JP7834982B2 - Adhesives and adhesive sheets for polyvinyl chloride - Google Patents

Description

This invention relates to an adhesive and adhesive sheet for polyvinyl chloride that can be suitably used for laminating polyvinyl chloride.

Labels and stickers applied to motorcycles, automobiles, signs, electrical appliances, etc., commonly use polyvinyl chloride (PVC) resin sheets as the base material due to their flexibility and weather resistance. The back of these PVC sheets has a release liner attached via an adhesive, while the surface of the sheet is printed. Such PVC adhesive sheets, especially those used outdoors on motorcycles, are prone to dirt and may come into contact with gasoline. As a result, the PVC may swell, and the resulting stress can overwhelm the adhesive, leading to lifting or peeling.

Furthermore, the surfaces to which adhesive sheets are applied, such as those on motorcycles, automobiles, signs, and electrical appliances, are almost always curved. Adhesive sheets must not peel off over time even when applied to curved surfaces (curved surface suitability), and in addition, they must be suitable for wet application to the bodies of motorcycles, automobiles, etc. (wet application suitability).

Furthermore, in recent years, from the perspective of reducing vehicle weight for energy conservation, resin parts are increasingly being used in vehicles. Marking film, which is a decorative adhesive sheet with colors, letters, patterns, etc. printed on a film, is increasingly being used on these resin parts. Marking film is made by laminating an adhesive layer onto a base material such as polyvinyl chloride, and is used by attaching the adhesive layer side to the substrate. In emerging countries, the automotive society is still expanding explosively, and the opportunities for marking film to be applied to rubber products such as tires and plastic products for interiors at mass retailers are also increasing dramatically.
The marking films used on these vehicles are frequently exposed to gasoline, so high gasoline resistance is required.

Regarding adhesives used in such marking films, Patent Document 1 describes the use of an adhesive in which the ratio of the mol% of carboxyl group-containing monomer to the average number of carbon atoms of the alkyl group of the alkyl acrylate is 1.5 to 4.5, and the gel fraction after crosslinking is 40 to 90%. Furthermore, Patent Document 2 discloses that in acrylic copolymers used in adhesives, gasoline resistance is improved by using acrylic copolymers obtained using carboxyl group-containing monomers and hydroxyl group-containing monomers.

Furthermore, in recent years, from a "sustainability" perspective, attempts are being made to utilize plant-derived bioethanol as an alternative fuel to gasoline, and the use of ethanol-blended gasoline in gasoline engine vehicles has also begun. This means that adhesive sheets are required to be resistant to both low-polarity gasoline and high-polarity ethanol. Therefore, the selection and design of materials for adhesive sheets, including the adhesive itself, have become more stringent.

In contrast, Patent Document 3 discloses that an adhesive layer formed from an adhesive composition containing an acrylic copolymer and a crosslinking agent, obtained by copolymerizing a monomer mixture containing alkyl (meth)acrylate and a high-concentration carboxyl group-containing unsaturated monomer applied to a vinyl chloride resin film, exhibits excellent resistance to mixed gasoline, with a weight-average molecular weight of 250,000 or more of the sol when immersed for 20 minutes in a mixed solvent of gasoline and ethanol (gasoline:ethanol = 90:10, volume ratio), so-called E10 fuel.

Japanese Patent Application Publication No. 10-46127 Japanese Patent Publication No. 2016-210863 Japanese Patent Publication No. 2019-70078

However, adhesive layers formed from acrylic copolymers obtained by copolymerizing high concentrations of carboxyl group-containing unsaturated monomers have a problem: they have high initial adhesive strength when bonded to an adherend, making them difficult to reposition, i.e., they have poor reworkability.

In other words, the problem that this invention aims to solve is to provide an adhesive sheet that possesses solvent resistance to low-polarity solvents, suitability for curved surfaces, suitability for water application, and also excellent reworkability, as well as an adhesive for polyvinyl chloride used to form the adhesive sheet.

The inventors of this invention, after diligent research to solve the above problems, discovered that the following adhesive for polyvinyl chloride can solve the problems, and thus completed the present invention.

That is, the first invention comprises an acrylic resin (A) and a curing agent (B),
The present invention relates to an adhesive for polyvinyl chloride, wherein the acrylic resin (A) is a copolymer of a monomer mixture containing 15 to 60% by mass of monomer (a1) represented by the following general formula (1) and 0.5 to 7% by mass of monomer (a2) having a carboxyl group, and the curing agent (B) is at least one selected from the group consisting of chelating curing agents, isocyanate curing agents, and epoxy curing agents.
(In general formula (1), ^R1 is an alkyl group having 1 to 2 carbon atoms, or a phenyl group; ^R2 is an alkylene group having 2 to 4 carbon atoms; ^R3 is a hydrogen atom or a methyl group; and n represents a repeating unit and is an integer from 1 to 4.)

Furthermore, the second invention relates to a monomer mixture containing an alkyl (meth)acrylate monomer (a3) having an alkyl group having 1 to 2 carbon atoms,
The present invention relates to an adhesive for polyvinyl chloride, wherein the content of (meth)acrylate alkyl ester monomer (a3) is 35 to 79.5% by mass in 100% by mass of the monomer mixture.

Furthermore, the third invention relates to a monomer mixture containing an alkyl (meth)acrylate monomer (a5) having an alkyl group having 4 to 13 carbon atoms,
The present invention relates to an adhesive for polyvinyl chloride in which the content of (meth)acrylate alkyl ester monomer (a5) is 0.5 to 10% by mass in 100% by mass of the monomer mixture.

Furthermore, the fourth invention relates to a monomer mixture containing a monomer (a4) having a hydroxyl group,
The present invention relates to an adhesive for polyvinyl chloride, wherein the content of monomer (a4) having a hydroxyl group is 0.5 to 7% by mass in 100% by mass of the monomer mixture.

Furthermore, the fifth invention relates to the polyvinyl chloride adhesive, wherein the acrylic resin (A) has a weight-average molecular weight of 200,000 to 600,000.

Furthermore, the sixth invention relates to the adhesive for polyvinyl chloride, wherein the curing agent (B) comprises a chelating curing agent or an epoxy curing agent.

Furthermore, the seventh invention relates to the adhesive for polyvinyl chloride, wherein the curing agent (B) comprises a chelating curing agent or an epoxy curing agent and an isocyanate curing agent.

Furthermore, the eighth invention relates to an adhesive sheet comprising a base material and an adhesive layer formed from the aforementioned polyvinyl chloride adhesive.

Furthermore, the ninth invention relates to the adhesive sheet, wherein the base material is polyvinyl chloride.

This invention provides an adhesive sheet that possesses solvent resistance to low-polarity solvents, suitability for curved surfaces, suitability for water application, and excellent reworkability, as well as an adhesive for polyvinyl chloride used to form the adhesive sheet.

The following terms are defined for the purposes of this invention. First, sheet, film, and tape are synonymous. (Meth)acrylic acid means acrylic acid and methacrylic acid. Monomer means an ethylenically unsaturated double bond-containing monomer. Adhesion refers to the surface to which the adhesive sheet is attached.
In this specification, adhesive for polyvinyl chloride, monomer (a1) represented by general formula (1), monomer (a2) having a carboxyl group, alkyl (meth)acrylate monomer having an alkyl group with 1 to 2 carbon atoms (a3), monomer (a4) having a hydroxyl group, and alkyl (meth)acrylate monomer having an alkyl group with 4 to 13 carbon atoms (a5) may be abbreviated as adhesive, monomer (a1), monomer (a2), monomer (a3), monomer (a4), and monomer (a5), respectively.
Unless otherwise noted, the various components mentioned herein may be used individually or in combination of two or more.
Furthermore, unless otherwise specified, the "~" indicating a numerical range includes both the lower and upper limits.
The embodiments of the present invention will be described in detail below, but the following description is merely an example (representative example) of the embodiments of the present invention, and the present invention is not limited to these contents unless it exceeds the gist of the invention.

Adhesives for polyvinyl chloride
The adhesive of the present invention comprises an acrylic resin (A) and a curing agent (B), wherein the acrylic resin (A) is a copolymer of a monomer mixture containing 15 to 60% by mass of a monomer (a1) represented by general formula (1) and 0.5 to 7% by mass of a monomer (a2) having a carboxyl group, and the curing agent (B) is at least one selected from the group consisting of chelating curing agents, isocyanate curing agents, and epoxy curing agents.
Monomer (a1) has 1 to 4 alkylene oxide groups, which imparts hydrophilicity and excellent resistance to hydrocarbons in gasoline. Monomer (a2) has carboxyl groups, which exhibit high adhesion to the adherend and act as crosslinking points with the curing agent. This adhesive uses an acrylic resin (A) in which monomer (a1) and monomer (a2) are in the above-mentioned specific ratio, and further combines it with the above-mentioned specific curing agent (B). As a result, it exhibits excellent resistance to solvents such as ethanol-mixed gasoline, excellent suitability for curved surfaces and wet application, and also has excellent reworkability without excessively high adhesive strength.
This adhesive contains at least an acrylic resin (A) and a curing agent (B), and may contain other components to the extent that it achieves the effects of the present invention.

<Acrylic resin (A)>
Acrylic resin (A) is a copolymer of a mixture containing at least 15 to 60% by mass of monomer (a1) and 0.5 to 7% by mass of monomer (a2) having a carboxyl group, and may further contain other monomers. In other words, acrylic resin (A) is a copolymer having at least constituent units derived from monomer (a1) and constituent units derived from monomer (a2), and may further contain constituent units derived from other monomers.

Other monomers besides monomers (a1) and (a2) include alkyl (meth)acrylate monomers having an alkyl group with 1 to 2 carbon atoms (a3), monomers having a hydroxyl group (a4), alkyl (meth)acrylate monomers having an alkyl group with 4 to 13 carbon atoms (a5), monomers having an amide bond, monomers having an epoxy group, monomers having an amino group, styrenes, vinyl ethers, vinyl esters, and the like.

Due to its excellent solvent resistance, suitability for curved surfaces, suitability for water application, and reworkability, the acrylic resin (A) is a copolymer of a monomer mixture containing 15 to 60% by mass of monomer (a1) represented by general formula (1) and 0.5 to 7% by mass of monomer (a2) having a carboxyl group.
Preferably, the copolymer is a monomer mixture further containing an alkyl (meth)acrylate monomer (a3) having an alkyl group having 1 to 2 carbon atoms.
Preferably, it is a copolymer of monomer mixtures further containing a monomer (a4) having a hydroxyl group.
Preferably, the copolymer is a monomer mixture further containing an alkyl (meth)acrylate monomer (a5) having an alkyl group having 4 to 13 carbon atoms.

In the present invention, the weight-average molecular weight (hereinafter also referred to as Mw) of the acrylic resin (A) may be 10,000 or more, but is preferably 150,000 to 700,000, more preferably 200,000 to 600,000, even more preferably 200,000 to 500,000, and particularly preferably 200,000 to 400,000, from the viewpoint of excellent solvent resistance, suitability for curved surfaces, suitability for water application, and reworkability.
A weight-average molecular weight of 150,000 or more ensures a sufficient number of crosslinking points with the curing agent (B), resulting in an adhesive that is less soluble in low-polarity solvents. On the other hand, a weight-average molecular weight of 700,000 or less is sufficient, providing excellent solvent resistance, suitability for curved surfaces, suitability for water application, and reworkability.
In this invention, the weight-average molecular weight is a polystyrene-based value measured by gel permeation chromatography (GPC), and specific details can be found in the examples described later.

[Monomer (a1)]
Monomer (a1) is a monomer represented by the following general formula (1).
By using monomers having such alkoxyalkylene oxide groups, alkoxypolyalkylene oxide groups, phenoxyalkylene oxide groups, or phenoxypolyalkylene oxide groups, polarity can be imparted, resulting in an adhesive layer that is less soluble in low-polarity solvents (such as gasoline). Preferably, the monomer is an alkyl (meth)acrylate monomer having an alkoxyalkylene oxide group.

(In general formula (1), ^R1 is an alkyl group having 1 to 2 carbon atoms, or a phenyl group; ^R2 is an alkylene group having 2 to 4 carbon atoms; ^R3 is a hydrogen atom or a methyl group; and n represents a repeating unit and is an integer from 1 to 4.)

^R1 is an alkyl group having 1 to 2 carbon atoms, or a phenyl group, specifically a methyl group, an ethyl group, or a phenyl group, and is preferably a methyl group or an ethyl group in terms of water resistance and resistance to low polarity solvents.
^R2 is an alkylene group having 2 to 4 carbon atoms, and may be linear or branched. Specifically, it is an ethylene group, a propylene group, or a butylene group, and is preferably an ethylene group from the viewpoint of water resistance and resistance to low polar solvents.
^R3 is either a hydrogen atom or a methyl group.
n represents the repeating unit and is an integer from 1 to 4. From the viewpoint of water resistance and resistance to low-polarity solvents, the repeating unit n is preferably 1.

The monomer (a1) content is 15 to 60% by mass, preferably 20 to 30% by mass. A higher monomer (a1) content results in better resistance to low-polarity solvents; therefore, 15% by mass or more is preferred. On the other hand, setting the monomer (a1) content to 60% by mass or less results in an adhesive with excellent water resistance and suitability for wet application.

Examples of monomers (a1) include 2-methoxyethyl (meth)acrylate, 2-ethoxyethyl (meth)acrylate, 2-phenoxyethyl (meth)acrylate, methoxypolyethylene glycol (meth)acrylate, ethoxypolyethylene glycol (meth)acrylate, methoxypolypropylene glycol (meth)acrylate, ethoxypolypropylene glycol (meth)acrylate, phenoxypolyethylene glycol (meth)acrylate, and phenoxypolypropylene glycol (meth)acrylate.
The repeating unit n of the polyalkylene glycol chain is an integer between 2 and 4.
Among these, 2-methoxyethyl (meth)acrylate or 2-ethoxyethyl (meth)acrylate is preferred from the viewpoint of balancing water resistance and resistance to low-polarity solvents, and 2-methoxyethyl (meth)acrylate is more preferred.

[Monomer (a2)]
Monomer (a2) is a monomer having a carboxyl group.
The monomer (a2) imparts tackiness to the acrylic resin (A), and maintains its tackiness and cohesive force even when swollen with a low-polarity solvent, thereby suppressing lifting and peeling from the adherend.
The monomer (a2) content is 0.5 to 7% by mass in 100% by mass of the monomer mixture constituting the acrylic resin. From the viewpoint of achieving both adhesive strength and reworkability, and storage stability, 1.0 to 6% by mass is preferred, 2 to 5% by mass is more preferred, and 2 to 4% by mass is even more preferred.
Within this range, it is possible to satisfy the desired adhesive properties while maintaining reworkability and storage stability.

Examples of monomers (a2) include (meth)acrylic acid, monohydroxyethyl acrylic phthalate, p-carboxybenzyl acrylic acid, ethylene oxide-modified acrylic phthalate (number of moles of ethylene oxide added: (2 to 18)), monohydroxyethyl acrylic succinate, β-carboxyethyl acrylate, itaconic acid, and the like.
Among these, (meth)acrylic acid is preferred from the viewpoint of imparting tackiness and cohesiveness when expanded by a low-polarity solvent.

[Monomer (a3)]
Monomer (a3) is an alkyl (meth)acrylate monomer having an alkyl group with 1 to 2 carbon atoms, excluding monomers (a1), (a2), and (a4).
It is preferable that the acrylic resin (A) further contains monomer (a3) as a monomer. By including monomer (a3), the cohesive strength of the acrylic resin is improved, and while maintaining the basic adhesive properties, solvent resistance and reworkability provided by monomer (a1) and solvent resistance to adhesive properties provided by monomer (a3) can be added.

When monomer (a3) is used, the content of monomer (a3) is preferably 35 to 79.5% by mass, more preferably 45 to 75% by mass, and even more preferably 55 to 70% by mass, of 100% by mass of the monomer mixture.
By using monomer (a3) within this range, cohesive force is imparted to the acrylic resin (A), maintaining cohesive force even in adhesives swollen with low-polarity solvents, and suppressing lifting and peeling of polyvinyl chloride film due to swelling.

Examples of monomers (a3) include methyl acrylate, ethyl acrylate, methyl methacrylate, and ethyl methacrylate.
Among these, methyl acrylate and ethyl acrylate, which have fewer carbon atoms in their side chains, are preferred from the viewpoint of low polarity solvents.

[Monomer (a4)]
Monomer (a4) is a monomer having a hydroxyl group.
It is preferable that the acrylic resin (A) further contains monomer (a4) as a monomer. By including monomer (a4), the cohesive force of the acrylic resin (A) is further enhanced, and the cohesive force of the adhesive layer swollen by the low-polarity solvent can be further maintained. In addition, the polarity of the adhesive can be made higher, which suppresses the dissolution of the adhesive in the low-polarity solvent and suppresses lifting and peeling of the polyvinyl chloride film due to swelling when immersed in the low-polarity solvent.
On the other hand, when prioritizing the storage stability of adhesives for polyvinyl chloride, it is preferable that the acrylic resin (A) does not contain monomer (a4) as a constituent monomer. Adhesives using acrylic resin (A) that do not contain monomer (a4) exhibit suppressed viscosity changes even during long-term storage.

When monomer (a4) is used, the content of monomer (a4) is preferably 0.5 to 7% by mass, more preferably 1 to 6% by mass, and even more preferably 2 to 5% by mass, based on 100% by mass of the monomer mixture.
By using monomer (a4) within this range, it is possible to impart both cohesive force and polarity to the acrylic resin (A) while maintaining its basic adhesive properties and solvent resistance. This allows the cohesive force to be maintained even in an adhesive layer swollen with a low-polarity solvent, suppressing lifting and peeling due to swelling of the polyvinyl chloride film. Furthermore, it is preferable because it can also provide excellent suitability for water application and curved surfaces.

Examples of monomers (a4) include hydroxyalkyl (meth)acrylates such as 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, 2-hydroxybutyl (meth)acrylate, 3-hydroxybutyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, 6-hydroxyhexyl (meth)acrylate, and 8-hydroxyoctyl (meth)acrylate; glycol mono(meth)acrylates such as polyethylene glycol mono(meth)acrylate, polypropylene glycol mono(meth)acrylate, and 1,4-cyclohexanedimethanol mono(meth)acrylate; caprolactone-modified (meth)acrylate; and N-hydroxyalkyl (meth)acrylamides such as N-hydroxymethyl (meth)acrylamide and N-hydroxyethyl (meth)acrylamide. Among these, 2-hydroxyethyl (meth)acrylate is preferred.

[Monomer (a5)]
Monomer (a5) is an alkyl (meth)acrylate monomer having an alkyl group with 4 to 13 carbon atoms, excluding monomers (a1), (a2), and (a4).
By further using monomer (a5) as a monomer constituting the acrylic resin (A), longer side chains can be introduced compared to monomers (a1) and (a3), thus providing cohesive force through entanglement of side chains. This cohesive force is maintained even in the adhesive layer swollen with a low-polarity solvent, and lifting and peeling due to swelling of the polyvinyl chloride film is suppressed, making it preferable. More preferably, an alkyl (meth)acrylate monomer having an alkyl group having 4 to 8 carbon atoms is used. The alkyl group may be linear or branched.

When monomer (a5) is used, the content of monomer (a5) is preferably 0.5 to 10% by mass, more preferably 1.5 to 8% by mass, and even more preferably 2.5 to 6% by mass, based on 100% by mass of the monomer mixture.
Using monomer (a5) within this range is preferable because it allows for the imparting of cohesive force through the entanglement of side chains, which maintains cohesive force even in an adhesive layer swollen with a low-polarity solvent, thereby suppressing lifting and peeling of the polyvinyl chloride film due to swelling.

Examples of monomers (a5) include, but are not limited to, n-butyl (meth)acrylate, isobutyl (meth)acrylate, tert-butyl (meth)acrylate, n-pentyl (meth)acrylate, neopentyl (meth)acrylate, n-hexyl (meth)acrylate, cyclohexyl (meth)acrylate, n-pentyl (meth)acrylate, n-octyl (meth)acrylate, isooctyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, n-nonyl (meth)acrylate, n-decyl (meth)acrylate, isodecyl (meth)acrylate, undecyl (meth)acrylate, dodecyl (meth)acrylate, and tridecyl (meth)acrylate.

The acrylic resin (A) is preferable to contain at least one of monomer (a3) and monomer (a5) because it is superior in solvent resistance, flexibility, water-bonding suitability, and reworkability.
By combining monomer (a1) with monomer (a3) and/or monomer (a5), resistance to both hydrophilic and hydrophobic solvents is improved, and excellent resistance to solvents such as ethanol-blended gasoline is also achieved.

[Other monomers]
The monomers constituting the acrylic resin (A) of the present invention may include other monomers other than monomers (a1), monomer (a2), monomer (a3), monomer (a4), and monomer (a5).
Other monomers include monomers having amide bonds, monomers having epoxy groups, monomers having amino groups, monomers having aromatic rings, styrenes, vinyl ethers, vinyl esters, and vinyl cyanides.

Examples of monomers having an amide bond include (meth)acrylamide compounds such as (meth)acrylamide, N-methyl(meth)acrylamide, N-isopropyl(meth)acrylamide, N,N-dimethyl(meth)acrylamide, N,N-diethyl(meth)acrylamide, N,N-dimethylaminopropyl(meth)acrylamide, diacetone(meth)acrylamide, and N-(butoxymethyl)acrylamide;
Examples include heterocyclic compounds such as N-vinylpyrrolidone, N-vinylcaprolactam, and acryloylmorpholine.

Examples of monomers containing an epoxy group include glycidyl (meth)acrylate, methylglycidyl (meth)acrylate, and 3,4-epoxycyclohexylmethyl (meth)acrylate.

Examples of monomers containing an amino group include monomethylaminoethyl (meth)acrylate, monoethylaminoethyl (meth)acrylate, monomethylaminopropyl (meth)acrylate, and monoethylaminopropyl (meth)acrylate, as well as other monoalkylamino esters of (meth)acrylate.

Examples of monomers containing aromatic rings include phenoxyethyl acrylate, benzyl acrylate, phenoxydiethylene glycol (meth)acrylate, and ethylene oxide-modified nonylphenol (meth)acrylate.

From the viewpoint of adhesion, film hardness, weather resistance, etc., the monomer having an aromatic ring is preferably present in an amount of 0 to 7% by mass of 100% by mass of the monomer mixture.

Other monomers include cyclohexyl (meth)acrylate, propyl (meth)acrylate, isopropyl (meth)acrylate, octadecyl (meth)acrylate, isooctadecyl (meth)acrylate; styrenes such as styrene and α-methylstyrene; vinyl ethers such as ethyl vinyl ether, n-propyl vinyl ether, isopropyl vinyl ether, n-butyl vinyl ether, and isobutyl vinyl ether; vinyl esters such as vinyl acetate and vinyl propionate; and vinyl cyanides such as acrylonitrile.

From the viewpoint of adhesiveness, the content of other monomers is preferably 0 to 10% by mass of 100% by mass of the monomer mixture. If the other monomers are monomers having amide bonds, epoxy groups, amino groups, or aromatic rings, their content is preferably 0 to 7% by mass, from the viewpoint of polarity, crosslinkability, and reactivity.

[Method for manufacturing acrylic resin (A)]
Acrylic resin (A) can be produced by adding a polymerization initiator to a monomer mixture and appropriately selecting known production methods such as solution polymerization, bulk polymerization, emulsion polymerization, and various radical polymerizations. Among these, solution polymerization is preferred because it allows for easy adjustment of the weight-average molecular weight of the acrylic resin.

The solvent used in the solution polymerization is preferably, for example, methyl acetate, ethyl acetate, n-butyl acetate, isobutyl acetate, toluene, xylene, hexane, acetone, methyl ethyl ketone, methyl isobutyl ketone, methanol, ethanol, n-propanol, and isopropanol, with methyl ethyl ketone being more preferred. The solvent can be used alone or in combination of two or more types.

The aforementioned solution polymerization is preferably carried out by adding approximately 0.001 to 1 part by mass of polymerization initiator to 100 parts by mass of the monomer mixture. Typically, polymerization can be carried out under a nitrogen atmosphere at a temperature of approximately 50°C to 90°C for 4 to 12 hours. Furthermore, during polymerization, a chain transfer agent can be used to appropriately adjust the weight-average molecular weight of the acrylic resin.

Azo compounds and organic peroxides can be used as polymerization initiators.

Examples of azo compounds include 2,2'-azobisisobutyronitrile, 2,2'-azobis(2-methylbutyronitrile), 1,1'-azobis(cyclohexane-1-carbonitride), 2,2'-azobis(2,4-dimethylvaleronitrile), 2,2'-azobis(2,4-dimethyl-4-methoxyvaleronitrile), dimethyl-2,2'-azobis(2-methylpropionate), 4,4'-azobis(4-cyanovaleric acid), 2,2'-azobis(2-hydroxymethylpropionitrile), and 2,2'-azobis(2-(2-imidazolin-2-yl)propane).

Examples of organic peroxides include benzoyl peroxide, t-butyl perbenzoate, cumene hydroperoxide, diisopropyl peroxydicarbonate, di-n-propyl peroxydicarbonate, di(2-ethoxyethyl) peroxydicarbonate, t-butyl peroxyneodecanoate, t-butyl peroxybivalate, (3,5,5-trimethylhexanoyl) peroxide, dipropionyl peroxide, and diacetyl peroxide. Polymerization initiators can be used alone or in combination of two or more types, and/or in combination with the following chain transfer agents.

Examples of chain transfer agents include n-dodecyl mercaptan, mercaptoisobutyl alcohol, mercaptoacetic acid, 2-mercaptoethanol, thioglycolic acid, 2-ethylhexyl thioglycolate, 2,3-dimercapto-1-propanol, glycidyl mercaptan, α-methylstyrene dimer, carbon tetrachloride, chloroform, and hydroquinone.
The chain transfer agent can be used in an amount of about 0.01 to 1 part by mass per 100 parts by mass of the monomer mixture.

<Hardening agent (B)>
The curing agent (B) is at least one selected from the group consisting of chelating curing agents, isocyanate curing agents, and epoxy curing agents.
By using the curing agent (B), the adhesive according to the present invention can be made to have excellent solvent resistance to low-polarity solvents, suitability for curved surfaces, suitability for water application, and reworkability, while also maintaining storage stability.
Of the curing agents (B), chelate curing agents or epoxy curing agents are preferred due to their superior solvent resistance to low-polarity solvents, suitability for curved surfaces, suitability for water application, reworkability, and storage stability.
Furthermore, to enhance the effect of the chelating curing agent or epoxy curing agent, it is preferable to use an isocyanate curing agent in combination with these curing agents. That is, it is preferable that curing agent (B) comprises a chelating curing agent and an isocyanate curing agent, or that curing agent (B) comprises an epoxy curing agent and an isocyanate curing agent.
When an isocyanate curing agent is used in combination, it is preferable to use 0.1 to 65 parts by mass of the isocyanate curing agent, and more preferably 0.5 to 25 parts by mass, per 100 parts by mass of the chelating curing agent or epoxy curing agent.

[Chelating agent]
Chelate curing agents include, for example, coordination compounds of polyvalent metals such as aluminum, iron, copper, zinc, tin, titanium, nickel, antimony, magnesium, vanadium, chromium, and zirconium with acetylacetone or ethyl acetoethyl acetate.

The chelating curing agent is preferably present in an amount of 0.1 to 5 parts by mass per 100 parts by mass of the acrylic resin (A). Including 0.1 to 5 parts by mass makes it easier to balance the cohesive force and adhesive force of the adhesive layer.

[Isocyanate hardening agent]
The isocyanate system includes, for example, adducts of isocyanate monomers such as tolylene diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate, xylylene diisocyanate, hydrogenated xylylene diisocyanate, diphenylmethane diisocyanate, hydrogenated diphenylmethane diisocyanate, tetramethyl xylylene diisocyanate, naphthalene diisocyanate, triphenylmethane triisocyanate, and polymethylene polyphenyl isocyanate, and polyol compounds such as trimethylolpropane; burettes of the isocyanate monomers; isocyanurates of the isocyanate monomers; and compounds having three or more isocyanate groups in the molecule, such as adducts of the isocyanate monomers and known polyether polyols or polyester polyols, acrylic polyols, polybutadiene polyols, polyisoprene polyols, etc.
Examples include isocyanate monomers such as tolylene diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate, xylylene diisocyanate, hydrogenated xylylene diisocyanate, diphenylmethane diisocyanate, hydrogenated diphenylmethane diisocyanate, tetramethyl xylylene diisocyanate, naphthalene diisocyanate, triphenylmethane triisocyanate, and polymethylene polyphenyl isocyanate; and compounds having two isocyanate groups in the molecule, such as the allophanate of hexamethylene diisocyanate (Coronate 2770: manufactured by Nippon Polyurethane Industry Co., Ltd.). Among these, the trimethylolpropane adduct of tolylene diisocyanate is preferred because it suppresses performance degradation due to plasticizer absorption. Note that the number of isocyanate groups is the average number.

The isocyanate curing agent is preferably present in an amount of 0.01 to 1 part by mass, more preferably 0.01 to 0.6 parts by mass, even more preferably 0.05 to 0.6 parts by mass, and particularly preferably 0.1 to 0.6 parts by mass, per 100 parts by mass of the acrylic resin (A). Including 0.01 to 1 part by mass can improve the deformation-following properties and processability of the PVC film. The isocyanate curing agent can be used in combination with an epoxy curing agent or a chelating curing agent. For example, the hydroxyl groups generated by the reaction of carboxyl groups with the epoxy curing agent can be further crosslinked with the isocyanate curing agent.

[Epoxy hardener]
Examples of epoxy curing agents include bisphenol A-epichlorohydrin type epoxy resins, ethylene glycol diglycidyl ether, polyethylene glycol diglycidyl ether, glycerin diglycidyl ether, glycerin triglycidyl ether, 1,6-hexanediol diglycidyl ether, trimethylolpropane triglycidyl ether, diglycidylaniline, N,N,N',N'-tetraglycidyl-m-xylylenediamine, 1,3-bis(N,N'-diglycidylaminomethyl)cyclohexane, and N,N,N',N'-tetraglycidylaminophenylmethane.

The epoxy curing agent is preferably present in an amount of 0.01 to 1 part by mass per 100 parts by mass of the acrylic resin (A). Including 0.01 to 1 part by mass makes it easier to balance the cohesive force and adhesive force of the adhesive layer.

<Adhesive-granting resin>
This adhesive may contain a tackifying resin to the extent that it can solve the problems of the present invention. Preferred tackifying resins include, for example, rosin resins, terpene resins, alicyclic hydrocarbon resins, aliphatic petroleum resins, aromatic petroleum resins, and alkylphenol formaldehyde resins (oil-based phenolic resins). The tackifying resin can be used alone or in combination of two or more. Including a tackifying resin in the adhesive makes it easier to improve the adhesive strength to polyvinyl chloride.

The tackifying resin is preferably present in an amount of 10 to 30 parts by mass per 100 parts by mass of acrylic resin (A).

The softening point of the tackifying resin is preferably 80 to 140°C. Setting the softening point to 80 to 140°C makes it easy to achieve both adhesive strength and cohesive strength.

<Other optional ingredients>
The adhesive of the present invention may also contain, to the extent that it can solve the problems of the present invention, various other resins, curing catalysts, silane coupling agents, ultraviolet absorbers, light stabilizers, antioxidants, ultraviolet absorbers, anti-aging agents, weather-resistant stabilizers, antistatic agents, defoamers, leveling agents, release agents, oils, softeners, dyes, pigments, fillers, and the like as optional components.

Adhesive sheet
The adhesive sheet of the present invention comprises an adhesive layer formed from the polyvinyl chloride adhesive of the present invention described above.
The adhesive sheet of the present invention is used to bond polyvinyl chloride and an adhesive layer, but the base material of the adhesive sheet may be polyvinyl chloride, or the adherend may be polyvinyl chloride.
In other words, the adhesive of the present invention is suitable as an adhesive for bonding polyvinyl chloride, and can be used in general labels and seals, adhesive optical films, elastic wall materials, waterproof coatings, flooring materials, sealants, molding materials, urethane foam (rigid, semi-rigid, flexible), sound-absorbing materials, vibration-damping materials, various molding materials, elastic fibers, artificial leather, synthetic leather, etc., but is not limited to these.
In particular, due to its excellent solvent resistance to low-polarity solvents, flexibility to curved surfaces, and suitability for water application, it can be suitably used as an adhesive sheet with a polyvinyl chloride base material, and as a marking film for resin parts of vehicles, etc., as the adherend.

The adhesive sheet preferably comprises a substrate and an adhesive layer formed from the adhesive of the present invention. Alternatively, it may be a double-sided adhesive sheet having adhesive layers on both sides of a core material, or a cast adhesive sheet consisting only of an adhesive layer without a substrate or core material. The adhesive layer can be formed by coating the adhesive onto a release sheet, drying it to form the adhesive layer, and then bonding it to the substrate. Alternatively, it can be formed by coating the adhesive onto the substrate and drying it.

<Polyvinyl chloride>
Polyvinyl chloride (PVC), used as a base material or adherend, is a resin produced by polymerizing vinyl chloride monomer and then softening and stabilizing it with additives such as plasticizers, which is then molded into a film. By adjusting the polymerization conditions and additives, it exhibits excellent durability, weather resistance, chemical resistance, transparency, and processability. Because it can be manufactured at a very low cost, it is widely used not only in industrial materials, building materials, and automotive components, but also as general consumer goods.
The thickness of the polyvinyl chloride film used as the base material can be appropriately selected depending on the application and is not particularly limited, but is preferably about 25 μm to 2000 mm. The shape of the polyvinyl chloride used as the adherend can be any shape according to the application of the polyvinyl chloride, and the adherend surface may be curved.

<Adhesive layer>
The adhesive layer comprises an acrylic resin (A) and a curing agent (B), and can be formed using an acrylic adhesive containing the acrylic resin (A) and the curing agent (B).
This type of adhesive layer provides good solvent resistance, suitability for curved surfaces, and suitability for wet application.

The viscosity of the adhesive may be adjusted by adding the solvent exemplified in solution polymerization during the coating process.

The method of applying the adhesive is not particularly limited and includes, for example, a Meyer bar, applicator, brush, spray, roller, gravure coater, die coater, lip coater, comma coater, knife coater, reverse coater, and spin coater. It is preferable to perform a drying process during coating. The drying apparatus is not particularly limited and includes, for example, a hot air dryer, infrared heater, and vacuum method. The drying temperature is usually around 60 to 140°C.

The thickness of the adhesive layer is preferably 1 to 300 μm, and more preferably 5 to 100 μm. Being within the range of 1 to 300 μm allows for appropriate adjustment of the adhesive properties.

<Base material>
The adhesive sheet substrate can be made from materials other than polyvinyl chloride, such as cellophane, other plastics, rubber, foam, cloth, rubberized cloth, resin-impregnated cloth, glass, and wood. The substrate can be in the form of a plate or a film, but a film is preferred because it is easier to handle. The substrate can be used alone or as a laminate of two or more materials.

Examples of the aforementioned other plastic substrates include polyolefins such as polyvinyl alcohol, triacetylcellulose, polypropylene, polyethylene, polycycloolefin, and ethylene-vinyl acetate copolymer; polyesters such as polyethylene terephthalate, polybutylene terephthalate, and polyethylene naphthalate; and polyurethane, polycarbonate, polynorbornene, polyarylate, polyacrylic, polyphenylene sulfide, polystyrene, polyamide, and polyimide.

The present invention will now be described in more detail with reference to examples, but the present invention is not limited thereto. In the examples, "parts" means "parts by mass," and "%" means "mass percent." Furthermore, the amounts in the table are in parts by mass, and for all substances except the solvent, the values are calculated based on non-volatile content. A blank space in the table indicates that the substance was not included.

The method for measuring the weight-average molecular weight (Mw) of acrylic resins is shown below.

[Measurement of weight-average molecular weight (Mw)]
The weight-average molecular weight (Mw) was measured using GPC. GPC is a liquid chromatography method that separates and quantifies substances dissolved in a solvent (THF; tetrahydrofuran) based on differences in their molecular size, and the Mw was determined in terms of polystyrene equivalent.
Device name: Manufactured by Shimadzu Corporation,
LC-GPC system "Prominence"
Columns: Four Tosoh GMHXL columns and one Tosoh HXL-H column connected in series. Mobile phase solvent: Tetrahydrofuran. Flow rate: 1.0 ml/min. Column temperature: 40°C.

The abbreviations for the materials used in the examples and comparative examples are listed below.
<Monomer (a1)>
MEA: 2-methoxyethyl acrylate EEA: 2-ethoxyethyl acrylate <monomer (a2)>
AA: Acrylic acid MAA: Methacrylic acid <monomer (a3)>
MA: Methyl acrylate (number of carbon atoms in the alkyl group: 1)
EA: Ethyl acrylate (number of carbon atoms in the alkyl group: 2)
<Monomer (a4)>
HEA: 2-hydroxyethyl acrylate HEMA: 2-hydroxyethyl methacrylate <monomer (a5)>
BA: Butyl acrylate (number of carbon atoms in the alkyl group: 4)
HA: n-hexyl acrylate (number of carbon atoms in the alkyl group: 6)
2EHA: 2-ethylhexyl acrylate (number of carbon atoms in the alkyl group: 8)
DA: n-dodecyl acrylate (number of carbon atoms in the alkyl group: 12)
<Hardening agent (B)>
Aluminum Chelate A: Aluminum trisacetylacetonate (manufactured by Kawaken Fine Chemicals Co., Ltd.)
TDI/TMP: Trimethylolpropane adduct of tolylene diisocyanate TETRAD-X: N,N,N',N'-tetraglycidyl-1,3-benzenedi(methanamine)
<Polymerization Initiator>
AIBN: 2,2'-azobisisobutyronitrile <chain transfer agent>
Chain transfer agent: 2-ethylhexyl thioglycolate

<Manufacturing of acrylic resins>
(Acrylic resin (A-1))
In a reaction vessel under a nitrogen atmosphere, 63.0 parts methyl acrylate, 10.0 parts butyl acrylate, 25.0 parts 2-methoxyethyl acrylate, 2.0 parts acrylic acid, 49.0 parts methyl ethyl ketone, and 0.04 parts 2,2'-azobisisobutyronitrile (hereinafter referred to as AIBN) were charged. The mixture was heated with stirring to confirm the start of the polymerization reaction, and the reaction was carried out at reflux temperature for 3 hours. Next, 0.03 parts AIBN were added to the reaction solution and the reaction was carried out for 1 hour, and then another 0.03 parts AIBN were added to the reaction solution and the reaction was continued for 3 hours. After that, the reaction vessel was cooled to obtain an acrylic resin (A-1) solution with a weight-average molecular weight of 320,000.

(Acrylic resins (A-2 to A-25), acrylic resins (AC-1 to AC-3))
Acrylic resins (A-2 to 25) and acrylic resins (AC-1) to acrylic resins (AC-1 to 3) were obtained in the same manner as acrylic resin (A-1), except that the monomer compositions and blending amounts (parts by mass) were as shown in Tables 1 to 4.

(Acrylic resin (A-26))
In a reaction vessel (hereinafter simply referred to as "reaction vessel") equipped with a stirrer, thermometer, reflux condenser, dropping device, and nitrogen inlet tube, 42 parts methyl ethyl ketone, 10.0 parts butyl acrylate, 61.0 parts methyl acrylate, 25.0 parts 2-methoxyethyl acrylate, 2.0 parts acrylic acid, 2.0 parts 2-hydroxyethyl acrylate, 0.04 parts 2,2'-azobisisobutyronitrile (hereinafter referred to as AIBN), and 0.05 parts 2-ethylhexyl thioglycolate were charged under a nitrogen atmosphere. The mixture was heated while stirring to confirm the start of the polymerization reaction, and the reaction was carried out at reflux temperature for 3 hours. Next, 0.03 parts AIBN was added to the reaction solution and the reaction was carried out for 1 hour, and then another 0.03 parts AIBN was added to the reaction solution and the reaction was continued for 3 hours. After that, the reaction vessel was cooled to obtain an acrylic resin (A-26) solution with a weight-average molecular weight of 150,000.

(Acrylic resin (A-27))
In a reaction vessel (hereinafter simply referred to as "reaction vessel") equipped with a stirrer, thermometer, reflux condenser, dropping device, and nitrogen inlet tube, 42 parts methyl ethyl ketone, 10.0 parts butyl acrylate, 61.0 parts methyl acrylate, 25.0 parts 2-methoxyethyl acrylate, 2.0 parts acrylic acid, 2.0 parts 2-hydroxyethyl acrylate, 0.04 parts 2,2'-azobisisobutyronitrile (hereinafter referred to as AIBN), and 0.03 parts 2-ethylhexyl thioglycolate were charged under a nitrogen atmosphere. The mixture was heated while stirring to confirm the start of the polymerization reaction, and the reaction was carried out at reflux temperature for 3 hours. Next, 0.03 parts AIBN was added to the reaction solution and the reaction was carried out for 1 hour, and then another 0.03 parts AIBN was added to the reaction solution and the reaction was continued for 3 hours. After that, the reaction vessel was cooled to obtain an acrylic resin (A-27) solution with a weight-average molecular weight of 200,000.

(Acrylic resin (A-28))
In a reaction vessel (hereinafter simply referred to as "reaction vessel") equipped with a stirrer, thermometer, reflux condenser, dropping device, and nitrogen inlet tube, 30 parts methyl ethyl ketone, 12 parts ethyl acetate, 10.0 parts butyl acrylate, 61.0 parts methyl acrylate, 25.0 parts 2-methoxyethyl acrylate, 2.0 parts acrylic acid, 2.0 parts 2-hydroxyethyl acrylate, and 0.04 parts 2,2'-azobisisobutyronitrile (hereinafter referred to as AIBN) were charged under a nitrogen atmosphere. The mixture was heated while stirring to confirm the start of the polymerization reaction, and the reaction was carried out at reflux temperature for 3 hours. Next, 0.03 parts AIBN were added to the reaction solution and the reaction was carried out for 1 hour, and then another 0.03 parts AIBN were added to the reaction solution and the reaction was continued for 3 hours. After that, the reaction vessel was cooled to obtain an acrylic resin (A-28) solution with a weight-average molecular weight of 600,000.

(Acrylic resin (A-29))
In a reaction vessel (hereinafter simply referred to as "reaction vessel") equipped with a stirrer, thermometer, reflux condenser, dropping device, and nitrogen inlet tube, 21 parts methyl ethyl ketone, 21 parts ethyl acetate, 10.0 parts butyl acrylate, 61.0 parts methyl acrylate, 25.0 parts 2-methoxyethyl acrylate, 2.0 parts acrylic acid, 2.0 parts 2-hydroxyethyl acrylate, and 0.04 parts 2,2'-azobisisobutyronitrile (hereinafter referred to as AIBN) were charged under a nitrogen atmosphere. The mixture was heated while stirring to confirm the start of the polymerization reaction, and the reaction was carried out at reflux temperature for 3 hours. Next, 0.03 parts AIBN were added to the reaction solution and the reaction was carried out for 1 hour, and then another 0.03 parts AIBN were added to the reaction solution and the reaction was continued for 3 hours. After that, the reaction vessel was cooled to obtain an acrylic resin (A-29) solution with a weight-average molecular weight of 700,000.

(Example 1)
In the acrylic resin solution obtained in the above synthesis example, 1.0 part of aluminum chelate A as a metal chelating agent and 0.2 parts of TDI/TMP (trimethylolpropane adduct of tolylene diisocyanate) as an isocyanate curing agent were added to 100 parts (non-volatile content) of acrylic resin (A-1) and an acrylic adhesive was obtained by further adjusting the non-volatile content to 50% with ethyl acetate as a solvent.
The aforementioned acrylic adhesive was applied to a commercially available release sheet with a thickness of 75 μm using a comma coater so that the thickness after drying was 25 μm, and an adhesive layer was formed by drying at 100°C for 2 minutes. Next, a 50 μm thick polyvinyl chloride film was laminated to this adhesive layer, and an adhesive sheet with the structure of "release sheet/adhesive layer/PVC sheet" was obtained by aging it for one week at a temperature of 25°C and 50% RH.

(Examples 2-33, Comparative Examples 1-5)
Except for changing the composition and proportions of the materials as shown in Tables 1 to 4, the same procedure as in Example 1 was followed to obtain the acrylic adhesives and adhesive sheets of Examples 2 to 33 and Comparative Examples 1 to 5, respectively.

<Evaluation of adhesive sheets>
The solvent resistance, flexibility to curved surfaces, suitability for wet application, and reworkability of the adhesive sheets were evaluated using the following methods. The results are shown in Tables 1 to 4.
The "melamine-coated board" used in the evaluation is a metal board with a surface coating of black melamine, measuring 70 mm in width and 150 mm in length.

<Solvent resistance>
The obtained adhesive sheet was cut to a size of 30 mm in width and 50 mm in length. Under conditions of 23°C and 50% relative humidity, the release sheet was peeled off the cut adhesive sheet, and the exposed adhesive layer was attached to a melamine-coated board. The board was pressed down once using a 2 kg roll and left for 24 hours to prepare a sample. The sample prepared under the above conditions was placed in a plastic container measuring 100 mm in width, 200 mm in length, and 50 mm in height. 100 g of E25 fuel (gasoline:ethanol = 75:25, volume ratio) adjusted to 23°C was then added to the plastic container and left to stand for 30 minutes under conditions of 23°C and 50% relative humidity. After that, the melamine-coated board to which the adhesive sheet was attached was removed, and the degree of lifting and peeling of the adhesive sheet from the melamine-coated board was visually evaluated.
Furthermore, the same solvent resistance tests were conducted by changing the temperature of the E25 fuel and the ambient temperature of the plastic container to 5°C, 10°C, and 35°C. The evaluation criteria are as follows.
[Evaluation Criteria]
○: No lifting or peeling of the adhesive sheet was observed. Good.
△: The adhesive sheet did not peel off, and fewer than 5 lifts were observed, but they returned to their original state after 1 hour of removal. Usable.
×: The adhesive sheet did not peel off, but lifting was observed, and it did not return to its original position after one hour of removal. Unusable and impractical.

<Suitability for curved surfaces>
The obtained adhesive sheet was prepared to a size of 25 mm in width and 100 mm in length. Next, under an atmosphere of 25°C and 50% relative humidity, the adhesive sheet was attached to a metal rod with a diameter of 60 mm along its circumference to prepare a sample. After the sample was left to stand for 7 days under an atmosphere of 25°C and 50% relative humidity, the degree of lifting and peeling of the edges of the adhesive sheet from the metal rod was visually evaluated. The evaluation criteria were as follows.
[Evaluation Criteria]
◎: No lifting or peeling observed at the edges of the adhesive sheet. Excellent.
○: There was some lifting or peeling at the edges of the adhesive sheet, less than 0.2 mm. This does not affect practical use and is in good condition.
△: There was lifting or peeling at the edges of the adhesive sheet, between 0.2 mm and 0.5 mm. Usable.
×: The adhesive sheet edges are lifted or peeling by 0.5 mm or more, making it unusable. Not practical for use.

<Suitability for wet application>
The obtained adhesive sheet was cut to a size of 25 mm in width and 100 mm in length. Under an atmosphere of 23°C and 50% relative humidity, the release sheet was peeled off the cut adhesive sheet, and the exposed adhesive layer was attached to a melamine-coated board whose entire surface had been sprayed with distilled water. A 2 kg roll was moved back and forth to allow water droplets to escape from the surface of the melamine-coated board, and after wiping off the excess water, the sample was left to stand for 24 hours after pressing. A standard sample was prepared in the same manner except that distilled water was not sprayed. The water-bonded sample and the standard sample were subjected to a peel test using a tensile testing machine under conditions of a peel speed of 300 mm/min and a peel angle of 180 degrees, and the peel strength (N/25 mm) was compared to determine the peel strength ratio compared to the standard (without water bonding).
Peel strength percentage (%) = Peel strength of wet-mounted sample / Peel strength of standard sample × 100
[Evaluation Criteria]
◎: Peel strength is 90% or more compared to the standard. Excellent.
○: Peel strength is 70% to less than 90% compared to the standard. No practical problems, good.
△: Peel strength is 50% or more but less than 70% of the standard. Usable.
×: Peel strength is less than 50% of the standard and therefore unusable. Not practical.

<Removability (Reworkability)>
The obtained adhesive sheet was cut to a size of 25 mm in width and 100 mm in length. Under conditions of 23°C and 50% relative humidity, the release sheet was peeled off the cut adhesive sheet, and the exposed adhesive layer was attached to a melamine-coated board to prepare a sample. Next, a test was conducted in which the adhesive sheet was peeled off by hand in a 180-degree direction immediately after attachment, and the sample after the hand-peel test was reattached to the melamine-coated board. The peeling force during hand-peeling and the appearance of the adhesive sheet after reattachment were evaluated on a four-point scale.
[Evaluation Criteria]
◎: It requires no force to peel it off the melamine-coated board, and no cosmetic defects are observed when it is reattached. Excellent.
○: It requires little force to peel off the melamine-coated board, and there are few cosmetic defects when reattached. Good.
△: Requires force to peel off melamine-coated boards, resulting in minimal cosmetic defects when re-bonded. Usable.
×: Requires force to peel off the melamine-coated board, and shows cosmetic defects when reattached. Not practical for use.

[Summary of Results]
Examples 1 to 6 in Table 1 are examples of adhesives for polyvinyl chloride using an acrylic resin (A) that does not contain monomer (a4). All of the adhesives in Examples 1 to 6 were shown to have excellent water-applicability and re-peelability, as well as good flexibility for curved surfaces and solvent resistance. Furthermore, the adhesives in Examples 1 to 6 showed good storage stability, with no significant increase in viscosity even when used after long-term storage.
Examples 7 to 33 in Tables 2 to 4 are examples of adhesives for polyvinyl chloride using an acrylic resin (A) containing monomer (a4). The adhesives using the acrylic resin (A) containing monomer (a4) all exhibited excellent water-bonding suitability and re-peelability, as well as good flexibility for curved surfaces and solvent resistance, with a particular tendency to show excellent solvent resistance at 35°C.
As described above, the adhesive for polyvinyl chloride of the present invention, which comprises an acrylic resin (A) and a curing agent (B), wherein the acrylic resin (A) is a copolymer of a monomer mixture containing 15 to 60% by mass of monomer (a1) represented by general formula (1) and 0.5 to 7% by mass of monomer (a2) having a carboxyl group, and the curing agent (B) is at least one selected from the group consisting of chelating curing agents, isocyanate curing agents, and epoxy curing agents, was confirmed to have better solvent resistance to low-polarity solvents (E25 fuel (gasoline:ethanol = 75:25, volume ratio), suitability for curved surfaces, suitability for wet application, and re-peelability compared to the adhesive of the comparative example, even when using a polyvinyl chloride film as the substrate.

Claims (7)

The material comprises an acrylic resin (A) and a hardening agent (B).
The acrylic resin (A) is a copolymer of monomer mixtures containing 15 to 60% by mass of monomer (a1) represented by the following general formula (1), 0.5 to 7% by mass of monomer (a2) having a carboxyl group, and 35 to 79.5% by mass of alkyl (meth)acrylate monomer (a3) having an alkyl group having 1 to 2 carbon atoms.
The curing agent (B) contains at least an isocyanate curing agent, and may also contain a chelating curing agent or an epoxy curing agent.
The isocyanate curing agent is contained in an amount of 0.01 to 1 part by mass per 100 parts by mass of acrylic resin (A).
When the chelating curing agent is included, the amount of the chelating curing agent is 0.1 to 5 parts by mass per 100 parts by mass of the acrylic resin (A) .
If the epoxy curing agent is included, the epoxy curing agent is present in an amount of 0.01 to 1 part by mass per 100 parts by mass of acrylic resin (A) as an adhesive for polyvinyl chloride.
(In general formula (1), ^R1 is an alkyl group having 1 to 2 carbon atoms, or a phenyl group; ^R2 is an alkylene group having 2 to 4 carbon atoms; ^R3 is a hydrogen atom or a methyl group; and n represents a repeating unit and is an integer from 1 to 4.) The monomer mixture contains an alkyl (meth)acrylate monomer (a5) having an alkyl group having 4 to 13 carbon atoms.
The polyvinyl chloride adhesive according to claim 1, wherein the content of (meth)acrylate alkyl ester monomer (a5) is 0.5 to 10% by mass in 100% by mass of the monomer mixture. The monomer mixture contains a monomer (a4) having a hydroxyl group,
The polyvinyl chloride adhesive according to claim 1 or 2, wherein the content of the monomer having a hydroxyl group (a4) is 0.5 to 7% by mass in 100% by mass of the monomer mixture. The acrylic resin (A) has a weight-average molecular weight of 200,000 to 600,000, as described in any one of claims 1 to 3, for use with polyvinyl chloride. The polyvinyl chloride adhesive according to any one of claims 1 to 4 , wherein the curing agent (B) comprises a chelating curing agent or an epoxy curing agent and an isocyanate curing agent. An adhesive sheet comprising a base material and an adhesive layer formed from the polyvinyl chloride adhesive described in any one of claims 1 to 5 . The adhesive sheet according to claim 6 , wherein the base material is polyvinyl chloride.