Adhesive agent and adhesive sheet

Abstract

An adhesive agent for applications in which the adhesive agent itself is to be recycled, wherein the gel fraction (gel fraction G1) after being immersed for 72 hours in ethyl acetate is 25% or greater, and the gel fraction (gel fraction G2) after being immersed for 72 hours in a solvent resulting from mixing ethyl acetate and acetylacetone at a mass ratio of 9:1 is 25% or less. With this adhesive agent and an adhesive sheet that is provided with an adhesive agent layer comprising this adhesive agent, the adhesive agent itself can be recycled. It also becomes possible to recycle an adherend or substrate to which the adhesive agent layer was adhered.

Description

Adhesives and adhesive sheets

[0001] The present invention relates to an adhesive and an adhesive sheet for applications in which the adhesive itself is recycled.

[0002] In recent years, there has been an increasing demand for the creation of a recycling-oriented society, and there is a demand for recyclability of various products and materials. Therefore, it is desirable that the adhesive in adhesive sheets can also be recycled.

[0003] Patent Document 1 discloses a recyclable adhesive composition for tack paper, which comprises a specific acrylic copolymer and a tertiary amino alcohol as main components, to which a nonionic surfactant and a fatty acid soap are added.

[0004] The adhesive composition (adhesive) described in Patent Document 1 can be easily disintegrated in water when repulped after use in tack paper, adhesive tape, etc. The adhesive does not flow out into wastewater but is redispersed and remains in the pulp, allowing recycled paper containing the adhesive to be produced.

[0005] Japanese Patent Application Publication No. 11-140408

[0006] However, the adhesive described in Patent Document 1 cannot be reused as an adhesive after the tack paper or adhesive tape is dissolved in water.

[0007] The present invention has been made in view of the above-mentioned circumstances, and has an object to provide a pressure-sensitive adhesive and a pressure-sensitive adhesive sheet that enable the pressure-sensitive adhesive itself to be recycled.

[0008] In order to achieve the above object, first, the present invention provides a pressure-sensitive adhesive for use in recycling the pressure-sensitive adhesive itself, characterized in that the gel fraction (gel fraction G1) after immersion in ethyl acetate for 72 hours is 25% or more, and the gel fraction (gel fraction G2) after immersion in a solvent in which ethyl acetate and acetylacetone are mixed in a mass ratio of 9:1 for 72 hours is 25% or less (Invention 1).

[0009] In the above invention (Invention 1), the gel fraction G1 is as described above, so that the adhesive layer exhibits suitable cohesive strength and can be reformed into a pressure-sensitive adhesive layer having suitable cohesive strength after dissolving in a solvent. Furthermore, the gel fraction G2 is as described above, so that the adhesive is easily soluble in solvents, particularly solvents containing acetylacetone. That is, the pressure-sensitive adhesive according to the above invention is easily dissolved in solvents and can then be reformed into a pressure-sensitive adhesive layer, so that the pressure-sensitive adhesive itself can be recycled. Furthermore, because the pressure-sensitive adhesive according to the above invention is easily soluble in solvents, the adhesive easily detaches from the adherend or substrate to which the pressure-sensitive adhesive layer is attached, which allows for the recycling of the adherend or substrate (limited to those that are not soluble in the above solvent).

[0010] In the above invention (Invention 1), it is preferable that the gel fraction (gel fraction G3) after dissolving the material in a solvent in which ethyl acetate and acetylacetone are mixed in a mass ratio of 1:1, drying at 120°C for 1 minute, and then immersing in ethyl acetate for 72 hours is 25% or more (Invention 2).

[0011] The pressure-sensitive adhesive according to the above inventions (Inventions 1 and 2) is preferably crosslinked with a metal chelate crosslinking agent (Invention 3).

[0012] In the above inventions (Inventions 1 to 3), an acrylic adhesive is preferred (Invention 4).

[0013] In the above invention (invention 4), the adhesive may be a solvent-based acrylic adhesive (invention 5), or a solventless acrylic adhesive (invention 6).

[0014] In the above invention (Invention 5), it is preferable that the adhesive strength of the adhesive layer made of the adhesive and having a thickness of 25 μm and a width of 25 mm to soda-lime glass is 1 N / 25 mm or more and 55 N / 25 mm or less (Invention 7).

[0015] In the above invention (Invention 6), it is preferable that the adhesive strength of the adhesive layer made of the adhesive and having a thickness of 25 μm and a width of 25 mm to soda lime glass is 10 N / 25 mm or more (Invention 8).

[0016] In the above invention (Invention 5), it is preferable that the adhesive strength of a 25 μm thick, 25 mm wide adhesive layer formed by dissolving ethyl acetate and acetylacetone in a 1:1 mass ratio solvent and then drying at 120° C. for 1 minute is 1 N / 25 mm or more and 55 N / 25 mm or less to soda-lime glass (Invention 9).

[0017] In the above invention (Invention 6), it is preferable that an adhesive layer having a thickness of 25 μm and a width of 25 mm, which is formed by dissolving the adhesive in a solvent in which ethyl acetate and acetylacetone are mixed in a mass ratio of 1:1 and then drying at 120° C. for 1 minute, has an adhesive strength to soda-lime glass of 10 N / 25 mm or more (Invention 10).

[0018] Secondly, the present invention provides a pressure-sensitive adhesive sheet including at least a pressure-sensitive adhesive layer, characterized in that the pressure-sensitive adhesive constituting the pressure-sensitive adhesive layer is the pressure-sensitive adhesive described above (Inventions 1 to 10) (Invention 11).

[0019] In the above invention (Invention 11), it is preferable that the adhesive sheet has two release sheets, and the adhesive layer is sandwiched between the release sheets so as to contact the release surfaces of the two release sheets (Invention 12).

[0020] According to the pressure-sensitive adhesive and pressure-sensitive adhesive sheet of the present invention, the pressure-sensitive adhesive itself can be recycled.

[0021] 1 is a cross-sectional view of a pressure-sensitive adhesive sheet according to one embodiment of the present invention, and FIG. 2 is a cross-sectional view of a pressure-sensitive adhesive sheet according to another embodiment of the present invention.

[0022] Hereinafter, an embodiment of the present invention will be described. [Adhesive] An adhesive according to one embodiment of the present invention is an adhesive for use in recycling the adhesive itself, and preferably has a gel fraction (hereinafter sometimes referred to as "gel fraction G1") of 25% or more after immersion in ethyl acetate for 72 hours, and a gel fraction (hereinafter sometimes referred to as "gel fraction G2") of 25% or less after immersion in a solvent containing a mixture of ethyl acetate and acetylacetone in a mass ratio of 9:1 for 72 hours. The method for measuring the gel fraction in this specification is as shown in the test examples described below.

[0023] The pressure-sensitive adhesive according to the present embodiment has a gel fraction G1 as described above, which allows it to exhibit a suitable cohesive strength as a pressure-sensitive adhesive layer and, after dissolving in a solvent, to be reformed into a pressure-sensitive adhesive layer having a suitable cohesive strength. Furthermore, the pressure-sensitive adhesive according to the present embodiment has a gel fraction G2 as described above, which allows it to be easily dissolved in a solvent, particularly a solvent containing acetylacetone. That is, the pressure-sensitive adhesive according to the present embodiment is easily dissolved in a solvent and can then be reformed into a pressure-sensitive adhesive layer, allowing the pressure-sensitive adhesive itself to be recycled. Furthermore, the pressure-sensitive adhesive according to the present embodiment is easily dissolved in a solvent, which allows it to be easily removed from an adherend or substrate to which the pressure-sensitive adhesive layer is attached, thereby enabling the recycling of the adherend or substrate (limited to those that are not soluble in the above solvent).

[0024] From the above viewpoints, the gel fraction G1 is preferably 30% or more, more preferably 35% or more, particularly preferably 40% or more, even more preferably 45% or more, and of these, preferably 50% or more. Furthermore, from the viewpoint of making it easier to set the gel fraction G2 within the above range, the upper limit of the gel fraction G1 is preferably 100% or less, more preferably 95% or less, particularly preferably 90% or less, and even more preferably 85% or less.

[0025] From the same viewpoint, the gel fraction G2 is preferably 22% or less, more preferably 20% or less, particularly preferably 18% or less, and even more preferably 17% or less. Also, from the viewpoint of making it easier to set the gel fraction G1 within the above range, the lower limit of the gel fraction G2 is preferably 0% or more, more preferably 1% or more, particularly preferably 2% or more, and even more preferably 3% or more.

[0026] The pressure-sensitive adhesive according to the present embodiment preferably has a gel fraction (hereinafter sometimes referred to as "gel fraction G3") of 25% or more after being dissolved in a solvent containing ethyl acetate and acetylacetone in a 1:1 mass ratio, dried at 120°C for 1 minute, and then immersed in ethyl acetate for 72 hours. Because the pressure-sensitive adhesive according to the present embodiment has the above-mentioned gel fraction G3, it can be reconstituted into a pressure-sensitive adhesive layer having suitable cohesive strength after being dissolved in a solvent, particularly a solvent containing acetylacetone.

[0027] From the above viewpoints, the gel fraction G3 is preferably 30% or more, more preferably 35% or more, particularly preferably 40% or more, even more preferably 45% or more, of which 50% or more is preferable, and especially preferably 55% or more. Furthermore, from the viewpoint of making it easier to set the gel fraction G2 within the above range, the upper limit of the gel fraction G3 is preferably 100% or less, more preferably 95% or less, particularly preferably 90% or less, and even more preferably 85% or less.

[0028] As a pressure-sensitive adhesive having the above physical properties, one that is crosslinked by a metal chelate crosslinking agent is preferred. The metal chelate crosslinking agent bonds with the base polymer of the pressure-sensitive adhesive through a coordinate bond to form a crosslinked structure. This coordinate bond is broken by the action of a specific solvent, particularly a solvent containing acetylacetone. Therefore, by contacting (immersing, etc.) the pressure-sensitive adhesive with the solvent, the metal chelate crosslinking agent and the base polymer separate, and the pressure-sensitive adhesive dissolves in the solvent. Then, by removing the solvent from the solution in which the pressure-sensitive adhesive has dissolved, the base polymer and the metal chelate crosslinking agent bond again through a coordinate bond to form a crosslinked structure. This re-forms a pressure-sensitive adhesive layer with suitable cohesive strength.

[0029] The solvent for dissolving the PSA (layer) during recycling is preferably acetylacetone or a mixed solvent containing acetylacetone. In the mixed solvent containing acetylacetone, components other than acetylacetone include, for example, aliphatic hydrocarbons such as hexane and heptane, aromatic hydrocarbons such as toluene and xylene, halogenated hydrocarbons such as methylene chloride and ethylene chloride, alcohols such as methanol, ethanol, propanol and butanol, ketones such as acetone, methyl ethyl ketone, 2-pentanone, diisobutyl ketone, cyclohexanone, isophorone and cyclohexanone, esters such as ethyl acetate and butyl acetate, cellosolves such as ethyl cellosolve, ethylene glycol mono-n-butyl ether, ethylene glycol mono-t-butyl ether and propylene glycol monomethyl ether, glycol ethers such as 3-methoxy-3-methylbutanol, 3-methoxy-3-methylbutyl acetate, diethylene glycol monobutyl ether and 1-ethoxy-2-propanol, and the like. One type can be used alone, or two or more types can be used in combination. Among the above, from the viewpoint of efficient solubility of the pressure-sensitive adhesive (layer), esters such as ethyl acetate and butyl acetate are preferred, with ethyl acetate being particularly preferred.

[0030] The mixing ratio (mass ratio) of acetylacetone to a component other than acetylacetone is preferably 0.1:99.9 to 99.9:0.1, more preferably 0.4:99.6 to 99.6:0.4, particularly preferably 0.8:99.2 to 99.2:0.8, and further preferably 1:99 to 99:1.

[0031] Examples of types of pressure-sensitive adhesives include acrylic pressure-sensitive adhesives, polyester pressure-sensitive adhesives, polyurethane pressure-sensitive adhesives, rubber pressure-sensitive adhesives, silicone pressure-sensitive adhesives, etc. Among these, acrylic pressure-sensitive adhesives are preferred. Acrylic pressure-sensitive adhesives crosslinked with a metal chelate crosslinking agent easily satisfy the above-mentioned physical properties, and can also exhibit good adhesive strength in the pressure-sensitive adhesive layer after recycling.

[0032] The acrylic adhesive may be any of emulsion type, solvent type, or solventless type, and may be either non-curable or curable by active energy rays. Among the above, from the viewpoint of easily satisfying the above-mentioned physical properties, solvent type or solventless type is preferable, and active energy ray non-curable is particularly preferable. Note that, from the viewpoint of SDGs, the material constituting the adhesive may be a material with a high biomass content, a material that can be recycled or reused, or a recycled or reused material.

[0033] The adhesive strength of a 25 μm thick, 25 mm wide adhesive layer made of the adhesive of this embodiment to soda lime glass is preferably 1 N / 25 mm or more, more preferably 5 N / 25 mm or more, and in the case of a solvent-based acrylic adhesive, it is preferably 10 N / 25 mm or more, particularly preferably 14 N / 25 mm or more, and even more preferably 17 N / 25 mm or more, and in the case of a solventless acrylic adhesive, it is preferably 10 N / 25 mm or more, particularly preferably 20 N / 25 mm or more, and even more preferably 30 N / 25 mm or more, of which 40 N / 25 mm or more is preferred, and especially preferably 50 N / 25 mm or more. This allows for good adhesive strength to be exhibited to a variety of adherends.

[0034] The adhesive strength is preferably 120 N / 25 mm or less, more preferably 95 N / 25 mm or less, and particularly preferably 75 N / 25 mm or less, and in the case of a solvent-based acrylic adhesive, it is preferably 55 N / 25 mm or less, more preferably 40 N / 25 mm or less, more preferably 30 N / 25 mm or less, and especially preferably 22 N / 25 mm or less, thereby achieving good reworkability.

[0035] Here, the adhesive strength in this specification basically refers to the adhesive strength measured by the 180-degree peel method in accordance with JIS Z0237:2009, and the measurement sample is 25 mm wide and 100 mm long, and the measurement sample is attached to an adherend, pressurized at 0.5 MPa and 50°C for 20 minutes, and then left to stand for 24 hours under conditions of normal pressure, 23°C, and 50% RH, and then measured at a peel speed of 300 mm / min.

[0036] The adhesive according to the present embodiment can obtain a relatively high adhesive strength by using a solvent-free acrylic adhesive, and can obtain a relatively low adhesive strength by using a solvent-based acrylic adhesive. In this way, by changing the type (polymerization method) of the acrylic adhesive, it is possible to control the adhesive strength to a desired level.

[0037] The adhesive layer of this embodiment, which is 25 μm thick and 25 mm wide, is formed by dissolving the adhesive (layer) in a 1:1 mass ratio of ethyl acetate and acetylacetone and then drying at 120° C. for 1 minute. The adhesive strength to soda-lime glass of this adhesive layer is preferably 1 N / 25 mm or more, more preferably 5 N / 25 mm or more. In the case of a solvent-based acrylic adhesive, it is preferably 10 N / 25 mm or more, particularly preferably 12 N / 25 mm or more, and even more preferably 15 N / 25 mm or more. In the case of a solventless acrylic adhesive, it is preferably 10 N / 25 mm or more, particularly preferably 20 N / 25 mm or more, and even more preferably 30 N / 25 mm or more, of which 40 N / 25 mm or more is preferred, and especially preferably 50 N / 25 mm or more. This allows the adhesive layer, even when re-formed after solvent dissolution, to exhibit good adhesive strength to various adherends.

[0038] The adhesive strength is preferably 120 N / 25 mm or less, more preferably 95 N / 25 mm or less, and particularly preferably 75 N / 25 mm or less, and in the case of a solvent-based acrylic adhesive, it is even more preferably 55 N / 25 mm or less, more preferably 40 N / 25 mm or less, and particularly preferably 30 N / 25 mm or less. This allows good reworkability even for an adhesive layer reformed after dissolution in a solvent.

[0039] Specifically, the pressure-sensitive adhesive in this embodiment is preferably a (meth)acrylic acid ester polymer (A) (corresponding to the main polymer of an acrylic pressure-sensitive adhesive) crosslinked with a metal chelate crosslinking agent (B). In this specification, (meth)acrylic acid means both acrylic acid and methacrylic acid. The same applies to other similar terms. Furthermore, the term "polymer" also includes the concept of "copolymer."

[0040] 1. Components (1) (Meth)acrylic acid ester polymer (A) The (meth)acrylic acid ester polymer (A) may be a polymer obtained by solution polymerization (solution polymer), a polymer obtained by polymerization without a solvent (solventless polymer), or an emulsion polymer. Among these, from the viewpoint of facilitating the attainment of the above-mentioned physical properties, a solution polymer or a solventless polymer is preferred. In the case of solution polymerization, a pressure-sensitive adhesive having a relatively low adhesive strength can be obtained, and in the case of solventless polymerization, a pressure-sensitive adhesive having a relatively high adhesive strength can be obtained.

[0041] The (meth)acrylic acid ester polymer (A) preferably contains a (meth)acrylic acid alkyl ester as a monomer unit constituting the polymer. This allows the polymer to exhibit good adhesiveness. The alkyl group may be linear or branched, or may have a cyclic structure.

[0042] From the viewpoint of adhesiveness, the (meth)acrylic acid alkyl ester is preferably a (meth)acrylic acid alkyl ester having an alkyl group with a carbon number of 1 to 20. Examples of (meth)acrylic acid alkyl esters having an alkyl group with a carbon number of 1 to 20 include methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, n-butyl (meth)acrylate, n-pentyl (meth)acrylate, isopentyl (meth)acrylate, n-hexyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, n-octyl (meth)acrylate, isooctyl (meth)acrylate, n-decyl (meth)acrylate, n-dodecyl (meth)acrylate, myristyl (meth)acrylate, palmityl (meth)acrylate, and stearyl (meth)acrylate. Among these, from the viewpoint of easily satisfying the above-mentioned physical properties, (meth)acrylic acid esters having an alkyl group containing 2 to 8 carbon atoms are preferred, with methyl (meth)acrylate, ethyl (meth)acrylate, n-butyl (meth)acrylate, and 2-ethylhexyl (meth)acrylate being particularly preferred, and methyl acrylate, ethyl acrylate, n-butyl acrylate, and 2-ethylhexyl acrylate being even more preferred. These may be used alone or in combination of two or more.

[0043] Among the above, in the case of solution polymerization, it is preferable to use a combination of n-butyl acrylate and methyl acrylate, or n-butyl acrylate alone. When n-butyl acrylate and methyl acrylate are used in combination, the blending ratio (by mass) thereof is preferably 99:1 to 1:1, more preferably 19:1 to 3:2, particularly preferably 9:1 to 7:3, and further preferably 17:3 to 3:1.

[0044] On the other hand, in the case of solventless polymerization, it is preferable to use a combination of 2-ethylhexyl acrylate and ethyl acrylate. When 2-ethylhexyl acrylate and ethyl acrylate are used in combination, the blending ratio (by mass) thereof is preferably 1:99 to 3:2, more preferably 1:19 to 1:1, particularly preferably 1:9 to 2:3, further preferably 3:17 to 7:13, and of these, preferably 1:4 to 3:7.

[0045] The (meth)acrylic acid ester polymer (A) preferably contains 70 to 100 mass% of (meth)acrylic acid alkyl ester as a monomer unit constituting the polymer, more preferably 75 to 99.5 mass%, and particularly preferably 80 to 99 mass%. In the case of solution polymerization, it is preferably contained in 84 to 98 mass%, particularly preferably 88 to 97 mass%, and even more preferably 92 to 96 mass%. In the case of solventless polymerization, it is preferably contained in 90 to 98.8 mass%, particularly preferably 90 to 98.6 mass%, and even more preferably 95 to 98.5 mass%. This makes it easier to satisfy the above-mentioned physical properties and allows other monomer components such as functional group-containing monomers to be introduced into the (meth)acrylic acid ester polymer (A) in suitable amounts.

[0046] The (meth)acrylic acid ester polymer (A) preferably contains a functional group-containing monomer having a functional group in the molecule as a monomer unit constituting the polymer. The presence of the functional group derived from the functional group-containing monomer allows the (meth)acrylic acid ester polymer (A) and the metal chelate crosslinking agent (B) to react and bond to form a crosslinked structure. In this case, the bond is a coordinate bond.

[0047] As the functional group-containing monomer, a monomer having a carboxy group in the molecule (carboxy group-containing monomer) is preferred from the viewpoint of reactivity with the metal chelate crosslinking agent (B). Further, as the functional group-containing monomer, a monomer having a functional group such as an amino group, a cyano group, or a sulfonyl group in the molecule can also be used.

[0048] Examples of carboxyl group-containing monomers include ethylenically unsaturated carboxylic acids such as acrylic acid, methacrylic acid, crotonic acid, maleic acid, itaconic acid, and citraconic acid. Among them, acrylic acid or methacrylic acid is preferred from the viewpoint of reactivity with the metal chelate crosslinking agent (B), and acrylic acid is particularly preferred. These may be used alone or in combination of two or more.

[0049] The (meth)acrylic acid ester polymer (A) preferably contains 0.1 to 30% by mass of functional group-containing monomer as a monomer unit constituting the polymer, more preferably 0.4 to 26% by mass, particularly preferably 0.8 to 22% by mass, and even more preferably 1 to 18% by mass. In the case of solution polymerization, the content is preferably 2 to 15% by mass, particularly preferably 3 to 13% by mass, and even more preferably 4 to 11% by mass. In the case of solventless polymerization, the content is preferably 1.2 to 12% by mass, particularly preferably 1.3 to 8% by mass, and even more preferably 1.4 to 4% by mass. This makes it possible to effectively improve the cohesion of the coating while maintaining good adhesion.

[0050] The (meth)acrylic acid ester polymer (A) may contain other monomers as monomer units constituting the polymer, if desired. Examples of other monomers include monomers containing a nitrogen atom in the molecule, monomers having an alicyclic structure in the molecule, monomers having an aromatic ring in the molecule, vinyl acetate, styrene, etc. These may be used alone or in combination of two or more.

[0051] From the viewpoint of the water resistance of the pressure-sensitive adhesive, it is preferable that the (meth)acrylic acid ester polymer (A) does not contain a (meth)acrylic acid alkoxyalkyl ester as a monomer unit constituting the polymer. Even if it does contain such an alkoxyalkyl ester, the content thereof is preferably 0.1% by mass or less, more preferably 0.01% by mass or less, and even more preferably 0.001% by mass or less.

[0052] The (meth)acrylic acid ester polymer (A) is preferably a linear polymer, since the linear polymer makes it easier for molecular chains to become entangled, and is expected to improve cohesive strength.

[0053] The polymerization mode of the (meth)acrylic acid ester polymer (A) may be a random copolymer or a block copolymer.

[0054] The weight-average molecular weight of the (meth)acrylic acid ester polymer (A) is preferably 100,000 to 3,000,000, more preferably 180,000 to 2,200,000, particularly preferably 260,000 to 1,500,000, further preferably 320,000 to 1,150,000, and of these, preferably 380,000 to 900,000. This makes it easier to satisfy the above-mentioned physical properties and improves solvent solubility. The weight-average molecular weight of the (meth)acrylic acid ester polymer in this specification is a value measured by gel permeation chromatography (GPC) in terms of standard polystyrene.

[0055] In the pressure-sensitive adhesive according to this embodiment, the (meth)acrylic acid ester polymer (A) may be used alone or in combination of two or more.

[0056] The content of the (meth)acrylic acid ester polymer (A) in the pressure-sensitive adhesive according to this embodiment is preferably 70 to 99.99% by mass, more preferably 80 to 99.95% by mass, particularly preferably 90 to 99.9% by mass, even more preferably 95 to 99.8% by mass, and most preferably 98 to 99.7% by mass, which allows for obtaining good adhesive strength while ensuring the content of the metal chelate crosslinker (B).

[0057] (2) Metal Chelate Crosslinking Agent (B) The metal chelate crosslinking agent (B) may be any metal chelate compound capable of forming a coordinate bond with the base polymer ((meth)acrylic acid ester polymer (A)) of the adhesive, and examples thereof include metal chelate compounds whose metal atoms are aluminum, zinc, iron, zirconium, titanium, tin, etc. Specific examples thereof include aluminum chelate compounds, zinc chelate compounds, iron chelate compounds, zirconium chelate compounds, titanium chelate compounds, tin chelate compounds, etc.

[0058] From the viewpoints of solvent solubility and adhesive re-formability, the metal chelate crosslinking agent (B) is preferably a metal chelate compound having a β-dicarbonyl compound as a ligand. Examples of the β-dicarbonyl compound include β-diketones such as acetylacetone and benzoylacetone, and β-ketoesters such as ethyl acetoacetate. Among these, acetylacetone or ethyl acetoacetate is preferred, with acetylacetone being particularly preferred.

[0059] Examples of aluminum chelate compounds include aluminum tris(acetylacetonate), monoacetylacetonate aluminum bis(isobutylacetoacetate) chelate, monoacetylacetonate aluminum bis(2-ethylhexylacetoacetate) chelate, monoacetylacetonate aluminum bis(dodecylacetoacetate) chelate, and monoacetylacetonate aluminum bis(oleylacetoacetate) chelate. These may be used alone or in combination of two or more. Of the above, aluminum tris(acetylacetonate) is particularly preferred from the viewpoints of solvent solubility and adhesive re-formability.

[0060] Examples of zinc chelate compounds include zinc(II) acetylacetonate, zinc(II) ethylacetoacetate, zinc(II) hexafluoroacetylacetonate, etc. Among these, zinc bis(acetylacetonate) is particularly preferred from the viewpoints of solvent solubility and adhesive re-formability.

[0061] Examples of iron chelate compounds include iron(III) acetylacetonate, tris(2,4-pentanedionato)iron(III), etc. Among these, iron(III) acetylacetonate is particularly preferred from the viewpoints of solvent solubility and adhesive re-formability.

[0062] Examples of metal chelate compounds other than those mentioned above include zirconium chelate compounds such as zirconium (IV) acetylacetonate, and titanium chelate compounds such as titanium tetrakis(acetylacetonate).

[0063] The content of the metal chelate crosslinker (B) in the pressure-sensitive adhesive according to this embodiment is preferably 0.01 to 10 parts by mass, more preferably 0.05 to 8 parts by mass, particularly preferably 0.1 to 4 parts by mass, and even more preferably 0.15 to 2 parts by mass, relative to 100 parts by mass of the base polymer ((meth)acrylic acid ester polymer (A)) of the pressure-sensitive adhesive. Furthermore, in the case of a solvent-based acrylic pressure-sensitive adhesive, the content is preferably 0.2 to 1 part by mass, more preferably 0.24 to 0.7 parts by mass, and particularly preferably 0.28 to 0.5 parts by mass. In the case of a solventless acrylic pressure-sensitive adhesive, the content is preferably 0.4 to 1.7 parts by mass, more preferably 0.7 to 1.5 parts by mass, and particularly preferably 1 to 1.3 parts by mass. This makes it easier to satisfy the above-mentioned physical properties, resulting in better solvent solubility and pressure-sensitive adhesive re-formability.

[0064] (3) Photopolymerization initiator (C) When ultraviolet light is used as the active energy ray for irradiating the (meth)acrylic acid ester polymer (A) in the solvent-free polymerization, it is preferable to further use a photopolymerization initiator (C). This allows efficient copolymerization without leaving any monomers constituting the (meth)acrylic acid ester polymer (A) remaining in the pressure-sensitive adhesive, and also reduces the polymerization and curing time and the exposure dose of the active energy ray.

[0065] Examples of the photopolymerization initiator (C) include benzoin, benzoin methyl ether, benzoin ethyl ether, benzoin isopropyl ether, benzoin-n-butyl ether, benzoin isobutyl ether, acetophenone, dimethylaminoacetophenone, 2,2-dimethoxy-2-phenylacetophenone, 2,2-diethoxy-2-phenylacetophenone, 2-hydroxy-2-methyl-1-phenylpropan-1-one, 1-hydroxycyclohexyl phenyl ketone, 2-methyl-1-[4-(methylthio)phenyl]-2-morpholino-propan-1-one, 4-(2-hydroxyethoxy)phenyl-2-(hydroxy-2-propyl)ketone, benzophenone, p-phenylbenzophenone, 4,4 Examples of suitable benzoxanthraquinones include 2-methyl-2-hydroxybenzoyl-1-methyl-1-methyl-2-methyl-1 ...

[0066] The amount of the photopolymerization initiator (C) used is preferably 0.01 to 10 parts by mass, more preferably 0.05 to 6 parts by mass, and even more preferably 0.1 to 3 parts by mass, particularly preferably 0.5 to 2 parts by mass, and even more preferably 0.8 to 1.5 parts by mass, relative to 100 parts by mass of the total amount of the monomers constituting the acrylic acid ester polymer (A). This makes it easier for the resulting pressure-sensitive adhesive to satisfy the above-mentioned physical properties.

[0067] (4) Other Components The adhesive according to the present embodiment may contain, if desired, various additives that are commonly used in acrylic adhesives, such as a silane coupling agent, an anti-rust agent, an ultraviolet absorber, an infrared absorber, an antistatic agent, a tackifier, an antioxidant, a light stabilizer, a softener, a refractive index adjuster, a colorant, and a filler.

[0068] 2. Production of Pressure-Sensitive Adhesive (1) In the Case of Solvent-Type Pressure-Sensitive Adhesive The solvent-type pressure-sensitive adhesive according to the present embodiment can be produced by preparing a pressure-sensitive adhesive composition (coating liquid) containing a (meth)acrylic acid ester polymer (A) and a metal chelate crosslinking agent (B), applying the pressure-sensitive adhesive composition to a desired object, and then crosslinking the (meth)acrylic acid ester polymer (A) and the metal chelate crosslinking agent (B).

[0069] The above-mentioned adhesive composition can be prepared by producing a (meth)acrylic acid ester polymer (A) by a solution polymerization method, mixing the obtained (meth)acrylic acid ester polymer (A) with a metal chelate crosslinking agent (B), and adding additives, etc., as desired.

[0070] When the (meth)acrylic acid ester polymer (A) is polymerized by solution polymerization, it is preferable to add the monomers constituting the polymer and, if desired, a polymerization initiator to a polymerization solvent and polymerize them by radical polymerization.

[0071] Examples of the polymerization solvent include ethyl acetate, n-butyl acetate, isobutyl acetate, toluene, acetone, hexane, and methyl ethyl ketone, and two or more of these may be used in combination.

[0072] Examples of the polymerization initiator include azo compounds, organic peroxides, etc., and two or more of them may be used in combination. In the polymerization step, the weight average molecular weight of the resulting polymer can be adjusted by adding a chain transfer agent such as 2-mercaptoethanol.

[0073] Once the (meth)acrylic acid ester polymer (A) is obtained as described above, the metal chelate crosslinking agent (B), and, if desired, additives, a dilution solvent, and the like are added to a solution of the (meth)acrylic acid ester polymer (A), and the mixture is thoroughly mixed to obtain a pressure-sensitive adhesive composition (coating liquid) diluted with a solvent.

[0074] Examples of the dilution solvent include aliphatic hydrocarbons such as hexane, heptane, and cyclohexane; aromatic hydrocarbons such as toluene and xylene; halogenated hydrocarbons such as methylene chloride and ethylene chloride; alcohols such as methanol, ethanol, propanol, butanol, and 1-methoxy-2-propanol; ketones such as acetone, methyl ethyl ketone, 2-pentanone, isophorone, and cyclohexanone; esters such as ethyl acetate and butyl acetate; and cellosolve-based solvents such as ethyl cellosolve.

[0075] The concentration and viscosity of the coating solution prepared in this manner are not particularly limited as long as they are within a range that allows coating, and can be appropriately selected depending on the situation. For example, the adhesive is diluted to a concentration of 10 to 60 mass %. The addition of a dilution solvent or the like is not a necessary condition for obtaining the coating solution, and as long as the adhesive has a viscosity that allows coating, the addition of a dilution solvent is not necessary. In this case, the adhesive becomes a coating solution in which the polymerization solvent for the (meth)acrylic acid ester polymer (A) is used as the dilution solvent.

[0076] After the adhesive composition is applied to a desired object, a heat treatment is preferably carried out to crosslink the (meth)acrylic acid ester polymer (A) and the metal chelate crosslinking agent (B).

[0077] The heating temperature in the heat treatment is preferably 60 to 150° C., particularly preferably 80 to 120° C. The heating time in the heat treatment is preferably 10 seconds to 10 minutes, particularly preferably 30 seconds to 5 minutes. This heat treatment can also serve as a drying treatment after application of the pressure-sensitive adhesive composition.

[0078] (2) In the case of solvent-free pressure-sensitive adhesive The solvent-free pressure-sensitive adhesive according to this embodiment can be produced by preparing a pressure-sensitive adhesive composition (coating liquid) containing an acrylic syrup containing a (meth)acrylic acid ester polymer (A) and monomers constituting the polymer, and a metal chelate crosslinking agent (B), applying the pressure-sensitive adhesive composition to a desired object, and then polymerizing the monomers in the acrylic syrup to obtain the (meth)acrylic acid ester polymer (A) and crosslinking the (meth)acrylic acid ester polymer (A) with the metal chelate crosslinking agent (B).

[0079] When the (meth)acrylic acid ester polymer (A) is polymerized in the absence of a solvent, it is preferable to irradiate a mixed liquid containing the monomers constituting the polymer and, if desired, the photopolymerization initiator (C) with active energy rays to polymerize a portion of the monomers, thereby preparing an acrylic syrup containing the (meth)acrylic acid ester polymer (A) and the monomers constituting the polymer.

[0080] The active energy ray refers to an electromagnetic wave or a charged particle beam that has an energy quantum, and specific examples thereof include ultraviolet rays, electron beams, etc. Among active energy rays, ultraviolet rays are particularly preferred because they are easy to handle.

[0081] The ultraviolet irradiation can be performed using a high-pressure mercury lamp, a Heraeus H lamp, a xenon lamp, or the like, and the amount of ultraviolet irradiation is set to an illuminance of 50 to 1000 mW / cm. ２ The light intensity is preferably 50 to 10,000 mJ / cm ２ is preferably 100 to 7000 mJ / cm ２ More preferably, it is 200 to 4000 mJ / cm ２ On the other hand, the electron beam irradiation can be carried out by an electron beam accelerator or the like, and the irradiation dose of the electron beam is preferably 10 to 1000 krad.

[0082] Once the acrylic syrup is obtained as described above, the metal chelate crosslinking agent (B) and, if desired, additives and the like are added and thoroughly mixed to obtain a pressure-sensitive adhesive composition (coating liquid).

[0083] After applying the pressure-sensitive adhesive composition to a desired object, it is preferable to again irradiate the composition with active energy rays in the same manner as above to polymerize the monomers in the acrylic syrup to obtain the (meth)acrylic acid ester polymer (A). Furthermore, a heat treatment may be performed to crosslink the (meth)acrylic acid ester polymer (A) with the metal chelate crosslinking agent (B). The heating temperature and heating time of the heat treatment are the same as those described for the solvent-based pressure-sensitive adhesive. The above-mentioned active energy ray irradiation and heat treatment may be performed separately or simultaneously.

[0084] 3. Physical Properties (Storage Modulus G') The storage modulus G' of the pressure-sensitive adhesive according to this embodiment at 23°C is preferably 0.001 to 2 MPa, more preferably 0.01 to 1 MPa, particularly preferably 0.05 to 0.5 MPa, and even more preferably 0.1 to 0.2 MPa. This allows for better solvent solubility and adhesive strength.

[0085] The storage modulus G' at 85°C of the pressure-sensitive adhesive according to this embodiment is preferably 0.001 to 1 MPa, more preferably 0.005 to 0.5 MPa, particularly preferably 0.01 to 0.2 MPa, and even more preferably 0.03 to 0.1 MPa. This allows for better solvent solubility and adhesive strength. The method for measuring the storage modulus in this specification is as shown in the test examples described below.

[0086] [Adhesive according to another embodiment] An adhesive according to another embodiment of the present invention is an adhesive crosslinked by a metal chelate crosslinking agent, and preferably has a gel fraction of 25% or more after immersion in ethyl acetate for 72 hours, and preferably has a gel fraction of 25% or less after immersion in a solvent in which ethyl acetate and acetylacetone are mixed at a mass ratio of 9:1 for 72 hours. The adhesive according to this embodiment is not limited to applications in which the adhesive itself is recycled, but the adhesive itself can be recycled.

[0087] [Adhesive Sheet] An adhesive sheet according to one embodiment of the present invention comprises at least an adhesive layer, and preferably comprises a release sheet laminated on one or both sides of the adhesive layer. A specific configuration of an example of the adhesive sheet according to this embodiment is shown in Figures 1 and 2.

[0088] 1. Structure As shown in FIG. 1 , the pressure-sensitive adhesive sheet 1A according to the first embodiment is composed of, from the bottom up, a release sheet 12, a pressure-sensitive adhesive layer 11 laminated on the release surface of the release sheet 12, and a substrate 13 laminated on the pressure-sensitive adhesive layer 11.

[0089] 2, the pressure-sensitive adhesive sheet 1B according to the second embodiment is composed of two release sheets 12a and 12b and a pressure-sensitive adhesive layer 11 sandwiched between the two release sheets 12a and 12b so as to be in contact with the release surfaces of the two release sheets 12a and 12b. In this specification, the release surface of a release sheet refers to the surface of the release sheet that has releasability, and includes both a surface that has been subjected to a release treatment and a surface that exhibits releasability even without being subjected to a release treatment.

[0090] In both pressure-sensitive adhesive sheets 1A and 1B, the pressure-sensitive adhesive layer 11 is made of the pressure-sensitive adhesive described above. The thickness of the pressure-sensitive adhesive layer 11 (measured in accordance with JIS K7130) is determined appropriately depending on the intended use of the pressure-sensitive adhesive sheets 1A and 1B, but is preferably 1 to 2000 μm, more preferably 4 to 1000 μm, particularly preferably 8 to 500 μm, even more preferably 12 to 100 μm, and even more preferably 15 to 60 μm, and especially preferably 20 to 35 μm. This allows for good adhesive strength and improved solvent solubility, improving the recyclability of the pressure-sensitive adhesive. The pressure-sensitive adhesive layer 11 may be formed as a single layer or as a laminate of multiple layers.

[0091] The substrate 13 is not particularly limited, and any substrate used as a substrate sheet for a normal pressure-sensitive adhesive sheet can be used. Examples of the plastic film include polyester films such as polyethylene terephthalate, polybutylene terephthalate, and polyethylene naphthalate; polyolefin films such as polyethylene and polypropylene; cellophane, diacetyl cellulose film, triacetyl cellulose film, acetyl cellulose butyrate film, polyvinyl chloride film, polyvinylidene chloride film, polyvinyl alcohol film, ethylene-vinyl acetate copolymer film, polystyrene film, polycarbonate film, polymethylpentene film, polysulfone film, polyether ether ketone film, polyether sulfone film, polyetherimide film, fluororesin film, polyamide film, acrylic resin film, polyurethane resin film, norbornene polymer film, cyclic olefin polymer film, cyclic conjugated diene polymer film, and vinyl alicyclic hydrocarbon polymer film, or laminated films thereof; woven or nonwoven fabrics using fibers such as rayon, acrylic, and polyester; papers such as fine paper, glassine paper, impregnated paper, and coated paper; metal foils such as aluminum and copper; foams such as urethane foam and polyethylene foam; and laminates of two or more of these. From the perspective of the SDGs, the substrate 13 may be made of a material with a high biomass content, a material that can be recycled or reused, or a recycled or reused material.

[0092] The substrate 13 may be a desired optical member, such as a polarizing plate (polarizing film), a polarizer, a retardation plate (retardation film), a viewing angle compensation film, a brightness enhancement film, a contrast enhancement film, a liquid crystal polymer film, a diffusion film, or a semi-transmissive reflective film.

[0093] The thickness of the substrate 13 varies depending on the type and application, but is usually preferably 10 to 300 μm, particularly preferably 20 to 200 μm, and further preferably 30 to 100 μm.

[0094] The release sheets 12, 12a, and 12b protect the adhesive layer 11 until the adhesive sheet 1 is used, and are peeled off when the adhesive sheet 1 (adhesive layer 11) is used.

[0095] Examples of materials that can be used as the release sheets 12, 12a, and 12b include polyethylene films, polypropylene films, polybutene films, polybutadiene films, polymethylpentene films, polyvinyl chloride films, vinyl chloride copolymer films, polyethylene terephthalate films, polyethylene naphthalate films, polybutylene terephthalate films, polyurethane films, ethylene vinyl acetate films, ionomer resin films, ethylene-(meth)acrylic acid copolymer films, ethylene-(meth)acrylic acid ester copolymer films, polystyrene films, polycarbonate films, polyimide films, and fluororesin films. Crosslinked films of these materials can also be used. Furthermore, laminated films of these materials may also be used. From the perspective of the SDGs, the release sheets 12, 12a, and 12b may be made of materials with a high biomass content, recyclable or reusable materials, or recycled or reused materials.

[0096] The release surfaces of the release sheets 12, 12a, and 12b (particularly the surfaces that come into contact with the pressure-sensitive adhesive layer 11) are preferably subjected to a release treatment. Examples of release agents used for the release treatment include alkyd-based, silicone-based, fluorine-based, unsaturated polyester-based, polyolefin-based, and wax-based release agents. It is preferable that one of the release sheets 12a and 12b be a heavy-release type release sheet with a high release strength, and the other be a light-release type release sheet with a low release strength.

[0097] There are no particular restrictions on the thickness of the release sheets 12, 12a, 12b, but it is generally preferred that the thickness be 20 to 200 μm, and more preferably 30 to 120 μm.

[0098] 2. Manufacturing Method (1) For Solvent-Based Pressure-Sensitive Adhesives To manufacture the pressure-sensitive adhesive sheet 1A, preferably, a coating liquid of the pressure-sensitive adhesive composition described above is applied to the release surface of the release sheet 12 to form a coating layer, and then the substrate 13 is laminated on the coating layer, and the pressure-sensitive adhesive composition in the coating layer is crosslinked to form the pressure-sensitive adhesive layer 11. Alternatively, a coating liquid of the pressure-sensitive adhesive composition described above is applied to one surface of the substrate 13 to form a coating layer, and then the release surface of the release sheet 12 is laminated on the coating layer, and then the pressure-sensitive adhesive composition in the coating layer is crosslinked to form the pressure-sensitive adhesive layer 11. Crosslinking of the pressure-sensitive adhesive composition is preferably carried out by the heat treatment described above.

[0099] In addition, to manufacture the above-mentioned adhesive sheet 1B, a coating solution of the adhesive composition described above is applied to the release surface of one of the release sheets 12a (or 12b) to form a coating layer, and then the release surface of the other release sheet 12b (or 12a) is superimposed on the coating layer, and the adhesive composition in the coating layer is crosslinked to form the adhesive layer 11.

[0100] The coating liquid can be applied by, for example, bar coating, knife coating, roll coating, blade coating, die coating, gravure coating, or the like.

[0101] (2) In the case of a solventless adhesive To produce the adhesive sheet 1A, preferably, a coating solution of the adhesive composition described above is applied to the release surface of the release sheet 12 to form a coating layer, and then the substrate 13 is laminated on the coating layer, and the adhesive composition in the coating layer is polymerized and crosslinked to form the adhesive layer 11. Alternatively, a coating solution of the adhesive composition described above is applied to one side of the substrate 13 to form a coating layer, and then the release surface of the release sheet 12 is laminated on the coating layer, and then the adhesive composition in the coating layer is polymerized and crosslinked to form the adhesive layer 11. The polymerization of the adhesive composition is preferably carried out by irradiation with the above-mentioned active energy rays. The crosslinking of the adhesive composition may also be carried out by the above-mentioned heat treatment.

[0102] In addition, to manufacture the above-mentioned adhesive sheet 1B, a coating solution of the adhesive composition described above is applied to the release surface of one of the release sheets 12a (or 12b) to form a coating layer, and then the release surface of the other release sheet 12b (or 12a) is superimposed on the coating layer, and the adhesive composition of the coating layer is polymerized and crosslinked to form the adhesive layer 11.

[0103] 3. Physical Properties (1) Chromaticity b* The chromaticity b* of the pressure-sensitive adhesive layer 11 in this embodiment, as defined by the CIE 1976 L*a*b* color system, is preferably -10 to 10, more preferably -5 to 5, particularly preferably -1 to 1, and even more preferably -0.1 to 0.1. This gives the pressure-sensitive adhesive layer a color that is close to colorless and transparent, making it suitable for optical applications (for displays). The method for measuring chromaticity b* in this specification is as shown in the test examples described below.

[0104] In the case where the pressure-sensitive adhesive layer 11 of this embodiment is dissolved in a solvent and a new pressure-sensitive adhesive layer is formed, the chromaticity b* of the pressure-sensitive adhesive layer is preferably in the same range as above.

[0105] (2) Haze Value The haze value of the pressure-sensitive adhesive layer 11 in this embodiment is preferably 10% or less, more preferably 5% or less, particularly preferably 1% or less, and even more preferably 0.5% or less. This provides very high transparency and makes the pressure-sensitive adhesive layer 11 suitable for optical applications (displays). On the other hand, the lower limit of the haze value of the pressure-sensitive adhesive layer 11 is not particularly restricted. The lower limit may be 0%, but is usually about 0.01% due to factors such as measurement accuracy. Here, the haze value in this specification is a value measured in accordance with JIS K7136:2000.

[0106] In the case where the pressure-sensitive adhesive layer 11 in this embodiment is dissolved in a solvent and a new pressure-sensitive adhesive layer is formed, the haze value of the pressure-sensitive adhesive layer is preferably in the same range as above.

[0107] (3) Total Light Transmittance The total light transmittance of the pressure-sensitive adhesive layer 11 in this embodiment is preferably 80% or more as a lower limit, more preferably 90% or more, particularly preferably 95% or more, and even more preferably 99% or more. This results in very high transparency and good visibility as a display. On the other hand, the upper limit of the total light transmittance of the pressure-sensitive adhesive layer 11 is not particularly limited, but is usually 100% or less. Here, the total light transmittance in this specification is a value measured in accordance with JIS K7361-1:1997.

[0108] In the case where the pressure-sensitive adhesive layer 11 in this embodiment is dissolved in a solvent and a pressure-sensitive adhesive layer is formed again, the total light transmittance of the pressure-sensitive adhesive layer is preferably in the same range as above.

[0109] (4) Holding power When the adhesive layer of the adhesive sheet (PET film / adhesive layer) according to this embodiment is attached to a vertical soda lime glass and subjected to a holding power test in accordance with JIS Z0237:2009, the deviation (total of absolute values; initial deviation D1) at both left and right corners under the conditions of a test temperature of 80 ° C, an attachment area of ​​10 mm × 10 mm, a load of 9.8 N, and a holding time of 500 hours is preferably 10 mm or less, more preferably 8 mm or less, particularly preferably 6 mm or less, and even more preferably 5 mm or less. This means that the holding power is excellent. Details of the holding power test in this specification are as shown in the test examples described below.

[0110] In the present embodiment, when the pressure-sensitive adhesive layer 11 is dissolved in a solvent and a pressure-sensitive adhesive layer is formed again, the above-mentioned displacement amount (total of absolute values; displacement amount after recycling D2) as a holding power of the pressure-sensitive adhesive layer is preferably 12 mm or less, more preferably 10 mm or less, particularly preferably 8 mm or less, even more preferably 6 mm or less, and most preferably 5 mm or less. This means that the pressure-sensitive adhesive layer after reforming also has excellent holding power.

[0111] The rate of change ((D2 / D1) x 100) (%) of the displacement D2 after recycling to the initial displacement D1 is preferably 200% or less, more preferably 175% or less, particularly preferably 150% or less, even more preferably 125% or less, and most preferably 110% or less. This means that the pressure-sensitive adhesive layer after reforming has the same holding power as the initial pressure-sensitive adhesive layer.

[0112] The above-described embodiments have been described to facilitate understanding of the present invention, and are not intended to limit the present invention. Therefore, each element disclosed in the above embodiments is intended to include all design modifications and equivalents that fall within the technical scope of the present invention.

[0113] For example, the release sheet 12 in the pressure-sensitive adhesive sheet 1A may be omitted, and either one of the release sheets 12a and 12b in the pressure-sensitive adhesive sheet 1B may be omitted.

[0114] In this specification, when it is stated that "X to Y" (X and Y are any numbers), it means "X or more and Y or less" unless otherwise specified, and also means "preferably greater than X" or "preferably smaller than Y". Furthermore, when it is stated that "X or more" (X is any number), it means "preferably greater than X" unless otherwise specified, and when it is stated that "Y or less" (Y is any number), it also means "preferably smaller than Y" unless otherwise specified.

[0115] The present invention will be explained in more detail below with reference to examples, but the scope of the present invention is not limited to these examples.

[0116] Example 1 1. Preparation of (meth)acrylic acid ester polymer (A) 75 parts by mass of n-butyl acrylate, 20 parts by mass of methyl acrylate, and 5 parts by mass of acrylic acid were copolymerized by solution polymerization to prepare a (meth)acrylic acid ester polymer (A). The molecular weight of this (meth)acrylic acid ester polymer (A) was measured by the method described below, and the weight average molecular weight (Mw) was found to be 800,000.

[0117] 2. Preparation of coating solution of adhesive composition 100 parts by mass of the (meth)acrylic acid ester polymer (A) obtained above and 0.2 parts by mass of aluminum tris(acetylacetonate) (B1) as the metal chelate crosslinking agent (B) were mixed in ethyl acetate and thoroughly stirred to obtain a coating solution of an adhesive composition.

[0118] Table 1 shows the formulations (solid content equivalent) of the pressure-sensitive adhesives when the (meth)acrylic acid ester polymer (A) is taken as 100 parts by mass (solid content equivalent). The details of the abbreviations and the like in Table 1 are as follows. [(Meth)acrylic acid ester polymer (A)] BA: n-butyl acrylate EA: ethyl acrylate 2HEA: 2-ethylhexyl acrylate MA: methyl acrylate AAc: acrylic acid [Metal chelate crosslinker (B)] B1: aluminum tris(acetylacetonate) B2: zinc bis(acetylacetonate) B3: iron(III) acetylacetonate [Other crosslinkers] Epoxy: 1,3-bis(N,N-diglycidylaminomethyl)cyclohexane (manufactured by Mitsubishi Gas Chemical Company, Inc., product name "TC-5") TDI: isocyanate-based crosslinker (manufactured by Mitsui Chemicals, Inc., product name "Takenate D-101E")

[0119] 3. Production of Pressure-Sensitive Adhesive Sheet The obtained coating solution of the pressure-sensitive adhesive composition was applied with an applicator to the release-treated surface of a heavy-release type release sheet R1, which was a polyethylene terephthalate film having one side treated with a silicone-based release agent for release, and then the sheet was heated at 120°C for 1 minute to form a pressure-sensitive adhesive layer.

[0120] Next, the adhesive layer on the release sheet R1 obtained above was bonded to a light-release type release sheet R2, one side of which was a polyethylene terephthalate film treated with a silicone-based release agent, so that the release-treated surface of the release sheet R2 was in contact with the adhesive layer, producing an adhesive sheet (initial adhesive sheet) consisting of release sheet R2 / adhesive layer (thickness: 25 μm) / release sheet R1.

[0121] The thickness of the pressure-sensitive adhesive layer was measured in accordance with JIS K7130 using a constant pressure thickness measuring device (manufactured by Teclock Corporation, product name "PG-02") (the same applies hereinafter). Furthermore, it was confirmed that the release strength of release sheet R1 was greater than that of release sheet R2 in the obtained pressure-sensitive adhesive sheet.

[0122] 4. Reforming the adhesive layer (manufacturing a recycled adhesive sheet) The release sheet R2 was peeled off from the initial adhesive sheet prepared above, and the exposed adhesive layer was attached to a glass plate. The release sheet R1 was then peeled off to obtain a laminate of adhesive layer / glass plate. The laminate was immersed in a solvent containing ethyl acetate and acetylacetone in a 1:1 mass ratio for 24 hours, and shaken and stirred to dissolve the adhesive. At this time, it was confirmed that no adhesive residue remained on the adhesive layer-attached surface of the glass plate.

[0123] The solvent in which the adhesive had been dissolved was then applied with an applicator to the release-treated surface of release sheet R1, followed by heat treatment at 120°C for 1 minute to reform the adhesive layer. Release sheet R2 was then attached so that the release-treated surface of release sheet R2 was in contact with the adhesive layer, yielding a recycled adhesive sheet consisting of release sheet R2 / adhesive layer (reformed) / release sheet R1. The thickness of the reformed adhesive layer was the same as that of the adhesive layer of the initial adhesive sheet.

[0124] Examples 2 to 4, 7 to 8, Comparative Examples 1 to 3 Initial pressure-sensitive adhesive sheets and recycled pressure-sensitive adhesive sheets were produced in the same manner as in Example 1, except that the type and ratio of each monomer constituting the (meth)acrylic acid ester polymer (A), the weight-average molecular weight of the (meth)acrylic acid ester polymer (A), and the type and blending amount of the crosslinking agent were changed as shown in Table 1. However, the initial pressure-sensitive adhesive sheets of Comparative Examples 2 and 3 were poor in recyclability, and a sufficient amount of adhesive elution was not obtained to produce a recycled pressure-sensitive adhesive sheet, so recycled pressure-sensitive adhesive sheets were produced with a pressure-sensitive adhesive layer thickness of 1 μm.

[0125] In Comparative Example 1, 0.05 parts by mass of 1,3-bis(N,N-diglycidylaminomethyl)cyclohexane, an epoxy-based crosslinking agent, was used as the crosslinking agent, and in Comparative Example 2, 0.1 parts by mass of an isocyanate-based crosslinking agent (manufactured by Mitsui Chemicals, Inc., product name "Takenate D-101E") was used as the crosslinking agent.

[0126] Example 5 78.5 parts by mass of ethyl acrylate, 20 parts by mass of 2-ethylhexyl acrylate, 1.5 parts by mass of acrylic acid, and 1.0 part by mass of bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide as a photopolymerization initiator (C) were mixed without solvent. This mixture was irradiated with active energy rays (ultraviolet rays; UV) under the conditions below to polymerize a portion of the monomers, thereby preparing an acrylic syrup containing a (meth)acrylic acid ester polymer (A) and the monomers that constitute the polymer.

[0127] <Activated energy ray irradiation conditions> - High pressure mercury lamp used - Illuminance 200 mW / cm ２ , light intensity 2000mJ / cm ２ ・UV illuminance and light intensity meter used is "UVPF-A1" manufactured by iGraphics

[0128] 100 parts by mass of the acrylic syrup obtained above and 0.9 parts by mass of aluminum tris(acetylacetonate) (B1) as the metal chelate crosslinking agent (B) were mixed and thoroughly stirred to obtain a coating liquid of an adhesive composition.

[0129] The obtained coating solution of the pressure-sensitive adhesive composition was applied to the release-treated surface of the release sheet R1 using an applicator. The coating film was then irradiated with active energy rays (ultraviolet rays; UV) under the same conditions as described above, and heated at 120°C for 1 minute to form a pressure-sensitive adhesive layer. At this stage, all of the monomers were polymerized to form a (meth)acrylic acid ester polymer (A). The weight-average molecular weight of the (meth)acrylic acid ester polymer (A) was that at this stage.

[0130] Thereafter, the adhesive layer on the release sheet R1 obtained above was bonded to release sheet R2 so that the release-treated surface of release sheet R2 was in contact with the adhesive layer, thereby producing an adhesive sheet (initial adhesive sheet) consisting of release sheet R2 / adhesive layer (thickness: 50 μm) / release sheet R1.

[0131] Further, a recycled adhesive sheet was produced in the same manner as in Example 1 from the initial adhesive sheet produced above.

[0132] Example 6 Initial adhesive sheets and recycled adhesive sheets were produced in the same manner as in Example 5, except that the blending amount of the metal chelate crosslinking agent (B) was changed as shown in Table 1.

[0133] Here, the weight average molecular weight (Mw) of the (meth)acrylic acid ester polymer (A) is a polystyrene-equivalent weight average molecular weight measured using gel permeation chromatography (GPC) under the following conditions (GPC measurement). <Measurement conditions> GPC measurement device: HLC-8020, manufactured by Tosoh Corporation GPC columns (passed in the following order): TSK guard column HXL-H, TSK gel GMHXL (x2), TSK gel G2000HXL, manufactured by Tosoh Corporation Measurement solvent: tetrahydrofuran Measurement temperature: 40°C

[0134] [Test Example 1] (Measurement of gel fraction) The initial adhesive sheets and recycled adhesive sheets produced in the Examples and Comparative Examples were cut to a size of 80 mm x 80 mm, the adhesive layer was wrapped in a polyester mesh (mesh size 200), and the mass was weighed using a precision balance. The mass of the mesh alone was subtracted to calculate the mass of the adhesive alone. This mass was designated M1.

[0135] Next, the adhesive wrapped in the polyester mesh was immersed for 72 hours at room temperature (23°C) in ethyl acetate (initial adhesive sheet and recycled adhesive sheet) or a solvent containing a 9:1 mass ratio of ethyl acetate and acetylacetone (initial adhesive sheet only). The adhesive was then removed and air-dried for 24 hours at 23°C and 50% relative humidity, and then dried in an oven at 80°C for 12 hours. After drying, the mass was weighed using a precision balance, and the mass of the adhesive alone was calculated by subtracting the mass of the mesh alone. This mass was designated M2. The gel fraction (%) was expressed as (M2 / M1) x 100. The gel fraction of the adhesive was calculated from this. Here, the gel fraction when the initial adhesive sheet was immersed in ethyl acetate was designated G1, the gel fraction when the initial adhesive sheet was immersed in the mixed solvent was designated G2, and the gel fraction when the recycled adhesive sheet was immersed in ethyl acetate was designated G3. The results are shown in Table 2.

[0136] [Test Example 2] (Measurement of adhesive strength) The release sheet R2 was peeled off from the initial adhesive sheet produced in the Examples and Comparative Examples, and the exposed adhesive layer was attached to an easy-adhesion layer of a polyethylene terephthalate (PET) film (manufactured by Toyobo Co., Ltd., product name "Cosmoshine A4360", thickness: 100 μm) having an easy-adhesion layer, to obtain a release sheet R1 / adhesive layer / PET film laminate. The obtained laminate was cut into a width of 25 mm and a length of 100 mm.

[0137] The release sheet R1 was peeled from the laminate under an environment of 23°C and 50% RH, and the exposed adhesive layer was attached to soda lime glass (manufactured by Nippon Sheet Glass Co., Ltd.). It was then pressurized for 20 minutes at 0.5 MPa and 50°C in an autoclave manufactured by Kurihara Manufacturing Co., Ltd. The sample was then left for 24 hours under conditions of 23°C and 50% RH, and used as a sample. The adhesive strength (initial adhesive strength; N / 25 mm) was measured using a tensile tester (manufactured by Orientec Co., Ltd., product name "Tensilon") at a peel rate of 300 mm / min and a peel angle of 180°. Measurements were conducted under conditions other than those described here in accordance with JIS Z0237:2022. The results are shown in Table 2.

[0138] The adhesive strength (adhesive strength after recycling: N / 25 mm) of the recycled adhesive sheets produced in the Examples and Comparative Examples was measured in the same manner as above. The results are shown in Table 2.

[0139] [Test Example 3] (Measurement of L*a*b*) For the adhesive layers of the initial adhesive sheets and recycled adhesive sheets produced in the examples and comparative examples, a simultaneous photometric spectrophotometer colorimeter (manufactured by Nippon Denshoku Industries Co., Ltd., product name "SQ2000") was used to measure the chromaticity b* (initial and post-recycled) defined by the CIE1976 L*a*b* color system in transmitted light. The results are shown in Table 2.

[0140] [Test Example 4] (Haze Value Measurement) The adhesive layer of the initial adhesive sheet and the recycled adhesive sheet produced in the Examples and Comparative Examples was attached to glass, and this was used as a measurement sample. After background measurement was performed on the glass, the haze value (%) (initial / post-recycled) of the measurement sample was measured using a haze meter (manufactured by Nippon Denshoku Industries Co., Ltd., product name "SH7000") in accordance with JIS K7136:2000. The results are shown in Table 2.

[0141] [Test Example 5] (Measurement of total light transmittance) The adhesive layers of the initial adhesive sheets and recycled adhesive sheets produced in the Examples and Comparative Examples were attached to glass to prepare measurement samples. After background measurement was performed on the glass, the total light transmittance (%) (initial and post-recycled) of the measurement samples was measured using a haze meter (manufactured by Nippon Denshoku Industries Co., Ltd., product name "SH7000") in accordance with JIS K7361-1:1997. The results are shown in Table 2.

[0142] [Test Example 6] (Measurement of storage modulus G') The pressure-sensitive adhesive layers of the initial pressure-sensitive adhesive sheets produced in the Examples and Comparative Examples were laminated in multiple layers to a thickness of 0.8 mm. A cylindrical body with a diameter of 8 mm (height of 0.8 mm) was punched out from the resulting pressure-sensitive adhesive layer laminate, and this was used as a sample.

[0143] The dynamic viscoelasticity of the above sample was measured in accordance with JIS K7244-1 using a viscoelasticity measuring device (manufactured by Anton Paar, product name "MCR302e") by the torsional shear method under the following conditions, and the storage modulus (G') (MPa) at 23°C and 85°C was measured. The results are shown in Table 2. Measurement frequency: 1 Hz Measurement temperature range: 0°C to 100°C Heating rate: 4°C / min

[0144] [Test Example 7] (Holding Power Test) The release sheet R2 was peeled off from the initial adhesive sheet produced in the Examples and Comparative Examples, and the exposed adhesive layer was attached to an easy-adhesion layer of a polyethylene terephthalate (PET) film (manufactured by Toyobo Co., Ltd., product name "Cosmoshine A4360", thickness: 100 μm) having an easy-adhesion layer, to obtain a release sheet R1 / adhesive layer / PET film laminate. The obtained laminate was cut into 10 mm × 10 mm.

[0145] Next, the release sheet R1 was peeled off from the laminate, and this was used as a sample. The exposed pressure-sensitive adhesive layer (10 mm x 10 mm) of this sample was attached to a soda lime glass plate (manufactured by Nippon Sheet Glass Co., Ltd.). The soda lime glass plate was then placed vertically on the ground, and the sample was left at 80°C for 500 hours while a load of 9.8 N was applied to the sample. The absolute values ​​of the displacement (mm) at both left and right corners were then measured, and the sum of the absolute values ​​of the left and right displacement (mm; initial displacement D1) was calculated. The results are shown in Table 2. Note that samples that fell during the test are referred to as "dropped."

[0146] The recycled PSA sheets produced in the Examples and Comparative Examples were also subjected to the holding power test in the same manner as above, and the sum of the absolute values ​​of the left and right misalignment amounts (mm; misalignment amount D2 after recycling) was calculated. The results are shown in Table 2.

[0147] The rate of change ((D2 / D1) x 100) (%) of the displacement D2 after recycling to the initial displacement D1 was also calculated. The results are shown in Table 2.

[0148] [Test Example 8] (Evaluation of Positional Stability) Based on the initial displacement D1 in Test Example 7, the positional stability of the pressure-sensitive adhesive layer was evaluated according to the following criteria. The results are shown in Table 2. ⊚: Initial displacement D1 is 5 mm or less; ◯: Initial displacement D1 is more than 5 mm but not more than 10 mm; ×: Initial displacement D1 is more than 10 mm

[0149] [Test Example 9] (Evaluation of Recyclability) Based on the rate of change ((D2 / D1) x 100) (%) of the displacement D2 after recycling to the initial displacement D1 in Test Example 7, the recyclability was evaluated according to the following criteria. The results are shown in Table 2. ⊚: rate of change is 100% or less ◯: rate of change is more than 100% and less than 200% ×: rate of change is more than 200%

[0150] Test Example 10 (Evaluation of Ease of Dismantling) A laminate of release sheet R1 / adhesive layer / PET film was prepared and cut into a size of 10 mm x 10 mm in the same manner as in Test Example 7. The release sheet R1 was peeled off from the laminate, and the exposed adhesive layer (10 mm x 10 mm) was attached to a soda-lime glass plate (manufactured by Nippon Sheet Glass Co., Ltd.) to prepare a sample.

[0151] The obtained sample was immersed in a solvent containing ethyl acetate and acetylacetone in a 9:1 mass ratio, and shaken and stirred for 48 hours. The adhesive (layer) was then evaluated for ease of dismantling (solvent solubility) based on the following criteria. The results are shown in Table 2. ⊚: The adhesive layer fell off from the glass plate and PET film within 2 hours. ◯: The adhesive layer fell off from the glass plate and PET film within more than 2 hours and within 48 hours. ×: The adhesive layer did not fall off from the glass plate and / or PET film within 48 hours.

[0152]

[0153]

[0154] As can be seen from Table 2, the adhesive layer (adhesive) of the adhesive sheet produced in the example was excellent in ease of dismantling and recyclability, and showed excellent adhesive strength even after recycling.

[0155] The pressure-sensitive adhesive and pressure-sensitive adhesive sheet according to the present invention are suitable for applications requiring recyclability.

[0156] DESCRIPTION OF SYMBOLS 1A, 1B... Adhesive sheet 11... Adhesive layer 12, 12a, 12b... Release sheet 13... Substrate

Claims

1. An adhesive intended for recycling, characterized in that the gel fraction after immersion in ethyl acetate for 72 hours is 25% or more, and the gel fraction after immersion in a solvent in which ethyl acetate and acetylacetone are mixed in a mass ratio of 9:1 for 72 hours is 25% or less.

2. The adhesive described in claim 1, characterized in that the gel fraction after dissolving in a solvent containing a 1:1 mass ratio of ethyl acetate and acetylacetone, drying at 120°C for 1 minute, and then immersing in ethyl acetate for 72 hours is 25% or more.

3. The adhesive according to claim 1, which is crosslinked with a metal chelate crosslinking agent.

4. The adhesive according to claim 1, which is an acrylic adhesive.

5. The adhesive according to claim 4, which is a solvent-based acrylic adhesive.

6. The adhesive according to claim 4, which is a solvent-free acrylic adhesive.

7. The adhesive described in claim 5, characterized in that the adhesive layer made of the adhesive and having a thickness of 25 μm and a width of 25 mm has an adhesive strength to soda lime glass of 1 N / 25 mm or more and 55 N / 25 mm or less.

8. The adhesive according to claim 6, characterized in that the adhesive layer made of said adhesive and having a thickness of 25 μm and a width of 25 mm has an adhesive strength to soda lime glass of 10 N / 25 mm or more.

9. The adhesive described in claim 5, characterized in that the adhesive layer is 25 μm thick and 25 mm wide and is formed by dissolving the adhesive in a solvent containing a 1:1 mixture of ethyl acetate and acetylacetone by mass ratio and then drying at 120°C for 1 minute, and the adhesive strength to soda-lime glass is 1 N / 25 mm or more and 55 N / 25 mm or less.

10. The adhesive described in claim 6, characterized in that the adhesive layer is 25 μm thick and 25 mm wide and is formed by dissolving the adhesive in a solvent containing a 1:1 mass ratio of ethyl acetate and acetylacetone and then drying at 120°C for 1 minute, and the adhesive strength to soda-lime glass is 10 N / 25 mm or more.

11. An adhesive sheet comprising at least an adhesive layer, wherein the adhesive constituting the adhesive layer is the adhesive described in any one of claims 1 to 10.

12. The adhesive sheet according to claim 11, characterized in that the adhesive sheet comprises two release sheets, and the adhesive layer is sandwiched between the release sheets so as to contact the release surfaces of the two release sheets.

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