Painted steel sheet, resin-metal joint, and method for manufacturing resin-metal joint

Abstract

To provide a coated steel plate which is excellent in water-resistant adhesion of a joined part to a resin composition, and scratch resistance of a coated film part, a resin-metal joined body obtained using the coated steel plate and a method for manufacturing the same.SOLUTION: There are provided a coated steel plate that has a steel plate and a coated film formed on the surface of the steel plate, wherein the coated film contains a resin containing either or both of a bisphenol type epoxy resin and a polyester resin, and a crosslinking agent containing either or both of a melamine curing agent and an isocyanate curing agent, in an IR spectrum by FT-IR of the coated film, an IR spectrum intensity ratio corresponding to an abundance ratio of a crosslinking agent having an unreacted group to a resin in the coated film is controlled within a predetermined range; and a resin-metal joined body obtained using the coated steel plate and a method for manufacturing the same.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a painted steel sheet suitable for bonding with a thermoplastic resin composition, a resin-metal joint, and a method for manufacturing a resin-metal joint. [Background technology]

[0002] Steel sheets are used in a variety of industrial products, including automobiles and electrical equipment. Steel sheets are sometimes used as resin-metal composites, and traditionally, these composites were manufactured by fitting the steel sheet and the resin composition together. However, this method involves many steps and has low productivity. Therefore, in recent years, the mainstream method for manufacturing resin-metal composites has become insert molding (injection molding), which joins the molded steel sheet and resin composition.

[0003] When manufacturing a resin-metal joint by insert molding, it is important to improve the adhesion between the steel plate and the molded resin composition. To further enhance adhesion through insert molding, it has been proposed to pre-form a predetermined coating on the surface of the steel plate, and for the polycarbonate unit-containing polyurethane resin in the coating to become compatible with the thermoplastic resin composition and bond firmly, thereby improving adhesion that cannot be achieved by anchor hardening alone, such as by roughening the surface of the steel plate (Patent Document 1). [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2013-226796 [Overview of the project] [Problems that the invention aims to solve]

[0005] In conventional resin-metal joints, adhesion can deteriorate when water vapor or moisture penetrates the joint interface, making it difficult to maintain adhesion over long periods. Furthermore, when using coatings such as those described in Patent Document 1, the mechanical strength of the coating in areas not bonded to the resin composition may be relatively low, and there is a need to improve the scratch resistance in those areas as well.

[0006] The present invention has been made in view of the above, and aims to provide a painted steel sheet that exhibits excellent water-resistant adhesion at the joint and scratch resistance of the painted film portion other than the joint when a resin composition is joined, as well as a resin-metal joint obtained using the painted steel sheet and a method for manufacturing the same. [Means for solving the problem]

[0007] The inventors of the present invention discovered that the above problems can be solved by forming a predetermined coating film on the surface of a steel plate, and after further investigation, completed the present invention.

[0008] In other words, the present invention relates to the following painted steel sheet having a predetermined coating film formed on the surface of the steel sheet, a resin-metal joint obtained using the same, and a method for manufacturing the same. (1) A painted steel sheet having a steel sheet and a coating film formed on the surface of the steel sheet, The coating film comprises a resin containing either or both a bisphenol-type epoxy resin and a polyester resin, and a crosslinking agent containing either or both a melamine curing agent and an isocyanate curing agent. A painted steel sheet characterized in that the IR spectrum of the aforementioned coating film measured by FT-IR has an IR spectral intensity ratio expressed by the following formula 1 of 12 to 30. ((I3+I4) / (I1+I2))×100...Equation 1 Here, I1 is 1600 (±10) cm -1 These are peak intensities originating from the -C=C- group of bisphenol-type epoxy resins. I2 is 1730 (±10) cm -1The peak intensity is derived from the ester group of the polyester resin. I3 is 800 (±10) cm -1 The peak intensity is derived from the amino group of the melamine curing agent. I4 is 1690 (±10) cm -1 The peak intensity in this sample originates from the -N=C=O isocyanate curing agent. (2) The painted steel sheet according to (1) above, characterized in that the resin contains a polyester resin. (3) The painted steel sheet according to (1) or (2) above, characterized in that the crosslinking agent contains both a melamine curing agent and an isocyanate curing agent. (4) The painted steel sheet according to any one of (1) to (3) above, characterized in that the coating film contains 5% by mass or less of carbon black. (5) Painted steel sheet as described in any one of the above items (1) to (4), A thermoplastic resin composition bonded onto the painted steel plate and A resin-metal composite comprising the crosslinking agent and having a covalent bond between the crosslinking agent and the thermoplastic resin composition. (6) The resin-metal joint according to (5) above, characterized in that the thermoplastic resin composition is a polyester elastomer. (7) The process of preparing the painted steel sheet described in any one of the above items (1) to (4), The process of inserting the painted steel sheet into an injection molding die, The process involves injecting a thermoplastic resin composition into the injection molding die to bond a molded body of the thermoplastic resin composition to the surface of the painted steel sheet. A method for manufacturing a resin-metal composite, characterized by including the following: [Effects of the Invention]

[0009] According to the present invention, it is possible to provide a painted steel sheet that exhibits excellent water-resistant adhesion at the joint and scratch resistance of the painted film portion other than the joint when resin compositions are joined, as well as a resin-metal joint obtained using the painted steel sheet and a method for manufacturing the same. [Modes for carrying out the invention]

[0010] Hereinafter, an embodiment of the present invention will be described.

[0011] [Painted Steel Sheet] The painted steel sheet according to the embodiment of the present invention has a steel sheet and a coating film formed on the surface of the steel sheet. Further, a chemical conversion film may be formed between the steel sheet and the coating film. Hereinafter, each element of the painted steel sheet will be described.

[0012] [Steel Sheet] The type of the steel sheet serving as the painting base material is not particularly limited. Examples of the steel sheet include cold-rolled steel sheets, galvanized steel sheets, Zn-Al alloy plated steel sheets, Zn-Al-Mg alloy plated steel sheets, Zn-Al-Mg-Si alloy plated steel sheets, aluminum plated steel sheets, stainless steel sheets (including austenitic, martensitic, ferritic, and ferritic-martensitic two-phase types), and the like. The steel sheet may be subjected to known pre-painting treatments such as degreasing and pickling as necessary. The thickness of the steel sheet is not particularly limited, but for the purpose of weight reduction of the final product, it is preferably, for example, 0.3 to 3.2 mm, and may be 0.5 mm or more, 1.0 mm or more, and / or 2.5 mm or less, 2.0 mm or less, or 1.6 mm or less.

[0013] [Chemical Conversion Film] As described above, a chemical conversion film may be formed between the steel sheet and the coating film of the painted steel sheet. The chemical conversion film is formed on the surface of the steel sheet and improves the adhesion of the coating film to the steel sheet and the corrosion resistance of the steel sheet. The chemical conversion film is usually formed on the entire surface of the steel sheet.

[0014] The type of the chemical conversion treatment for forming the chemical conversion film is not particularly limited. Examples of the chemical conversion treatment include chromate treatment, chromium-free treatment, phosphate treatment, and the like. The adhesion amount of the chemical conversion film formed by the chemical conversion treatment is not particularly limited as long as it is within a range effective for improving the coating film adhesion and corrosion resistance. For example, in the case of a chromate film or a chromium-free film, 10 to 500 mg / m 2The amount of coating applied should be adjusted to stay within this range. Also, in the case of phosphate coatings, the application rate is 0.1-5 g / m². 2 You can adjust the amount of adhesive to achieve this result.

[0015] [coating film] The coating film contains a resin which includes either or both of a bisphenol-type epoxy resin and a polyester resin, or either or both of these resins, and a crosslinking agent which includes either or both of a melamine curing agent and an isocyanate curing agent, or either or both of these resins. By including a required amount of crosslinking agent with unreacted groups, the water-resistant adhesion of the molded article of the thermoplastic resin composition to the steel sheet is improved. The coating film may further contain optional additives other than the bisphenol-type epoxy resin, polyester resin, melamine curing agent, and isocyanate curing agent, such as organic components. The coating film, like a chemical conversion coating, is usually formed over the entire surface of the steel sheet.

[0016] Bisphenol-type epoxy resins have bisphenol-type epoxy units in their molecular chains, and polyester resins have polyester units in their molecular chains. In resin-metal joints, bonding strength is improved by intermolecular forces between the bisphenol-type epoxy units or polyester units in the resin and the thermoplastic resin composition. The total content of bisphenol-type epoxy resin and polyester resin is not particularly limited, but may be, for example, 50% or more by mass, 60% or more by mass, 70% or more by mass, or 80% or more by mass relative to the solid content of the coating film, and / or 95% or less by mass or 90% or less by mass. The solid content of the coating film refers to all components excluding the solvent used during coating film preparation.

[0017] The melamine curing agent and / or isocyanate curing agent, used as a crosslinking agent, partially undergoes a crosslinking reaction with the bisphenol-type epoxy resin and / or polyester resin during coating formation, thereby forming a coating with excellent scratch resistance. The crosslinking agent with unreacted groups that did not react with the resin during coating formation, i.e., the melamine curing agent and / or isocyanate curing agent, undergoes a crosslinking reaction with the thermoplastic resin composition during bonding of the painted steel sheet and the thermoplastic resin composition, thereby improving the water-resistant adhesion of the joint after bonding. The crosslinking agent used for coating formation has multiple reactive groups in a single molecule. It is presumed that the greatest improvement in water-resistant adhesion is achieved when some of the reactive groups crosslink with the resin during coating formation, and the remaining unreacted groups undergo a crosslinking reaction during bonding with the thermoplastic resin composition. In embodiments of the present invention, as described above, by appropriately providing a crosslinking agent having unreacted groups when bonding with the thermoplastic resin composition, the crosslinking agent can be sufficiently activated both during film formation and during bonding with the thermoplastic resin composition. As a result, both the scratch resistance of the coating film and the water-resistant adhesion at the joint with the thermoplastic resin composition can be significantly improved.

[0018] The ratio of bisphenol-type epoxy resin and polyester resin (either one or both) to crosslinking agents (either melamine curing agent and isocyanate curing agent) that have unreacted groups that did not react with these resins during coating formation, in the coating film formed on the steel plate, is controlled so that the ratio of IR spectral intensity obtained by FT-IR (Fourier transform infrared spectroscopy) of the coating film is 12 to 30 for every 100 parts of resin, more specifically, for every 100 parts of either the bisphenol-type epoxy resin and the polyester resin. If the ratio of crosslinking agents with unreacted groups to the above resins in the coating film is too low, sufficient water-resistant adhesion at the joints cannot be obtained. In particular, if the ratio of crosslinking agents with unreacted groups is low due to a small amount of crosslinking agent blended, the crosslinking density of the coating film may also be insufficient, and in this case, the scratch resistance of the coating film will also decrease. Conversely, if the ratio of crosslinking agents with unreacted groups to the resins is too high, water-resistant adhesion will also decrease. Although the reason is not clear, it is presumed that the water-resistant adhesion between the coating film and the thermoplastic resin composition is obtained by the combined action of the intermolecular forces between the resin in the coating film and the thermoplastic resin composition, and the covalent bonds formed by the crosslinking reaction between the crosslinking agent with unreacted groups remaining in the coating film and the thermoplastic resin composition. There is a limit to the crosslinking points between the crosslinking agent and the thermoplastic resin composition, and covalent bonds cannot be obtained beyond a certain point. Furthermore, since the intermolecular forces due to the entanglement of molecular chains are weaker in the crosslinking agent than in the resin in the coating film, it is thought that if the proportion of crosslinking agent becomes too high, the action by covalent bonding will saturate, while the intermolecular forces between the resin in the coating film and the thermoplastic resin composition will decrease, making it impossible to obtain the desired water-resistant adhesion. In particular, if the proportion of crosslinking agent with unreacted groups is high due to a large amount of crosslinking agent, the crosslinking density of the coating film may become too high. In this case, the coating film may become hard and brittle, and properties such as the adhesion to bent parts of the coating film may also decrease.From the above viewpoints, the IR spectrum intensity ratio, which is an index of the abundance ratio of the crosslinking agent having unreacted groups, is preferably 15 to 28 of the crosslinking agent having unreacted groups with respect to 100 of the resin in the coating film, more preferably 16 to 25 or 16 to 24 of the crosslinking agent having unreacted groups, and most preferably 18 to 22 or 18 to 21 of the crosslinking agent having unreacted groups.

[0019] The IR spectrum intensity ratio is represented by the following formula 1 using the peak intensities I1 to I4 of peaks P1 to P4 in each obtained frequency band by measuring the coating film surface of the coated steel sheet by FT-IR (Fourier transform infrared spectroscopy), and means the abundance ratio of the crosslinking agent having unreacted groups with respect to the molecular chains of either one or both of the resins, more specifically, a bisphenol type epoxy resin and a polyester resin, in the coating film. P1: 1600 (±10) cm -1 Peak derived from -C=C- of bisphenol type epoxy resin at P2: 1730 (±10) cm -1 Peak derived from the ester group of the polyester resin at P3: 800 (±10) cm -1 Peak derived from the amino group of the melamine curing agent at P4: 1690 (±10) cm -1 Peak derived from -N=C=O of the isocyanate curing agent at ((I3 + I4) / (I1 + I2)) × 100 ··· Formula 1

[0020] The bisphenol-type epoxy resin is a resin having a bisphenol-type epoxy skeleton, and may be, for example, a bisphenol A-type epoxy resin, a bisphenol F-type epoxy resin, etc. The number average molecular weight of the bisphenol-type epoxy resin is not particularly limited, but from the viewpoint of film elongation, it is preferably 200 to 2500, which is commonly used for paints. When the number average molecular weight of the bisphenol-type epoxy resin is 200 or more, a coating film with good elongation is easily obtained, and the corrosion resistance of the processed part of the coated steel sheet is also easily improved. If the number average molecular weight of the bisphenol-type epoxy resin is too high, the viscosity of the raw material paint may become too high, impairing the workability of the coating. In view of the above, the number average molecular weight of the bisphenol-type epoxy resin is preferably 200 to 2500, and more preferably 400 to 2000. By incorporating a large amount of crosslinking agent at the crosslinking points of the bisphenol-type epoxy resin, the crosslinking agent with unreacted groups can be left in the coating film, and the remaining crosslinking agent with unreacted groups can contribute to bonding with the subsequent thermoplastic resin composition. The crosslinking points of the bisphenol-type epoxy resin can be calculated from the number-average molecular weight and epoxy equivalent of the resin, and it is preferable to blend 1.1 to 1.2 times the amount of crosslinking agent for each calculated crosslinking point. The epoxy equivalent is not particularly limited, but generally, for bisphenol-type epoxy resins, the epoxy equivalent is 180 to 3300.

[0021] The polyester resin is a resin having a polyester skeleton, and from the viewpoint of improving the heat resistance of the resulting coating film, it is preferable that it also has an aromatic skeleton. The number average molecular weight of the polyester resin is not particularly limited, but from the viewpoint of coating film elongation, it is preferable that it is 3,000 to 18,000, which is commonly used for paints. When the number average molecular weight of the polyester resin is 3,000 or more, a coating film with good elongation is easily obtained, and the corrosion resistance of the processed part of the coated steel sheet is also easily improved. If the number average molecular weight of the polyester resin is too high, the viscosity of the raw material paint may become too high, impairing the workability of the coating. In view of the above, the number average molecular weight of the polyester resin is preferably 3,000 to 18,000, and more preferably 5,000 to 15,000. By incorporating a large amount of crosslinking agent at the crosslinking points of the polyester resin, the crosslinking agent with unreacted groups can remain in the coating film, and the remaining crosslinking agent with unreacted groups can contribute to bonding with the subsequent thermoplastic resin composition. The crosslinking points of the polyester resin can be calculated from the number-average molecular weight and hydroxyl value of the resin, and it is preferable to blend 1.1 to 1.2 times the amount of crosslinking agent for each calculated crosslinking point. The hydroxyl value is not particularly limited, but is generally 10 to 80 mgKOH / g for polyester resins.

[0022] The type of melamine curing agent is not particularly limited, but alkoxy melamine resins can be used as general melamine curing agents. Specifically, examples include methylated melamine resins, ethylated melamine resins, n-butylated melamine resins, and isobutylated melamine resins, which use methoxy, ethoxy, n-butoxy, isobutoxy, etc., as alkoxy groups.

[0023] The aforementioned isocyanate curing agent is preferably a blocked polyisocyanate compound due to the storage stability of the raw material paint. As the blocked polyisocyanate compound, it is preferable to use aliphatic polyfunctional isocyanates such as non-yellowing hexamethylene diisocyanate (HMDI), alicyclic aliphatic polyfunctional isocyanates such as isophorone diisocyanate (IPDI), and polyisocyanate compounds such as diphenylmethane-4,4'-diisocyanate (MDI) and hydrogenated MDI in which the functional groups are partially or completely blocked.

[0024] Examples of blocking agents for the blocked polyisocyanate compound include oxime-based blocking agents such as acetone oxime, cyclohexanone oxime, acetophenone oxime, and benzophenone oxime, as well as blocking agents such as methyl acetoacetate, ethyl acetoacetate, and methanol.

[0025] The melamine curing agent and the isocyanate curing agent can be used individually as crosslinking agents, or multiple different types of melamine curing agents can be used in combination. In particular, by combining two or more crosslinking agents with different crosslinking temperatures, it is possible to leave behind crosslinking agents with unreacted groups when bonding with the thermoplastic resin composition. In this case, it is preferable that the crosslinking temperature at the time of film formation of the coating film and the crosslinking temperature with the remaining crosslinking agent when bonding with the thermoplastic resin composition do not overlap. Specifically, it is preferable that the crosslinking temperature at the time of film formation of the coating film is 180 to 220°C, and the crosslinking temperature with the remaining crosslinking agent when bonding with the thermoplastic resin composition is 220 to 350°C.

[0026] The aforementioned coating film may contain other additives as needed, for example, any amount of either or both of a rust-preventive pigment or a coloring pigment may be included. The rust-preventive pigment is added to impart corrosion resistance to the coating film. The amount added is preferably 1 to 29% by mass of the solid content of the coating film. Examples of rust-preventive pigments include chromate-based pigments such as strontium chromate and zinc chromate, and chromium-free pigments such as phosphomolybdic acid and phosphovanadic acid. The rust-preventive pigment may also be calcium ion-exchange silica, etc. Any type of coloring pigment can be used as long as it achieves the purpose of imparting color. For example, the coloring pigment may contain or may be carbon black, etc. The content of the coloring pigment or carbon black may be 1% by mass or more, 1.5% by mass or more, or 2% by mass or more, and / or 10% by mass or less, 8% by mass or less, or 5% by mass or less, based on the solid content of the coating film.

[0027] In the painted steel sheet according to the embodiment of the present invention, a portion of the crosslinking agent undergoes a crosslinking reaction with the resin during coating film formation, thereby achieving excellent scratch resistance even in the coated portion other than the joint with the thermoplastic resin composition molded body. Therefore, the thickness of the coating film is not particularly limited, but from the viewpoint of further improving the scratch resistance of the coating film other than the joint with the thermoplastic resin composition molded body, the thickness of the coating film is preferably 2 μm or more. On the other hand, the upper limit of the coating film thickness is not particularly limited in terms of adhesive strength and coating film scratch resistance, but if the film thickness becomes too thick, not only will the coating cost per steel sheet area increase, but the baking and drying time after resin liquid application will also increase. In particular, in the continuous manufacturing process of a continuous coating line, the line speed will decrease, productivity will decrease, and as a result, manufacturing costs will increase. Therefore, a coating film thickness of 25 μm or less is preferable. From the perspective of both coating film scratch resistance and manufacturing cost, a more desirable resin coating film adhesion amount is preferably in the range of 3 to 20 μm or 3 to 15 μm.

[0028] [Painting Method] Any conventional method can be used to form a coating on a steel sheet. From the viewpoint of productivity, methods applicable to coil or sheet-shaped steel sheets, such as bar coaters, roll coaters, and curtain flow coaters, are particularly preferred. A coating can be applied to the steel sheet using a bar coater or the like, using a paint prepared by mixing and dispersing a resin, crosslinking agent, and other optional additives with any suitable solvent.

[0029] The baking conditions during coating film formation must be appropriately controlled to ensure that the amount of crosslinking agent with unreacted groups in the coating film is in the predetermined ratio. The baking temperature during coating film formation must be set within a predetermined temperature range in accordance with the crosslinking agent. If the temperature falls below a certain level, the crosslinking reaction in the coating film will not occur, resulting in insufficient mechanical properties such as coating film hardness. On the other hand, if the temperature exceeds the predetermined level, the crosslinking agent will undergo not only a crosslinking reaction with the resin in the coating film but also a self-condensation reaction, which consumes the crosslinking agent used when bonding with the thermoplastic resin composition, resulting in insufficient water-resistant adhesion at the joint between the coating film and the thermoplastic resin composition. Furthermore, the baking time during coating film formation must also be set within a predetermined range. If the baking time is too short, the crosslinking reaction in the coating film will not occur sufficiently. On the other hand, if the baking time is too long, a self-condensation reaction of the crosslinking agent will occur, resulting in insufficient water-resistant adhesion at the joint between the coating film and the thermoplastic resin composition. When different types of crosslinking agents are used to differentiate the functions of crosslinking during coating film formation and crosslinking with the thermoplastic resin composition during bonding, the baking temperature during coating film formation must not exceed the temperature at which bonding occurs with the thermoplastic resin composition. If the baking temperature during coating film formation exceeds the bonding temperature, the crosslinking agent used during bonding with the thermoplastic resin composition will be consumed during coating film formation, and sufficient water-resistant adhesion cannot be obtained at the joint between the coating film and the thermoplastic resin composition.

[0030] Specifically, the curing temperature must be controlled so that the maximum achievable plate temperature is 180-230°C. If the maximum achievable plate temperature is below 180°C, insufficient crosslinking, i.e., insufficient crosslinking density, results in poor scratch resistance. In addition, due to insufficient crosslinking, the coating film does not harden sufficiently, causing the IR spectral intensity ratio represented by Equation 1 to exceed 30, which is high. This can lead to a decrease in the scratch resistance of the coating film, as well as a decrease in properties such as adhesion to bent parts of the coating film, and even a decrease in water-resistant adhesion at the joint with the thermoplastic resin composition. On the other hand, if the maximum achievable plate temperature exceeds 230°C, over-curing causes the crosslinking agent present in the coating film to be consumed by self-condensation reactions, and insufficient crosslinking agent remains to be used in the reaction with the thermoplastic resin composition, resulting in an inability to obtain the desired water-resistant adhesion. From the above viewpoint, a maximum achievable plate temperature of 190-220°C is preferable.

[0031] The curing time for the coating film must also be within the range of approximately 20 to 60 seconds. If the curing time is too short, crosslinking will be insufficient, resulting in poor scratch resistance. In addition, if the coating film does not harden sufficiently due to such insufficient crosslinking, the IR spectral intensity ratio represented by Equation 1 will exceed 30, and in addition to a decrease in the scratch resistance of the coating film, it may also lead to a decrease in properties such as the adhesion of the coating film to the bent part, and furthermore, a decrease in the water-resistant adhesion of the joint with the thermoplastic resin composition. On the other hand, if the curing time is too long, the crosslinking agent present in the coating film will be consumed in self-condensation reactions, and insufficient crosslinking agent will remain to be used in the reaction with the thermoplastic resin composition, so the desired water-resistant adhesion cannot be obtained. From the above viewpoint, a curing time of 20 to 40 seconds is preferable. If multiple crosslinking agents with different crosslinking temperatures, selected from either the melamine curing agent or the isocyanate curing agent, or both, are used, and two or more types of crosslinking agents are blended so that the crosslinking temperature during film formation of the coating film does not overlap with the crosslinking temperature with the thermoplastic resin composition, the baking temperature during film formation must not reach the crosslinking temperature with the thermoplastic resin composition. If all the crosslinking agents are consumed in the self-condensation reaction during film formation of the coating film, and no crosslinking agents with unreacted groups remain in the coating film, the crosslinking reaction with the thermoplastic resin composition does not occur, and the desired water-resistant adhesion cannot be obtained.

[0032] [Resin metal joint] As described above, the painted steel sheet has a sufficient amount of crosslinking agent with unreacted groups remaining in the coating film. Therefore, when it is joined to a thermoplastic resin composition to produce a resin-metal joint, the water-resistant adhesion of the joint can be significantly improved by the covalent bonds formed by the crosslinking reaction between the crosslinking agent with unreacted groups and the thermoplastic resin composition, in addition to the intermolecular forces between the resin in the coating film and the thermoplastic resin composition. More specifically, the covalent bonds formed at the interface between the painted steel sheet and the thermoplastic resin composition. The type of thermoplastic resin that constitutes the molded article of the thermoplastic resin composition used in the resin-metal joint is not particularly limited. Examples of thermoplastic resins include acrylonitrile-butadiene-styrene (ABS) resins, polyethylene terephthalate (PET) resins, polybutylene terephthalate (PBT) resins, polycarbonate (PC) resins, polyamide (PA) resins, polyphenylene sulfide (PPS) resins, polyester elastomers, or combinations thereof.

[0033] When the resin in the coating on the steel plate is a polyester resin, the thermoplastic resin is preferably a polyester elastomer. The polyester units of the resin in the coating and the polyester units in the thermoplastic resin composition are compatible when joined, and the bonding strength can be improved by the entanglement of molecular chains.

[0034] The thermoplastic resin composition may contain inorganic fillers, thermoplastic resin fillers, etc., from the viewpoint of molding shrinkage rate, material strength, mechanical strength, scratch resistance, etc.

[0035] Inorganic fillers improve the rigidity of molded articles made from thermoplastic resin compositions. The type of inorganic filler is not particularly limited, and known substances can be used. Examples of inorganic fillers include fibrous fillers such as glass fibers, carbon fibers, and aramid resins; powder fillers such as carbon black, calcium carbonate, calcium silicate, magnesium carbonate, silica, talc, glass, clay, lignin, mica, quartz powder, and glass spheres; and pulverized carbon fibers and aramid fibers. The amount of inorganic filler added is not particularly limited, but is preferably in the range of 5 to 50% by mass. Inorganic fillers may be used alone or in combination of two or more types.

[0036] Thermoplastic resin fillers improve the impact resistance of molded articles of thermoplastic resin compositions. The type of thermoplastic resin filler is not particularly limited. Examples of thermoplastic resin fillers include at least one granular additive selected from the group consisting of acrylonitrile-butadiene-styrene resins, polyethylene terephthalate resins, polybutylene terephthalate resins, polycarbonate resins, polystyrene resins, polyphenylene ether resins, and polyolefin resins. Thermoplastic resin fillers may be used alone or in combination of two or more types.

[0037] [How to check for covalent bonds] The covalent bond between the crosslinking agent and the thermoplastic resin composition is confirmed by the Soxhlet extraction method. By extracting the resin-metal composite using a solvent that dissolves the thermoplastic resin composition, if a covalent bond exists, the thermoplastic resin composition remains on the surface of the coating. On the other hand, if a covalent bond does not exist, the thermoplastic resin composition dissolves completely, and no thermoplastic resin composition remains on the coating.

[0038] [Manufacturing method for resin-metal composites] The aforementioned resin-metal joint can be manufactured, for example, by: 1) a first step of preparing the painted steel sheet described above; 2) a second step of inserting the painted steel sheet into an injection molding die; and 3) a third step of injecting a thermoplastic resin composition into the injection molding die to bond a molded body of the thermoplastic resin composition to the surface of the painted steel sheet. Each step will be described below.

[0039] (1) First step In the first step, the painted steel sheet is prepared according to the procedure described above.

[0040] (2)Second process In the second step, the painted steel sheet prepared in the first step is inserted into the injection molding die. The painted steel sheet may be processed into the desired shape by press working or other methods.

[0041] (3) Third step In the third step, a high-temperature thermoplastic resin composition is injected under high pressure into the injection mold into which the painted steel sheet was inserted in the second step. At this time, it is preferable to provide a gas vent in the injection mold to allow the thermoplastic resin composition to flow smoothly. The high-temperature thermoplastic resin composition comes into contact with the coating film formed on the surface of the painted steel sheet. The temperature of the injection mold is preferably near the melting point of the thermoplastic resin composition. The injection temperature of the thermoplastic resin composition must be above 230°C. If it is not above the film formation temperature of the coating film, the reaction with the crosslinking agent having remaining unreacted groups will not proceed easily, the crosslinking reaction between the thermoplastic resin composition and the coating film will not occur, and covalent bonds will not be formed, so sufficient water-resistant adhesion cannot be obtained at the joint between the thermoplastic resin composition and the coating film. After injecting the thermoplastic resin composition, there is no particular limit on the bonding time with the steel sheet, but 5 seconds or more is preferable, and 10 seconds or more is more preferable. While this is related to the crosslinking agent and bonding temperature, if the bond is extremely short, the crosslinking reaction between the residual crosslinking agent in the coating film and the thermoplastic resin composition may not occur, potentially resulting in insufficient water-resistant adhesion.

[0042] After injection molding is complete, the mold is opened and demolded to obtain a resin-metal joint. The resin-metal joint obtained by injection molding may be annealed after molding to eliminate internal distortion caused by molding shrinkage.

[0043] By following the above procedure, a resin-metal joint according to an embodiment of the present invention can be manufactured.

[0044] As described above, a resin-metal joint can be manufactured by bonding a molded body of a thermoplastic resin composition to the surface of a painted steel sheet according to an embodiment of the present invention. In the painted steel sheet according to an embodiment of the present invention, a predetermined coating film is formed on the joint portion, which has excellent adhesion to both the steel sheet and the thermoplastic resin composition, and on the portion other than the joint portion, which has excellent scratch resistance.

[0045] The present invention will be described in detail below with reference to examples, but the present invention is not limited to these examples. [Examples]

[0046] In this example, painted steel sheets were prepared, and the proportion of crosslinking agent containing unreacted groups in the coating formed on the surface of the painted steel sheets was measured. The scratch resistance and adhesion of the coating to the bent portion were also investigated. Furthermore, a resin-metal joint was manufactured using the prepared painted steel sheets, and the water-resistant adhesion of the joint was investigated.

[0047] [Manufacturing of painted steel sheets] As a base material for painted steel sheets, the amount of plating deposited per side of the sheet is 45 g / m². 2 Hot-dip Zn-11 mass%Al-3 mass%Mg alloy plated steel sheets and hot-dip Zn plated steel sheets were prepared. Cold-rolled steel sheets (SPCC) with a thickness of 0.5 mm were used as the base material for the plated steel sheets.

[0048] First, the cold-rolled steel sheet is degreased, and then a chromium-free chemical conversion treatment consisting of tannic acid (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.), silane coupling agent (KBE-403, manufactured by Shin-Etsu Silicone Co., Ltd.), silica microparticles (Snowtex O, manufactured by Nissan Chemical Corporation), and polyester resin (Vylonal MD1480, manufactured by Toyobo Co., Ltd.) is applied. After drying, the amount of treatment coating per unit area is 100 mg / m². 2 The material was then coated with a bar coat, baked and dried in a 60°C drying oven, and subjected to a pre-painting chemical treatment.

[0049] Next, paints were prepared by kneading and dispersing resins (polyester resin and / or bisphenol-type epoxy resin), crosslinking agents (melamine curing agent and / or isocyanate curing agent), other additives, and solvents according to the paint formulations shown in Table 1. Byron GK-810 (manufactured by Toyobo Co., Ltd.) was used as the polyester resin, EPICLON 850 (bisphenol A type epoxy resin, manufactured by DIC Corporation) as the bisphenol-type epoxy resin, CYMEL303LF (methylated melamine resin, manufactured by Ornex Japan Co., Ltd.) and Uban 20SB (n-butylated melamine resin, manufactured by Mitsui Chemicals, Inc.) as the melamine curing agents, and Coronate BI-301 (manufactured by Tosoh Corporation) and MF-B60B (manufactured by Asahi Kasei Corporation) as blocked polyisocyanate compounds were used as the isocyanate curing agents.

[0050] [Table 1]

[0051] The coated substrate, which had undergone the aforementioned chromium-free chemical conversion treatment, was coated with a bar coat under the conditions shown in Table 2, and then baked in a hot air dryer set to 300°C for an arbitrary time to reach a predetermined steel sheet temperature. Immediately afterward, it was water-coated to produce a coated steel sheet.

[0052] [Measurement of the proportion of crosslinking agents containing unreacted groups in the coating film] The surface of the painted steel sheet obtained by the above preparation was subjected to infrared absorption spectroscopy using a Fourier transform infrared spectrometer (micro-IR, Agilent Technologies 610FTIR microscope) to measure its infrared absorption spectrum. Using the peak intensities I1 to I4 of the peaks P1 to P4 detected in the following wavelength band, the ratio of crosslinking agent with unreacted groups to the resin in the coating film (IR spectral intensity ratio) was calculated using Equation 1 below. P1: 1600 (±10) cm -1 Peaks originating from -C=C- in bisphenol-type epoxy resins. P2: 1730 (±10) cm -1 Peak originating from the ester group of the polyester resin. P3: 800 (±10) cm -1 Peak derived from the amino group of the melamine curing agent. P4: 1690 (±10) cm -1 The peak originates from the -N=C=O of the isocyanate curing agent. ((I3+I4) / ((I1+I2))×100...Equation 1

[0053] [Scratch resistance of the coating] As described above, a 10-yen coin with no serrations was placed perpendicularly against the painted steel sheet and applied pressure equivalent to 1 kg. The surface of the painted sheet was then slid horizontally, and the degree of peeling of the painted sheet was observed to determine the scratch resistance of the painted sheet. Sheets with no abnormalities were judged as passing (○), and those with scratches or peeling of the painted sheet were judged as failing (×).

[0054] [Adhesion of the coating film to the bent portion] The test specimen was bent 180° with the paint film of the aforementioned painted steel sheet facing outwards, and a steel sheet of the same thickness as the test specimen was placed between them. Adhesive tape was attached to the apex of the bend in the painted steel sheet, and the degree of delamination after quickly peeling off the tape was evaluated. A specimen was judged as passing (○) if no delamination of the paint film occurred at all, and failing (×) if any delamination of the paint film was observed.

[0055] [Thermoplastic resin composition] As thermoplastic resin compositions, we prepared polyester elastomer (Hytrel SB654, melting temperature 160°C, manufactured by Toray DuPont Co., Ltd.), polyethylene terephthalate (Novaduran 5710F40, melting temperature 240°C, manufactured by Mitsubishi Engineering Plastics Co., Ltd.), and polyphenylene sulfide (1130MF1, melting temperature 310°C, manufactured by Polyplastics Co., Ltd.).

[0056] [Fabrication of resin-metal composites] A painted steel sheet was inserted into an injection molding die, and a molten thermoplastic resin composition was injected into the injection molding die. The volume of the portion of the injection molding die into which the thermoplastic resin composition was introduced was 30 mm wide x 100 mm long x 4 mm thick, and the paint film and the thermoplastic resin composition were in close contact in a region of 30 mm wide x 30 mm long. The thermoplastic resin composition was heated to a temperature at which it could be injected with sufficient fluidity and would not decompose thermally (molten temperature of the thermoplastic resin + 20 to 80°C), injected into the injection molding die, held at the injection temperature for a predetermined time, then the die was opened and air-cooled to solidify the thermoplastic resin composition, thereby obtaining a resin-metal joint between the painted steel sheet and the thermoplastic resin.

[0057] [Evaluation of water-resistant adhesion of resin-metal joints] The resin-metal joint was immersed in pure water at room temperature for 24 hours. The joint, consisting of a painted steel plate and a thermoplastic resin composition, was then pulled in the same plane at a speed of 100 mm / min, and the strength at which it broke was measured as the joint strength. A joint strength of 1.0 MPa or higher was considered a pass (○), and a joint strength of less than 1.0 MPa was considered a fail (×).

[0058] [Presence or absence of covalent bond] The covalent bond between the crosslinking agent and the thermoplastic resin composition was confirmed by the Soxhlet extraction method. The resin-metal composite was extracted using a predetermined solvent in which the thermoplastic resin composition dissolves. If the thermoplastic resin composition remained on the surface of the coating film, it was determined that a covalent bond existed between the crosslinking agent and the thermoplastic resin composition. On the other hand, if the thermoplastic resin composition completely dissolved and no thermoplastic resin composition remained on the coating film, it was determined that a covalent bond did not exist between the crosslinking agent and the thermoplastic resin composition.

[0059] Table 2 shows the measurement and evaluation results for the manufacturing conditions of the painted steel sheet, the manufacturing conditions of the resin-metal joint, the ratio of crosslinking agent with unreacted groups relative to the resin in the coating (i.e., the IR spectral intensity ratio represented by Equation 1), the scratch resistance of the coating, the adhesion of the coating to the bent parts, and the water-resistant adhesion of the resin-metal joint.

[0060] [Table 2]

[0061] Referring to Table 2, Comparative Examples 36-38 had low IR spectral intensity ratios represented by Equation 1, resulting in unsatisfactory scratch resistance of the coating film and poor water-resistant adhesion at the joint with the thermoplastic resin composition. This is thought to be due to a low amount of crosslinking agent in the paint and insufficient crosslinking density in the coating film. Related to this, the proportion of crosslinking agent with unreacted groups was also low, and it is thought that a sufficient crosslinking reaction did not occur when the coated steel sheet and the thermoplastic resin composition were joined. Comparative Examples 39-41 had high IR spectral intensity ratios represented by Equation 1, resulting in unsatisfactory adhesion of the coating film to the bent section and poor water-resistant adhesion at the joint with the thermoplastic resin composition. This is thought to be due to an excessive amount of crosslinking agent in the paint, which made the coating film hard and brittle, reducing adhesion to the bent section. Furthermore, it is thought that the covalent bonding action due to the high proportion of crosslinking agent with unreacted groups became saturated, while the intermolecular forces due to entanglement of molecular chains decreased due to the low resin ratio of the coating film, resulting in poor water-resistant adhesion. Comparative Examples 42-44 had high IR spectral intensity ratios represented by Formula 1, and failed to meet all performance standards for scratch resistance of the coating, adhesion to bent sections, and water-resistant adhesion of the joint with the thermoplastic resin composition. This is thought to be due to insufficient curing of the coating because the maximum temperature reached on the steel sheet during coating baking was low, and the baking time was also short. Comparative Examples 45-47 had low IR spectral intensity ratios represented by Formula 1, and failed to meet the water-resistant adhesion of the joint between the painted steel sheet and the thermoplastic resin composition. This is thought to be because the maximum temperature reached on the steel sheet during coating baking was too high, causing the self-condensation reaction of the crosslinking agent to proceed, resulting in an insufficient amount of crosslinking agent used for covalent bonding with the thermoplastic resin composition. In addition, although not shown in Table 2, Soxhlet extraction confirmed the presence of covalent bonding between the crosslinking agent and the thermoplastic resin composition in all of the inventive examples in Table 2, while the presence of such covalent bonding was not confirmed in Comparative Examples 36, 37, 38, 45, 46, and 47. [Industrial applicability]

[0062] The painted steel sheet and the resin-metal joint of the painted steel sheet and the thermoplastic resin composition according to the embodiment of the present invention are useful in areas where long-term adhesion is required and molding processes such as bending and flexing are necessary, because the paint film of the painted steel sheet has excellent scratch resistance and water-resistant adhesion with the thermoplastic resin. For example, they are useful in inverter cases and ECU (engine control unit) cases of automobiles, and precision electronic component cases of electrical products.

Claims

1. A painted steel sheet having a steel sheet and a coating film formed on the surface of the steel sheet, The coating film comprises a resin containing either or both a bisphenol-type epoxy resin and a polyester resin, and a crosslinking agent containing either or both a melamine curing agent and an isocyanate curing agent. The bisphenol-type epoxy resin is a bisphenol A-type epoxy resin, the polyester resin has a number average molecular weight of 5,000 to 15,000 and a hydroxyl value of 10 to 80 mg KOH / g, the melamine curing agent is an alkoxymelamine resin, and the isocyanate curing agent is a blocked polyisocyanate compound. A painted steel sheet characterized in that the IR spectrum of the coating film measured by FT-IR has an IR spectral intensity ratio represented by the following formula 1 of 12 to 30. ((I3+I4) / (I1+I2))×100...Formula 1 Here, I1 is 1600 (±10) cm -1 The peak intensity originates from the -C=C- junction of the bisphenol-type epoxy resin. I2 is 1730 (±10) cm -1 The peak intensity is derived from the ester group of the polyester resin. I3 is 800 (±10) cm -1 The peak intensity is derived from the amino group of the melamine curing agent. I4 is 1690 (±10) cm -1 This peak intensity originates from the -N=C=O isocyanate curing agent.

2. The painted steel sheet according to claim 1, characterized in that the resin contains a polyester resin.

3. The painted steel sheet according to claim 1 or 2, characterized in that the crosslinking agent includes both a melamine curing agent and an isocyanate curing agent.

4. The painted steel sheet according to any one of claims 1 to 3, characterized in that the coating film contains 5% by mass or less of carbon black.

5. A painted steel sheet according to any one of claims 1 to 4, A thermoplastic resin composition bonded onto the painted steel plate and A resin-metal composite comprising the crosslinking agent and having a covalent bond between the crosslinking agent and the thermoplastic resin composition.

6. The resin-metal bond according to claim 5, characterized in that the thermoplastic resin composition is an acrylonitrile-butadiene-styrene resin, a polyethylene terephthalate resin, a polybutylene terephthalate resin, a polycarbonate resin, a polyamide resin, a polyphenylene sulfide resin, a polyester elastomer, or a combination thereof.

7. The resin-metal bond according to claim 6, characterized in that the thermoplastic resin composition is a polyester elastomer.

8. A step of preparing a painted steel sheet according to any one of claims 1 to 4, The process of inserting the painted steel sheet into an injection molding die, The process involves injecting a thermoplastic resin composition into the injection molding die to bond a molded body of the thermoplastic resin composition to the surface of the painted steel sheet. A method for manufacturing a resin-metal composite, characterized by including the following:

Images