JP3130754B2 - Thin-film organic composite steel sheet with excellent corrosion resistance and solvent resistance - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an organic composite steel sheet coated with a thin film, and more particularly to an organic composite steel sheet having excellent corrosion resistance and solvent resistance as a steel sheet for automobiles.

[0002]

2. Description of the Related Art In recent years, there has been an increasing market need for zinc-coated steel sheets for automobiles and home appliances for higher quality, higher functionality, and lower cost, and research and development of new products in response thereto. Is being actively conducted. However, since galvanized steel sheets rely on corrosion resistance due to sacrificial corrosion protection, there is a problem that if corrosion resistance is further improved, an increase in coating weight cannot be avoided, resulting in deterioration of press workability and spot weldability. . Also,
As a rust-preventive steel sheet with a low coating weight, a plated steel sheet or a multilayer coated steel sheet in which zinc and nickel, cobalt, chromium, iron, manganese, and the like are alloyed have been developed. However, it does not have sufficient corrosion resistance to a hem portion or a bag structure portion exposed to more severe corrosion conditions in an automobile body. Under such circumstances, an organic composite steel sheet having improved corrosion resistance by coating the surface with a thin organic resin has been developed and put into practical use. However, even with these organic composite steel sheets, at present, it is not possible to satisfy all the quality performances such as workability, spot weldability, cationic electrodeposition property, and solvent resistance.

[0003] Patents relating to organic composite steel sheets include, for example,
There is Japanese Patent Publication No. 64-11830. The present invention forms a hardly soluble chromate film having a water-soluble content of 5% or less as 10 to 150 mg / m ² as a first layer of a steel sheet subjected to zinc plating, aluminum plating, or zinc-based composite alloy plating. An organic material having excellent press workability, weldability, electrodeposition coating properties, and corrosion resistance, characterized in that a coating composition having the following ratio is formed as a second layer on the upper layer, and the coating composition is formed into a thin film of 0.3 to 5 μm as a solid film. The present invention relates to a composite steel sheet. (A) a bisphenol-type epoxy resin having a number-average molecular weight of 300 to 100,000 in a coating solid content of 30% by weight or more,
(B) at least one curing agent selected from the group consisting of a polyisocyanate compound and a blocked polyisocyanate compound in a weight ratio of 1 to the solid content of the epoxy resin;
/ 10 to 20/10, (C) average particle size 0.1 to 100 μm
m of fumed silica in the solid content of the paint is 5 to 50% by weight,
(D) A coating composition containing a ketone-based organic solvent in an amount of 40% by weight or more based on the weight of the coating material and a solid content of the coating material of 10 to 50% by weight. However, there is a problem in the curability of the coating film, and the solvent resistance is poor.

Japanese Patent Application Laid-Open No. 5-15845 discloses that a zinc-plated steel sheet has a chromate layer on the surface thereof, and a modified epoxy resin (A) comprising an epoxy resin, a polyfunctional amine and a monoisocyanate as an upper layer. ,
An outer surface having an organic film having a thickness of 0.2 to 2 μm in which a predetermined ratio of a rust-preventive additive is added to a block urethane-modified epoxy resin obtained by mixing block urethane (B) comprising a polyisocyanate and a blocking agent at a predetermined ratio; The present invention relates to an organic composite coated steel sheet having excellent rust and sharpness. However, since the polyol portion of the film is easily dissolved in the solvent, the solvent resistance is still poor.

[0005]

SUMMARY OF THE INVENTION The present invention is intended to improve the corrosion resistance and solvent resistance of an organic composite steel sheet without deteriorating performances such as press workability, spot weldability and cationic electrodeposition. .

[0006]

In order to solve the above-mentioned problems, the present invention provides a steel sheet having a chromate film as a first layer on a zinc-based plating surface of 10 to 150 mg / m ² , and a coating composition shown below as a second layer. The product is a solid film of 0.3 to 3.
The present invention has led to the invention of a thin-film organic composite steel sheet having excellent corrosion resistance and solvent resistance characterized by being formed at 0 μm. (A) a bisphenol-type epoxy resin having a number-average molecular weight of 300 to 100,000 in a coating solid content of 30% by weight or more,
(B) A tetraisocyanate diisocyanate compound is used as a curing agent, and the weight ratio to the epoxy resin solids is 1/2.
0 to 1/1, (C) fumed silica having an average primary particle diameter of 7 to 100 nm is 5 to 50% by weight based on the solid content of the paint,
(D) A coating composition containing a ketone-based organic solvent in an amount of 40% by weight or more based on the weight of the coating material and a solid content of the coating material of 10 to 50% by weight.

In the coating composition, the ketone organic solvent (D) may be at least one selected from the group consisting of methyl isobutyl ketone, acetone and cyclohexanone. However, if the solid content of the paint is 10% by weight or less, it is difficult to secure a film thickness due to insufficient solid content. on the other hand,
If it exceeds 50% by weight, the paint viscosity will be too high.
Disturbs painting workability. Further, in the coating composition, a lubricant is contained in an amount of 0.1 to 10% by weight based on the solid content, so that press workability can be further improved.

In the present invention, a zinc or zinc-based alloy-plated steel sheet having a chromate film specified as described above is composed of a bisphenol-type epoxy resin, a diisocyanate compound of 4 to hexamer, a fumed silica, and a ketone-based organic solvent. Characterized by a thin film coating of an organic solvent-based coating composition, by coating this thin film, without reducing press workability, spot weldability, cationic electrodeposition, corrosion resistance, solvent resistance It is an improvement. This is based on the discovery that the use of a tetra- to hexamer diisocyanate compound in the coating composition constituting the coating film of the present invention dramatically improves the solvent resistance after curing. .

The zinc or zinc-based alloy coated steel sheet applied to the present invention includes electrogalvanized steel sheet, zinc-iron alloy coated steel sheet, zinc-nickel alloy coated steel sheet,
A zinc-cobalt alloy-plated steel sheet, a zinc-nickel-cobalt alloy-plated steel sheet, or the like is applied, and a known plating method may be used.

[0010]

Functions and Examples The effects and the range of action of the constituent factors of the present invention will be described below based on experimental results. (1) Base zinc-coated steel sheet After degreasing and pickling a low carbon steel sheet with a thickness of 0.8 mm,
Zinc-nickel plating is reduced to 20 by a known plating method.
g / m ² to prepare a base zinc-coated steel sheet. (2) Chromate film The chromate film used in the present invention is located between the base plating layer and the organic film and improves the adhesion between the plating film and the organic film. As a result, it is important for ensuring the corrosion resistance of the organic composite steel sheet. Plays an important role. The type of chromate may be any of the known electrolytic chromate and coating type chromate, and the treatment method is not particularly limited.

When the amount of the chromate film adhered is less than 10 mg / m ^{2 in} terms of the total amount of chromium, the adhesion to the upper organic film is poor due to the insufficient amount of the adhered chromate film. On the other hand, the total chromium content was 150 mg /
exceed m ^2, significant adhesion lowering of plating film and the organic film from the cohesive failure of the chromate film. From the above, the adhesion amount of the chromate film must be in the range of 10 to 150 mg / m ² . FIG. 1 shows the relationship between the amount of deposited chromium and the adhesion. 2 m after hot water test (50 ° C. × 10 days) as the chromium adhesion amount on the horizontal axis and the adhesiveness index on the vertical axis
It is a score of the m-goban test. From these results, the Goban score was 10 to 150 mg / m of chromium adhesion.
The range of ² is good, and it can be seen that the adhesion to the upper organic film is good within the range of the chromium adhesion amount.

(3) Organic film The base resin used for forming the organic film of the present invention is:
It is a bisphenol type epoxy resin having a molecular weight of 300 to 100,000. The molecular formula is

[0013]

Embedded image

## EQU1 ## The part indicated by A is> C (CH ₃ )
A case of ₂ gives particularly favorable results. When the number average molecular weight is less than 300, the polymerization does not proceed sufficiently in the curing reaction, and thus the corrosion resistance is insufficient. On the other hand, the molecular weight is 100,000
In the above, since the reaction sites of the epoxy resin are relatively small, the corrosion resistance is also poor due to insufficient crosslinking density.

FIG. 2 shows the effect of the molecular weight of the bisphenol type epoxy resin on corrosion resistance. The horizontal axis is the molecular weight of the epoxy resin, and the vertical axis is the cycle corrosion test (S
(ST-dry-wet repetition test) The red rust generation area after 300 cycles is taken. From the results, the area of red rust occurrence after 300 cycles of the cyclic corrosion test was 20%.
It can be seen that the following regions having excellent corrosion resistance are in the molecular weight range of 300 to 100,000 of the bisphenol type epoxy resin. If the thickness of the organic film is less than 0.3 μm, the corrosion resistance is insufficient, and if it exceeds 3.0 μm, the spot weldability is reduced, so that it must be in the range of 0.3 to 3.0 μm.

FIG. 3 shows the effect of the thickness of the organic film on the corrosion resistance. The horizontal axis shows the thickness of the organic film, and the vertical axis shows the area of red rust occurrence after 300 cycles of the cyclic corrosion test. From these results, the cycle corrosion test 3
It can be seen that the area of excellent corrosion resistance where the area of red rust occurrence after 00 cycles is 20% or less is 0.3 μm or more in thickness of the organic film. FIG. 4 shows the effect of the thickness of the organic film on spot weldability. The horizontal axis plots the thickness of the organic film, and the vertical axis plots the number of continuous welding points during spot welding. From these results, the number of continuous welding points is 300
The excellent region of 0 or more points has a thickness of the organic film of 0.3 to 3.
It can be seen that it is in the range of 0 μm.

The compounding amount of the epoxy resin is 30
If the amount is less than 30% by weight, the binder effect of the epoxy resin on the fumed silica is reduced, and film formation becomes difficult. Next, the curing agent is 4-6
A diisocyanate compound. As the diisocyanate compound, an aliphatic or alicyclic diisocyanate compound is mainly used, and examples thereof include hexamethylene diisocyanate, isophorone diisocyanate, and tolylene diisocyanate. In use, these isocyanate compounds may be used after being blocked. However, the adduct formed by adding to the isocyanate group as a blocking agent is stable at room temperature,
In addition, it is necessary that the resin is dissociated at the time of baking the coating film to regenerate free isocyanate groups.

Examples of such blocking agents include lactam blocking agents (such as ε-caprolactam and γ-butyrolactam), oxime blocking agents (such as methyl ethyl ketoxime and cyclohexanone oxime), and alcohol blocking agents (such as methanol, ethanol and isobutyl alcohol). ), Phenol-based blocking agents (phenol, p-tert-butylphenol, cresol, etc.) and ester-based blocking agents (ethyl acetoacetate, methyl acetoacetate, etc.). Oxime, ethyl acetoacetate and the like are preferred. A film formed using a tetramer to hexamer diisocyanate compound is much more excellent in solvent resistance than a dimer isocyanate compound.

FIG. 5 shows the relationship between the number of hexamethylene diisocyanates and the solvent resistance. The horizontal axis indicates the number of hexamethylene diisocyanate monomers, and the vertical axis indicates the rating of solvent resistance to methyl ethyl ketone. From this result,
When the number of hexamethylene diisocyanates is 4 or more,
It can be seen that an organic film having excellent solvent resistance can be obtained. The amount of the above curing agent is based on the solid content of the epoxy resin.
The weight ratio is 1/20 to 1/1. This results in a maximum reached coating temperature of 130-190 ° C. and a baking time of 5-6
Sufficient reaction is performed under baking conditions of about 0 seconds. When the mixing ratio of the curing agent and the epoxy resin is less than 1/20, the crosslinking reaction is insufficient and the solvent resistance and the corrosion resistance are insufficient. Conversely, when the ratio is 1/1 or more, the water resistance and alkali resistance of the organic film are reduced, so that the adhesion to the overcoat film is insufficient, so that the corrosion resistance is poor.

FIG. 6 shows the effect of the mixing ratio of the curing agent and the epoxy resin on the solvent resistance. The horizontal axis shows the mixing ratio of the curing agent and the epoxy resin, and the vertical axis shows the solvent resistance to methyl ethyl ketone. From this result,
It can be seen that when the mixing ratio of the curing agent and the epoxy resin is 1/20 or more, an organic film having excellent solvent resistance can be obtained.

FIG. 7 shows the corrosion resistance after coating of the sample subjected to the cationic electrodeposition of 20 μm. The abscissa shows the blending amount of the curing agent and the epoxy resin, and the ordinate shows the swollen width from the cross cut portion after 10 days of the warm salt water test. From this result,
The mixing ratio of the curing agent and the epoxy resin is 1/20 to 1/1,
It can be seen that an organic film excellent in corrosion resistance after painting with a swollen width from the cross cut portion of 2 mm or less can be obtained. Further, in the present invention, fumed silica having an average particle diameter of primary particles of 7 to 100 nm as a rust-preventive pigment is contained in the paint solid content in order to impart high corrosion resistance without harmful substances being eluted in the degreasing / chemical conversion treatment bath. 5 to 50% by weight is added. If the average primary particle diameter of the fumed silica exceeds 100 nm, the corrosion resistance is reduced. Further, fumed silica having an average primary particle diameter of less than 7 nm is not supplied stably.

FIG. 8 shows the effect of the average primary particle size of fumed silica on corrosion resistance. The horizontal axis indicates the average primary particle diameter of the fumed silica, and the vertical axis indicates the red rust generation area after 300 cycles of the cyclic corrosion test. From this result, the average primary particle diameter of the fumed silica was 10
At 0 nm or less, it is understood that the corrosion resistance is excellent. The amount of the fumed silica is 5 to 50% by weight in the solid content of the paint.
Is suitable. If the amount of fumed silica is less than 5% by weight, the corrosion resistance is insufficient due to insufficient rust-preventing ability of the fumed silica. become worse.

FIG. 9 shows the effect of the amount of fumed silica on the corrosion resistance. The horizontal axis represents the blended amount of fumed silica, and the vertical axis represents the area of red rust occurrence after 300 cycles of the cyclic corrosion test. From these results, it can be seen that the corrosion resistance is excellent when the amount of fumed silica is in the range of 5 to 50% by weight. From the viewpoint of press moldability, the coating composition of the present invention contains a polyolefin,
Lubricants such as carboxylate esters, metal carboxylate, and polyalkylene glycols, and lubricant powders such as molybdenum disulfide, silicone compounds, and fluorine compounds may be used. Further, it is preferable to further improve the processability by adding 0.1 to 10% by weight based on the solid content of the paint.

FIG. 10 shows the effect of the addition of polyethylene, which is a polyolefin lubricant, on cylindrical press workability. The abscissa indicates the amount of polyethylene added, and the ordinate indicates the weight reduction during cylindrical pressing. It can be seen that the addition reduces the weight loss due to powdering and galling during processing and improves the press workability. However, the addition of more than 10% by weight does not improve the effect, but rather leads to a decrease in other properties such as corrosion resistance. Therefore, the added amount of polyethylene is preferably in the range of 0.1 to 10% by weight.

[0025]

As described above, the organic composite steel sheet of the present invention having the above-mentioned problems with the conventional organic composite steel sheet in terms of corrosion resistance and solvent resistance, such as press formability, continuous spot weldability, and cationic electrodeposition property, has had problems in quality. This is an epoch-making organic composite steel sheet that has been improved without deteriorating its performance, and sufficiently satisfies the needs of the market.

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship between the amount of chromium deposited and adhesion.

FIG. 2 is a diagram showing the effect of the molecular weight of a bisphenol-type epoxy resin on corrosion resistance;

FIG. 3 is a diagram showing the effect of the thickness of an organic film on corrosion resistance.

FIG. 4 is a diagram showing the effect of the thickness of an organic film on spot weldability.

FIG. 5 is a diagram showing the relationship between the number of hexamethylene diisocyanates and the solvent resistance;

FIG. 6 is a diagram showing the effect of the mixing ratio of a curing agent and an epoxy resin on solvent resistance.

FIG. 7 is a view showing the corrosion resistance of a sample subjected to cationic electrodeposition 20 μm after coating,

FIG. 8 is a diagram showing the effect of the average primary particle size of fumed silica on corrosion resistance;

FIG. 9 is a graph showing the effect of the amount of fumed silica on corrosion resistance.

FIG. 10 is a graph showing the effect of the addition of polyethylene, which is a polyolefin-based lubricant, on cylindrical press workability.

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ Identification code FI B05D 7/14 B05D 7/14 L 7/24 302 7/24 302T 302U 303 303B C09D 175/04 C09D 175/04 C23C 22/26 C23C 22/26 (56) References JP-A-5-309334 (JP, A) JP-A-3-57639 (JP, A) JP-B-4-28539 (JP, B2) (58) Fields studied (Int .Cl. ⁷ , DB name) B32B 15/08 B05D 7/14 C09D 175/04

Claims (1)

(57) [Claims] 1. A steel sheet having a chromate film of 10 to 150 mg / m ² as a first layer on the surface of a zinc-based plating, and a coating composition shown below as a solid film of the following composition as a second layer.
Corrosion resistance characterized by being formed in a range of 3 to 3.0 μm,
Thin organic composite steel sheet with excellent solvent resistance. (A) a bisphenol-type epoxy resin having a number-average molecular weight of 300 to 100,000 in a coating solid content of 30% by weight or more,
(B) A tetraisocyanate diisocyanate compound is used as a curing agent, and the weight ratio to the epoxy resin solids is 1/2.
0 to 1/1, (C) fumed silica having an average primary particle diameter of 7 to 100 nm is 5 to 50% by weight based on the solid content of the paint,
(D) A coating composition containing a ketone-based organic solvent in an amount of 40% by weight or more based on the weight of the coating material and a solid content of the coating material of 10 to 50% by weight.