JPH059410A - Clear anticorrosive coating composition - Google Patents

Abstract

PURPOSE:To provide a coating composition which gives a clear coating film excellent in filiform corrosion resistance without impairing the metallic sheen on the substrate metal surface. CONSTITUTION:The title composition comprises, as major components, a poly(vinyl acetal) resin, an epoxy resin, and a metal phosphate, with the amount of the phosphate being 1-5 pts. A crosslinking agent is preferably added to these components.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a transparent anticorrosion coating composition which can be applied to a metal surface to prevent thread rusting without damaging the metallic luster of the background.

[0002]

2. Description of the Related Art Recently, along with the demand for weight reduction of vehicle bodies, the proportion of automobile parts such as wheels, brake cylinders, radiators and engine cylinder heads made of aluminum-based or magnesium-based light alloys is increasing. In particular, the so-called aluminum wheels in which the wheels are made of an aluminum-based alloy have been remarkably popular, and are widely used including those having excellent design.

Generally, an aluminum wheel is produced by molding an aluminum alloy into a wheel shape by means such as casting or forging, and then cutting the necessary portions of the inner and outer surfaces. In order to impart beauty and weather resistance, The entire surface or part of it is painted. This coating is usually undercoated with a paint containing color pigments, metal powder pigments, rust preventive pigments, extender pigments, etc., and depending on the type, it is partially cut again to maintain the exposed metallic luster of the background for a long time. Therefore, a step of overcoating with a colorless and transparent clear coating once or twice is adopted.

Among them, as the undercoat paint, a wheel having a composition containing rust preventive pigments such as strontium chromate, zinc chromate and basic lead sulfate, and a phosphomolybdic acid pigment known as a pollution-free pigment is used. The corrosion resistance of is greatly improved. However, the thermosetting acrylic melamine resin-based clear coating that has been conventionally used for topcoating is excellent in terms of transparency, weather resistance, adhesion, etc., but it has excellent corrosion resistance when the coating film is scratched. There are difficulties. For this reason, improvements in coating film forming components have been promoted in combination with polyester resins and epoxy resins, but they have not yet reached the level of corrosion resistance comparable to undercoat paints containing anticorrosion pigments.

In order to solve such a problem, when painting an aluminum wheel, a portion of the outer surface to be painted which is to be colored is painted with an acrylic coloring paint, and then the entire outer surface to be painted is made transparent. A method of clear coating with a silicone-based paint (Japanese Patent Publication No. 3-2024), a coating composition containing 0.05 to 20 parts by weight of a specific hydroxide with respect to 100 parts by weight of a film-forming component (Japanese Patent Publication No. 61-44902). Gazette) has been proposed.

Further, a resin composition having a group capable of reacting with a hydroxyl group of the resin by heating with a water-soluble polyvinyl alcohol resin and / or a water-soluble polyvinyl acetal resin as a metal pretreatment agent, an oxyacid salt and / or
Alternatively, a thermosetting metal pretreatment coating composition comprising a metal oxide and phosphoric acid (JP-A-56-70062) has been proposed.

[0007]

DISCLOSURE OF THE INVENTION The inventors of the present invention have earnestly studied to solve the above-mentioned problems by using a coating composition different from the prior art. As a result, a polyvinyl acetal resin and an epoxy resin are provided with a specific amount of metal phosphate. The present invention has been developed by confirming the fact that the paints of the compositions formulated in combination exhibit extremely excellent transparent anticorrosion performance.

Therefore, an object of the present invention is to provide a coating excellent in transparency, corrosion resistance, and water resistance without damaging the metallic luster (indicating reflected light of interference fringes of rainbow color) peculiar to the cutting surface of aluminum-based or magnesium-based alloy. It is intended to provide a transparent anticorrosive coating composition capable of forming a film.

[0009]

The transparent anticorrosive coating composition according to the present invention for achieving the above object is defined by the following (A).
Characteristically, the main component is (C). (A) Polyvinyl acetal resin (B) Epoxy resin (C) Metal phosphate 1-5 PHR (PHR: wt% per resin solid content)

The polyvinyl acetal resin of the component (A) which constitutes the present invention is obtained by acetalizing polyvinyl alcohol and aldehydes, and includes polyvinyl formal resin, polyvinyl acetoacetal resin,
Examples thereof include polyvinyl propional resin, polyvinyl butyral resin, polyvinyl hexyl resin, and the like. Among these, for the purpose of the present invention, the average degree of polymerization of 300 to 2000, from the viewpoint of the water resistance of the coating film, the adhesion to the metal surface, etc.
A polyvinyl butyral resin having a butyralization degree of 60 to 75 mol% is preferably used.

As the epoxy resin as the component (B), a general-purpose epoxy resin composed of a compound containing two or more epoxy groups in one molecule can be applied. In particular,
For example, bisphenol A, bisphenol F, 1,1,2,2
-Tetrakis (4'-hydroxyphenyl) ethane glycidyl ether epoxy resin of polyphenol compounds such as catechol, resorcin, hydroquinone, phloroglucin polyhydric phenol glycidyl ether epoxy resin; ethylene glycol, butanediol, glycerol, erythritol, Glycidyl ether-based epoxy resins of polyhydric alcohols such as polyoxyalkylene glycols; novolak type epoxy resins; cycloaliphatic epoxy resins such as vinylcyclohexene dioxide, limonene dioxide, dicyclopentadiene dioxide; phthalic acid, cyclohexane-1 , 2 Polycarboxylic acid ester condensation products such as dicarboxylic acid, polyglycidyl ester epoxy resin; polyglycidyl amine epoxy resin, etc. That. Of these epoxy resins, it is preferable to add a glycidyl ether type epoxy resin of a polyphenol compound or a novolac type epoxy resin. Particularly, from bisphenol A or bisphenol F having a number average molecular weight of 800 to 3000 and a glass transition point of 25 to 70 ° C. If a glycidyl ether epoxy resin is added, a more preferable corrosion resistance effect is given.

As the metal phosphate as the component (C) of the present invention, for example, chromium phosphate, nickel phosphate, calcium phosphate, cobalt phosphate, manganese phosphate, zinc phosphate, iron phosphate and the like are applied. However, of these,
The most effective substance in terms of adhesion, colorability and transparency is chromium phosphate.

A cross-linking agent can be further added as the component (D) to the above-mentioned components (A), (B) and (C), and this addition can further improve water resistance, corrosion resistance and the like. . Suitable crosslinking agents are amino resins and blocked isocyanate compounds, but these can be used alone or in combination of both components. When the amino resin and the blocked isocyanate compound are used in combination, the respective components complement each other's drawbacks of mutual reactivity and a well-balanced crosslinking agent can be obtained.

Among them, the amino resin is, for example, melamine resin or benzoguanamine resin, preferably melamine resin such as n-butylated melamine resin or isobutylated melamine resin whose alkoxy group is methoxy group, ethoxy group or n-butoxy group. Group, an i-butoxy group, and other alkoxymethylmelamine resins. These resins can be usually obtained by subjecting melamine to an addition reaction or an addition condensation reaction of aldehydes such as formaldehyde and paraformaldehyde, and etherifying the aldehyde with a monohydric alcohol having 1 to 4 carbon atoms.

Further, the blocked isocyanate compound is
The following blocking agent blocks the reaction of the isocyanate group. As the blocking agent (dissociation temperature in ()), methanol (180-185 ° C), ethanol (180-185 ° C)
185 ° C.), n-pentanol (205 to 210 ° C.), ethylene chlorohydrin (155 to 160 ° C.), isopropyl alcohol (170 to 175 ° C.), and other alcohols; phenol (140
Phenols such as p-nitrophenol (120 to 125 ° C) and cresol (135 to 140 ° C); acetylacetone (105 to 110 ° C), ethyl acetoacetate (125 to 130).
C.), MEK oxime (150 to 155.degree. C.), .epsilon.-caprolactam (140 to 144.degree. C.) and the like. A blocking agent having a dissociation temperature of 100 to 180 ° C., more preferably 120 to 160 ° C. is preferably selected and used. As the isocyanate compound, one or more kinds of polyfunctional isocyanates such as TDI-based, MDI-based, HDI (hexamethylene diisocyanate) -based, IPDI (isophorone diisocyanate) -based and the like are appropriately used. If necessary, a tin-based catalyst is applied and reacted.

The blending ratio (parts by weight) of (A) polyvinyl acetal resin and (B) epoxy resin in the present invention is
(A) / (B) 10 / 90-60 / 40, preferably 20 / 80-30
It is preferable to set it in the range of / 70. When the compounding ratio of the component (A) is less than 10/90, the internal stress tends to increase and the adhesiveness and the thread rust suppressing power tend to decrease. When it exceeds 60/40, the adhesive force decreases due to deterioration in water resistance. Does not provide sufficient anticorrosion effect.

The metal phosphate as the component (C) is set to 1 to 5 PHR, preferably 2 to 4 PHR as a blending ratio with respect to the resin solid contents of the above (A) and (B). The specification of the blending range is an important requirement of the present invention, and if it is less than 1 PHR, the amount of the metal phosphate is insufficient to passivate the metal surface of the base material, and effective adhesion and corrosion resistance are imparted. On the other hand, if it exceeds 5 PHR, the stability of the coating composition deteriorates and the appearance of the coating film shifts to a chromatic color, making it impossible to form a transparent top coat.

When the crosslinking agent (D) is added to the above components, the ratio (part by weight) of {(A) + (B)} / (D) is preferably 60/40 to 90/10, more preferably It is preferably set in the range of 70/30 to 80/20. If the addition ratio of the cross-linking agent (D) to the components (A) + (B) exceeds 40%, the cross-linking will proceed too much and the coating film formed will become brittle, and if it is less than 10%, the water resistance will be insufficient.

The transparent anticorrosion coating composition of the present invention comprises the above-mentioned component (A) polyvinyl acetal resin, (B) epoxy resin and (C) metal phosphate, or (D) cross-linking agent added thereto. Although it is manufactured with sufficient stirring, the coating process of the coating composition on the metal surface will be described below by taking an aluminum wheel as an example. First, necessary portions of the inner and outer surfaces of a wheel cast into a predetermined shape are cut, and surface treatment is performed with a chemical conversion treatment agent (chromium is mainly used). Next, the transparent anticorrosion coating composition of the present invention is applied, and an undercoat containing a coloring pigment or a metal powder pigment for imparting cosmetic properties is applied on the entire outer surface, and both coatings are applied simultaneously or individually. Let dry and harden. Here, the paint anticorrosive coating composition of the present invention may be omitted, and the paint anticorrosive coating composition of the present invention may be used by replacing it with a general undercoat containing a general anticorrosive pigment. An aluminum wheel of a type that creates an aluminum metal luster portion for the purpose of beauty is further partially cut, and again subjected to a surface treatment with a chemical conversion treatment agent. Next, the transparent anticorrosion coating composition of the present invention is spray-coated on the entire outer surface including the non-cutting portion. The thickness of the coating film to be formed is preferably 5 to 15 μm, and if it is thicker than this, the internal stress increases and the adhesion and flexibility decrease,
As it becomes thinner, the ability of passivation decreases. On the transparent anticorrosion coating composition of the present invention, a clear coating composed of a conventional acrylic / melamine-based resin containing an ultraviolet absorber for improving weather resistance and beauty is applied, and simultaneously or individually, dried and cured. It can also be done.

[0020]

[Function] The effect of imparting anticorrosion property to the light alloy material is mainly to prevent the occurrence of thread rust. However, in view of the coating resin, the effect of preventing thread rust is O ₂ , H ₂ O, Reduces ion permeability, enhances adhesion to the substrate,
Increase the film thickness. It is necessary to make a tough coating film having a high Young's modulus and to reduce internal stress. In the coating composition of the present invention, the effect of preventing the thread rust property is effectively achieved by the synergistic action of suppressing the internal stress of the coating film and passivating the surface.

Although thread rust and internal stress seem to be unrelated, it can be understood that the relationship between internal stress, adhesiveness, and thread rust is related via adhesiveness. In other words, the internal stress of the coating film increases due to environmental conditions over time compared to the initial stage of film formation, so a coating film with a large internal stress causes adhesion deterioration between the material and the coating film, and the progression of thread rust is easy. Becomes In the composition of the present invention, when the resin-based component is composed of (A) polyvinyl acetal resin and (B) epoxy resin, it has a function of suppressing phenomena such as increase of internal stress due to crosslinking and deterioration of adhesion. Is a requirement for That is, while the epoxy resin has excellent corrosion resistance, it has a tendency to have a large internal stress, but the polyvinyl acetal resin to be blended skillfully acts as a buffer to suppress the internal stress. Through this action, a tough coating film with low internal stress and high adhesion is formed. Further, the polyvinyl acetal resin effectively functions as a binder that causes the metal phosphate as the component (C) to undergo a diffusion reaction in the coating material.

The function of passivating the substrate surface is brought about by the diffusing metal phosphate (C) in the resin component. That is, when the coating composition of the present invention is applied to a metal surface, a phosphate film is formed by the phosphoric acid component contained in the coating to passivate the surface layer. This passivation layer, in combination with the action of the above-mentioned resin-based components, promotes the adhesion of the top coating film and remarkably maintains the anticorrosion effect.

These effects can be further improved by adding the crosslinking agent (D) to the components (A) to (C).

[0024]

EXAMPLES Examples of the present invention will be described below in comparison with comparative examples.

Examples 1 to 22, Comparative Examples 1 to 6 Various polyvinyl acetal resins, epoxy resins and metal phosphates were blended in the amounts (parts by weight) shown in Table 1 (Examples) and Table 2 (Comparative Examples). Then, the mixture was sufficiently stirred to obtain a clear paint. The types of each component indicated by symbols in Table 1 and Table 2 are as follows. (A) Polyvinyl acetal resin A-1: Polyvinyl butyral [S-REC B BL-3: manufactured by Sekisui Chemical Co., Ltd.] A-2: Polyvinyl formal [S-REC BX-2: manufactured by Sekisui Chemical Co., Ltd.] A-3 : Polyvinyl acetoacetal [S-REC KS-1:
Sekisui Chemical Co., Ltd.) (B) Epoxy resin B-1: Glycidyl ether epoxy [Epotote YD-0
14: Toto Kasei Co., Ltd.] B-2: Novolac type epoxy [Epotote YDCN 704: Toto Kasei Co., Ltd.] (C) Metal phosphate C-1: Chromium phosphate C-2: Calcium phosphate C-3 : Zinc phosphate C-4: Manganese phosphate (D) crosslinking agent D-1: Blocked isocyanate [DESMODUL BL-3175:
Sumitomo Bayer Urethane] D-2: Melamine resin [Uban 20SE-60: manufactured by Mitsui Toatsu Chemicals, Inc.]

A machined aluminum wheel is treated with a chemical conversion treatment agent [Chromium conversion treatment agent "Alsurf 1000" manufactured by Nippon Paint Co., Ltd.], and then the above clear paint is dried to a thickness of about 10 μm. Like spray painting,
The coating was cured by heating at 150 ° C for 20 minutes. The appearance, yarn rust resistance and water resistance of each of the obtained coating films were evaluated, and the results are shown in Table 3 (Examples) and Table 4 (Comparative Examples).

The evaluation method was as follows. (1) Appearance of the coating film Transparency: The transparency of the coating film was judged by visual observation according to the following evaluation criteria. ○: Completely transparent △: Slightly colored, but with a metallic feel of the aluminum wheel ×… Severely colored, no metallic feel of the aluminum wheel

(2) Evaluation of thread rust resistance test After making a linear cut of 3 cm reaching the aluminum substrate with a cutter knife, dip salt in 1N hydrochloric acid water (35 ° C.) containing 5% by weight concentration, and immediately Take it out and dry it indoors until there are no water drops. Then, temperature 40-45 ℃, humidity 70
It was put out in a thermo-hygrostat kept at 75% RH for 72 hours and taken out. With respect to this product, the number and length of thread rusts generated in the cut portion were determined based on the following evaluation criteria, and the thread rust resistance was determined. ◎… No rust occurs, or only rust points with a length of 1 mm or less ○… 2 or less thread rusts with a maximum length of 3 mm or less △… 5 or less thread rusts with a maximum length of 6 mm or less ×… Not applicable (3) Water resistance test It was immersed in a constant temperature water bath maintained at an evaluation temperature of 38 to 40 ° C., taken out every 24 hours, and the whiteness of the coating film surface was evaluated for water resistance by the following evaluation criteria. ◎… No whitening even after immersion for 360 hours ○… No whitening after immersion for 240 hours ×… Blanching is observed after immersion for less than 240 hours

[0029]

[Table 1]

[0030]

[Table 2]

[0031]

[Table 3]

[0032]

[Table 4]

As can be seen from the examination of Tables 1 to 4, the examples in which the transparent anticorrosive coating composition of the present invention was applied showed good results in both coating appearance, thread rust resistance and water resistance.
Comparative examples using coating compositions that deviate from the requirements of the present invention show poor results in any of the coating film appearance, yarn rust resistance, and water resistance,
It was not possible to combine both performances.

Examples 23 to 30 A machined aluminum wheel was subjected to chemical conversion treatment in the same manner as in Example 1, and the transparent anticorrosive coating composition of the present invention obtained in Example was spray-coated on the aluminum wheel to obtain a dry coating film. The undercoating was applied to a thickness of 15 μm, and the mixture was heat-cured at 150 ° C. for 20 minutes. Furthermore, a thermosetting acrylic melamine resin-based clear paint [Nippon Paint Co., Ltd., “Super Rack 5000 AW
-2 clear] was spray-coated to a dry film thickness of about 30 μm and heated at 150 ° C. for 20 minutes to cure the coating film.
The obtained coating film was evaluated in the same manner as in Example 1, and the results are shown in Table 5.

[0035]

[Table 5]

[0036]

As described above, according to the present invention, there is provided a coating composition which is transparent and has excellent anticorrosion property, which can be applied to the surface of a metal surface to effectively prevent the generation of thread rust without spoiling the metallic luster. can do. Therefore, it is useful for the purpose of transparent coating of automobile parts such as wheels manufactured from light alloy materials such as aluminum and magnesium.

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor, Yasunori Hamada             4-12 Minami-Shinagawa, Shinagawa-ku, Tokyo Japan             Paint Co., Ltd. Tokyo office (72) Inventor Hiroyuki Noguchi             30 Ikeshita, Ano-cho, Toyoake-shi, Aichi Asahi Sol             Bent Industry Co., Ltd.

Claims (2)

[Claims] 1. A transparent anticorrosion coating composition comprising the following main components (A) to (C). (A) Polyvinyl acetal resin (B) Epoxy resin (C) Metal phosphate 1-5 PHR 2. A transparent anticorrosive coating composition having a composition in which (D) a crosslinking agent is added to (A) to (C) of claim 1.