An adhesive composition and an adhesive sheet using the adhesive composition.

Abstract

To provide a pressure-sensitive adhesive composition which has excellent adhesive strength to low-polarity adherends, heat-resistant adhesive strength, and heat-resistant retention, and is excellent in transparency and coating film appearance during processing. [Solution] An adhesive composition comprising an acrylic copolymer (A) having a glass transition temperature of 0°C or lower and a weight average molecular weight of 400,000 to 1,500,000, an acrylic copolymer (B) having a glass transition temperature of 30°C or higher and a weight average molecular weight of 5,000 to 200,000, and a curing agent (C), wherein the copolymer (A) is a copolymer of a monomer mixture containing 0.5% by mass or more and 10% by mass or less of a nitrogen-containing monomer (a1) based on 100% by mass of the monomer mixture, and the copolymer (B) is a copolymer of a monomer mixture containing a (meth)acrylic monomer (b1) having a glass transition temperature of 80°C or higher and a (meth)acrylic monomer (b2) having a carboxy group, and the content of the copolymer (B) relative to 100 parts by mass of the copolymer (A) is 5 parts by mass or more and 50 parts by mass or less.

Description

[Technical Field]

[0001] The present invention relates to a pressure-sensitive adhesive composition and a pressure-sensitive adhesive sheet using the pressure-sensitive adhesive composition. [Background technology]

[0002] Because of their ease of handling, adhesive sheets with adhesive layers formed from adhesives are widely used in a wide range of fields, including label and medical applications. The substrates used for adhesive sheets vary widely depending on the application, with some being highly polar, such as stainless steel or polyester, and others being low-polar, such as polyolefin. However, low-polarity substrates generally tend to be difficult to adhere to, and adhesives that can ensure adhesion to, for example, polyolefin-based resins are in demand. Furthermore, adhesives used in applications expected to be used in a variety of environments, such as marking films, window films, automotive components, and optical displays, require excellent coating transparency, excellent processability to prevent lifting or peeling at the adhesive interface between the adherend and the adhesive layer during processing, and excellent heat-resistant adhesion and heat retention to prevent peeling or misalignment even in high-temperature environments.

[0003] One example is an adhesive used to attach decorative sheets to substrates used to decorate the exterior surfaces of molded products such as automotive interior and exterior parts, keyboards, home appliances, smartphones, housing construction materials, furniture, musical instruments, and Shinkansen window frames. Decorating molded products using decorative sheets typically requires a heating process, so various durability properties are required in high-temperature environments. Furthermore, in recent years, there has been an increase in the use of polypropylene resin (hereinafter referred to as PP resin), which has a low specific gravity and excellent thermoformability, as a substrate for decorative sheets, and therefore adhesion to PP resin is also required.

[0004] Patent Document 1 discloses a pressure-sensitive adhesive composition for decorative films, which contains a carboxyl group-containing (meth)acrylic polymer (A) in which the content of structural units derived from a monomer having a carboxyl group is in the range of 0.5% by mass to 6% by mass relative to all structural units, a (meth)acrylic polymer (B) derived from a monomer having at least one functional group selected from an amino group and a hydroxyl group, and a crosslinking agent, wherein the content ratio of the (meth)acrylic polymer (A) to the (meth)acrylic polymer (B) is in the range of 100 / 5 to 100 / 30 by mass. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 2019-73640 Summary of the Invention [Problem to be solved by the invention]

[0006] The inventors have conducted studies and found that the pressure-sensitive adhesive composition described in Patent Document 1 does not exhibit satisfactory performance when peeled off from PP resin, which is a low-polarity adherend, due to the effects of zipping and the like.

[0007] In other words, no pressure-sensitive adhesive composition has been developed to date that has excellent adhesive strength to low-polarity adherends, heat-resistant adhesive strength, and heat-resistant retention, as well as excellent processability that does not cause poor appearance during processing and transparency of the coating film. [Means for solving the problem]

[0008] The present inventors have conducted extensive research to solve the above problems and have completed the present invention. The pressure-sensitive adhesive composition according to the present invention and the pressure-sensitive adhesive sheet using the pressure-sensitive adhesive composition have the following configurations [1] to [9].

[0009] [1] A pressure-sensitive adhesive composition comprising an acrylic copolymer (A) having a glass transition temperature of 0°C or lower, an acrylic copolymer (B) having a glass transition temperature of 30°C or higher, and a curing agent (C), wherein the acrylic copolymer (A) is a copolymer of a monomer mixture containing 0.5% by mass to 10% by mass of a nitrogen-containing monomer (a1) based on 100% by mass of the monomer mixture, and the acrylic copolymer (B) is a copolymer of a monomer mixture containing a (meth)acrylic monomer (b1) having a glass transition temperature of 80°C or higher and a (meth)acrylic monomer (b2) having a carboxy group, wherein the weight-average molecular weight of the acrylic copolymer (A) is in the range of 400,000 to 1,500,000, and the weight-average molecular weight of the acrylic copolymer (B) is in the range of 5,000 to 200,000, and the content of the acrylic copolymer (B) per 100 parts by mass of the acrylic copolymer (A) is 5 parts by mass to 50 parts by mass.

[0010] [2] The pressure-sensitive adhesive composition according to [1], wherein the monomer mixture constituting the acrylic copolymer (A) further comprises an alkyl(meth)acrylic monomer (a2) having an alkyl group with 7 or less carbon atoms and an alkyl(meth)acrylic monomer (a3) ​​having an alkyl group with 8 or more carbon atoms, and the content of the monomer (a2) is 5% by mass or more and 60% by mass or less, and the content of the monomer (a3) ​​is 30% by mass or more and 90% by mass or less, in 100% by mass of the monomer mixture.

[0011] [3] The pressure-sensitive adhesive composition according to [1] or [2], wherein the monomer mixture constituting the acrylic copolymer (A) further contains a (meth)acrylic monomer (a4) having a hydroxy group, and the content of the monomer (a4) in 100% by mass of the monomer mixture is 0.05% by mass or more and 0.5% by mass or less.

[0012] [4] The pressure-sensitive adhesive composition according to any one of [1] to [3], wherein the monomer mixture constituting the acrylic copolymer (B) further contains a (meth)acrylic monomer (b3) having no carboxy group and having a glass transition temperature of 0°C or lower, and the content of the monomer (b3) in 100% by mass of the monomer mixture is 1% by mass or more and 40% by mass or less.

[0013] [5] The pressure-sensitive adhesive composition according to any one of [1] to [4], wherein the monomer mixture constituting the acrylic copolymer (B) further contains a (meth)acrylic monomer (b4) having a hydroxy group, and the content of the monomer (b4) in 100% by mass of the monomer mixture is 0.01% by mass or more and 5% by mass or less.

[0014] [6] The pressure-sensitive adhesive composition according to any one of [1] to [5], wherein the glass transition temperature of the acrylic copolymer (A) is −50° C. or higher and 0° C. or lower.

[0015] [7] The pressure-sensitive adhesive composition according to any one of [1] to [6], wherein the acrylic copolymer (B) has a glass transition temperature of 30°C or higher and lower than 180°C.

[0016] [8] The pressure-sensitive adhesive composition according to any one of [1] to [7], further comprising 1 part by mass or more and 30 parts by mass or less of a tackifier resin per 100 parts by mass of the acrylic copolymer (A).

[0017] [9] A pressure-sensitive adhesive sheet having a pressure-sensitive adhesive layer made of the pressure-sensitive adhesive composition according to any one of [1] to [8]. [Effects of the Invention]

[0018] The present invention makes it possible to provide a pressure-sensitive adhesive composition that exhibits excellent adhesion to low-polarity adherends, heat-resistant adhesion, and heat-resistant retention, and that exhibits excellent processability and transparency of the coating film, without causing poor appearance during processing, and a pressure-sensitive adhesive sheet using the pressure-sensitive adhesive composition. It has been found that when a pressure-sensitive adhesive layer is formed from a pressure-sensitive adhesive composition containing a predetermined (meth)acrylic copolymer (A) and a predetermined (meth)acrylic copolymer (B), it is possible to impart, in the decoration of a molded article, properties that are excellent in adhesion to low-polarity adherends, heat-resistant adhesion, and heat-resistant retention, and that are excellent in coating film transparency and processability. DETAILED DESCRIPTION OF THE INVENTION

[0019] The pressure-sensitive adhesive composition of the present invention and the pressure-sensitive adhesive sheet using the pressure-sensitive adhesive composition will be described below, but the present invention is not limited thereto. In this specification, the term "(meth)acrylic acid ester" includes acrylic acid ester and methacrylic acid ester, and the term "(meth)acryloyl group" includes acryloyl group and methacryloyl group. The term "monomer" refers to a monomer having an ethylenically unsaturated group. Furthermore, in this specification, a numerical range specified using "to" includes the numerical values ​​before and after "to" as the lower and upper limit values ​​of the range. Furthermore, "film" and "sheet" are not distinguished by thickness. In other words, in this specification, "sheet" includes thin film-like objects, and "film" includes thick sheet-like objects. Furthermore, both "sheet" and "film" include laminates. Furthermore, the term "adherend" refers to a counterpart to which the pressure-sensitive adhesive layer of the pressure-sensitive adhesive sheet is attached. Unless otherwise noted, the various components appearing in this specification may be used independently either alone or in combination of two or more.

[0020] The pressure-sensitive adhesive composition of the present invention contains an acrylic copolymer (A), an acrylic copolymer (B), and a curing agent (C). By containing acrylic copolymer (A) and acrylic copolymer (B) which has a smaller weight-average molecular weight than acrylic copolymer (A) and has a high affinity for low-polarity adherends, the acrylic copolymer (B) can be appropriately localized on the outermost surface (the surface that comes into contact with the adherend) of the adhesive layer while maintaining transparency when the adhesive layer is formed.Furthermore, due to the interaction between the specific monomers that make up acrylic copolymer (A) and acrylic copolymer (B), the localization of acrylic copolymer (B) can be maintained even in high-temperature environments, resulting in exceptional effects such as excellent adhesion to low-polarity adherends, heat-resistant adhesion, and heat-resistant retention, as well as excellent transparency of the coating film and processability that does not cause poor appearance during processing. Each component will be described below.

[0021] <Acrylic copolymer (A)> The acrylic copolymer (A) is a copolymer of a monomer mixture containing 0.5% by mass or more and 10% by mass or less of a nitrogen-containing monomer (a1) based on 100% by mass of the monomer mixture constituting the acrylic copolymer (A), and is an acrylic copolymer having a weight-average molecular weight in the range of 400,000 to 1,500,000 and a glass transition temperature of 0°C or less.

[0022] [Nitrogen-containing monomer (a1)] The nitrogen-containing monomer (a1) (hereinafter also referred to as the monomer (a1)) plays a role in improving the compatibility between the acrylic copolymer (A) and the acrylic copolymer (B).

[0023] Examples of the nitrogen-containing monomer (a1) include: N-vinyl lactams such as N-methylvinylpyrrolidone, N-vinylcaprolactam, N-vinylpyrrolidone, N-vinyl-2-pyrrolidone, and N-(meth)acryloylpyrrolidone; cyclic amides such as vinylpyridine, vinylpyrimidine, vinylpiperazine, vinylpyrazine, vinylpyrrole, vinylimidazole, vinyloxazole, vinylmorpholine, and (meth)acryloylmorpholine; Amino group-containing (meth)acrylic monomers such as 2-(dimethylamino)methyl (meth)acrylate, 2-(diethylamino)ethyl (meth)acrylate, 3-(dimethylamino)propyl (meth)acrylate, and 2-(dimethylamino)dimethylaminoethyl (meth)acrylate; N-vinylcarboxylic acid amides such as N-vinylformamide and N-vinylacetamide; (meth)acrylamides such as (meth)acrylamide and substituted (meth)acrylamides; etc. Examples of substituted (meth)acrylamides include N-alkyl (meth)acrylamides such as N-methyl (meth)acrylamide, N-ethyl (meth)acrylamide, N-propyl (meth)acrylamide, N-isopropyl (meth)acrylamide, Nn-butyl (meth)acrylamide, diacetone (meth)acrylamide, and N,N'-methylenebis(meth)acrylamide; N,N-dialkyl(meth)acrylamides such as N,N-dimethyl(meth)acrylamide, N,N-diethyl(meth)acrylamide, N,N-dipropyl(meth)acrylamide, N,N-ethylmethyl(meth)acrylamide, and N,N-diallyl(meth)acrylamide; aminoalkyl(meth)acrylamides such as N-[3-(dimethylamino)propyl](meth)acrylamide, aminomethyl(meth)acrylamide, and aminoethyl(meth)acrylamide; hydroxyalkyl(meth)acrylamides such as N-hydroxymethyl(meth)acrylamide and N-hydroxyethyl(meth)acrylamide; alkoxyalkyl(meth)acrylamides such as N-methoxymethyl(meth)acrylamide and N-(n-butoxymethyl)(meth)acrylamide; Examples include: Among these, from the viewpoint of ease of handling, the nitrogen-containing monomer (a1) preferably includes at least one selected from the group consisting of 2-(dimethylamino)ethyl (meth)acrylate and 4-acryloylmorpholine.

[0024] The content of monomer (a1) is 0.5% by mass or more and 10% by mass or less based on 100% by mass of the monomer mixture. It is preferably 1% by mass or more and 8% by mass or less, and more preferably 1.5% by mass or more and 5% by mass or less. A content of 0.5% by mass or more can improve compatibility with the acrylic copolymer (B), and a content of 10% by mass or less can appropriately localize the acrylic copolymer (B) at the outermost surface of the pressure-sensitive adhesive layer.

[0025] The monomer mixture constituting the acrylic copolymer (A) preferably contains, in addition to monomer (a1), an alkyl(meth)acrylic monomer (a2) having an alkyl group with less than 7 carbon atoms and an alkyl(meth)acrylic monomer (a3) ​​having an alkyl group with 8 or more carbon atoms. The inclusion of monomer (a1), monomer (a2), and monomer (a3) ​​provides a good balance between the wettability of the adhesive to a low-polarity adherend and the cohesive strength of the adhesive, which is preferable. In this specification, the alkyl groups in monomer (a2) and monomer (a3) ​​are linear or branched, and cyclic alkyl groups are excluded.

[0026] [Alkyl (meth)acrylic monomer (a2) having an alkyl group with 7 or less carbon atoms] The alkyl(meth)acrylic monomer (a2) having an alkyl group with 7 or less carbon atoms (hereinafter also referred to as monomer (a2)) contributes to adjusting the cohesive strength of the pressure-sensitive adhesive layer.

[0027] Examples of the alkyl(meth)acrylic monomer (a2) having an alkyl group with 7 or less carbon atoms include methyl(meth)acrylate, ethyl(meth)acrylate, n-propyl(meth)acrylate, isopropyl(meth)acrylate, n-butyl(meth)acrylate, isobutyl acrylate, t-butyl acrylate, pentyl(meth)acrylate, hexyl(meth)acrylate, and n-heptyl(meth)acrylate.

[0028] As the monomer (a2), methyl (meth)acrylate and n-butyl (meth)acrylate are preferred from the viewpoint of the cohesive strength of the adhesive. The monomer (a2) may be used alone or in combination of two or more.

[0029] The content of monomer (a2) is preferably 5% by mass or more and 60% by mass or less based on 100% by mass of the monomer mixture. More preferably, it is 6% by mass or more and 50% by mass or less, and even more preferably, it is 10% by mass or more and 40% by mass or less. When it is 5% by mass or more, the adhesive strength is excellent in high temperature environments, and when it is 60% by mass or less, the adhesive strength is high even at room temperature.

[0030] [Alkyl (meth)acrylic monomer (a3) ​​having an alkyl group with 8 or more carbon atoms] The alkyl(meth)acrylic monomer (a3) ​​having an alkyl group with 8 or more carbon atoms (hereinafter also referred to as the monomer (a3)) contributes to wettability to a low-polarity adherend.

[0031] As the alkyl(meth)acrylic monomer (a3) ​​having an alkyl group with 8 or more carbon atoms, an alkyl(meth)acrylic monomer having an alkyl group with 8 to 18 carbon atoms is preferred from the viewpoint of wettability to a low-polarity adherend.

[0032] Examples of the alkyl(meth)acrylic monomer (a3) ​​having an alkyl group with 8 or more carbon atoms include 2-ethylhexyl(meth)acrylate, n-octyl(meth)acrylate, isooctyl(meth)acrylate, nonyl(meth)acrylate, isononyl(meth)acrylate, decyl(meth)acrylate, isodecyl(meth)acrylate, undeca(meth)acrylate, lauryl(meth)acrylate, stearyl(meth)acrylate, and isostearyl(meth)acrylate.

[0033] As the monomer (a3), 2-ethylhexyl (meth)acrylate and lauryl (meth)acrylate are preferred from the viewpoint of wettability to a low-polarity adherend. The monomer (a3) ​​may be used alone or in combination of two or more.

[0034] The content of monomer (a3) ​​in 100% by mass of the monomer mixture is preferably 30.0% by mass or more and 90.0% by mass or less, more preferably 35.0% by mass or more and 90.0% by mass or less, and even more preferably 40.0% by mass or more and 88.0% by mass or less. By being 30.0% by mass or more and 90.0% by mass or less, excellent wettability to low-polarity adherends is achieved.

[0035] The monomer mixture constituting the acrylic copolymer (A) preferably further contains a (meth)acrylic monomer (a4) having a hydroxy group, which is preferable because the inclusion of the monomer (a4) increases the cohesive strength of the pressure-sensitive adhesive composition and provides a reaction site with the isocyanate-based curing agent.

[0036] [(Meth)acrylic monomer (a4) having a hydroxy group] Examples of the (meth)acrylic monomer (a4) having a hydroxy group (hereinafter also referred to as monomer (a4)) include 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, and 4-hydroxybutyl (meth)acrylate, and these can be used alone or in combination of two or more.

[0037] The content of monomer (a4) is preferably 0.05% by mass or more and 0.5% by mass or less based on 100% by mass of the monomer mixture. It is more preferably 0.1% by mass or more and 0.5% by mass or less, and even more preferably 0.1% by mass or more and 0.2% by mass or less. By setting the content of monomer (a4) to 0.05% by mass or more, the cohesive strength of the pressure-sensitive adhesive composition is increased, preventing lifting and peeling after storage in a high-temperature atmosphere. By setting the content of monomer (a4) to 0.5% by mass or less, adhesion to polyolefin adherends such as polypropylene can be ensured.

[0038] [Other monomers] The monomer mixture constituting the acrylic copolymer (A) may contain other monomers as optional components in addition to the monomers (a1) to (a4) as long as the effects of the present invention are not impaired. Examples include (meth)acrylic acid ester monomers having a cyclic alkyl group but no functional group (e.g., cyclohexyl (meth)acrylate, isobornyl (meth)acrylate), alkoxy (meth)acrylic acid esters such as 2-methoxyethyl (meth)acrylate, vinyl monomers such as vinyl acetate and styrene, and monomers having a glycidyl group such as glycidyl (meth)acrylate.

[0039] The content of other monomers is preferably 15% by mass or less, more preferably 12% by mass or less, even more preferably 9% by mass or less, particularly preferably 6% by mass or less, and most preferably 3% by mass or less, based on 100% by mass of the monomer mixture.

[0040] (Weight average molecular weight (Mw)) The weight-average molecular weight of the acrylic copolymer (A) is 400,000 to 1,500,000. 600,000 to 1,200,000 is preferred, and 600,000 to 1,000,000 is more preferred. By keeping the weight-average molecular weight within the above range, it becomes easier to achieve both adhesion and holding power to polyolefin substrates and cross-cut test (processability). The weight-average molecular weight is a polystyrene-equivalent value measured by gel permeation chromatography (GPC). Details will be described in the Examples.

[0041] (glass transition temperature) The glass transition temperature (Tg) of the acrylic copolymer (A) is 0°C or lower. When the glass transition temperature (Tg) of the acrylic copolymer (A) is 0°C or lower, the pressure-sensitive adhesive layer does not become too hard and can exhibit high initial adhesive strength and adhesive strength over time. Furthermore, from the viewpoint of further improving the adhesive strength of the pressure-sensitive adhesive layer (i.e., initial adhesive strength and adhesive strength over time), the glass transition temperature (Tg) of the acrylic copolymer (A) is preferably -65°C or higher and 0°C or lower, more preferably -60°C or higher and 0°C or lower, and even more preferably -50°C or higher and -10°C or lower.

[0042] The glass transition temperature (Tg) of the acrylic copolymer (A) is a value obtained by converting the absolute temperature (K) calculated from the following formula 1 into Celsius temperature (°C). 1 / Tg=m1 / Tg1+m2 / Tg2+...+m(k-1) / Tg(k-1)+mk / Tg k (Equation 1)

[0043] In formula 1, Tg1, Tg2, . . . , Tg(k-1), and Tgk respectively represent the glass transition temperatures (Tg) expressed in absolute temperature (K) when each monomer constituting the acrylic copolymer (A) is made into a homopolymer. m1, m2, . . . , m(k-1), and mk respectively represent the mole fractions of each monomer constituting the acrylic copolymer (A). m1+m2+···+m(k-1)+mk=1. Note that absolute temperature (K) can be converted to Celsius temperature (℃) by subtracting 273 from it, and Celsius temperature (℃) can be converted to absolute temperature (K) by adding 273 to it.

[0044] "Glass transition temperature (Tg) of a homopolymer" refers to the glass transition temperature (Tg) of a homopolymer produced by polymerizing the monomer alone. This can be obtained from information from the vendor of the monomer or from the values ​​listed in "Polymer Handbook 3rd Edition" (A Wiley-Interscience Publication, 1989).

[0045] The glass transition temperatures (Tg) of homopolymers expressed in Celsius degrees (°C) are as follows: 2-ethylhexyl acrylate (2-EHA) -76°C, 2-ethylhexyl methacrylate (2-EHMA) -10°C, n-butyl acrylate (n-BA) -57°C, methyl acrylate (MA) 5°C, methyl methacrylate (MMA) 103°C, lauryl acrylate (LA) -23°C, styrene (St) 100°C, isobornyl methacrylate (IBMX) 180°C, 2-hydroxyethyl acrylate (HEA) -15°C, 2-hydroxyethyl methacrylate (HEMA) 55°C, acrylic acid (AA) 163°C, dimethylaminoethyl methacrylate (DM) 18°C, and 4-acryloylmorpholine (ACMO) 145°C.

[0046] The glass transition temperature (Tg) of the acrylic copolymer (A) can be appropriately adjusted, for example, by using monomers that have different glass transition temperatures (Tg) when made into homopolymers.

[0047] <Acrylic copolymer (B)> The acrylic copolymer (B) is a copolymer of a monomer mixture containing a (meth)acrylic monomer (b1) having a glass transition temperature of 80°C or higher and a (meth)acrylic monomer (b2) having a carboxy group, and is an acrylic copolymer having a weight average molecular weight in the range of 5,000 to 200,000 and a glass transition temperature of 30°C or higher.

[0048] [(Meth)acrylic monomer (b1) having a glass transition temperature of 80°C or higher] A (meth)acrylic monomer (b1) having a glass transition temperature of 80°C or higher (hereinafter also referred to as monomer (b1)) contributes to wettability and cohesive strength to a low-polarity adherend, i.e., initial adhesive strength and heat-resistant adhesive strength. In this specification, even if the monomer has a glass transition temperature of 80°C or higher, if it has a carboxy group, it is classified as a (meth)acrylic monomer (b2) having a carboxy group.

[0049] Examples of (meth)acrylic monomers (b1) having a homopolymer glass transition temperature (Tg) of 80° C. or higher include methyl methacrylate (MMA, Tg: 103° C.), isobornyl methacrylate (IBXMA, Tg: 180° C.), and isobornyl acrylate (Tg: 96° C.). Among these, isobornyl methacrylate (IBXMA, Tg: 180° C.) is preferred as the monomer (b1) from the viewpoint of the balance between cohesive strength and wettability to low-polarity adherends.

[0050] The content of the monomer (b1) is 40% by mass or more and 95% by mass or less in 100% by mass of the monomer mixture constituting the acrylic copolymer (B). It is preferably 45% by mass or more and 95% by mass or less, and more preferably 55% by mass or more and 95% by mass or less. A content of 40% by mass or more can improve wettability to low-polarity adherends, and a content of 95% by mass or less can impart cohesive strength.

[0051] [(Meth)acrylic monomer (b2) having a carboxy group] The (meth)acrylic monomer (b2) having a carboxy group (hereinafter also referred to as monomer (b2)) forms an ionic bond with the nitrogen-containing monomer (a1) constituting the acrylic copolymer (A), and plays a role in maintaining the localization of the acrylic copolymer (B) on the outermost surface of the pressure-sensitive adhesive layer even in a high-temperature environment. Examples of the monomer (b2) include acrylic acid, methacrylic acid, and monomers represented by the following formula: CH2=CR1-CO-O-(C2H4-COO-)nH Here, R1 represents hydrogen or a methyl group, and n represents an integer of 1 or more.

[0052] The content of the monomer (b2) is 1% by mass or more and 10% by mass or less based on 100% by mass of the monomer mixture constituting the acrylic copolymer (B). It is preferably 1% by mass or more and 7% by mass or less, and more preferably 1% by mass or more and 5% by mass or less. By being 1% by mass or more and 10% by mass or less, the acrylic copolymer (B) can be localized on the outermost surface of the pressure-sensitive adhesive layer even in a high-temperature environment.

[0053] The monomer mixture constituting the acrylic copolymer (B) may contain, in addition to the monomer (b1) and the monomer (b2), a (meth)acrylic monomer (b3) having a glass transition temperature of 0°C or lower, as needed.

[0054] [(meth)acrylic monomer (b3) having a glass transition temperature of 0°C or lower] The inclusion of a (meth)acrylic monomer (b3) (hereinafter also referred to as monomer (b3)) having a glass transition temperature of 0°C or lower can enhance affinity with the acrylic copolymer (A) containing the alkyl (meth)acrylate monomers (a2) and (a3) ​​as constituent monomers. This improves the compatibility between the acrylic copolymer (B) and the acrylic copolymer (A), allowing the acrylic copolymer (B) to be appropriately localized at the outermost surface of the pressure-sensitive adhesive layer. When the acrylic copolymer (B) is appropriately localized at the outermost surface of the pressure-sensitive adhesive layer, wettability with the adherend is improved, allowing the pressure-sensitive adhesive layer to adhere to the adherend with high adhesive strength. Therefore, the pressure-sensitive adhesive layer can exhibit high initial adhesive strength. Furthermore, when the acrylic copolymer (B) is appropriately compatible with the acrylic copolymer (A), for example, a decrease in holding power due to poor compatibility is prevented, resulting in high durability. In this specification, even if the glass transition temperature is 0° C. or lower, if it has a carboxy group, it is classified as a (meth)acrylic monomer (b2) having a carboxy group.

[0055] The (meth)acrylic monomer (b3) having a homopolymer glass transition temperature (Tg) of 0°C or less is preferably an unsubstituted alkyl(meth)acrylic monomer, and the type thereof is not particularly limited. The alkyl group of the alkyl(meth)acrylic monomer may be linear, branched, or cyclic. The number of carbon atoms in the alkyl group is preferably in the range of 1 to 18, more preferably 1 to 12, from the viewpoints of, for example, the adhesive strength of the pressure-sensitive adhesive layer and the adhesion between the pressure-sensitive adhesive layer and the substrate.

[0056] Examples of the alkyl(meth)acrylic monomer (b3) having a homopolymer glass transition temperature (Tg) of 0°C or lower include ethyl acrylate (EA, Tg: -27°C), 2-ethylhexyl methacrylate (2EHMA, Tg: -10°C), n-butyl acrylate (n-BA, Tg: -57°C), 2-ethylhexyl acrylate (2EHA, Tg: -76°C), and n-octyl acrylate (Tg: -80°C). Among these, the alkyl(meth)acrylic monomer (b3) having a homopolymer glass transition temperature (Tg) of 0°C or less is preferably at least one selected from the group consisting of ethyl acrylate (EA), 2-ethylhexyl methacrylate (2EHMA), and n-butyl acrylate (n-BA), from the viewpoint of the balance between wettability and cohesive strength, and more preferably 2-ethylhexyl methacrylate (2EHMA).

[0057] The monomer mixture constituting the acrylic copolymer (B) may contain only one kind or two or more kinds of alkyl(meth)acrylic monomers (b3) whose homopolymer has a glass transition temperature (Tg) of 0°C or lower.

[0058] The content of monomer (b3) is preferably 1% by mass or more and 40% by mass or less based on 100% by mass of the monomer mixture. It is more preferably 5% by mass or more and 40% by mass or less, and even more preferably 5% by mass or more and 35% by mass or less. A content of 1% by mass or more can improve wettability to low-polarity adherends, while a content of 40% by mass or less relatively increases the glass transition temperature (Tg) of the acrylic copolymer (B), thereby suppressing a decrease in viscoelasticity (i.e., a decrease in cohesive strength) of the pressure-sensitive adhesive layer due to high-temperature heating.

[0059] The monomer mixture constituting the acrylic copolymer (B) may further contain a (meth)acrylic monomer (b4) having a hydroxy group, if necessary.

[0060] [(Meth)acrylic monomer (b4) having a hydroxy group] Examples of (meth)acrylic monomers (b4) having a hydroxy group (hereinafter also referred to as monomer (b4)) include 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, etc., and these can be used alone or in combination of two or more.

[0061] The content of monomer (b4) is preferably 0.01% by mass or more and 5% by mass or less based on 100% by mass of the monomer mixture. It is more preferably 0.1% by mass or more and 5% by mass or less, and even more preferably 0.1% by mass or more and 1% by mass or less. By making the content of monomer (b4) 0.01% by mass or more, lifting and peeling after leaving it in a high-temperature atmosphere can be prevented. By making the content of monomer (b4) 5 parts by mass or less, adhesion to low-polarity adherends such as polypropylene can be ensured.

[0062] [Other monomers] The monomer mixture constituting the acrylic copolymer (B) may contain other monomers in addition to the monomers (b1) to (b4) as optional components, provided that the effects of the present invention are not impaired. Examples include alkoxy (meth)acrylic acid esters such as 2-methoxyethyl (meth)acrylate, vinyl monomers such as vinyl acetate and styrene, and monomers having a glycidyl group such as glycidyl (meth)acrylate.

[0063] The content of other monomers is preferably 15% by mass or less, more preferably 12% by mass or less, even more preferably 9% by mass or less, particularly preferably 6% by mass or less, and most preferably 3% by mass or less, based on 100% by mass of the monomer mixture constituting the acrylic copolymer (B).

[0064] (Weight average molecular weight (Mw)) The weight-average molecular weight (Mw) of the acrylic copolymer (B) is in the range of 5,000 to 200,000, preferably 10,000 to 150,000, and more preferably 10,000 to 50,000. When the weight-average molecular weight (Mw) of the acrylic copolymer (B) is 5,000 or more, the pressure-sensitive adhesive composition can exhibit appropriate cohesive strength. Furthermore, when the weight-average molecular weight (Mw) is 5,000 or more, the compatibility between the acrylic copolymer (B) and the acrylic copolymer (A) tends not to be excessive, and therefore, when the pressure-sensitive adhesive layer is formed, the acrylic copolymer (B) is appropriately localized on the surface of the pressure-sensitive adhesive layer. When the acrylic copolymer (B) is appropriately localized on the surface of the pressure-sensitive adhesive layer, the wettability with the adherend is improved, allowing the pressure-sensitive adhesive layer to adhere to the adherend with high adhesive strength, and therefore the pressure-sensitive adhesive layer can exhibit high initial adhesive strength. Furthermore, when the weight-average molecular weight (Mw) of the acrylic copolymer (B) is 200,000 or less, the cohesive strength of the pressure-sensitive adhesive composition is not excessively high, and the wettability with the adherend is good, so that the pressure-sensitive adhesive layer adheres to the adherend with high adhesive strength, and therefore the pressure-sensitive adhesive layer can exhibit high initial adhesive strength.

[0065] The weight average molecular weight (Mw) of the acrylic copolymer (B) can be adjusted to a desired value by adjusting the reaction temperature, reaction time, amount of organic solvent used, type of polymerization initiator, amount of polymerization initiator used, and the like.

[0066] [Glass transition temperature (Tg)] The glass transition temperature (Tg) of the acrylic copolymer (B) is 30°C or higher. When the glass transition temperature (Tg) of the acrylic copolymer (B) is 30°C or higher, a decrease in viscoelasticity (i.e., a decrease in cohesive strength) due to high-temperature heating is suppressed. The pressure-sensitive adhesive layer can exhibit high initial adhesive strength and adhesive strength over time by having appropriate viscoelasticity. The upper limit of the glass transition temperature is not particularly limited, but since the highest Tg of the homopolymer of available (meth)acrylic monomers is 180°C, it is practically below 180°C. Furthermore, from the viewpoint of cohesive strength, the glass transition temperature (Tg) of the acrylic copolymer (B) is preferably 30°C or higher but lower than 180°C, more preferably 55°C or higher but lower than 180°C, and even more preferably 70°C or higher but lower than 180°C.

[0067] The glass transition temperature (Tg) of the acrylic copolymer (B) can be appropriately adjusted, for example, by using monomers that have different glass transition temperatures (Tg) when made into homopolymers.

[0068] The glass transition temperature (Tg) of the acrylic copolymer (B) is a value calculated by the same method as for the glass transition temperature (Tg) of the acrylic copolymer (A) described above.

[0069] (Production of acrylic copolymer (A) and acrylic copolymer (B)) The acrylic copolymer (A) and the acrylic copolymer (B) can be produced by polymerizing the above-mentioned monomer mixture. The polymerization can be carried out by known polymerization methods such as solution polymerization, bulk polymerization, emulsion polymerization, and suspension polymerization, but solution polymerization is preferred. Solvents used in solution polymerization are preferably, for example, acetone, methyl acetate, ethyl acetate, toluene, xylene, anisole, methyl ethyl ketone, and cyclohexanone. The polymerization temperature is preferably a boiling point reaction at 60 to 120°C. The polymerization time is preferably about 5 to 12 hours.

[0070] The polymerization initiator used for the polymerization is preferably a radical polymerization initiator, and the radical polymerization initiator is generally a peroxide or an azo compound. Examples of peroxides include dialkyl peroxides such as di-t-butyl peroxide, dicumyl peroxide, t-butylcumyl peroxide, α,α'-bis(t-butylperoxy-m-isopropyl)benzene, and 2,5-di(t-butylperoxy)hexyne-3; Peroxyesters such as t-butyl peroxybenzoate, t-butyl peroxyacetate, and 2,5-dimethyl-2,5-di(benzoylperoxy)hexane; ketone peroxides such as cyclohexanone peroxide, 3,3,5-trimethylcyclohexanone peroxide, and methylcyclohexanone peroxide; Peroxyketals such as 2,2-bis(4,4-di-t-butylperoxycyclohexyl)propane, 1,1-bis(t-butylperoxy)3,3,5-trimethylcyclohexane, 1,1-bis(t-butylperoxy)cyclohexane, and n-butyl-4,4-bis(t-butylperoxy)valerate; Hydroperoxides such as cumene hydroperoxide, diisopropylbenzene hydroperoxide, and 2,5-dimethylcyclohexane-2,5-dihydroperoxide; diacyl peroxides such as benzoyl peroxide, decanoyl peroxide, lauroyl peroxide, and 2,4-dichlorobenzoyl peroxide; Examples include peroxydicarbonates such as bis(t-butylcyclohexyl) peroxydicarbonate.

[0071] Examples of the azo compound include 2,2'-azobisbutyronitrile such as 2,2'-azobisisobutyronitrile (abbreviation: AIBN) and 2,2'-azobis(2-methylbutyronitrile); 2,2'-azobisvaleronitrile such as 2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile) and 2,2'-azobis(2,4-dimethylvaleronitrile); 2,2'-azobispropionitriles such as 2,2'-azobis(2-hydroxymethylpropionitrile); Examples include 1,1'-azobis-1-alkanenitriles such as 1,1'-azobis(cyclohexane-1-carbonitrile).

[0072] The polymerization initiator is preferably used in an amount of 0.01 to 10 parts by mass, more preferably 0.1 to 2 parts by mass, based on 100 parts by mass of the monomer mixture.

[0073] <Curing agent (C)> The curing agent (C) is preferably one that reacts with the functional groups in the acrylic copolymer (A) and / or (B) by heat or the like to form bonds.

[0074] As the curing agent (C), one or more types can be appropriately selected from known curing agents such as epoxy-based curing agents, isocyanate-based curing agents, oxazoline-based curing agents, aziridine-based curing agents, carbodiimide-based curing agents, metal chelate-based curing agents, and melamine-based curing agents, taking into consideration the reactivity with the functional groups of the acrylic copolymer in the pressure-sensitive adhesive composition. From the viewpoint of removability, it is preferable to include an isocyanate-based curing agent, and it is more preferable to use an isocyanate-based curing agent and an epoxy-based curing agent in combination.

[0075] The isocyanate curing agent is an isocyanate having two or more isocyanate groups. Examples of preferred isocyanates include aromatic polyisocyanates, aliphatic polyisocyanates, araliphatic polyisocyanates, alicyclic polyisocyanates, and their biuret, nurate, and adduct forms. From the viewpoint of yellowing resistance, aliphatic polyisocyanates, alicyclic polyisocyanates, and their biuret, nurate, and adduct forms are more preferred. The amount of the isocyanate curing agent is preferably 0.1 to 5 parts by mass, more preferably 0.1 to 2 parts by mass, per 100 parts by mass of the acrylic copolymer (A).

[0076] The biuret compound is a self-condensation product having a biuret bond formed by self-condensation of an isocyanate monomer, such as a biuret compound of hexamethylene diisocyanate.

[0077] The nurate derivative is a trimer of an isocyanate monomer, such as a trimer of hexamethylene diisocyanate, a trimer of isophorone diisocyanate, or a trimer of tolylene diisocyanate.

[0078] The adduct is a bifunctional or higher isocyanate compound obtained by reacting an isocyanate monomer with a bifunctional or higher low-molecular-weight active hydrogen-containing compound. Examples of the adduct include a compound obtained by reacting trimethylolpropane with hexamethylene diisocyanate, a compound obtained by reacting trimethylolpropane with tolylene diisocyanate, a compound obtained by reacting trimethylolpropane with xylylene diisocyanate, a compound obtained by reacting trimethylolpropane with isophorone diisocyanate, and a compound obtained by reacting 1,6-hexanediol with hexamethylene diisocyanate.

[0079] The isocyanate compound is preferably a trifunctional isocyanate compound from the viewpoint of forming a sufficient crosslinked structure. The isocyanate compound is more preferably an adduct or nurate, which is a reaction product of an isocyanate monomer and a trifunctional low-molecular-weight active hydrogen-containing compound. The isocyanate compound is preferably a trimethylolpropane adduct of hexamethylene diisocyanate, a nurate of hexamethylene diisocyanate, a trimethylolpropane adduct of tolylene diisocyanate, a nurate of tolylene diisocyanate, a trimethylolpropane adduct of isophorone diisocyanate, or a nurate of isophorone diisocyanate, and more preferably a trimethylolpropane adduct of hexamethylene diisocyanate, a trimethylolpropane adduct of tolylene diisocyanate, or a trimethylolpropane adduct of isophorone diisocyanate.

[0080] Examples of epoxy curing agents include N,N,N',N'-tetraglycidyl-m-xylylenediamine, tetraglycidyl-3,3'-diaminodiphenyl sulfone, tetraglycidyldiaminodiphenylmethane, 1,3-bis(N,N'-diglycidylaminomethyl)cyclohexane, N,N,N',N'-tetraglycidylaminophenylmethane (all of which are examples of tetrafunctional epoxy compounds), glycerin diglycidyl ether, 1,6-hexanediol diglycidyl ether, etc., and one or more of these can be selected as appropriate. The amount of epoxy curing agent is preferably 0.1 to 10 parts by mass, more preferably 0.3 to 5 parts by mass, and most preferably 0.5 to 2 parts by mass, per 100 parts by mass of the acrylic copolymer (A).

[0081] The oxazoline-based curing agent has two or more oxazoline groups in the molecule and may be a low molecular weight compound or a polymer. Examples of the low molecular weight oxazoline-based curing agent include 2-vinyl-2-oxazoline, 2-vinyl-4-methyl-2-oxazoline, 2-vinyl-5-methyl-2-oxazoline, 2-isopropenyl-2-oxazoline, 2-isopropenyl-4-methyl-2-oxazoline, 2-isopropenyl-5-ethyl-2-oxazoline, 2,2'-bis-(2-oxazoline), 2,2'-methylene-bis-(2-oxazoline), 2,2'-ethylene-bis-(2-oxazoline), 2,2'-trimethylene-bis-(2-oxazoline), and 2,2'-tetramethylene-bis-(2-oxazoline). Examples of oxazoline-based curing agents for polymers include 2,2'-hexamethylene-bis-(2-oxazoline), 2,2'-octamethylene-bis-(2-oxazoline), 2,2'-ethylene-bis-(4,4'-dimethyl-2-oxazoline), 2,2'-p-phenylene-bis-(2-oxazoline), 2,2'-m-phenylene-bis-(2-oxazoline), 2,2'-m-phenylene-bis-(4,4'-dimethyl-2-oxazoline), 2,2'-(1,3-phenylene)-bis-(2-oxazoline), bis-(2-oxazolinylcyclohexane) sulfide, and bis-(2-oxazolinylnorbornane) sulfide. Examples of oxazoline-based curing agents for polymers include polymers that contain an addition-polymerizable oxazoline as an essential component. Examples of the addition-polymerizable oxazoline include 2-vinyl-2-oxazoline, 2-vinyl-4-methyl-2-oxazoline, 2-vinyl-5-methyl-2-oxazoline, 2-isopropenyl-2-oxazoline, 2-isopropenyl-4-methyl-2-oxazoline, 2-isopropenyl-5-methyl-2-oxazoline, 2-isopropenyl-5-ethyl-2-oxazoline, etc. The amount of the oxazoline-based curing agent is preferably 0.1 to 10 parts by mass per 100 parts by mass of the acrylic copolymer (A).

[0082] Examples of the aziridine curing agent include N,N'-diphenylmethane-4,4'-bis(1-aziridinecarboxite), tris-2,4,6-(1-aziridinyl)-1,3,5-triazine, 4,4'-bis(ethyleneiminocarbonylamino)diphenylmethane, etc. The amount of the aziridine curing agent is preferably 0.1 to 2 parts by mass per 100 parts by mass of the acrylic copolymer (A).

[0083] The carbodiimide curing agent is preferably a high-molecular-weight polycarbodiimide produced by a decarboxylation condensation reaction of a diisocyanate compound in the presence of a carbodiimidization catalyst. Commercially available high-molecular-weight polycarbodiimides are preferably the Carbodilite series from Nisshinbo Industries. Among these, Carbodilite V-03, 07, and 09 are preferred due to their excellent compatibility with organic solvents. The amount of the carbodiimide curing agent is preferably 0.1 to 10 parts by mass per 100 parts by mass of the acrylic copolymer (A).

[0084] The metal chelate curing agent is preferably a coordination compound of a polyvalent metal such as aluminum, iron, copper, zinc, tin, titanium, nickel, antimony, magnesium, vanadium, chromium, or zirconium with acetylacetone or ethyl acetoacetate. Examples of metal chelates include aluminum ethyl acetoacetate diisopropylate, aluminum trisacetylacetonate, aluminum bisethyl acetoacetate monoacetylacetonate, and aluminum alkyl acetoacetate diisopropylate. The amount of the metal chelate curing agent is preferably 0.1 to 10 parts by mass, more preferably 0.3 to 5 parts by mass, and most preferably 0.5 to 2 parts by mass, per 100 parts by mass of the acrylic copolymer (A).

[0085] Typical examples of the melamine-based curing agent include fully alkyl-etherified melamine resins, methylol group-type melamine resins, imino group-type melamine resins partially containing imino groups, etc. The amount of the melamine-based curing agent is preferably 0.1 to 10 parts by mass per 100 parts by mass of the acrylic copolymer (A).

[0086] <Pressure-sensitive adhesive composition> The pressure-sensitive adhesive composition of the present invention comprises an acrylic copolymer (A) having a glass transition temperature of 0°C or lower, an acrylic copolymer (B) having a glass transition temperature of 30°C or higher, and a curing agent (C), and the content of the acrylic copolymer (B) per 100 parts by mass of the acrylic copolymer (A) is 5 parts by mass or more and 50 parts by mass or less. When the content of acrylic copolymer (B) relative to 100 parts by mass of acrylic copolymer (A) is 5 parts by mass or more, the acrylic copolymer (B) is appropriately localized on the surface of the pressure-sensitive adhesive layer during heating. When the acrylic copolymer (B) is appropriately localized on the surface of the pressure-sensitive adhesive layer, wettability with the adherend is improved, allowing the pressure-sensitive adhesive layer to adhere to the adherend with high adhesive strength. Therefore, the pressure-sensitive adhesive layer can exhibit high initial adhesive strength. Furthermore, when the content of acrylic copolymer (B) relative to 100 parts by mass of acrylic copolymer (A) is 50 parts by mass or less, the acrylic copolymers (A) and (B) maintain appropriate compatibility, preventing, for example, a decrease in adhesive strength due to poor compatibility. Therefore, the pressure-sensitive adhesive layer can exhibit high adhesive strength over time. The content of acrylic copolymer (B) relative to 100 parts by mass of acrylic copolymer (A) is preferably 10 parts by mass or more and 40 parts by mass or less, more preferably 20 parts by mass or more and 40 parts by mass or less.

[0087] (tackifying resin) The pressure-sensitive adhesive composition of the present invention preferably further contains a tackifying resin. Examples of tackifying resins include aliphatic petroleum resins, aromatic petroleum resins, synthetic hydrocarbon resins, terpene resins, rosin resins (rosin, polymerized rosin, hydrogenated rosin, and their esters with glycerin, pentaerythritol, etc., resin acid dimers, etc.), acrylic resins, etc. Among these, rosin resins, terpene resins, and synthetic hydrocarbon resins are preferred, and synthetic hydrocarbon resins are more preferred, from the viewpoints of adhesiveness, suppression of foaming, lifting, and peeling when left in a high-temperature environment or a high-temperature, high-humidity environment, ability to be attached to curved surfaces, and constant-load peel resistance.

[0088] Examples of aliphatic petroleum resins include Quinton B170 manufactured by Zeon Corporation; examples of aromatic petroleum resins include Nippon Oil Neopolymer L-90 manufactured by JXTG; examples of aliphatic / aromatic petroleum resins include FTR6100 manufactured by Mitsui Chemicals, Inc.; and examples of rosin derivatives include Sylvatac RE85 manufactured by Arizona Chemical Company and Super Ester A-75 manufactured by Arakawa Chemical Industries, Ltd.

[0089] Examples of synthetic hydrocarbon resins include aliphatic petroleum resins, aromatic petroleum resins, aliphatic / aromatic petroleum resins, hydrogenated petroleum resins, coumarone-indene resins, and phenol resins.

[0090] Examples of terpene resins include α-pinene resins, β-pinene resins, dipentene resins, aromatic modified terpene resins, hydrogenated terpene resins, terpene phenol resins, acid modified terpene resins, styrenated terpene resins, and styrene-aliphatic hydrocarbon copolymer resins.

[0091] Examples of rosin-based resins include rosin ester, polymerized rosin, hydrogenated rosin, disproportionated rosin, maleic acid-modified rosin, fumaric acid-modified rosin, rosin phenolic resin, and natural rosin.

[0092] The amount of tackifier resin blended is preferably 1 to 30 parts by mass, more preferably 1 to 25 parts by mass, and most preferably 1 to 20 parts by mass per 100 parts by mass of the acrylic copolymer (A). By using an amount of 1 part by mass or more, it is easy to ensure adhesiveness, ability to be attached to curved surfaces, and constant-load peel resistance, while by using an amount of 30 parts by mass or less, it is easy to prevent yellowing when left in a high-temperature environment or outdoors for a long period of time, and peeling and yellowing due to light, without impairing adhesiveness to polyolefins.

[0093] The softening point of the tackifier resin is preferably 90 to 160°C, more preferably 90 to 140°C, and most preferably 90 to 135°C. By setting the softening point to 90°C or higher, it becomes easier to ensure that foaming, lifting, and peeling are suppressed when left in a high-temperature environment or a high-temperature, high-humidity environment, and that adhesion to curved surfaces and constant-load peel resistance are ensured. By setting the softening point to 160°C or lower, it becomes easier to ensure adhesion to polyolefin-based adherends such as polypropylene, and it becomes easier to prevent peeling of the PSA sheet due to light when exposed outdoors for long periods of time. The softening point of the tackifier resin was measured in accordance with JIS K 6863. Specifically, after filling a ring base with the tackifier resin, a ball (specified in JIS B1501: steel ball / diameter 9.53 mm / mass 3.5±0.05 g) was placed on it, and the temperature at which the ball dropped at a heating rate of 5°C / °C using an automatic bulb-type softening point tester ASP-KG4 (manufactured by Meitex Co., Ltd.) was taken as the melting point.

[0094] <Adhesive sheet> The pressure-sensitive adhesive sheet comprises a pressure-sensitive adhesive layer made of the pressure-sensitive adhesive composition of the present invention.

[0095] The pressure-sensitive adhesive sheet of the present invention preferably has a configuration in which a release film is attached to at least one surface of the pressure-sensitive adhesive layer, specifically, a configuration in which release films are formed on both surfaces of the pressure-sensitive adhesive layer, or a configuration in which a release film is formed on one surface of the pressure-sensitive adhesive layer and a light-transmitting substrate is provided on the other surface of the pressure-sensitive adhesive, and the pressure-sensitive adhesive layer is formed from the pressure-sensitive adhesive composition of the present invention.

[0096] <Adhesive layer> The pressure-sensitive adhesive layer of the pressure-sensitive adhesive sheet of the present invention preferably has a gel fraction of 20 to 60% by mass, more preferably 25 to 55% by mass, even more preferably 30 to 50% by mass, and most preferably 35 to 45% by mass. A gel fraction of 20% by mass or more improves the cohesive strength of the pressure-sensitive adhesive composition, resulting in a tough pressure-sensitive adhesive layer, making it easier to ensure stress relaxation, and making it easier to suppress lifting and peeling in high-temperature environments. A gel fraction of 60% by mass or less makes it easier to obtain a flexible pressure-sensitive adhesive layer, making it easier to ensure adhesion to polyolefin substrates. Generally, the gel fraction of a polymer is equal to the degree of crosslinking, and the more crosslinked portions in the polymer, the higher the gel fraction. The gel fraction (the amount of crosslinked structure introduced) can be adjusted to a desired range by the method for introducing the crosslinked structure, the type and amount of the crosslinking agent, etc.

[0097] When applying the pressure-sensitive adhesive composition, the viscosity can be adjusted by adding an appropriate liquid medium. Specific examples include hydrocarbon solvents such as toluene, xylene, hexane, and heptane; ester solvents such as ethyl acetate and butyl acetate; ketone solvents such as acetone and methyl ethyl ketone; halogenated hydrocarbon solvents such as dichloromethane and chloroform; ether solvents such as diethyl ether, methoxytoluene, and dioxane, and other hydrocarbon solvents. However, water and alcohols must be used carefully because they may inhibit the reaction between the acrylic copolymer (A) and the isocyanate curing agent.

[0098] The coating method is not particularly limited, and examples thereof include various coating methods using a Mayer bar, applicator, brush, spray, roller, gravure coater, die coater, lip coater, comma coater, knife coater, reverse coater, spin coater, etc. The drying and curing method is also not particularly limited, and examples thereof include hot air drying, infrared rays, reduced pressure methods, and methods using active energy rays, but hot air or steam heating at 60 to 180°C is preferred from the viewpoint of outgassing resistance.

[0099] The thickness of the adhesive layer is preferably 2 to 1000 μm, more preferably 5 to 500 μm, still more preferably 10 to 100 μm, and most preferably 20 to 50 μm. The adhesive layer may be in the form of a single layer or a laminate of two or more layers.

[0100] The pressure-sensitive adhesive composition of the present invention has excellent adhesion to low-polarity substrates, heat-resistant adhesion, heat-resistant retention, and durability that does not cause poor appearance even in high-temperature environments, and therefore can be suitably used as a pressure-sensitive adhesive composition for decorative sheets.

[0101] <Decorative sheet> The decorative sheet comprises a substrate and a pressure-sensitive adhesive layer made of the pressure-sensitive adhesive composition. If necessary, the exposed surface of the pressure-sensitive adhesive layer can be covered with a release sheet. The release sheet is peeled off when the pressure-sensitive adhesive sheet is attached to an adherend. The substrate is not particularly limited, and examples thereof include resin sheets, paper, and metal foils. The substrate may also be a laminated sheet in which one or more layers are laminated on at least one surface of the substrate. If necessary, the surface of the substrate on which the pressure-sensitive adhesive layer is to be formed may be subjected to an adhesion-enhancing treatment such as corona discharge treatment or application of an anchor coating agent. The release sheet is not particularly limited, and a known release sheet can be used, in which a known release treatment such as application of a release agent is applied to the surface of a substrate sheet such as a resin sheet or paper.

[0102] Examples of the substrate include plastic films such as polyester (polyethylene terephthalate, polyethylene naphthalate), polycarbonate, polymethyl methacrylate, polyethylene, polypropylene, cyclic olefin, polyvinyl chloride, polyurethane, polyamide, and ethylene-vinyl acetate copolymer; inorganic materials such as glass plates; nonwoven fabrics; and paper.

[0103] <Release film> The release film is not particularly limited, but a transparent plastic substrate can be suitably used. Examples of materials for the transparent plastic substrate include polyesters such as polyethylene terephthalate (PET), acrylic resins such as polymethyl methacrylate (PMMA), and plastic materials such as polycarbonate, triacetyl cellulose, polysulfone, polyarylate, and polycycloolefin. The plastic materials can be used alone or in combination of two or more.

[0104] Among the transparent plastic substrates described above, a transparent plastic substrate having excellent heat resistance, i.e., a transparent plastic substrate that is suppressed or prevented from deforming under severe conditions such as high temperature, high temperature and high humidity, etc. PET films or sheets are particularly suitable as the transparent plastic substrate.

[0105] The thickness of the transparent plastic substrate is not particularly limited, and is, for example, preferably from 10 to 200 μm, more preferably from 25 to 150 μm.

[0106] The decorative sheet using the pressure-sensitive adhesive composition can be suitably used for attachment to an adherend by vacuum forming or vacuum-pressure forming. Conventional known methods can be used for vacuum forming or vacuum-pressure forming.

[0107] <Method of manufacturing decorative sheet> A method for producing a decorative sheet having a pressure-sensitive adhesive layer made from the pressure-sensitive adhesive composition will now be described. However, conventionally known methods can be used as appropriate, and the present invention is not limited to the following production method. For example, when using a pressure-sensitive adhesive composition prepared by dissolving in a solvent, the pressure-sensitive adhesive composition diluted with a solvent is coated onto a sheet having a surface layer and a decorative layer formed thereon by knife coating, bar coating, blade coating, doctor coating, roll coating, cast coating, or the like to form a coating film. The sheet can then be heated and dried as needed to form a pressure-sensitive adhesive layer on the sheet. On the other hand, when using a pressure-sensitive adhesive composition prepared by a kneading method using heat, the pressure-sensitive adhesive layer can be formed on the sheet using a hot-melt coater, which can soften the pressure-sensitive adhesive composition by preheating. A decorative sheet having a pressure-sensitive adhesive layer made from the pressure-sensitive adhesive composition exhibits moderate adhesive strength at room temperature (e.g., 25°C) and excellent adhesiveness at high temperatures (e.g., 100 to 150°C), and can maintain these properties for long periods of time at high temperatures (e.g., 80°C for 7 days). Therefore, a decorative sheet having an adhesive layer made of the present adhesive composition can be suitably used for decorating the interior and exterior of aircraft, automobiles, building materials, nursing care and medical fields, electrical appliances, electronic components, smartphones, furniture, musical instruments, Shinkansen window frames, and the like, for example, by three-dimensional surface decoration (Threeimension Overlay Method: TOM molding), a type of vacuum and compressed air molding.

[0108] <Decorative structure and method for manufacturing the same> A decorative sheet having a pressure-sensitive adhesive layer made of the pressure-sensitive adhesive composition can be attached to an adherend to form a decorative structure. The shape of the adherend is not particularly limited, and various shapes of adherends (e.g., three-dimensional molded products) can be used.

[0109] More specifically, the decorative structure may 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 forms the surface of the decorated molded product, and any material conventionally known in the field of decorative sheets may be used as appropriate. The surface layer may be composed 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 used to decorate the object to be decorated, and various materials may be used depending on the intended use. The decorative layer may contain, for example, at least one resin selected from the group consisting of olefin-based resins, styrene-based resins, and vinyl chloride resins. The decorative layer may also optionally include other layers, such as a design layer, a bulk layer, and a bonding layer. The number of layers of the decorative sheet, the type, arrangement, thickness, etc. of each layer may be selected as appropriate 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. [Example]

[0110] The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to these examples. In the examples, unless otherwise specified, "parts" means "parts by mass" and "%" means "% by mass." The blending amounts in the tables are in parts by mass, and amounts other than the solvent are calculated as non-volatile contents. Blank spaces in the tables indicate that no blending was performed. The method for measuring the weight average molecular weight of the acrylic copolymer is as follows.

[0111] <Measurement of weight average molecular weight (Mw)> The weight-average molecular weight (Mw) was measured by gel permeation chromatography (GPC). The instrument used was a GPC instrument manufactured by Shimadzu Corporation: an LC-GPC system "Prominence." The columns used were TSKgel α-M manufactured by Tosoh Corporation, with two columns connected in series. N,N-dimethylformamide (DMF) was used as the eluent, and measurements were carried out at 40°C. Mw was determined by conversion using polystyrene with a known Mw as the standard substance.

[0112] <Production Example of Acrylic Copolymer (A)> (Acrylic copolymer (A-1)) Using a reactor equipped with a stirrer, reflux condenser, nitrogen inlet, thermometer, and dropping tube, 0.5 parts of dimethylaminoethyl methacrylate (DAM) as monomer (a1), 10 parts of methyl acrylate (MA) as monomer (a2), 89.3 parts of 2-ethylhexyl acrylate (2EHA) as monomer (a3), 0.2 parts of 2-hydroxyethyl acrylate (HEA) as monomer (a4), 0.02 parts of 2,2-azobisisobutyronitrile (AIBN) as a polymerization initiator, and 110 parts of ethyl acetate as a solvent were charged into a reaction vessel and polymerized under a nitrogen atmosphere at 70 °C for 5 hours. The viscosity was adjusted appropriately by dropwise addition of ethyl acetate during polymerization. After the reaction was completed, the mixture was cooled and diluted with ethyl acetate to obtain an acrylic copolymer solution (A-1) with a nonvolatile content of 40.0 ± 0.5%. The weight-average molecular weight (Mw) of the resulting acrylic copolymer is shown in Table 1.

[0113] (Acrylic copolymers (A-2 to A-27, A'-1 to A'-5)) Acrylic copolymers (A-2 to A-27, A'-1 to A'-5) were synthesized in the same manner as for the production of acrylic copolymer (A-1), except that the compositions and blending amounts (parts by mass) were changed to those shown in Tables 1 and 2 and the amount of solvent was appropriately adjusted. The weight-average molecular weights and glass transition temperatures of the obtained acrylic copolymers are also shown in the tables.

[0114] [Table 1]

[0115] [Table 2]

[0116] Details of each monomer listed in Tables 1 and 2 above are as follows: "DAM": 2-(dimethylamino)ethyl methacrylate (Tg of homopolymer: 18°C) "ACMO": 4-acryloylmorpholine (Tg of homopolymer: 145°C) "MA": methyl acrylate (Tg of homopolymer: 5°C) "BA": n-butyl acrylate (Tg of homopolymer: -57°C) "2EHA": 2-ethylhexyl acrylate (Tg of homopolymer: -76°C) "2EHMA": 2-ethylhexyl methacrylate (Tg of homopolymer: -10°C) "LA": ​​Lauryl acrylate (Tg of homopolymer: -23°C) "HEA": 2-hydroxyethyl acrylate (Tg of homopolymer: -15°C)

[0117] <Production Example of Acrylic Copolymer (B)> (Acrylic copolymer (B-1)) Using a reactor equipped with a stirrer, reflux condenser, nitrogen inlet tube, thermometer, and dropping tube, 99 parts of isobornyl methacrylate (IBXMA) as monomer (b1), 1 part of acrylic acid as monomer (a2), 0.02 parts of 2,2-azobisisobutyronitrile (AIBN) as a polymerization initiator, and 150 parts of methyl ethyl ketone as a solvent were charged into a reaction vessel, and polymerized at 70 ° C. for 5 hours under a nitrogen atmosphere. After the reaction was completed, the mixture was cooled and diluted with ethyl acetate to obtain an acrylic copolymer solution (B-1) with a non-volatile content of 40.0 ± 0.5%. The weight average molecular weight (Mw) of the obtained acrylic copolymer is shown in the table.

[0118] (Acrylic Copolymers (B-2 to B-19, B'-1 to B'-4)) Acrylic copolymers (B-2 to A-19, B'-1 to B'-4) were synthesized in the same manner as for the acrylic copolymer (B-1), except that the compositions and blending amounts (parts by mass) were changed to those shown in Tables 3 and 4 and the amount of solvent was adjusted appropriately. The weight-average molecular weights and glass transition temperatures of the obtained acrylic copolymers are also shown in the tables.

[0119] [Table 3]

[0120] [Table 4]

[0121] Details of each monomer listed in Tables 3 and 4 above are as follows: "IBXMA": Isobornyl methacrylate (Tg of homopolymer: 180°C) "MMA": Methyl methacrylate (Tg of homopolymer: 103°C) "AA": acrylic acid (Tg of homopolymer: 163°C) "2EHMA": 2-ethylhexyl methacrylate (Tg of homopolymer: -10°C) "HEMA": 2-hydroxyethyl methacrylate (Tg of homopolymer: 55°C) "St": Styrene (Tg of homopolymer: 100°C)

[0122] Example 1 A pressure-sensitive adhesive composition was obtained by blending 100 parts of the acrylic copolymer (A-1), 30 parts of the acrylic copolymer (B-1), and 0.2 parts of "Coronate L" (manufactured by Tosoh Corporation, trimethylolpropane-modified toluene diisocyanate) as the curing agent (C). The mixture was diluted with ethyl acetate to a non-volatile content of 35.0±0.5%. The obtained adhesive composition was applied to a 38 μm thick polyethylene terephthalate release sheet (Cerapeel MF, manufactured by Toray Advanced Film Co., Ltd.) using a comma coater so that the dried thickness would be 50 μm, and the coated sheet was dried at 100° C. for 2 minutes to obtain an adhesive sheet. Next, one side of a 100 μm thick polyethylene terephthalate film (hereinafter referred to as PET film, product name: A-4300, manufactured by Toyobo Co., Ltd.) was bonded to the adhesive surface of this adhesive sheet to prepare a laminate consisting of "release sheet / adhesive layer / PET film." The resulting laminate was then aged for 1 week under conditions of a temperature of 25° C. and a relative humidity of 55%, to obtain a test laminate.

[0123] <Examples 2 to 70 and Comparative Examples 1 to 11> As shown in Tables 5 and 6, test laminates were obtained in the same manner as in Example 1, except that the types and blending amounts of the acrylic copolymer, curing agent, and tackifier resin were changed.

[0124] [Gel fraction measurement method] A 30 mm × 100 mm sample for gel fraction measurement was cut from the pressure-sensitive adhesive sheet obtained above, and the release sheet was peeled off. The gel fraction of the pressure-sensitive adhesive layer was determined using the following formula 1 as the mass fraction (unit: mass%) of the insoluble components after immersion of the pressure-sensitive adhesive layer in ethyl acetate at 50°C for 1 day relative to the pressure-sensitive adhesive layer before immersion. (Formula 1) Gel fraction (mass%) = (Y / X) × 100 X = mass of adhesive layer before immersion (g) Y = mass of adhesive layer after immersion (g)

[0125] [Table 5]

[0126] [Table 6]

[0127] The materials used in the examples and comparative examples are listed below. <Curing agent (C)> "Coronate L": Tosoh Corporation, trimethylolpropane-modified toluene diisocyanate "TETRAD-X": N,N,N',N'-tetraglycidyl-m-xylenediamine manufactured by Mitsubishi Gas Chemical Company, Inc.

[0128] <Tackifying resin> M-90: "Alcon M-90", manufactured by Arakawa Chemical Industries, Ltd., hydrogenated petroleum resin, softening point: 90°C M-100: "Alcon M-100", manufactured by Arakawa Chemical Industries, Ltd., hydrogenated petroleum resin, softening point: 100°C M-115: "Alcon M-115", manufactured by Arakawa Chemical Industries, Ltd., hydrogenated petroleum resin, softening point: 115°C M-135: "Alcon M-135", manufactured by Arakawa Chemical Industries, Ltd., hydrogenated petroleum resin, softening point: 135°C P-140: "Alcon P-140", manufactured by Arakawa Chemical Industries, Ltd., hydrogenated petroleum resin, softening point: 140°C A-100: "Super Ester A-100", manufactured by Arakawa Chemical Industries, Ltd., rosin ester, softening point: 100°C PX 1000: "YS Resin PX1000", manufactured by Yasuhara Chemical Co., Ltd., terpene resin, softening point: 100°C

[0129] <Evaluation of test laminate> The adhesive strength, heat-resistant adhesive strength, heat-resistant retention, transparency, and cross-cut test (processability) of the test laminate obtained from the pressure-sensitive adhesive composition of the present invention were evaluated by the following methods. The results are shown in Tables 7 and 8.

[0130] <Adhesive strength> The test laminate obtained above was cut into a size of 25 mm wide x 100 mm long. The release sheet was peeled off, and the adhesive layer was attached to polypropylene (PP) at 23°C and 50% relative humidity (hereinafter referred to as 23°C-50%RH) and pressed with a roll according to JIS Z-0237. After pressing, the components were held in an autoclave at 50°C and 5 atmospheres for 20 minutes to adhere to each other, yielding a measurement sample. After 24 hours, the adhesive strength (peel angle 180°, peel speed 300 mm / min; unit N / 25 mm width) was measured using a universal tensile tester (permanent adhesive strength). Separately, the adherend was left to stand in an 80°C environment for 2 hours, and the adhesive strength was measured in the same manner as above (80°C adhesive strength). The permanent adhesive strength and 80°C adhesive strength values ​​were evaluated according to the following criteria. [Evaluation criteria] A: Adhesive strength of 20.0N / 25mm or more (excellent) B: Adhesive strength is 15.0N / 25mm or more and less than 20.0N / 25mm (good). C: Adhesion strength is 10.0N / 25mm or more and less than 15.0N / 25mm (fairly good). D: Adhesive strength is less than 10.0N / 25mm (poor)

[0131] <Holding force> The resulting test laminate was prepared in a size of 25 mm wide x 150 mm long. The release sheet was peeled off, and the exposed adhesive layer was polished. The adhesive was then attached to a 25 mm wide x 25 mm wide section of the bottom edge of a 30 mm wide x 150 mm long polypropylene plate. After one back-and-forth press with a 2 kg roll, a 1 kg load was applied to the bottom edge of the test laminate in an 80°C atmosphere and the laminate was left for 70,000 seconds to measure the holding strength (in accordance with JIS Z0237:2000). Evaluation was based on the following criteria: The length by which the top edge of the test laminate's adhesive surface shifted downward from its original position was measured. [Evaluation criteria] A: Displacement length less than 3 mm: Good B: Misalignment length is 3mm or more but less than 7mm: Fairly good C: The deviation is 7mm or more and less than 10mm: Usable D: If the deviation is 10mm or more or if it falls, it cannot be used.

[0132] <Transparency> The obtained test laminate was visually inspected and evaluated based on the following evaluation criteria. A: The whole is transparent: Good B: There are some slightly cloudy areas: Usable C: The whole is cloudy: Unusable

[0133] <Cross-cut evaluation (processability)> The test laminate obtained above was attached to the frame of a TOM molding machine (NGF molding machine, manufactured by Fuse Vacuum Co., Ltd.), and a PP resin mortar cup with an inner diameter of 66 mm, an outer diameter of 70 mm, and a height of 38 mm was placed inside the frame. The PP resin mortar cup was then covered with the test laminate, resulting in a decorated structure. The top and curved surfaces of this decorated structure were cut crosswise with a cutter and left in an 80°C environment for 1000 hours. The presence or absence of lifting or peeling was visually inspected and evaluated according to the following criteria. [Evaluation criteria] A: No lifting or peeling is observed (Excellent) B: At least one of lifting and peeling is observed, and the size or distance is 0.5 mm or less (good). C: At least one of bubbling, lifting, and peeling is observed, and the size or distance is more than 0.5 mm and 1 mm or less (fairly good). D: At least one of bubbling, lifting, and peeling is observed, and the size or distance is more than 1 mm and 1.5 mm or less (poor).

[0134] [Table 7]

[0135] [Table 8]

[0136] As shown in Tables 7 and 8, the test laminates obtained from the pressure-sensitive adhesive compositions of the present invention exhibited excellent results in adhesion to olefin-based low-polarity substrates, heat-resistant adhesion, heat-resistant retention, transparency, and cross-cut test (appearance of coating film during processing). On the other hand, the pressure-sensitive adhesive compositions of the comparative examples were unable to satisfy all of the above. From these results, it can be said that the pressure-sensitive adhesive composition of the present invention can be suitably used for applications in which the adherend is an olefin-based low-polarity substrate and use in a high-temperature environment is expected, such as the formation of a sheet for decorative molding.

Claims

1. A pressure-sensitive adhesive composition comprising: an acrylic copolymer (A) having a glass transition temperature of −19.4° C. or lower; an acrylic copolymer (B) having a glass transition temperature of 48.9° C. or higher; and a curing agent (C), The acrylic copolymer (A) is a copolymer of a monomer mixture containing 0.5% by mass or more and 10% by mass or less of a nitrogen-containing monomer (a1) based on 100% by mass of the monomer mixture, the monomer mixture constituting the acrylic copolymer (A) further contains an alkyl(meth)acrylic monomer (a2) having an alkyl group with 7 or less carbon atoms, an alkyl(meth)acrylic monomer (a3) ​​having an alkyl group with 8 or more carbon atoms, and a (meth)acrylic monomer (a4) having a hydroxy group, the total content of the monomer (a2) and the monomer (a3) ​​is 89.8% by mass or more and 99.3% by mass or less, based on 100% by mass of the monomer mixture; the acrylic copolymer (B) is a copolymer of a monomer mixture containing a (meth)acrylic monomer (b1) having a glass transition temperature of 80°C or higher and a (meth)acrylic monomer (b2) having a carboxy group, the weight average molecular weight of the acrylic copolymer (A) is in the range of 400,000 to 1,500,000, and the weight average molecular weight of the acrylic copolymer (B) is in the range of 5,000 to 200,000; the content of the acrylic copolymer (B) relative to 100 parts by mass of the acrylic copolymer (A) is 5 parts by mass or more and 50 parts by mass or less, The pressure-sensitive adhesive composition, wherein the curing agent (C) contains an isocyanate-based curing agent.

2. 2. The pressure-sensitive adhesive composition according to claim 1, wherein the content of the monomer (a2) is 5% by mass or more and 60% by mass or less, and the content of the monomer (a3) ​​is 30% by mass or more and 90% by mass or less, relative to 100% by mass of the monomer mixture constituting the acrylic copolymer (A).

3. The pressure-sensitive adhesive composition according to claim 1, wherein the content of the monomer (a4) is 0.05% by mass or more and 0.5% by mass or less relative to 100% by mass of the monomer mixture constituting the acrylic copolymer (A).

4. the monomer mixture constituting the acrylic copolymer (B) further contains a (meth)acrylic monomer (b3) having a glass transition temperature of 0°C or lower, The pressure-sensitive adhesive composition according to claim 1 , wherein a content of the monomer (b3) in 100% by mass of the monomer mixture is 1% by mass or more and 40% by mass or less.

5. the monomer mixture constituting the acrylic copolymer (B) further contains a (meth)acrylic monomer (b4) having a hydroxy group, The pressure-sensitive adhesive composition according to claim 1 , wherein the content of the monomer (b4) is 0.01% by mass or more and 5% by mass or less relative to 100% by mass of the monomer mixture.

6. 2. The pressure-sensitive adhesive composition according to claim 1, wherein the acrylic copolymer (A) has a glass transition temperature of −50° C. or higher and −19.4° C. or lower.

7. The pressure-sensitive adhesive composition according to claim 1, wherein the acrylic copolymer (B) has a glass transition temperature of 48.9°C or higher and lower than 180°C.

8. The pressure-sensitive adhesive composition according to claim 1 , further comprising 1 part by mass or more and 30 parts by mass or less of a tackifier resin relative to 100 parts by mass of the acrylic copolymer (A).

9. A pressure-sensitive adhesive sheet having a pressure-sensitive adhesive layer comprising the pressure-sensitive adhesive composition according to any one of claims 1 to 8.

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