JPS63235496A - Coating method - Google Patents

Abstract

PURPOSE:To form a double electrodeposited film having excellent corrosion resistance, etc., by electrodepositing a cationic electrodeposition paint consisting essentially of a pigment having more than a specified oil absorption on only the lower part of an automobile body, and the electrodepositing a secondary electrodeposition paint having a specified characteristic on the whole surface of the automobile body while the first pint is not cured. CONSTITUTION:Only the lower part of the automobile body is electrodeposited by using a cationic electrodeposition paint I consisting of a resin A and a pigment B, >=5wt.% of which has >=100 oil absorption, and wherein both materials are mixed to control the total oil absorption of the pigment B to 1,000-10,000 based on 100g resin A. An emulsion type cationic electrodeposition paint II having a specified minimum electrodeposition current density is the electrodeposited on the whole automobile body as the second layer while the paint I is not cured. The deterioration of the coated surface condition on the general part other than the edge part, which is the defect of the first electrodeposited film, can be prevented, and the deterioration in the smoothness and brightness of the overcoated film coated on the electrodeposited film is also prevented. Moreover, the corrosion resistance of the edge part of the automobile body and the filiform corrosion resistance are remarkably improved, and a coated film having excellent surface smoothness can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides corrosion protection for the car body by applying two coats of cationic electrocoating paint to the car body, especially at edges such as cut corners, bends, and protrusions. The present invention relates to an electrodeposition coating method for forming a composite electrodeposition coating film having excellent properties and thread rust resistance.

Electrodeposition coating has traditionally been used in the field of car body painting, as compared to organic solvent-based air spray painting or electrostatic spray painting, it has better coverage of the paint on the car body and provides a relatively uniform film thickness. It has been widely put into practical use because it has the advantage of being easy to obtain. Particularly in recent years, cationic electrodeposition has been replaced by 7-ion electrodeposition in fields where corrosion resistance is important, such as automobile bodies, because it forms a coating film with excellent corrosion resistance.

However, even if cationic electrodeposition coating is performed, the desired film thickness cannot be obtained, especially on the edges of automobile bodies, because the formed cationic electrodeposition coating undergoes thermal fluidization during heating and curing.
As a result, there is a defect in that the covering properties of the edge portions are poor and the anticorrosion properties are reduced.

However, measures to improve these defects include using rust-proof steel plates for the object to be coated, and applying an organic solvent-based paint with good anti-corrosion properties to the edges using a roller or brush called Edge Coat. However, it is not always satisfactory in terms of cost and productivity.Also, electrodeposition paints are being used as a method to improve the corrosion resistance of the edges of objects to be coated and the thread rust resistance of electrodeposition coatings. For example, Japanese Patent Publication No. 60-7716 filed by the present applicant proposes a method of applying two coats.
The first layer of electrodeposition paint contains conductive powder and has a volume specific electrical resistance value of I×10' to 1×10 LIΩ.
・A method has been proposed in which a second layer of electrodeposition paint is applied using an electrodeposition paint that can form a coating film of cm. Because the paint contains conductive powder, the workability of the electrodeposition paint is poor, resulting in uneven coating and roughness of the coating, so it is recommended not to apply the electrodeposition paint a second time. Even if it is coated over and over again, there is a problem that the corrosion resistance and smoothness of the coating film other than the edges are poor.

Furthermore, as a method of improving the corrosion resistance of the edge portion of the object to be coated, the first electrodeposition paint is applied at a low voltage of 10 to 100V at 0.0V.
A method has also been proposed in which an electrodeposition film is formed only on the edges by performing electrodeposition coating for a short time of 5 to 2 seconds, and then a second electrodeposition coating is applied (Japanese Patent Application Laid-Open No. 61-1
95998), however, this method
It is usually difficult to apply two coats of electrodeposition paint to form a smooth painted surface without uneven coating, so the first coat is applied to form a coating only on the edges of the object. The area other than the edges is painted with the second electrodeposition paint, but 1
Since the time required for the first electrodeposition paint to be applied is as short as 0.5 to 2 seconds, it is extremely difficult to accurately control the coating process, and the objects to be coated, such as car bodies, are transferred to the electrodeposition paint bath using a conveyor. It is difficult to apply it on a practical line where it is transported and electrocoated. In addition, since the object to be coated generally has a complex shape, it is actually difficult to form a coating film only on the edges in the first electrodeposition coating, and a coating film may also be formed on areas other than the edges. Therefore, there is a problem that the smoothness of the coating film after the second electrodeposition coating is poor.

Therefore, the present inventors have solved the problems of the prior art as described above, and have achieved a method that does not impair the coating surface condition such as the smoothness of the coating film even if the cationic electrodeposition paint is applied twice, and that As a result of intensive research to develop an electrodeposition coating method that can form an electrodeposition coating film with excellent coating performance such as corrosion resistance and thread rust resistance on the edges of By electrocoating only the lower part of the car body as the first layer with a cationic electrodeposition paint containing 100 or more pigments as an essential component, the corrosion resistance of the edges of the car body is mainly improved. By electrodepositing an emulsion-type cationic electrodeposition paint having a specific minimum electrodeposition current density as a second layer over the entire surface of the automobile body without curing the electrodeposition coating of the second layer.

The disadvantages of the first layer of electrodeposited coating are the deterioration of the coating surface condition (especially smoothness) in general areas other than the edges, and the smoothness and sharpness of the top coat applied on top of the electrodeposition coating. As a result, the corrosion resistance and thread rust resistance of the edges of automobile bodies are significantly improved, and it is possible to form a coating film with excellent coating surface smoothness. The invention was completed.

Thus, according to one invention, resin (A) and at least one
at least 5% by weight of the pigment (B) consists of a pigment with an oil absorption of 100 or more, and the total oil absorption of the pigment CB) is 1, After performing cationic electrodeposition coating only on the lower part of the car body using the cationic electrodeposition paint (I) which is blended within the range of 000 to 10.000, the resin (C ) and pigment (D), the minimum electrodeposition current density is 0.7 mA/cm" or less, the degree of emulsion is 80% by weight or more, and the total oil absorption amount of pigment (D) is less than the cationic electrodeposition paint ( A coating method is provided, which comprises electrodepositing a smaller emulsion-type cationic electrodeposition paint (IT) in I) on the entire surface of the automobile body, and then heating and curing it to form a composite electrodeposition coating film. .

In the present invention, cationic electrodeposition paints (I) and (rr)
are resin binders commonly used in electrodeposition coatings as their resin binder components (A) and (C), for example polyamine resins such as amine-added epoxy resins,
For example, (1) adducts of polyepoxides with primary mono- and polyamines, 2-edge mono- and polyamines, or 1.2-class mixed polyamines (e.g., U.S. Pat. No. 3,984,299)
Adducts of Ti1l polyepoxides with 2-edge polyamines having 7 ketiminated primary amino groups (see e.g. U.S. Pat. No. 4,017,438), 0+D polyepoxides and ketiminated 1 It may contain a reaction product obtained by etherification with a hydroxy compound having an amino group (see, for example, Japanese Patent Application Laid-open No. 59-43013).

In addition, when the composite cured coating film formed by the present invention is required to have good weather resistance, the resin binder (A) and/or (
As C), an amine group-containing or nonionic acrylic resin with excellent weatherability may be used in combination with the amine-added epoxy resin.
C) may be an amino group-containing acrylic resin alone.

The above-mentioned amine-added epoxy resin can be cured using a polyisocyanate compound blocked with an alcohol to form an electrodeposited coating.

It is also possible to use amine-added epoxy resins that can be cured without using blocked incyanate compounds; for example, by adding β-hydroxyalkyl carbamate to a polyepoxide material (see Publication No. 155470); Curable type resin [e.g. JP-A-55-8043
Reference No. 6] may also be used.

Preparation of a cationic aqueous dispersion using the resin binder described above is usually carried out by neutralizing the resin binder with a water-soluble organic acid such as formic acid, acetic acid, or lactic acid. The cationic electrodeposition paint containing the aqueous dispersion obtained as a main component further contains a pigment, and in the cationic electrodeposition paint (I) used for electrodeposition coating only the lower part of the automobile body, as the pigment (B), At least 5% by weight of pigment (B) is a pigment with an oil absorption of 100 or more, preferably 150 or more.
, preferably 10% to 95% by weight, more preferably 20% to 90% by weight.
The amount of pigment (CB) to be blended is such that the total oil absorption amount of pigment (B) is 1,000 to 10,000, preferably 500 to 9,000, more preferably 3,000 to 7,000, per 100 g of the resin component. The amount is within the range of 000.

Examples of pigments with an oil absorption of 1 too or more that are blended as an essential pigment component in the above-mentioned cationic electrodeposition paint (I) include silicon dioxide pigments such as anhydrous silicon dioxide and hydrated amorphous silicon dioxide, and carbon pigments. It is preferably a silicon dioxide pigment. In the present invention, if the carbon-based pigment is used alone as the pigment with an oil absorption of 100 or more, since this pigment has conductivity, the coating workability of the cationic electrodeposition paint (I) will be somewhat reduced.
Preferably, it is blended in combination with a silicon dioxide pigment. In this case, the combined weight ratio of the silicon dioxide-based lII4 material and the carbon-based pigment is 9515 to 60/4.
0, preferably within the range of 90/10 to 70/30.

As a commercially available silicon dioxide pigment having an oil absorption of 100 or more, for example, Nippon Aerosil Co., Ltd.'s product name "Erosil 2" is available.
00J (oil absorption 143-183), Fuji Davison's product name "Syroid 161J (oil absorption 128-13
5), "Thyroid 244" (oil absorption 270-330)
, "Thyroid 308" (oil absorption 170-220), "
Thyroid 404'' (oil absorption 170-230), r Thyroid 978'' (oil absorption 180-230), etc. Carbon pigments include furnace type or channel type carbon black (which is usually used as a black pigment). The oil absorption amount is usually 100 to 130).
For example, the product name of Columbia Carbon Co., Ltd. in the United States is “Carbon B”.
Examples include AGJ.

In addition to the above-mentioned pigments having an oil absorption of 100 or more, the pigments to be added to the cationic electrodeposition paint (I) used in the present invention include pigments commonly used in electrodeposition paints, such as red iron, titanium white, and other inorganic pigments. Colored pigment; talc,
Pigment (B) is an extender pigment such as clay or calcium carbonate.
The total oil absorption amount for 100 g of resin (A) is t, ooo~
They can be used in combination as long as they fall within the range of 10.000.

The total oil absorption amount of the pigment in the present invention is JIS K5101.
-78 (Pigment Test Method) as measured by the following method.

First, the oil absorption amount of each pigment is determined as follows.

Accurately weigh a specified amount of sample to the nearest 10 mg on parchment paper and transfer it to a frosted glass plate. Calculate the amount of boiled linseed oil required from the expected oil absorption of the sample in advance, and weigh about 90% of it. Take the sample from the microbiulet to the tip of the probe, and immediately use this spatula to thoroughly knead the sample on the frosted glass plate and the boiled linseed oil for about 5 minutes.

Next, add 1-2 drops of boiled linseed oil to the spatula and mix evenly. The process of adding 1-2 drops of oil to the boiled flax and kneading it is repeated, and the end point is when the whole sample becomes a hard, uniform, putty-like mass for the first time, and the boiled flax has been added to the boiled flax until then. Read the amount of oil to the nearest 0.01-.

Next, the oil absorption amount is calculated using the following formula.

Here, A: Oil absorption amount S: Mass of sample (g) L: Amount of boiled linseed oil used (g) Also, the total oil absorption amount is determined by the formula above for each pigment formulation i (g) blended for 100 g of resin. This value is calculated as the sum of the values obtained by multiplying the oil absorption values of each pigment.

Among the pigments (B) used in the cationic electrodeposition paint (I) of the present invention, when the amount of the pigment with an oil absorption of 100 or more is less than 5% by weight of the pigment (B), the cationic electrodeposition paint (II)
) tends to be difficult to deposit.

In addition, in the present invention, if the total oil absorption amount of the pigment (B) in the cationic electrodeposition paint (I) is less than 1,000, the covering of the edge portion, which is the object of the present invention, will not be sufficient, and
When it exceeds 0,000, pigment aggregation tends to occur, resulting in problems such as filter clogging and coating surface abnormalities (blurring, skin irritation, etc.) due to pigment agglomerates.

In the present invention, after the electrodeposition coating is applied only to the lower part of the car body, it is not necessarily necessary to wash the electrodeposition coating with water, but it is usually preferable to wash the electrodeposition coating with water (washing with shower water or immersion).The washing water is deionized. Water, Ultrafiltration filtrate, purified water by reverse osmosis, etc. can be used.

This washing with water further improves the corrosion resistance of the edges and forms an electrodeposited coating film that is substantially free of pinhole defects.

Next, as the cationic electrodeposition paint (II), resin (C
) and pigment (D), and the minimum electrodeposition current density is 0, 7.
mA/cm2 or less, preferably 0.5mA/am2
It is more preferably 0.3 mA/cm2 or less and the degree of emulsion is 80% by weight or more.

It is preferably 85% by weight, more preferably 90% by weight or more, and the total oil absorption amount of the pigment is higher than that of the cationic electrodeposition paint (I).
), more preferably less than t, ooo, an electrodeposition paint of any composition can be used without particular limitation.

The above-mentioned minimum deposition current density is a value measured by the following method.

A platinum plate with a surface area of 1 cm2 and an insulated back side was used as the opposite electrode to the object to be coated, and the plate was placed so that the two surfaces faced each other.
Placed in the electrodeposition paint bath at a distance of 5 cm. 28°
C1 A constant current was applied without stirring, and the time and voltage were recorded.The current density was changed every 0 to 05 raA/am2, and the voltage suddenly increased due to the increase in resistance due to electrodeposition of the paint for 3 minutes or 3 minutes. The current density when the current density occurs in the vicinity of more than 1 minute is defined as the minimum deposition current density.

Furthermore, the degree of emulsion of the cationic electrodeposition paint (n) described above is an index representing the proportion (wt%) of particles truly suspended as particles in the electrodeposition paint, and is determined by the following procedure. .

First, about 35cc of 15-20% by weight clear emulsion was placed in a cell and sealed.
Centrifuge the separated sample for 60 minutes at
Remove c with a pipette, dry at 120°C for 1 hour, and measure the nonvolatile content Nr (%).

Next, turn the cell upside down, drain off the supernatant, and add another 10
Invert for a minute and remove the supernatant layer. After homogenizing the remaining sediment layer with a glass rod, 1.5 to 2.0 g was accurately weighed, and 120 g
Dry at ℃ for 1 hour and measure non-volatile content N2 (%).

Next, approximately 2 cc of the clear emulsion was accurately weighed, dried at 120°C for 1 hour, and the nonvolatile content No. (%) was measured. The degree of emulsionization is a value determined by the following formula.

1 = Luci, degree of conversion (weight%) = N, 22\L and x
io.

No (N2-N□) In the present invention, if the minimum electrodeposition current density of the cationic electrodeposition paint (■) exceeds 0.7 A/cm2, it becomes difficult to secure a film thickness that provides smoothness of the coated surface. .

In addition, if the degree of emulsion is less than 80% by weight, the second layer of electrodeposited coating will be less than 1% in the lower part of the car body.
It tends to mix with the electrodeposition coating of the layers, resulting in a decrease in both edge corrosion resistance and coating surface smoothness.

The automobile body in the coating method of the present invention is a passenger car, bus, truck, motorcycle, microbus, etc.
The lower part has many edges and is often exposed to a harsh corrosive environment.

The method of painting an automobile body according to the present invention uses cationic paint CI.
), and then coated the entire surface of the body with cationic paint (II) without curing.
) and then heated to cure both coatings simultaneously. That is, only the lower part of the car body is immersed in a bath of cationic electrodeposition paint (I), and the body is coated with cationic electrodeposition, then taken out of the bath, washed with deionized water, etc., and then air-dried as necessary. After that,
The above automobile body is completely immersed in a bath of cationic electrodeposition paint (RL) and subjected to cationic electrodeposition coating.Then, after washing with water, heating is performed to cure the above-mentioned electrodeposition coating film.

As the method and apparatus for electrocoating the bipolar thionic electrodeposition paint in the present invention, the per se known methods and apparatuses conventionally used in cathodic electrodeposition coating can be used. In this case, it is desirable to use the car body as a cathode and a carbon plate as an anode.

Electrodeposition coating conditions that can be used are not particularly limited, but generally bath temperature: 20 to 30°C, voltage: lOO~
400V (preferably 200-300V), current density:
0.01-3A/dm2. Energization time: 30 seconds to 10 minutes,
Pole area ratio (A/C): 6/1 to l/6, distance between poles: 1
It is desirable to electrodeposit to a thickness of 0 to 100 cm under stirring.

The film thickness (dry state) of the first layer of electrodeposition coating film formed only on the lower part of the automobile body by the above-mentioned electrodeposition coating method is 5.
~30μ, preferably within the range of lO~ZSU, and on top of that, the film thickness (dry state! s) of the second layer electrodeposition coating film formed on the entire surface of the automobile body is preferably 5~70JJj. Conveniently, the range is from 10 to 50.

In the present invention, it is preferable to perform the second layer electrodeposition coating while the first layer electrodeposition coating is uncured, from the viewpoint of forming a composite coating film and from the viewpoint of adhesion. Yes, it is a necessary condition, but the first electrodeposition coating film is heated at 120°C for about 1
It should be understood that the term "uncured state" in the present invention also includes a semi-cured state. be.

The electrodeposited coating film formed on the car body is about 1% after cleaning.
It is hardened by baking at a temperature of 50 to about 180°C. Regarding the thickness of the electrodeposited coating, it is generally desirable that the total thickness of the lower part of the car body where the first and second layers overlap is within the range of 15 to 80 coats from the economical point of view. The thus painted automobile body can be finished by appropriately applying intermediate coats and top coats as necessary.

When the cationic electrocoating paints (I) and (II) are electrocoated as described above based on the method of the present invention, the second electrocoating film becomes the first layer on the lower part of the car body. It is deposited on the surface of the electrodeposition coating film, and a coating film is formed in a multilayer state including a first electrodeposition coating layer and a second electrodeposition coating layer.

In other words, the first layer of electrodeposition coating containing pigments with high oil absorption covers the edges, and the second layer of electrodeposition coating improves coating surface smoothness and uniform film formation. to share the load. As a result, the multilayer coating film has excellent corrosion resistance at the edges and has an excellent coating surface condition with no pinhole defects.

According to the electrodeposition coating method of the present invention, the corrosion resistance of the edges of an automobile body, which has been a weak point of conventional electrodeposition coatings, is significantly improved, and the coated surface is also excellent in smoothness.

Examples Hereinafter, the present invention will be specifically explained with reference to Examples and Comparative Examples. 0 parts and % mean parts by weight and % by weight.

Example 1 The lower part of an automobile body was placed in a bath of a cationic electrodeposition paint (paint 1-A) having the following composition using a thermosetting water-soluble epoxy polyamino resin composition equivalent to Nireclone No. 9000 manufactured by Kansai Paint Co., Ltd. Only the sample was immersed and electrodeposition was applied under the conditions shown in Table 1.

Next, take the body out of the bath and wash it with water, then
The above automobile body was completely immersed in the following cationic electrodeposition paint (paint 1-B) bath, and the entire surface was electrodeposited under the conditions shown in Table 1.
After washing with water, it was heated to about 180°C to form a cured coating film. Table 2 shows the test results for this coating.

Resin: Pigment = 100:49 Total oil absorption 8,765 0 1 An epoxy polyamino resin with a resin base number of 80 is made water-soluble with acetic acid at a neutralization equivalent of 0.55.

No. 2 Manufactured by Asahi Carbon Co., Ltd. Product name: “Carbon ASMJ Attack 3”
Water-containing amorphous silicon dioxide pigment (oil absorption 300) manufactured by Fuji Davison Co., Ltd., product name.

Palm 4 An epoxy polyamine resin with a resin base number of 80 was water-dispersed with acetic acid at a neutralization equivalent of 0.24, and the minimum electrodeposition current density was 0.50 mA/cm2 and the degree of emulsion was 85% by weight.

Example 2 Only the lower part of the car body was immersed in a bath of cationic electrodeposition paint (paint 2-A), which is equivalent to Nireclone No. 9000 manufactured by Kansai Paint Co., Ltd., using a thermosetting water-soluble epoxy polyamino resin. After applying electrodeposition under the conditions of 1, washing with water, and then using Kansai Bait Co., Ltd., Nireclone No. 9600.
The above automobile body was completely immersed in a bath of cationic electrodeposition paint (Paint 2-B) using an epoxy resin containing thermosetting and curing acrylic resin as a substitute, and electrocoated under the conditions shown in Table 1. After washing with water, it was heated to about 180°C to form a cured coating film.

Table 2 shows the test results for this coating.

Coating 1 Shishi-3 Resin: Pigment = 100:18 Total oil absorption 2,050 Coating 1 Shishi 2 J Zero 5 A water-soluble acrylic modified epoxy resin with a resin base number of 45 was dissolved in water at a neutralization equivalent of 0.2 with acetic acid. In the case of dispersion, the minimum deposition current density is 0, 20 + Alc layer 2, and the degree of emulsion is 92% by weight.

Example 3 The lower part of the car body was immersed in a bath of cationic electrodeposition paint (paint 3-A), which is equivalent to Nireclone No. 9000 manufactured by Kansai Paint Co., Ltd., using thermosetting water-soluble epoxy polyamide resin. After electrodeposition painting under the following conditions, wash with water,
The above automobile body was fully immersed in the cationic electrodeposition paint (paint 2-B) bath used in Example 2, and electrodeposition was applied under the conditions shown in Table 1. After washing with water, the coating was baked at about 180°C and hardened. formed. -Test results for this coating are shown in Table 2.

Resin: Pigment = 100:17 Total oil absorption 4,580 0 An epoxy polyamino resin with a resin base number of 80 is water-solubilized with formic acid at a neutralization equivalent of 0.55.

Example 4 Nireclone No. manufactured by Kansai Paint Co., Ltd. using a thermosetting water-soluble epoxy polyamino resin composition.

Cationic electrodeposition paint (Paint 4-A) equivalent to 9400
Only the lower part of the car body was immersed in the bath, and after electrodeposition coating was performed under the conditions shown in Table 1, it was washed with water and a cationic electrodeposition paint (paint 4-
B) The above automobile body was completely immersed in a bath and electrocoated under the conditions shown in Table 1, and after washing with water, it was baked at about 180° C. to form a cured coating film.

Table 2 shows the test results for this coating.

Coating L-3 Resin: Pigment = 100:45 Total oil absorption: 4,085 Off An epoxy polyamino resin with a resin base number of 80 is water-solubilized with hydroxyacetic acid at a neutralization equivalent of 0.6.

＼, 1 per coating 8 Epoxy polyamino resin with a resin base value of 80 is water-dispersed with acetic acid at a neutralization equivalent of 0.15, and the minimum electrodeposition current density is 0.29 mA/c112 and emulsion degree is 94 weight Indicates the value in %.

Comparative Example 1 Under the conditions shown in Table 1, coating material 1-B of Example 1 was used to electrodeposit only the lower part of an automobile body, and after washing with water, the automobile body was further coated in coating material 1-B in an uncured state. It was completely immersed, electrocoated, washed with water, and baked.

Table 1 shows the test results for this coating film.

Comparative Example 2 A cationic electrodeposition paint with the same formulation as Paint 2-A of Example 2, using 5 g of titanium oxide (oil absorbing i22) instead of 5 g of siloid to make the total oil absorption of the pigment 555, was applied only to the lower part of the car body. A composite cured coating film was formed in the same manner as in Example 2 except that the electrodeposition coating was carried out. Table 2 shows the test results for this coating.

Comparative Example 3 A composite cured coating film was formed in the same manner as in Example 1, except that a cationic electrodeposition paint containing the following pigment components with a total oil absorption of 856 was used instead of Paint 1-A in Example 1. . Table 2 shows the test results for this coating.

Comparative Example 4 The epoxy polyamino resin of Paint 2-H used in Example 3 was water-solubilized with acetic acid at a neutralization equivalent of 0.5, and the minimum electrodeposition current density was 0.9 A/cm" and the degree of emulsion was 75. Example 3 except using f-paint in weight%
A composite cured coating film was formed in the same manner as above. Table 2 shows the test results for this coating.

[Test Method] Thread Rust Resistance: Using a 0 single-edged safety razor that complies with ASTM-02803-69T thread rust test, make an X-shaped cut on the specimen diagonally reaching the substrate and place it in a salt spray tester for 24 hours. . After washing with salt water spray, thoroughly wash with ionized water, and before the specimen dries, place it in a thermostatic chamber at a temperature of 50 ± 2 °C and a humidity of 8.
The test was maintained at 5±2% RH and continued for 720 hours. Intermediate checks were made at 240 hours and 480 hours during the test to check for the occurrence of thread rust, and if thread rust was clearly observed, its length was measured.

Corrosion resistance at 45° joint: An SPC mild steel plate is processed at an angle of 45°, subjected to surface treatment Bonderite #3004 treatment, and subjected to predetermined electrodeposition coating, and used for the test. The corrosion resistance test was based on JIS Z2371 salt spray test. The test continued for up to 720 hours. 24 during the exam
Intermediate checks were carried out at 0 and 480 hours to determine the presence or absence of black rust.

Corrosion resistance of general parts: Based on JIS 22371 salt spray test. Check for black rust and blistering on the paint film in general areas where no cuts are made. maximum 2
Lasted for 000 hours. At the 1000th hour, the test was checked to see if black rust or blisters were present.

Accelerated Weathering Resistance: Tested for 100 hours using a Sunshine Weather-Ometer, and expressed as the gloss retention rate (60' gloss) of the coating film.

0: Gloss retention rate 80% or more

Claims (1)

[Claims] 1. Consisting of a resin (A) and at least one pigment (B), at least 5% by weight of the pigment (B) has an oil absorption of 1
00 or more, and the total oil absorption amount of pigment (B) is 1,000 to 10,000 per 100 g of resin (A).
After performing cationic electrodeposition coating only on the lower part of the car body using a cationic electrodeposition paint (I) that is formulated to fall within the range of , the resin (C) and pigment (D) remain uncured. ), with a minimum deposition current density of 0.7 mA/c.
m^2 or less and an emulsion degree of 80% by weight or more, and the total oil absorption of the pigment (D) is smaller than that of the cationic electrodeposition paint (I), and the above-mentioned automobile body is used. A coating method characterized by applying cationic electrodeposition coating to the entire surface of the surface, and then heating and curing to form a composite electrodeposition coating film.