JP2025030110A - Pressure-sensitive adhesive composition for decorative sheet, decorative sheet, decorative structure, and method for producing same - Google Patents

Abstract

To provide an adhesive composition for a decorative sheet, a decorative sheet, a decorative structure, and a manufacturing method thereof, which are used for attaching to an adherend by a vacuum forming method or a vacuum and pressure forming method, and which allow repositioning before heating, realize durability under high-temperature or high-temperature and high-humidity conditions, follow surface irregularities, and can impart an excellent appearance to a molded product.SOLUTION: The adhesive composition for a decorative sheet used for attaching to an adherend by a vacuum forming method or a vacuum and pressure forming method comprises: (A) a (meth)acrylate copolymer that is a copolymer of a monomer mixture containing respective specific amounts of (a) an alkyl (meth)acrylate monomer where an alkyl group has 8 carbon atoms and (b) a monomer having a carboxyl group; and (B) a curing agent.SELECTED DRAWING: None

Description

The present invention relates to a pressure-sensitive adhesive composition for decorative sheets used for attachment to adherends by vacuum forming or vacuum/pressure forming, a decorative sheet, a decorative structure, and a method for producing the same.

To improve the appearance quality of molded products such as automobile interior and exterior parts, keyboards, home appliances, smartphones, housing materials, furniture, musical instruments, and Shinkansen window frames, the exterior surfaces of the molded products are decorated with decorative films (decorative sheets). Known methods for decorating molded products having three-dimensional shapes with decorative sheets include insert molding, in-mold molding, vacuum molding, and vacuum-compressed air molding.

The insert molding method is a method in which a decorative sheet is first formed into a predetermined shape and inserted into a mold, and then heated molten resin is pressurized and poured into the mold for injection molding, and then cooled and solidified to produce a decorated molded product in which the decorative sheet and the molded product are integrated.
In-mold molding is a method of producing a decorated molded product by setting a film with a printed design in a mold with a release layer between them, injecting heated molten resin and allowing it to cool and solidify, and then peeling off the film to transfer the design.
In this way, in the insert molding method and the in-mold molding method, the molded article is decorated at the same time as it is molded, and a design is formed on the exterior surface of the molded article.

On the other hand, vacuum forming is a method in which the decorative sheet is softened by applying heat and the air between the decorative sheet and the molded product is removed from the molded product side, creating a near-vacuum state and adhering the decorative sheet to the molded product.
In addition, the vacuum and pressure forming method creates a similar vacuum state, and then applies air pressure from the decorative sheet side above the molded product to adhere the decorative sheet to the molded product.
Thus, the vacuum forming method and the vacuum pressure forming method are methods for decorating a molded product by attaching a decorative sheet at room temperature after the molded product is completed and heating it. In other words, the decorative sheet is attached to the exterior surface of the molded product in a separate operation from the molding of the molded product, so that a single device can be used to attach the decorative sheet to molded products of various shapes. In addition, the vacuum forming method and the vacuum pressure forming method are widely used because they allow continuous coating from the front to the back of the molded product edge, i.e., wrap-around coating, which is difficult to achieve with the in-mold molding method and the like.

Patent Document 1 discloses a molded product in which an adhesive film having an adhesive layer containing a (meth)acrylic polymer that contains a specific proportion of carboxyl groups and has a glass transition temperature of 25°C or lower, and a (meth)acrylic polymer that contains a specific proportion of amino groups and has a glass transition temperature of 75°C or higher is bonded by heat and pressure.

Patent Document 2 discloses a pressure-sensitive adhesive composition that contains two types of (meth)acrylic polymers in a specific ratio, each having a glass transition temperature in a specific range.

Patent Document 3 discloses a decorative molding film including an adhesive layer that is characterized by containing a copolymer of methyl acrylate and an alkyl acrylate ester containing an alkoxy group having four carbon atoms, and 1 to 40 parts by weight of an alkyl (meth)acrylate ester having a glass transition temperature of 115°C or less and a weight average molecular weight of 5,000 to 100,000 per 100 parts by weight of the copolymer.

JP 2009-035588 A International Publication No. 2021/049480 JP 2017-132205 A

Normally, when a decorative sheet is attached to a molded product by vacuum forming or vacuum/pressure forming, it cannot be reattached after heat forming, so it is often reattached before molding. For this reason, it is essential that it can be easily reattached. In addition, since it is expected that decorated molded products having three-dimensional shapes will be used in a variety of environments, it is important that the sheet conforms to complex shapes having curved surfaces and uneven parts, and that it has durability that does not cause poor appearance such as lifting or peeling at the adhesive interface with the adhesive layer even in high temperature or high temperature/high humidity environments. There is a demand for an adhesive composition for decorative sheets that can satisfy each of these performances in a well-balanced manner.

The adhesive layer described in Patent Document 1 has excellent adhesion at high temperatures, but has a high glass transition temperature, which means that the decorative sheet cannot be temporarily attached to the substrate before vacuum forming or vacuum-pressure forming.

The adhesive composition of Patent Document 2 has excellent repositionability and coating appearance, but does not mention durability in high temperature environments, and is therefore insufficient in terms of practical performance.

The decorative molding film in Patent Document 3 achieves adhesion, dimensional stability, and conformability to curved surfaces in environments up to 80°C, but in high-temperature environments of 100°C or higher, or when applied to more complex uneven surfaces, samples can float or peel off, resulting in poor appearance.

In other words, no adhesive composition for decorative sheets has been developed to date that can provide excellent repositionability before heating, durability at high temperatures or high temperature and humidity, conformability to uneven surfaces, and an excellent appearance to molded products.

The present inventors conducted extensive research to solve the above problems and arrived at the present invention.
That is, the adhesive composition for decorative sheets to be used for attachment to an adherend by a vacuum forming method or a vacuum/pressure forming method comprises a (meth)acrylic acid ester copolymer (A) and a curing agent (B), the (meth)acrylic acid ester copolymer (A) is a copolymer of a monomer mixture, the monomer mixture contains a (meth)acrylic acid alkyl ester monomer (a) having an alkyl group with 8 carbon atoms and a monomer (b) having at least one carboxy group, the content of the (meth)acrylic acid alkyl ester monomer (a) having an alkyl group with 8 carbon atoms is 50 to 98 mass% in 100 mass% of the monomer mixture, and the content of the monomer (b) having a carboxy group is more than 0 mass% and less than 12 mass% in 100 mass% of the monomer mixture.

The present invention makes it possible to provide a pressure-sensitive adhesive composition for decorative sheets, decorative sheets, and decorative structures, which are excellent in repositionability before heating, in conformity with irregularities, in durability at high temperatures or high temperatures and high humidity, and in appearance to molded articles, and which are also excellent in conformity with irregularities, as well as a method for producing the same, in decorating molded articles.
The present inventors have found that when a pressure-sensitive adhesive layer is formed from a pressure-sensitive adhesive composition containing a (meth)acrylic acid ester copolymer (A) containing a specified (meth)acrylate and a curing agent (B), it is possible to highly simultaneously achieve various performance properties, such as repositionability before heating, durability in high temperature or high temperature/high humidity environments, excellent appearance, and unevenness-following ability.

The pressure-sensitive adhesive composition for decorative sheets, the decorative sheet, the decorative structure, and the method for producing a decorative structure according to the present disclosure have the following configurations [1] to [7].
[1] A composition comprising a (meth)acrylic acid ester copolymer (A) and a curing agent (B),
The (meth)acrylic acid ester copolymer (A) is a copolymer of a monomer mixture,
The monomer mixture contains a (meth)acrylic acid alkyl ester monomer (a) having an alkyl group with 8 carbon atoms and a monomer (b) having at least one type of carboxy group, and is characterized in that the content of the (meth)acrylic acid alkyl ester monomer (a) having an alkyl group with 8 carbon atoms is 50 to 98 mass% in 100 mass% of the monomer mixture, and the content of the monomer (b) having a carboxy group is more than 0 mass% and less than 12 mass% in 100 mass% of the monomer mixture.
A pressure-sensitive adhesive composition for decorative sheets used for attachment to an adherend by vacuum forming or vacuum/pressure forming.
[2] The pressure-sensitive adhesive composition for decorative sheets according to [1], wherein the curing agent (B) is an epoxy-based curing agent.
[3] The pressure-sensitive adhesive composition for decorative sheets according to [1] or [2], characterized in that the monomer mixture further contains at least one of (c) an alkyl (meth)acrylate monomer having 1 to 3 carbon atoms and (d) an alkyl (meth)acrylate monomer having 4 carbon atoms and a homopolymer glass transition temperature of -30°C or higher, and the total content thereof is 1 to 45 mass% in 100 mass% of the monomer mixture.
[4] A decorative sheet comprising a substrate and an adhesive layer made of the adhesive composition for decorative sheets according to any one of [1] to [3].
[5] A decorative structure comprising an adherend and the decorative sheet according to [4].
[6] A method for producing a decorative structure according to [5], characterized in that the decorative sheet is integrated with an adherend by vacuum forming or vacuum-pressure forming to form a decorative structure.

The present disclosure is described in detail below. Needless to say, other embodiments are also included within the scope of the present disclosure as long as they conform to the spirit of the present disclosure.

In this disclosure, "sheet" refers to a flexible laminate, and includes thin laminates known as "films."

In this disclosure, "(meth)acrylic" means either or both of "acrylic" and "methacrylic", and "(meth)acrylate" means either or both of "acrylate" and "methacrylate".

In this disclosure, a numerical range specified using "~" is intended to include the numerical values before and after "~" as the lower and upper limit values of the range.

In this disclosure, "Mw" is the weight average molecular weight in terms of polystyrene measured by gel permeation chromatography (GPC). "Mn" is the number average molecular weight in terms of polystyrene measured by GPC.
The weight average molecular weight and number average molecular weight can be measured by the method described in the section [Examples].

In this disclosure, "(meth)acrylic acid alkyl ester monomer (a) having 8 carbon atoms in the alkyl group" may be abbreviated as "monomer (a)", "monomer (b) having a carboxy group" may be abbreviated as "monomer (b)", "(meth)acrylic acid alkyl ester monomer (c) having 1 to 3 carbon atoms in the alkyl group" may be abbreviated as "monomer (c)", and "(meth)acrylic acid alkyl ester monomer (d) having a homopolymer glass transition temperature of -30°C or higher and having 4 carbon atoms in the alkyl group" may be abbreviated as "monomer (d)".

The following describes in detail the embodiments of the present invention, but the following description is merely one example (representative example) of the embodiment of the present invention, and the present invention is not limited to these details as long as it does not exceed the gist of the invention.

<Adhesive Composition>
The pressure-sensitive adhesive composition comprises a (meth)acrylic acid ester copolymer (A) and a curing agent (B), the (meth)acrylic acid ester copolymer (A) being a copolymer of a monomer mixture, the monomer mixture containing a (meth)acrylic acid alkyl ester monomer (a) having an alkyl group with 8 carbon atoms and a monomer (b) having a carboxy group, the content of the (meth)acrylic acid ester monomer (a) having an alkyl group with 8 carbon atoms being 50 to 98 mass% in 100 mass% of the monomer mixture, and the content of the monomer (b) having a carboxy group being more than 0 mass% and less than 12 mass% in 100 mass% of the monomer mixture.

<(Meth)acrylic acid ester copolymer (A)>
The (meth)acrylic acid ester copolymer (A) (hereinafter also referred to as copolymer (A)) contained in the present pressure-sensitive adhesive composition is a copolymer obtained by polymerizing all or a part of a mixture of monomers containing a (meth)acrylic acid ester.

The content of (meth)acrylic acid alkyl ester monomer (a) having an alkyl group with 8 carbon atoms in 100% by mass of the monomer mixture is 50 to 98% by mass, more preferably 55 to 80% by mass, and particularly preferably 60% by mass or more and less than 70% by mass. By keeping it within the above range, it is possible to impart excellent unevenness-following properties, durability under high temperature and high humidity, and repositionability.

Examples of monomer (a) include 2-ethylhexyl (meth)acrylate, isooctyl (meth)acrylate, and normal octyl (meth)acrylate. These may be used alone or in combination of two or more. Among these, those having an alkyl group with a branched structure are preferred, and 2-ethylhexyl (meth)acrylate is particularly preferred from the viewpoint of conformability to uneven surfaces.

In 100% by mass of the monomer mixture, the content of monomer (b) having a carboxy group is more than 0% by mass and less than 12% by mass, more preferably less than 10% by mass, even more preferably 6% by mass or less, and particularly preferably less than 5% by mass. The lower limit is preferably 1% by mass or more. By keeping it within the above range, it is possible to achieve a high degree of compatibility between adhesive strength, durability in high temperature or high temperature/high humidity environments, and repositionability.

Examples of monomer (b) include (meth)acrylic acid, 2-carboxyethyl acrylate, itaconic acid, maleic acid, styrenesulfonic acid, 2-(meth)acryloyloxyethyl succinic acid, 2-(meth)acryloyloxyethyl phthalic acid, and 2-acryloyloxyethyl hexahydrophthalic acid. These may be used alone or in combination of two or more. Among these, (meth)acrylic acid and 2-carboxyethyl acrylate are preferred from the viewpoint of polymerizability.

In order to adjust the compatibility with other components, the dispersibility of other components, the adhesion to the substrate and the adherend, the crosslinkability, and the like, monomers other than the monomers (a) and (b) can also be used in the synthesis of the copolymer (A).
More specifically, in the present invention, it is preferable that the monomer mixture constituting the copolymer (A) further contains at least one of (c) a (meth)acrylic acid alkyl ester monomer having an alkyl group with 1 to 3 carbon atoms and (d) a (meth)acrylic acid alkyl ester monomer having an alkyl group with 4 carbon atoms and having a glass transition temperature of −30° C. or higher when the homopolymer forms the monomer.

Examples of monomer (c) include methyl (meth)acrylate, ethyl (meth)acrylate, and propyl (meth)acrylate. Examples of monomer (d) include n-butyl methacrylate (Tg: 20°C), isobutyl acrylate (Tg: -26°C), isobutyl methacrylate (Tg: 48°C), tert-butyl acrylate (Tg: 14°C), and tert-butyl methacrylate (Tg: 107°C). These may be used alone or in combination of two or more. Among them, monomer (c) is preferred from the viewpoints of curved surface, repositionability, and unevenness-following ability, and methyl (meth)acrylate and ethyl (meth)acrylate are particularly preferred.

In 100% by mass of the monomer mixture, the total content of monomer (c) and monomer (d) is preferably 1 to 45% by mass, more preferably 11 to 40% by mass, and particularly preferably 15 to 35% by mass. By keeping it within the above range, durability in high temperature and high temperature/high humidity environments can be improved.

In the present invention, the glass transition temperature (Tg) of the copolymer (A) is a value calculated based on the following formula (1) (Fox formula).
1/Tg=W1/Tg1+W2/Tg2+...+Wn/Tgn (Formula 1)
[In formula (1), Tg is the Tg (unit: K) of the acrylic polymer (A), Tgi (i = 1, 2, ... n) is the Tg (unit: K) when the radical polymerizable monomer i forms a homopolymer, and Wi (i = 1, 2, ... n) is the mass fraction of the radical polymerizable monomer i in the total monomer components. For the Tg of the homopolymer, a published value such as a literature value or a catalog value is used.]
The above formula (1) is a calculation formula when the copolymer (A) is composed of n types of monomer components, namely, Monomer 1, Monomer 2, . . . , Monomer n.

Furthermore, examples of other monomers that can be included in the monomer mixture constituting copolymer (A) include the following:

Vinyl monomers such as vinyl acetate, vinyl propionate, styrene, vinyl laurate, vinyl caprate, vinyl caprylate, and vinyl nonanoate;
(meth)acrylic acid alkyl ester monomers having an alkyl group having 4 to 26 carbon atoms, such as n-butyl acrylate, (meth)acrylic acid, isopentyl (meth)acrylate, 2-methylbutyl (meth)acrylate, 4-methyl-2-pentyl (meth)acrylate, heptyl (meth)acrylate, nonyl (meth)acrylate, isononyl (meth)acrylate, decyl (meth)acrylate, isodecyl (meth)acrylate, undecyl (meth)acrylate, lauryl (meth)acrylate, tridecyl (meth)acrylate, tetradecyl (meth)acrylate, pentadecyl (meth)acrylate, hexadecyl (meth)acrylate, heptadecyl (meth)acrylate, stearyl (meth)acrylate, isostearyl (meth)acrylate, behenyl (meth)acrylate, eicosanyl (meth)acrylate, and hexacosanyl (meth)acrylate (excluding (meth)acrylic acid ester monomers (a) and (d));
(meth)acrylic acid ester monomers having an aliphatic ring, such as cyclohexyl (meth)acrylate, 4-t-butylcyclohexyl (meth)acrylate, isobornyl (meth)acrylate, dicyclopentenyl (meth)acrylate, dicyclopentenyloxyethyl (meth)acrylate, dicyclopentanyl (meth)acrylate, and 3,3,5-trimethylcyclohexyl (meth)acrylate;
(meth)acrylic acid ester monomers having an aromatic ring, such as benzyl (meth)acrylate, 2-phenoxyethyl (meth)acrylate, 2-(o-phenylphenoxy)ethyl (meth)acrylate, and nonylphenoxy polyethylene glycol (meth)acrylate;
(meth)acrylic acid ester monomers having an alkoxy group, such as 2-methoxymethyl (meth)acrylate, 2-methoxyethyl (meth)acrylate, 2-ethoxymethyl (meth)acrylate, 2-ethoxyethyl (meth)acrylate, 3-methoxypropyl (meth)acrylate, 3-ethoxypropyl (meth)acrylate, 4-methoxybutyl (meth)acrylate, and 4-ethoxybutyl (meth)acrylate;
(meth)acrylic acid ester-based monomers having a polyether chain, such as methoxypolyethylene glycol (meth)acrylate, ethoxypolyethylene glycol (meth)acrylate, methoxypolypropylene glycol (meth)acrylate, ethoxypolypropylene glycol (meth)acrylate, phenoxypolyethylene glycol (meth)acrylate, and phenoxypolypropylene glycol (meth)acrylate;
N-vinyl cyclic amide monomers such as N-vinyl-2-pyrrolidone, N-vinyl-2-piperidone, N-vinyl-3-morpholinone, N-vinyl-2-caprolactam, N-vinyl-1,3-oxazin-2-one, and N-vinyl-3,5-morpholinedione;
(meth)acrylamide-based monomers such as (meth)acrylamide, N-alkyl(meth)acrylamide, N,N-dialkyl(meth)acrylamide, N-hydroxyalkyl(meth)acrylamide, and N-alkoxyalkyl(meth)acrylamide;
N,N-dialkyl(meth)acrylamide monomers such as N,N-dimethyl(meth)acrylamide, N,N-diethyl(meth)acrylamide, N,N-dipropyl(meth)acrylamide, N,N-diisopropyl(meth)acrylamide, N,N-di(n-butyl)(meth)acrylamide, and N,N-di(t-butyl)(meth)acrylamide;
N-hydroxyalkyl(meth)acrylamide monomers such as N-methylol(meth)acrylamide, N-(2)-hydroxyethyl)(meth)acrylamide, N-(2-hydroxypropyl)(meth)acrylamide, N-(1-hydroxypropyl)(meth)acrylamide, N-(3-hydroxypropyl)(meth)acrylamide, N-(2-hydroxybutyl)(meth)acrylamide, N-(3-hydroxybutyl)(meth)acrylamide, N-(4-hydroxybutyl)(meth)acrylamide, and N-methyl-N-2-hydroxyethyl(meth)acrylamide;
N-alkoxyalkyl(meth)acrylamides such as N-methoxymethyl(meth)acrylamide and N-butoxymethyl(meth)acrylamide;
Monomers having an acidic group other than a carboxy group, such as 2-(meth)acryloyloxyethyl acid phosphate and 2-methacryloyloxyethyl acid phosphate;
Examples of the monomer include (meth)acrylic acid ester monomers having a hydroxyl group, such as 2-hydroxyethyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, 3-chloro-2-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, 6-hydroxyhexyl (meth)acrylate, 8-hydroxyoctyl (meth)acrylate, and polyalkylene glycol mono(meth)acrylate, as well as allyl ether monomers having a hydroxyl group, such as polyalkylene glycol allyl ether.
These may or may not be contained in the monomer mixture constituting the copolymer (A) as required.

The content of other monomers is not particularly limited, but from the viewpoint of adhesive strength, it is preferable that it is 0% by mass or more and 30% by mass or less.

The polymerization form of the (meth)acrylic acid ester copolymer (A) is not particularly limited. That is, the copolymer (A) may be any of an alternating copolymer, a random copolymer, a block copolymer, and a graft copolymer of each monomer including an (meth)acrylic acid alkyl ester. For example, the block copolymer may be a diblock or a triblock.

Copolymer (A) can be synthesized by a conventionally known method, for example, by a known polymerization method such as solution polymerization, bulk polymerization, emulsion polymerization, or suspension polymerization. Copolymer (A) can also be synthesized by a known polymerization method such as living radical polymerization or active energy ray polymerization. In this case, the monomer mixture for synthesizing copolymer (A) can contain a photopolymerization initiator or a conventionally known additive. As the active energy ray, ultraviolet light, visible light, X-rays, gamma rays, or electron beams can be used. In addition, light sources such as low pressure mercury lamps, medium pressure mercury lamps, high pressure mercury lamps, ultra-high pressure mercury lamps, xenon lamps, metal halide lamps, electrodeless lamps, and LEDs can be used for ultraviolet irradiation.

The blending ratio of each structural unit (monomer component) of copolymer (A) can be determined using various instruments such as nuclear magnetic resonance analysis (NMR) and gas chromatography (GC).

The weight average molecular weight (Mw) of the (meth)acrylic acid ester copolymer (A) is preferably 600,000 to 2,000,000, more preferably 800,000 to 1,600,000, and even more preferably more than 1,000,000 to 1,500,000. By having the Mw of the copolymer (A) in the above range, sufficient durability can be obtained.

<Curing Agent (B)>
This composition contains a curing agent (B). By providing a crosslinking group to the (meth)acrylic acid ester copolymer (A) and crosslinking it with the curing agent (B), the cohesive strength, adhesive strength, and durability at high temperature or at high temperature and high humidity of the adhesive layer can be improved.

Examples of the curing agent contained in the pressure-sensitive adhesive composition include isocyanate-based curing agents, epoxy-based curing agents, aziridine-based curing agents, metal chelate-based curing agents, amine-based curing agents, acrylate-based curing agents, etc. Among these, isocyanate-based curing agents, epoxy-based curing agents, and metal chelate-based curing agents are preferred, and epoxy-based curing agents and metal chelate-based curing agents are more preferred from the viewpoint of repositionability, and epoxy-based curing agents are particularly preferred from the viewpoint of durability in high-temperature environments.
It is preferable to use one type of curing agent alone.

Examples of isocyanate-based curing agents include adducts of diisocyanates such as tolylene diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate, xylylene diisocyanate, hydrogenated xylylene diisocyanate, diphenylmethane diisocyanate, hydrogenated diphenylmethane diisocyanate, tetramethylxylylene diisocyanate, naphthalene diisocyanate, triphenylmethane triisocyanate, and polymethylene polyphenyl isocyanate with polyol compounds such as trimethylolpropane, as well as their biuret forms and their isocyanurates, as well as adducts of the above diisocyanates with polyether polyols, polyester polyols, acrylic polyols, polybutadiene polyols, and polyisocyanates. Compounds having three or more isocyanate groups in the molecule, such as adducts with any of polyols such as ene polyols and polyisoprene polyols; diisocyanates such as tolylene diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate, xylylene diisocyanate, hydrogenated xylylene diisocyanate, diphenylmethane diisocyanate, hydrogenated diphenylmethane diisocyanate, tetramethyl xylylene diisocyanate, naphthalene diisocyanate, triphenylmethane triisocyanate, and polymethylene polyphenyl isocyanate, as well as compounds having two isocyanate groups in the molecule, such as allophanate compounds of hexamethylene diisocyanate; and the like. Among these, trimethylolpropane adducts of tolylene diisocyanate are preferred because the adhesive properties can be easily adjusted. The number of isocyanate groups is the average number.

Epoxy curing agents include, for example, bisphenol A-epichlorohydrin type epoxy resins, ethylene glycol diglycidyl ether, polyethylene glycol diglycidyl ether, glycerin diglycidyl ether, glycerin triglycidyl ether, 1,6-hexanediol diglycidyl ether, trimethylolpropane triglycidyl ether, diglycidylaniline, N,N,N',N'-tetraglycidyl-m-xylylenediamine, 1,3-bis(N,N'-diglycidylaminomethyl)cyclohexane, and N,N,N',N'-tetraglycidylaminophenylmethane. From the viewpoint of curability and durability, N,N,N',N'-tetraglycidyl-m-xylylenediamine and 1,3-bis(N,N'-diglycidylaminomethyl)cyclohexane are particularly preferred.

Examples of aziridine-based curing agents include β-aziridinyl propionate), trimethylolpropane tris(β-aziridinyl propionate), etc. Commercially available products include "TAZO" and "TAZM" manufactured by Sogo Pharmaceutical Co., Ltd., and "ChemiTite PZ-33" manufactured by Nippon Shokubai Co., Ltd.

Metal chelate curing agents include, for example, coordination compounds of polyvalent metals such as aluminum, iron, copper, zinc, tin, titanium, nickel, antimony, magnesium, vanadium, chromium, and zirconium with acetylacetone or ethyl acetoacetate.

Examples of amine-based hardeners include hexamethylenediamine, triethyldiamine, polyethyleneimine, hexamethylenetetramine, diethylenetriamine, triethyltetramine, isophoronediamine, amino resins, and methylene resins.

Examples of acrylate-based curing agents include multifunctional acrylates such as 1,6-hexanediol acrylate.

The content of the curing agent (B) may be changed as appropriate depending on the desired physical properties such as the type of copolymer (A) and the degree of crosslinking, but from the viewpoints of adhesive strength, repositionability, and durability at high temperatures, it is preferably 0.01 to 3 parts by mass, more preferably 0.015 to 1 part by mass, even more preferably 0.018 to 0.5 parts by mass or less, and particularly preferably 0.02 to 0.06 parts by mass, per 100 parts by mass of copolymer (A). In particular, by setting the content of the epoxy compound as the curing agent within the above range, it is possible to achieve a high degree of both adhesion and heat resistance in a high-temperature environment.

The degree of crosslinking of the pressure-sensitive adhesive composition of the present invention is indicated by the gel fraction, and the method for measuring it is as shown in the Examples. There is no particular limit to the gel fraction, but it is preferably 40 to 70%, and more preferably 50 to 60%. By keeping it within the above range, it is possible to achieve a high degree of both repositionability and durability in high-temperature environments.

<Organic Solvent (C)>
The adhesive composition may contain an organic solvent (C). By containing the organic solvent (C), the adhesive composition can be easily adjusted to a suitable viscosity for coating the adhesive composition on the substrate, and the coating property can be improved. In addition, while a UV-curing system that coats and cures an adhesive composition without a solvent generally tends to have a non-uniform degree of crosslinking in the coating due to oxygen inhibition, UV lamp distance, etc., a thermosetting system that contains an organic solvent (C) can easily exhibit properties such as unevenness-following ability because a crosslinking reaction occurs uniformly in the coating.
As the organic solvent (C), the solvent used in the polymerization of the (meth)acrylic acid ester copolymer (A) may be used as it is, or a further solvent may be added, and any suitable solvent may be used.

Examples of the organic solvent (C) include methyl acetate, ethyl acetate, n-butyl acetate, isobutyl acetate, toluene, xylene, hexane, acetone, methyl ethyl ketone, methyl isobutyl ketone, methanol, ethanol, n-propanol, and isopropanol. These may be used alone or in combination of two or more. The viscosity of the adhesive composition can be adjusted by adding these organic solvents, or the viscosity can be reduced by heating the adhesive composition.
From the viewpoint of solubility and drying property, it is preferable to use an organic solvent having an SP value of 9 or more. Examples of organic solvents having an SP value of 9 or more include ethyl acetate, butyl acetate, methyl ethyl ketone, isopropanol, etc. Among these, it is more preferable to use ethyl acetate.
On the other hand, from the viewpoint of decreasing the viscosity of the pressure-sensitive adhesive composition, it is preferable to use an organic solvent having an SP value of less than 9. Examples of organic solvents having an SP value of less than 9 include hexane, heptane, and octane.

In addition to the copolymer (A) and the curing agent (B), the adhesive composition may contain any additives, such as a tackifier, a resin other than the copolymer (A), a plasticizer, a silane coupling agent, a leveling agent, inorganic and/or organic fine particles, an ultraviolet absorber, a light stabilizer, and an antioxidant, as necessary. The amount of these additives added can be appropriately selected within a range in which the effects of the present disclosure can be obtained, and it is preferable to appropriately set the content of the copolymer (A) within the above range, for example.

Resins other than the (meth)acrylic acid ester copolymer (A) include known thermoplastic resins, such as low-density polyethylene, very low-density polyethylene, low-crystalline (amorphous) polypropylene, ionomer resins, ethylene copolymers such as ethylene-vinyl acetate copolymer, ethylene-(meth)acrylic acid copolymer, ethylene-(meth)acrylic acid ester-maleic anhydride copolymer, and ethylene-glycidyl methacrylate copolymer, as well as olefin-based copolymers such as polyolefin-modified polymers, as well as thermoplastic elastomers such as butadiene-based elastomers, ester-based elastomers, styrene-based elastomers, styrene-butadiene-based elastomers, styrene-isoprene-based elastomers, and polyurethane-based elastomers, as well as thermoplastic polyesters, as well as polyamide-based resins such as polyamide-based copolymers, polyurethanes, polystyrene-based resins, cellophane, polyacrylonitrile, and polyvinyl chloride-based resins such as vinyl chloride-vinyl acetate copolymers. These resins may be used alone or in combination of two or more. The content of these other resins in the solid content of the composition is preferably less than 10% by mass. If the content of these resins is less than 10% by mass, good compatibility is easily obtained, and appropriate adhesive strength can be easily obtained.

A tackifier (tackifier resin) can be used in the adhesive composition to adjust the adhesive strength. Tackifiers are not particularly limited, but examples include rosin-based resins, terpene-based resins, synthetic hydrocarbon-based resins, etc.

Examples of rosin-based resins include rosin esters, polymerized rosin, hydrogenated rosin, disproportionated rosin, maleic acid modified rosin, fumaric acid modified rosin, rosin phenolic resin, and natural rosin.

Examples of terpene resins include α-pinene resin, β-pinene resin, dipentene resin, aromatic modified terpene resin, hydrogenated terpene resin, terpene phenol resin, acid modified terpene resin, styrenated terpene resin, and styrene-aliphatic hydrocarbon copolymer resin.

Examples of synthetic hydrocarbon resins include coumarone resins, coumarone-indene resins, styrene resins, xylene resins, phenol resins, and petroleum resins.

From the viewpoint of obtaining good adhesion to the object to which it is applied, it is preferable to use rosin-based resins and terpene-based resins as tackifiers. Tackifiers may be used alone or in combination of two or more types.

The content of the tackifier is preferably less than 30 parts by mass, more preferably 25 parts by mass or less, and even more preferably 20 parts by mass or less, per 100 parts by mass of the acrylic acid ester copolymer (A). By making the content of the tackifier less than 30 parts by mass, it is easy to obtain good compatibility, appropriate re-tensioning properties, and excellent appearance of the molded product.

The present pressure-sensitive adhesive composition preferably contains the (meth)acrylic acid ester copolymer (A) as a main component, which refers to the component that is present in the largest amount among all the components contained in the target object (here, the present composition).
Specifically, the content of the (meth)acrylic acid ester copolymer (A) in 100% by mass of the solid content of the present adhesive composition is preferably 70 to 100% by mass, more preferably 80 to 100% by mass, and even more preferably 90 to 99% by mass. When the (meth)acrylic acid ester copolymer (A) is contained in 70% by mass or more in 100% by mass of the solid content of the present adhesive composition, it is possible to easily obtain an appropriate adhesive strength that allows repositioning at room temperature and is free of air bubble entrapment and foreign matter contamination.

<Decorative sheet>
The adhesive composition described above can be preferably used for a decorative sheet. The decorative sheet according to the present invention (hereinafter also referred to as the decorative sheet) comprises a substrate and an adhesive layer made of a cured product of the adhesive composition. If necessary, the exposed surface of the adhesive layer can be covered with a release sheet. The release sheet is peeled off when the adhesive sheet is attached to an adherend.
The substrate is not particularly limited, and may be a resin sheet, paper, metal foil, etc. The substrate may be a laminated sheet in which any one or more layers are laminated on at least one surface of the substrate. The surface of the substrate on which any adhesive layer is formed may be subjected to an easy-adhesion treatment such as corona discharge treatment and application of an anchor coating agent, if necessary.
The release sheet is not particularly limited, and any known release sheet can be used, which is obtained by subjecting the surface of a base sheet such as a resin sheet or paper to a known release treatment such as coating with a release agent.

The method for producing the present decorative sheet having an adhesive layer can be appropriately used by a conventionally known method, and is not particularly limited. For example, the present composition, which is diluted with a solvent as necessary, is coated on a sheet on which a surface layer and a decorative layer are formed by knife coating, bar coating, blade coating, doctor coating, roll coating, cast coating, etc. to form a coating film. Then, the adhesive layer (solidified product) can be formed on the sheet by heating and drying or curing as necessary.
In addition, when the copolymer (A) contained in the present composition needs to be crosslinked by active energy rays, the present composition is applied onto a sheet, and then the sheet is irradiated with active energy rays to form a pressure-sensitive adhesive layer (crosslinked product). At that time, for example, a belt conveyor type ultraviolet ray irradiation device is used to irradiate the pressure-sensitive adhesive composition applied onto the sheet with ultraviolet rays to obtain a pressure-sensitive adhesive layer. The amount of ultraviolet irradiation can be, for example, 500 mJ/ ^cm2 or more and 5000 mJ/cm2 or ^less .
This decorative sheet has a suitable adhesive strength at room temperature (e.g., 25°C), has excellent adhesive properties when bonded at high temperatures (e.g., 100 to 130°C), and can maintain its properties for a long period of time at high temperatures (e.g., 100°C for 7 days). Therefore, this decorative sheet can be applied to the decoration of aircraft, automobiles, interior and exterior of building materials, nursing care and medical fields, electrical appliances, electronic parts, smartphones, furniture, musical instruments, Shinkansen window frame walls, etc., by three-dimensional surface decoration (Three Dimension Overlay Method: TOM molding), which is a type of vacuum and compressed air molding.

<Decorative structure and method for producing same>
The decorative structure according to the present disclosure (hereinafter also referred to as the present structure) can be formed by attaching a decorative sheet to an adherend using the present pressure-sensitive adhesive composition. The adherend is not particularly limited, and various shapes of objects (e.g., molded products having three-dimensional shapes) can be used. The decorative sheet using the present pressure-sensitive adhesive composition is used for attachment to an adherend by a vacuum forming method and a vacuum-pressure forming method. Conventionally known methods can be appropriately applied for the vacuum forming method and the vacuum-pressure forming method.

More specifically, the present decorated structure can include, for example, a surface layer, a decorative layer, and a pressure-sensitive adhesive layer in this order.
The surface layer is a layer that becomes the surface of the decorated molded product, and any layer that is conventionally known in the field of decorative sheets can be used as appropriate. The surface layer can be made of various resins, such as acrylic resin, polyurethane, fluororesin, and polyvinyl chloride. The exposed surface of the surface layer may also be embossed.
The decorative layer is for decorating the object to be decorated, and various materials can be used depending on the intended use. The decorative layer can contain at least one resin selected from the group consisting of olefin resins, styrene resins, and vinyl chloride resins. The decorative layer may further contain other layers such as a design layer, a bulk layer, and a bonding layer as optional elements. The number of layers of the decorative sheet, the type, arrangement, thickness, etc. of each layer can be appropriately selected and are not particularly limited.
The adhesive layer is a layer for attaching a decorative sheet to an adherend (e.g., a molded product formed into a three-dimensional shape), and can be formed by drying the adhesive composition (e.g., after applying it to a substrate) or by crosslinking it by irradiating it with ultraviolet light, etc.

Next, further details will be explained by showing examples, but the present disclosure is not limited to these examples. In the examples, unless otherwise specified, "parts" means "parts by mass" and "%" means "% by mass". Also, the compounding amounts in the tables are in parts by mass.

<Measurement of weight average molecular weight (Mw)>
The Mw of the (meth)acrylic acid ester copolymer (A) was measured by the following method. The copolymer (A) dissolved in tetrahydrofuran (THF) was subjected to liquid chromatography for separating and quantifying based on the difference in molecular size, and the Mw was specified. The measurement device used was a LC-GPC system "Prominence" (product name) of GPC manufactured by Shimadzu Corporation. The weight average molecular weight was determined in terms of polystyrene. The columns were four GMHXL columns manufactured by Tosoh Corporation (product name: GMHXL) and one HXL-H column manufactured by Tosoh Corporation (product name: HXL-H) connected in series, and the measurement was performed at a flow rate of 1.0 ml/min and a column temperature of 40°C.

[Production Example of (Meth)Acrylic Acid Ester Copolymer (A)]
<Synthesis Example 1>
A reaction vessel (hereinafter sometimes simply referred to as "reaction vessel") equipped with a stirrer, a thermometer, a reflux condenser, a dropping funnel, and a nitrogen inlet tube was charged with 34 parts of 2-ethylhexyl acrylate, 14 parts of ethyl acrylate, 2 parts of acrylic acid, 63 parts of acetone, and 0.005 parts of 2,2'-azobisisobutyronitrile (AIBN), and the air in the reaction vessel was then replaced with nitrogen gas. Furthermore, 34 parts of 2-ethylhexyl acrylate, 14 parts of ethyl acrylate, 2 parts of acrylic acid, 17 parts of acetone, 10 parts of ethyl acetate, and 0.005 parts of 2,2'-azobisisobutyronitrile (AIBN) were charged into the dropping funnel. Next, the reaction vessel was heated to 80°C while stirring under a nitrogen atmosphere, and the reaction was started. The contents of the dropping funnel were added dropwise over 1.5 hours, and the polymerization reaction was carried out for 5 hours at reflux temperature under a nitrogen atmosphere. After the reaction was completed, the mixture was cooled and diluted with ethyl acetate to obtain an acrylic polymer solution having a non-volatile content of 30%. The weight average molecular weight (Mw) of the acrylic polymer was 1,420,000.

<Synthesis Example 2>
A reaction vessel (hereinafter sometimes simply referred to as "reaction vessel") equipped with a stirrer, a thermometer, a reflux condenser, a dropping funnel, and a nitrogen inlet tube was charged with 34 parts of 2-ethylhexyl acrylate, 14 parts of ethyl acrylate, 2 parts of acrylic acid, 63 parts of ethyl acetate, and 0.005 parts of 2,2'-azobisisobutyronitrile (AIBN), and the air in the reaction vessel was then replaced with nitrogen gas. Furthermore, 34 parts of 2-ethylhexyl acrylate, 14 parts of ethyl acrylate, 2 parts of acrylic acid, 27 parts of ethyl acetate, and 0.005 parts of 2,2'-azobisisobutyronitrile (AIBN) were charged into the dropping funnel. Next, the reaction vessel was heated to 80°C while stirring under a nitrogen atmosphere, and the reaction was started. The contents of the dropping funnel were added dropwise over 1.5 hours, while the polymerization reaction was carried out for 5 hours at reflux temperature under a nitrogen atmosphere. After the reaction was completed, the mixture was cooled and diluted with ethyl acetate to obtain an acrylic polymer solution having a non-volatile content of 30%. The weight average molecular weight (Mw) of the acrylic polymer was 950,000.

<Synthesis Example 3>
A reaction vessel (hereinafter sometimes simply referred to as "reaction vessel") equipped with a stirrer, a thermometer, a reflux condenser, a dropping funnel, and a nitrogen inlet tube was charged with 34 parts of 2-ethylhexyl acrylate, 14 parts of ethyl acrylate, 2 parts of acrylic acid, 53 parts of ethyl acetate, 10 parts of methyl ethyl ketone, and 0.005 parts of 2,2'-azobisisobutyronitrile (AIBN), and the air in the reaction vessel was then replaced with nitrogen gas. Furthermore, 34 parts of 2-ethylhexyl acrylate, 14 parts of ethyl acrylate, 2 parts of acrylic acid, 27 parts of ethyl acetate, and 0.005 parts of 2,2'-azobisisobutyronitrile (AIBN) were charged into the dropping funnel. Next, the reaction vessel was heated to 80°C while stirring under a nitrogen atmosphere, and the reaction was started. The contents of the dropping funnel were added dropwise over 1.5 hours, and the polymerization reaction was carried out for 5 hours at reflux temperature under a nitrogen atmosphere. After the reaction was completed, the mixture was cooled and diluted with ethyl acetate to obtain an acrylic polymer solution having a non-volatile content of 30%. The weight average molecular weight (Mw) of the acrylic polymer was 780,000.

<Synthesis Example 4>
A reaction vessel (hereinafter sometimes simply referred to as "reaction vessel") equipped with a stirrer, a thermometer, a reflux condenser, a dropping funnel, and a nitrogen inlet tube was charged with 34 parts of 2-ethylhexyl acrylate, 14 parts of ethyl acrylate, 2 parts of acrylic acid, 63 parts of acetone, and 0.005 parts of 2,2'-azobisisobutyronitrile (AIBN), and the air in the reaction vessel was then replaced with nitrogen gas. Furthermore, 34 parts of 2-ethylhexyl acrylate, 12.5 parts of ethyl acrylate, 3.5 parts of acrylic acid, 17 parts of acetone, 10 parts of ethyl acetate, and 0.005 parts of 2,2'-azobisisobutyronitrile (AIBN) were charged into the dropping funnel. Next, the reaction vessel was heated to 80°C while stirring under a nitrogen atmosphere to start the reaction, and the polymerization reaction was carried out for 5 hours at reflux temperature under a nitrogen atmosphere while dropping the contents of the dropping funnel over 1.5 hours. After the reaction was completed, the mixture was cooled and diluted with ethyl acetate to obtain an acrylic polymer solution having a non-volatile content of 30%. The weight average molecular weight (Mw) of the acrylic polymer was 1,420,000.

<Synthesis Example 5>
A reaction vessel (hereinafter sometimes simply referred to as "reaction vessel") equipped with a stirrer, a thermometer, a reflux condenser, a dropping funnel, and a nitrogen inlet tube was charged with 34 parts of 2-ethylhexyl acrylate, 14 parts of ethyl acrylate, 2 parts of acrylic acid, 63 parts of acetone, and 0.005 parts of 2,2'-azobisisobutyronitrile (AIBN), and the air in the reaction vessel was then replaced with nitrogen gas. Furthermore, 34 parts of 2-ethylhexyl acrylate, 10 parts of ethyl acrylate, 6 parts of acrylic acid, 17 parts of acetone, 10 parts of ethyl acetate, and 0.005 parts of 2,2'-azobisisobutyronitrile (AIBN) were charged into the dropping funnel. Next, the reaction vessel was heated to 80°C while stirring under a nitrogen atmosphere, and the reaction was started. The contents of the dropping funnel were added dropwise over 1.5 hours, and the polymerization reaction was carried out for 5 hours at reflux temperature under a nitrogen atmosphere. After the reaction was completed, the mixture was cooled and diluted with ethyl acetate to obtain an acrylic polymer solution having a non-volatile content of 30%. The weight average molecular weight (Mw) of the acrylic polymer was 1,380,000.

<Synthesis Example 6>
A reaction vessel (hereinafter sometimes simply referred to as "reaction vessel") equipped with a stirrer, a thermometer, a reflux condenser, a dropping funnel, and a nitrogen inlet tube was charged with 34 parts of 2-ethylhexyl acrylate, 14 parts of ethyl acrylate, 2 parts of acrylic acid, 63 parts of acetone, and 0.005 parts of 2,2'-azobisisobutyronitrile (AIBN), and the air in the reaction vessel was then replaced with nitrogen gas. Furthermore, 34 parts of 2-ethylhexyl acrylate, 6.5 parts of ethyl acrylate, 9.5 parts of acrylic acid, 17 parts of acetone, 10 parts of ethyl acetate, and 0.005 parts of 2,2'-azobisisobutyronitrile (AIBN) were charged into the dropping funnel. Next, the reaction vessel was heated to 80°C while stirring under a nitrogen atmosphere, and after starting the reaction, the contents of the dropping funnel were dropped over 1.5 hours, while performing a polymerization reaction at reflux temperature under a nitrogen atmosphere for 5 hours. After the reaction was completed, the mixture was cooled and diluted with ethyl acetate to obtain an acrylic polymer solution having a non-volatile content of 30%. The weight average molecular weight (Mw) of the acrylic polymer was 1,350,000.

<Synthesis Example 7>
A reaction vessel (hereinafter sometimes simply referred to as "reaction vessel") equipped with a stirrer, a thermometer, a reflux condenser, a dropping funnel, and a nitrogen inlet tube was charged with 34 parts of 2-ethylhexyl acrylate, 14 parts of ethyl acrylate, 2 parts of acrylic acid, 63 parts of acetone, and 0.005 parts of 2,2'-azobisisobutyronitrile (AIBN), and the air in the reaction vessel was then replaced with nitrogen gas. Furthermore, 34 parts of 2-ethylhexyl acrylate, 13.5 parts of ethyl acrylate, 2.5 parts of 2-carboxyethyl acrylate, 17 parts of acetone, 10 parts of ethyl acetate, and 0.005 parts of 2,2'-azobisisobutyronitrile (AIBN) were charged into the dropping funnel. Next, the reaction vessel was heated to 80°C while stirring under a nitrogen atmosphere, and the reaction was started. The contents of the dropping funnel were added dropwise over 1.5 hours, and the polymerization reaction was carried out for 5 hours at reflux temperature under a nitrogen atmosphere. After the reaction was completed, the mixture was cooled and diluted with ethyl acetate to obtain an acrylic polymer solution having a non-volatile content of 30%. The weight average molecular weight (Mw) of the acrylic polymer was 1,400,000.

<Synthesis Example 8>
A reaction vessel (hereinafter sometimes simply referred to as "reaction vessel") equipped with a stirrer, a thermometer, a reflux condenser, a dropping funnel, and a nitrogen inlet tube was charged with 26 parts of 2-ethylhexyl acrylate, 4 parts of n-butyl acrylate, 17 parts of ethyl acrylate, 2 parts of acrylic acid, 30 parts of acetone, 30 parts of ethyl acetate, and 0.005 parts of 2,2'-azobisisobutyronitrile (AIBN), and the air in the reaction vessel was then replaced with nitrogen gas. Further, the dropping funnel was charged with 27 parts of 2-ethylhexyl acrylate, 4 parts of n-butyl acrylate, 18 parts of ethyl acrylate, 2 parts of acrylic acid, 10 parts of acetone, 10 parts of ethyl acetate, and 0.005 parts of 2,2'-azobisisobutyronitrile (AIBN). Next, the inside of the reaction vessel was heated to 80°C while stirring under a nitrogen atmosphere to start the reaction, and the contents of the dropping funnel were added dropwise over 1.5 hours while the polymerization reaction was carried out at reflux temperature under a nitrogen atmosphere for 5 hours. After the reaction was completed, the mixture was cooled and diluted with ethyl acetate to obtain an acrylic polymer solution with a non-volatile content of 30%. The weight average molecular weight (Mw) of the acrylic polymer was 1,220,000.

<Synthesis Example 9>
A reaction vessel (hereinafter sometimes simply referred to as "reaction vessel") equipped with a stirrer, a thermometer, a reflux condenser, a dropping funnel, and a nitrogen inlet tube was charged with 30 parts of 2-ethylhexyl acrylate, 17 parts of ethyl acrylate, 2 parts of acrylic acid, 30 parts of acetone, 30 parts of ethyl acetate, and 0.005 parts of 2,2'-azobisisobutyronitrile (AIBN), and the air in the reaction vessel was then replaced with nitrogen gas. Furthermore, 30 parts of 2-ethylhexyl acrylate, 1 part of n-butyl acrylate, 18 parts of ethyl acrylate, 2 parts of acrylic acid, 10 parts of acetone, 10 parts of ethyl acetate, and 0.005 parts of 2,2'-azobisisobutyronitrile (AIBN) were charged into the dropping funnel. Next, the reaction vessel was heated to 80°C while stirring under a nitrogen atmosphere to start the reaction, and the polymerization reaction was carried out for 5 hours at reflux temperature under a nitrogen atmosphere while dropping the contents of the dropping funnel over 1.5 hours. After the reaction was completed, the mixture was cooled and diluted with ethyl acetate to obtain an acrylic polymer solution having a non-volatile content of 30%. The weight average molecular weight (Mw) of the acrylic polymer was 1,200,000.

<Synthesis Example 10>
A reaction vessel (hereinafter sometimes simply referred to as "reaction vessel") equipped with a stirrer, a thermometer, a reflux condenser, a dropping funnel, and a nitrogen inlet tube was charged with 31 parts of 2-ethylhexyl acrylate, 17 parts of ethyl acrylate, 2 parts of acrylic acid, 30 parts of acetone, 30 parts of ethyl acetate, and 0.005 parts of 2,2'-azobisisobutyronitrile (AIBN), and the air in the reaction vessel was then replaced with nitrogen gas. Furthermore, 31 parts of 2-ethylhexyl acrylate, 17 parts of ethyl acrylate, 2 parts of acrylic acid, 10 parts of acetone, 10 parts of ethyl acetate, and 0.005 parts of 2,2'-azobisisobutyronitrile (AIBN) were charged into the dropping funnel. Next, the reaction vessel was heated to 80°C while stirring under a nitrogen atmosphere, and the reaction was started. The contents of the dropping funnel were added dropwise over 1.5 hours, and the polymerization reaction was carried out for 5 hours at reflux temperature under a nitrogen atmosphere. After the reaction was completed, the mixture was cooled and diluted with ethyl acetate to obtain an acrylic polymer solution having a non-volatile content of 30%. The weight average molecular weight (Mw) of the acrylic polymer was 1,200,000.

<Synthesis Example 11>
A reaction vessel (hereinafter sometimes simply referred to as "reaction vessel") equipped with a stirrer, a thermometer, a reflux condenser, a dropping funnel, and a nitrogen inlet tube was charged with 37 parts of 2-ethylhexyl acrylate, 10 parts of ethyl acrylate, 2 parts of acrylic acid, 30 parts of acetone, 30 parts of ethyl acetate, and 0.005 parts of 2,2'-azobisisobutyronitrile (AIBN), and the air in the reaction vessel was then replaced with nitrogen gas. Furthermore, 38 parts of 2-ethylhexyl acrylate, 11 parts of ethyl acrylate, 2 parts of acrylic acid, 10 parts of acetone, 10 parts of ethyl acetate, and 0.005 parts of 2,2'-azobisisobutyronitrile (AIBN) were charged into the dropping funnel. Next, the reaction vessel was heated to 80°C while stirring under a nitrogen atmosphere, and the reaction was started. The contents of the dropping funnel were added dropwise over 1.5 hours, and the polymerization reaction was carried out for 5 hours at reflux temperature under a nitrogen atmosphere. After the reaction was completed, the mixture was cooled and diluted with ethyl acetate to obtain an acrylic polymer solution having a non-volatile content of 30%. The weight average molecular weight (Mw) of the acrylic polymer was 1,100,000.

<Synthesis Example 12>
A reaction vessel (hereinafter sometimes simply referred to as "reaction vessel") equipped with a stirrer, a thermometer, a reflux condenser, a dropping funnel, and a nitrogen inlet tube was charged with 40 parts of 2-ethylhexyl acrylate, 8 parts of ethyl acrylate, 1.5 parts of acrylic acid, 30 parts of acetone, 30 parts of ethyl acetate, and 0.005 parts of 2,2'-azobisisobutyronitrile (AIBN), and the air in the reaction vessel was then replaced with nitrogen gas. Furthermore, 41 parts of 2-ethylhexyl acrylate, 8 parts of ethyl acrylate, 1.5 parts of acrylic acid, 10 parts of acetone, 10 parts of ethyl acetate, and 0.005 parts of 2,2'-azobisisobutyronitrile (AIBN) were charged into the dropping funnel. Next, the reaction vessel was heated to 80°C while stirring under a nitrogen atmosphere, and after starting the reaction, the contents of the dropping funnel were added dropwise over 1.5 hours, and a polymerization reaction was carried out for 5 hours at reflux temperature under a nitrogen atmosphere. After the reaction was completed, the mixture was cooled and diluted with ethyl acetate to obtain an acrylic polymer solution having a non-volatile content of 30%. The weight average molecular weight (Mw) of the acrylic polymer was 1,050,000.

<Synthesis Example 13>
A reaction vessel (hereinafter sometimes simply referred to as "reaction vessel") equipped with a stirrer, a thermometer, a reflux condenser, a dropping funnel, and a nitrogen inlet tube was charged with 35 parts of 2-ethylhexyl acrylate, 13.5 parts of n-butyl acrylate, 0.25 parts of ethyl acrylate, 2 parts of acrylic acid, 60 parts of acetone, and 0.005 parts of 2,2'-azobisisobutyronitrile (AIBN), and then the air in the reaction vessel was replaced with nitrogen gas. Furthermore, the dropping funnel was charged with 34 parts of 2-ethylhexyl acrylate, 13 parts of n-butyl acrylate, 2 parts of acrylic acid, 0.25 parts of ethyl acrylate, 10 parts of acetone, 10 parts of ethyl acetate, and 0.005 parts of 2,2'-azobisisobutyronitrile (AIBN). Next, the inside of the reaction vessel was heated to 80°C while stirring under a nitrogen atmosphere to start the reaction, and the contents of the dropping funnel were added dropwise over 1.5 hours while the polymerization reaction was carried out at reflux temperature under a nitrogen atmosphere for 5 hours. After the reaction was completed, the mixture was cooled and diluted with ethyl acetate to obtain an acrylic polymer solution with a non-volatile content of 30%. The weight average molecular weight (Mw) of the acrylic polymer was 1,180,000.

<Synthesis Example 14>
A reaction vessel (hereinafter sometimes simply referred to as "reaction vessel") equipped with a stirrer, a thermometer, a reflux condenser, a dropping funnel, and a nitrogen inlet tube was charged with 35 parts of 2-ethylhexyl acrylate, 9 parts of n-butyl acrylate, 4 parts of ethyl acrylate, 2 parts of acrylic acid, 60 parts of acetone, and 0.005 parts of 2,2'-azobisisobutyronitrile (AIBN), and the air in the reaction vessel was then replaced with nitrogen gas. Furthermore, 34 parts of 2-ethylhexyl acrylate, 10 parts of n-butyl acrylate, 4 parts of ethyl acrylate, 2 parts of acrylic acid, 10 parts of acetone, 10 parts of ethyl acetate, and 0.005 parts of 2,2'-azobisisobutyronitrile (AIBN) were charged into the dropping funnel. Next, the reaction vessel was heated to 80°C while stirring under a nitrogen atmosphere, and the reaction was started. The contents of the dropping funnel were added dropwise over 1.5 hours, and the polymerization reaction was carried out for 5 hours at reflux temperature under a nitrogen atmosphere. After the reaction was completed, the mixture was cooled and diluted with ethyl acetate to obtain an acrylic polymer solution having a non-volatile content of 30%. The weight average molecular weight (Mw) of the acrylic polymer was 1,250,000.

<Synthesis Example 15>
A reaction vessel (hereinafter sometimes simply referred to as "reaction vessel") equipped with a stirrer, a thermometer, a reflux condenser, a dropping funnel, and a nitrogen inlet tube was charged with 35 parts of 2-ethylhexyl acrylate, 8 parts of n-butyl acrylate, 6 parts of ethyl acrylate, 1.5 parts of acrylic acid, 60 parts of acetone, and 0.005 parts of 2,2'-azobisisobutyronitrile (AIBN), and the air in the reaction vessel was then replaced with nitrogen gas. Further, the dropping funnel was charged with 34 parts of 2-ethylhexyl acrylate, 8 parts of n-butyl acrylate, 6 parts of ethyl acrylate, 1.5 parts of acrylic acid, 10 parts of acetone, 10 parts of ethyl acetate, and 0.005 parts of 2,2'-azobisisobutyronitrile (AIBN). Next, the inside of the reaction vessel was heated to 80°C while stirring under a nitrogen atmosphere to start the reaction, and the contents of the dropping funnel were added dropwise over 1.5 hours while the polymerization reaction was carried out at reflux temperature under a nitrogen atmosphere for 5 hours. After the reaction was completed, the mixture was cooled and diluted with ethyl acetate to obtain an acrylic polymer solution with a non-volatile content of 30%. The weight average molecular weight (Mw) of the acrylic polymer was 1.28 million.

<Synthesis Example 16>
A reaction vessel (hereinafter sometimes simply referred to as "reaction vessel") equipped with a stirrer, a thermometer, a reflux condenser, a dropping funnel, and a nitrogen inlet tube was charged with 28 parts of 2-ethylhexyl acrylate, 20 parts of ethyl acrylate, 1.5 parts of acrylic acid, 60 parts of acetone, and 0.005 parts of 2,2'-azobisisobutyronitrile (AIBN), and the air in the reaction vessel was then replaced with nitrogen gas. Furthermore, 22 parts of 2-ethylhexyl acrylate, 2 parts of butyl acrylate, 20 parts of ethyl acrylate, 1.5 parts of acrylic acid, 10 parts of acetone, 10 parts of ethyl acetate, and 0.005 parts of 2,2'-azobisisobutyronitrile (AIBN) were charged into the dropping funnel. Next, the reaction vessel was heated to 80°C while stirring under a nitrogen atmosphere, and the reaction was started. The contents of the dropping funnel were added dropwise over 1.5 hours, and the polymerization reaction was carried out for 5 hours at reflux temperature under a nitrogen atmosphere. After the reaction was completed, the mixture was cooled and diluted with ethyl acetate to obtain an acrylic polymer solution having a non-volatile content of 30%. The weight average molecular weight (Mw) of the acrylic polymer was 1,360,000.

<Synthesis Example 17>
A reaction vessel (hereinafter sometimes simply referred to as "reaction vessel") equipped with a stirrer, a thermometer, a reflux condenser, a dropping funnel, and a nitrogen inlet tube was charged with 27 parts of 2-ethylhexyl acrylate, 22 parts of ethyl acrylate, 1 part of acrylic acid, 60 parts of acetone, and 0.005 parts of 2,2'-azobisisobutyronitrile (AIBN), and the air in the reaction vessel was then replaced with nitrogen gas. Furthermore, 26 parts of 2-ethylhexyl acrylate, 23 parts of ethyl acrylate, 1 part of acrylic acid, 10 parts of acetone, 10 parts of ethyl acetate, and 0.005 parts of 2,2'-azobisisobutyronitrile (AIBN) were charged into the dropping funnel. Next, the reaction vessel was heated to 80°C while stirring under a nitrogen atmosphere, and the reaction was started. The contents of the dropping funnel were added dropwise over 1.5 hours, and the polymerization reaction was carried out for 5 hours at reflux temperature under a nitrogen atmosphere. After the reaction was completed, the mixture was cooled and diluted with ethyl acetate to obtain an acrylic polymer solution having a non-volatile content of 30%. The weight average molecular weight (Mw) of the acrylic polymer was 1,370,000.

<Synthesis Example 18>
A reaction vessel (hereinafter sometimes simply referred to as "reaction vessel") equipped with a stirrer, a thermometer, a reflux condenser, a dropping funnel, and a nitrogen inlet tube was charged with 34 parts of 2-ethylhexyl acrylate, 9 parts of methyl acrylate, 4 parts of ethyl acrylate, 2 parts of acrylic acid, 60 parts of acetone, and 0.005 parts of 2,2'-azobisisobutyronitrile (AIBN), and the air in the reaction vessel was then replaced with nitrogen gas. Furthermore, 35 parts of 2-ethylhexyl acrylate, 9 parts of methyl acrylate, 5 parts of ethyl acrylate, 2 parts of acrylic acid, 10 parts of acetone, 10 parts of ethyl acetate, and 0.005 parts of 2,2'-azobisisobutyronitrile (AIBN) were charged into the dropping funnel. Next, the reaction vessel was heated to 80°C while stirring under a nitrogen atmosphere, and the reaction was started. The contents of the dropping funnel were added dropwise over 1.5 hours, and the polymerization reaction was carried out for 5 hours at reflux temperature under a nitrogen atmosphere. After the reaction was completed, the mixture was cooled and diluted with ethyl acetate to obtain an acrylic polymer solution having a non-volatile content of 30%. The weight average molecular weight (Mw) of the acrylic polymer was 1,380,000.

<Synthesis Example 19>
A reaction vessel (hereinafter sometimes simply referred to as "reaction vessel") equipped with a stirrer, a thermometer, a reflux condenser, a dropping funnel, and a nitrogen inlet tube was charged with 34 parts of 2-ethylhexyl acrylate, 4 parts of isobutyl acrylate, 9 parts of n-butyl acrylate, 2 parts of acrylic acid, 60 parts of acetone, and 0.005 parts of 2,2'-azobisisobutyronitrile (AIBN), and then the air in the reaction vessel was replaced with nitrogen gas. Further, the dropping funnel was charged with 35 parts of 2-ethylhexyl acrylate, 4 parts of isobutyl acrylate, 10 parts of n-butyl acrylate, 2 parts of acrylic acid, 10 parts of acetone, 10 parts of ethyl acetate, and 0.005 parts of 2,2'-azobisisobutyronitrile (AIBN). Next, the inside of the reaction vessel was heated to 80°C while stirring under a nitrogen atmosphere to start the reaction, and the contents of the dropping funnel were added dropwise over 1.5 hours while the polymerization reaction was carried out at reflux temperature under a nitrogen atmosphere for 5 hours. After the reaction was completed, the mixture was cooled and diluted with ethyl acetate to obtain an acrylic polymer solution with a non-volatile content of 30%. The weight average molecular weight (Mw) of the acrylic polymer was 1,210,000.

<Synthesis Example 20>
A reaction vessel (hereinafter sometimes simply referred to as "reaction vessel") equipped with a stirrer, a thermometer, a reflux condenser, a dropping funnel, and a nitrogen inlet tube was charged with 34 parts of 2-ethylhexyl acrylate, 4 parts of tert-butyl acrylate, 9 parts of n-butyl acrylate, 2 parts of acrylic acid, 60 parts of acetone, and 0.005 parts of 2,2'-azobisisobutyronitrile (AIBN), and then the air in the reaction vessel was replaced with nitrogen gas. Further, 35 parts of 2-ethylhexyl acrylate, 4 parts of tert-butyl acrylate, 10 parts of n-butyl acrylate, 2 parts of acrylic acid, 10 parts of acetone, 10 parts of ethyl acetate, and 0.005 parts of 2,2'-azobisisobutyronitrile (AIBN) were charged into the dropping funnel. Next, the inside of the reaction vessel was heated to 80°C while stirring under a nitrogen atmosphere to start the reaction, and the contents of the dropping funnel were added dropwise over 1.5 hours while carrying out a polymerization reaction at reflux temperature under a nitrogen atmosphere for 5 hours. After the reaction was completed, the mixture was cooled and diluted with ethyl acetate to obtain an acrylic polymer solution with a non-volatile content of 30%. The weight average molecular weight (Mw) of the acrylic polymer was 1.2 million.

<Synthesis Example 21>
A reaction vessel (hereinafter sometimes simply referred to as "reaction vessel") equipped with a stirrer, a thermometer, a reflux condenser, a dropping funnel, and a nitrogen inlet tube was charged with 44 parts of 2-ethylhexyl acrylate, 6 parts of acrylic acid, 60 parts of acetone, and 0.005 parts of 2,2'-azobisisobutyronitrile (AIBN), and the air in the reaction vessel was then replaced with nitrogen gas. Furthermore, 45 parts of 2-ethylhexyl acrylate, 5 parts of acrylic acid, 10 parts of acetone, 10 parts of ethyl acetate, and 0.005 parts of 2,2'-azobisisobutyronitrile (AIBN) were charged into the dropping funnel. Next, the reaction vessel was heated to 80°C while stirring under a nitrogen atmosphere to start the reaction, and the contents of the dropping funnel were added dropwise over 1.5 hours, while performing a polymerization reaction at reflux temperature under a nitrogen atmosphere for 5 hours. After the reaction was completed, the mixture was cooled and diluted with ethyl acetate to obtain an acrylic polymer solution with a non-volatile content of 30%. The weight average molecular weight (Mw) of the acrylic polymer was 1,100,000.

<Synthesis Example 22>
A reaction vessel (hereinafter sometimes simply referred to as "reaction vessel") equipped with a stirrer, a thermometer, a reflux condenser, a dropping funnel, and a nitrogen inlet tube was charged with 25 parts of 2-ethylhexyl acrylate, 25 parts of ethyl acrylate, 1.5 parts of acrylic acid, 30 parts of acetone, 30 parts of ethyl acetate, and 0.005 parts of 2,2'-azobisisobutyronitrile (AIBN), and the air in the reaction vessel was then replaced with nitrogen gas. Furthermore, 25 parts of 2-ethylhexyl acrylate, 22 parts of ethyl acrylate, 1.5 parts of acrylic acid, 20 parts of ethyl acetate, and 0.005 parts of 2,2'-azobisisobutyronitrile (AIBN) were charged into the dropping funnel. Next, the reaction vessel was heated to 80°C while stirring under a nitrogen atmosphere, and after starting the reaction, the contents of the dropping funnel were dropped over 1.5 hours, while performing a polymerization reaction at reflux temperature under a nitrogen atmosphere for 5 hours. After the reaction was completed, the mixture was cooled and diluted with ethyl acetate to obtain an acrylic polymer solution having a non-volatile content of 30%. The weight average molecular weight (Mw) of the acrylic polymer was 1,220,000.

<Synthesis Example 23>
A reaction vessel (hereinafter sometimes simply referred to as "reaction vessel") equipped with a stirrer, a thermometer, a reflux condenser, a dropping funnel, and a nitrogen inlet tube was charged with 34 parts of n-octyl acrylate, 14 parts of ethyl acrylate, 2 parts of acrylic acid, 63 parts of acetone, and 0.005 parts of 2,2'-azobisisobutyronitrile (AIBN), and the air in the reaction vessel was then replaced with nitrogen gas. Furthermore, 34 parts of n-octyl acrylate, 14 parts of ethyl acrylate, 2 parts of acrylic acid, 17 parts of acetone, 10 parts of ethyl acetate, and 0.005 parts of 2,2'-azobisisobutyronitrile (AIBN) were charged into the dropping funnel. Next, the reaction vessel was heated to 80°C while stirring under a nitrogen atmosphere, and the reaction was started. The contents of the dropping funnel were added dropwise over 1.5 hours, and the polymerization reaction was carried out for 5 hours at reflux temperature under a nitrogen atmosphere. After the reaction was completed, the mixture was cooled and diluted with ethyl acetate to obtain an acrylic polymer solution having a non-volatile content of 30%. The weight average molecular weight (Mw) of the acrylic polymer was 1,490,000.

<Synthesis Example 24>
A reaction vessel (hereinafter sometimes simply referred to as "reaction vessel") equipped with a stirrer, a thermometer, a reflux condenser, a dropping funnel, and a nitrogen inlet tube was charged with 34 parts of 2-ethylhexyl acrylate, 14 parts of ethyl acrylate, 0.4 parts of acrylic acid, 63 parts of acetone, and 0.005 parts of 2,2'-azobisisobutyronitrile (AIBN), and the air in the reaction vessel was then replaced with nitrogen gas. Furthermore, 34 parts of 2-ethylhexyl acrylate, 3.2 parts of butyl acrylate, 14 parts of ethyl acrylate, 0.4 parts of acrylic acid, 17 parts of acetone, 10 parts of ethyl acetate, and 0.005 parts of 2,2'-azobisisobutyronitrile (AIBN) were charged into the dropping funnel. Next, the reaction vessel was heated to 80°C while stirring under a nitrogen atmosphere, and the reaction was started. The contents of the dropping funnel were added dropwise over 1.5 hours, and the polymerization reaction was carried out for 5 hours at reflux temperature under a nitrogen atmosphere. After the reaction was completed, the mixture was cooled and diluted with ethyl acetate to obtain an acrylic polymer solution having a non-volatile content of 30%. The weight average molecular weight (Mw) of the acrylic polymer was 1,480,000.
<Synthesis Example 25>
A reaction vessel (hereinafter sometimes simply referred to as "reaction vessel") equipped with a stirrer, a thermometer, a reflux condenser, a dropping funnel, and a nitrogen inlet tube was charged with 34 parts of 2-ethylhexyl acrylate, 14 parts of ethyl acrylate, 0.5 parts of acrylic acid, 63 parts of acetone, and 0.005 parts of 2,2'-azobisisobutyronitrile (AIBN), and the air in the reaction vessel was then replaced with nitrogen gas. Furthermore, 34 parts of 2-ethylhexyl acrylate, 3 parts of butyl acrylate, 14 parts of ethyl acrylate, 0.5 parts of acrylic acid, 17 parts of acetone, 10 parts of ethyl acetate, and 0.005 parts of 2,2'-azobisisobutyronitrile (AIBN) were charged into the dropping funnel. Next, the reaction vessel was heated to 80°C while stirring under a nitrogen atmosphere, and the reaction was started. The contents of the dropping funnel were added dropwise over 1.5 hours, and the polymerization reaction was carried out for 5 hours at reflux temperature under a nitrogen atmosphere. After the reaction was completed, the mixture was cooled and diluted with ethyl acetate to obtain an acrylic polymer solution having a non-volatile content of 30%. The weight average molecular weight (Mw) of the acrylic polymer was 1,480,000.

Comparative Synthesis Example 1
A reaction vessel (hereinafter sometimes simply referred to as "reaction vessel") equipped with a stirrer, a thermometer, a reflux condenser, a dropping funnel, and a nitrogen inlet tube was charged with 20 parts of 2-ethylhexyl acrylate, 25 parts of n-butyl acrylate, 4 parts of acrylic acid, 30 parts of acetone, 30 parts of ethyl acetate, and 0.005 parts of 2,2'-azobisisobutyronitrile (AIBN), and the air in the reaction vessel was then replaced with nitrogen gas. Furthermore, 20 parts of 2-ethylhexyl acrylate, 26 parts of n-butyl acrylate, 5 parts of acrylic acid, 20 parts of ethyl acetate, and 0.005 parts of 2,2'-azobisisobutyronitrile (AIBN) were charged into the dropping funnel. Next, the reaction vessel was heated to 80°C while stirring under a nitrogen atmosphere, and after starting the reaction, the contents of the dropping funnel were added dropwise over 1.5 hours, while performing a polymerization reaction at reflux temperature under a nitrogen atmosphere for 5 hours. After the reaction was completed, the mixture was cooled and diluted with ethyl acetate to obtain an acrylic polymer solution having a non-volatile content of 30%. The weight average molecular weight (Mw) of the acrylic polymer was 1,100,000.

Comparative Synthesis Example 2
A reaction vessel (hereinafter sometimes simply referred to as "reaction vessel") equipped with a stirrer, a thermometer, a reflux condenser, a dropping funnel, and a nitrogen inlet tube was charged with 42 parts of 2-ethylhexyl acrylate, 7 parts of acrylic acid, 60 parts of acetone, and 0.005 parts of 2,2'-azobisisobutyronitrile (AIBN), and the air in the reaction vessel was then replaced with nitrogen gas. Furthermore, 43 parts of 2-ethylhexyl acrylate, 8 parts of acrylic acid, 20 parts of ethyl acetate, and 0.005 parts of 2,2'-azobisisobutyronitrile (AIBN) were charged into the dropping funnel. Next, the reaction vessel was heated to 80°C while stirring under a nitrogen atmosphere to start the reaction, and the contents of the dropping funnel were added dropwise over 1.5 hours, while performing a polymerization reaction at reflux temperature under a nitrogen atmosphere for 5 hours. After the reaction was completed, the mixture was cooled and diluted with ethyl acetate to obtain an acrylic polymer solution with a non-volatile content of 30%. The weight average molecular weight (Mw) of the acrylic polymer was 1,070,000.

Comparative Synthesis Example 3
A reaction vessel (hereinafter sometimes simply referred to as "reaction vessel") equipped with a stirrer, a thermometer, a reflux condenser, a dropping funnel, and a nitrogen inlet tube was charged with 42 parts of 2-ethylhexyl acrylate, 5 parts of n-butyl acrylate, 2 parts of 2-hydroxyethyl acrylate, 60 parts of acetone, and 0.005 parts of 2,2'-azobisisobutyronitrile (AIBN), and the air in the reaction vessel was then replaced with nitrogen gas. Furthermore, 43 parts of 2-ethylhexyl acrylate, 6 parts of n-butyl acrylate, 2 parts of 2-hydroxyethyl acrylate, 20 parts of ethyl acetate, and 0.005 parts of 2,2'-azobisisobutyronitrile (AIBN) were charged into the dropping funnel. Next, the reaction vessel was heated to 80°C while stirring under a nitrogen atmosphere, and the reaction was started. The contents of the dropping funnel were added dropwise over 1.5 hours, and the polymerization reaction was carried out for 5 hours at reflux temperature under a nitrogen atmosphere. After the reaction was completed, the mixture was cooled and diluted with ethyl acetate to obtain an acrylic polymer solution having a non-volatile content of 30%. The weight average molecular weight (Mw) of the acrylic polymer was 1,050,000.

Comparative Synthesis Example 4
A reaction vessel (hereinafter sometimes simply referred to as "reaction vessel") equipped with a stirrer, a thermometer, a reflux condenser, a dropping funnel, and a nitrogen inlet tube was charged with 49 parts of 2-ethylhexyl acrylate, 0.25 parts of ethyl acrylate, 0.25 parts of acrylic acid, 60 parts of acetone, and 0.005 parts of 2,2'-azobisisobutyronitrile (AIBN), and the air in the reaction vessel was then replaced with nitrogen gas. Furthermore, 50 parts of 2-ethylhexyl acrylate, 0.25 parts of ethyl acrylate, 0.25 parts of acrylic acid, 20 parts of ethyl acetate, and 0.005 parts of 2,2'-azobisisobutyronitrile (AIBN) were charged into the dropping funnel. Next, the reaction vessel was heated to 80°C while stirring under a nitrogen atmosphere, and the reaction was started. The contents of the dropping funnel were added dropwise over 1.5 hours, and the polymerization reaction was carried out for 5 hours at reflux temperature under a nitrogen atmosphere. After the reaction was completed, the mixture was cooled and diluted with ethyl acetate to obtain an acrylic polymer solution having a non-volatile content of 30%. The weight average molecular weight (Mw) of the acrylic polymer was 1,130,000.

Example compounds are specifically shown in Table 1 below, but are not limited to these.

The abbreviations in Table 1 are described below.
[Monomer (a)]
2EHA: 2-ethylhexyl acrylate NOA: n-octyl acrylate [monomer (b)]
AA: acrylic acid BCEA: 2-carboxyethyl acrylate [monomer (c)]
MA: methyl acrylate EA: ethyl acrylate [monomer (d)]
IBA: isobutyl acrylate (Tg: 48° C.)
TBA: tert-butyl acrylate (Tg: 14° C.)
[Other monomers]
BA: n-butyl acrylate HEA: 2-hydroxyethyl acrylate

Example 1
100 parts of the copolymer (Synthesis Example 1) obtained in the above Production Example was mixed with 0.05 parts (non-volatile content) of TETRAD-X (manufactured by Mitsubishi Gas Chemical Company, Inc.) as a curing agent (B), and ethyl acetate was further added as a solvent to adjust the non-volatile content to 25%, thereby obtaining a pressure-sensitive adhesive composition of Example 1. The obtained pressure-sensitive adhesive composition was evaluated according to the measurement and evaluation methods shown below.

<Preparation of Decorative Sheet>
The obtained adhesive composition was applied to a release-treated surface of a polyethylene terephthalate (PET) film (thickness 38 μm) having one side subjected to a release treatment so that the thickness after drying would be 25 μm. Then, after drying at 105° C. for 2 minutes, the coating film was attached to a commercially available polyvinyl chloride film and aged for 7 days in an atmosphere of 23° C. and relative humidity 50% (hereinafter referred to as 50% RH) to produce a decorative sheet.

<Gel Fraction>
A test piece with a width of 3 mm and a length of 100 mm was cut out from the decorative sheet obtained above, and its weight was measured. The release sheet was peeled off from the test piece, and the exposed adhesive layer was attached to a 300 mesh stainless steel wire mesh, which was folded so that the test piece would not peel off, and the test piece was wrapped in the wire mesh and immersed in ethyl acetate as an extraction solution and left to stand at 50°C for 24 hours. After immersion, the wire mesh was removed, washed with a small amount of ethyl acetate, and dried at 100°C for 30 minutes, and then its weight was measured. The gel fraction was calculated by the following formula (2).
(Gel fraction)={(W2−W0−W3)/(W1−W0−W3)}×100 (Equation 2)
W0: Weight of wire mesh W1: Weight of wire mesh + test piece W2: Weight of wire mesh + test piece after drying W3: Weight of substrate of test piece

(SUS adhesion)
After curing for 5 hours under an atmosphere of 23°C-50% RH, a test piece with a width of 25 mm and a length of 150 mm was cut out from the decorative sheet. The release sheet was peeled off from the cut decorative sheet to expose the adhesive layer, which was then attached to the surface of a stainless steel plate (SUS304) and pressed with a 2 kg roll once. The sheet was then left for 24 hours under an atmosphere of 23°C-50% RH. Next, the adhesive strength (adhesive strength before heating) was measured using a tensile tester (Tensilon: manufactured by Orientec Co., Ltd.) in accordance with JIS Z0237, at a peel speed of 300 mm/min and a peel angle of 180°. The evaluation criteria are as follows.
[Evaluation Criteria]
◎: 13N/25mm or more, excellent.
◯: 8 N/25 mm or more and less than 13 N/25 mm, good.
△: 1N/25mm or more and less than 8N/25mm, usable.
×: Less than 1N/25mm, not practical.

(PP adhesive strength)
After curing for 5 hours under an atmosphere of 23°C-50% RH, a test piece with a width of 25 mm and a length of 150 mm was cut out from the decorative sheet. The release sheet was peeled off from the cut decorative sheet, and the exposed adhesive layer was attached to the surface of a polypropylene plate and pressed with a 2 kg roll once. Then, the sheet was left in an atmosphere of 23°C-50% RH for 24 hours. Next, the adhesive strength (adhesive strength before heating) was measured using a tensile tester (Tensilon: manufactured by Orientec Co., Ltd.) in accordance with JIS Z0237 under the conditions of a peel speed of 300 mm/min and a peel angle of 180°. The evaluation criteria are as follows.
[Evaluation Criteria]
◎: 8N/25mm or more, excellent.
○: 5N/25mm or more and less than 8N/25mm, good.
△: 1N/25mm or more and less than 5N/25mm, usable.
×: Less than 1N/25mm, not practical.

(Adhesion to SUS curved surfaces)
After curing for 5 hours in an atmosphere of 23°C-50% RH, a test piece 25 mm wide and 150 mm long was cut out from the decorative sheet. The release sheet was peeled off from the cut decorative sheet, and the exposed adhesive layer was attached to the surface of a stainless steel (SUS304) cylinder with a diameter of 10 mm and sufficiently pressed. After that, the sheet was left in an atmosphere of 23°C-50% RH for 24 hours, and the length of peeling at the end was measured. The evaluation criteria were as follows. The peeling at the end was the sum of the peeling lengths at both ends, and the peeling length at one end was the average of the peeling lengths on the left and right.
[Evaluation Criteria]
◎: Less than 2 mm, excellent.
○: 2 mm or more and less than 3 mm, good.
△: 3 mm or more and less than 5 mm, usable.
×: 5 mm or more, not practical.

(PP curved surface attachment)
After curing for 5 hours in an atmosphere of 23°C-50% RH, a test piece 25 mm wide and 150 mm long was cut out from the decorative sheet. The release sheet was peeled off from the cut decorative sheet, and the exposed adhesive layer was attached to the surface of a polypropylene cylinder with a diameter of 10 mm and sufficiently pressed. After that, the sheet was left for 24 hours in an atmosphere of 23°C-50% RH, and the length of peeling at the end was measured. The evaluation criteria were as follows. The peeling at the end was the sum of the peeling lengths at both ends, and the peeling length at one end was the average of the peeling lengths on the left and right.
[Evaluation Criteria]
◎: Less than 3 mm, excellent.
◯: 3 mm or more and less than 5 mm, good.
△: 5 mm or more and less than 7 mm, usable.
×: 7 mm or more, not practical.

<Repositionability>
The decorative sheet obtained above was evaluated for re-attachability to an acrylonitrile butadiene styrene (hereinafter referred to as ABS) resin plate when attached to the frame of a TOM molding machine (NGF molding machine, manufactured by Fuse Vacuum Co., Ltd.) based on the following evaluation criteria.
(Evaluation Criteria)
: No peeling marks or peeling sounds were observed on the sample surface. Very good.
Good: No peeling marks remained on the sample surface, but there was a peeling sound.
△: Peel marks remained partially on the sample surface. Practical use possible.
×: Peel marks remained on the entire surface of the sample. Not suitable for practical use.

<Unevenness-following ability>
The decorative sheet obtained above was attached to the frame of a TOM molding machine (NGF molding machine, manufactured by Fuse Vacuum Co., Ltd.), and then a melamine plate with a width of 7 cm and a length of 15 cm was set in the frame, and a square pyramid-shaped ABS resin with an upper side of 20 mm, a base side of 40 mm, and a height of 20 mm was set on top of the melamine plate. The melamine plate and the ABS resin were then molded so as to cover them with the decorative sheet, to obtain a decorative structure. The surface condition after molding was evaluated based on the following evaluation criteria.
(Evaluation Criteria)
◎: No lifting or peeling was observed on any surface. Very good.
Good: No lifting is observed on the uneven surface, but some peeling occurs at the edge.
△: Some lifting was observed on uneven surfaces, and peeling occurred at the edges.
×: Lifting occurred on all uneven surfaces, and peeling occurred on the entire surface. Not practical.

<Durability after molding>
The decorative sheet obtained above was attached to the frame of a TOM molding machine (NGF molding machine, manufactured by Fuse Vacuum Co., Ltd.), and then an ABS resin plate of 10 cm x 7 cm x 5 mm thickness was set in the frame. The ABS resin plate was then covered with the decorative sheet and molded at 130°C to obtain a decorative structure. The molded product obtained was then left for 24 hours in a 23°C-50%RH atmosphere, and then a 5 cm long cross was made on the surface of the molded product. The molded product was then left for 168 hours in a high temperature or high temperature and high humidity environment (80°C/90°C/80°C-95%RH), and then evaluated for lifting and displacement based on the following evaluation criteria.
(Evaluation Criteria)
⊚: No lifting or peeling, but some misalignment of less than 1 mm occurred. Very good.
◯: No lifting or peeling occurred, but a displacement of 1 mm or more and less than 3 mm occurred.
△: No lifting or peeling occurred, but there was a displacement of 3 mm or more and less than 10 mm.
×: Lifting or peeling occurs. Not practical.

(Examples 2 to 34, Comparative Examples 1 to 4)
Except for changing the types and amounts of the (meth)acrylic acid ester copolymer (A) and the curing agent (B) and additives contained in the adhesive composition to those shown in Table 2, the adhesive compositions and decorative sheets of Examples 2 to 34 and Comparative Examples 1 to 4 were obtained in the same manner as in Example 1. The exemplified compounds are specifically shown in Table 2, but are not limited thereto. The evaluation results of the obtained decorative sheets of Examples 2 to 34 and Comparative Examples 1 to 4 are shown in Table 3.

The abbreviations in Table 2 are described below.
[Curing agent (B)]
TETRAD-X: Epoxy curing agent (manufactured by Mitsubishi Gas Chemical Company, Inc., N,N,N',N'-tetraglycidyl-m-xylylenediamine)
AL: Metal chelate-based curing agent (Kawaken Fine Chemicals Co., Ltd., aluminum trisacetylacetonate)
TDI-TMP: Isocyanate-based curing agent, trimethylolpropane adduct of tolylene diisocyanate [additive]
AcAc: acetylacetone

The present invention is not limited to the above-described embodiments and examples, and design changes are possible as long as they do not deviate from the spirit of the present invention.

The adhesive composition for decorative sheets of the present invention was excellent in adhesive strength, curved surface attachment property, reattachment property, unevenness-following property, and durability under high temperature or high temperature and high humidity environment, as shown in Examples 1 to 34 in Table 3. In contrast, Comparative Examples 1 to 4 in Table 3 were found to be inferior in the above-mentioned physical properties.
The composition having the above-mentioned excellent properties can be suitably used for decoration of, for example, aircraft, automobiles, railways, interior and exterior building materials, nursing and medical fields, electrical appliances, etc., and its use is not particularly limited, but is preferably used for automobile interior and exterior parts, home appliances, smartphones, and housing building materials, more preferably used for automobile interior and exterior parts and housing building materials, and particularly preferably used for automobile interior and exterior parts. In addition, insert molding, in-mold molding, vacuum molding, and vacuum pressure molding are known as methods for decoration using decorative sheets, and are not particularly limited, but vacuum molding and vacuum pressure molding are particularly preferred.

(Examples 2 to 34, Comparative Examples 1 to 4)
The adhesive compositions and decorative sheets of Examples 2 to 34 and Comparative Examples 1 to 4 were obtained in the same manner as in Example 1, except that the types and amounts of the (meth)acrylic acid ester copolymer (A), the curing agent (B), and the additives contained in the adhesive composition were changed to those shown in Table 2.
The exemplified compounds are specifically shown in Table 2, but are not limited to these. The evaluation results of the decorative sheets obtained in Examples 2 to 34 and Comparative Examples 1 to 4 are shown in Table 3.
However, Examples 18, 26 to 28, and 31 to 32 are reference examples.

(Examples 2 to 34, Comparative Examples 1 to 4)
The adhesive compositions and decorative sheets of Examples 2 to 34 and Comparative Examples 1 to 4 were obtained in the same manner as in Example 1, except that the types and amounts of the (meth)acrylic acid ester copolymer (A), the curing agent (B), and the additives contained in the adhesive composition were changed to those shown in Table 2.
The exemplified compounds are specifically shown in Table 2, but are not limited to these. The evaluation results of the decorative sheets obtained in Examples 2 to 34 and Comparative Examples 1 to 4 are shown in Table 3.
However, Examples 13, 17 to 18, 21 to 22, 26 to 28, and 31 to 32 are reference examples.

Claims (6)

Contains a (meth)acrylic acid ester copolymer (A) and a curing agent (B),
The (meth)acrylic acid ester copolymer (A) is a copolymer of a monomer mixture,
The monomer mixture contains (a) an alkyl (meth)acrylate monomer having an alkyl group with 8 carbon atoms and (b) a monomer having a carboxy group,
the content of the (meth)acrylic acid alkyl ester monomer (a) having an alkyl group with 8 carbon atoms is 50 to 98 mass% in 100 mass% of the monomer mixture;
The content of the monomer (b) having a carboxy group is more than 0 mass% and less than 12 mass% in 100 mass% of the monomer mixture.
A pressure-sensitive adhesive composition for decorative sheets used for attachment to an adherend by vacuum forming or vacuum/pressure forming. The adhesive composition for decorative sheets according to claim 1, characterized in that the curing agent (B) is an epoxy-based curing agent. The adhesive composition for decorative sheets according to claim 1, characterized in that the monomer mixture further contains at least one of (c) an alkyl ester monomer having 1 to 3 carbon atoms in the alkyl group and (d) an alkyl ester monomer having 4 carbon atoms in the alkyl group and having a homopolymer glass transition temperature of -30°C or higher, and the total content of these monomers is 1 to 45% by mass in 100% by mass of the monomer mixture. A decorative sheet comprising a substrate and an adhesive layer made of the adhesive composition for decorative sheets according to any one of claims 1 to 3. A decorative structure comprising an adherend and the decorative sheet according to claim 4. The method for producing a decorative structure according to claim 5, wherein the decorative sheet is integrated with an adherend by vacuum forming or vacuum/pressure forming to form the decorative structure.