JP2002047445A - Coating material composition and coated steel sheet made by using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a coating material composition which gives excellent marring resistance by thick coating and can form a coating film free from the occurrence of thermal cracks on a film at a formed and processed part. SOLUTION: The coating material composition consists of (A) 20-90 pts.wt. hydroxylated polyester resin having a number-average molecular weight of 1,000 to 10,000, a glass transition temperature of -40 to 50 deg.C and a hydroxyl value of 5-100 mgKOH/g, (B) 3-50 pts.wt. blocked polyisocyanate compound, (C) 3-30 pts.wt. epoxy resin having a number-average molecular weight of 300 to 3,000, and (D) 3-30 pts.wt. of at least one crystalline resin selected from hydroxylated crystalline polyester resin each having a number-average molecular weight of 500 to 10,000 and hydroxylated crystalline polyurethane resins, wherein the equivalent ratio of the blocked isocyanato groups of the blocked polyisocyanate compound (B) to the total hydroxyl groups of the polyester resin (A) and the crystalline resin (D) is (0.8 to 1.2):1.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a coating composition which is capable of thick film coating and which does not cause cracks in the coating film of the processed portion even when the formed portion such as a bent portion is heated. And a coated steel sheet coated with a top coat.

[0002]

2. Description of the Related Art
In the continuous coating line such as coil coating, as an undercoat paint for painted steel sheets for home appliances and building materials to be painted,
Epoxy paints and polyester melamine paints are used. At a line speed of 50 m / min or more, these undercoating materials are cured at high temperature and short time, so that a wobble (foaming of the coating film) is liable to occur. Manufacturing is difficult, and usually about 5 μm.

[0003] As an example of a polyurethane coating, JP-A-20
JP-A-00-7988 discloses a polyurethane overcoating composition capable of forming a thick film coating, but is not intended for undercoating.

In general, an undercoat composition comprising a polyester polyol and a blocked polyisocyanate masked with ε-caprolactam can be formed to a dry film thickness of 25 μm or more, but is inferior in adhesion to materials and corrosion resistance. It is enough. In addition, in a composition emphasizing processability, the coating film has a low Tg, and exhibits a rubbery region at a temperature higher than Tg.
The steel plate coated with this undercoat is molded at room temperature,
When a steel sheet is heated to a temperature equal to or higher than Tg, the stretched rubber contracts when heated, and cracks occur in the coating film of the processed portion due to thermodynamic characteristics. In particular, this phenomenon appears remarkably in a polyurethane crosslinked coating film having a uniform network structure.

[0005] The present invention relates to a polyurethane coating for coated steel sheets, which has no undercoating even with a dry film thickness of 25 μm or more, and has excellent adhesion to the material and excellent corrosion resistance by a single undercoating and baking at a high temperature for a short time. Can form a membrane,
By obtaining a polyurethane undercoat that does not cause cracks in the coating film of the molded part even when heated to 85 ° C or higher, and by combining an overcoat, a good balance between hardness and workability is obtained, and of course, An object of the present invention is to obtain a coated steel sheet having high weather resistance without cracks in a coating film of a molded part.

[0006]

Means for Solving the Problems As a result of intensive studies, the present inventors have found that, in a polyurethane undercoat, a hydroxyl group-containing polyester resin, a block polyisocyanate compound, a low molecular weight epoxy resin, a hydroxyl group-containing crystalline polyester resin and a hydroxyl group-containing crystal are used. The present inventors have found that the above object can be achieved by a coating containing at least one crystalline resin selected from hydrophilic polyurethane resins, and have reached the present invention.

That is, the present invention provides (A) a number average molecular weight of 1,
000-10,000, glass transition temperature -40 ° C-50 ° C, hydroxyl value 5
20 to 90 parts by weight of a hydroxyl group-containing polyester resin of 100 to 100 mg KOH / g, (B) a blocked polyisocyanate compound 3 to 50
Parts by weight, (C) epoxy resin with a number average molecular weight of 300 to 3,000
A coating composition comprising 3 to 30 parts by weight of 3 to 30 parts by weight and (D) at least one kind of crystalline resin selected from a hydroxyl group-containing crystalline polyester resin and a hydroxyl group-containing crystalline polyurethane resin having a number average molecular weight of 500 to 10,000. The equivalent ratio of the blocked isocyanate group of the blocked polyisocyanate compound (B) / the total hydroxyl group of the polyester resin (A) and the crystalline resin (D) is in the range of 0.8 to 1.2. And a coating composition characterized by the following.

The present invention also provides an undercoat film of the coating composition according to claim 1 on an untreated or treated galvanized steel sheet, zinc alloy plated steel sheet or aluminum plated steel sheet. And on the undercoat film,
It is intended to provide a thick-film type coated steel sheet provided with a topcoat composition.

Hereinafter, the coating composition of the present invention will be described in more detail.

Hydroxyl-Containing Polyester Resin (A) The hydroxyl-containing polyester resin as the component (A) in the composition of the present invention is a polyester resin having a hydroxyl group in the molecule, and includes an oil-free polyester resin, an oil-modified alkyd resin, Modified products of these resins, for example, urethane-modified polyester resins, urethane-modified alkyd resins, epoxy-modified polyester resins, and the like.

The above oil-free polyester resin comprises 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 and the like The above dibasic acids and lower alkyl esterified products of these acids are mainly used, and if necessary, monobasic acids such as benzoic acid, crotonic acid, Pt-butyl benzoic acid, trimellitic anhydride, methylcyclohexene A tribasic or higher polybasic acid such as tricarboxylic acid and pyromellitic anhydride 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 dihydric alcohol can be used in combination. These polyhydric alcohols can be used alone or in combination of two or more. As the acid component, isophthalic acid, terephthalic acid, and lower alkyl esters of these acids are particularly preferred.

The alkyd resin is obtained by reacting an oil fatty acid by a method known per se in addition to the acid component and the alcohol component of the above-mentioned oil-free polyester resin. Soybean oil fatty acids, linseed oil fatty acids, safflower oil fatty acids, tall oil fatty acids, dehydrated castor oil fatty acids, kiri oil fatty acids and the like can be mentioned. The oil length of alkyd resin is less than 30%, especially
It is preferably about 5 to 20%.

The urethane-modified polyester resin includes:
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, reacts with a polyisocyanate compound by a method known per se. Examples include: Further, the urethane-modified alkyd resin, the alkyd resin, or a low molecular weight alkyd resin obtained by reacting each component used in the production of the alkyd resin was reacted with a polyisocyanate compound by a method known per se. Things are included. Polyisocyanate compounds that can be used when producing urethane-modified polyester resin and urethane-modified alkyd resin include hexamethylene diisocyanate, isophorone diisocyanate, xylylene diisocyanate, tolylene diisocyanate, 4,4'-diphenylmethane diisocyanate, 4,4
'-Methylenebis (cyclohexyl isocyanate),
2,4,6-triisocyanatotoluene and the like. In general, as the urethane-modified resin, those having a modification degree in which 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 suitably used.

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

Further, a hydroxyl group-containing polyester resin (A)
Is the 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 paint at the time of coating and the solid content of the paint at the time of coating, 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 ° C. to 30 ° C., and has a hydroxyl value of 5 to 100 mg KOH / g, preferably
It is a hydroxyl-containing polyester resin in the range of .about.70 mg KOH / g.

The hydroxyl group-containing polyester resin (A) comprises 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 parts by weight based on 100 parts by weight of the total resin solid content.
0 parts by weight, for example, 30 to 80 parts by weight. In the present invention, the glass transition temperature (Tg) is determined by differential thermal analysis (DTA).
The number average molecular weight was 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 as the component (B) in the composition of the present invention is a compound obtained by masking an isocyanate group of the polyisocyanate compound with a blocking agent.

The polyisocyanate compound includes:
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 4,4
Organic diisocyanates themselves such as aromatic diisocyanates such as 4'-diphenylmethane diisocyanate, or adducts of each of these organic diisocyanates with polyhydric alcohols, low molecular weight polyester resins or water, or of each of the organic diisocyanates as described above Cyclic polymers, further, isocyanurates, biurets, and the like can be given.

Examples of the blocking agent for blocking the isocyanate group include phenols such as phenol, cresol and xylenol;
-Butylphenol, n-nonylphenol, di-n-
Alkylphenols such as propylphenol and isopropylcresol; lactams such as ε-caprolactam, δ-valerolactam, γ-butyrolactam, β-propiolactam; 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, propylene glyceryl monomethyl ether, and benzyl alcohol Oximes such as formamidoxime, acetoaldoxime, acetoxime, methyl ethyl ketoxime, diacetyl monooxime, benzophenone oxime, cyclohexane oxime; active methylenes such as dimethyl malonate, diethyl malonate, ethyl acetoacetate, ethyl acetoacetate, methyl acetoacetate, and acetylacetone Imidazoles such as imidazole and 2-methylimidazole; triazo Triazoles; pyrazoles; 3,5-dimethylpyrazoles; pyrazoles; diphenylamines, carbazoles, di-n-propylamines, diisopropylamines; amines; bisulfites such as sodium bisulfite blocking agents; One type or two or more types can be suitably used.

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

Blocked polyisocyanate compound (B)
Reacts with the hydroxyl groups contained in the hydroxyl group-containing polyester resin (A), epoxy resin (C) and crystalline resin (D) to form a polyurethane bond. In the present invention, the blocked polyisocyanate compound (B) is blended in an amount of 3 to 50 parts by weight based on 100 parts by weight of the total resin solids. Specifically, the blocked polyisocyanate compound (B)
Is blended so that the equivalent ratio of the blocked isocyanate group / to the total hydroxyl group of the polyester resin (A) and the crystalline resin (D) is in the range of 0.8 to 1.2. When the above ratio is less than 0.8, the crosslink density of the coating film is low, the curability is reduced, 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 and the coating film is coated. When the water resistance of the film is reduced and the corrosion resistance is apt to be deteriorated, on the other hand, when the above ratio is larger than 1.2, the workability of the coated film is reduced, and after coating, unnecessary by the isocyanate groups remaining in the coated film. This causes a problem that a hardening reaction proceeds and cracks in a processed portion of the coating film progress with time. Since the hydroxyl value of the epoxy resin (C) is very small, it may or may not be included in the equivalent calculation of the NCO / OH ratio.

Epoxy Resin (C) As the epoxy resin which is the component (C) in the composition of the present invention, bisphenol A type epoxy resin, phenol novolak type epoxy resin, cresol novolak type epoxy resin and many epoxy groups in the molecule are used. Various epoxy resins such as a phenol glyoxal type ethoxy resin having the same can be mentioned. The epoxy resin,
The number average molecular weight is 300 to 3,000, and the lower the viscosity, that is, the lower the molecular weight, the better to suppress the occurrence of armpits, and the number average molecular weight is preferably in the range of 300 to 1,000.

Commercially available bisphenol A type epoxy resins include Epicoat 825, -827, 828, 834, -1001, 1007 (manufactured by Yuka Shell Epoxy Co., Ltd.), Epototo YD -127 and -128 (manufactured by Toto Kasei Co., Ltd.).

As commercial products of the above novolak type epoxy resin, phenol novolak type is available as Epicoat 152, EP-154 (both manufactured by Yuka Shell Epoxy Co., Ltd.), EPPN-201 (Nippon Kayaku ( Co., Ltd.) and Epototo YDPN-638 (manufactured by Toto Kasei Co., Ltd.). As a cresol novolac type, Epicoat 180S6
5, -180H65 (all, manufactured by Yuka Shell Epoxy Co., Ltd.), EOCN-102S, EOCN-103S, EOCN-104S (all, manufactured by Nippon Kayaku Co., Ltd.), Epototo YDCN-70
1, -702, -703, and -704 (manufactured by Toto Kasei Co., Ltd.) and the like. In addition, Epototh ZX-1071T and ZX-1
015, ZX-1247 and YDG-414S (all manufactured by Toto Kasei Co., Ltd.).

In the present invention, the epoxy resin (C) is
It is contained in an amount of 3 to 30 parts by weight, preferably 10 to 20 parts by weight, based on 100 parts by weight of the total resin solids. If the amount is less than 3 parts by weight, there is no improvement in corrosion resistance and adhesion to the material, and if it exceeds 30 parts by weight, the scratch resistance decreases.

Crystalline Resin (D) Examples of the hydroxyl group-containing crystalline polyester resin as the component (D) in the composition of the present invention include polycaprolactone diol and polycarbonate diol, and have a number average molecular weight of 500 to 10,000, preferably 1,000. Within the range of from 4,000 to 4,000, the sharper the molecular weight distribution, the better, and the ratio of weight average molecular weight (Mw) / number average molecular weight (Mn) is preferably 1.5 or less.

Commercially available hydroxyl group-containing crystalline polyester resins include Praxel 205, -210, -220 and
-240, -210N, -220N, -CD205, -CD21
0, the same -CD220 (Daicel Chemical Industries, Ltd.), desmophen
C200 (Sumitomo Bayer Urethane Co., Ltd.) and the like.

The hydroxyl group-containing crystalline polyurethane resin includes those obtained by reacting the hydroxyl group-containing crystalline polyester resin with the polyisocyanate compound by a known method. For example, a resin composed of polycaprolactone diol having a number average molecular weight of 2,000 and 4,4'-diphenylmethane diisocyanate can be mentioned.

In the synthesis of the hydroxyl group-containing crystalline polyurethane resin, the equivalent ratio of hydroxyl group / isocyanate group is about 1.0.
5-10, preferably 1.5-5, 40 ° C-140
C., preferably at 60.degree. C. to 100.degree. For this reaction, an organometallic catalyst such as a tin-based, lead-based, zinc-based, or iron-based catalyst may be used.

The crystalline resin (D) is blended in order to prevent cracks in the coating film of the processed part when the formed part such as a bent part is heated. That is, in the rubber-like region, the hard segments made of microcrystals are loosened, and the stress of the coating film applied during the molding process is reduced. The crystalline resin (D) is solid, partially crystalline (solid), or waxy at 20 ° C. and has a crystallization peak temperature of 0%.
° C or higher.

The crystalline resin (D) has a total resin solid content of 100
It is contained in an amount of 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. If the amount exceeds 30 parts by weight, the crosslink density of the coating film is reduced, and the coating film properties such as scratch resistance and hardness are reduced.

The coating composition of the present invention can consist essentially of a hydroxyl group-containing polyester resin (A), a blocked polyisocyanate compound (B), an epoxy resin (C) and a crystalline resin (D). , Rust-preventive pigments, organic solvents are blended, if necessary, curing catalyst, pigments such as titanium white, talc, alumina silica, amino resin as a curing aid, defoamer known per se for paints,
Additives such as a coating surface adjuster and an antisettling agent may be contained.

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

[0034] Calcium ion-exchanged amorphous silica fine particles (hereinafter sometimes abbreviated as "ion-exchanged silica") are silica fine particles in which calcium ions are introduced into a fine porous silica carrier by ion exchange. is there. It is considered that the ion-exchanged silica blended in the coating is ion-exchanged with H ⁺ ions that have permeated the coating, and calcium ions, which are complex-preventing species ions, are released 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, and the like.
What can dissolve or disperse a hydroxyl group-containing polyester resin (A), a block polyisocyanate compound (B), an epoxy resin (C) and a crystalline resin (D) can be used,
Specifically, for example, toluene, xylene, hydrocarbon solvents such as high-boiling petroleum hydrocarbons, methyl ethyl ketone, methyl isobutyl ketone, cyclohexanone, ketone solvents such as isophorone, ethyl acetate, butyl acetate,
Ester solvents such as ethylene glycol monoethyl ether acetate and diethylene glycol monoethyl ether acetate; alcohol solvents such as methanol, ethanol, isopropanol and butanol; ethers such as ethylene glycol monoethyl ether, ethylene glycol monobutyl ether and diethylene glycol monobutyl ether Alcohol solvents and the like can be mentioned, and these can be used alone or as a mixture of two or more.

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

When these curing catalysts are blended, the 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 amount of resin solids. Is preferred.

The above-mentioned curing aid is compounded for improving curability and modifying a relatively soft urethane coating film. Suitable curing aids include melamine, urea, benzoguanamine, acetoguanamine, and the like. Steroganamin,
Methylolated amino resins obtained by reacting an amino component such as spiroganamin or dicyandiamide with an aldehyde are exemplified. As the aldehyde used in the above reaction, formaldehyde, paraformaldehyde,
Acetaldehyde, benzaldehyde and the like can be mentioned.
Further, those obtained by etherifying the above methylolated amino resin 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,
-Ethylhexanol and the like.

Of the above-mentioned amino resins, melamine resins are preferable. 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 preferable that the mixed melamine resin contains not less than 60% by weight and not more than 40% by weight of the butyl etherified melamine resin.

Specific examples of the melamine resin include, for example, Cymel 300, 303, 325, 327, 350, 730,
736 and 738 (both are Mitsui Cytec Corporation)
Melan 522, 523 (all manufactured by Hitachi Chemical Co., Ltd.), Nikaratsuta MSOOl, MX430, MX650 (all manufactured by Sanwa Chemical Co., Ltd.), Sumimar M-5
5, methyl etherified melamine resins such as M-100, M-40S (all, manufactured by Sumitomo Chemical Co., Ltd.), Regimin 740, and 747 (all, manufactured by Monsanto Co.); 225 (all made by Mitsui)
Super Becamine J820-60, L-117-60, L-109
Butyl etherified melamine resins such as -65, 47-508-60, L-118-60, and G821-60 (all manufactured by Dainippon Ink and Chemicals, Inc.); Cymel 232, 266, XV-514, 1130 (all manufactured by Mitsui Cytec Co., Ltd.), Nikarac MX500, MX600, MS35, MS
95 (both manufactured by Sanwa Chemical Co., Ltd.), Regimin
Examples thereof include mixed etherified melamine resins of methyl ether and butyl ether such as 753 and 755 (all of which are manufactured by Monsanto Co., Ltd.) and Sumimar M-66B (manufactured by Sumitomo Chemical Co., Ltd.). 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 aids is usually preferably in the range of 3 to 10 parts by weight based on 100 parts by weight of the total amount of resin solids. .

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. The steel sheets to be coated include cold-rolled steel sheets, hot-dip galvanized steel sheets, electro-galvanized steel sheets, zinc alloy-plated steel sheets, and aluminum-plated steel 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 a chemical conversion treatment such as a phosphate treatment or a chromate treatment. Steel sheets, zinc alloy plated steel sheets, and aluminum plated steel sheets are preferred.

The composition of the present invention can be coated on the steel plate by a known method such as a roll coating method, a spray method, a brush coating method, and a dipping method. Drying of the coating film may be appropriately set depending on the type of resin used, etc., but when continuously baking what is applied by a coil coating method or the like, the material reaching maximum temperature is usually 160 to 2
It is baked for 15 to 60 seconds at a temperature of 50 ° C, preferably 180 to 230 ° C. 80-140 ° C for baking in batch
Baking for 10 to 30 minutes.

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

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

The coated steel sheet can be suitably obtained by applying the undercoat paint of the present invention by a roll coat method, baking, and then applying an overcoat paint by a roll coat method and baking. The top coat forming the top coat is not particularly limited. Examples of topcoats include polyurethane paints, polyester melamine paints, acrylic paints, fluorine paints and silicone polyester paints. Examples of the polyurethane topcoat include the above-mentioned hydroxyl group-containing polyester resin (A) and the above-mentioned blocked polyisocyanate compound (B), and the blocked isocyanate group of the blocked polyisocyanate compound (B) / the hydroxyl group of the hydroxyl group-containing polyester resin (A). A polyurethane topcoat having an equivalent ratio in the range of 0.8 to 1.2 can be mentioned. The blocked polyisocyanate compound (B) is preferably at least one compound selected from aliphatic and alicyclic blocked polyisocyanate compounds from the viewpoint of weather resistance.

This top coating composition can be substantially composed of the above-mentioned hydroxyl group-containing polyester resin (A) and the above-mentioned blocked polyisocyanate compound (B), but is usually compounded with an organic solvent and, if necessary, further. , Organic resin powders, inorganic powders, curing catalysts, coloring pigments, glittering pigments, polyethylene wax, carnauba wax, lanolin wax, lubricating agents, defoamers, coating surface modifiers, sediments known per se for paints An additive such as an inhibitor may be contained.

Examples of the resin type 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% in total of the resin solid content.
Usually, it is preferably in the range of 3 to 20 parts by weight based on parts by weight.

The types of the inorganic powder include curk,
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 inorganic powder, silica is particularly 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,
Preferably it is in the range of 0.3 to 15 parts by weight.

The coated steel sheet of the present invention is excellent in weather resistance and scratch resistance due to thick film coating, and can exhibit excellent coating film performance in which a coating film in a molded portion does not generate thermal cracks.

[0053]

The present invention will be described more specifically with reference to the following examples.

Examples 1 to 9 and Comparative Examples 1 to 5 Each raw material was blended 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 was indicated by the weight of the supply form.
The numbers in parentheses are the ratios to 100 parts by weight of the total resin solids. Preparation of the undercoat paint of each Example and Comparative Example was performed as follows.

The hydroxyl group-containing polyester resin (A), a pigment and an appropriate amount of a solvent were blended and dispersed with a red devil until the particle gauge became 5 μm or less. Next, a block polyisocyanate compound, an epoxy resin, a crystalline resin, a catalyst, an additive, and a remaining amount of a solvent were blended, and the mixture was stirred with a disper to prepare a coating composition.

Hereinafter, the raw materials used in Examples and Comparative Examples will be shown.

Hydroxyl-containing polyester resin (A) Alkinol VPLS2013: Sumitomo Bayer Urethane Co., Ltd.
Made, oil-free polyester resin, hydroxyl value = about 90
mgKOH / g (solid content), number average molecular weight = 2500 Alkinol 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.
-Caprolactam block TDI prepolymer Sumidur BL3175 manufactured by Sumitomo Bayer Urethane Co., Ltd.
Made, MEK oxime block HDI isocyanurate

Epoxy Resin (C) Epicoat 828: Yuka Shell Epoxy Co., Ltd., bisphenol A type epoxy resin Epototo YDPN-638: Toto Kasei Co., Ltd., novolak type epoxy resin crystalline resin (D) Placcel 220N: Polycaprolactone diol, manufactured by Daicel Chemical Industries, Ltd., hydroxyl value = about 55 mgKOH / g
(Solid content), number average molecular weight = 2000, Mw / Mn = 1.2
4. Peak crystallization temperature = 18 ° C

Hydroxyl-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 inlet tube was set to 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 under a nitrogen atmosphere. The resulting polyol is
Hydroxyl value 25.2mgKOH / g, solid content 70%, viscosity 8
It was 00 mPa.s / 25 ° C and the crystallization peak temperature was 4 ° C.

Catalyst dibutyltin dilaurate

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

Additive Acronal 4F: CAB-53-1: manufactured by BASF manufactured by Eastman Chemical Co., Ltd.

The top coat is prepared by blending a hydroxyl group-containing polyester resin (A), a pigment (titanium oxide) and an appropriate amount of a solvent based on Table B, and using a red devil to obtain a particle gauge of 5 μm or less. Dispersed. 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 blended, and the mixture was stirred with a disper to prepare a polyurethane overcoat composition.

The coating compositions shown in Table A were coated on a chromate-treated hot-dip galvanized steel sheet having a thickness of 0.5 mm, and the waki limit and coin scratch resistance were examined. Also,
On the steel sheet coated with the undercoat composition to a dry film thickness of 30 μm, an overcoat paint was applied to a dry film thickness of 20 μm, and the solvent resistance, cross-cut adhesion, T-bend test, and heat crack resistance test were performed. Was. The coating conditions are as follows.

Undercoat: 241 ° C. × 60 seconds (PM
T. ) Top coat: 232 ° C. × 60 seconds (PMT)

The test method is described below. The undercoat for waki limit undercoat was baked under the above conditions and indicated by the maximum film thickness (μm) in which no waki was recognized.

On a coated steel sheet baked with a coin scratch-resistant undercoat under the above conditions,
It was scratched with a 10-yen coin to which an excess weight of 5 kg was added, and evaluated according to the following criteria according to the state of peeling of the coating film. 〇: No base exposure is observed. Δ: Base exposure is slightly observed. X: Base exposure is noticeably observed.

A 1 kg / cm ² load was applied to the coated surface with a gauze impregnated with methyl ethyl ketone in a room at 20 ° C. on a coated steel plate in which the solvent-resistant base coat and the top coat were baked under the above conditions. The state of the coated surface after reciprocating 100 times over a length of about 5 cm was visually evaluated according to the following criteria. 〇: No change is observed on the coated surface △: No whitening or swelling of the coating is observed, but scratches are observed on the coated surface ×: Whitening or swelling of the coating is observed

Cross-cut adhesion A JIS K-5400 8.5.2 (199) was coated on a coated steel sheet in which an undercoat paint and an overcoat paint were baked under the above conditions.
The evaluation was carried out 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 cut and the square at a glance. 8 points: There is a slight peeling at the intersection of the cut and no peel at a glance of the square. The area of the missing part is within 5% of the total square area. 6 points: Both sides of the cut are peeled off from the intersection, and the area of the defective part is 5% to 15% of the total square area. 4 points: The width of the peel due to the cut is Wide, the area of the defect is 15% to 35% of the total square area. 2 points: The width of the peeling due to the cut is wider than 4 points, and the area of the defect is 35% to 65% of the total square area. 0 points: The peeling Area is 65% or more of the total square area

T-bend test At a room temperature of 20 ° C., a 180 ° bending process was performed with a spacer having the same thickness as that of the test piece interposed therebetween. Pinholes and cracks were checked. The evaluation was performed by changing the number of spacers and displaying the minimum number of spacers at the time when minute pinholes and cracks did not occur. When the minimum number of spacers is 0, it is 0T, 1 is 1T, 2 is 2T, and 3 is 3T.

Heat Crack Resistance Test The same sample subjected to the T-bend test was left in an oven at 85 ° C. for 24 hours, and the surface of the coating film in the processed portion was observed with a 30-fold loupe to check for fine pinholes and cracks in the coating film. The presence or absence was checked and evaluated according to the following criteria. 〇: No fine pinholes or cracks were observed on the coating film surface. Δ: Slight pinholes were observed on the coating film surface. D: Many fine pinholes were observed on the coating film surface, and cracks were also observed.

[0073]

[Table 1]

[0074]

[Table 2]

[0075]

Table 3 Table B Topcoat paint formulation SV # 200: Solvesso # 200 MPA: Methoxypropyl acetate

[0076]

EFFECT OF THE INVENTION The coating composition of the present invention forms a coating film having excellent adhesion to a material and excellent corrosion resistance even after a single undercoating and baking at a high temperature for a short time at a dry film thickness of 25 μm or more. It is possible to form a polyurethane undercoat film in which cracks do not occur in the coating film of the molded part even when heated to 85 ° C. or higher.

By combining the coating composition of the present invention with a top coating, hardness and workability are well balanced, and, of course, there is no crack in the coating film of the molded part when heated.
A highly weather-resistant coated steel sheet can be obtained.

Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat II (reference) C09D 5/00 C09D 5/00 D 175/04 175/04 F term (reference) 4F100 AA21H AB03B AB10B AB18B AB31B AK41A AK51A AK53A AL05A AL06A BA03 BA07 BA10B BA10C CA13 CC00B EH71B GB07 GB48 JA05A JA07A JA11A JB02 JB20A JK06 JK12 JK15 JL09 YY00A 4J038 DB002 DB062 DB072 DD001 DD002 DG002 DG111 DG302 GA03 MA13 MA14 NA11 PC02

Claims (3)

[Claims] (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, and a hydroxyl value of 5 to 100 mgKOH / g, and (B) a blocked polyisocyanate compound 3 to 50 parts by weight, (C) 3 to 30 parts by weight of an epoxy resin having a number average molecular weight of 300 to 3,000, and (D) a hydroxyl group-containing crystalline polyester resin and a hydroxyl group-containing crystalline polyurethane resin having a number average molecular weight of 500 to 10,000. A coating composition comprising 3 to 30 parts by weight of at least one crystalline resin, wherein the total of the blocked isocyanate group / hydroxyl-containing polyester resin (A) of the blocked polyisocyanate compound (B) and the crystalline resin (D) A coating composition, wherein the equivalent ratio of the hydroxyl groups obtained is in the range of 0.8 to 1.2. 2. The coating composition according to claim 1, wherein the coating composition has a dry film thickness of 25 μm or more. 3. An undercoat film of the coating composition according to claim 1 is provided on a galvanized steel sheet, a zinc alloy-plated steel sheet or an aluminum-plated steel sheet, and an overcoat is formed on the undercoat film. A coated steel sheet provided with a coating composition.