JP2024064940A - Adhesive composition and adhesive sheet for foldable displays - Google Patents

Abstract

[Problem] The present invention aims to provide an adhesive composition for a foldable display that achieves both bending resistance and high adhesive strength, while suppressing changes in adhesive strength to a release film over time. [Solution] The composition includes a (meth)acrylic copolymer (A) containing a monomer (A1) having a functional group and a (meth)acrylic acid alkyl ester monomer (A2) as structural units, a (meth)acrylic tackifier (B) containing a monomer (B1) having a functional group, a (meth)acrylic acid alkyl ester monomer (B2) having a homopolymer glass transition temperature (Tg) of -50°C or lower, and a (meth)acrylic acid (cyclo)alkyl ester monomer (B3) having a homopolymer glass transition temperature of 40°C or higher, and a crosslinking agent (C). [Selected Figure] None

Description

The present invention relates to an adhesive composition and an adhesive sheet for foldable displays.

In recent years, portable electronic devices equipped with a foldable display (so-called foldable display) are becoming more and more popular. In a foldable display, bending is repeatedly performed at the same place, and the adhesive sheet may peel off from the adherend at the bending point. Therefore, it is required that the adhesive composition used for the foldable display can form an adhesive layer with excellent bending resistance. It is generally known that the bending resistance is better when the stress relaxation property of the adhesive layer is high. As an example of a design that makes the adhesive layer softer, the lower the glass transition temperature (Tg) of the (meth)acrylic copolymer in the adhesive composition, the more excellent the bending resistance of the adhesive layer can be formed. However, when the adhesive layer is designed to be soft, the bending resistance is improved, but on the other hand, there is a tendency for the problem of a decrease in cohesive force and a large decrease in adhesive strength to occur. For the above reasons, since bending resistance and adhesive strength are in a trade-off relationship, it has been difficult to achieve performance improvements in both.

Patent Document 1 proposes a pressure-sensitive adhesive sheet containing an acrylic base polymer with a crosslinked structure and an acrylic oligomer with a glass transition temperature (Tg) of 60°C or higher as a method for improving bending resistance without reducing adhesive strength.

JP 2020-122140 A

However, the (meth)acrylic oligomer used in the adhesive composition of Patent Document 1 has a glass transition temperature (Tg) of 60° C. or higher (i.e., a high Tg), and therefore has poor stress relaxation properties, and when stress accumulates due to repeated folding over a long period of time, it is prone to poor appearance such as lifting and peeling. Therefore, the technology of Patent Document 1 cannot meet the high requirements for folding resistance. Furthermore, adhesive sheets made from adhesive layers containing high Tg (meth)acrylic oligomers tend to have increased adhesive strength to release films over time. This is thought to be due to the fact that the molecular weight of the (meth)acrylic oligomer is very low compared to the (meth)acrylic base polymer, so it is prone to migrate to the surface of the adhesive layer over time, and further, the (meth)acrylic oligomer component that has migrated to the surface of the adhesive layer has a high Tg, which causes an excessive increase in cohesive strength at the interface between the release film and the adhesive layer. In other words, if a long time has passed since the adhesive sheet was processed into one and then transported, etc., a great deal of force is required to peel the adhesive sheet from the release film, which may cause problems during the process. Thus, the technology of Patent Document 1 does not adequately achieve both bending resistance and high adhesive strength, and there is a problem in that the adhesive strength to the release film increases over time.

Therefore, the present invention aims to provide an adhesive composition for foldable displays that combines the two properties of bending resistance and high adhesive strength, while suppressing changes over time in adhesive strength to release films.

In order to achieve the object of the present invention, the adhesive composition for a foldable display of the present invention comprises a (meth)acrylic copolymer (A) containing, as structural units, a monomer (A1) having a functional group and a (meth)acrylic acid alkyl ester monomer (A2); a (meth)acrylic taconamide copolymer (B) containing, as structural units, a monomer (B1) having a functional group, a (meth)acrylic acid alkyl ester monomer (B2) having a homopolymer glass transition temperature (Tg) of -50°C or lower, and a (meth)acrylic acid (cyclo)alkyl ester monomer (B3) having a homopolymer glass transition temperature of 40°C or higher. The adhesive composition for foldable displays includes a tackifier (B) and a crosslinking agent (C), and the glass transition temperature (Tg) of the (meth)acrylic copolymer (A) is -50°C or lower, and the glass transition temperature (Tg) of the (meth)acrylic tackifier (B) is -10°C or higher and 40°C or lower, where the glass transition temperature (Tg) of the (meth)acrylic copolymer (A) and the glass transition temperature (Tg) of the (meth)acrylic tackifier (B) are calculated based on the FOX formula, and the adhesive layer made of the adhesive composition for foldable displays has an adhesive strength of 6.0 N/25 mm or higher, where the adhesive strength is a value measured in accordance with the provisions of JIS Z0237:2009.

The present invention provides an adhesive composition for foldable displays that combines the two properties of bending resistance and high adhesive strength, while suppressing changes over time in adhesive strength to a release film.

The embodiments are described in detail below. Note that the following embodiments do not limit the invention according to the claims, and not all combinations of features described in the embodiments are necessarily essential to the invention. Two or more features among the multiple features described in the embodiments may be combined in any desired manner.

<Adhesive composition for foldable displays>
The adhesive composition for a foldable display of the present invention includes a (meth)acrylic copolymer (A) containing a monomer (A1) having a functional group and a (meth)acrylic acid alkyl ester monomer (A2) as structural units, a monomer (B1) having a functional group, a (meth)acrylic acid alkyl ester monomer (B2) having a homopolymer glass transition temperature (Tg) of -50 ° C. or less, and a (meth)acrylic acid (cyclo) alkyl ester monomer (B3) having a homopolymer glass transition temperature of 40 ° C. or more as structural units, and a (meth)acrylic tackifier (B), and a crosslinking agent (C). The adhesive composition for a foldable display of the present invention may further include a crosslinking reaction accelerator in addition to the above components (A) to (C).

The adhesive composition for foldable displays contains 0.5 to 20 parts by mass, preferably 0.5 to 14 parts by mass, of (meth)acrylic tackifier (B) per 100 parts by mass of (meth)acrylic copolymer (A). Here, if the adhesive composition for foldable displays contains more than 20 parts by mass of (meth)acrylic tackifier (B) per 100 parts by mass of (meth)acrylic copolymer (A), the cohesive force of the adhesive layer becomes too large, and the stress relaxation property decreases, resulting in a decrease in bending resistance. On the other hand, if the adhesive composition for foldable displays contains less than 0.5 parts by mass of (meth)acrylic tackifier (B) per 100 parts by mass of (meth)acrylic copolymer (A), the cohesive force of the adhesive layer decreases, resulting in a decrease in adhesive strength.

((Meth)acrylic copolymer (A))
The (meth)acrylic copolymer (A) contains a monomer (A1) having a functional group and a (meth)acrylic acid alkyl ester monomer (A2) as constituent units. Here, when the (meth)acrylic copolymer (A) does not contain a monomer (A1) having a functional group as a constituent unit and contains only the (meth)acrylic acid alkyl ester monomer (A2) as a constituent unit, the (meth)acrylic copolymer (A) does not have a component that reacts with the crosslinking agent (C), so it cannot form a crosslinked structure. Therefore, it cannot exert an appropriate cohesive force, and a phenomenon of adhesive residue due to cohesive failure occurs during peeling. Furthermore, it cannot form a crosslinked structure with the (meth)acrylic tackifier (B) via the crosslinking agent (C), so the (meth)acrylic tackifier (B) is easily transferred to the pressure-sensitive adhesive layer surface (film surface), which causes a problem of an increase in adhesive force over time to the release film. In this specification, the term "component (A1)" refers to the monomer (A1) having a functional group, and the term "component (A2)" refers to the alkyl (meth)acrylate monomer (A2).

The (meth)acrylic copolymer (A) contains 0.1 to 8.0 parts by mass, preferably 0.2 to 5.0 parts by mass, of a monomer (A1) having a functional group per 100 parts by mass of the (meth)acrylic copolymer (A). Here, if the (meth)acrylic copolymer (A) contains more than 8.0 parts by mass of the monomer (A1) having a functional group per 100 parts by mass of the (meth)acrylic copolymer (A), the crosslink density of the adhesive layer becomes too large, and the adhesive strength and bending resistance of the adhesive layer decrease. On the other hand, if the (meth)acrylic copolymer (A) contains less than 0.1 parts by mass of the monomer (A1) having a functional group per 100 parts by mass of the (meth)acrylic copolymer (A), the crosslink density of the adhesive layer becomes very small, and the adhesive layer cannot exert an appropriate cohesive strength, and the phenomenon of adhesive residue due to cohesive failure occurs during peeling. Furthermore, since the crosslinking structure with the (meth)acrylic tackifier (B) via the crosslinking agent (C) cannot be sufficiently formed, the (meth)acrylic tackifier (B) is likely to migrate to the surface of the adhesive layer (film surface), causing a problem of increased adhesive strength to the release film over time. This requires a great deal of force to peel the adhesive sheet from the release film, which can cause problems during work.

The (meth)acrylic copolymer (A) contains 50 to 99.9 parts by mass, preferably 70 to 99.0 parts by mass, of (meth)acrylic acid alkyl ester monomer (A2) per 100 parts by mass of the (meth)acrylic copolymer (A).

The (meth)acrylic copolymer (A) contains a (meth)acrylic acid alkyl ester monomer (A2) as the main monomer component. In this specification, "(meth)acrylic" means "acrylic" and "methacrylic", and "(meth)acrylate" means "acrylate" and "methacrylate".

(Monomer (A1) having a functional group)
The monomer (A1) having a functional group is not limited to a specific type of monomer containing a functional group, so long as it is a monomer containing a functional group. The monomer (A1) having a functional group is a polar monomer such as a hydroxyl group-containing monomer, a carboxyl group-containing monomer, a nitrogen-containing monomer, or an epoxy group-containing monomer.

The monomer (A1) having a functional group includes hydroxyl group-containing monomers such as 2-hydroxyethyl (meth)acrylate and 4-hydroxybutyl (meth)acrylate, carboxyl group-containing monomers such as (meth)acrylic acid and β-carboxyethyl (meth)acrylate, nitrogen-containing monomers such as (meth)acrylamide and dimethylaminopropyl (meth)acrylamide, and epoxy group-containing monomers such as glycidyl (meth)acrylate and 4-hydroxybutyl (meth)acrylate glycidyl ether. The monomer (A1) having a functional group is preferably 4-hydroxybutyl acrylate (Tg; -32°C), 2-hydroxyethyl acrylate (Tg; -15°C), and acrylic acid (Tg; 106°C). The monomer (A1) having a functional group may include only one of the above, or may include two or more of them.

From the viewpoint of improving adhesive strength and bending resistance, and suppressing the rate of change in adhesive strength to a release film, the monomer (A1) having a functional group is preferably a hydroxyl group-containing monomer. In addition, the monomer containing the functional group described above reacts with an isocyanate-based crosslinking agent and an epoxy-based crosslinking agent, etc., and therefore it becomes possible to introduce a crosslinked structure into the acrylic copolymer (A). This is thought to lead to improved cohesive strength and adhesion of the adhesive layer.

((Meth)acrylic acid alkyl ester monomer (A2))
The type of (meth)acrylic acid alkyl ester monomer (A2) is not particularly limited. The alkyl group of the (meth)acrylic acid alkyl ester monomer (A2) may be either linear or branched. The alkyl group preferably has 1 to 18 carbon atoms. In order to lower the glass transition temperature (Tg) of the (meth)acrylic copolymer (A), the glass transition temperature (Tg) of the homopolymer of the (meth)acrylic acid alkyl ester monomer (A2) is preferably −50° C. or lower.

The (meth)acrylic acid alkyl ester monomer (A2) includes 2-ethylhexyl acrylate (Tg; -70°C), n-hexyl acrylate (Tg; -65°C), n-octyl acrylate (Tg; -65°C), isononyl acrylate (Tg; -60°C), n-nonyl acrylate (Tg; -58°C), isooctyl acrylate (Tg; -58°C), butyl acrylate (Tg; -52°C), n-dodecyl methacrylate (Tg; -65°C), and the like. The (meth)acrylic acid alkyl ester monomer (A2) is preferably butyl acrylate and 2-ethylhexyl acrylate. The (meth)acrylic acid alkyl ester monomer (A2) may include only one of the above, or may include two or more of the above.

(Physical Properties)
The (meth)acrylic copolymer (A) has a glass transition temperature (Tg) of -50°C or lower. The method for calculating the glass transition temperature (Tg) of the (meth)acrylic copolymer (A) will be described later. When the glass transition temperature (Tg) of the (meth)acrylic copolymer (A) exceeds -50°C, the stress relaxation property of the pressure-sensitive adhesive layer is reduced, resulting in reduced bending resistance.

The weight average molecular weight of the (meth)acrylic copolymer (A) is 500,000 or more and 2,500,000 or less, preferably 700,000 or more and 2,000,000 or less, and more preferably 800,000 or more and 1,800,000 or less.

(Polymerization method, etc.)
The (meth)acrylic copolymer (A) can be produced by ordinary solution polymerization, bulk polymerization, emulsion polymerization, suspension polymerization, etc. In particular, it is preferable to produce the (meth)acrylic copolymer (A) by using solution polymerization that can obtain the (meth)acrylic copolymer (A) as a solution. This allows the (meth)acrylic copolymer (A) in a solution state to be used as is in the production of the pressure-sensitive adhesive composition for foldable displays of the present invention. Solvents used in solution polymerization include organic solvents such as, for example, ethyl acetate, toluene, n-hexane, acetone, and methyl ethyl ketone.

(Polymerization initiator)
Examples of the polymerization initiator used when polymerizing the above components (A1) and (A2) include oil-soluble organic peroxides such as 2,4-dichlorobenzoyl peroxide, t-butyl peroxypivalate, benzoyl peroxide, o-methylbenzoyl peroxide, bis-3,5,5-trimethylhexanoyl peroxide, octanoyl peroxide, t-butylperoxy-2-ethylhexanoate, cyclohexanone peroxide, methyl ethyl ketone peroxide, dicumyl peroxide, lauroyl peroxide, diisopropylbenzene hydroperoxide, t-butyl hydroperoxide, and t-butyl peroxide, and oil-soluble azo compounds such as 2,2'-azobisisobutyronitrile, 2,2'-azobis(2,4-dimethylvaleronitrile), and 2,2'-(2,4-dimethyl-4-methoxyvaleronitrile). The oil-soluble polymerization initiator is preferably an oil-soluble azo compound. The oil-soluble polymerization initiator may contain only one type of the above-mentioned oil-soluble polymerization initiators, or may contain two or more types of the above-mentioned oil-soluble polymerization initiators.

(Chain Transfer Agent)
In order to adjust the molecular weight of the (meth)acrylic copolymer (A), a chain transfer agent can be appropriately used. Examples of the chain transfer agent include mercaptans such as octyl thioglycolate, methoxybutyl thioglycolate, octyl mercaptopropionate, methoxybutyl mercaptopropionate, stearyl mercaptan, lauryl mercaptan, and the like, and α-methylstyrene dimer, and the like, and one of the above may be used alone or two or more may be used in combination.

(Other additives)
In addition to the above-mentioned components (A1) and (A2), the (meth)acrylic copolymer (A) may contain additives for adjusting adhesive strength and other properties, etc. Specific examples of the additives include tackifiers, plasticizers, silane coupling agents, antioxidants, and antistatic agents.

((Meth)acrylic tackifier (B))
The (meth)acrylic tackifier (B) contains, as structural units, a monomer (B1) having a functional group, a (meth)acrylic acid alkyl ester monomer (B2) having a homopolymer glass transition temperature (Tg) of −50° C. or lower, and a (meth)acrylic acid (cyclo)alkyl ester monomer (B3) having a homopolymer glass transition temperature of 40° C. or higher.

Here, if the (meth)acrylic tackifier (B) does not contain the above-mentioned (B1) component as a constituent unit, but contains the above-mentioned (B2) component and (B3) component as constituent units, the above-mentioned (B2) component cannot form a crosslinked structure, and further cannot form a crosslinked structure with the (meth)acrylic copolymer (A) via the crosslinking agent (C), so that the (meth)acrylic tackifier (B) is likely to migrate to the pressure-sensitive adhesive layer surface (film surface), resulting in a problem of an increase in adhesive strength to the release film over time.

In addition, if the (meth)acrylic tackifier (B) does not contain the above-mentioned (B2) component as a constituent unit or if the content is low and the glass transition temperature (Tg) of the (meth)acrylic tackifier (B) exceeds 40°C, the stress relaxation properties of the (meth)acrylic tackifier (B) are insufficient, and the bending resistance tends to decrease. On the other hand, if the (meth)acrylic tackifier (B) does not contain the above-mentioned (B3) component as a constituent unit or if the content is low and the glass transition temperature (Tg) of the (meth)acrylic tackifier (B) is less than -10°C, the cohesive strength of the (meth)acrylic tackifier (B) is insufficient, and the adhesive strength tends to decrease.

In this specification, "(B1) component" refers to a monomer (B1) having a functional group, "(B2) component" refers to a (meth)acrylic acid alkyl ester monomer (B2) whose homopolymer has a glass transition temperature (Tg) of -50°C or lower, and "(B3) component" refers to a (meth)acrylic acid (cyclo)alkyl ester monomer (B3) whose homopolymer has a glass transition temperature of 40°C or higher.

The (meth)acrylic tackifier (B) contains 0.1 to 20 parts by mass, preferably 1 to 17 parts by mass, of a monomer (B1) having a functional group per 100 parts by mass of the (meth)acrylic tackifier (B). Here, if the (meth)acrylic tackifier (B) contains more than 20 parts by mass of the (B1) component per 100 parts by mass of the (meth)acrylic tackifier (B), the crosslinking density increases and the migration of the (meth)acrylic tackifier (B) to the pressure-sensitive adhesive layer surface is greatly reduced. As a result, the effect of improving the cohesive force at the interface between the pressure-sensitive adhesive layer and the adherend is not fully exerted, and the adhesive force is reduced, resulting in a reduced practicality as an adhesive for foldable displays. On the other hand, if the (meth)acrylic tackifier (B) contains less than 0.1 parts by mass of the (B1) component per 100 parts by mass of the (meth)acrylic tackifier (B), a crosslinked structure cannot be formed with the (meth)acrylic copolymer (A) via the crosslinking agent (C), and the (meth)acrylic tackifier (B) tends to migrate excessively to the pressure-sensitive adhesive layer surface (film surface), resulting in a problem of increased adhesive strength to the release film over time.

The (meth)acrylic tackifier (B) contains 10 to 60 parts by mass, preferably 10 to 55 parts by mass, more preferably 10 to 45 parts by mass of a (meth)acrylic acid alkyl ester monomer (B2) having a homopolymer glass transition temperature (Tg) of -50°C or less per 100 parts by mass of the (meth)acrylic tackifier (B). Here, if the (meth)acrylic tackifier (B) contains more than 60 parts by mass of the (B2) component per 100 parts by mass of the (meth)acrylic tackifier (B), the cohesive force of the (meth)acrylic tackifier (B) is insufficient, and therefore the adhesive strength tends to decrease. On the other hand, if the (meth)acrylic tackifier (B) contains less than 10 parts by mass of the (B2) component per 100 parts by mass of the (meth)acrylic tackifier (B), the stress relaxation property of the acrylic tackifier (B) is insufficient, and therefore the bending resistance tends to decrease.

The (meth)acrylic tackifier (B) contains 35 to 85 parts by mass, preferably 40 to 85 parts by mass, and more preferably 50 to 85 parts by mass of a (meth)acrylic acid alkyl ester monomer (B3) having a glass transition temperature of 40°C or higher when in a homopolymer form per 100 parts by mass of the (meth)acrylic tackifier (B). Here, if the (meth)acrylic tackifier (B) contains more than 85 parts by mass of the (B3) component per 100 parts by mass of the (meth)acrylic tackifier (B), the stress relaxation properties of the acrylic tackifier (B) are insufficient, and therefore the bending resistance tends to decrease. On the other hand, if the (meth)acrylic tackifier (B) contains less than 35 parts by mass of the (B3) component per 100 parts by mass of the (meth)acrylic tackifier (B), the cohesive strength of the meth)acrylic tackifier (B) is insufficient, and therefore the adhesive strength tends to decrease.

In this specification, "(cyclo)alkyl" refers to "alkyl" and "cycloalkyl", and "(meth)acrylic acid (cyclo)alkyl ester monomer" refers to "acrylic acid alkyl ester monomer", "methacrylic acid alkyl ester monomer", "acrylic acid cycloalkyl ester monomer", and "methacrylic acid cycloalkyl ester monomer".

(Monomer (B1) having a functional group)
The monomer (B1) having a functional group is not limited to a specific type of monomer containing a functional group, so long as it is a monomer containing a functional group. The monomer (B1) having a functional group is a polar monomer such as a hydroxyl group-containing monomer, a carboxyl group-containing monomer, a nitrogen-containing monomer, or an epoxy group-containing monomer.

The monomer (B1) having a functional group includes a hydroxyl group-containing monomer such as 2-hydroxyethyl (meth)acrylate and 4-hydroxybutyl (meth)acrylate, a carboxyl group-containing monomer such as (meth)acrylic acid and β-carboxyethyl (meth)acrylate, a nitrogen-containing monomer such as (meth)acrylamide and dimethylaminopropyl (meth)acrylamide, an epoxy group-containing monomer such as glycidyl (meth)acrylate and 4-hydroxybutyl (meth)acrylate glycidyl ether, and the like. The monomer (B1) having a functional group is preferably 4-hydroxybutyl acrylate (Tg; -32°C), 2-hydroxyethyl acrylate (Tg; -15°C) and acrylic acid (Tg; 106°C). The monomer (B1) having a functional group may contain only one of the above, or may contain two or more of them. In addition, from the viewpoint of compatibility with the (meth)acrylic copolymer (A), it is preferable that the monomer (B1) having a functional group is a monomer having the same functional group as the monomer (A1) having a functional group.

In addition, the monomer containing the functional group described above reacts with an isocyanate-based crosslinking agent and an epoxy-based crosslinking agent, etc., making it possible to introduce a crosslinked structure into the (meth)acrylic tackifier (B). Furthermore, the (meth)acrylic tackifier (B) can form a crosslinked structure with the (meth)acrylic copolymer (A) via an isocyanate crosslinking agent, etc. This suppresses the migration of the (meth)acrylic tackifier (B) to the pressure-sensitive adhesive layer surface (film surface), thereby resolving the problem of increased adhesive strength to the release film over time.

((Meth)acrylic acid alkyl ester monomer (B2))
The type of (meth)acrylic acid alkyl ester monomer (B2) is not particularly limited as long as it is a (meth)acrylic acid alkyl ester monomer having a homopolymer glass transition temperature (Tg) of -50°C or less. The alkyl group of the (meth)acrylic acid alkyl ester monomer (B2) may be either linear or branched. The number of carbon atoms of the alkyl group is preferably 1 to 18. In addition, it is preferable that the difference in glass transition temperature (Tg) between the above-mentioned (B2) component and the (B3) component, which are the constituent units of the (meth)acrylic tackifier (B), is large. Furthermore, it is more preferable to use the same (meth)acrylic acid alkyl ester monomer (B2) as the (meth)acrylic acid alkyl ester monomer (A2) from the viewpoint of compatibility with the (meth)acrylic copolymer (A).

(Meth)acrylic acid alkyl ester monomer (B2) includes 2-ethylhexyl acrylate (Tg; -70°C), n-hexyl acrylate (Tg; -65°C), n-octyl acrylate (Tg; -65°C), isononyl acrylate (Tg; -60°C), n-nonyl acrylate (Tg; -58°C), isooctyl acrylate (Tg; -58°C), butyl acrylate (Tg; -52°C), n-dodecyl methacrylate (Tg; -65°C), etc. (Meth)acrylic acid alkyl ester monomer (B2) is preferably butyl acrylate and 2-ethylhexyl acrylate. (Meth)acrylic acid alkyl ester monomer (B2) may include only one of the above, or may include two or more. Furthermore, from the viewpoint of compatibility with the (meth)acrylic copolymer (A), it is more preferable that the (meth)acrylic acid alkyl ester monomer (B2) contains one or more monomers similar to the (meth)acrylic acid alkyl ester monomer (A2).

((Meth)acrylic acid (cyclo)alkyl ester monomer (B3))
The type of (meth)acrylic acid (cyclo)alkyl ester monomer (B3) is not particularly limited as long as the homopolymer has a glass transition temperature of 40° C. or higher. The alkyl group of the (meth)acrylic acid (cyclo)alkyl ester monomer (B3) may be linear, branched, or cyclic. The (meth)acrylic acid (cyclo)alkyl ester monomer (B3) includes a monomer having a saturated hydrocarbon group containing a cyclic structure. In addition, the difference in glass transition temperature (Tg) between the above-mentioned (B2) component and (B3) component, which are the constituent units of the (meth)acrylic tackifier (B), is preferably large. Therefore, the glass transition temperature (Tg) of the homopolymer of the (meth)acrylic acid (cyclo)alkyl ester monomer (B3) is preferably high, preferably 40° C. or higher, and more preferably 80° C. or higher.

The (meth)acrylic acid (cyclo)alkyl ester monomer (B3) includes methyl methacrylate (Tg; 105°C), i-butyl methacrylate (Tg; 48°C), t-butyl methacrylate (Tg; 107°C), cyclohexyl methacrylate (Tg; 100°C), isobornyl methacrylate (Tg; 155°C), isobornyl acrylate (Tg; 96°C), ethyl methacrylate (Tg; 65°C), and the like. The (meth)acrylic acid (cyclo)alkyl ester monomer (B3) is preferably methyl methacrylate, t-butyl methacrylate, or cyclohexyl methacrylate, from the viewpoint of high glass transition temperature (Tg) and excellent compatibility. The (meth)acrylic acid (cyclo)alkyl ester monomer (B3) may include only one of the above, or may include two or more of them.

(Physical Properties)
The glass transition temperature (Tg) of the (meth)acrylic tackifier (B) is −10° C. or higher and 40° C. or lower. The method for calculating the glass transition temperature (Tg) of the (meth)acrylic tackifier (B) will be described later. When the glass transition temperature (Tg) of the (meth)acrylic tackifier (B) exceeds 40° C., the stress relaxation property of the pressure-sensitive adhesive layer decreases, and therefore the bending resistance of the pressure-sensitive adhesive layer decreases. On the other hand, when the glass transition temperature (Tg) of the (meth)acrylic tackifier (B) is lower than −10° C., the cohesive force of the pressure-sensitive adhesive layer decreases, and therefore the adhesive force of the pressure-sensitive adhesive layer decreases.

The weight-average molecular weight of the (meth)acrylic tackifier (B) is 5,000 or more and 150,000 or less. Here, if the weight-average molecular weight of the (meth)acrylic tackifier (B) exceeds 150,000, the migration of the (meth)acrylic tackifier (B) to the surface of the adhesive layer is greatly reduced, so that the effect of improving the cohesive force at the interface between the adhesive layer and the adherend is not fully exerted, and the adhesive strength is reduced. On the other hand, if the weight-average molecular weight of the (meth)acrylic tackifier (B) is less than 5,000, the migration of the (meth)acrylic tackifier (B) to the surface of the adhesive layer is excessively large, resulting in a problem of an increase in adhesive strength to the release film over time.

(Polymerization method, etc.)
The (meth)acrylic tackifier (B) can be produced by ordinary solution polymerization, bulk polymerization, emulsion polymerization, suspension polymerization, etc. In particular, it is preferable to produce the (meth)acrylic tackifier (B) by using solution polymerization that can obtain the (meth)acrylic tackifier (B) as a solution. This allows the (meth)acrylic tackifier (B) in a solution state to be used as is in the production of the pressure-sensitive adhesive composition for foldable displays of the present invention. Solvents used in solution polymerization include organic solvents such as, for example, ethyl acetate, toluene, n-hexane, acetone, and methyl ethyl ketone.

(Polymerization initiator)
Examples of the polymerization initiator used when polymerizing the above components (B1) to (B3) include oil-soluble organic peroxides such as 2,4-dichlorobenzoyl peroxide, t-butyl peroxypivalate, benzoyl peroxide, o-methylbenzoyl peroxide, bis-3,5,5-trimethylhexanoyl peroxide, octanoyl peroxide, t-butylperoxy-2-ethylhexanoate, cyclohexanone peroxide, methyl ethyl ketone peroxide, dicumyl peroxide, lauroyl peroxide, diisopropylbenzene hydroperoxide, t-butyl hydroperoxide, and t-butyl peroxide, and oil-soluble azo compounds such as 2,2'-azobisisobutyronitrile, 2,2'-azobis(2,4-dimethylvaleronitrile), and 2,2'-(2,4-dimethyl-4-methoxyvaleronitrile). The oil-soluble polymerization initiator is preferably an oil-soluble azo compound. The oil-soluble polymerization initiator may contain only one type of the above-mentioned oil-soluble polymerization initiator, or may contain two or more types of the above-mentioned oil-soluble polymerization initiator.

(Chain Transfer Agent)
In order to adjust the molecular weight of the (meth)acrylic tackifier (B), a chain transfer agent can be appropriately used. Examples of the chain transfer agent include mercaptans such as octyl thioglycolate, methoxybutyl thioglycolate, octyl mercaptopropionate, methoxybutyl mercaptopropionate, stearyl mercaptan, lauryl mercaptan, and the like, and α-methylstyrene dimer, and the like, and one of the above may be used alone or two or more may be used in combination.

(Other additives)
In addition to the above components (B1) to (B3), the (meth)acrylic tackifier (B) may contain additives for adjusting adhesive strength and other performance, etc. Specific examples of the additives include tackifiers, plasticizers, silane coupling agents, antioxidants, and antistatic agents.

(Crosslinking Agent (C))
The adhesive composition for foldable displays contains 0.01 to 10 parts by mass, preferably 0.05 to 5 parts by mass, of a crosslinking agent (C) relative to 100 parts by mass of a (meth)acrylic copolymer (A). The type of crosslinking agent (C) is not particularly limited, and includes isocyanate-based crosslinking agents, epoxy-based crosslinking agents, aziridine-based crosslinking agents, melamine-based crosslinking agents, and polyfunctional metal chelates. The crosslinking agent (C) may contain only one of the above, or may contain two or more types. In addition, from the viewpoint of improving the adhesive strength and bending resistance of the adhesive composition for foldable displays in a well-balanced manner, it is preferably an isocyanate-based crosslinking agent.

The isocyanate-based crosslinking agent is a polyisocyanate compound having two or more isocyanate groups in one molecule. Examples of the polyisocyanate compound include adducts of pentamethylene diisocyanate, hexamethylene diisocyanate, xylylene diisocyanate, tolylene diisocyanate, isophorone diisocyanate, etc. with trimethylolpropane, etc., isocyanurate compounds, biuret-type compounds, etc. The polyisocyanate compound is preferably a compound of tolylene diisocyanate and xylylene diisocyanate.

(Crosslinking reaction accelerator)
In order to shorten the crosslinking reaction period, the adhesive composition for foldable displays may contain 0.01 to 10 parts by mass, preferably 0.05 to 5 parts by mass, of a crosslinking reaction accelerator relative to 100 parts by mass of the (meth)acrylic copolymer (A) in addition to the crosslinking agent (C). The type of the crosslinking reaction accelerator is not particularly limited, and is, for example, a metal chelate compound such as an aluminum chelate, a zirconium chelate, or a titanium chelate.

<Applications>
The adhesive composition for a foldable display of the present invention is, for example, cured to form an adhesive layer and used for adhesion of a foldable display. The term "foldable display" refers to a flexible display having an arbitrary radius of curvature designed to be repeatedly folded and unfolded like paper. The foldable display is used, for example, in a folding smartphone and a foldable phone. The adhesive layer is included in the foldable display and is formed, for example, on a base film (also called an optical member). The adhesive layer may be present, for example, on one or both sides of a polarizing plate, as an example of an optical member. As described above, the adhesive composition for a foldable display of the present invention can form an adhesive layer that realizes improved bending resistance and adhesive strength, and suppression of the rate of change in adhesive strength against a release film, and therefore can be suitably used, for example, for bonding optical members constituting a foldable display, but is not limited thereto. For example, the adhesive layer of the present invention can also be applied to bonding optical members constituting a display having a simple curved display or a display having a complex shape without folding the display.

The thickness of the adhesive layer in the optical member of the present invention is not particularly limited and is set appropriately depending on, for example, the types of the optical member and the foldable display, and the materials of the optical member and the foldable display. The thickness of the adhesive layer according to one embodiment is in the range of 1 μm to 100 μm, and preferably in the range of 5 μm to 50 μm.

Optical components include, for example, components constituting devices such as image display devices and input devices (so-called optical devices) or components used in these devices. Optical components include polarizing plates, AG (Anti-Glare) polarizing plates, wave plates, retardation plates including 1/2 and 1/4 wave plates, viewing angle compensation films, optical compensation films, brightness enhancement films, light guide plates, reflective films, anti-reflection films, transparent conductive films such as ITO (Indium-Tin Oxide) films, prism sheets, lens sheets, diffusion plates, etc.

Materials for optical components include polyester resins, acetate resins, polyethersulfone resins, polycarbonate resins, polyamide resins, polyimide resins, polyolefin resins, acrylic resins, polyvinyl chloride resins, ABS (Acrylonitrile Butadiene Styrene) resins, fluorine resins, etc.

<Production of adhesive composition for foldable display, etc.>
Table 1 shows the explanation of the abbreviations of the monomers in Tables 2 and 3. Table 2 shows the raw material composition and physical properties of the (meth)acrylic copolymer (A). Table 3 shows the raw material composition and physical properties of the (meth)acrylic tackifier (B). Table 4 shows the raw material composition of the adhesive composition for foldable displays of Examples 1 to 19. Table 5 shows the raw material composition of the adhesive composition for foldable displays of Comparative Examples 1 to 13. Table 6 shows the performance evaluation results for the two types of adhesive sheets of Examples 1 to 19. Table 7 shows the performance evaluation results for the two types of adhesive sheets of Comparative Examples 1 to 13.

(Production Example A-1: Production of (meth)acrylic copolymer (A) solution)
Based on the raw material composition of A-1 in Table 2, nitrogen gas was sealed in a reaction apparatus equipped with a stirrer, a thermometer, a reflux condenser, and a nitrogen inlet tube, and then 2 parts by mass of 4-hydroxybutyl acrylate (4HBA) as component (A1), 49 parts by mass of butyl acrylate (BA) as component (A2), and 49 parts by mass of 2-ethylhexyl acrylate (2EHA) were added, and 0.05 parts by mass of a polymerization initiator (azobisisobutyronitrile), 50 parts by mass of ethyl acetate as a reaction solvent, and 30 parts by mass of acetone were added. While stirring, these were reacted for 5 hours at the reflux temperature of ethyl acetate. After the reaction was completed, the mixture was diluted with ethyl acetate and cooled to room temperature to obtain a (meth)acrylic copolymer (A) solution.

(Production Examples A-2 to A-7: Production of (meth)acrylic copolymer (A) solution)
Based on the raw material compositions of Production Examples A-2 to A-7 in Table 2 and the same production process as Production Example A-1, (meth)acrylic copolymer (A) solutions of Production Examples A-2 to A-7 were produced. Note that the types and amounts of the reaction solvent, polymerization initiator, and chain transfer agent were adjusted to achieve the target weight average molecular weight (Mw) of the (meth)acrylic copolymer (A) of each of Production Examples A-2 to A-7.

(Production Example B-1: Production of (meth)acrylic tackifier (B) solution)
Based on the raw material composition of B-1 in Table 3, nitrogen gas was sealed in a reaction apparatus equipped with a stirrer, a thermometer, a reflux condenser, a nitrogen inlet tube, and a dropping device, and then 50 parts by mass of methyl ethyl ketone and 10 parts by mass of ethyl acetate were added as a reaction solvent, and the mixture was heated and refluxed at a temperature of about 90 ° C. for 10 minutes. Thereafter, 1.0 part by mass of a polymerization initiator (azobisisobutyronitrile) was added, and a solution obtained by mixing 5.0 parts by mass of 4-hydroxybutyl acrylate (4HBA) as the (B1) component, 30 parts by mass of butyl acrylate (BA) as the (B2) component, and 65 parts by mass of methyl methacrylate (MMA) as the (B3) component was added from the dropping device over 90 minutes. After the addition was completed, the mixture was heated and refluxed for another 5 hours to complete the polymerization reaction. After the reaction was completed, the mixture was diluted with ethyl acetate and cooled to room temperature to obtain a (meth)acrylic tackifier (B-1) solution.

(Production Examples B-2 to B-21: Production of (meth)acrylic tackifier (B) solutions)
Based on the raw material compositions of Production Examples B-2 to B-21 in Table 3 and the same production process as Production Example B-1, (meth)acrylic tackifier (B) solutions of Production Examples B-2 to B-21 were produced. Note that the types and amounts of the reaction solvent, polymerization initiator, and chain transfer agent were adjusted to achieve the target weight average molecular weight (Mw) of the (meth)acrylic tackifier (B) of each of Production Examples B-2 to B-21.

(Method of measuring weight average molecular weight (Mw))
The weight average molecular weight (Mw) shown in Tables 2 to 3 is a polystyrene-equivalent molecular weight measured by Gel Permeation Chromatography (GPC). Specifically, the (meth)acrylic copolymers (A) of Production Examples A-1 to A-7 and the (meth)acrylic tackifiers (B) of Production Examples B-1 to B-21 were dried at room temperature to obtain coating films, which were dissolved in tetrahydrofuran and measured with a high performance liquid chromatograph (Tosoh Corporation; HLC-8320GPC, column; KF-G + KF-806 x 2, detector; differential refractometer (RI), measurement temperature; 40 ° C., eluent; tetrahydrofuran, flow rate; 1 mL / min, injection amount; 100 μL) to obtain the weight average molecular weight (Mw) in terms of polystyrene.

(Method of measuring glass transition temperature (Tg))
The method for measuring the glass transition temperature (Tg) of the (meth)acrylic copolymer (A) and the (meth)acrylic tackifier (B) will be described. The glass transition temperature (Tg) in the present invention is a theoretical value calculated from the glass transition temperature (Tg) of each homopolymer of the monomer components of the (meth)acrylic copolymer (A) and the (meth)acrylic tackifier (B) and the FOX formula (see formula 1 below).

1/Tg=W1/Tg1+W2/Tg2+...Wn/Tgn (Formula 1)
Here, the glass transition temperature (Tg) represents the glass transition temperature (unit: K) of a copolymer obtained by copolymerizing n types of monomer components (monomers 1 to n). W1, W2, ... Wn represent the mass fraction of each monomer (1, 2, ... n) with respect to the total amount of the n types of monomer components. Tg1, Tg2, ... Tgn represent the glass transition temperature (unit: K) of a homopolymer of each monomer (1, 2, ... n).

For example, the glass transition temperatures (Tg) of the homopolymers of the monomer components of the (meth)acrylic copolymers (A) and (meth)acrylic tackifiers (B) shown in Tables 2 and 3 are as shown in Table 1. Using the glass transition temperatures (Tg) of the homopolymers of each monomer shown in Table 1, the glass transition temperatures (Tg) of the (meth)acrylic copolymers (A) of Production Examples A-1 to A-7 and the (meth)acrylic tackifiers (B) of Production Examples B-1 to B-21 were calculated.

The glass transition temperature (Tg) of each homopolymer of the monomer components is a value measured by DSC (Differential Scanning Calorimetry).

(Example 1: Production of adhesive composition for foldable displays)
Based on the raw material composition of Example 1 in Table 4, 10 parts by mass (solid content equivalent) of the (meth)acrylic tackifier (B) of Production Example B-1 was added to 100 parts by mass (solid content equivalent) of the (meth)acrylic copolymer (A) of Production Example A-1, 0.3 parts by mass of an adduct type of toluene diisocyanate (manufactured by Mitsui Chemicals, Inc., product name: Takenate D101E (45EA)) which is an isocyanate compound as a crosslinking agent (C), and 0.2 parts by mass of aluminum trisacetylacetonate (manufactured by Kawaken Fine Chemicals Co., Ltd., product name: Aluminum Chelate A) which is a metal chelate compound as a crosslinking accelerator, and mixed thoroughly to obtain a pressure-sensitive adhesive composition for foldable displays of Example 1.

(Preparation of adhesive sheet for evaluating adhesive strength)
The adhesive composition for foldable displays of Example 1 was applied to a 38 μm thick polyethylene terephthalate (PET) film (corresponding to the substrate of the adhesive sheet) whose surface was silicone-treated so that the thickness of the adhesive layer after drying for 120 seconds in a dryer at 120 ° C. was 10 μm (dry). After drying (after hardening of the adhesive composition for foldable displays), a 50 μm thick PET film was attached to the coated surface of the 38 μm thick PET film, and the adhesive sheet for adhesive strength evaluation was produced by curing for 4 days in an atmosphere of 60 ° C.

(Preparation of Adhesive Sheet for Bending Test)
In addition, a pressure-sensitive adhesive sheet for a bending test was separately prepared, in which the thickness of the pressure-sensitive adhesive layer after drying (after curing of the pressure-sensitive adhesive composition for a foldable display) was 50 μm. Note that the steps other than the step of adjusting the thickness of the pressure-sensitive adhesive layer after drying to 50 μm were the same as the steps for preparing the pressure-sensitive adhesive sheet for adhesive strength evaluation, and therefore detailed explanations will be omitted.

The method for producing the adhesive sheets for adhesive strength evaluation and the adhesive sheets for foldability testing using the adhesive compositions for foldable displays of Examples 2 to 19 and Comparative Examples 1 to 13 is the same as the method for producing the adhesive sheet of Example 1, so a detailed description is omitted.

<Test Method>
The adhesive sheets for adhesive strength evaluation and the adhesive sheets for bending property test of each of Examples 1 to 19 and Comparative Examples 1 to 13 were used to carry out the tests of the following Test Examples 1 to 3 to evaluate the performance.

(Test Example 1: Measurement of adhesive strength to glass)
The adhesive sheet for adhesive strength evaluation was measured at 180° peeling adhesion to a glass plate in an environment of 23 ° C. and 50% RH according to the provisions of JIS Z0237:2009. The glass plate is an example of an adherend related to the performance evaluation of adhesive strength, and is not limited thereto. The adherend may be a member made of a material different from glass. Specifically, the adhesive sheet for adhesive strength evaluation was cut to a width of 25 mm, the release film was peeled off, and then the sheet was attached to a glass plate and pressed once with a pressure roller under a load of 2 kg to prepare a test piece. The adhesive strength to the glass plate was measured by peeling the adhesive strength evaluation sheet from the glass plate at a peeling speed of 300 mm / min for the test piece left at 23 ° C. for 24 hours after pressing. The adhesive strength of the adhesive sheet for adhesive strength evaluation to glass was evaluated based on the following evaluation criteria. For example, if the measured adhesive strength is 6.0 N / 25 mm or more, it means that there is no practical problem in adhesion to the optical member of the foldable display. On the other hand, if the measured adhesive strength is less than 6.0 N/25 mm, this means that practical problems will arise in adhering the foldable display to the optical component.
(Evaluation criteria)
◯: The measured adhesive strength is 6.0 N/25 mm or more. In this case, no practical problems arise.
Δ: The measured adhesive strength is 6.0 N/25 mm or more, but the phenomenon of cohesive failure is observed upon peeling. In this case, a practical problem occurs.
×: The measured adhesive strength is less than 6.0 N/25 mm. In this case, a practical problem occurs.

(Test Example 2: Measurement of rate of change in adhesive strength against release film)
The adhesive strength of the adhesive sheet for adhesive strength evaluation was measured at 180° peeling against the release film in an environment of 23 ° C. and 50% RH in accordance with the provisions of JIS Z0237:2009. Specifically, the adhesive sheet for adhesive strength evaluation was cut to a width of 25 mm, and the back side of the release film was attached to a glass plate as a support to prepare a test piece. Thereafter, the adhesive sheet for adhesive strength evaluation was peeled off at a peeling speed of 300 mm / min, and the adhesive strength against the release film was measured. Furthermore, in order to confirm the change over time in the adhesive strength against the release film, the adhesive sheet for adhesive strength evaluation cut to a width of 25 mm was left at 50 ° C. for 14 days as an accelerated condition, and the same measurement was performed. Here, the adhesive strength of the adhesive sheet for adhesive strength evaluation against the release film before being left at 50 ° C. for 14 days was defined as the "initial" adhesive strength (referred to as initial (gf / 25 mm) in Tables 6 and 7). The adhesive strength of the adhesive sheet for adhesive strength evaluation to the release film after standing at 50° C. for 14 days was defined as the adhesive strength "after change over time" (referred to as "after change over time (gf/25 mm)" in Tables 6 and 7). The rate of change in adhesive strength was calculated using the following formula 2.
Rate of change in adhesive strength (%)=Adhesive strength after aging (gf/25 mm)/Initial adhesive strength (gf/25 mm)×100 (Formula 2)

The rate of change in the adhesive strength of the adhesive sheet for adhesive strength evaluation against the release film was evaluated based on the following evaluation criteria. For example, when the rate of change is 150% or less, the rate of change is small and the increase in adhesive strength against the release film is suppressed, so there is no concern about deterioration of workability. On the other hand, when the rate of change is greater than 150%, the adhesive strength against the release film increases over time, so a large force is required to peel the adhesive sheet from the release film, and there is a concern about deterioration of workability.
(Evaluation criteria)
◯: The rate of change is 150% or less. In this case, there is no concern about deterioration of workability.
×: The rate of change is greater than 150%. In this case, there is a concern that the workability may deteriorate.

(Test Example 3: Bending Property Test)
After the release film was peeled off from the foldability test adhesive sheet, it was attached to a 50 μm thick polyimide (PI) film (Toray DuPont Co., Ltd., product name: Kapton 200H) and pressed with a hand roller. This sample was cut into a size of 25 mm x 100 mm to obtain a test piece. The test piece was attached to a folding tester (Yuasa System Equipment Co., Ltd., product name: Tension-Free (registered trademark) Folding Clamshell-type) so that the polyimide film was on the inside and the PET film was on the outside, and a folding test was performed under the following conditions.
(Test condition)
Measurement temperature: 23°C, 50% RH
Bend radius: 2 mm
Bending angle: 180°
Bending speed: 2 seconds/cycle Number of bending cycles: 300,000 cycles

After the test, the folded portion of the test piece was observed and the folding resistance was evaluated visually.
(Evaluation criteria)
⊚: No lifting or peeling was observed in the test piece. In this case, there is no problem in practical use, and the sheet can be suitably used as a foldable pressure-sensitive adhesive sheet.
◯: Peeling was observed in only a small area of the test piece. In this case, there was no problem in practical use, and the sheet could be suitably used as a foldable pressure-sensitive adhesive sheet.
×: Lifting or peeling is clearly observed in the test piece. In this case, the sheet is not suitable as a foldable pressure-sensitive adhesive sheet and cannot be used.

<Test Results>
The test results of Examples 1 to 19 and Comparative Examples 1 to 13 will be described with reference to Tables 6 and 7. First, Examples 1 to 19 were excellent in all evaluation items, including adhesive strength (against glass), suppression of change rate of adhesive strength (against release film), and bending resistance. On the other hand, Comparative Examples 1 to 13 only satisfied the performance for some of the items among all evaluation items. Below, a comparison between Example 1 and Comparative Examples 1 to 13 and the characteristics of Examples 1 to 19 will be described.

(Comparison between Example 1 and Comparative Example 1)
The adhesive strength (to glass) of Example 1 was improved compared to Comparative Example 1 (wherein (meth)acrylic tackifier (B) was not contained). On the other hand, the adhesive layer of Comparative Example 1 lacked cohesive strength, and therefore the adhesive strength (to glass) of Comparative Example 1 was reduced.

(Comparison between Example 1 and Comparative Example 2)
The folding resistance of Example 1 was improved compared to Comparative Example 2 (wherein the blending amount of (meth)acrylic tackifier (B) exceeded the amount specified in the present invention). On the other hand, the pressure-sensitive adhesive layer of Comparative Example 2 had too large a cohesive force, which reduced the stress relaxation properties, and therefore the folding resistance of Comparative Example 2 was reduced.

(Comparison between Example 1 and Comparative Example 3)
The adhesive strength (against glass) of Example 1 was improved compared to Comparative Example 3 (wherein the weight average molecular weight (Mw) of the (meth)acrylic tackifier (B) was greater than the weight average molecular weight (Mw) specified in the present invention). On the other hand, the adhesive strength (against glass) of Comparative Example 3 was reduced because the migration of the (meth)acrylic tackifier (B) to the adhesive layer surface was significantly reduced in the adhesive layer of Comparative Example 3.

(Comparison between Example 1 and Comparative Example 4)
The rate of change in adhesive strength (against release film) in Example 1 is smaller than that in Comparative Example 4 (wherein the weight average molecular weight (Mw) of the (meth)acrylic tackifier (B) is smaller than the weight average molecular weight (Mw) specified in the present invention), and the change in adhesive strength (against release film) over time is suppressed. On the other hand, the adhesive layer in Comparative Example 4 has excessively large migration of the (meth)acrylic tackifier (B) to the adhesive layer surface, so that the rate of change in adhesive strength (against release film) in Comparative Example 4 is larger than the rate of change specified in the present invention.

(Comparison between Example 1 and Comparative Example 5)
The folding resistance of Example 1 was improved compared to Comparative Example 5 (wherein the glass transition temperature (Tg) of the (meth)acrylic tackifier (B) was higher than the glass transition temperature (Tg) specified in the present invention). On the other hand, the pressure-sensitive adhesive layer of Comparative Example 5 had a reduced stress relaxation property, and therefore the folding resistance of Comparative Example 5 was reduced.

(Comparison between Example 1 and Comparative Example 6)
The adhesive strength (against glass) of Example 1 was improved compared to Comparative Example 6 (wherein the glass transition temperature (Tg) of the (meth)acrylic tackifier (B) was lower than the glass transition temperature (Tg) specified in the present invention). On the other hand, the adhesive strength (against glass) of Comparative Example 6 was reduced because the cohesive strength of the adhesive layer of Comparative Example 6 was reduced.

(Comparison between Example 1 and Comparative Example 7)
The adhesive strength (against glass) and bending resistance of Example 1 were improved compared to Comparative Example 7 (wherein the (meth)acrylic tackifier (B) did not contain the components (B2) and (B3)). On the other hand, the adhesive layer of Comparative Example 7 had reduced stress relaxation properties and cohesive strength, and therefore the adhesive strength (against glass) and bending resistance of Comparative Example 7 were reduced.

(Comparison between Example 1 and Comparative Example 8)
The rate of change in adhesive strength (to release film) in Example 1 is smaller than that in Comparative Example 8 (wherein the (meth)acrylic tackifier (B) does not contain the (B1) component), and the change in adhesive strength (to release film) over time is suppressed. On the other hand, the adhesive layer in Comparative Example 8 has excessively large migration of the (meth)acrylic tackifier (B) to the adhesive layer surface, so the rate of change in adhesive strength (to release film) in Comparative Example 8 is larger than the rate of change specified in the present invention.

(Comparison between Example 1 and Comparative Example 9)
The adhesive strength (against glass) of Example 1 was improved compared to Comparative Example 9 (wherein the (meth)acrylic tackifier (B) contained a (B1) component exceeding the upper limit value specified in the present invention). On the other hand, the adhesive strength (against glass) of Comparative Example 9 was reduced because the migration of the (meth)acrylic tackifier (B) to the adhesive layer surface was significantly reduced in the adhesive layer of Comparative Example 9.

(Comparison between Example 1 and Comparative Example 10)
The folding resistance of Example 1 was improved compared to Comparative Example 10 (wherein the glass transition temperature (Tg) of the (meth)acrylic copolymer (A) exceeded the glass transition temperature (Tg) specified in the present invention). On the other hand, the pressure-sensitive adhesive layer of Comparative Example 10 had a reduced stress relaxation property, and therefore the folding resistance of Comparative Example 10 was reduced.

(Comparison between Example 1 and Comparative Example 11)
The rate of change in adhesive strength (against release film) in Example 1 is smaller than that in Comparative Example 11 (when the (meth)acrylic copolymer (A) does not contain the (A1) component), and the change in adhesive strength (against release film) over time is suppressed. On the other hand, in the adhesive layer of Comparative Example 11, the (meth)acrylic copolymer (A) cannot form a crosslinked structure with the (meth)acrylic tackifier (B) via the crosslinking agent (C), so the migration of the (meth)acrylic tackifier (B) to the adhesive layer surface (film surface) becomes excessively large, and the rate of change in adhesive strength (against release film) in Comparative Example 11 is larger than the rate of change specified in the present invention. Furthermore, since the cohesive strength is reduced, the phenomenon of cohesive failure was confirmed during peeling.

(Comparison between Example 1 and Comparative Example 12)
The adhesive strength (against glass) and bending resistance of Example 1 were improved compared to Comparative Example 12 (wherein the (meth)acrylic copolymer (A) contained an (A1) component in an amount greater than that specified in the present invention). On the other hand, the adhesive strength (against glass) and bending resistance of Comparative Example 12 were reduced because the crosslinking density of the adhesive layer of Comparative Example 12 was too high.

(Comparison between Example 1 and Comparative Example 13)
The folding resistance of Example 1 was improved compared to Comparative Example 13 (wherein the (meth)acrylic copolymer (A) contained a carboxyl group-containing monomer as the (A1) component, and the (meth)acrylic tackifier (B) did not contain the (B2) component but contained TBMA as the (B3) component). On the other hand, the pressure-sensitive adhesive layer of Comparative Example 13 had a reduced stress relaxation property, and therefore the folding resistance of Comparative Example 13 was reduced.

(Features of Examples 1 to 5 and 12)
Examples 1 to 5 and 12 were well-balanced and excellent in all evaluation items including adhesive strength (against glass), rate of change in adhesive strength (against release film), and bending resistance.

(Features of Examples 6 and 7)
Since the (meth)acrylic tackifier (B) contains the component (B3) in an amount equal to the upper limit of the range specified in the present invention, the stress relaxation property is reduced, and therefore the bending resistance is somewhat reduced.

(Features of Example 8)
Since the (meth)acrylic tackifier (B) contains the (B1) component in the lower limit of the blending amount specified in the present invention, the migration of the (meth)acrylic tackifier (B) to the pressure-sensitive adhesive layer surface is increased, and the rate of change in adhesive strength (against release film) is somewhat large.

(Features of Example 9)
Since the (meth)acrylic tackifier (B) contains the (B1) component in the upper limit of the blending amount specified in the present invention, the crosslinking density of the (meth)acrylic tackifier (B) increases and the migration to the pressure-sensitive adhesive layer surface decreases. As a result, the adhesive strength (against glass) of Example 9 is lower than that of Example 1.

(Features of the Tenth Example)
Since the weight average molecular weight (Mw) of the (meth)acrylic tackifier (B) is the lower limit specified in the present invention, the migration of the (meth)acrylic tackifier (B) to the pressure-sensitive adhesive layer surface is increased, and the rate of change in adhesive strength (against release film) is somewhat large.

(Features of Example 11)
Since the weight average molecular weight (Mw) of the (meth)acrylic tackifier (B) is the upper limit value specified in the present invention, the migration of the (meth)acrylic tackifier (B) to the pressure-sensitive adhesive layer surface is reduced, and the adhesive strength (against glass) is lower than that of Example 1.

(Features of Example 13)
Since the glass transition temperature (Tg) of the (meth)acrylic tackifier (B) is within the lower limit of the range specified in the present invention, the cohesive strength of the adhesive layer is small. As a result, the adhesive strength (against glass) of Example 13 is lower than that of Example 1.

(Features of Example 14)
Since the amount of the (meth)acrylic tackifier (B) relative to the (meth)acrylic copolymer (A) is the lower limit specified in the present invention, the adhesive strength (against glass) of Example 14 is lower than that of Example 1.

(Features of Example 15)
Since the blending amount of the (meth)acrylic tackifier (B) relative to the (meth)acrylic copolymer (A) is the upper limit value specified in the present invention, the adhesive strength (to glass) of Example 15 is high. On the other hand, since the cohesive strength of the adhesive layer is large, the bending resistance is slightly decreased.

(Features of Example 16)
Since the glass transition temperature (Tg) of the (meth)acrylic copolymer (A) is the upper limit specified in the present invention, the adhesive strength (to glass) is high while the bending resistance is slightly decreased.

(Features of Example 17)
Even when the weight average molecular weight (Mw) of the (meth)acrylic copolymer (A) was smaller than the weight average molecular weight (Mw) of the (meth)acrylic copolymer (A) in Examples 1 to 5, there was no difference in performance between Example 17 and Examples 1 to 5.

(Features of Example 18)
Even when the (meth)acrylic copolymer (A) contained a carboxyl group-containing monomer (AAC) as the component (A1), there was no difference in performance between Example 18 and Examples 1 to 5.

(Features of Example 19)
Even when the (meth)acrylic copolymer (A) contains a carboxyl group-containing monomer (AAC) as the (A1) component and the (meth)acrylic tackifier (B) also contains a carboxyl group-containing monomer (AAC) as the (B1) component, there was no difference in performance between Example 19 and Examples 1 to 5.

As described above, the adhesive layer formed from the adhesive composition for foldable displays of the present invention has the remarkable effect of achieving both bending resistance and high adhesive strength, while suppressing changes in adhesive strength to release films over time.

The invention is not limited to the above-described embodiment, and various modifications and variations are possible within the scope of the invention.

Claims (10)

A pressure-sensitive adhesive composition for a foldable display,
A (meth)acrylic copolymer (A) containing, as structural units, a monomer (A1) having a functional group and a (meth)acrylic acid alkyl ester monomer (A2);
a (meth)acrylic tackifier (B) containing, as structural units, a monomer (B1) having a functional group, a (meth)acrylic acid alkyl ester monomer (B2) having a homopolymer glass transition temperature (Tg) of −50° C. or lower, and a (meth)acrylic acid (cyclo)alkyl ester monomer (B3) having a homopolymer glass transition temperature of 40° C. or higher;
A crosslinking agent (C),
The (meth)acrylic copolymer (A) has a glass transition temperature (Tg) of −50° C. or lower;
The (meth)acrylic tackifier (B) has a glass transition temperature (Tg) of −10° C. or more and 40° C. or less;
Here, the glass transition temperature (Tg) of the (meth)acrylic copolymer (A) and the glass transition temperature (Tg) of the (meth)acrylic tackifier (B) are calculated based on the FOX equation,
The adhesive strength of the adhesive layer made of the adhesive composition for a foldable display is 6.0 N/25 mm or more,
Here, the adhesive strength is a value measured in accordance with the provisions of JIS Z0237:2009.
Adhesive composition for foldable displays. The (meth)acrylic tackifier (B) is contained in an amount of 0.5 to 20 parts by mass relative to 100 parts by mass of the (meth)acrylic copolymer (A).
The pressure-sensitive adhesive composition for a foldable display according to claim 1 . The weight average molecular weight of the (meth)acrylic copolymer (A) is 500,000 or more and 2,500,000 or less,
The weight average molecular weight of the (meth)acrylic tackifier (B) is 5,000 or more and 150,000 or less.
The pressure-sensitive adhesive composition for a foldable display according to claim 1 . The (meth)acrylic tackifier (B) contains, per 100 parts by mass of the (meth)acrylic tackifier (B), 0.1 to 20 parts by mass of the monomer (B1) having the functional group, 10 to 60 parts by mass of the (meth)acrylic acid alkyl ester monomer (B2), and 35 to 85 parts by mass of the (meth)acrylic acid (cyclo)alkyl ester monomer (B3),
the monomer (B1) having a functional group includes at least one of a hydroxyl group-containing monomer, a carboxyl group-containing monomer, a nitrogen-containing monomer, and an epoxy group-containing monomer;
The (meth)acrylic acid alkyl ester monomer (B2) includes at least one of 2-ethylhexyl acrylate (Tg; -70°C), n-hexyl acrylate (Tg; -65°C), n-octyl acrylate (Tg; -65°C), isononyl acrylate (Tg; -60°C), n-nonyl acrylate (Tg; -58°C), isooctyl acrylate (Tg; -58°C), butyl acrylate (Tg; -52°C), and n-dodecyl methacrylate (Tg; -65°C);
The (meth)acrylic acid (cyclo)alkyl ester monomer (B3) includes at least one of methyl methacrylate (Tg; 105° C.), i-butyl methacrylate (Tg; 48° C.), t-butyl methacrylate (Tg; 107° C.), cyclohexyl methacrylate (Tg; 100° C.), isobornyl methacrylate (Tg; 155° C.), isobornyl acrylate (Tg; 96° C.), and ethyl methacrylate (Tg; 65° C.).
The pressure-sensitive adhesive composition for a foldable display according to claim 1 . The monomer (B1) having a functional group includes at least one of a hydroxy group-containing monomer such as 2-hydroxyethyl (meth)acrylate or 4-hydroxybutyl (meth)acrylate, a carboxyl group-containing monomer such as (meth)acrylic acid or β-carboxyethyl (meth)acrylate, a nitrogen-containing monomer such as (meth)acrylamide or dimethylaminopropyl (meth)acrylamide, and an epoxy group-containing monomer such as glycidyl (meth)acrylate or 4-hydroxybutyl (meth)acrylate glycidyl ether,
The (meth)acrylic acid alkyl ester monomer (B2) includes at least one of 2-ethylhexyl acrylate (Tg; -70°C), n-hexyl acrylate (Tg; -65°C), n-octyl acrylate (Tg; -65°C), isononyl acrylate (Tg; -60°C), n-nonyl acrylate (Tg; -58°C), isooctyl acrylate (Tg; -58°C), butyl acrylate (Tg; -52°C), and n-dodecyl methacrylate (Tg; -65°C);
The (meth)acrylic acid (cyclo)alkyl ester monomer (B3) includes at least one of methyl methacrylate (Tg; 105° C.), i-butyl methacrylate (Tg; 48° C.), t-butyl methacrylate (Tg; 107° C.), cyclohexyl methacrylate (Tg; 100° C.), isobornyl methacrylate (Tg; 155° C.), isobornyl acrylate (Tg; 96° C.), and ethyl methacrylate (Tg; 65° C.).
The pressure-sensitive adhesive composition for a foldable display according to claim 1 . The (meth)acrylic copolymer (A) contains, per 100 parts by mass of the (meth)acrylic copolymer (A), 0.1 to 8.0 parts by mass of the monomer (A1) having the functional group and 50 to 99.9 parts by mass of the (meth)acrylic acid alkyl ester monomer (A2);
the monomer (A1) having a functional group includes at least one of a hydroxyl group-containing monomer, a carboxyl group-containing monomer, a nitrogen-containing monomer, and an epoxy group-containing monomer,
The (meth)acrylic acid alkyl ester monomer (A2) includes at least one of 2-ethylhexyl acrylate (Tg; -70°C), n-hexyl acrylate (Tg; -65°C), n-octyl acrylate (Tg; -65°C), isononyl acrylate (Tg; -60°C), n-nonyl acrylate (Tg; -58°C), isooctyl acrylate (Tg; -58°C), butyl acrylate (Tg; -52°C), and n-dodecyl methacrylate (Tg; -65°C).
The pressure-sensitive adhesive composition for a foldable display according to claim 1 . The monomer (A1) having a functional group includes at least one of a hydroxy group-containing monomer such as 2-hydroxyethyl (meth)acrylate or 4-hydroxybutyl (meth)acrylate, a carboxyl group-containing monomer such as (meth)acrylic acid or β-carboxyethyl (meth)acrylate, a nitrogen-containing monomer such as (meth)acrylamide or dimethylaminopropyl (meth)acrylamide, and an epoxy group-containing monomer such as glycidyl (meth)acrylate or 4-hydroxybutyl (meth)acrylate glycidyl ether,
The (meth)acrylic acid alkyl ester monomer (A2) includes at least one of 2-ethylhexyl acrylate (Tg; -70°C), n-hexyl acrylate (Tg; -65°C), n-octyl acrylate (Tg; -65°C), isononyl acrylate (Tg; -60°C), n-nonyl acrylate (Tg; -58°C), isooctyl acrylate (Tg; -58°C), butyl acrylate (Tg; -52°C), and n-dodecyl methacrylate (Tg; -65°C).
The pressure-sensitive adhesive composition for a foldable display according to claim 1 . The pressure-sensitive adhesive composition for a foldable display contains 0.01 to 10 parts by mass of a crosslinking agent (C) relative to 100 parts by mass of a (meth)acrylic copolymer (A),
The pressure-sensitive adhesive composition for a foldable display according to claim 1 . A substrate;
A pressure-sensitive adhesive layer comprising the pressure-sensitive adhesive composition for a foldable display according to any one of claims 1 to 8 on the substrate,
Adhesive sheet. When the adherend is a release film, the rate of change in adhesive strength of the pressure-sensitive adhesive sheet to the release film is 150% or less;
Here, the rate of change in adhesive strength of the pressure-sensitive adhesive sheet to the release film is a value calculated by the following formula:
Rate of change in adhesive strength (%) = adhesive strength after aging (gf/25 mm) / initial adhesive strength (gf/25 mm) × 100,
The pressure-sensitive adhesive sheet according to claim 9.