JP7537264B2 - Adhesive composition, adhesive, and adhesive sheet - Google Patents

Description

The present invention relates to an adhesive composition, an adhesive obtained by crosslinking the adhesive composition, and an adhesive sheet having an adhesive layer made of the adhesive. The adhesive composition of the present invention can be suitably used as an adhesive for a multi-stage curing adhesive sheet that is cured in multiple stages, and excellent adhesive properties can be obtained even in a low crosslinking state after primary curing.

In recent years, touch panels that combine a display and a position input device have come into widespread use in mobile devices such as televisions, PC monitors, laptops, mobile phones, smartphones, and tablet terminals, and capacitive touch panels in particular are on the rise.
A touch panel is usually composed of a display made of an organic electroluminescence (EL) or liquid crystal, a transparent conductive film substrate (ITO substrate), and a protective film (protective glass), and a transparent adhesive sheet is used to bond these components together.

Since transparent adhesive sheets require bonding components of various shapes, the adhesive for the transparent adhesive sheet is bonded in a low-crosslinking state before it is completely cured, and sufficient adhesive properties are required in the process until the adhesive layer is completely cured.
Specifically, when bonding to a component with a complex shape, bonding defects such as the transparent adhesive sheet sticking to a place other than the intended place are likely to occur, so a transparent adhesive sheet with low tackiness is required even in a low crosslinking state. Furthermore, in order to fix components with complex shapes to each other, it is required to suppress peeling of the transparent adhesive sheet from the component when stress is applied. Therefore, a transparent adhesive sheet with high constant load holding power is required even in a low crosslinking state.
Therefore, adhesives for transparent adhesive sheets are required to have not only adhesive properties such as adhesive strength after complete curing, but also excellent adhesive properties in a low crosslinked state after primary curing. Generally, primary curing is performed by crosslinking through heat or irradiation with active energy rays, and complete curing is performed by crosslinking and curing through irradiation with active energy rays.
Examples of pressure-sensitive adhesive sheets using such multi-stage curing pressure-sensitive adhesives include the pressure-sensitive adhesive sheets described in Patent Documents 1 to 3.

Patent Document 1 discloses that by further using an organic solvent that is easily volatilized under general drying conditions in a solvent-based adhesive made of an acrylic resin, and blending an ethylenically unsaturated monomer that is less likely to volatilize in a specific ratio, it is possible to apply a thick coating and obtain an adhesive layer with a clean coating surface.
Furthermore, Patent Document 2 discloses that in order to form a three-dimensional network structure of a solvent-based acrylate-based pressure-sensitive adhesive without using a crosslinking agent, a hydrogen abstraction photoinitiator is used and light irradiation is performed after the coating and drying process, thereby eliminating the aging process.
Furthermore, Patent Document 3 discloses that a pressure-sensitive adhesive having excellent step conformability and blister resistance can be obtained by using an acrylic resin having a high glass transition temperature.
However, none of Patent Documents 1 to 3 takes into consideration the adhesive properties in a low crosslinking state during primary curing.

JP 2012-111939 A JP 2017-210542 A International Publication No. 2017/022770

The problem that the present invention aims to solve is to provide an adhesive composition that can be used for multi-stage curing and that provides excellent adhesive properties even in a low crosslinked state after primary curing, and further to provide an adhesive obtained by crosslinking the adhesive composition, and an adhesive sheet having an adhesive layer made of the adhesive.

In order to solve the above problems, the present inventors conducted intensive research and found that by using a pressure-sensitive adhesive composition containing an acrylic resin obtained by using copolymer components of a specific composition, excellent adhesive properties such as low tackiness and high constant load retention can be obtained even in a low crosslinked state after primary curing.
Specifically, the present invention has the following configuration.

(1) A pressure-sensitive adhesive composition comprising an acrylic resin (A) and a crosslinking agent (B),
the acrylic resin (A) is a copolymer obtained by polymerizing a copolymerization component (a) containing 10 to 30 % by weight of an alkyl acrylate (a1) having a glass transition temperature of −27 to 50° C. when a homopolymer is formed, 10 to 40 % by weight of an alkyl methacrylate (a2) having a glass transition temperature of 0 to 100° C. when a homopolymer is formed , 10 to 30% by weight of a hydroxyl group-containing (meth)acrylic acid ester monomer (a3 ), and 35 to 60% by weight of 2-ethylhexyl acrylate, and the content ratio (a1/a2) of the alkyl acrylate (a1) to the alkyl methacrylate (a2) is 20/80 to 55/45 by weight,
A pressure-sensitive adhesive composition, characterized in that the acrylic resin (A) has a glass transition temperature of -10 to 15°C.

(2) An adhesive characterized in that the above adhesive composition is crosslinked.

(3) An adhesive sheet having an adhesive layer made of the above adhesive.

The adhesive composition of the present invention has excellent adhesive properties, such as low tackiness even in a low crosslinking state after primary curing and high constant load retention, improving workability and reliability. For this reason, it is particularly useful as an adhesive or adhesive sheet for use in touch panels and image display devices.

The present invention will be described in detail below, but these are merely examples of preferred embodiments.
In the present invention, "(meth)acrylic" means acrylic or methacrylic, "(meth)acryloyl" means acryloyl or methacryloyl, and "(meth)acrylate" means acrylate or methacrylate, respectively, and "acrylic resin" means a resin obtained by polymerizing a monomer component containing at least one (meth)acrylic monomer. Furthermore, "sheet" conceptually includes a sheet, a film, and a tape.

<Adhesive Composition>
The pressure-sensitive adhesive composition of the present invention contains an acrylic resin (A) and a crosslinking agent (B).
The acrylic resin (A) is a copolymer obtained by polymerizing a copolymerization component (a) containing at least an alkyl acrylate (a1), an alkyl methacrylate (a2), and a hydroxyl group-containing monomer (a3).
Each component will be described below.

[Alkyl acrylate (a1)]
The alkyl acrylate (a1) has a glass transition temperature (hereinafter referred to as "Tg") of -27 to 50° C., preferably -10 to 45° C., and more preferably -5 to 10° C. When the Tg is within this range, the effects of the present invention can be obtained.
The Tg of the alkyl acrylate (a1) is the glass transition temperature of a homopolymer made of the alkyl acrylate (a1), and a standard analytical value described in the Wiley publication "POLYMER HANDBOOK" or the like can be used.

Examples of such alkyl acrylates (a1) include methyl acrylate (Tg = 8°C), ethyl acrylate (Tg = -22°C), isobutyl acrylate (Tg = -26°C), t-butyl acrylate (Tg = 41°C), and cyclohexyl acrylate (Tg = 15°C). Among these, methyl acrylate is preferred from the viewpoint of adhesive properties. One selected from these may be used alone, or two or more may be used in combination.

The content of the alkyl acrylate (a1) in the copolymerization component (a) is 5% by weight or more, preferably 8 to 50% by weight, more preferably 10 to 30% by weight, particularly preferably 12 to 28% by weight, and especially preferably 15 to 25% by weight.
If the content is too low, the adhesive properties in the primary cured state tend to decrease, whereas if the content is too high, the adhesive properties after complete curing tend to decrease.

[Alkyl methacrylate (a2)]
The alkyl methacrylate (a2) has a Tg of 0 to 100° C., preferably 20 to 85° C., and more preferably 40 to 70° C. When the Tg is within this range, the effects of the present invention can be obtained.
The Tg of the alkyl methacrylate (a2) is the glass transition temperature of a homopolymer made of the alkyl methacrylate (a2), and standard analytical values described in the Wiley publication "POLYMER HANDBOOK" or the like can be used.

Examples of such alkyl methacrylates (a2) include ethyl methacrylate (Tg = 65°C), n-butyl methacrylate (Tg = 20°C), isobutyl methacrylate (Tg = 48°C), cyclohexyl methacrylate (Tg = 66°C), etc. One selected from these may be used alone, or two or more may be used in combination. Of these, ethyl methacrylate and isobutyl methacrylate are preferred.

The content of the alkyl methacrylate (a2) in the copolymerization component (a) is 10% by weight or more, preferably 12 to 40% by weight, and more preferably 15 to 30% by weight. If the content is too low, the adhesive properties in the primary cured state tend to decrease, and if the content is too high, the adhesive properties after complete curing tend to decrease.

In the copolymerization component (a) used in the present invention, the content ratio (a1/a2) of the alkyl acrylate (a1) and the alkyl methacrylate (a2) is preferably 20/80 to 55/45, more preferably 30/70 to 52/48, and particularly preferably 35/65 to 50/50, by weight.
When the content ratio of (a1/a2) is within the above range, the adhesive properties in the primary cured state are excellent.

The total content of the alkyl acrylate (a1) and the alkyl methacrylate (a2) in the copolymerization component (a) is preferably 30 to 70% by weight, more preferably 38 to 65% by weight, and particularly preferably 40 to 60% by weight.
If the total content of both monomers is within the above range, the adhesive properties in a low crosslinked state in the primary curing state become good, which is preferable.

[Hydroxyl Group-Containing (Meth)Acrylic Acid Ester Monomer (a3)]
The hydroxyl group-containing (meth)acrylic acid ester monomer is a (meth)acrylic acid ester having one or more hydroxyl groups. Examples of such monomer (a3) include hydroxyalkyl acrylates such as 2-hydroxyethyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, 5-hydroxypentyl (meth)acrylate, 6-hydroxyhexyl (meth)acrylate, and 8-hydroxyoctyl (meth)acrylate; caprolactone-modified monomers such as caprolactone-modified 2-hydroxyethyl (meth)acrylate; diethylene glycol ( Examples of the monomers include oxyalkylene-modified monomers such as 2-acryloyloxyethyl-2-hydroxyethylphthalate, N-methylol(meth)acrylamide, and hydroxyethylacrylamide; secondary hydroxyl group-containing monomers such as 2-hydroxypropyl(meth)acrylate, 2-hydroxybutyl(meth)acrylate, and 3-chloro2-hydroxypropyl(meth)acrylate; and tertiary hydroxyl group-containing monomers such as 2,2-dimethyl2-hydroxyethyl(meth)acrylate. Among these, 2-hydroxyethyl(meth)acrylate and 4-hydroxybutyl(meth)acrylate are preferred, and 2-hydroxyethyl(meth)acrylate is particularly preferred, in that they contain less impurities such as di(meth)acrylates and are easy to produce. One selected from these may be used alone, or two or more may be used in combination.

The content of the hydroxyl group-containing (meth)acrylic acid ester monomer (a3) in the copolymerization component (a) is preferably 5% by weight or more, more preferably 10 to 40% by weight, and particularly preferably 12 to 30% by weight. If the content is too low, the moist heat resistance tends to decrease, and if the content is too high, the acrylic resin is more likely to undergo a self-crosslinking reaction, which tends to decrease the heat resistance.

In addition, the hydroxyl group-containing (meth)acrylic acid ester monomer (a3) used in the present invention is preferably one having a lower content of di(meth)acrylate contained as an impurity in the hydroxyl group-containing (meth)acrylic acid ester monomer (a3). Specifically, it is preferable to use a hydroxyl group-containing (meth)acrylic acid ester monomer (a3) having a di(meth)acrylate content of 0.5% by weight or less, more preferably 0.2% by weight or less, particularly preferably 0.1% by weight or less, and even more preferably 0% by weight.

In addition, hydroxyl group-containing (meth)acrylic acid ester monomer (a3) often contains acrylic acid as an impurity, and the content of acrylic acid in the hydroxyl group-containing (meth)acrylic acid ester monomer (a3) is usually about 0.001 to 0.5% by weight, and it is preferable to use a smaller amount.

[(Meth)acrylic acid ester monomer (a4) containing an active energy ray-curable structure]
The acrylic resin (A) used in the present invention may also have an active energy ray crosslinkable structural site.
The active energy ray crosslinkable structural moiety is a structural moiety that can react with a part of the acrylic resin (A) or other curing components contained in the pressure-sensitive adhesive composition upon irradiation with active energy rays to form a crosslinked structure.
In the present invention, the active energy ray crosslinkable structural moiety is preferably a benzophenone-based crosslinkable structural moiety, which has high reactivity and is excellent in improving cohesive strength.

Therefore, in the present invention, it is preferred that the copolymerization component (a) for polymerizing the acrylic resin (A) further contains an active energy ray-crosslinkable structural moiety-containing (meth)acrylic acid ester monomer (a4).
As the (meth)acrylic acid ester monomer (a4) having an active energy ray crosslinkable structural moiety, a (meth)acrylic acid ester monomer having a benzophenone-based crosslinkable structural moiety is preferred in that it can form an efficient crosslinked structure by active energy rays such as ultraviolet rays and electron beams, and examples thereof include 4-(meth)acryloyloxybenzophenone.

The content of the (meth)acrylic acid ester monomer (a4) containing an active energy ray crosslinkable structure is preferably 0.01 to 5% by weight in the copolymerization component (a), and the content of the (meth)acrylic acid ester monomer having a benzophenone-based crosslinkable structure is preferably 0.01 to 5% by weight in the copolymerization component (a), more preferably 0.1 to 2% by weight, and even more preferably 0.2 to 1% by weight. If the content is too small, the holding power when forming a crosslinked structure by active energy rays tends to decrease, and further, when forming a crosslinked structure to produce a processable adhesive sheet, a large amount of active energy radiation is required, and a large amount of energy is required when producing the adhesive sheet, which tends to make efficient production difficult. Also, if the content is too large, the cohesive force of the entire system tends to increase too much, and the adhesive strength tends to decrease.

In addition, when introducing an active energy ray crosslinkable structural moiety into the acrylic resin (A), in addition to copolymerizing the (meth)acrylic acid ester monomer (a4) containing an active energy ray crosslinkable structural moiety, it is also possible to introduce an ethylenically unsaturated group as an active energy ray crosslinkable structural moiety by incorporating a hydroxyl group in the acrylic resin (A) and reacting the hydroxyl group with an isocyanate compound containing an ethylenically unsaturated group.

[Ethylenically unsaturated monomer (a5)]
In the present invention, the copolymerization component (a) may further contain, if necessary, a copolymerizable ethylenically unsaturated monomer (a5) other than the monomers (a1) to (a4).
Examples of other copolymerizable ethylenically unsaturated monomers (a5) include alkyl (meth)acrylates such as n-butyl acrylate and 2-ethylhexyl (meth)acrylate (excluding monomers (a1) and (a2)); aromatic ring monomers such as phenyl (meth)acrylate, benzyl (meth)acrylate, phenoxyethyl (meth)acrylate, phenyl diethylene glycol (meth)acrylate, phenoxy polyethylene glycol (meth)acrylate, phenoxy polyethylene glycol-polypropylene glycol-(meth)acrylate, orthophenylphenoxyethyl (meth)acrylate, and nonylphenol ethylene oxide adduct (meth)acrylate. Monomers containing: cyclohexyloxyalkyl (meth)acrylate, tert-butylcyclohexyloxyethyl (meth)acrylate, isobornyl (meth)acrylate, dicyclopentanyl (meth)acrylate, and other alicyclic monomers; 2-methoxyethyl (meth)acrylate, 2-ethoxyethyl (meth)acrylate, 3-methoxybutyl (meth)acrylate, 2-butoxyethyl (meth)acrylate, 2-butoxydiethylene glycol (meth)acrylate, methoxydiethylene glycol (meth)acrylate, methoxytriethylene glycol (meth)acrylate, ethoxydiethylene glycol (meth)acrylate, methoxydipropylene glycol (meth)acrylate, Ether chain-containing monomers such as (meth)acrylate, methoxypolyethylene glycol (meth)acrylate, octoxypolyethylene glycol-polypropylene glycol mono(meth)acrylate, lauroxypolyethylene glycol mono(meth)acrylate, and stearoxypolyethylene glycol mono(meth)acrylate; carboxy group-containing monomers such as (meth)acrylic acid, acrylic acid dimers such as β-carboxyethyl acrylate, crotonic acid, maleic acid, maleic anhydride, fumaric acid, citraconic acid, glutaconic acid, itaconic acid, N-glycolic acid, and cinnamic acid; (meth)acrylamide, N-(n-butoxyalkyl)(meth)acrylamide, N,N-dimethyl(meth)acrylamide, N ... ) Amide group-containing monomers such as acrylamide, N,N-diethyl(meth)acrylamide, and N,N-dimethylaminoalkyl(meth)acrylamide; and other monomers such as acrylonitrile, methacrylonitrile, styrene, α-methylstyrene, vinyl acetate, vinyl propionate, vinyl stearate, vinyl chloride, vinylidene chloride, alkyl vinyl ethers, vinyl toluene, vinyl pyridine, vinyl pyrrolidone, itaconic acid dialkyl esters, fumaric acid dialkyl esters, allyl alcohol, acrylic chloride, methyl vinyl ketone, N-acrylamidomethyltrimethylammonium chloride, allyltrimethylammonium chloride, and dimethylallyl vinyl ketone.
One selected from these may be used alone, or two or more may be used in combination.

In addition, when the purpose is to increase the molecular weight of the acrylic resin (A), compounds having two or more ethylenically unsaturated groups, such as ethylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate, triethylene glycol di(meth)acrylate, polyethylene glycol di(meth)acrylate, propylene glycol di(meth)acrylate, and divinylbenzene, can also be used in combination.

The content of the other copolymerizable ethylenically unsaturated monomer (a5) in the copolymerization component (a) is usually 60% by weight or less, preferably 48% by weight or less, and more preferably 38% by weight or less.
If the content is too high, the adhesive properties at low crosslinking levels tend to decrease.

[Acrylic resin (A)]
The acrylic resin (A) used in the present invention can be produced by copolymerizing the copolymerization component (a) containing the above-mentioned alkyl acrylate (a1), alkyl methacrylate (a2), and hydroxyl group-containing (meth)acrylic acid ester monomer (a3) as essential components, and further containing the above-mentioned optional polymerization components (a4) and (a5), as necessary.

As the polymerization method for the acrylic resin (A), for example, a conventionally known polymerization method such as solution polymerization, suspension polymerization, bulk polymerization, emulsion polymerization, etc. can be used. In the present invention, however, production by solution polymerization is preferred in that the acrylic resin (A) can be safely and stably produced with any monomer composition.
An example of a preferred method for producing the acrylic resin (A) used in the present invention will be described below.

First, copolymerization component (a) and a polymerization initiator are mixed or dropped into an organic solvent to carry out solution polymerization.

Examples of organic solvents used in the polymerization reaction include aromatic hydrocarbons such as toluene and xylene; aliphatic hydrocarbons such as n-hexane; esters such as methyl acetate, ethyl acetate, and butyl acetate; aliphatic alcohols such as methyl alcohol, ethyl alcohol, n-propyl alcohol, and isopropyl alcohol; ketones such as acetone, methyl ethyl ketone, methyl isobutyl ketone, and cyclohexanone; aliphatic ethers such as dimethyl ether and diethyl ether; aliphatic halogenated hydrocarbons such as methylene chloride and ethylene chloride; and cyclic ethers such as tetrahydrofuran. One selected from these may be used alone, or two or more may be used in combination. Among these solvents, esters and ketones are preferred, and ethyl acetate and acetone are particularly preferred.

As the polymerization initiator used in the above polymerization reaction, azo-based polymerization initiators and peroxide-based polymerization initiators, which are ordinary radical polymerization initiators, can be used.
Examples of the azo polymerization initiator include 2,2'-azobis(2-methylbutyronitrile), 2,2'-azobisisobutyronitrile, (1-phenylethyl)azodiphenylmethane, 2,2'-azobis(2,4-dimethylvaleronitrile), 2,2'-azobis(2-cyclopropylpropionitrile), and 2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile).
Examples of the peroxide polymerization initiator include benzoyl peroxide, di-tert-butyl peroxide, cumene hydroperoxide, lauroyl peroxide, tert-butyl peroxypivalate, tert-hexyl peroxypivalate, tert-hexyl peroxyneodecanoate, diisopropyl peroxycarbonate, and diisobutyryl peroxide.
One selected from these may be used alone, or two or more may be used in combination.
Of these, azo-based polymerization initiators are preferred, and 2,2'-azobisisobutyronitrile and 2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile) are more preferred.

The amount of the polymerization initiator used is usually 0.001 to 10 parts by weight, preferably 0.1 to 8 parts by weight, more preferably 0.5 to 6 parts by weight, particularly preferably 1 to 4 parts by weight, even more preferably 1.5 to 3 parts by weight, and most preferably 2 to 2.5 parts by weight, per 100 parts by weight of the copolymerization component (a). If the amount of the polymerization initiator used is too small, the polymerization rate of the acrylic resin (A) tends to decrease, the amount of residual monomers tends to increase, and the weight average molecular weight of the acrylic resin (A) tends to increase, while if the amount used is too large, the acrylic resin (A) tends to gel.

The polymerization conditions for the solution polymerization are not particularly limited, and the polymerization can be carried out according to conventionally known polymerization conditions. For example, the copolymerization component (a) and the polymerization initiator can be mixed or dropped into the organic solvent, and the polymerization can be carried out under the specified polymerization conditions.

The polymerization temperature in the above polymerization reaction is usually 40 to 120°C, but in the present invention, 50 to 90°C is preferred in terms of ensuring a stable reaction. If the polymerization temperature is too high, the acrylic resin (A) tends to gel easily, and if it is too low, the activity of the polymerization initiator decreases, so the polymerization rate decreases and the amount of residual monomers tends to increase.

The polymerization time in the polymerization reaction is not particularly limited, but is preferably 0.5 hours or more, preferably 1 hour or more, more preferably 2 hours or more, and particularly preferably 5 hours or more from the final addition of the polymerization initiator.
The polymerization reaction is preferably carried out while refluxing the solvent in order to facilitate heat removal.
In this manner, the acrylic resin (A) used in the present invention can be produced.

As described above, the acrylic resin (A) used in the present invention can be produced, and the weight-average molecular weight of the acrylic resin (A) is preferably from 10,000 to 1,000,000, more preferably from 50,000 to 400,000, particularly preferably from 100,000 to 300,000, and even more preferably from 150,000 to 280,000. If the weight-average molecular weight is too large, the viscosity becomes too high, and the coatability and handling properties tend to decrease, whereas if the weight-average molecular weight is too small, the cohesive force decreases, and the adhesive properties tend to decrease.
The weight average molecular weight of the acrylic resin (A) is the weight average molecular weight at the time of completion of production, that is, the weight average molecular weight of the acrylic resin (A) that has not been subjected to heating or the like after production.

The degree of dispersion (weight average molecular weight/number average molecular weight) of the acrylic resin (A) is preferably 15 or less, more preferably 10 or less, particularly preferably 7 or less, and even more preferably 5 or less. If the degree of dispersion is too high, the durability of the adhesive layer tends to decrease and foaming and the like tends to occur easily, and if the degree of dispersion is too low, the handleability tends to decrease. The lower limit of the degree of dispersion is usually 1.1 in view of the limitations of production.

The weight average molecular weight is a weight average molecular weight based on standard polystyrene molecular weight conversion, and is measured by using three columns in series: Shodex GPC KF-806L (exclusion limit molecular weight: 2×10 ⁷ , separation range: 100 to 2×10 ⁷ , theoretical plate number: 10,000 plates/column, packing material: styrene-divinylbenzene copolymer, packing particle size: 10 μm) in a high performance liquid chromatograph (Waters Japan, "Waters 2695 (main body)" and "Waters 2414 (detector)"), and the number average molecular weight can also be measured using the same method. The degree of dispersion is determined from the weight average molecular weight and the number average molecular weight.

The acrylic resin (A) used in the present invention has a glass transition temperature (Tg) of -10 to 15°C, preferably -7 to 12°C, more preferably -5 to 10°C, and particularly preferably 0 to 8°C. If the glass transition temperature is too high, the adhesive strength tends to decrease due to a decrease in the step-following ability and adhesion of the adhesive layer. If the glass transition temperature is too low, the adhesive properties at low cross-linking tend to decrease.

The glass transition temperature (Tg) of the acrylic resin (A) is determined by the following measurement method.
The release sheet is peeled off from the pressure-sensitive adhesive sheet before irradiation with active energy rays, which will be described later, and a plurality of pressure-sensitive adhesive sheets are laminated to prepare a pressure-sensitive adhesive sheet having a thickness of about 800 μm in an uncrosslinked state. The dynamic viscoelasticity of the prepared sheet is measured under the following conditions, and the temperature at which the loss tangent (loss modulus G″/storage modulus G′=tan δ) becomes maximum is read and regarded as the glass transition temperature (Tg) of the acrylic resin (A).

(Measurement conditions)
Measuring equipment: Dynamic viscoelasticity measuring device (product name: DVA-225, manufactured by IT Measurement and Control Co., Ltd.)
Deformation mode: shear Strain: 0.1%
Measurement temperature: -100~60℃
Measurement frequency: 1Hz

The adhesive composition of the present invention preferably contains the acrylic resin (A) in an amount of 80% by weight or more, more preferably 90 to 99.9% by weight, and particularly preferably 92 to 99.9% by weight, based on the total weight of the adhesive composition.

[Crosslinking agent (B)]
The pressure-sensitive adhesive composition of the present invention contains a crosslinking agent (B). As the crosslinking agent (B), it is preferable to contain an active energy ray crosslinking agent (B1) such as a monofunctional crosslinking agent or a polyfunctional crosslinking agent and/or a thermal crosslinking agent (B2).
When the crosslinking agent (B) contains only the active energy ray crosslinking agent (B1), multi-stage curing is possible by controlling the amount of active energy rays, and when the crosslinking agent (B) contains the active energy ray crosslinking agent (B1) and the heat curing agent (B2), multi-stage curing is possible by using heat curing and active energy ray curing in combination. By controlling the crosslinking reaction in this way, the cohesive force of the entire pressure-sensitive adhesive layer can be adjusted, and stable adhesive properties can be obtained after primary curing or complete curing.

(Active Energy Ray Crosslinking Agent (B1))
Examples of the active energy ray crosslinking agent (B1) include a monofunctional crosslinking agent containing one ethylenically unsaturated group in one molecule, and a polyfunctional crosslinking agent (B1-1) containing two or more ethylenically unsaturated groups in one molecule, and the polyfunctional crosslinking agent (B1-1) is particularly preferred.

Examples of the polyfunctional crosslinking agent (B1-1) include hexanediol di(meth)acrylate, butanediol di(meth)acrylate, 1,9-nonanediol di(meth)acrylate, 1,10-decanediol di(meth)acrylate, (poly)ethylene glycol mono(meth)acrylate, (poly)butylene glycol mono(meth)acrylate, (poly)ethylene glycol di(meth)acrylate, (poly)propylene glycol di(meth)acrylate, (poly)tetramethylene glycol di(meth)acrylate, (poly)pentamethylene glycol di(meth)acrylate, and (poly)hexamethylene glycol di(meth)acrylate. acrylate, neopentyl glycol di(meth)acrylate, pentaerythritol di(meth)acrylate, pentaerythritol tri(meth)acrylate, dipentaerythritol hexa(meth)acrylate, trimethylolpropane tri(meth)acrylate, EO-modified trimethylolpropane tri(meth)acrylate, glycerin tri(meth)acrylate, EO-modified glycerin tri(meth)acrylate, tetramethylolmethane tri(meth)acrylate, isocyanuric acid ethylene oxide-modified tri(meth)acrylate, allyl (meth)acrylate, vinyl (meth)acrylate, and urethane (meth)acrylate.
The polyfunctional crosslinking agents (B1-1) may be used alone or in combination of two or more. Among them, in terms of the balance of adhesive properties after curing, (meth)acrylates containing two ethylenically unsaturated groups are preferred, and (poly)ethylene glycol di(meth)acrylate, (poly)propylene glycol di(meth)acrylate, and (poly)tetramethylene glycol di(meth)acrylate are particularly preferred.

The content of the active energy ray crosslinking agent (B1) is usually preferably 0.01 to 20 parts by weight, more preferably 0.1 to 10 parts by weight, and particularly preferably 0.5 to 7.5 parts by weight, based on 100 parts by weight of the acrylic resin (A).
If the amount of the active energy ray crosslinking agent (B1) is too small, the cohesive strength tends to be insufficient and sufficient durability tends to be difficult to obtain, whereas if the amount is too large, the adhesive properties during primary curing tend to decrease.

(Thermal Crosslinking Agent (B2))
The thermal crosslinking agent (B2) used in the present invention exhibits excellent adhesive strength by reacting mainly with the functional group derived from the functional group-containing monomer, which is a constituent monomer of the acrylic resin (A), and examples thereof include isocyanate-based crosslinking agents, epoxy-based crosslinking agents, aziridine-based crosslinking agents, melamine-based crosslinking agents, aldehyde-based crosslinking agents, amine-based crosslinking agents, and metal chelate-based crosslinking agents. Among these, isocyanate-based crosslinking agents (B2-1) are preferably used in terms of improving adhesion to the substrate and reactivity with the acrylic resin (A).

Examples of the isocyanate crosslinking agent (B2-1) include tolylene diisocyanate compounds such as 2,4-tolylene diisocyanate and 2,6-tolylene diisocyanate, xylylene diisocyanate compounds such as 1,3-xylylene diisocyanate, 1,4-xylylene diisocyanate and tetramethyl xylylene diisocyanate, aromatic isocyanate compounds such as 1,5-naphthalene diisocyanate and triphenylmethane triisocyanate; hexamethylene diisocyanate compounds such as hexamethylene diisocyanate and trimethylhexamethylene diisocyanate, and aliphatic isocyanate compounds such as lysine diisocyanate; alicyclic isocyanate compounds such as isophorone diisocyanate; and adducts of these isocyanate compounds with polyol compounds such as trimethylolpropane; biuret compounds and isocyanurate compounds of these isocyanate compounds; and the like.
Among the above-mentioned isocyanate-based crosslinking agents (B2-1), it is preferable to use an aromatic isocyanate-based compound, particularly preferably a tolylene diisocyanate-based compound, in terms of excellent reactivity, and it is preferable to use an aliphatic isocyanate-based compound, particularly preferably a hexamethylene diisocyanate-based compound, in terms of suppressing yellowing.

Examples of the epoxy crosslinking agent include bisphenol A-epichlorohydrin type epoxy resins, ethylene glycol diglycidyl ether, polyethylene glycol diglycidyl ether, glycerin diglycidyl ether, glycerin triglycidyl ether, 1,6-hexanediol diglycidyl ether, trimethylolpropane triglycidyl ether, sorbitol polyglycidyl ether, polyglycerol polyglycidyl ether, pentaerythritol polyglycidyl erythritol, diglycerol polyglycidyl ether, etc.

Examples of the aziridine-based crosslinking agent include tetramethylolmethane-tri-β-aziridinyl propionate, trimethylolpropane-tri-β-aziridinyl propionate, N,N'-diphenylmethane-4,4'-bis(1-aziridinecarboxamide), N,N'-hexamethylene-1,6-bis(1-aziridinecarboxamide), etc.

Examples of the melamine-based crosslinking agent include hexamethoxymethylmelamine, hexaethoxymethylmelamine, hexapropoxymethylmelamine, hexaptoxymethylmelamine, hexapentyloxymethylmelamine, hexahexyloxymethylmelamine, melamine resins, etc.

Examples of the aldehyde crosslinking agents include glyoxal, malondialdehyde, succindialdehyde, maleic dialdehyde, glutaric dialdehyde, formaldehyde, acetaldehyde, and benzaldehyde.

Examples of the amine-based crosslinking agent include hexamethylenediamine, triethyldiamine, polyethyleneimine, hexamethylenetetraamine, diethylenetriamine, triethyltetraamine, isophoronediamine, amino resins, polyamides, etc.

Examples of the metal chelate crosslinking agent include acetylacetone and acetoacetyl ester coordination compounds of polyvalent metals such as aluminum, iron, copper, zinc, tin, titanium, nickel, antimony, magnesium, vanadium, chromium, and zirconium.

These thermal crosslinking agents (B2) may be used alone or in combination of two or more.

The content of the thermal crosslinking agent (B2) is usually preferably 0.001 to 5 parts by weight, more preferably 0.02 to 1 part by weight, and particularly preferably 0.05 to 0.5 parts by weight, based on 100 parts by weight of the acrylic resin (A).
If the amount of the thermal crosslinking agent (B2) is too small, the cohesive strength will be insufficient and the adhesive properties will tend to decrease during primary curing, whereas if the amount is too large, the adhesive strength will tend to decrease during complete curing.

The content of the crosslinking agent (B) is usually 20 parts by weight or less, preferably 0.001 to 15 parts by weight, and more preferably 0.1 to 10 parts by weight, per 100 parts by weight of the acrylic resin (A). If there is too much crosslinking agent, the adhesive strength tends to decrease. If there is too little, the adhesive properties under high temperature conditions tend to decrease.

As the crosslinking agent (B), it is preferable to use an active energy ray crosslinking agent (B1) and a thermal crosslinking agent (B2) in combination. When used in combination, the content ratio (weight ratio) of the active energy ray crosslinking agent (B1) to the thermal crosslinking agent (B2) is preferably 100/1 to 100/50, and more preferably 100/4 to 100/10.

[Photopolymerization initiator]
The pressure-sensitive adhesive composition of the present invention can be crosslinked (cured) to form a pressure-sensitive adhesive, and may further contain a photopolymerization initiator in order to efficiently carry out crosslinking. In particular, when the acrylic resin (A) does not have the above-mentioned active energy ray crosslinkable structural portion, it is preferable to blend a photopolymerization initiator.

Such photopolymerization initiators are not particularly limited as long as they generate radicals by the action of active energy rays, and examples thereof include photopolymerization initiators such as acetophenones, benzoins, benzophenones, thioxanthones, and acylphosphine oxides. These photopolymerization initiators can be used alone or in combination of two or more. Among these photopolymerization initiators, it is preferable to use hydrogen abstraction type benzophenones and intramolecular cleavage type acetophenones, because they can efficiently crosslink between molecules or within molecules.

Examples of benzophenone photopolymerization initiators include benzophenone, 4-methylbenzophenone, 2,4,6-trimethylmethylbenzophenone, benzoylbenzoic acid, 3,3'-dimethyl-4-methoxybenzophenone, polyvinylbenzophenone, etc. One type selected from these may be used alone, or two or more types may be used in combination.

Examples of intramolecular cleavage type acetophenone photopolymerization initiators include oxyphenyl-acetic acid 2-[2-oxo-2-phenylacetoxyethoxy]ethyl ester.

Other photopolymerization initiators include, for example, acetophenones such as diethoxyacetophenone, 2-hydroxy-2-methyl-1-phenylpropan-1-one, benzyl dimethyl ketal, 4-(2-hydroxyethoxy)phenyl-(2-hydroxy-2-propyl)ketone, 1-hydroxycyclohexyl phenyl ketone, 2-methyl-2-morpholino(4-thiomethylphenyl)propan-1-one, 2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)butanone, and 2-hydroxy-2-methyl-1-[4-(1-methylvinyl)phenyl]propanone oligomer; benzoin, benzoin methyl ether, benzoin ethyl ether, benzoin isopropyl ether, Benzoins such as benzoin isobutyl ether; thioxanthones such as 2-isopropylthioxanthone, 4-isopropylthioxanthone, 2,4-diethylthioxanthone, 2,4-dichlorothioxanthone, 1-chloro-4-propoxythioxanthone, and 2-(3-dimethylamino-2-hydroxy)-3,4-dimethyl-9H-thioxanthone-9-one mesochloride; and acylphosphine oxides such as 2,4,6-trimethylbenzoyl-diphenylphosphine oxide, bis(2,6-dimethoxybenzoyl)-2,4,4-trimethyl-pentylphosphine oxide, and bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide.

The content of the photopolymerization initiator is preferably 0.01 to 10 parts by weight, more preferably 0.1 to 5 parts by weight, and particularly preferably 0.5 to 2 parts by weight, per 100 parts by weight of the acrylic resin (A). If the content of the photopolymerization initiator is within the above range, sufficient curability can be obtained, for example, even if the acrylic resin (A) does not have the above-mentioned active energy ray crosslinkable structural portion.

In addition, as an auxiliary for these photopolymerization initiators, it is possible to use in combination triethanolamine, triisopropanolamine, 4,4'-dimethylaminobenzophenone (Michler's ketone), 4,4'-diethylaminobenzophenone, 2-dimethylaminoethylbenzoic acid, ethyl 4-dimethylaminobenzoate, (n-butoxy)ethyl 4-dimethylaminobenzoate, isoamyl 4-dimethylaminobenzoate, 2-ethylhexyl 4-dimethylaminobenzoate, 2,4-diethylthioxanthone, 2,4-diisopropylthioxanthone, etc. One selected from these may be used alone, or two or more may be used in combination.

[Carbodiimide Compounds]
In terms of heat resistance, it is preferable that the pressure-sensitive adhesive composition of the present invention further contains a carbodiimide compound as a component other than the acrylic resin (A) and the crosslinking agent (B).
Examples of the carbodiimide-based compound include monocarbodiimides such as bis(2,6-diisopropylphenyl)carbodiimide, dicyclohexylcarbodiimide, diisopropylcarbodiimide, dimethylcarbodiimide, diisobutylcarbodiimide, dioctylcarbodiimide, t-butylisopropylcarbodiimide, diphenylcarbodiimide, di-t-butylcarbodiimide, and didodecylcarbodiimide, polycarbodiimides having a plurality of carbodiimides, and cyclic carbodiimides. One of these may be used alone, or two or more may be used in combination. Among these, monocarbodiimide-based compounds, and further bis(2,6-diisopropylphenyl)carbodiimide are preferred from the viewpoint of heat resistance.

The content of the carbodiimide compound in the present invention is preferably 0.01 to 10 parts by weight, more preferably 0.1 to 5 parts by weight, particularly preferably 0.2 to 2 parts by weight, and further particularly preferably 0.3 to 1 part by weight, based on 100 parts by weight of the acrylic resin (A).
If the content of the carbodiimide compound is too low, the thermal stability of the acrylic resin (A) tends to decrease, whereas if the content is too high, the durability tends to decrease due to the effects of bleed-out and the like.

Furthermore, the pressure-sensitive adhesive composition of the present invention may contain other pressure-sensitive adhesives, or may contain conventionally known additives such as crosslinking accelerators, antistatic agents, tackifiers, and functional dyes, if necessary.
The content of other adhesives and additives is preferably 10 parts by weight or less, and more preferably 5 parts by weight or less, based on 100 parts by weight of the acrylic resin (A).

Thus, the adhesive composition of the present invention can be obtained by mixing the acrylic resin (A), the crosslinking agent (B), and, if necessary, the photopolymerization initiator and other optional components. The mixing method is not particularly limited, and various methods can be used, such as a method of mixing each component at once, or a method of mixing the optional components and then mixing the remaining components at once or sequentially.

<Adhesive>
The pressure-sensitive adhesive composition of the present invention becomes the pressure-sensitive adhesive according to the present invention when the pressure-sensitive adhesive composition is crosslinked (cured) or, when the acrylic resin (A) has an active energy ray-crosslinkable structural site, the acrylic resin (A) contained in the pressure-sensitive adhesive composition forms at least one of an intramolecular and an intermolecular crosslinked structure upon irradiation with active energy rays.
The pressure-sensitive adhesive of the present invention has a multi-stage curing property that allows it to be cured in multiple stages, and is in a low crosslinking state by a primary curing before complete curing. Although complete curing and primary curing cannot be clearly distinguished from each other, they can be distinguished by the difference in gel fraction and dynamic viscoelasticity.
The curing means in both the primary curing step and the complete curing step is not particularly limited, and may be either heating or irradiation with active energy rays. The primary curing step may be carried out in multiple steps, or multi-stage curing may be carried out to achieve a complete cured state.
The pressure-sensitive adhesive of the present invention has excellent adhesive properties after primary curing, and is suitably used for bonding optical members constituting touch panels, image display devices, and the like.

<Adhesive sheet>
A pressure-sensitive adhesive layer made of the pressure-sensitive adhesive of the present invention can be provided on a substrate sheet to form a pressure-sensitive adhesive sheet, and the pressure-sensitive adhesive layer can be provided on a release sheet to form a double-sided pressure-sensitive adhesive sheet. Furthermore, a pressure-sensitive adhesive layer can be formed on a release sheet instead of the substrate sheet, and a release sheet can be attached to the opposite side of the pressure-sensitive adhesive layer to produce a substrate-less double-sided pressure-sensitive adhesive sheet. In addition, a pressure-sensitive adhesive layer can be further formed on the formed pressure-sensitive adhesive layer to form a thick pressure-sensitive adhesive layer.
When the obtained pressure-sensitive adhesive sheet or double-sided pressure-sensitive adhesive sheet is used, the release sheet is peeled off from the pressure-sensitive adhesive layer.

Methods for producing the above-mentioned adhesive sheet include, for example, (i) a method in which the adhesive composition of the present invention is dissolved in a solvent and coated to form an adhesive sheet, and (ii) a method in which the adhesive composition of the present invention is melted by heating to form an adhesive sheet.

First, the method (i) will be described.
When the pressure-sensitive adhesive composition of the present invention is dissolved in a solvent and applied to prepare a pressure-sensitive adhesive sheet, the concentration of the coating solution containing the pressure-sensitive adhesive composition of the present invention is adjusted with an appropriate organic solvent, and the coating solution is directly applied onto a substrate sheet. Then, the coating solution is dried, for example, by heating at 80 to 105°C for 0.5 to 10 minutes, and the resulting product is attached to a substrate sheet or a release sheet. Then, the pressure-sensitive adhesive composition is crosslinked (cured) by aging, and further, if necessary, is irradiated with active energy rays, whereby a pressure-sensitive adhesive sheet having a pressure-sensitive adhesive layer made of the pressure-sensitive adhesive can be produced.

As the organic solvent used to adjust the concentration, the organic solvents listed as those used in the polymerization reaction of the acrylic resin (A) can be used. The concentration of the adhesive composition is usually 20 to 60% by weight, preferably 30 to 50% by weight, as solids content.

Next, the method (ii) will be described.
When the pressure-sensitive adhesive composition of the present invention is melted by heating to form a pressure-sensitive adhesive sheet, the composition is applied in the molten state to one or both sides of a substrate sheet and then cooled, or the composition is extrusion laminated onto the substrate sheet using a T-die or the like to form a pressure-sensitive adhesive layer of a predetermined thickness on one or both sides of the substrate sheet. Next, a release sheet can be attached to the surface of the pressure-sensitive adhesive layer as necessary to produce a pressure-sensitive adhesive sheet.
In addition, after forming the pressure-sensitive adhesive layer on the base sheet, if necessary, an active energy ray irradiation treatment may be carried out, and further aging may be carried out to produce a pressure-sensitive adhesive sheet having a pressure-sensitive adhesive layer in which the pressure-sensitive adhesive composition is cured (crosslinked).
Furthermore, a substrate-less double-sided pressure-sensitive adhesive sheet can also be produced by forming a pressure-sensitive adhesive layer on a release sheet and laminating a release sheet to the opposite side of the pressure-sensitive adhesive layer.
When the obtained pressure-sensitive adhesive sheet or double-sided pressure-sensitive adhesive sheet is used, the release sheet is peeled off from the pressure-sensitive adhesive layer.

Examples of base sheets include polyester resins such as polyethylene naphthalate, polyethylene terephthalate, polybutylene terephthalate, and polyethylene terephthalate/isophthalate copolymer; polyolefin resins such as polyethylene, polypropylene, and polymethylpentene; polyethylene fluoride resins such as polyvinyl fluoride, polyvinylidene fluoride, and polyethylene fluoride; polyamides such as nylon 6 and nylon 6,6; vinyl polymers such as polyvinyl chloride, polyvinyl chloride/vinyl acetate copolymer, ethylene-vinyl acetate copolymer, ethylene-vinyl alcohol copolymer, polyvinyl alcohol, and vinylon; cellulose resins such as cellulose triacetate and cellophane; acrylic resins such as polymethyl methacrylate, polyethyl methacrylate, polyethyl acrylate, and polybutyl acrylate; polystyrene; polycarbonate; polyarylate; and synthetic resin sheets such as polyimide; metal foils such as aluminum, copper, and iron; paper such as fine paper and glassine paper; and woven and nonwoven fabrics made of glass fiber, natural fibers, and synthetic fibers. These base sheets can be used as a single layer or as a multi-layered body in which two or more types are laminated. Among these, synthetic resin sheets are preferred from the standpoint of weight reduction, etc.

As the release sheet, for example, the various synthetic resin sheets, papers, woven fabrics, nonwoven fabrics, etc. exemplified above as the base sheet can be used that have been subjected to a release treatment. As the release sheet, for example, it is preferable to use a silicone-based release sheet.

The method for applying the pressure-sensitive adhesive composition is not particularly limited, and examples include roll coating, die coating, gravure coating, comma coating, slot coating, and screen printing.

The above-mentioned active energy rays may include light rays such as far ultraviolet rays, ultraviolet rays, near ultraviolet rays, and infrared rays; electromagnetic waves such as X-rays and gamma rays; electron beams; proton beams; and neutron beams. However, curing with ultraviolet rays is preferred in terms of curing speed, availability of irradiation equipment, cost, etc.

The gel fraction of the adhesive layer of the adhesive sheet before it is completely cured is preferably 5 to 60% by weight, more preferably 10 to 50% by weight, and particularly preferably 20 to 48% by weight, because it allows easy bonding regardless of the shape of the adherend, and the adhesive layer can hold the adherend after bonding.

The gel fraction of the adhesive layer of the adhesive sheet after complete curing is preferably 40 to 100% by weight, more preferably 45 to 90% by weight, and particularly preferably 48 to 80% by weight, from the standpoint of durability and adhesive strength. If the gel fraction is too low, the cohesive strength decreases, which tends to reduce durability. However, if the gel fraction is too high, the cohesive strength increases, which tends to reduce adhesive strength.

The gel fraction can be adjusted to fall within the above range by, for example, adjusting the amount of active energy ray irradiation or the content of active energy ray crosslinkable structural moieties in the acrylic resin (A), or by adjusting the type and amount of the crosslinking agent (B) or photopolymerization initiator.

The gel fraction is a measure of the degree of crosslinking (degree of hardening) and is calculated, for example, by the following method. That is, an adhesive sheet (without a release sheet) consisting of an adhesive layer formed on a polymeric sheet (e.g., polyethylene terephthalate (PET) film, etc.) serving as a substrate is wrapped in a 200-mesh SUS wire mesh and immersed in toluene maintained at 23°C for 24 hours, and the weight percentage of the undissolved adhesive component remaining in the wire mesh is taken as the gel fraction. However, the weight of the substrate is subtracted from the weight before and after dissolution in toluene to calculate the gel fraction.

The thickness of the adhesive layer of the adhesive sheet is usually preferably 50 to 3000 μm, more preferably 75 to 1000 μm, and particularly preferably 100 to 350 μm. If the adhesive layer is too thin, the impact absorption properties tend to decrease, and if it is too thick, the overall thickness tends to increase when it is attached to an optical component, for example, and the practicality tends to decrease.

In addition, the thickness of the adhesive layer in this invention is a value obtained by subtracting the measured thickness of the components other than the adhesive layer from the measured thickness of the entire adhesive layer-containing laminate using Mitutoyo Corporation's "ID-C112B."

The pressure-sensitive adhesive layer of the pressure-sensitive adhesive sheet of the present invention preferably has a haze value of 2% or less when the thickness of the pressure-sensitive adhesive layer is 100 μm, more preferably 0 to 1.5%, and particularly preferably 0 to 1%. If the haze value is too high, the pressure-sensitive adhesive layer tends to whiten and lose transparency.
The haze value was calculated by measuring the diffuse transmittance and total light transmittance using a HAZE MATER NDH4000 (manufactured by Nippon Denshoku Industries Co., Ltd.) and substituting the obtained diffuse transmittance (DT) and total light transmittance (TT) values into the following formula 1. This machine complies with JIS K7361-1.
[Formula 1] Haze value (%) = (DT/TT) x 100

In the present invention, the pressure-sensitive adhesive layer is laminated on an optical member to obtain an optical member with a pressure-sensitive adhesive layer. For example, the pressure-sensitive adhesive layer surface of the pressure-sensitive adhesive sheet of the present invention, in which the pressure-sensitive adhesive layer is formed on a release sheet, is attached to an optical member, and then the release sheet is peeled off to obtain an optical member with a pressure-sensitive adhesive layer. In addition, optical members can be bonded to each other using the double-sided pressure-sensitive adhesive sheet.

The optical components include components that make up touch panels and image display devices, such as displays (organic electroluminescence, liquid crystal), transparent conductive film substrates (ITO substrates), protective films (glass), transparent antennas (films), transparent wiring, etc.

The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to the following examples as long as it does not exceed the gist of the invention. In the examples, "parts" and "%" refer to weight standards. The weight average molecular weight and glass transition temperature of the acrylic resin (A) were measured according to the methods described above.

[Production Example 1: Production of acrylic resin [A-1]]
A 2 L flask equipped with a condenser was charged with 24 parts of ethyl acetate (boiling point 77° C.) as a polymerization solvent, 16 parts of methyl ethyl ketone (boiling point 80° C.), 0.01 parts of 2,2′-azobis(2,4-dimethylvaleronitrile) (ADVN: 10-hour half-life temperature 52° C.) as a polymerization initiator, and 10% of 100 parts of a previously mixed monomer solution (a mixed solution of 15 parts of methyl acrylate (MA: a1, Tg=−7° C.), 25 parts of ethyl methacrylate (EMA: a2, Tg=65° C.), 15 parts of 2-hydroxyethyl acrylate (HEA: a3), and 45 parts of 2-ethylhexyl acrylate (2EHA: a5)), and the mixture was heated to reflux in the flask, and then 0.2 parts of ADVN and the remaining 90% of the monomer solution were added dropwise over 3 hours. After 30 minutes from the dropwise addition, a mixture of 10 parts of ethyl acetate and 0.17 parts of ADVN was added dropwise over 1 hour to react, thereby obtaining a solution of acrylic resin [A-1] (weight average molecular weight: 260,000, dispersity: 3.05, glass transition temperature: -2°C).

[Production Example 2, Production Example 3, Comparative Production Example 1]
Acrylic resins [A-2], [A-3] and [A'-1] were produced in the same manner as in Production Example 1, except that the copolymerization components of the acrylic resin were as shown in Table 1.
The acrylic resin [A-2] had a weight average molecular weight of 260,000, a dispersity of 2.97, and a glass transition temperature of -7°C.
The acrylic resin [A-3] had a weight average molecular weight of 180,000, a dispersity of 2.73, and a glass transition temperature of 5°C.
The acrylic resin [A'-1] had a weight average molecular weight of 260,000, a dispersity of 3.45, and a glass transition temperature of -13°C.

*MA: Methyl acrylate, EMA: Ethyl methacrylate, HEA: 2-hydroxyethyl acrylate, 2EHA: 2-ethylhexyl acrylate

Example 1
100 parts (solid content equivalent) of the acrylic resin [A-1] solution were mixed with 0.3 parts (solid content equivalent) of Coronate L55E (manufactured by Tosoh Corporation) as a crosslinking agent, and 5.0 parts (solid content equivalent) of polypropylene glycol #400 diacrylate (NK Ester APG400) (manufactured by Shin-Nakamura Chemical Co., Ltd.) as a photopolymerization initiator, 0.25 parts (solid content equivalent) of Omnirad 184 (manufactured by IGM Resins B.V.) and 0.75 parts (solid content equivalent) of Omnirad 754 (manufactured by IGM Resins B.V.) to obtain a pressure-sensitive adhesive composition. The obtained pressure-sensitive adhesive composition solution was adjusted to a solid content concentration of 50% with ethyl acetate, and applied to a polyester-based release sheet so that the thickness after drying was about 50 μm, and dried at 100 ° C. for 5 minutes to form a pressure-sensitive adhesive composition layer.

Two polyester release sheets on which the pressure-sensitive adhesive composition layers were formed in this way were prepared, and the two pressure-sensitive adhesive composition layers were laminated facing each other. The laminated pressure-sensitive adhesive composition layers were sandwiched between the polyester release sheets on both sides and allowed to stand at 40° C. for three days to form a pressure-sensitive adhesive layer (primary curing), and a substrate-less double-sided pressure-sensitive adhesive sheet with a pressure-sensitive adhesive layer thickness of 100 μm was obtained.
In addition, the release sheet on one side was peeled off from the adhesive layer of the substrate-less double-sided adhesive sheet obtained above, and the exposed adhesive layer side was pressed against an easy-adhesion treated polyethylene terephthalate (PET) sheet (thickness 125 μm) to obtain a PET sheet with an adhesive layer having a thickness of 100 μm.

<Examples 2 to 4, Comparative Example 1>
A substrate-less double-sided pressure-sensitive adhesive sheet and a PET sheet with a pressure-sensitive adhesive layer were obtained in the same manner as in Example 1, except that the acrylic resin [A-1] was changed to the acrylic resin [A-2], the acrylic resin [A-3] or the acrylic resin [A'-1].
The components of each adhesive composition are summarized in Table 2.

[Gel fraction: before complete curing (after primary curing)]
The substrateless double-sided pressure-sensitive adhesive sheet was cut to 40 mm x 40 mm, and then left to stand for 30 minutes under conditions of 23 ° C. x 50% RH, and one release sheet was peeled off, and the exposed pressure-sensitive adhesive layer side was attached to a 50 mm x 100 mm SUS mesh sheet (200 mesh). The remaining release sheet was peeled off, and the pressure-sensitive adhesive layer was wrapped in the SUS mesh sheet by folding back the center in the longitudinal direction of the SUS mesh sheet. The gel fraction (%) was measured based on the weight change when the sheet was immersed for 24 hours in a sealed container containing 250 g of toluene kept at 23 ° C.

[Gel fraction: after complete curing]
The above substrate-less double-sided pressure-sensitive adhesive sheet was irradiated with ultraviolet light using a high-pressure mercury UV irradiation device at a peak irradiance of 150 mW/cm ² and an accumulated exposure dose of 3000 mJ/cm ² (1000 mJ/cm ² × 3 passes), then cut into 40 mm × 40 mm pieces and left to stand for 30 minutes under conditions of 23 ° C. × 50% RH. Then, one release sheet was peeled off, and the exposed pressure-sensitive adhesive layer side was attached to a 50 mm × 100 mm SUS mesh sheet (200 mesh). The remaining release sheet was peeled off, and the pressure-sensitive adhesive layer was wrapped in the SUS mesh sheet by folding back the center in the longitudinal direction of the SUS mesh sheet. The gel fraction (%) was measured based on the weight change when the sheet was immersed for 24 hours in a sealed container containing 250 g of toluene maintained at 23 ° C.
The gel fractions before complete curing (after the primary curing) and after complete curing are summarized in Table 2.

[Probe tack: before complete curing (after primary curing)]
The PET sheet with the pressure-sensitive adhesive layer was cut to a size of 12 mm wide x 12 mm long, the release sheet was peeled off, and the probe tack (unit: N/ ² kPa) was measured using a probe tack tester (Probe Tack Tester TE-6001, manufactured by Tester Sangyo Co., Ltd.) under conditions of pressure time of 1 second, application pressure of 20.4 gf/cm2, push-in speed of 120 mm/min, pull-up speed of 600 mm/min, and probe diameter of 5.1 mm (diameter). The evaluation criteria were as follows. The results are summarized in Table 3.
(Evaluation Criteria)
◎: Probe tack (unit: N/2kPa) is less than 3. ○: Probe tack (unit: N/2kPa) is 3 or more and less than 6. △: Probe tack (unit: N/2kPa) is 6 or more and less than 9. ×: Probe tack (unit: N/2kPa) is 9 or more.

[Constant load holding power (50℃): Before complete curing (after primary curing)]
The PET sheet with the adhesive layer was cut to a size of 25 mm wide x 75 mm long (adhesive layer portion: 25 mm wide x 50 mm long + non-adhesive layer portion: 25 mm wide x 25 mm long), and a release sheet was attached. The exposed adhesive layer side was pressed and attached to a stainless steel plate (SUS304) by reciprocating a 2 kg roller (attached area: 25 mm x 50 mm) and left to stand in an atmosphere at 50°C for 20 minutes. A 50 g weight was hung from the longitudinal end of a portion (area 25 mm x 25 mm) and a load of 50 g was applied in a direction at 90° to the plane of the stainless steel plate. The plate was left to stand in this state for 60 minutes, and the PET sheet was The peeled distance was measured and the evaluation criteria were as follows. The results are summarized in Table 3.
(Evaluation Criteria)
◎: Peeling distance is less than 5 mm. ○: Peeling distance is 5 mm or more but less than 10 mm. △: Peeling distance is 10 mm or more but less than 50 mm. ×: Falling.

[180 degree peel strength (23°C): after complete curing]
The PET sheet with the adhesive layer was cut into a size of 25 mm wide x 100 mm long, and irradiated with ultraviolet light using a high-pressure mercury UV irradiation device at a peak irradiance of 150 mW/cm ² and an accumulated exposure dose of 3000 mJ/cm ² (1000 mJ/cm ² x 3 passes), and then the release sheet was peeled off. The exposed adhesive layer side was pressed and attached to non-alkali glass (Corning's "Eagle XG", thickness 1.1 mm) using a 2 kg rubber roller in two round trips under an atmosphere of 23°C and 50% RH, and left to stand for 30 minutes under the same atmosphere. After this, the 180° peel strength (N/25 mm) was measured at room temperature (23°C) and a peel speed of 300 mm/min. The results are summarized in Table 3.

[Holding power (80℃): after complete curing]
The PET sheet with the adhesive layer was cut into a size of 25 mm x 50 mm, and was irradiated with a high-pressure mercury UV irradiator at a peak irradiance of 150 mW/cm ² and an accumulated exposure of 3000 mJ/cm ² (1000 mJ/cm ² x 3 After irradiating the adhesive layer with ultraviolet light using a 2 kg roller, the release sheet was peeled off. The exposed adhesive layer side was then pressed and attached to a stainless steel plate (SUS304) using a 2 kg roller. The area of the adhesive layer was 25 mm x 25 mm. The retention strength was measured by applying a load of 1 kg to the sample for 24 hours in an atmosphere at 80° C. using a creep tester (TESTER SANGYO CO., LTD., Retention Strength Tester BE-501 with Thermo-Humidity Chamber). The evaluation criteria were as follows: The results are summarized in Table 3.
(Evaluation Criteria)
◯: No misalignment △: Misalignment is less than 1.0 mm ×: Misalignment is 1.0 mm or more, or the product has fallen

[Constant load holding power (80℃): after complete curing]
The PET sheet with the adhesive layer was cut into a size of 25 mm wide x 75 mm long (adhesive layer portion: 25 mm wide x 50 mm long + non-adhesive layer portion: 25 mm wide x 25 mm long), and irradiated with high-pressure mercury UV. The adhesive layer was exposed to ultraviolet light at a peak irradiance of 150 mW/cm ² and an accumulated exposure dose of 3000 mJ/cm ² (1000 mJ/cm ² × 3 passes) using the apparatus, and then the release sheet was peeled off. The film was pressed and attached to a stainless steel plate (SUS304) by reciprocating a 2 kg roller (attached area: 25 mm x 50 mm) and left to stand for 20 minutes in an atmosphere at 80°C. After that, the length of the non-attached portion (area: 25 mm x 25 mm) was A 50 g weight was hung from the end in the thickness direction, and a load of 50 g was applied in a direction at 90° to the plane of the stainless steel plate. The plate was left in this state for 60 minutes, and the distance the PET sheet peeled off was measured. The evaluation criteria were as follows. The results are summarized in Table 3.
(Evaluation Criteria)
○: Peeling distance is less than 10 mm △: Peeling distance is 10 mm or more and less than 50 mm ×: Fall

[Moisture and heat resistance: after complete curing]
The PET sheet with the adhesive layer was cut into a size of 30 mm x 50 mm, and irradiated with ultraviolet light using a high-pressure mercury UV irradiation device at a peak irradiance of 150 mW/ ^cm2 and an accumulated exposure dose of 3000 mJ/ ^cm2 (1000 mJ/ ^cm2 x 3 passes), and then the release sheet was peeled off. The exposed adhesive layer side was attached to alkali-free glass (Corning's "Eagle XG", thickness 1.1 mm), and then autoclaved (50°C, 0.5 MPa, 20 minutes), and left to stand for 30 minutes in an atmosphere of 23°C and 50% RH to prepare a test piece having a configuration of "alkali-free glass/adhesive layer/PET".

Using the obtained test pieces, a moist heat resistance test was carried out for 7 days (168 hours) in an atmosphere of 60° C. and 90% RH, and the haze value was measured before and after the moist heat resistance test.
The haze value was calculated by measuring the diffuse transmittance and total light transmittance using a HAZE MATER NDH4000 (manufactured by Nippon Denshoku Industries Co., Ltd.) and substituting the obtained diffuse transmittance (DT) and total light transmittance (TT) values into the following formula 1. Furthermore, the increase rate (%) of the haze value was calculated from the following formula 2. This machine complies with JIS K7361-1. The results are summarized in Table 3.
[Formula 1] Haze value (%) = (DT/TT) x 100
[Formula 2] Haze value increase rate (%)={(haze value after moist heat resistance test−haze value before moist heat resistance test)/haze value before moist heat resistance test}×100
(Evaluation Criteria)
○: Increase rate is less than 100% ×: Increase rate is 100% or more

The adhesive sheets obtained using the adhesive compositions of Examples 1 to 4 had excellent adhesive properties, such as low tackiness and high constant load retention, even in a low crosslinking state before complete curing (after primary curing), and also had excellent adhesive properties after complete curing.
On the other hand, the adhesive sheet obtained using the adhesive composition of Comparative Example 1, in which the glass transition temperature of the acrylic resin (A) was outside the range of -10 to 15°C, had inferior adhesive properties in a low crosslinked state after primary curing compared to Examples 1 to 4.

The adhesive obtained using the adhesive composition of the present invention has excellent adhesive properties in a low crosslinked state after primary curing, and is particularly useful as an adhesive used for bonding optical members constituting touch panels and image display devices, and for sealing organic EL displays.

Claims (7)

A pressure-sensitive adhesive composition comprising an acrylic resin (A) and a crosslinking agent (B),
The acrylic resin (A) is
10 to 30 % by weight of an alkyl acrylate (a1) having a glass transition temperature of −27 to 50° C. when a homopolymer is formed,
10 to 40 % by weight of an alkyl methacrylate (a2) having a glass transition temperature of 0 to 100° C. when a homopolymer is formed,
10 to 30% by weight of a hydroxyl group -containing (meth)acrylic acid ester monomer (a3), and
Contains 35 to 60% by weight of 2-ethylhexyl acrylate ,
a copolymer obtained by polymerizing a copolymerization component (a) having a weight ratio (a1/a2) of an alkyl acrylate (a1) and an alkyl methacrylate (a2) of 20/80 to 55/45;
A pressure-sensitive adhesive composition, characterized in that the acrylic resin (A) has a glass transition temperature of -10 to 15°C. 2. The pressure-sensitive adhesive composition according to claim 1 , wherein the acrylic resin (A) has a weight average molecular weight of 50,000 to 400,000. 3. The pressure-sensitive adhesive composition according to claim 1, further comprising, as the crosslinking agent (B), an active energy ray crosslinking agent (B1) and a thermal crosslinking agent (B2). A pressure-sensitive adhesive obtained by crosslinking the pressure-sensitive adhesive composition according to any one of claims 1 to 3 . 5. The pressure-sensitive adhesive according to claim 4 , wherein the crosslinking is carried out by irradiation with active energy rays. A pressure-sensitive adhesive sheet comprising a pressure-sensitive adhesive layer comprising the pressure-sensitive adhesive according to claim 4 or 5 . The pressure-sensitive adhesive sheet according to claim 6 , wherein the pressure-sensitive adhesive layer has a multi-stage curing property in which the pressure-sensitive adhesive layer is cured in a plurality of stages.