Water-based adhesives and adhesive sheets

Abstract

The present invention provides a water-based adhesive and an adhesive sheet having low-temperature adhesion, initial adhesion and penetration resistance. The water-based adhesive of the present invention contains a (meth)acrylic copolymer (A) polymerized in the presence of an emulsifier, an alkali metal compound (B) and an adhesion-imparting resin (C) in specific amounts. The (meth)acrylic copolymer (A) contains structural units derived from a (meth)acrylic acid alkyl ester monomer (a1) and a specific amount of a carboxyl-containing monomer (a2), and the alkali metal compound (B) is a compound that does not belong to the emulsifier and does not have an alkyl group with a carbon number of 8 or more, a cyclic skeleton with a carbon number of 6 or more, and an alkylene oxide. The alkali metal of the alkali metal compound contains 10 mol to 100 mol relative to 100 mol of carboxyl groups in the (meth)acrylic copolymer.

Description

[0001] This application claims the benefit of priority based on Japanese patent application No. 2020-100887, filed on June 10, 2020, the disclosure of which is hereby incorporated by reference in its entirety. Technical Field

[0002] The present invention relates to an aqueous adhesive containing a (meth)acrylic acid-based copolymer and also to an adhesive sheet having an adhesive layer formed of the aqueous adhesive. Background Art

[0003] Solvent-based adhesives using organic solvents have been the mainstream adhesives used in various applications such as automobiles, furniture, electronic equipment, advertising materials, and buildings. However, recently, from the perspective of reducing environmental load, water-based adhesives using water as a dispersion medium have been sought, and substitution from solvent-based to water-based adhesives has been promoted.

[0004] Patent Document 1 discloses an acrylic emulsion adhesive composition, which includes an acrylic emulsion copolymer, a crosslinking agent, and an alkali metal hydroxide and / or an alkaline earth metal hydroxide. In addition, Patent Document 2 discloses an aqueous peelable adhesive, which includes two acrylic particles having a D50 average particle size in a specific range and a crosslinking agent. In addition, Patent Document 3 discloses an aqueous pressure-sensitive adhesive composition, which is formed by adding a specific amount of at least one salt selected from an alkali metal salt or ammonium salt of sulfuric acid or nitric acid to 100 parts by weight of the resin component of the pressure-sensitive adhesive resin aqueous dispersion.

[0005] [Prior art literature]

[0006] [Patent Document]

[0007] Patent Document 1: Japanese Patent Application Publication No. 2010-254961

[0008] Patent Document 2: Japanese Patent Application Publication No. 2015-105353

[0009] Patent Document 3: Japanese Patent Laid-Open No. 2-138380 Summary of the invention

[0010] [Problems to be solved by the invention]

[0011] Adhesive sheets generally include a laminate including an adhesive layer formed by an adhesive and a substrate layer such as paper or film, and are used as, for example, decorative sheets, masking films, and display labels. Adhesive sheets are required to have excellent adhesion and cohesion, and excellent quality. However, adhesive sheets manufactured using water-based adhesives have inferior properties compared to adhesive sheets manufactured using solvent-based adhesives.

[0012] Furthermore, there is a phenomenon (hereinafter referred to as penetration resistance) in which the components of the adhesive layer are transferred to the substrate layer, thereby causing degradation of the substrate layer (e.g., spotty stains, discoloration (blackening, etc.)), and it is required to improve it. By increasing the rigid component of the (meth)acrylic copolymer, the penetration resistance of the substrate can be improved to a certain extent, but if the rigid component is increased in the (meth)acrylic copolymer, there is a paradoxical phenomenon that the initial viscosity decreases, and it is urgently desired to develop an adhesive sheet that meets these characteristics. In addition, in order to expand the use of adhesive sheets using water-based adhesives, it is strongly required to improve the low-temperature adhesion.

[0013] Furthermore, in the above, the problem regarding the resistance to penetration into the substrate is described, but the same problem may also occur in a layer of the adhesive sheet other than the substrate or an adherend.

[0014] The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide a water-based adhesive and an adhesive sheet having low-temperature adhesive strength, initial tack, and permeation resistance.

[0015] [Technical means to solve the problem]

[0016] The present inventors have conducted diligent studies and have found that the problems of the present invention can be solved in the following aspects, thereby completing the present invention.

[0017] [1]: A water-based adhesive comprising a (meth)acrylic acid copolymer obtained by emulsion polymerization in the presence of an emulsifier, an alkali metal compound, and a tackifier resin,

[0018] The (meth)acrylic copolymer comprises structural units derived from a (meth)acrylic acid alkyl ester monomer and a carboxyl group-containing monomer.

[0019] The carboxyl group-containing monomer is used in an amount of 0.5 to 5 parts by mass based on 100 parts by mass of the monomer mixture.

[0020] The alkali metal compound is a compound that does not belong to the emulsifier and does not have an alkyl group with 8 or more carbon atoms, a cyclic skeleton with 6 or more carbon atoms, and an alkylene oxide.

[0021] The alkali metal compound contains 10 mol to 100 mol of alkali metal based on 100 mol of carboxyl groups in the (meth)acrylic acid-based copolymer.

[0022] The tackifier resin is contained in an amount of 1 to 10 parts by mass based on 100 parts by mass of the (meth)acrylic copolymer.

[0023] The water-based adhesive does not contain a covalent crosslinking agent for the (meth)acrylic copolymer, and does not contain a compound having a divalent or higher valent cation.

[0024] [2]: The water-based adhesive according to [1], wherein the (meth)acrylic copolymer is obtained by polymerizing a monomer mixture containing the (meth)acrylic acid alkyl ester monomer and the carboxyl group-containing monomer in the presence of the alkali metal compound.

[0025] [3]: The water-based adhesive according to [1] or [2], wherein the (meth)acrylic copolymer is obtained by polymerizing a monomer mixture containing the (meth)acrylic acid alkyl ester monomer and the carboxyl group-containing monomer in the presence of the tackifier resin.

[0026] [4]: The water-based adhesive according to any one of [1] to [3], wherein the tackifier resin contains a terpene resin.

[0027] [5]: The water-based adhesive according to any one of [1] to [4], wherein the alkali metal compound is an alkali metal salt, and the alkali metal is sodium.

[0028] [6]: The water-based adhesive according to any one of [1] to [5], wherein the average particle size D50 of the (meth)acrylic copolymer is 0.1 μm to 0.6 μm.

[0029] [7]: An adhesive sheet comprising a substrate and an adhesive layer laminated on at least one surface of the substrate,

[0030] The adhesive layer is formed using the aqueous adhesive according to any one of [1] to [6].

[0031] [Effects of the Invention]

[0032] According to the present invention, there is provided an excellent effect of providing a water-based adhesive and an adhesive sheet having low-temperature adhesive strength, initial tack and permeation resistance. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 This is a schematic explanatory diagram showing an example of pseudo-crosslinking between an alkali metal compound (B) and a carboxyl group of a (meth)acrylic acid-based copolymer (A).

[0034] Figure 2 This is a diagram for explaining the effect of adding an alkali metal compound (B) on the gel fraction. DETAILED DESCRIPTION

[0035] Hereinafter, an example of an embodiment to which the present invention is applied is described. However, the present invention is not limited to the embodiments and variations, and other embodiments may also fall within the scope of the present invention as long as they conform to the gist of the present invention. In addition, the numerical values ​​"A to B" specified in this specification refer to the ranges satisfying numerical value A and a value greater than numerical value A and numerical value B and a value less than numerical value B. In addition, the numerical values ​​specified in this specification are values ​​obtained by the methods disclosed in the embodiments or examples. In addition, the sheets, films, and tapes in this specification have the same meaning. For the purpose of clear explanation, the following description and drawings are appropriately simplified. As long as there are no special annotations, the various components appearing in this specification can be used independently and separately, or two or more can be used in combination.

[0036] The water-based adhesive of the present invention contains a (meth)acrylic copolymer (A) polymerized in the presence of an emulsifier, an alkali metal compound (B) and an adhesion-imparting resin (C). Moreover, the (meth)acrylic copolymer (A) contains structural units derived from a (meth)acrylic acid alkyl ester monomer (a1) and a carboxyl group-containing monomer (a2). The carboxyl group-containing monomer (a2) is used in an amount of 0.5 to 5 parts by mass relative to 100 parts by mass of the monomer mixture of the (meth)acrylic copolymer (A). In addition, the monomer mixture in this specification does not contain a reactive emulsifier described below.

[0037] The alkali metal compound (B) is a compound that does not belong to the above emulsifier and does not have any of an alkyl group having 8 or more carbon atoms, a cyclic skeleton having 6 or more carbon atoms, and an alkylene oxide. The alkali metal compound (B) contains 10 mol to 100 mol of alkali metal with respect to 100 mol of carboxyl groups in the (meth)acrylic copolymer. The tackifier resin (C) contains 1 to 10 parts by mass with respect to 100 parts by mass of the (meth)acrylic copolymer (A).

[0038] The water-based adhesive does not contain a covalent crosslinking agent for the (meth)acrylic copolymer (A), and does not contain a compound having a divalent or higher cation. In addition, in the water-based adhesive, at least a portion of the carboxyl groups of the structural units derived from the carboxyl group-containing monomer (a2) form a simulated crosslink with the cation of the alkali metal (b1) in the alkali metal compound (B). Figure 1 Schematic diagram of simulated crosslinking in the water-based adhesive of the present embodiment is shown in FIG. In the example of the figure, an example of using trisodium citrate as the alkali metal compound (B) is shown. If the alkali metal compound (B) is added during polymerization or after the (meth) acrylic copolymer (A) is polymerized ( Figure 1 In the example of trisodium citrate) as an additive, the cation of the alkali metal (b1) (in Figure 1In the example of sodium ions) and the carboxyl groups in the (meth)acrylic copolymer (A) form simulated crosslinks (refer to Figure 1 ).

[0039] The so-called emulsifier refers to a surfactant used to form an emulsion of an aqueous medium and a monomer containing an alkyl (meth)acrylate monomer (a1) and a carboxyl group-containing monomer (a2), and is preferably a surfactant that does not adversely affect the polymerization reaction. The "emulsifier" is not particularly limited as long as it is an emulsifier commonly used in emulsion polymerization reactions, and may be a compound containing an alkali metal.

[0040] The water-based adhesive of the present invention uses the above-mentioned (meth) acrylic copolymer (A) having a specific structure, the above-mentioned specific alkali metal compound (B) and the adhesion-imparting resin (C) in specific amounts, respectively, and does not contain a covalent cross-linking agent and does not contain a compound having a divalent or higher cation, thereby having low-temperature adhesion, initial adhesion and penetration resistance.

[0041] The reason for thinking that these can be achieved is that a specific amount of a specific acrylic copolymer (A) and an adhesion-imparting resin (C) are used, the content of a specific alkali metal compound (B) is set to a specific range, and then the cross-linking agent formed by a divalent or higher cation and a covalent cross-linking agent are not deployed in the water-based adhesive, thereby improving the cohesive force based on the simulated cross-linking, improving the permeability, and improving the holding power of the adhesive layer. It is believed that by using a specific amount of a compound that does not belong to an emulsifier, does not contain an alkyl group with a carbon number of 8 or more, does not contain a cyclic structure with a carbon number of 6 or more, and does not contain an alkyl oxide as an alkali metal compound (B), the cluster-like ions of the carboxyl group of the acrylic copolymer (A) can be urged to gather together, thereby promoting simulated cross-linking. In addition, it is believed that by such simulated cross-linking, the softness of the adhesive layer is appropriate, the initial viscosity is good, and the low-temperature adhesion can be exerted. In addition, the coating stability can also be excellent. Therefore, it is also suitable for use in cold areas such as -10°C.

[0042] [(Meth)acrylic acid copolymer (A)]

[0043] The (meth)acrylic copolymer (A) is a resin polymerized in the presence of an emulsifier, and is a copolymer containing a structural unit derived from a (meth)acrylic monomer as a main component (70% by mass or more). The (meth)acrylic copolymer (A) may contain a structural unit derived from a vinyl monomer other than a (meth)acrylic monomer, or may contain only a structural unit derived from a (meth)acrylic monomer.

[0044] Here, the so-called (meth)acrylic acid refers to at least one of acrylic acid and methacrylic acid. In addition, a (meth)acrylic monomer refers to a monomer having a (meth)acryloyl group in the molecule. The so-called (meth)acryloyl group refers to an acryloyl group, a methacryloyl group or a mixture of the two. Furthermore, a vinyl monomer refers to a monomer having a polymerizable carbon-carbon double bond in the molecule. In addition, a structural unit derived from a (meth)acrylic monomer refers to a structural unit having a portion of a carbon-carbon single bond formed by polymerization of a carbon-carbon double bond of a (meth)acrylic monomer that can undergo free radical polymerization, and a structural unit derived from a vinyl monomer refers to a structural unit having a portion of a carbon-carbon single bond formed by polymerization of a carbon-carbon double bond of a vinyl monomer that can undergo free radical polymerization.

[0045] The (meth)acrylic copolymer (A) has at least structural units derived from a (meth)acrylic acid alkyl ester monomer (a1) (hereinafter also referred to as monomer (a1)) and a carboxyl-containing monomer (a2) (hereinafter also referred to as monomer (a2)). The (meth)acrylic copolymer (A) may contain only these monomers, or may contain other monomers (a3) ​​that can be copolymerized with the monomers (a1) and (a2). As other monomers (a3), vinyl monomers and (meth)acrylic acid monomers can be exemplified. The carboxyl-containing monomer (a2) may be an acrylic acid monomer or a vinyl monomer. In addition, when a reactive emulsifier is used as an emulsifier, since the reactive emulsifier has a vinyl unsaturated group, it reacts with the monomers (a1) and (a2) during polymerization, and the emulsifier is introduced into the (meth)acrylic copolymer (A). In addition, it is assumed that the other monomers (a3) ​​do not contain a reactive emulsifier.

[0046] The (meth) alkyl acrylate monomer (a1) is an alkyl acrylate, an alkyl methacrylate, or a mixture of the two. The alkyl group may be linear or branched. The (meth) alkyl acrylate monomer (a1) is preferably 70 to 99.5 parts by mass in 100 parts by mass of the monomer mixture. More preferably, it is 80 to 99.5 parts by mass, and further preferably, it is 90 to 99.5 parts by mass. By setting it to 70 to 99.5 parts by mass, a material with a better balance between adhesion and cohesion is formed.

[0047] The (meth)acrylic acid alkyl ester monomer (a1) is preferably a monomer having an alkyl group with a carbon number of 1 to 10, more preferably 1 to 9, and further preferably 1 to 8. Preferably, the monomer (a1) having an alkyl group with a carbon number of 1 to 10 is contained in the monomer mixture in an amount of 70 parts by mass or more, more preferably 80 parts by mass or more, and further preferably 90 parts by mass or more. By using a monomer (a1) having an alkyl group with a carbon number of 1 to 10, the cohesive force is made appropriate, thereby obtaining a more excellent holding force.

[0048] Preferred examples of (meth) alkyl acrylate monomers (a1) include: methyl (meth) acrylate, ethyl (meth) acrylate, n-propyl (meth) acrylate, isopropyl (meth) acrylate, n-butyl (meth) acrylate, isobutyl (meth) acrylate, tert-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, decyl (meth) acrylate, isodecyl (meth) acrylate, n-lauryl (meth) acrylate, n-stearyl (meth) acrylate, etc. Among these monomers, alkyl (meth) acrylate is preferably used in combination with 2-ethylhexyl acrylate, butyl acrylate and methyl methacrylate. If methyl methacrylate is used, cohesion is improved, and if 2-ethylhexyl acrylate is used, low temperature properties are further improved. In addition, by using 2-ethylhexyl acrylate and methyl methacrylate in combination, it is easy to achieve a balance between low temperature properties and cohesion. When methyl methacrylate is used, it is preferably used in an amount of 1 to 19 parts by mass, more preferably 3 to 15 parts by mass, based on 100 parts by mass of the monomer mixture.

[0049] The carboxyl-containing monomer (a2) refers to a compound having a polymerizable carbon-carbon double bond such as a (meth)acryloyl group and a carboxyl group. Specific examples include acrylic acid, methacrylic acid, itaconic acid, maleic acid, maleic anhydride, crotonic acid, citraconic acid, β-carboxyethyl acrylate, fumaric acid, fumaric anhydride, etc. Among these, acrylic acid is preferred.

[0050] The amount of the carboxyl group-containing monomer (a2) used is 0.5 to 5 parts by mass relative to 100 parts by mass of the monomer mixture. The range is more preferably 0.5 to 4 parts by mass, and further preferably 0.5 to 3 parts by mass. By setting it to 0.5 to 5 parts by mass, the polymerization stability, initial viscosity and cohesive force can be improved.

[0051] Examples of the vinyl monomers in other monomers (a3) ​​include aromatic monomers such as styrene, α-methylstyrene, benzyl methacrylate, and vinyl toluene; vinyl esters such as vinyl acetate; amide group-containing monomers such as heterocyclic vinyl compounds such as vinyl pyrrolidone, acrylamide, and diacetone (meth)acrylamide; cyano group-containing monomers such as acrylonitrile; and polyfunctional vinyl monomers such as diallyl phthalate.

[0052] Examples of the (meth)acrylic monomer in the other monomers (a3) ​​include: glycidyl methacrylate and other glycidyl group-containing monomers; hydroxyl group-containing monomers such as 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, and 6-hydroxyhexyl (meth)acrylate; and amino group-containing monomers such as aminomethyl (meth)acrylate, dimethylaminomethyl (meth)acrylate, dimethylaminoethyl (meth)acrylate, and dimethylaminopropyl (meth)acrylate.

[0053] The glass transition temperature (Tg) of the (meth)acrylic copolymer (A) is preferably -70°C to -30°C, more preferably -65°C to -40°C, and still more preferably -60°C to -45°C. By using a (meth)acrylic copolymer (A) having a Tg of -70°C to -30°C, the adhesiveness to the adherend can be further improved, and the low temperature resistance can be further improved.

[0054] The glass transition temperature (Tg) of the (meth)acrylic copolymer is a calculated value obtained from the following formula (1).

[0055] Equation (1)

[0056] 1 / Tg=[(W 1 / Tg 1 )+(W 2 / Tg 2 )+...(W n / Tg n )] / 100

[0057] Among them, W 1 : Mass % of monomer 1, Tg 1 : Glass transition temperature (K) of the homopolymer that can be formed only from monomer 1, W 2 : Mass % of monomer 2, Tg 2 : Glass transition temperature (K) of the homopolymer that can be formed only from monomer 2, W n : Mass % of monomer n, Tg n : The glass transition temperature (K) of a homopolymer that can be formed only from monomer n. 1 +W 2 +...+W n = 100. In addition, only the monomer is used in the calculation of Tg, and no reactive emulsifier is included even if copolymerization is possible.

[0058] In this specification, the glass transition temperature of the homopolymer refers to "Kitaoka Kyozou, "Introduction to Synthetic Resins for Coatings", Polymer Publishing Society, published on May 25, 1974".

[0059] The average particle size D50 of the (meth)acrylic copolymer (A) is not particularly limited, but is preferably 0.1 μm to 0.6 μm. More preferably, it is 0.2 μm to 0.5 μm. By being within the range of 0.1 μm to 0.6 μm, the viscosity of the emulsion can be easily adjusted, and the coating property of the adhesive can be improved.

[0060] The D50 average particle size can be measured using a light scattering particle size measuring device "Microtrac 9320HRA" (trade name); manufactured by Nikkiso Co., Ltd., etc. In addition, the measurement is diluted with water, and the measurement concentration is adjusted to be within the optimal concentration meter width displayed on the screen.

[0061] [Alkali metal compound (B)]

[0062] The alkali metal compound (B) is a compound that is not an emulsifier and does not have any of an alkyl group having 8 or more carbon atoms, a cyclic skeleton having 6 or more carbon atoms, and an alkylene oxide. At least a portion of the carboxyl groups in the (meth)acrylic copolymer (A) forms a pseudo crosslink between molecules with the cation (b2) of the alkali metal (b1) of the alkali metal compound (B). It is believed that by using an alkali metal compound (B) that does not have any of an alkyl group having 8 or more carbon atoms, a cyclic skeleton having 6 or more carbon atoms, and an alkylene oxide, the aggregation of cluster ions with the carboxyl groups of the (meth)acrylic copolymer (A) can be promoted.

[0063] The molecular weight of the alkali metal compound (B) is not particularly limited, but from the viewpoint of promoting simulated crosslinking with the carboxyl group of the (meth)acrylic copolymer (A), it is preferred that the alkali metal compound (B) does not have a large hydrophobic group such as a hydrophobic group having 8 or more carbon atoms, or does not have a hydrophobic group itself. The molecular weight of the alkali metal compound (B) is preferably 10 to 700, more preferably 10 to 400.

[0064] The alkali metal (b1) of the alkali metal compound (B) contains 10 mol to 100 mol of the carboxyl group in the (meth)acrylic copolymer (A). It is more preferably 20 mol to 100 mol, and even more preferably 30 mol to 100 mol. By containing 10 mol to 100 mol, the effect of achieving both the conflicting properties of cohesive force and initial viscosity can be obtained.

[0065] Examples of the alkali metal compound (B) include alkali metal salts and hydroxides of alkali metals. Examples of the alkali metal salt include alkali metal salts of oxoacids, alkali metal salts of polyphosphoric acids, alkali metal salts of sulfuric acid, alkali metal salts of nitrates, alkali metal salts of carbonates, and alkali metal salts of acetates. Examples of the alkali metal of the alkali metal compound (B) include Li, Na, and K. Among these, sodium is more preferred.

[0066] As a preferred example of the alkali metal compound (B), compounds showing alkalinity in an aqueous solution of the alkali metal compound (B) can be cited. As an alkali metal compound (B) showing alkalinity, an alkali metal salt of a weak acid or an alkali metal hydroxide can be exemplified. By using a salt of a weak acid, an alkali metal ion or / and an alkali metal compound (B) can be obtained that easily stays on the surface of the polymer particles and easily forms a simulated crosslinking effect with the carboxyl groups present on the surface of the particles. As a preferred example of a weak acid salt, an alkali metal salt of an oxoacid can be cited. As a preferred example of an alkali metal compound (B), a form containing at least one of an alkali metal salt of an oxoacid and an alkali metal salt of polyphosphoric acid can be cited.

[0067] Specific examples of weak acid salts of alkali metal compounds (B) include sodium citrate, sodium tripolyphosphate, sodium hexametaphosphate, and sodium pyrophosphate. In addition, examples of alkali metal hydroxides include sodium hydroxide. In addition, examples include compounds obtained by replacing sodium with lithium or potassium in these compounds.

[0068] [Tackifying resin (C)]

[0069] The tackifier resin (C) (solid content) contains 1 to 10 parts by mass relative to 100 parts by mass of the (meth)acrylic copolymer (A). It is more preferably 1 to 8 parts by mass, and further preferably 1 to 5 parts by mass. By using 1 to 10 parts by mass of the tackifier resin (C) and further using a specific amount of the specific alkali metal compound (B), both the penetration resistance and the initial tack can be improved.

[0070] The tackifier resin (C) is preferably mainly a natural resin or a synthetic resin.

[0071] The natural resin is preferably, for example, a rosin resin (rosin, rosin ester, rosin phenol, colorless rosin derivatives, etc.), a terpene resin (terpene, aromatic modified terpene, terpene phenol, etc.), or the like.

[0072] The synthetic resin is preferably, for example, a petroleum resin (including styrene resin, α-methylstyrene resin, aliphatic resin, etc.), or other resins (including phenol resin, xylene resin, ketone resin, etc.). Among these, rosin resin, terpene resin, and petroleum resin are preferred, and rosin resin is more preferred. If rosin resin is used, the time stability of viscosity, re-peelability, and adhesion can be further improved. In addition, the adhesion imparting resin is preferably used when a solid type is dissolved in an alkyl acrylate monomer for polymerization. Among these, terpene resin is particularly preferred.

[0073] The softening point of the tackifier resin (C) is preferably 80° C. to 120° C., more preferably 80° C. to 110° C., and still more preferably 85° C. to 105° C. By combining a terpene resin having a softening point of 80° C. to 120° C. with a specific amount of the alkali metal compound (B), low temperature properties can be more effectively improved.

[0074] [Water-based adhesive]

[0075] The water-based adhesive may be formulated with various additives commonly used in water-based adhesives, such as a defoamer, a wetting agent, a coloring pigment, a thickener, a plasticizer, an antioxidant, an ultraviolet absorber, a preservative, and the like, as needed.

[0076] The gel fraction of the water-based adhesive is preferably 20% to 60% by mass, and more preferably 30% to 50% by mass. By keeping the gel fraction within the range of 20% to 60% by mass, it is possible to achieve a higher level of both cohesive force and anchoring to the substrate. The gel fraction can be easily adjusted by adding an alkali metal compound (B) during or after the polymerization of the (meth)acrylic copolymer. By not containing a covalent crosslinking agent or a compound having a divalent or higher cation in the water-based adhesive, a specific amount of the alkali metal compound (B) is used, such as Figure 2 As shown, the gel fraction can be easily adjusted. In addition, the gel fraction in the figure is an image diagram for explaining the change of the gel fraction value, and the numerical value is not limited to the above-mentioned graph.

[0077] The method for measuring the gel fraction (mass %) in this specification is as follows. That is, after applying the water-based adhesive on a polyethylene terephthalate (PET) film in a manner such that the dry film thickness becomes about 20 μm, it is dried at 100°C for 2 minutes. Next, the mass of a 200 mesh metal mesh is measured (its mass is set as M). The adhesive sheet is cut into a size of 5cm×5cm, and the mass of the test piece attached to the 200 mesh metal mesh is measured (its mass is set as A). In addition, 200 mesh is the mesh specified by Japanese Industrial Standards (JIS) G-3555.

[0078] The test piece was left in 50 ml of ethyl acetate at 50° C. for one day, then taken out, dried at 100° C. for 20 minutes, and then the mass was measured (the mass was referred to as T).

[0079] Next, the polyethylene terephthalate (PET) film was taken out from the test piece, the adhesive layer was removed using ethyl acetate, and the mass of the PET film was measured (the mass was designated as K). The obtained value was substituted into the following formula (2) to determine the gel fraction.

[0080] Equation (2)(TMK)×100 / (AMK)

[0081] [Method for producing water-based adhesive]

[0082] The (meth)acrylic copolymer (A) is formed by emulsion polymerization in the presence of an emulsifier. An emulsifier is used in the emulsion polymerization. The emulsifier can use at least one of a reactive emulsifier and a non-reactive emulsifier. Among these, a reactive emulsifier is more preferred from the viewpoint of water resistance. Examples of reactive emulsifiers include emulsifiers having ethylenically unsaturated groups. The non-reactive emulsifier is preferably an emulsifier having 7 to 50 repeating numbers of ethylene oxide units (hereinafter referred to as EO (ethyleneoxide) unit numbers). A reactive emulsifier and a non-reactive emulsifier may also be used in combination.

[0083] Examples of the reactive emulsifier include anionic reactive emulsifiers such as sodium vinyl sulfonate, ammonium polyoxyethylene acrylate sulfate, sodium polyoxyethylene methacrylate sulfonate, ammonium polyoxyethylene alkenylphenyl sulfonate, sodium polyoxyethylene alkenylphenyl sulfate, sodium allyl alkyl sulfosuccinate, and sodium polyoxypropylene methacrylate sulfonate; and nonionic reactive emulsifiers such as polyoxyethylene alkenylphenyl ether and polyoxyethylene methacryloyl ether.

[0084] Examples of the non-reactive emulsifier include anionic emulsifiers such as dodecylbenzene sulfonate, lauryl sulfate, alkyl diphenyl ether sulfonate, dialkyl sulfosuccinate, polyoxyethylene alkyl phenyl ether sulfate, polyoxyethylene polycyclic phenyl ether sulfate, and polyoxyethylene alkyl ether sulfate; polyoxyethylene alkyl phenyl ethers such as polyoxyethylene nonylphenyl ether and polyoxyethylene octylphenyl ether; polyoxyethylene alkyl ethers such as polyoxyethylene lauryl ether, polyoxyethylene stearyl ether, and polyoxyethylene oleyl ether; polyoxyethylene polycyclic phenyl ethers such as polyoxyethylene distyrenated phenyl ether; and nonionic emulsifiers such as polyoxyethylene sorbitan fatty acid esters.

[0085] The emulsifier is preferably used in an amount of 0.1 to 5 parts by mass, more preferably 0.5 to 3 parts by mass, based on 100 parts by mass of the monomer mixture. When the amount is within the range of 0.1 to 5 parts by mass, adhesion and polymerization stability can be improved.

[0086] The polymerization initiator used in the emulsion polymerization is preferably a water-soluble polymerization initiator. The water-soluble polymerization initiator is preferably a peroxide. A reducing agent may also be combined with the peroxide. Examples of the peroxide include persulfates such as potassium persulfate, ammonium persulfate, sodium persulfate, and tert-butyl hydroperoxide.

[0087] The amount of the water-soluble polymerization initiator used is preferably 0.1 to 1 part by mass, more preferably 0.2 to 0.8 parts by mass, relative to 100 parts by mass of the monomer mixture. 0.1 to 1 part by mass can improve emulsion polymerizability.

[0088] When the acrylic copolymer (A) is subjected to emulsion polymerization, a chain transfer agent may be used to adjust the molecular weight. Chain transfer agents are generally known to have compounds having a thiol group or a hydroxyl group, for example. As compounds having a thiol group, for example, thiols such as lauryl mercaptan, 2-mercaptoethanol, dodecyl mercaptan, and mercaptosuccinic acid, alkyl mercaptopropionates such as n-butyl mercaptopropionate or octyl mercaptopropionate, and alkoxyalkyl mercaptopropionates such as methoxybutyl mercaptopropionate. As compounds having a hydroxyl group, alcohols such as methanol, n-propanol, isopropanol, tert-butyl alcohol, and benzyl alcohol may be cited. Among these, alcohols such as isopropanol are preferably used from the viewpoint of further reducing the odor and condensate of the adhesive.

[0089] The solvent used for emulsion polymerization is preferably water, but a hydrophilic solvent such as alcohol may be used in combination as long as the effect of the present invention is obtained.

[0090] The acrylic copolymer (A) is preferably neutralized with a basic compound. The basic compound is preferably ammonia or amines such as monoethylamine, dimethylethanolamine, and methylpropanolamine.

[0091] The alkali metal compound (B) may be added during the polymerization of the acrylic copolymer (A) or may be blended after the polymerization. From the viewpoint of achieving better initial viscosity, it is preferably added during the polymerization of the acrylic copolymer (A).

[0092] The tackifier resin (C) may be added during the polymerization of the acrylic copolymer (A) or may be blended after the polymerization. From the viewpoint of achieving better low-temperature properties, it is preferably added during the polymerization of the acrylic copolymer (A).

[0093] [Adhesive sheet]

[0094] The water-based adhesive of the present embodiment is preferably formed by being coated on a substrate (X) to form an adhesive layer (Y), and is used as an adhesive sheet (Z) including a substrate (X). In addition, the water-based adhesive can also be coated on a peelable sheet and laminated with the substrate (X). The substrate (X) of the adhesive sheet (Z) can use general raw materials such as paper, plastic and synthetic paper. The adhesive layer (Y) is stacked on at least one side of the substrate (X). In addition, in addition to being stacked on the entire surface of the principal surface of the substrate (X), it can also be stacked on a part.

[0095] The manufacturing method of the adhesive sheet of the present invention is not limited, and can be manufactured by applying a water-based adhesive on a peelable sheet and laminating it with a substrate after drying. Alternatively, it can also be manufactured by directly applying a water-based adhesive sheet on a substrate (X) and laminating it with a peelable sheet after drying.

[0096] Examples of paper as the substrate (X) include woodfree paper, coated paper, glossy paper, coated paper, impregnated paper, and synthetic paper. Examples of plastics as the substrate (X) include polyolefins such as polyvinyl alcohol, triacetyl cellulose, polypropylene, polyethylene, polycycloolefins, and ethylene-vinyl acetate copolymers; polyesters such as polyethylene terephthalate, polybutylene terephthalate, and polyethylene naphthalate; polycarbonate, polynorbornene, polyarylate, polyacrylic acid, polyphenylene sulfide, polystyrene, polyamide, polyimide, epoxy polyethylene, and polyvinyl chloride.

[0097] As the releasable sheet, paper or plastic film subjected to a releasable treatment with silicone or the like can be used.

[0098] The adhesive can be applied using a known coating device such as a notch wheel coater, blade coater, roll coater such as a gravure coater, slot die coater, lip coater, curtain coater, etc. The thickness of the adhesive layer is preferably 0.1 μm to 200 μm.

[0099] The adhesive sheet of the present invention may further include an anchor layer, a printing layer, an outer coating layer, and the like.

[0100] [Example]

[0101] Hereinafter, the present invention will be described in detail with reference to Examples and Comparative Examples, but the present invention is not limited to the following Examples. In the Examples, "parts" means "parts by mass" and "%" means "% by mass".

[0102] <Example 1>

[0103] As (meth)acrylic acid alkyl ester monomers, 28.5 parts of 2-ethylhexyl acrylate, 65.0 parts of butyl acrylate, 5.0 parts of methyl methacrylate, and 1.5 parts of acrylic acid as a carboxyl group-containing monomer were used, and to these were added 0.05 parts of octyl thioglycolate (OTG) (2-ethylhexyl thioglycolate) as a chain transfer agent and 3.0 parts of "YS resin PX-1000" (terpene resin, manufactured by Yasuhara Chemical Co., Ltd.) as a tackifier resin. Furthermore, 1.5 parts of trisodium citrate as an alkali metal compound (50 mol of alkali metal per 100 mol of acrylic acid) and 5.0 parts of a 20% aqueous solution of "Aquaron KH-10" (polyoxyethylene-1-(allyloxymethyl) alkyl ether sulfate ammonium salt, manufactured by Daiichi Kogyo Seiyaku Co., Ltd.) as an anionic reactive emulsifier were added to 33.5 parts of deionized water and stirred to obtain an emulsion with a pH of 5. The obtained emulsion was placed in a dropping funnel.

[0104] In addition, 19.6 parts of deionized water were added to a four-necked flask equipped with a stirrer, a cooling tube, a thermometer and the dropping funnel, and the inside of the flask was replaced with nitrogen. The internal temperature was heated to 70°C while stirring. Thereafter, 8.3 parts of potassium persulfate with a concentration of 3% were added, and the reaction was started. While the internal temperature was maintained at 70°C, the emulsion was dripped for 240 minutes, and the internal temperature was maintained at 70°C while further stirring, and the reaction was continued for 1 hour in the state. Thereafter, 0.6 parts of potassium persulfate with a concentration of 3% were added twice every 30 minutes, and the reaction was continued for 1 hour. Thereafter, cooling was performed to obtain an emulsion of an acrylic copolymer having a non-volatile component concentration of 60% and an average particle size D50 of 0.3 μm.

[0105] Next, 0.5 parts of 25% ammonia water as a neutralizer, 0.3 parts of "Adekanate B-940" (manufactured by ADEKA) as a defoamer, 0.02 parts of "Rebanakkusu BX-150" (manufactured by Changrong Chemical Co., Ltd.) as a preservative, 0.4 parts of "Pelex OT-P" (manufactured by Kao Corporation) as a leveling agent, and 0.2 parts of "Adekanol UH-541VF" (manufactured by ADEKA) as a urethane thickener were added to obtain a water-based adhesive. The viscosity of the water-based adhesive was 1000 mPa·s at 3 / 60 revolutions of a BL type viscometer.

[0106] (Example 2 to Example 22, Comparative Example 1 to Comparative Example 14)

[0107] The water-based adhesive is manufactured by the same method as in Example 1 except that the compounds / mixing amounts are changed to those shown in Tables 1 to 6. The compounds recorded in the acrylic copolymer column in Tables 1, 3, and 5 are compounds used when polymerizing the (meth)acrylic copolymer, and the compounds recorded in the additive column in Tables 2, 4, and 6 are compounds mixed when preparing the water-based adhesive after polymerizing the (meth)acrylic copolymer. In addition, the content of alkali metal compounds, divalent cations, and crosslinking agents represents the mol of alkali metal per 100 mol of carboxyl groups in the (meth)acrylic copolymer. On the other hand, the adhesion-imparting resin is the mass parts of the solid component (non-volatile component) converted to 100 mass parts of the (meth)acrylic copolymer during synthesis, and the additive column represents the mass parts of the emulsion.

[0108] The abbreviations of Tables 1 to 6 are as follows.

[0109] ·EHA: 2-Ethylhexyl acrylate

[0110] BA: Butyl acrylate

[0111] MMA: Methyl methacrylate

[0112] AA: Acrylic acid

[0113] MAA: Methacrylic acid

[0114] ·PX-1000: "YS resin PX-1000" (terpene resin, softening point 100°C)

[0115] ·TSR-903 (terpene resin, softening point 160°C)

[0116] ·S-100 (rosin resin, softening point 100℃)

[0117] ·D-160: "Pensel D-160" (rosin resin, softening point 160°C)

[0118] ·FTR-6100 (petroleum resin, softening point 100℃)

[0119] Reactive emulsifier: KH-10: "Aquaron KH-10" (anionic reactive emulsifier, polyoxyethylene-1-(allyloxymethyl) alkyl ether sulfate ammonium salt non-volatile matter 100%, manufactured by Daiichi Kogyo Seiyaku Co., Ltd.)

[0120] Non-reactive emulsifier: RA9612: "Newcol RA9612" (anionic non-reactive emulsifier, polyoxyethylene alkyl ether sulfate ammonium salt, non-volatile content 25%, manufactured by Nippon Emulsifier Co., Ltd.)

[0121] T-1050: "NANOLET T-1050" (terpene resin emulsion, non-volatile content 50%, softening point 100°C)

[0122] ·TSR-5903 (terpene resin emulsion, non-volatile content 50%, softening point 160°C)

[0123] NS-100H (rosin resin emulsion, non-volatile content 50%, softening point 100°C)

[0124] ·E-788: "Super Ester E-788" (rosin resin emulsion, non-volatile content 50%, softening point 160°C)

[0125] Solution No. 1: "Zinc Oxide Solution No. 1" (zinc oxide (crosslinking agent), non-volatile content 25%, manufactured by BASF)

[0126] EX-313: "Denacol EX-313" (epoxy compound (crosslinking agent), non-volatile content 100%, manufactured by Nagase ChemteX)

[0127] [Preparation of adhesive sheet (PET film)]

[0128] The water-based adhesive obtained in each embodiment and comparative example was applied to a polyethylene terephthalate (PET) film in a manner such that the dry film thickness was about 20 μm, and then dried at 100°C for 2 minutes to form an adhesive layer. Subsequently, a peelable sheet (glassine paper) with a thickness of 80 μm was attached to the adhesive layer, and aged for 24 hours at a temperature of 23°C and a relative humidity of 50%, thereby obtaining adhesive sheets with a structure of "peelable sheet / adhesive layer / PET film". The obtained adhesive sheet was used and the gel fraction was measured using the method described in the specification. The results of each embodiment and comparative example are shown in Table 2, Table 4, and Table 6.

[0129] [Production of adhesive sheet (coated paper)]

[0130] The water-based adhesive obtained in each example and comparative example was applied to a 65 μm thick peelable sheet (glass paper) using an applicator so that the thickness after drying was about 30 μm, and dried at 100° C. for 2 minutes to form an adhesive layer. Subsequently, a 70 μm thick substrate (coated paper) was attached to the adhesive layer and aged for 24 hours at a temperature of 23° C. and a relative humidity of 50%, thereby obtaining adhesive sheets having a structure of "peelable sheet / adhesive layer / coated paper", and the following tests (1) to (4) were performed.

[0131] [Test method]

[0132] (1) Adhesion strength at -10℃

[0133] The adhesive sheets of each embodiment and comparative example were cut into a size of 100 mm in length and 25 mm in width under 23°C and 50% RH as samples. After being placed under -10°C for 24 hours, the peelable sheet was peeled off from the sample, and the exposed adhesive layer was pasted on a high-density polyethylene plate (hereinafter referred to as PE (polyethylene) plate). A 2 kg roller was moved back and forth once, and the adhesive strength was measured shortly after 24 hours under -10°C. In addition, the adhesive strength was measured using a tensile tester at a peeling speed of 300 mm / min and a peeling angle of 90°.

[0134] The -10°C adhesive strength was evaluated based on the following criteria.

[0135] ++++: The adhesive force is 5.0 N / 25 mm or more, which is extremely good.

[0136] +++: The adhesive strength is 4.0 N / 25 mm or more and less than 5.0 N / 25 mm, which is good.

[0137] ++: The adhesive strength is 3.0 N / 25 mm or more and less than 4.0 N / 25 mm, which is good.

[0138] +: The adhesive strength is 2.0 N / 25 mm or more and less than 3.0 N / 25 mm, which is practical.

[0139] NG: The adhesive force is less than 2.0N / 25mm and cannot be used practically.

[0140] (2) Initial viscosity

[0141] The adhesive sheets of each embodiment and comparative example were cut into a size of 280mm in length and 25mm in width at 23℃50%RH as a sample. The sample was made into a ring with the adhesive surface as the outer side and placed on the upper chuck of the tensile tester. It was contacted with the PE plate of the adherend at a speed of 300mm / min until the upper and lower gaps of the ring sample became 60mm. After the gap reached 60mm, it was peeled off at a speed of 300mm / min after 15 seconds, and the maximum adhesion was measured.

[0142] The initial viscosity was evaluated according to the following criteria.

[0143] ++++: Adhesion is 8.0 N / 25 mm or more. Very good.

[0144] +++: Adhesion strength is 7.0 N / 25 mm or more and less than 8.0 N / 25 mm. Good.

[0145] ++: Adhesion strength is 6.0 N / 25 mm or more and less than 7.0 N / 25 mm. Good.

[0146] +: Adhesion strength is 4.0 N / 25 mm or more and less than 6.0 N / 25 mm. Practical.

[0147] NG: Adhesion strength is less than 4.0N / 25mm. Not practical.

[0148] (3) Retention

[0149] The adhesive sheets of each embodiment and comparative example were cut into a size of 100 mm in length and 25 mm in width under 23°C 50% RH environment as samples. Then, the peelable sheet of the sample with an area of ​​25 mm in length and 25 mm in width was peeled off, and the exposed adhesive layer was pasted on stainless steel (SUS) and pressed back and forth with a 2 kg roller. For the attached sample, a load of 1 kg was applied under 40°C-dry conditions as the measurement environment, and after leaving it for 20 minutes, it was measured for 20 hours.

[0150] The holding power was evaluated according to the following criteria.

[0151] ++++: Staying power is more than 20 hours. Very good.

[0152] +++: The retention time is 10 hours or more and less than 20 hours. Good.

[0153] ++: Staying power is 5 hours or more and less than 10 hours. Good.

[0154] +: The retention time is more than 1 hour and less than 5 hours. It is practical.

[0155] NG: The retention time is less than 1 hour. Not practical.

[0156] (4) Penetration resistance

[0157] The adhesive sheets of each example and comparative example were cut into a size of 100 mm in length and 100 mm in width to prepare a sample. The sample was sandwiched between glass plates and subjected to a pressure of 90 g / cm 2 The adhesive sheet was placed in a 70°C dry environment for two weeks with a load of 1000 g. The appearance of the adhesive sheet was visually evaluated to see if there was any change.

[0158] The penetration resistance was evaluated according to the following criteria.

[0159] ++++: Discoloration is observed on less than 10% of the adhesive sheet surface. Very good.

[0160] +++: Discoloration is observed on 10% or more and less than 20% of the adhesive sheet surface. Good.

[0161] ++: Discoloration is observed on 20% or more and less than 30% of the adhesive sheet surface. Good.

[0162] +: Discoloration is visible over 30% or more and less than 40% of the adhesive sheet surface. Practical.

[0163] NG: Discoloration is visible on more than 40% of the adhesive sheet surface. Not practical.

[0164] (5) Coating stability

[0165] The obtained water-based adhesive was placed in an environment of 40° C., and the viscosity after one month was measured. The viscosity was compared with the viscosity before the placement, and the stability of the coating solution was evaluated.

[0166] The coating liquid stability was evaluated according to the following criteria.

[0167] ++++: Viscosity change is less than ±10%. Very good.

[0168] +++: Viscosity change is ±10% or more and less than ±20%. Good.

[0169] +: Viscosity change is more than ±20% and less than ±30%. Practical.

[0170] NG: Viscosity changes by more than ±30%, or adhesive aggregates. Not practical.

[0171] [Table 1]

[0172]

[0173] *Non-volatile content of the compound based on 100 parts of acrylic copolymer

[0174] [Table 2]

[0175]

[0176] *Non-volatile content of the compound based on 100 parts of acrylic copolymer

[0177] [Table 3]

[0178]

[0179] [Table 4]

[0180]

[0181]

[0182]

[0183] From the results of Table 2, Table 4, and Table 6, it can be confirmed that the embodiments using the water-based adhesive of the present invention can obtain excellent results in low-temperature adhesion, initial viscosity, and penetration resistance. Furthermore, it is confirmed that the retention is excellent. In addition, it is confirmed that the coating liquid stability is excellent. On the other hand, it is confirmed that the comparative examples do not obtain satisfactory results in any aspect of low-temperature adhesion, initial viscosity, and penetration resistance.

[0184] [Industrial Applicability]

[0185] The adhesive sheet of the present invention can be used in a wide range of applications, including delivery labels, invoice labels for industrial and household use, masking tapes for building materials and household use, protective films, decorative sheets, and the like.

Claims

1. A water-based adhesive comprising a (meth)acrylic acid copolymer obtained by emulsion polymerization in the presence of an emulsifier, an alkali metal compound, and a tackifier resin, The (meth)acrylic copolymer comprises structural units derived from a (meth)acrylic acid alkyl ester monomer and a carboxyl group-containing monomer. The (meth)acrylic acid alkyl ester monomer comprises at least one of 2-ethylhexyl acrylate and methyl methacrylate, The carboxyl group-containing monomer is used in an amount of 0.5 to 5 parts by mass based on 100 parts by mass of the monomer mixture. The alkali metal compound is a compound that does not belong to the emulsifier and does not have an alkyl group with a carbon number of 8 or more, a cyclic skeleton with a carbon number of 6 or more, and an alkylene oxide, and includes at least one of sodium citrate and sodium tripolyphosphate. The alkali metal compound contains 30 mol to 100 mol of alkali metal based on 100 mol of carboxyl groups in the (meth)acrylic acid-based copolymer. The tackifier resin is contained in an amount of 1 to 10 parts by mass based on 100 parts by mass of the (meth)acrylic copolymer. The water-based adhesive does not contain a covalent crosslinking agent for the (meth)acrylic copolymer, and does not contain a compound having a divalent or higher valent cation.

2. The water-based adhesive according to claim 1, in, The (meth)acrylic copolymer is obtained by polymerizing a monomer mixture including the (meth)acrylic acid alkyl ester monomer and the carboxyl group-containing monomer in the presence of the alkali metal compound.

3. The water-based adhesive according to claim 1 or 2, in, The (meth)acrylic copolymer is obtained by polymerizing a monomer mixture containing the (meth)acrylic acid alkyl ester monomer and the carboxyl group-containing monomer in the presence of the tackifier resin.

4. The water-based adhesive according to claim 1 or 2, in, The adhesion-imparting resin comprises a terpene resin.

5. The water-based adhesive according to claim 1 or 2, in, The (meth)acrylic copolymer has an average particle size D50 of 0.1 μm to 0.6 μm.

6. An adhesive sheet comprising a substrate and an adhesive layer laminated on at least one surface of the substrate, The adhesive layer is formed using the water-based adhesive according to any one of claims 1 to 5.

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