JPH0357639A - Highly corrosion-resistant organic coated steel plate - Google Patents

Abstract

PURPOSE:To develop good electrodeposition painting properties, chromium solution resistance, corrosion resistance and film close adhesiveness by applying chromate treatment to a zinc plated steel plate in a specific adhesion amount and forming a specific org. composite film thereon as an upper layer. CONSTITUTION:Zinc plating or zinc type alloy plating is applied and a chromium solution containing trivalent chromium, zinc oxide and a silica sol is applied to the plating layer in an adhesion amount of 5 - 500 mg/m<2> on the basis of metal chromium and baked. Bisphenol A and a diisocyanate compound are successively reacted with a composition consisting of an epichlorohydrin/ bisphenol A type epoxy resin and polyalkylene glycol diglycidyl ether in a wt. ration of 100/300 - 100/10 to prepare an urethane modified epoxy resin having an epoxy equivalent of 1,000 - 5,000 and alkanolamine is added to the epoxy group of said resin to form a resin composition while 10 - 150 pts.wt. of silica is compounded with 100 pts.wt. of said resin composition to prepare a paint composition. This paint composition is applied to the chromate layer in an adhesion amount of 0.1 - 4.0g/m<2> as a solid film.

Description

[Detailed Description of the Invention] Industrial Application Fields The present invention provides cationic electrodeposition coating properties for use in automobile companies;
This invention relates to a rust-preventive organic coated steel sheet that has excellent chromium elution properties, workability, weldability, corrosion resistance, etc.

In recent years, increasing the corrosion resistance of automobile steel sheets has attracted attention as a social demand, and various rust-proof steel sheets have been proposed to meet this challenge, and are gradually becoming established.

Some of these antirust steel sheets are surface-treated by hot-dip galvanizing, hot-dip zinc-based alloy plating, electrolytic galvanizing, electrolytic zinc-based alloy plating, organic film-based zinc-rich coating, and the like.

When a steel plate is applied to a bag structure or a bent part (hemming part) of an automobile body plate, the surface thereof is required to have a high degree of corrosion resistance, and the above-mentioned surface-treated steel plate does not have sufficient corrosion resistance. Therefore, a composite N-covered steel sheet was developed in which a plated steel sheet is coated with an organic coating. However, even in this anti-rust coated steel sheet, there is a problem in that the film peels off in the processed parts such as press forming, and the corrosion resistance deteriorates.

Furthermore, for the purpose of these improvements, steel sheets that can be electrodeposited with a thin film (thickness 0.3 to 3 μm) that does not use any conductive pigments have been proposed, for example, in JP-A-62-289274 and JP-A-63-22637. No. 63-35798, etc.

Problems to be Solved by the Invention> These anti-rust coated steel sheets have high corrosion resistance, weldability,
Although improvements were observed in press formability, appearance during electrodeposition, and water-resistant secondary adhesion after electrodeposition, all of these were based on the premise that the film was sufficiently crosslinked with a crosslinking agent.

Furthermore, although these coated steel sheets are excellent rust-proof steel sheets, chromate is often used as a base treatment for organic coatings in order to improve corrosion resistance.

When normal chromate is used as a base treatment, chromium is eluted during the alkaline degreasing and chemical conversion treatment during the blank assembly - alkaline degreasing - chemical conversion treatment - electrodeposition painting - intermediate coating -> top coating process, causing problems in waste liquid treatment.

In addition, when the base treatment is normally done using chromate,
In a high-temperature environment (immersion in warm pure water at 40° C. for 10 days), deterioration of so-called water-resistant secondary adhesion occurs, resulting in peeling at the interface of the chromate-organic film.

Furthermore, in recent years, materials with so-called bake-hardenability, which have a low yield strength before press forming and increase in yield strength when baked and painted, are being used. is required to be 150°C or less, and it is preferable that the time to reach the baking temperature be as short as possible.

However, since the above-mentioned paint system basically assumes sufficient cross-linking, sufficient cross-linking is not achieved by low-temperature rapid heating, and the film is eluted or soft-swelled due to the alkali generated at the interface during cationic electrodeposition coating. There was a problem in that the appearance after painting and the adhesion of the paint deteriorated.

The present invention has been made based on the above-mentioned points, and an object of the present invention is to provide a highly corrosion-resistant organic coated steel sheet that has good electrodeposition coating properties, chromium elution resistance, corrosion resistance, and coating film adhesion even when rapidly heated at low temperatures. The purpose is to

<Means for Solving the Problems> In order to achieve the above object, the present invention provides zinc plating or zinc-based alloy plating on at least one side of a steel sheet, and also provides a molarity of trivalent chromium and total chromium. Ratio (C r
”/T-Cr) from 0.05 to 0.7, zinc oxide was added at a molar ratio of 4 to 25 (T-Cr/ZnO) to the total chromium amount, and silica sol was added to the total chromium amount. chromium solution containing 50 to 300% is 5 to 500 mg/m2 in terms of metallic chromium.
It was coated and baked in a coating amount of
A resin composition in which an alkanolamine is added to the epoxy group of a urethane-modified epoxy resin having an epoxy equivalent of 1,000 to 5,000, which is obtained by reacting bisphenol A with a composition having a ratio of 300 to 100/10 and further reacting with a diisocyanate compound. A highly corrosion-resistant organic coating characterized by forming a coating composition containing 100 parts by weight and 10 to 150 parts by weight of silica as a solid component at a coating weight of 0.1 to 4.0 g/m2 as a solid film. It provides steel plates.

The organic coated steel sheet provided by the present invention has a good chromium fixation rate, high corrosion resistance, good electrodeposition coating properties, and good coating film adhesion.

In the present invention, as described above, the resin is given adhesiveness and hydrophilicity by reacting the alkylene glycol skeleton, which is a lignophilic skeleton, with bisphenol A+ grade, which has excellent adhesiveness, and furthermore, the resin is given adhesiveness and hydrophilicity by diisocyanate. molecularization,
Furthermore, due to the reinforcing effect of the film due to the interaction between the added alkanolamine and the silica, it basically exhibits sufficient alkali resistance even without a crosslinking reaction.

That is, even under low-temperature rapid baking conditions, the resin film does not elute or become soft swollen due to alkali generated at the interface during cationic electrodeposition coating, and exhibits good coating adhesion. Also,
By imparting wood-philicity, the wetting property between the electrodeposition paint and the resin film is improved, and the electrical resistance at the early stage of the electrodeposition coating is lowered, resulting in a uniform appearance of the electrodeposition coating.

The present invention will be explained in more detail below.

In the present invention, the plated steel sheets used include electrolytic galvanized steel sheets, hot-dip galvanized steel sheets, and electrolytic zinc alloy plated steel sheets (Zn-Ni, Zn-Fe% Zn-Aj!
, Zn-Mn, etc.), alloyed hot-dip zinc plating (Zn-Mn, etc.),
AJI, Zn-FIB, ZnMg)
, a hot-dip aluminized steel plate, a dispersion-plated steel plate, or a multi-layer plated steel plate obtained by applying these in multiple layers may be used.

A chromate treatment is performed on the plated steel sheet in order to improve the adhesion of the organic film described below and, in turn, improve its corrosion resistance. This chromate treatment is carried out at a concentration of 5 to 500 mg/II+2 as metallic chromium. 5Qlg
/II12, not only the corrosion resistance is insufficient, but also the adhesion with the organic film is poor, and 500 mg/II1"
If it is too thick, workability and weldability deteriorate, so it is not preferable.

5 IOg/m2 or more and less than 10 mg/m', the adhesion is good but the corrosion resistance is slightly lacking;
If it is less than 0.0 mg/m2 and more than 200 lIlg/m', the workability is good but there are some problems in weldability, so a coating amount of 10 to 200 lIg/m2 is desirable.

The composition of the treatment liquid used for chromate treatment is such that the molar ratio of trivalent chromium to total chromium (Cr"/T-Cr) is 0.
．． 05 to 0.7, zinc oxide was added at a molar ratio of 4 to 25 to the total chromium amount (T - Cr / ZnO), and silica sol was added at a molar ratio of 50 to 300 to the total chromium amount. % is preferable.

Molar ratio of trivalent chromium to total chromium (Cr 3 +/
If T - C r ) is less than 0.05, water-soluble Cr6+ is present in excess in the chromate film, and the amount of chromium eluted increases dramatically. Furthermore, if the molar ratio of trivalent chromium to total chromium (Cr"/T-Cr) exceeds 0.7, the amount of chromium eluted will decrease, but the corrosion resistance of the chromate film will deteriorate, and the liquid itself will also deteriorate. It loses stability, gels over time, and solidifies into a pudding-like consistency, so it cannot be used in production.

By adding zinc oxide, it is possible to reduce the amount of chromium eluted without deteriorating corrosion resistance. The mechanism by which zinc oxide improves chromium elution resistance is not clear, but it is thought to be as follows. That is, the eluted chromium is Cr'', which dissolves in the aqueous solution in the form of Cry4'-. Here, the isocharged point of ZnO is at pH 9.3.
~10.3, and the surface of ZnO is positively charged at pH 9 or lower. Therefore, since Cr042- is electrostatically adsorbed on the surface of ZnO, elution of Cry4'- to the outside of the film is suppressed. Molar ratio of zinc oxide to total chromium amount (TC
r/ZnO) is less than 4, the pH of the chromate solution is ZnO
The liquid itself loses its stability, gels over time, and hardens into a pudding-like consistency, making it unusable for manufacturing purposes. Molar ratio of zinc oxide to total chromium amount (T-Cr/Zn
When O) exceeds 25, Zn electrostatically adsorbs Cry4”-.
Since the amount of O is insufficient, no improvement in chromium elution resistance is observed, which is not preferable.

Silica sol significantly improves the corrosion resistance of chromate coatings. The corrosion resistance improvement mechanism of silica in chromate is thought to be as follows.

Chromium Eiwa oxide is positively charged and is said to have anion selectivity, making it difficult for cations to pass through, but easy for anions to pass through. The surface of silica has a pH of 3 to 11.
is negatively charged. From this, it is thought that by adding silica to chromate, the film itself loses its anion permeability and becomes cation selective.

That is, OH- (plated surface) generated in the cathode reaction becomes difficult to permeate through the chromate film, thereby suppressing the cathode reaction. If the silica sol accounts for less than 50% of the total chromium content, no improvement in corrosion resistance will be observed, which is undesirable. In addition, silica sol is 3% of the total chromium content.
If it exceeds 0.00%, some silica precipitates begin to form in the chromate treatment solution, which deteriorates the uniform coating of the chromate film, which is not preferable.

Next, when forming an organic film on the upper layer of the chromate film, the optimum conditions for the film composition will be described below.

When reacting bisphenol A with a mixture of epichlorohydrin-bisphenol A type epoxy resin and polyalkylene glycol diglycidyl ether, the weight ratio of epichlorohydrin-bisphenol A type epoxy resin/polyalkylene gelicol diglycidyl ether is 100/300 to 10. 0/1 Must be 0. If the weight ratio of epichlorohydrin-bisphenol A type epoxy resin/borealkylene glycol diglycidyl ether is less than 100/300, the resin will have wood-philic properties and the painted appearance after electrodeposition will be good. Since the ratio of the epichlorohydrin-bisphenol A type epoxy resin, which has good adhesion coating properties, is relatively reduced, the adhesion between the electrodeposition coating film and the organic film is reduced. If the weight ratio of epichlorohydrin-bisphenol A type epoxy resin/polyalkylene glycol diglycidyl ether is larger than 100/10, the proportion of hydrophilic polyalkylene glycol diglycidyl ether will be relatively reduced, resulting in poor wood-philicity. is insufficient, and the appearance of the paint after electrodeposition becomes poor.

Examples of shrimp chlorohydrin-bisphenol A epoxy resins used for these include Epikote 828, 1001, 1004, 1007, and 1 manufactured by Shell Chemical Co., Ltd.
Commercially available products such as 009 can be used.

Examples of polyalkylene glycol diglycidyl ether include polyethylene glycol diglycidyl ether, polypropylene glycol diglycidyl ether, polybutylene glycol diglycidyl ether, neopentyl glycol diglycidyl ether, 1.
6-hexanediol diglycidyl ether, glycerin diglycidyl ether, etc. can be used.

The resin composition thus obtained has an epoxy equivalent of io
A urethanized epoxy resin having a molecular weight of 0 to 5000 is obtained. If the epoxy equivalent is less than 1000, the resulting resin has a small molecular weight, so sufficient alkali resistance cannot be obtained, and the film may be eluted, swelled, or softened during electrodeposition coating. Moreover, when the epoxy equivalent is more than 5000, the concentration of epoxy groups is too low, the amount of alkanolamine added is small, and a sufficient reinforcing effect by silica cannot be obtained.

The diisocyanate compound used is an aliphatic, alicyclic, or aromatic compound having at least two isocyanate groups in one molecule, or a compound obtained by partially reacting these compounds with a polyhydric alcohol. For example, m- or p-phenylene diisocyanate, 2,
4- or 2,6-tolylene diisocyanate or p
- Xylene diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate alone or in mixture, or polyhydric alcohols (dihydric alcohols such as ethylene glycol, propylene glycol, glycerin, trimethylolpropane, pentaerythritol, sorbitol, diventaerythritol, etc.) is a reaction product with alcohols) containing at least 2
Examples include compounds in which isocyanate remains.

The epoxy equivalent obtained in this way is 1000 to 500.
An alkanolamine is further added to the epoxy group of the urethanized epoxy resin No. 0. The amount of the main alkanol added to the epoxy group of the urethanized epoxy resin is preferably 0.5 to 1.0 mol per 1 mol of the epoxy group. By adding 0.5 mole or more of alkanolamine to 1 mole of epoxy group, a sufficient reinforcing effect with silica can be obtained, which improves the effect of preventing swelling of the film due to alkali generated at the interface during electrodeposition. The effect of preventing deterioration of film adhesion is improved. If the amount of alkanolamine added exceeds 1.0 mole relative to the epoxy group, there will be an excess of alkanolamine, and these will not be involved in the reinforcing effect with silica, so adding more than 1.0 mole is not practical. has no meaning.

Examples of alkanolamines include monoalkanolamines such as ethylethanolamine and ethanolamine, and dialkanolamines such as diethanolamine, dibropanolamine and dibutanolamine.
Dialkanolamines are preferred. The reason for this is that dialkanolamine can introduce more primary hydroxyl groups, and as a result, the reinforcing effect with silica becomes greater, and the low-temperature baking properties are further improved.

Furthermore, in the present invention, silica can be included in the resin composition film to improve the anticorrosion effect. Silica has a weight ratio of base resin/silica (as solid content) of 1
0 0/1 0 to 100/150, preferably 1oo
/2o to 100/100. If the weight ratio of base resin/silica exceeds 100/10, no improvement in corrosion resistance can be expected, and if it is less than 1007150, the adhesion of the paint film after two coats and processability will decrease. So I don't like it.

As the silica used, both colloidal silica and fumed silica can be used.

When applying the organic composite coating composition of the present invention as described above to the plated steel plate, the coating method may be roll coating, spray coating, shower coating, etc. teeth! OO~200
℃ is sufficient, but when considering the bake hardenability (BH property) of the steel plate, it is preferably 150℃ or less. The dry film thickness of the organic composite coating composition, that is, the amount of solid film deposited, is 0.1 to 4.0 g/m', preferably 0.3 to 2.0 g/m'.
g/m' is good. This is because if the deposition amount is less than 0.1 g/m2, sufficient corrosion resistance cannot be obtained, and if it exceeds 4.0 g/m2, a stable uniform appearance of electrodeposition coating and good resistance weldability cannot be obtained.

Further, a known curing agent may be added to the organic composite coating composition of the present invention as described above. Examples of the curing agent include melamine resins, urea resins, amino resins such as penzoguanamine, and blocked isocyanates.

Example The present invention will be specifically described below based on examples.

(Example) Preparation of monochromate treatment solution Any amount of reducing agent (phosphoric acid) in monochromate (2M at a time)
is added to increase the ratio of trivalent chromium to total chromium in the liquid from 0 to 1.
Change up to. Furthermore, while adjusting the pH of the solution to 1 to 1.9 while adding phosphoric acid, 0.03M zinc oxide was added.
- Add 0.8M and stabilize. Further, immediately before application, 0 to 350% by weight of silica colloid (acidic stable type: Snowtex manufactured by Nissan Chemical) is added to the total amount of chromium, and after standing for 24 hours, the state of the liquid is observed. As a result, those that gelled and those that did not were determined, and those that did not were coated with a roll coater to achieve various chromate adhesion amounts.

- Preparation of resin paint - (Example No. 81) Polyethylene glycol diglycidyl ether (manufactured by Kyoeisha Yushi Co., Ltd., trade name "Epolite 200EJ. Epoxy equivalent = 200)" 10
0g and bisphenol A type epoxy resin (manufactured by Shell Chemical Co., Ltd., trade name "Ebicoat 1001", epoxy m = 5
Add 50g of bisphenol A to the mixture with 100g of oo), react at 150°C for 6 hours, and after confirming that unreacted phenolic hydroxyl groups have disappeared, add 125g of xylol.
, 125 g of methyl ethyl ketone was added and cooled to obtain raw material A.

Next, after heating raw material A to 80° C., 20 g of tolylene diisocyanate was removed and reacted for 1 hour. After the dropwise addition was completed, the reaction was continued while keeping the temperature at 80° C. for 3 hours. The end point of the reaction is determined by the absorption of isocyanate groups (2 2 70 c
The point where m-') disappears. In this way, a urethanized epoxy resin having an epoxy equivalent of 1000 was obtained. Subsequently, 13.3 g of diethanolamine was added and reacted at 80° C. for 2 hours.

Colloidal silica dispersed in an organic solvent was mixed with the obtained urethanized epoxy resin at a base resin/silica weight ratio of 70/30 to prepare a paint. Next, this paint is subjected to a degreasing treatment and the chromate treatment (total chromium amount = 20 to 1
00mg/m2) coated Zn-Ni plated steel sheet (Ni
A coating with an average thickness of 1.5 g/m2 was formed by applying the coating using a bar coater to a coating material (content = 12%, coating amount 20 g/m2) and baking at a plate temperature of 150°C for 30 seconds. An organic coated steel sheet was produced.

Hereinafter, Example No. 1 shown in Table 1 as the present invention was prepared by changing the original plate, the amount of chromate coating, and the resin composition. 1~No. 99 and No. 99 as a comparative example. 1 00 NNo. 1 1 9 test steel plates were manufactured.

The chromium elution property, ED property (electrodeposition coating property), coating film adhesion after two coats, corrosion resistance, and workability of the thus obtained steel plate were evaluated by the following methods.

(Chromium elution) Perform normal spray-type alkaline degreasing treatment, followed by phosphate treatment (Nippon Parkerizing Co., Ltd. Valbond 3020), and analyze the amount of chromium deposited before and after alkaline degreasing and phosphate treatment using fluorescent X-ray analysis. The amount of chromium eluted (the difference between the amount of chromium eluted before IA burial and the amount of chromium eluted after treatment) was determined by the method.

(ED property) Power Totsubu U-100 (manufactured by Nippon Paint ■) was energized for 180 seconds at a voltage of toov and a bath temperature of 28°C, and then heated at 170°C.
The electrodeposited film was baked for 20 minutes to give a thickness of 20 um, and the number of craters generated on the electrodeposited film was measured and evaluated.

◎: No crater generation O: 1 to 6 pieces/cm ” △: 7 to 10 pieces/cm 2 1) After the ED painting, the test steel plate was further spray-painted with Amilac No. 002 manufactured by Kansai Paint Co., Ltd. to a thickness of 30 μm.
A coated steel plate was produced. Each test steel plate was immersed in warm pure water at 40°C for 240 hours. After that, after taking it out, 10
Within minutes, scratch 100 pieces on the grid at 2mm intervals with a cutter knife, and use adhesive tape (cellophane tape) to
A peel test was conducted. Evaluation was performed by measuring the number of peeled pieces.

◎． No peeling ○; Number of pieces peeled off: 1 or less △; Number of pieces peeled off: 2 to 10
4 hours → drying at 60°C, 2 hours → wet at 50°C, R895%
, 2 hours), and the area where red rust occurred after 200 cycles was measured and evaluated.

◎; No red rust occurrence ○; Red rust occurrence less than 10% △; Red rust occurrence 10-50% ×; Red rust occurrence more than 50% (workability) For each test steel plate, punching diameter 90 mmφ, wrinkle pressing pressure 1 ton, punch diameter 50 mmφ A cylindrical drawing with a drawing height of 25 mm was performed, and the processed portion was peeled off with a cellophane tape, and the amount of peeled film (mg/circumference) was measured.

◎: Amount of film peeled off: less than 1 mg ○: Amount of film peeled off: 1 or more and less than 2 mg Δ: Film peeled off: 1111ffi2 or more and less than 5 mg ×: Amount of film peeled off: 5 mg or more The evaluation results of each test are shown in Table 1.

The epoxy equivalent in the table is the epoxy equivalent of the urethane-modified epoxy resin in which A, B, and C (described later) were reacted.

Further, the molar ratio is the number of moles of alkanolamine per equivalent of epoxy group in the urethane-modified epoxy resin.

Also, A, 口, C, and 2 are the following types of numbers.

As is clear from Table 1, Example No. 1, which is an example of the present invention.
Steel plates Nos. 1 to 99 are Comparative Example No. Compared to No. 100 to No. 119 steel sheets, the stability of the chromate solution is better in all cases, there is no chromium elution, and the ED resistance (external change after electrodeposition) and water resistance are 2.
The adhesion, corrosion resistance and workability were all good.

<Effects of the Invention> As explained above, the organic coated steel sheet of the present invention has good chromium elution resistance, ED properties (electrodeposition coating properties), coating film adhesion, and corrosion resistance even during low-temperature rapid heating, and is suitable for automobiles. It shows extremely excellent performance as a coated steel sheet for industrial use.

same

Claims (1)

[Claims] (1) At least one side of the steel sheet is galvanized or zinc-based alloy plated, and the molar ratio of trivalent chromium to total chromium (Cr^3^+/T-
Cr) is set to 0.05 to 0.7, zinc oxide is added at a molar ratio of 4 to 25 (T-Cr/ZnO) to the total chromium amount, and silica sol is added to the total chromium amount at a molar ratio of 4 to 25. ~
A chromium solution containing 300% is converted to 5% in terms of metallic chromium.
~ After coating and baking with a coating amount of 500mg/m^2,
Epichlorohydrin-bisphenol A type epoxy resin and polyalkylene glycol diglycidyl ether in a weight ratio of 100/300 to
Silica was added to 100 parts by weight of a resin composition in which an alkanolamine was added to the epoxy group of a urethane-modified epoxy resin having an epoxy equivalent of 1000 to 5000, which was obtained by reacting bisphenol A with a 100/10 composition and further reacting with a diisocyanate compound. 10-150 as solid content
0.1 parts by weight of the coating composition as a solid film
A highly corrosion-resistant organic coated steel sheet formed with a coating amount of ~4.0 g/m^2.