Paint for cans and cans made using the same

Abstract

This invention provides a paint for cans that suppresses flow marks that occur during painting with a roll coater, and can obtain a smooth, excellent coating appearance. [Solution] A paint for cans comprising polymer particles (A), water, and an organic solvent (B), wherein the volume average particle diameter D of the polymer particles (A) 50 A paint for cans having a particle size of 0.40 μm or more and 0.70 μm or less, a shear viscosity of 0.50 Pa·s or more and 2.30 Pa·s or less at a shear rate of 0.1 / s, and a shear viscosity of 0.30 Pa·s or more and 1.00 Pa·s or less at a shear rate of 1.0 / s.

Description

[Technical Field]

[0001] This invention relates to paint for cans and cans made using the same. [Background technology]

[0002] Methods for applying paint to cans include roll coating, coil coating, and spray coating, and the choice of method depends on the application and container type. Among these, roll coating is the most commonly used and cost-effective method for obtaining flat coated objects because its structure and operation are relatively simple, and it allows for coating one sheet at a time.

[0003] On the other hand, paints for cans have been moving towards water-based systems for a long time, from the standpoint of resource conservation, energy saving, and environmental pollution. Water-based paints for cans are widely used, consisting of water, particulate resin that can be dispersed in water, organic solvents, lubricants such as waxes, and other additives; in other words, water-dispersible paints.

[0004] Roll coating is commonly used in organic solvent-based paints and water-based paints using water-soluble resins, but is not widely used in water-dispersible paints. Water-dispersible paints are frequently used in coil coating and spray coating.

[0005] Patent Document 1 discloses a method for producing a multi-layer coating film that has a smooth appearance even when wet-on-wet coating is performed, by using an aqueous coating composition as the undercoat coating composition and specifying the shear viscosity of the undercoat coating under a high shear rate. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2024-92625 [Overview of the project] [Problems that the invention aims to solve]

[0007] However, because these water-based paints are aqueous dispersions, their viscosity possesses a unique non-Newtonian fluidity. Therefore, when applying paint using a roll coater, where the shear rate from the coating equipment is considered slow, it is difficult to obtain a smooth painted surface. In light of these circumstances, the present invention aims to provide a paint for cans that can suppress flow marks that occur during painting with a roll coater and obtain a smooth, excellent coating appearance. [Means for solving the problem]

[0008] [1]: A paint for cans comprising polymer particles (A), water and an organic solvent (B), Volume average particle diameter D of the polymer particles (A) 50 The size is between 0.40 μm and 0.70 μm. The shear viscosity at a shear rate of 0.1 / s is between 0.50 Pa·s and 2.30 Pa·s, and the shear viscosity at a shear rate of 1.0 / s is between 0.30 Pa·s and 1.00 Pa·s. Paint for cans. [2]: A paint for cans according to [1], containing the organic solvent (B) in a proportion of 10% by mass or more and 30% by mass or less. [3]: The paint for cans of [1], wherein the organic solvent (B) is composed of at least two types of organic solvents. [4]: The paint for cans of [1] wherein at least one of the organic solvents (B) is a glycol solvent. [5]: The paint for cans according to [1], wherein the polymer forming the polymer particles (A) is an acrylic-modified epoxy resin. [6]: A can made by coating metal with any of the can paints listed in [1] to [5]. [Effects of the Invention]

[0009] The present invention makes it possible to provide a paint for cans that can suppress flow marks that occur during coating with a roll coater, and obtain a smooth, excellent coating film appearance. [Modes for carrying out the invention]

[0010] The paint for cans of the present invention comprises polymer particles (A) and water or an organic solvent (B) that disperses them, and additional additives such as curing agents, surfactants, defoamers, and waxes may be added as needed. The present invention is characterized in that the average particle size of the polymer particles (A) and the shear viscosity at shear rates of 0.1 / s and 1.0 / s are within a specific range. Therefore, when the paint for cans is applied with a roll coater, it exhibits excellent paintability, and a smooth coating appearance with inconspicuous flow marks can be obtained.

[0011] The components constituting the paint for cans of the present invention are described in detail below. The polymers constituting the polymer particles (A) can include epoxy resins, acrylic resins, polyester resins, urethane resins, and acrylic-modified epoxy resins, which will be described later. Among these, epoxy resins are preferred, and specifically, acrylic-modified epoxy resins modified with acrylic resin or acrylic monomers are preferably used to improve affinity for water. In this invention, "acrylic monomer" is synonymous with "ethylenically unsaturated monomer".

[0012] (Epoxy resin) The epoxy resin should be of the bisphenol, novolac, naphthalene, or biphenyl type. Among these, bisphenol A type epoxy resin is more preferred when considering the film hardness, retort resistance, processability, and adhesion required for can coatings. The epoxy resin should preferably have a weight-average molecular weight of 2,500 to 70,000. If the weight-average molecular weight is less than 2,500, the amount of unreacted substances such as bisphenol A will be large, and the physical properties such as the durability of the coating film may not be sufficiently obtained. On the other hand, if the weight-average molecular weight exceeds 70,000, sufficient adhesion between the coating film and the substrate metal may not be obtained. Examples of commercially available bisphenol A type epoxy resins include JER1007, JER1009, and JER1010 from Mitsubishi Chemical Corporation; Epotote YD-017, YD-019, YD-020G, and YD-7910 from Nippon Steel Chemical & Material Co., Ltd.; NPES-607, NPES-619A, NPES-629, NPES-639, NPES-907, and NPES-909 from Nanya Plastics Industry Co., Ltd.; EPOTEC YD-017, YD-019, YD-907, and YD-909 from Aditya Birla; and YD-017 and YD-019 from Guoto Chemical Co., Ltd.

[0013] (Acrylic-modified epoxy resin) Since epoxy resin cannot dissolve or disperse in water on its own, a stable water-based paint can be obtained by modifying it with an acrylic resin or acrylic monomer containing a carboxyl group. Acrylic-modified epoxy resin is a composite resin obtained using an ethylenically unsaturated monomer and an epoxy resin, and can be obtained, for example, by the following method. An epoxy resin is dissolved in an organic solvent, and an ethylenically unsaturated monomer is polymerized in the solution using a radical polymerization initiator. Simultaneously with the polymerization reaction, grafting of the monomer or its polymer into the epoxy resin occurs, yielding an acrylic-modified epoxy resin in which the acrylic resin is grafted onto the epoxy resin. Next, if necessary, a basic compound or the like, which is a reaction catalyst, may be added to carry out an esterification reaction between the carboxy group in the produced acrylic resin part and the epoxy group in the epoxy resin. Alternatively, an acrylic resin having a carboxyl group may be obtained by copolymerizing an ethylenically unsaturated monomer in advance, and an acrylic-modified epoxy resin may also be obtained by subjecting a part of the carboxyl group and a part of the epoxy group in the epoxy resin to an esterification reaction in the presence of a basic compound.

[0014] (Ethylenically unsaturated monomer) In the present invention, it is preferable to use a carboxyl group-containing monomer and another ethylenically unsaturated monomer as the ethylenically unsaturated monomer. Examples of the carboxyl group-containing monomer include (meth)acrylic acid [“acrylic acid” and “methacrylic acid” are collectively referred to as “(meth)acrylic acid”. The same shall apply hereinafter.], maleic acid, maleic anhydride, itaconic acid, fumaric acid, and the like. Examples of the other ethylenically unsaturated monomers include alkyl (meth)acrylates such as methyl (meth)acrylate, ethyl (meth)acrylate, isopropyl (meth)acrylate, n-propyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, t-butyl (meth)acrylate, hexyl (meth)acrylate, 2-ethylhexyl (meth)acrylate; hydroxyalkyl (meth)acrylates such as hydroxymethyl (meth)acrylate, 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, hydroxypentyl (meth)acrylate, hydroxyhexyl (meth)acrylate; aromatic monomers such as styrene and methylstyrene; N-hydroxyalkyl (meth)acrylamides such as N-hydroxymethyl (meth)acrylamide, N-hydroxyethyl (meth)acrylamide, N-hydroxybutyl (meth)acrylamide; N-alkoxyalkyl (meth)acrylamides such as N-methoxymethyl (meth)acrylamide, N-ethoxymethyl (meth)acrylamide, N-(n-, iso)butoxymethyl (meth)acrylamide, N-methoxyethyl (meth)acrylamide, N-ethoxyethyl (meth)acrylamide, N-(n-, iso)butoxyethyl (meth)acrylamide; and (meth)acrylamide and the like can be mentioned.

[0015] Considering the solution stability or dispersion stability of the acrylic-modified epoxy resin, the coating suitability of the paint, the workability when forming a coating film, and the adhesion to metals, the ethylenically unsaturated monomer is preferably blended so that the carboxyl group concentration is 2.7 to 7.1 mmol / g. When the carboxyl group concentration is less than 2.7 mmol / g, the water dispersibility of the polymer particles may decrease and the storage stability as an aqueous paint may be impaired. On the other hand, when the carboxyl group concentration is greater than 7.1 mmol / g, it may be difficult to obtain workability, metal adhesion, and durability when forming a coating film. The carboxyl group concentration in the present invention is a theoretical value obtained from the amount of carboxyl group-containing monomer contained in the blending composition of the ethylenically unsaturated monomer, and refers to the number of moles of carboxyl groups present per unit mass of the monomer composition.

[0016] The ethylenically unsaturated monomer is preferably used so that the mass ratio of the ethylenically unsaturated monomer / epoxy resin is 10 / 90 to 50 / 50 in terms of the mass ratio to the epoxy resin. When the mass ratio of the ethylenically unsaturated monomer is less than 10, the stability of the solution or dispersion may decrease or the coating property may be inferior. Also, when the mass ratio of the ethylenically unsaturated monomer is greater than 50, the workability, adhesion to the substrate, and durability when forming a coating film may be inferior. As the radical polymerization initiator used when polymerizing the ethylenically unsaturated monomer, for example, organic peroxides, persulfates, azobis compounds, etc. can be used. Also, a redox system combining an oxidizing agent and a reducing agent can be adopted. In the present invention, an organic peroxide-based initiator is preferred, and particularly benzoyl peroxide is preferred.

[0017] The radical polymerization initiator is preferably used in an amount of 1 to 10 parts by mass, and more preferably 1 to 6 parts by mass, per 100 parts by mass of total ethylenically unsaturated monomers. The reaction conditions, such as temperature and time during polymerization, are not special and known conditions can be used. It is important to adjust the reaction conditions appropriately to obtain an acrylic-modified epoxy resin with optimal viscosity characteristics when used as an aqueous paint. Furthermore, polymerization of ethylenically unsaturated monomers is preferably carried out in an organic solvent.

[0018] Furthermore, in the above method, examples of basic compounds used as catalysts during the esterification reaction include alcoholamines such as dimethylethanolamine (also known as dimethylaminoethanol), ethanolamine, diethanolamine, and aminomethylpropanol, as well as alkylamines such as trimethylamine, triethylamine, and butylamine, morpholine, and ammonia.

[0019] The basic compound is preferably used in an amount of 5 to 60 mol%, and more preferably 30 to 50 mol%, relative to 100 mol% of carboxyl groups in the resin. Furthermore, the reaction conditions such as temperature and time during the esterification reaction are not special and can be carried out using known conditions. However, it is important to adjust the reaction conditions appropriately to obtain an acrylic-modified epoxy resin with optimal fluidity of the resulting aqueous paint.

[0020] The obtained acrylic-modified epoxy resin can be obtained as an aqueous dispersion using the same methods as conventional methods. Specifically, this involves neutralizing the carboxyl groups present in the acrylic-modified epoxy resin with a basic compound to impart hydrophilicity. More specifically, examples include adding a basic compound to the acrylic-modified epoxy resin and then adding an aqueous medium such as water to form an aqueous dispersion, or adding an aqueous medium such as water containing a basic compound to the acrylic-modified epoxy resin to form an aqueous dispersion. As the basic compound used for neutralization, known compounds such as the amines mentioned above can be used. In this way, polymer particles (A) having acrylic-modified epoxy resin as a component can be obtained.

[0021] (Organic solvent (B)) The can coating of the present invention preferably contains 10 to 30% by mass of an organic solvent, and more preferably consists of at least two or more types of organic solvents, for the purpose of improving the paintability of the obtained coating, including the solubility of the epoxy resin during manufacturing, and ensuring the storage stability of the coating. Regarding organic solvent (B), the present invention preferably consists of two types of organic solvents, namely organic solvent b-1 and organic solvent b-2. Organic solvent b-1 is preferably glycol monoethers that can dissolve epoxy resin and are also soluble in water, possessing both hydrophobic and hydrophilic properties. Specifically, examples include ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, ethylene glycol monohexyl ether, diethylene glycol monoethyl ether, diethylene glycol monobutyl ether, methyl propylene glycol, methyl propylene diglycol, propyl propylene glycol, propyl propylene diglycol, butyl propylene glycol, and dipropylene glycol monomethyl ether.

[0022] As the organic solvent b-2, highly hydrophilic solvents such as those listed below are preferred. Specifically, alcohols such as methanol, ethanol, n-propyl alcohol, iso-propyl alcohol, n-butyl alcohol, iso-butyl alcohol, sec-butyl alcohol, tert-butyl alcohol, n-amyl alcohol, sec-amyl alcohol, n-octanol, 2-ethyl-1-hexanol, and tridecanol; Examples of glycols include ethylene glycol, propylene glycol, diethylene glycol, 1,2-butylene glycol, 1,3-butylene glycol, and 1,4-butylene glycol. In particular, coatings formed using glycol solvents improve the opening properties of coatings that cover metal can lids, for example. In the present invention, from the viewpoint of the solubility of epoxy resin, the concentration of reaction solids during acrylic modification, the storage stability of aqueous paints, and the solvent evaporation rate during painting, it is desirable that the can paint of the present invention contains 5 to 20% by mass of organic solvent b-1, more preferably 7 to 15% by mass, and 1 to 15% by mass of organic solvent b-2, more preferably 5 to 10% by mass.

[0023] (Hardening agent) The can coating of the present invention may further contain one or more curing agents, such as phenolic resins, to improve the toughness and metal adhesion of the coating film, as needed. In addition to self-crosslinking, phenolic resins can react with carboxyl groups in acrylic-modified epoxy resins. Furthermore, if the acrylic-modified epoxy resin contains hydroxyl groups, the phenolic resin can also react with those hydroxyl groups. Moreover, if the ethylenically unsaturated monomer includes an amide monomer, and the acrylic-modified epoxy resin has crosslinkable functional groups derived from this amide monomer, the phenolic resin can also react with these crosslinkable functional groups. Examples of phenolic resins used in the present invention include Sumilite Resin PR-55317 and PR-55819 from Sumitomo Bakelite Co., Ltd., Phenodur PR-371, PR-411, PR-516, PR-521, PR-566, PR-612, and VPR-1785 from Allnex, and Curaphen 40-424 B50, 40-720 B72, 40-809 B60, and 40-872 B80 from Bitrez. When using a curing agent, it is preferable to use 0.5 to 20 parts by mass, and more preferably 1 to 10 parts by mass, per 100 parts by mass of the main component, such as acrylic-modified epoxy resin.

[0024] (Other ingredients) The paint for cans in this invention may optionally contain surfactants, defoamers, waxes, etc.

[0025] (viscosity) The paint for cans of the present invention must be pre-adjusted to a viscosity suitable for application with a roll coater. The viscosity is preferably 40 seconds to 120 seconds, more preferably 50 seconds to 100 seconds, and even more preferably 50 seconds to 80 seconds, at No. 4 Ford Cup and 25°C. If the viscosity is less than 40 seconds, the paint is more likely to drip from between the rolls of the roll coater, making proper coating impossible. On the other hand, if the viscosity exceeds 120 seconds, streaks are more likely to appear on the rolls, making it difficult to obtain a smooth and clean painted appearance. (Non-volatile content concentration) The non-volatile content concentration is preferably 20-45% by mass, and more preferably 25-40% by mass. If the non-volatile content concentration is too low, it becomes difficult to secure the required amount of paint film with a roll coater, and if the non-volatile content concentration is too high, the viscosity of the paint increases, making it difficult to obtain a smooth and beautiful painted appearance.

[0026] (Volume average particle diameter D) 50 ) Volume-average particle diameter D in the present invention 50 This can be measured by conventionally known methods such as laser diffraction. Specifically, it is measured using a Microtrac MT3000II manufactured by Nikkiso Co., Ltd., and D 50 The value obtained is the particle size corresponding to the volume at which the cumulative frequency is 50% in the particle size distribution. Water was added to the above apparatus, and water-based paint was added so that the TR was 0.95 to 0.8, and the measurement was performed. The measurement conditions were: particle conditions: permeability: permeable, refractive index 1.44, shape: non-spherical; solvent conditions: refractive index 1.333. The measurement time was 20 seconds, and the number of cumulative measurements was 2. Volume average particle diameter D of polymer particles (A) 50To adjust the particle size, one example is to adjust the structure of the acrylic-modified epoxy resin that constitutes it. For example, to reduce the average particle size, one can (1) increase the molecular weight of the acrylic resin portion, (2) increase the mass ratio of the acrylic resin portion, (3) use an epoxy resin with a low epoxy equivalent, or (4) increase the reaction rate in the esterification reaction between the carboxyl group and the epoxy group. Conversely, to increase the average particle size, one can adjust it in the opposite direction of (1) to (4) above. (Shear viscosity) In this invention, viscosity at specific shear rates was measured under 25°C conditions using an Anton Paar Physica MCR301 rheometer with a 50 mm diameter and a 0.5 ° cone rotor. The viscosity was measured after holding for 60 seconds at a shear rate of 0.1 / s and after holding for 20 seconds at a shear rate of 1.0 / s. Water-based paints, which disperse polymer particles (A) in water, tend to be thixotropic and are less likely to become neutonian liquids. In particular, the volume-average particle diameter D of polymer particles (A) 50 When the volume average particle size D of polymer particles (A) is less than 0.40 μm, for example, the shear viscosity at a shear rate of 0.1 / s tends to exceed 2.30 Pa·s, making flow marks more noticeable after coating with a roll coater. 50 When the particle size exceeds 0.70 μm, for example, the shear viscosity at a shear rate of 0.1 / s tends to fall below 0.50 Pa·s, making the flow marks after coating by a roll coater less noticeable. However, on the other hand, the dispersed polymer particles tend to settle more easily, reducing the storage stability of the paint and making it more likely for defects such as bumps to appear on the surface of the formed coating film to occur. If the shear viscosity at a shear rate of 0.1 / s is between 0.50 Pa·s and 2.30 Pa·s, and the shear viscosity at a shear rate of 1.0 / s is between 0.30 Pa·s and 1.00 Pa·s, it becomes easier to obtain a coating film with less noticeable flow marks in roll coating.

[0027] The can coating in the present invention can be applied to various metal substrates, and a metal packaging material or component thereof having a coating layer formed from the coating on a metal plate can be obtained. Examples of substrates include various untreated or surface-treated metal plates such as aluminum plates, tin-free plates, and tinplate plates, metal plates coated with a primer, or PET film-coated metal plates obtained by laminating a polyester film (also called PET film) onto these metal plates. The metal packaging material in the present invention is preferably used for metal cans that contain beverages, food, etc. Types of cans include DI cans (cans manufactured by the Drawing & Ironing method), DR cans (cans manufactured by the Drawing & Redrawing method), various 3-piece cans, and film-laminated cans. It can also be applied to can lids such as the top lid and bottom lid of these cans. The can coating of the present invention may be applied to these substrates, baked and cured, and then further deformed. Through various processing steps, a packaging material can be obtained.

[0028] (Painting method) Examples of painting methods include roll coating, coil coating, spray coating, curtain coating, spin coating, die coating, and dip coating, and are not particularly limited; however, the paint for cans of the present invention is particularly preferred for roll coating.

[0029] (Baking conditions) When the paint for cans of the present invention is applied and baked, a dry film is formed, and if the paint contains a hardening agent, a cured coating film is obtained. For baking, electric ovens, infrared ovens, gas ovens, etc., can be used as appropriate. The baking temperature is preferably 170-220°C, and more preferably 180-200°C. The baking time is preferably 5-15 minutes, and more preferably 8-12 minutes. If the baking temperature exceeds 220°C or the baking time is longer than 15 minutes, the coating film will easily become yellowish and discolored, which is undesirable for the appearance of the can. Also, if the baking temperature is lower than 170°C or the baking time is shorter than 5 minutes, the drying and curing properties of the paint will be insufficient, and the performance of the coating film, such as retort resistance and hardness, will decrease. [Examples]

[0030] Examples and comparative examples of the present invention are described below. Unless otherwise specified, "parts" means "parts by mass" and "%" means "mass%" in terms of the units of blending. Unless otherwise specified, the blending amounts of the raw materials (excluding solvents) listed in the Examples section and the Table are calculated on a non-volatile content basis.

[0031] [Manufacturing Example 1] <Manufacturing of acrylic resin solution (A)> A reaction vessel equipped with a stirrer, thermometer, reflux condenser, dropping tank, and nitrogen gas inlet tube contains eth 100 parts of lenglycol monobutyl ether were charged and heated to 125°C to dissolve. While maintaining the temperature in the reaction vessel at 125°C, a mixture consisting of 40 parts methacrylic acid, 25 parts styrene, 25 parts methyl methacrylate, 10 parts ethyl acrylate, and 3.4 parts dibenzoyl peroxide was continuously added dropwise from a dropping vessel over a period of 2 hours.

[0032] One hour and two hours after the end of the dropwise addition, 0.3 parts of dibenzoyl peroxide were added, and the reaction was continued for 3 hours from the end of the dropwise addition. Thereafter, 100 parts of n-butyl alcohol were added to obtain a product with a non-volatile content of 35%. This was designated as acrylic resin solution (A).

[0033] [Example 1] <Paint manufacturing> (1) 62 parts of ethylene glycol monobutyl ether (2) n-butyl alcohol 11 parts (3) JER1009 (Bisphenol A type epoxy resin, manufactured by Mitsubishi Chemical Corporation) 210 copies (4) Methacrylic acid 28 parts (5) Ethyl acrylate 7 parts (6) Styrene 35 parts (7) Dibenzoyl peroxide 5.4 parts (8) 42 parts of ethylene glycol monobutyl ether (9) n-butyl alcohol 22.1 parts (10) n-butyl alcohol 32 parts (11) Dimethylaminoethanol 11.9 parts (12) Ion-exchanged water 414 parts (13) Phenolic resin 14 parts

[0034] (1) to (3) were placed in a four-necked flask, and the epoxy resin was dissolved at 120°C to 130°C. (4) to (8) were mixed in a separate container beforehand and placed in a dropping funnel. After confirming that the epoxy resin had dissolved, the dropping was started from the dropping funnel and the entire amount was added dropwise over 1 hour. After the dropping was complete, (9) was added, and the reaction was continued for another 2 hours. Once the reaction was complete, the heater was turned off, (10) was added, and after cooling to 90°C, (11) was added and stirred for 10 minutes. (12) was placed in a dropping funnel and added dropwise over 90 minutes. After the dropwise addition was complete, Bitrez Curaphen 40-809 B60 was added as (13) while stirring. Ford Cup #4 was adjusted with deionized water to achieve a viscosity in the range of 60 to 80 seconds at 25°C, resulting in a water-based paint with a non-volatile content of 31.5%.

[0035] [Example 2] In Example 1 described above, a water-based paint was obtained in the same manner as in Example 1, except that n-butyl alcohol in (2), (9), and (10) was replaced with ethylene glycol. [Example 3] A water-based paint was obtained in the same manner as in Example 1, except that n-butyl alcohol in (2), (9), and (10) was replaced with propylene glycol, and (5) was increased to 24.5 parts and (6) to 17.5 parts. [Example 4] In Example 1 described above, a water-based paint was obtained in the same manner as in Example 1, except that n-butyl alcohol in (2), (9), and (10) was replaced with diethylene glycol. [Example 5] A water-based paint was obtained in the same manner as in Example 1, except that n-butyl alcohol in (2), (9), and (10) was replaced with 1,4-butylene glycol. [Example 6] A water-based paint was obtained in the same manner as in Example 1, except that n-butyl alcohol in (2), (9), and (10) was replaced with 1,2-butylene glycol, and (4) was used in 35 parts and (6) in 28 parts. [Example 7] <Paint manufacturing> (1) 62 parts of ethylene glycol monobutyl ether (2) n-butyl alcohol 11 parts (3) JER1009 (manufactured by Mitsubishi Chemical Corporation) 210 copies (4) Methacrylic acid 28 parts (5) Ethyl acrylate 7 parts (6) Styrene 35 parts (7) Dibenzoyl peroxide 5.4 parts (8) 42 parts of ethylene glycol monobutyl ether (9) n-butyl alcohol 22.1 parts (10) n-butyl alcohol 32 parts (11) Dimethylaminoethanol 0.58 parts (12) Deionized water 0.58 parts (13) Dimethylaminoethanol 11.3 parts (14) Ion-exchanged water 414 parts (15) Phenolic resin 14 parts

[0036] (1) to (3) were placed in a four-necked flask, and the epoxy resin was dissolved at 120°C to 130°C. (4) to (8) were mixed in a separate container beforehand and placed in a dropping funnel. After confirming that the epoxy resin had dissolved, the dropping was started from the dropping funnel and the entire amount was added over 1 hour. After the dropping was complete, (9) was added and the reaction was continued for another 2 hours. After the reaction was complete, (10) was added and cooled, and the mixture of (11) and (12) that had been mixed in a separate container beforehand was added, and the reaction was carried out at 80°C for 1.5 hours. After the reaction was complete, the heater was turned off, and (13) and (14) that had been mixed in a separate container beforehand were placed in a dropping funnel and added over 90 minutes. After the dropping was complete, Bitrez Curaphen 40-809 B60 was added as (15) while stirring. Ford Cup #4 was adjusted with deionized water to achieve a viscosity in the range of 60 to 80 seconds at 25°C, resulting in a water-based paint with a non-volatile content of 31.0%. [Example 8] Except for changing n-butyl alcohol to propylene glycol in (2), (9), and (10) of Example 7, the preparation was carried out in the same manner as in Example 7, and as a result, an aqueous paint with a non-volatile content of 30.9% was obtained. [Example 9] Except for using 4.3 parts of dibenzoyl peroxide in (7) of Example 7, the preparation was carried out in the same manner as in Example 7, and as a result, an aqueous paint with a non-volatile content of 27.3% was obtained. [Example 10] In a reaction vessel similar to that of Example 1, 210 parts of JER1009 and 202 parts of the acrylic resin solution (A) obtained in Production Example 1 were charged, and the temperature was raised to 125°C to dissolve. After confirming dissolution, the mixture was cooled to 90°C, and then 0.5 parts of dimethylaminoethanol and 0.5 parts of deionized water were added in a mixture, and the esterification reaction was carried out at 90°C for 1.5 hours. After the reaction was complete, a mixture of 10.7 parts of dimethylaminoethanol and 414 parts of deionized water was added dropwise over 90 minutes. After the dropwise addition was complete, Curaphen 40-809 B60 manufactured by Bitrez was charged while stirring. Ford Cup #4 was adjusted with deionized water to achieve a viscosity in the range of 60 to 80 seconds at 25°C, resulting in a water-based paint with a non-volatile content of 30.7%.

[0037] [Comparative Example 1] In the above Example 7, an aqueous paint was obtained in the same manner as in Example 7, except that 12.0 parts of (11) was used and (13) was not used. [Comparative Example 2] In the above Example 7, an aqueous paint was obtained in the same manner as in Example 7, except that (2), (9), and (10) were changed from n-butyl alcohol to propylene glycol, 12.0 parts of (11) was used, and (13) was not used. [Comparative Example 3] In the above Example 7, an aqueous paint was obtained in the same manner as in Example 7, except that 3.8 parts of (7), 1.5 parts each of (11) and (12), and 10.4 parts of (13) were used.

[0038] [Evaluation of Paint Properties] For the paints obtained in each Example and Comparative Example, the properties in the following items were evaluated. [Volume Average Particle Diameter D 50 Measured using Microtrac MT3000II manufactured by Nikkiso Co., Ltd., and read the value of D 50 (The particle diameter corresponding to the volume with a cumulative frequency of 50% in the particle size distribution). [Shear Viscosity] Measured using Physica MCR301 manufactured by Anton Parr, and read the values of shear viscosity (unit: Pa·s) at shear rates of 0.1 (1 / s) and 1.0 (1 / s). [Viscosity] Measured the viscosity at 25 °C using Ford Cup No. 4. (Unit: seconds) [Non-Volatile Content Concentration] Put about 1 g of the paint in an aluminum dish, precisely weighed to the fourth decimal place, and calculated by precisely weighing the mass after baking in an electric oven at 200 °C for 10 minutes. [Flowability] The paint was dripped onto an A4-sized aluminum plate, the entire surface of the aluminum plate was painted with a hand roller, and the state of the painted surface was visually evaluated after baking in an electric oven at 200 °C for 2 minutes. [Evaluation Criteria] ​○ (Good): A smooth painted surface has been achieved. × (Not practical): The painted surface is severely wavy.

[0039] [Evaluation of physical properties] The following preparations were made to evaluate the physical properties of each water-based paint obtained in the above examples and comparative examples. As a base material, 5182 aluminum sheet material (0.24 mm thick) for can lids was cut to an appropriate size, and a wire bar coater was used to apply a dry coating with a film thickness of 4 g / m². 2 ~5g / m 2 The coating was applied in this manner. After application, the coated board was dried and baked in a gas oven at 190°C for 10 minutes. Each of the obtained painted boards was cut to the required size in order to perform the physical property tests listed below. [Opening Test] A painted board was cut to a size of 50 mm x 50 mm to serve as a test panel. Using a press, the painted surface of the test panel was given a textured surface resembling the shape of a typical stay-on tab opening found on beverage cans, which was then used as an evaluation sample. This sample was immersed in water and retorted in a retort kettle at 121°C for 90 minutes. After that, an aluminum plate was peeled off from the unpainted side of the sample, following the shape of the opening, and the opening was magnified under a microscope and visually inspected. Poor opening properties tend to result in paint film remaining around the opening, and the width of the paint film overflowing into the opening becomes larger. Good opening properties mean that the paint film does not overflow into the opening at all, or if it does, the overflow is very small. Specifically, the width of the overflowing paint film was measured and evaluated according to the evaluation criteria below. 5: The maximum width of the overhanging paint film is less than 100 μm (very good) 4: The maximum width of the overflowing coating is 100 μm or more and less than 200 μm (good). 3: The maximum width of the overflowing paint film is 200 μm or more and less than 500 μm (practical). 2: The maximum width of the overflowing coating is 500 μm or more and less than 800 μm (practical depending on the conditions). 1: The maximum width of the overflowing paint film is 800 μm or more (not practical). [Coating film appearance (retort resistance)] The test specimens were immersed in tap water and subjected to retort treatment at 121°C for 90 minutes. After treatment, the test specimens were air-dried, and the condition of the coating, more specifically whether or not the coating had whitened, was visually evaluated.

[0040] The evaluation results of each paint obtained in the examples and comparative examples are shown in Tables 1 and 2. 。 However, Examples 1, 7, 9, and 10 are for reference only. Na The abbreviations used in the table are as follows: BC: Ethylene glycol monobutyl ether n-BuOH: n-butyl alcohol EG: Ethylene glycol PG: Propylene glycol DEG: Diethylene glycol 1,4-BD:1,4-Butylene glycol 1,2-BD:1,2-Butylene glycol

[0041] [Table 1]

[0042] [Table 2]

Claims

1. A paint for cans comprising polymer particles (A), water, and an organic solvent (B), The polymer forming the polymer particles (A) is an acrylic-modified epoxy resin. The aforementioned organic solvent (B) contains 1 to 15% by mass of a glycol-based organic solvent in the paint for cans. The volume-average particle diameter D of the polymer particles (A) ５０ The particle size is between 0.40 μm and 0.70 μm. The shear viscosity at 25°C and a shear rate of 0.1 / s is 0.50 Pa·s or more and 2.30 Pa·s or less, and the shear viscosity at 25°C and a shear rate of 1.0 / s is 0.30 Pa·s or more and 1.00 Pa·s or less. Paint for cans.

2. The paint for cans according to claim 1, comprising the organic solvent (B) in a proportion of 10% by mass or more and 30% by mass or less.

3. The paint for cans according to claim 1, wherein the organic solvent (B) is composed of at least two types of organic solvents.

4. A can in which metal is coated using a paint for cans according to any one of claims 1 to 3.

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