JP6121289B2 - Antifouling resin coated aluminum plate - Google Patents

Description

  The present invention relates to a resin-coated aluminum plate used for vehicles, building materials and the like.

  Aluminum plates are lightweight and have excellent corrosion resistance, and are therefore used in vehicles and building materials. Surfaces of vehicles and building materials used outdoors are contaminated by accumulation of dust floating in the atmosphere, combustion products in automobile exhaust gas, and the like. In particular, during rainy weather, dust and combustion products are carried by the rain and flow down on the surface (hereinafter also referred to simply as the surface) of vehicles and building materials. At that time, contaminants adhere to the surface along the rainwater path, and when dried, rain streak stains form on the surface. In order not to impair the aesthetic appearance, it is necessary to carry out regular cleaning. Therefore, an antifouling coating film having a self-cleaning function is formed on the surface by applying an antifouling paint for the purpose of reducing the number of cleanings and time. The self-cleaning function means that when dirt components adhere to the surface of a hydrophilic coating film formed of an antifouling paint, if rainwater or the like further adheres, the rainwater is placed between the dirt components and the hydrophilic coating film. Refers to the function of removing dirt components together with rainwater.

  By the way, when an aluminum plate is used for a vehicle, a building material, or the like, it is often processed into a shape suitable for the purpose, if necessary. An aluminum plate may be processed into a predetermined shape in advance, and then an antifouling coating film as described above may be formed on the surface of the processed product. However, the process of forming the coating film lengthens the production process of the target aluminum material, which is economically disadvantageous compared to the processing of the precoat material.

  Therefore, Patent Document 1 discloses an antifouling resistance pre-coated aluminum plate in which a chemical conversion film is formed on the surface of an aluminum plate and a two-layer coating film is further formed thereon. In this pre-coated aluminum plate, a coating containing a polyester resin having a number average molecular weight in the range of 20,000 to 30,000 and a polydispersity in the range of 2 to 10 as a base resin is baked to obtain a coating thickness of 3 to 13 μm. A base resin made of an acrylic resin, colloidal silica, a silicone compound containing an alkoxy group, and a paint containing one or more of pigments or dyes are baked to form an undercoat layer of 8 to 20 μm. The film thickness is the top coat layer. In the pre-coated aluminum plate of Patent Document 1, the undercoat layer uses a high molecular polyester resin having a number average molecular weight of 20,000 to 30,000 and a polydispersity in the range of 2 to 10 for the undercoat layer. Can be stretched and not cracked, and the bending workability can be improved. However, recently, when an antifouling resin-coated aluminum plate is studied for outdoor use, not only bending workability but also new corrosion resistance of a bent part is required. When the undercoat layer is not cracked, it is expected that the corrosion resistance of the bent portion is excellent as compared with the case where the undercoat layer is cracked.

JP 2006-247906 A

  However, in the CASS test 24 hours, the former and the latter are equivalent, but in the CASS test 48 hours, the corrosion resistance was poor when the former, that is, the undercoat layer was not cracked. Here, the corrosion resistance is not the depth of point-like corrosion, but refers to the fact that the component causing the corrosion has entered under the coating film, so that the coating film is lifted (bulging and cracking seen from the outside). . In the former case, water, chloride ions, copper ions, etc. penetrate into the cracked portion of the overcoat layer and penetrate into the undercoat layer, then reach the chemical conversion film and the aluminum plate, and corrosion proceeds. As a result, it is considered that gas is generated under the coating film, swelling occurs and cracks are expanded. In the latter case, water, chloride ions, copper ions, etc. that have entered the cracked portion of the undercoat layer reach the surface of the chemical conversion film, and corrosion is considered to proceed starting from the cracked portion of the undercoat layer. Therefore, the generated gas is considered to be easily diffused into the atmosphere. Therefore, if the time for the corrosion test is increased, in the former case, swelling is increased, and thus it is considered that the corrosion resistance in visual evaluation is inferior. Moreover, in the precoat aluminum plate of patent document 1, when a top coat layer is applied with a roll coater, an appearance (bubble drawing) is generated such that bubbles are drawn on the surface of the top coat layer, and the quality of the appearance is deteriorated and the bent portion is processed. The problem of poor corrosion resistance remained.

  An object of the present invention is to provide an antifouling resin-coated aluminum plate that can prevent deterioration in appearance quality and is excellent in anticorrosion and bending workability, as well as corrosion resistance of a bent part.

  An antifouling resin-coated aluminum plate according to the present invention includes an aluminum plate, a chemical conversion film formed on at least one surface of the aluminum plate, and a resin film formed on the chemical conversion film. A coated aluminum plate, wherein the resin film has a two-layer structure composed of an overcoat layer and an undercoat layer, the overcoat layer contains a silica-containing acrylic resin and acrylic particles, and the undercoat layer has a glass transition temperature. Including a polyester resin having a number average molecular weight of 10,000 to 18000, the film thickness of the overcoat layer being film thickness A, and the film thickness of the undercoat layer being film thickness B. The film thickness B is 9 μm to 19 μm, and the ratio of the film thickness A to the film thickness B (film thickness A / film thickness B) is 0.8 or less.

  The ratio of the film thickness A to the film thickness B (film thickness A / film thickness B) is 0.2 to 0.8, the film thickness A is 2 μm to 10 μm, and the film thickness A and the The total thickness (A + B) of the film thickness B is preferably 15 μm to 25 μm.

  Moreover, it is preferable that the average particle diameter of the acrylic particles is 0.002 μm to 0.5 μm.

  The antifouling resin-coated aluminum plate according to the present invention uses a polyester resin having a low glass transition temperature and a number average molecular weight of 10,000 to 18,000 for the undercoat layer, and further the film thickness of the undercoat layer and the overcoat layer and the undercoat layer. By limiting the film thickness ratio of the layers, even if the overcoat layer is cracked after bending, the undercoat layer follows the deformation of the aluminum plate, and the crack of the overcoat layer is small. Excellent appearance.

  By controlling the molecular weight of the polyester-based resin used for the undercoat layer, the crosslink density is increased, the crosslink structure of the undercoat layer after bending is made dense, and the barrier property against penetration of chloride ions, moisture, etc. is improved. Since chloride ions, moisture and the like entering from the cracked portion of the topcoat layer do not easily penetrate into the undercoat layer, aluminum corrosion is difficult to start, and the corrosion resistance of the bent portion is improved.

  Further, by incorporating acrylic particles in the topcoat layer, the foaming disappears and the coating appearance is improved.

  By interacting with each other, the corrosion resistance of the bent portion of the antifouling resin-coated aluminum plate according to the present invention is remarkably improved.

(A) Aluminum plate The aluminum used in the present invention is preferably a 5000 series aluminum alloy, a 3000 series aluminum alloy, or a 1000 series aluminum alloy among aluminum alloys for rolling. The plate thickness is preferably 0.4 mm or more and 1.2 mm or less. An aluminum alloy plate (referred to as an aluminum plate in the present invention) is obtained by chamfering a rectangular parallelepiped slab (rolling ingot), hot-rolling at a high temperature of about 400 ° C. or higher, and cooling at room temperature. Manufactured by hot rolling.

(B) Chemical conversion film In this invention, a chemical conversion film is formed in the at least one surface of an aluminum plate. In forming the chemical conversion film, first, the cold-rolled aluminum plate is degreased. As the degreasing treatment, an alkaline degreasing treatment is preferable. The alkaline degreasing solution is a solution in which a surfactant and an alkali builder added to ensure degreasing properties are dissolved or dispersed in water. The surfactant has an effect of emulsifying and removing the oil remaining on the surface of the aluminum plate after cold rolling, and preventing the emulsified and dispersed oil from reattaching to the surface of the aluminum plate. Examples of such surfactants include polyoxyethylene surfactants such as polyoxyethylene alkyl ether and polyoxyethylene alkylphenyl ether, and higher alcohol surfactants. Alkali builders have the effect of chelating polyvalent cations such as calcium ions and magnesium ions present in hard water or mixed in alkali builders to prevent insolubilization of the surfactant. Examples of the alkali builder include alkali metal carbonates such as sodium carbonate and potassium carbonate, alkali metal hydroxides such as sodium hydroxide, alkali metal phosphates such as sodium phosphate and sodium hydrogen phosphate, and alkalis such as sodium silicate. Metal silicate or the like is used. The alkaline degreasing solution is preferably sprayed on the surface of the aluminum plate after cold rolling by a spray method. It is preferable to perform a washing treatment with water immediately after the alkaline degreasing treatment. The purpose of this is to prevent contamination with an alkaline degreasing solution in the subsequent process and to remove alkali components, surfactants, reaction residues, and the like remaining on the surface of the aluminum plate. A method of spraying clean water or ion-exchanged water on the surface of an aluminum plate is usually applied to the washing treatment with water. Then, in order to adjust the surface of an aluminum plate as needed, you may perform a pickling process.

Subsequently, the aluminum plate degreased as described above is subjected to chemical conversion treatment. As the chemical conversion treatment, phosphoric acid chromate treatment is preferable from the viewpoint of corrosion resistance and economy. The phosphoric acid chromate treatment solution is a solution obtained by mixing a main agent containing chromic acid and orthophosphoric acid and an auxiliary agent containing fluoride and diluting with water. Examples of the fluoride include hydrofluoric acid, hexafluorosilicic acid, hexafluorozirconic acid, hexafluorotitanic acid, and tetrafluoroboric acid. The chemical conversion treatment liquid is preferably sprayed on the surface of the degreased aluminum plate by a spray method. It is preferable to perform a washing treatment with water immediately after the phosphoric acid chromate treatment. The amount of the phosphate chromate film formed on the surface of the aluminum plate is preferably 10 mg / m ² or more and 50 mg / m ² or less in terms of the amount of metal element Cr. If it is less than 10 mg / m ² , the corrosion resistance is insufficient, and if it exceeds 50 mg / m ² , the effect of corrosion resistance is saturated and the cost increases. The amount of Cr is measured by fluorescent X-ray elemental analysis (JIS K 0119).

(C) Resin film In the present invention, a resin film is formed on a chemical conversion film, and the resin film has a two-layer structure including an overcoat layer and an undercoat layer.

(C-1) Undercoat layer In the present invention, the undercoat layer contains a polyester resin having a glass transition temperature of 7 ° C to 20 ° C and a number average molecular weight of 10,000 to 18,000.

(C-1-1) Polyester resin The polyester resin is superior to the epoxy resin usually used for the undercoat layer, and has excellent film pick-up properties. Can be secured. The polyester-based resin can be obtained by polycondensation using an acid component mainly composed of a polyvalent carboxylic acid and an alcohol component mainly composed of a polyhydric alcohol as raw materials. Acid components include phthalic acid, trimet acid and their anhydrides, succinic acid, adipic acid, azelaic acid, sebacic acid, dodecanedicarboxylic acid, saturated aliphatic dicarboxylic acids such as 1,4-cyclohexanedicarboxylic acid, and their anhydrides. Or a mixture thereof. Examples of the alcohol component include ethylene glycol, 1,3-propanediol, 1,4-butanediol, 1,5-pentanediol, 1,5-hexanediol, diethylene glycol, triethylene glycol, 1,4-cyclohexanediol, , 4-cyclohexanedimethanol, 1,2-propanediol, neopentyl glycol, 1,2-butanediol, 1,3-butanediol, 1,2-pentanediol, 2,3-pentanediol, 1,4- Pentanediol, 1,4-hexanediol, 2,5-hexanediol, 3-methyl-1,5-pentanediol, 1,2-dodecanediol, 1,2-octadecanediol, trimethylolpropane, glycerin, pentaerythritol Or a mixture of these It is possible.

  The polyester resin used in the present invention has a number average molecular weight of 10,000 to 18,000, and a glass transition temperature of 7 to 20 ° C. As will be described later, the overcoat layer in the present invention is hard for exhibiting excellent antifouling properties, and cracks occur when the processing conditions are severe after press bending. However, the number average molecular weight and glass transition temperature of the polyester resin contained in the undercoat layer are within the above ranges, so that when the aluminum plate is processed, the undercoat layer follows the deformation of the aluminum plate, The magnitude | size of a crack can be made small and the external appearance of the bending process part by visual observation becomes favorable. When the number average molecular weight exceeds 18000, the molecular weight between cross-linking points becomes large, the molecular chain forming the cross-linked structure is extended by tensile deformation by bending, and the space between the cross-linking points is expanded. Therefore, moisture, chloride ions, copper ions, etc. are likely to penetrate into the undercoat layer. As a result, the arrival time of chloride ions, copper ions, etc. under the resin film is short, and the corrosion of aluminum progresses, so that the undercoat layer is pushed up and swells and cracks. Inferior. On the other hand, when the number average molecular weight is less than 10,000, the molecular weight between cross-linking points is small and the expansion between the cross-linking points is small, so that moisture, chloride ions, copper ions, etc. do not easily penetrate into the undercoat layer. However, since the elongation of the undercoat layer is small, the undercoat layer is cracked after bending and the crack of the overcoat layer is also increased. That is, bending workability is inferior. Further, moisture, chloride ions, copper ions, etc. that have entered the cracked portions of the overcoat layer and undercoat layer reach the chemical conversion film and aluminum, and corrosion progresses. Compared with an unprocessed part, the adhesiveness of an undercoat layer and a chemical conversion film falls in a bending process part. Since the aluminum corrosion of the cracked portion also proceeds, swelling and peeling of the resin film are recognized, and the corrosion resistance of the bent portion is inferior. The number average molecular weight is dissolved in THF as an eluent using a gel permeation chromatograph (GPC), detected with a UV detector (254 nm), and measured in terms of standard polystyrene. The molecular weight between the crosslinking points is inversely proportional to the crosslinking density. The crosslinking density was measured using a dynamic viscoelasticity measuring device at a frequency of 1 Hz, a strain of 0.05%, a temperature increase rate of 3 ° C./minute, and a temperature. The minimum value of the dynamic storage rubber elastic modulus is obtained from the temperature dependence curve of the storage elastic modulus of the free film in the range of −20 ° C. to 180 ° C., and can be calculated by the following relational expression.

Crosslink density = {dynamic storage modulus / (3 × R × T)}
R: Gas constant T: Measurement temperature The crosslink density in the undercoat layer is preferably 9.5 × 10 ^{−5 to} 1.0 × 10 ⁻² mol / cc. More preferably, it is 5.0 * 10 < ^-4 > -5.0 * 10 < ^-3 > mol / cc. Moreover, the molecular weight between crosslinking points in the undercoat layer is the reciprocal of the crosslinking density, and is preferably 1.0 × 10 ^{2 to} 1.1 × 10 ⁴ . More preferably, it is 2.0 × 10 ^{2 to} 2.0 × 10 ³ . When the crosslink density is less than 9.5 × 10 ⁻⁵ mol / cc, the space between the crosslink points is expanded, and the corrosion resistance of the bent portion may be inferior. On the other hand, when the crosslink density exceeds 1.0 × 10 ⁻² mol / cc, the undercoat layer is insufficiently stretched, the bending workability is inferior, and the corrosion resistance of the bent part may be inferior.

  The glass transition temperature is a temperature at which the physical properties of the resin change. The resin becomes a glassy hard state at a temperature lower than the glass transition temperature, and has a rubbery elasticity at a temperature exceeding the glass transition temperature. The processing temperature for bending is usually 25 to 30 ° C. When the glass transition temperature of the polyester resin in the present invention is less than 7 ° C., the polyester resin at the temperature at the time of bending is in a state having rubber-like elasticity. The elongation of the undercoat layer at the bending temperature is significantly larger than the elongation of the aluminum alloy material or the hard overcoat layer. Adhesion is lowered, the undercoat layer is easily peeled off from the chemical conversion film, and the corrosion resistance of the bent portion is inferior. When the glass transition temperature of the polyester resin in the present invention exceeds 20 ° C., the elongation of the undercoat layer at the temperature at the time of bending is equal to or less than the elongation of the aluminum alloy material or the overcoat layer. Following the deformation, the undercoat layer breaks. That is, bending workability is inferior. Further, since corrosion of the aluminum in the cracked portion proceeds, the corrosion resistance of the bent portion is also inferior. The glass transition temperature is measured by differential scanning calorimetry (DSC) by raising the temperature from 0 ° C. to 200 ° C. at 10 ° C./min in a nitrogen atmosphere.

(C-1-2) Curing agent The undercoat layer in the present invention may contain a curing agent. An amino resin such as a melamine resin or a urea resin is used as the curing agent. It is preferable to use a melamine resin having excellent water resistance. The melamine resin is produced by heating and polycondensing methylolmelamine obtained by condensing melamine and formaldehyde under alkaline conditions. Examples of the melamine resin include isobutyl alcohol-modified melamine resin, butylated melamine resin, butylated urea melamine resin, isobutylated melamine resin, methylated melamine resin, and a mixture thereof.

  The blending ratio (weight ratio) of the polyester resin and the curing agent is preferably 98: 2 to 70:30. When the blending ratio of the curing agent is less than 2% by weight with respect to 100% by weight of the total amount of the polyester resin and the curing agent, the curing becomes insufficient, and the bending workability and the corrosion resistance of the bent portion may be inferior. When the blending ratio of the curing agent exceeds 30% by weight with respect to the total of 100% by weight of the polyester resin and the curing agent, the unreacted material of the curing agent increases, and the bending workability and the corrosion resistance of the bent portion may be inferior. is there.

(C-1-3) Additive The undercoat layer in the invention preferably contains a pigment such as a white pigment, a black pigment, or a blue pigment as appropriate. In order to improve the paintability, extender pigments, leveling agents and the like may be appropriately contained.

(C-1-4) Film thickness B
The thickness B of the undercoat layer in the present invention is 9 to 19 μm. When the film thickness B of the undercoat layer is less than 9 μm, the film thickness of the undercoat layer is reduced by bending, so that the barrier property of the undercoat layer is insufficient and the corrosion resistance of the bent portion is inferior. When the thickness B of the undercoat layer exceeds 19 μm, the amount of deformation of the undercoat layer at the time of bending increases, and the undercoat layer breaks due to the influence of cracking of the overcoat layer, and the corrosion resistance of the bent portion is inferior.

(C-2) Topcoat layer In the present invention, the topcoat layer contains silica-containing acrylic resin and acrylic particles as essential components.

(C-2-1) Silica-containing acrylic resin The silica-containing acrylic resin used in the present invention is based on an acrylic resin and contains colloidal silica. In the outermost layer of the topcoat layer, colloidal silica is concentrated and has a dense structure. Further, since the top coat layer has a high gel fraction of 92% or more, moisture, chloride ions, etc. are difficult to permeate, and the corrosion resistance of the resin film that is not cracked or unprocessed by the bending process is remarkably excellent. The acrylic resin is an acrylic ester or methacrylic ester polymer and is an amorphous synthetic resin. Examples of the acrylate ester include methyl acrylate, ethyl acrylate, butyl acrylate, 2-dimethylaminoethyl acrylate, 2-hydroxyethyl acrylate, and the like. In addition, methacrylic acid esters include methyl methacrylate, ethyl methacrylate, n-butyl methacrylate, i-butyl methacrylate, t-butyl methacrylate, 2-ethylhexyl methacrylate, lauryl methacrylate, dodecyl methacrylate, methacrylic acid. Stearyl etc. can be mentioned. Moreover, acrylic acid ester or methacrylic acid ester can be copolymerized with acrylic acid, methacrylic acid, hydroxyalkyl acrylate, hydroxyalkyl methacrylate, or the like. In addition, there is no particular problem even if a fluorine resin, a urethane resin or the like is blended.

  Colloidal silica is added to the acrylic resin as described above to impart hydrophilicity. Colloidal silica is concentrated in the outermost layer of the topcoat layer, and is uniformly distributed below the outermost layer. The colloidal silica concentrated in the outermost layer can densify the structure of the surface of the resin film, prevent entry of dirt, and can form silanol to bring about hydrophilicity. When hydrophilicity is imparted to the topcoat layer, rainwater or the like flows smoothly and is difficult to accumulate, and attached dirt is easily removed.

  The content of colloidal silica affects the degree of hydrophilicity and processability. The hydrophilicity is usually evaluated by a contact angle with water, and the contact angle is desirably 50 ° or less. When the contact angle exceeds 50 °, that is, when the hydrophilicity is low, attached rainwater or the like is likely to be accumulated, and the removal of dirt is hindered. Thus, in order to ensure hydrophilicity such that the contact angle is 50 ° or less, the content of colloidal silica is 1% by weight or more and 20% by weight or less with respect to 100% by weight of the acrylic resin. preferable. If the amount is less than 1% by weight based on 100% by weight of the acrylic resin, the hydrophilic effect cannot be sufficiently obtained. On the other hand, even if it exceeds 20% by weight with respect to 100% by weight of the acrylic resin, there is no further improvement in hydrophilicity, and only the cost is increased. On the other hand, if the content of colloidal silica exceeds 20% by weight, the starting point of resin film cracking increases, and cracking is likely to occur during processing.

  The average particle size of the colloidal silica is preferably 0.1 μm or less. The particle size of colloidal silica affects processability. Colloidal silica having a large particle size is a starting point for cracking of the topcoat layer.

(C-2-2) Acrylic Particles Acrylic particles are fine particles synthesized by emulsion polymerization of acrylic monomers such as acrylic acid esters and methacrylic acid esters. It may be copolymerized with other vinyl monomers such as styrene. Since these acrylic particles are distributed inside the topcoat layer after baking and drying and do not bleed to the outermost layer of the topcoat layer, the antifouling property is not adversely affected.

  In the present invention, foaming occurs when acrylic particles are not added to the paint for the top coat layer. If foaming is present in the bent portion, the crack of the top coat layer becomes large and the undercoat layer is also cracked locally, so that the corrosion resistance of the bent portion is inferior. This bubbling is due to the paint running out when the paint is picked up by the roll. An antifoaming agent is usually added in order to eliminate the foam generated in the overcoat layer coating, but even if an antifoaming agent is added to the overcoat layer coating, no effect is observed. On the other hand, when acrylic particles are added to the paint for the topcoat layer in an amount of 0.1% by weight or more with respect to 100% by weight of the acrylic resin, the acrylic particles are moderately dispersed with each other and weak among the acrylic particles. Interaction occurs. Thereby, the structural viscosity of the paint for top coat layers improves. Therefore, the elongation of the topcoat layer coating is increased as compared with the case where no acrylic particles are added to the topcoat layer coating. By improving the elasticity of the liquid film of the paint for the top coat layer, the paint for the top coat layer does not run out of liquid and foaming does not occur after drying. On the other hand, when the content of acrylic particles exceeds 2.3% by weight with respect to 100% by weight of the acrylic resin, the effect is saturated and the cost increases. The elongation of the paint is an elongation measured when a paint whose viscosity is adjusted to 30 seconds is dipped in a circular frame and pulled up at 1 cm / second. The effect which suppresses foaming is recognized as elongation of a coating material is 6.5 mm or more.

  The average particle diameter of the acrylic particles is preferably in the range of 0.002 μm to 0.5 μm. When the average particle size is less than 0.002 μm, acrylic particles aggregate non-uniformly, so that the structural viscosity of the paint cannot be improved, foaming occurs, and the corrosion resistance of the bent portion may be inferior. . When the average particle diameter exceeds 0.5 μm, the acrylic particles serve as a starting point for cracking of the overcoat layer, and the bending workability may be inferior, and the corrosion resistance of the bent part may be inferior. The average particle diameter is measured by a dynamic light scattering method. Use a dynamic light scattering photometer (Zetasizer Nano ZS90 manufactured by Malvern Co., Ltd.) with a measurement temperature of 25 ° C, a measurement time of 10 seconds, and a measurement count of 5 times. The average particle diameter is calculated as an average value of 5 measurements.

(C-2-3) Curing Agent The overcoat layer in the present invention may contain a curing agent. It is preferable to use a butylated melamine resin, an isocyanate resin, an etherified melamine resin, or the like as the curing agent. As the isocyanate resin, a polyisocyanate compound, a polyhydric alcohol adduct of a polyisocyanate compound, or the like is used. The isocyanate group may be blocked with a blocking agent.

  The mixing ratio (weight ratio) of the silica-containing acrylic resin and the curing agent is preferably 95: 5 to 55:45. If the blending ratio of the curing agent is less than 5% by weight with respect to 100% by weight of the total of the silica-containing acrylic resin and the curing agent, the antifouling property may be inferior. If the blending ratio of the curing agent exceeds 45% by weight with respect to a total of 100% by weight of the silica-containing acrylic resin and the curing agent, bending workability may be inferior.

(C-2-4) Additives The topcoat layer in the invention may contain a pigment or dimethylpolysiloxane in order to satisfy the design properties according to the application. Examples of the pigment include inorganic pigments such as titanium oxide and carbon black, and organic pigments such as cyanine green and cyanine blue.

(C-2-5) Film thickness A
In the present invention, the film thickness A of the overcoat layer is preferably in the range of 2 μm to 10 μm. When the film thickness A is less than 2 μm, the influence of the color of the undercoat layer on the color tone after coating the overcoat layer becomes strong, color matching becomes difficult, and appearance quality may be inferior. On the other hand, when the film thickness A exceeds 10 μm, the degree of roping and cocoon skin becomes severe, and the appearance quality may be inferior. The total thickness (A + B) of the topcoat layer and the undercoat layer is preferably 15 μm to 25 μm. When the total thickness (A + B) is less than 15 μm, the corrosion resistance is poor. On the other hand, if the total thickness (A + B) exceeds 25 μm, bending workability may be inferior. The ratio of film thickness A to film thickness B (film thickness A / film thickness B) is 0.8 or less. Preferably, it is the range of 0.2-0.8. When the ratio of the film thickness A to the film thickness B exceeds 0.8, for example, when the overcoat layer is 8 μm and the undercoat layer is 9 μm, or when the overcoat layer is 9 μm and the undercoat layer is 9 μm or 11 μm The crack of the overcoat layer becomes large, and the undercoat layer breaks without enduring. That is, the film thickness of the overcoat layer becomes relatively large, the crack of the overcoat layer becomes large, and the corrosion resistance of the bent portion is inferior. When the ratio of the film thickness A to the film thickness B is less than 0.2, for example, when the film thickness of the overcoat layer is 2 μm and the film thickness of the undercoat layer is 19 μm, 17 μm, or 14 μm, the color of the undercoat layer is overcoated The effect on the color tone after painting the layer becomes stronger. That is, since the film thickness of the overcoat layer occupying the total thickness is thin, it is difficult to match the color required according to the application, and the appearance quality may be inferior.

(C-3) Coating method In the present invention, after forming a chemical conversion film on the surface of the aluminum plate, an undercoat layer coating material in which the composition for forming the undercoat layer is dissolved and dispersed in an organic solvent is applied, and further heated. And bake and harden. On the undercoat layer, an overcoat layer coating material in which the composition for forming the overcoat layer is dissolved and dispersed in an organic solvent is applied and further heated to be baked and cured.

  The organic solvent is not particularly limited as long as each component can be dissolved or dispersed, and examples thereof include acetone, methyl ethyl ketone, cyclohexanone, ethanol, ethylene glycol monoethyl ether, propylene glycol ethyl ether, and the like. It is preferable to add so that the heating residue is 20 to 80%. If the heating residue is less than 20%, foaming or the like may occur during baking, and the resin film may not be formed uniformly. When the heating residue exceeds 80%, the viscosity of the paint becomes high, and it is difficult to handle and it may be difficult to apply the paint uniformly. As a method for applying the coating composition for the undercoat layer and the coating composition for the topcoat layer prepared by dissolving and dispersing the composition for forming the undercoat layer and the topcoat layer in an organic solvent, a roll coater method excellent in uniformity and productivity is preferable. . Examples of the roll coater method include a reverse coat method, a natural coat method, and a gravure roll method. Moreover, as a method of drying after applying the paint, it is preferable to use a hot air drying method, an induction heating method, or a near infrared heating method. It is preferable to produce the baking temperature at a material reaching temperature (PMT) of 200 to 300 ° C. When the baking temperature is less than PMT 200 ° C., the antifouling property may be inferior. When the baking temperature exceeds PMT 300 ° C., the resin may be decomposed due to overbaking.

  Examples and Comparative Examples are described below, but are not limited to these Examples.

(Examples 1-13 and Comparative Examples 1-8)
After each component constituting the undercoat layer shown in Tables 1 and 2 is dissolved and dispersed in an organic solvent, titanium oxide is added so that the ratio of solid content and titanium oxide constituting the undercoat layer is 100: 16. As a result, a paint for an undercoat layer was obtained. Similarly, after each component constituting the top coat layer shown in Tables 1 and 2 is dissolved and dispersed in an organic solvent, the titanium oxide is adjusted so that the ratio of solid content and titanium oxide constituting the top coat layer is 100: 16. Was added to obtain a paint for the topcoat layer. An aluminum alloy plate (5052-H34 material, 1.0 mm thickness) is alkali degreased, washed with water, subjected to chemical conversion treatment (phosphate chromate), and then applied with a roll coater for an undercoat layer paint and an overcoat layer paint, Heating was performed in a hot air drying furnace to obtain a resin-coated aluminum plate shown in Table 1. The film thickness was measured by an eddy current method according to JIS K 5600-1-7. The baking temperature of the undercoat layer was PMT 210 ° C., and the baking temperature of the overcoat layer was PMT 232 ° C.

The silica-containing acrylic resin contained in the overcoat layer is obtained by adding 0.08 μm colloidal silica to an acrylic resin. The content of colloidal silica was 3% by weight with respect to 100% by weight of the acrylic resin. The number average molecular weight of the polyester resin contained in the undercoat layer was measured using a gel permeation chromatograph (GPC) in accordance with JIS K7252. The glass transition temperature was measured by differential scanning calorimetry (DSC) based on JIS K7121. The average particle diameter of the acrylic particles was measured by a dynamic light scattering method based on JIS Z8826. The amount of chemical conversion film was measured by fluorescent X-ray elemental analysis and found to be 30 mg / m ² in terms of the amount of metal element Cr.

  The resin-coated aluminum plate thus obtained was evaluated for appearance, bending workability, corrosion resistance of the bent portion, and antifouling property. The evaluation results are shown in Tables 1 and 2.

<Appearance>
The appearance of the resin-coated aluminum plate was visually evaluated. ○ was accepted and x was rejected.
○: No foaming is observed.
X: Foaming is recognized.
<Bending workability>
The resin-coated aluminum plate was subjected to press bending (1.5R), and the appearance of the bent portion of the obtained sample was visually evaluated. Further, the appearance after the cellophane tape was bonded and peeled was visually evaluated. ○ was accepted and x was rejected.
a) Resin film cracking ◯: No cracking is observed.
X: A crack is recognized.
b) Adhesiveness ○: No peeling.
X: There is peeling.
<Corrosion resistance of bent parts>
Using a sample obtained by press-bending (1.5R) the resin-coated aluminum plate, a CASS test was performed for 48 hours, and the state of the resin film in the bent portion was visually evaluated. ○ and △ were accepted and x was rejected.
○: Slight swelling is observed, but no peeling is observed.
(Triangle | delta): Although swelling is recognized, there are few, and peeling and a crack are partially recognized.
X: Many swellings, cracks or peelings are observed.
<Anti-fouling>
The resin-coated aluminum plate was subjected to an exposure test in an industrial area, and the appearance of stains and the removability of stains were evaluated after 6 months. In addition, regarding the stain appearance, a color difference (ΔE) before and after the exposure test was measured using a spectral colorimeter. Further, regarding the stain removability, the color tone of the surface after wiping with Kimwipe (registered trademark) containing water was measured using a spectrocolorimeter, and the color difference (ΔE) before and after the exposure test was calculated. ○ and △ were accepted and x was rejected.
a) Appearance of dirt ○: Slightly but dirt is observed (ΔE ≦ 2).
Δ: Dirt is slightly recognized (2 <ΔE ≦ 4).
X: Dirt is remarkably recognized (4 <ΔE).
b) Dirt removability ○: Dirt is completely removed (ΔE ≦ 1).
Δ: Dirt is slightly removed (1 <ΔE ≦ 2).
X: Dirt remains significantly (2 <ΔE).

  In the resin-coated aluminum plates obtained in Examples 1 to 13, the overcoat layer includes a silica-containing acrylic resin and acrylic particles, and the undercoat layer has a glass transition temperature of 7 to 20 ° C. and a number average. Including a polyester resin having a molecular weight of 10,000 to 18000, the film thickness B of the undercoat layer is 9 to 19 μm, and the ratio of the film thickness A to the film thickness B (film thickness A / film thickness B) is 0.8 or less. It is. Appearance, bending workability, corrosion resistance of the bent portion, and antifouling properties were all good.

  In Comparative Example 1, since the glass transition temperature of the polyester resin contained in the undercoat layer exceeds 20 ° C., the bending workability and the corrosion resistance of the bent portion were inferior.

  In Comparative Example 2, since the molecular weight of the polyester resin contained in the undercoat layer was less than 10,000, the bending workability and the corrosion resistance of the bent portion were inferior.

  In Comparative Example 3, since the molecular weight of the polyester resin contained in the undercoat layer exceeded 18000, the corrosion resistance of the bent portion was inferior.

  In Comparative Example 4, since the glass transition temperature of the polyester resin contained in the undercoat layer was less than 7 ° C., the bending workability and the corrosion resistance of the bent portion were inferior.

  In Comparative Example 5, since the thickness of the undercoat layer exceeded 19 μm, the bending workability and the corrosion resistance of the bent portion were inferior.

  In Comparative Example 6, since the thickness of the undercoat layer was less than 9 μm, the corrosion resistance of the bent portion was inferior.

  In Comparative Example 7, since the film thickness ratio between the topcoat layer and the undercoat layer exceeded 0.8, the bending workability and the corrosion resistance of the bent portion were inferior.

  Since the comparative example 8 did not contain acrylic particles in the overcoat layer, the appearance, bending workability, and corrosion resistance of the bent portion were inferior.

Claims (3)

An antifouling resin-coated aluminum plate comprising an aluminum plate, a chemical conversion film formed on at least one surface of the aluminum plate, and a resin film formed on the chemical conversion film,
The resin film has a two-layer structure composed of an overcoat layer and an undercoat layer. The overcoat layer contains a silica-containing acrylic resin and acrylic particles, and the undercoat layer has a glass transition temperature of 7 ° C to 20 ° C. And a polyester-based resin having a number average molecular weight of 10,000 to 18,000, and when the film thickness of the overcoat layer is A and the film thickness of the undercoat layer is B, the film thickness B is 9 μm. An antifouling resin-coated aluminum plate having a thickness of 19 μm and a ratio of the film thickness A to the film thickness B (film thickness A / film thickness B) is 0.8 or less.   The ratio of the film thickness A to the film thickness B (film thickness A / film thickness B) is 0.2 to 0.8, the film thickness A is 2 μm to 10 μm, and the film thickness A and the film thickness. 2. The antifouling resin-coated aluminum plate according to claim 1, wherein the total thickness (A + B) of B is 15 to 25 [mu] m.   3. The antifouling resin-coated aluminum plate according to claim 1, wherein an average particle diameter of the acrylic particles is 0.002 μm to 0.5 μm.