JPH01288374A - Film forming method - Google Patents

Abstract

PURPOSE:To improve chipping resistance, corrosion resistance and physical characteristics by forming a composite film consisting of an electrodeposition film and a top coat film on the surface to be coated on a metal while forming a film having a specific composition between the films of said composite film. CONSTITUTION:A composition based on an isocyanate modified polybutadiene resin A having two or more free isocyanate groups on an average in one molecule thereof and a number average MW of 500-20,000 and a polyol component B having two or more hydroxy groups on an average in one molecule thereof and a number average MW of 150-50,000 is prepared. An electrodeposition film and a top coat film or the electrodeposition film, an intermediate coat film and the top coat film are formed on the surface to be coated of a metal and the aforementioned composition is applied to at least one of the gaps between the film layers of the composite film to form a film having static glass transition temp. of -100-0 deg.C.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for forming a coating film with excellent chipping resistance, corrosion resistance and physical performance on metal products, particularly automobile body skins and parts thereof.

In painting the outer panels of automobile bodies, attention is being paid to the problem of impact peeling and the accompanying corrosion of raw metals. For example, in cold regions of Europe and the United States, to prevent road surfaces from freezing, gravel is often laid with a large amount of relatively coarsely crushed rock salt set on fire. Rock salt particles and pebbles thrown up by the wheels collide with the paint surface, and the impact often causes the paint film to peel off locally from the car body, a so-called "chipping peeling" phenomenon. As a result of this phenomenon, the metal surface of the impact area on the outer surface of the vehicle body is exposed, rusting occurs, and corrosion progresses. Usually, paint film peeling due to chipping peels off often on the bottom of the car body and around the suspension, 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 peeling and the progression of corrosion caused by it, various studies have been carried out on chemical conversion treatments on the surface of the metal base of car body outer panels, as well as electrodeposition paints, intermediate coats, and top coats. No solution has been found yet.

Therefore, in order to improve the above-mentioned problems, the present inventors
To provide a method for forming a coating film with excellent chipping resistance, physical properties, and corrosion resistance without deteriorating the finished appearance in a coating system for automobile body exterior panels consisting of electrodeposition paint, intermediate coating paint, and top coating paint. As a result of extensive research into this objective, we have now completed the present invention.

Thus, according to the present invention, in a multilayer coating film consisting of an electrodeposition coating film and a topcoat coating film, or an electrodeposition coating film, an intermediate coating film and a topcoat coating film, these multilayer coating films are formed on a metal surface to be coated. (A) an isocyanate-modified polybutadiene resin containing an average of two or more free isocyanate groups in one molecule and having a number average molecular weight of 500 to 20,000, and (B) an isocyanate-modified polybutadiene resin in at least one layer between the layers. A coating film with a static glass transition temperature of 1100 to 0°C is formed using a composition whose main component is a polyol component containing an average of two or more hydroxyl groups and a number average molecular weight of 150 to 50,000. Provided is a coating film forming method characterized by:

A feature of the present invention is the step of sequentially applying an electrodeposition paint, an intermediate coating (this intermediate coating may be omitted), and a top coating to a metal product.
In addition, a coating film (hereinafter referred to as "composition for low Tg") consisting of a composition having the above-mentioned specific composition and physical properties (hereinafter referred to as "composition for low Tg")
It forms a low Tg coating film. As a result, it is possible to form a coating film with significantly superior chipping resistance, corrosion resistance, and physical performance. That is, the glass transition temperature (Tg) formed using a composition mainly consisting of the above two components (A) and (B) is -100 to 0°C.
The low Tg coating film adjusted to the range of When formed between any or all of the layers of the paint film, intermediate coat film, and/or top coat film, even if a strong impact force from rock salt, pebbles, etc. is applied, most or all of the impact energy is absorbed by the low Tg paint film. It is absorbed into the film, eliminating problems such as chipping and peeling. In other words, the low Tg coating film exhibits a buffering effect on external impact force and a repulsion effect due to the cohesive force of the coating film, significantly improving chipping resistance and preventing rusting and corrosion of metal products due to chipping and peeling. , and rock salt,
It also eliminates the deterioration of the top coat due to collisions with pebbles, etc.

The coating method of the present invention will be specifically explained below.

Product having a metal surface to be coated: Any material is not subject to any restrictions as long as it is a material to be coated on which a coating film is formed by the method of the present invention and has a metal surface that can be coated by electrodeposition. Examples include iron, copper, aluminum, tin, zinc, alloys containing these metals, and plated or vapor-deposited products of these metals and alloys, and specifically, passenger cars, buses, and trucks made using these metals. , the outer panels of motorcycles, etc. Further, it is preferable that the metal coating surface of the product is previously subjected to a chemical conversion treatment with a phosphate or chromate.

Electrodeposition paint: This is to be applied to the metal surface of the product, and any known cationic or anionic electrodeposition paint can be used.

First, cationic electrodeposition paints are based on resins with basic amine groups or onium salt resins, and are neutralized with acids.
This includes a cathode-deposited thermosetting electrodeposition coating material that is made water-soluble (water-dispersed), and is applied using the object to be coated as a cathode.

On the other hand, anionic electrodeposition paints are anodic electrodeposition paints that are mainly based on resins with carboxyl groups and are neutralized with basic compounds to make them water-soluble (moisture oxidation). Painted as an anode.

Low Tg composition: A coating composition for forming a low Tg coating film, which has the following components: (A) contains an average of 2 or more free isocyanate groups per molecule and has a number average molecular weight; isocyanate-modified polybutadiene resin having a molecular weight of 500 to 20,000, and (B) a polyol component containing an average of two or more hydroxyl groups per molecule and a number average molecular weight of 150 to 50,000, and having a static Glass transition temperature is 1100~0
It is a composition capable of forming a coating film at a temperature of °C.

(A) contains an average of 2 or more free isocyanate groups per molecule and has a number average molecular weight of 500 to 2
0,000 isocyanate-modified polybutadiene resin (
(hereinafter abbreviated as rNco-PBJ) includes, for example, a resin obtained by reacting a polybutadiene resin containing a hydroxyl group with a polyisocyanate compound.

Polybutadiene resin having hydroxyl groups is obtained by introducing hydroxyl groups into a base polymer mainly composed of butadiene, and the butadiene units in the base polymer are cis-1,4 type, trans-1,4 type, and 1.2 type. 7011 may be any of the cis-1,4 type and trans-1,4 type.
It is preferable to use a polybutadiene resin containing at least 80% by weight, particularly at least 80% by weight, in order to improve chipping resistance. Such polybutadiene resins include not only homopolymers of butadiene but also copolymers of butadiene and other monomers, and other monomers include ethylene, fluoropylene, pentadiene, cyclopentadiene, norbornene, etc. The monomer content is generally 4% based on the weight of the copolymer.
It is preferably 0% by weight or less. The polybutadiene resin has a number average molecular weight of 200 to 19,000, particularly 1
Preferably, it is within the range of 200 to 5000. Furthermore, the introduction of hydroxyl groups into the polybutadiene base can be carried out, for example, by reacting the polybutadiene base resin with ethylene oxide.

NGO-PB is obtained by reacting the hydroxyl groups of the above polybutadiene resin with a polyisocyanate compound so that each molecule has an average of two or more free isocyanate groups. Specifically, (i) a polybutadiene resin with an equimolar amount of a polyisocyanate compound added to the hydroxyl groups, and (j) a polybutadiene resin that is made to have a high molecular weight via a polyisocyanate compound. Examples include those in which an equimolar amount of a polyisocyanate compound is added to a hydroxyl group, and in any of these cases, it is important that each molecule contains an average of two or more free isocyanate groups.

A polyisocyanate compound is a compound having 2ffl1 or more isocyanate groups in one molecule, such as tolylene diisocyanate, diphenylmethane diisocyanate, and a reaction product of 3 moles of these diisocyanate compounds and 1 mole of a trihydric alcohol such as trimethylolpropane. , polymers (multimers) of these diisocyanate compounds, aromatic polyisocyanate compounds such as reaction products consisting of 3 mol of tolylene diisocyanate and 2 mol of hexamethylene diisocyanate: hexamethylene diisocyanate, lysine diisocyanate, trimethyl hexa Aliphatic polyisocyanate compounds such as methylene diisocyanate, dimer acid diisocyanate, reaction products of hexamethylene diisocyanate and trimethylolpropane, and reaction products of hexamethylene diisocyanate and water: 4,4'-methylenebis(cyclohexyl isocyanate), methylcyclohexane -2.4(
Alternatively, alicyclic polyisocyanate compounds such as 2,6)-diisocyanate, 1,3-(isocyanatomethyl)cyclohexane, and inphorone diisocyanate can be mentioned. In the present invention, one or more selected from these are used, and among these, diphenylmethane diisocyanate, or its 2-3f1 form,
Preferred are inphorone diisocyanate, tolylene diisocyanate, and the like.

NGO-PB has an average of 2 or more isocyanate groups, preferably 3 to 4 isocyanate groups in one molecule, and has a number average molecular weight of 5.
00-20,000, preferably 1,000-io,o
oo, more preferably within the range of 1500 to 6000; if the number of free isocyanate groups is less than 2, curability, water resistance, etc. will decrease, and if the number average molecular weight is less than 500, the coating film will deteriorate. is undesirable because it exhibits tackiness, and on the other hand, if it exceeds 20,000, the smoothness of the coating film decreases.

Component (B), a polyol component containing an average of two or more hydroxyl groups per molecule and having a number average molecular weight of 150 to 50°000, undergoes a three-dimensional crosslinking and curing reaction with component (A). This includes both linear types and branched types.

A linear type polyol component is a polyol component whose main skeleton is linear and has two or more hydroxyl groups at its terminal and/or pendant side chain. Ester reaction products with alcohols; ■Polyether polyols (e.g., polyethylene glycol, polybutylene glycol, polypropylene glycol, etc.), ■εε-caprolactone ring-opening polymers; ■Bisphenol A or its combination with polyhydric alcohols (e.g., ethylene glycol, (butylene glycol, etc.), ■ - General % formula % [In the formula, R is a hydrocarbon group, and n is a number in a range compatible with the above molecular weight]; ■ Polyalkylene polycarbonate represented by the formula %; ■ hydroxyl group Contains polybutadiene (fat;■
Tricyclodecane dimetatool; ■Hydroxypivalic acid neopentyl glycol ester; ■2.2.4-
Examples include trimethyl-1,3-pentadiol.

A branched type polyol component is a polyol component whose main skeleton is branched and has two or more hydroxyl groups at the terminal and/or pendant side chain of the skeleton, and specifically, it is a polyol component having a branched main skeleton and two or more hydroxyl groups at the end of the skeleton and/or a pendant side chain. Polyoxypolyol obtained by ring-opening addition reaction of caprolactone to (trimethylolpropane, pentaerythritol, etc.); ■ Branched polyester polyol obtained using dibasic acid and polyhydric alcohol of dihydric and trihydric or higher (generally within the range of 10 to 200).

These polyol components [component (B)] have two or more hydroxyl groups, preferably 2 to 4, in one molecule, and have a number average molecular weight of 150 to 50,000, preferably 200 to
5,000, more preferably within the range of 200 to 1,000; if the number of hydroxyl groups is less than 2, the crosslinking curability of the coating film will be insufficient, and if the number average molecular weight is less than 150, the pigment dispersion stability will deteriorate, NC0-PB If it exceeds 50,000, on the other hand, the smoothness will deteriorate, which is not preferable.

As the polyol component, one or more types selected from the above-mentioned examples can be used, but among them, linear type ones are preferable, and in particular, ester reaction products (especially those containing cyclohexane dimetatool) , ring-opened polymers of ε-caprolactone (particularly 3-1O-mers), bisphenol A.

It is preferable to use polyalkylene polycarbonate or the like.

The ratio of component (A) to component (B) is 0.00 molar ratio of hydroxyl group in component (A)/isocyanate group in component (B).
It is preferably in the range of 2 to 5.0, particularly 0.5 to 2.0.

The composition for low Tg has the above-mentioned components (A) and (B) as main components, but can further contain organic solvents, pigments, plasticizers, additives, etc. as necessary.

The organic solvent is preferably one that does not have active hydrogen that reacts with the isocyanate group of component (A), such as benzene, toluene, xylene, and Swazol #1000, Swazol #1500, and Swazol #, which have higher Buddha points. Aromatic hydrocarbons such as 1800 (all Maruzen Oil products, trade names); aliphatic hydrocarbons such as hexane, heptane, octane, and decane; ditrichlorethylene, perchloroethylene, dichloroethylene, dichloroethane, dichlorobenzene For example, chlorinated hydrocarbons such as esters, ketones, ethers, and other organic solvents can also be used. Pigments include coloring pigments (e.g., titanium white, carbon black, etc.), extender pigments (e.g., asbestos, talc, clay, barida, silica, etc.), and rust preventive pigments (e.g., zinc powder, lead zinc oxide, lead chromate). , zinc chromate, etc.), conductive powders (carbon black, etc.), metallic pigments, etc., and examples of the plasticizer include dioctyl phthalate, triglycidyl phosphate, and butyl sebacate.

Furthermore, additives such as anti-sagging agents (for example, aluminum stearate, silica, basic calcium sulfonate, etc.), curing accelerators (for example, dibutyltin oxide, dibutyltin laurate, etc.), and ultraviolet absorbers can also be blended.

The glass transition temperature (Tg) of the cured coating film formed by the composition is -1OO to 0°C1, preferably =80 to -
It is important that the temperature be within the range of 30°C, particularly preferably -80 to -50°C.

When the glass transition temperature of the low Tg coating film is adjusted within the above range, the adhesion between the coating film and other adjacent coating films is excellent, and the coating formed on the surface of the coating film is The technical effect is that the impact resistance (including chipping resistance) of the film is significantly improved.

If the glass transition temperature of the coating film is higher than 0°C, the coating shrinks significantly when it cools from the curing temperature (usually 60 to 150°C) to room temperature, and it also lacks flexibility, making it difficult to improve chipping resistance. becomes.

On the other hand, if the glass transition temperature is lower than 1100° C., the water vapor permeability of the coating film increases, resulting in a decrease in adhesion and water resistance, which is not preferable.

The glass transition temperature of the cured coating film formed by the low Tg composition (low Tg coating film) was measured using a differential scanning calorimeter (DSC).
), and the composition was applied onto a glass plate with a doctor blade or the like, heated at 80°C for 30 minutes to cure, and then the coating was peeled off and samples were prepared by collecting lO~20B. , was carried out.

For low Tg compositions, it is preferable to separate components (A) and (B) in advance and mix the same components immediately before use (painting) (within 8 hours, preferably within 4 hours). Organic solvents, pigments, plasticizers, additives, etc. to be used depending on the conditions can be blended in advance with either or both of the components (A) and (B).

The low Tg composition prepared by mixing the above-mentioned components is applied to the above-mentioned product having a metal surface by a conventional coating method such as spray coating, electrostatic coating, dip coating, or brush coating. can do. Although the coating film thickness can be arbitrarily selected depending on the purpose, it is generally preferred that the cured coating film has a thickness of 1 to 50 μm, particularly 10 to 20 μm.

Furthermore, although the low Tg coating film can be cured at room temperature, it is also possible to cure it by heating.
It is usually preferred to cure at a temperature of about 50 to about 200°C, particularly 60 to 150°C.

Intermediate paint: This is applied to the electrodeposited surface or low Tg surface as needed, and is a well-known medium with excellent adhesion, smoothness, sharpness, overbake resistance, weather resistance, etc. Paint can be used. Specifically, oil length 30
% or less of kerosene, an ultra-short pre-alkyd resin, or an oil-free polyester resin, and an amino resin as vehicle main components.

These alkyd resins and polyester resins preferably have a hydroxyl value of 60 to 140 and an acid value of 300 or less, and use unsaturated oil or unsaturated fatty acid as the modified oil. Number 1 to 5) etherified melamine resin,
Urea resin, benzoguanamine resin, etc. are suitable. The blending ratio of these two resins is based on the solid content weight, and the alkyd resin and/or oil-free polyester resin 65%
~85%, especially 70-80%, and amino resin 35-1
It is preferably 5%, especially 30-20%. Furthermore, at least a portion of the amino tree can be replaced with a polyisocyanate compound or a blocked polyisocyanate compound.

Further, the form of the intermediate coating is most preferably an organic solution type, but it may also be a non-aqueous dispersion type, a high solid type, an aqueous solution type, an aqueous dispersion type, a powder type, etc. using the above vehicle component. I can't help it. In the present invention, the intermediate coating film hardness (pencil hardness) is generally preferably in the range of 3B or higher.

Furthermore, extender pigments, coloring pigments, other paint additives, and the like can be added to the intermediate paint as necessary.

Topcoat 2 A paint that is applied to the electrodeposited surface or intermediate coated surface described above.Specifically, it has properties such as finished appearance (sharpness, smoothness, gloss, etc.), weather resistance (glossy retention, color retention, etc.) Any known plastic or metal paint that forms a coating film with excellent chemical resistance, water resistance, moisture resistance, hardening properties, etc.) (such as chalk resistance), chemical resistance, water resistance, moisture resistance, hardening properties, etc. can be used, such as amino-acrylic resin, Examples include paints containing amino-alkyd resins, amino-polyester resins, and the like as vehicle-forming components. The form of these paints is not particularly limited, and any form such as organic solution type, non-aqueous dispersion type, aqueous (dispersion) molding, powder type, and high solid type can be used. Drying or curing of the coating film is carried out by drying at room temperature, heating drying, irradiation with active energy rays, etc.

The top coat used in the present invention may be either an enamel paint in which metallic pigments and/or colored pigments are blended into a paint whose main component is the vehicle described above, or a clear paint that does not contain any or almost any of these pigments. Examples of methods for forming a topcoat film using these paints include the following methods:
■Metallic pigment, metallic paint made by blending coloring pigments as necessary, or solid color paint blended by coloring pigments is applied and heated to cure (metallic or solid color by 1 coat 1 bake method) finishing).

■Paint with metallic paint or solid color paint,
After being heated and cured, a clear coating is applied and then heated and cured again (metallic or solid color finish using 2-coat, 2-bake method).

■Paint with metallic paint or solid color paint,
A method in which a clear paint is applied together and then heated to cure both coatings at the same time (metallic or solid color finish using the 2-coat I-beta method).

The coating film forming method according to the present invention can be carried out using the above-described coating material, for example, as described below.

First, an electrodeposition paint is applied to a product having a metal surface to be coated. The thickness of the coating film after curing is preferably in the range of 10 to 40 microns, and the coating is not particularly limited and can be carried out using a normal electrodeposition coating method. It is preferable that the electrodeposited coating film is cured by heating prior to the next coating process, and the suitable heating temperature is usually about 120 to 180°C.

Next, a low Tg composition or intermediate coating is applied to the electrodeposited surface. There are no particular restrictions on the coating method of the composition for low Tg.
The coating can be carried out by airless spraying, air spraying, electrostatic coating, dip coating, etc., and the film thickness is suitably in the range of 1 to 50 microns, particularly 10 to 25 microns, based on the cured coating. The coating film cures at room temperature, but at 50-200°C, especially at 60-1
It is preferable to heat and cure at 30°C.

On the other hand, the intermediate coating can be applied in the same manner as for the low Tg composition, and the coating film thickness is preferably in the range of lO to 50μ, preferably 20 to 40μ, based on the cured coating. Preferably, the curing temperature of the coating varies depending on the vehicle components, but is, for example, about 60 to about 140°C, particularly 80°C.
Preferably, a temperature in the range from 120°C to 120°C is used.

In a system in which a low Tg composition is applied to an electric blue coating surface, an intermediate coating or a top coating is applied to the low Tg coating surface. The intermediate coating is applied in the same manner as described above, and the top coating is preferably applied by spray coating, electrostatic coating, or the like.

In addition, the coating film thickness is based on the dry coating film, in the case of (1) above, the range is 25 to 40μ, and in the cases of
The range of .mu. is preferably 25 to 50 .mu. for the clear paint. The temperature for heating and curing can be arbitrarily selected depending on the vehicle components, but for example, the temperature is about 60 to about 160℃.
It is preferable to heat at 80 to 140°C for 10 to 40 minutes.

In the system that has undergone the steps of electrodeposition coating - coating of low Tg composition - intermediate coating, a top coat is applied to the intermediate coated surface, or
Alternatively, it is also possible to apply the low Tg composition again on the intermediate coated surface and then apply the top coat. These coatings can be performed as described above.

Further, in a system that has undergone the electrodeposition coating-intermediate coating process, a low Tg composition is applied to the intermediate coating surface, and then a top coating is applied. These coatings can also be performed in the same manner as described above.

The above coating process in the coating film forming method of the present invention is summarized as follows.

(1) Metal products - Electrodeposition coating - Coating of composition for low Tg - Top coat, (2) Metal products - Electrodeposition coating - Coating of composition for low Tg - Intermediate coating - Top coat, (3) Metal Products - Electrodeposition coating - Coating of composition for low Tg - Intermediate coating - Coating of composition for low Tg - Top coat, (4) Metal products - Electrodeposition coating - Intermediate coating - Coating of composition for low Tg - Top coat.

The performance of the multilayer coating film formed according to the method of the present invention is excellent in finished appearance (e.g., smoothness, gloss, sharpness, etc.), water resistance, weather resistance, etc., as well as chipping resistance, corrosion resistance, and physical properties. It is characterized by significantly improved physical properties.

Next, the present invention will be explained in more detail with reference to Examples and Comparative Examples.

■Preparation of sample (1) Metal member: Steel plate (size 300 x 90 x 0.8 mm) chemically treated with Bonderite #3030 (manufactured by Nippon Barker Rising Co., Ltd., a zinc phosphate metal surface treatment agent).

(2) Electrodeposition paint: Cationic electrodeposition paint (P) (Kansai Paint Frame, epoxy polyamide type, trade name "Nileclone 9200").

(3) Low Tg composition: It was manufactured by mixing the components shown in Table 1 below.

In Table 1, (A-1) NGO- containing an average of 3.2 isocyanate groups per molecule and a number average molecular weight of about 4500;
PB (manufactured by Idemitsu Petrochemical ■, product name: Unimax P)
, the content of type 1.4 in polybutadiene was 80%, and 4.4'-diphenylmethane diisocyanate was used as the polyisocyanate).

(A-2) 3 moles of polybutadiene having hydroxyl groups at both ends (manufactured by Idemitsu Oil Company, trade name, LM-10, 1°4 type content 88% by weight, number average molecular weight 1400) and biuret in hexamethylene A product obtained by adding 3 moles of inphorone diisocyanate to a reaction product with 1 mole of diisocyanate adduct type triisocyanate (manufactured by Asahi Glass ■, trade name, "Duranate 24-A"). It has an average of 3 isocyanate groups in one molecule, and the number average molecular weight is 5450° (B, -1).
-2) K-FLEX 88 (manufactured by King Industries, trade name, polyester containing cyclohexane dimetatool, number average molecular weight 435, hydroxyl value 250 mgK OHI3).

Table 1 (*l) Free isocyanate groups in component (A) and (B
) Equivalence ratio with the hydroxyl group in the component.

(*2) Measured by the above method on a coating film cured by heating at 0°C for 30 minutes.

(4) Intermediate paint: The intermediate paint (S) is Amirac N-2 Sealer (manufactured by Kansai Paint Hakuba, amino polyester resin intermediate paint)
.

(5) Top coat: T-1 Amyluck White (manufactured by Kansai Paint ■, amino alkyd resin top coat paint, ■ color paint for coat 1 bake, pencil hardness H (20°C)).

T-2: Magikron Silver (manufactured by Kansai Paint ■, aminoacrylic resin topcoat paint, 2-coat silver metallic paint for baking, pencil hardness H (20'O)).

T-3 = Magikron Clear (manufactured by Kansai Paint ■, aminoacrylic resin top coat, 2-coat clear paint for baking, pencil hardness H (20°O)).

■, Examples, Comparative Examples Using the various paints prepared as described above, the following 2.
Coating was performed according to the coating process shown in the table, and a coating film performance test was conducted on each coated plate obtained. The results of these coating processes and performance tests are shown in Table 2.

In Table 2, the electrodeposition paint was applied to a thickness of 15 μm based on the cured film and heated at 170° C. for 30 minutes.

The low Tg composition was applied to a cured film thickness of 14 to 20 microns, and cured by heating at 100°C for 20 minutes.

Apply the intermediate paint so that the cured film thickness is 30μ, and
It was heated at 40° C. for 20 minutes (this step may be omitted). Then, as a top coat, T-1 was applied to a thickness of 35μ based on the cured film, heated at 140°C for 30 minutes, and T-2 was applied to a thickness of 15μ based on the cured film, and air-dried. Thereafter, T-3 was coated with a cured coating film to a thickness of 30 μm, and then heated at 140° C. for 30 minutes to cure both T-2 and T-3 coatings.

The performance test method in Table 2 is as follows.

(Book 1) Chipping resistance: ■Test equipment: Q-G-R Veragrometer (product of Q Panel Company) ■Stone that can be sprayed: Crushed stone with a diameter of approximately 15-20m/m ■Capacity of stone that can be sprayed: Approximately 500 - ■ Spraying with air pressure of approximately 4 kg/cm'■ Temperature during test: approximately 20°C Mount the test specimen on a specimen holder, and spraying approximately 4 kg/cm'' with air pressure of approximately 500 mβ. After the crushed stone was fired onto the test piece, the condition of the painted surface was evaluated.

Coated surface condition ○ (good): Very few scratches due to impact are observed in a part of the top coat, and no peeling of the intermediate coat or electrodeposited coat is observed.

Δ (slightly poor) Many scratches and peeling due to impact were observed in the second and intermediate coats, and peeling was also observed here and there in the electrodeposited paint.

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

C-2) Salt water spray resistance After carrying out the same procedure as above (*I), the test piece was JISZ2
371 for 288 hours, then an adhesive cellophane tape is attached to the painted surface, and after rapid peeling off, the presence or absence of rust from the impacted area, corrosion state, peeling of the paint film, etc. are observed.

Salt spray resistance ○: Almost no rust, corrosion, or peeling of the paint film is observed.

Δ: Slightly more rust, corrosion, and peeling of the paint film are observed.

×: Significant rust, corrosion, and paint peeling occurred.

(*3) Adhesion: JIS K5400. 100 goblets of size 1 x 1 mm are made on the paint film according to 6.15 of 1979, adhesive cellophane tape is attached to the surface of the goblets, and the coated surface is evaluated after being rapidly peeled off.

○: No removability of paint film △: Slight peeling of paint film ×: Much peeling of paint film (ne 4) Impact resistance: -10°C atmosphere according to JIS K5400-1979 6.13.38 method Do it below.

A weight weighing 5002 kg was dropped from a height of 50 cm to examine damage to the paint film.

O: No abnormality at all △: Slight cracking and peeling occurs. ×: Many cracks and peeling occur.

Claims (1)

[Scope of Claims] A method for forming an electrodeposition coating film and a topcoat film, or a multilayer coating film consisting of an electrodeposition coating film, an intermediate coating film, and a topcoat film on a metal surface to be coated. At least one of the interlayers of the layer coating film contains (A) an isocyanate-modified polybutadiene resin containing an average of two or more free isocyanate groups per molecule and having a number average molecular weight of 500 to 20,000, and (B) Using a composition whose main component is a polyol component containing an average of two or more hydroxyl groups per molecule and a number average molecular weight of 150 to 50,000, coating with a static glass transition temperature of -100 to 0°C is used. A coating film forming method characterized by forming a film.