JPS61120673A - Method for painting steel plate - Google Patents

Abstract

PURPOSE:To enhance scratch resistance and chipping resistance, by forming a cationic electrodeposition paint film, a barrier coat film, an intermediate coat film and a top coat film, each of which has specific film capacity, to a steel plate. CONSTITUTION:A cationic electrodeposition paint, wherein 40-150pts.wt. of pigment is compounded with 100pts.wt. of a resin solid, is applied to a steel plate subjected to chemical forming treatment and cured under heating. Thereafter, a composition based on a modified polyolefin resin, of which the static glass transition temp. of a foamed film is -30--60 deg.C and the tensile breaking strength elongation is 200-1,000% in an atmosphere of -20 deg.C, is applied to the cured paint film as barrier coat. Subsequently, an intermediate coat comprising org. solvent type thermosetting resin is applied to the barrier coat surface and, thereafter, a top coat forming an ultra-hard paint film excellent in finishing appearance and weatherability and having good scratch resistance is applied.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a coating method for forming a coating film with excellent scratch resistance, chipping resistance, edge corrosion resistance, and physical performance on a steel plate, such as a steel plate for an automobile body. It is.

In the automotive industry, emphasis is placed on the durability of paint films, especially problems such as deterioration of the appearance of the paint film due to scratches, deterioration of the corrosion resistance of the paint film due to peeling of lacquer, progression of corrosion of steel plates, and the tendency for rust to form on the edges. It is being done.

Painted surfaces finished in dark colors such as black with excellent image clarity can be cleaned using a car wash brush, polishing compound, or dirt from driving.
Because of K, the occurrence of scratches is more noticeable.

Conventionally, the occurrence of such scratches was prevented by making the topcoat film harder, but on the other hand, the coating film became brittle and the impact resistance, chipping resistance described below, etc. were significantly reduced. It has been difficult to form a well-balanced coating film with excellent properties.

In addition, in cold regions of Europe and the United States, gravel mixed with a large amount of relatively coarsely crushed rock salt is often laid down to prevent road surfaces from freezing during the winter, and vehicles traveling on this type of road are often covered with gravel mixed with large amounts of rock salt. Rock salt particles and pebbles thrown up by the wheels collide with the paint surface of the vehicle, and the impact often causes the paint film to peel off locally from the vehicle body, a so-called "chipping." Due to the phenomenon,
The metal surface of the impact area on the outer surface of the car body is exposed, and rust quickly develops and corrosion progresses. Usually, paint film peeling due to chipping occurs more often on the bottom of the car body and undercarriages, but it is known that it also occurs on the hood and roof, and localized corrosion becomes quite noticeable after about six months to a year.

In order to prevent this chipping and the progression of corrosion caused by it, various studies have been made on chemical conversion treatments for the surface of the external metal base of the car body, as well as electrocoating paints, intermediate coats, and top coats.

For example, in chemical conversion treatments, the use of iron phosphate coatings and zinc phosphate coatings with different crystal forms has been considered, but it is difficult to sufficiently improve the adhesion of coatings on impact areas with such chemical conversion treatments. be. In addition, various studies have been conducted on the resin and/or pigment contained in electrodeposition paints and topcoat paints, but so far no one has been found that has a sufficient adhesion-improving effect to withstand chipping. .

Ninth, an inorganic foil-like pigment such as sericite or talcum powder is contained in the intermediate coating composition, thereby mitigating and/or dispersing the impact force caused by shearing in the intermediate coating layer due to the inorganic foil-like pigment. Alternatively, peeling can be caused locally within the intermediate coating layer or at the boundary between the electrocoated paint surface and the intermediate coating film, thus preventing damage to the electrocoating coating film and creating a scratch-free surface. The purpose of this method is to ensure that the electro-deposition paint film maintains its rust-preventing function, but the impact force applied to the outside surface of the car body is not constant and may be large, so these methods require that the intermediate coating If an impact force that exceeds the mitigation and dispersion ability due to shear within the film layer is applied, the impact force cannot be stopped at the intermediate coating layer and the affected area will damage all the coating films, including the electrodeposited coating. Another disadvantage is that the entire coating film peels off from the surface of the metal substrate, and as a result, the affected area quickly develops rust and corrosion progresses.

Furthermore, many steel plate products, such as automobile bodies, have many end faces of steel plates (for example, fenders, fenders, etc.).
, door panels, panel hoods, panel roofs, panel trunk lids, side parts, lower edges, back surfaces of body parts, etc.),
This end face has a sharp shape different from the other flat parts.

Therefore, it was inevitable that the applied coating material would melt and flow during heating and curing at the edge portions, resulting in a coating film that was not thinner than that on flat portions, and particularly at acute corner portions, the coating film would be extremely thin. As a result, the anti-corrosion properties of the end faces, including the sharp corners, are significantly inferior to those of the flat parts, and rust tends to occur easily from the end faces, and improvements in this respect are strongly desired.

Therefore, in order to improve the above-mentioned problems, the present inventors
Scratch resistance, chipping resistance, edge rust prevention, and physical performance have been improved without degrading the finished appearance obtained by conventional steel plate coating systems consisting of electrocoating paint, intermediate coating paint, and top coating paint. As a result of intensive research aimed at providing a coating method for forming a coating film, the present invention was completed.

That is, according to the present invention, in the painting process of sequentially applying a cationic electrodeposition paint, an intermediate coat paint, and a top coat paint to a steel plate, (1) a cationic electrodeposition paint containing a pigment at a high concentration is used as the cationic electrodeposition paint; , (2) Prior to applying the intermediate coating paint, apply a modified polyolefin resin whose main component is a modified polyolefin resin having a static glass transition temperature of -30 to -60°C to the surface of the cationic electrodeposition coating film. Apply a barrier coat in advance,
And (3) there is provided a steel plate coating method characterized in that a coating that forms an ultra-hard coating is used as the top coating.

The feature of the present invention is that the above-mentioned requirements (1) to (3) are satisfied in the coating step 5 in which a cationic electrodeposition paint, an intermediate paint, and a top coat are sequentially applied. As a result, it was possible to significantly improve the scratch resistance, chipping resistance, edge rust prevention, physical performance, etc. of the coating film produced by the coating process.

That is, since the top nine layers of the coating film formed according to the present invention are extremely hard, they almost completely prevent the occurrence of scratches caused by car wash brushes, polishing dung, sand dust, etc. I was able to do that.

In addition, a barrier coating film with a glass transition temperature adjusted to -30 to -60°C (more preferably, as described below),
The tensile breaking strength elongation rate of the coating film is 20 at -20°C.
(adjusted to 0 to 1000%) is more flexible than the above-mentioned medium MB coating film aimed at improving chipping resistance.
λ also has unique viscoelasticity due to the modified polyolefin resin. Therefore, even if a strong impact force from rock salt, pebbles, etc. is applied to the surface of the intermediate coat, paint film, or top coat formed through a barrier coat with such physical properties, most or all of the impact energy will be absorbed. is absorbed into the barrier coat film and does not spread to the electrodeposition film of the F layer, eliminating impact peeling of the electrodeposition film and drastically reducing physical damage caused by strong impact to the top coat film. was completed. Tsu'! By interposing the above-mentioned barrier coating film, impact peeling of the coating film due to chipping can be prevented, and rusting and corrosion of the steel material can be eliminated.

Next, the pigment content in conventional cationic electrodeposition paints is practically no more than 35 parts by weight per 100 parts by weight of resin solids, and if more is added than this, the smoothness of the coating film will be affected, and the sharpness of the topcoat will be affected. It is rarely used because it reduces performance. On the other hand, the present inventors investigated the possibility of forming a thick cationic electrodeposition coating on the end face of steel materials to improve corrosion resistance, and found that a cationic electrodeposition coating containing a high concentration of pigment was used. They found that when painted, a thick coating film is formed on the edges, improving the corrosion resistance of the flat areas and edges. It was recognized.

Therefore, the present inventors further improved the corrosion resistance of the edge part, and also improved the smoothness, sharpness, etc. of the coating film (as a result of continued research, we found that All of these defects can be eliminated by applying the barrier coat having a specific composition and properties on the surface of the electrodeposition coating film, and then applying intermediate and top coats. That is, all of these defects can be eliminated. When a barrier coat with the above-mentioned physical properties is applied to a cationic electrodeposited coating film containing a large amount of pigment, which has poor smoothness and image clarity, minute particles of the electrodeposited coating film are applied. It is thought that this is because it penetrates into the uneven areas, reducing the suction of the intermediate paint, improving smoothness and image clarity, as well as improving edge surface rust prevention.

The coating method of the present invention will now be explained in detail.

Steel plate: Any material to be coated by the method of the present invention that has a metal surface that can be coated by cationic electrodeposition is not subject to any restrictions. Examples include iron, copper, aluminum, tin, zinc, alloys containing these metals, and plated or vapor-deposited products of these metals and alloys. Specifically, passenger cars, trucks, There are car bodies such as safari cars and motorcycles. Further, it is preferable that the steel sheet is previously subjected to a chemical conversion treatment with a phosphate or chromate prior to coating with a cationic electrodeposition paint.

Cationic Electrodeposition Paint 2 This is an electrodeposition paint for coating the above-mentioned steel plate, and is a paint containing a higher concentration of pigment than the commonly used cationic electrodeposition paint.

The cationic electrodeposition paint uses a basic water-dispersed resin that can be neutralized with an organic or inorganic acid, such as an epoxy, acrylic, or polybutadiene resin that has a large number of amine groups in its resin skeleton. It is a water-based paint (resin is not limited to this), and the resin is mixed with a neutralizing agent, a pigment, a hydrophilic solvent, water, and if necessary, a curing agent, a crosslinking agent, an additive, etc. It is made into paint. Examples of neutralizing agents for neutralizing and water-soluble (dispersing) the above basic water-dispersible resin (usually used by dissolving it in a hydrophilic solvent) include acetic acid, hydroxyl acetic acid, propionic acid, butyric acid, lactic acid, glycine, etc. Organic acids such as sulfuric acid, hydrochloric acid, phosphoric acid, and other inorganic acids can be used. The amount of the neutralizing agent to be blended is based on the base number of the resin (approximately 30 to 20
0) A range of 0.1 to 0.4 in terms of neutralization equivalent to K is appropriate.

Further, the amount of pigment blended in the cationic electrodeposition paint used in the present invention is usually very large, and specifically, 40 to 150 parts by weight, preferably 55 parts by weight per 100 parts by weight of resin solid content. ~100 parts by weight, more preferably 60-8
5 parts by weight. Pigments that can be blended into the cationic electrodeposition paint are not particularly limited, and may include coloring pigments, extender pigments, antirust pigments, etc. that are known per se. Examples include zinc pigment, antimony white, cyclic lead sulfate, basic Lead carbonate, titanium white, lithopone, lead silicate, zircon oxide, carbon black, graphite, black iron oxide, aniline black, cuprous oxide, cadmium red, from per million, red iron, pigment red, pigment violet, pigment orange, base lead chromate, yellow lead, ocher, cadmium yellow,
Strontium chromate, titanium yellow, litharge, pigment yellow, pigment green, zinc rim, chromium green, chromium oxide, phthalocyanine green, group growth, dark blue back,
7 Nurocyanine blue, pigment blue, cobalt purple, pigment violet, zinc powder, zinc oxide, red lead,
Lead cyanamide, calcium leadate, zinc yellow, silicon carbide, aluminum powder, asbestin, alumina, clay, diatomaceous supernatant lime, gypsum, talc, barium carbonate, precipitated calcium carbonate, calcium carbonate, precipitated barium sulfate, pallite, Examples include bentonite, white carbon, glass beads, etc., and these can be used alone or in combination of two or more. The solid content concentration is about 5~
Dilute with deionized water etc. to 40% by weight, and adjust the pH.
The above-mentioned steel plate is electrodeposited using a conventional method while maintaining the viscosity within the range of 5.5 to 8.0. The thickness of the electrodeposition coating film is not particularly limited, but it is preferably 10 to 40 μm for a cured coating film.
The coating film is cured by heating to about 140-210°C.

Barrier coat: A composition to be applied to the above cationic electrodeposited surface, the static glass transition temperature of the formed coating being -30
The main component is a modified polyolefin resin having a temperature of -60°C (preferably -40 to -55°C). That is, as the modified polyolefin resin, for example, propylene-ethylene copolymer (in molar ratio, 40-80:60-2
0% is preferred), and the chlorination rate of chlorinated polyolefin is approximately 1%.
~60%)), preferably 10 to 20 parts by weight (all based on 100 parts by weight of the copolymer), or 100 parts by weight of the above propylene-ethylene copolymer. 0.1 to 50 parts by weight of maleic acid or maleic anhydride, preferably 0.3 to 20 parts by weight
Examples include resins that are graft-polymerized in parts by weight. In the present invention, if these hexamodified polyolefin resins themselves have a static glass transition temperature within the above range, they can be used as a barrier coat by themselves; If it is desired to change the static glass transition temperature, a viscosity imparting agent can be added as necessary. The viscosity imparting agent includes, for example, rosin, petroleum resin (coumaron), ester gum, holiftadiene, epoxy-modified polybutadiene, low molecular weight aliphatic epoxy resin, low molecular weight aliphatic bisphenol type, which has good compatibility with the modified polyolefin resin. Examples include epoxy resin, polyoxytetramethylene glycol, vinyl acetate-modified polyethylene, etc., and the blending amount thereof is preferably 91 to 50 parts by weight per 100 parts by weight of the above-mentioned modified polyolefin resin.

In order to improve the coating properties of the barrier coat, it is preferable to dissolve or disperse the above components in an organic solvent. Examples of organic solvents include aromatic hydrocarbons such as benzene, toluene, and xylene, hexane, heptane, Examples include aliphatic hydrocarbons such as octane and decane, and chlorinated hydrocarbons such as trichloroethylene, perfluoroethylene, dichloroethylene, dichloroethane, and dichlorobenzene.

In the present invention, regarding the formed coating film of the barrier coat,
Although it is essential that the static glass transition temperature is within the above range, it is further preferable that the tensile strength elongation at break of the coating film is 200 to 1000% in an atmosphere of 120°C. In addition, the static glass transition temperature of the formed coating film is -30
If the temperature rises, the above object of the present invention cannot be achieved, and -
If the temperature is higher than 60°C, the performance of the coating film, especially water resistance, adhesion, etc. will deteriorate, which is not preferable. Furthermore, extender pigments, coloring pigments (excluding anticorrosion pigments), etc. may be added to the barrier coat. The blending amount of these pigments is #) 10 to 100 parts by weight of modified polyolefin resin.
100 parts by weight is preferred.0 In the present invention, these barrier coats are applied to the surface of the cationic electrodeposition coating, but the coating method is not established, such as spray coating, brush coating, dipping coating, melt coating, When electrostatic coating is used, the coating film thickness is preferably 1 to 20 μm, particularly I/c 5 to 10 μm, depending on the coating film formed.

Note that the static glass transition temperature of the formed coating film of the barrier coat used in the present invention was determined using a differential scanning calorimeter (Daini Seikokin DS).
The tensile strength elongation at break is the value measured using a universal tensile tester with a constant temperature bath (Takasu Seisakusho Autograph 8-D model). The strength was 20 wux and the tensile speed was 20 u/min. The samples used for these measurements were coated on a tin plate to a thickness of 25 μm with the barrier coat added to the formed coating film.
After baking at 120° C. for 30 minutes, it was isolated by the mercury amalgam method and used.

Before applying the intermediate coat paint to the barrier coat coating surface, it is preferable to bake the barrier coat in advance, but it is also possible to apply the intermediate coat paint wet-on-wet without baking. Baking temperature is 80
~160°C, especially 80-130°C are suitable.

Intermediate paint: As a paint to be applied to the above-mentioned barrier coated surface, any known intermediate paint with excellent adhesion, smoothness, sharpness, overbake resistance, weather resistance, etc. can be used. Specifically, organic solution-type thermosetting intermediate coating paints whose vehicle main components are short oils with an oil length of 30 inches or less, ultra-short alkyd resins, or oil-free polyester resins and amino resins are mentioned. These alkyd resins and polyester resins preferably have a hydroxyl value of 60 to 1401, an acid value of 5 to 201, and use unsaturated oil (or unsaturated fatty acid) as the modified oil.
Suitable are alkyl (carbon number 1 to 5) etherified melamine resins, urea resins, benzoguanamine resins, and the like.

These compounding ratios are based on the solid content weight of alkyd resin and/or oil-free polyester resin.
85%, especially 70-80%, amine resin 35-15%,
In particular, it is preferably 30 to 20 inches. Furthermore, the above amine resin can be replaced with a polyisocyanate compound or a blocked polyinsyanate compound. Also,
The form of the core paint is organic soluble.

Molding is most preferable, but non-aqueous dispersion, vine lid type, aqueous solution type, aqueous dispersion type, etc. using the above vehicle components may also be used. In the present invention, the hardness (pencil hardness) of the intermediate coating film is preferably in the range of 3B to 2H. Furthermore, the intermediate coating paint includes extender pigments, colored pigments,
Additives for other paints.

Machining etc. can be added as necessary.

In the present invention, the intermediate coating paint can be applied to the barrier coat surface in the same manner as the barrier coat, and the coating thickness is preferably in the range of 10 to 50 μm even for a cured coating film. The curing temperature varies depending on the vehicle components, and when curing by heating, it is 80 to 170°C, especially 1
It is preferable to heat at a temperature of 20 to 150°C.

Top coat paint: A paint that is applied to the intermediate coat surface, and in the present invention, it is a paint that has excellent finished appearance (sharpness, smoothness, gloss, etc.), weather resistance (glossy retention, yoking resistance, etc.), etc. Moreover, a well-known coating material is used which forms an ultra-hard coating film with good scratch resistance.

In the present invention, an ultra-hard coating film refers to a cured coating film that has a hardness of 4H to 9H (20
℃) coating film. This test method involves holding a test plate that has been painted and hardened on a glass plate at 20°C, and using a sharpened pencil (Mitsubishi drafting pencil “Uni#”) with a flattened tip and a sharp angle at a 45 degree angle. The hardness of the pencil was evaluated by holding it and moving it about 1 cm (3 seconds/ctrt) while pressing it against the coated surface, which is strong enough not to break, and to determine the hardness of the hardest pencil that leaves no trace of scratches caused by lead!IK.

The top coating that can be used in the present invention is not particularly limited as long as it has the hardness and performance described above, and examples include amine, acrylic resin, amine/alkyd resin, amino/polyester resin, amino/fluorine resin, etc. Crosslinking using vehicle components such as resins, amine/silicone polyester resins, unsaturated polyester resins, incyanate/acrylic resins, incyanate polyester resins, incyanate/7-component resins, unsaturated acrylic resins, etc. Examples include hardening paints. The form of these paints is not particularly limited, and may be an organic solution type, a non-aqueous dispersion type, or a water-soluble (
Can be used in dispersion molding, powder molding, high solid molding, etc. The coating film is formed by drying at room temperature, heating, or irradiation with active energy rays such as electron beams or ultraviolet rays.

The top coat paints used in the present invention are classified into enamel paints, which are mixtures of metallic pigments or colored pigments into paints using the above-mentioned vehicle main component, and clear paints, which contain no or almost no such pigments. Then, as a method of forming a top coat film using these paints, for example, (1) Applying a metallic paint containing a metallic pigment, if necessary a colored pigment, or a solid color paint containing a colored pigment; Heat cure 5 (metallic or solid color finish using 1 coat 1 beta method).

■ After applying metallic paint or solid color paint and heating and curing, apply clear paint and heat and harden again (metallic or solid color finish using 2-coat, 2-bake method).

■ After applying a metallic paint or a solid color paint, and subsequently applying a clear paint, both coatings are cured simultaneously by heating (metallic or solid color finish using the 2-coat 1-beta method).

These top coat paints are preferably applied by spray painting, electrostatic painting, etc. In addition, the coating film thickness is based on the dry coating film, 25 to 40μ for the above ■, and for the above ■ and ■,
10-3 for metallic paints and solid color paints
0μ and clear paint preferably have a thickness of 25 to 50μ. Heating conditions can be arbitrarily selected depending on the vehicle components, but preferably 80C to 170C, particularly 120 to 150C for 10 to 40 minutes.

As described above, the properties of the coating film formed on the steel plate by cationic electrodeposition coating, barrier coating coating, intermediate coating coating, and top coating coating are determined by the finished appearance (e.g., smoothness, gloss, etc.).
It has good properties such as sharpness, water resistance, weather resistance, etc., but it has particularly improved scratch resistance, chipping resistance, corrosion resistance of the end face, and physical properties.

Next, examples and comparative examples related to the present invention will be described.

0 ■ Sample (1) Steel plate: A steel plate (size 300 mm) in which the angle between the end face and the flat part is 90 degrees, chemically treated with Bonderite φ3030 (manufactured by Nihon Parkerizing Co., Ltd., zinc phosphate type).
(2) Cationic electrodeposition paint: It had the composition shown in Table 1 below and was manufactured according to a conventional method. However, 1st (3) Barrier coat (A): Organic liquid of resin made by graft polymerizing maleic acid to propylene-ethylene copolymer (static glass transition temperature -43℃, tensile strength elongation at -20℃) rate 410%).

(B) 2 An organic liquid of a resin (static glass transition temperature -52°C, tensile strength elongation at break at -20°C 700%) in which 10 parts by weight of Silozin is mixed per 100 parts by weight of the resin of (A).

(C): Organic liquid of resin made by graft polymerizing maleic acid to propylene-ethylene cot(T&) (static glass transition temperature +5°C) 0 (4) Intermediate coating: Amilac N-27-Lar (Kansai Paint Co., Ltd. (manufactured by Kansai Paint Co., Ltd., amino polyester resin intermediate coating) (5) Super hard top coating (A): Magikron Black (manufactured by Kansai Paint Co., Ltd., amino acrylic resin upper MB paint, pencil hardness 5H) (B): Magi Cron Clear H (Kansai Paint Co., Ltd.)
(C): Radicure P Black (manufactured by Kansai Paint Co., Ltd.): Aminoacrylic resin top coat paint, 2-coat 1-bake clear paint, pencil hardness 5H) (C): Radicure P Black (manufactured by Kansai Paint Co., Ltd.); Pencil hardness 6H
) Examples and Comparative Examples Using the above samples, steel plates were coated with cationic electrodeposition paint, barrier coat, intermediate coat paint, and ultra-hard top coat paint as shown in Table 2.

In Table 2, (1) All film thicknesses should be the same as the cured coating.

(2) In Example 2, the intermediate coat was applied without baking the barrier coat, and the top coat of Example 3.5.7.8 and Comparative Example 3.4.6 was applied in two coats. This was based on the 1-bake method, and after applying the top coat paint (A), it was left to stand at room temperature for 15 minutes, and then the top coat paint (B) was applied.

(3) The top coat films in Example 4.6 and Comparative Example 2 were cured by irradiating with an electron beam of 6 Mrad at 270 Key.

Performance Test Results A coating film performance test was conducted using the coated plates coated in the above examples and comparative examples.The results are shown in Table FA3.

The test method for the coating film in Table 3 is as follows.

(*l) Chipping resistance: (1) Test equipment: Flying stone tester JA-400 type (gravelometer) (Suga Test Instruments Co., Ltd. Shakyu product) (2)
Stones to be sprayed: Crushed stones with a diameter of approximately 15 to 20rr (3 tons) Capacity of stones to be sprayed: approximately 59Qm (4) Air pressure for spraying approximately 4 to 9 cm (5) Temperature during test: approximately 20 ℃ The test piece was placed on a test piece holding stand, and crushed stone particles of about 5 QQal were sprayed onto the test piece using air pressure of about 4 to 9/an, and then the coated surface condition and salt spray resistance were evaluated. The condition of the painted surface was visually observed and evaluated using the following criteria.

O (Good): Only a few scratches due to impact were observed on a part of the top coat, and no peeling of the electrodeposited film was observed.

Δ (slightly poor): Many scratches due to impact were observed on the top MB and intermediate coat, and peeling of the electrodeposited film was also observed here and there.

× (Poor): Most of the top coat and intermediate coat have peeled off, and the electrodeposited film on the impact area including the impact area and its surrounding area has peeled off.

The salt spray resistance of the test piece was rated 24 according to JIS Z2371.
A salt spray test was conducted for 0 hours, and the presence or absence of rust from the impact area and the state of corrosion were observed.

(*2) Impact resistance: Tested in accordance with JIS K5400-1979 6.113B method in an atmosphere at 0°C.

A weight weighing 500 f was dropped from a height of 50 cWL to examine damage to the paint film.

(*3) Adhesion: A coated MK goblin was made according to JIS K5400-1979 6.15, adhesive cellophane tape was attached to the surface, and the coated surface was evaluated after being quickly peeled off.

(*4) Water resistance: The coated surface was evaluated after being immersed in water at 40° C. for 1 day.

(*5) Smoothness: Visual judgment (*6) Edge corrosion resistance: A salt spray test was carried out for 480 hours in the same manner as in (*1) above, and the condition of the coated surface at the edge of the steel plate was visually judged.

(*7) Thread rust resistance: Cut the coating film in two diagonal lines with a cutter to reach the substrate, place it in a salt spray tester according to JIS Z2371 for 48 hours, and then clean the coating surface with deionized water. After cleaning, place the thermostatic toilet box (temperature 40±2℃, humidity 85±2℃)
%), the occurrence of yarn rust after being placed in the yarn for 480 hours was investigated.

Record the average length and density of thread rust, and check if there are 2 to 3 or less thread rusts with a length of 10 or more within 101 cm of the length of the cut part.F.

5-6 inside and outside M 10 or more D Evaluate using as a guideline.

(*8) Scapping resistance: Soaked in warm water at 40℃ for 120 hours, then soaked at 20℃ for 4 hours.
After drying for an hour, use No. 6 crushed stone 3001 at 4 kli/c.
The coated plates that had been chipped (also straight-cut) with ``RRT'' were subjected to 3 times a week (soaked in 5-chip saline solution (30'C) for 2 hours → left at -20℃ for 1 hour → exposed outdoors for 45 hours). This was treated as one cycle, and the condition of the painted surface (particularly rust, blisters, etc.) after 1 o take was investigated.

(*9) Scratch resistance: At 20°C, layer four sheets of cheesecloth on the coated surface of a test plate held horizontally, and place a 1 to 9 weight scale (for top-pan balances with a flat bottom) on top of them. 5 crrt in diameter), hold the edge of the cheesecloth, and roll it over the test piece at a speed of 2ocR/sec for 20 minutes.
After reciprocating, the condition of the painted surface was evaluated. Almost no Qtd scratches are observed.

Δ has slightly more scratches. × Many scratches can be seen.

(*10) Pencil hardness: The hardest pencil hardness at 20°C that leaves no trace of the pencil field.

Claims (1)

[Claims] In a painting process in which a cationic electrodeposition paint, an intermediate coating paint, and a topcoat paint are sequentially applied to a steel plate, (1) a cationic electrodeposition paint containing a pigment at a high concentration is used as the cationic electrodeposition paint; ( 2) Prior to applying the intermediate coating paint, the static glass transition temperature of the formed coating film is -3 on the surface of the cationic electrodeposition coating film.
A steel plate characterized in that a barrier coat mainly composed of a modified polyolefin resin having a temperature of 0 to -60°C is applied in advance, and (3) a paint that forms an ultra-hard film is used as the top coat. Painting method.