JP3259583B2 - Organic composite coated steel sheet with excellent rust resistance and 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 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, an organic composite coated steel sheet in which a chromate film is formed on the surface of a galvanized steel sheet and a resin film having a thickness of about 1 μm is formed thereon (see, for example, JP-A-1-80522) Etc.) have been proposed and put into practical use. This organic composite coated steel sheet has been widely applied to automobile bodies because it has much better corrosion resistance in the unpainted state than other surface-treated steel sheets, and also has good weldability and powdering resistance.

Recently, studies have been made on the improvement of characteristics of organic composite coated steel sheets.
In No. 9, for the purpose of improving electrodeposition coating properties,
A resin film containing a polyamine graft resin and / or a polyimine graft resin obtained by graft-polymerizing an organic solvent-soluble binder resin in which a monomer containing an aromatic hydroxy compound is formed, and a hydrophilic polyamine and / or polyimine resin; An organic composite coated steel sheet having the same has been proposed.

[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 was found that the above-mentioned organic composite coated steel sheet disclosed in Japanese Patent Application Laid-Open No. 3-268939 cannot necessarily exhibit sufficient performance with respect to this rust resistance. This is because a hardener is not added as an essential component to the organic film of the coated steel sheet, and even if it is added, a sufficient crosslink density can be obtained with a known hardener (resole-type phenol resin, polyisocyanate compound, etc.). It is thought that it is not.

[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. In view of the above situation, an object of the present invention is to provide an organic composite coated steel sheet having excellent corrosion resistance (perforation resistance), paint adhesion, etc., as well as excellent rust resistance and electrodeposition coating properties. I do.

[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 the following [A] and [A] B], a coating composition containing the components of [C] and [D] is adhered, and the dry film thickness is 0.2 to 3.0 μm.
An organic composite coated steel sheet having excellent rust resistance and electrodeposition coating properties, characterized by forming a resin film of m. [A] A number average molecular weight of 600 to 1 formed from a monomer containing an aromatic hydroxy compound in an amount of 25 mol% or more.
100,000 parts by weight of an organic solvent-soluble binder resin of 00000 [B] A number average molecular weight of 600 to 100,0, formed from a monomer containing 25 mol% or more of an amine compound.
10 to 150 parts by weight of a hydrophilic polyamine and / or polyimine resin of No. 00, or a hydrophilic polyamine and / or polyimine resin formed from a monomer containing at least 25 mol% of an amine compound, with the binder resin [A] or the binder A polyamine graft resin and / or a polyimine graft resin having a number average molecular weight of 600 to 100,000 obtained by graft polymerization of another resin compatible with the resin [A] {(10 to 150) × (total of the graft resin) weight)/
(Weight of polyamine and / or polyimine resin part in graft resin)｝ part by weight [C] 5 to 80 parts by weight of polyfunctional polyisocyanate compound having at least 6 isocyanate groups in one molecule [D] Rust preventive additive To ([A] + [B] + [C]) /
Ratio of 90/10 to 40/90 by weight ratio of [D]

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

(4) In the organic composite-coated steel sheet of (1), (2) or (3), the polyfunctional polyisocyanate compound [C] is hexamethylene diisocyanate having at least 6 isocyanate groups in one molecule. Organic composite coated steel sheet which is excellent in rust resistance and electrodeposition coating property.

[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, a paint containing the following binder resin [A], hydrophilic resin [B], polyfunctional polyisocyanate compound [C] and rust preventive additive [D] in an organic solvent. By applying the composition and drying by heating, a resin film having a thickness of 0.2 to 3.0 μm is formed. The binder resin [A] was formed from a monomer containing at least 25 mol% of an aromatic hydroxy compound.
It is composed of one or more organic solvent-soluble resins having a number average molecular weight of 600 to 100,000. The remaining monomers that may comprise from 0 to less than 75 mole percent of the binder resin may be monomers used in the manufacture of conventional coating resins, such as epihalohydrins, dicarboxylic acids, polyisocyanates, formaldehyde, amine compounds, and the like.

The aromatic hydroxy compound includes monohydric or polyhydric phenols, bisphenols and derivatives thereof. Specifically, monohydric phenolic compounds such as phenol, cresol, thymol, and vinylphenol, dihydric phenolic compounds such as catechol, resorcin, hydroquinone, and orcin, trihydric phenolic compounds such as pyrogallol and hydroxyhydroquinone, bisphenol A And bisphenol-based compounds such as bisphenol AD. As the binder resin [A], a resin having no reactivity with the hydrophilic polyamine and / or polyimine resin [B] is used. For example,
When the binder resin [A] is of an epoxy resin type obtained by reacting an epihydroxyhydrin with an aromatic hydroxy compound, it is necessary that the resin does not substantially contain an epoxy group. If the binder resin contains an epoxy group, it reacts with the polyamine and / or polyimine resin, and the desired coating properties cannot be obtained.

Specific examples of the binder resin [A] are shown below, but are not limited thereto. (A1) A resin substantially free of epoxy groups, which is obtained by reacting a bisphenol type and / or resorcinol type epoxy resin with a secondary amine compound (eg, dialkylamine, dialkanolamine, alkylalkanolamine, etc.). (A2) An epoxy ester resin substantially free of an epoxy group, which is obtained by reacting an aliphatic or aromatic dicarboxylic acid with a bisphenol type and / or resorcinol type epoxy resin in the presence or absence of an amine catalyst. Preferred dicarboxylic acids are alkylenedicarboxylic acids, especially azelaic acid. A tricarboxylic or higher carboxylic acid can be used in combination with the dicarboxylic acid.

(A3) Of the resins (A1) and / or (A2), a partially blocked polyisocyanate compound is added to a resin having a primary hydroxyl group introduced by an alkanolamine compound having a primary hydroxyl group. Urethane resin obtained by the reaction. The partially blocked polyisocyanate is preferably one obtained by incompletely protecting an aliphatic or alicyclic diisocyanate with a conventional isocyanate protecting agent. (A4) Polyphenoxy resin. This is obtained by subjecting an aromatic hydroxy compound and epihalohydrin to polycondensation in almost equivalent amounts in the presence of an alkali catalyst, substantially free of epoxy groups, usually having a number average molecular weight of 8,000 to 10
It is a resin of 000.

(A5) Novolak type phenol resin.
This is a resin obtained by condensing phenols and aldehydes in the presence of an acidic catalyst. (A6) Polyalkenyl phenol resin. This is because, for example, 25 mol% or more of alkenylphenol such as vinylphenol, dimethylvinylphenol, isopropenylphenol and isobutenylphenol and other α,
It is obtained by polymerizing a β-ethylenically unsaturated monomer with 0 to less than 75 mol% by various polymerization methods such as thermal polymerization, radical polymerization and cationic polymerization.

Among these resins, (A1) to (A1)
The resin of 3) is preferable, and the use of the urethane resin of (A3) is particularly preferable because a resin film having more excellent adhesion can be obtained. When the content of the aromatic hydroxy compound in the monomer constituting the binder resin [A] is less than 25 mol%, the formed film has reduced water resistance, corrosion resistance, and chemical resistance. When the number average molecular weight of the binder resin [A] is less than 600, the water resistance, corrosion resistance, and chemical resistance of the formed resin film become insufficient. On the other hand, when the number average molecular weight exceeds 100,000,
As the solubility in the organic solvent decreases, the viscosity of the resin liquid increases, so that the coating workability decreases.

Next, the hydrophilic resin [B] used in the present invention
Is a polyamine and / or polyimine resin formed from a monomer containing an amine compound in an amount of 25 mol% or more, all of which have a number average molecular weight in the range of 600 to 100,000, or the polyamine and / or polyimine resin described above as a binder. It is a polyamine graft resin and / or a polyimine graft resin obtained by graft-polymerizing the resin [A] or another resin compatible with the binder resin [A], and one or more of these can be used. The polyamine and / or polyimine resin is substantially free of acid amide and acid imide bonds.

The amine compound used in an amount of 25 mol% or more when forming the hydrophilic resin [B] is an aliphatic, aromatic, or heterocyclic primary, secondary or tertiary amine or quaternary ammonium compound. Is included. These amine compounds themselves form the main chain of the resin or are present in the resin as pendant (side chain) groups attached to the main chain. The remaining monomers that may comprise from 0 to less than 75 mol% of the hydrophilic polyamine and / or polyimine may be, for example, α, β-ethylenically unsaturated compounds, epoxy compounds, isocyanate compounds, and the like.

Specific examples of the hydrophilic resin [B] are shown below, but are not limited thereto. (B1) An acrylic resin-based copolymer formed from a monomer mixture containing 25 mol% or more of an amino-containing acrylic or vinyl-based monomer. Examples of the amino group-containing monomer include dialkylaminoalkyl acrylate or methacrylates, vinylpyridines, and the like, and an acrylic resin-based copolymer obtained by polymerizing these with another monomer as desired.

(B2) A resin obtained by reacting a polyfunctional epoxy compound with a primary or secondary amine compound occupying 25 mol% or more of the monomer. Examples of the polyfunctional epoxy resin include a resin obtained by reacting a polyvalent glycidyl ether such as glycerol polyglycidyl ether with a polyvalent isocyanate such as isophorone diisocyanate, and α, such as glycidyl methacrylate.
Resins obtained by polymerization of a β-ethylenically unsaturated glycidyl monomer are exemplified. As the primary or secondary amine, an amine compound such as an aliphatic or heterocyclic ring can be used. (B3) Polyethyleneimine which is a polymer of ethyleneimine. (B4) Obtained by graft-polymerizing the polyamine and / or polyimine resin of (B1) to (B3) described above to the binder resin [A] or another resin compatible with the binder resin [A]. , Polyamine and / or polyimine graft resins.

In the coating composition used in the present invention, 10 to 150 parts by weight of the hydrophilic resin [B] is blended with respect to 100 parts by weight of the binder resin [A]. However,
When using a polyamine graft resin and / or a polyimine graft resin, the polyamine and / or
Alternatively, it is used so that the amount of the polyimine moiety falls within the above range. That is, in this case, {(10 to 150) × (total weight of graft resin) / (weight of polyamine and / or polyimine resin portion in graft resin)} weight based on 100 parts by weight of binder resin [A]. The above graft resin is blended in the amount of parts. The presence of the hydrophilic resin in the binder resin film allows the electrodeposition coating liquid to quickly penetrate into the resin film during electrodeposition coating, lowering the electrical resistance of the resin film and enabling good electrodeposition. The uniformity of the deposited coating is significantly improved.

The effect of improving the electrodeposition coating property is that the amine compound in the monomer constituting the hydrophilic resin [B] is less than 25 mol% or the compounding ratio of the hydrophilic resin [B] (the graft resin). In this case, when the blending ratio of the polyamine and / or polyimine portion, the same applies hereinafter) is less than 10 parts by weight with respect to 100 parts by weight of the binder resin [A], it is not sufficiently obtained. In addition, hydrophilic resin [B]
If the compounding ratio exceeds 150 parts by weight with respect to 100 parts by weight of the binder resin [A], the adhesion of the electrodeposition coating film tends to decrease. The number average molecular weight of the hydrophilic polyamine or polyimine resin is in the range of 600 to 100,000. If the molecular weight is less than 600, the water resistance and corrosion resistance of the coating will be reduced, while if the molecular weight exceeds 100,000, the solubility in the solvent will be reduced, making it difficult to obtain a uniform composition.

Further, among the hydrophilic resins [B], a resin having an amino or imino group in a side chain group, a polyamine resin and / or a polyimine resin and a binder resin [A] or a binder resin [A] and When a graft resin with a soluble resin is used, a general-purpose organic solvent for paint can be used, and even when a large amount of a rust preventive additive such as silica is contained, low viscosity and excellent paint workability are obtained. It is particularly suitable for the purposes of the present invention as it forms a low cost coating composition.

In the present invention, as a means for improving rust resistance, a curing agent is added as an essential component, and a specific polyfunctional polyisocyanate compound [C] is added as the curing agent. A urethanization reaction is caused between hydroxyl groups in the polyisocyanate and isocyanate groups in the polyisocyanate. The polyisocyanate compound used as a curing agent for the resin film of the present invention has six or more isocyanate groups in one molecule (these isocyanate groups may be blocked) for the purpose of improving rust resistance. Having a polyfunctional isocyanate. 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 the 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 is 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 [C] as such a curing agent can be used as a binder resin [A] 10
It is blended in an amount of 5 to 80 parts by weight (solid content), preferably 10 to 50 parts by weight based on 0 parts by weight (solid content).
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, when the amount is more than 80 parts by weight, the unreacted residual isocyanate absorbs water, and not only has no effect on rust resistance but also impairs 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. 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 purpose of providing the resin composition film as described above in the present invention is as follows. In order to obtain high corrosion resistance (perforation resistance) and electrodeposition coating property, a specific amount of hydrophilic polyamine and / or polyimine resin is present in the resin composition. Due to the presence of such a hydrophilic resin, the film hydrophilicity of the steel sheet is improved, so that the permeability of the electrodeposition coating liquid into the resin film is improved, and as a result, the current distribution at the time of electrodeposition becomes uniform, so that the uniformity is obtained. An electrodeposition coating is formed. In general, when a hydrophilic resin is blended, the water resistance of the resin film tends to decrease, but in the present invention, by using a specific binder resin, a highly durable film can be obtained despite the presence of the hydrophilic resin. Can be. Also,
In general, the selection range of the solvent in which the hydrophilic resin and the binder resin are compatible with each other is very narrow, and an expensive solvent is required.
In addition, it is usual that the viscosity is significantly increased by the addition of a rust preventive additive as described later. However, in the present invention, the use of a polyamine or polyimine resin or a grafted resin thereof as a hydrophilic resin makes it possible to use a usual coating solvent and does not cause a significant increase in viscosity. By employing a polyfunctional polyisocyanate compound having at least 6 or more isocyanate groups in one molecule as a curing agent, a highly crosslinked density film having excellent rust resistance can be obtained.

As the organic solvent used in the coating composition, one or a mixture of two or more of the organic solvents usually used in the coating industry can be used, but for the purpose, alcohol solvents having a high boiling point should be avoided. Is preferred. This example, ethylene glycol or diethylene glycol monoalkyl ethers, alcohols having C ₅ or more primary hydroxyl group. Such solvents are
Inhibits the curing reaction of the film. Recommended solvents include
Examples include hydrocarbon solvents, ketone solvents (methyl isobutyl ketone, acetone, cifurohexanone, isophorone, etc.), SELL solvents and ether solvents, and alcohols having a low molecular weight of ₄ or less, or secondary or tertiary hydroxyl groups. Are also suitable.

In the present invention, a 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 a wide variety of processes for producing hydrophobic silica, and typical ones are reactions of organic solvents such as alcohols, ketones and esters, silanes, silazanes, polysiloxanes and the like. Examples of the method include a reaction pressurization 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 a resin composition comprising the above-mentioned specific binder resin, hydrophilic resin and polyfunctional polyisocyanate compound. Due to the synergistic effect of the anticorrosion 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 as follows: (binder resin [A] + hydrophilic resin [B] + polyfunctional polyisocyanate compound [C]) / (silica + slightly soluble chromate) = 90/10 to 40/60 silica / poorly soluble chromate = 35/5 to 1/39, the most excellent corrosion resistance can be obtained. Here, (binder resin [A] + hydrophilic resin [B] + polyfunctional polyisocyanate compound [C]) / (silica + poorly soluble chromate) is 90/10
If it exceeds, the anticorrosion effect of the silica and the hardly soluble chromate salt is not sufficiently exhibited, and the rust resistance is inferior. On the other hand, if the ratio is less than 40/60, the above-mentioned characteristics cannot be sufficiently obtained due to a decrease in the resin component. Also, even if the ratio of silica / poorly soluble chromate exceeds 35/5,
If less than this, 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 a hexafunctional polyisocyanate compound of isophorone diisocyanate was prepared by changing the ratio of silica and a sparingly soluble chromate. 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 (a puncture resistance test 2) when a hexamethylene diisocyanate-based hexafunctional polyisocyanate compound is 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 the hardly soluble chromate are the main additive components in the resin composition. In addition, a silane coupling agent, a coloring pigment (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 above-mentioned organic film 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-described coating composition to a galvanized steel sheet, the coating composition is usually applied by a roll coater method, but after being applied by a dipping method or a spray method, an air knife method or a roll drawing method is applied. 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.

If the baking temperature is lower than 80 ° C., the crosslinking of the film does not proceed, and sufficient corrosion resistance cannot be obtained.
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.

[0050]

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 binder resin, hydrophilic resin and curing agent (polyisocyanate) shown in the table were prepared by the following method.

[Synthesis Example of Resin] Production of Binder Resin A1 Epichlorohydrin-bisphenol A type epoxy resin (number average molecular weight: 2900, epoxy equivalent: 2000,
After dissolving 1000 parts of bisphenol A (48 mol%) in 190 parts of xylol and 238 parts of cellosolve acetate, 53.5 parts (0.5 mol) of diethanolamine.
And reacted at 145 ° C. for 6 hours. When no epoxy group was detected by infrared spectroscopy, xylol 4 was added.
20 parts and 260 parts of methyl ethyl ketone were added, and the mixture was cooled to obtain a varnish of binder resin A1 (solid content: 49%).

Production of Binder Resin A2 Epichlorohydrin-bisphenol A type epoxy resin (number average molecular weight: 2900, epoxy equivalent: 2000,
After dissolving 1000 parts of bisphenol A (48 mol%) in 190 parts of xylol and 238 parts of cellosolve acetate, 42.4 parts (0.21 mol) of sebacic acid and 8.3 parts (0.08 mol) of diethanolamine were added.
The reaction was carried out at 145 ° C. for 6 hours, and no epoxy group was detected by infrared spectroscopy.
When OH (mg / g) is reached, xylol 420
And 260 parts of methyl ethyl ketone, and the mixture was cooled to obtain a varnish of binder resin A2 (solid content: 49%).

Production of Hydrophilic Resin B1 351 parts of cellosolve acetate were charged into a reaction vessel equipped with a dropping funnel, a cooling pipe, a nitrogen introduction pipe, a thermometer and a stirring blade, and the temperature was raised to 120 ° C. On the other hand, 585 parts of glycidyl methacrylate and 13 parts of t-butylperoxy-2-ethylhexanoate were charged into a dropping funnel, and the contents of the dropping funnel were dropped at 120 ° C. in a nitrogen atmosphere into the reaction vessel at a constant speed over 3 hours. did. After completion of the dropwise addition, the temperature was maintained at 120 ° C. for 30 minutes. Then, 49 parts of cellosolve acetate and t-butylperoxy-2-ethylhexanoate 2
Was charged into a dropping funnel at 120 ° C. under a nitrogen atmosphere, and the content was dropped at a constant speed over 1 hour. After dropping, 1 hour 1
The temperature was maintained at 20 ° C. to complete the polymerization. Thus, a polyfunctional epoxy compound having a solid content of 59.8% and a number average molecular weight of 9,800 was obtained. The above-mentioned polyfunctional epoxy compound 60 was placed in a reaction vessel equipped with a cooling pipe, a nitrogen introduction pipe, a thermometer and a stirring blade.
2.1 parts and 185 parts of diethylamine were charged at 90 ° C.
For 5 hours to carry out a reaction. In addition, xylene 8
After stirring 0 parts, stirring was carried out for 30 minutes, and the solid content was 50 minutes.
% Of xylene to obtain a varnish of the hydrophilic resin B1.

Production of Hydrophilic Resin B2 400 parts of xylene was charged into a reaction vessel equipped with a cooling pipe, a nitrogen introduction pipe, a thermometer and a stirring blade, and the temperature was raised to 130 ° C. On the other hand, 150 parts of styrene, methyl methacrylate 32
0 parts, 45 parts of 2-hydroxyethyl, n-methacrylic acid
60 parts of butyl, 20 parts of acrylic acid, and 5 parts of t-butylperoxy-2-ethylhexanoate were charged into a dropping funnel, and the content of the dropping funnel was dropped at 130 ° C. under a nitrogen atmosphere at a constant speed over 3 hours. . After completion of the dropwise addition, the temperature was maintained at 130 ° C. for 1 hour to complete the polymerization. This gives 60% solids,
Number average molecular weight 2700, solid content acid value 26.2, OH value 3
A carboxyl group-containing acrylic resin-based prepolymer of 2.4 was obtained. The above-mentioned polyfunctional epoxy compound was placed in a reaction vessel equipped with a cooling pipe, a nitrogen introduction pipe, a thermometer and a stirring blade.
2.1 parts and 166 parts of diethylamine were charged at 90 ° C.
For 5 hours to form a polyamine resin. Next, 996 parts of the acrylic resin-based prepolymer obtained above were charged and stirred at 90 ° C. for 3 hours to obtain a varnish of the hydrophilic resin B2.

[Curing Agent] Hexafunctional isocyanate (Curing agent a) 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 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%.

Tetrafunctional Isocyanate (Curing Agent b) 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 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%.

Trifunctional isocyanate (curing agent c) 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 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%.

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. 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 performed 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 continuous 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

(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 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

[0067]

[Table 1]

[0068]

[Table 2]

[0069]

[Table 3]

[0070]

[Table 4]

[0071]

[Table 5]

[0072]

[Table 6]

[0073]

[Table 7]

[0074]

[Table 8]

[0075]

[Table 9]

[0076]

[Table 10]

[0077]

[Table 11]

[0078]

[Table 12]

[0079]

[Table 13]

[0080]

[Table 14]

[0081]

[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.

[0084]

The organic composite coated steel sheet of the present invention described above is excellent in rust resistance, puncture resistance and electrodeposition paintability, and also excellent in paint adhesion and weldability. It is extremely useful as a surface-treated steel sheet for vehicle 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-3-268939 (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 the following [A], [B] ], A coating composition containing the components [C] and [D] is adhered, and the dry film thickness is 0.2 to 3.0 μm.
Organic composite coated steel sheet with excellent rust resistance and electrodeposition coating properties, characterized by forming a resin film of [A] A number average molecular weight of 600 to 1 formed from a monomer containing an aromatic hydroxy compound in an amount of 25 mol% or more.
100,000 parts by weight of an organic solvent-soluble binder resin of 00000 [B] A number average molecular weight of 600 to 100,0, formed from a monomer containing 25 mol% or more of an amine compound.
10 to 150 parts by weight of a hydrophilic polyamine and / or polyimine resin of No. 00, or a hydrophilic polyamine and / or polyimine resin formed from a monomer containing at least 25 mol% of an amine compound, with the binder resin [A] or the binder A polyamine graft resin and / or a polyimine graft resin having a number average molecular weight of 600 to 100,000 obtained by graft polymerization of another resin compatible with the resin [A] {(10 to 150) × (total of the graft resin) weight)/
(Weight of polyamine and / or polyimine resin part in graft resin)｝ part by weight [C] 5 to 80 parts by weight of polyfunctional polyisocyanate compound having at least 6 isocyanate groups in one molecule [D] Rust preventive additive To ([A] + [B] + [C]) /
Ratio of 90/10 to 40/90 by weight ratio of [D] 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 and / or a hardly soluble chromate. 3. A rust preventive additive [D] constituting a resin film.
2. The organic composite coated steel sheet according to claim 1, which is a silica and a sparingly soluble chromate having the following weight ratios. Silica / poorly soluble chromate = 35/5/1/39 4. A polyfunctional polyisocyanate compound [C]
4. The organic composite coated steel sheet according to claim 1, 2 or 3, which is a polyfunctional hexamethylene diisocyanate having at least 6 isocyanate groups in one molecule.