JP3259581B2 - Organic composite coated steel sheet with excellent rust resistance and cationic electrodeposition coating properties - 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 which is suitable for automobile bodies and has excellent rust resistance and excellent cationic electrodeposition coating properties.

[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 Unexamined Patent Publication No. 2-185436 discloses an organic composite having excellent corrosion resistance, which has a chromate film on the surface of a Zn-based plated steel sheet and an organic resin film having a thickness of about 1 μm on the surface. Coated steel sheets have been proposed. The organic coating of the organic composite coated steel sheet disclosed in JP-A-2-185436 has the following features.

(1) Epichlorohydrin having excellent corrosion resistance
Based on bisphenol A type epoxy resin, (2) reacting alkylene glycol which is a hydrophilic skeleton in the skeleton of bisphenol A for the purpose of improving electrodeposition coating property,
(3) Polymerization (urethane modification) with diisocyanate for the purpose of improving alkali resistance, and (4) alkanolamine is provided in anticipation of interaction with silica.
Since the resin film of this organic composite coated steel sheet is a urethane-modified epoxy resin as a means of polymerizing,
The curing agent for bake hardening is not an essential component, but merely describes that a known curing agent (such as an amino resin or a general blocked isocyanate) can be used if necessary.

[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 organic 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 Japanese Patent Application Laid-Open No. 2-185436 cannot necessarily exhibit sufficient performance with respect to the rust resistance. This is considered to be because the hardening agent was not added as an essential component to the organic coating of the coated steel sheet, and even if it was added, a sufficient crosslink density was not obtained with a known hardening agent.

[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 organic resin, the rust resistance of the organic composite coated steel sheet is reduced. No specific measures are taken to improve it. The present invention has been made in view of the above circumstances, and aims to provide an organic composite coated steel sheet which has excellent properties such as corrosion resistance (perforation resistance) and paint adhesion, as well as excellent rust resistance and cationic electrodeposition coating properties. And

[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 metallic chromium is formed on the surface of a Zn-based plated steel sheet, and an epichlorohydrin-bisphenol A type epoxy resin and a polyalkylene are formed on the chromate film. Glycol diglycidyl ether in a weight ratio (epichlorohydrin-bisphenol A type epoxy resin / polyalkylene glycol diglycidyl ether) of 100
Bisphenol A is reacted with a mixture mixed at a ratio of / 300 to 100/10, and an epoxy equivalent obtained by further reacting with an isocyanate compound is 1000 to 500.
With respect to 100 parts by weight (solid content) of a base resin [A] obtained by adding an alkanolamine to an epoxy group of a urethane-modified epoxy resin of No. 0, at least 6
5 to 80 parts by weight (solid content) of a polyfunctional polyisocyanate compound [B] having two isocyanate groups, and a rust preventive additive [C] of ([A] + [B]) / [C] Characterized in that a resin composition containing 90/10 to 40/60 by weight per minute was applied to form a resin film having a dry film thickness of 0.2 to 3.0 μm. Composite coated steel sheet with excellent properties and cationic electrodeposition coating properties.

(2) In the organic composite coated steel sheet of the above (1), the rust-preventive additive [C] constituting the organic film is silica or / and a hardly soluble chromate, and is resistant to rust and cationic electrodeposition. Organic composite coated steel sheet with excellent paintability. (3) In the organic composite coated steel sheet of the above (1), the rust-preventive additive [C] constituting the organic film is silica and a sparingly soluble chromate having the following weight ratio, and is resistant to rust and cationic electrodeposition. Organic composite coated steel sheet with excellent paintability. Silica / poorly soluble chromate = 35/5/1/39

(4) In the organic composite coated steel sheet according to the above (1), (2) or (3), the polyfunctional polyisocyanate compound [B] is a hexamethylene diisocyanate having at least six isocyanate groups in one molecule. An organic composite coated steel sheet which is a multifunctional material and has excellent rust resistance and cationic electrodeposition coating properties.

[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. Hereinafter, the details of the present invention and the reasons for limitation will be described.

[0011] As the base zinc-coated steel sheet,
Galvanized steel sheet, Zn-Ni alloy plated steel sheet, Zn-F
e alloy plated steel sheet, Zn-Mn alloy plated steel sheet, Zn-
Al alloy plated steel sheet, Zn-Cr alloy plated 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 plating in which metal oxide, hardly soluble chromate, polymer, etc. are dispersed and plated Steel plates and the like 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 used among an electrolysis method, a melting method, and a gas phase method can be adopted, but the electrolysis method is advantageous from the viewpoint of selectivity of the underlying cold-rolled steel sheet.

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, epichlorohydrin-bisphenol A type epoxy resin and polyalkylene glycol diglycidyl ether are added in a weight ratio (epichlorohydrin-bisphenol A type epoxy resin / polyalkylene glycol diglycidyl ether) of 10%.
An epoxy equivalent obtained by reacting bisphenol A with a mixture mixed at a ratio of 0/300 to 100/10 and further reacting with an isocyanate compound is 1000 to 50.
A polyfunctional polyisocyanate having at least 6 isocyanate groups per molecule as a curing agent for 100 parts by weight (solid content) of a base resin [A] obtained by adding an alkanolamine to an epoxy group of a urethane-modified epoxy resin of No. 00 The compound [B] contains 5 to 80 parts by weight (solid content), and the rust preventive additive [C] is added to ([A] + [B]) /
A resin film to which a resin composition containing 90/10 to 40/60 by weight of the solid content of [C] is adhered is formed.

The epichlorohydrin-bisphenol A type epoxy resin used for the resin film is a condensate obtained by subjecting bisphenol A and epichlorohydrin to a condensation reaction. As the epoxy resin, besides epichlorohydrin-bisphenol A type epoxy resin, there is also a resin consisting of only an aliphatic epoxy resin or an alicyclic epoxy resin structure, but in the present invention, in order to obtain excellent corrosion resistance, the epoxy resin of the above condensate is used. Is used. Specific examples (commercially available) of epichlorohydrin-bisphenol A type epoxy resin include, for example, Epicoat 828, 1001, 1004,
And 1007 and 1009 (all manufactured by Shell Chemical Co., Ltd.). These can be used alone or in combination.

In reacting bisphenol A with a mixture of epichlorohydrin-bisphenol A type epoxy resin and polyalkylene glycol diglycidyl ether, the weight ratio of epichlorohydrin-bisphenol A type epoxy resin / polyalkylene glycol diglycidyl ether in the mixture is 100 / If it is less than 300, hydrophilicity is imparted to the resin, and although the appearance of the coating film after electrodeposition coating is improved, epichlorohydrin which improves coating adhesion is obtained.
Since the mixing ratio of the bisphenol A type epoxy resin is relatively low, the adhesion between the electrodeposition coating film and the resin film is reduced. On the other hand, when the weight ratio of epichlorohydrin-bisphenol A type epoxy resin / polyalkylene glycol diglycidyl ether exceeds 100/10, the compounding ratio of the hydrophilic polyalkylene glycol diglycidyl ether becomes relatively low, so that the hydrophilicity becomes low. Insufficient, and the appearance of the coating film after electrodeposition coating deteriorates.

Examples of the polyalkylene glycol diglycidyl ether include polyethylene glycol diglycidyl ether, polypropylene glycol diglycidyl ether, polybutylene glycol diglycidyl ether, neopentyl glycol diglycidyl ether, and 1,6-hexanediol diglycidyl ether. ,
Glycerin diglycidyl ether or the like can be used. By reacting bisphenol A with a mixture of epichlorohydrin-bisphenol A type epoxy resin and polyalkylene glycol diglycidyl ether,
A film excellent in electrodeposition coating property can be obtained.

By reacting the thus obtained resin composition with an isocyanate compound, a urethane-modified epoxy resin having an epoxy equivalent of 1,000 to 5,000 is obtained. If the epoxy equivalent of the urethane-modified epoxy resin is less than 1,000, sufficient alkali resistance cannot be obtained due to the small molecular weight of the obtained resin, and elution, swelling, and softening of the film occur during electrodeposition coating. On the other hand, when the epoxy equivalent is 50
If it exceeds 00, the amount of alkanolamine to be added is small because the concentration of the epoxy group is too low, and sufficient rust resistance cannot be obtained.

The isocyanate compound used is an aliphatic, alicyclic or aromatic compound having at least two isocyanate groups in one molecule, or a compound obtained by partially reacting such a compound with a polyhydric alcohol. For example, m- or p-phenylene diisocyanate,
2,4- or 2,6-tolylene diisocyanate or p-xylene diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate alone or in a mixture, or a polyhydric alcohol (a dihydric alcohol such as ethylene glycol and propylene glycol, glycerin, Reaction products with methylolpropane, pentaerythritol, sorbitol, polypentahydric alcohols such as dipentaerythrol, and at least 2
And a compound in which two isocyanates remain.

In particular, from the viewpoint of rust resistance, a polyfunctional compound having at least 3 (preferably 4 or more, more preferably 6 or more) isocyanate groups in one molecule, similarly to the curing agent described later. It is preferable to use a polyisocyanate compound. Specific examples of these polyfunctional polyisocyanate compounds will be described in the description of the curing agent. The polyfunctional polyisocyanate compound used for urethane modification and the polyfunctional polyisocyanate compound used for the curing agent may be the same compound or different compounds.

The epoxy equivalent thus obtained is 1
Alkanolamine is further added to the epoxy group of the urethane-modified epoxy resin of 000 to 5000 to obtain a base resin [A]. The amount of the alkanolamine added to the epoxy group of the urethane-modified epoxy resin is preferably 0.5 to 1.0 mol per 1 mol of the epoxy group. Alkanolamine was added in an amount of 0.1 to 1 mole of epoxy group.
By adding 5 mol or more, a sufficient reinforcing effect with silica can be obtained, so that the effect of suppressing film swelling due to alkali generated at the interface during electrodeposition is improved, and the effect of preventing deterioration of coating film adhesion is improved. On the other hand, if the added amount of alkanolamine exceeds 1.0 mole per 1 mole of epoxy group, the excess alkanolamine is not added to the epoxy group, which is not only uneconomical but also causes unreacted components in the film. It reduces corrosion resistance and secondary adhesion.

The alkanolamines used include:
Examples thereof include monoalkanolamines such as ethylethanolamine and ethanolamine and dialkanolamines such as diethanolamine, dipropanolamine and dibutanolamine. Of these, dialkanolamine is particularly preferable. The reason for this is that dialkanolamine can introduce more primary hydroxyl groups into the base resin than other alkanolamines, and as a result,
This is because the reinforcing effect due to the interaction between the graded hydroxyl group and silica is increased, and the low-temperature seizure is further improved.

The base resin [A] thus obtained.
Has sufficient properties such as corrosion resistance and paint adhesion as a resin composition for forming a film as it is, but cannot provide satisfactory rust resistance. Therefore, in the present invention, a specific polyfunctional polyisocyanate compound [B] is further added as a curing agent to the base resin [A] obtained by adding an alkanolamine to the urethane-modified epoxy resin, and the hydroxyl group in the base resin and the polyisocyanate in the polyisocyanate are added. A urethanization reaction occurs between the isocyanate group and the isocyanate group.

The polyisocyanate compound used as a curing agent for the resin film of the present invention has at least 6 isocyanate groups in one molecule (even if these isocyanate groups are blocked) for the purpose of improving rust resistance. Good). That is, 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 isocyanate compound having three or more isocyanate groups in one molecule, more preferably a polyfunctional isocyanate compound having four or more isocyanate groups, and still more preferably a polyfunctional isocyanate compound having six or more isocyanate groups, It has been found that rust resistance, which is much better than isocyanate 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 used in the present invention may be a mixture of homologous compounds having different numbers of isocyanate groups in one molecule. Further, the above polyfunctional polyisocyanate compound is
More than one kind 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 at least two isocyanate groups in one molecule. There is a compound obtained by reacting a compound having a group with a polyhydric alcohol, or a compound such as a buret type adduct 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, and hexanetriol; hexamethylene diisocyanate, isophorone diisocyanate, Burette type adducts such as diisocyanate, xylylene diisocyanate, 4,4'-diphenylmethane diisocyanate, 4,4'-methylenebis (cyclohexyl isocyanate) or And the like Soshianuru 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
An alicyclic diisocyanate compound such as-(or 1,4-) di (isocyanatomethyl) cyclohexane; and xylylene diisocyanate, tolylene diisocyanate, m
And aromatic diisocyanate compounds such as-(or p-) phenyl diisocyanate, diphenylmethane diisocyanate, and bis (4-isocyanatophenyl) sulfone.

In order to stably store the film formed product, it is necessary to protect the isocyanate of the 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 [B] as a curing agent is used in an amount of 5 to 80 parts by weight (solid content), preferably 10 to 50 parts by weight, based on 100 parts by weight (solid content) of the base resin [A]. It is blended in 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 but also impairs the corrosion resistance (perforation resistance) and adhesion.

Further, a monohydric alcohol having 1 to 5 carbon atoms is reacted with 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. The alkyl etherified amino resin may be used in combination with the above isocyanate compound. It should be noted that the resin is sufficiently cross-linked with 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. As the curing promoting catalyst, for example, N-ethylmorpholine, dibutyltin dilaurate, cobalt naphthenate, stannous chloride, zinc naphthenate, bismuth nitrate and the like can be used. In addition, in order to slightly improve the physical properties such as adhesion, a known acrylic,
Resins such as alkyds and polyesters can also be added.

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 [C] is added to the resin film. As the rust preventive additive [C], 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.

The sparingly soluble chromate added to the organic film is dissolved in a very small amount in a corrosive environment to form
It is thought to release chromic ions of valence and suppress corrosion of the zinc-based plated steel sheet by the same mechanism as the chromate film. Barium chromate (BaCrO ₄ ), strontium chromate (SrCrO ₄ ), calcium chromate (CaCr)
O _4), chromic acid zinc (ZnCrO ₄ · 4Zn (O
H) ₂ ), potassium zinc chromate (K ₂ O.4ZnO.4)
Fine powders such as CrO ₃ .3H ₂ O) and lead chromate (PbCrO ₄ ) can be used. It is also possible to use a mixture of two or more of the hardly soluble chromates described above. However, from the viewpoint of corrosion resistance, it is preferable to use barium chromate or strontium chromate, which can be expected to have a self-healing effect by chromate ions over a long period of time. Further, from the viewpoint of minimizing the elution of water-soluble chromium from the organic film in the pretreatment step of coating the automobile, barium chromate having low solubility in water is preferable.

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 nonvolatile content is as follows: (base resin [A] + polyfunctional polyisocyanate compound [B]) / (silica + slightly soluble chromate = 90/10)
When 40/60 silica / poorly soluble chromate = 35/5 to 1/39, the most excellent corrosion resistance can be obtained. Here, when (base resin [A] + polyfunctional polyisocyanate compound [B]) / (silica + poorly soluble chromate) exceeds 90/10, the anticorrosion effect of silica and the sparingly soluble chromate is sufficient. Since it is not exhibited, it has poor rust resistance. On the other hand, if it is less than 40/60,
The effect of the epoxy resin as a binder becomes insufficient, and the adhesiveness of the paint deteriorates. Further, if the ratio of silica / slightly soluble chromate exceeds 35/5, or if it is less than 1/39, the synergistic effect becomes insufficient, and the rust resistance of the resin slightly deteriorates.

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. About the obtained organic composite coated steel sheet, perforation resistance, rust resistance, paint adhesion,
Each test for weldability and electrodeposition paintability was performed. The results are shown in Tables 5 to 15. 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 base resin and curing agent (polyisocyanate) shown in the table were prepared by the following method.

[Production of Base Resin] Base resin A1 polyethylene glycol diglycidyl ether (trade name Denacol EX-830 manufactured by Nagase Kasei Kogyo Co., Ltd.)
Epoxy equivalent 262) 100 g and epichlorohydrin-
50 g of bisphenol A was added to a mixture of 100 g of bisphenol A type epoxy resin (trade name: Epicoat 1001: epoxy equivalent 500, manufactured by Yuka Shell Epoxy Co., Ltd.)
Was added and reacted at 150 ° C. for 6 hours. After confirming that unreacted phenolic hydroxyl groups had disappeared, xylol 125
g and 125 g of methyl ethyl ketone were added thereto, followed by cooling to obtain a product X. Next, the product X was heated to 80 ° C. and reacted while dropping 20 g of tolylene diisocyanate over 1 hour. After the addition, 8 hours for 3 hours
The reaction was carried out while keeping the temperature at 0 ° C, and the epoxy equivalent was 1785.
To obtain a urethane-modified epoxy resin. The end point of the reaction is determined by absorption of isocyanate groups (infrared spectrophotometer).

(Equation 1) ) Disappears. Then, diethanolamine 1
4.7 g was added and reacted at 80 ° C. for 2 hours.
I got

Base resin A2 1,6 hexanediol diglycidyl ether (trade name Denacol EX-212 manufactured by Nagase Kasei Kogyo Co., Ltd.)
Epoxy equivalent 150) 100 g and epichlorohydrin-
A mixture of 100 g of bisphenol A type epoxy resin (trade name: Epicoat 1004, manufactured by Yuka Shell Epoxy Co., Ltd., epoxy equivalent: 1000) was mixed with bisphenol A75.
g was added and reacted at 150 ° C. for 6 hours. After confirming that unreacted phenolic hydroxyl groups had disappeared, xylol 12
5 g and 125 g of methyl ethyl ketone were added and cooled to obtain a product Y. Next, the product Y was heated to 80 ° C. and reacted while dropping 20 g of tolylene diisocyanate over 1 hour. After the completion of the dropwise addition, the reaction was further continued for 3 hours while keeping the temperature at 80 ° C., and the epoxy equivalent was 25
A urethane modified epoxy resin of No. 00 was obtained. The end point of the reaction is determined by absorption of isocyanate groups (infrared spectrophotometer).

(Equation 2) ) Disappears. Next, 9.0 g of diethanolamine was added and reacted at 80 ° C. for 2 hours to obtain a base resin A2.
I got

[Curing Agent] Hexafunctional isocyanate (Curing agent B1) 222 parts of isophorone diisocyanate and 34 parts of methyl isobutyl ketone were weighed into a reaction vessel equipped with a thermometer, a stirrer and a reflux condenser with a dropping funnel. After 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 3) No absorption by isocyanate groups was confirmed, and 50.4 parts of butyl cellosolve was added to obtain a curing agent B1. The active ingredient of the curing agent B1 was 80%.

Tetrafunctional Isocyanate (Curing Agent B2) 222 parts of isophorone diisocyanate and 34 parts of methyl isobutyl ketone were weighed and uniformly dissolved in a reaction vessel equipped with a thermometer, a stirrer and a reflux condenser with a dropping funnel. Thereafter, 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 4) No absorption by isocyanate groups was confirmed, and 52 parts of butyl cellosolve was added to obtain a curing agent B2. The active ingredient of this curing agent B2 was 80%.

Trifunctional isocyanate (curing agent B3) Duranate TPA-100 (HMD) was placed in a reaction vessel 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 5) No absorption by isocyanate groups was confirmed, and 47 parts of butyl cellosolve was added to obtain a curing agent B3. The active ingredient of this curing agent B3 was 90%.

Bifunctional Isocyanate (Curing Agent B4) Takenate B-870N (MEK oxime block of IPDI; manufactured by Takeda Pharmaceutical Co., Ltd.) was used as the curing agent B4. Hexamethylene diisocyanate-based hexafunctional isocyanate (curing agent B5) Hexamethylene diisocyanate-based hexafunctional isocyanate compound duranate MF-B80M (HMD
An I-type oxime block of a hexafunctional isocyanate (manufactured by Asahi Kasei Corporation) was used as the curing agent B5.

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 edges and the back surface 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

(E) Electrodeposition properties After the test piece was degreased in the same manner as in the corrosion resistance test, electrodeposition coating (20 μm in thickness) was performed with an electrodeposition coating power top U-600 manufactured by Nippon Paint Co., Ltd. Bake at 165 ° C. for 25 minutes. The electrodeposition coating property was evaluated by visual inspection of the coating appearance according to the following criteria. ◎: Smooth and uniform appearance ：: Very good skin appearance with very slight skin roughness △: Good skin roughness or craters ×: Many craters or non-electrodepositable

(D) Weldability A continuous hitting test was conducted with a CF type electrode, a pressure of 200 kgf, a conduction time of 10 cycles / 50 Hz, and a welding current of 10 kA, and the continuous hitting number 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

(E) Electrodeposition properties After the test piece was degreased in the same manner as in the corrosion resistance test, electrodeposition coating (20 μm in film thickness) was performed using an electrodeposition coating power top U-600 manufactured by Nippon Paint Co., Ltd. Bake at 165 ° C. for 25 minutes. The electrodeposition paintability is determined by visual inspection of the paint appearance.
Evaluation was made according to the following criteria. ◎: Smooth and uniform appearance ：: Very good skin appearance with very slight skin roughness △: Good skin roughness or craters ×: Many craters or non-electrodepositable

[0063]

[Table 1]

[0064]

[Table 2]

[0065]

[Table 3]

[0066]

[Table 4]

[0067]

[Table 5]

[0068]

[Table 6]

[0069]

[Table 7]

[0070]

[Table 8]

[0071]

[Table 9]

[0072]

[Table 10]

[0073]

[Table 11]

[0074]

[Table 12]

[0075]

[Table 13]

[0076]

[Table 14]

[0077]

[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 exhibits 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.

[0080]

The organic composite coated steel sheet of the present invention described above is excellent in rust resistance, puncture resistance and cationic electrodeposition coating property, and also excellent in 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-2-185436 (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 an adhesion amount 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 epichlorohydrin-bisphenol A type epoxy resin and polyalkylene glycol are formed on the chromate film. Diglycidyl ether in a weight ratio (epichlorohydrin-bisphenol A type epoxy resin / polyalkylene glycol diglycidyl ether) of 100 /
An epoxy equivalent obtained by reacting bisphenol A with a mixture mixed at a ratio of 300 to 100/10 and further reacting with an isocyanate compound is 1000 to 5000.
For 100 parts by weight (solid content) of a base resin [A] obtained by adding an alkanolamine to an epoxy group of a urethane-modified epoxy resin, at least 6
5 to 80 parts by weight (solid content) of a polyfunctional polyisocyanate compound [B] having an isocyanate group of (a), and a rust preventive additive [C] of ([A] + [B]) / [C] solid content Rust resistance, characterized in that a resin composition containing 90/10 to 40/60 by weight in weight ratio was applied to form a resin film having a dry film thickness of 0.2 to 3.0 μm. And organic composite coated steel sheet with excellent cationic electrodeposition coating properties. 2. A rust preventive additive [C] constituting an organic film.
The organic composite coated steel sheet according to claim 1, which is silica or / and a sparingly soluble chromate, having excellent rust resistance and cationic electrodeposition coating properties. 3. An antirust additive [C] constituting an organic film.
Is an organic composite coated steel sheet having excellent rust resistance and cationic electrodeposition coating properties according to claim 1, wherein the weight ratio is silica and a hardly soluble chromate having the following weight ratios. Silica / poorly soluble chromate = 35/5/1/39 4. A polyfunctional polyisocyanate compound [B]
Is a polyfunctional hexamethylene diisocyanate having at least six isocyanate groups in one molecule, and is an organic composite coated steel sheet excellent in rust resistance and cationic electrodeposition coating properties according to claim 1, 2 or 3. .