JPH03150396A - Method for coating steel sheet by electrodeposition with superior resistance to pinhole due to gas - Google Patents

Abstract

PURPOSE:To prevent the occurrence of pinholes due to gas at the time of coating by electrodeposition by sticking a coating film of a blend of specified modified epoxy resin with paint to a steel sheet pretreated before coating, raising voltage to a prescribed value in cationic electrodeposition paint and keeping the raised voltage. CONSTITUTION:A paint compsn. contg. 100 pts.wt. modified epoxy resin and 10-150 pts.wt. (expressed in terms of solid matter) silica is prepd. The modified epoxy resin is obtd. by allowing 100 pts.wt. epichlorohydrin-bisphenol A type epoxy resin to react with 10-100 pts.wt. isocyanate compd. and adding dialkanolamine to the resulting urethane modified epoxy resin having 1,000-5,000 epoxy equiv. by 0.5-1.0mol per 1 equiv. of epoxy group. The compsn. is stuck to the surface of a Zn alloy plated steel sheet pretreated before coating so as to form a solid coating film of 0.3-2.0mum thickness. Voltage is raised to 300V or above within 10sec in cationic electrodeposition paint with the steel sheet as the cathode and the raised voltage is kept for a prescribed time. An org. resin coating film free from pinholes due to gas can be formed.

Description

[Detailed description of the invention] Industrial application field> The present invention provides a highly corrosion-resistant surface-treated steel sheet for automobiles that prevents the occurrence of gas pinholes during cationic electrodeposition coating of Zn-based alloy plated steel sheets and has a good paint finish. This invention relates to an electrodeposition coating method.

<Prior art> In cationic electrodeposition coating as a brimer coating widely used on automobiles, cationic paint particles are deposited on the surface of the object to be coated during electrodeposition, and at the same time H2 is generated by electrolysis of water, which is a medium.
Because gas is generated simultaneously, gas pinholes may occur. This tendency tends to occur on surface-treated steel sheets that have been subjected to alloy plating. Here, alloy plating refers to Zn-Ni in electroplating, Zn-Fe.

Zn-Ni-Co, Zn-Fe-Co, etc., and alloyed hot-dip Zn plating.

A method of applying Fe-based plating on alloy plating has been disclosed as a method for preventing the generation of gas pinholes during cationic electrodeposition coating (for example, JP-A No. 5B-15554,
(See Japanese Patent Application Laid-Open No. 61-253397).

Problems to be Solved by the Invention> However, Fe-based plating has a drawback as a rust-preventing steel sheet in that red rust is likely to occur, and Fe-based plating has low precipitation efficiency and It also has the disadvantage of requiring a large amount of cost to return.

On the other hand, the mechanism by which gas pinholes occur is that, as is already widely acknowledged, a large amount of H2 gas is generated locally due to current concentration on some potential unevenness on the surface of the steel sheet, which interferes with the adhesion of electrodeposited paint. It is speculated that the reason why this phenomenon is more likely to occur in the case of alloy plating is because there are areas where alloy precipitation is uneven.

The present invention has been made in view of the above circumstances, and by applying an insulating film containing urethane-modified epoxy resin and silica on an alloy-plated steel plate, the voltage resistance is increased, and unevenness during the initial rapid voltage rise is improved. The purpose of the present invention is to provide a method for electrodeposition coating of surface-treated steel sheets that prevents severe current concentration.

<Means for solving the problem> The following experiment was conducted regarding electrodeposition coating on alloy-plated steel sheets. First, 200 to 400
The boost time was set to 10 seconds or less depending on the final voltage reached by v.

In Zn-Ni, Zn-Fe (electroplated and hot-dipped) alloy plated steel sheets, gas pinholes began to occur when the voltage exceeded 250V, and gradually became more severe as the voltage was increased.

On the other hand, when an insulating film made of a combination of urethane-modified epoxy resin and silica is applied on the alloy-plated steel sheet, gas pinholes occur at voltages below 200 to 300V, and gas pinholes occur at voltages of 300V or higher, contrary to the case of alloy plating alone. It was found that gas pinholes were less likely to occur.

It was also found that regarding the relationship between the thickness of the organic insulating film and the voltage, the lower the voltage, the thinner the film needs to be, and the higher the voltage, the thicker the film needs to be.

From the above findings, high productivity can be obtained by using an electrodeposition device under a high voltage of 300 V or higher and by applying an organic coating on an alloy-plated steel sheet.

That is, in order to achieve the above object, according to the first aspect of the present invention, after the surface of a Zn-based alloy plated steel sheet is subjected to painting pretreatment, epichlorohydrin-bisphenol A type is applied to the surface of the steel sheet after the pretreatment. 0.5 to 1.0 mol of dialkanolamine is added to 1 equivalent of epoxy group of a urethane-modified epoxy resin having an epoxy equivalent of 1000 to 5000, which is obtained by reacting 10 to 100 parts by weight of an isocyanate compound to 6100 parts by weight of an epoxy tree. A coating composition containing 10 to -150 parts by weight of silica as a solid content is deposited on 100 parts by weight of the modified epoxy resin as a solid film of 0.3 to 2.0 μm, and this organic coated steel sheet is used as a cathode to conduct a cationic electrode. In the paint,
After increasing the voltage to a specified voltage of 300v or more within 10 seconds,
It is possible to provide a method for electrodepositing a steel plate with excellent gas pinhole resistance, which is characterized by maintaining the voltage at the voltage for a predetermined period of time.

According to the second aspect of the present invention, in obtaining the urethane-modified epoxy resin, before reacting the isocyanate compound, epichlorohydrin-bisphenol A type epoxy resin and polyalkylene glycol diglycidyl ether are mixed in a weight ratio of Zoo /300~100/1
Provided is a method for electrocoating a steel plate with excellent gas pinhole resistance, which involves reacting a composition with bisphenol A having the following properties:

The present invention will be explained in more detail below. When applying an organic film to an alloy-plated steel sheet, it is preferable to perform a base treatment such as phosphate or chromate as a pretreatment as known in the art. As the organic insulating film, Electric sign paint (epoxy resin)
Epoxy resin is used because of its compatibility with

The first aspect of the present invention will be explained.

First, the epichlorohydrin-bisphenol A type epoxy resin used in the present invention refers to a condensate obtained by subjecting only bisphenol A and epichlorohydrin to a condensation reaction.

In addition to epichlorohydrin-bisphenol A type epoxy resin, epoxy resins include those consisting only of aliphatic epoxy resins, alicyclic epoxy resin structures, copolymerizations of the above epoxy resins and bisphenol A type epoxy resins, and dicarboxylic acid , epoxy esters produced by reaction with monocarboxylic acids, etc., but when paying attention to the corrosion resistance of processed parts, it is best to use epichlorohydrin-bisphenol A type.

A specific example is Epicote 828. Commercial products such as 1001, 1004, 1007, and 1009 (all manufactured by Shell Chemical Co., Ltd.) may be used, and these may be used alone or as a mixture. Processability and alkali resistance are imparted to such resins by polymerization. By reacting isocyanate compounds in order to
A urethanized epoxy resin having an epoxy equivalent weight of 1000 to 5000 is obtained.

When reacting the isocyanate compound with the epichlorohydrin-bisphenol A epoxy resin, the weight ratio is preferably 10 to 100 parts by weight per 1001 parts of the epichlorohydrin-bisphenol A epoxy resin.

If the amount of isocyanate compound is less than 10 parts by weight per 100 parts by weight of epichlorohydrin-bisphenol A epoxy resin, not only will processability be insufficient, but polymerization will be insufficient, resulting in failure to obtain good alkali resistance and electrodeposition. Elution and soft swelling of the film sometimes occur, which deteriorates the adhesion of the coating after electrodeposition, which is undesirable.
If it exceeds 0.00 parts by weight, the polymerization of the resin will progress too much.
This is also undesirable because it inevitably increases the viscosity of the paint and reduces the paintability.

The isocyanate compound used is an aliphatic compound, an alicyclic compound having at least two isocyanate groups in one molecule,
or an aromatic compound, or a compound obtained by partially reacting these compounds with a polyhydric alcohol, such as m- or p-phenylene diisocyanate; 2.
4- or 2.6-tolylene diisocyanate, p-xylene diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate alone or in mixture, or polyhydric alcohols (dihydric alcohols such as ethylene glycol and propylene glycol, glycerin, trimethylolpropane) , pentaerythritol,
It is a reaction product with polyhydric alcohols such as sorbitol and dipentaerythritol, and contains at least 2
Examples include compounds in which isocyanate groups remain.

The reaction between the epichlorohydrin-bisphenol A type epoxy resin and the isocyanate compound can be carried out without a catalyst, but if necessary, a known catalyst such as a tertiary amine, an organic compound, etc. can be added.

In addition, the epoxy equivalent of the obtained urethanized epoxy resin must be in the range of 1000 to 5000. If the epoxy equivalent is less than 1000, the molecular weight of the resin is small, so sufficient alkali resistance cannot be obtained, and during electrodeposition, Elution and soft swelling of the film occur. Furthermore, if the epoxy equivalent is more than 5,000, 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.

A dialkanolamine is further added to the epoxy group of the urethanized epoxy resin having an epoxy equivalent of 1,000 to 5,000 thus obtained. The amount of dialkanolamine added to the epoxy group of the above urethanized epoxy resin is preferably 0.5 to 1.0 mol per 1 mol of epoxy group. 0.5 mol of dialkanolamine per 1 mol of epoxy group. By adding the above, sufficient reinforcing effect with silica can be obtained.
When the amount of dialkanolamine added exceeds 1.0 mol based on the epoxy group, the effect of preventing swelling of the film due to alkali generated at the interface during electrodeposition is improved, and the effect of preventing deterioration of coating film tightness is improved. The excess amount is not added to the epoxy group, which is not only uneconomical, but also remains in the coating film as an unreacted amount, reducing corrosion resistance and secondary adhesion.

Examples of dialkanolamines used include jetanolamine, cypropanolamine, dibutanolamine, and the like.

The composite resin thus obtained further contains silica to improve the anticorrosion effect.
It is blended at a ratio of 0 to 150 parts by weight. If the silica content is less than 10 parts by weight, no improvement in corrosion resistance can be expected, and if the silica content is greater than 150 parts by weight, the adhesion of the coating film after two coats will be reduced. This results in a decrease in properties and workability.

Examples of the silica used include colloidal silica and fumed silica, and any of them may be used.

The coating composition obtained above is applied to the surface of the pretreated steel sheet. The coating method may be roll coating, spraying, shower coating, etc., or during heat treatment for drying and curing. The board temperature is 100~
Although 200°C is sufficient, it hardens sufficiently even at temperatures below 150°C, so when applied to BH steel sheets,
A remarkable effect can be obtained without impairing the B)I properties of the steel sheet.

The dry coating pressure of the resin composition, that is, the amount of solid film deposited, needs to be 0.3 to 2.0 μm.

If it is less than 0.3 μm, the coverage is poor and potential unevenness occurs.
Current concentration tends to occur. Also, if the thickness exceeds 2.0 μm, the insulation is too high and non-uniform breakdown occurs, resulting in current concentration and gas pinholes. 0.3 to 2
．． If the thickness is 0 μm, the film will be destroyed uniformly during electrodeposition, and uniform electrodeposition will be possible.

Next, using the plate coated steel plate obtained above as a cathode,
Electrodeposition coating in cationic electrodeposition paint.

As a cationic electrodeposition paint, for example, Power Top U-1
00 (manufactured by Nippon Paint), Electron 9450 (
(manufactured by Kansai Paint), but is not limited to these.

The electrodeposition coating is performed by reaching a predetermined voltage of 300 V or more within 10 seconds and then maintaining the voltage for a predetermined time.
When performing electrodeposition coating at less than 300V, it is necessary to reduce the thickness of the organic coating to less than 0.3μm.Generally, the upper limit of the applied voltage is about 400V, and the voltage application time is 2 to 300V.
It takes about 3 minutes.

Next, a second aspect of the present invention will be described. Note that explanations of parts common to the first aspect will be omitted.

The difference from the first embodiment is that before reacting the isocyanate compound to obtain the urethane-modified epoxy resin used in the present invention, epichlorohydrin-bisphenol A
The only point is that polyalkylene glycol diglycidyl ether is blended into the type epoxy resin and bisphenol A is reacted with this.

First, bisphenol A is reacted with a mixture of epichlorohydrin-bisphenol A type epoxy resin and polyalkylene gelicol diglycidyl ether.
It is used to copolymerize the type epoxy resin and polyalkylene glycol diglycidyl ether, and is added in an amount sufficient for the copolymerization reaction.

The weight ratio of epichlorohydrin-bisphenol A type epoxy/polyalkylene glycol diglycidyl ether should be from 1007300 to 100/10;
If this weight ratio is less than 1007300, the resin will have wood-philic properties and the appearance of the electrodeposition coating will be improved, but the proportion of the shrimp chlorbisphenol A type epoxy resin, which has good coating film adhesion, will be relatively reduced. Therefore, the adhesion of the electrodeposited film decreases.

When the weight ratio is greater than 100/10, the proportion of hydrophilic fullkylene glycol diglycidyl ether is relatively reduced, resulting in insufficient wood-philicity and poor appearance after electrodeposition.

Examples of the polyalkylene glycol diglycidyl ether include polyethylene glycol diglycidyl ether, polypropylene glycol diglycidyl ether, polybutylene glycol diglycidyl ether, polybutylene glycol diglycidyl ether, 1.6-
Examples include hexanediol diglycidyl ether and glycerin diglycidyl ether.

<Example> The present invention will be specifically explained below based on Examples.

(Example 1) (A) Production of isocyanate compound 528 parts of hexamethylene diisocyanate and 620 parts of methyl isobutyl ketone were added to a reaction apparatus equipped with a reflux condenser, a stirring device, a thermometer, and a nitrogen gas blowing device, and the mixture was uniformly dispersed. After dissolving and heating to 80℃, glycerin: 92
1 part was gradually added dropwise over 1 hour, and the reaction was further carried out at 100° C. for 4 hours to obtain isocyanate compound A with a nonvolatile content of 50%. The isocyanate equivalent of this compound A was 207 in terms of solid content.

(8) Production of gaseous resin Epikote 1007 (epoxy resin manufactured by Shell Chemical Co., Ltd.: epoxy equivalent = 2000) = 20
00 parts and toluene: 1000 parts were added, and the temperature was raised to 80°C to form a homogeneous solution. Next, 600 parts (solid content) of the above-mentioned isocyanate compound A was gradually added dropwise over 1 hour, and the mixture was further heated at 80°C for 3 hours. Made it react. The end point of the reaction was determined by the absorption of isocyanate groups (2270 cm) using an infrared spectrophotometer.
In this way, a urethanized epoxy resin with an epoxy equivalent of 2,600 was obtained.

Subsequently, 105 g of jetanolamine was added to this urethanized epoxy resin and reacted at 80° C. for 2 hours.
Colloidal silica dispersed in an organic solvent was mixed with the urethanized epoxy resin thus obtained at a weight ratio of gaseous resin/silica = 0.730 to prepare a coating liquid.

Next, this coating liquid was subjected to degreasing treatment and coating type chromate treatment (Vercolen LN4513 manufactured by Nippon Parkerizing ■).
H) Zn-Nj plated steel sheet (Ni content 12%
, coated with a bar coater on a plating amount of 20g/■2),
By baking at a furnace temperature of 150℃ for 30 seconds, the resin film thickness becomes 0.1.0.3.0.5.1.0.2.0.2.5μm.
An organic coated steel sheet on which a solid film layer was formed was manufactured.

Using each organic coated steel plate as a cathode and using Power Top U-100 (manufactured by Nippon Paint ■) as an electrodeposition paint, the voltage was increased to 100V, 200V, 300V, and 400V within 10 seconds, and then held at the voltage for taO seconds. Electrodeposition coating was performed and the occurrence of gas pinholes was investigated.

The results are shown in the table below along with those without organic coating.
In addition, the table shows O: No Nigas pinholes, ¥: Few Nigas pinholes, and X: Many Nigas pinholes.

--Electrodeposition voltage (v) 1 All of the resin film thicknesses of 073 to 2□0 μm were free of gas pinholes at an electrodeposition voltage of 300 V or higher.

(Example 2) Polyethylene glycol diglycidyl ether (Ebolite 200E manufactured by Kyoeisha Yushi, epoxy equivalent = 200) 1
00g and bisphenol A type epoxy resin (Epicoat 1001 manufactured by Shell Chemical, epoxy equivalent = soo) 100
Add 50g of bisphenol A to the mixture of g and heat at 150°C.
After reacting for 6 hours and confirming that unreacted phenolic hydroxyl groups had disappeared, 125 g of xylene and 125 g of methyl ethyl ketone were added and cooled to obtain product B.

Next, after heating the product B to 80°C, 20 g of tolylene diisocyanate was added dropwise over 1 hour to react. After the addition, the reaction was continued while keeping the temperature at 80°C for 3 hours. The end point of the reaction was , the point at which the absorption of isocyanate groups (227 Ocm-'') disappears using an infrared spectrophotometric system, and thus the epoxy equivalent = 1
000 urethanized epoxy resin was obtained. Next, 13.3 g of jetanolamine was added and the mixture was heated at 80°C for 2 hours.
The resulting urethanized epoxy resin was reacted for a time, and colloidal silica dispersed in an organic solvent was added to the base resin/silica =
A coating liquid was prepared by mixing at a weight ratio of 0,730.

This coating liquid was then subjected to degreasing treatment and electrochromate treatment (Cry, + Colloidal Sin).

An organic coated steel plate was produced in the same manner as in Example 1, except that a Zn-Ni plated steel plate coated with a bath (described in JP-A No. 62-278298) was used, and electrodeposition coating was performed.

The table below shows the occurrence of gas pinholes along with those without organic coating.

1 1 Electrodeposition voltage (v) 1 1 l 1001200530014001
1 membrane 10.5IΔ 1Δ lo 1011 12
51・1・1Δ 1Δ 1 Resin film thickness is 0.3 to 2.
All samples with a diameter of 0 μm had no gas pinholes at an electrodeposition voltage of 300 V or higher.

(Example 3) A Zn-Fe plated steel plate pretreated in the same manner as in Example 2 was used, and a gas pin was applied in the same manner as in Example 2, except that Elekron 9450 (manufactured by Kansai Paint Co., Ltd.) was used as the electrodeposition paint. We investigated the occurrence of holes.

The results are shown in the table below.

■ 1 Electrodeposition voltage (V) Single layer 1100 cool 300 cool
In all cases where the ol resin film thickness was 0.3 to 2.0 μm, there was no gas pinhole at an electrodeposition voltage of 300 V or higher.

(Example 4) Same as Example 2, except that an alloyed hot-dip galvanized steel sheet that had been pretreated in the same manner as in Example 2 was used, and Elekron 9450 (manufactured by Kansai Paint V/J) was used as the electrodeposition paint. We investigated the occurrence of gas pinholes.

I I Electrodeposition voltage (V) 1-
Shi00 l 200 l 300 Shi00 1
Thickness l ・, Ol ・ 1 ・ IOI
All of the O1 resin film thicknesses of 0.3 to 2.0 μm had no gas pinholes at an electrodeposition voltage of 300 V or higher.

<Effects of the Invention> Since the present invention is configured as described above, alloy-plated steel sheets are prone to gas pinhole defects in single-hour rapid pressure increase electrodeposition coating, but according to the present invention, gas pinhole defects are likely to occur. By applying an organic resin coating, this can be prevented and high productivity can be obtained.

Patent applicant: Kawasaki Steel Corporation Representative Patent Attorney Nozomi Watanabe Same Patent Attorney Haruko Miwa

Claims (2)

[Claims] (1) After applying pre-painting treatment to the surface of a Zn-based alloy plated steel sheet, 10 to 100 parts by weight of an isocyanate compound is reacted with 100 parts by weight of epichlorohydrin-bisphenol A type epoxy resin on the surface of the pretreated steel sheet. 100 parts by weight of a modified epoxy resin with 0.5 to 1.0 mol of dialkanolamine added to 1 equivalent of the epoxy group of a urethane modified epoxy resin with an epoxy equivalent of 1000 to 5000, and 10 to 150 parts of silica as a solid content. The paint composition mixed in parts by weight was deposited as a solid film of 0.3 to 2.0 μm, and this organic coated steel sheet was used as a cathode in a cationic electrodeposition paint.
After increasing the voltage to a specified voltage of 300V or more within 10 seconds,
A method for electrodepositing a steel plate with excellent gas pinhole resistance, the method comprising maintaining the voltage at the voltage for a predetermined period of time. (2) In obtaining the urethane-modified epoxy resin according to claim 1, before reacting the isocyanate compound,
2. The highly resistant gas pinhole-resistant composition according to claim 1, wherein bisphenol A is reacted with a composition comprising epichlorohydrin-bisphenol A type epoxy resin and polyalkylene glycol diglycidyl ether in a weight ratio of 100/300 to 100/10. Reactive electrodeposition coating method.