Adhesives, hot melt adhesives and labels

Abstract

To provide an adhesive which has excellent adhesivity at the time of use, allows sticking bodies such as a label to stick strongly to an adherend such as a container, and when, on the other hand, the sticking body becomes unnecessary, the sticking body can easily be peeled and removed from the adherend by contact with an alkaline aqueous solution, and further which has excellent thermal stability.SOLUTION: The adhesive of the present invention comprises a (meth)acrylic copolymer having, as a whole, an acid value of 30 to 98 mgKOH / g and has a water swelling ratio (alkali expansion coefficient / water expansion coefficient) of 1.3 or more, which is a ratio of an alkali expansion coefficient after immersion in an alkaline aqueous solution of 23°C for 24 hours, followed by immersion in an alkaline aqueous solution of 80°C for 2 hours to a water expansion coefficient after immersion in water of 23°C for 24 hours, followed by immersion in water of 80°C for 2 hours.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to an adhesive, a hot melt adhesive, and a label. [Background technology]

[0002] Conventionally, containers made of synthetic resins such as polyethylene terephthalate have been widely used to store soft drinks, liquid seasonings, cosmetics, etc. Under the enforcement of the Containers and Packaging Recycling Law, containers are collected for recycling after use. The collected containers are crushed into flakes, washed with an alkaline aqueous solution containing an alkali metal hydroxide such as sodium hydroxide, dried, and processed into pellets as needed to be used as recycled raw materials.

[0003] An adhesive such as a label is attached to the outer periphery of a container via an adhesive. The adhesive that attaches the label to the outer periphery of the container must have sufficient adhesiveness to prevent the label from peeling off from the outer periphery of the container while the container is in use, but must also be able to easily peel off and remove the label attached to the outer periphery of the container when the container is recycled, which are contradictory performance requirements.

[0004] The following adhesive has been disclosed as an adhesive for attaching a label to the outer periphery of a container: Patent Document 1 discloses an alkali-dispersible hot-melt adhesive containing (A) a thermoplastic block copolymer that is a copolymer of a vinyl aromatic hydrocarbon and a conjugated diene compound, (B) a tackifier resin, and (C) at least one selected from the group consisting of fatty acids and their derivatives, wherein the thermoplastic block copolymer (A) contains (A1) a styrene-based block copolymer having a styrene content of 40% by weight or more.

[0005] Patent Document 2 discloses a pressure-sensitive adhesive composition comprising a copolymer comprising (a) 30 to 80% by weight of vinyl monomer units represented by formula (I) or (II), (b) 10 to 45% by weight of vinyl monomer units having a carboxyl group in the molecule and having a glass transition temperature of a homopolymer of −20°C or lower, and (c) 5 to 60% by weight of (meth)acrylic acid alkyl ester monomer units having an alkyl group having 4 to 12 carbon atoms, and further comprising (d) 0 to 10 parts by weight of other radically polymerizable monomer units based on 100 parts by weight in total of the monomer units (a) to (c).

[0006] Patent Document 3 discloses an alkali-peelable pressure-sensitive adhesive composition containing: a predetermined alkyl acrylate; 100 parts by mass of an acrylic polymer obtained by copolymerizing 40 to 85% by mass of the alkyl acrylate and a carboxyl group-containing radically polymerizable monomer; 3 to 30 parts by mass of an acrylic polymer obtained by copolymerizing a nitrogen-containing radically polymerizable monomer and a (meth)acrylic alkyl ester and having a molecular weight of 1,000 to 50,000 and a glass transition temperature of 0 to 150°C; and (C) a polyfunctional crosslinking agent. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 2020-73642 [Patent Document 2] Japanese Patent Application Publication No. 8-73827 [Patent Document 3] Japanese Patent Application Laid-Open No. 2012-153789 Summary of the Invention [Problem to be solved by the invention]

[0008] However, when a label attached to the outer periphery of a container using the adhesives of Patent Documents 1 and 2 is immersed in an alkaline aqueous solution to remove the label, the adhesive disperses or dissolves in the alkaline aqueous solution, contaminating the alkaline aqueous solution, making it difficult to reuse the alkaline aqueous solution and requiring frequent replacement of the alkaline aqueous solution.

[0009] Furthermore, when a container with a label attached is immersed in a contaminated alkaline solution, the adhesive dispersed in the alkaline solution adheres to the surface of the container from which the label has been removed, which creates another problem: the container must be washed after the label has been removed.

[0010] Furthermore, the alkali-peelable pressure-sensitive adhesive composition of Patent Document 3 contains nitrogen-containing radically polymerizable monomer units, and therefore has the problem of being prone to coloration and gelation when heated and melted, and of having low thermal stability.

[0011] The present invention provides a pressure-sensitive adhesive that has excellent adhesive properties during use and can firmly adhere a label or other adhesive object to an adherend such as a container, and that, when the adhesive object is no longer needed, can be easily peeled and removed from the adherend by contacting it with an alkaline aqueous solution while reducing contamination by the alkaline aqueous solution, and that also has excellent thermal stability. [Means for solving the problem]

[0012] The adhesive of the present invention contains a (meth)acrylic copolymer having a total acid value of 30 to 98 mgKOH / g, and has an alkalinity of 100% or less after immersion in an alkaline aqueous solution at 23°C for 24 hours and then immersion in an alkaline aqueous solution at 80°C for 2 hours. Swelling The rate of water absorption after 24 hours in 23°C water and 2 hours in 80°C water Swelling rate (alkali Swelling rate / water Swelling The ratio is 1.3 or higher.

[0013] The adhesive was immersed in an alkaline aqueous solution at 23°C for 24 hours, and then immersed in an alkaline aqueous solution at 80°C for 2 hours. Swelling The rate of water absorption after 24 hours in 23°C water and 2 hours in 80°C water Swelling Swelling ratio (alkaline Swelling rate / water Swelling The swelling ratio (hereinafter sometimes simply referred to as "swelling ratio") of the PSA is 1.3 or higher. By having a swelling ratio of 1.3 or higher, the PSA swells significantly when immersed in an alkaline aqueous solution, reducing its adhesiveness and providing excellent releasability. The PSA swells when immersed in an alkaline aqueous solution, thereby exhibiting releasability, and this reduces contamination of the alkaline aqueous solution caused by the PSA dispersing or dissolving in the alkaline aqueous solution. The adhesive can easily reduce its adhesiveness by contacting it with an alkaline aqueous solution at a low temperature of around 80°C, and there is no need to include a surfactant in the alkaline aqueous solution.

[0014] The swelling ratio of the adhesive is 1.3 or more, preferably 1.35 or more, and more preferably 1.4 or more. The swelling ratio of the adhesive is preferably 5.0 or less, more preferably 4.5 or less, more preferably 4.0 or less, more preferably 3.5 or less, and more preferably 3.0 or less. When the swelling ratio of the adhesive is 1.3 or more, it swells upon immersion in an alkaline aqueous solution and exhibits excellent releasability. When the swelling ratio of the adhesive is 5.0 or less, contamination of the alkaline aqueous solution due to the adhesive being dispersed or dissolved in the alkaline aqueous solution can be reduced.

[0015] The swelling ratio of an adhesive is measured as follows: An alkaline aqueous solution with a sodium hydroxide concentration of 2% by mass is prepared. The alkaline aqueous solution is maintained at 23°C. A first test piece consisting of approximately 0.2 g of adhesive is prepared, and the initial mass W1 of this first test piece is measured. The first test piece is immersed in the alkaline aqueous solution at 23°C for 24 hours. Thereafter, the alkaline aqueous solution is heated to 80°C, and the first test piece is immersed in the alkaline aqueous solution at 80°C for two hours. The first test piece is removed from the alkaline aqueous solution, and the mass W2 of the first test piece after alkali immersion is measured. The alkali swelling ratio R1 of the first test piece is calculated using the following formula: Alkali swelling ratio R1 (%) = 100 × (W2 - W1) / W1

[0016] Separately, a second test piece consisting of approximately 0.2 g of adhesive is prepared, and the initial mass W3 of this second test piece is measured. The second test piece is immersed in ion-exchanged water at 23°C for 24 hours. Thereafter, the ion-exchanged water is heated to 80°C, and the second test piece is immersed in the 80°C ion-exchanged water for two consecutive hours. The second test piece is removed from the ion-exchanged water, and the mass W4 of the second test piece after immersion in ion-exchanged water is measured. The water swelling ratio R2 of the second test piece is calculated using the following formula: Water swelling rate R2(%)=100×(W4-W3) / W3

[0017] The swelling ratio R is calculated based on the alkali swelling ratio R1 and the ion-exchanged water swelling ratio R2. Swelling ratio R = Alkali swelling ratio R1 / Ion-exchanged water swelling ratio R2

[0018] The pressure-sensitive adhesive contains a (meth)acrylic copolymer having a total acid value of 30 to 98 mgKOH / g. The term "(meth)acrylic copolymer" encompasses both acrylic and methacrylic copolymers. The content of the (meth)acrylic copolymer in the pressure-sensitive adhesive is preferably 50% by mass or more, more preferably 60% by mass or more, more preferably 70% by mass or more, more preferably 80% by mass or more, more preferably 90% by mass or more, more preferably 95% by mass or more, and more preferably 100% by mass. By containing a (meth)acrylic copolymer having a total acid value of 30 to 98 mgKOH / g, the pressure-sensitive adhesive has excellent adhesiveness and excellent thermal stability during use. In the present invention, the acid value of the (meth)acrylic copolymer refers to the number of milligrams of potassium hydroxide required to neutralize the acid contained in 1 g of the pressure-sensitive adhesive, and is a value measured in accordance with JIS K2501.

[0019] The acid value of the entire (meth)acrylic copolymer is 30 mgKOH / g or more, preferably 39 mgKOH / g or more, and preferably 45 mgKOH / g or more. The acid value of the entire (meth)acrylic copolymer is 98 mgKOH / g or less, preferably 90 mgKOH / g or less, more preferably 80 mgKOH / g or less, and preferably 70 mgKOH / g or less. When the acid value of the entire (meth)acrylic copolymer is 30 mgKOH / g or more, the PSA has excellent adhesiveness during use, and after use, when the PSA is brought into contact with an alkaline aqueous solution, the PSA smoothly absorbs and retains the alkaline aqueous solution and swells, thereby easily reducing the adhesiveness of the PSA. When the acid value of the entire (meth)acrylic copolymer is 98 mgKOH / g or less, when the PSA is brought into contact with an alkaline aqueous solution, dispersion and dissolution of the PSA in the alkaline aqueous solution are reduced, thereby reducing contamination of the alkaline aqueous solution, and the PSA has excellent thermal stability.

[0020] The adhesive maintains excellent adhesiveness during use by adjusting the acid value of the entire (meth)acrylic copolymer to 30 to 98 mgKOH / g and the swelling ratio of the adhesive to 1.3 or more. After use, by contacting the adhesive with an alkaline aqueous solution, the adhesive swells significantly, reducing the adhesiveness and allowing it to be easily peeled from the adherend, while reducing dispersion or dissolution in the alkaline aqueous solution and reducing contamination of the alkaline aqueous solution.

[0021] The monomer constituting the (meth)acrylic copolymer is not particularly limited, and examples thereof include alkyl (meth)acrylates and monomers having a carboxy group. The (meth)acrylic monomers may be used alone or in combination of two or more. The (meth)acrylate refers to acrylate or methacrylate.

[0022] The alkyl(meth)acrylate is not particularly limited, and examples thereof include ethyl(meth)acrylate, n-propyl(meth)acrylate, n-butyl(meth)acrylate, isobutyl(meth)acrylate, sec-butyl(meth)acrylate, n-hexyl(meth)acrylate, 2-ethylhexyl(meth)acrylate, n-octyl(meth)acrylate, isooctyl(meth)acrylate, n-nonyl(meth)acrylate, isononyl(meth)acrylate, n-decyl(meth)acrylate, isodecyl(meth)acrylate, and lauryl(meth)methacrylate. Of these, n-propyl(meth)acrylate, n-butyl(meth)acrylate, isobutyl(meth)acrylate, sec-butyl(meth)acrylate, n-hexyl(meth)acrylate, and 2-ethylhexyl(meth)acrylate are preferred, and butyl(meth)acrylate is more preferred.

[0023] The alkyl group refers to the atomic group remaining after removing (pulling out) one hydrogen atom from an aliphatic saturated hydrocarbon. The hydrogen of the alkyl group of the alkyl (meth)acrylate is not substituted with another atom or atomic group, and the alkyl group may be either linear or branched, but a linear alkyl group is preferred because the resulting adhesive has excellent adhesive strength.

[0024] The number of carbon atoms in the alkyl group of the alkyl (meth)acrylate is preferably 1 or more, more preferably 2 or more, more preferably 3 or more, and more preferably 4 or more. The number of carbon atoms in the alkyl group of the alkyl (meth)acrylate is 12 or less, preferably 10 or less, more preferably 8 or less, and more preferably 6 or less. When the alkyl group of the alkyl (meth)acrylate has 2 or more carbon atoms, the adhesive has excellent adhesiveness. When the alkyl group of the alkyl (meth)acrylate has 12 or less carbon atoms, the adhesive has excellent adhesiveness, and when the adhesive is brought into contact with an alkaline aqueous solution, the adhesive easily swells, and the adhesiveness of the adhesive can be easily reduced.

[0025] The content of alkyl (meth)acrylate units in the (meth)acrylic copolymer is preferably 80% by mass or more, more preferably 85% by mass or more, and even more preferably 90% by mass or more. The content of alkyl (meth)acrylate units in the (meth)acrylic copolymer is preferably 98% by mass or less, and more preferably 95% by mass or less. When the content of alkyl (meth)acrylate units is 80% by mass or more, the adhesive has excellent adhesiveness. When the content of alkyl (meth)acrylate units is 98% by mass or less, the adhesive not only has excellent adhesiveness, but also can be easily swelled by contact with an alkaline aqueous solution, thereby easily reducing the adhesiveness of the adhesive.

[0026] The (meth)acrylic copolymer preferably contains a monomer having a carboxy group (-COOH). The monomer containing a carboxy group (-COOH) preferably does not contain an ester bond, since the resulting adhesive has excellent adhesive properties.

[0027] When the (meth)acrylic copolymer contains a monomer unit having a carboxy group in the molecule, the adhesive has excellent adhesiveness. On the other hand, by contacting the adhesive with an alkaline aqueous solution, the alkaline aqueous solution is smoothly absorbed and retained in the adhesive, and the adhesive easily swells, thereby easily reducing the adhesiveness of the adhesive.

[0028] The monomer having a carboxy group in the molecule is not particularly limited, and examples thereof include acrylic monomers such as acrylic acid, methacrylic acid, β-carboxyethyl acrylate, and β-carboxyethyl methacrylate, maleic acid, and fumaric acid. Acrylic monomers having a carboxy group are preferred, and acrylic acid and methacrylic acid are more preferred.

[0029] The content of monomer units having a carboxy group in the molecule in the (meth)acrylic copolymer is preferably 4% by mass or more, more preferably 4.5% by mass or more, more preferably 5% by mass or more, and even more preferably 5.5% by mass or more. The content of monomer units having a carboxy group in the molecule in the (meth)acrylic copolymer is more preferably 12% by mass or less, more preferably 10% by mass or less, and even more preferably 7% by mass or less. When the content of monomer units having a carboxy group in the molecule is 4% by mass or more, the PSA has excellent adhesive properties, and when the PSA is brought into contact with an alkaline aqueous solution, the PSA smoothly absorbs and retains the alkaline aqueous solution, easily swelling the PSA, and the adhesive properties of the PSA can be easily reduced. When the content of monomer units having a carboxy group in the molecule is 12% by mass or less, the (meth)acrylic copolymer has an appropriate glass transition temperature, and the PSA has excellent adhesive properties and thermal stability.

[0030] The (meth)acrylic copolymer preferably contains an ultraviolet-crosslinkable group-containing monomer unit. When the (meth)acrylic copolymer contains an ultraviolet-crosslinkable group-containing monomer unit, a crosslinked structure is introduced into the (meth)acrylic copolymer by ultraviolet irradiation, making the crosslinked (meth)acrylic copolymer less soluble in an alkaline aqueous solution, and making it possible to easily retain the alkaline aqueous solution in the (meth)acrylic copolymer, which facilitates swelling of the adhesive, and allows the swelling of the adhesive to smoothly proceed upon contact with the alkaline aqueous solution, thereby easily reducing the adhesiveness of the adhesive.

[0031] The ultraviolet crosslinkable group-containing monomer refers to a monomer having an ultraviolet crosslinkable group that can form a chemical bond and crosslink when irradiated with ultraviolet light.

[0032] The ultraviolet crosslinkable group is not particularly limited and examples thereof include a thiol group, a glycidyl group, an oxetanyl group, a vinyl group, a (meth)acryloyl group, a benzophenone group, a benzoin group, and a thioxanthone group, with a benzophenone group being preferred, and a benzophenone group being more preferred. Note that (meth)acryloyl means methacryloyl or acryloyl.

[0033] The ultraviolet crosslinkable group-containing monomer is not particularly limited, and examples thereof include glycidyl (meth)acrylate, 4-hydroxybutyl acrylate glycidyl ether, 4-(meth)acryloyloxybenzophenone, 4-[2-((meth)acryloyloxy)ethoxy]benzophenone, 4-(meth)acryloyloxy-4'-methoxybenzophenone, 4-(meth)acryloyloxyethoxy-4'-methoxybenzophenone, 4-(meth)acryloyloxy-4'-bromobenzophenone, and 4-(meth)acryloyloxyethoxy-4'-bromobenzophenone, with 4-(meth)acryloyloxybenzophenone and 4-[2-((meth)acryloyloxy)ethoxy]benzophenone being preferred. The ultraviolet crosslinkable group-containing monomer may be used alone or in combination of two or more. Note that (meth)acryloyloxy refers to methacryloyloxy or acryloyloxy.

[0034] The content of the ultraviolet-crosslinkable group-containing monomer unit in the (meth)acrylic copolymer is preferably 0.01% by mass or more, more preferably 0.05% by mass or more, and even more preferably 0.1% by mass or more. When the content of the ultraviolet-crosslinkable group-containing monomer unit is 0.01% by mass or more, a crosslinked structure is introduced into the (meth)acrylic copolymer, and a retention space for stably retaining an alkaline aqueous solution can be formed within the crosslinked body of the (meth)acrylic copolymer. By contacting the pressure-sensitive adhesive with an alkaline aqueous solution, the alkaline aqueous solution can be easily retained within the crosslinked body of the (meth)acrylic copolymer, and the pressure-sensitive adhesive can be smoothly swollen.

[0035] The content of the ultraviolet-crosslinkable group-containing monomer unit in the (meth)acrylic copolymer is preferably 1% by mass or less, more preferably 0.8% by mass or less, more preferably 0.6% by mass or less, and still more preferably 0.4% by mass or less. When the content of the ultraviolet-crosslinkable group-containing monomer unit is 1% by mass or less, the pressure-sensitive adhesive exhibits excellent adhesiveness after crosslinking by ultraviolet irradiation.

[0036] The above describes the introduction of a crosslinked structure into the (meth)acrylic copolymer by incorporating a UV-crosslinkable group-containing monomer into the (meth)acrylic copolymer. However, as a method for introducing a crosslinked structure into the (meth)acrylic copolymer, instead of incorporating a UV-crosslinkable group-containing monomer, a hydroxyl group-containing monomer unit may be incorporated into the (meth)acrylic copolymer, and a crosslinked structure may be introduced using a curing agent containing an isocyanate compound having an isocyanate group. Alternatively, a hydroxyl group- or carboxyl group-containing monomer unit may be incorporated into the (meth)acrylic copolymer, and a crosslinked structure may be introduced using a metal chelate crosslinking agent. Alternatively, a peroxide may be added to the (meth)acrylic copolymer, and the resulting mixture may be heated in the presence of a crosslinking aid (e.g., divinylbenzene, trimethylolpropane trimethacrylate, 1,9-nonanediol dimethacrylate, etc.) as needed.

[0037] As described above, the PSA contains a (meth)acrylic copolymer having an overall acid value within a predetermined range. However, the (meth)acrylic copolymer may be a mixture of two types of (meth)acrylic copolymers having acid values ​​within a predetermined range, as long as the overall acid value of the (meth)acrylic copolymer is within the predetermined range.

[0038] The pressure-sensitive adhesive is preferably a mixture of two types of (meth)acrylic copolymers having acid values ​​within a predetermined range, and the (meth)acrylic copolymer preferably contains two types of (meth)acrylic copolymers having different affinities with alkaline aqueous solutions.

[0039] The (meth)acrylic copolymer is preferably a mixture of a (meth)acrylic copolymer (B) having a high acid value and a (meth)acrylic copolymer (A) having an acid value lower than that of the (meth)acrylic copolymer (B).

[0040] The (meth)acrylic copolymer (B) has excellent affinity for alkaline aqueous solutions, allowing it to absorb and retain the alkaline aqueous solution and swell easily, while the (meth)acrylic copolymer (A) has lower affinity for alkaline aqueous solutions than the (meth)acrylic copolymer (B) and prevents it from dissolving or dispersing in the alkaline aqueous solution. This allows the adhesive to smoothly absorb and retain the alkaline aqueous solution, swell, and reduce adhesion without dissolving or dispersing in the alkaline aqueous solution.

[0041] The acid value of the (meth)acrylic copolymer (A) is preferably 15 mgKOH / g or more, more preferably 20 mgKOH / g or more, and more preferably 30 mgKOH / g or more. The acid value of the (meth)acrylic copolymer (A) is preferably less than 80 mgKOH / g, more preferably 70 mgKOH / g or less, more preferably 65 mgKOH / g or less, more preferably 59 mgKOH / g or less, and more preferably 55 mgKOH / g or less. When the acid value of the (meth)acrylic copolymer (A) is 15 mgKOH / g or more, when the PSA is brought into contact with an alkaline aqueous solution, the PSA smoothly absorbs and retains the alkaline aqueous solution and swells, thereby easily reducing and improving the adhesiveness of the PSA, making the PSA more suitable for use as a hot-melt PSA. When the acid value of the (meth)acrylic copolymer (A) is 80 mgKOH / g or less, dispersion and dissolution in an alkaline aqueous solution are reduced, and contamination of the alkaline aqueous solution can be reduced. In addition, the adhesive has low melt viscosity when heated and melted and excellent thermal stability, making it suitable for use as a hot-melt adhesive.

[0042] The acid value of the (meth)acrylic copolymer (B) is preferably 60 mgKOH / g or more, more preferably 70 mgKOH / g or more, more preferably 80 mgKOH / g or more, and more preferably 90 mgKOH / g or more. The acid value of the (meth)acrylic copolymer (B) is preferably 150 mgKOH / g or less, more preferably 140 mgKOH / g or less, more preferably 130 mgKOH / g or less, more preferably 120 mgKOH / g or less, and more preferably 115 mgKOH / g or less. When the acid value of the (meth)acrylic copolymer (B) is 60 mgKOH / g or more, when the PSA is brought into contact with an alkaline aqueous solution, the PSA smoothly absorbs and retains the alkaline aqueous solution, swelling, and the adhesiveness of the PSA can be easily reduced. When the acid value of the (meth)acrylic copolymer (B) is 150 mgKOH / g or less, dispersion and dissolution in the alkaline aqueous solution can be reduced, thereby reducing contamination of the alkaline aqueous solution.

[0043] The weight-average molecular weight of the (meth)acrylic copolymer (A) is preferably 80,000 or more, more preferably 100,000 or more, more preferably 110,000 or more, more preferably 120,000 or more, more preferably 130,000 or more, and more preferably 140,000 or more. The weight-average molecular weight of the (meth)acrylic copolymer (A) is preferably 300,000 or less, more preferably 250,000 or less, and more preferably 200,000 or less. When the weight-average molecular weight of the (meth)acrylic copolymer (A) is 80,000 or more, dispersion and dissolution in an alkaline aqueous solution can be reduced, and contamination of the alkaline aqueous solution can be reduced. When the weight-average molecular weight of the (meth)acrylic copolymer (A) is 300,000 or less, the pressure-sensitive adhesive can be more suitably used as a hot-melt pressure-sensitive adhesive.

[0044] The weight-average molecular weight of the (meth)acrylic copolymer (B) is preferably 2,000 or more, more preferably 10,000 or more, more preferably 11,000 or more, more preferably 12,000 or more, and more preferably 13,000 or more. The weight-average molecular weight of the (meth)acrylic copolymer (B) is preferably 50,000 or less, more preferably 40,000 or less, and more preferably 30,000 or less. When the weight-average molecular weight of the (meth)acrylic copolymer (B) is 2,000 or more, dispersion and dissolution in an alkaline aqueous solution are reduced, and contamination of the alkaline aqueous solution can be reduced. When the weight-average molecular weight of the (meth)acrylic copolymer (B) is 50,000 or less, the molecular movement of the (meth)acrylic copolymer (B) in the (meth)acrylic copolymer is smooth. When the pressure-sensitive adhesive is brought into contact with an alkaline aqueous solution, the pressure-sensitive adhesive smoothly absorbs and retains the alkaline aqueous solution, swells, and the adhesiveness of the pressure-sensitive adhesive can be easily reduced.

[0045] The molecular weight distribution (Mw / Mn) of the (meth)acrylic copolymer (A) is preferably 10.0 or less, more preferably 6.0 or less, and even more preferably 5.0 or less. When the (meth)acrylic copolymer (A) has a molecular weight distribution (Mw / Mn) of 10.0 or less, dispersion and dissolution in an alkaline aqueous solution can be reduced, thereby reducing contamination of the alkaline aqueous solution.

[0046] The molecular weight distribution (Mw / Mn) of the (meth)acrylic copolymer (B) is preferably 5.0 or less, more preferably 3.0 or less, and even more preferably 2.0 or less. When the (meth)acrylic copolymer (B) has a molecular weight distribution (Mw / Mn) of 5.0 or less, contamination of the alkaline aqueous solution due to dispersion or dissolution of the PSA in the alkaline aqueous solution can be reduced.

[0047] The weight-average molecular weight and number-average molecular weight of the (meth)acrylic copolymer are values ​​measured by GPC (gel permeation chromatography) in terms of polystyrene. Specifically, 0.01 g of the (meth)acrylic copolymer is collected and placed in a test tube. THF (tetrahydrofuran) is added to the test tube to dilute the (meth)acrylic copolymer 500 times, and the mixture is filtered to prepare a measurement sample.

[0048] Using this measurement sample, the weight average molecular weight Mw and number average molecular weight Mn of the (meth)acrylic copolymer can be measured by GPC.

[0049] The weight average molecular weight Mw and number average molecular weight Mn of the (meth)acrylic copolymer can be measured, for example, using the following measuring device and under the following measuring conditions. Measurement equipment: Waters ACQUITY APC system Measurement conditions Column: Waters HSPgel™ HR MB-M Mobile phase: Tetrahydrofuran 0.5mL / min Detector: RI detector Standard material: polystyrene SEC temperature: 40℃

[0050] The (meth)acrylic copolymer (B) preferably does not contain a UV-crosslinkable group-containing monomer unit. By not introducing a crosslinked structure into the (meth)acrylic copolymer (B), the (meth)acrylic copolymer (B), which has a high affinity for alkaline aqueous solutions, can improve its ability to absorb and retain the alkaline aqueous solution. By contacting the pressure-sensitive adhesive with the alkaline aqueous solution, the (meth)acrylic copolymer (B) can smoothly absorb and retain the alkaline aqueous solution, smoothly swelling the pressure-sensitive adhesive and reducing the adhesiveness of the pressure-sensitive adhesive.

[0051] On the other hand, the (meth)acrylic copolymer (A) preferably contains a monomer containing an ultraviolet-crosslinkable group. By introducing a crosslinked structure into the (meth)acrylic copolymer (A), the dispersion and dissolution of the pressure-sensitive adhesive in an alkaline aqueous solution can be reduced, and contamination of the alkaline aqueous solution by the pressure-sensitive adhesive can be reduced.

[0052] The content of the ultraviolet-crosslinkable group-containing monomer unit in the (meth)acrylic copolymer (A) is preferably 0.01% by mass or more, more preferably 0.05% by mass or more, and even more preferably 0.1% by mass or more. When the content of the ultraviolet-crosslinkable group-containing monomer unit is 0.01% by mass or more, a crosslinked structure is introduced into the (meth)acrylic copolymer, and a retention space for stably retaining an alkaline aqueous solution can be formed within the crosslinked (meth)acrylic copolymer. By contacting the pressure-sensitive adhesive with an alkaline aqueous solution, the alkaline aqueous solution can be easily retained within the crosslinked (meth)acrylic copolymer, and the pressure-sensitive adhesive can be smoothly swollen.

[0053] The content of the ultraviolet-crosslinkable group-containing monomer unit in the (meth)acrylic copolymer (A) is preferably 1% by mass or less, more preferably 0.8% by mass or less, more preferably 0.6% by mass or less, and still more preferably 0.4% by mass or less. When the content of the ultraviolet-crosslinkable group-containing monomer unit is 1% by mass or less, the pressure-sensitive adhesive exhibits excellent adhesiveness after crosslinking by ultraviolet irradiation.

[0054] When the (meth)acrylic copolymer contains (meth)acrylic copolymers (A) and (B), the content of (meth)acrylic copolymer (B) in the (meth)acrylic copolymer is preferably 1 part by mass or more, more preferably 3 parts by mass or more, and more preferably 5 parts by mass or more, per 100 parts by mass of (meth)acrylic copolymer (A). The content of (meth)acrylic copolymer (B) in the (meth)acrylic copolymer is preferably 30 parts by mass or less, more preferably 20 parts by mass or less, more preferably 15 parts by mass or less, and more preferably 13 parts by mass or less, per 100 parts by mass of (meth)acrylic copolymer (A). When the content of (meth)acrylic copolymer (B) is 1 part by mass or more, when the pressure-sensitive adhesive is brought into contact with an alkaline aqueous solution, the pressure-sensitive adhesive smoothly absorbs and retains the alkaline aqueous solution inside, swells, and the adhesiveness of the pressure-sensitive adhesive can be easily reduced. When the content of the (meth)acrylic copolymer (B) is 20 parts by mass or less, dispersion and dissolution in the alkaline aqueous solution are reduced, and contamination of the alkaline aqueous solution can be reduced.

[0055] When the (meth)acrylic copolymer is crosslinked, the gel fraction of the crosslinked (meth)acrylic copolymer is preferably 60% by mass or more, more preferably 65% ​​by mass or more. The gel fraction of the (meth)acrylic copolymer is preferably 80% by mass or less, more preferably 78% by mass or less. When the gel fraction of the crosslinked (meth)acrylic copolymer is 60% by mass or more, dispersion and dissolution in alkaline aqueous solutions are reduced, and contamination of the alkaline aqueous solutions can be reduced. When the gel fraction of the crosslinked (meth)acrylic copolymer is 80% by mass or less, the adhesive has sufficient wettability to adherends and label substrates and exhibits excellent adhesive properties.

[0056] The gel fraction of the crosslinked (meth)acrylic copolymer is determined by immersing Xg of the crosslinked (meth)acrylic copolymer in Yg of tetrahydrofuran at 23°C for 24 hours. The amount of tetrahydrofuran, Yg, is 20 times the amount of Xg of the crosslinked (meth)acrylic copolymer. The insoluble matter is filtered through a 200-mesh wire netting, and the residue on the wire netting is vacuum-dried at 80°C for 2 hours. The mass, Zg, of the dried residue on the wire netting is measured, and the value is calculated based on the following formula: Gel fraction (mass%) = 100 × Z / X

[0057] [Additives] The (meth)acrylic copolymer may contain additives such as a tackifier, an ultraviolet polymerization initiator, a plasticizer, an antioxidant, a colorant, a flame retardant, and an antistatic agent, within limits that do not impair its physical properties.

[0058] [(Meth)acrylic copolymer] The (meth)acrylic copolymer is produced by a general-purpose radical polymerization method, for example, by radically polymerizing a monomer composition containing an alkyl(meth)acrylate and a monomer having a carboxy group, and optionally a monomer containing an ultraviolet crosslinkable group, in a solvent using a radical polymerization initiator, optionally in the presence of a molecular weight modifier.

[0059] The acid value of the (meth)acrylic copolymer can be controlled, for example, by adjusting the content of the monomer unit having a carboxy group contained in the (meth)acrylic copolymer. By increasing (decreasing) the content of the monomer unit having a carboxy group in the (meth)acrylic copolymer, the acid value of the (meth)acrylic copolymer can be increased (decreased).

[0060] The molecular weight modifier is not particularly limited, and examples thereof include monofunctional thiol compounds such as n-dodecyl mercaptan and stearyl-3-mercaptopropionate, and polyfunctional thiol compounds such as trimethylolpropane tris(3-mercaptopropionate), trimethylolpropane tris(3-mercaptopropionate)tris(3-mercaptopropionyloxy)ethyl isocyanurate, pentaerythritol tetrakis(3-mercaptopropionate), and dipentaerythritol hexakis(3-mercaptopropionate).

[0061] The radical polymerization initiator may be any known polymerization initiator used in radical polymerization. Examples of the radical polymerization initiator include photoradical polymerization initiators such as benzophenone, α-hydroxy-α,α'-dimethyl-acetophenone, 4-(2-hydroxyethoxy)phenyl(2-hydroxy-2-propyl)ketone, 2,2-dimethoxy-1,2-diphenylethan-1-one, 1-hydroxy-cyclohexyl-phenyl-ketone, and 2-methyl-1[4-(methylthio)phenyl]-2-morpholinopropan-1-one, and thermal cleavage radical polymerization initiators such as t-hexyl peroxypivalate, benzoyl peroxide, cumene hydroperoxide, diisopropyl peroxydicarbonate, cumyl peroxyneodecanoate, cumyl peroxyoctoate, and azobisisobutyronitrile. The radical polymerization initiators may be used alone or in combination of two or more.

[0062] The solvent used in radical polymerization is not particularly limited as long as it is a solvent that has been conventionally used in radical polymerization. Examples thereof include ester solvents such as ethyl acetate, propyl acetate, and butyl acetate, with ethyl acetate being preferred.

[0063] When the (meth)acrylic copolymer is a mixture of two types of (meth)acrylic copolymers (A) and (B) having different acid values, the (meth)acrylic copolymer (A) and the (meth)acrylic copolymer (B) may be produced by polymerization, and the resulting (meth)acrylic copolymers (A) and (B) may be mixed together. The (meth)acrylic copolymer (A) [(B)] may also be polymerized in a reaction solution containing the (meth)acrylic copolymer (B) [(A)].

[0064] The method for adjusting the acid value of the (meth)acrylic copolymer is not particularly limited, and can be carried out, for example, by adjusting the content of the monomer unit having a carboxy group contained in the (meth)acrylic copolymer. By increasing (decreasing) the content of the monomer unit having a carboxy group contained in the (meth)acrylic copolymer, the acid value of the (meth)acrylic copolymer can be increased (decreased).

[0065] The (meth)acrylic copolymer is preferably used as an adhesive, particularly as an adhesive for attaching an adhesive such as a label to an adherend such as a container. The adhesive can be preferably used as a hot melt adhesive that does not contain a solvent (e.g., organic solvents such as alcohol, xylene, toluene, benzene, ethyl acetate, propyl acetate, and butyl acetate, water, etc.).

[0066] The procedure for attaching a label to the outer periphery of a container using an adhesive containing a (meth)acrylic copolymer is described below. The label to be attached to the outer periphery of a container is formed in a strip shape having a length slightly longer than the outer periphery of the container. The label includes, for example, a sheet-like label base containing a synthetic resin (e.g., polyethylene terephthalate, polyolefin resin, etc.) and an adhesive layer containing an adhesive, laminated and integrated onto one surface of the label base. The adhesive is applied to one of the longitudinal ends of the label, after heating as necessary. Thereafter, the label is wrapped around the outer periphery of the container, and both longitudinal ends of the label are overlapped with each other via the adhesive. After that, the adhesive is crosslinked as necessary to bond both ends of the label together, thereby attaching the label to the outer periphery of the container in a wrapped state. Alternatively, an adhesive may be applied to one side of the label substrate, preferably the entire surface of the label substrate, after heating as necessary, and the adhesive may be subjected to a crosslinking treatment as necessary to form an adhesive layer on one side of the label substrate, and the label may be attached and integrated to the outer peripheral surface of the container using the adhesive layer formed on that side.

[0067] Examples of crosslinking treatments for adhesives include (1) a method of crosslinking an adhesive by irradiating it with ionizing radiation (e.g., alpha rays, beta rays, electron beams, ultraviolet rays, etc.), (2) a method of crosslinking an adhesive by adding a curing agent, and (3) a method of crosslinking an adhesive by heating it.

[0068] A pressure-sensitive adhesive containing a (meth)acrylic copolymer has excellent adhesive properties under normal conditions, and can stably adhesively integrate desired members together over a long period of time.

[0069] On the other hand, adhesives such as labels attached to the outer periphery of adherends such as containers are peeled off and removed from the adherend after use or after a predetermined period of time has passed in order to recycle the adherend, etc. The adhesive properties of the pressure-sensitive adhesive can be easily reduced by contacting it with an alkaline aqueous solution, and the adhesive can be easily peeled off and removed from the adherend.

[0070] The alkaline aqueous solution is not particularly limited, and examples thereof include an aqueous tetramethylammonium hydroxide solution, an aqueous potassium hydroxide solution, and an aqueous sodium hydroxide solution.

[0071] The (meth)acrylic copolymer contained in the pressure-sensitive adhesive has excellent adhesiveness in normal state, and the pressure-sensitive adhesive member can be attached or adhered integrally to a desired location on an adherend.

[0072] On the other hand, when a pressure-sensitive adhesive containing a (meth)acrylic copolymer is brought into contact with an alkaline aqueous solution, it can smoothly absorb and retain the alkaline aqueous solution inside, causing it to swell, while reducing its solubility and dispersion in the alkaline aqueous solution. This easily reduces the adhesiveness of the pressure-sensitive adhesive, making it possible to easily peel an adhesive object such as a label from an adherend such as a container.

[0073] The PSA has reduced solubility and dispersibility in the alkaline aqueous solution, reducing contamination of the alkaline aqueous solution. This allows the alkaline aqueous solution to be used for a long period of time, and also makes it possible to generally prevent the PSA contained in the alkaline aqueous solution from adhering to the surface of an adherend that has been brought into contact with the alkaline aqueous solution. This eliminates the need for a cleaning step to remove the PSA from the adherend, allowing the adherend to be smoothly subjected to subsequent steps. [Effects of the Invention]

[0074] The adhesive of the present invention has excellent adhesiveness under normal conditions, and the adhesiveness can be easily reduced by contacting it with an alkaline aqueous solution.

[0075] The adhesive of the present invention has reduced solubility and dispersibility in alkaline aqueous solutions, which generally prevents contamination of the alkaline aqueous solution and enables the alkaline aqueous solution to be used over a long period of time. It also generally prevents adhesive deposits from adhering to components that have come into contact with the alkaline aqueous solution, allowing subsequent processes to be carried out smoothly on the adherend, such as a container. DETAILED DESCRIPTION OF THE INVENTION

[0076] The present invention will be described in more detail below using examples, but the present invention is not limited thereto. Specific numerical values ​​of blending ratios (content ratios), physical property values, parameters, etc. used in the following description can be replaced with the upper limit values ​​(numeric values ​​defined as "equal to or less than") or lower limit values ​​(numeric values ​​defined as "equal to or greater than") of the corresponding blending ratios (content ratios), physical property values, parameters, etc. described in the "Summary of the Invention." [Example]

[0077] The following compounds were used in the examples and comparative examples.

[0078] [Synthesis of (meth)acrylic copolymer (A)] [(Meth)acrylic monomers] Butyl acrylate Acrylic acid 4-Acryloyloxybenzophenone

[0079] [Molecular weight regulator] n-Dodecyl mercaptan

[0080] [Thermal cleavage type radical polymerization initiator] t-Hexyl peroxypivalate (NOF Corporation, trade name "Perhexyl PV")

[0081] [Dilution solvent] Ethyl acetate

[0082] [Synthesis of (meth)acrylic copolymer (B)] [(Meth)acrylic monomers] Butyl acrylate Acrylic acid Methyl methacrylate

[0083] [Molecular weight regulator] n-Dodecyl mercaptan

[0084] [Thermal cleavage type radical polymerization initiator] Azobisisobutyronitrile (AIBN)

[0085] [Dilution solvent] Ethyl acetate

[0086] [Synthesis of (meth)acrylic copolymer (A)] A reaction liquid containing a (meth)acrylic monomer composition containing butyl acrylate, acrylic acid, and 4-acryloyloxybenzophenone, n-dodecyl mercaptan as a molecular weight modifier, and ethyl acetate as a dilution solvent in the prescribed amounts shown in Table 1 was supplied to a 2 L separable flask equipped with a stirrer, a condenser, a thermometer, and a nitrogen gas inlet, and the reaction liquid was stirred at a rotation speed of 100 rpm.

[0087] After the atmosphere in the separable flask was replaced with nitrogen gas, the reaction solution was heated to 96°C using a water bath. Next, a predetermined amount of t-hexyl peroxypivalate (manufactured by NOF Corporation, trade name "Perhexyl PV") was added as a thermal cleavage radical polymerization initiator to the separable flask, as shown in Table 1, and radical polymerization was carried out for 6 hours, after which the radical polymerization reaction was terminated. The reaction solution was left at 130°C under normal pressure for 2 hours, and then the solvent was removed at 130°C under reduced pressure for 2 hours, yielding a (meth)acrylic copolymer (A). The content (% by mass) of the monomer units constituting the (meth)acrylic copolymer (A) was the same as the content (% by mass) of the monomers contained in the acrylic monomer composition.

[0088] The number average molecular weight Mn, weight average molecular weight Mw, molecular weight distribution (Mw / Mn) and acid value of the obtained (meth)acrylic copolymer (A) were measured in the same manner as above. The results are shown in Table 1.

[0089] [Synthesis of (meth)acrylic copolymer (B)] A reaction liquid containing a (meth)acrylic monomer composition containing butyl acrylate, acrylic acid, and methyl methacrylate, n-dodecyl mercaptan as a molecular weight modifier, and ethyl acetate as a dilution solvent in the prescribed amounts shown in Table 2 was supplied to a 2 L separable flask equipped with a stirrer, a condenser, a thermometer, and a nitrogen gas inlet, and the reaction liquid was stirred at a rotation speed of 100 rpm.

[0090] After the atmosphere in the separable flask was replaced with nitrogen gas, the reaction solution was heated to 75°C using a water bath. Next, a predetermined amount of azobisisobutyronitrile as a thermal cleavage radical polymerization initiator shown in Table 2 was added to the separable flask, and radical polymerization was carried out for 6 hours, after which the radical polymerization reaction was terminated. The reaction solution was left at 130°C under normal pressure for 2 hours, and then the solvent was removed at 130°C under reduced pressure for 2 hours, thereby obtaining a (meth)acrylic copolymer (B). The content (% by mass) of the monomer units constituting the (meth)acrylic copolymer (B) was the same as the content (% by mass) of the monomers contained in the acrylic monomer composition.

[0091] The number average molecular weight Mn, weight average molecular weight Mw, molecular weight distribution (Mw / Mn) and acid value of the obtained (meth)acrylic copolymer (B) were measured in the same manner as above. The results are shown in Table 2.

[0092] (Examples 1 to 7, Comparative Examples 1 to 4) The (meth)acrylic copolymers (A) and (B) were heated to 130°C, and then the (meth)acrylic copolymers (A) and (B) were uniformly mixed in the predetermined amounts shown in Table 3 to prepare a pressure-sensitive adhesive containing the (meth)acrylic copolymer.

[0093] The acid value and swelling ratio (alkali) of the obtained (meth)acrylic copolymer were Swelling rate / water Swelling The results are shown in Table 3.

[0094] The resulting pressure-sensitive adhesive was measured for PET peel strength, alkaline hot water peelability, melt viscosity at 130°C, thermal stability, stain resistance and gel fraction in the following manner. The results are shown in Table 3.

[0095] [Peel strength against PET] After heating the adhesive to 130°C, the molten adhesive was coated onto a biaxially oriented polypropylene film (hereinafter referred to as "OPP film") with a width of 100 mm, a length of 250 mm, and a thickness of 60 μm to a thickness of 16 μm.

[0096] Next, an ultraviolet irradiation device [Heraeus (formerly Fusion UV Systems) product name "Light Hammer 6" (using an H bulb)] was used to irradiate the sample at an irradiation intensity of approximately 48 mW / cm 2 , the cumulative light intensity is approximately 100mJ / cm 2 The adhesive was irradiated with ultraviolet light (UV-C) to crosslink the adhesive [(meth)acrylic copolymer] (using the product name "UV Power Puck II" manufactured by EIT INSTRUMENT MARKETS).

[0097] A test film was prepared by laminating a 16 μm thick adhesive layer onto an OPP film, and this test film was cut into strips 15 mm wide to prepare test pieces.

[0098] Separately, an amorphous polyethylene terephthalate plate (hereinafter referred to as "PET plate") was prepared, and its surface was wiped and degreased with a mixed solvent of hexane and acetone (3:1). A test piece was attached to the surface of the PET plate using the adhesive layer, and then a 2 kg hand roller was rolled back and forth on the test piece to create a laminate. The laminate was left to cure in an atmosphere at 23°C for 24 hours.

[0099] Next, the test piece was peeled from the PET plate at an angle of 180° at a speed of 300 mm / min using a peel strength measuring device (Shimadzu Corporation, product name "Autograph AGS-100NX") to measure the peel strength (N / 25 mm).

[0100] [Alkaline hot water peelability] Test pieces were prepared in the same manner as when measuring the peel strength from PET. A 300 μm thick polyethylene terephthalate plate was prepared. The test piece was attached to the surface of the polyethylene terephthalate plate with an adhesive layer to prepare a test specimen, which was then left to cure in an atmosphere at 23°C for 3 days. Ten flat square sample pieces with sides of 7 mm were cut out from the test specimen.

[0101] An alkaline aqueous solution was prepared by dissolving 10.0 parts by mass of sodium hydroxide in 500 parts by mass of water.

[0102] The alkaline aqueous solution was supplied to a metal can and stirred at a rotation speed of 200 rpm using a stirrer. Next, the alkaline aqueous solution was heated to 80°C using a water bath, and 10 sample pieces were placed in the alkaline aqueous solution and stirred at a rotation speed of 200 rpm for 10 minutes. After stirring for 10 minutes, the sample pieces were removed from the alkaline aqueous solution and placed in 200 parts by mass of water to remove the alkaline aqueous solution adhering to the sample pieces.

[0103] For each sample piece, the area of ​​the test piece peeled off from the polyethylene terephthalate plate [peeled area (mm 2 The sum of the peeled areas of the test pieces of each sample piece was calculated. In Comparative Example 6, the pressure-sensitive adhesive layer of the test piece dissolved in the alkaline aqueous solution.

[0104] [Hot melt coatability (melt viscosity at 130°C)] The measuring equipment shown below was prepared. 13 g of adhesive was sampled and placed in an aluminum cylinder mounted inside the Thermosel. The temperature was set to 130°C to melt the adhesive. The melt viscosity was measured over 30 minutes using a spindle 4-29. The value after 30 minutes of measurement was read and used as the melt viscosity at 130°C. Measuring instrument: DV-E Viscometer (Brookfield) Thermosel (Brookfield)

[0105] [Thermal stability] The initial melt viscosity of the pressure-sensitive adhesive at 130°C was measured using the same method as in the measurement of "Melt viscosity at 130°C" above. Next, 13 g of the pressure-sensitive adhesive was sampled and left in an atmosphere at 130°C for 7 days. For the pressure-sensitive adhesive left in the atmosphere at 130°C, the melt viscosity of the pressure-sensitive adhesive after heating at 130°C was measured using the same method as in the measurement of "Melt viscosity at 130°C" above. The rate of change in melt viscosity was calculated based on the following formula. Change rate of melt viscosity (%) = 100 x (melt viscosity after heating - initial melt viscosity) / initial melt viscosity

[0106] [Gel fraction] A polyethylene terephthalate film with a release treatment on one side was prepared. The adhesive was heated to 130°C to melt it. The molten adhesive was applied to the release-treated surface of the polyethylene terephthalate film to a thickness of 16 μm. An ultraviolet irradiation device [Heraeus (formerly Fusion UV Systems) product name "Light Hammer 6" (using an H bulb)] was used to apply the adhesive at an irradiation intensity of approximately 48 mW / cm. 2 , the cumulative light intensity is approximately 100mJ / cm 2 The adhesive was irradiated with ultraviolet light (UV-C) to crosslink the adhesive [(meth)acrylic copolymer] (using the product name "UV Power Puck II" manufactured by EIT INSTRUMENT MARKETS).

[0107] 0.2 g of the crosslinked pressure-sensitive adhesive was immersed for 24 hours in 200 g of tetrahydrofuran at 23° C. The insoluble matter was filtered through a 200-mesh wire netting, and the residue on the wire netting was vacuum-dried at 80° C. for 2 hours. The mass Zg of the dried residue on the wire netting was measured, and the gel fraction (mass%) was calculated according to the following formula. Gel fraction (mass%) = 100 × Z / 0.2

[0108] [Stain resistance] A polyethylene terephthalate film with a release treatment on one side was prepared. The adhesive was heated to 130°C to melt it. The molten adhesive was applied to the release-treated surface of the polyethylene terephthalate film to a thickness of 16 μm. An ultraviolet irradiation device [Heraeus (formerly Fusion UV Systems) product name "Light Hammer 6" (using an H bulb)] was used to apply the adhesive at an irradiation intensity of approximately 48 mW / cm. 2 , the cumulative light intensity is approximately 100mJ / cm 2 The adhesive was irradiated with ultraviolet light (UV-C) to crosslink the adhesive [(meth)acrylic copolymer] (using the product name "UV Power Puck II" manufactured by EIT INSTRUMENT MARKETS).

[0109] An alkaline aqueous solution was prepared by dissolving 10.0 parts by mass of sodium hydroxide in 500 parts by mass of water. 0.2 g of a crosslinked pressure-sensitive adhesive was immersed in 200 g of alkaline aqueous solution at 23°C for 24 hours. The insoluble matter was filtered through a 200-mesh wire netting, and the residue on the wire netting was vacuum-dried at 80°C for 2 hours. The mass (Pg) of the dried residue on the wire netting was measured, and the alkaline gel fraction (mass%) was calculated using the following formula, which was used as the contamination resistance. The higher the alkaline gel fraction, the more likely it is that the alkaline aqueous solution will be less susceptible to contamination. Alkaline gel fraction (mass%) = 100 × P / 0.2

[0110] [Table 1]

[0111] [Table 2]

[0112] [Table 3]

Claims

1. the (meth)acrylic copolymer has a total acid value of 30 to 98 mgKOH / g, and the swelling ratio (alkali swelling ratio / water swelling ratio) of the alkali swelling ratio after immersion in an alkaline aqueous solution at 23°C for 24 hours and then immersion in an alkaline aqueous solution at 80°C for 2 hours to the water swelling ratio after immersion in water at 23°C for 24 hours and then immersion in water at 80°C for 2 hours is 1.3 or more; The (meth)acrylic copolymer is a (meth)acrylic copolymer (B) having an acid value of 60 to 150 mgKOH / g and not containing an ultraviolet crosslinkable group-containing monomer unit; a (meth)acrylic copolymer (A) having an acid value lower than that of the (meth)acrylic copolymer (B), ie, 15 mgKOH / g or more and less than 80 mgKOH / g, and containing an ultraviolet-crosslinkable group-containing monomer unit; The (meth)acrylic copolymer is a pressure-sensitive adhesive characterized in that it contains 1 to 30 parts by mass of the (meth)acrylic copolymer (B) per 100 parts by mass of the (meth)acrylic copolymer (A).

2. The adhesive described in claim 1, characterized in that the weight average molecular weight of the (meth)acrylic copolymer (B) is 2,000 to 50,000.

3. An adhesive as described in claim 1 or claim 2, characterized in that the gel fraction of the crosslinked body of the (meth)acrylic copolymer is 60 to 80 mass %.

4. The adhesive described in Claim 1, characterized in that the ultraviolet-crosslinkable group-containing monomer has a benzophenone group.

5. A hot melt pressure-sensitive adhesive comprising the pressure-sensitive adhesive according to any one of claims 1 to 4.

6. A label comprising a label substrate and the adhesive layer according to any one of claims 1 to 4 laminated integrally on one surface of the label substrate.

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