JP3497818B2 - Paint composition and coated steel sheet using the same - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is capable of thick film coating and is excellent in moldability at low temperatures.
The present invention relates to a coating composition having excellent heat resistance in which cracks do not occur in a coating film in a processed part even when heated to 0 ° C. or higher, and a coated steel sheet coated with the coating composition and a top coating composition. The coated steel sheet of the present invention is suitable for, for example, home electric appliances and building materials,
It can also be used for automobiles. In particular,
In building material applications, it is effective as a substitute for PVC-coated steel sheet, which has a concern about the generation of dioxin these days.

[0002]

2. Description of the Related Art Conventionally, epoxy paints and polyester / melamine paints have been used as undercoat paints for painted steel sheets for household appliances and building materials in continuous coating lines for coil coating and the like. These undercoat paints are
At a line speed of 50 m / min or more, a high temperature and a short time of curing can easily cause cracking (foaming of the coating film), and it is generally about 5 μm per coating baking, and the film thickness is 25
It is difficult to produce a dry coating film with a thickness of μm or more.

Generally, a urethane undercoat coating composition comprising a polyesterpolyol and a blocked polyisocyanate masked with ε-caprolactam has a dry film thickness of 25.
It can be formed to a thickness of μm or more, but it is insufficient in terms of adhesion to the material and corrosion resistance.

Further, in the formulation of the urethane undercoating coating composition with an emphasis on processability, the Tg of the coating film is low, and a rubbery region is exhibited at a temperature of Tg or higher. When a steel sheet coated with this undercoat paint is molded at room temperature and heated to a temperature above Tg, the stretched rubber shrinks when heated, causing cracks in the coating film of the processed portion, which is called a flower. Bloom phenomenon occurs. In particular, this phenomenon remarkably appears in a polyurethane cross-linked coating film having a uniform network structure.

When used in applications such as PVC coated steel sheets and pre-coated steel sheets for automobiles, they have excellent processability at low temperatures, good scratch resistance of coating films, high heat resistance and no flowering phenomenon. Required.

Japanese Unexamined Patent Publication No. 2000-7988 discloses a polyurethane coating composition capable of forming a thick film coating film. However, this coating composition is a polyurethane top-coating coating composition and is not used for undercoating, and the flowering phenomenon has not been improved.

[0007]

DISCLOSURE OF THE INVENTION The present invention solves the above-mentioned problems of the prior art and eliminates the occurrence of armpits even at a dry film thickness of 25 μm or more by one undercoating and high temperature short time baking. It can form a coating film with excellent adhesion to materials, corrosion resistance, and scratch resistance. It also has excellent low-temperature processability at 0 ° C, and does not crack in the coating film at the molding part even when heated to 100 ° C or higher. By obtaining a polyurethane undercoat and combining the topcoat, a good balance of hardness and workability can be obtained, and of course there is no crack in the coating film of the molding part during heating, and a coated steel sheet with high weather resistance can be obtained. The purpose is to obtain.

[0008]

Means for Solving the Problems As a result of intensive studies, the inventors of the present invention have conducted a low temperature processing in a polyurethane undercoat paint by adding a hydroxyl group-containing crystalline resin to a basic formulation from a hydroxyl group-containing polyester resin and a block polyisocyanate compound. It has been found that the above object can be achieved by improving the properties and flower blooming phenomenon and further improving the scratch resistance by blending an appropriate epoxy resin.

That is, the present invention relates to (A) number average molecular weight
1,000-10,000, glass transition temperature -40 ° C-50 ° C, crystallization peak temperature less than 0 ° C, hydroxyl group-containing polyester resin having hydroxyl value of 5-100 mgKOH / g, 20-90 parts by weight, (B) blocked polyisocyanate compound 3-50 Parts by weight, (C) 2 to 30 parts by weight of an epoxy resin having a number average molecular weight of more than 3,000 , and (D)
Crystallization peak temperature 0 ° C or higher, number average molecular weight 500-10,000
A coating composition comprising 3 to 30 parts by weight of the hydroxyl group-containing crystalline resin, wherein the blocked polyisocyanate compound (B) is a total of the blocked isocyanate group / the hydroxyl group-containing polyester resin (A) and the crystalline resin (D). The coating composition is characterized in that the equivalent ratio of the generated hydroxyl groups is in the range of 0.8 to 1.2.

Further, according to the present invention, an untreated or treated uncoated or coated galvanized steel sheet, zinc alloy-plated steel sheet or aluminum-plated steel sheet is provided with an undercoat coating film of the coating composition according to claim 1. On the undercoat coating film,
The present invention provides a thick film type coated steel sheet which is provided with a top coating composition.

The coating composition of the present invention will be described in more detail below.

Hydroxyl group-containing polyester resin (A) The hydroxyl group-containing polyester resin which is the component (A) in the composition of the present invention is a polyester resin having a hydroxyl group in the molecule, and is an oil-free polyester resin, an oil-modified alkyd resin, or Examples thereof include modified products of these resins, such as urethane-modified polyester resin, urethane-modified alkyd resin, and epoxy-modified polyester resin.

The oil-free polyester resin is composed of an esterified product of a polybasic acid component and a polyhydric alcohol component. As the polybasic acid component, for example, one selected from phthalic anhydride, isophthalic acid, terephthalic acid, tetrahydrophthalic anhydride, hexahydrophthalic anhydride, succinic acid, fumaric acid, adibic acid, sebacic acid, maleic anhydride, etc. The above dibasic acids and lower alkyl esterified products of these acids are mainly used, and if necessary, benzoic acid, crotonic acid, monobasic acids such as P-t-putylbenzoic acid, trimellitic anhydride, and methylcyclohexene. A tribasic or higher polybasic acid such as tricarboxylic acid or pyromellitic dianhydride is used in combination. Examples of the polyhydric alcohol component include ethylene glycol, diethylene glycol, propylene glycol, 1,4-butanediol, neopentyl glycol, 3-methylpentanediol,
Dihydric alcohols such as 1,4-hexanediol and 1,6-hexanediol are mainly used, and if necessary, trihydric or higher polyhydric alcohols such as glycerin, trimethylolethane, trimethylolpropane and pentaerythritol are used. A polyhydric alcohol can be used in combination. These polyhydric alcohols can be used alone or in admixture of two or more. As the acid component, isophthalic acid, terephthalic acid, and lower alkyl ester products of these acids are particularly preferable.

The alkyd resin is obtained by reacting an oil fatty acid by a method known per se in addition to the acid component and alcohol component of the oil-free polyester resin, and examples of the oil fatty acid include coconut oil fatty acid and large oil fatty acid. Examples include soybean oil fatty acid, linseed oil fatty acid, safflower oil fatty acid, tall oil fatty acid, dehydrated castor oil fatty acid, tung oil fatty acid and the like. Oil length of alkyd resin is 30% or less, especially
It is preferably about 5 to 20%.

As the urethane-modified polyester resin,
The oil-free polyester resin, or a low molecular weight oil-free polyester resin obtained by reacting an acid component and an alcohol component used in the production of the oil-free polyester resin, is reacted with a polyisocyanate compound by a method known per se. Some of them are the ones. Further, the urethane-modified alkyd resin is obtained by reacting the alkyd resin, or a low molecular weight alkyd resin obtained by reacting each component used in the production of the alkyd resin, with a polyisocyanate compound by a method known per se. Things are included. Examples of the polyisocyanate compound that can be used when producing the urethane-modified polyester resin and the urethane-modified alkyd resin include hexamethylene diisocyanate, isophorone diisocyanate, xylylene diisocyanate, tolylene diisocyanate, 4,4′-diphenylmethane diisocyanate, 4,4
'-Methylenebis (cyclohexyl isocyanate),
Examples include 2,4,6-triisocyanatotoluene. As the urethane-modified resin, generally, those having a modification degree such that the amount of the polyisocyanate compound forming the urethane-modified resin is 30% by weight or less based on the urethane-modified resin can be preferably used.

Among the polyester resins described above, an oil-free polyester resin is particularly preferable.

Further, a hydroxyl group-containing polyester resin (A)
Is a balance of physical properties such as workability and hardness of the obtained coating film,
The number average molecular weight is in the range of 1,000 to 10,000, preferably 3,000 to 7,000, in view of the viscosity of the coating material and the solid content of the coating material, which affects the generation of armpits, and the glass transition temperature (T
g) is in the range of -40 ° C to 50 ° C, preferably -20 to 30 ° C, and has a hydroxyl value of 5 to 100 mgKOH / g, preferably 20.
It is a hydroxyl group-containing polyester resin in the range of up to 70 mgKOH / g.

The hydroxyl group-containing polyester resin (A) is 2
It is liquid at 0 ° C and has a crystallization peak temperature of less than 0 ° C, for example, less than -20 ° C. The amount of the hydroxyl group-containing polyester resin (A) is 20 to 9 in 100 parts by weight of the total resin solid content.
It is 0 part by weight, for example, 30 to 80 parts by weight. In the present invention, the glass transition temperature (Tg) is the differential thermal analysis (DTA)
The number average molecular weight is measured by gel permeation chromatography (GPC) using a standard polystyrene calibration curve. The crystallization peak temperature is
It is measured using a differential scanning calorimeter (DSC).

Blocked Polyisocyanate Compound (B) The blocked polyisocyanate compound which is the component (B) in the composition of the present invention is a compound in which the isocyanate group of the polyisocyanate compound is masked by a blocking agent.

As the above polyisocyanate compound,
Aliphatic diisocyanates such as hexamethylene diisocyanate or trimethylhexamethylene diisocyanate; cycloaliphatic diisocyanates such as hydrogenated xylylene diisocyanate, hydrogenated diphenylmethane diisocyanate or isophorone diisocyanate; tolylene diisocyanate or 4,
Organic diisocyanates such as aromatic diisocyanates such as 4'-diphenylmethane diisocyanate itself, or adducts of each of these organic diisocyanates with polyhydric alcohols, low molecular weight polyester resins, water, etc., or each of the organic diisocyanates described above. Examples thereof include cyclized polymers, isocyanurate, and biuret.

Examples of the blocking agent for blocking the isocyanate group include phenols such as phenol, cresol and xylenol; n-propylphenol, n
-Butylphenol, n-nonylphenol, di-n-
Alkylphenols such as propylphenol and isopropylcresol; ε-caprolactam, δ-valerolactam, γ-putyrolactam, β-propiolactam and other lactams; methanol, ethanol, n- or i-
Alcohols such as propyl alcohol, n-, i- or t-butyl alcohol, ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, propyleneglycol monomethyl ether, benzyl alcohol Oxime such as formamidoxime, acetoaldoxime, acetoxime, methylethylketoxime, diacetylmonooxime, benzophenone oxime, cyclohexaneoxime; active methylene such as dimethyl malonate, diethyl malonate, ethyl acetoacetate, methyl acetoacetate and acetylacetone System; imidazole such as imidazole and 2-methylimidazole system; triazo Such as triazole type; pyrazole, 3,5-dimethylpyrazole and other pyrazole type; diphenylamine, carbazole, di-n-propylamine, diisopropylamine and other amine type; bisulfite blocking agent such as sodium bisulfite One kind or two or more kinds can be used suitably.

By mixing the polyisocyanate compound and the blocking agent and heating the mixture, the isocyanate group of the polyisocyanate compound can be easily blocked. By using reactive methyl ethyl ketoxime or ε-caprolactam among these blocking agents, it is possible to improve the armpit resistance when the coating film is cured by heating.

Blocked polyisocyanate compound (B)
Reacts with the hydroxyl group contained in the hydroxyl group-containing polyester resin (A), the epoxy resin (C), and the crystalline resin (D) to form a polyurethane bond. In the present invention, the blocked polyisocyanate compound (B) is added in an amount of 3 to 50 parts by weight based on 100 parts by weight of the total resin solid content. Specifically, the blocked polyisocyanate compound (B)
The blocked isocyanate group / the polyester resin (A) and the crystalline resin (D) are mixed so that the equivalent ratio of the total hydroxyl groups is in the range of 0.8 to 1.2. When the above ratio is less than 0.8, the cross-linking density of the coating film is low, the curability is deteriorated, the physical properties of the coating film such as solvent resistance and hardness are insufficient, and the number of hydroxyl groups remaining in the coating film is increased, resulting in a decrease in the coating property. If the water resistance of the film decreases and corrosion resistance easily deteriorates, on the other hand, if the above ratio exceeds 1.2, the processability of the coating film decreases, and unnecessary due to the isocyanate groups remaining in the coating film after coating. However, there is a problem in that the hardening reaction progresses and cracks in the processed portion of the coating film progress over time. Since the epoxy resin (C) has a very low hydroxyl value, it may or may not be included in the NCO / OH ratio equivalent calculation.

Epoxy Resin (C) The epoxy resin which is the component (C) in the composition of the present invention includes bisphenol A type epoxy resin, bisphenol F type epoxy resin, phenol novolac type epoxy resin, cresol novolac type epoxy resin and intramolecular Various epoxy resins such as phenol glyoxal type epoxy resin having a large number of epoxy groups can be mentioned. The above-mentioned epoxy resin has a number average molecular weight of 2,000 or more, preferably 3,000 or more, and particularly 4,000 or more in order to suppress smoke generation.

Commercially available bisphenol A type epoxy resins include Epicoat 1007, Epicoat 1009, Epicoat 1010 (produced by Yuka Shell Epoxy Co., Ltd.), Epotote YD-017, Epotote YD-019, (Toto Kasei Co., Ltd.). )
Manufactured).

Commercially available products of the above novolac type epoxy resin are phenol novolac type, Epicoat 18
0 (all above, manufactured by Yuka Shell Epoxy Co., Ltd.), Epotote YDPN-701 (manufactured by Tohto Kasei Co., Ltd.), etc., and as a cresol novolac type, Epicoat 180S6
5, Epicoat 180H65 (all above, Yuka Shell Epoxy Co., Ltd.), Epotote YDCN-701, Epotote YDC
N-702, Epotote YDCN-703, Epotote YDCN-704
(Manufactured by Tohto Kasei Co., Ltd.) and the like. In addition, Epotote ZX-1071T, Epotote ZX-1015, Epotote ZX-1
247, Epotote YDG-414S, Epotote YDF-2004, Epotote ST-5100 (all above are manufactured by Toto Kasei Co., Ltd.)
And so on.

In the present invention, the epoxy resin (C) is
2 to 30 parts by weight, preferably 2 to 10 parts by weight are contained in 100 parts by weight of the total resin solid content. If the blending amount is less than 2 parts by weight, the corrosion resistance, scratch resistance and adhesion to the material are not improved, and if it exceeds 30 parts by weight, the viscosity of the coating material becomes high, which causes cracking during thick film coating. Absent.

Crystalline Resin (D) As the hydroxyl group-containing crystalline resin which is the component (D) in the composition of the present invention, polyester polyols such as polycaprolactone diol, polycarbonate diol, polyethylene adipate, polytetramethylene adipate,
Polyhexamethylene adipate and the like, polyether polyols such as polyethylene glycol and polytetramethylene glycol, and the like, having a number average molecular weight of 500 to 10,000
Within the range of 0, preferably 1,000 to 4,000, the sharper the molecular weight distribution is, the better, and the weight average molecular weight (Mw) / number average molecular weight (Mn) ratio is preferably 1.5 or less. Two or more types can be used and used suitably.

Commercially available products of the above-mentioned hydroxyl group-containing crystalline resin are polycaprolactone diol type, Praxel
205, Praxel 210, Praxel 220, Praxel 240,
Praxel 210N, Praxel 220N (Daicel Chemical Industry
As a polycarbonate diol type, Praxel-CD205, Praxel-CD210, Praxel-CD220
(Daicel Chemical Industry Co., Ltd.), Desmophen C200 (Sumitomo Bayer Urethane Co., Ltd.), etc. as polyethylene adipate / polytetramethylene adipate / polyhexamethylene adipate type Brucolan 2000M, Brucolan 2001K, Brucolan 2028I (Sumitomo Bayer Urethane Co., Ltd.) ) Etc., polyethylene glycol type, PEG1000, PEG2000 (Sanyo Chemical Industry Co., Ltd.), etc., polytetramethylene glycol type, PTMG1000, PTMG2000 (Sanyo Chemical Industry Co., Ltd.),
And so on.

Further, the hydroxyl group-containing crystalline polyurethane resin includes a compound obtained by reacting the hydroxyl group-containing crystalline polyester resin / polyether resin with the polyisocyanate compound by a known method. For example, number average molecular weight
A resin consisting of 2,000 polycaprolactone diols and 4,4'-diphenylmethane diisocyanate can be mentioned.

The above hydroxyl group-containing crystalline polyurethane resin is synthesized in such a manner that the equivalent ratio of hydroxyl group / isocyanate group is about 1.0.
5-10, preferably 1.5-5, 40 ° C-140
The reaction is carried out at a temperature of 60 ° C, preferably 60 ° C to 100 ° C. In this reaction, a tin-based, lead-based, zinc-based, iron-based or other organometallic catalyst may be used.

The crystalline resin (D) is added in order to prevent cracks from being generated in the coating film of the processed part when the molded part such as bending is heated. That is, in the rubber-like region, the hard segment made of fine crystals of the crystalline resin is loosened and plastically deformed, so that the stress of the coating film applied during the molding process is relaxed. The crystalline resin (D) is 2
It is solid at 0 ° C., partially crystalline (solid), or waxy, and has a crystallization peak temperature of 0 ° C. or higher.

The crystalline resin (D) has a total resin solid content of 100.
The content is 3 to 30 parts by weight, preferably 5 to 20 parts by weight. If the amount is less than 3 parts by weight, the effect of preventing cracks is not sufficient, and if it exceeds 30 parts by weight, the crosslink density of the coating film decreases, and the physical properties of the coating film such as scratch resistance and hardness decrease.

The coating composition of the present invention may consist essentially of a hydroxyl group-containing polyester resin (A), a blocked polyisocyanate compound (B), an epoxy resin (C) and a crystalline resin (D), but , A rust preventive pigment, an organic solvent is further added, and if necessary, a curing catalyst, pigments such as titanium white, talc, and alumina silica, an amino resin as a curing aid, a defoaming agent known per se for paints,
It may contain additives such as a paint surface modifier and an anti-settling agent.

Examples of the above-mentioned rust preventive pigments include strontium chromate and calcium ion-exchanged amorphous silica. Commercially available products include SHIELDEX (Sealdex, registered trademark) C303, SHIELDEX-AC3, SHIELDEX-AC5 (Grace Japan. (Manufactured by Co., Ltd.) and the like.

Calcium ion-exchanged amorphous silica fine particles (hereinafter sometimes abbreviated as "ion-exchange silica") are silica fine particles in which calcium ions are introduced by ion exchange into a fine porous silica carrier. is there. It is considered that the ion-exchange silica compounded in the coating film is ion-exchanged with H ⁺ ions that have permeated the coating film, releasing calcium ions, which are complexing species ions, to protect the metal surface.

The above-mentioned organic solvent is added as necessary for improving the coating property of the composition of the present invention.
What can dissolve or disperse the hydroxyl group-containing polyester resin (A), the block polyisocyanate compound (B), the epoxy resin (C) and the crystalline resin (D) can be used,
Specifically, for example, toluene, xylene, hydrocarbon solvents such as high-boiling petroleum hydrocarbons, methyl ethyl ketone, methyl isoptyl ketone, cyclohexanone, ketone-based bulking agents such as isophorone, ethyl acetate, butyl acetate,
Ester-based solvents such as ethylene glycol monoethyl ether acetate and diethylene glycol monoethyl ether acetate, alcohol-based solvents such as methanol, ethanol, isopropanol and butanol, ethers such as ethylene glycol monoethyl ether, ethylene glycol monoeptyl ether and diethylene glycol monoeptyl ether Examples thereof include alcohol solvents, which may be used alone or in admixture of two or more.

The above-mentioned curing catalyst is added as needed to accelerate the urethanization reaction. Suitable curing catalysts include, for example, tin octylate, dibutyltin di (2-ethylhexanoate), Examples of the organic metal catalyst include dioctyl di (2-ethylhexanoate), dioctyl diacetate, dibutyltin dilaurate, dibutyltin oxide, dioctyltin oxide and lead 2-ethylhexanoate.

When these curing catalysts are compounded, the compounding amount of the curing catalyst is usually in the range of 0.1 to 2.0 parts by weight based on 100 parts by weight of the total resin solid content. Is preferred.

The above-mentioned curing aid is added for improving the curability and modifying the relatively soft coating film of urethane. Suitable curing aids are melamine, urea, benzoguanamine, acetoguanamine, Steroganamin,
Examples thereof include a methylolated amino resin obtained by reacting an amino component such as spiroguanamine and dicyandiamide with an aldehyde. As the aldehyde used in the above reaction, formaldehyde, paraformaldehyde,
Examples thereof include acetaldehyde and benzaldehyde.
Further, the above methylolated amino resin etherified with a suitable alcohol can also be used as the amino resin. Examples of alcohols used for etherification include methyl alcohol, ethyl alcohol, n-propyl alcohol, isopropyl alcohol, n-butyl alcohol, isobutyl alcohol, 2-ethylbutanol, 2
-Ethylhexanol and the like.

Among the above amino resins, melamine resin is preferred, and among them, methyl etherified melamine resin, mixed etherified melamine resin of methyl ether and butyl ether, or methyl etherified melamine resin or the above mixed etherified melamine resin is preferable. It is preferably a mixed melamine resin containing 60% by weight or more and 40% by weight or less of a butyl etherified melamine resin.

Specific examples of the melamine resin include, for example, Cymel 300, Cymel 303, Cymel 325, Cymel 3
27, Cymel 350, Cymel 730, Cymel 736, Cymel 738 (all manufactured by Mitsui Cytec Co., Ltd.), Melan 522, Melan 523 (all manufactured by Hitachi Chemical Co., Ltd.)
Made), Nikara ivy MSOOl, Nikara ivy MX430, Nikara ivy
MX650 (all manufactured by Sanwa Chemical Co., Ltd.), Sumimar M-55, Sumimar M-100, Sumimar M-40S
(These are all manufactured by Sumitomo Chemical Co., Ltd.), Regimin 740,
Methyl etherified melamine resin such as Regimin 747 (all manufactured by Monsanto); Uban 20SE, Uban 225 (all manufactured by Mitsui Co., Ltd.), Super Beckamine J820-60, Super Beckamine L-117- 60, Super Beckamine L-109-65, Super Beckamine 47-50
Butyl etherified melamine resins such as 8-60, Super Beckamine L-118-60, Super Beckamine G821-60 (all manufactured by Dainippon Ink and Chemicals, Inc.); Cymel 232, Super Beckamine 266, Super Beckamine XV-514, Super Beckamine 1130 (above, all manufactured by Mitsui Cytec Co., Ltd.), Nicalax MX500, Nicalax MX600, Nicalacs MS35, Nicalacs MS95 (above,
Mixed etherified melamine of methyl ether and butyl ether such as Sanwa Chemical Co., Ltd., Regimin 753, Regimin 755 (all of them are manufactured by Monsanto Co., Ltd.), and Sumimar M-66B (Sumitomo Chemical Co., Ltd.). Resin etc. can be mentioned. These melamine resins can be used alone or as a mixture of two or more.

When these curing aids are blended, the amount of the curing aid is usually preferably in the range of 3 to 10 parts by weight with respect to 100 parts by weight of the total resin solid content. .

Next, a coated steel sheet using the composition of the present invention will be described.

The composition of the present invention can be used, for example, as an undercoat paint for steel sheets. Examples of steel sheets to be coated include cold rolled steel sheets, hot dip galvanized steel sheets, electrogalvanized steel sheets, zinc alloy plated steel sheets, and aluminum plated sheets. Examples include steel sheets, stainless steel sheets, copper-plated steel sheets, tin-plated steel sheets, and steel sheets obtained by subjecting these steel sheets to chemical conversion treatment such as phosphate treatment or chromate treatment. A steel plate, a zinc alloy-plated steel plate, and an aluminum-plated steel plate are preferable.

The composition of the present invention can be coated on the above steel sheet by a known method such as a roll coating method, a spraying method, a brush coating method and a dipping method. Drying of the coating film may be appropriately set according to the type of resin used, but when continuously baking the one coated by the coil coating method or the like, the maximum temperature that the material reaches is usually 160 to 2
It is baked for 15 to 60 seconds under the condition of 50 ° C, preferably 180 to 230 ° C. 80 to 140 ° C when baking in batch
It can also be done by baking for 10 to 30 minutes.

The film thickness of the coating film obtained from the composition of the present invention is
Although not particularly limited, even in the continuous coating line of the coil coating method, a dry film thickness of 25 μm is possible with one coating baking, and it is preferably used in the range of 25 to 40 μm.

The coated steel sheet of the present invention comprises a galvanized steel sheet, a zinc alloy plated steel sheet, and an aluminum plated steel sheet, which may be subjected to chemical conversion treatment, provided with an undercoat coating film of the above-mentioned undercoat coating composition of the present invention, An overcoat coating film is provided on the undercoat coating film. The undercoat film thickness is usually 2
˜40 μm, preferably 25 to 40 μm, and the film thickness of the top coating film is usually 8 to 35 μm, preferably 10 to 35 μm.

This coated steel sheet can be suitably obtained by applying the undercoat paint of the present invention by the roll coating method and baking it, and then applying the top coating material by the roll coating method and baking it. Any type of top-coat paint may be used for forming the top-coat coating film. Examples of top-coat paints include polyurethane paints, polyester melamine paints, acrylic paints, fluorine paints and silicone polyester paints. As an example of the polyurethane top coat, it is composed of the above-mentioned hydroxyl group-containing polyester resin (A) and the above-mentioned block polyisocyanate compound (B), and the block isocyanate group of the block polyisocyanate compound (B) / the hydroxyl group of the hydroxyl group-containing polyester resin (A) Mention may be made of polyurethane topcoats having an equivalence ratio in the range of 0.8 to 1.2. The block polyisocyanate compound (B) is preferably at least one compound selected from aliphatic and alicyclic block polyisocyanate compounds from the viewpoint of weather resistance.

The top coating composition can consist essentially of the hydroxyl group-containing polyester resin (A) and the blocked polyisocyanate compound (B), but usually contains an organic solvent and, if necessary, further comprises an organic solvent. , Organic resin powders and inorganic powders, curing catalysts, coloring pigments, luster pigments, polyethylene waxes known per se for paints, carnauba wax, lanolin wax, lubricity-imparting agents, defoaming agents, surface modifiers, sedimentation It may contain additives such as an inhibitor.

Examples of the resin species of the organic resin powder include polyamide resins such as nylon 6, nylon 11 and nylon 12, polyimide resins, polypropylene resins, polyvinylidene fluoride resins, polytetrafluoroethylene,
Examples thereof include polyacrylonitrile resin, acrylic resin, polyurethane resin, phenol resin, silicone resin and urea resin, and these can be used alone or in combination.

When these organic resin powders are blended, the blending amount of the organic resin powder is usually 100
Usually, it is preferably in the range of 3 to 20 parts by weight with respect to parts by weight.

As the types of the above-mentioned inorganic powder, kurk,
Clay, silica, mica, alumina, barium sulfate, calcium carbonate and the like can be mentioned, and these can be used alone or in combination. As the above-mentioned inorganic powder, silica is preferable. When blending these inorganic powders, the blending amount of the inorganic powder is usually 100 parts by weight of the total amount of resin solids,
It is preferably in the range of 0.3 to 15 parts by weight.

The coated steel sheet of the present invention is excellent in weather resistance, scratch resistance due to thick film coating, excellent in low-temperature processability, and exhibits excellent coating film performance in which thermal cracks do not occur in the coating film in the molded portion. You can

[0055]

EXAMPLES The present invention will be described in more detail below with reference to examples.

Examples 1 to 7 and Comparative Examples 1 to 6 The respective raw materials were mixed in the proportions shown in Table A to prepare coating compositions of Examples and Comparative Examples according to the present invention. The composition of each raw material is indicated by the weight of the supply form. (
The numbers in parentheses are ratios with respect to 100 parts by weight of the total resin solid content. Preparation of the undercoat paint of each Example and Comparative Example,
The procedure was as follows.

A hydroxyl group-containing polyester resin (A), a pigment and an appropriate amount of a solvent were mixed and dispersed with a red devil until the particle size became 5 μm or less with a particle gauge. Next, the block polyisocyanate compound (B), the epoxy resin (C), the crystalline resin (D), the catalyst, the additive, and the remaining amount of the solvent are mixed,
A coating composition was prepared by stirring with a disper.

Raw materials used in Examples and Comparative Examples are shown below.

Hydroxyl group-containing polyester resin (A) alkynol VPLS2326: Sumitomo Bayer Urethane Co., Ltd.
Made, oil-free polyester resin, hydroxyl value = about 35
mgKOH / g (solid content), number average molecular weight = 3000, crystallization peak temperature = less than −50 ° C.

Block polyisocyanate compound (B) Desmodur BL1265: Sumitomo Bayer Urethane Co., Ltd.
Ε-caprolactam block TDI prepolymer Sumijour BL3175: Sumitomo Bayer Urethane Co., Ltd.
Made, MEK oxime block HDI isocyanurate

Epoxy resin (C) Epotote YD019: manufactured by Tohto Kasei Co., Ltd., bisphenol A type epoxy resin number average molecular weight = 4300 Epicoat 828: manufactured by Yuka Shell Epoxy Co., bisphenol A type epoxy resin, number average molecular weight = 400 crystalline resin (D) Praxel 220N: manufactured by Daicel Chemical Industries, Ltd., polycaprolactone diol, hydroxyl value = about 55 mgKOH / g
(Solid content), number average molecular weight = 2000, Mw / Mn = 1.2
4, crystallization peak temperature = 18 ° C

Hydroxyl group-containing crystalline polyurethane resin OH00
01: A method for producing the OH0001 resin will be described below. A four-necked flask equipped with a stirrer, a thermometer, a reflux condenser, and a nitrogen blowing tube was placed in a nitrogen atmosphere, and 672 parts of polycaprolactone diol (number average molecular weight 2000), 4,
28 parts of 4′-diphenylmethane diisocyanate and 300 parts of methoxypropyl acetate were charged and reacted at 80 ° C. for 6 hours in a nitrogen atmosphere. The obtained polyol is
Hydroxyl value 25.2 mg KOH / g, solid content concentration 70%, viscosity 8
The temperature was 00 mPa.s / 25 ° C and the crystallization peak temperature was 4 ° C.

Catalyst Dibutyltin dilaurate

Pigment Strontium chromate: manufactured by Kikuchi Color Co., Ltd. titanium oxide CR97: manufactured by Ishihara Sangyo Co., Ltd. Micron White # 5000A: manufactured by Hayashi Kasei Co., Ltd.

Additives Acronal 4F: BASF CAB-53-1: Eastman Chemical Japan Co., Ltd. Solvent Solveso # 200: Exxon Chemical Co., Ltd.

Topcoat paint The topcoat paint was prepared according to Table B by blending a hydroxyl group-containing polyester resin (A), a pigment (titanium oxide) and an appropriate amount of solvent, and using Red Devil, a particle gauge of 5 μm or less was obtained. Dispersed up to. Next, the remaining amount of the hydroxyl group-containing polyester resin (A), the blocked polyisocyanate compound (B), the catalyst, the additive, and the remaining amount of the solvent were mixed and stirred with a disper to prepare a polyurethane top coating composition.

The coating composition shown in Table A was coated on a chromate-treated 0.5 mm thick hot-dip galvanized steel sheet, and its armpit limit and coin scratch resistance were examined. Also,
The undercoat paint composition was applied to a dry film thickness of 30 μm, and the top paint was applied to a dry film thickness of 20 μm, and solvent resistance, cross-cut adhesion, T-bend test, and heat crack resistance test were conducted. It was The coating conditions are as follows.

[0068] Undercoat: 241 ° C x 60 seconds (PMT) Topcoat: 232 ° C x 60 seconds (PMT)

The test methods are shown below. Limit of underarm The undercoat paint was baked under the above conditions, and the maximum film thickness (μm) where no underarm was observed was displayed.

Smoke-Producing Property The undercoat paint was baked under the above conditions, and the condition of the coated plate immediately after baking was visually inspected and evaluated according to the following criteria. ○: No smoke generation was confirmed ×: Smoke generation was confirmed

Coin scratch resistance On a coated steel sheet baked with the undercoat paint under the above conditions,
It was scratched with a 10-yen coin to which an excess weight of 5 kg was added, and the state of peeling of the coating film was evaluated according to the following criteria. ◯: No exposed base material △: Slightly exposed base material ×: Significantly exposed base material

Solvent resistance On a coated steel sheet on which the undercoat paint and the topcoat paint were baked under the above conditions, in a room at 20 ° C., 1 kg was applied to the coated surface with gauze soaked with methyl ethyl ketone (MEK).
/ Cm ^{2 with} a load of 100 for a length of about 5 cm
The state of the coated surface after reciprocating once was visually evaluated according to the following criteria. ◯: No change was observed on the coating surface Δ: No whitening or swelling of the coating film was observed, but scratches were observed on the coating surface ×: Whitening or swelling of the coating film was observed

Cross-cut Adhesiveness A coated steel sheet obtained by baking the undercoat paint and the topcoat paint under the above-mentioned conditions is subjected to JISK-5400 8.5.2 (199).
The test was performed according to the adhesion test specified in 9) and evaluated according to the following criteria. 10 points: Each cut is fine and smooth on both sides, and there is no peeling at the intersection of the cuts and the square at a glance. 8 points: There is slight peeling at the intersection of the cuts and there is no peeling at the square at a glance. The area of the defect is within 5% of the total square area 6 points: There is peeling at both sides of the cut and the intersection, and the area of the defect is 5% to 15% of the total square area 4 points: The width of peeling due to the cut is Wide, the area of the defective portion is 15% to 35% of the total square area 2: The width of peeling due to the cut is wider than 4 points, and the area of the defective portion is 35% to 65% of the total square area 0 point: Peeling Area is 65% or more of the total square area

T bend test At room temperature of 0 ° C. and 20 ° C., after performing a 180 degree bending process with a spacer having the same thickness as the test piece sandwiched,
The coating film surface of the processed portion was observed with a 30-fold magnifying glass to examine the presence or absence of fine pinholes and cracks in the coating film. The evaluation was performed by changing the number of spacers and displaying the minimum number of spacers when fine pinholes and cracks did not occur. When the minimum number of spacers is 0, it is 0T, when it is 1T, 2
The number of sheets is 2T, and the number of sheets is 3T.

Heat-resistant crack resistance test The same sample subjected to the T-bend test was immersed in boiling water for 4 hours, and the surface of the coating film in the processed portion was observed with a 30-fold magnifying glass to check for fine pinholes and cracks in the coating film. It was examined and evaluated according to the following criteria. ◯: No fine pinholes or cracks were observed on the surface of the coating film. Δ: Minute pinholes were slightly observed on the coating film surface. X: Many fine pinholes were observed on the coating film surface, and cracks were also observed.

[0076]

[Table 1]

[0077]

[Table 2]

[0078]

[Table 3] Table B Topcoat paint mix SV # 200: Solvesso # 200 MPA: Methoxypropyl acetate

[0079]

EFFECTS OF THE INVENTION The coating composition of the present invention forms a coating film which is excellent in adhesion to materials and corrosion resistance even after dry film thickness of 25 μm or more by one undercoating and baking at high temperature for a short time. it can. Further, it is possible to form a polyurethane undercoating film which is excellent in processability at low temperatures and which does not cause cracks in the coating film in the molding part even when heated to 100 ° C. or higher.

By combining the coating composition of the present invention with an overcoating material, the hardness and workability are well balanced, and of course, there is no generation of cracks in the coating film of the molding processed portion upon heating,
A coated steel sheet with high weather resistance can be obtained.

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI B32B 15/08 104 B32B 15/08 104Z C09D 163/00 C09D 163/00 C23C 28/00 C23C 28/00 C (58) Field (Int.Cl. ⁷ , DB name) C09D 5/00-201/10 B05D 7/14 B05D 7/24 B32B 15/08

Claims (4)

(57) [Claims] 1. (A) 20 to 90 parts by weight of a hydroxyl group-containing polyester resin having a number average molecular weight of 1,000 to 10,000, a glass transition temperature of -40 ° C. to 50 ° C., a crystallization peak temperature of less than 0 ° C., and a hydroxyl value of 5 to 100 mgKOH / g. , (B) 3 to 50 parts by weight of a blocked polyisocyanate compound, (C) 2 to 30 parts by weight of an epoxy resin having a number average molecular weight of more than 3,000 , and (D) a crystallization peak temperature of 0 ° C. or higher, a number average molecular weight of 500 to 1
A coating composition comprising 3 to 30 parts by weight of a hydroxyl-containing crystalline resin of 0,000, wherein the blocked polyisocyanate compound (B) has a blocked isocyanate group / the hydroxyl-containing polyester resin (A) and a crystalline resin (D). A coating composition, wherein the equivalent ratio of the total hydroxyl groups is in the range of 0.8 to 1.2. 2. The coating composition according to claim 1, which has a dry film thickness of 25 μm or more. 3. The coating composition according to claim 1, which has high workability even at low temperature and can maintain the moldability of the processed portion even in a heat resistance test. 4. An undercoat coating film of the coating composition according to claim 1 is provided on a zinc-plated steel plate, a zinc alloy-plated steel plate or an aluminum-plated steel plate, and the topcoat film is overcoated. A coated steel sheet provided with a coating composition.