JP3259582B2 - 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, for example, Japanese Unexamined Patent Publication No. Hei 4-236786 discloses an organic composite having excellent corrosion resistance, which has a coating type chromate film on the surface of a Zn-based plated steel sheet and a resin film having a thickness of about 1 μm thereon. Coated steel sheets have been proposed. The organic composite coated steel sheet disclosed in Japanese Patent Application Laid-Open No. Hei 4-236786 is characterized in that a hydrophilic polyamide resin is used to secure the smoothness of the electrodeposition coating film, and a silica fine powder is used to ensure adhesion and corrosion resistance. In order to improve the electrodeposition coating property while ensuring good lubrication by combining a polyamide resin and silica fine powder, a high corrosion resistant epoxy resin binder is used as an essential component, respectively, and these high corrosion resistant epoxy resin, hydrophilic polyamide It has a resin film made of a solvent-type organic polymer resin containing a resin and silica fine powder.

[0003]

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-described organic composite coated steel sheet disclosed in Japanese Patent Application Laid-Open No. Hei 4-236786 cannot always exhibit sufficient performance with respect to this rust resistance. This is because the resin film of the coated steel sheet does not contain any curing agent, or even if it is added, only a known curing agent (resole type phenol resin, melamine resin, general polyisocyanate) is added, This is probably because the crosslink density during bake hardening is not sufficiently high.

[0004] 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 is excellent not only in characteristics such as corrosion resistance (perforation resistance), paint adhesion and electrodeposition paintability, but also in rust resistance. And

[0005]

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 chromium metal is formed on the surface of a Zn-based plated steel sheet, and a urethane epoxy having a number average molecular weight of 300 to 100,000 is formed on the chromate film. The hydrophilic polyamide resin [B] having a degree of polymerization of 50 to 1000 is added to the ester resin [A] 100 parts by weight (solid content).
10 parts by weight (solid content), 5 to 80 parts by weight (solid content) of a polyfunctional polyisocyanate compound [C] having at least 6 isocyanate groups in one molecule, and a rust preventive additive [D] is ([A] + [B] + [C]) /
An organic solvent-based coating composition containing 90/10 to 40/60 by weight of the solid content [D] was applied to form a resin film having a dry film thickness of 0.2 to 3.0 µm. Organic composite coated steel sheet with excellent rust resistance and electrodeposition coating properties.

(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, and has a rust-resistant and electrodeposition coating. 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 according to (1) to (3), the polyfunctional polyisocyanate compound [C] is a polyfunctional hexamethylene diisocyanate having at least six isocyanate groups in one molecule. Organic composite coated steel sheet with excellent rust resistance and electrodeposition coating properties.

[0009]

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

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

The chromate film formed on the surface of the galvanized steel sheet suppresses the corrosion of the galvanized steel sheet by a self-repairing action of chromate ions of hexavalent chromium. If the amount of the chromate film adhered is less than 10 mg / m ^{2 in} terms of chromium metal, sufficient corrosion resistance cannot be expected, while if it exceeds 200 mg / m ² , the weldability deteriorates. For this reason, the adhesion amount of the chromate film is set to 10 to ²⁰⁰ mg / m ^{2 in} terms of chromium metal. 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.

[0012] 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. .

The upper layer of the chromate film has a degree of polymerization of 5 based on 100 parts by weight (solid content) of a urethane epoxy ester resin [A] having a number average molecular weight of 300 to 100,000.
0 to 1000 of the hydrophilic polyamide resin [B] is 1 to 11
0 to 5 parts by weight (solid content), 5 to 80 parts by weight (solid content) of a polyfunctional polyisocyanate compound [C] having at least 6 isocyanate groups in one molecule, and rust preventive addition An organic solvent-based coating composition containing the agent [D] in a ratio of 90/10 to 40/60 by weight of the solid content of ([A] + [B] + [C]) / [D] is applied. As a result, a resin film is formed.

The urethane epoxy resin [A], which is a resin for a binder, includes an epoxy resin containing 50% by weight or more of phenol in a molecule as a solid content;
There is an epoxy ester resin obtained by reacting a dicarboxylic acid in the presence or absence of an amine catalyst with the epoxy resin, a urethane epoxy resin obtained by reacting a partially blocked isocyanate compound with the epoxy ester resin, and the like. These urethanized epoxy ester resins are used alone or in combination. When the number average molecular weight of the urethane epoxy resin [A] is less than 300, adhesion and corrosion resistance are reduced. On the other hand, the number average molecular weight which can be used as a coating by dissolving a resin in an organic solvent is limited to 100,000.

The hydrophilic polyamide resin [B] forms the most important skeleton in the resin film, is a highly hydrophilic and highly hygroscopic organic resin having many acid amide bonds, and has a high degree of polymerization and a very high molecular weight. It has great features. As the hydrophilic polyamide resin, nylon 6, nylon 6
6 or other copolymers of nylon with them; polyether polyol-modified nylon, polyesterol-modified nylon or polybutadiene polyol-modified nylon; aromatic polyamides such as polymetaphenylene isophthalamide and polyparaphenylene terephthalamide. be able to. By including such a hydrophilic polyamide resin in the resin film, the electrodeposition liquid penetrates into the coating film during the electrodeposition coating, and the electric resistance value of the coating film decreases, and the coating electrodeposition property and the electrodeposition coating film Appearance (prevention of generation of gas pins, yuzu skin, etc.) is significantly improved. The addition of a polyamide resin to the resin film is also desirable for imparting flexibility and high workability to the resin film.

The fact that the degree of polymerization of the hydrophilic polyamide resin is large and high molecular weight is effective in preventing the resin film from swelling and dissolving during pretreatment of electrodeposition coating (particularly during alkaline degreasing) or during electrodeposition coating. is there. If the degree of polymerization of the hydrophilic polyamide resin is less than 50, a uniform appearance after coating cannot be obtained because the resin film is dissolved during alkali degreasing or electrodeposition coating before electrodeposition coating, and a decrease in acid resistance is also observed. The degree of polymerization is 1
If it exceeds 000, the molecular weight becomes too high, which is not suitable for the present invention. The degree of polymerization of the hydrophilic polyamide resin is 50 to 5
A range of 00 is particularly preferred. If the blending amount (solid content) of the hydrophilic polyamide resin [B] is less than 1 part by weight with respect to 100 parts by weight (solid content) of the urethane-modified epoxy ester resin [A], the uniform electrodeposition property (prevention of generation of gas pins and yuzu skin) Cannot be expected to be sufficiently improved. On the other hand, if the compounding amount exceeds 110 parts by weight, the adhesion of the electrodeposition coating film is undesirably reduced.

In the present invention, as a means for ensuring excellent rust resistance, a curing agent is added as an essential component, and a specific polyfunctional polyisocyanate compound [C] is added as the curing agent, and It is characterized in that a urethanization reaction is caused between a hydroxyl group and an isocyanate group in the polyisocyanate.

The polyisocyanate compound used as a curing agent for a resin film of the present invention has at least six 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 4
It has been found that a polyfunctional isocyanate compound having at least three, particularly preferably at least six, isocyanate groups can impart much better rust resistance than monoisocyanate compounds or diisocyanate compounds.

Among polyfunctional polyisocyanate compounds having six or more isocyanate groups in one molecule, a polyfunctional hexamethylene diisocyanate is particularly effective for rust resistance because it is highly functional. 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
Aromatic diisocyanate compounds such as-(or p-) phenylene diisocyanate, diphenylmethane diisocyanate, bis (4-isocyanatophenyl) sulfone and the like can be mentioned.

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.

If the amount (solid content) of the polyfunctional polyisocyanate compound [C] is less than 5 parts by weight per 100 parts by weight (solid content) of the urethane-containing epoxy ester resin [A], the cross-linking density of the formed film is 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. The amount of the polyfunctional polyisocyanate compound is particularly preferably in the range of 10 to 50 parts by weight.

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.

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. For the purpose, a high boiling alcohol solvent is used. Is preferably avoided. 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. Recommended solvents include hydrocarbons, ketones (methyl isobutyl ketone,
Acetone, cifurohexanone, isophorone, etc., S-based solvents, ether-based solvents, and alcohols having a low molecular weight of ₄ or less or alcohols having a secondary or tertiary hydroxyl group are also suitable.

In the present invention, a 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 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.

Further, 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 are mixed in a specific ratio in a resin composition comprising the above specific base resin and a 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, in terms of the weight ratio of the silica and the sparingly soluble chromate to the non-volatile content, (urethane epoxy resin [A] + hydrophilic polyamide resin [B] + polyfunctional polyisocyanate compound [C]) / (silica + poor solubility) 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 (urethane epoxy resin [A] + hydrophilic polyamide resin [B] + polyfunctional polyisocyanate compound [C]) / (silica + poorly soluble chromate) exceeds 90/10, silica and Since the anti-corrosion effect of the hardly soluble chromate is not sufficiently exhibited, rust resistance is poor. On the other hand, if it is less than 40/60,
The effect of the resin as a binder becomes insufficient, and the paint adhesion and the like deteriorate. 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) composed of 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 salt. 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.

FIG. 4 shows the unpainted corrosion resistance (Heat 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 sparingly soluble chromate 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 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 for applying the above-mentioned 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 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.

If the baking temperature is lower than 80 ° C., the film does not cross-link 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.

[0040]

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

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

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

(3) Organic Resin Table 2 shows the organic resins used. The base resin (urethane-epoxy resin) and curing agent (polyisocyanate) shown in the table were prepared by the following method.

[Production of Urethane Epoxy Ester Resin] Production of Isocyanate Compound A reactor equipped with a reflux condenser, stirrer, thermometer and nitrogen gas blowing device was charged with 168 parts of hexamethylene diisocyanate and 168 parts of methyl isobutyl ketone. After weighing, uniformly dissolving and raising the temperature to 80 ° C., a solution of 87 parts of methyl ethyl ketone oxime and 87 parts of methyl isobutyl ketone was added dropwise over 1 hour, and further added at 80 ° C.
For 3 hours to obtain an isocyanate compound X having a nonvolatile content of 50%. The isocyanate equivalent of this isocyanate compound X was 510.

Urethane-modified epoxy ester resin A1 A reactor equipped with a reflux condenser, a stirrer, a thermometer and a nitrogen gas blower was used to prepare 250 parts of Epicoat 1001 (product of Yuka Shell Epoxy Co., Ltd., epoxy equivalent: 500) as xylol. After dissolving in 50 parts and 63 parts of cellosolve acetate, 39.2 parts of azelaic acid and 4.3 parts of diethanolamine were added, and the mixture was reacted at 145 ° C. for 6 hours.
When the acid value of the resin solids reached 1.1, 110 parts of xylol and 68 parts of methyl ethyl ketone were added and cooled. Next, the isocyanate compound X obtained above
200 parts (solid content) and cellosolve acetate 200
Was added, and the mixture was reacted at 100 ° C. for 3 hours. Then, 100 parts of isopropyl alcohol was added, and the mixture was cooled to obtain a urethane epoxy resin A1.

Urethane-modified epoxy ester resin A2 475 parts of Epicoat 1004 (epoxy equivalent: 950, manufactured by Yuka Shell Epoxy Co., Ltd.) was supplied to a reactor equipped with a reflux condenser, a stirrer, a thermometer, and a nitrogen gas blowing device. After dissolving in 95 parts and 119 parts of cellosolve acetate, 29.2 parts of adipic acid and 8.3 parts of diethanolamine were added, and the mixture was reacted at 145 ° C. for 6 hours.
When the acid value of the resin solid content reached 1.1, 209 parts of xylol and 13 parts of methyl ethyl ketone were added and cooled. Next, the isocyanate compound X obtained above
150 parts (solid content) and cellosolve acetate 150
Was added, and the mixture was reacted at 100 ° C. for 3 hours. Then, 100 parts of isopropyl alcohol was added, and the mixture was cooled to obtain a urethane epoxy resin A2.

[Curing Agent] Hexafunctional isocyanate (curing agent C1) 222 parts of isophorone diisocyanate and 34 parts of methyl isobutyl ketone were weighed and placed in 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 were 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 C1. The active ingredient of the curing agent C1 was 80%.

Tetrafunctional Isocyanate (Curing Agent C2) 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. over 2 hours. Thereafter, 34 parts of pentaerythritol 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 2) No absorption by isocyanate groups was confirmed, and 52 parts of butyl cellosolve was added to obtain a curing agent C2. The active ingredient of this curing agent C2 was 80%.

Trifunctional isocyanate (curing agent C3) 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. It was dropped over 2 hours. Then, an IR measurement of the reaction product was performed,

(Equation 3) No absorption by isocyanate groups was confirmed, and 47 parts of butyl cellosolve was added to obtain a curing agent C3. The active ingredient of this curing agent C3 was 90%.

Bifunctional Isocyanate (Curing Agent C4) Takenate B-870N (MEK oxime block of IPDI; manufactured by Takeda Pharmaceutical Co., Ltd.) was used as the curing agent C4. Hexamethylene diisocyanate hexafunctional isocyanate (curing agent C5) Hexamethylene diisocyanate hexafunctional isocyanate compound duranate MF-B80M (HMD
I-based oxime block of hexafunctional isocyanate: manufactured by Asahi Kasei Corporation) was used as the curing agent C5.

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

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

(E) Electrocoating property After the test piece was degreased in the same manner as in the corrosion resistance test, electrodeposition coating (20 μm in 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

[0056]

[Table 1]

[0057]

[Table 2]

[0058]

[Table 3]

[0059]

[Table 4]

[0060]

[Table 5]

[0061]

[Table 6]

[0062]

[Table 7]

[0063]

[Table 8]

[0064]

[Table 9]

[0065]

[Table 10]

[0066]

[Table 11]

[0067]

[Table 12]

[0068]

[Table 13]

[0069]

[Table 14]

[0070]

[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.

The organic composite coated steel sheet of the present invention described above is excellent in rust resistance and puncture resistance, and has good paint adhesion,
Since it has excellent electrodeposition coating properties and weldability, it is extremely useful as a surface-treated steel sheet particularly for an automobile body.

[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-4-236786 (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 a number average molecular weight of 3
The polymerization degree is 50 with respect to 100 parts by weight (solid content) of the urethane-containing epoxy ester resin [A] of 100 to 100,000.
From 1000 to 1000 hydrophilic polyamide resin [B]
5 parts by weight (solid content), 5 to 80 parts by weight (solid content) of a polyfunctional polyisocyanate compound [C] having at least 6 isocyanate groups in one molecule, and a rust preventive additive An organic solvent-based coating composition containing [D] in a ratio of 90/10 to 40/60 by weight of a solid content of ([A] + [B] + [C]) / [D] is applied. 0.2 dry film thickness
Characterized in that a resin film of up to 3.0 μm was formed,
Organic composite coated steel sheet with excellent rust resistance and electrodeposition coating properties. 2. A rust preventive additive [D] constituting a resin film.
2. The organic composite coated steel sheet according to claim 1, wherein the steel sheet is silica or / and a sparingly soluble chromate. 3. 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]
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 electrodeposition coating properties according to claim 1, 2 or 3. .