Adhesive composition, adhesive, and adhesive sheet

Abstract

To provide an adhesive composition which is excellent in bending durability and high temperature reliability while being excellent in adhesive force to an adherend, and is also excellent in shape restorability after an adhesive layer has been deformed.SOLUTION: An adhesive composition contains an acrylic resin (A), in which the acrylic resin (A) contains at least two acrylic resins having different weight average molecular weights, a weight average molecular weight (M1) of the acrylic resin (A1) having the highest weight average molecular weight is 800,000-1,600,000, a weight average molecular weight (M2) of the acrylic resin (A2) having the lowest weight average molecular weight is 700,000-1,500,000, a content of a structural site derived from a carboxy group-containing monomer in the acrylic resin (A) is 0.1 wt.% or less, and a haze of the acrylic resin (A) is 2.0% or less.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to an adhesive composition, an adhesive, and an adhesive sheet. Specifically, the present invention relates to an adhesive composition, an adhesive, and an adhesive sheet that have excellent adhesion to an adherend, excellent bending durability (durability when repeatedly bent) and reliability in a high-temperature environment (hereinafter sometimes referred to as "high-temperature reliability"), and further excellent shape recoverability (hereinafter sometimes referred to as "shape recoverability") after the adhesive layer is deformed.

Background Art

[0002] In recent years, in mobile devices such as televisions, monitors for personal computers, notebook computers, mobile phones, tablet terminals, and wearable terminals, a protective layer formed of a plastic sheet or the like is usually provided on the viewing side of the display, and a space (air layer) is provided between the display and the protective layer to prevent damage to the display due to external impact. However, there is a problem that reflection occurs at the interface between the protective layer and the air layer and at the interface between the air layer and the display, causing a decrease in visibility. Therefore, in recent years, in order to ensure impact resistance while improving visibility and further thinning mobile devices (plastic sheets), an impact-absorbing adhesive layer is used instead of the air layer.

[0003] In order for the adhesive layer to exhibit sufficient impact absorption performance, it is necessary to have a certain thickness. Therefore, using a solvent-based acrylic adhesive that has been commonly used in the past for thick coating applications, a hot melt adhesive that can be a solvent-free adhesive, and an active energy ray-curable adhesive have been proposed (for example, see Patent Document 1). Among solvent-free adhesives, hot melt adhesives do not require a drying process for volatilizing the solvent after coating, and an adhesive layer can be efficiently obtained in a short time even when thick coating is performed.

Prior Art Documents

Patent Documents

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2014-214280 [Summary of the Invention] [Problems to be Solved by the Invention]

[0005] In recent years, from the viewpoints of higher functionality, multifunctionality, and further diversification of designs of mobile devices, curving and flexibilizing of flat displays have been demanded. Along with this, adhesives having durability for repeated bending such as the substrate not cracking even when bent and no cracks entering the substrate even when repeatedly bent, and durability in a bent state such as reliability in a high-temperature and high-humidity environment in a bent state are required. Furthermore, from the viewpoint of suppressing deformation of the adhesive layer after bending, it is also required to quickly recover from deformation (strain) caused by an external force.

[0006] However, in the technology disclosed in the above Patent Document 1, these bending durabilities are not considered, and improvement as an adhesive composition is required. In addition, an adhesive having a low elastic modulus can be considered from the viewpoint of improving bending durability, but there is a concern that only this may reduce the adhesive strength and the like. Furthermore, further improvement is required to make all of the resilience to deformation of the adhesive layer, bending durability, adhesive strength, reliability in a high-temperature environment, etc. excellent in a well-balanced manner.

[0007] Therefore, in the present invention, under such a background, an object is to provide an adhesive composition that is excellent in adhesive strength to an adherend, excellent in bending durability and high-temperature reliability, and further excellent in shape recoverability after the adhesive layer is deformed. [Means for Solving the Problems]

[0008] However, as a result of intensive research in view of such circumstances, the present inventor has found that in an adhesive composition containing an acrylic resin, at least two acrylic resins having different weight average molecular weights are contained, and all of them are acrylic resins having a relatively high molecular weight, and the structural part derived from a carboxy group-containing monomer is also small. By making the haze of the acrylic resin 2.0% or less, it has excellent adhesiveness to an adherend, excellent bending durability and high-temperature reliability, and further excellent shape recoverability after the adhesive layer is deformed. Thus, the present invention has been completed.

[0009] That is, the present invention is an adhesive composition containing an acrylic resin (A), wherein the acrylic resin (A) contains at least two acrylic resins having different weight average molecular weights, and the weight average molecular weight (M1) of the acrylic resin (A1) having the highest weight average molecular weight is 800,000 to 2,000,000, and the weight average molecular weight (M2) of the acrylic resin (A2) having the lowest weight average molecular weight is 700,000 to 1,500,000. The content of the structural part derived from the carboxy group-containing monomer in the acrylic resin (A) is 0.1% by weight or less, and the haze of the acrylic resin (A) is 2.0% or less. The first gist of the present invention is an adhesive composition characterized by the above.

[0010] Furthermore, in the present invention, the second gist is an adhesive using the adhesive composition of the first gist, and the third gist is an adhesive sheet having an adhesive layer using the adhesive composition of the first gist.

[0011] In an adhesive composition using an acrylic resin, blending acrylic resins with each other has been conventionally practiced. Generally, for the purpose of segregating specific components on the surface of the adhesive layer, a low-molecular-weight acrylic resin, such as one having a weight-average molecular weight of about 100 to 10,000, is often blended with a high-molecular-weight acrylic resin, and it is usually difficult to blend acrylic resins having relatively high molecular weights with each other. That is, blending acrylic resins having high molecular weights generally makes it difficult to control compatibility, and thus it is not usually practiced. However, it has been unexpectedly found that blending two or more kinds of acrylic resins having relatively high molecular weights results in an adhesive composition excellent in adhesive strength, bending durability, high-temperature reliability, and shape recoverability of the adhesive layer.

Advantages of the Invention

[0012] The adhesive composition of the present invention is an adhesive composition containing an acrylic resin (A), wherein the acrylic resin (A) contains at least two kinds of acrylic resins having different weight-average molecular weights, the weight-average molecular weight (M1) of the acrylic resin (A1) having the highest weight-average molecular weight is 800,000 to 2,000,000, the weight-average molecular weight (M2) of the acrylic resin (A2) having the lowest weight-average molecular weight is 700,000 to 1,500,000, the content of the structural site derived from the carboxy group-containing monomer in the acrylic resin (A) is 0.1% by weight or less, and the haze of the acrylic resin (A) is 2.0% or less. Therefore, the adhesive composition is excellent in adhesive strength to an adherend, excellent in bending durability and high-temperature reliability, and further excellent in shape recoverability after the adhesive layer is deformed. Therefore, it is particularly useful as an adhesive or an adhesive sheet used for touch panels, image display devices, etc., especially for touch panels and image display devices such as foldable smartphones.

Embodiments for Carrying Out the Invention

[0013] Hereinafter, the present invention will be described in detail, but these are examples of desirable embodiments. In the present invention, “(meth)acryl” means acrylic or methacrylic, “(meth)acryloyl” means acryloyl or methacryloyl, and “(meth)acrylate” means acrylate or methacrylate, respectively.

[0014] The pressure-sensitive adhesive composition of the present invention contains an acrylic resin (A). First, the acrylic resin (A) will be described.

[0015] <Acrylic resin (A)> In the present invention, the above acrylic resin (A) contains at least two acrylic resins having different weight average molecular weights from the viewpoints of adhesive strength, bending durability, and shape recovery property, and further, all of the acrylic resins have relatively large molecular weights.

[0016] In the present invention, further, the glass transition temperature (T1) read from the temperature at which the loss tangent of the dynamic viscoelasticity of the acrylic resin (A1) having the highest weight average molecular weight is maximized, and the glass transition temperature (T2) read from the temperature at which the loss tangent of the dynamic viscoelasticity of the acrylic resin (A2) having the lowest weight average molecular weight is maximized, it is preferable from the viewpoint of compatibility that the temperature difference therebetween is 20°C or less. The above glass transition temperature can be read from the temperature at which the loss tangent of the dynamic viscoelasticity is maximized, and more specifically, it is measured under the conditions used in the examples described later.

[0017] The acrylic resin (A) containing at least two acrylic resins having different weight average molecular weights is usually obtained by mixing acrylic resins having different weight average molecular weights produced separately. The acrylic resin (A) containing at least two acrylic resins having different weight average molecular weights can also be obtained, for example, by devising a polymerization method such as two-stage polymerization.

[0018] When mixing acrylic resins with different weight-average molecular weights produced separately, the number of acrylic resins is usually 2 to 4, preferably 2 to 3, and particularly preferably 2. The productivity and economy tend to decrease as the number of acrylic resins contained in the acrylic resin (A) increases. First, the acrylic resin used in the present invention will be described.

[0019] The above acrylic resin is preferably obtained by polymerizing a polymerization component containing a hydroxyl group-containing monomer (a1), more preferably further containing at least one copolymerizable monomer (a2) selected from (meth)acrylic acid alkyl ester monomers and vinyl ester monomers having an alkyl group with 5 to 14 carbon atoms (excluding (a1)), at least one copolymerizable monomer (a3) selected from (meth)acrylic acid alkyl ester monomers and vinyl ester monomers having an alkyl group with 1 to 4 carbon atoms (excluding (a1) and (a2)), and, if necessary, a functional group-containing ethylenically unsaturated monomer (a4) (excluding (a1)), and other copolymerizable monomers (a5) as polymerization components.

[0020] 〈Hydroxyl group-containing monomer (a1)〉 Examples of the hydroxyl group-containing monomer (a1) include hydroxy (meth) 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; oxyalkylene-modified monomers such as diethylene glycol (meth) acrylate and polyethylene glycol (meth) acrylate; primary hydroxyl group-containing monomers such as 2-acryloyloxyethyl-2-hydroxyethyl phthalate; secondary hydroxyl group-containing monomers such as 2-hydroxypropyl (meth) acrylate, 2-hydroxybutyl (meth) acrylate, and 3-chloro-2-hydroxypropyl (meth) acrylate; and tertiary hydroxyl group-containing monomers such as 2,2-dimethyl-2-hydroxyethyl (meth) acrylate. These can be used alone or in combination of two or more.

[0021] Among the above hydroxyl group-containing monomers (a1), 2-hydroxyethyl (meth) acrylate, 4-hydroxybutyl (meth) acrylate, 2-hydroxypropyl (meth) acrylate, particularly 2-hydroxyethyl (meth) acrylate and 4-hydroxybutyl (meth) acrylate, are particularly preferably used in terms of excellent balance between heat and humidity resistance and heat resistance.

[0022] In addition, at least one of the acrylic resins contained in the acrylic resin (A) preferably uses a hydroxyl group-containing monomer containing 2-hydroxyethyl (meth) acrylate and 4-hydroxybutyl (meth) acrylate as a polymerization component in terms of achieving both adhesion and bending durability, and it is particularly preferable that the hydroxyl group-containing monomer contains only 2-hydroxy (meth) acrylate and 4-hydroxybutyl (meth) acrylate.

[0023] In the present invention, the polymerization ratio of the above 2-hydroxyethyl (meth)acrylate and 4-hydroxybutyl (meth)acrylate [2-hydroxyethyl (meth)acrylate / 4-hydroxybutyl (meth)acrylate] is preferably 95 / 5 to 30 / 70, more preferably 80 / 20 to 40 / 60, particularly preferably 75 / 25 to 45 / 65, especially preferably 70 / 30 to 50 / 50, on a weight basis. If the amount of 2-hydroxyethyl (meth)acrylate is too small, the adhesive strength when used as an adhesive tends to decrease, and if it is too large, the bending durability when used as an adhesive tends to decrease.

[0024] In addition, as the hydroxyl group-containing monomer (a1) used in the present invention, the lower the content ratio of di(meth)acrylate contained as an impurity in the hydroxyl group-containing monomer (a1), the more preferable it is. Specifically, it is preferable to use those having a content of 0.5% by weight or less, particularly preferably 0.2% by weight or less, more preferably 0.1% by weight or less, and most preferably 0% by weight.

[0025] In the present invention, the content of the hydroxyl group-containing monomer (a1) is usually 5 to 60% by weight, preferably 7 to 40% by weight, particularly preferably 8 to 30% by weight, more preferably 11 to 25% by weight, especially preferably 12 to 20% by weight, based on the total polymerization components. If such a content is too small, the wet heat resistance when used as an adhesive tends to decrease, and if it is too large, the self-crosslinking reaction of the acrylic resin tends to occur easily, and the heat resistance tends to decrease.

[0026] <At least one copolymerizable monomer (a2) selected from (meth)acrylic acid alkyl ester monomers and vinyl ester monomers having an alkyl group having 5 to 14 carbon atoms> In the present invention, as the polymerization component, a copolymerizable monomer which has a structure in which hydrogen abstraction easily occurs in a high energy state such as high temperature or ultraviolet irradiation, and as a result, crosslinking is easily formed, and after copolymerization, it is possible to lower the glass transition temperature of the acrylic resin. It is preferable to contain a copolymerizable monomer, and particularly preferably, at least one copolymerizable monomer (a2) selected from (meth)acrylic acid alkyl ester monomers and vinyl ester monomers having an alkyl group having 5 to 14 carbon atoms. More preferably, it is a (meth)acrylic acid alkyl ester monomer having an alkyl group having 5 to 14 carbon atoms and having a branched structure, and particularly preferably 2-ethylhexyl (meth)acrylate. These can be used alone or in combination of two or more.

[0027] The content of the copolymerizable monomer (a2) is preferably 15 to 90% by weight based on the total polymerization components, particularly preferably 20 to 85% by weight, more preferably 30 to 80% by weight, particularly preferably 40 to 75% by weight, and most preferably 45 to 70% by weight. When such a content is too small, the step followability and durability when used as an adhesive tend to decrease. On the other hand, when there is too much of the copolymerizable monomer (a2), the adhesive strength when used as an adhesive tends to decrease.

[0028] <At least one copolymerizable monomer (a3) selected from (meth)acrylic acid alkyl ester monomers and vinyl ester monomers having an alkyl group having 1 to 4 carbon atoms> In the present invention, as the polymerization component, it is preferable to contain at least one copolymerizable monomer (a3) (excluding (a1) and (a2)) selected from (meth)acrylic acid alkyl ester monomers and vinyl ester monomers having an alkyl group having 1 to 4 carbon atoms from the viewpoint of improving the cohesive force, further improving the adhesive strength when used as an adhesive, and improving the reliability in a high temperature environment.

[0029] Examples of the copolymerizable monomer (a3) include methyl (meth) acrylate, ethyl (meth) acrylate, n-butyl (meth) acrylate, t-butyl (meth) acrylate, isobutyl (meth) acrylate, n-propyl (meth) acrylate, isopropyl (meth) acrylate, vinyl propionate, vinyl acetate, and the like. These copolymerizable monomers (a3) may be used alone or in combination of two or more. Among the copolymerizable monomers (a3), it is preferable to use methyl (meth) acrylate, ethyl (meth) acrylate, or t-butyl (meth) acrylate from the viewpoint of improving the cohesive force when used as an adhesive.

[0030] Among the copolymerizable monomers (a3), it is preferable to use at least one of methyl (meth) acrylate and ethyl (meth) acrylate, i.e., (meth) acrylate (a3-1), from the viewpoint of further exerting the effects of the present invention.

[0031] The content of the copolymerizable monomer (a3) is preferably 5 to 70% by weight, particularly preferably 10 to 60% by weight, and still more preferably 15 to 45% by weight based on the total polymerization components. If the content of the copolymerizable monomer (a3) is too small, the adhesive strength tends to decrease when used as an adhesive. If it is too large, the bending durability tends to decrease when used as an adhesive.

[0032] Among the components (a3), when using at least one of methyl (meth) acrylate and ethyl (meth) acrylate, i.e., (meth) acrylate (a3-1), the content is preferably 5 to 40% by weight, particularly preferably 7 to 30% by weight, and still more preferably 10 to 25% by weight based on the total polymerization components. If the content of (a3-1) is too large, the handleability during processing tends to decrease due to an increase in viscosity. If it is too small, the adhesive strength tends to decrease when used as an adhesive.

[0033] 〈Functional group-containing ethylenically unsaturated monomer (a4)〉 In the present invention, a functional group-containing ethylenically unsaturated monomer (a4) (excluding (a1)) can be used as a polymerization component of the acrylic resin as needed.

[0034] Examples of the functional group-containing ethylenically unsaturated monomer (a4) include a functional group-containing monomer having a nitrogen atom, an acetoacetyl group-containing monomer, an isocyanate group-containing monomer, a glycidyl group-containing monomer, and the like. Among these, a functional group-containing monomer having a nitrogen atom is preferable in terms of imparting cohesive force and crosslinking promoting action, and particularly preferably an amino group-containing monomer or an amide group-containing monomer, and more preferably an amino group-containing monomer.

[0035] Examples of the amino group-containing monomer include primary amino group-containing (meth)acrylates such as aminomethyl (meth)acrylate and aminoethyl (meth)acrylate; secondary amino group-containing (meth)acrylates such as t-butylaminoethyl (meth)acrylate and t-butylaminopropyl (meth)acrylate; tertiary amino group-containing (meth)acrylates such as ethylaminoethyl (meth)acrylate, dimethylaminoethyl (meth)acrylate, diethylaminoethyl (meth)acrylate, dimethylaminopropyl (meth)acrylate, diethylaminopropyl (meth)acrylate, and dimethylaminopropyl acrylamide; and the like.

[0036] Examples of the amide group-containing monomer include (meth)acrylamide; N-alkyl(meth)acrylamides such as N-methyl(meth)acrylamide, N-ethyl(meth)acrylamide, N-propyl(meth)acrylamide, N-n-butyl(meth)acrylamide, diacetone(meth)acrylamide, and N,N'-methylenebis(meth)acrylamide; N,N-dialkyl(meth)acrylamides such as N,N-dimethyl(meth)acrylamide, N,N-diethyl(meth)acrylamide, N,N-dipropyl(meth)acrylamide, N,N-ethylmethylacrylamide, and N,N-diallyl(meth)acrylamide; hydroxyalkyl(meth)acrylamides such as N-hydroxymethyl(meth)acrylamide and N-hydroxyethyl(meth)acrylamide; alkoxyalkyl(meth)acrylamides such as N-methoxymethyl(meth)acrylamide and N-(n-butoxymethyl)(meth)acrylamide; and the like.

[0037] Examples of the acetoacetyl group-containing monomer include 2-(acetoacetoxy)ethyl (meth)acrylate and allyl acetoacetate.

[0038] Examples of the isocyanate group-containing monomer include 2-acryloyloxyethyl isocyanate, 2-methacryloyloxyethyl isocyanate, and their alkylene oxide adducts.

[0039] Examples of the glycidyl group-containing monomer include glycidyl (meth)acrylate and allyl glycidyl (meth)acrylate.

[0040] These functional group-containing ethylenically unsaturated monomers (a4) may be used alone or in combination of two or more.

[0041] The content of the above functional group-containing ethylenically unsaturated monomer (a4) is preferably 30% by weight or less, particularly preferably 20% by weight or less, more preferably 10% by weight or less, and especially preferably 5% by weight or less, based on the total polymerization components. The lower limit is usually 0% by weight. If the content of the functional group-containing ethylenically unsaturated monomer (a4) is too large, the heat resistance of the resin tends to decrease.

[0042] In addition to the above, carboxy group-containing monomers such as (meth)acrylic acid can also be mentioned as the functional group-containing ethylenically unsaturated monomer (a4). However, in the present invention, the content of the structural site derived from the carboxy group-containing monomer in the acrylic resin is 0.1% by weight or less, preferably 0.05% by weight or less, more preferably 0.01% by weight or less, and it is preferably not substantially contained. If there are too many structural sites derived from such carboxy group-containing monomers, corrosion may occur when the adherend is a metal, or the adhesive composition may thicken or gel over time, resulting in a tendency for the stability over time to decrease. Note that thickening can be confirmed by measuring the viscosity change over time at room temperature or in a 40°C atmosphere.

[0043] 〈Other copolymerizable monomer (a5)〉 In the present invention, other copolymerizable monomers (a5) can be used as needed as the copolymerization components of the acrylic resin.

[0044] Examples of the above-mentioned other copolymerizable monomer (a5) include aromatic (meth)acrylate 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, nonylphenol ethylene oxide adduct (meth)acrylate, and monomers such as acrylonitrile, methacrylonitrile, styrene, α-methylstyrene, vinyl stearate, vinyl chloride, vinylidene chloride, alkyl vinyl ether, vinyl toluene, vinyl pyridine, vinyl pyrrolidone, dialkyl itaconate, dialkyl fumarate, allyl alcohol, acrylic chloride, methyl vinyl ketone, N-acrylamidomethyl trimethyl ammonium chloride, allyl trimethyl ammonium chloride, dimethyl allyl vinyl ketone. These can be used alone or in combination of two or more.

[0045] When aiming to increase the molecular weight of the acrylic resin, for example, 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, divinylbenzene can be used in a small amount in combination. At this time, these compounds having two or more ethylenically unsaturated groups have high reactivity and usually do not remain unreacted when used as a polymerization component of the acrylic resin. However, if the usage amount is too large, these compounds having two or more ethylenically unsaturated groups will remain unreacted, and the acrylic resin tends to gel.

[0046] The content of the above-mentioned other copolymerizable monomer (a5) is preferably 50% by weight or less, particularly preferably 40% by weight or less, and still more preferably 20% by weight or less, based on the total polymerization components. The lower limit is usually 0% by weight. If the content ratio of the above-mentioned other copolymerizable monomer (a5) is too high, the resin may thicken over time, resulting in a decrease in stability over time or a tendency for the adhesive strength to decrease.

[0047] The acrylic resin used in the present invention can be produced by appropriately selecting the above-mentioned polymerization components so as to obtain a desired glass transition temperature and polymerizing them.

[0048] As the above-mentioned polymerization method, for example, conventionally known polymerization methods such as solution polymerization, suspension polymerization, bulk polymerization, and emulsion polymerization can be used. In the present invention, production by solution polymerization is preferable in that an acrylic resin can be produced safely, stably, and with an arbitrary monomer composition. Hereinafter, an example of a preferable production method of the acrylic resin used in the present invention will be shown.

[0049] First, in an organic solvent, the above-mentioned polymerization components and a polymerization initiator are mixed or dropped, and solution polymerization is carried out to obtain an acrylic resin solution.

[0050] 〔Organic solvent〕 Examples of the organic solvent used in the above polymerization reaction include aromatic hydrocarbons such as toluene and xylene, aliphatic hydrocarbons such as hexane, esters such as ethyl acetate and butyl acetate, aliphatic alcohols such as N-propyl alcohol and isopropyl alcohol, and ketones such as acetone, methyl ethyl ketone, methyl isobutyl ketone, and cyclohexanone. These can be used alone or in combination of two or more. Among these solvents, it is preferable to use ethyl acetate, acetone, methyl ethyl ketone, and methyl acetate from the viewpoints of solubility, drying property during coating, price, etc., and particularly preferably to use ethyl acetate and acetone.

[0051] 〔Polymerization initiator〕 As the polymerization initiator used in the above polymerization reaction, an azo-based polymerization initiator or a peroxide-based polymerization initiator, which are ordinary radical polymerization initiators, can be used. Examples of the azo-based 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), 2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile), etc. Examples of the peroxide-based polymerization initiator include benzoyl peroxide, di-t-butyl peroxide, cumene hydroperoxide, lauroyl peroxide, t-butyl peroxypivalate, t-hexyl peroxypivalate, t-hexyl peroxyneodecanoate, diisopropyl peroxydicarbonate, diisobutyryl peroxide, etc. These can be used alone or in combination of two or more.

[0052] In the production of the above acrylic resin, it is preferable to use an organic solvent with a boiling point of 80 °C or lower as the reaction solvent for solution polymerization and carry out the polymerization at a relatively low temperature. At this time, if a polymerization initiator with a high 10-hour half-life temperature is used, the polymerization initiator is likely to remain. When the polymerization initiator remains, the stability of the acrylic resin solution over time decreases, and gelation of the acrylic resin tends to occur.

[0053] Therefore, from the viewpoint of improving the stability over time of the acrylic resin solution, it is preferable to use a polymerization initiator having a 10-hour half-life temperature of 70°C or lower among the above polymerization initiators. Among them, particularly preferably, 2,2'-azobis(2-methylbutyronitrile) (67°C), 2,2'-azobisisobutyronitrile (65°C), 2,2'-azobis(2,4-dimethylvaleronitrile) (52°C), 2,2'-azobis(2-cyclopropylpropionitrile) (49.6°C), 2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile) (30°C), t-butylperoxypivalate (54.6°C), t-hexylperoxypivalate (53.2°C), t-hexylperoxyneodecanoate (44.5°C), diisopropylperoxycarbonate (40.5°C), diisobutyrylperoxide (32.7°C). More preferably, 2,2'-azobis(2,4-dimethylvaleronitrile) (52°C) and t-hexylperoxypivalate (53.2°C). The numerical value in the parentheses described following each of the above compound names is the 10-hour half-life temperature of each compound.

[0054] The amount of the above polymerization initiator used is usually 0.001 to 10 parts by weight, preferably 0.1 to 8 parts by weight, particularly preferably 0.5 to 6 parts by weight, more preferably 1 to 4 parts by weight, especially preferably 1.5 to 3 parts by weight, and most preferably 2 to 2.5 parts by weight with respect to 100 parts by weight of the polymerization components. If the amount of the above polymerization initiator used is too small, the polymerization rate of the acrylic resin tends to decrease and the residual monomer tends to increase. If the amount used is too large, gelation of the acrylic resin may occur, etc., and the stability over time of the acrylic resin solution tends to decrease.

[0055] 〔Polymerization conditions, etc.〕 Regarding the polymerization conditions for solution polymerization, polymerization may be carried out according to conventionally known polymerization conditions. For example, in a solvent, the polymerization components and the polymerization initiator can be mixed or dropped and polymerized under predetermined polymerization conditions.

[0056] The polymerization temperature in the above polymerization reaction is usually 40 to 120°C. However, in the present invention, from the viewpoint of enabling stable reaction, 50 to 90°C is preferable, particularly preferably 55 to 75°C, and still more preferably 60 to 70°C. If the polymerization temperature is too high, the acrylic resin tends to gel easily. If it is too low, the activity of the polymerization initiator decreases, resulting in a decrease in the polymerization rate and an increase in residual monomers.

[0057] Also, the polymerization time in the polymerization reaction (when the subsequent forced heating is carried out, it is the time until the start of forced heating) is not particularly limited, but it is preferably 0.5 hours or more from the addition of the last polymerization initiator, particularly preferably 1 hour or more, still more preferably 2 hours or more, and especially preferably 5 hours or more. The upper limit of the polymerization time is usually 72 hours. In addition, the polymerization reaction is preferably carried out while refluxing the solvent in terms of easy heat removal.

[0058] In the production of the above acrylic resin, in order to reduce the amount of residual polymerization initiator, it is preferable to thermally decompose the polymerization initiator by forced heating.

[0059] The above forced heating temperature is preferably carried out at a temperature higher than the 10-hour half-life temperature of the above polymerization initiator. Specifically, it is usually 40 to 150°C, preferably 55 to 130°C from the viewpoint of gelation suppression, and particularly preferably 75 to 95°C. If the forced heating temperature is too high, the acrylic resin tends to turn yellow. If it is too low, the polymerization components and polymerization initiator remain, and the gelation and thickening of the acrylic resin occur, resulting in a tendency for the temporal stability and thermal stability to decrease. Thus, an acrylic resin solution can be obtained.

[0060] In the present invention, the acrylic resins (A1) and (A2) preferably contain a structural moiety derived from the hydroxyl group-containing monomer (a1), and the content of the structural moiety derived from the hydroxyl group-containing monomer (a1) in the acrylic resin is usually 5 to 60% by weight, preferably 7 to 40% by weight, particularly preferably 8 to 30% by weight, more preferably 11 to 25% by weight, and especially preferably 12 to 20% by weight based on the total polymerization components.

[0061] In addition, the acrylic resins (A1) and (A2) preferably contain, in addition to the structural moiety derived from the hydroxyl group-containing monomer (a1), a structural moiety derived from at least one of methyl (meth)acrylate and ethyl (meth)acrylate, i.e., (meth)acrylate (a3-1). The content of the structural moiety derived from such (meth)acrylate (a3-1) is usually 5 to 40% by weight, preferably 7 to 30% by weight, and particularly preferably 10 to 25% by weight based on the acrylic resin.

[0062] In the acrylic resin (A1), it is particularly preferable that the structural moieties derived from methyl (meth)acrylate and ethyl (meth)acrylate are contained as (meth)acrylate (a3-1) from the viewpoint of excellent adhesive strength.

[0063] In addition, in the acrylic resin (A2), it is particularly preferable that the structural moiety derived from ethyl (meth)acrylate is contained as (meth)acrylate (a3-1) from the viewpoints of excellent bending durability and adhesive strength.

[0064] Here, the proportion (composition ratio) of the structural moieties derived from the respective components of the acrylic resin can be determined by, for example, NMR.

[0065] In the present invention, the weight average molecular weight (M1) of the acrylic resin (A1) is from 800,000 to 2,000,000, more preferably from 900,000 to 1,500,000, particularly preferably from 1,000,000 to 1,400,000, and still more preferably from 1,050,000 to 1,300,000. If such a weight average molecular weight is too small, the cohesive force decreases, and the reliability in a high-temperature environment tends to decrease. If it is too large, the viscosity becomes too high, and the coatability and handleability tend to decrease.

[0066] Also, the weight average molecular weight (M2) of the acrylic resin (A2) is from 700,000 to 1,500,000, more preferably from 750,000 to 1,400,000, particularly preferably from 800,000 to 1,300,000, and still more preferably from 850,000 to 1,250,000. If such a weight average molecular weight is too small, the cohesive force decreases, and the reliability in a high-temperature environment tends to decrease. If it is too large, the viscosity becomes too high, and the coatability and handleability tend to decrease.

[0067] The dispersity (weight average molecular weight / number average molecular weight) of the acrylic resins (A1) and (A2) is preferably 15 or less, particularly preferably 10 or less, still more preferably 7 or less, and especially preferably 5 or less. If such a dispersity is too high, the durability performance of the adhesive layer tends to decrease, and foaming or the like is likely to occur. If it is too low, the handleability tends to decrease. The lower limit of the dispersity is usually 1.1 from the viewpoint of manufacturing limitations.

[0068] Further, the difference (|M1 - M2|) between the weight average molecular weight (M1) of the acrylic resin (A1) and the weight average molecular weight (M2) of the acrylic resin (A2) is preferably 700,000 or less, more preferably 500,000 or less, and particularly preferably 300,000 or less in terms of adhesive strength and reliability in a high-temperature environment. If outside the above range, either the adhesive strength or the reliability in a high-temperature environment tends to decrease.

[0069] Also, the weight average molecular weight (M) of the acrylic resin (A) is preferably 700,000 to 2,000,000 from the viewpoint of reliability in a high-temperature environment, more preferably 800,000 to 1,500,000, and particularly preferably 900,000 to 1,200,000. If such a weight average molecular weight (M) is too low, the reliability in a high-temperature environment tends to decrease, and if it is too high, the adhesive strength tends to decrease.

[0070] In addition, the weight average molecular weight in the present invention is the weight average molecular weight in terms of standard polystyrene molecular weight, and for high-performance liquid chromatography (manufactured by Tosoh Corporation, "HLC-8320GPC"), column: TSKgel GMHXL (exclusion limit molecular weight: 4×10 8 , separation range: 100 to 4×10 8 , theoretical plate number: 14,000 plates / column, filler material: styrene-divinylbenzene copolymer, filler particle size: 9 μm, column size: 7.8 mm I.D.×30 cm) and three columns: TSKgel G2000HXL (exclusion limit molecular weight: 1×10 4 , separation range: 100 to 1×10 4 , theoretical plate number: 16,000 plates / column, filler material: styrene-divinylbenzene copolymer, filler particle size: 5 μm, column size: 7.8 mm I.D.×30 cm) are used in series, and it is measured in this way. The number average molecular weight can also be measured using the same method. In addition, the dispersity is obtained from the weight average molecular weight and the number average molecular weight.

[0071] Also, in the present invention, the glass transition temperature (T1) of the acrylic resin (A1) is preferably -30 to -10°C, more preferably -25 to -12°C, and particularly preferably -23 to -15°C. If the glass transition temperature is too low, the adhesive strength tends to decrease, and if the glass transition temperature is too high, the bending durability tends to decrease.

[0072] The glass transition temperature (T2) of the above acrylic resin (A2) is preferably -40 to -20°C, more preferably -35 to -23°C, and particularly preferably -30 to -25°C. If the glass transition temperature is too low, the adhesive strength and reliability in a high-temperature environment tend to decrease. If the glass transition temperature is too high, the bending durability tends to decrease.

[0073] Furthermore, from the viewpoint of the compatibility of the acrylic resin, and more specifically, the balance between the adhesive strength and the bending durability, the temperature difference between the glass transition temperature (T1) read from the temperature at which the loss tangent of the dynamic viscoelasticity of the above acrylic resin (A1) is maximum and the glass transition temperature (T2) read from the temperature at which the loss tangent of the dynamic viscoelasticity of the acrylic resin (A2) is maximum is preferably 20°C or less, more preferably 15°C or less, particularly preferably 10°C or less, and even more preferably 8°C or less. If the temperature difference of the glass transition temperature is too large, the compatibility of the resin tends to decrease. The lower limit value of such a temperature difference is preferably 3°C, particularly preferably 4°C. If the temperature difference is too small, either the adhesive strength or the bending durability tends to decrease.

[0074] In the present invention, it is more preferable that the glass transition temperature (T1) of the acrylic resin (A1) in the above acrylic resin (A) is higher than the glass transition temperature (T2) of the acrylic resin (A2).

[0075] The glass transition temperature (T) of the acrylic resin (A) is preferably -40 to -15°C, more preferably -35 to -17°C, and particularly preferably -30 to -18°C in terms of the adhesive strength and the bending durability. If such a glass transition temperature is too low, the adhesive strength and the reliability in a high-temperature environment tend to decrease. If it is too high, the bending durability tends to decrease.

[0076] In the present invention, when the acrylic resin (A1) with a higher weight-average molecular weight and the acrylic resin (A2) with a lower weight-average molecular weight are included, particularly when the glass transition temperature (T1) of the acrylic resin (A1) is higher than the glass transition temperature (T2) of the acrylic resin (A2), the content ratio [(A1) / (A2)] of the acrylic resin (A1) and the acrylic resin (A2) is preferably 70 / 30 to 30 / 70, more preferably 65 / 35 to 40 / 60, and particularly preferably 60 / 40 to 50 / 50 in terms of weight ratio from the viewpoint of achieving both adhesiveness and bending durability. If the content of the acrylic resin (A1) is too small [the content of the acrylic resin (A2) is too large], the adhesiveness tends to decrease and the reliability in a high-temperature and high-humidity environment tends to deteriorate. If the content of the acrylic resin (A1) is too large [the content of the acrylic resin (A2) is too small], the bending durability tends to decrease.

[0077] Furthermore, the weight ratio [(a1) amount of (A1):(a1) amount of (A2)] of the amount of the hydroxyl group-containing monomer (a1) constituting the acrylic resin (A1) to the amount of the hydroxyl group-containing monomer (a1) constituting the acrylic resin (A2) is preferably 3:1 to 1:3, particularly preferably 2:1 to 1:2, and more preferably 1:1 to 1:1.5. If the amount of the hydroxyl group-containing monomer (a1) constituting the acrylic resin (A1) is too small, the adhesiveness when used as an adhesive decreases. If the amount of the hydroxyl group-containing monomer (a1) is too large, the bending durability when used as an adhesive tends to decrease. If the difference in the amount of the hydroxyl group-containing monomer (a1) between the two is too large, the compatibility of the resin tends to decrease and the moisture and heat resistance also tends to decrease.

[0078] Also, the total content ratio of the acrylic resin (A1) and the acrylic resin (A2) to the acrylic resin (A) is usually 50% by weight or more, preferably 80% by weight or more, more preferably 90% by weight or more, and still more preferably 100% by weight.

[0079] The acid value of the above acrylic resin (A) is preferably 0.001 to 2 mgKOH / g, particularly preferably 0.001 to 1 mgKOH / g, and still more preferably 0.001 to 0.5 mgKOH / g. If the acid value is too high, depending on the type of adherend, there is a tendency to corrode the adherend.

[0080] When adjusting the acid value of the above acrylic resin (A) to the above range, it is preferable that the content of the structural part derived from the carboxy group-containing monomer is 0.1% by weight or less, particularly preferably 0.05% by weight or less, still more preferably 0.01% by weight or less, and more preferably not substantially contained.

[0081] Here, the acid value in the present invention is determined by neutralization titration based on JIS K 0070.

[0082] The acrylic resin (A) is the main component of the pressure-sensitive adhesive composition of the present invention. That is, the content of the acrylic resin (A) in the pressure-sensitive adhesive composition is preferably 70% by weight or more based on the whole from the viewpoint of reliability during durability, particularly preferably 80 to 99.9% by weight, and still more preferably 90 to 98% by weight.

[0083] In the present invention, at least two acrylic resins having different weight average molecular weights are included in the above acrylic resin (A). As a method for confirming these, for example, using tetrahydrofuran (THF), the acrylic resin (A) [or the pressure-sensitive adhesive composition] is dissolved to prepare a sample solution of about 0.1% by weight. This sample solution can be confirmed by analyzing it by gradient method using a high performance liquid chromatograph (HPLC) with an ODS (octadecylsilyl) column. Examples of the above gradient method include a method of using acetonitrile and THF as the mobile phase and changing the mixing ratio of acetonitrile and THF.

[0084] In the present invention, it is important that the haze of the acrylic resin (A) is 2.0% or less, preferably 1.0% or less, more preferably 0.5% or less. If the haze of such an acrylic resin (A) is too large, it becomes difficult to apply it to the bonding of optical members. The lower limit value of the haze is usually 0.01%. Here, the haze of the acrylic resin (A) can be measured as follows.

[0085] 〔Haze〕 An acrylic resin (A) solution is applied to a polyester release sheet and dried to produce a pressure-sensitive adhesive sheet with a thickness of 100 μm in an uncrosslinked state. After bonding the pressure-sensitive adhesive layer side of the above pressure-sensitive adhesive sheet to non-alkali glass (Corning's "Eagle XG", thickness 1.1 mm), autoclave treatment (50 °C, 0.5 MPa, 20 minutes) is performed, and it is left standing for 30 minutes under the conditions of 23 °C × 50% RH, and then the release sheet is peeled off to produce a test piece having a structure of "non-alkali glass / pressure-sensitive adhesive layer". Using the above-obtained test piece, the haze value is measured as follows. The haze value is measured by measuring the diffuse transmittance and the total light transmittance using HAZE MATER NDH4000 (manufactured by Nippon Denshoku Industries Co., Ltd.), and substituting the obtained values of the diffuse transmittance (DT) and the total light transmittance (TT) into the following formula to calculate the haze value. Note that this machine complies with JIS K7361-1. [Formula] Haze value (%) = (DT / TT) × 100

[0086] <Photoinitiator> The pressure-sensitive adhesive composition of the present invention becomes a pressure-sensitive adhesive layer when cured. However, when curing is performed by active energy rays described later, it is preferable to use a photoinitiator in that the reaction during active energy ray irradiation can be stabilized.

[0087] Such a photopolymerization initiator is not particularly limited as long as it generates radicals by the action of light. For example, photopolymerization initiators such as acetophenones, benzoins, benzophenones, thioxanthones, acylphosphine oxides, etc. can be mentioned. 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 acetophenone photopolymerization initiators from the viewpoint of efficient crosslinking between molecules or within molecules.

[0088] The blending amount of such a photopolymerization initiator is preferably 0.01 to 10 parts by weight, particularly preferably 0.1 to 5 parts by weight, and even more preferably 0.5 to 2 parts by weight with respect to 100 parts by weight of the acrylic resin (A). If the blending amount is too small, the curing rate tends to decrease or the curing becomes insufficient. If it is too large, the curability does not improve and the economy tends to decrease.

[0089] In addition, as auxiliaries for these photopolymerization initiators, for example, triethanolamine, triisopropanolamine, 4,4'-dimethylaminobenzophenone (Michler's ketone), 4,4'-diethylaminobenzophenone, 2-dimethylaminoethyl benzoate, ethyl 4-dimethylaminobenzoate, (n-butoxy)ethyl 4-dimethylaminobenzoate, isoamyl 4-dimethylaminobenzoate, 2-ethylhexyl 4-dimethylaminobenzoate, 2,4-diethylthioxanthone, 2,4-diisopropylthioxanthone, etc. can also be used in combination. These auxiliaries may be used alone or in combination of two or more.

[0090] <Active energy ray curable monomer> In addition, when curing is carried out by active energy rays, it is preferable to use an active energy ray curable monomer, whereby the cohesive force of the entire adhesive layer can be adjusted and stable adhesive physical properties can be obtained.

[0091] As the active energy ray curable monomer, a polyfunctional monomer or oligomer containing two or more ethylenically unsaturated groups in one molecule is preferred. For example, hexanediol di(meth)acrylate, butanediol di(meth)acrylate, (poly)ethylene glycol di(meth)acrylate, (poly)propylene glycol di(meth)acrylate, neopentyl glycol di(meth)acrylate, pentaerythritol di(meth)acrylate, pentaerythritol tri(meth)acrylate, dipentaerythritol hexa(meth)acrylate, trimethylolpropane tri(meth)acrylate, tetramethylolmethane tri(meth)acrylate, ethylene oxide modified tri(meth)acrylate of isocyanuric acid, allyl (meth)acrylate, vinyl (meth)acrylate, urethane (meth)acrylate, etc. may be mentioned. The above polyfunctional monomers can be used alone or in combination of two or more.

[0092] The number of ethylenically unsaturated groups of the above active energy ray curable monomer is preferably 2 to 4, and more preferably 2 to 3.

[0093] The blending amount of such an active energy ray curable monomer is preferably 0 to 100 parts by weight, and particularly preferably 2.5 to 50 parts by weight with respect to 100 parts by weight of the acrylic resin (A).

[0094] <Other optional components> As the constituent components of the pressure-sensitive adhesive composition of the present invention, in addition to the above photopolymerization initiator and active energy ray curable monomer, other optional components may be included.

[0095] Examples of other optional components include carbodiimide compounds, antioxidants, plasticizers, tackifiers, crosslinking agents, crosslinking accelerators, silane coupling agents, antistatic agents, functional dyes, etc. These can be used alone or in combination of two or more. The content of other optional components may be in a range that does not impair the effects of the present invention, and is preferably 10% by weight or less, and more preferably 0.1 to 5% by weight in the pressure-sensitive adhesive composition.

[0096] Also, when a carbodiimide compound is blended as other components, it is preferable to blend it into the acrylic resin (A) prior to the photopolymerization initiator, the active energy ray curable monomer, and other optional components from the viewpoint of the chemical stability of the resulting resin composition. Particularly preferably, the acrylic resin (A) and the carbodiimide compound are mixed at 0 to 140 °C, and more preferably at 20 to 100 °C.

[0097] Since the pressure-sensitive adhesive layer obtained by curing the above pressure-sensitive adhesive composition is excellent in adhesion to an adherend, bending durability, and reliability under a high-temperature environment, it is useful as a pressure-sensitive adhesive layer of a pressure-sensitive adhesive sheet. Further, the above pressure-sensitive adhesive composition is also useful when used as a material component of a pressure-sensitive adhesive.

[0098] <Adhesive sheet> The pressure-sensitive adhesive composition of the present invention is preferably used as a pressure-sensitive adhesive sheet having a pressure-sensitive adhesive layer obtained by curing this on a base sheet, a double-sided pressure-sensitive adhesive sheet having a pressure-sensitive adhesive layer provided on a release sheet, and an optical member with a pressure-sensitive adhesive layer having a pressure-sensitive adhesive layer provided on an optical member. Examples of the above curing method include a method of curing with active energy rays, a method of curing by crosslinking using a crosslinking agent, and a method combining these. Among them, the method of curing with active energy rays is preferable in that it enables two-stage curing and eliminates the need for an aging treatment.

[0099] The above adhesive sheet can be produced, for example, as follows. In the present invention, the term "sheet" is not particularly distinguished from "film" or "tape", and is described to include these.

[0100] First, a method of applying the pressure-sensitive adhesive composition diluted with a solvent to one or both sides of a base sheet and then drying it, or a method of melting the pressure-sensitive adhesive composition by heating and extrusion laminating it onto the base sheet using a T-die or the like, etc., is used to form a pressure-sensitive adhesive layer on one or both sides of the base sheet to have a predetermined thickness. Subsequently, a pressure-sensitive adhesive sheet can be produced by laminating a release sheet onto the pressure-sensitive adhesive layer surface as needed.

[0101] Also, after forming a pressure-sensitive adhesive layer on a base sheet, a pressure-sensitive adhesive sheet having a pressure-sensitive adhesive layer in which the pressure-sensitive adhesive composition is cured (crosslinked) can be produced by performing an active energy ray irradiation treatment as needed and further aging.

[0102] In addition, a double-sided pressure-sensitive adhesive sheet without a base material can also be produced by forming a pressure-sensitive adhesive layer on a release sheet and laminating a release sheet on the opposite pressure-sensitive adhesive layer surface. When in use, the obtained pressure-sensitive adhesive sheet or double-sided pressure-sensitive adhesive sheet is used by peeling the release sheet from the pressure-sensitive adhesive layer.

[0103] Examples of the base sheet include sheets made of at least one synthetic resin selected from the group consisting of polyester resins such as polyethylene naphthalate, polyethylene terephthalate, polybutylene terephthalate, and polyethylene terephthalate / isophthalate copolymer; polyolefin resins such as polyethylene, polypropylene, and polymethylpentene; polyfluoroethylene resins such as polyvinyl fluoride, polyvinylidene fluoride, and polyfluoroethylene; 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 triacetate cellulose and cellophane; acrylic resins such as polymethyl methacrylate, polyethyl methacrylate, polyethyl acrylate, and polybutyl acrylate; polystyrene; polycarbonate; polyarylate; and polyimide; metal foils such as aluminum, copper, and iron; papers such as high-quality paper and glassine paper; and woven or non-woven fabrics made of glass fibers, natural fibers, synthetic fibers, etc. These base sheets can be used as single-layer sheets or as multi-layer sheets in which two or more types are laminated. Among these, sheets made of synthetic resin are preferred from the viewpoint of weight reduction and the like.

[0104] Furthermore, as the release sheet, for example, those obtained by subjecting various synthetic resin sheets, papers, cloths, non-woven fabrics, etc. exemplified by the above support base material to a release treatment can be used. As the release sheet, a silicone-based release sheet is preferably used.

[0105] In addition, the coating method of the pressure-sensitive adhesive composition is not particularly limited as long as it is a general coating method, and examples thereof include methods such as roll coating, die coating, gravure coating, comma coating, and screen printing.

[0106] By irradiating active energy rays, the acrylic resin (A) in the pressure-sensitive adhesive composition forms a crosslinked structure in at least one of intramolecular and intermolecular.

[0107] As the active energy rays, for example, light rays such as far ultraviolet rays, ultraviolet rays, near ultraviolet rays, and infrared rays, electromagnetic waves such as X-rays and γ-rays, and electron beams, proton beams, neutron beams, etc. can be used. However, ultraviolet rays are preferred in terms of curing speed, availability of irradiation devices, price, etc.

[0108] When irradiating with ultraviolet rays for curing, a high-pressure mercury lamp, an ultra-high-pressure mercury lamp, a carbon arc lamp, a metal halide lamp, a xenon lamp, a chemical lamp, an electrodeless discharge lamp, an LED lamp, etc. that emit light in the wavelength range of 150 to 450 nm is used, and usually 30 to 3000 mJ / cm 2 , preferably 100 to 1500 mJ / cm 2 of ultraviolet rays may be irradiated.

[0109] The above aging treatment is preferably carried out particularly when a crosslinking agent is used in the adhesive composition. As the conditions of the above aging treatment, the temperature is usually room temperature (23°C) to 100°C, and the time is usually 1 to 30 days. Specifically, for example, it can be carried out under conditions such as 1 to 20 days at 23°C, preferably 3 to 10 days at 23°C, 1 to 7 days at 40°C, etc.

[0110] And in the present invention, by laminating and forming the above adhesive layer on an optical member, an optical member with an adhesive layer can be obtained. Also, the optical members can be bonded together using the above double-sided adhesive sheet.

[0111] Regarding the gel fraction of the adhesive layer of the above adhesive sheet, it is preferably 20 to 100% by weight from the viewpoints of durability performance and adhesive strength, particularly preferably 50 to 90% by weight, and especially preferably 70 to 85% by weight. If the gel fraction is too low, the cohesive force decreases, and the durability tends to decrease. Also, if the gel fraction is too high, the adhesive strength tends to decrease, and the bending durability and the shape recovery of the adhesive layer tend to decrease.

[0112] The gel fraction serves as an indicator of the degree of crosslinking (degree of curing). For example, it is calculated by the following method. That is, an adhesive sheet (one without a separator) in which an adhesive layer is formed on a polymer sheet serving as a base material (e.g., a polyethylene terephthalate (PET) film, etc.) is wrapped with a 200-mesh SUS wire mesh and immersed in toluene at 23°C for 24 hours. The weight percentage of the insoluble adhesive layer remaining in the wire mesh with respect to the weight of the adhesive layer before the toluene immersion is defined as the gel fraction. However, the weight of the base material is subtracted.

[0113] In adjusting the gel fraction to the above range, for example, it can be achieved by adjusting the active energy ray irradiation dose, the amount of photoinitiator, the type and amount of the active energy ray curable monomer, and when using a crosslinking agent, by adjusting the type and amount of the crosslinking agent, etc.

[0114] The thickness of the adhesive layer of the above adhesive sheet is usually preferably 15 to 3000 μm, more preferably 20 to 1000 μm, and particularly preferably 50 to 350 μm. If the thickness of the adhesive layer is too thin, the shock absorbency tends to decrease, and if it is too thick, the thickness of the entire optical member increases and the practicality tends to decrease.

[0115] In the present invention, the film thickness is a value obtained by subtracting the measured value of the thickness of the constituent members other than the adhesive layer from the measured value of the thickness of the entire laminate containing the adhesive layer using "ID-C112B" manufactured by Mitutoyo Corporation.

[0116] The adhesive obtained by curing the adhesive composition of the present invention is excellent in adhesive strength, bending durability, reliability in a high-temperature environment, and shape recoverability of the adhesive. Therefore, it can be suitably used for double-sided adhesion applications and as an adhesive having impact resistance and strong adhesiveness. Specifically, it is useful as an adhesive component for applications such as attaching optical sheets such as glass, ITO transparent electrode sheets, polyethylene terephthalate (PET), polycarbonate (PC), polymethyl methacrylate (PMMA), and clear polyimide (CPI), polarizing plates, retardation plates, optical compensation films, and brightness enhancement films. Furthermore, it can be suitably used for image display devices such as touch panels including these optical members, particularly touch panels and image display devices such as foldable smartphones. In addition, the adhesive composition of the present invention can also be used as an adhesive for various labels and a masking adhesive, and is particularly suitably used for electronic component applications and the like.

Examples

[0117] Hereinafter, the present invention will be described more specifically with reference to examples, but the present invention is not limited to the following examples as long as the gist thereof is not exceeded. In the examples, “parts” and “%” mean weight basis. Also, with respect to the weight average molecular weight and haze of the acrylic resin, they were measured according to the measurement methods of the weight average molecular weight and haze of the acrylic resin described above.

[0118] Also, with respect to the measurement of the glass transition temperature of the acrylic resin, it was measured as follows. Note that the content of the structural site of the acrylic resin after completion (after polymerization) is substantially the same as the blending content of the polymerization components.

[0119] <Glass transition temperature (Tg) of acrylic resin> An acrylic resin solution before solvent removal was applied to a polyester release sheet and dried, and then laminated to produce a pressure-sensitive adhesive sheet with a thickness of about 650 μm in an uncrosslinked state. The dynamic viscoelasticity of the produced pressure-sensitive adhesive sheet was measured under the following conditions, and the temperature at which the loss tangent (loss elastic modulus G" / storage elastic modulus G' = tanδ) was maximized was read and taken as the glass transition temperature (Tg) of the acrylic resin. · Measuring instrument: DVA-225 (manufactured by IT Measurement and Control Co., Ltd.) · Deformation mode: shear · strain: 0.1% · Measuring temperature: -100 to 20 °C · Measuring frequency: 1 Hz

[0120] Prior to the examples, an acrylic resin was produced as follows.

[0121] 〈Production of Acrylic Resins (A-1) to (A-5)〉 Into a 2 L flask equipped with a cooler, 93 parts of ethyl acetate (boiling point 77 °C) as a polymerization solvent, 20 parts of acetone (boiling point 56 °C), 0.01 part of 2,2'-azobisisobutyronitrile (AIBN, half-life temperature 65 °C) as a polymerization initiator, and 100 parts of a polymerization component having the composition shown in Table 1 were heated to reflux in the flask. After 60 minutes, 20 parts of ethyl acetate and 0.01 part of AIBN were added dropwise over 2 hours and reacted to obtain an acrylic resin solution. The weight average molecular weights and glass transition temperatures of the obtained acrylic resins (A-1) to (A-5) were as shown in Table 1.

[0122] 〈Production of Acrylic Resins (A'-1) to (A'-3)〉 Into a 2 L flask equipped with a cooler, 35 parts of ethyl acetate (boiling point 77 °C) as a polymerization solvent and 0.01 part of 2,2'-azobis(2,4-dimethylvaleronitrile) (ADVN, half-life temperature 52 °C) as a polymerization initiator were heated to reflux in the flask. A solution prepared by premixing 100 parts of a polymerization component having the composition shown in Table 1, 10 parts of acetone, and 0.15 part of ADVN was added dropwise over 3 hours. After the dropwise addition, 10 parts of ethyl acetate and 0.14 part of ADVN were added dropwise over 1 hour and reacted to obtain an acrylic resin solution. The weight-average molecular weights and glass transition temperatures of the obtained acrylic resins (A'-1) to (A'-3) were as shown in Table 1.

[0123]

Table 1

[0124] Using the acrylic resins (A-1) to (A-5), (A'-1) to (A'-3) obtained above, adhesive compositions of Examples and Comparative Examples were produced.

[0125] <Example 1> An acrylic resin (A-i) solution was obtained by mixing an acrylic resin (A-1) solution (60 parts in terms of solid content) and an acrylic resin (A-3) solution (40 parts in terms of solid content). The difference in weight-average molecular weight between the acrylic resin (A-1) and the acrylic resin (A-3) in the acrylic resin (A-i) was 270,000, the difference in glass transition temperature was 4°C, the weight-average molecular weight of the acrylic resin (A-i) was 1,010,000, and the glass transition temperature was -22°C. Also, the haze of the acrylic resin (A-i) was 0.2%. To the above acrylic resin (A-i) solution (100 parts in terms of solid content), 7.5 parts of polypropylene glycol #400 diacrylate (NK Ester APG400, manufactured by Shin-Nakamura Chemical Co., Ltd.), 0.25 part of Omnirad 184 (manufactured by IGM Resins B.V.) and 0.75 part of Omnirad 754 (manufactured by IGM Resins B.V.) as a photopolymerization initiator were mixed to obtain an adhesive composition [I-1].

[0126] <Example 2> An acrylic resin (A-4) solution (40 parts in terms of solid content) and an acrylic resin (A-2) solution (60 parts in terms of solid content) were mixed to obtain an acrylic resin (A-iii) solution. The difference in weight-average molecular weight between the acrylic resin (A-4) and the acrylic resin (A-2) in the acrylic resin (A-iii) was 20,000, and the difference in glass transition temperature was 6 °C. The weight-average molecular weight of the acrylic resin (A-iii) was 1.1 million, and the glass transition temperature was -22 °C. Also, the haze of the acrylic resin (A-iii) was 0.3%. To 100 parts (in terms of solid content) of the above acrylic resin (A-iii) solution, 5.0 parts of polypropylene glycol #400 diacrylate (NK Ester APG400, manufactured by Shin-Nakamura Chemical Co., Ltd.), 0.25 part of Omnirad 184 (manufactured by IGM Resins B.V.) and 0.75 part of Omnirad 754 (manufactured by IGM Resins B.V.) as a photopolymerization initiator were mixed to obtain an adhesive composition [I-2].

[0127] <Example 3> An acrylic resin (A-2) solution (60 parts in terms of solid content) and an acrylic resin (A-5) solution (40 parts in terms of solid content) were mixed to obtain an acrylic resin (A-iv) solution. The difference in weight-average molecular weight between the acrylic resin (A-2) and the acrylic resin (A-5) in the acrylic resin (A-iv) was 60,000, and the difference in glass transition temperature was 6 °C. The weight-average molecular weight of the acrylic resin (A-iv) was 1.03 million, and the glass transition temperature was -23 °C. Also, the haze of the acrylic resin (A-iv) was 0.3%. To 100 parts (in terms of solid content) of the above acrylic resin (A-iv) solution, 5.0 parts of polypropylene glycol #400 diacrylate (NK Ester APG400, manufactured by Shin-Nakamura Chemical Co., Ltd.), 0.25 part of Omnirad 184 (manufactured by IGM Resins B.V.) and 0.75 part of Omnirad 754 (manufactured by IGM Resins B.V.) as a photopolymerization initiator were mixed to obtain an adhesive composition [I-3].

[0128] <Comparative Example 1> To 100 parts (in terms of solid content) of an acrylic resin (A'-1) solution, 5.0 parts of trimethylolpropane triacrylate, 0.25 part of Omnirad 184 (manufactured by IGM Resins B.V.) and 0.50 part of Omnirad 754 (manufactured by IGM Resins B.V.) as a photopolymerization initiator were mixed to obtain an adhesive composition [I'-1].

[0129] <Comparative Example 2> An acrylic resin (A'-3) solution (45 parts in terms of solid content) and an acrylic resin (A'-2) solution (55 parts in terms of solid content) were mixed to obtain an acrylic resin (A'-ii) solution. The difference in weight average molecular weight between the acrylic resin (A'-3) and the acrylic resin (A'-2) in the acrylic resin (A-ii) was 80,000, the difference in glass transition temperature was 4 °C, the weight average molecular weight of the acrylic resin (A'-ii) was 560,000, and the glass transition temperature was -22 °C. Also, the haze of the acrylic resin (A'-ii) was 0.1%. To 100 parts (in terms of solid content) of the above acrylic resin (A'-ii) solution, 5.0 parts of trimethylolpropane triacrylate, 0.25 part of Omnirad 184 (manufactured by IGM Resins B.V.) and 0.75 part of Omnirad 754 (manufactured by IGM Resins B.V.) as a photopolymerization initiator were mixed to obtain an adhesive composition [I'-2].

[0130] <Comparative Example 3> In Comparative Example 2, the same procedure was carried out except that 5.0 parts of trimethylolpropane triacrylate was changed to 10.0 parts of trimethylolpropane triacrylate to obtain an adhesive composition [I'-3].

[0131] The formulation compositions of the adhesive compositions of Examples 1 to 3 and Comparative Examples 1 to 3 obtained above are shown in Table 2 below.

[0132]

Table 2

[0133] Using the obtained adhesive compositions of the examples and comparative examples, a substrate-free double-sided adhesive sheet and a PET sheet with an adhesive layer were produced as follows.

[0134] <Production of Substrate-Free Double-Sided Adhesive Sheet and PET Sheet with Adhesive Layer> The above-obtained adhesive composition solution (solid content concentration 30%) was 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 an adhesive layer. After laminating three obtained adhesive layers, they were sandwiched between polyester-based release sheets, and further irradiated with ultraviolet rays using a high-pressure mercury UV irradiation device at a peak illuminance of 150 mW / cm 2 and an integrated exposure amount of 1000 mJ / cm 2 (500 mJ / cm 2 ×2 passes) to obtain a substrate-free double-sided adhesive sheet. At this time, the adhesive composition becomes an adhesive. Also, the release sheet on one side was peeled off from the adhesive layer of the above-obtained substrate-free double-sided adhesive sheet, and it was pressed against an easily adherable polyethylene terephthalate (PET) sheet (thickness 125 μm) to obtain a PET sheet with an adhesive layer having an adhesive layer thickness of about 150 μm.

[0135] Using the above-obtained substrate-free double-sided adhesive sheets and PET sheets with adhesive layers of the examples and comparative examples, evaluations of gel fraction, adhesive strength, bending durability, high-temperature reliability (high-temperature adhesive strength, 80°C constant load holding force, 80°C holding force), shape recovery, and stress relaxation (shear stress, relaxation rate) were performed. The results are shown in Table 3 below.

[0136] 〔Gel Fraction〕 After cutting the above substrate-free double-sided adhesive sheet into a size of 40 mm × 40 mm, it was irradiated with ultraviolet rays using a high-pressure mercury UV irradiation device at a peak illuminance of 150 mW / cm 2 and an integrated exposure amount of 2000 mJ / cm 2 (1000 mJ / cm 2Ultraviolet irradiation was carried out in a 2-pass manner, and after standing for 30 minutes under the conditions of 23 °C and 50% RH, one release sheet was peeled off, and the adhesive layer side was bonded to a SUS mesh sheet (200 mesh) with a size of 50 mm × 100 mm. Then, the other release sheet was peeled off, and after folding back from the center with respect to the longitudinal direction of the SUS mesh sheet to wrap the sample, it was immersed in a sealed container containing 250 g of toluene for 24 hours. The weight percentage of the insoluble adhesive layer remaining on the SUS mesh sheet with respect to the weight of the adhesive layer before toluene immersion was defined as the gel fraction (%).

[0137] 〔Adhesive strength〕 For the PET sheet with the above-mentioned adhesive layer, it was cut into a size of 25 mm in width × 100 mm in length, and irradiated with ultraviolet rays using a high-pressure mercury UV irradiation device with a peak illuminance of 150 mW / cm 2 and an integrated exposure amount of 2000 mJ / cm 2 (1000 mJ / cm 2 ×2 passes). After ultraviolet irradiation, the release sheet was peeled off, and the adhesive layer side was pressure-bonded to non-alkali glass (Corning's "Eagle XG", thickness 1.1 mm) with a 2 kg rubber roller reciprocating twice under an atmosphere of 23 °C and 50% RH. After standing for 30 minutes under the conditions of 23 °C × 50% RH, the 180-degree peel strength (N / 25 mm) was measured at a peel rate of 300 mm / min at room temperature (23 °C).

[0138] 〔Bending durability〕 For the PET sheet with the above-mentioned adhesive layer, the adhesive layer side of the adhesive sheet was pressed against a transparent polyimide sheet (thickness 50 μm) under an environment of 23 °C and 50% RH to obtain a test piece with a layer structure of "PET sheet / adhesive layer / polyimide sheet". After that, for the above test piece, ultraviolet irradiation was carried out from the PET sheet side using a high-pressure mercury UV irradiation device with a peak illuminance of 150 mW / cm 2 and an integrated exposure amount of 2000 mJ / cm 2 (1000 mJ / cm 2 ×2 passes). After ultraviolet irradiation, it was cut into a size of 40 mm in width × 120 mm in length, and the following repeated bending test was carried out. The repeated bending test was carried out with the PET sheet side facing inward under the environment of 23°C and 50% RH, and the test conditions are as follows. [Test conditions] · Test equipment: Flat specimen unloaded U-shaped expansion and contraction tester DLDM111LH (manufactured by Yuasa System Equipment Co., Ltd.) · Bending speed: 45 times / min · Bending radius: 2.5 mm In such repeated bending tests, the number of times without visual appearance change was measured and evaluated according to the following criteria. (Evaluation criteria) ◎... 200,000 times or more ○... 100,000 times or more and less than 200,000 times ×... Less than 100,000 times

[0139] [High-temperature adhesion] For the above PET sheet with an adhesive layer, it was cut into a size of 10 mm in width × 100 mm in length, and irradiated with ultraviolet rays by a high-pressure mercury UV irradiation device with a peak illuminance of 150 mW / cm 2 and an integrated exposure amount of 2000 mJ / cm 2 (1000 mJ / cm 2 × 2 passes). After ultraviolet irradiation, the release sheet was peeled off, and the adhesive layer side was pasted onto non-alkali glass (Corning's "Eagle XG", thickness 1.1 mm) with a 2 kg rubber roller reciprocating twice in an atmosphere of 23°C and 50% RH. After that, autoclave treatment (50°C, 0.5 MPa, 20 minutes) was carried out, and after standing for 60 minutes under the condition of 60°C, the peel strength (N / 10 mm) at a peel speed of 300 mm / min at 180 degrees was measured at 60°C. The evaluation criteria are as follows. (Evaluation criteria) ◎... 7.0 N or more 〇... 6.0 N or more and less than 7.0 N △... 5.0 N or more and less than 6.0 N ×... Less than 5.0 N

[0140] [Constant load holding force] Regarding the PET sheet with the above-mentioned adhesive layer, it was cut into a size of 25 mm × 75 mm (adhesive layer part: 25 mm × 50 mm + non-adhesive layer part: 25 mm × 25 mm), and irradiated with ultraviolet rays using a high-pressure mercury UV irradiation device at a peak illuminance of 150 mW / cm 2 and an integrated exposure dose of 2000 mJ / cm 2 (1000 mJ / cm 2 × 2 passes). After that, the release sheet was peeled off, and it was pressure-bonded to a stainless steel plate (SUS304) by reciprocating a 2 kg roller (bonding area: 25 mm × 50 mm), and left standing for 20 minutes in an 80°C atmosphere. Then, a 50 g weight was hung at the end of the non-bonded part (area: 25 mm × 25 mm), a 50 g load was applied in a direction 90° to the plane of the stainless steel plate, and it was left standing in that state for 60 minutes, and the distance at which the PET sheet peeled off was measured. The evaluation criteria are as follows. (Evaluation criteria) ◎···Peeling distance is less than 1 mm 〇···Peeling distance is 1 mm or more and less than 25 mm △···Peeling distance is 25 mm or more and 50 mm or less ×···Falling off

[0141] 〔Retention force〕 Regarding the PET sheet with the above-mentioned adhesive layer, it was cut into a size of 25 mm × 50 mm, and irradiated with ultraviolet rays using a high-pressure mercury UV irradiation device at a peak illuminance of 150 mW / cm 2 and an integrated exposure dose of 2000 mJ / cm 2 (1000 mJ / cm 2 × 2 passes). After that, the release sheet was peeled off, and it was pressure-bonded to a stainless steel plate (SUS304) by reciprocating a 2 kg roller (bonding area: 25 mm × 25 mm), and the retention force was measured using a creep tester (manufactured by Tester Sangyo Co., Ltd., holding force tester BE-501 with a high-temperature and high-humidity chamber) by applying a load of 1 kg over 24 hours in an 80°C atmosphere. The evaluation criteria are as follows. (Evaluation criteria) ◎···No displacement 〇···Displacement is less than 0.1 mm △···Displacement is 0.1 mm or more and less than 1.0 mm ×···Displacement is 1.0 mm or more, or falling off

[0142] [Shape recoverability after the adhesive layer is deformed] The substrate-free double-sided adhesive sheet was laminated to a thickness of about 650 μm, and ultraviolet irradiation was performed using a high-pressure mercury UV irradiation device with a peak illuminance of 150 mW / cm 2 and an integrated exposure amount of 2000 mJ / cm 2 (1000 mJ / cm 2 × 2 passes), and then it was cut into a size of 25 mm in diameter to prepare a measurement sample. One release sheet of the sample was peeled off, bonded to a measurement jig (parallel plate), the other release sheet was peeled off, and it was bonded onto the lower plate of a rotational rheometer (Rotational Rheometer MCR301 manufactured by Anton Paar). After stretching by 200% under the condition of 23°C, the stress was released, and the strains 1 minute and 5 minutes after the stress release were measured respectively. The recovery rate (%) was calculated from the following formula and evaluated according to the following criteria. Recovery rate (%) = [(200 - strain) / 200] × 100 (Evaluation criteria) ◎ ··· The recovery rate 1 minute after is 55% or more, and the recovery rate 5 minutes after is 75% or more ○ ··· The recovery rate 1 minute after is 55% or more, and the recovery rate 5 minutes after is 65% or more and less than 75% × ··· The recovery rate 1 minute after is less than 55%

[0143] [Stress relaxation property] The substrate-free double-sided adhesive sheet was laminated to a thickness of about 650 μm, and ultraviolet irradiation was performed using a high-pressure mercury UV irradiation device with a peak illuminance of 150 mW / cm 2 and an integrated exposure amount of 2000 mJ / cm 2 (1000 mJ / cm 2 × 2 passes), and then it was cut into a size of 25 mm in diameter to prepare a measurement sample. One release sheet of the sample was peeled off, bonded to a measurement jig (parallel plate), the other release sheet was peeled off, and it was bonded onto the lower plate of a rotational rheometer (Rotational Rheometer MCR301 manufactured by Anton Paar). After stretching by 200% under the condition of 23°C, the stress was controlled to maintain a strain of 200%. When the strain reached 200%, it was set as 0 minute, and the shear stress 10 minutes later was measured. The stress relaxation rate was calculated by the following formula. Stress relaxation rate (%) = [(Shear stress at 0 minutes - Shear stress after 10 minutes) / Shear stress at 0 minutes] × 100

[0144] [Table 3]

[0145] As can be seen from Table 3 above, in Examples 1 to 3, all of the adhesive strength, bending durability, high-temperature reliability, shape recovery after deformation of the adhesive layer, and stress relaxation were excellently balanced. On the other hand, in Comparative Examples 1 to 3, none were excellently balanced in various physical properties aimed at by the present invention.

Industrial Applicability

[0146] The adhesive composition of the present invention is excellent in the adhesive strength to an adherend, excellent in bending durability and high-temperature reliability, and further excellent in the shape recovery after deformation of the adhesive layer. Therefore, it can be suitably used for touch panels, image display devices, etc., and impact-absorbing sheets, etc.

Claims

1. An adhesive composition containing an acrylic resin (A), wherein the acrylic resin (A) contains at least two acrylic resins having different weight average molecular weights, the weight average molecular weight (M1) of the acrylic resin (A1) having the highest weight average molecular weight is 800,000 to 2,000,000, and the weight average molecular weight (M2) of the acrylic resin (A2) having the lowest weight average molecular weight is 700,000 to 1,500,000, the glass transition temperature (T) read from the temperature at which the loss tangent of the dynamic viscoelasticity of the acrylic resin (A) is maximum is -40 to -15 °C, the glass transition temperature (T1) read from the temperature at which the loss tangent of the dynamic viscoelasticity of the acrylic resin (A1) is maximum is -30 to -10 °C, the glass transition temperature (T2) read from the temperature at which the loss tangent of the dynamic viscoelasticity of the acrylic resin (A2) is maximum is lower than the above T1 and is -40 to -20 °C, the content ratio [A1 / A2] of the acrylic resin (A1) and the acrylic resin (A2) is 70 / 30 to 30 / 70 by weight ratio, the content of the structural site derived from the carboxy group-containing monomer in the acrylic resin (A) is 0.1% by weight or less, the content of the structural site derived from the hydroxyl group-containing monomer (a1) in the acrylic resin (A) is 5 to 20% by weight based on the total polymerization components, the content of the structural site derived from at least one copolymerizable monomer (a2) selected from (meth)acrylic acid alkyl ester monomers and vinyl ester monomers having an alkyl group with 5 to 14 carbon atoms is 40 to 85% by weight based on the total polymerization components, An adhesive composition characterized in that the haze of the acrylic resin (A) is 2.0% or less.

2. The adhesive composition according to Claim 1, characterized in that the difference (|M1 - M2|) between the weight average molecular weight (M1) of the acrylic resin (A1) and the weight average molecular weight (M2) of the acrylic resin (A2) is 700,000 or less.

3. The adhesive composition according to Claim 1 or 2, characterized in that the weight average molecular weight (M) of the acrylic resin (A) is 700,000 to 2,000,000.

4. The temperature difference between the glass transition temperature (T1) read from the temperature at which the loss tangent of the dynamic viscoelasticity of the acrylic resin (A1) is maximum and the glass transition temperature (T2) read from the temperature at which the loss tangent of the dynamic viscoelasticity of the acrylic resin (A2) is maximum is 20°C or less. The pressure-sensitive adhesive composition according to any one of claims 1 to 3, characterized in that.

5. At least one of the acrylic resins contained in the acrylic resin (A) is an acrylic resin copolymerized with a hydroxyl group-containing monomer containing 2-hydroxyethyl (meth)acrylate and 4-hydroxybutyl (meth)acrylate as polymerization components. The pressure-sensitive adhesive composition according to any one of claims 1 to 4, characterized in that.

6. A pressure-sensitive adhesive characterized by using the pressure-sensitive adhesive composition according to any one of claims 1 to 5.

7. A pressure-sensitive adhesive sheet characterized by having a pressure-sensitive adhesive layer formed using the pressure-sensitive adhesive composition according to any one of claims 1 to 5.

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