JP6085503B2 - Manufacturing method of painted metal plate - Google Patents

Description

  The present invention relates to a coated metal plate and a method for manufacturing the same.

  The painted metal plate is generally excellent in processability, designability and durability, and is used in various applications. The coated metal plate is generally composed of a metal plate, an undercoat film, and a topcoat film that are stacked in this order. A coated metal plate containing an epoxy resin in the undercoat film and an acrylic resin in the topcoat film is known (see, for example, Patent Documents 1 and 2).

JP-A-6-262139 JP-A-8-73806

  In general, an epoxy resin is excellent in adhesion to a metal plate, and an acrylic resin is excellent in weather resistance and transparency. However, as described in Patent Document 1, an overcoat film (hereinafter, referred to as “epoxy resin undercoat film”) containing an acrylic resin on an undercoat film composed of an epoxy resin (hereinafter also referred to as “epoxy resin undercoat film”). In the case of forming an “acrylic resin topcoat”, the adhesion of the epoxy resin basecoat to the acrylic resin topcoat may be insufficient. For this reason, when such a coated metal plate is used as a pre-coated metal plate, the top coat film may be peeled off from the undercoat film.

  On the other hand, as proposed in Patent Document 2, when the undercoat film is formed of a resin composition in which an acrylic resin is mixed with an epoxy resin, the adhesion of the undercoat film to the top coat film of the acrylic resin is increased. . However, since the undercoating film contains an acrylic resin, the adhesion of the undercoating film to the metal plate may be reduced, and the metal plate may be corroded.

  For this reason, establishment of the technique which raises the adhesiveness of the undercoat film with respect to a topcoat film is desired, without mixing an acrylic resin with the epoxy resin of an undercoat film.

  The present invention has been made in view of the above point, and a coated metal having sufficient adhesion of an undercoat film to an acrylic resin topcoat film without blending an acrylic resin into the undercoat film of an epoxy resin. The purpose is to provide a board.

  The present inventors have found that an undercoat film composed of an epoxy resin having a specific molecular weight has sufficient adhesion to an acrylic resin topcoat film, and completed the present invention.

That is, this invention relates to the following coated metal plates.
[1] A metal plate, an undercoat film composed of an epoxy resin which is a reaction product of a prepolymer and a curing agent on the surface of the metal plate, and an overcoat containing an acrylic resin on the surface of the undercoat film A coated metal plate having a coating film, wherein the prepolymer has a weight average molecular weight of 10,000 or more and a molecular weight distribution of the prepolymer is 4.5 or less.
[2] The painted metal sheet according to [1], wherein the number average molecular weight of the prepolymer is 2300 to 6000.
[3] The painted metal sheet according to [1] or [2], wherein the number ratio of the molecular weight of the prepolymer of 1000 or less is 6.5% or less.
[4] The painted metal sheet according to any one of [1] to [3], wherein the number ratio of molecules having a molecular weight of 500 or less of the prepolymer is 3.0% or less.
[5] The coated metal plate according to any one of [1] to [4], wherein the curing agent is a melamine resin or a polyisocyanate compound.

Moreover, this invention relates to the manufacturing method of the following coated metal plates.
[6] A step of applying a coating for an undercoating film containing the prepolymer and the curing agent to the surface of the metal plate and solidifying to form a first coating film; and A step of forming a second coating film by applying a coating for a top coating film containing an acrylic resin and a solvent to the surface; heating the metal plate having the second coating film; and Curing both of the film and the second coating film, wherein the prepolymer has a weight average molecular weight of 10,000 or more, and the molecular weight distribution of the prepolymer is 4.5 or less. A method for producing a metal plate.

  ADVANTAGE OF THE INVENTION According to this invention, even if it does not mix | blend an acrylic resin with an undercoat film of an epoxy resin, it is possible to provide the coating metal plate which has sufficient adhesiveness of the undercoat film with respect to the overcoat film of an acrylic resin. . Therefore, according to the present invention, it is possible to provide a coated metal plate having an undercoat coating film in which the resin component is composed solely of an epoxy resin and a top coat film in which the resin component is composed only of an acrylic resin.

  In the coated metal plate according to the present invention, the metal plate, the undercoat film and the topcoat film are overlapped in this order.

  The kind of the metal plate is not particularly limited. Examples of metal plates include cold-rolled steel sheets, galvanized steel sheets, Zn-Al alloy-plated steel sheets, Zn-Al-Mg alloy-plated steel sheets, aluminum-plated steel sheets, stainless steel sheets (austenite, martensite, ferrite, Martensite two-phase system), aluminum plates, aluminum alloy plates, copper plates and the like. The metal plate may be subjected to known coating pretreatments such as degreasing and pickling. The metal plate is preferably a plated steel plate from the viewpoint of corrosion resistance, weight reduction and cost effectiveness.

From the viewpoint of improving the corrosion resistance, a chemical conversion treatment film may be formed on the surface of the metal plate. The type of chemical conversion treatment 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 coating is not particularly limited as long as it is within a range effective for improving coating film adhesion and suppressing corrosion. For example, in the case of a chromate film, the adhesion amount may be adjusted so that the total Cr conversion adhesion amount is 5 to 100 mg / m ² . In the case of a chromium-free coating, the Ti-Mo composite coating has a total Ti and Mo equivalent deposit of 5 to 500 mg / m ² , and the fluoroacid-based coating has a fluorine equivalent deposit or a total metal element equivalent deposit of 3 to 100 mg / m ² . it may be adjusted the coating weight to be within the range of m ^2. Moreover, in the case of a phosphate film, what is necessary is just to adjust the said adhesion amount so that the phosphorus conversion adhesion amount may be 0.1-5 g / m < ² >.

  The chemical conversion film can be formed by a known method. For example, the chemical conversion treatment film can be formed by applying a chemical conversion treatment solution to the surface of a metal plate by a method such as a roll coating method, a spin coating method, or a spraying method, and then drying. The drying temperature and drying time are not particularly limited as long as moisture can be evaporated. From the viewpoint of productivity, the drying temperature is preferably a temperature at which the ultimate temperature of the metal plate is 60 to 150 ° C., and the drying time is preferably in the range of 2 to 10 seconds.

  The said undercoat coating film is comprised by the epoxy resin on the surface of the said metal plate or the said chemical conversion treatment film. The epoxy resin is a reaction product of a prepolymer and a curing agent. The type of epoxy resin may be one type or two or more types.

  The prepolymer is a polymer having a plurality of epoxy groups. The number of epoxy groups in the prepolymer can be appropriately determined as long as the effects of the present invention are obtained. For example, the epoxy value of the prepolymer is 0.03 or less. The epoxy value is the number of moles of epoxy groups in 100 g of the prepolymer.

  Examples of the prepolymer include bisphenol derivatives having a plurality of epoxy groups. Examples of such bisphenol derivatives include copolymers of bisphenol and epichlorohydrin. Examples of the bisphenol include bisphenol A, bisphenol AP, bisphenol AF, bisphenol B, bisphenol BP, bisphenol C, bisphenol E, bisphenol F, bisphenol G, bisphenol M, bisphenol S, bisphenol P, bisphenol PH, bisphenol TMC and bisphenol. Z is included.

  The prepolymer has a weight average molecular weight (Mw) of 10,000 or more. If the Mw of the prepolymer is smaller than 10,000, the acrylic resin in the topcoat film is not sufficiently entangled with the epoxy resin in the undercoat film, and the adhesion of the undercoat film to the topcoat film may be insufficient. is there. The upper limit of Mw of the prepolymer is not particularly limited from the viewpoint of obtaining sufficient coating film adhesion of the coated metal plate, but is determined to be, for example, 50,000 according to the preparation and coating properties of the coating film for the undercoat film. Can do. From the above viewpoint, the Mw of the prepolymer is preferably 10,000 to 50,000, and more preferably 10,000 to 20,000.

  The prepolymer has a molecular weight distribution (Mw / Mn) of 4.5 or less. When the molecular weight distribution of the prepolymer is larger than 4.5, the adhesion of the undercoat film to the topcoat film may be insufficient. The lower limit of the molecular weight distribution of the prepolymer is not particularly limited from the viewpoint of obtaining sufficient coating film adhesion of the coated metal plate, but is determined to be, for example, 2.0 from the viewpoint of easy availability of the prepolymer. Can do. The molecular weight distribution of the prepolymer is preferably 2.0 to 4.5 from the above viewpoint.

  The number average molecular weight (Mn) of the prepolymer is preferably 2300 to 6000 from the viewpoint of further improving the adhesion of the undercoat film to the topcoat film. If the Mn of the prepolymer is too small with respect to the above range, the molecular weight distribution described above may not be realized, and the adhesion of the undercoat film to the topcoat film may be insufficient. Even if the Mn of the prepolymer is too large with respect to the above range, the molecular weight variation in the prepolymer is reduced, and it becomes difficult to obtain a prepolymer having such a molecular weight in a necessary amount or at low cost. Mn of the prepolymer is preferably 2500 to 5000 from the above viewpoint.

  The number ratio of the prepolymer molecules having a molecular weight of 1000 or less is preferably 6.5% or less from the viewpoint of further improving the adhesion of the undercoat film to the topcoat film. When the number ratio of the prepolymer is larger than 6.5%, the effect of enhancing the adhesion of the undercoat film to the topcoat film by the curing agent described later may not be sufficiently obtained. From the above viewpoint, the number ratio of the prepolymer is more preferably 5.5% or less.

  The number ratio of the prepolymer molecules having a molecular weight of 500 or less is preferably 3.0% or less from the viewpoint of further improving the adhesion of the undercoat film to the topcoat film. When the number ratio of the prepolymer is larger than 3.0%, the curing agent described later may not sufficiently obtain the effect of improving the adhesion of the undercoat film to the topcoat film. From the above viewpoint, the number ratio of the prepolymer is more preferably 2.5% or less.

  The Mw and Mn of the prepolymer are measured, for example, by gel permeation chromatography (GPC). The number ratio of the prepolymer is, for example, dissolving and removing a lower molecular prepolymer by washing the prepolymer with a solvent, or synthesizing a higher molecular weight prepolymer by living polymerization, It can be adjusted by.

  Examples of the curing agent include a compound having two or more functional groups chemically bonded to an epoxy group. Examples of the functional group include an isocyanate group and an amino group. Examples of the curing agent having such a functional group include polyisocyanate compounds and melamine resins. The higher the reactivity of the curing agent with the epoxy group, the better the adhesion of the undercoat film to the topcoat film. From such a viewpoint, the curing agent is preferably a polyisocyanate compound or a melamine resin, and more preferably a melamine resin. Examples of the polyisocyanate compound include Duranate (registered trademark of Asahi Kasei Chemicals Corporation) MF-K60B and MF-B60B (manufactured by Asahi Kasei Chemicals Corporation). Examples of the melamine resin include imino group type, methylol imino group type, methylol group type and fully alkyl group type melamine resin, and more specifically, Cymel (registered trademark of Cytech Technology) 303, 300, 370, 325 and 235 (Cytech Technology) are included.

  In the epoxy resin, the amount of the curing agent used is 100 parts by mass of the prepolymer from the viewpoint of forming an undercoat film having sufficient strength, and from the viewpoint of further improving the adhesion of the undercoat film to the topcoat film. It is preferable that it is 5-50 mass parts with respect to it, and it is more preferable that it is 10-30 mass parts. If the amount of the curing agent used is too small, the strength of the undercoat coating film may be insufficient. When there is too much usage-amount of a hardening | curing agent, the said adhesive improvement effect by a hardening | curing agent may not fully be acquired. The usage-amount of the hardening | curing agent in an epoxy resin can be confirmed with well-known analyzers, such as NMR, IR, and GC-MS.

  Although the film thickness of an undercoat coating film is not specifically limited, For example, it is 0.5-50 micrometers.

  The top coat film contains an acrylic resin and is formed on the surface of the undercoat film. The type of acrylic resin may be one type or two or more types. Examples of the acrylic resin include a polymer of (meth) acrylic ester such as polymethyl methacrylate resin (PMMA), and an acrylic modified resin such as acrylic silicon resin. The acrylic resin can be suitably selected from known acrylic resins that are usually blended in paints for top coat films. Although the film thickness of top coat film is not specifically limited, For example, it is 0.5-300 micrometers.

  The top coating film may be a coating film composed of an acrylic resin alone, but may contain other resins from the viewpoint of imparting properties such as processability and antifouling properties to the top coating film. . Examples of other resins include fluororesins and polyester resins. The content of the acrylic resin and the other resin is appropriately determined within the range in which the construction of the top coat film by the acrylic resin and the provision of the characteristics by the other resin are obtained. For example, when an acrylic resin and a fluororesin are used in combination, the ratio (Ac / Ep) of the mass (Ac) of the acrylic resin to the mass (Ep) of the fluororesin is 0.05 or more. Moreover, the top coat film may further contain the curing agent described above.

  The fluororesin is blended in the top coat film for the purpose of improving antifouling properties and weather resistance. Examples of fluororesins include resins obtained by polymerizing monomers containing fluorine-containing olefins, such as difluorinated fluororesins, trifluorinated fluororesins, and tetrafluoride fluororesins. . Examples of such a fluororesin include a polyvinylidene fluoride (PVDF) -based fluororesin (for example, Kyner (registered trademark of Arkema)), a fluorotriethylene / vinyl ether (FEVE) alternating copolymer (for example, Lumiflon ( Asahi Glass Co., Ltd.) and tetrafluoroethylene / vinyl copolymer (for example, Zeffle (registered trademark of Daikin Industries, Ltd.)).

  In addition to the resin described above, the undercoat coating film and the top coating film may contain further materials within the range where the effects of the present invention can be obtained. Examples of such further materials include rust preventive pigments, colored pigments, extender pigments, aggregates, lubricants, gloss modifiers, UV absorbers, light stabilizers and antioxidants.

  The rust preventive pigment is blended in the undercoat coating film for the purpose of improving the corrosion resistance of the coated metal plate. Examples of rust preventive pigments include zinc phosphate, zinc phosphite, magnesium magnesium phosphate, magnesium phosphate, magnesium phosphite, silica, calcium ion exchanged silica, zirconium phosphate, aluminum trihydrogen phosphate, zinc oxide Zinc phosphomolybdate, barium metaborate, vanadic acid, calcium vanadate, manganese phosphate, strontium chromate and zinc chromate.

  The extender pigment is blended in the undercoat film or the topcoat film for the purpose of adjusting the physical properties of the coating film or the coating material. Examples of extender pigments include barium sulfate, titanium oxide, silica and calcium carbonate.

  The color pigment is blended in the undercoat film or the topcoat film for the purpose of imparting design properties. Examples of the color pigment include titanium oxide, carbon black, chromium oxide, iron oxide, bengara, titanium yellow, cobalt blue, cobalt green, aniline black, phthalocyanine blue, metallic pigment, pearl pigment, and thermal barrier pigment.

  Aggregates are blended into the top coat to improve the mechanical strength, abrasion resistance, and scratch resistance of the coating. Examples of aggregates include inorganic aggregates and organic aggregates having an average particle size of 7 μm or more. Examples of inorganic aggregates include silica and glass beads. Examples of the organic aggregate include spherical beads made of a resin such as urea resin, polyacrylonitrile, polytetrafluoroethylene, polyamide (nylon), and polyester.

  The lubricant is blended in the top coat film for the purpose of imparting lubricity during processing of the coated metal plate. Examples of the lubricant include organic waxes such as fluorine wax, polyethylene wax, styrene wax, and polypropylene wax, and inorganic lubricants such as molybdenum disulfide and talc.

  The gloss modifier is blended in the top coat film for the purpose of adjusting gloss. Examples of the gloss adjusting agent include an inorganic gloss adjusting agent and an organic gloss adjusting agent having an average particle diameter of 1 to 6 μm. Examples of the inorganic gloss adjusting agent include silica and glass beads. Examples of the organic gloss modifier include spherical beads made of a resin such as polyacrylonitrile and polytetrafluoroethylene.

  An ultraviolet absorber, a light stabilizer and an antioxidant are mainly blended in the topcoat film for the purpose of improving weather resistance and stability. Examples of ultraviolet absorbers include benzotriazole ultraviolet absorbers and hydroxyphenyltriazine ultraviolet absorbers.

  Examples of light stabilizers include tetrakis (1,2,2,6,6-pentamethyl-4-piperidyl) 1,2,3,4-butanetetracarboxylate, tetrakis (1,2,2,6,6). -Pentamethyl-4-piperidyl) butane-1,2,3,4-butanetetracarboxylate, 1,2,2,6,6-pentamethyl-4-piperidyl and tridecyl-1,2,3,4-butanetetra Carboxylate, condensate of 2,2,6,6-tetramethyl-piperidinol, tridecyl alcohol and 1,2,3,4-butanetetracarboxylic acid, 1,2,3,4-butanetetracarboxylic acid and 2,2,6,6-tetramethyl-4-piperidinol and β, β, β, β-tetramethyl-3,9- (2,4,8,10-tetraoxaspiro [5,5] undecane)- Jieta Condensate with diol, bis (2,2,6,6-tetramethyl-4-piperidyl) decanedioate, 1,2,2,6,6-pentamethyl-4-piperidyl methacrylate, 2,2,6 , 6-tetramethyl-4-piperidylmethacrylate and hindered amine light stabilizers such as (bis (1-undecanoxy-2,2,6,6-tetramethylpiperidin-4-yl) carbonate).

  Examples of antioxidants include vitamins such as ascorbic acids and tocopherols, stearyl-β- (3,5-di-t-butyl-4-hydroxyphenyl) propionate and pentaerythritol tetrakis [3- (3,5 Phenolic antioxidants such as -di-t-butyl-4-hydroxyphenyl) propionate], and tris (2,4-t-butylphenyl) phosphite and cyclic neopentanetetraylbis (2,6- Phosphite-based antioxidants such as di-t-butyl-4-methylphenyl) phosphite.

  The coated metal plate according to the present invention is applied to the surface of the metal plate, for example, by applying a coating for an undercoat film containing the prepolymer and the curing agent, and solidifying to form a first coating film. A second step of forming a second coating film by applying a coating for a top coating film containing an acrylic resin and a solvent to the surface of the first coating film, and the second process And a third step of heating both the first coating film and the second coating film by heating the metal plate having the coating film.

  The undercoat coating film contains the prepolymer and the curing agent. The paint for undercoat films may further contain other materials blended in the undercoat paint film. Moreover, the coating material for undercoat films may contain the solvent further from a viewpoint of an applicability improvement, for example. Examples of the solvent include aromatic solvents such as toluene and xylene, alkane solvents such as hexane and cyclohexane, ketone solvents such as acetone, alcohol solvents such as ethanol and 2-propanol, and mixtures thereof. The content of the component in the undercoat paint is determined based on the content of the component in the undercoat film.

  The top coat film contains the acrylic resin and a solvent. The paint for top coat film may further contain other materials blended in the top coat film. Moreover, the solvent in the paint for top coat films can be suitably selected from the solvents described above. Content of the said component in the coating material for top coat films is determined based on content of the said component in top coat film.

  In the first step, the first coating film is formed, for example, by heating a metal plate to which a paint for undercoat coating film is applied. The first coating film may be an undercoat paint layer that has been completely cured so that the hardness does not increase any more. In this case, sufficient adhesion with the second coating film can be obtained. There may not be. For this reason, it is preferable that a 1st coating film is a layer of the coating material for undercoat films hardened to such an extent that the room for further hardening is left. On the surface of such a first coating film, it is possible to sufficiently apply a coating material for a top coating film, and when the second coating film described later is formed, The acrylic resin in the second coating film is more strongly entangled with the epoxy resin and the prepolymer, and the prepolymer in the first coating film reacts with the curing agent in the second coating film. ,There is expected. Moreover, solidification of such a paint for undercoat films can be adjusted by the compounding ratio of the curing agent with respect to the prepolymer in the paint, the ultimate temperature during heating of the metal plate, or the heating time. The end point of the solidification can be appropriately determined depending on the hardness of the first coating film, for example.

  In the second step, the top coat paint is applied to the solidified undercoat paint film by a known method such as roll coating, curtain flow coating, spray coating, or dip coating. The film thickness of the top coat film is adjusted by, for example, the coating amount of the paint for the top coat film. The surface of said 1st coating film swells with the solvent in the coating material for top coat films, and the acrylic resin in a 2nd coating film can act with respect to the epoxy resin in a 1st coating film.

  In the third step, both the first coating film and the second coating film are cured. The curing of both coatings is completed by the third step. Curing of both coating films can be adjusted by the ultimate temperature or heating time when the metal plate is heated. Moreover, the end point of hardening of both coating films is determined by measuring the hardness of the undercoat film and top coat film which are formed, for example. For example, the end point of curing of both coating films is determined after the increase in hardness accompanying heating of both coating films has stopped.

  Said manufacturing method may further include other processes in the range in which the effect of this invention is acquired. Examples of such other processes include a process of pre-coating the metal plate and a process of forming the coated metal plate with a mold after the coating film is cured. Said other process may be performed by the well-known method in manufacture of a coating metal plate.

  The coated metal plate according to the present invention has a top coat film composed of an acrylic resin on the surface of an undercoat film composed of an epoxy resin, but even if the undercoat film does not contain an acrylic resin, High adhesion of the undercoat film to the film is obtained. The reason is considered as follows.

  First, the prepolymer has a weight average molecular weight of 10,000 or more. For this reason, the magnitude | size of the epoxy resin molecule | numerator which comprises undercoat film also becomes large. Since the epoxy resin is large, the individual networks of the epoxy resin network structure are also large. Therefore, when the top coat film is formed on the surface of the undercoat film, the acrylic resin constituting the top coat film is sufficiently entangled with the network of the epoxy resin at the interface between the top coat film and the undercoat film. It is considered that the adhesion of the undercoat film to the topcoat film is improved.

  The prepolymer has a relatively narrow molecular weight distribution of 4.5 or less. That is, the prepolymer contains a lot of larger prepolymer and the content of the smaller prepolymer is small. A small prepolymer forms a small network when it becomes an epoxy resin. However, since such a small polymer is sufficiently small, individual networks of the above-mentioned network structure of the epoxy resin are not easily reduced. Therefore, it is considered that the adhesion of the undercoat film to the topcoat film is enhanced by the entanglement of the acrylic resin with the epoxy resin as described above over the entire surface of the undercoat film.

  Furthermore, prepolymers are generally less reactive as they are larger. For this reason, if there are few small prepolymers, at the time of baking of an under_coat_film, both a prepolymer and a hardening | curing agent remain in an unreacted state easily. If the unreacted prepolymer or curing agent in the undercoat film reacts with a specific functional group (for example, hydroxyl group) of the hardener or acrylic resin in the topcoat film during baking of the topcoat film, A covalent bond is formed with the coating film. Further, even when the acrylic resin in the topcoat film does not have a reactive site with the unreacted curing agent in the undercoat film, the hydroxyl group of the unreacted prepolymer in the undercoat film and the hydroxyl group in the acrylic resin Can form hydrogen bonds. It is considered that the adhesion of the undercoat film to the topcoat film is also enhanced by such a covalent bond or hydrogen bond.

  As described above, the coated metal plate according to the present invention has high coating film adhesion. And since the coating metal plate which concerns on this invention has a top coat film of an acrylic resin, a weather resistance, a designability, etc. are favorable. Therefore, the coated metal plate according to the present invention can be used in various applications as an industrial product such as an electronic device such as a mobile phone or an exterior building material, or as a pre-coated metal plate.

  EXAMPLES Hereinafter, although this invention is demonstrated in detail with reference to an Example, this invention is not limited by these Examples.

[1. Synthesis of prepolymer]
100 parts by mass of bisphenol A and about 200 to 800 parts by mass of epichlorohydrin were copolymerized by a dehydrohalogenation reaction in the presence of an alkali catalyst to obtain resins A to F as prepolymers. The weight average molecular weight (Mw) and number average molecular weight (Mn) of the obtained prepolymer were measured by a gel permeation chromatography (GPC) method. GPC apparatus HLC-8010 manufactured by Tosoh Corporation was used as the GPC apparatus, two columns of Shodex K-800D and K805L were used as the column, and chloroform was used as the eluent. The values of weight average molecular weight (Mw) and number average molecular weight (Mn) were converted from standard polystyrene. Furthermore, the molecular weight distribution (PDI) ((Mw) / (Mn)), the number ratio of molecules having a molecular weight of 1000 or less, and the number ratio of molecules having a molecular weight of 500 or less were determined. The results are shown in Table 1,

[2. Preparation of metal plate]
Using a hot-dip galvanized steel sheet with a plate thickness of 0.5 mm and a single-sided coating weight of 90 g / m ² , a fluoroacid-based chromium-free chemical conversion treatment liquid is applied to the surface of the galvanized layer in accordance with a conventional method with a fluorine-equivalent coating weight. A plated steel sheet having a fluoroacid-based film was prepared by coating at 10 mg / m ² .

[3. Preparation of paint for undercoat film]
Curing agents A to G shown in Table 2 below were prepared. Cymel 303, 300, 370, 325 and 235 are manufactured by Cytec Technology, and “Cymel” is a registered trademark of the same company. Duranate MF-K60B and MF-B60B are both manufactured by Asahi Kasei Chemicals Corporation, and “Duranate” is a registered trademark of the company.

  90 parts by mass of prepolymer A and 10 parts by mass of curing agent A were mixed to prepare paint 1 for an undercoat film. Moreover, except having changed the combination of a prepolymer and a hardening | curing agent as shown in Table 3, it carried out similarly to the coating material 1 for undercoat films, and prepared the coating materials 2-24 for undercoat films, respectively.

[4. Preparation of paint for top coat film]
A paint for top coat was prepared. The composition of this paint was as follows.
Acrylic resin 10 parts by mass Fluorine resin 40 parts by mass Solvent 50 parts by mass The acrylic resin was polymethyl methacrylate (PMMA), the fluororesin was polyvinylidene fluoride (PVDF), and the solvent was isophorone.

[5. Preparation of painted metal plate]
On the surface of the plated steel sheet, the undercoat paint 1 was applied in a dry film thickness of 5 μm. Then, the plated steel sheet is heated so that the temperature reached by the plated steel sheet is 215 ° C. for 40 seconds so that the prepolymer A and the curing agent A in the paint 1 do not react, and the coating film of the paint 1 is solidified. did. Next, the above-described paint for the top coat film was applied to the surface of the solidified paint film 1 so as to have a dry film thickness of 16 μm. Next, the reached temperature of the plated steel sheet becomes 60 seconds at 240 ° C. so that the prepolymer A in the paint 1 is completely reacted and the top coat film is cured on the plated steel sheet coated with the paint for the top coat film. And immediately cooled with water to completely cure both the undercoat film and the topcoat film. Thus, a coated metal plate 1 was produced.

  Coated metal plates 2 to 24 were produced in the same manner as the painted metal plate 1 except that the paints 2 to 24 were used in place of the paint 1.

[6. Evaluation of painted metal plate]
(1) Peel test (Adhesion test 1)
The painted metal plate 1 is folded until the surface (coating side) of the painted metal plate 1 is the outside, the back surface of the painted metal plate 1 is the inside, and the back surfaces are in close contact with each other. The film was observed for the presence or absence of peeling when adhered and peeled, and evaluated according to the following criteria.
○: No peeling ×: With peeling

(2) Swelling test in high temperature and high humidity environment (Adhesion test 2)
Using a test machine called CCHC (Cleveland Condensing Humidity Cabinet: ASTM D-2247-87-Type A2), the surface of the coated metal plate 1 is faced down, and the coated metal plate 1 is filled with hot water at 80 ° C. The surface of the coated metal plate 1 was exposed to steam for a certain period (240 hours). After the test, the occurrence of blisters in the coating film of the coated metal plate 1 was evaluated according to the criteria of ASTM D714-56. That is, the size (average diameter) and density of each blister are evaluated by comparing the coating film of the coated metal plate 1 with a standard judgment photograph, and the evaluation result is indicated by a grade symbol. The size is 8 (diameter approx. 1 mm), 6 (diameter approx. 2 mm), 4 (diameter approx. 3 mm) and 2 (diameter approx. 5 mm) in this order, and the density is F, FM, M, MD from the smallest. , D is classified into 5 stages, and 10 if there is no blister.

(3) Corrosion resistance test According to JIS K5600-7-1, the end surface part of the coated metal plate 1 was sealed, and 5 mass% saline was continuously sprayed at 35 ° C for a certain period (240 hours). Thereafter, the swelling of the coating film at the flat part (p-plane part of the coating film) was evaluated in accordance with the above-mentioned criteria of ASTM D714-56. Moreover, the corrosion state (presence or absence of rust) of the metal plate at the flat part was observed and evaluated.

  The said test of each of the coated metal plates 2 to 24 was performed in the same manner as the painted metal plate 1, and the coated metal plates 2 to 24 were evaluated. The results are shown in Table 4 and Table 5.

  As apparent from Table 1 and Table 4, in the coated metal plates 1 to 15 using the prepolymers A to C having a weight average molecular weight of 10,000 or more and a molecular weight distribution of 4.5 or less, the type of the curing agent is Regardless, the coating film adhesion and corrosion resistance were high. As is clear from Table 1, all of the prepolymers A to C have a low content of low molecular components. Therefore, it can be seen that the epoxy resin undercoat formed from a prepolymer having a relatively large molecular weight and a low content of low molecular components has high adhesion to the acrylic resin topcoat.

  On the other hand, as is apparent from Tables 1 and 5, the coated metal plate 16 using prepolymers D and F having prepolymer weight average molecular weights of several thousands and molecular weight distributions of 4.5 and 3.1. 18-24, both coating film adhesion and corrosion resistance were low even if the molecular weight distribution was 4.5 or less. Further, even when the weight average molecular weight is 10,000 or more, the coated metal plate 17 using the prepolymer E having a large molecular weight distribution of 14.0 has a large number ratio of molecules having a molecular weight of 1000 or less, which is about 40%. Both resistance and corrosion resistance were much lower.

  Further, in the coated metal plates 23 and 24 having an epoxy resin undercoat film formed of the prepolymer D and a polyisocyanate compound curing agent, compared to the coated metal plates 16 and 19 to 22, the coating film adhesion and Both corrosion resistances were even lower.

  When the part in which the peeling of the coating film occurred in the coated metal plates 16 to 24 was observed with a magnifying glass, the top coating film was peeled off at the interface with the undercoat coating film. Moreover, when the part which the blister of the coating film in the coated metal plates 16-24 generate | occur | produced was observed with the loupe, all had the blister generate | occur | produced in the interface of undercoat and topcoat.

  From the above, the epoxy resin primer film formed from a prepolymer having a relatively low molecular weight has sufficient adhesion to the acrylic resin primer film even though the molecular weight distribution of the prepolymer is relatively narrow. For this reason, it turns out that the workability and corrosion resistance of the coated metal plate are insufficient.

An epoxy resin is known to have excellent properties as a resin for an undercoat film of a coated metal plate, and an acrylic resin is known to have excellent properties as a resin for a topcoat film. According to the present invention, the undercoat coating on the top coat without mixing an acrylic resin with an undercoat comprising an epoxy resin and without mixing an epoxy resin with an overcoat containing an acrylic resin. High adhesion of the film can be obtained. Therefore, it is possible to provide a highly versatile painted metal sheet at a lower cost, and further expansion of demand and application of the coated metal sheet is expected.

Claims (5)

A metal plate, an undercoat film composed of an epoxy resin which is a reaction product of a prepolymer and a curing agent on the surface of the metal plate, and an overcoat film containing an acrylic resin on the surface of the undercoat film A method of manufacturing a coated metal plate having
Applying a precoating coating material containing the prepolymer and the curing agent to the surface of the metal plate, and solidifying to form a first coating;
On the surface of the first coating film, a step of forming a second coating film by applying a top coating film paint containing an acrylic resin and a solvent; and
Heating the metal plate having the second coating film, and curing both the first coating film and the second coating film,
The prepolymer has a weight average molecular weight of 10,000 or more,
The method for producing a coated metal sheet, wherein the prepolymer has a molecular weight distribution of 4.5 or less. The number average molecular weight of the prepolymer is from 2300 to 6000, method for manufacturing a coated metal sheet according to claim 1. The method for producing a coated metal sheet according to claim 1 or 2 , wherein the number ratio of molecules having a molecular weight of 1000 or less in the prepolymer is 6.5% or less. The method for producing a coated metal plate according to any one of claims 1 to 3 , wherein a number ratio of molecules having a molecular weight of 500 or less of the prepolymer is 3.0% or less. Wherein the curing agent is a melamine resin or a polyisocyanate compound, method for manufacturing a coated metal sheet according to any one of claims 1-4.