JP2006175815A - Coated metal sheet excellent in processability and antistaining properties - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a precoated metal sheet further enhanced in the antistaining properties of a precoated metal sheet of a conventional technique and also having good processability at the same time. <P>SOLUTION: The precoated metal sheet is constituted by arranging the outermost surface layer and the lower layer arranged on the outermost surface layer and characterized in that the outermost surface layer contains an amino resin, the concentration of the amino resin in the surface part of the outermost surface layer is higher than that of the aminoplast resin in the interfacial part with the lower layer, the lower layer contains the amino resin and the concentration of the amino resin of the interfacial part of the outermost surface layer in the lower layer is higher than that of the amino resin in the interfacial part on the side of the metal sheet in the lower layer. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a coated metal plate excellent in workability and contamination resistance used for home appliances, building materials, automobiles and the like.

  Recently, in the production lines for home appliances, building materials, automobiles, etc., pre-coated metal sheets (hereinafter referred to as painted metal sheets) have been developed due to needs such as improvement of working environment, release from pollution problems, and improvement of production efficiency by omitting processes. (Abbreviated) is increasing.

  The coated metal sheet must be formed and processed, so it requires a high degree of workability, while satisfying the traditionally required performance such as hardness and stain resistance. The design was technically difficult.

  In JP-A-8-168723, the coating film structure is analyzed and analyzed by glow discharge emission spectroscopy (GDS) and X-ray photoelectron spectroscopy (XPS). It is described that the coating film has a coating film excellent in processability, stain resistance and hardness. In the Examples, an undercoat layer is provided on a metal plate, and a polyester resin-based coating film containing an aminoplast resin is formed thereon as the outermost layer.

  The coated metal plate having good processability and contamination resistance described in this publication is a coated metal plate having a polyester resin-based coating film formed on the outermost layer. The conditions of A) to (C) are satisfied.

(A) When the element concentration distribution in the depth direction of the polyester resin coating film is measured by high frequency discharge type glow discharge emission spectrometry (hereinafter referred to as high frequency GDS), the polyester resin coating film is in contact with the air. The spectral intensity of nitrogen on the surface is 1.2 times or more of the spectral intensity of nitrogen at the interface opposite to air.

(B) When the element concentration distribution in the depth direction of the polyester resin coating film is measured by high frequency GDS, the depth from the interface with the air is measured from 1 / 10th of the thickness of the polyester resin coating film. An increase in the spectral intensity of nitrogen is seen toward the air interface.

(C) When XPS the interface with air, the area of the spectrum component derived from the melamine ring of the melamine resin in the carbon spectrum is 30% or more of the area of the total carbon spectrum.

  In this coating film, the nitrogen concentration in the coating film increases toward the surface, and it is considerably concentrated on the extreme surface of the coating film. Nitrogen is mainly derived from nitrogen in the aminoplast resin in the coating film, and the concentration of aminoplast resin is different in the depth direction of the coating film, and the concentration increases toward the surface. And The amino blast resin concentration inclined toward the surface provides a coating film in which workability, hardness and stain resistance are highly compatible.

  However, it was found that this stain resistance is affected by the underlying layer under this coating. If the hardness (crosslinking degree) of the surface of the underlayer is low, the melamine moves to the lower layer and the melamine concentration in the outermost layer decreases. As a result, the melamine concentration in the outermost layer decreases, and the hardness and contamination resistance of the outermost layer are expected. Lower than what you do. In order to increase the crosslink density on the surface of the underlayer, it is necessary to sufficiently cure the base layer after coating, and then to coat and cure the outermost layer (so-called 2-coat 2-bake type). In the case of this coating method, the crosslink density on the surface of the underlayer is increased, but the interlaminar adhesion with the outermost layer is deteriorated and it is easy to peel off. In order to solve this problem, at present, the outermost layer is applied by being cured so that the crosslinking density of the underlayer does not increase so much.

JP-A-8-168723

  It is an object of the present invention to provide a precoated metal sheet that further improves the stain resistance of the prior art precoated metal sheet and at the same time has good workability.

  The present invention has solved the problems of the cross-linking density and interlayer adhesion of the surface of the base layer of the pre-coated metal sheet of the prior art described above by simultaneously curing the outermost layer and the base layer.

That is, the present invention
(1) A pre-coated metal plate having an outermost layer disposed on a metal plate and a lower layer disposed under the outermost layer,
The outermost layer contains an aminoblast resin, and the concentration of the aminoblast resin on the surface portion in the outermost layer is higher than the concentration in the interface portion with the lower layer,
The pre-coated metal sheet, wherein the lower layer includes an amino blast resin, and the concentration of the amino blast resin in the interface portion on the outermost layer side in the lower layer is higher than the concentration in the interface portion on the metal plate side in the lower layer.

  (2) The precoated metal sheet according to (1), wherein Ra at the interface between the lower layer and the outermost layer is 0.3 to 0.6 μm.

  (3) The precoated metal sheet according to (1) or (2), wherein the value of N / (C + O + N) measured by GDS at the interface portion on the outermost layer side in the lower layer is 0.4 or more.

  (4) The coating solution for forming the outermost layer and the lower layer both contains a polyester resin having a hydroxyl value of 40 or less, a methylated melamine resin, and an acid catalyst neutralized with a volatile base (1) The precoat metal plate in any one of-(3).

  (5) The method for producing a precoated metal sheet according to any one of (1) to (4), wherein the lower layer and the outermost layer are applied simultaneously in multiple layers, or both layers are simultaneously cured after wet-on-wet application.

(6) A pre-coated metal plate having an outermost layer disposed on the metal plate and a lower layer disposed under the outermost layer,
The outermost layer contains an aminoblast resin, and the concentration of the aminoblast resin on the surface portion in the outermost layer is higher than the concentration in the interface portion with the lower layer,
A precoated metal sheet that is simultaneously cured after the lower layer and the outermost layer are applied.

  By simultaneously curing the outermost layer and the lower layer, the crosslink density of the interface portion between the outermost layer and the lower layer in the lower layer is increased, and the workability, hardness, and stain resistance can be met to meet the high demands of consumers. A particularly excellent coated metal plate can be provided. If simultaneous curing is used, the coating may be simultaneous multilayer coating or wet-on-wet coating.

  By making the component constitution of the lower layer the same formulation as that of the outermost layer and forming a gradient of aminoblast resin concentration in the lower layer, the stain resistance and workability of the entire coating film are further improved.

  By simultaneously curing the outermost layer and the lower layer using the simultaneous multilayer coating and the wet-on-wet method, it is possible to provide high interlayer adhesion by controlling the Ra of the interlayer interface to be increased.

  The polyester resin-based coating film used for the outermost layer means a coating film containing a polyester resin and an aminoplast resin as essential components.

  The polyester resin is a resin having an ester group in the resin, and is called oil-free polyester resin, alkyd resin, linear high molecular weight polyester resin, or branched high molecular weight polyester resin. The polyester resin used for the outermost layer is a resin having a hydroxyl value of 40 or less, preferably a hydroxyl value of 2 to 30. When the hydroxyl value is larger than the above range, workability such as cutting and bending is lowered, which is not preferable.

  Another resin, for example, an epoxy resin, a urethane resin, an acrylic resin, a fluororesin, a silicon resin, an amide resin, an imide resin, or the like may be mixed with the polyester resin.

  Aminoplast resins are melamine resins, benzoguanamine resins, and the like, and examples of melamine resins include methylated melamine resins, butylated melamine resins, and mixed etherified melamine resins such as butyl-methyl. A methylated melamine resin is preferred.

  In addition to the aminoplast resin, there may be added a crosslinking agent component having a group that reacts with a component added to the coating film, such as an isocyanate group, an epoxy group, an amino group, or a polyester resin.

  In the outermost layer composition of the present invention, an acid catalyst is blended in addition to the above resins. The acid catalyst that can be used is not particularly limited, and examples thereof include dodecylbenzenesulfonic acid and toluenesulfonic acid. It is preferred to use an acid catalyst that has been blocked by neutralization with a volatile base. Although it does not specifically limit as a volatile base, Ammonia, a triethylamine, a dimethylethanolamine etc. are mentioned.

  For polyester resin coatings, coloring pigments such as titanium oxide, petals, barium sulfate, silica, cyanine blue, additives such as extender pigments, resin beads, aluminum flakes, mica, antifoaming agents, leveling agents, etc. These additives can be added as necessary.

  The film thickness of the outermost layer is arbitrary, but in a coated metal plate, a dry film thickness of about 1 to 50 μm, particularly 5 to 30 μm is common.

  As for the outermost polyester resin coating film of the present invention, the concentration of nitrogen in the surface portion in contact with the air in the outermost layer is higher than the nitrogen concentration in the interface portion with the lower layer in the analysis using high frequency GDS. It is necessary to be. Nitrogen is mainly derived from nitrogen in the aminoplast resin in the coating film. The concentration of the aminoplast resin is different in the depth direction of the coating film, and the concentration increases toward the surface. Is shown. Therefore, a high crosslink density in the surface portion provides high hardness and stain resistance, and a workability is provided by having a gradient in which the crosslink density decreases toward the interface portion.

  In general, the lower layer component disposed under the outermost layer can be a thermosetting resin such as a urea resin, a melamine resin, an epoxy resin, an unsaturated polyester resin, an alkyd resin, or a urethane resin.

  In the present invention, the lower layer preferably has the same component structure as the outermost layer. That is, it is preferable that the lower layer contains an amino blast resin, and the concentration of the amino blast resin in the interface portion on the outermost layer side in the lower layer is higher than the concentration in the interface portion on the metal plate side in the lower layer. By adopting this configuration, a gradient of aminoblast resin concentration is formed even in the lower layer, the cross-linking density of the interface portion on the outermost layer side of the lower layer is further increased, high hardness and contamination resistance are provided, and the metal plate side Further processability is provided by having a gradient that reduces the crosslink density towards. In this way, the stain resistance and workability of the entire coating film can be further improved. Specifically, good results were obtained when the nitrogen concentration at the interface portion on the outermost layer side in the lower layer represented by the value of N / (C + O + N) was 0.4 or more.

  The film thickness of the lower layer is arbitrary, but in a coated metal plate, a dry film thickness of about 1 to 30 μm, particularly 3 to 12 μm is common.

  Examples of the metal plate include a steel plate, an aluminum plate, a stainless steel plate, a titanium plate, and a copper plate. Among these, as examples of steel plates, cold rolled steel plates, hot rolled steel plates, hot dip galvanized steel plates, alloyed hot dip galvanized steel plates, electrogalvanized steel plates, zinc-iron alloy plated steel plates, zinc-aluminum alloy plated steel plates, aluminum plated steel plates Chrome-plated steel sheet, nickel-plated steel sheet, zinc-nickel alloy-plated steel sheet, tin-plated steel sheet and the like.

  Various coating pretreatments can be performed before applying the lower layer to the metal plate. For example, cleaning with water or solvent, degreasing, etc. to clean the surface of the object to be coated, corona discharge treatment, flame treatment, etc., processing to improve adhesion by generating polar groups on the surface, zinc phosphate treatment, chromate treatment , Composite oxide coating treatment, chromate-free pretreatment, metal treatment, brushing, grinding and other treatments to remove irregularities and obstruct surface adhesion, pickling, alkali washing Chemical treatments such as these, or a combination of these can be performed. The conditions for the pretreatment for coating can be selected as appropriate.

  In the present invention, the number of coating layers to be formed is not limited at all, and if necessary, after forming several coating layers, the lower layer and the outermost layer according to the present invention may be formed. The thickness of each coating layer, the resin composition, the pigment composition, etc. are not limited.

  For the application of the lower layer and the outermost layer, so-called multilayer simultaneous application or a wet-on-wet method is used. As a method of applying the outermost layer on the lower layer by wet-on-wet, the lower layer is formed using a roll coater, roller curtain or curtain coating machine, spray coating machine, etc., and then roller curtain coating is performed thereon. An application process for forming the outermost layer using slit-type curtain coating or spray coating may be considered. The coating method of each layer is not particularly limited, but from the viewpoint of coating film appearance and film thickness control, the outermost layer coating method is preferably a coating method that does not contact an already formed lower layer coating film. In the present invention, it is preferable to use a curtain coater for coating the lower layer and the outermost layer. The method of simultaneous multi-layer coating is not particularly limited, and slide hopper type curtain coating, slide coating, die feed type curtain coater, die coater, multilayer feed type coater, and the like can be used. In particular, it is possible to use a slide hopper type curtain coating in which a lower layer and an outermost layer are superposed in two layers and discharged from a die, and the two layers are simultaneously coated on a metal plate. As the coating method of the present invention, it is preferable to use slide hopper type curtain coating.

  Specifically, as a coating machine for coating the outermost layer and the lower layer, a slide hopper type curtain coating apparatus disclosed in Japanese Patent Publication No. 49-24133, which is used for photographic photosensitive materials, can be used. A schematic diagram of this slide hopper type curtain coating machine is shown in FIG. The slide hopper 1 is provided with a paint supply hole 8 and a slit 6 through which three layers of paint are quantitatively sent out by a gear pump or the like. A chain-like curtain guide 3 is provided so as to be in contact with both end portions of the lip portion 7A of the slide surface 7. A paint pan 5 is installed below the lip 7 </ b> A, and the curtain guide 3 hangs down to the bottom of the paint pan 5. The paint P is uniformly supplied to the slide surface 7 through the slit 6 from each paint supply hole 8 of the slide hopper 1, and is laminated on the slide surface 7. Since the laminated paint is spread by the curtain guide 3 when falling to the paint pan 5 at the tip end (lip part 7A) of the slide surface 7, it flows as a uniform liquid film in the width direction as the curtain 4 of paint. By passing a strip-shaped metal plate such as the steel strip 2 through this liquid film, a plurality of layers of paint can be applied simultaneously on the surface of the steel strip 2.

  After applying the lower layer and the outermost layer on the metal plate, the two layers are simultaneously dried and cured. As a drying method, a known method can be applied, for example, natural drying, heating air heating, induction heating, infrared heating or the like to heat and dry the coating film, or radiation such as ultraviolet rays or electron beams to the coating film. There are a method of drying and curing by irradiation, a method of drying and curing the coating film by passing through a booth filled with a catalyst, a method of combining these, etc., which can be selected according to the type of paint applied it can.

  In this way, when the lower layer and the outermost layer are simultaneously dried and cured, the crosslink density on the lower layer surface is not as high as in the case of two coats and two baked, but to prevent the melamine in the outermost layer from moving to the lower layer. Sufficient crosslink density is obtained.

  The roughness (Ra) of the interface between the lower layer and the outermost layer is preferably in the range of 0.3 to 0.6 μm. The interface between the lower layer and the upper layer of the coating film prepared in the so-called 2-coat 2-bake method in which the lower layer is applied and then dried and cured, and then the outermost layer is applied and dried and cured is usually less than 0.3 μm. This is because the lower surface is sufficiently leveled and smooth. When the roughness of the interface is too small, the adhesion with the outermost layer is deteriorated at the time of molding after coating, and it is easy to peel off. When the lower layer and the outermost layer are simultaneously cured and the roughness (Ra) of the interface between the two layers is controlled within the range of about 0.3 to about 0.6 μm, the adhesion between the lower layer and the outermost layer is secured. . Such a large interface roughness of the coating film of the present invention is realized by simultaneously applying the lower layer and the outermost layer or applying them by a wet-on-wet method, and then simultaneously drying them.

  The drying conditions may be appropriately selected according to the content of the paint, but conditions such as a hot air furnace, an induction heating furnace, a near-infrared furnace, etc., with a maximum plate temperature of 150 to 250 ° C. and an arrival time of about 10 to 200 seconds are common. is there.

Measurement by high frequency GDS The coating film thus formed is analyzed by using high frequency GDS. The measurement conditions of the high frequency GDS are not particularly limited. For example, the discharge power is 20 to 60 W, the argon flow rate is 150 to 350 ml / min, the photomultiplier voltage for nitrogen detection is 450 to 650 V, and the sampling interval is 0.3 to 2.5 seconds. To do.

  Note that these values are fixed for a single measurement and are not changed during the measurement. In order to know the position of the interface between the outermost coating layer and its lower layer, and the metal plate, the undercoat layer or the intermediate coat layer on the spectrum, other elements may be measured together. For example, when the lower layer is a metal plate, the spectrum of the metal is measured, and when the lower layer is an undercoat film layer, the characteristic of the undercoat film layer, for example, Cr contained in the rust preventive pigment is measured. The interface is easy to distinguish.

  From the spectrum obtained by high-frequency GDS measurement, the spectral intensity Ia of nitrogen on the surface of the polyester resin coating film in contact with air and the spectral intensity Ii of nitrogen at the interface opposite to air are obtained. The spectrum of the surface in contact with air may be slightly disturbed due to unstable discharge. In this case, as shown in FIG. 2, the spectrum inside the coating is directed toward the surface in contact with air. And extrapolate.

  FIG. 2 shows an example in which the outermost layer of the precoated metal sheet of the present invention is measured by high frequency GDS. The nitrogen concentration in the coating film increases toward the surface, which means that the coating film is considerably concentrated on the extreme surface. Nitrogen is mainly derived from nitrogen in the aminoplast resin in the coating film. The concentration of the aminoplast resin is different in the depth direction of the coating film, and the concentration increases toward the surface. Is shown.

Measuring the roughness of the interface between the lower layer and the outermost layer by cutting the pre-coated metal plate of the present invention of each Example, embedding in the resin, and polishing, thereby smoothing the cross section perpendicular to the surface of the coating film, After taking a picture with a 3500 times scanning microscope, the roughness Ra of the interface was evaluated.
Ra of the interface is covered with a transparent sheet used for OHP from the top of the photograph, and the unevenness of the interface is traced precisely. Then, as shown in FIG. 3, the area of the vertical line is measured with an image processing apparatus. The average value can be obtained from the following equation.

  Furthermore, in order to easily measure the Ra of the interface, the transparent sheet used for the OHP is covered from the top of the photograph, the unevenness of the interface is traced precisely, the average value is subtracted, and the transparent sheet along the unevenness Ra may be obtained by measuring the weight of the uneven portions above and below the average value and converting the weight into an average length. The average roughness (Ra) of the interface in each of the inventive examples was in the range of 0.3 μm to 0.6 μm.

[Invention Example 1]
An electrogalvanized steel sheet (single-sided coating weight 20 g / m ² ) was applied with 100 mg / m ² of chromate-free treatment E300N (manufactured by Nihon Parkerizing Co., Ltd.) (adhesion amount after drying), and dried at 70 ° C. Prepared.

Component polyester resin of outermost layer : number average molecular weight 12000, glass transition temperature (Tg) 20 ° C., hydroxyl value 5.
Fully alkylated methylated melamine (degree of polymerization 1.3): 35 parts by weight per 100 parts by weight of polyester resin.
Acid catalyst: 1.5 parts by weight of dodecylbenzenesulfonic acid with respect to 100 parts by weight of polyethylene resin.
Lower component polyester resin: number average molecular weight 15000, glass transition temperature (Tg) 25 ° C., hydroxyl value 8.
Fully alkylated methylated melamine (degree of polymerization 1.3): 35 parts by weight per 100 parts by weight of polyester resin.
Acid catalyst: 1.5 parts by weight of dodecylbenzenesulfonic acid with respect to 100 parts by weight of polyethylene resin.

  On the electrogalvanized steel sheet prepared as described above, using a slide hopper type curtain coating device, the lower layer and the outermost layer were applied simultaneously, and the galvanized steel sheet with the paint applied was applied to both layers in an induction heating furnace. Were baked at the same time.

[Invention Example 2]
The lower layer was applied with a roll coater, and the outermost layer was applied with a wet-on-wet with a curtain coater, and then both layers were simultaneously baked in an induction heating furnace.

[Comparative Example 1]
The lower layer and the outermost layer having the same components as in Invention Example 1 were used. The lower layer was applied on a prepared electrogalvanized steel sheet with a roll coater and then dried and cured, and then the outermost layer was applied and cured.

[Comparative Example 2]
The same as Example 1, except that an acid catalyst not neutralized with a volatile base was used.

The top coat film of the precoated metal plate thus obtained was analyzed by high frequency GDS.
Measurement of high-frequency GDS High-frequency GDS from the coating surface to the vicinity of the interface with the steel sheet was measured using System 3860 manufactured by Rigaku Denki Kogyo. The discharge power was 30 W, the sampling interval was 2 seconds, the nitrogen concentration was calculated from the integrated intensity of C, O, and N atoms by the following formula, and the nitrogen concentration distribution in the depth direction was examined. The values of the photomultiplier voltage (V) were set to C: 480, O: 900, and N: 640. In GDS, since the exact concentration ratio of each element cannot be quantified, the relative concentration ratio was determined by the following equation.
Nitrogen concentration (mol%) = (integral intensity of N) × 100 / (sum of integral intensities of N, C, O)
The spot diameter of GDS at this time was 5 mmφ.

[Evaluation of workability]
The processability of the precoated metal plate was evaluated at 20 ° C. The pre-coated metal plate was bent at 180 degrees, and the cracks generated in the bent portion were observed with a 20-fold magnifier, and the limit number of Ts where no cracks occurred was displayed. For example, 2T means a case where two plates having the same thickness as the pre-coated metal plate are sandwiched in the bent portion, and 0T means a case where the plate is bent 180 degrees without sandwiching a plate having the same thickness. This workability was evaluated at a temperature of 20 ° C.

[Evaluation of interlayer adhesion]
The prepared pre-coated metal plate was subjected to cylindrical drawing with a hydraulic Erichsen type press working tester. The cylindrical drawing was performed at a drawing ratio of 2.3. A cut was applied to the body part, a tape was applied, and then the film was peeled off to observe the presence or absence of peeling, and evaluated according to the following criteria.
◎: No peeling ○: Slight peeling △: Peeling at the cut part ×: Peeling at other than the cutting part

[Evaluation of contamination resistance]
The stain resistance was evaluated using magic ink. After drawing a line on the coating film surface with red magic ink, it was left at 20 ° C. for 24 hours, and the trace of the line after wiping with ethanol was visually evaluated and evaluated according to the following criteria.
◎: There is no trace of the line ○: Some trace of the line remains △: The trace of the line remains clearly ×: The line cannot be wiped off with ethanol

Stain resistance was evaluated using mustard. A commercially available SB Japanese style mustard (trademark) was dissolved in water (90% by mass), placed on the coating film, left to stand at 30 ° C. for 24 hours, washed with water, and visually evaluated for the scar mark after the following criteria. It was evaluated with.
◎: No trace ○: Some trace remains △: Clear trace remains ×: Almost no trace

The stain resistance was evaluated using a blue dye. Methyl violet was dissolved in acetone until saturated. Each precoated metal plate of the example was immersed in this solution. The metal plate was taken out after 24 hours, and the color residue after being washed with water was visually evaluated and evaluated according to the following criteria.
◎: No trace ○: Some trace remains △: Clear trace remains ×: Almost no trace

Hereafter, the evaluation result of the created precoat metal plate is described in the table.

  Inventive Examples 1 and 2 both showed good results in workability, interlayer adhesion, and contamination resistance. The average roughness (Ra) of the interfaces in these examples was 0.5 μm for all.

  In Comparative Example 1, the average roughness (Ra) of the interface is as small as 0.1 μm. For this reason, interlayer adhesion was very poor.

  Comparative Example 2 is an example where no acid catalyst neutralized and blocked with a volatile base was used. As can be seen from N / (C + O + N) by GDS, there is no melamine concentration gradient in the outermost layer or the lower layer. For this reason, contamination resistance and workability are deteriorated.

The schematic of the slide hopper type coating machine used by this invention. It is drawing which shows the method of calculating | requiring Ia, Ib, and Ii from a high frequency GDS chart. It is a figure explaining the Ra evaluation method of a coating-film interface.

Explanation of symbols

1 Die 2 Steel strip 3 Curtain guide 4 Curtain 5 Paint pan 6 Slit 7 Slide surface 8 Paint supply hole

Claims (6)

A pre-coated metal plate having an outermost layer disposed on the metal plate and a lower layer disposed under the outermost layer,
The outermost layer contains an aminoblast resin, and the concentration of the aminoblast resin on the surface portion in the outermost layer is higher than the concentration in the interface portion with the lower layer,
The pre-coated metal sheet, wherein the lower layer includes an amino blast resin, and the concentration of the amino blast resin in the interface portion on the outermost layer side in the lower layer is higher than the concentration in the interface portion on the metal plate side in the lower layer.   The precoated metal sheet according to claim 1, wherein Ra at the interface between the lower layer and the outermost layer is 0.3 to 0.6 μm.   3. The precoated metal sheet according to claim 1, wherein a value of N / (C + O + N) measured by GDS of the interface portion on the outermost layer side in the lower layer is 0.4 or more.   The coating liquid for forming the outermost layer and the lower layer both contains a polyester resin having a hydroxyl value of 40 or less, a methylated melamine resin, and an acid catalyst neutralized with a volatile base. The precoat metal plate as described in any one of Claims.   The method for producing a precoated metal sheet according to any one of claims 1 to 4, wherein the lower layer and the outermost layer are applied simultaneously in multiple layers, or both layers are simultaneously cured after wet-on-wet application. A pre-coated metal plate having an outermost layer disposed on the metal plate and a lower layer disposed under the outermost layer,
The outermost layer contains an aminoblast resin, and the concentration of the aminoblast resin on the surface portion in the outermost layer is higher than the concentration in the interface portion with the lower layer,
A precoated metal sheet that is simultaneously cured after the lower layer and the outermost layer are applied.

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