JP3259580B2 - Organic composite coated steel sheet with excellent rust resistance - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an organic composite coated steel sheet suitable for an automobile body and having excellent rust resistance.

[0002]

2. Description of the Related Art In recent years, in cold regions such as North America and Northern Europe, corrosion of automobile bodies due to salts for preventing road freezing sprayed in winter has become a major social problem. For this reason, there is an increasing tendency to use a surface-treated steel sheet having excellent corrosion resistance in place of a conventional cold-rolled steel sheet as one of the measures for preventing rust of an automobile body. As such a surface-treated steel sheet, for example, Japanese Patent Publication No. 6-4311, etc., an organic composite coating excellent in corrosion resistance having a Zn-plated steel sheet having a chromate film on the surface and a resin film having a thickness of about 1 μm on the surface. Steel plates have been proposed. The resin film of the organic composite coated steel sheet disclosed in Japanese Patent Publication No. 6-4311 has the following features.

(1) Epichlorohydrin having excellent corrosion resistance
Based on bisphenol A type epoxy resin, (2) by adding dialkanolamine in the skeleton of the epoxy resin, to improve alkali resistance by interaction with silica, (3) amino resin as a curing agent, In particular, benzoguanamine resin was used to make the composition curable at low temperatures, and (4) a silane coupling agent was added for the purpose of improving paint adhesion.

[0004]

On the other hand, recently, corrosion resistance in an environment in which iron rust coexists in a corrosive environment (hereinafter referred to as rust resistance) has begun to be a problem (CAMP-ISIJ vo).
l.5 (1992), p.1693). That is, it has been pointed out that when the organic composite coated steel sheet is used in such an environment, iron rust adheres to the surface of the resin film, thereby deteriorating the excellent corrosion resistance inherent in the organic composite coated steel sheet. However, it has been found that the above-mentioned organic composite coated steel sheet disclosed in JP-A-6-4311 cannot necessarily exhibit sufficient performance with respect to the rust resistance. This is because the resin film of the coated steel plate uses an amino resin (benzoguanamine resin, urea-formaldehyde condensation polymer, monomeric and polymeric melamine resin, etc.) as a curing agent, and the film using such a curing agent is used. It is considered that a sufficient crosslink density cannot be obtained.

[0005] A known document, "GALVATECH '92, p.3
72 '' describes that when the crosslinking agent density is reduced by reducing the amount of the curing agent in the resin film, the rust resistance of the organic composite coated steel sheet is reduced. No specific measures are taken to improve it. In view of such a situation, an object of the present invention is to provide an organic composite coated steel sheet having excellent properties such as corrosion resistance (perforation resistance) and paint adhesion, as well as excellent rust resistance.

[0006]

Means for Solving the Problems The organic composite coated steel sheet of the present invention has been intensively studied by the present inventors to achieve the above object, and as a result, a hardening agent is required to improve the rust resistance. It has been found that the two conditions of using silica and a sparingly soluble chromate as rust preventive additives are extremely effective as a rust preventive additive by multifunctionalizing a polyisocyanate compound. That is, the organic composite coated steel sheet of the present invention has the following configuration.

(1) A chromate film having an adhesion amount of 5 to 200 mg / m ^{2 in} terms of metal chromium is formed on the surface of a Zn-based plated steel sheet, and 500 to 500
With respect to 100 parts by weight (solid content) of a modified epoxy resin [A] to which 0.8 to 1.0 mol of dialkanolamine is added to 1 equivalent of epoxy group of an epichlorohydrin-bisphenol A type epoxy resin having an epoxy equivalent of 0 0.5 to 10 parts by weight of the silane coupling agent [B] and 5 to 80 parts of the polyfunctional polyisocyanate compound [C] having at least 6 isocyanate groups in one molecule.
Parts by weight and further contains a rust inhibitor [D] ([A] +
[B] + [C]) / [D] solid content by weight ratio of 90/1
An organic composite coated steel sheet having excellent rust resistance, characterized in that a resin film having a dry film thickness of 0.2 to 3.0 [mu] m is formed by adhering a resin composition containing at a ratio of 0 to 40/60. .

(2) In the organic composite coated steel sheet of the above (1), the rust preventive additive [D] constituting the resin film is silica or / and an organic solvent excellent in rust resistance which is hardly soluble chromate. Composite coated steel sheet. (3) In the organic composite coated steel sheet of the above (1), the rust preventive additive [D] constituting the resin film is silica having the following weight ratio and a hardly soluble chromate, which has excellent rust resistance. Composite coated steel sheet. Silica / poorly soluble chromate = 35/5/1/39

(4) In the organic composite coated steel sheet according to the above (1) to (3), the polyfunctional polyisocyanate compound [C] is a polyfunctional hexamethylene diisocyanate having at least six isocyanate groups in one molecule. Organic composite coated steel sheet with excellent rust resistance.

[0010]

[Function] A resin film formed as a second layer on a chromate film formed on the surface of a zinc-based plated steel sheet suppresses excessive elution of hexavalent chromate ions in the chromate film into a corrosive environment. In particular, in the present invention, it is formed by reacting a specific base resin with a polyfunctional polyisocyanate compound having at least 6 isocyanate groups in one molecule as a curing agent. The effect of the high cross-linking density resin film and the rust preventive additive added in the resin film in a specific ratio (particularly, the synergistic effect when silica and sparingly soluble chromate are added in combination), Compared with the composite coated steel sheet, it has significantly improved rust resistance.

The details of the present invention and the reasons for the limitation will be described below. As the base zinc-coated steel sheet, a zinc-coated steel sheet, a Zn-Ni alloy-coated steel sheet, a Zn-Fe alloy-coated steel sheet, a Zn-Mn alloy-coated steel sheet, a Zn-Al alloy-coated steel sheet, a Zn-Cr alloy-coated steel sheet, Zn -Co
-Cr alloy-plated steel sheet, Zn-Cr-Ni alloy-plated steel sheet, Zn-Cr-Fe alloy-plated steel sheet, and zinc-based composite coated steel sheet in which metal oxide, hardly soluble chromate, polymer, etc. are dispersedly coated Can be mentioned. Further, it may be a multi-layer plated steel sheet obtained by plating two or more layers of the same kind or different kinds of the above plating. As a plating method, any method that can be performed among an electrolysis method, a melting method, and a gas phase method can be adopted.
From the selectivity of the base cold-rolled steel sheet, the electrolytic method is advantageous.

The chromate film formed on the surface of the galvanized steel sheet suppresses corrosion of the galvanized steel sheet by a self-repairing action of chromate ions of hexavalent chromium. If the amount of the chromate film is less than 5 mg / m ^{2 in} terms of chromium metal, sufficient corrosion resistance cannot be expected, while if it exceeds 200 mg / m ² , the weldability is deteriorated. For this reason, the adhesion amount of the chromate film is 5 to 200 mg / m ^{2 in} terms of metal chromium. Further, in order to satisfy even higher corrosion resistance and weldability, the content is preferably in the range of ²⁰ to 100 mg / m ^{2 in} terms of chromium metal. As a chromate treatment for forming the chromate film, any of a reaction type, an electrolytic type, and a coating type can be applied. From the viewpoint of corrosion resistance, a coating type containing a large amount of hexavalent chromium chromate ions in the chromate film is preferable.

[0013] The coating type chromate treatment comprises a treatment solution containing a partially reduced aqueous solution of chromic acid as a main component and, if necessary, one or more of the following components added thereto. And dried without washing. Water-soluble or water-dispersible organic resins such as acrylic resins and polyester resins Colloids and / or powders of oxides such as silica, alumina, titania, zirconia, and zinc oxide Acids such as molybdic acid, tungstic acid, and vanadic acid Or its salts Phosphoric acid such as phosphoric acid and polyphosphoric acid Zirconium fluoride, silicide fluoride, fluoride such as titanium fluoride Metal ion such as zinc ion Conductive fine powder such as iron phosphide, antimony-doped tin oxide Hydrogen fluoride Silane coupling agent In the application type chromate treatment, the treatment liquid is usually applied by a roll coater method, but after applying by a dipping method or a spray method, it is also possible to adjust the application amount by an air knife method or a roll drawing method. .

On the upper layer of the chromate film, 500 to 50
100 parts by weight (solid content) of a modified epoxy resin [A] to which 0.8 to 1.0 mol of dialkanolamine is added to 1 equivalent of epoxy group of an epichlorohydrin-bisphenol A type epoxy resin having an epoxy equivalent of 00 0.5 to 10 parts by weight of the silane coupling agent [B] and 5 to 80 parts of the polyfunctional polyisocyanate compound [C] having at least 6 isocyanate groups in one molecule.
Parts by weight and further contains a rust inhibitor [D] ([A] +
[B] + [C]) / [D] solid content by weight ratio of 90/1
A resin film to which a resin composition containing 0 to 40/60 is adhered is formed.

The epichlorohydrin-bisphenol A type epoxy resin having an epoxy equivalent of 500 to 5000 used for the resin film is a condensate obtained by subjecting bisphenol A and epichlorohydrin to a condensation reaction. As the epoxy resin, epichlorohydrin-bisphenol A
In addition to the type epoxy resin, there is, for example, a resin having only an aliphatic epoxy resin or an alicyclic epoxy resin structure. In the present invention, the epoxy resin of the above condensate is used in order to obtain excellent corrosion resistance.

Specific examples (commercially available) of epichlorohydrin-bisphenol A type epoxy resin include Epicoat 1010, 1009, 1007, 1004, 1001
(All manufactured by Shell Chemical Co., Ltd.), and these can be used alone or in combination. If the epoxy equivalent of the epichlorohydrin-bisphenol A type epoxy resin is less than 500, the molecular weight of the resin is too low to obtain sufficient alkali resistance, and the coating film adhesion after electrodeposition coating and the rust resistance are reduced. On the other hand, when the epoxy equivalent is 50
If it exceeds 00, the amount of the alkanolamine to be added to the epoxy group is reduced, and the reinforcing effect with silica and the crosslinking density at the time of curing with the polyfunctional polyisocyanate compound are not sufficient. In addition, the perforation resistance and the electrodeposition coating property are reduced.

The modified epoxy resin [A] can be obtained by adding 0.8 to 1.0 mol of dialkanolamine to 1 equivalent of the epoxy group of the epichlorohydrin-bisphenol A type epoxy resin as described above. If the addition amount of dialkanolamine is less than 0.8 mol per equivalent of epoxy group, the crosslinking density and the reinforcing effect with silica cannot be sufficiently obtained when curing from polyfunctional polyisocyanate, so that the rust resistance is increased. And the coating film swells due to the high alkali at the interface during electrodeposition coating, and the coating film adhesion deteriorates. On the other hand, if the amount of dialkanolamine added exceeds 1.0 mol per 1 equivalent of epoxy group, excess dialkanolamine not added to the epoxy group and not involved in the reinforcing effect with silica will be present, Not only is it not economically desirable, but dialkanolamine remains as an unreacted component in the resin film, and thus reduces rust resistance, puncture resistance, secondary adhesion, and the like.

[0018] Such a modified epoxy resin has a
Epichlorohydrin having an epoxy equivalent of 5000
It is obtained by adding dialkanolamine to a bisphenol A type epoxy resin and reacting at room temperature to 100 ° C. for 3 to 5 hours. Examples of the dialkanolamine include diethanolamine, dipropanolamine, dibutanolamine and the like. Dialkanolamine can introduce a large number of primary hydroxyl groups into the epichlorohydrin-bisphenol A type epoxy resin, and can increase the crosslink density of the coating due to the reinforcing effect due to the interaction between the primary hydroxyl groups and silica.

The modified epoxy resin [A] is mixed with a silane coupling agent [B], a polyfunctional polyisocyanate compound [C], and a rust preventive additive [D] to form a resin composition to form a resin film. can get. The silane coupling agent [B] not only acts as a cross-linking agent by a dehydration condensation reaction with a hydroxyl group in the modified epoxy resin, but also enhances the adhesion between the undercoat chromate film and the bond between the chromate film and the resin film. It acts to strengthen the force and consequently contributes to the improvement of coating film adhesion. In particular, the silane coupling agent works effectively in the case of a resin having a primary hydroxyl group at a terminal as used in the present invention. That is, the silane coupling agent enhances the composite of the resin and silica, but the resin used in the present invention has a high reactivity with the silane coupling agent because it has a primary hydroxyl group at a terminal, and the silica and the silica have a high reactivity. The composite is further strengthened, thereby significantly improving rust resistance and puncture resistance.

Among the silane coupling agents, those having a molecular structure shown below have good compatibility with the coating composition and liquid stability, and enhance the bonding strength between the chromate film and the resin film, and Improves adhesion. That is, it is preferable to use a silane coupling agent represented by the following general formula (I) as the silane coupling agent. X _4-n Si- (OR) _n (I) where n: 0 to 4 (n is an integer) X: a functional group that reacts with an organic substance OR: a hydrolyzable group (a methoxy group, an ethoxy group, etc.) And X are preferably a vinyl group, a mercapto group, an epoxy group, and a chloro group. On the other hand, a coupling agent containing an amino group as X cannot be used because it increases the viscosity when added to a paint.

As the silane coupling agent, epoxy silane, amino silane, vinyl silane, mercapto silane and the like are generally known. However, as described above, amino silane has poor compatibility with the resin used in the present invention. Is not suitable. As a specific example of the coupling agent used in the present invention, β- (3,4-
Epoxycyclohexyl) ethyltrimethoxysilane,
γ-glycidoxypropyltrimethoxysilane and the like, vinyltriethoxysilane, vinyltris (2-methoxyethoxy) silane and γ-methacryloxypropyltrimethoxysilane as vinylsilane, and γ-mercaptopropyltrimethoxy as mercaptosilane Silane and the like can each be mentioned.

The amount of the silane coupling agent is 0.5 to 1 based on 100 parts by weight (solid content) of the modified epoxy resin.
0 parts by weight (solid content), preferably 1 to 5 parts by weight.
If the blending ratio of the silane coupling agent to the modified epoxy resin is less than 0.5 part by weight, the effect of adding the coupling agent cannot be sufficiently obtained, and the adhesion between the resin film and the chromate film is insufficient, so that the adhesion is poor. It does not improve and has poor rust resistance and puncture resistance. On the other hand, if the mixing ratio exceeds 10 parts by weight, an unreacted portion may be generated, and this unreacted portion is not preferable because it causes a poor adhesion.

In the present invention, a curing agent is added as an essential component to the modified epoxy resin [A] for the purpose of improving rust resistance, and a specific isocyanate compound, that is, a polyfunctional polyisocyanate is used as the curing agent. Using the compound [C], the resin film is cured by a urethanization reaction between the hydroxyl group of the modified epoxy resin and the isocyanate group in the polyfunctional polyisocyanate compound.

The polyfunctional polyisocyanate compound has at least six isocyanate groups in one molecule (these isocyanate groups may be blocked) for the purpose of improving rust resistance. A monoisocyanate compound having one isocyanate group in one molecule or a diisocyanate compound having two isocyanate groups in one molecule cannot sufficiently impart rust resistance to the resin film. On the other hand, in the present invention, a polyfunctional polyisocyanate compound having at least 3 isocyanate groups in one molecule, more preferably a polyfunctional polyisocyanate compound having 4 or more, particularly preferably 6 or more isocyanate groups is used. It has been found that rust resistance, which is far superior to monoisocyanate compounds and diisocyanate compounds, can be imparted.

Further, among polyfunctional polyisocyanate compounds having 6 or more isocyanate groups in one molecule, a polyfunctional compound of hexamethylene diisocyanate is particularly effective for rust resistance since it is resistant to rust. The polyfunctional polyisocyanate compound may be a mixture of homologous compounds having different numbers of isocyanate groups in one molecule, or two or more of the above polyfunctional polyisocyanate compounds may be used in combination.

Examples of such a polyfunctional polyisocyanate compound having three or more isocyanate groups in one molecule include a compound having three or more isocyanate groups in one molecule and an isocyanate compound having at least two isocyanate groups. Is reacted with a polyhydric alcohol, or a compound such as a buret type adduct thereof or an isocyanuric ring type adduct thereof. For example, triphenylmethane-4,4 ', 4 "-triisocyanate, 1,3,5-triisocyanatobenzene, 2,4,4
Polyisocyanate compounds having three or more isocyanate groups such as 6-triisocyanatotoluene, 4,4'-dimethyldiphenylmethane-2,2 ', 5,5'-tetraisocyanate; ethylene glycol, propylene glycol, 1,4 An adduct obtained by reacting a polyisocyanate compound in such an amount that an isocyanate group becomes excessive with respect to a hydroxyl group of a polyol such as butylene glycol, polyalkylene glycol, trimethylolpropane, hexanetriol; hexamethylene diisocyanate, isophorone diisocyanate, Burette type adducts such as diisocyanate, xylylene diisocyanate, 4,4'-diphenylmethane diisocyanate, and 4,4'-methylenebis (cyclohexyl isocyanate), And the like Anuru ring type adducts.

In the adduct obtained by reacting an amount of the polyisocyanate compound in which the amount of the isocyanate group becomes excessive with respect to the hydroxyl group of the polyol, the polyisocyanate compound may be a polyisocyanate compound having three or more isocyanate groups. Aliphatic diisocyanate compounds such as hexamethylene diisocyanate, 1,4-tetramethylene diisocyanate, dimer acid diisocyanate, and lysine diisocyanate; isophorone diisocyanate, 4,4'-methylene bis (cyclohexyl isocyanate), methylcyclohexane-
2,4- (or -2,6-) diisocyanate, 1,3
Alicyclic diisocyanate compounds such as-(or 1,4-) di (isocyanatomethyl) cyclohexane; and xylylene diisocyanate, tolylene diisocyanate,
Examples thereof include aromatic diisocyanate compounds such as m- (or p-) phenylene diisocyanate, diphenylmethane diisocyanate, and bis (4-isocyanatophenyl) sulfone.

In order to stably store the resin composition for forming a film, it is necessary to protect the isocyanate as a curing agent. As this method, a protection method in which a protective group (blocking agent) is eliminated during heat curing and an isocyanate group is regenerated can be employed. Examples of the protective agent (blocking agent) include: (1) aliphatic monoalcohols such as methanol, ethanol, propanol, butanol, and octyl alcohol; and (2) monoethers of ethylene glycol and / or diethylene glycol, for example, methyl and ethyl. Monoethers such as, propyl (n-, iso) and butyl (n-, iso, sec); (3) aromatic alcohols such as phenol and cresol; (4) oximes such as acetoxime and methyl ethyl ketone oxime. By reacting one or more kinds with the isocyanate compound, an isocyanate compound protected stably at least at room temperature is obtained.

The polyfunctional polyisocyanate compound [C] as such a curing agent is used in an amount of 5 to 80 parts by weight (solid content), preferably 10 to 100 parts by weight (solid content) of the modified epoxy resin as the base resin. It is blended at a ratio of ５０50 parts by weight. If the amount of the curing agent is less than 5 parts by weight, the crosslinked density of the formed film becomes insufficient and the effect of improving rust resistance is small. On the other hand, if the amount is more than 80 parts by weight, the unreacted residual isocyanate absorbs water and not only has no effect on the rust resistance and also impairs the corrosion resistance (perforation resistance) and adhesion.

Further, a monohydric alcohol having 1 to 5 carbon atoms is reacted with a part or all of a methylol compound obtained by reacting formaldehyde with at least one selected from melamine, urea and benzoguanamine as a crosslinking agent. May be used in combination with the above polyfunctional polyisocyanate compound. The resin is sufficiently cross-linked by the above-mentioned cross-linking agent. However, in order to further increase the low-temperature cross-linking property, it is desirable to use a known curing promoting catalyst. Examples of the curing acceleration catalyst include N-ethylmorpholine, dibutyltin dilaurate,
Cobalt naphthenate, stannous chloride, zinc naphthenate,
Bismuth nitrate or the like can be used. In addition, a known resin such as acryl, alkyd, or polyester can be added to the resin composition for the purpose of slightly improving physical properties such as adhesion.

The film-forming composition of the present invention can be used as a water-dispersible or water-soluble composition by neutralizing a base of an epoxy resin as a base resin with a low molecular acid.
When used as a coating material for a BH steel sheet that requires low-temperature drying at a plate temperature of 250 ° C. or less, particularly 170 ° C. or less, it is dissolved in an organic solvent without performing such a neutralization operation. More desirably, it is used as a reduced composition. That is, in a water-dispersed or water-soluble composition, an acidic compound required for water-solubilization forms a salt in the film, easily absorbs moisture in and under the film in a humid environment, and is dried at a low temperature. Corrosion resistance and adhesion tend to be slightly inferior, for example, because a sufficiently strong film cannot be obtained under the conditions.

As the kind of the organic solvent, one or a mixture of two or more kinds of organic solvents usually used in the paint industry can be used. For that purpose, it is preferable to avoid a high boiling alcohol solvent. . This example, ethylene glycol or diethylene glycol monoalkyl ethers, alcohols having C ₅ or more primary hydroxyl group. Such a solvent inhibits the curing reaction of the film. Examples of the recommended solvent include hydrocarbon solvents, ketone solvents (methyl isobutyl ketone, acetone, cifurohexanone, isophorone, etc.), SELL solvents, ether solvents, and alcohols having a low molecular weight of ₄ or less;
Alternatively, alcohols having a secondary or tertiary hydroxyl group are also suitable.

In the present invention, the rust preventive additive [D] is added to the resin film. As the rust preventive additive [D], silica and a sparingly soluble chromate salt are particularly preferable. Either one of these silicas and the hardly soluble chromate may be added alone, but by adding both in a specific ratio, excellent rust resistance and puncture resistance can be realized. Silica has the effect of promoting the formation of basic zinc chloride, which is effective in suppressing corrosion among the corrosion products of galvanized steel sheets, and dissolves a small amount in a corrosive environment to form silicate ions. It is presumed that the anticorrosion effect is exhibited by functioning as a mold corrosion inhibitor.

The silica used in the present invention includes dry silica (for example, AEROS manufactured by Nippon Aerosil Co., Ltd.).
IL 130, AEROSIL 200, AEROSIL
300, AEROSIL 380, AEROSIL R
972, AEROSIL R811, AEROSIL R
805, etc.), and organosilica sols (for example, MA-ST, IPA-ST, NBA-S manufactured by Nissan Chemical Industries, Ltd.)
T, IBA-ST, EG-ST, XBA-ST, ETC
-ST, DMAC-ST, etc.), sedimentation wet silica (for example, T-32 (S), K-41, F- manufactured by Tokuyama Soda Co., Ltd.)
80), gel wet silica (for example, Syloid 244, Syloid 150, Syloid 72, Syloid 65, SHIELDEX, etc., manufactured by Fuji Devison Chemical Co., Ltd.)
And the like. Further, two or more kinds of the above-mentioned silicas can be used as a mixture.

These silicas include hydrophilic silica and hydrophobic silica. Among these, hydrophilic silica can be expected to have an effect of improving rust resistance, but as described later, hydrophobic silica significantly improves rust resistance. Hydrophilic silica has a hydroxyl group (

Embedded image ) And shows hydrophilicity. Since the silanol group is highly reactive, it easily reacts with various organic compounds, and can organize the silica surface. The hydrophobic silica is obtained by partially or almost completely substituting a silanol group on the surface of the hydrophilic silica with a methyl group, an alkyl group, or the like to make the silica surface hydrophobic.

There are various methods for producing hydrophobic silica, and typical examples thereof include reactions of organic solvents such as alcohols, ketones, and esters, silanes, silazanes, and polysiloxanes. Examples of the method include a reaction pressure method in an organic solvent, a catalyst heating method, and the like. Silica has an excellent anticorrosion effect, but hydrophobic silica is particularly effective in improving rust resistance. The reason is that hydrophilic silica is apt to cause penetration of iron ions or iron oxide in iron rust due to its strong hydrophilicity, which is presumed to be the reason that it has little effect on improving rust resistance. For this reason, in the present invention, it is preferable to employ hydrophobic silica.

Further, the hardly soluble black added to the organic film
By dissolving trace amounts of oxalate in a corrosive environment, 6
Releases chromate ions of valence and is similar to chromate film.
It is thought that the mechanism will prevent corrosion of galvanized steel sheet
It is. The hardly soluble chromate used in the present invention includes
Barium romate (BaCrO) _Four), Strontium chromate
Um (SrCrO)_Four), Calcium chromate (CaCr)
O_Four), Zinc chromate (ZnCrO)_Four・ 4Zn (O
H)_Two), Potassium potassium chromate (K_TwoO ・ 4ZnO ・ 4
CrO_Three・ 3H_TwoO), lead chromate (PbCrO)_Four)
Fine powder can be used. In addition, the hardly soluble
It is also possible to use a mixture of two or more
You. However, from the viewpoint of corrosion resistance,
Chromic acid burrs that can be expected to have a self-healing effect by acid ions
It is preferable to use strontium chromate
No. In addition, organic coatings are used in the pre-painting process of automobiles.
Minimize elution of water-soluble chromium from inside
Chromic acid with low solubility in water
Barium is preferred.

In the present invention, silica and a sparingly soluble chromate salt are blended in a specific ratio into the above-mentioned resin composition comprising the specific base resin and the polyfunctional polyisocyanate compound, so that the synergistic effect of both anticorrosive effects can be obtained. By effect
The most excellent rust resistance can be realized. That is, the weight ratio of silica and the hardly soluble chromate to the non-volatile content is (modified epoxy resin [A] + silane coupling agent [B] + polyfunctional polyisocyanate compound [C]) /
(Silica + poorly soluble chromate) = 90/10 to 40/6
0 Silica / poorly soluble chromate = 35/5 to 1/39, the most excellent corrosion resistance can be obtained. Here, (modified epoxy resin [A] +
Silane coupling agent [B] + polyfunctional polyisocyanate compound [C]) / (silica + poorly soluble chromate) is 9
If it exceeds 0/10, the anticorrosive effect of the silica and the hardly soluble chromate salt is not sufficiently exhibited, and the rust resistance is poor. On the other hand, when the ratio is less than 40/60, the effect of the epoxy resin as a binder is insufficient, and the adhesiveness of the paint is deteriorated. In addition, the silica / poorly soluble chromate is 35/5.
If the ratio exceeds 1, or if the ratio is less than 1/39, the synergistic effect becomes insufficient, and the rust resistance is slightly deteriorated.

As described above, silica suppresses corrosion due to rust due to the effect of promoting the formation of stable corrosion products, while sparingly soluble chromate removes defective portions of the organic film formed by rust. It repairs by the effect of hexavalent chromate ions. By using silica and sparingly soluble chromate, which have different mechanisms of inhibiting corrosion due to rust, excellent rust resistance is obtained for the first time. That was achieved.

FIG. 1 shows that an organic resin (No. 2 in Table 2) comprising the above-mentioned specific base resin and an isophorone diisocyanate-based hexafunctional polyisocyanate compound was prepared by changing the ratio of silica and a sparingly soluble chromate to the same. Corrosion resistance (evaluation after 200 cycles of puncture resistance test) and rust resistance (Rust resistance test 7)
Evaluation after the cycle) and the weight ratio of silica / poorly soluble chromate are shown. According to this, when the weight ratio of silica / poorly soluble chromate exceeds 35/5, the rust resistance deteriorates, whereas when it is less than 1/39, the normal unpainted corrosion resistance (perforation resistance) deteriorates. I do. Therefore, the compounding ratio of silica / poorly soluble chromate is preferably 35/5 to 1/39.
In order to obtain the most excellent corrosion resistance (corrosive rust resistance and puncture resistance), it is preferable to set it in the range of 20/20 to 5/35. FIG. 2 shows the relationship between the rust resistance and the weight ratio of silica / poorly soluble chromate when a conventional diisocyanate compound (HMDI) is used as a curing agent for comparison. From FIGS. 1 and 2, it can be seen that the present invention has been achieved for the first time by the synergistic effect of the combination of the polyfunctional polyisocyanate compound, the specific ratio of silica and the sparingly soluble chromate.

Further, FIG. 4 shows the unpainted corrosion resistance (perforation resistance test 2) when a hexamethylene diisocyanate-based hexafunctional polyisocyanate compound was used as a curing agent.
The relationship between the rust resistance (evaluation after 00 cycles) and rust resistance (evaluation after 15 cycles of rust resistance test) and the weight ratio of silica / poorly soluble chromate is shown. According to this,
As in the case of using the isophorone diisocyanate-based 6-functional polyisocyanate compound, the mixing ratio of silica / poorly soluble chromate is preferably 35/5 to 1/39,
In particular, when importance is attached to rust resistance, 20/20 to 1 /
In order to obtain the most excellent corrosion resistance (corrosion resistance and puncture resistance), it is 20/20.
It is preferably in the range of 5 to 35. Further, examples described later (for example, comparison between No. 70 and No. 73)
As is clear from the description, when the hexamethylene diisocyanate-based hexafunctional polyisocyanate compound is compared with the isophorone diisocyanate-based hexafunctional polyisocyanate compound, when the mixing ratio of silica / poorly soluble chromate is the same, The use of a hexamethylene diisocyanate-based hexafunctional polyisocyanate compound provides more excellent rust resistance.

In the present invention, the above-mentioned silica and sparingly soluble chromate salt are the main additive components in the resin composition. In addition, silane coupling agents, coloring pigments (for example, condensed polycyclic Organic pigments, phthalocyanine organic pigments, etc.), coloring dyes (eg, azo dyes, azo metal complex dyes, etc.), rust-preventive pigments (eg, aluminum trihydrogen diphosphate, aluminum phosphomolybdate, zinc phosphate, etc.) ), A conductive pigment (for example, iron phosphide, antimony-doped tin oxide, etc.), a surfactant, and the like. In addition, workability can be improved by adding at least one of the following lubricants. Polyolefin wax (polyethylene wax, polypropylene wax, etc.) Fluororesin (ethylene tetrafluoride resin, ethylene tetrafluoride-hexafluoropropylene copolymer resin, ethylene fluoride
(Perfluoroalkyl vinyl ether copolymer resin, ethylene tetrafluoride-ethylene copolymer resin, ethylene trifluoride ethylene resin, vinylidene fluoride resin, etc.) Graphite Molybdenum disulfide Boron nitride You may.

The organic film as described above is coated on the chromate film in an amount of 0.2 to 3.0 μm, preferably 0.5 to 2.0 μm.
m. If the film thickness is less than 0.2 μm, sufficient rust resistance cannot be obtained, while 3.0 μm
If it exceeds, weldability (particularly continuous hitting property) is reduced. FIG.
The results of examining the relationship between the thickness of the organic film and the spot weldability (continuous spotting property) are shown in FIG. According to this, the film thickness is 3.
It can be seen that when the thickness exceeds 0 μm, the spot weldability decreases.

As a method of applying the above-mentioned coating composition to a galvanized steel sheet, the coating composition is usually applied by a roll coater method. However, after applying by a dipping method or a spraying method, an air knife method or a roll drawing method is used. Can also adjust the amount of application. Further, as a heat treatment method after applying the coating composition, a hot blast furnace, a high-frequency induction heating furnace, an infrared furnace, or the like can be used. The heat treatment is performed at an ultimate plate temperature of 80 to 250 ° C, preferably 100 to 200 ° C.
It is desirable to perform within the range. Further, when the present invention is applied to a BH steel sheet, a heat treatment at 150 ° C. or less is desirable. The steel sheet of the present invention has a great feature that it is obtained by such low-temperature baking.

When the baking temperature is lower than 80 ° C., the film does not crosslink, and it is not possible to obtain sufficient corrosion resistance.
High-temperature baking exceeding 50 ° C. deteriorates corrosion resistance. This is due to the volatilization of water contained in the chromate film component and the hydroxyl group (

Embedded image ) The rapid progress of the dehydration-condensation reaction between the two causes the chromate film to break down due to the occurrence of cracks in the chromate film, and the reduction of hexavalent chromium to proceed to reduce the passivating effect of hexavalent chromium. It is presumed to be due to The car body is subjected to cationic electrodeposition coating, but the chromate film + the organic film has a wet electric resistance of 200.
If it exceeds kΩ / cm ² , there is a problem that the cationic electrodeposition coating is not formed well. Therefore, in the steel sheet of the present invention mainly used for automobile bodies, the wet resistance of the chromate film + resin composition film is 200 kΩ / cm ² or less. It is preferable to form both films so as to keep the film thickness at a minimum.

The present invention includes a steel sheet having the above-described coating structure on both sides or one side. Embodiments of the steel sheet of the present invention include, for example, the following. (1) One side: Plating film-Chromate film-Organic film One surface: Fe surface (2) One surface: Plating film-Chromate film-Organic film One surface: Plating film (3) Both surfaces: Plating film-Chromate film-Organic film The organic composite coated steel sheet of the invention is not limited to automobiles,
It can also be used for home appliances and building materials.

[0047]

EXAMPLES As an organic composite coated steel sheet for an automobile body, a zinc-based coated steel sheet was alkali-degreased, washed with water, dried, subjected to chromate treatment, and then coated with a coating composition using a roll coater and baked. With respect to the obtained organic composite coated steel sheet, each test was performed for resistance to puncture, resistance to rust, paint adhesion and weldability. The results are shown in Tables 5 to 15.
Shown in The manufacturing conditions of this example are as follows. (1) Zinc-based plated steel sheet A cold-rolled steel sheet having a thickness of 0.8 mm and a surface roughness (Ra) of 1.0 μm was subjected to various zinc-based platings and used as an original plate for processing.
(See Table 1)

(2) Chromate treatment Coating type chromate treatment A chromate treatment solution having the following composition was applied by a roll coater and dried without washing with water. The adhesion amount of the chromate layer was adjusted by changing the peripheral speed ratio between the pickup roll of the roll coater and the applicator roll. Chromic anhydride: 20 g / l Phosphate ion: 4 g / l Zirconium fluoride ion: 1 g / l Zinc ion: 1 g / l Hexavalent chromium / trivalent chromium: 3/3 (weight ratio) Chromic anhydride / zirconium fluoride Ion: 20/1
(Weight ratio)

Electrolytic chromate treatment Chromic anhydride 30 g / l, sulfuric acid 0.2 g / l, bath temperature 40
A zinc-plated steel sheet was subjected to cathodic electrolysis at a current density of 10 A / dm ² using a treatment liquid at a temperature of 10 ° C., followed by washing with water and drying.
The adhesion amount of the chromate layer was adjusted by controlling the amount of electricity in the cathodic electrolysis treatment. Reaction type chromate treatment Chromic anhydride 30 g / l, phosphoric acid 10 g / l, NaF
Using a treatment liquid of 0.5 g / l, K ₂ TiF ₆ 4 g / l, and a bath temperature of 60 ° C., the zinc-plated steel sheet was spray-treated, washed with water and dried. The amount of the chromate layer deposited was adjusted by changing the treatment time.

(3) Organic Resin Table 2 shows the organic resins used. The modified epoxy resin and curing agent (polyisocyanate) shown in the table were prepared by the following method. [Modified Epoxy Resin] (1) Production of Modified Epoxy Resin A1 Epicoat 1009 (epoxy resin manufactured by Yuka Kagaku Shell Epoxy Co., Ltd.) was added to a reactor equipped with a stirrer, reflux condenser, thermometer and liquid dropping device; (Molecular weight about 3750)
1880 g (0.5 mol) and methyl isobutyl ketone /
1000 g of a mixed solvent of xylene = 1/1 (weight ratio) was weighed, stirred and heated, and uniformly dissolved at the boiling point of the solvent. After that, the mixture was cooled to 70 ° C.
70 g of (propanol) amine were added dropwise over a period of 30 minutes. During this time, the reaction temperature was maintained at 70 ° C. After completion of the dropwise addition, the mixture was kept at 120 ° C. for 2 hours to complete the reaction, thereby obtaining a modified epoxy resin A1. The effective component of the modified epoxy resin A1 was 66%.

(2) Production of Modified Epoxy Resin A2 Epicoat 1004 (epoxy resin manufactured by Yuka Shell Epoxy Co., Ltd .; molecular weight of about 16)
00) 1600 g and xylene 1000 g are weighed and 10
The mixture was heated to 0 ° C., and 65 g of diethanolamine and 30 g of monoethanolamine collected in a liquid dropping apparatus were added thereto for 3 hours.
Dropping took 0 minutes. Thereafter, the temperature was maintained at 120 ° C. for 2 hours to complete the reaction, and a modified epoxy resin A2 was obtained. The effective component of the modified epoxy resin A2 was 63%.

[Curing Agent] (1) Hexafunctional isocyanate (Curing agent a) 222 parts of isophorone diisocyanate and 34 parts of methyl isobutyl ketone were weighed into a reactor equipped with a thermometer, a stirrer and a reflux condenser with a dropping funnel. After taking and dissolving uniformly, 87 parts of methyl ethyl ketone oxime was dropped from the dropping funnel into the stirred isocyanate solution kept at 70 ° C. for 2 hours. Thereafter, 30.4 parts of sorbitol was added, the temperature was raised to 120 ° C, and the reaction was performed at 120 ° C. Then, an IR measurement of the reaction product was performed,

(Equation 1) No absorption by isocyanate groups was confirmed, and 50.4 parts of butyl cellosolve was added to obtain a curing agent a. The active ingredient of the curing agent a was 80%.

(2) Tetrafunctional isocyanate (curing agent b) 222 parts of isophorone diisocyanate and 34 parts of methyl isobutyl ketone were weighed and measured in a reactor equipped with a thermometer, a stirrer and a reflux condenser with a dropping funnel. Then, 87 parts of methyl ethyl ketone oxime was dropped from the dropping funnel into the stirred isocyanate solution kept at 70 ° C. for 2 hours. Thereafter, 34 parts of pentaerythritol was added and the temperature was raised to 120 ° C.
Was reacted. Then, an IR measurement of the reaction product was performed,

(Equation 2) No absorption by isocyanate groups was confirmed, and 52 parts of butyl cellosolve was added to obtain a curing agent b. The active ingredient of this curing agent b was 80%.

(3) Trifunctional isocyanate (curing agent c) Duranate TPA-100 (HMD) was placed in a reactor equipped with a thermometer, a stirrer and a reflux condenser with a dropping funnel.
550 parts of isocyanuric ring type I (manufactured by Asahi Kasei Corporation) and 34 parts of methyl isobutyl ketone are weighed and uniformly dissolved, and then 270 parts of methyl ethyl ketone oxime is stirred at 70 ° C. from the dropping funnel at 70 ° C. The solution was dropped into the solution over 2 hours. Then, an IR measurement of the reaction product was performed,

(Equation 3) No absorption by an isocyanate group was confirmed, and 47 parts of butyl cellosolve was added to obtain a curing agent c. The active ingredient of this curing agent c was 90%.

(4) Bifunctional isocyanate (curing agent d) Takenate B-870N (MEK oxime block of IPDI; manufactured by Takeda Pharmaceutical Co., Ltd.) was used as curing agent d. (5) Hexamethylene diisocyanate-based hexafunctional isocyanate (curing agent e) Hexamethylene diisocyanate-based hexafunctional isocyanate compound Duranate MF-B80M (HMD
I-type oxime block of hexafunctional isocyanate: Asahi Kasei Kogyo Co., Ltd.) was used as the curing agent e.

The evaluation method of each characteristic is as follows. (A) Corrosion resistance test (perforation resistance) After tape-sealing the edge and back surface of the unpainted test piece, a cross cut was made on the lower half surface of the test piece, and the corrosion promotion test of the following composite corrosion test cycle was performed. And the degree of progress of corrosion after 200 cycles. 5% NaCl spraying / 35 ° C (4 hours) ↓ Drying / 60 ° C (2 hours) ↓ 95% RH wet / 50 ° C (4 hours) The evaluation criteria are as follows. ◎: No red rust generated ○ +: Red rust area ratio less than 5% ○: Red rust area ratio 5% or more and less than 10% ○-: Red rust area ratio 10% or more, less than 20% △: Red rust area ratio 20% or more, 50% Less than ×: Red rust area ratio 50% or more

(B) Rust-Resistant Resistance After the edge and the back of the unpainted test piece were tape-sealed, an accelerated corrosion test was performed by the following composite corrosion test cycle in the presence of iron rust. 7 for 1-69
The degree of rust generation after the cycle was measured by using Example No. 70 ~
For 124, the degree of rusting after 15 cycles was evaluated. In the presence of iron rust (* Note) 5% NaCl immersion, 50 ° C (18 hours) ↓ 95% RH wet, 50 ° C (3 hours) ↓ Drying, 60 ° C (3 hours) (* Note) Iron rust supply method: A cold rolled steel sheet having an area of 10 cm ² per liter of salt water was immersed. The evaluation criteria are as follows. ◎: No red rust occurred ○: Red rust area ratio less than 10% ○-: Red rust area ratio 10% or more, less than 20% △: Red rust area ratio 20%, less than 50% ×: Red rust area ratio 50% or more

(C) Paint adhesion After the test piece was degreased with alkali, Nippon Paint Co., Ltd.
At 600, electrodeposition coating (25 μm thickness) was performed, and then overcoating (35 μm thickness) was performed with Luga Bake B-531 manufactured by Kansai Paint Co., Ltd. These tests were immersed in ion exchange water at 40 ° C. for 240 hours. Next, the test piece was taken out and allowed to stand at room temperature for 24 hours. Then, 100 pieces of grids were cut at intervals of 2 mm on the coating film, the adhesive tape was adhered and peeled off, and the peeling rate of the coating film was evaluated. The evaluation criteria are as follows. ◎: no peeling ○: peeling rate less than 3% △: peeling rate 3% or more and less than 10% ×: peeling rate 10% or more

(D) Weldability A continuous hitting test was carried out with a CF electrode, a pressure of 200 kgf, a conduction time of 10 cycles / 50 Hz, and a welding current of 10 kA, and the number of hits was evaluated. The evaluation criteria are as follows. ◎: 2000 points or more ○: 1000 points or more and less than 2000 points ×: less than 1000 points

[0060]

[Table 1]

[0061]

[Table 2]

[0062]

[Table 3]

[0063]

[Table 4]

[0064]

[Table 5]

[0065]

[Table 6]

[0066]

[Table 7]

[0067]

[Table 8]

[0068]

[Table 9]

[0069]

[Table 10]

[0070]

[Table 11]

[0071]

[Table 12]

[0072]

[Table 13]

[0073]

[Table 14]

[0074]

[Table 15]

* 1 Emission: Example of the present invention, ratio: Comparative example * 2 * 3 Adhesion amount of chromate in terms of metal chrome * 4 No. of organic resin listed in Table 2 * 5 No. of silica listed in Table 3. * 6 No. of the hardly soluble chromate described in Table 4. * 7 Non-volatile content weight ratio * 8 Non-volatile content weight ratio

As is clear from the above examples, the organic composite coated steel sheet of the present invention shows excellent rust resistance. Ma
In addition, in Example No. 70 and Example Nos.
Comparison of No. 73 shows that hexamethylene diisocyanate-based hexafunctional polyisocyanate compound as the curing agent can provide more excellent rust resistance than using isophorone diisocyanate-based hexafunctional polyisocyanate compound.

[0077]

The organic composite coated steel sheet of the present invention as described above has extremely excellent rust resistance and puncture resistance, and also has excellent paint adhesion and weldability. It is extremely useful as a surface-treated steel sheet for automobile bodies.

[Brief description of the drawings]

FIG. 1 shows the addition of silica and a sparingly soluble chromate to an organic resin composed of a specific base resin defined by the present invention and a polyfunctional polyisocyanate compound (isophorone diisocyanate-based hexafunctional polyisocyanate compound) at different ratios. Weight ratio of silica / sparingly soluble chromate and the usual unpainted corrosion resistance and rust resistance (Rust rust resistance test 7)
Graph showing the relationship with

FIG. 2 shows the weight ratio of silica / slightly soluble chromate and the rust resistance when silica and sparingly soluble chromate are added in a different ratio to an organic resin using a conventional diisocyanate compound as a curing agent. Graph showing the relationship with

FIG. 3 is a graph showing the relationship between the thickness of an organic film and weldability.

FIG. 4 is a graph showing the relationship between the ratio of silica and the sparingly soluble chromate added to an organic resin comprising a specific base resin defined by the present invention and a hexamethylene diisocyanate-based hexafunctional polyisocyanate compound at a different ratio. Graph showing the relationship between the weight ratio of hardly soluble chromate and ordinary unpainted corrosion resistance and rust resistance (evaluation after 15 cycles of rust resistance test).

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-6-235071 (JP, A) JP-A-3-130141 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) B05D 7/14 B32B 15/08 C23C 22/24 C23C 22/30

Claims (4)

(57) [Claims] 1. A chromate film having a coating weight of 5 to 200 mg / m ^{2 in} terms of metallic chromium is formed on the surface of a Zn-based plated steel sheet, and 500 to 5000
Of the epoxy group of the epichlorohydrin-bisphenol A type epoxy resin having an epoxy equivalent of 0.1.
For 100 parts by weight (solid content) of the modified epoxy resin [A] to which 8 to 1.0 mol of dialkanolamine is added,
0.5 to 10 parts by weight of the silane coupling agent [B],
It contains 5-80 parts by weight of a polyfunctional polyisocyanate compound [C] having at least 6 isocyanate groups in a molecule, and further contains a rust preventive additive [D] as ([A] + [B] +
[C]) / [D] 90/10 to 40 by weight of solid content
An organic composite coated steel sheet having excellent rust resistance, characterized in that a resin film having a dry film thickness of 0.2 to 3.0 [mu] m was formed by adhering a resin composition containing the resin composition at a ratio of / 60. 2. A rust preventive additive [D] constituting a resin film.
2. The organic composite coated steel sheet according to claim 1, wherein the steel sheet is silica or / and a sparingly soluble chromate. 3. The organic composite coating having excellent rust resistance according to claim 1, wherein the rust preventive additive [D] constituting the resin organic film is silica and a sparingly soluble chromate having the following weight ratios. steel sheet. Silica / poorly soluble chromate = 35/5/1/39 4. A polyfunctional polyisocyanate compound [C]
4. The organic composite coated steel sheet having excellent rust resistance according to claim 1, which is a polyfunctional hexamethylene diisocyanate having at least six isocyanate groups in one molecule.