JP2005169648A - Urethane coated metal sheet and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain a urethane coated metal sheet improved in ply separation resistance and excellent in humidity resistance and scratch resistance. <P>SOLUTION: A topcoating film 6 based on a urethane resin is laminated on the undercoating film 4, which is formed from urethane coating compounded with thermoplastic resin particles 3 and provided on a base material/metal sheet 1 and a part of the thermoplastic resin particles 3 dispersed in the undercoating film 4 enters the topcoating film 6 in a wedge shape to form projections 3p. A coating composition prepared by compounding the thermoplastic resin particles 3 with an average particle size of 5-100 μm with the urethane coating is used for forming the undercoating film 4. The coating composition is applied to the metal sheet 1 and baked at the maximum arriving sheet temperature of 200-260°C to form the undercoating film 4 and the urethane coating is again applied and baked to form the topcoating film 6 wherein the wedge-shaped projections 3p enter. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a coated metal plate that is excellent in interlayer adhesion of a coating film and is used for exterior materials, interior materials, surface materials, and the like.

  Metal plates provided with a vinyl chloride coating film that easily imparts various patterns and patterns are used for exterior materials, interior materials, cover materials, and the like. However, the use of vinyl chloride tends to be avoided because it adversely affects the environment, including the dioxin problem, and urethane resin coatings are beginning to be used as coatings in place of vinyl chloride in coated metal plates. The urethane resin coating film is a coating film formed by cross-linking and curing by producing a urethane bond (-NHCOO-) by a polyaddition reaction between a polyol and an isocyanate (curing agent). In the paint film using urethane paint, workability and durability equivalent to those of PVC steel sheet are obtained by undercoating and overcoating a thick paint film with urethane paint.

In the urethane resin coating film having a two-layer structure, the lower layer coating film is made soft in order to ensure processability, and the upper layer coating film is made a hard coating film in order to improve the weather resistance. Since the coating film hardness differs between the lower layer coating film and the upper layer coating film, when shear strain is applied from the coating film surface, the strain tends to concentrate on the interface between the lower layer coating film and the upper layer coating film. Therefore, if the interlayer adhesion between the lower layer coating film / upper layer coating film is insufficient, the upper layer coating film may be peeled off due to contact with a foreign substance or the like during handling or forming of the coated metal plate.
By the way, when forming a coating film using the urethane paint which mix | blended the thermoplastic resin particle | grains with the present inventors, the uneven | corrugated pattern similar to the skin tone applied to the vinyl chloride coating metal plate by embossing is expressed. It has been found and introduced as a coated metal plate with improved design properties (Patent Document 1). The appearance of the uneven pattern is considered to be caused by remelting of the thermoplastic resin particles in a state constrained by the surface tension of the urethane paint when the urethane resin coating film is baked.
JP 2002-003780 A

The inventors of the present invention investigated and examined in detail the process of forming a coating film from a urethane paint containing thermoplastic resin particles. As a result, the remelting of the thermoplastic resin particles during baking of the coating film is not limited to the patterning of the coating film surface, and further improves the interlayer adhesion of the lower coating film / upper coating film when the upper coating film is laminated. I found that it was also effective.
The present invention has been completed on the basis of knowledge about remelting of thermoplastic resin particles, and a part of the remelted thermoplastic resin particles is wedged into the upper layer coating film / An object of the present invention is to provide a coated metal sheet having improved interlayer adhesion, moisture resistance and scratch resistance of the upper coating film.

The coated metal plate of the present invention has an upper layer coating film based on a urethane resin laminated on a base material / metal plate to a lower layer coating film formed with a urethane coating compounded with thermoplastic resin particles. A part of the thermoplastic resin particles dispersed in the film is characterized by forming a wedge-like protrusion in the upper coating film.
The thermoplastic resin particles are preferably resin particles adjusted to have a melting temperature lower than the firing temperature of the upper layer coating film. The lower layer coating may be provided directly on the substrate / metal plate that has been pre-painted, but in order to ensure the adhesion of the coating to the substrate / metal plate, it is an epoxy coating with excellent adhesion and epoxy modification. It is provided via an undercoat film such as a polyester film or a polyester film.
For the formation of the lower layer coating film, a coating composition is used in which thermoplastic resin particles having an average particle size of 5 to 100 μm are blended with a urethane coating composed of a polyester resin and an isocyanate curing agent. The lower layer coating film is formed by applying the coating composition to the substrate / metal plate and baking it at a maximum reached plate temperature of 200 to 260 ° C. Next, a urethane paint composed of a polyester resin and an isocyanate curing agent is applied, and an upper layer coating film is formed by baking at a maximum plate temperature of 200 to 260 ° C.

In the present invention, the lower layer coating film and the upper layer coating film are physically meshed in addition to the chemical affinity of the lower layer coating film / upper layer coating film by utilizing the phenomenon expressed by the combination of the urethane coating and the thermoplastic resin particles. As a result, the adhesion between the layers is improved, and a urethane-coated metal plate having excellent moisture resistance and scratch resistance is obtained.
It is guessed as follows that the interlayer adhesion of the lower layer coating film / upper layer coating film is improved by the blending of the thermoplastic resin particles, and it is also confirmed in Examples described later.

When a urethane paint containing thermoplastic resin particles is applied to the metal plate 1, a wet coating film 2 in which the thermoplastic resin particles 3 are dispersed is formed (FIG. 1a). Since the thermoplastic resin particles 3 have substantially the same specific gravity as the urethane paint, the thermoplastic resin particles 3 are taken into the coating film 2 to form a surface in which a portion of the thermoplastic resin particles 3 is a convex portion 2p.
The thermoplastic resin particles 3 are softened or melted and fluidized at the time of heating when the wet coating film 2 is baked and dried to form a dry coating film 4, but the surroundings are restrained by the surface tension of the urethane paint. The plastic resin particles 3 are flattened (FIG. 1b). Due to the flattening of the thermoplastic resin particles 3, the surface of the wet coating film 2 is drawn to form a recess 4 d. The urethane coating film on the flattened thermoplastic resin particles 3 is also thinned, and the thermoplastic resin particles 3 are positioned in the vicinity of the surface of the coating film 4.

Next, a urethane coating for forming an upper layer coating is applied on the coating 4 and a wet upper coating 5 is stacked. The urethane coating includes the thinned coating on the thermoplastic resin particles 13. It swells with the solvent present (FIG. 1c). When the wet upper coating film 5 is baked in this state, the thermoplastic resin particles 3 are remelted and the lower coating film 4 is thermally contracted. The shrinkage force F generated in the lower layer coating film 4 acts as a force that pushes up the thermoplastic resin particles 3 in a melt-softened state. As a result, a part of the thermoplastic resin particles 3 is pushed into the upper coating film 6 in the middle of curing as wedge-shaped protrusions 3p (FIG. 1d).
Since the lower layer coating film 4 and the upper layer coating film 6 are mechanically engaged with each other by the wedge-shaped protrusions 3p, the adhesion (delamination resistance) of the upper layer coating film 6 to the lower layer coating film 4 is improved. The upper coating film 6 is pinned by the wedge-shaped protrusions 3p distributed at the interface between the lower coating film 4 and the upper coating film 6, and the stress applied to the upper coating film 6 is dispersed. This is considered to be the cause of improving the resistance and scratch resistance.

Although the coating raw plate which can be used does not impose special restrictions on the material, there are a plated steel plate, a stainless steel plate, an aluminum plate, an aluminum alloy plate, and the like. Examples of the plated steel sheet include a Zn plated steel sheet, an Al plated steel sheet, a Zn—Al alloy plated steel sheet, a Zn—Al—Mg alloy plated steel sheet, and a Cu plated steel sheet. Prior to painting, pre-coating treatments such as degreasing, washing, replacement treatment, and chemical conversion treatment are appropriately performed.
Although a lower layer coating film may be directly formed by applying a urethane coating to a pre-painted coating original plate, it is preferable to interpose an undercoat coating film in order to improve coating film adhesion. When providing an undercoat, epoxy resin, epoxy-modified polyester, or polyester resin paints containing rust-preventive pigments such as strontium chromate are used for the undercoat. By baking, an undercoat film having a dry film thickness of about 5 μm is formed.

  The lower layer coating film and the urethane coating for forming the upper layer coating film are prepared by blending an isocyanate curing agent with a polyester resin having a hydroxyl group at the terminal (polyester polyol). As the polyester resin, a resin having a number average molecular weight of 1500 to 35000 (preferably 3000 to 20000) and a glass transition temperature Tg of 0 to 80 ° C. is used. As the isocyanate curing agent, one or more of TDI (triresin isocyanate), IPDI (isophorone diisocyanate), HDI (hexamethylene diisocyanate), MDI (diphenylmethane diisocyanate), XDI (xylylene resin diisocyanate), etc. are used. Blocking agents such as methyl ketone oxime, ε-caprolactam, ethyl acetoacetate and phenol are used in combination. In the one-pack type urethane paint, it is necessary to inactivate an isocyanate group rich in reactivity with water with a blocking agent.

  In the urethane coating for forming the lower layer coating film, thermoplastic resin particles are preferably blended at a ratio of 1 to 10% by mass. The average particle diameter of the thermoplastic resin particles is appropriately selected in the range of 5 to 100 μm depending on the film thickness of the lower layer coating film. When forming a relatively thin coating film having a thickness of about 10 μm, a relatively small diameter thermoplastic resin particle having an average particle size of about 5 to 30 μm is preferable, and a relatively thick coating film having a thickness of about 50 μm is formed. In this case, thermoplastic resin particles having a relatively large diameter with an average particle size of about 70 to 80 μm are preferable. In any case, in order to distribute the flat thermoplastic resin particles capable of forming wedge-shaped projections in the vicinity of the surface of the lower layer coating film, the film thickness of the lower layer coating film is 15 to 40 μm, and the average particle diameter of the thermoplastic resin particles It is preferable to determine the blending amount of the thermoplastic resin particles in the range of 2 to 10% by mass.

As the thermoplastic resin particles, polyamide resin, acrylic resin, fluororesin, polyethylene resin, polypropylene resin, or the like can be used. For example, when a polyamide resin is used, transparent particles or colored resins having a basic composition of nylon 12 and an average particle diameter of 5 to 100 μm (preferably 40 to 60 μm) are suitable. The thermoplastic resin particles are preferably a resin whose melting temperature is adjusted in the range of 160 to 180 ° C. so that it is melted by heating during baking and drying of the urethane paint.
The urethane coating for forming the lower layer coating film is baked at a maximum plate temperature of 200 to 260 ° C. (preferably 230 to 250 ° C.) after being applied to the surface of the metal plate. By heating, the isocyanate blocking agent is dissociated, the reaction between the isocyanate and the polyester polyol proceeds, and a lower layer coating film is formed. Further, the thermoplastic resin particles are appropriately melted and flattened by heating in the temperature range. Although there is a dislike of yellowing of the coating film at an excessively high baking temperature, it is preferable to cure the lower coating film at a temperature equal to or higher than the melting temperature of the thermoplastic particles by selecting a blocked isocyanate (curing agent).

  After forming the lower layer coating film, the upper layer coating film is laminated on the lower layer coating film by applying and baking a urethane coating for forming the upper layer coating film. The urethane paint is prepared by blending a curing agent such as hexamethylene diisocyanate and isophorone diisocyanate and a blocking agent such as ε-caprolactam into a polyester polyol composed of a polyhydric alcohol and a polybasic acid. By blending a color pigment, a metallic pigment or the like as necessary, a color tone that meets the needs can be imparted to the upper layer coating film.

  The urethane paint for forming the upper coating film is applied by a roll coating method, a curtain coating method, or the like, and baked at a maximum plate temperature of 200 to 260 ° C. In the baking of the upper layer coating film, the isocyanate blocking agent is dissociated in the same manner as the lower layer coating film, and the upper layer coating film is formed by the reaction between the isocyanate and the polyester polyol. 250 ° C.). During baking of the upper layer coating, the lower layer coating shrinks and the curing reaction further proceeds and the residual solvent volatilizes.

When firing the upper layer coating film, the thermoplastic resin particles in the vicinity of the surface layer of the lower layer coating film are remelted, and the shrinkage force F acts to break the thin film on the particle surface and flow into the upper layer coating film during curing, Part of the thermoplastic resin particles becomes wedge-shaped protrusions and is bitten by the upper coating film after curing. The greater the number of wedge-shaped protrusions distributed at the interface between the lower layer coating film and the upper layer coating film, the better the interlayer adhesion of the upper layer coating film to the lower layer coating film.
The number of wedge-shaped protrusions depends on the resin type of the lower layer coating, the resin type of the thermoplastic resin particles, the particle size, etc., but in the system in which commercially available nylon particles are blended with the polyester resin, the blending ratio (mass%) The number of wedge-shaped projections (number / mm) approximately equal to is observed at the interface between the lower layer coating film and the upper layer coating film. The influence of wedge-shaped projections on interlaminar adhesion becomes significant at 2 pieces / mm (4 / mm ² in terms of unit area) or more, and the number of wedge-shaped projections is a thermoplastic resin for urethane coatings for forming lower layer coatings. It is adjusted by the amount of particles.

  A urethane paint (residue by weight heating 50%) was prepared by blending HDI (curing agent) and ε-caprolactam (blocking agent) into a polyester resin having a number average molecular weight of 5000 to 6000. Polyurethane particles (nylon 12 particles: manufactured by Daicel Huls Co., Ltd., melting temperature: 170 ° C.) having an average particle size of 40 to 60 μm manufactured by the reprecipitation method are used as thermoplastic resin particles, and urethane paints with various blending ratio The coating composition for lower layer coating film formation was prepared by adding to.

  A 55% Al—Zn alloy-plated steel sheet was used as the coating original sheet. After degreasing and chemical conversion of the coating original plate, an epoxy resin paint was applied and baked to form an undercoat film having a dry film thickness of 5 μm. After applying a polyamide particle-containing urethane coating to the undercoat and forming a lower layer coating with a film thickness of 20 μm by baking at a maximum reached plate temperature of 240 ° C., apply a urethane coating without the addition of polyamide particles to achieve the same maximum reached plate temperature. An upper coating film having a thickness of 20 μm was formed by baking at 240 ° C.

  When the surface layer cross section of the prepared coated steel sheet was observed, wedge-shaped protrusions extending from the thermoplastic resin particles in the lower coating film into the upper coating film were observed, and the number of wedge-shaped protrusions was the urethane for forming the lower coating film. As the number of thermoplastic resin particles blended in the coating composition increased, a larger number of wedge-shaped protrusions were detected in the observed cross section. The number of wedge-shaped protrusions extending from the lower coating film to the upper coating film was counted in a microscopic observation field, and the formation state of the wedge-shaped protrusions was converted per 1 cm in length along the interface between the lower coating film and the upper coating film. Quantified numerically.

A test piece was cut out from the obtained coated steel sheet and subjected to a wet test and a coin scratch test.
In the wet test, after leaving in a high temperature and humid atmosphere of 70 ° C. and 98% RH for 1000 hours, the presence or absence of coating film swelling was examined by visual observation of the coating film surface. Moisture resistance was evaluated with a test piece where remarkable coating film swelling occurred as x, a test piece where coating film swelling progressed to some extent was indicated as Δ, and a test piece where coating film swelling was not detected as ○.
In the coins scratch test, a 10-yen coin was pressed against the surface of the test piece with a load of 1 kg, the surface of the test piece was scratched at a scratching speed of 10 mm / sec, and the peeling state of the coating film was examined by visual observation. Scratch resistance was evaluated with x indicating a test piece in which significant peeling of the coating film occurred, Δ for a test piece in which coating film peeling was detected, and ○ for a test piece in which coating film peeling had hardly occurred.

  As can be seen from the investigation results in Table 1, when the upper urethane film was laminated on the lower urethane film in which the thermoplastic resin particles were dispersed, both the moisture resistance and scratch resistance were excellent. In particular, when thermoplastic resin particles are blended in a urethane coating for forming a lower layer coating at a ratio of 2% by mass or more, the number of wedge-shaped protrusions extending from the lower layer coating to the upper layer coating increases, resulting in a remarkable property improvement effect. Obtained. On the other hand, in the comparative example in which the lower layer coating film was formed with a urethane paint without addition of thermoplastic resin particles, remarkable coating film swelling was observed in the wet test, and the scratch resistance was poor.

  In the above example, the polyamide particles are dispersed in the lower layer coating film. However, as long as the thermoplastic resin particles having an appropriate melting temperature are used as well as the polyamide particles, the thermoplastic resin particles are regenerated during the firing of the upper layer coating film. The wedge-shaped protrusion which melted and extended from the lower layer coating film to the upper layer coating film was formed. Since the wedge-shaped protrusions extend to the upper coating film, the delamination resistance, moisture resistance, and scratch resistance are improved in the same manner as when the polyamide particles are dispersed.

  As described above, the upper coating film in which the wedge-shaped protrusions extend from the thermoplastic resin particles in the lower coating film is obtained by applying and baking the urethane coating on the lower coating film in which the thermoplastic resin particles are dispersed. It is formed. Since the lower coating film and the upper coating film mechanically mesh with each other with wedge-shaped protrusions, physical bonding can be expected in addition to mutual bonding between the lower coating film and the upper coating film due to chemical affinity, resulting in delamination It becomes a hard urethane resin coating. The urethane-coated metal sheet obtained in this way is excellent in delamination resistance, moisture resistance and scratch resistance, and can be easily imparted with patterns and patterns. Therefore, it can be used as an exterior material, interior material, Used in a wide range of fields as a cover material.

The conceptual diagram explaining the process in which the wedge-shaped protrusion which penetrates into an upper layer coating film from the thermoplastic resin particle disperse | distributed to the lower layer coating film is formed

Explanation of symbols

1: Metal plate (painted original plate) 2: Wet lower layer coating film 2p: Convex part generated in lower layer coating film 3: Thermoplastic resin particles 3p: Wedge-like projections 4: Dry lower layer coating film 4d: Concave part generated in lower layer coating film 5: Wet upper layer coating film 6: Upper layer coating during curing

Claims (4)

  The upper layer coating film based on urethane resin is laminated on the lower layer coating film formed with urethane paint blended with thermoplastic resin particles, and a part of the thermoplastic resin particles dispersed in the lower layer coating film is the upper layer. A urethane-coated metal sheet characterized by forming wedge-like protrusions in the coating film.   The urethane-coated metal sheet according to claim 1, wherein thermoplastic resin particles having a melting temperature lower than the firing temperature of the upper layer coating are dispersed in the lower layer coating.   The urethane-coated metal sheet according to claim 1, wherein the lower layer coating film is an intermediate coating film provided on the substrate / metal plate via the undercoat coating film.   A paint composition in which thermoplastic resin particles having an average particle size of 5 to 100 μm are mixed with a urethane paint composed of a polyester resin and an isocyanate curing agent is applied to a base material / metal plate, and the lower layer is baked at a maximum plate temperature of 200 to 260 ° C. After forming a coating film, a urethane paint comprising a polyester resin and an isocyanate curing agent is applied, and an upper layer coating film is formed by baking at a maximum plate temperature of 200 to 260 ° C. .

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