JP7806955B1 - An adhesive composition and an adhesive sheet using the adhesive composition. - Google Patents

Abstract

The present invention provides a pressure-sensitive adhesive composition that has excellent adhesion to low-polarity adherends, heat-resistant adhesion, and heat-resistant retention, as well as excellent 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; 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.
[Selection diagram] None

Description

The present invention relates to a pressure-sensitive adhesive composition and a pressure-sensitive adhesive sheet using the pressure-sensitive adhesive composition.

Adhesive sheets with an adhesive layer formed from an adhesive are easy to handle and are 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 there is a demand for adhesives that can ensure adhesion to polyolefin resins, for example. 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-resistant retention to prevent peeling or misalignment, even in high-temperature environments.

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

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% 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.

Japanese Patent Application Laid-Open No. 2019-73640

The inventors' investigations revealed that the pressure-sensitive adhesive composition described in Patent Document 1 did not exhibit satisfactory performance when peeled off from PP resin, a low-polarity adherend, due to effects such as zipping.

In other words, no pressure-sensitive adhesive composition has been developed to date that exhibits excellent adhesion to low-polarity substrates, heat-resistant adhesion, and heat-resistant retention, as well as excellent processability that prevents coating film transparency and poor appearance during processing.

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].

[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 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 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 or more and 50 parts by mass or less.

[2] The pressure-sensitive adhesive composition according to [1], wherein 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 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, based on 100% by mass of the monomer mixture.

[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.

[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.

[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.

[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.

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

[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).

[9] A pressure-sensitive adhesive sheet having a pressure-sensitive adhesive layer made of the pressure-sensitive adhesive composition described in any one of [1] to [8].

The present invention makes it possible to provide a pressure-sensitive adhesive composition that exhibits excellent adhesion, heat-resistant adhesion, and heat-resistant retention to low-polarity adherends, as well as excellent processability that prevents coating film transparency and poor appearance during processing, and a pressure-sensitive adhesive sheet using the pressure-sensitive adhesive composition. It has been discovered that when a pressure-sensitive adhesive layer is formed from a pressure-sensitive adhesive composition containing a specified (meth)acrylic copolymer (A) and a specified (meth)acrylic copolymer (B), it is possible to impart excellent adhesion, heat-resistant adhesion, and heat-resistant retention to low-polarity adherends, as well as excellent coating film transparency and processability, when decorating molded articles.

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 both acrylic acid ester and methacrylic acid ester, and the term "(meth)acryloyl group" includes both 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 range of the lower and upper limits. 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.

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 the acrylic copolymer (A) and has a high affinity for low-polarity adherends, the acrylic copolymer (B) can be localized appropriately on the outermost surface (the surface that comes into contact with the adherend) of the pressure-sensitive adhesive layer while maintaining transparency when the pressure-sensitive adhesive layer is formed, and further, due to the interaction between the specific monomers that constitute the acrylic copolymer (A) and the acrylic copolymer (B), the localization of the 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.

<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) relative to 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.

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

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, N-n-butyl(meth)acrylamide, diacetone(meth)acrylamide, and N,N'-methylenebis(meth)acrylamide;
N,N-dialkyl(meth)acrylamides such as N,N-dimethyl(meth)acrylamide, N,N-diethyl(meth)acrylamide, N,N-dipropyl(meth)acrylamide, N,N-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:
In particular, 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.

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), while a content of 10% by mass or less can ensure that the acrylic copolymer (B) is appropriately localized at the outermost surface of the pressure-sensitive adhesive layer.

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) is preferred because it provides a good balance between the wettability of the adhesive to a low-polarity adherend and the cohesive strength of the adhesive. Note that, in this specification, the alkyl groups in monomer (a2) and monomer (a3) are linear or branched, and cyclic alkyl groups are excluded.

[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.

Examples of alkyl(meth)acrylic monomers (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, isobutylacrylate, t-butyl acrylate, pentyl(meth)acrylate, hexyl(meth)acrylate, and n-heptyl(meth)acrylate.

Methyl (meth)acrylate and n-butyl (meth)acrylate are preferred as monomer (a2) from the viewpoint of the cohesive strength of the adhesive. Monomer (a2) may be used alone or in combination of two or more types.

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. It is more preferably 6% by mass or more and 50% by mass or less, and even more preferably 10% by mass or more and 40% by mass or less. A content of 5% by mass or more ensures excellent retention of adhesive strength in high-temperature environments, while a content of 60% by mass or less ensures high adhesive strength even at room temperature.

[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 monomer (a3)) contributes to wettability to low-polarity adherends.

As for 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 low-polarity substrates.

Examples of alkyl(meth)acrylic monomers (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.

As the monomer (a3), 2-ethylhexyl (meth)acrylate and lauryl (meth)acrylate are preferred from the viewpoint of wettability to low-polarity substrates. Monomer (a3) may be used alone or in combination of two or more types.

The content of monomer (a3) is preferably 30.0% by mass or more and 90.0% by mass or less, based on 100% by mass of the monomer mixture. It is 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. Having a content of 30.0% by mass or more and 90.0% by mass or less provides excellent wettability to low-polarity adherends.

The monomer mixture constituting the acrylic copolymer (A) preferably further contains a (meth)acrylic monomer (a4) having a hydroxy group. The inclusion of monomer (a4) is preferred because it increases the cohesive strength of the pressure-sensitive adhesive composition and also serves as a reaction site with the isocyanate-based curing agent.

[(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.

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.

[Other monomers]
The monomer mixture constituting the acrylic copolymer (A) may contain, as an optional component, other monomers 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-based (meth)acrylic acid esters such as 2-methoxyethyl (meth)acrylate, vinyl-based monomers such as vinyl acetate and styrene, and monomers having a glycidyl group such as glycidyl (meth)acrylate.

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.

(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 are described in the Examples.

(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.

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 (Formula 1)

In Formula 1, Tg1, Tg2, ..., Tg(k-1), and Tgk each represent the glass transition temperature (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 each represent the molar fraction 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 (°C) by subtracting 273 from the absolute temperature (K), and Celsius temperature (°C) can be converted to absolute temperature (K) by adding 273 to the Celsius temperature (°C).

"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 provided by the vendor of the various monomers, or from the values listed in "Polymer Handbook 3rd Edition" (A Wiley-Interscience Publication, 1989).

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

The glass transition temperature (Tg) of the acrylic copolymer (A) can be adjusted appropriately, for example, by using a monomer that has a different glass transition temperature (Tg) when made into a homopolymer.

<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 has a weight-average molecular weight in the range of 5,000 to 200,000 and a glass transition temperature of 30°C or higher.

[(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.

Examples of (meth)acrylic monomers (b1) whose homopolymers have a 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 monomer (b1) from the viewpoint of the balance between cohesive strength and wettability to low-polarity adherends.

The content of monomer (b1) is 40% by mass or more and 95% by mass or less, based on 100% by mass of the monomer mixture constituting 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, while a content of 95% by mass or less can impart cohesive strength.

[(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:
CH ₂ =CR ₁ -CO-O-(C ₂ H ₄ -COO-)nH
Here, _R1 represents hydrogen or a methyl group, and n represents an integer of 1 or more.

The content of monomer (b2) is from 1 to 10% by mass, based on 100% by mass of the monomer mixture constituting acrylic copolymer (B). It is preferably from 1 to 7% by mass, and more preferably from 1 to 5% by mass. By having it be from 1 to 10% by mass, acrylic copolymer (B) can be localized on the outermost surface of the pressure-sensitive adhesive layer, even in high-temperature environments.

The monomer mixture constituting the acrylic copolymer (B) may optionally 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.

[(Meth)acrylic monomer (b3) having a glass transition temperature of 0°C or lower]
By including a (meth)acrylic monomer (b3) (hereinafter also referred to as monomer (b3)) having a glass transition temperature of 0°C or lower, the affinity with the acrylic copolymer (A) containing the alkyl (meth)acrylate monomer (a2) and monomer (a3) as constituent monomers can be increased, thereby improving the compatibility between the acrylic copolymer (B) and the acrylic copolymer (A), and allowing the acrylic copolymer (B) to be appropriately localized on the outermost surface of the pressure-sensitive adhesive layer. When the acrylic copolymer (B) is appropriately localized on 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, thereby providing 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.

The (meth)acrylic monomer (b3) whose homopolymer has a 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. Furthermore, 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.

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), and more preferably 2-ethylhexyl methacrylate (2EHMA), from the viewpoint of the balance between wettability and cohesive strength.

The monomer mixture constituting the acrylic copolymer (B) may contain only one type, or two or more types, of alkyl(meth)acrylic monomer (b3) whose homopolymer has a glass transition temperature (Tg) of 0°C or lower.

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 the viscoelasticity of the pressure-sensitive adhesive layer (i.e., a decrease in cohesive strength) due to high-temperature heating.

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

[(Meth)acrylic monomer (b4) having a hydroxy group]
Examples of the (meth)acrylic monomer (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, and 4-hydroxybutyl (meth)acrylate, and these can be used alone or in combination of two or more.

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 setting the content of monomer (b4) to 0.01% by mass or more, lifting and peeling after storage in a high-temperature atmosphere can be prevented. By setting the content of monomer (b4) to 5 parts by mass or less, adhesion to low-polarity adherends such as polypropylene can be ensured.

[Other monomers]
The monomer mixture constituting the acrylic copolymer (B) may contain, as optional components, other monomers in addition to the monomers (b1) to (b4) described above, 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.

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).

(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 in the range of 10,000 to 150,000, and more preferably in the range of 10,000 to 50,000. When the weight-average molecular weight (Mw) of the acrylic copolymer (B) is 5,000 or more, the PSA composition can exhibit appropriate cohesive strength. Furthermore, when the weight-average molecular weight (Mw) is 5,000 or more, the PSA composition tends to be less compatible with the acrylic copolymer (B) and the acrylic copolymer (A), and therefore, when the PSA layer is formed, the acrylic copolymer (B) is appropriately localized on the surface of the PSA 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, and the pressure-sensitive adhesive layer can be attached to the adherend with high adhesive strength, thereby allowing the pressure-sensitive adhesive layer to 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 does not become 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.

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

[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.

The glass transition temperature (Tg) of the acrylic copolymer (B) can be adjusted appropriately, for example, by using a monomer that has a different glass transition temperature (Tg) when made into a homopolymer.

The glass transition temperature (Tg) of the acrylic copolymer (B) is a value calculated in the same manner as the glass transition temperature (Tg) of the acrylic copolymer (A) described above.

(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. Examples of solvents used in solution polymerization include 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.

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.

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'-azobispropionitrile such as 2,2'-azobis(2-hydroxymethylpropionitrile);
Examples include 1,1'-azobis-1-alkanenitriles such as 1,1'-azobis(cyclohexane-1-carbonitrile).

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, per 100 parts by mass of the monomer mixture.

<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.

As the curing agent (C), one or more curing agents 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 their reactivity with the functional groups of the acrylic copolymer in the pressure-sensitive adhesive composition. However, from the perspective of removability, it is preferable to include an isocyanate-based curing agent, and it is even more preferable to use an isocyanate-based curing agent in combination with an epoxy-based curing agent.

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 perspective of yellowing resistance, aliphatic polyisocyanates, alicyclic polyisocyanates, and their biuret, nurate, and adduct forms are even more preferred. The amount of 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).

The biuret compound is a self-condensation product having a biuret bond formed by the self-condensation of an isocyanate monomer. An example of a biuret compound is the biuret compound of hexamethylene diisocyanate.

The nurate derivative is a trimer of an isocyanate monomer. Examples include a trimer of hexamethylene diisocyanate, a trimer of isophorone diisocyanate, and a trimer of tolylene diisocyanate.

The adduct is a difunctional or higher isocyanate compound obtained by reacting an isocyanate monomer with a difunctional or higher low-molecular-weight active hydrogen-containing compound. Examples of adducts 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.

From the viewpoint of forming a sufficient crosslinked structure, trifunctional isocyanate compounds are preferred. More preferred isocyanate compounds are adducts and nurates, which are reaction products of an isocyanate monomer and a trifunctional low-molecular-weight active hydrogen-containing compound. Preferred isocyanate compounds are trimethylolpropane adducts of hexamethylene diisocyanate, nurates of hexamethylene diisocyanate, trimethylolpropane adducts of tolylene diisocyanate, nurates of tolylene diisocyanate, trimethylolpropane adducts of isophorone diisocyanate, and nurates of isophorone diisocyanate, and more preferred are trimethylolpropane adducts of hexamethylene diisocyanate, trimethylolpropane adducts of tolylene diisocyanate, and trimethylolpropane adducts of isophorone diisocyanate.

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, and the like, 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).

Oxazoline-based curing agents have two or more oxazoline groups in the molecule and may be low molecular weight compounds or polymers. Examples of low molecular weight oxazoline-based curing agents 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). , 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, bis-(2-oxazolinylnorbornane) sulfide, etc. Examples of polymeric oxazoline-based curing agents include polymers containing 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, and 2-isopropenyl-5-ethyl-2-oxazoline. 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).

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

The carbodiimide curing agent is preferably a high-molecular-weight polycarbodiimide produced by the 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 carbodiimide curing agent is preferably 0.1 to 10 parts by mass per 100 parts by mass of the acrylic copolymer (A).

Preferred metal chelate curing agents are 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 metal chelates include aluminum ethyl acetoacetate diisopropylate, aluminum trisacetylacetonate, aluminum bisethyl acetoacetate monoacetylacetonate, and aluminum alkyl acetoacetate diisopropylate. The amount of metal chelate curing agent per 100 parts by weight of acrylic copolymer (A) is preferably 0.1 to 10 parts by weight, more preferably 0.3 to 5 parts by weight, and most preferably 0.5 to 2 parts by weight.

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

<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) 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.
When the content of acrylic copolymer (B) per 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) per 100 parts by mass of acrylic copolymer (A) is 50 parts by mass or less, the acrylic copolymer (A) and the acrylic copolymer (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) per 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.

(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.

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 SylvatacRE85 manufactured by Arizona Chemical Company and Superester A-75 manufactured by Arakawa Chemical Industries, Ltd.

Examples of synthetic hydrocarbon resins include aliphatic petroleum resins, aromatic petroleum resins, aliphatic/aromatic petroleum resins, hydrogenated petroleum resins, coumarone-indene resins, and phenolic resins.

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 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.

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 acrylic copolymer (A). An amount of 1 part by mass or more makes it easier to ensure adhesion, ability to be attached to curved surfaces, and constant-load peel resistance, while an amount of 30 parts by mass or less makes it easier to prevent yellowing when left in a high-temperature environment or outdoors for long periods of time, as well as peeling and yellowing due to light, without impairing adhesion to polyolefins.

The softening point of the tackifier resin is preferably 90 to 160°C. 90 to 140°C is more preferred, and 90 to 135°C is most preferred. By setting the softening point to 90°C or higher, it becomes easier to ensure suppression of foaming, lifting, and peeling when left in a high-temperature environment or a high-temperature, high-humidity environment, as well as curved surface adhesion and constant-load peel resistance. By setting the softening point to 160°C or lower, it becomes easier to ensure adhesion to polyolefin-based adherends such as polypropylene, and 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 temperature increase rate of 5°C/°C using a bulb-type automatic softening point tester ASP-KG4 model (manufactured by Matex Co., Ltd.) was taken as the melting point.

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

The pressure-sensitive adhesive sheet of the present invention preferably has a configuration in which a release film is bonded to at least one surface of the pressure-sensitive adhesive layer. Specifically, the pressure-sensitive adhesive sheet has either 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.

<Adhesive Layer>
The gel fraction of the pressure-sensitive adhesive layer of the pressure-sensitive adhesive sheet of the present invention is preferably 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 suppressing 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 and 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.

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 alcohol should be used with caution, as they may inhibit the reaction between the acrylic copolymer (A) and the isocyanate curing agent.

There are no particular restrictions on the coating method, and various methods can be used, such as with a Mayer bar, applicator, brush, spray, roller, gravure coater, die coater, lip coater, comma coater, knife coater, reverse coater, and spin coater. There are also no particular restrictions on the drying and curing method, and examples include hot air drying, infrared rays, reduced pressure methods, and active energy rays, but from the perspective of outgassing resistance, hot air or steam heating at 60 to 180°C is preferred.

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

The pressure-sensitive adhesive composition of the present invention has excellent adhesion to low-polarity adherends, heat-resistant adhesion, heat-resistant retention, and durability that prevents poor appearance even in high-temperature environments, making it suitable for use as a pressure-sensitive adhesive composition for decorative sheets.

<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.

Examples of substrates 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.

<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.

Of the transparent plastic substrates mentioned above, those with excellent heat resistance, i.e., those that are inhibited or prevented from deforming under harsh conditions such as high temperature or high temperature and humidity, are suitable for use as release films. PET films or sheets are particularly suitable as transparent plastic substrates.

The thickness of the transparent plastic substrate is not particularly limited, but is preferably 10 to 200 μm, and more preferably 25 to 150 μm.

Decorative sheets made using this pressure-sensitive adhesive composition can be suitably used for attachment to adherends using vacuum forming and vacuum-pressure forming. Conventional vacuum forming and vacuum-pressure forming methods can be used as appropriate.

<Method of manufacturing decorative sheet>
A method for producing a decorative sheet having a pressure-sensitive adhesive layer made from the present pressure-sensitive adhesive composition is described below. 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 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 present pressure-sensitive adhesive composition exhibits moderate adhesive strength at room temperature (e.g., 25°C) and excellent adhesion at high temperatures (e.g., 100-150°C), and can maintain its properties at high temperatures for long periods of time (e.g., 80°C for 7 days). Therefore, a decorative sheet having a pressure-sensitive adhesive layer made of the present pressure-sensitive adhesive composition can be suitably used for decorating the interior and exterior of aircraft, automobiles, building materials, the nursing and medical fields, electrical appliances, electronic components, smartphones, furniture, musical instruments, Shinkansen window frames, and the like, for example, by three-dimensional surface decoration (Three Dimension Overlay Method: TOM molding), which is a type of vacuum and compressed air forming.

<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 substrates (e.g., three-dimensional molded products) can be used.

More specifically, the decorative structure may comprise, for example, a surface layer, a decorative layer, and an adhesive layer, in this order. The surface layer is the layer that forms the surface of the decorated molded product, and any conventionally known material in the field of decorative sheets may be used. 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 contain other layers, such as a design layer, bulk layer, or bonding layer. The number of layers of the decorative sheet, the type, arrangement, and thickness of each layer may be selected as appropriate and are not particularly limited. The adhesive layer is a layer used to attach the decorative sheet to an adherend (for example, a molded product formed into a three-dimensional shape), and can be formed by drying the adhesive composition (for example, after applying it to a substrate) or by crosslinking it by irradiating it with ultraviolet light, etc.

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". In addition, the blending amounts in the tables are in parts by mass, and the amounts other than the solvent are calculated as non-volatile contents. Note that blank spaces in the tables indicate that no blending has been performed.
The method for measuring the weight average molecular weight of the acrylic copolymer is as follows.

<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: LC-GPC system "Prominence". The column used was a 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 made at 40°C. The Mw was determined by conversion using polystyrene with a known Mw as the standard substance.

<Production Example of Acrylic Copolymer (A)>
(Acrylic Copolymer (A-1))
Using a reactor equipped with a stirrer, reflux condenser, nitrogen inlet tube, 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 for 5 hours at 70 ° C. under a nitrogen atmosphere, and the viscosity was appropriately adjusted by dropwise addition of ethyl acetate during polymerization. After completion of the reaction, the mixture was cooled and diluted with ethyl acetate to obtain an acrylic copolymer solution (A-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 Table 1.

(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.

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)

<Production Example of Acrylic Copolymer (B)>
(Acrylic Copolymer (B-1))
Using a reaction apparatus equipped with a stirrer, reflux condenser, nitrogen inlet pipe, 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 polymerization was carried out at 70°C for 5 hours under a nitrogen atmosphere. After completion of the reaction, 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.

(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 production of 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 appropriately adjusted. The weight-average molecular weights and glass transition temperatures of the obtained acrylic copolymers are also shown in the tables.

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.)

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), and diluting the mixture with ethyl acetate to a nonvolatile 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 then dried for 2 minutes at 100° C. 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," and the obtained 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.

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

[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 (g) of the adhesive layer before immersion
Y = mass (g) of the adhesive layer after immersion

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

<Tackifying Resin>
M-90: "Alcon M-90", manufactured by Arakawa Chemical Industries, Ltd., hydrogenated petroleum resin, softening point: 90°C
M-100: "Arcon 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: "Arcon 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

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

<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 pressure-sensitive adhesive layer was attached to polypropylene (PP) in an atmosphere of 23°C and 50% relative humidity (hereinafter referred to as 23°C-50%RH), and pressure-bonded with a roll in accordance with JIS Z-0237. After pressure-bonding, the components were held in an autoclave at 50°C and 5 atmospheres for 20 minutes to adhere to each other, thereby obtaining a measurement sample. After 24 hours, the adhesive strength (peel angle 180°, peel rate 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 (adhesive strength at 80°C). The permanent adhesive strength and adhesive strength at 80°C were evaluated according to the following criteria.
[Evaluation criteria]
A: Adhesive strength is 20.0 N/25 mm or more (excellent).
B: Adhesive strength is 15.0 N/25 mm or more and less than 20.0 N/25 mm (good).
C: Adhesion strength is 10.0 N/25 mm or more and less than 15.0 N/25 mm (fairly good).
D: Adhesive strength less than 10.0 N/25 mm (poor)

<Holding power>
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 attached to a 25 mm wide x 25 mm wide portion of the lower end of a 30 mm wide x 150 mm long polypropylene plate. After one round trip of pressure bonding with a 2 kg roll, a 1 kg load was applied to the lower end of the test laminate in an 80°C atmosphere and left for 70,000 seconds to measure the holding power (in accordance with JIS Z0237:2000). Evaluation was performed by measuring the distance the upper end of the test laminate's adhesive surface shifted downward from its original position, and the evaluation was based on the following criteria.
[Evaluation criteria]
A: The deviation is less than 3 mm: Good B: The deviation is 3 mm or more but less than 7 mm: Fairly good C: The deviation is 7 mm or more but less than 10 mm: Usable D: The deviation is 10 mm or more or has fallen: Unusable

<Transparency>
The obtained test laminate was visually inspected and evaluated based on the following evaluation criteria.
A: Entirely transparent: Good B: Some areas are slightly cloudy: Usable C: Entirely cloudy: Unusable

<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 set inside the frame. The PP resin mortar cup was then covered with the test laminate to obtain a decorative structure. The top and curved surfaces of this decorative structure were cross-cut with a cutter, and the structure was 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 was 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).

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 (9)

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 . 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). 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 in 100% by mass of the monomer mixture constituting the acrylic copolymer (A). 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. 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 in 100% by mass of the monomer mixture. 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. 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. 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 per 100 parts by mass of the acrylic copolymer (A). A pressure-sensitive adhesive sheet having a pressure-sensitive adhesive layer made of the pressure-sensitive adhesive composition according to any one of claims 1 to 8.