Alkali-peelable labels

Abstract

To provide a label that is easy to peel off by alkali peeling treatment even when it has been stuck on metal containers such as SUS.SOLUTION: An alkali-peelable label comprises a resin substrate and an adhesive layer. The adhesive layer comprises an acrylic copolymer and a compound with an acid value of 30 mgKOH / g or more. The acrylic copolymer is obtained by copolymerization of a monomer mixture that contains an acid group-containing vinyl monomer beyond 0.5 mass%.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to an alkali-peelable label. [Background technology]

[0002] In recent years, the reuse of empty containers has been attracting attention due to environmental considerations. When reusing containers, a cleaning process is required to remove the labels attached to the containers. In the cleaning process for removing the labels, the containers are immersed in alkaline water to remove the labels (for example, Patent Document 1). Labels that can be peeled off from the adherend by alkaline water are also called alkali-peelable labels.

[0003] Alkali-peelable labels are generally formed by laminating a pressure-sensitive adhesive layer on a substrate, and resin substrates have been widely used for alkali-peelable labels.

[0004] Various studies have been conducted on the adhesive layer of such acrylic peelable labels. For example, Patent Document 1 discloses an alkali-soluble adhesive composition in which a metal chelate is blended with a copolymer containing an alkyl (meth)acrylate ester, a (meth)acrylate ester having an ether bond, and an unsaturated monomer containing a carboxyl group and / or a hydroxyl group. In this composition, the (meth)acrylate ester having an ether bond imparts hydrophilicity to the adhesive composition, and the unsaturated monomer containing a carboxyl group and / or a hydroxyl group increases solubility in an alkaline aqueous solution and serves as a crosslinking point for the metal chelate compound, which is a crosslinking agent.

[0005] In view of the fact that the pressure-sensitive adhesive of Patent Document 1 is alkali-soluble and may contaminate alkaline water, Patent Document 2 discloses an alkali-removable pressure-sensitive adhesive composition having a specific monomer composition that undergoes interfacial peeling between the pressure-sensitive adhesive layer and the adherend surface, such as a glass container, in an alkaline aqueous solution. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 6-184508 [Patent Document 2] Japanese Patent Application Laid-Open No. 2012-153789 Summary of the Invention [Problem to be solved by the invention]

[0007] In recent years, with the increasing awareness of the environment, not only glass containers such as beer bottles and liquor bottles are being reused, but also metal containers such as stainless steel. Because glass surfaces are very smooth, labels attached to them tend to peel off easily, but compared to glass, alkaline stripping treatment is not as easy on adherends such as metal containers.

[0008] Therefore, an object of the present invention is to provide a label that can be easily peeled off by alkaline peeling treatment even from metal containers such as SUS. [Means for solving the problem]

[0009] The above object of the present invention can be achieved by the following means.

[0010] 1. An alkali-peelable label comprising a resin substrate and an adhesive layer, wherein the adhesive layer comprises an acrylic copolymer and a compound having an acid value of 30 mgKOH / g or more, and the acrylic copolymer is obtained by copolymerizing a monomer mixture containing more than 0.5 mass% of an acid group-containing vinyl monomer.

[0011] 2. The alkali-peelable label according to 1., wherein the compound is at least one selected from the group consisting of tackifiers, anionic surfactants, and polyester resins.

[0012] 3. The alkali-peelable label according to 1. or 2., wherein the acrylic copolymer is an emulsion-based polymer.

[0013] 4. An alkali-peelable label according to 2. or 3., wherein the anionic surfactant is a polymeric surfactant.

[0014] 5. The alkali-peelable label according to any one of 2. to 4., wherein the adhesive composition for forming the adhesive layer contains an emulsion of a tackifier and / or an aqueous polyester resin.

[0015] 6. The alkali-peelable label according to any one of 1. to 5., wherein the acid groups in the acrylic copolymer are not blocked with a crosslinking agent.

[0016] 7. The alkali-peelable label according to any one of 1. to 6., wherein the content of the compound is 1 to 50 parts by mass per 100 parts by mass of the acrylic copolymer.

[0017] 8. The alkali-peelable label according to any one of 1. to 7., wherein the monomer mixture contains 50 mass % or more of n-butyl (meth)acrylate and / or 2-ethylhexyl (meth)acrylate.

[0018] 9. An alkali-peelable label according to any one of 1. to 8., wherein the acid groups in the compound having an acid value of 30 mgKOH / g or more are not blocked with a neutralizing agent.

[0019] 10. A method for removing a label from an adherend, comprising contacting an adherend to which an alkali-peelable label according to any one of 1. to 9. has been affixed with alkaline water. [Effects of the Invention]

[0020] According to the present invention, it is possible to provide a label that can be easily peeled off by alkaline peeling treatment even from metal containers such as SUS in a cleaning process using alkaline water. [Brief explanation of the drawings]

[0021] [Figure 1]1 is a cross-sectional view showing an embodiment of an alkali-peelable label of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0022] One aspect of the present invention is an alkali-peelable label comprising a resin substrate and an adhesive layer, wherein the adhesive layer comprises an acrylic copolymer and a compound having an acid value of 30 mgKOH / g or more, and the acrylic copolymer is obtained by copolymerizing a monomer mixture containing more than 0.5 mass% of an acid group-containing vinyl monomer.

[0023] This configuration allows the label to be easily peeled off from metal containers such as stainless steel when immersed in alkaline water. The present invention requires two essential components: a certain amount or more of an acid group-containing vinyl monomer that constitutes the acrylic copolymer, and the addition of a compound with a specific acid value. If even one of these requirements is missing, the alkali-peelable label will be difficult to peel off from the adherend (e.g., a stainless steel adherend) even when immersed in alkaline water (see the comparative example below).

[0024] The mechanism by which the effects of the present invention are achieved by the configuration of the present invention is presumed to be as follows.

[0025] In this embodiment, the pressure-sensitive adhesive composition contains more than 0.5% by mass of an acid group-containing vinyl monomer among all monomers used to form the acrylic copolymer. It is believed that the presence of a certain amount of acid groups in the acrylic copolymer neutralizes the acid groups when immersed in alkaline water, improving the hydrophilicity of the pressure-sensitive adhesive layer and facilitating peeling of the label from the adherend. However, increasing the amount of acid group-containing vinyl monomer may reduce the stability of the dispersion or reduce the adhesive properties. For this reason, in the present application, a compound with a high acid value is used in combination with the acrylic copolymer. This neutralizes the acid groups in the compound with a high acid value when immersed in alkaline water, further improving hydrophilicity and facilitating peeling of the label from the adherend.

[0026] The above assumption does not limit the technical scope of the present invention in any way.

[0027] The present invention will be described in detail below.

[0028] In this specification, the range "X to Y" means "X or more and Y or less." Unless otherwise specified, operations and measurements of physical properties are performed at room temperature (20 to 25°C) and a relative humidity of 45 to 55% RH. In this specification, "(meth)acrylic acid" refers to "acrylic acid or methacrylic acid," and "(meth)acrylate" refers to "acrylate or methacrylate."

[0029] The concept of label includes things called tapes, sheets, films, etc.

[0030] Alkali-peelable labels are those that can be at least partially peeled off from an adherend when immersed in alkaline water (aqueous sodium hydroxide solution with a pH of 13) at 60°C for 10 minutes.Alkali-peelable labels are typically used in applications where the label is peeled off from an adherend by contacting it with an alkali.

[0031] FIG. 1 is a cross-sectional schematic diagram showing one embodiment of the alkali-peelable label of the present invention. The drawings are exaggerated for ease of explanation, and the dimensional proportions of the components in the drawings may differ from the actual proportions. In FIG. 1, the alkali-peelable label 10 is composed of a resin substrate 11, an adhesive layer 12, and a release liner 13. The release liner 13 is a component that protects the adhesive layer 12 and prevents a decrease in adhesiveness. The release liner 13 is peeled from the alkali-peelable label when it is attached to an adherend. Therefore, an alkali-peelable label that does not have a release liner 13 is also within the technical scope of the present invention. The alkali-peelable label 10 also has a printed portion 14. The printed portion 14 is optional, and an alkali-peelable label that does not have the printed portion 14 is also within the technical scope of the present invention.

[0032] In the embodiment shown in FIG. 1, the alkali-peelable label 10 is composed of a printed portion 14, a resin substrate 11, an adhesive layer 12, and a release liner 13. However, as long as the resin substrate 11, adhesive layer 12, and release liner 13 are arranged in this order, other functional layers (e.g., a primer layer between the resin substrate and the adhesive layer to improve adhesion, or a print-receiving layer between the resin substrate and the printed portion) may be present between or on each layer.

[0033] The adhesive strength of the alkali-peelable label to the adherend is preferably 4.0 N / 25 mm or more, and more preferably 6.0 N / 25 mm or more. Having an adhesive strength to the adherend that is equal to or greater than the above-mentioned lower limit ensures adhesion during normal use. Furthermore, the adhesive strength of the alkali-peelable label to the adherend is preferably 20.0 N / 25 mm or less, and more preferably 18.0 N / 25 mm or less. Having an adhesive strength that is equal to or less than the above-mentioned upper limit makes the label more likely to peel off when immersed in alkaline water. The adhesive strength to the adherend can be measured by the method described in the Examples below.

[0034] Each component constituting the alkali-peelable label will now be described.

[0035] [Resin substrate] The resin substrate may be, for example, a film made of at least one resin selected from polyolefins such as polyethylene and polypropylene, polyesters such as polyethylene terephthalate, polybutylene terephthalate, and polyethylene naphthalate, polyvinyl chloride, polystyrene, polyurethane, polycarbonate, polyamide, polyimide, polyetherimide, poly(meth)acrylic acid ester, polybutene, polybutadiene, polymethylpentene, acrylic urethane, ethylene-vinyl acetate copolymer, ethylene-(meth)acrylic acid copolymer, ethylene-(meth)acrylic acid ester copolymer, ABS resin, ionomer resin, and various thermoplastic elastomers, or a laminate film of one or more types selected from these.

[0036] The resin substrate may contain, as needed, stabilizers, lubricants, fillers, colorants, processing aids, softeners, metal powders, antifogging agents, ultraviolet absorbers, antioxidants, antistatic agents, flame retardants, etc. Examples of stabilizers that may be used include Ba-Zn-based, Cd-Ba-based, and Sn-based stabilizers, and these may be used in combination with epoxidized soybean oil, epoxy resins, etc. Examples of softeners that may be used include ethylene / vinyl acetate copolymers and ethylene / vinyl acetate / carbon monoxide copolymers.

[0037] The resin substrate may be a stretched film such as a uniaxially stretched film or a biaxially stretched film, or may be a non-stretched film, or may be formed by a casting method or the like using a processing material.

[0038] The thickness of the resin substrate may be set as appropriate, but considering the alkali peelability and mechanical strength of the label, it is preferably 10 to 500 μm, more preferably 20 to 100 μm.

[0039] [Adhesive layer] The pressure-sensitive adhesive layer is formed from a pressure-sensitive adhesive composition containing an acrylic copolymer.

[0040] (Adhesive composition) <Acrylic copolymer> The pressure-sensitive adhesive composition contains an acrylic copolymer obtained by copolymerizing a monomer mixture containing more than 0.5% by mass of an acid group-containing vinyl monomer.

[0041] The monomer mixture contains alkyl (meth)acrylate as the main monomer component, and optionally contains a monomer copolymerizable with alkyl (meth)acrylate (copolymerizable monomer). Here, the main component refers to 50% by mass or more of the monomers, preferably 65% ​​by mass or more, and more preferably 85% by mass or more.

[0042] The alkyl group of the alkyl (meth)acrylate may be any of a linear, branched, or cyclic alkyl group, and is preferably an alkyl group having 1 to 24 carbon atoms, and more preferably an alkyl group having 1 to 18 carbon atoms.

[0043] Examples of alkyl (meth)acrylates include methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, isopropyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, n-pentyl (meth)acrylate, n-hexyl (meth)acrylate, cyclohexyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, isooctyl (meth)acrylate, decyl (meth)acrylate, dodecyl (meth)acrylate, myristyl (meth)acrylate, palmityl (meth)acrylate, and stearyl (meth)acrylate. These may be used alone or in combination of two or more. Among these, considering stripping with alkaline water, it is preferable to use n-butyl (meth)acrylate and / or 2-ethylhexyl (meth)acrylate as the alkyl (meth)acrylate, and it is more preferable to use 2-ethylhexyl (meth)acrylate because this can shorten the stripping time with alkaline water, and it is even more preferable to use 2-ethylhexyl acrylate. In one preferred embodiment, the monomer mixture contains 50 mass% or more of n-butyl (meth)acrylate and / or 2-ethylhexyl (meth)acrylate (preferably 2-ethylhexyl (meth)acrylate, more preferably 2-ethylhexyl acrylate), and further, it is preferable that the monomer mixture contains 50 to 99.5 mass%, 70 to 97 mass%, and 80 to 95 mass%, respectively, of n-butyl (meth)acrylate and / or 2-ethylhexyl (meth)acrylate (preferably 2-ethylhexyl (meth)acrylate, more preferably 2-ethylhexyl acrylate). When n-butyl (meth)acrylate and / or 2-ethylhexyl (meth)acrylate is used, the amount is preferably 50 to 99.5 mass %, more preferably 70 to 97 mass %, and even more preferably 80 to 95 mass %, based on the total amount of alkyl (meth)acrylate and acid group-containing vinyl polymer.

[0044] From the viewpoint of adhesiveness, it is preferable to use methyl (meth)acrylate in combination with n-butyl (meth)acrylate and / or 2-ethylhexyl (meth)acrylate, and it is more preferable to use methyl methacrylate in combination. The content of methyl (meth)acrylate is preferably 0.5 to 20 mass%, more preferably 1 to 20 mass%, even more preferably 1 to 15 mass%, and even more preferably 5 to 15 mass%, based on the total amount of monomers.

[0045] In this embodiment, an acid group-containing vinyl monomer is used to facilitate peeling with alkaline water. Examples of the acid group-containing vinyl monomer include carboxyl group-containing vinyl monomers (compounds having an ethylenic double bond and a carboxyl group in the same molecule) such as (meth)acrylic acid, maleic acid, fumaric acid, crotonic acid, itaconic acid, myristoleic acid, palmitoleic acid, and oleic acid; carboxylic acid anhydride group-containing vinyl monomers (compounds having an ethylenic double bond and a carboxylic acid anhydride group in the same molecule) such as maleic anhydride and itaconic anhydride; acrylic monomers having a phosphoric acid group such as 2-methacryloyloxyethyl diphenyl phosphate (meth)acrylate, trimethacryloyloxyethyl phosphate (meth)acrylate, and triacryloyloxyethyl phosphate (meth)acrylate; and acrylic monomers having a sulfonic acid group such as sodium sulfopropyl (meth)acrylate, sodium 2-sulfoethyl (meth)acrylate, and sodium 2-acrylamido-2-methylpropanesulfonate.

[0046] Among these, the acid group-containing vinyl monomer is preferably a carboxyl group-containing vinyl monomer, more preferably (meth)acrylic acid, and even more preferably acrylic acid, since peeling with alkaline water is effectively carried out.

[0047] The acid group-containing vinyl monomer may be used alone or in combination of two or more kinds.

[0048] The content of the acid group-containing vinyl monomer in the total monomers exceeds 0.5% by mass. When the content of the acid group-containing vinyl monomer is 0.5% by mass or less, the film is less likely to peel from the adherend even when immersed in alkaline water. The content of the acid group-containing vinyl monomer in the total monomers is preferably 1.0% by mass or more, more preferably 2.0% by mass or more, and even more preferably 3.0% by mass or more. From the viewpoint of production, the upper limit of the content of the acid group-containing vinyl monomer in the total monomers is usually 10% by mass or less, and may be less than 5% by mass or 4.0% by mass or less.

[0049] The monomer mixture may also contain other copolymerizable monomers copolymerizable with alkyl (meth)acrylate. Examples of copolymerizable monomers copolymerizable with alkyl (meth)acrylate include hydroxyalkyl (meth)acrylates such as 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, and 4-hydroxybutyl (meth)acrylate, and hydroxyl group-containing monomers such as glycerin dimethacrylate; amide group-containing monomers such as acrylamide, methacrylamide, N-vinylpyrrolidone, and N,N-dimethylacrylamide; amino group-containing monomers such as aminoethyl (meth)acrylate and (meth)acryloylmorpholine; aromatic vinyl compounds such as styrene and substituted styrene; cyano group-containing monomers such as acrylonitrile; and vinyl esters such as vinyl acetate. These may be used alone or in combination of two or more.

[0050] When other copolymerizable monomers are used, they are preferably contained in an amount of 5% by mass or less (lower limit: 0% by mass) of the total monomers, more preferably 3% by mass or less, even more preferably 1% by mass or less, and particularly preferably substantially free. Here, "substantially free" means that the inclusion of impurities is permitted, and specifically means 0.01% by mass or less, or even 0% by mass.

[0051] Considering the performance as a display label at room temperature (not easily peeled off), the glass transition temperature of the acrylic copolymer is preferably −55 to −20° C. The glass transition temperature can be adjusted by the type and content of the monomers constituting the acrylic copolymer.

[0052] The glass transition temperature of the acrylic copolymer is calculated from the Tgn of each of the constituent polymers constituting the copolymer according to the Fox equation below.

[0053] Fox formula: 1 / Tg = Σ(Wn / Tgn) Tg: Calculated Tg of the polymer (K) Wn: weight fraction of monomer n Tgn: Glass transition temperature (K) of the homopolymer of monomer n The Tg value (Tgn) of a homopolymer of monomer n is described, for example, in technical documents from monomer manufacturers such as Nippon Shokubai Co., Ltd., Mitsubishi Chemical Corporation, and Toagosei Co., Ltd., in Polymer Data Handbook (published by Baifukan, edited by the Society of Polymer Science (Basics), first edition January 1986), and in Polymer Handbook 4th edition (J. Brandrup, E. H. Immergut, E. A. Grulke, published in 1999, Wiley-Interscience).

[0054] The content of the acrylic copolymer in the pressure-sensitive adhesive composition is preferably 50 to 99 mass %, more preferably 60 to 99 mass %.

[0055] The method for producing the acrylic copolymer is not particularly limited, and conventionally known methods such as solution polymerization using a polymerization initiator, emulsion polymerization, suspension polymerization, reverse-phase suspension polymerization, thin-film polymerization, and spray polymerization can be used. In addition to the method of initiating polymerization using a polymerization initiator, a method of initiating polymerization by irradiation with radiation, electron beams, ultraviolet rays, or the like can also be employed. Among these, emulsion polymerization is preferred because it further enhances the effects of the present invention. That is, in a preferred embodiment of the present invention, the acrylic copolymer is an emulsion polymer.

[0056] When the acrylic copolymer is an emulsion polymer, it is preferable that the acid groups in the acrylic copolymer are not blocked with a crosslinking agent. Emulsion polymers are preferred because they are easy to obtain high molecular weight polymers, exhibit adhesive properties without using a crosslinking agent, and are not blocked with the acid groups because this improves the releasability from the adherend with alkaline water.

[0057] An example of the emulsion polymerization method is a method in which an emulsifier and a polymerization initiator are added to a monomer mixture containing the above-mentioned monomers, and emulsion polymerization is carried out.

[0058] In emulsion polymerization, from the viewpoint of polymerization stability, it is preferable that the emulsifier (or a part of the emulsifier) ​​is dissolved in the monomer mixture or that the monomer mixture is made into an O / W emulsion in advance.

[0059] The procedure for carrying out emulsion polymerization may be, for example, the following methods (1) to (3). (1) The entire amount of the monomer mixture, emulsifier, water, etc. is charged, the temperature is raised, and the polymerization initiator dissolved in water is added dropwise or in portions to carry out polymerization. (2) Water, an emulsifier, and a portion of the monomer mixture are charged into a reaction vessel, and the temperature is raised. Then, a polymerization initiator dissolved in water is added dropwise or in portions to proceed with the polymerization reaction. Then, the remaining monomer mixture is added dropwise or in portions to continue the polymerization. (3) A polymerization initiator dissolved in water is placed in a reaction vessel, and after the temperature is raised, an emulsion consisting of a monomer mixture, an emulsifier, and water is added dropwise or in portions to carry out polymerization.

[0060] The emulsifier is not particularly limited, but from the viewpoint of improving the dispersion stability of the emulsion polymer, anionic emulsifiers or nonionic emulsifiers are preferred, and anionic emulsifiers are more preferred.

[0061] Examples of anionic emulsifiers include sodium lauryl sulfate, ammonium lauryl sulfate, sodium dodecylbenzenesulfonate, sodium polyoxyethylene alkyl ether sulfate, sodium polyoxyethylene alkylphenyl ether sulfate, ammonium polyoxyalkylene alkenyl ether sulfate, sodium polyoxyethylene alkyl ether sulfate, and allyl alkyl sulfosuccinate salts. Examples of nonionic emulsifiers include polyoxyethylene alkyl ether and polyoxyethylene alkylphenyl ether. These emulsifiers may be used alone or in combination of two or more.

[0062] The amount of emulsifier added is preferably 0.5 to 12 parts by mass, more preferably 0.5 to 8 parts by mass, and even more preferably 0.7 to 6 parts by mass, per 100 parts by mass of the monomer mixture, from the viewpoint of stability of the emulsion polymerization reaction and prevention of deterioration of physical properties due to remaining unreacted emulsifier.

[0063] The emulsifier may be added directly to a solution prepared by adding water to the monomer mixture, or may be added in advance to a polymerization vessel, or both may be used.

[0064] The polymerization initiator may be either water-soluble or oil-soluble. Examples include azo compounds such as 2,2'-azobis(2-methylpropionamidine) dihydrochloride and 2,2'-azobis(2-amidinopropane) dihydrochloride; persulfates such as potassium persulfate, sodium persulfate, and ammonium persulfate; and peroxides such as benzoyl peroxide, t-butyl hydroperoxide, and hydrogen peroxide. Redox initiators such as a combination of a persulfate and sodium bisulfite or a combination of a peroxide and sodium ascorbate may also be used. The polymerization initiators may be used alone or in combination of two or more. Among these, persulfates or redox initiators are preferred from the viewpoint of excellent polymerization stability.

[0065] The amount of polymerization initiator added is preferably 0.01 to 6 parts by mass, more preferably 0.03 to 4 parts by mass, and even more preferably 0.1 to 2 parts by mass, relative to 100 parts by mass of the monomer mixture, from the viewpoint of accelerating the polymerization rate.

[0066] The polymerization initiator may be added to the reaction vessel in advance, may be added immediately before the start of polymerization, may be added in multiple batches after the start of polymerization, may be added to the monomer mixture in advance, or may be added to an emulsion of the monomer mixture after preparing the emulsion.

[0067] During emulsion polymerization, a known chain transfer agent or pH buffer may be further added.

[0068] The water used in emulsion polymerization is preferably ion-exchanged water, and the amount of water used is preferably 30 to 400 parts by mass, more preferably 35 to 200 parts by mass, and even more preferably 40 to 150 parts by mass, per 100 parts by mass of the monomer mixture.

[0069] It is preferable to further add aqueous ammonia, various water-soluble amines, or aqueous alkali solutions such as aqueous sodium hydroxide and aqueous potassium hydroxide to the emulsion polymer dispersion obtained by emulsion polymerization to adjust the pH to 5 to 9 (preferably pH 6 to 8.5).

[0070] The solid content concentration of the emulsion polymer dispersion is preferably 10 to 80 mass %, more preferably 25 to 70 mass %, and even more preferably 45 to 65 mass %.

[0071] The viscosity of the emulsion polymer dispersion at 25° C. is preferably 50 to 12,000 mPa·s, more preferably 100 to 10,000 mPa·s, and even more preferably 200 to 9,000 mPa·s. In this specification, the viscosity is a value measured using a B-type rotational viscometer.

[0072] The emulsion polymer has an emulsion shape (particle shape) in which the emulsion polymer is dispersed. In this case, the average particle size of the emulsion polymer is preferably 500 nm or less, more preferably 300 nm or less, and even more preferably 200 nm or less, because this improves water resistance. The average particle size of the emulsion polymer is usually 50 nm or more, and more preferably 100 nm or more. Here, the average particle size of the emulsion polymer is the volume-based median size measured by laser diffraction dispersion.

[0073] <Compound having an acid value of 30 mgKOH / g or more (hereinafter, the compound having an acid value of 30 mgKOH / g or more will also be referred to as compound A)> The acid value of compound A is 30 mgKOH / g or more, preferably 50 mgKOH / g or more, and may be 100 mgKOH / g or more, 150 mgKOH / g or more, or even 200 mgKOH / g or more. In particular, when compound A is a tackifier and / or anionic surfactant, or even when compound A is a tackifier, the acid value is preferably 100 mgKOH / g or more, more preferably 150 mgKOH / g or more, and even more preferably 200 mgKOH / g or more, because this accelerates peeling in alkaline water. The upper limit of the acid value of compound A is preferably 700 mgKOH / g or less, more preferably 650 mgKOH / g or less, and even more preferably 600 mgKOH / g or less, because this allows for high adhesion even to non-polar adherends and allows for stable storage of the emulsion polymer before coating.

[0074] Note that compound A exerts its hydrophilic properties by neutralizing the acid groups when it comes into contact with alkaline water, and therefore, in the pressure-sensitive adhesive layer before coming into contact with alkaline water, it is preferable that the acid groups are not (all) blocked by, for example, a neutralizing agent.

[0075] The acid value is defined as the number of milligrams of potassium hydroxide required to neutralize the acid contained in 1 g of the solid content of the compound, and is a value measured in accordance with the potentiometric titration method described in JIS K 0070:1992.

[0076] The amount of compound A added in the pressure-sensitive adhesive composition is appropriately determined depending on the type of compound A. The amount of compound A added in the pressure-sensitive adhesive composition is, for example, 1 to 50% by mass relative to the acrylic copolymer.

[0077] The compound A may be used alone or in combination of two or more kinds.

[0078] The compound having an acid value of 30 mgKOH / g or more preferably has a molecular weight (mass average molecular weight) of 100 to 50,000. By having the molecular weight in this range, heat resistance and high adhesive strength can be obtained. The upper limit of the molecular weight (mass average molecular weight) is preferably 20,000 or less, 15,000 or less, and 10,000 or less, in that order. The lower limit of the molecular weight (mass average molecular weight) of the polymer is preferably, for example, 100 or more, 300 or more, and 500 or more, in that order. Within these ranges, alkali peelability can be further improved. The mass average molecular weight can be a value measured by GPC.

[0079] Compound A is preferably at least one selected from the group consisting of a tackifier, an anionic surfactant, and a polyester resin. Compound A may be a tackifier and / or an anionic surfactant. When compound A is a tackifier and / or a polyester resin, adhesive strength can be further improved. Therefore, compound A is more preferably a tackifier and / or a polyester resin, and particularly preferably a tackifier.

[0080] (tackifier) Examples of tackifiers having an acid value of 30 mgKOH / g or more include rosin resins. Accordingly, in one embodiment of the present invention, the PSA layer comprises at least one selected from the group consisting of rosin resins, anionic surfactants, and polyester resins, each having an acid value of 30 mgKOH / g or more. In yet another embodiment of the present invention, the PSA layer comprises at least one selected from the group consisting of rosin resins, anionic polymeric surfactants, and polyester resins, each having an acid value of 30 mgKOH / g or more. Examples of rosin resins include unmodified rosins such as gum rosin, wood rosin, and tall oil rosin; modified rosins obtained by modifying unmodified rosin through one or more treatments selected from hydrogenation, disproportionation, polymerization, and chemical modification; and various rosin derivatives. Examples of rosin derivatives include rosin esters obtained by esterifying unmodified rosin or modified rosin with alcohols; unsaturated fatty acid modified rosins obtained by modifying unmodified rosin or modified rosin with unsaturated fatty acids; unsaturated fatty acid modified rosin esters obtained by modifying rosin esters with unsaturated fatty acids; rosin alcohols obtained by reducing the carboxyl groups of unmodified rosin, modified rosin, unsaturated fatty acid modified rosin, or unsaturated fatty acid modified rosin esters; unmodified rosin, modified rosin, and various rosins. metal salts of rosins (particularly rosin esters) such as rosin derivatives; rosin phenolic resins obtained by adding phenol to rosins such as unmodified rosin, modified rosin, and various rosin derivatives using an acid catalyst and then thermally polymerizing the rosin; acid-modified rosins obtained by modifying unmodified rosin or modified rosin with acids such as acrylic acid, fumaric acid, or maleic acid; acid-modified rosin esters obtained by modifying rosin esters with acids such as acrylic acid, fumaric acid, or maleic acid, and polymerized hydrogenated rosins obtained by hydrogenating these.

[0081] When compound A is a tackifier, the amount of compound A added in the pressure-sensitive adhesive composition is, from the viewpoint of alkali peelability, preferably 1 part by mass or more, more preferably 5 parts by mass or more, even more preferably 10 parts by mass or more, and particularly preferably 15 parts by mass or more, relative to 100 parts by mass of the acrylic copolymer. Furthermore, when compound A is a tackifier, from the viewpoint of water resistance and transparency, the amount is preferably 50 parts by mass or less, more preferably less than 40 parts by mass, and even more preferably 30 parts by mass or less, relative to 100 parts by mass of the acrylic copolymer.

[0082] When the acrylic copolymer is an emulsion polymer, the tackifier can be added by dissolving the tackifier in the acrylic monomer and polymerizing it, or by emulsifying the tackifier alone and mixing it with an emulsion polymer obtained by polymerizing the acrylic monomer. Among these methods, emulsifying the tackifier alone and mixing it with an emulsion obtained by polymerizing the acrylic monomer is preferred because it results in a pressure-sensitive adhesive layer with high cohesive strength. That is, it is preferred that the pressure-sensitive adhesive composition for forming the pressure-sensitive adhesive layer contains an emulsion of the tackifier. Conventional emulsification methods can be used to emulsify the tackifier. For example, solvent-based emulsification, solventless emulsification, or phase inversion emulsification can be used. Conventional emulsifiers can be used to emulsify the tackifier, including one or more of anionic, nonionic, and cationic emulsifiers, as well as synthetic polymer emulsifiers. Among these, anionic emulsifiers are preferred from the viewpoint of emulsification properties. Specific examples of anionic emulsifiers include those described in the section on acrylic copolymers. The amount of emulsifier used is, for example, 1 to 10 parts by mass per 100 parts by mass of the tackifier.

[0083] (anionic surfactants) The anionic surfactant that can be used as compound A is preferably a polymeric surfactant obtained by copolymerizing a monomer component that contains a compound having an alkyl group with 12 or more carbon atoms and a radically polymerizable unsaturated double bond in the molecule, and a compound having an acid group such as a carboxyl group, sulfonic acid group, or phosphate group and a radically polymerizable unsaturated double bond in the molecule, and may further contain other compounds having radically polymerizable unsaturated double bonds as necessary, in order to further enhance the effects of the present invention. Here, the polymeric surfactant refers to one having a mass-average molecular weight of 1,000 or more, preferably 3,000 or more.

[0084] Examples of compounds having an alkyl group with 12 or more carbon atoms in the molecule and a radically polymerizable unsaturated double bond include alkyl (meth)acrylate compounds with 12 or more carbon atoms, such as lauryl (meth)acrylate and stearyl (meth)acrylate, and α-olefins with 14 or more carbon atoms, such as 1-tetradecene, 1-hexadecene, 1-octadecene, 1-eicosene, 1-docosene, 1-tetracosene, 1-octacosene, and 1-triacontene. There are no particular limitations on the upper limit of the number of carbon atoms in the alkyl group with 12 or more carbon atoms, but it is usually about 30 carbon atoms.

[0085] Furthermore, examples of compounds having an acid group and a radically polymerizable unsaturated double bond in the molecule include carboxyl group-containing unsaturated monomers such as (meth)acrylic acid, crotonic acid, maleic anhydride, maleic acid, monoalkyl ester compounds of maleic acid, itaconic acid, monoalkyl ester compounds of itaconic acid, citraconic acid, and monoalkyl ester compounds of citraconic acid; sulfone group-containing unsaturated monomers such as 2-acrylamido-2-methyl-1-propanesulfonic acid, 2-methacrylamido-2-methyl-1-propanesulfonic acid, and styrenesulfonic acid; and phosphate group-containing unsaturated monomers such as acid phosphonyl (meth)acrylate and acid phosphonylethyl (meth)acrylate.

[0086] Other compounds having a radically polymerizable unsaturated double bond include compounds having an alkyl group with 11 or less carbon atoms in the molecule and a radically polymerizable unsaturated double bond, such as methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, butyl (meth)acrylate, and hexyl (meth)acrylate; styrene-based monomer compounds such as styrene, α-methylstyrene, and vinyltoluene; and acrylamide-based monomer compounds.

[0087] In the monomer component consisting of the above compounds, the content of the compound having an alkyl group with 12 or more carbon atoms in the molecule and a radically polymerizable unsaturated double bond is preferably 5% by mass or more, and the upper limit thereof is preferably about 60% by mass.

[0088] In addition, some of the acid groups of the anionic surfactant may exist as salts. However, if they are all neutralized, it is difficult to obtain the desired effect when immersed in alkaline water, so partial neutralization is preferred. Examples of salts include ammonium salts, potassium salts, sodium salts, and lithium salts. Hereinafter, when simply referring to an acid group, this also includes its salts. (For example, when referring to a sulfonic acid group, this also includes a sulfonic acid (salt) group.)

[0089] Specific examples of polymeric anionic surfactants include the following: polymeric surfactants having sulfonic acid groups, such as polystyrene sulfonic acid, styrene / styrene sulfonic acid copolymer, 2-(meth)acryloylamino-2,2-dimethylethanesulfonic acid / (meth)acrylic acid copolymer, 2-(meth)acryloylamino-2,2-dimethylethanesulfonic acid / (meth)acrylic acid / acrylamide copolymer, naphthalenesulfonic acid formaldehyde condensate, methylnaphthalenesulfonic acid formaldehyde condensate, and dimethylnaphthalenesulfonic acid formaldehyde condensate; Polymeric surfactants having a sulfate ester group, such as 2-hydroxyethyl (meth)acrylate sulfate, 2-hydroxyethyl (meth)acrylate / 2-hydroxyethyl (meth)acrylate sulfate copolymer, and sulfated poly{2-hydroxyethyl (meth)acrylate}; phosphonic acids (salts), such as poly{(meth)acryloyloxyethyl phosphonic acid}, 2-hydroxyethyl (meth)acrylate / (meth)acryloyloxyethyl phosphonic acid copolymer, and naphthalene phosphonic acid formaldehyde condensate. polymeric surfactants having a phosphate group such as poly{2-hydroxyethyl (meth)acrylate phosphate ester}, 2-hydroxyethyl (meth)acrylate / 2-hydroxyethyl (meth)acrylate phosphate ester copolymer, and phosphate ester of poly{2-hydroxyethyl (meth)acrylate}; polymeric surfactants having a carboxylic acid group such as poly(meth)acrylic acid, (meth)acrylic acid-maleic acid copolymer, (meth)acrylic acid-itaconic acid copolymer, (meth)acrylic acid-fumaric acid copolymer, (meth)acrylic acid / vinyl acetate copolymer and 2-hydroxyethyl methacrylate / (meth)acrylic acid copolymer, carboxymethylated product of poly{2-hydroxyethyl (meth)acrylate}, carboxymethyl cellulose, carboxymethyl methyl cellulose, carboxymethyl ethyl cellulose, benzoic acid formaldehyde condensate, benzoic acid-phenol-formaldehyde condensate, and polyalkylene glycol (meth)acrylic acid ester / methacrylic acid copolymer.These polymer surfactants may be used alone or in combination of two or more.

[0090] Among these, from the viewpoint of the effects of the present invention, polymeric surfactants having a carboxylic acid (salt) group are preferred, and poly(meth)acrylic acid (salt) is particularly preferred.

[0091] When compound A is an anionic surfactant, the amount of compound A added to the pressure-sensitive adhesive composition is preferably 0.5 parts by mass or more, and more preferably 1 part by mass, per 100 parts by mass of the acrylic copolymer from the viewpoint of alkali peelability. Also, when compound A is an anionic surfactant, the amount is preferably 20 parts by mass or less, and more preferably 10 parts by mass or less, per 100 parts by mass of the acrylic copolymer from the viewpoint of adhesiveness.

[0092] (Polyester resin) Examples of polyester resins include copolymers obtained by polycondensation of an alcohol component and a carboxylic acid component, and modified products of such copolymers.

[0093] Examples of modified copolymers include polyurethane-modified polyester resins obtained by reacting hydroxyl groups at the terminals of a copolymer obtained by polycondensation of an alcohol component and a carboxylic acid component with a polyisocyanate compound. In the present invention, such modified polyester resins are also included in the "polyester resin."

[0094] As the alcohol component, a polyhydric alcohol having two or more hydroxyl groups in one molecule can be used.

[0095] Specific examples of the alcohol component include glycols such as ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, propylene glycol, dipropylene glycol, 1,4-butanediol, 1,3-butanediol, 2,3-butanediol, 1,2-butanediol, 3-methyl-1,2-butanediol, 1,2-pentanediol, 1,5-pentanediol, 1,4-pentanediol, 2,4-pentanediol, 3-methyl-4,5-pentanediol, 2,2,4-trimethyl-1,3-pentanediol, 1,6-hexanediol, 1,5-hexanediol, 1,4-hexanediol, 2,5-hexanediol, neopentyl glycol, and hydroxypivalic acid neopentyl glycol ester. Examples of suitable alcohol components include polylactone diols obtained by adding lactones such as ε-caprolactone to these glycols, polyester diols such as bis(hydroxyethyl) terephthalate, dihydric cyclic alcohols such as 1,3-cyclohexanedimethanol, 1,4-cyclohexanedimethanol, tricyclodecane dimethanol, hydrogenated bisphenol A, hydrogenated bisphenol F, spiroglycol, and dihydroxymethyltricyclodecane, ethylene oxide and propylene oxide adducts of bisphenol A, and trihydric or higher polyhydric alcohols such as glycerin, trimethylolpropane, trimethylolethane, diglycerin, triglycerin, 1,2,6-hexanetriol, pentaerythritol, dipentaerythritol, sorbitol, and mannitol. These alcohol components may be used alone or in combination of two or more.

[0096] As the carboxylic acid component, a polybasic acid having two or more carboxyl groups in one molecule can be used.

[0097] Specific examples of the carboxylic acid component include phthalic acid, isophthalic acid, terephthalic acid, naphthalenedicarboxylic acid, 4,4-diphenyldicarboxylic acid, diphenylmethane-4,4'-dicarboxylic acid, succinic acid, adipic acid, azelaic acid, sebacic acid, HET acid, maleic acid, and the like. Examples of the carboxylic acid component include dicarboxylic acids and anhydrides thereof, such as cyclohexane-1,3-dicarboxylic acid, cyclohexane-1,4-dicarboxylic acid, hexahydrophthalic acid, hexahydroisophthalic acid, hexahydroterephthalic acid, and methylhexahydrophthalic acid; tricarboxylic acids and anhydrides thereof, such as trimellitic acid, pyromellitic acid, trimesic acid, methylcyclohexene tricarboxylic acid, hexahydrotrimellitic acid, and tetrachlorohexene tricarboxylic acid; and tetracarboxylic acids and anhydrides thereof, such as 1,2,4,5-cyclohexanetetracarboxylic acid, 1,2,3,4-butanetetracarboxylic acid, 1,2,3,4-cyclobutanetetracarboxylic acid, 1,2,3,4-cyclopentanetetracarboxylic acid, and pyromellitic acid. These carboxylic acid components may be used alone or in combination of two or more.

[0098] When the acrylic copolymer is an emulsion polymer, the polyester resin may be added by dissolving the polyester resin in an acrylic monomer and polymerizing the resulting solution, or by mixing the aqueous polyester resin with an emulsion polymer obtained by polymerizing the acrylic monomer. Among these methods, mixing the aqueous polyester resin with an emulsion obtained by polymerizing the acrylic monomer is preferred because it results in a pressure-sensitive adhesive layer with high cohesive strength. In other words, it is preferred that the pressure-sensitive adhesive composition for forming the pressure-sensitive adhesive layer contains an aqueous polyester resin.

[0099] "Aqueous polyester resin" refers to a polyester resin that can be dissolved in an aqueous solvent to form an aqueous solution, or a polyester resin that can be dispersed as an emulsion in an aqueous solvent to form an aqueous dispersion. The use of such an "aqueous" polyester resin makes it possible to reduce the amount of volatile organic compounds emitted during coating. Here, an aqueous solvent refers to one that contains 60% by mass or more of water (up to 100% by mass), preferably 70% by mass or more, more preferably 85% by mass or more, and most preferably 95% by mass or more of aqueous solvent.

[0100] Components other than water contained in the aqueous solvent include water-soluble organic solvents, such as methanol, ethanol, isopropanol, butanol, acetone, methyl ethyl ketone, dimethylformamide, methyl cellosolve, tetrahydrofuran, and ethylene glycol mono-t-butyl ether.

[0101] In one embodiment of the present invention, in order to prepare an aqueous dispersion in which the aqueous polyester resin is dispersed as an emulsion in water, a small amount of an emulsifier, a surfactant, or the like may be used within a range that does not impair the effects of the present invention.

[0102] However, low-molecular-weight components such as emulsifiers and surfactants may be localized in the pressure-sensitive adhesive layer, resulting in reduced adhesiveness and possibly reduced interlayer adhesion. From the viewpoint of suppressing this phenomenon, in one embodiment of the present invention, the aqueous polyester resin is preferably a self-emulsifying aqueous polyester resin.

[0103] If the resin is a self-emulsifying aqueous polyester resin, it is possible to form an emulsion without using low-molecular-weight components such as emulsifiers or surfactants, which cause a decrease in interlayer adhesion, and therefore the interlayer adhesion of the resulting PSA sheet can be further improved. Note that "self-emulsifying" means that some kind of hydrophilic group is chemically introduced into the resin skeleton, and the resin itself has emulsifying ability, without the need to add an emulsifier or surfactant.

[0104] From the viewpoint of adjusting the acid value within the above range, the aqueous polyester resin used in one embodiment of the present invention preferably has a structural unit derived from a polybasic acid having three or more carboxyl groups in one molecule, and more preferably has a structural unit derived from a tricarboxylic acid or an anhydride of a tricarboxylic acid.

[0105] The aqueous polyester resin used in one embodiment of the present invention is preferably an aqueous polyester resin containing a carboxyl group.

[0106] From the viewpoint of improving the solubility or dispersibility in water and interlayer adhesion with the substrate and / or pressure-sensitive adhesive layer, the number average molecular weight (Mn) of the aqueous polyester resin is preferably 1,000 to 15,000, more preferably 1,500 to 10,000, and even more preferably 2,000 to 5,000. If the Mn of the aqueous polyester resin is in such a relatively low molecular weight range, the solubility or dispersibility in water is sufficiently high, and the coating liquid can be stored stably for a long period of time, which is preferable.

[0107] The glass transition temperature (Tg) of the aqueous polyester resin may be, for example, 10 to 70°C, or 30 to 65°C.

[0108] In the present invention, the number average molecular weight (Mn) is a value calculated as a standard polystyrene as measured by gel permeation chromatography (GPC), and specifically, is a value measured according to the method described below. (1) Number average molecular weight (Mn) Measurements are carried out using a gel permeation chromatograph (manufactured by Tosoh Corporation, product name "HLC-8320GPC") under the conditions below, and the values ​​measured are converted into standard polystyrene equivalents. Columns: "TSK guard column super HH", "TSK gel super HM-H (x2)", "TSK gel super HM-H" (all manufactured by Tosoh Corporation) Column temperature: 40℃ Developing solvent: tetrahydrofuran ·Flow rate: 1.0mL / min In the present invention, the glass transition temperature (Tg) can be specifically measured by the method described below. (2) Glass transition temperature (Tg) Measurements are performed in accordance with JIS K 7121:2012 using a differential scanning calorimeter (manufactured by TA Instruments Japan, product name "DSC Q2000") at a heating rate of 20°C / min.

[0109] When compound A is a polyester resin, the amount of compound A added in the pressure-sensitive adhesive composition is preferably 1 part by mass or more, more preferably 3 parts by mass or more, and even more preferably 5 parts by mass or more, relative to 100 parts by mass of the acrylic copolymer, from the viewpoint of alkali peelability. Furthermore, when compound A is a polyester resin, the amount is preferably 15 parts by mass or less, more preferably less than 12 parts by mass, and even more preferably 10 parts by mass or less, relative to 100 parts by mass of the acrylic copolymer, from the viewpoint of water resistance and transparency.

[0110] <Other ingredients> The pressure-sensitive adhesive composition may contain a crosslinking agent. Known crosslinking agents can be used as the crosslinking agent. Examples include, but are not limited to, isocyanate-based crosslinking agents, epoxy-based crosslinking agents, metal chelate-based crosslinking agents, aziridine-based crosslinking agents, oxazoline-based crosslinking agents, and carbodiimide-based crosslinking agents. Among these, isocyanate-based crosslinking agents are preferred from the viewpoint of reactivity. In particular, when the acrylic copolymer is an emulsion-based polymer, a pressure-sensitive adhesive composition that is substantially free of a crosslinking agent is also suitable. Here, "substantially free" means that the presence of impurities is permitted, specifically, 0.01 parts by mass or less, or even 0 parts by mass, per 100 parts by mass of the acrylic copolymer.

[0111] The crosslinking agent may be used alone or in the form of a mixture of two or more kinds.

[0112] When a crosslinking agent is added, the amount of the crosslinking agent added is preferably 0.1 to 10 parts by mass, more preferably 0.5 to 5 parts by mass, relative to 100 parts by mass of the acrylic copolymer.

[0113] The pressure-sensitive adhesive composition may further contain other conventionally known additives. Examples of such additives include fillers, pigments, ultraviolet absorbers, etc. Examples of fillers include zinc oxide, silica, calcium carbonate, etc.

[0114] <Formation method> The method for forming the adhesive layer is not particularly limited, and the adhesive layer may be formed by directly coating a substrate with an adhesive composition (for example, a mixture of an acrylic copolymer emulsion and an emulsion of a tackifier (compound A), a mixture of an acrylic copolymer emulsion and an anionic surfactant (compound A), or a mixture of an acrylic copolymer emulsion and an aqueous polyester resin (compound A)), or by forming an adhesive layer on a release liner and then laminating this to the substrate. Specifically, the adhesive layer may be formed by coating a release liner with an adhesive composition, drying it, and then transferring the adhesive layer made of the adhesive composition to the substrate.

[0115] The method for applying the pressure-sensitive adhesive composition to the substrate or release liner is not particularly limited, and the composition can be applied using a known coating device such as a roll coater, knife coater, air knife coater, bar coater, blade coater, slot die coater, lip coater, gravure coater, etc. Drying conditions are not particularly limited, and drying is usually performed at 60 to 150°C for 10 to 60 seconds.

[0116] The thickness of the pressure-sensitive adhesive layer (film thickness after drying) is usually 5 to 100 μm, preferably 10 to 50 μm.

[0117] [Release liner] The release liner is not particularly limited, but examples thereof include paper such as fine paper, glassine paper, clay-coated paper, and polyethylene-laminated paper; polyester films such as polyethylene terephthalate, polybutylene terephthalate, and polyethylene naphthalate; and plastic films such as polyolefin films such as polypropylene and polyethylene.

[0118] The thickness of the release liner is usually about 10 to 400 μm. The surface of the release liner may be provided with a layer of a release agent made of silicone or the like to improve the releasability of the pressure-sensitive adhesive layer. When such a layer is provided, the thickness of the layer is usually about 0.01 to 5 μm.

[0119] [Printing Department] There are no particular limitations on how the printed portion is formed, and it can be formed by, for example, flexographic printing, offset printing, letterpress printing, gravure printing, screen printing, etc., which are suitable for mass production of a fixed pattern, etc. On the other hand, thermal transfer, inkjet, electrophotographic (electrostatic), etc. methods are suitable for printing variable information such as a serial number.

[0120] The ink used to form the printed portion is not particularly limited, and examples thereof include oil-based ink, water-based ink, and photocurable ink (ultraviolet-curable ink, electron beam-curable ink).

[0121] The printed portion may be provided on the entire surface or only on a part of the surface.

[0122] (Application) The alkali-peelable label of this embodiment is peeled off by alkaline water. Therefore, the present invention also provides a method for peeling a label from an adherend, which comprises contacting the adherend to which the alkali-peelable label is affixed with alkaline water.

[0123] The substrate to which the alkali-peelable label of this embodiment can be affixed is not particularly limited, and examples include glass containers such as beer bottles and liquor bottles; resin containers such as body soap containers, shampoo bottle containers, and dishwashing detergent containers (resins include polyethylene terephthalate (PET), polyethylene (PP), and polypropylene (PP)); and stainless steel containers such as seasoning containers. The alkali-peelable label of the present invention can be easily peeled off with alkali even from substrates that are difficult to peel, such as stainless steel containers. [Example]

[0124] The effects of the present invention will be explained using the following examples and comparative examples. In the examples, the units "parts" and "%" are sometimes used, but unless otherwise specified, they represent "parts by mass" or "% by mass." Unless otherwise specified, each operation is carried out at room temperature (25°C).

[0125] Example 1 1. Formation of adhesive layer <Preparation of Pressure-Sensitive Adhesive Composition> A reaction vessel equipped with a stirrer, a thermometer, a reflux condenser, and a dropping funnel was charged with 90 parts by mass of 2-ethylhexyl acrylate (2EHA), 6.5 parts by mass of methyl methacrylate (MMA), and 3.5 parts by mass of acrylic acid (AA) as raw material monomers, 4.0 parts by mass (0.8 parts by mass in terms of solids content) of polyoxyalkylene alkenyl ether ammonium sulfate ("Latemul (registered trademark) PD-104" manufactured by Kao Corporation) as an emulsifier, and 35 parts by mass of degassed ion-exchanged water, and the mixture was stirred to prepare an emulsion.

[0126] Separately, 40 parts by mass of degassed ion-exchanged water was added to a reaction vessel equipped with a stirrer, thermometer, reflux condenser, and dropping funnel, and the temperature was raised to 80°C. Next, the emulsion was transferred to the dropping funnel and added dropwise over 4 hours. Concurrently, 4.0 parts by mass of a 3.0% by mass potassium persulfate aqueous solution was added dropwise as a polymerization initiator solution, and emulsion polymerization was carried out at a reaction temperature of 80°C. After completion of the addition, the mixture was aged at 80°C for 2 hours to obtain an emulsion polymerization composition. The mixture was then cooled to room temperature, and the pH was adjusted to 7.0 with 25% by mass ammonia water and 25% by mass sodium hydroxide aqueous solution to obtain an aqueous dispersion of an acrylic copolymer. The average emulsion particle size of the aqueous dispersion was 150 nm.

[0127] 100 parts by mass of "KR-120" (acid value 320 mgKOH / g) manufactured by Arakawa Chemical Industries, Ltd. as a rosin-based tackifier was melted at 180°C for approximately 1 hour and then cooled to 140°C. 5.0 parts by mass of sodium lauryl sulfate ("Monogen Y-100" manufactured by Daiichi Kogyo Seiyaku Co., Ltd.) as an emulsifier was then added, and the mixture was kneaded under pressure at 120°C for 90 minutes. 105 parts by mass of ion-exchanged water was then added and stirred for 10 minutes to obtain a tackifier emulsion with a solids concentration of 50% by mass.

[0128] To the resulting aqueous dispersion of the acrylic copolymer, 20 parts by mass (solid content) of the prepared tackifier emulsion was added per 100 parts by mass (solid content) of the acrylic copolymer to obtain a pressure-sensitive adhesive composition.

[0129] The resulting adhesive composition was coated onto a release liner using a knife coater so that the film thickness after drying would be 20 μm. The adhesive layer surface was transferred onto a resin substrate (OPP film, 50 μm) and left to stand at 23°C for 1 week to produce an alkali-peelable label.

[0130] Example 2 An alkali-peelable label was produced in the same manner as in Example 1, except that the raw material monomers were changed to 2EHA / MMA / AA=90 / 8.0 / 2.0 (parts by mass).

[0131] Example 3 An alkali-peelable label was produced in the same manner as in Example 1, except that the amount of the rosin-based tackifier added was changed to 10 parts by mass.

[0132] Example 4 An alkali-peelable label was produced in the same manner as in Example 1, except that the amount of rosin-based tackifier added was changed to 40 parts by mass.

[0133] Example 5 An alkali-peelable label was prepared in the same manner as in Example 1, except that the rosin-based tackifier was changed to "KR-610" (acid value 170 mgKOH / g) manufactured by Arakawa Chemical Industries, Ltd.

[0134] Example 6 An alkali-peelable label was prepared in the same manner as in Example 1, except that the amount of emulsifier added during acrylic copolymer polymerization was changed to 2.0 parts by mass (0.4 parts by mass in terms of solid content) and an aqueous dispersion of acrylic copolymer with an average emulsion particle size of 320 nm was used.

[0135] Example 7 An alkali-peelable label was prepared in the same manner as in Example 1, except that 3 parts by mass of an anionic surfactant, which is an ammonium salt of polycarboxylic acid (SN Dispersant 5468 manufactured by San Nopco, acid value 500 mg KOH / g), was used instead of 20 parts by mass of the rosin-based tackifier.

[0136] Example 8 An alkali-peelable label was prepared in the same manner as in Example 1, except that 6 parts by mass (solids content equivalent) of an aqueous polyester resin (acid value 50 mgKOH / g, solids content 25% by mass, number average molecular weight 3,000, glass transition temperature 46°C) was used instead of 20 parts by mass of the rosin-based tackifier.

[0137] (Comparative Example 1) An alkali-peelable label was produced in the same manner as in Example 1, except that no rosin-based tackifier was added.

[0138] (Comparative Example 2) An alkali-peelable label was prepared in the same manner as in Example 1, except that the rosin-based tackifier was changed to "KE-311" (acid value 5 mgKOH / g) manufactured by Arakawa Chemical Industries, Ltd.

[0139] (Comparative Example 3) An alkali-peelable label was produced in the same manner as in Example 1, except that the raw material monomers were changed to 2EHA / MMA / AA=90 / 9.5 / 0.5 (parts by mass).

[0140] [Evaluation method 1: Delabeling test (alkaline water)] The alkali-peelable labels (5 cm x 5 cm) obtained in the examples and comparative examples were attached to SUS plates as adherends and left to age for 24 hours in an environment of 23°C and 50% RH. The labels were then immersed in sodium hydroxide solution at pH 13.0 and 60°C, stirred at 300 rpm, and the time it took for the labels to peel off was measured (maximum 15 minutes). The results are shown in Table 1.

[0141] [Evaluation method 2: Adhesion test] The alkali-peelable labels obtained in the Examples and Comparative Examples were left standing for one day under standard conditions (23°C, 50% RH), the release liner was peeled off, and the adhesive layer was attached to a SUS plate. After leaving the labels standing for one day under standard conditions, the adhesive strength was measured. Specifically, the alkali-peelable labels were peeled off in a 180° direction at a test speed of 300 mm / min using a tensile tester, and the adhesive strength was measured. The values ​​were converted to peel force per 25 mm of label width (N / 25 mm). The results are shown in Table 1.

[0142] [Evaluation method 3: Water resistance test] The alkali-peelable labels (5 cm x 5 cm) obtained in the examples and comparative examples were attached to a glass plate and left to cure for 24 hours in an environment of 23°C and 50% RH. They were then immersed in water for 24 hours, and the whitening of the adhesive surface was evaluated according to the following criteria. The results are shown in Table 1.

[0143] ◎: No change ○: Very slight whitening is observed △: Some whitening is observed, but this does not pose a problem in practical use.

[0144] [Evaluation method 4: Transparency test] The alkali-peelable labels (5 cm x 5 cm) obtained in the examples and comparative examples were attached to a glass plate and left to cure for 24 hours in an environment of 23°C and 50% RH. The transparency of the adhesive surface was then evaluated on a 5-point scale. A higher number indicates greater transparency, and a lower number indicates greater opacity. A score of 3 or higher indicates no problems in practical use. The results are shown in Table 1.

[0145] [Table 1]

[0146] From the above results, the acrylic peelable labels of the Examples easily peeled off when immersed in alkaline water at 60°C, even when the adherend was SUS. Comparative Example 1, which did not contain a compound with an acid value of 30 mgKOH / g or more, and Comparative Example 2, which contained a compound with an acid value of less than 30 mgKOH / g, required time for label peeling to occur when immersed in alkaline water at 60°C. Furthermore, Comparative Example 3, which contained 0.5% by mass or less of an acid group-containing vinyl monomer, did not experience label peeling even after 15 minutes. Furthermore, all Examples exhibited excellent water resistance and label transparency. The adhesive strengths of all Examples and Comparative Examples were comparable. [Explanation of symbols]

[0147] 10 Alkali peelable labels, 11 Resin substrate, 12 adhesive layer, 13 release liner, 14 Printing Department.

Claims

1. An alkali-peelable label comprising a resin substrate and a pressure-sensitive adhesive layer, the pressure-sensitive adhesive layer contains an acrylic copolymer and a compound having an acid value of 30 mgKOH / g or more, The acrylic copolymer is obtained by copolymerizing a monomer mixture consisting of an alkyl (meth)acrylate, an acid group-containing vinyl monomer, and, if necessary, a monomer copolymerizable with the alkyl (meth)acrylate, and the monomer mixture contains the acid group-containing vinyl monomer in an amount of 1% by mass or more but less than 5% by mass of all monomers, and the monomer copolymerizable with the alkyl (meth)acrylate accounts for 5% by mass or less.

2. 2. The alkali-peelable label according to claim 1, wherein the compound is at least one selected from the group consisting of a tackifier, an anionic surfactant, and a polyester resin.

3. 3. The alkali-peelable label according to claim 2, wherein the anionic surfactant is a polymeric surfactant.

4. 3. The alkali-peelable label according to claim 2, wherein the adhesive composition for forming the adhesive layer comprises an emulsion of a tackifier and / or an aqueous polyester resin.

5. An alkali-peelable label comprising a resin substrate and an adhesive layer, the pressure-sensitive adhesive layer contains an acrylic copolymer and a compound having an acid value of 30 mgKOH / g or more, The acrylic copolymer is obtained by copolymerizing a monomer mixture containing an acid group-containing vinyl monomer in an amount of 1% by mass or more, The alkali-peelable label, wherein the compound is a polyester resin.

6. An alkali-peelable label as described in Claim 5, wherein the polyester-based resin is an aqueous polyester-based resin.

7. An alkali-peelable label comprising a resin substrate and an adhesive layer, the pressure-sensitive adhesive layer contains an acrylic copolymer and a compound having an acid value of 30 mgKOH / g or more, The acrylic copolymer is obtained by copolymerizing a monomer mixture containing an acid group-containing vinyl monomer in an amount of 1% by mass or more and less than 5% by mass, An alkali-peelable label, wherein the compound is a polymeric anionic surfactant.

8. 8. The alkali-peelable label according to claim 1, wherein the acrylic copolymer is an emulsion-based polymer.

9. 8. The alkali-peelable label according to claim 1, wherein the acid groups in the acrylic copolymer are not blocked by a crosslinking agent.

10. 8. The alkali-peelable label according to claim 1, wherein the content of the compound is 1 to 50 parts by mass per 100 parts by mass of the acrylic copolymer.

11. 8. The alkali-peelable label according to claim 1, 5 or 7, wherein the monomer mixture contains 50% by mass or more of n-butyl (meth)acrylate and / or 2-ethylhexyl (meth)acrylate.

12. 8. An alkali-peelable label according to claim 1, 5 or 7, wherein the acid groups in the compound having an acid value of 30 mgKOH / g or more are not blocked with a neutralizing agent.

13. A method for removing an alkali-peelable label from an adherend, comprising contacting the adherend to which the alkali-peelable label according to claim 1, 5 or 7 has been affixed with alkaline water.

Images