Coating composition for laminated steel sheet

Abstract

This coating composition for a laminated steel sheet comprises: a (meth)acrylic-based resin (A) which contains a constituent unit derived from a monomer having a carboxy group, at a ratio of 6-18% by mass based on all the constituent units, and which has a glass transition temperature of 70-100°C; an epoxy resin (B) having an epoxy equivalent of 400-800 g / eq; and a curing agent (C) having a carboxyl group, and a functional group (c) capable of reacting with an epoxy group. The ratio of the mass content of the (meth)acrylic-based resin (A) to the mass content of the epoxy resin (B) is 76 / 24 to 93 / 7, and the ratio of the number of moles of the functional group (c) in the curing agent (C) to the total number of moles of the carboxy group in the (meth)acrylic-based resin (A) and the epoxy group in the epoxy resin (B) is 0.39-1.30.

Description

Coating composition for laminated steel sheets

[0001] This disclosure relates to a coating composition for laminated steel sheets.

[0002] High efficiency and miniaturization for motors are crucial, requiring motor core materials to have high magnetic permeability and low iron loss. Core processing presents challenges due to the degradation of core magnetic properties. Furthermore, low noise and vibration in motors are increasingly important from the perspective of protecting the living environment, and high-precision positioning control is required for motors. Generally, laminated steel sheets, formed by stacking electromagnetic steel sheets, are used for motor cores.

[0003] Traditionally, welding, riveting, and bolting have been used to fasten electrical steel sheets together during the manufacturing of laminated steel sheets. However, these fastening methods result in a deterioration of magnetic properties. Furthermore, laminated steel sheets manufactured using these fastening methods often lack sufficient mechanical rigidity, raising concerns about noise and vibration. Therefore, in recent years, a method has been adopted for manufacturing laminated steel sheets using electrical steel sheets with a pre-formed adhesive coating on their surface (also called "adhesive-coated electrical steel sheets"). In this method, first, a coating agent for forming an adhesive coating (also called "adhesive-coating agent") is applied to the surface of the electrical steel sheet, and then dried at high temperature to produce an adhesive-coated electrical steel sheet. After punching or shearing the prepared adhesive-coated electrical steel sheets, they are laminated and then fixed together by high temperature and pressure. Laminated steel sheets manufactured using adhesive-coated electromagnetic steel sheets tend to have less degradation of magnetic properties due to processing during fixing compared to laminated steel sheets manufactured by fixing electromagnetic steel sheets together by welding, and also have higher core rigidity, thus being less prone to noise and vibration problems (see Kaido Riki, Takeda Kazutoshi, Wakisaka Takeaki, and Mizogami Masato, "On the various properties of adhesive-coated electromagnetic steel sheet cores," Journal of the Institute of Electrical Engineers of Japan D, Vol. 119, No. 7, pp. 1010-1015, 1999).

[0004] Various reports have been made on electromagnetic steel sheets with adhesive coatings. For example, Japanese Unexamined Patent Publication No. Hei 7-308990 discloses that on at least one surface of an electromagnetic steel sheet, (A) an acrylic resin having a glass transition point of 80°C to 130°C and containing a functional group capable of reacting with an epoxy group in the molecule, and (B) an epoxy resin having a melting point or softening point of 70°C to 140°C as main components, wherein the mass ratio of resin solid content of component (A) to component (B) is 95 / 5 to 70 / 30, and the gelation rate (so-called degree of curing) of the mixed resin is 10% by mass to 90% by mass. Further, Japanese Unexamined Patent Publication No. Hei 2-208034 discloses that on the surface of an electromagnetic steel sheet, (A) a thermoplastic acrylic resin emulsion having a glass transition temperature of 60°C or higher, (B) an epoxy resin emulsion, (C) an amine-based epoxy resin curing agent that reacts with component (B) upon heating, and (D) a film-forming aid having a solubility in water at 20°C of 20 g / 100 ml or less and a solubility in water at 20°C of 0.5 g / 100 ml or more as main components, wherein the mass ratio of resin solid content of component (A) to component (B) is 70 / 30 to 95 / 5, the method for producing a laminated iron core comprises a step of applying an aqueous heat-adhesive insulating coating composition having the above ratio.

[0005] In addition to functioning as an adhesive for fixing laminated electromagnetic steel sheets, the adhesive coating is also required to function as an insulating coating formed on the surface of the electromagnetic steel sheet. Generally, insulating coatings are required to have insulation properties and corrosion resistance. Therefore, the coating agent for forming an adhesive coating is required to have excellent film-forming properties that can form an adhesive coating without cissing and cracks on the surface of the electromagnetic steel sheet. A high adhesive force can be expected from an adhesive coating with high smoothness without cissing and cracks. In addition, since laminated steel sheets are used in high-temperature environments, the coating agent for forming an adhesive coating is also required to be capable of forming an adhesive coating that exhibits high adhesive force in high-temperature environments.

[0006] In relation to these points, the heat-bondable surface-coated electromagnetic steel sheet described in Japanese Patent Publication No. 7-308990 uses a mixed resin of acrylic resin and epoxy resin as the resin for forming the adhesive film. However, since both the glass transition temperature of the acrylic resin and the melting point or softening point of the epoxy resin are high, it is considered that cracks are likely to occur in the adhesive film after it has been baked onto the electromagnetic steel sheet. On the other hand, the water-based heat-bondable insulating coating composition described in Japanese Patent Publication No. 2-208034 contains acrylic resin, epoxy resin, epoxy resin curing agent, etc. However, since the epoxy resin curing agent is a tertiary amine compound, the acrylic resin and epoxy resin do not crosslink, and a dense crosslinked structure is not formed. In order to increase the adhesive strength of the adhesive film in a high-temperature environment, it is necessary to increase the cohesive force of the resin. Since the cohesive force of the resin is expressed by the formation of a dense crosslinked structure, it is considered difficult to form an adhesive film that exhibits high adhesive strength in a high-temperature environment with the water-based heat-bondable insulating coating composition described in Japanese Patent Publication No. 2-208034.

[0007] This disclosure has been made in view of the circumstances described above. One embodiment of this disclosure aims to solve the problem of providing a coating composition for laminated steel sheets that can form a coating film exhibiting high shear adhesion in a high-temperature environment and has excellent film-forming properties.

[0008] The following embodiments are included as specific means for solving the problem: <1> A (meth)acrylic resin (A) containing constituent units derived from monomers having carboxyl groups in a proportion of 6% to 18% by mass relative to the total constituent units, and having a glass transition temperature of 70°C to 100°C; an epoxy resin (B) having an epoxy equivalent of 400 g / eq to 800 g / eq; and a curing agent (C) having a functional group (c) that can react with carboxyl groups and epoxy groups, wherein the ratio of the content mass of the (meth)acrylic resin (A) to the content mass of the epoxy resin (B) [i.e., (meth)acrylic resin A coating composition for laminated steel sheets, wherein the ratio of the mass content of fat (A) to the mass content of epoxy resin (B) is 76 / 24 to 93 / 7, and the ratio of the number of moles of the functional group (c) in the curing agent (C) to the total number of moles of carboxyl groups in the (meth)acrylic resin (A) and epoxy groups in the epoxy resin (B) [i.e., the number of moles of the functional group (c) in the curing agent (C) / the total number of moles of carboxyl groups in the (meth)acrylic resin (A) and epoxy groups in the epoxy resin (B)] is 0.39 to 1.30. <2> The coating composition for laminated steel sheets according to <1>, wherein the functional group (c) in the curing agent (C) contains at least one of a primary amino group and a secondary amino group. <3> The coating composition for laminated steel sheets according to <1> or <2>, wherein the (meth)acrylic resin (A) contains constituent units derived from an alkyl (meth)acrylate monomer. <4> The coating composition for laminated steel sheets according to any one of <1> to <3>, wherein the (meth)acrylic resin (A) contains constituent units derived from a monomer having a ring structure. <5> The coating composition for laminated steel sheets according to <4>, wherein the constituent units derived from the monomer having a ring structure are constituent units derived from styrene. <6> The coating composition for laminated steel sheets according to any one of <1> to <5>, wherein the shape of the (meth)acrylic resin (A) is particulate. <7> The coating composition for laminated steel sheets according to any one of <1> to <6>, wherein the shape of the epoxy resin (B) is particulate. <8> The coating composition for laminated steel sheets according to <6> or <7>, further comprising water.<9> A coating composition for laminated steel sheets according to any one of <1> to <8>, wherein the gel fraction after heating at 170°C for 90 seconds and then at 230°C for 60 seconds is 90% by mass or more.

[0009] According to one embodiment of the present disclosure, a coating composition for laminated steel sheets is provided that enables the formation of a coating film exhibiting high shear adhesion in a high-temperature environment and has excellent film-forming properties.

[0010] The coating compositions for laminated steel sheets of this disclosure will be described in detail below. The requirements described below may be based on typical embodiments of this disclosure, but this disclosure is not limited to such embodiments and can be modified as appropriate within the scope of the purpose of this disclosure.

[0011] In this disclosure, numerical ranges indicated using "~" mean ranges that include the numerical values ​​before and after "~" as the lower and upper limits, respectively. In numerical ranges described in stages in this disclosure, the upper or lower limit stated in one numerical range may be replaced with the upper or lower limit of another numerical range described in stages. Furthermore, in numerical ranges described in this disclosure, the upper or lower limit stated in one numerical range may be replaced with the values ​​shown in the examples.

[0012] In this disclosure, a combination of two or more preferred embodiments is a more preferred embodiment.

[0013] In this disclosure, the amount of each component in the composition means the total amount of any multiple substances present in the composition, unless otherwise specified, if there are multiple substances corresponding to each component in the composition.

[0014] In this disclosure, unless otherwise specified, "solids" means components other than the solvent contained in the composition, and "solvent" means water and organic solvents. For example, if the solvent contained in the composition is only water, then "solids" means components other than water contained in the composition; if the solvent contained in the composition is only an organic solvent, then "solids" means components other than the organic solvent contained in the composition; and if the solvent contained in the composition is both water and an organic solvent, then "solids" means components other than water and organic solvents contained in the composition.

[0015] In this disclosure, "(meth)acrylic monomer" means a monomer having a (meth)acryloyl group. In this disclosure, "(meth)acrylic resin" means a resin that contains constituent units derived from a (meth)acrylic monomer, and in which the proportion of constituent units derived from a (meth)acrylic monomer is 50% by mass or more.

[0016] In this disclosure, "(meth)acrylic" is a term that encompasses both "acrylic" and "methacrylic," "(meth)acrylate" is a term that encompasses both "acrylate" and "methacrylate," and "(meth)acryloyl" is a term that encompasses both "acryloyl" and "methacryloyl."

[0017] In this disclosure, "n-" means normal, "i-" means iso, "s-" means secondary, and "t-" means tertiary.

[0018] In this disclosure, "constituent units derived from monomers" means constituent units formed by the addition polymerization of monomers.

[0019] In this disclosure, "mass%" and "weight%" are synonymous, and "parts by mass" and "parts by weight" are synonymous. Also, in this disclosure, "monomer" and "monomer" are synonymous, and "polymer" and "polymer" are synonymous.

[0020] In this disclosure, the term "process" includes not only independent processes but also processes that cannot be clearly distinguished from other processes, as long as their intended purpose is achieved.

[0021] [Coating Composition for Laminated Steel Sheets] The coating composition for laminated steel sheets of the present disclosure (hereinafter also simply referred to as "coating composition") comprises a (meth)acrylic resin (A) containing constituent units derived from monomers having carboxyl groups in a proportion of 6% to 18% by mass relative to the total constituent units, and having a glass transition temperature of 70°C to 100°C; an epoxy resin (B) having an epoxy equivalent of 400 g / eq to 800 g / eq; and a curing agent (C) having a functional group (c) that can react with carboxyl groups and epoxy groups, wherein the ratio of the mass content of the (meth)acrylic resin (A) to the mass content of the epoxy resin (B) is 76 / 24 to 93 / 7, and the ratio of the number of moles of the functional group (c) in the curing agent (C) to the total number of moles of carboxyl groups in the (meth)acrylic resin (A) and epoxy groups in the epoxy resin (B) is 0.39 to 1.30.

[0022] The coating composition of this disclosure is a coating composition used for laminated steel sheets, and more specifically, a composition for coating the surface of electrical steel sheets used in the manufacture of laminated steel sheets. The coating composition of this disclosure, having the above-described configuration, can form a coating film that exhibits high shear adhesion in high-temperature environments and has excellent film-forming properties.

[0023] [(meth)acrylic resin (A)] The coating composition of this disclosure contains a (meth)acrylic resin (A) which contains constituent units derived from monomers having carboxyl groups in a proportion of 6% to 18% by mass relative to the total constituent units, and has a glass transition temperature of 70°C to 100°C. In this disclosure, "a (meth)acrylic resin (A) which contains constituent units derived from monomers having carboxyl groups in a proportion of 6% to 18% by mass relative to the total constituent units, and has a glass transition temperature of 70°C to 100°C" is also simply referred to as "(meth)acrylic resin (A)". The coating composition of this disclosure may contain one type of (meth)acrylic resin (A) alone, or two or more types.

[0024] In the coating composition of this disclosure, the (meth)acrylic resin (A) may exist in a dissolved state in the medium or in a dispersed state in the medium. When the (meth)acrylic resin (A) is dispersed in the medium, the shape of the (meth)acrylic resin (A) may be particulate. Generally, coating compositions tend to have difficulty forming films when the resin is in particulate form. In contrast, the coating composition of this disclosure tends to have excellent film-forming properties even when the (meth)acrylic resin (A) is in particulate form.

[0025] <Constituent units derived from monomers having carboxyl groups> The (meth)acrylic resin (A) contains constituent units derived from monomers having carboxyl groups in a proportion of 6% to 18% by mass relative to the total number of constituent units. The type of monomer having carboxyl groups is not particularly limited. Examples of monomers having carboxyl groups include monomers having at least one carboxyl group and an ethylenically unsaturated group in one molecule. Examples of ethylenically unsaturated groups include vinyl groups, allyl groups, vinylphenyl groups, (meth)acrylamide groups, and (meth)acryloyl groups.

[0026] Specific examples of monomers having a carboxyl group include (meth)acrylic acid, crotonic acid, maleic acid, fumaric acid, itaconic acid, glutaconic acid, citraconic acid, ω-carboxy-polycaprolactone mono(meth)acrylate [e.g., ω-carboxy-polycaprolactone (n≈2) monoacrylate], and succinic acid derivatives (e.g., 2-acryloyloxyethyl succinic acid). The monomer having a carboxyl group preferably contains (meth)acrylic acid, more preferably (meth)acrylic acid, and even more preferably methacrylic acid.

[0027] The (meth)acrylic resin (A) may contain one or more constituent units derived from monomers having a carboxyl group.

[0028] The content of constituent units derived from monomers having carboxyl groups in (meth)acrylic resin (A) is 6% to 18% by mass relative to the total constituent units of (meth)acrylic resin (A). When the content of constituent units derived from monomers having carboxyl groups in (meth)acrylic resin (A) is 6% by mass or more relative to the total constituent units of (meth)acrylic resin (A), the crosslinking reaction between (meth)acrylic resin (A) and curing agent (C) is sufficiently carried out, and the cohesive force of the resin is sufficiently increased. As a result, the coating film tends to exhibit high shear adhesion in high-temperature environments. From this viewpoint, the content of constituent units derived from monomers having carboxyl groups in (meth)acrylic resin (A) is preferably 7% by mass or more, and more preferably 8% by mass or more, relative to the total constituent units of (meth)acrylic resin (A). When the content of constituent units derived from monomers having carboxyl groups in (meth)acrylic resin (A) is 18% by mass or less relative to the total constituent units of (meth)acrylic resin (A), the crosslinking reaction between (meth)acrylic resin (A) and curing agent (C) does not proceed excessively when the coating composition is baked onto the electrical steel sheet. As a result, the coating composition tends to exhibit excellent film-forming properties. Furthermore, when the content of constituent units derived from monomers having carboxyl groups in (meth)acrylic resin (A) is 18% by mass or less relative to the total constituent units of (meth)acrylic resin (A), the gel fraction of the coating film after baking onto the electrical steel sheet does not increase excessively, so the coating films fuse together sufficiently when the electrical steel sheets are heat-pressed together. As a result, the coating film tends to exhibit high shear adhesion in high-temperature environments. From this viewpoint, the content of constituent units derived from monomers having carboxyl groups in the (meth)acrylic resin (A) is preferably 15% by mass or less, and more preferably 12% by mass or less, relative to the total constituent units of the (meth)acrylic resin (A). In some embodiments, the content of constituent units derived from monomers having carboxyl groups in the (meth)acrylic resin (A) may be 7% by mass to 15% by mass, 7% by mass to 12% by mass, or 8% by mass to 12% by mass, relative to the total constituent units of the (meth)acrylic resin (A).

[0029] <Constituent units derived from (meth)acrylate alkyl ester monomers> The (meth)acrylic resin (A) preferably contains constituent units derived from (meth)acrylate alkyl ester monomers, for example, from the viewpoint of adjusting the glass transition temperature of the (meth)acrylic resin (A). In this disclosure, "(meth)acrylate alkyl ester monomer" refers to an (meth)acrylate alkyl ester monomer that does not have a carboxyl group. In other words, in this disclosure, "(meth)acrylate alkyl ester monomer having a carboxyl group" is classified as a monomer having a carboxyl group.

[0030] The type of alkyl (meth)acrylate monomer is not particularly limited. The alkyl (meth)acrylate monomer may be an alkyl acrylate monomer or an alkyl methacrylate monomer. The alkyl group of the alkyl (meth)acrylate monomer may be unsubstituted or substituted, but it is preferable that it be unsubstituted. The alkyl group of the alkyl (meth)acrylate monomer may be linear, branched, or cyclic. The number of carbon atoms in the alkyl group of the alkyl (meth)acrylate monomer is preferably 1 to 18, more preferably 1 to 12, even more preferably 1 to 8, and particularly preferably 1 to 4.

[0031] Specific examples of alkyl (meth)acrylate monomers include methyl (meth)acrylate, ethyl (meth)acrylate, n-butyl (meth)acrylate, i-butyl (meth)acrylate, s-butyl (meth)acrylate, t-butyl (meth)acrylate, n-octyl (meth)acrylate, i-octyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, n-nonyl (meth)acrylate, i-nonyl (meth)acrylate, n-decyl (meth)acrylate, stearyl (meth)acrylate, lauryl (meth)acrylate, cyclohexyl (meth)acrylate, and isobornyl (meth)acrylate.

[0032] The alkyl (meth)acrylate monomer preferably contains at least one of methyl methacrylate and n-butyl acrylate, more preferably at least one of methyl methacrylate and n-butyl acrylate, and even more preferably methyl methacrylate and n-butyl acrylate.

[0033] If the (meth)acrylic resin (A) contains constituent units derived from alkyl (meth)acrylate monomers, it may contain one type of constituent unit derived from alkyl (meth)acrylate monomers alone, or it may contain two or more types.

[0034] When the (meth)acrylic resin (A) contains constituent units derived from alkyl (meth)acrylate monomers, the content of constituent units derived from alkyl (meth)acrylate monomers in the (meth)acrylic resin (A) is not particularly limited, but for example, it is preferably 50% to 94% by mass, more preferably 52% to 75% by mass, and even more preferably 55% to 60% by mass, relative to the total constituent units of the (meth)acrylic resin (A). A content of constituent units derived from alkyl (meth)acrylate monomers in the (meth)acrylic resin (A) of 50% by mass or more relative to the total constituent units of the (meth)acrylic resin (A) means that constituent units derived from alkyl (meth)acrylate monomers are included as the main constituent units of the (meth)acrylic resin (A).

[0035] <Other constituent units> The (meth)acrylic resin (A) may contain constituent units that do not fall under either the constituent units derived from monomers having a carboxyl group or the constituent units derived from alkyl (meth)acrylate monomers (so-called other constituent units).

[0036] Other constituent units include, for example, monomers having hydroxyl groups, such as 2-hydroxyethyl (meth)acrylate and 4-hydroxybutyl (meth)acrylate; constituent units having aromatic rings, such as benzyl (meth)acrylate and phenoxyethyl (meth)acrylate; constituent units having alkoxyalkyl (meth)acrylates, such as methoxyethyl (meth)acrylate and ethoxyethyl (meth)acrylate; constituent units having aromatic monovinyls, such as styrene, α-methylstyrene, t-butylstyrene, p-chlorostyrene, chloromethylstyrene, and vinyltoluene; and constituent units having vinyl esters, such as vinyl formate, vinyl acetate, vinyl propionate, and vinyl versatate.

[0037] Other preferred structural units include, for example, those derived from monomers having a ring structure, from the viewpoint of the strength and gloss of the coating film. Examples of structural units derived from monomers having a ring structure include those derived from (meth)acrylate having the above-mentioned aromatic ring, and those derived from the above-mentioned aromatic monovinyl. Of the structural units derived from monomers having a ring structure, those derived from styrene are preferred, for example, from the viewpoint of the strength of the coating film.

[0038] If the (meth)acrylic resin (A) contains other constituent units, it may contain one type of other constituent unit alone or two or more types.

[0039] If the (meth)acrylic resin (A) contains other constituent units, the content of the other constituent units in the (meth)acrylic resin (A) can be set appropriately within a range that does not impair the effect of the coating composition of this disclosure. For example, if the (meth)acrylic resin (A) contains constituent units derived from monomers having a ring structure as other constituent units, the content of constituent units derived from monomers having a ring structure in the (meth)acrylic resin (A) may be 20% to 40% by mass relative to the total constituent units of the (meth)acrylic resin (A).

[0040] <<Glass transition temperature of (meth)acrylic resin (A)>> The glass transition temperature of (meth)acrylic resin (A) is 70°C to 100°C. When the glass transition temperature of (meth)acrylic resin (A) is 70°C or higher, a coating film with a relatively high elastic modulus is formed. As a result, the coating film tends to exhibit high shear adhesion in high-temperature environments. From this viewpoint, the glass transition temperature of (meth)acrylic resin (A) is preferably 75°C or higher, and more preferably 80°C or higher. When the glass transition temperature of (meth)acrylic resin (A) is 100°C or lower, the (meth)acrylic resin (A) fuses sufficiently when the coating composition is baked onto the electrical steel sheet. As a result, the coating composition tends to exhibit excellent film-forming properties. From this viewpoint, the glass transition temperature of (meth)acrylic resin (A) is preferably 95°C or lower, and more preferably 90°C or lower. In one embodiment, the glass transition temperature of the (meth)acrylic resin (A) may be 75°C to 95°C, 80°C to 95°C, or 80°C to 90°C.

[0041] In this disclosure, the glass transition temperature of the (meth)acrylic resin is measured using a differential scanning calorimetry (DSC). The specific measurement method is as follows: A liquid containing the (meth)acrylic resin is applied to release paper using a 4 mil (101.6 μm) applicator. The applied liquid containing the (meth)acrylic resin is then dried (drying temperature: 105°C, drying time: 10 minutes) to obtain a dried (meth)acrylic resin product. 10 mg of this dried product is placed in an aluminum sample pan (e.g., Tzero Pan (product name) manufactured by T.A. Instrument Japan Co., Ltd.), sealed with an aluminum lid (e.g., Tzero Hermetic Lid (product name) manufactured by T.A. Instrument Japan Co., Ltd.), and the glass transition temperature is measured using a differential scanning calorimetry under the following measurement conditions. In this measurement, the glass transition temperature is defined as the temperature at the intersection of a straight line extending the low-temperature baseline of the obtained DSC curve toward the high-temperature side and a tangent line drawn at the point where the slope of the curve representing the stepwise transition of the glass transition is maximum. As a differential scanning calorimeter, for example, a differential scanning calorimeter manufactured by T.A. Instruments Japan Co., Ltd. (product name: Discovery DSC 2500) can be used. However, the differential scanning calorimeter is not limited to this.

[0042] -Measurement Conditions- Atmosphere: Under air Measurement temperature range: -50°C to 150°C Heating rate: 10°C / min Standard material: Empty sample pan

[0043] The glass transition temperature of (meth)acrylic resin (A) can be adjusted to a desired value, for example, by changing the type and proportion of monomers used as raw materials for (meth)acrylic resin (A).

[0044] <<Content of (meth)acrylic resin (A)>> The content of (meth)acrylic resin (A) in the coating composition of the present disclosure is not particularly limited, but for example, it is preferably 66% to 86% by mass, more preferably 72% to 84% by mass, and even more preferably 74% to 82% by mass, based on the total solid content in the coating composition.

[0045] In the present disclosure, the "total solid content in the coating composition" means the mass of the residue obtained by removing the solvent from the coating composition.

[0046] [Method for producing (meth)acrylic resin (A)] The method for producing (meth)acrylic resin (A) is not particularly limited, as long as the aforementioned (meth)acrylic resin (A) can be produced. The (meth)acrylic resin (A) can be produced, for example, by an emulsion polymerization method. Examples of the emulsion polymerization method for producing the (meth)acrylic resin (A) include the methods [1] to [3] shown below. In the present disclosure, a monomer that forms a constituent unit of the (meth)acrylic resin (A) is also referred to as a "monomer component".

[0047] [1] A method in which a monomer component, a surfactant, and water are charged into a reactor equipped with a thermometer, a stirrer, a raw material introduction pipe, a reflux condenser, a nitrogen introduction pipe, etc., the temperature is increased while stirring under a nitrogen stream, and then a polymerization initiator, a reducing agent, etc. are added as appropriate to allow the emulsion polymerization reaction to proceed (the so-called one-pot charging method). [2] A method in which a surfactant and water are charged into a reactor equipped with a thermometer, a stirrer, a raw material introduction pipe, a reflux condenser, a nitrogen introduction pipe, etc., the temperature is increased while stirring under a nitrogen stream, then the monomer component is added dropwise, and a polymerization initiator, a reducing agent, etc. are added as appropriate to allow the emulsion polymerization reaction to proceed (the so-called monomer dropping method). [3] After putting a monomer component into a container, stirring is performed to prepare a monomer mixture. After putting water and a surfactant into another container, stirring is performed to prepare an aqueous surfactant solution. The pre-emulsion is prepared by stirring and emulsifying while adding the mixture of the monomer component to the prepared aqueous surfactant solution. A part of the pre-emulsion and water are charged into a reactor equipped with a thermometer, a stirrer, a raw material introduction pipe, a reflux condenser, a nitrogen introduction pipe, etc., and the temperature is increased while stirring under a nitrogen stream. After the temperature in the reactor stabilizes, the remaining pre-emulsion is added dropwise while a polymerization initiator, a reducing agent, etc. are appropriately added dropwise into the reactor, to allow the emulsion polymerization reaction to proceed (the so-called emulsified monomer dropping method). Among these, as the emulsion polymerization method for producing the (meth)acrylic resin (A), for example, from the viewpoint of industrial productivity, the emulsified monomer dropping method of the above [3] is preferred.

[0048] The (meth)acrylic resin (A) obtained by the above emulsion polymerization method is obtained in the form of a dispersion dispersed in a medium containing at least water and a surfactant.

[0049] Since details of the monomer components are as described above, and details of water and the surfactant are as described later, description thereof is omitted here.

[0050] The polymerization initiator is not particularly limited as long as it is used in ordinary emulsion polymerization. Examples of the polymerization initiator include persulfates, organic peroxides and azo compounds. Specific examples of the persulfate include ammonium persulfate (also referred to as "ammonium peroxodisulfate"), sodium persulfate, and potassium persulfate. Specific examples of the organic peroxide include t-butyl hydroperoxide, benzoyl peroxide, lauroyl peroxide, caproyl peroxide, di-i-propyl peroxydicarbonate, di-2-ethylhexyl peroxydicarbonate, and t-butyl peroxypivalate. Specific examples of the azo compound include 2,2'-azobisisobutyronitrile, 2,2'-azobis(2,4-dimethylvaleronitrile), 2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile), 1,1'-azobis(cyclohexane-1-carbonitrile), and dimethyl 2,2'-azobis(isobutyrate).

[0051] In the emulsion polymerization method for producing the (meth)acrylic resin (A), one type of polymerization initiator may be used alone, or two or more types thereof may be used.

[0052] The polymerization initiator is used in a commonly employed amount. The used amount of the polymerization initiator is preferably 0.05 parts by mass to 1.00 parts by mass, for example, per 100 parts by mass of the total monomer components.

[0053] The reducing agent is not particularly limited, as long as it is one that is commonly used in emulsion polymerization. Examples of reducing agents include sodium metabisulfite, sodium sulfite, sodium bisulfite, sodium pyrosulfite, sodium pyrophosphate, sodium hydroxymethanesulfinate, thioglycolic acid, and sodium thiosulfate.

[0054] In the emulsion polymerization method for producing (meth)acrylic resin (A), one reducing agent may be used alone, or two or more reducing agents may be used.

[0055] The reducing agent is used in the amount typically used. Preferably, the amount of reducing agent used is 0.05 parts by mass to 1.00 parts by mass per 100 parts by mass of the total monomer components.

[0056] In the emulsion polymerization method for producing (meth)acrylic resin (A), the polymerization temperature is, for example, 50°C to 80°C, preferably 60°C to 70°C. The polymerization time is, for example, 5 hours to 9 hours, preferably 6 hours to 8 hours.

[0057] [Epoxy resin (B)] The coating composition of this disclosure comprises epoxy resin (B) having an epoxy equivalent of 400 g / eq to 800 g / eq. In this disclosure, "epoxy resin (B) having an epoxy equivalent of 400 g / eq to 800 g / eq" is also simply referred to as "epoxy resin (B)".

[0058] The coating composition of this disclosure may contain one epoxy resin (B) alone or two or more epoxy resins.

[0059] In the coating composition of this disclosure, the epoxy resin (B) may exist in a dissolved state in the medium or in a dispersed state in the medium. When the epoxy resin (B) is dispersed in the medium, the epoxy resin (B) may take the form of particulate matter. Generally, coating compositions tend to have difficulty forming films when the resin is in the form of particulate matter. In contrast, the coating composition of this disclosure tends to have excellent film-forming properties even when the epoxy resin (B) is in the form of particulate matter.

[0060] The epoxy resin (B) has an epoxy equivalent of 400 g / eq to 800 g / eq. When the epoxy equivalent of epoxy resin (B) is 400 g / eq or more, the gel fraction of the coating film after baking onto the electrical steel sheet does not increase excessively, so that the coating films fuse together sufficiently when the electrical steel sheets are heat-pressed together. As a result, the coating film tends to exhibit high shear adhesion in high-temperature environments. From this viewpoint, the epoxy equivalent of epoxy resin (B) is preferably 420 g / eq or more, more preferably 450 g / eq or more, and even more preferably 500 g / eq or more. When the epoxy equivalent of epoxy resin (B) is 800 g / eq or less, the epoxy resin (B) fuses together sufficiently when the coating composition is baked onto the electrical steel sheet. As a result, the coating composition tends to exhibit excellent film-forming properties. From this viewpoint, the epoxy equivalent of epoxy resin (B) is preferably 750 g / eq or less, more preferably 680 g / eq or less, and even more preferably 600 g / eq or less. In some embodiments, the epoxy equivalent of epoxy resin (B) may be 420 g / eq to 750 g / eq, 450 g / eq to 680 g / eq, or 500 g / eq to 600 g / eq.

[0061] In this disclosure, the epoxy equivalent of the epoxy resin is measured by a method in accordance with JIS K 7236:2009. When using a commercially available epoxy resin, the epoxy equivalent of the epoxy resin shall be based on the catalog value of the commercially available product.

[0062] The type of epoxy resin (B) is not particularly limited as long as it satisfies the epoxy equivalent requirement. Examples of epoxy resin (B) include glycidyl ether type epoxy resins obtained by the reaction of a compound having a hydroxyl group in its molecule with epichlorohydrin; glycidyl ester type epoxy resins obtained by the reaction of a compound having a carboxyl group in its molecule with epichlorohydrin; glycidylamine type epoxy resins obtained by the reaction of a compound having an amino group in its molecule with epichlorohydrin; and the like. Examples of glycidyl ether type epoxy resins include bisphenol type epoxy resins obtained by the reaction of bisphenols such as bisphenol A, bisphenol F, bisphenol AD, bisphenol S, and hydrogenated bisphenol A with epichlorohydrin; novolac type epoxy resins obtained by the reaction of novolac resins such as phenol novolac and o-cresol novolac with epichlorohydrin; and the like. Examples of glycidyl ester type epoxy resins include diglycidyl phthalate, diglycidyl hexahydrophthalate, and diglycidyl dimer acid. Examples of glycidylamine-type epoxy resins include tetraglycidyldiaminodiphenylmethane, tetraglycidylxylenediamine, bis(diglycidylaminomethyl)cyclohexane, and triglycidyl isocyanurate.

[0063] Commercially available epoxy resins can be used as epoxy resin (B). Examples of commercially available epoxy resins (B) include "BECKOPOX VEP2381w / 55WA" (epoxy equivalent: 500g / eq) manufactured by Daicel Ornex Co., Ltd., "YUKAREZIN NE-3012" (epoxy equivalent: 625g / eq) manufactured by Yoshimura Oil Chemical Co., Ltd., and "ADEKA REZIN EM-101-50" (epoxy equivalent: 505g / eq) manufactured by ADEKA Corporation. "BECKOPOX," "YUKAREZIN," and "ADEKA REZIN" are all registered trademarks.

[0064] <<Mass Ratio of (Meth)acrylic Resin (A) to Epoxy Resin (B)>> The ratio of the mass content of (meth)acrylic resin (A) to the mass content of epoxy resin (B) in the coating composition of this disclosure [i.e., mass content of (meth)acrylic resin (A) / mass content of epoxy resin (B); hereinafter also referred to as "mass content ratio (A) / (B)"] is 76 / 24 to 93 / 7. When the mass content ratio (A) / (B) is 76 / 24 or higher, the gel fraction of the coating film after baking onto the electrical steel sheet does not increase excessively, so that the coating films fuse together sufficiently when the electrical steel sheets are heat-pressed together. Therefore, the coating film tends to exhibit high shear adhesion in a high-temperature environment. From this viewpoint, the mass content ratio (A) / (B) is preferably 80 / 20 or higher, and more preferably 85 / 15 or higher. When the mass ratio of (A) / (B) is 93 / 7 or less, the (meth)acrylic resin (A) and epoxy resin (B) fuse sufficiently when the coating composition is baked onto the electrical steel sheet. As a result, the coating composition tends to exhibit excellent film-forming properties. From this viewpoint, it is preferable that the mass ratio of (A) / (B) is 90 / 10 or less. In some embodiments, the mass ratio of (A) / (B) may be 80 / 20 to 93 / 7, 80 / 20 to 90 / 10, or 85 / 15 to 90 / 10.

[0065] [Curing agent (C)] The coating composition of the present disclosure comprises a curing agent (C) having a functional group (c) that can react with carboxyl groups and epoxy groups. In the present disclosure, "curing agent (C) having a functional group (c) that can react with carboxyl groups and epoxy groups" is also simply referred to as "curing agent (C)", and "functional group (c) that can react with carboxyl groups and epoxy groups" is also simply referred to as "functional group (c)".

[0066] The coating composition of this disclosure may contain one curing agent (C) alone, or it may contain two or more curing agents.

[0067] The functional group (c) in the curing agent (C) contributes to crosslinking with the (meth)acrylic resin (A) and the epoxy resin (B). Examples of the functional group (c) in the curing agent (C) include amino groups (e.g., primary and secondary amino groups) and thiol groups. From the viewpoint of reactivity with carboxyl groups, for example, the functional group (c) preferably contains at least one of a primary amino group and a secondary amino group, and more preferably at least one of a primary amino group and a secondary amino group.

[0068] Examples of curing agents having an amino group (so-called amine-based curing agents) include triethylenetetramine, 3,3'-diaminodiphenylsulfone, 4,4'-diaminodiphenylsulfone, 4,4'-diaminodiphenylmethane, 4,4'-diaminodiphenyl ether, 4,4'-diamino-3,3'-dimethoxybiphenyl, 4,4'-diaminophenylbenzoate, 1,3-diaminonaphthalene, 1,4-diaminonaphthalene, 1,5-diaminonaphthalene, 1,8-diaminonaphthalene, 1,3-diaminobenzene, 1,4-diaminobenzene, 4,4'-diaminobenzanilide, trimethylene-bis-4-aminobenzoate, and dicyandiamide.

[0069] Examples of curing agents containing thiol groups (so-called thiol-based curing agents) include trimethylolpropane tris(3-mercaptopropionate), pentaerythritol tetrakis(3-mercaptobutyrate), and tris(3-mercaptopropyl) isocyanurate.

[0070] The functional group equivalent of the curing agent (C) is not particularly limited, but is preferably 90 g / eq to 400 g / eq, more preferably 100 g / eq to 300 g / eq, and even more preferably 150 g / eq to 200 g / eq. In some embodiments, the functional group equivalent of the curing agent (C) may be 90 g / eq to 200 g / eq, 100 g / eq to 180 g / eq, or 120 g / eq to 160 g / eq. The functional group equivalent of the curing agent (C) refers to the active hydrogen equivalent when the functional group (c) is an amino group [i.e., when the curing agent (C) is an amine-based curing agent], and refers to the SH equivalent when the functional group (c) is a thiol group [i.e., when the curing agent (C) is a thiol-based curing agent].

[0071] In this disclosure, the active hydrogen equivalent of the amine-based curing agent is calculated based on the amine value measured in accordance with the potentiometric titration method described in JIS K 7237:1995. When using a commercially available amine-based curing agent, the catalog value of the commercially available product is given priority for determining the active hydrogen equivalent of the amine-based curing agent.

[0072] In this disclosure, the SH equivalent of the thiol-based curing agent is calculated based on the SH value measured by iodine titration. When using a commercially available thiol-based curing agent, the SH equivalent of the commercially available product shall be based on the catalog value.

[0073] Commercially available products can be used as the curing agent (C). Examples of commercially available curing agents (C) include "Fujicure FXH-940," an amine-based curing agent manufactured by T&K TOKA Corporation; "BECKOPOX EH616W / 67WA," an amine-based curing agent manufactured by Daicel Ornex Corporation; and "Yukarezin HD-03," an amine-based curing agent manufactured by Yoshimura Oil Chemical Co., Ltd. "Fujicure," "BECKOPOX," and "Yukarezin" are all registered trademarks.

[0074] <<Ratio of moles of functional group (c) in curing agent (C) to the total number of moles of carboxyl groups in (meth)acrylic resin (A) and epoxy groups in epoxy resin (B)>> The coating composition of the present disclosure has a ratio of moles of functional group (c) in curing agent (C) to the total number of moles of carboxyl groups in (meth)acrylic resin (A) and epoxy groups in epoxy resin (B) [i.e., number of moles of functional group (c) in curing agent (C) / total number of moles of carboxyl groups in (meth)acrylic resin (A) and epoxy groups in epoxy resin (B)] of 0.39 to 1.30. When the ratio of the number of moles of functional groups (c) in the curing agent (C) to the total number of moles of carboxyl groups in the (meth)acrylic resin (A) and epoxy groups in the epoxy resin (B) is 0.39 or higher, the functional groups (c) react sufficiently with the carboxyl groups in the (meth)acrylic resin (A) and epoxy groups in the epoxy resin (B) to form a crosslinked structure, resulting in a coating film with a high gel fraction after heat-pressing the electrical steel sheets together and a sufficiently high cohesive force of the resin. As a result, the coating film tends to exhibit high shear adhesion in high-temperature environments. From this viewpoint, the ratio of the number of moles of functional groups (c) in the curing agent (C) to the total number of moles of carboxyl groups in the (meth)acrylic resin (A) and epoxy groups in the epoxy resin (B) is preferably 0.40 or higher, more preferably 0.50 or higher, and even more preferably 0.60 or higher. When the ratio of the number of moles of functional groups (c) in the curing agent (C) to the total number of moles of carboxyl groups in the (meth)acrylic resin (A) and epoxy groups in the epoxy resin (B) is 1.30 or less, the crosslinking reaction mediated by the curing agent (C) proceeds efficiently, resulting in a high gel fraction after heat-pressing the electrical steel sheets together, and a coating film with sufficiently high resin cohesive force is formed. As a result, the coating film tends to exhibit high shear adhesion in high-temperature environments. From this viewpoint, the ratio of the number of moles of functional groups (c) in the curing agent (C) to the total number of moles of carboxyl groups in the (meth)acrylic resin (A) and epoxy groups in the epoxy resin (B) is preferably 1.15 or less, more preferably 1.00 or less, even more preferably 0.90 or less, and particularly preferably 0.80 or less.In one embodiment, the ratio of the number of moles of functional groups (c) in the curing agent (C) to the total number of moles of carboxyl groups in the (meth)acrylic resin (A) and epoxy groups in the epoxy resin (B) may be 0.40 to 1.15, 0.40 to 1.00, 0.40 to 0.90, 0.40 to 0.80, 0.50 to 1.15, 0.50 to 1.00, 0.50 to 0.90, 0.50 to 0.80, 0.60 to 1.15, 0.60 to 1.00, 0.60 to 0.90, or 0.60 to 0.80.

[0075] The number of moles of carboxyl groups in a (meth)acrylic resin [unit: mmol] is calculated using the following formula (1): Number of moles of carboxyl groups in a (meth)acrylic resin [unit: mmol] = [Content of constituent units derived from monomers having carboxyl groups in the (meth)acrylic resin [unit: mass%]] / 100 × [Amount of (meth)acrylic resin [unit: g]] / [Molecular weight of constituent units derived from monomers having carboxyl groups [unit: g / mol]] × [Number of carboxyl groups in constituent units derived from monomers having carboxyl groups] × 1000 ... (1)

[0076] The molecular weights of the constituent units derived from monomers containing a carboxyl group are, for example, 72.06 g / mol for acrylic acid and 86.09 g / mol for methacrylic acid.

[0077] The number of moles of epoxy groups in epoxy resin [unit: mmol] is calculated using the following formula (2): Number of moles of epoxy groups in epoxy resin [unit: mmol] = [Amount of epoxy resin [unit: g]] / [Epoxy equivalent of epoxy resin [unit: g / eq]] × 1000 ... (2)

[0078] The number of moles of functional group (c) in the curing agent [unit: mmol] is calculated using the following formula (3): Number of moles of functional group (c) in the curing agent [unit: mmol] = [Amount of curing agent [unit: g]] / [Equivalent amount of functional group in the curing agent [unit: g / eq]] × 1000 ... (3)

[0079] [Water] The coating compositions of this disclosure may contain water. If the coating compositions of this disclosure contain water, the (meth)acrylic resin (A) and epoxy resin (B) may be present in the coating composition in the form of dispersed particles. The water is not particularly limited, but from the viewpoint of having fewer impurities, for example, deionized water, distilled water, and purified water are preferred.

[0080] If the coating composition of this disclosure contains water, the water content is not particularly limited and can be set as appropriate depending on the purpose, as long as it does not impair the effect of the coating composition of this disclosure.

[0081] [Organic Solvents] The coating compositions of this disclosure may contain organic solvents. If the coating compositions of this disclosure contain organic solvents, the (meth)acrylic resin (A) and epoxy resin (B) may be present in the coating composition in the form of dispersed particles and / or dissolved particles. Examples of organic solvents include water-miscible organic solvents and water-immiscible organic solvents. The organic solvent may be a water-miscible organic solvent, a water-immiscible organic solvent, or a mixed solvent of a water-miscible organic solvent and a water-immiscible organic solvent.

[0082] Examples of water-miscible organic solvents include monohydric alcohol compounds such as methanol, ethanol, propanol, and isopropanol; polyhydric alcohol compounds such as glycerin, ethylene glycol, diethylene glycol, and propylene glycol; and glycol derivatives such as ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, diethylene glycol monomethyl ether, diethylene glycol monobutyl ether, propylene glycol monomethyl ether, propylene glycol monobutyl ether, and dipropylene glycol monomethyl ether.

[0083] Examples of water-immiscible organic solvents include aromatic hydrocarbon compounds such as benzene, toluene, ethylbenzene, n-propylbenzene, t-butylbenzene, o-xylene, m-xylene, p-xylene, tetralin, decalin, and aromatic naphtha; aliphatic or alicyclic hydrocarbon compounds such as n-hexane, n-heptane, n-octane, i-octane, n-decane, dipentene, petroleum spirits, petroleum naphtha, and turpentine oil; ester compounds such as methyl acetate, ethyl acetate, n-butyl acetate, n-amyl acetate, 2-hydroxyethyl acetate, 2-butoxyethyl acetate, 3-methoxybutyl acetate, and methyl benzoate; and ketone compounds such as acetone, methyl ethyl ketone, methyl-i-butyl ketone, isophorone, cyclohexanone, and methylcyclohexanone.

[0084] If the coating composition of this disclosure contains an organic solvent, it may contain one organic solvent alone or two or more organic solvents.

[0085] If the coating composition of this disclosure contains an organic solvent, the content of the organic solvent is not particularly limited and can be set as appropriate depending on the purpose, as long as it does not impair the effect of the coating composition of this disclosure.

[0086] [Surfactants] The coating compositions of this disclosure may contain surfactants. The type of surfactant is not particularly limited. Examples of surfactants include anionic surfactants, cationic surfactants, amphoteric surfactants, and nonionic surfactants. When the shape of the (meth)acrylic resin (A) is particulate, the surfactant is preferably at least one of anionic surfactants and nonionic surfactants, and more preferably anionic surfactants and nonionic surfactants, from the viewpoint of the manufacturing stability of the (meth)acrylic resin (A).

[0087] Examples of anionic surfactants include polyoxyalkylene polycyclic phenyl ether sulfates, such as polyoxyethylene distyrenated phenyl ether sulfate ammonium; polyoxyalkylene polycyclic phenyl ether sulfates, such as polyoxyethylene distyrenated phenyl ether sulfate ammonium; polyoxyalkylene alkyl phenyl ether sulfates, such as polyoxyethylene nonylphenyl ether sulfate sodium; polyoxyalkylene alkyl ether sulfates, such as polyoxyethylene lauryl ether sulfate sodium; polyoxyalkylene alkyl ether sulfates; alkyl phosphate esters; and others. Examples of nonionic surfactants include polyoxyalkylene alkyl ethers, such as polyoxyethylene oleyl ether and polyoxyethylene lauryl ether; and polyoxyalkylene styrene-derived phenyl ethers, such as polyoxyethylene styrene-derived phenyl ether.

[0088] Commercially available surfactants can be used. Examples of commercially available surfactants include "Neoperex G-65," an anionic surfactant manufactured by Kao Corporation; "Neugen EA-197D," a nonionic surfactant manufactured by Daiichi Kogyo Seiyaku Co., Ltd.; and "Emulgen A-60," a nonionic surfactant manufactured by Kao Corporation. "Neoperex," "Neugen," and "Emulgen" are all registered trademarks.

[0089] If the coating composition of this disclosure contains a surfactant, it may contain one surfactant alone or two or more surfactants.

[0090] If the coating composition of this disclosure contains a surfactant, the surfactant content is not particularly limited and can be set as appropriate depending on the purpose, as long as it does not impair the effect of the coating composition of this disclosure.

[0091] [Other Components] The coating composition of this disclosure may contain components other than those described above (so-called other components) as necessary, to the extent that it does not impair its effect. Examples of other components include various additives such as preservatives, wetting agents, and defoaming agents. Other components may include resins such as (meth)acrylic resins other than (meth)acrylic resin (A), epoxy resins other than epoxy resin (B), and curing agents other than curing agent (C).

[0092] [Gel fraction of coating composition after heating] The coating composition of this disclosure preferably has a gel fraction of 85% by mass or more, more preferably 88% by mass or more, and even more preferably 90% by mass or more, after being heated at 170°C for 90 seconds and then at 230°C for 60 seconds, from the viewpoint of the cohesive force of the coating film. The upper limit is not particularly limited and may be, for example, 100% by mass.

[0093] The coating composition of this disclosure, for example, from the viewpoint of shear adhesion, preferably has a gel fraction of 15% to 62% by mass after heating at 170°C for 90 seconds and before heating at 230°C for 60 seconds, more preferably 20% to 50% by mass, and even more preferably 30% to 40% by mass.

[0094] The gel fraction of the coating composition after heating according to this disclosure is the proportion of solvent-insoluble matter measured using tetrahydrofuran as the extraction solvent. In this disclosure, the gel fraction of the coating composition after heating is measured by the following method: The coating composition is applied to release paper to form a coating film. The formed coating film is then dried at room temperature (25°C) for 24 hours, and then heated and dried at a predetermined temperature for a predetermined time to obtain a resin film with release paper. The release paper is then peeled off from the resin film with release paper to obtain a resin film. Next, 0.2 g ± 0.01 g of the resin film is weighed into a glass container. Next, 20 g of tetrahydrofuran (THF) is added to the glass container containing the resin film, and the container is sealed. The container is then left to stand for 3 days in an environment of 23°C and 50% RH to immerse the resin film in THF. The sample obtained from this immersion is used as the measurement sample. Next, the measurement sample is filtered using a 1000-mesh stainless steel mesh. Next, the wire mesh used to filter the sample is dried at 100°C for 3 hours. Then, the mass of the dried wire mesh is measured. The gel fraction of the resin film is then calculated using the following formula. In the following formula, A is the mass of the weighed resin film (in g), B is the mass of the wire mesh before filtering the sample (in g), and C is the mass of the wire mesh after filtering the sample (in g). Gel fraction of resin film [unit: mass %] = (C - B) / A × 100

[0095] [Applications of the Coating Composition] The coating composition of this disclosure is a composition used to coat the surface of electrical steel sheets used in the manufacture of laminated steel sheets. The coating composition of this disclosure is capable of forming a coating film that exhibits high shear adhesion in high-temperature environments and has excellent film-forming properties. Therefore, a specific application is to fix electrical steel sheets together when manufacturing laminated steel sheets. The coating composition of this disclosure is suitable, for example, as a composition for forming an adhesive film on an adhesive-coated electrical steel sheet used in the manufacture of laminated steel sheets.

[0096] [Electromagnetic Steel Sheet] An electromagnetic steel sheet is a steel sheet on which an adhesive coating is formed, and is not particularly limited. The electromagnetic steel sheet may be a non-oriented electromagnetic steel sheet or a grain-oriented electromagnetic steel sheet. Specifically, as the electromagnetic steel sheet, for example, a non-oriented electromagnetic steel sheet of JIS C 2552:2014, a grain-oriented electromagnetic steel sheet of JIS C 2553:2019, or a non-oriented thin electromagnetic steel sheet or a grain-oriented thin electromagnetic steel sheet of JIS C 2558:2015 can be used.

[0097] [Method for Manufacturing the Coating Composition] The method for manufacturing the coating composition of this disclosure is not particularly limited. The coating composition of this disclosure can be manufactured, for example, by mixing a (meth)acrylic resin (A), an epoxy resin (B), and a curing agent (C) such that the ratio of the mass content of the (meth)acrylic resin (A) to the mass content of the epoxy resin (B) [i.e., mass content of (meth)acrylic resin (A) / mass content of epoxy resin (B)] is 76 / 24 to 93 / 7, and the ratio of the number of moles of functional groups (c) in the curing agent (C) to the total number of moles of carboxyl groups in the (meth)acrylic resin (A) and epoxy groups in the epoxy resin (B) [i.e., number of moles of functional groups (c) in the curing agent (C) / total number of moles of carboxyl groups in the (meth)acrylic resin (A) and epoxy groups in the epoxy resin (B)] is 0.39 to 1.30.

[0098] The mixing method is not particularly limited. For example, mixing can be done by stirring. A general stirring device or agitator can be used for stirring. The stirring temperature is not particularly limited, but is preferably 20°C to 30°C.

[0099] The coating compositions of this disclosure will be described in more detail below with reference to examples. This disclosure is not limited to the following examples unless it exceeds the spirit of the disclosure.

[0100] In the following examples, the glass transition temperature (Tg) of the (meth)acrylic resin was measured using a differential scanning calorimeter (product name: Discovery DSC 2500) manufactured by T.A. Instrument Japan Co., Ltd., by the method described above. The epoxy equivalent of the epoxy resin and the active hydrogen equivalent of the curing agent were both catalog values. The number of moles of carboxyl groups in the (meth)acrylic resin was determined by formula (1) described above. The number of moles of epoxy groups in the epoxy resin was determined by formula (2) described above. The number of moles of functional group (c) in the curing agent was determined by formula (3) described above.

[0101] [Preparation of Coating Composition] [Example 1] 55.6 parts by mass of n-butyl acrylate [n-BA; alkyl acrylate monomer], 176.0 parts by mass of methyl methacrylate [MMA; alkyl methacrylate monomer], 128.4 parts by mass of styrene [St; other monomers], and 40 parts by mass of methacrylic acid [MAA; monomer having a carboxyl group] were placed in a stainless steel container, and then stirred to prepare a monomer mixture. Next, in a separate stainless steel container, 248.6 parts by mass of deionized water, 2.7 parts by mass of the nonionic surfactant Neugen EA-197D (manufactured by Daiichi Kogyo Seiyaku Co., Ltd.), 4.1 parts by mass of the nonionic surfactant Emulgen (registered trademark) A-60 (manufactured by Kao Corporation), and 5.4 parts by mass of the anionic surfactant Neoperex G-65 (manufactured by Kao Corporation) were added and stirred to prepare an aqueous surfactant solution. Next, the monomer mixture prepared above was gradually added to the aqueous surfactant solution and emulsified using a stirrer to prepare a preemulsion. This prepared preemulsion was used for polymerization. Polymerization was carried out in a constant temperature bath using a 7 L (liter; the same applies hereafter) flask equipped with a reflux condenser, a stirrer, a nitrogen gas inlet tube, a pre-emulsion dropper pump (product name: High-Cera Pump V-10, manufactured by Iwaki Co., Ltd.), and a polymerization initiator addition device (product name: Quantitative Liquid Delivery Pump MP-2000, manufactured by Tokyo Rika Kikai Co., Ltd.). Specifically, polymerization was carried out as follows: 162.4 parts by mass of deionized water were placed in the flask, and then a portion of the pre-emulsion prepared above (19.8 parts by mass) was added. Nitrogen gas was blown into the flask at a flow rate of 300 ml / min, and the internal temperature of the flask was raised to a desired reaction temperature (standard conditions: 62°C) while stirring at a desired rotation speed (standard conditions: 150 rpm (revolutions per minute; the same applies hereafter)). After the internal temperature stabilized, 3.1 parts by mass of an 18% by mass aqueous solution of ammonium peroxodisulfate [polymerization initiator] and 3.1 parts by mass of a 15% by mass aqueous solution of sodium bisulfite [reducing agent] were added, and the nitrogen gas flow rate was adjusted to 50 ml / min.After confirming the temperature rise in the flask, the remaining pre-emulsion (641.0 parts by mass) was added dropwise over 180 minutes, while 84.5 parts by mass of 1.8% by mass ammonium peroxodisulfate aqueous solution [polymerization initiator] and 84.5 parts by mass of 1.5% by mass sodium bisulfite aqueous solution [reducing agent] were added dropwise over 240 minutes. Thirty minutes after the completion of the addition of ammonium peroxodisulfate aqueous solution and sodium bisulfite, 6.4 parts by mass of 6.9% by mass t-butyl hydroperoxide aqueous solution [polymerization initiator] and 6.4 parts by mass of 4.4% by mass sodium hydroxymethanesulfinate aqueous solution [reducing agent] were added dropwise over 30 minutes. The polymerization reaction was completed by cooling to 30°C 150 minutes after the completion of the pre-emulsion addition. To the emulsion polymer obtained by polymerization, 1.3 parts by mass of a preservative [product name: Topside 350, manufactured by Permachem Asia Co., Ltd.] and 1.3 parts by mass of a wetting agent [product name: Surfinol® 440, manufactured by Nisshin Chemical Industry Co., Ltd.] were added to obtain an aqueous dispersion of (meth)acrylic resin with a solid content concentration of 40.0% by mass. In the obtained aqueous dispersion, it was confirmed that particulate (meth)acrylic resin was present in a dispersed state in water.

[0102] 230.8 parts by mass of the aqueous dispersion of the (meth)acrylic resin obtained above (90.0 parts by mass as resin solids; 90.0 g), 18.2 parts by mass of epoxy A [product name: BECKOPOX VEP2381w / 55WA, manufactured by Daicel Ornex Co., Ltd.] (10.0 parts by mass as resin solids; 10.0 g), and 10.8 parts by mass of curing agent A [product name: Fujicure FXH-940, manufactured by T&K Toka Co., Ltd.] (8.3 parts by mass as solids; 8.3 g) were mixed, and then an appropriate amount of deionized water was added to obtain a coating composition with a solid content concentration of 32.0% by mass.

[0103] [Examples 2-9] The coating compositions of Examples 2-9 were obtained in the same manner as in Example 1, except that the composition of the coating composition other than the preservative and wetting agent was as shown in Table 1.

[0104] [Examples 10-17] The coating compositions of Examples 10-17 were obtained in the same manner as in Example 1, except that the composition of the coating composition other than the preservative and wetting agent was as shown in Table 2.

[0105] [Comparative Examples 1-6] Each of the coating compositions in Comparative Examples 1-6 was obtained in the same manner as in Example 1, except that the composition of the coating composition other than the preservative and wetting agent was as shown in Table 3.

[0106] [Comparative Examples 7-11] The coating compositions of Comparative Examples 7-11 were obtained in the same manner as in Example 1, except that the composition of the coating composition other than the preservative and wetting agent was as shown in Table 4.

[0107] [Measurement and Evaluation] 1. Measurement of Gel Fraction (1) Preparation of Resin Film with Release Paper (1-1) Resin Film X with Release Paper The coating composition was applied onto release paper using an 8 mil (203.2 μm) applicator to form a coating film. The formed coating film was then dried at room temperature (25°C) for 24 hours, and then further dried at 170°C for 90 seconds to obtain Resin Film X with Release Paper.

[0108] (1-2) A resin film Y with release paper was coated onto release paper using an 8 mil (203.2 μm) applicator to form a coating film. The formed coating film was then dried at room temperature (25°C) for 24 hours, then dried at 170°C for 90 seconds, and then dried at 230°C for 60 seconds to obtain a resin film Y with release paper.

[0109] (2) Measurement of Gel Fraction The release paper was peeled off from resin film X and resin film Y with release paper to obtain resin film X and resin film Y. The gel fraction was measured using the obtained resin film X and resin film Y. In the following measurements, "resin film X and resin film Y" will be collectively referred to as "resin film". 0.2 g ± 0.01 g of resin film was weighed into a glass bottle. Next, 20 g of tetrahydrofuran (THF) was added to the glass bottle containing the resin film, and after sealing, the bottle was left to stand for 3 days in an environment of ambient temperature of 23°C and 50% RH, immersing the resin film in THF. The material obtained from this immersion is called the sample. Next, the sample was filtered using a 1000 mesh stainless steel wire mesh. Next, the wire mesh after filtering the sample was dried at 100°C for 3 hours. Next, the mass of the wire mesh after drying was measured. The gel fraction was then calculated using the following formula. In the following formula, A is the mass of the weighed resin film (in g), B is the mass of the wire mesh before filtering the sample (in g), and C is the mass of the wire mesh after filtering the sample (in g). Gel fraction of the resin film [in mass %] = (C - B) / A × 100

[0110] 2. Film-forming properties A 2 cm x 10 cm electrical steel sheet [product number: 30HX1600, manufactured by Nippon Steel Corporation] was prepared. The coating composition was applied to the surface of the prepared electrical steel sheet. The amount of coating composition applied was such that the film thickness after drying was 2 μm. Next, the applied coating composition was dried by heating it at 170°C for 90 seconds using a hot air circulation dryer to produce an electrical steel sheet with a coating layer. Next, the surface of the coating layer of the produced electrical steel sheet with a coating layer was observed using an optical microscope to check for the presence and degree of cishing and cracking. Based on the results of the check, an evaluation was performed according to the evaluation criteria below. The evaluation results are shown in Tables 1 to 4. In the evaluation criteria below, "A" and "B" are at a level that does not pose a practical problem, and "A" is the most preferable.

[0111] -Evaluation Criteria- A: No defects or cracks were observed. B: Defects and cracks were observed, but no defects or cracks were observed in the area inside 3 mm from the edge of the electrical steel sheet. C: Defects and cracks were observed in the area inside 3 mm from the edge of the electrical steel sheet.

[0112] 3. Shear Adhesion Strength in High-Temperature Environments Two 2cm x 10cm electrical steel sheets [product number: 30HX1600, manufactured by Nippon Steel Corporation] were prepared. A coating composition was applied to the surface of the two prepared electrical steel sheets. The amount of coating composition applied was such that the film thickness after drying was 2 μm. Next, the applied coating composition was dried by heating it at 170°C for 90 seconds using a hot air circulation dryer to produce two electrical steel sheets with a coating layer. Next, the two produced electrical steel sheets with a coating layer were arranged so that the overlapping area of ​​the coating layers was 2cm x 2cm, and thermocompression bonding (temperature: 230°C, time: 60 seconds, pressure: 3 MPa) was performed to obtain a laminate. The obtained laminate was used as an evaluation sample. Next, a tensile shear adhesion strength test was performed on the evaluation sample in an environment of 150°C. Tensile shear bond strength tests were conducted in accordance with JIS K 6850:1999. Based on the measured shear bond strength, evaluation was performed according to the evaluation criteria below. The measured values ​​and evaluation results are shown in Tables 1 to 4. If the measured shear bond strength was 0.80 MPa or higher, it was judged to be acceptable for practical use.

[0113]

[0114]

[0115]

[0116]

[0117] In Tables 1-4, for convenience, components corresponding to "(meth)acrylic resin (A)" are denoted as "Component (A)", components corresponding to "epoxy resin (B)" are denoted as "Component (B)", and components corresponding to "curing agent (C)" are denoted as "Component (C)". All blending amounts listed in Tables 1-4 are based on solid content. In Tables 1-4, "-" means that there is no corresponding component in that column.

[0118] Details of each component listed in Tables 1-4 are as follows: "n-BA": n-butyl acrylate (alkyl acrylate monomer) "MMA": methyl methacrylate (alkyl methacrylate monomer) "St": styrene (other monomer) "MAA": methacrylic acid (monomer with a carboxyl group) "AA": acrylic acid (monomer with a carboxyl group)

[0119] "Epoxy A": BEKOPOX VEP2381w / 55WA (product name), manufactured by Daicel Ornex Co., Ltd. "Epoxy B": ADEKA Resin EM-101-50 (product name), manufactured by ADEKA Corporation "Epoxy C": BEKOPOX EP2350w / 60WA (product name), manufactured by Daicel Ornex Co., Ltd. "Epoxy D": BEKOPOX EP2307w / 45WAMP (product name), manufactured by Daicel Ornex Co., Ltd. "Epoxy E": BEKOPOX EP2340w / 56WA (product name), manufactured by Daicel Ornex Co., Ltd. "Epoxy F": Yuka Resin NE-320 (product name), manufactured by Yoshimura Oil Chemical Co., Ltd. "Epoxy G": Yuka Resin NE-3012 (product name), manufactured by Yoshimura Oil Chemical Co., Ltd. "Hardening Agent A": Fujicure FXH-940 (product name; amine-based hardener), manufactured by T&K TOKA Corporation "Hardening Agent B": BECKOPOX EH616W / 67WA (product name; amine-based hardener), manufactured by Daicel Ornex Corporation "Hardening Agent C": Yucarezin HD-03 (product name; amine-based hardener), manufactured by Yoshimura Oil Chemical Co., Ltd. "Amine A": Triethylamine (product name), manufactured by Tokyo Chemical Industry Co., Ltd.

[0120] As shown in Tables 1-4, the coating films formed by the coating compositions of the examples were found to have higher shear adhesion under high-temperature conditions compared with the coating films formed by the coating compositions of the comparative examples. Furthermore, the coating compositions of the examples were found to have superior film-forming properties compared with the coating compositions of the comparative examples.

[0121] The disclosures of Japanese Patent Application No. 2025-056956, filed on 28 March 2025, and Japanese Patent Application No. 2026-005404, filed on 15 January 2026, are incorporated herein by reference in their entirety. All documents, patent applications, and technical standards described herein are incorporated by reference to the same extent as if each individual document, patent application, and technical standard were specifically and individually noted.

Claims

1. A coating composition for laminated steel sheets comprising: a (meth)acrylic resin (A) containing constituent units derived from monomers having carboxyl groups in a proportion of 6% to 18% by mass relative to the total constituent units, and having a glass transition temperature of 70°C to 100°C; an epoxy resin (B) having an epoxy equivalent of 400 g / eq to 800 g / eq; and a curing agent (C) having a functional group (c) that can react with carboxyl groups and epoxy groups, wherein the ratio of the mass content of the (meth)acrylic resin (A) to the mass content of the epoxy resin (B) is 76 / 24 to 93 / 7, and the ratio of the number of moles of the functional group (c) in the curing agent (C) to the total number of moles of carboxyl groups in the (meth)acrylic resin (A) and epoxy groups in the epoxy resin (B) is 0.39 to 1.

30.

2. The coating composition for laminated steel sheets according to claim 1, wherein the functional group (c) in the curing agent (C) contains at least one of a primary amino group and a secondary amino group.

3. The coating composition for laminated steel sheets according to claim 1, wherein the (meth)acrylic resin (A) comprises constituent units derived from an alkyl (meth)acrylate monomer.

4. The coating composition for laminated steel sheets according to claim 1, wherein the (meth)acrylic resin (A) comprises constituent units derived from monomers having a ring structure.

5. The coating composition for laminated steel sheets according to claim 4, wherein the constituent unit derived from the monomer having a ring structure is a constituent unit derived from styrene.

6. The coating composition for laminated steel sheets according to claim 1, wherein the shape of the (meth)acrylic resin (A) is particulate.

7. The coating composition for laminated steel sheets according to claim 1, wherein the epoxy resin (B) is in the form of particles.

8. The coating composition for laminated steel sheets according to claim 6 or claim 7, further comprising water.

9. The coating composition for laminated steel sheets according to claim 1, wherein the gel fraction after heating at 170°C for 90 seconds and then at 230°C for 60 seconds is 90% by mass or more.

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