices. Materials used for these members include glass, polycarbonate, polyethylene terephthalate, polymethyl methacrylate, polyethylene naphthalate, cycloolefin polymer, triacetyl cellulose, polyimide, and cellulose acylate. Among these, it is preferable that the first adherend is a touch panel using glass, polycarbonate, or polymethyl methacrylate, and the second adherend is an image display device such as a polarizing plate or an organic EL display.

Examples of active energy rays to be irradiated to the laminate include ultraviolet rays, electron beams, visible light, X-rays, ion beams, etc., and among these, from the viewpoint of versatility, ultraviolet rays or electron beams are preferred, and ultraviolet rays are particularly preferred. Examples of light sources for ultraviolet rays that can be used include high-pressure mercury lamps, low-pressure mercury lamps, ultra-high-pressure mercury lamps, metal halide lamps, carbon arcs, xenon arcs, and electrodeless ultraviolet lamps.

In particular, since the pressure-sensitive adhesive sheet of the present invention contains a specified amount of self-cleavage type photoradical initiator (D), it has high reactivity even when using an LED lamp (e.g., 365 nm), and post-curing in the laminate proceeds easily.

Therefore, for laminates including the pressure-sensitive adhesive sheet of the present invention, in addition to conventional high-pressure mercury or metal halide UV lamps, LED lamps can also be used, making it easier to manufacture the laminate and reducing introduction costs. In other words, laminates including the pressure-sensitive adhesive sheet of the present invention can have stable performance regardless of the type of lamp used for post-curing.

Furthermore, because the content of these initiators is adjusted to an appropriate range, even if the adhesive sheet is exposed to UV light for an extended period of time after post-curing, the excessive reaction of the hydrogen abstraction initiator is suppressed, and the adhesive sheet does not become too hard, significantly suppressing the deterioration of stress relaxation properties and the generation of air bubbles.

The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

Example 1
According to the formulation shown in Table 1, a pressure-sensitive adhesive composition was prepared. Specifically, a pressure-sensitive adhesive composition was prepared by mixing 100 parts by mass, calculated as solid content, of "OP-9200-3" manufactured by AICA Co., Ltd. (ethyl acetate solution having a solid content concentration of 40% by mass) as the (meth)acrylic copolymer (A), 3 parts by mass of propoxylated pentaerythritol tri- and tetraacrylate (ATM-4PL manufactured by Shin-Nakamura Chemical Co., Ltd.) as the crosslinking agent (B), 0.2 parts by mass of 4-methylbenzophenone ("SPEED CURE MBP" manufactured by LAMBSON, hereinafter referred to as "4MBP") as the hydrogen abstraction type photoinitiator (C), and 0.5 parts by mass of 2,4,6-trimethylbenzoyl-diphenylphosphine oxide ("Omnirad TPO H" manufactured by IGM RESINS B.V.) as the self-cleavage type photoradical initiator (D).

This adhesive composition was applied uniformly with an applicator to the surface of a 38 μm-thick polyethylene terephthalate film (first release sheet) (Oji F-Tex Co., Ltd.: 38RL-07(2)) with a release layer treated with a silicone-based release agent, so that the coating thickness after drying would be 150 μm. It was then dried for 10 minutes in an air-circulating thermostatic oven at 100°C to form an adhesive layer on the surface of the first release sheet. Next, a 38 μm-thick second release sheet (Oji F-Tex Co., Ltd.: 38RL-07(L)) was laminated to the surface of this adhesive layer, to obtain a double-sided adhesive sheet with release sheet having a first release sheet/adhesive layer/second release sheet configuration in which the adhesive layer is sandwiched between a pair of release sheets with different release strengths.

Example 2
A double-sided pressure-sensitive adhesive sheet with a release sheet was obtained in the same manner as in Example 1, except that the amounts of the (meth)acrylic copolymer (A), the crosslinking agent (B), the hydrogen abstraction type photoinitiator (C), and the self-cleavage type photoradical initiator (D) used were changed as shown in Table 1.

Example 3
A double-sided pressure-sensitive adhesive sheet with a release sheet was obtained in the same manner as in Example 1, except that the crosslinking agent (B) was changed to "Viscoat #230" manufactured by Osaka Organic Chemical Industry Co., Ltd. and the amounts of the (meth)acrylic copolymer (A), crosslinking agent (B), hydrogen abstraction type photoinitiator (C), and self-cleavage type photoradical initiator (D) used were changed as shown in Table 1.

Example 4
A double-sided pressure-sensitive adhesive sheet with a release sheet was obtained in the same manner as in Example 1, except that the amounts of the (meth)acrylic copolymer (A), the crosslinking agent (B), the hydrogen abstraction type photoinitiator (C), and the self-cleavage type photoradical initiator (D) used were changed as shown in Table 1.

Example 5
A double-sided pressure-sensitive adhesive sheet with a release sheet was obtained in the same manner as in Example 1, except that 5 parts by mass of "MEDOL-10" manufactured by Osaka Organic Chemical Industry Co., Ltd. was used as a monofunctional monomer and the amounts of the (meth)acrylic copolymer (A), crosslinking agent (B), hydrogen abstraction type photoinitiator (C), and self-cleavage type photoradical initiator (D) used were changed as shown in Table 1.

Example 6
A double-sided pressure-sensitive adhesive sheet with a release sheet was obtained in the same manner as in Example 1, except that the self-cleavage type photoradical initiator (D) was changed to "Omnirad 184" manufactured by IGM RESINS B.V., 0.1 part by mass of "UVS-2171" manufactured by Kawasaki Chemical Industries, Ltd. was used in combination as a sensitizer, and the amounts of the (meth)acrylic copolymer (A), crosslinking agent (B), hydrogen abstraction type photoinitiator (C), and self-cleavage type photoradical initiator (D) used were changed as shown in Table 1.

(Example 7)
A double-sided pressure-sensitive adhesive sheet with a release sheet was obtained in the same manner as in Example 1, except that the amounts of the (meth)acrylic copolymer (A), the crosslinking agent (B), the hydrogen abstraction type photoinitiator (C), and the self-cleavage type photoradical initiator (D) used were changed as shown in Table 1.

(Example 8)
A double-sided pressure-sensitive adhesive sheet with a release sheet was obtained in the same manner as in Example 1, except that the amounts of the (meth)acrylic copolymer (A), the crosslinking agent (B), the hydrogen abstraction type photoinitiator (C), and the self-cleavage type photoradical initiator (D) used were changed as shown in Table 1.

(Comparative Example 1)
A double-sided pressure-sensitive adhesive sheet with a release sheet was obtained in the same manner as in Example 1, except that the amounts of the (meth)acrylic copolymer (A), the crosslinking agent (B), the hydrogen abstraction type photoinitiator (C), and the self-cleavage type photoradical initiator (D) used were changed as shown in Table 1.

(Comparative Example 2)
A double-sided pressure-sensitive adhesive sheet with a release sheet was obtained in the same manner as in Example 1, except that the amounts of the (meth)acrylic copolymer (A), the crosslinking agent (B), the hydrogen abstraction type photoinitiator (C), and the self-cleavage type photoradical initiator (D) used were changed as shown in Table 1.

(evaluation)
<S (HPML); 500% tensile stress>
S (HPML) was measured as follows. First, the double-sided pressure-sensitive adhesive sheet with release sheet obtained in the examples and comparative examples was cut out to a length of 50 mm and a width (width direction) of A mm. At this time, the value of A in the width direction was determined to be the thickness (mm) of the pressure-sensitive adhesive sheet x A mm = 6 ^mm2 . Next, post-curing was performed by irradiating light using a high-pressure mercury lamp with an illuminance of ¹⁶⁰ mW/cm2 so that the accumulated light amount was 3000 mJ/ ^cm2 . Thereafter, the first release sheet was peeled off, and only the pressure-sensitive adhesive sheet was peeled off from the second release sheet while rolling it in the width direction to obtain a cylindrical sample with a circle diameter of 2.8 mm and a height of 50 mm. The regions up to 10 mm from the upper and lower ends of the cylindrical sample were sandwiched between two PET films (total of four) with a thickness of 188 μm, a length of 25 mm, and a width of 50 mm. The area sandwiched between the two PET films was used as the chuck of the tensile tester, and the chuck distance was fixed at 30 mm. The specimen was then pulled at a tensile speed of 300 mm/min under an environment of a measurement temperature of 23° C. and a relative humidity of 50% until the tensile elongation reached 500%. The stress value at this time was designated as S(HPML).

<S(LED); 500% tensile stress>
The double-sided pressure-sensitive adhesive sheet with release sheet obtained in the examples and comparative examples was cut out to a length of 50 mm and a width (width direction) of A mm separately from the measurement of S (HPML). At this time, the value of A in the width direction was determined to be the thickness (mm) of the pressure-sensitive adhesive sheet x A mm = 6 ^mm2 . Next, the sheet was post-cured by irradiating light using an LED lamp with an illuminance of 160 mW/ ^cm2 and a wavelength of 365 nm so that the accumulated light amount was 3000 mJ/ ^cm2 . Thereafter, the first release sheet was peeled off, and only the pressure-sensitive adhesive sheet was peeled off from the second release sheet while rolling it in the width direction to obtain a cylindrical sample with a circle diameter of 2.8 mm and a height of 50 mm. The regions up to 10 mm from the upper and lower ends of the cylindrical sample were sandwiched between two PET films (total of four) with a thickness of 188 μm, a length of 25 mm, and a width of 50 mm. The area sandwiched between the two PET films was used as the chuck of the tensile tester, and the chuck distance was fixed at 30 mm. The specimen was then pulled at a tensile speed of 300 mm/min under an environment of a measurement temperature of 23° C. and a relative humidity of 50% until the tensile elongation reached 500%. The stress value at this time was designated as S(LED).

<Gel fraction (initial)>
The adhesive sheet was peeled off from the double-sided adhesive sheet with release sheet obtained in the Examples and Comparative Examples, and about 0.1 g of the adhesive sheet (only the adhesive layer) was collected in a sample bottle, to which 30 ml of ethyl acetate was added and shaken for 24 hours. The contents of the sample bottle were then filtered through a 150 mesh stainless steel wire screen, and the residue on the wire screen was dried at 100°C for 1 hour to measure the dry weight W (g). From the obtained dry weight, the following formula was calculated: gel fraction (%) = (dry mass W / collected mass of adhesive sheet) x 100
The value calculated from the above was taken as the gel fraction (initial).

<Gel fraction (after post-curing)>
In the measurement of S(HPML) described above, the gel fraction (after post-curing) was measured in the same manner as the gel fraction (initial), except that about 0.1 g of the post-cured PSA sheet was placed in a sample bottle.

<Gel fraction (after UV exposure)>
The gel fraction (after UV exposure) was measured in the same manner as for the gel fraction (initial), except that a QUV accelerated weather resistance tester manufactured by Q-Lab was used to measure the gel fraction (after UV exposure) by irradiating the second release sheet side of the pressure-sensitive adhesive sheet with UVA light of 0.77 W ^m2 at 60°C for 16 hours and then collecting about 0.1 g of the pressure-sensitive adhesive sheet in a sample bottle.

<Evaluation of step conformability (after post-curing)>
On the surface of a glass plate (length 90 mm x width 50 mm x thickness 0.5 mm), ultraviolet-curable ink was screen-printed in a frame shape (length 90 mm x width 50 mm, width 5 mm) so that the coating thickness was 5 μm. Then, the printed ultraviolet-curable ink was irradiated with ultraviolet light to cure. This process was repeated a predetermined number of times to obtain a printed stepped glass (first adherend) having a stepped portion with a thickness of 50 μm. Next, a second adherend was produced by the following procedure. First, a polarizing plate with an adhesive layer (polarizing plate manufactured by Samsung SDI: "2144") having a total thickness of 80 μm including the polarizing plate and the paste was cut into a length of 90 mm x width of 50 mm. Next, a circular hole with a diameter of 4 mm was made at a position 15 mm from the top end and 10 mm from the left end, and then the glass plate with a hole (length 90 mm x width 50 mm x thickness 0.5 mm) was attached to another glass plate from the above to obtain a glass plate with a polarizing plate (second adherend).

Next, the double-sided pressure-sensitive adhesive sheet with release sheet obtained in the examples and comparative examples was cut into a shape of 90 mm long x 50 mm wide, the first release sheet was peeled off, and the pressure-sensitive adhesive sheet (adhesive layer) was attached to the entire surface of the first adherend using a laminator (IKO-650EMT, manufactured by Yubon Co., Ltd.). Thereafter, the second release sheet was peeled off, and the polarizing plate side of a glass plate with a hole (90 mm long x 50 mm wide x 0.5 mm thick) was attached to the exposed pressure-sensitive adhesive sheet (adhesive layer) using a vacuum laminator (JE2020B-MVH, manufactured by Joyo Engineering Co., Ltd.). The lamination conditions at this time were 40°C, weak pressure of 0.6 kN, strong pressure of 1.2 kN, vacuum pressure of 100 Pa, and pressure retention time of 10 seconds. Next, a degassing treatment (autoclave treatment: 60°C, 0.5 MPa, 30 minutes) was performed. Thereafter, ultraviolet light was irradiated from the printed step glass side using a high pressure mercury lamp with an illuminance of 160 mW/ ^cm2 so that the cumulative light amount was 3000 mJ/ ^cm2 to obtain a laminate. The printed step portion of the laminate and the hole portion of the polarizing plate (collectively referred to as the step portion) were observed under a microscope (magnification: 25 times) and evaluated according to the following criteria.
◯: No air bubbles were observed in the stepped portion, and the stepped portion was completely filled.
×: Air bubbles were observed in the stepped portion, and the stepped portion was not filled.

<Stress relaxation evaluation (after post-curing)>
The laminate produced in the step conformability evaluation (after post-curing) was placed in a thermal shock tester and subjected to 100 cycles of 30 minutes at 80° C. and 30 minutes at −40° C. The step portion of the laminate, mainly the second adherend, was observed under a microscope (magnification: 25 times) and evaluated according to the following criteria.
◯: No air bubbles were observed in the stepped portion, and the stepped portion was completely filled.
×: Air bubbles were observed in the stepped portion, and the stepped portion was not filled.

<Stress relaxation evaluation (after UV exposure)>
The laminate prepared for the step followability evaluation (after post-curing) was placed in a QUV tester (manufactured by Q-LAB) and irradiated with ultraviolet light (UV-A) at 0.77 W/m ² and 60° C. for 16 hours from the printed step glass side, and then the laminate was placed in a thermal shock tester and treated for 100 cycles of 30 minutes at 80° C. and 30 minutes at −40° C. The step portions of the laminate, mainly of the second adherend, which had been subjected to the thermal shock test after ultraviolet exposure in this manner, were observed under a microscope (magnification: 25x) and evaluated according to the following criteria.
◯: No air bubbles were observed in the stepped portion, and the stepped portion was completely filled.
×: Air bubbles were observed in the stepped portion, and the stepped portion was not filled.

<Quality variation>
◯: The R value was within the range of 0.8 to 1.2, and the PSA sheet was capable of providing products with stable quality regardless of the type of lamp.
x: The value of R was outside the range of 0.8 to 1.2, and the adhesive sheet caused quality variations depending on the type of lamp.

Table 1 shows the synthesis conditions (types of components and their blending conditions) and evaluation results for the adhesive sheets of each Example and Comparative Example. In the table, the blending amount of each component is expressed in parts by mass, and the gel fraction in %.

The adhesive sheet obtained in the examples had each component blended in a specified amount, and the value of R = S (LED) / S (HTML) was in the range of 0.8 to 1.2, and the curing properties were stable regardless of the type of lamp, and the adhesive sheet had excellent step-following properties and stress relaxation properties. In particular, the adhesive sheet had excellent stress relaxation properties even after exposure to ultraviolet light.

In contrast, Comparative Example 1 contained too much crosslinking agent (B), so the value of R did not fall within the specified range, resulting in a large variation in the quality of the product cured with high-pressure mercury and LED, and also had poor stress relaxation properties. Comparative Example 2 contained too much hydrogen abstraction type photoinitiator (C), so the value of R fell within the specified range, but the gel fraction became too high after exposure to ultraviolet light, resulting in a decrease in stress relaxation properties after exposure to ultraviolet light. Comparative Example 3 did not contain hydrogen abstraction type photoinitiator (C), so curing was insufficient and bubbles were generated due to insufficient cohesion. Comparative Example 4 contained too much abstraction type photoinitiator (C), so the gel fraction became too high after exposure to ultraviolet light, resulting in a decrease in stress relaxation properties after exposure to ultraviolet light. Comparative Example 5 contained too little self-cleavage type photoradical initiator (D), so it did not cure sufficiently, causing a decrease in adhesion, etc., and the value of R also became small, resulting in a large variation in the quality of the product cured with high-pressure mercury and LED.

Claims (5)

A (meth)acrylic copolymer (A),
A crosslinking agent (B) having two or more acryloyl groups;
A hydrogen abstraction type photoinitiator (C);
and a self-cleavage type photoradical initiator (D),
The hydrogen abstraction type photoinitiator (C) is at least one selected from the group consisting of benzophenone, benzoylbenzoic acid, methyl benzoylbenzoate, 4-phenylbenzophenone, hydroxybenzophenone, 3,3'-dimethyl-4-methoxybenzophenone, 2,4,6-trimethylbenzophenone, 4-methylbenzophenone, thioxanthone, 2-chlorothioxanthone, 2-methylthioxanthone, 2,4-dimethylthioxanthone, isopropylthioxanthone, camphorquinone, dibenzosuberone, 2-ethylanthraquinone, 3,3',4,4'-tetra(t-butylperoxycarbonyl)benzophenone, and 9,10-phenanthrenequinone;
The self-cleavage type photoradical initiator (D) has an absorption wavelength peak in the wavelength range of 220 to 450 nm,
The crosslinking agent (B) is contained in an amount of less than 5 parts by mass relative to 100 parts by mass of the (meth)acrylic copolymer (A),
the hydrogen abstraction type photoinitiator (C) is contained in an amount of 0.3 parts by mass or less based on 100 parts by mass of the (meth)acrylic copolymer (A);
The self-cleavage type photoradical initiator (D) is contained in an amount of 0.2 parts by mass or more per 100 parts by mass of the (meth)acrylic copolymer (A),
The following formula (1)
R=S(LED)/S(HTML) (1)
(Here, S(HPML) indicates the stress value [N/mm2] when an adhesive sheet is irradiated with light from a high-pressure mercury lamp with an illuminance of 160 mW/ ^cm2 to an accumulated light amount of 3000 mJ/ ^cm2 , and then pulled at a speed of 300 mm/min until the tensile elongation reaches 500%, and S(LED) indicates the stress value [N/ ^mm2 ] when an adhesive sheet is irradiated with light from an LED lamp with an illuminance of 160 mW/ ^cm2 and a wavelength of 365 nm to an accumulated light amount of 3000 mJ/ ^cm2 , and then pulled at a speed of ³⁰⁰ mm/min until the tensile elongation reaches 500%)
The pressure-sensitive adhesive sheet has an R value of 0.8 to 1.2. The adhesive sheet according to claim 1, wherein the (meth)acrylic copolymer (A) has a glass transition temperature of -50°C to -40°C and a mass average molecular weight of 250,000 to 450,000. The pressure-sensitive adhesive sheet according to claim 1 or 2, wherein the (meth)acrylic copolymer (A) does not contain an acryloyl group in its structural units. The adhesive sheet according to any one of claims 1 to 3, which does not contain a thermal crosslinking agent. A laminate comprising the adhesive sheet according to any one of claims 1 to 4.