Water-based overprint varnish

Abstract

To provide a water-based overprint varnish for a shrink label, excellent in adhesion, moist friction properties and anti-blocking properties, and capable of forming a coating film suppressed in whitening at heat shrinkage on various substrates to be printed.SOLUTION: A water-based overprint varnish for a shrink label contains a binder resin, water and a water-soluble organic solvent. The binder resin is a polymer which includes a constitutional unit (i) represented by the following formula (1), and having a carboxy group, and has a number-average molecular weight of 15,000-50,000, and at least a part of the carboxy group in the constitutional unit (i) is neutralized by an alkaline substance and is self-emulsified to form emulsion particles.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a water-based overprint varnish for shrink labels. [Background technology]

[0002] In recent years, packaging means such as paper containers, steel cans, aluminum cans, glass containers, and plastic containers have been used to take advantage of the characteristics of each container to package beverages, cosmetics, daily necessities, etc. In order to enable consumers to easily distinguish a company's products from similar products of other companies, many innovations have been made in the color tone, display method, shape, etc. of these packaging means.

[0003] Shrink labels are widely used to package various items of various shapes (Patent Document 1). Shrink labels are, for example, cylindrical heat-shrinkable plastic films printed with a desired image or the like, which can be placed on the surface of a container or the like and then heat-shrunk to attach and package the item. Examples of heat-shrinkable plastic films that have been used include shrinkable polyvinyl chloride, shrinkable polystyrene, shrinkable polyethylene terephthalate, and shrinkable polypropylene.

[0004] Shrink labels are printed using inks containing solvents and binder resins suited to the characteristics of various films. For example, for shrinkable polyvinyl chloride, shrinkable polystyrene, and shrinkable polyethylene phthalate, inks containing an organic solvent and a binder resin that combines acrylic resin and nitrocellulose have been proposed (Patent Document 2). For shrinkable polypropylene, inks containing toluene and chlorinated polypropylene as a binder resin have been used.

[0005] In recent years, there has been a demand for the development of environmentally friendly products, and water-based inks in particular have been attracting attention. There is also a demand for water-based inks to be used for printing on resin film substrates, and many studies have been conducted on the transition from oil-based gravure inks to water-based flexographic inks (Patent Documents 3 and 4). [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2004-226468 [Patent Document 2] Japanese Patent Application Laid-Open No. 2009-286974 [Patent Document 3] Japanese Patent Application Publication No. 2018-131548 [Patent Document 4] Japanese Patent Application Publication No. 2019-203051 Summary of the Invention [Problem to be solved by the invention]

[0007] Water-based inks are environmentally friendly because they contain virtually no volatile organic solvents, but the resulting coating often has issues with its abrasion resistance in a wet state (wet abrasion resistance). In practice, when shrink labels are transported in a wet state, the ink coating can easily peel off.

[0008] Furthermore, if the binder resin in the overprint (OP) varnish or ink for shrink labels has a relatively high glass transition temperature (Tg), the transparency of the coating film may be impaired and the film may be prone to whitening when the shrink label is heated and shrunk. This whitening can often be prevented by using a binder resin with a relatively low glass transition temperature. However, using a binder resin with a relatively low glass transition temperature can easily reduce the coating film's blocking resistance, leading to problems such as coating films sticking together.

[0009] The present invention has been made in view of the problems associated with the prior art, and an object of the present invention is to provide an aqueous overprint varnish for shrink labels that is capable of forming a coating film on a variety of printing substrates that has excellent adhesion, wet friction resistance, and blocking resistance, and that is inhibited from whitening during heat shrinkage. [Means for solving the problem]

[0010] That is, according to the present invention, the following water-based OP varnish is provided. [1] An aqueous overprint varnish for shrink labels containing a binder resin, water, and a water-soluble organic solvent, wherein the binder resin is a polymer having a number average molecular weight of 15,000 to 50,000, and including 20 to 40% by mass of structural units (i) having a carboxy group represented by the following general formula (1), 10 to 40% by mass of structural units (ii) derived from styrene, 10 to 40% by mass of structural units (iii) derived from a first alkyl methacrylate having a homopolymer glass transition temperature of 80°C or higher, and 10 to 40% by mass of structural units (iv) derived from an alkyl acrylate having a homopolymer glass transition temperature of 0°C or lower, and wherein at least a portion of the carboxy groups in the structural units (i) are neutralized with at least one alkaline substance selected from the group consisting of ammonia, dimethylaminoethanol, and aminomethylpropanol, thereby self-emulsifying the binder resin and forming emulsion particles having a number average particle size of 50 to 200 nm.

[0011] TIFF2025168807000001.tif45170 (In the general formula (1), R1 represents a hydrogen atom or an alkyl group having 1 to 3 carbon atoms, "Polymer" represents a polymer moiety having a number average molecular weight of 3,000 to 10,000, which comprises 40 to 60 mass% of structural units (a) derived from methyl methacrylate, 10 to 30 mass% of structural units (b) derived from a second alkyl methacrylate having an alkyl group having 8 to 18 carbon atoms, 0.1 to 20 mass% of structural units (c) derived from a hydroxyalkyl methacrylate having a hydroxyalkyl group having 2 to 4 carbon atoms, and 10 to 30 mass% of structural units (d) derived from methacrylic acid, and p represents any number of repeating units.)

[0012] [2] The aqueous overprint varnish according to [1], wherein the first alkyl methacrylate is at least one selected from the group consisting of methyl methacrylate and isobornyl methacrylate, the alkyl acrylate is at least one selected from the group consisting of butyl acrylate, 2-ethylhexyl acrylate, and lauryl acrylate, the second alkyl methacrylate is at least one selected from the group consisting of dodecyl methacrylate and tridecyl acrylate, and the hydroxyalkyl methacrylate is 2-hydroxyethyl methacrylate. [3] The aqueous overprint varnish according to [1] or [2] above, further containing at least one aqueous crosslinking agent selected from the group consisting of epoxy-based crosslinking agents, isocyanate-based crosslinking agents, oxazoline-based crosslinking agents, aziridine-based crosslinking agents, and carbodiimide-based crosslinking agents. [4] The aqueous overprint varnish according to any one of [1] to [3] above, further comprising a wax component, wherein the wax component is emulsion particles formed by neutralizing the carboxyl groups of at least one polymer having a carboxyl group selected from the group consisting of polyethylene, polypropylene, poly-α-olefin, silicone, and copolymers thereof with an alkaline substance, followed by self-emulsification. [Effects of the Invention]

[0013] The present invention has been made in consideration of the problems associated with the prior art, and its object is to provide an aqueous overprint varnish for shrink labels that is capable of forming a coating film on a variety of printing substrates that has excellent adhesion, wet friction resistance, and blocking resistance, and that is inhibited from whitening during heat shrinkage. DETAILED DESCRIPTION OF THE INVENTION

[0014] <Water-based overprint (OP) varnish> The following describes embodiments of the present invention, but the present invention is not limited to these embodiments. One embodiment of the aqueous overprint varnish (hereinafter also referred to as "aqueous OP varnish") of the present invention is an aqueous liquid composition useful as a material for producing shrink labels, containing a binder resin, water, and a water-soluble organic solvent. The binder resin is a polymer having a number average molecular weight of 15,000 to 50,000, and including a carboxyl group-containing structural unit (i) represented by the following general formula (1), a styrene-derived structural unit (ii), a first alkyl methacrylate-derived structural unit (iii) having a homopolymer glass transition temperature of 80°C or higher, and an alkyl acrylate-derived structural unit (iv) having a homopolymer glass transition temperature of 0°C or lower. Note that the binder resin is preferably a polymer substantially composed only of structural units (i) to (iv). This binder resin is self-emulsified by neutralizing at least a portion of the carboxyl groups in the structural unit (i) with at least one alkaline substance selected from the group consisting of ammonia, dimethylaminoethanol, and aminomethylpropanol, to form emulsion particles with a number average particle size of 50 to 200 nm.

[0015] TIFF2025168807000002.tif45170 (In the general formula (1), R1 represents a hydrogen atom or an alkyl group having 1 to 3 carbon atoms, "Polymer" represents a polymer moiety having a number average molecular weight of 3,000 to 10,000, which comprises 40 to 60 mass% of structural units (a) derived from methyl methacrylate, 10 to 30 mass% of structural units (b) derived from a second alkyl methacrylate having an alkyl group having 8 to 18 carbon atoms, 0.1 to 20 mass% of structural units (c) derived from a hydroxyalkyl methacrylate having a hydroxyalkyl group having 2 to 4 carbon atoms, and 10 to 30 mass% of structural units (d) derived from methacrylic acid, and p represents any number of repeating units.)

[0016] (binder resin) The binder resin is the main component of the film (coating) formed by applying and drying an aqueous OP varnish, and is a polymer component that can exhibit effects such as adhesion to substrates such as plastic films, as well as abrasion resistance, water resistance, chemical resistance, blocking resistance, solvent resistance, and surface protection. The binder resin contains structural unit (i) and is a so-called graft copolymer, having a structure in which the polymer portion (polymer (A)) in structural unit (i) is grafted to a polymer of unsaturated bonds in structural unit (i) and a polymer chain (polymer (B)) that is the main chain formed by structural units (ii) to (iv).

[0017] Polymer (A) contains carboxyl groups derived from methacrylic acid. At least a portion of these carboxyl groups are neutralized with an alkaline substance, ionized, and dissolved in water. The water-insoluble polymer (B) then forms particles. This results in emulsion particles in which the binder resin is self-dispersed in the aqueous medium. When the aqueous OP varnish of this embodiment containing this binder resin is applied to a substrate such as a plastic film, the carboxyl groups and hydroxyl groups in polymer (A) exhibit adhesion to the substrate. Furthermore, polymer (B) not only exhibits adhesion to the substrate, but also exhibits water resistance due to its water insolubility. The structural units that make up polymer (B) form a coating film with excellent properties such as flexibility and abrasion resistance.

[0018] Furthermore, by using a crosslinking agent in combination, the crosslinking agent reacts with the hydroxyl and carboxyl groups in the polymer (A) to form a three-dimensional network structure, thereby improving the physical properties of the coating film. When a crosslinking agent is used in combination with a conventional linear binder resin, the main chain of the binder resin crosslinks to form a network structure, which tends to inhibit molecular motion. This makes it difficult to follow the shrinkage of the substrate during shrinkage, which can easily lead to defects such as whitening and cracking. Furthermore, using a soft polymer as the binder resin to follow the shrinkage of the substrate can easily reduce durability, such as abrasion resistance. In contrast, the binder resin used in the aqueous OP varnish of this embodiment is a graft copolymer with crosslinkable graft chains, so even when used in combination with a crosslinking agent, the flexibility of the uncrosslinked polymer main chain is fully exhibited. This prevents defects during shrinkage, such as whitening and cracking, and allows for the formation of a good coating film.

[0019] [Constituent unit (i)] The binder resin is a polymer containing a structural unit (i) having a carboxy group, represented by the following general formula (1): In general formula (1), at least a portion of the carboxy groups in the polymer portion represented by "Polymer" are neutralized with an alkaline substance and ionized. This allows the binder resin to be emulsified in an aqueous medium to form emulsion particles.

[0020] TIFF2025168807000003.tif45170 (In the general formula (1), R1 represents a hydrogen atom or an alkyl group having 1 to 3 carbon atoms, "Polymer" represents a polymer moiety having a number average molecular weight of 3,000 to 10,000, which comprises 40 to 60 mass% of structural units (a) derived from methyl methacrylate, 10 to 30 mass% of structural units (b) derived from a second alkyl methacrylate having an alkyl group having 8 to 18 carbon atoms, 0.1 to 20 mass% of structural units (c) derived from a hydroxyalkyl methacrylate having a hydroxyalkyl group having 2 to 4 carbon atoms, and 10 to 30 mass% of structural units (d) derived from methacrylic acid, and p represents any number of repeating units.)

[0021] The content of the structural unit (i) in the binder resin is 20 to 40% by mass, preferably 25 to 35% by mass (where the total of the structural units (i) to (iv) is 100% by mass). If the content of the structural unit (i) is less than 20% by mass, the amount of water-soluble polymer chains is small, which can result in large particle sizes of emulsion particles formed by self-emulsification, or the polymer can precipitate without emulsifying. On the other hand, if the content of the structural unit (i) is more than 40% by mass, the macromonomer that forms the structural unit (i) is likely to remain without being polymerized. Furthermore, the proportion of the main chain of the graft copolymer is reduced, which can lead to whitening during shrinking.

[0022] [Macromonomer] The structural unit (i) in the binder resin is a structural unit derived from a macromonomer represented by the following general formula (2): In other words, the structural unit (i) is formed by polymerizing the macromonomer represented by the following general formula (2) as a monomer.

[0023] TIFF2025168807000004.tif44170

[0024] In general formula (2), R1 represents a hydrogen atom or an alkyl group having 1 to 3 carbon atoms. Furthermore, in general formula (2), "Polymer" represents a polymer moiety having a number average molecular weight of 3,000 to 10,000, which comprises 40 to 60 mass% of structural units (a) derived from methyl methacrylate, 10 to 30 mass% of structural units (b) derived from a second alkyl methacrylate having an alkyl group having 8 to 18 carbon atoms, 0.1 to 20 mass% of structural units (c) derived from a hydroxyalkyl methacrylate having a hydroxyalkyl group having 2 to 4 carbon atoms, and 10 to 30 mass% of structural units (d) derived from methacrylic acid. R1 and "Polymer" in general formula (2) are synonymous with R1 and "Polymer" in general formula (1).

[0025] The macromonomer can be obtained by radical polymerization of the monomer that constitutes the polymer portion represented by "Polymer" in general formula (2) in the presence of a chain transfer agent and an azo-based or peroxide-based radical polymerization initiator. Bromomethylacrylic acid, methylbromomethylacrylate, ethylbromomethylacrylate, and propylbromomethylacrylate can be used as the chain transfer agent. By using the chain transfer agent, the desired macromonomer can be obtained, which has an ethylenically unsaturated bond derived from the chain transfer agent at one end.

[0026] When synthesizing a macromonomer having an unsaturated bond using a chain transfer agent, methacrylic acid or methacrylate is used as a monomer without substantially using acrylate or styrene. When acrylate or styrene is used as a monomer, the unsaturated bond in the chain transfer agent polymerizes with the acrylate or styrene, making it impossible to obtain a macromonomer having an unsaturated bond at one end. In contrast, when methacrylic acid or methacrylate is used as a monomer, the resulting tertiary carbon radical is less likely to react with the unsaturated bond in the chain transfer agent due to steric hindrance, and the unsaturated bond is maintained, making it possible to obtain a macromonomer having an unsaturated bond at one end.

[0027] The molecular weight of the resulting macromonomer can be controlled by adjusting the amount of chain transfer agent used. For example, a macromonomer with a target molecular weight can be obtained by using 1 to 10 parts by mass of chain transfer agent per 100 parts by mass of the total monomers. Radical polymerization can be carried out by solution polymerization, suspension polymerization, emulsion polymerization, or the like. Among these, it is preferable to synthesize the macromonomer by solution polymerization using an aqueous organic solvent contained in the aqueous OP varnish as the polymerization solvent.

[0028] In general formula (2), "Polymer" refers to a polymer portion (polymer (A)) containing a structural unit (a) derived from methyl methacrylate, a structural unit (b) derived from a second alkyl methacrylate having an alkyl group of 8 to 18 carbon atoms, a structural unit (c) derived from a hydroxyalkyl methacrylate having a hydroxyalkyl group of 2 to 4 carbon atoms, and a structural unit (d) derived from methacrylic acid. The methyl methacrylate constituting the structural unit (a) is selected from the perspective of cost, etc. The content of the structural unit (a) in the polymer portion (polymer (A)) is 40 to 60 mass%, and preferably 45 to 55 mass% (where the total of the structural units (a) to (d) is 100 mass%). If the content of the structural unit (a) is less than 40 mass%, the viscosity is likely to increase excessively and this can be disadvantageous in terms of cost. On the other hand, if the content of the structural unit (a) exceeds 60% by mass, the proportion of other structural units will be relatively reduced, and the glass transition temperature (Tg) of the polymer (A) may become too high, which may result in reduced shrinkability.

[0029] The second alkyl methacrylate constituting the structural unit (b) has an alkyl group having 8 to 18 carbon atoms. The second alkyl methacrylate contributes to adjusting the flexibility and glass transition temperature (Tg) of the polymer (A) and is effective in plasticizing the polymer (A). Examples of the second alkyl methacrylate include octyl methacrylate, 2-ethylhexyl methacrylate, decyl methacrylate, isodecyl methacrylate, trimethyldecyl methacrylate, dodecyl methacrylate, isododecyl methacrylate, tridecyl methacrylate, hexadecyl methacrylate, stearyl methacrylate, and isostearyl methacrylate. Among these, it is preferable that the second alkyl methacrylate be at least one selected from the group consisting of dodecyl methacrylate and tridecyl acrylate, as this can effectively lower the glass transition temperature (Tg) of the polymer (A) formed and is also preferable from the viewpoints of ease of availability and plasticization.

[0030] The content of the structural unit (b) in the polymer portion (polymer (A)) is 10 to 30% by mass, and preferably 15 to 25% by mass. If the content of the structural unit (b) is less than 10% by mass, the flexibility will be insufficient. On the other hand, if the content of the structural unit (b) is more than 30% by mass, the polymer will be too soft and tacky. Furthermore, since the second alkyl methacrylate has a long-chain alkyl group, the hydrophobicity of the polymer (A) will be too strong, and the adhesion to the substrate will be likely to decrease.

[0031] The hydroxyalkyl methacrylate constituting the structural unit (c) has a hydroxyalkyl group having 2 to 4 carbon atoms. That is, a hydroxyl group is present in the structural unit (c). Therefore, the structural unit (c) contributes to improving adhesion to the substrate through hydrogen bonding, and also serves as a reaction site with the crosslinking agent when a crosslinking agent is used. Examples of hydroxyalkyl methacrylates include 2-hydroxyethyl methacrylate, 2-hydroxypropyl methacrylate, 3-hydroxypropyl methacrylate, 4-hydroxybutyl methacrylate, and 3-hydroxybutyl methacrylate. Of these, from the viewpoint of versatility, etc., the hydroxyalkyl methacrylate is preferably 2-hydroxyethyl methacrylate, which has a primary hydroxyl group.

[0032] The content of the structural unit (c) in the polymer portion (polymer (A)) is 0.1 to 20% by mass, preferably 0.5 to 15% by mass. If the content of the structural unit (c) is less than 0.1% by mass, the effect of introducing a hydroxyl group cannot be obtained. On the other hand, if the content of the structural unit (c) is more than 20% by mass, there are too many reaction sites with the crosslinking agent, which makes intramolecular crosslinking more likely to occur and requires the addition of an excess of crosslinking agent. This may result in a decrease in shrinkability and durability of the coating film.

[0033] The structural unit (d) is a structural unit derived from methacrylic acid. Because structural unit (d) contains a carboxyl group, structural unit (d) contributes to improving adhesion to the substrate through hydrogen bonding and also serves as a reaction site with the crosslinking agent when a crosslinking agent is used. Furthermore, by neutralizing and ionizing at least a portion of the carboxyl group with an alkaline substance, polymer (A) can be dissolved in water.

[0034] The content of the structural unit (d) in the polymer portion (polymer (A)) is 10 to 30% by mass, preferably 15 to 25% by mass. If the content of the structural unit (d) is less than 10% by mass, the hydrophilicity will be insufficient even after neutralization with an alkaline substance, making it difficult to disperse the binder resin in an aqueous medium. On the other hand, if the content of the structural unit (d) is more than 30% by mass, the hydrophilicity will be too high, and the water resistance of the formed coating film will decrease. Furthermore, because the water solubility will be too high, gelation or high viscosity may occur when attempting to disperse the binder resin in an aqueous medium.

[0035] The number-average molecular weight of the macromonomer, measured by gel permeation chromatography (GPC) in terms of polystyrene, is 3,000 to 10,000, preferably 4,000 to 8,000. If the number-average molecular weight of the macromonomer is less than 3,000, the molecular weight is too small, resulting in a decrease in the water dispersibility of the binder resin. On the other hand, if the number-average molecular weight of the macromonomer is more than 10,000, the viscosity increases excessively and the number of graft chains that can be introduced into the main chain decreases, resulting in a decrease in the water dispersibility of the binder resin.

[0036] As described above, a macromonomer can be obtained by radically polymerizing the monomers that constitute the polymer portion (polymer (A)) in the presence of a chain transfer agent and a radical polymerization initiator. Examples of the radical polymerization initiator that can be used include azo-based initiators such as azobisisobutyronitrile and azobisisovaleric acid, and peroxide-based initiators such as benzoyl peroxide. The amount of radical polymerization initiator used is preferably 0.05 to 30 parts by mass per 100 parts by mass of the chain transfer agent. The radicals generated by cleavage of the radical polymerization initiator react with the monomer to produce a polymer with terminal radicals. After the generated polymer with terminal radicals reacts with the chain transfer agent, the bromo radical is eliminated to form an unsaturated bond. The bromo radical then reacts with the monomer to produce a polymer, and the terminal radical of the generated polymer reacts with the chain transfer agent. This reaction mechanism is repeated to produce the desired macromonomer. If the amount of radical polymerization initiator used is too large, it is likely that a large number of polymer chains that do not react with the chain transfer agent and do not have an unsaturated bond at their terminals will be produced. Therefore, it is preferable that the amount of radical polymerization initiator used is less than the amount of chain transfer agent used.

[0037] In the case of solution polymerization, conventionally known organic solvents can be used as the polymerization solvent. It is preferable to use the same water-soluble organic solvent as used in aqueous OP varnishes. By using the same water-soluble organic solvent as used in aqueous OP varnishes as the polymerization solvent, the resulting reaction product can be neutralized with an alkaline substance to form an emulsion, which can then be used directly in aqueous OP varnishes.

[0038] Water-soluble organic solvents that can be used as polymerization solvents include alcohol-based solvents, ether-based solvents, glycol ether-based solvents, and amide-based solvents. Examples of alcohol-based solvents include ethanol, propanol, isopropanol, butanol, and isobutanol. Examples of ether-based solvents include tetrahydrofuran, methyltetrahydrofuran, methylcyclopentane ether, and dioxolane. Examples of glycol ether-based solvents include ethylene glycol monobutyl ether, diethylene glycol monobutyl ether, triethylene glycol monobutyl ether, propylene glycol monomethyl ether, propylene glycol monopropyl ether, dipropylene glycol monomethyl ether, tripropylene glycol monomethyl ether, ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, and methoxymethylbutanol. Examples of amide-based solvents include dimethylformamide, dimethylacetamide, diethylacetamide, pyrrolidone, N-methylpyrrolidone, 3-methoxy-N,N-dimethylpropanamide, and 3-butoxy-N,N-dimethylpropanamide. Among these, from the viewpoint of drying properties, it is preferable to use a water-soluble organic solvent having a boiling point of 200°C or less, and it is more preferable to use a water-soluble organic solvent having a boiling point of 100°C or less.

[0039] After polymerization, the macromonomer can be extracted by drying or precipitating in a poor solvent, etc. From the viewpoint of process simplification and cost, it is preferable to carry out polymerization in a water-soluble organic solvent used in an aqueous OP varnish and use the polymerized solution in the next step without extracting the produced macromonomer.

[0040] [Constituent unit (ii)] The binder resin is a polymer containing a structural unit (ii) derived from styrene. Styrene is highly versatile, and its use allows the introduction of aromatic rings into the binder resin, improving adhesion to the substrate and increasing the glass transition temperature (Tg) of the binder resin. Furthermore, the use of styrene as a monomer prevents the polymerization reaction rate from increasing excessively, allowing for the production of a polymer containing uniform structural units. Furthermore, styrene does not need to be polymerized by dropwise addition; it can be charged and polymerized together with other monomers, thereby simplifying the process.

[0041] The content of the structural unit (ii) in the binder resin is 10 to 40% by mass, and preferably 20 to 30% by mass. If the content of the structural unit (ii) is less than 10% by mass, the effect of introducing the structural unit (ii) cannot be obtained. On the other hand, if the content of the structural unit (ii) is more than 40% by mass, the content of other structural units becomes relatively small, and the desired effect cannot be obtained.

[0042] [Constituent unit (iii)] The binder resin is a polymer containing a structural unit (iii) derived from a first alkyl methacrylate. The first alkyl methacrylate is a monomer such as an alkyl methacrylate, a cycloalkyl methacrylate, or an alkylcycloalkyl methacrylate, whose homopolymer has a glass transition temperature (Tg) of 80°C or higher. Use of the first alkyl methacrylate can improve adhesion to the substrate and abrasion resistance, and can also adjust the glass transition temperature (Tg) of the main chain of the binder resin.

[0043] The glass transition temperature (Tg) of a homopolymer can be obtained from various literature, such as various academic journals, polymer handbooks, and catalogs of monomer manufacturers. Alternatively, the glass transition temperature (Tg) of a homopolymer may be a value obtained by thermal analysis. The "glass transition temperature (Tg) of a homopolymer" in this specification is the value described in the "Polymer Handbook, 4th Edition."

[0044] Examples of the first alkyl methacrylate include methyl methacrylate (105), t-butyl methacrylate (122), trimethylcyclohexyl methacrylate (101), isobornyl methacrylate (150), and adamantyl methacrylate (183) (the number in parentheses is the glass transition temperature (Tg (°C)) of the homopolymer). Among these, it is preferable that the first alkyl methacrylate be at least one selected from the group consisting of methyl methacrylate and isobornyl methacrylate, because of its easy availability and low cost.

[0045] The content of the structural unit (iii) in the binder resin is 10 to 40% by mass, preferably 20 to 30% by mass. If the content of the structural unit (iii) is less than 10% by mass, the effect of introducing the structural unit (iii) cannot be obtained. On the other hand, if the content of the structural unit (iii) is more than 40% by mass, the polymerizability of the first alkyl methacrylate with other monomers is somewhat low, resulting in a decrease in the polymerization rate and a tendency for macromonomers to remain.

[0046] [Constituent unit (iv)] The binder resin is a polymer containing a component (iv) derived from an alkyl acrylate. The alkyl acrylate is a monomer whose homopolymer has a glass transition temperature of 0°C or lower, preferably -10°C or lower. The use of this alkyl acrylate softens and plasticizes the coating film, improving its shrinkability and preventing whitening and cracking. Furthermore, the glass transition temperature (Tg) of the binder resin's main chain can be adjusted.

[0047] Examples of alkyl acrylates include ethyl acrylate (-22), butyl acrylate (-54), isobutyl acrylate (-22), hexyl acrylate (-57), octyl acrylate (-65), 2-octyl acrylate (-45), 2-ethylhexyl acrylate (-50), and lauryl acrylate (-3) (the number in parentheses is the glass transition temperature (Tg (°C)) of the homopolymer). Among these, from the viewpoints of easy availability and low cost, as well as the ability to soften and plasticize the coating film with a small amount, it is preferable that the alkyl acrylate is at least one selected from the group consisting of butyl acrylate, 2-ethylhexyl acrylate, and lauryl acrylate.

[0048] The content of the structural unit (iv) in the binder resin is 10 to 30% by mass, and preferably 15 to 25% by mass. If the content of the structural unit (iv) is less than 10% by mass, the effect of introducing the structural unit (iv) cannot be obtained. On the other hand, if the content of the structural unit (iv) is more than 3% by mass, the formed coating film becomes too soft, and blocking and tackiness are likely to occur.

[0049] [Binder resin] By polymerizing the monomers that make up the structural units (i) to (iv) using a conventionally known radical polymerization initiator, it is possible to obtain a binder resin that is a graft copolymer in which the polymer portion of the structural unit (i) is grafted to the main chain. The monomer (macromonomer) that makes up the structural unit (i) is preferably used in a state dissolved in the polymerization solvent used to polymerize the macromonomer. The polymerization solvent used to polymerize the monomers that make up the structural units (i) to (iv) is preferably a water-soluble organic solvent used in aqueous OP varnish, similar to the polymerization solvent used to polymerize the macromonomer.

[0050] The polystyrene-equivalent number-average molecular weight of the binder resin, measured by gel permeation chromatography (GPC) using an organic solvent such as tetrahydrofuran as a developing solvent, is 15,000 to 50,000, preferably 17,000 to 40,000. If the number-average molecular weight of the binder resin is less than 15,000, the molecular weight is too small, resulting in insufficient physical properties of the coating film. On the other hand, if the number-average molecular weight of the binder resin is more than 50,000, the molecular weight is too large, resulting in reduced water dispersibility, precipitation, or high viscosity.

[0051] The molecular weight distribution (PDI = weight average molecular weight (Mw) / number average molecular weight (Mn)) of the binder resin is preferably 1.5 to 3.0, and more preferably 1.8 to 2.5. If the molecular weight distribution is too broad, emulsifiability may be easily reduced and precipitation may be easily generated.

[0052] An alkaline substance is added to the binder resin solution obtained by polymerization to neutralize and ionize at least a portion of the carboxy groups in the structural unit (i). This allows the polymer portion of the structural unit (i) to dissolve in water, and the binder resin is granulated (self-emulsified) by the water-insoluble main chain, forming emulsion particles. The alkaline substance used is at least one selected from the group consisting of ammonia, dimethylaminoethanol, and aminomethylpropanol. Neutralizing at least a portion of the carboxy groups using these alkaline substances can improve the drying properties of the coating film.

[0053] The number-average particle size of the emulsion particles measured by a light scattering method is 50 to 200 nm, preferably 80 to 150 nm. If the number-average particle size of the emulsion particles is less than 50 nm, the particle size may be too small, resulting in an excessively high viscosity. On the other hand, if the number-average particle size of the emulsion particles is more than 200 nm, clogging may occur easily during application (printing), and defects such as streaks may easily occur in the coating film.

[0054] The binder resin can be self-emulsified by adding an aqueous solution of an alkaline substance to a solution (reaction liquid) containing a polymer formed by solution polymerization. Alternatively, the binder resin can be self-emulsified by taking out the polymer formed by solution polymerization, adding it to an aqueous solution of an alkaline substance, and then adding a water-soluble organic solvent as needed to dissolve the polymer.

[0055] The content of the binder resin in the aqueous OP varnish is preferably 5 to 50% by mass, and more preferably 10 to 40% by mass, based on the total amount of the aqueous OP varnish. Furthermore, the aqueous OP varnish may further contain other resins in addition to the above-mentioned binder resins, as necessary. Examples of other resins include aqueous polyurethane resins, aqueous polyester resins, aqueous acrylic resins, aqueous styrene-acrylic resins, aqueous styrene-maleic anhydride resins, aqueous cellulose-based resins, and aqueous vinyl chloride copolymer resins.

[0056] (water) The aqueous OP varnish contains water. As the water, ion-exchanged water, distilled water, purified water, etc. are preferably used. The content of water in the aqueous OP varnish is preferably 30 to 70 mass % based on the total amount of the aqueous OP varnish.

[0057] (Water-soluble organic solvent) The aqueous OP varnish contains a water-soluble organic solvent. It is preferable to use a water-soluble organic solvent that has good wettability with a plastic film suitable for use as a substrate. Furthermore, it is preferable to use a water-soluble organic solvent that can function as a leveling agent for the substrate surface and also as a film-forming aid that improves the film-forming properties of the binder resin. Examples of the water-soluble organic solvent include the same water-soluble organic solvents as those described above that can be used as polymerization solvents.

[0058] Further examples of the water-soluble organic solvent include ketone-based solvents such as acetone and methyl ethyl ketone; glycol-based solvents such as ethylene glycol, diethylene glycol, propylene glycol, and dipropylene glycol; glycerin-based solvents such as glycerin, diglycerin, and ethylene oxide adducts of glycerin; urea-based solvents such as tetramethylurea and dimethylimidazolidinone; and carbonate-based solvents such as ethylene carbonate and dimethyl carbonate.

[0059] The content of the water-soluble organic solvent in the aqueous OP varnish is preferably 1 to 30% by mass, and more preferably 3 to 20% by mass, based on the total amount of the aqueous OP varnish. If the content of the water-soluble organic solvent is less than 1% by mass, leveling may decrease, resulting in a slight decrease in coating film properties (adhesion, wet rub resistance). On the other hand, if the content of the water-soluble organic solvent is more than 30% by mass, poor drying of the coating film may occur more easily, resulting in a slight decrease in coating film properties (adhesion, wet rub resistance, blocking resistance).

[0060] (Crosslinking agent) The aqueous OP varnish preferably further contains a crosslinking agent. The crosslinking agent reacts with hydroxyl groups and carboxyl groups in the polymer (A) (graft chain) of the binder resin to form a three-dimensional network structure. Therefore, the inclusion of a crosslinking agent can improve the durability of the coating film formed, its adhesion to the substrate, and its abrasion resistance. The crosslinking agent preferably is an aqueous crosslinking agent. Furthermore, the crosslinking agent preferably has a functional group that reacts with hydroxyl groups and carboxyl groups. Examples of the functional group include an epoxy group, an isocyanate group, a blocked isocyanate group, an oxazoline group, a carbodiimide group, and an aziridinyl group. The aqueous crosslinking agent having such a functional group is preferably at least one selected from the group consisting of an epoxy-based crosslinking agent, an isocyanate-based crosslinking agent, an oxazoline-based crosslinking agent, an aziridine-based crosslinking agent, and a carbodiimide-based crosslinking agent.

[0061] As the crosslinking agent, commercially available conventional crosslinking agents can be used. The amount of crosslinking agent added is preferably 0.1 to 8 parts by mass, and more preferably 0.5 to 5 parts by mass, per 100 parts by mass of aqueous OP varnish. If the amount of crosslinking agent added is less than 0.1 parts by mass per 100 parts by mass of aqueous OP varnish, the crosslinking reaction will not proceed sufficiently, and the physical properties of the resulting coating will not be significantly improved. On the other hand, if the amount of crosslinking agent added is more than 8 parts by mass per 100 parts by mass of aqueous OP varnish, the viscosity of the aqueous OP varnish may increase excessively during printing, and the blocking resistance of the resulting coating may be slightly reduced.

[0062] Commercially available crosslinking agents include, under the trade names below, isocyanate-based crosslinking agents such as Duranate WB40-100, WB40-80D, WT20-100, WT30-100, WL70-100, and WE50-100 (all manufactured by Asahi Kasei Corporation); oxazoline-based crosslinking agents such as Epocross K-2010E, K-2020E, K-2030E, WS-300, WS-500, and WS-700 (all manufactured by Nippon Shokubai Co., Ltd.); and Carbodilite V-02, V-02-L2, SV-02, and V-04. Examples of suitable crosslinking agents include carbodiimide-based crosslinking agents such as V-10, E-02, and E-05 (all manufactured by Nisshinbo Chemical Inc.); epoxy-based crosslinking agents such as Denacol EX-612, EX-614, EX-614B, EX-622, EX-313, EX-314, EX-421, EX-521, EX-321, EX-321L, and EX-411 (all manufactured by Nagase ChemteX Corporation); and aziridine-based crosslinking agents such as Chemitite PZ-33 and DZ-22E (all manufactured by Nippon Shokubai Co., Ltd.).

[0063] (wax component) The aqueous OP varnish preferably further contains a wax component. By including the wax component, the wet friction resistance of the coating film formed can be further improved. As the wax component, emulsion particles formed by neutralizing the carboxyl groups of at least one carboxyl group-containing polymer selected from the group consisting of polyethylene, polypropylene, poly-α-olefin, silicone, and copolymers thereof with an alkaline substance and then self-emulsifying are preferably used.

[0064] The number-average particle size of the wax component emulsion particles measured by light scattering is preferably 0.5 to 10 μm, and more preferably 2 to 8 μm. The content of the wax component in the aqueous OP varnish is preferably 1 to 7 mass% based on the total amount of the aqueous OP varnish. Examples of commercially available wax components include Chemipearl W100, W200, W300, W308, W310, W400, W410, W500, W700, and W800 (all manufactured by Mitsui Chemicals, Inc.).

[0065] (additives) The aqueous OP varnish of this embodiment can further contain various additives as needed, such as organic solvents other than the water-soluble organic solvents described above, leveling agents, surface tension adjusters, pH adjusters, rheology adjusters, UV absorbers, light stabilizers, antioxidants, dyes, pigments, fillers, pigment dispersants, thickeners, antifoaming agents, antifungal agents, antistatic agents, metal fine particles, and magnetic powders.

[0066] (Water-based OP varnish manufacturing method) The aqueous OP varnish of this embodiment can be produced by a conventional method. For example, a mixture is obtained by mixing water, a water-soluble organic solvent, and a binder resin. A disperser or homogenizer is preferably used for mixing. The mixture is then diluted with water and a water-soluble organic solvent as needed, and the mixture is passed through a filter to remove foreign matter and precipitates, thereby obtaining the desired aqueous OP varnish.

[0067] (Printing base material) The aqueous OP varnish of this embodiment is suitable as an overprint varnish for shrink labels. A plastic film that shrinks upon heat treatment can be used as the substrate (printed substrate) that forms the constituent material of the shrink label. Among these, it is preferable to use a heat-shrinkable plastic film that has high mechanical and chemical strength and good printability. Examples of such heat-shrinkable plastic films include oriented polyester films, shrinkable polyvinyl chloride films, shrinkable polystyrene films, shrinkable polyethylene terephthalate films, and shrinkable polypropylene films. The surface (printing surface) of the plastic film may be untreated or may be subjected to a surface treatment such as plasma treatment, corona treatment, radiation treatment, or silane coupling treatment. The plastic film may have a single-layer structure or a multilayer structure. Furthermore, the plastic film may be subjected to aluminum vapor deposition or transparent vapor deposition. [Example]

[0068] The present invention will be specifically described below based on examples, but the present invention is not limited to these examples. In the examples and comparative examples, "parts" and "%" are by mass unless otherwise specified.

[0069] <Synthesis of macromonomer> (Synthesis Example 1) 52 parts of diethylene glycol monobutyl ether (BDG), 52 parts of isopropyl alcohol (IPA), 55 parts of methyl methacrylate (MMA), 20 parts of lauryl methacrylate (LMA), 10 parts of 2-hydroxyethyl methacrylate (HEMA), 15 parts of methacrylic acid (MAA), 4 parts of ethyl α-bromomethyl acrylate (EBMA), and 1 part of azobisisobutyronitrile (AIBN) were placed in a reaction vessel and bubbled with nitrogen. Polymerization was carried out at 80 °C for 8 hours to form macromonomer MM-1, yielding a solution of macromonomer MM-1. A portion of the resulting solution was sampled and dried in a dryer at 180 °C until it reached a constant weight. The solids content was measured to be 50.1%, confirming that most of the polymerization had occurred. Furthermore, the polystyrene equivalent number average molecular weight (Mn) measured by gel permeation chromatography (GPC) equipped with a differential refractive index detector (RI) using tetrahydrofuran (THF) as a developing solvent was 6,000, and the molecular weight distribution (PDI = weight average molecular weight (Mw) / number average molecular weight (Mn)) was 1.60. Furthermore, when a GPC equipped with an ultraviolet detector (UV, wavelength 254 nm) was used, almost no peaks were observed. The solid obtained by drying was dissolved in a deuterated chloroform solvent and analyzed by a proton nuclear magnetic resonance spectrometer ( 1 NMR was measured using H-NMR, and the results confirmed the presence of peaks at 5.5 ppm and 6.1 ppm, which correspond to EBMA.

[0070] (Synthesis Examples 2 to 11, Comparative Synthesis Examples 1 to 5) Liquid macromonomers MM-2 to 11 and MM-H1 to 5 were obtained in the same manner as in Synthesis Example 1, except that the types and amounts (unit: parts) of each component were used as shown in Tables 1 to 3. The meanings of the abbreviations in Table 2 are as follows. SLMA: A mixture of dodecyl methacrylate and tridecyl methacrylate 2EHMA: 2-Ethylhexyl methacrylate HBMA: 4-hydroxybutyl methacrylate

[0071] TIFF2025168807000005.tif82170

[0072] TIFF2025168807000006.tif104170

[0073] TIFF2025168807000007.tif83170

[0074] <Production of binder resin> (Production Example 1) A reaction vessel was charged with 60 parts of a solution of macromonomer MM-1 (30 parts solids), 70 parts of IPA, 30 parts of styrene (St), 25 parts of MMA, 15 parts of butyl acrylate (BA), and 1.0 part of t-butylperoxy-2-ethylhexanoate (PB) (trade name "Perbutyl O" manufactured by NOF Corporation). Polymerization was carried out at 75°C for 8 hours to form a polymer, yielding a polymer solution. The conversion rate was approximately 100%. Analysis by GPC equipped with an RI revealed a monomodal peak, and no peak attributable to macromonomer MM-1 was detected. The Mn of the resulting polymer was 19,000, and the PDI was 1.9. Analysis by GPC equipped with a UV detector also revealed a large absorption peak. The Mn of the resulting polymer was 19,200, and the PDI was 1.9. This suggests that the absorption peak was detected because the styrene (St) used became a component of the polymer and absorbed ultraviolet light. Furthermore, the molecular weights calculated from the peaks detected by RI and UV were nearly identical, indicating that macromonomer MM-1 had polymerized uniformly. Next, a mixture of 3.3 parts of 28% aqueous ammonia (amount of ammonia: 1.1 molar equivalents of the methacrylic acid constituting macromonomer MM-1) and 132.4 parts of water was gradually added, allowing for self-emulsification, resulting in a transparent, translucent emulsion containing binder resin BB-1. The solids content of the resulting liquid (emulsion) was 29.8%, and the number-average particle size of the emulsion particles measured using a dynamic light scattering particle size distribution analyzer (nanoSAQRA, manufactured by Otsuka Electronics Co., Ltd.) was 106 nm. Furthermore, the viscosity of the liquid measured using a BM-type viscometer was 1.5 Pa·s, and the pH was 8.6.

[0075] (Comparative Manufacturing Example 1) A polymer solution was obtained by polymer formation in the same manner as in Production Example 1, except that a solution of macromonomer MM-H1 was used instead of the solution of macromonomer MM-1. The polymerization rate was approximately 100%. Analysis by GPC equipped with RI revealed two overlapping peaks. The Mn of the resulting polymer was 23,600, and the PDI was 3.86. Analysis by GPC equipped with UV revealed a single peak. The Mn of the resulting polymer was 19,800, and the PDI was 2.51. This is believed to have resulted in a mixture of macromonomer MM-H1 and a copolymer of St, MMA, and BA. Next, 28% aqueous ammonia was added in the same manner as in Production Example 1. The solution thickened, the polymer precipitated, and a cloudy liquid (containing binder resin BB-H1) was formed; therefore, an emulsion could not be obtained. Furthermore, the solid content could not be measured.

[0076] (Production Examples 2 to 5, Comparative Production Examples 2 and 3) Liquids containing binder resins BB-2 to BB-5 and BB-H2 to BB-H3 were obtained in the same manner as in Production Example 1, except that solutions of macromonomers MM-2 to MM-5 and MM-H2 to MM-H3 were used instead of the solution of macromonomer MM-1. The properties of the obtained binder resins and the evaluation results of "emulsification" are shown in Table 4. The evaluation criteria for "emulsification" are as follows: ○: Self-emulsification, no precipitation observed. Δ: Emulsification occurred, but the particle size of the emulsion particles formed was large and some precipitates were formed. ×: Self-emulsification was not achieved and a large amount of precipitate was formed.

[0077] TIFF2025168807000008.tif95170

[0078] (Production Examples 6 to 13, Comparative Production Examples 4 to 11) Liquids containing binder resins BB-6 to 13 and BB-H4 to 11 were obtained in the same manner as in Production Example 1 described above, except that the polymerization compositions (units: parts) shown in Tables 5 and 6 were used. The properties of the obtained binder resins and the evaluation results of "emulsification" are shown in Tables 5 and 6. The meanings of the abbreviations in Tables 5 and 6 are also shown below. In Comparative Production Example 6, significant heat was generated during polymerization, causing the polymerization system to thicken, so the polymerization was abandoned midway. It is presumed that the absence of styrene caused the polymerization rate to increase excessively, resulting in heat generation. MM: Macromonomer NV: solids CHMA: Cyclohexyl methacrylate 2EHA: 2-Ethylhexyl acrylate ·LA: Lauryl acrylate

[0079] TIFF2025168807000009.tif191170

[0080] TIFF2025168807000010.tif173170

[0081] <Production of water-based OP varnish> Example 1 270 parts of a liquid containing binder resin BB-1, 22.5 parts of water, 6 parts of a polyethylene wax dispersion (trade name "Chemipearl W500" manufactured by Mitsui Chemicals, Inc.), 0.6 parts of a defoamer (trade name "Tegoformex 805N" manufactured by Evonik), 0.3 parts of a surfactant (trade name "Tegowet 500" manufactured by Evonik), and 0.6 parts of a thickener (trade name "SN Thickener 623N" manufactured by San Nopco) were mixed and thoroughly stirred using a disperser to obtain a mixture. 100 parts of the resulting mixture was mixed with 5 parts of crosslinker A (aqueous epoxy crosslinker, trade name "Denacol EX-612" manufactured by Nagase Chemtex Co., Ltd.), and then diluted with tap water to a viscosity of 15 seconds at 25 °C as measured using a Zahn Cup #4 (manufactured by Rigo Co., Ltd.). This gave aqueous OP varnish SO-1.

[0082] (Examples 2 to 16, Comparative Examples 1 to 6) Aqueous OP varnishes SO-2 to SO-16 and SO-H1 to SO-H6 were obtained in the same manner as in Example 1, except that the binder resins and crosslinking agents shown in Table 7 were used. Details of crosslinking agents B and C in Table 7 are shown below. Crosslinker B: Aqueous aziridine crosslinker, trade name "ChemiTite PZ-33", manufactured by Nippon Shokubai Co., Ltd. Crosslinking agent C: Water-based isocyanate crosslinking agent, product name "Duranate WB40-100", manufactured by Asahi Kasei Corporation

[0083] <Shrink label manufacturing> A heat-shrinkable plastic film (trade name "HST", manufactured by Gunze Co., Ltd.) laminated with a polystyrene film and a polyethylene terephthalate film and having its surface corona-treated was prepared as the printing substrate (substrate film). A water-based color ink (trade name "Hydrick FCF Series", manufactured by Dainichiseika Color & Chemicals Mfg. Co., Ltd.) was diluted with tap water to a viscosity of 14 seconds at 25°C, measured using a Zahn Cup #4 (manufactured by Rigo Co., Ltd.), to prepare a color ink for coating. An anilox roll (cell volume 4.5 cm) was used. 3 / m 2 The prepared color inks for coating were applied onto the substrate film using a flexographic hand proofer equipped with a flexographic hand proofer as an applicator to form a color ink layer.

[0084] Next, an anilox roll (cell volume 8.5 cm 3 / m 2 A flexographic hand proofer equipped with a flexographic printer was used as an applicator to apply a water-based OP varnish onto the color ink layer that had been formed. The varnish was then dried at 25°C for 48 hours to form a coating film, yielding a test coated product (shrink label).

[0085] <Evaluation> (adhesion) After thoroughly drying the shrink label using a hair dryer, cellophane tape (trade name "Cellotape (registered trademark)" manufactured by Nichiban Co., Ltd.) was firmly pressed against the solid portion of the coating film and then peeled off. The degree of peeling of the coating film was then visually observed, and the adhesion of the coating film was evaluated according to the evaluation criteria shown below. The results are shown in Table 7. In the evaluation criteria shown below, "◎" and "○" were considered to be usable levels (passed). ⊚: The coating did not peel off at all. ○: The coating film peeled off slightly. Δ: The area of ​​the peeled coating film was smaller than the area of ​​the coating film that remained unpeeled. ×: The area of ​​the peeled coating film was larger than the area of ​​the coating film that remained unpeeled.

[0086] (blocking resistance) Untreated base film and shrink label are laminated together at 7kg / cm 2 The shrink label was left in a thermostatic chamber at 40°C for 24 hours with a load of 0.01 mm. The peel resistance when peeling the untreated film from the shrink label and the appearance of the coating film on the shrink label were then checked, and the blocking resistance of the coating film was evaluated according to the following evaluation criteria. The results are shown in Table 7. Of the evaluation criteria shown below, "◎" and "○" were considered usable (passed). ⊚: There was no transfer of the coating film to the untreated film, and there was no peeling resistance. ◯: There was almost no transfer of the coating film to the untreated film, and only slight peel resistance was felt. Δ: Some transfer of the coating to the untreated film was observed, and some resistance to peeling was felt. Δ×: Transfer of the coating film to the untreated film was observed, and peeling resistance was felt. ×: Violent transfer of the coating film to the untreated film was observed, and strong peel resistance was felt.

[0087] (Wet friction resistance) Using a Gakushin-type rub fastness tester (product name "RT-300", manufactured by Daiei Scientific Co., Ltd.), the coating surface of the shrink label was rubbed back and forth 100 times with a damp white cloth at a load of 200 g. After 100 strokes, the condition of the coating was visually observed, and the wet rub resistance of the coating was evaluated according to the following evaluation criteria. The results are shown in Table 7. In the evaluation criteria shown below, "◎" and "○" were considered usable levels (passed). ⊚: The coating did not peel off at all. ○: The coating film peeled off slightly. Δ: The area of ​​the peeled coating film was smaller than the area of ​​the coating film that remained unpeeled. ×: The area of ​​the peeled coating film was larger than the area of ​​the coating film that remained unpeeled.

[0088] (shrinkage, whitening, cracking) Test coated products (shrink labels) were produced in the same manner as in the "Production of Shrink Labels" section above, except that no color ink layer was formed. The produced shrink labels were immersed in 85°C hot water for 5 seconds to shrink them by 50%, yielding shrink films. The haze value of the shrink films (the ratio (%) of diffuse light transmittance to total light transmittance) was measured using an integrating sphere haze meter. The coating film was also checked for cracks. The coating film was then evaluated for shrinkage whitening and cracks according to the following evaluation criteria. Of the evaluation criteria shown below, "◎" and "〇" were considered usable levels (passed). The results are shown in Table 7. ⊚: The haze value was less than 30% and no cracks were observed. Good: The haze value was 30% or more and less than 40%, and no cracks were observed. △: The haze value was 30% or more and less than 40%, but cracks were observed. ×: The haze value was 40% or more, and cracks were also observed.

[0089] TIFF2025168807000011.tif170170 [Industrial Applicability]

[0090] The aqueous OP varnish of the present invention is useful as an OP varnish for shrink labels and is suitable as a material for printing on shrink labels for drinking water and food. The binder resin used in the aqueous OP varnish of the present invention is also useful as a coating component for inks for shrink labels.

Claims

1. An aqueous overprint varnish for a shrink label, comprising a binder resin, water, and a water-soluble organic solvent, the binder resin is a polymer having a number average molecular weight of 15,000 to 50,000, which comprises 20 to 40% by mass of a structural unit (i) having a carboxy group represented by the following general formula (1), 10 to 40% by mass of a structural unit (ii) derived from styrene, 10 to 40% by mass of a structural unit (iii) derived from a first alkyl methacrylate having a homopolymer glass transition temperature of 80°C or higher, and 10 to 40% by mass of a structural unit (iv) derived from an alkyl acrylate having a homopolymer glass transition temperature of 0°C or lower; The aqueous overprint varnish is a binder resin in which at least a portion of the carboxyl groups in the structural unit (i) are neutralized with at least one alkaline substance selected from the group consisting of ammonia, dimethylaminoethanol, and aminomethylpropanol, thereby self-emulsifying and forming emulsion particles having a number average particle size of 50 to 200 nm. (In the general formula (1), R １ represents a hydrogen atom or an alkyl group having 1 to 3 carbon atoms, "Polymer" represents a polymer moiety having a number average molecular weight of 3,000 to 10,000, which comprises 40 to 60 mass% of structural units (a) derived from methyl methacrylate, 10 to 30 mass% of structural units (b) derived from a second alkyl methacrylate having an alkyl group having 8 to 18 carbon atoms, 0.1 to 20 mass% of structural units (c) derived from a hydroxyalkyl methacrylate having a hydroxyalkyl group having 2 to 4 carbon atoms, and 10 to 30 mass% of structural units (d) derived from methacrylic acid, and p represents any number of repeating units.

2. the first alkyl methacrylate is at least one selected from the group consisting of methyl methacrylate and isobornyl methacrylate, the alkyl acrylate is at least one selected from the group consisting of butyl acrylate, 2-ethylhexyl acrylate, and lauryl acrylate; the second alkyl methacrylate is at least one selected from the group consisting of dodecyl methacrylate and tridecyl acrylate; 2. The aqueous overprint varnish according to claim 1, wherein the hydroxyalkyl methacrylate is 2-hydroxyethyl methacrylate.

3. 3. The aqueous overprint varnish according to claim 1, further comprising at least one aqueous crosslinking agent selected from the group consisting of an epoxy-based crosslinking agent, an isocyanate-based crosslinking agent, an oxazoline-based crosslinking agent, an aziridine-based crosslinking agent, and a carbodiimide-based crosslinking agent.

4. Further containing a wax component, 3. The aqueous overprint varnish according to claim 1, wherein the wax component is emulsion particles formed by neutralizing the carboxyl groups of at least one polymer having a carboxyl group selected from the group consisting of polyethylene, polypropylene, poly-α-olefin, silicone, and copolymers thereof with an alkaline substance, followed by self-emulsification.

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