Plural layer coating film-forming method

Details Plural layer coating film-forming method Plural layer coating film-forming method

2000-07-07

2002-09-10

Mayekar, K. (Department: 1741)

Stock material or miscellaneous articles

All metal or with adjacent metals

Composite; i.e., plural, adjacent, spatially distinct metal...

C204S484000, C204S500000

Reexamination Certificate

active

06447931

ABSTRACT:

The present invention relates to a novel plural layer coating film-forming method comprising electrodepositing in order a cationically electrodepositable coating material and an anionically electrodepositable coating material on a coated article.
It is publicly know that after a cationically electrodepositable coating material is applied on a coated article and then heated and cured, an intermediate coating material and a top coating material of an organic solvent base are applied in order by a spraying or electrostatic way to form a plural coating film. The plural coating film thus formed is excellent in a smoothness and employed in many fields. In recent years, however, it is strongly required to cut down use amounts of organic solvents in order to prevent environmental pollution and to elevate a coating efficiency for shortening the steps and save energy and labor.
An object of the present invention is to provide a novel plural layer coating film-forming method satisfying the requirements described above.
Intensive researches repeated by the present inventors have resulted in finding this time that the object described above can be achieved by applying a cationically electrodepositable coating material (A) showing a specific volume resistivity value on a coated article and applying an anionically electrodepositable coating material (B) on a cured coating film thereof to form a plural layer coating film, and they have come to complete the present invention.
Thus, provided according to the present invention is a plural layer coating film-forming method characterized by applying a cationically electrodepositable coating material (A) which provides a cured coating film having a volume resistivity value of 10
12
&OHgr;.cm or less on a coated article and then applying an anionically electrodepositable coating material (B) on a cured coating film surface thereof.
A method for recoating twice electrodepositable coating materials to form a plural layer coating film include various methods excluding the method of the present invention described above, but it is difficult to achieve the object described above by any of them.
That is, a plural layer coating film formed by applying a cationically electrodepositable coating material on a heated and cured coating film of a cationically electrodepositable coating material is inferior in a weatherability, a finished appearance and a hardness as compared with those of a plural layer coating film formed by the method of the present invention. Also, a plural layer coating film formed by applying a cationically electrodepositable coating material on a heated and cured coating film of an anionically electrodepositable coating material is markedly inferior in a corrosion resistance and further inferior as well in a weatherability, a finished appearance and a hardness as compared with those of a plural layer coating film formed by the method of the present invention. Further, a plural layer coating film formed by applying an anionically electrodepositable coating material on a heated and cured coating film of an anionically electrodepositable coating material is markedly inferior in a corrosion resistance and further inferior as well in a finished appearance as compared with those of a plural layer coating film formed by the method of the present invention.
In contrast with this, the plural layer coating film formed by the method of the present invention is excellent in a finished appearance such as a smoothness and a glossiness and a coating film performance such as a weatherability and a corrosion resistance and provides the marked effect that application of a top coating material can be omitted.
The plural layer coating film-forming method of the present invention shall be explained below in further details.
Coated Article
A coated article to which the method of the present invention is applied shall not specifically be restricted as long as it has an electroconductive surface on which cationic electrodeposition coating can be carried out. The method of the present invention is particularly useful for coating on outside plate parts and inside plate parts of car bodies of passenger cars, buses, trucks and two wheelers.
Cationically Electrodepositable Coating Material (A)
The cationically electrodepositable coating material (A) is a coating material which can be electrodeposited directly on the coated article described above, and a cationically electrodepositable coating material which forms a coating film having a volume resistivity value of 10
12
&OHgr;.cm or less is used therefor. To be specific, included is, for example, a cationically electrodepositable coating material which comprises a cationic resin and a conductive agent for controlling the volume resistivity value of the coating film in the range described above and which is prepared by mixing and dispersing these components in an aqueous medium.
Known resins usually used for a cationically electrodepositable coating material can be used for the cationic resin, and capable of being suitably used is, for example, a resinous composition comprising a base resin having a hydroxyl group and a cationizable group and a cross-linking agent such as a block polyisocyanate compound. The base resin used in this case includes, for example, reaction products of epoxy resins with cationizing agents; products obtained by protonating polycondensation products (refer to U.S. Pat. No. 2,450,940) of polycarboxylic acids and polyamines with acids; products obtained by protonating polyaddition products of polyisocyanate compounds, polyols and mono- or polyamines with acids; products obtained by protonating copolymers of acryl base or vinyl base monomers containing a hydroxyl group and an amino group with acids (refer to Japanese Patent Publication No. 12395/1970 and Japanese Patent Publication No. 12396/1970); and products obtained by protonating adducts of polycarboxylic acids to alkyleneimines with acids (refer to U.S. Pat. No. 3,403,088). Among them, a base resin obtained by reacting a cationizing agent with an epoxy resin obtained by reacting a polyphenol compound with epichlorohydrin is particularly preferred because of an excellent corrosion resistance thereof.
This epoxy resin has at least two epoxy groups in a molecule and has suitably a number average molecular weight falling in a range of 400 or more, particularly 400 to 4000, more particularly 800 to 2000 and an epoxy equivalent falling in a range of 190 to 2000, particularly 400 to 1000.
The polyphenol compound used for preparing the epoxy resin described above includes, for example, bis(4-hydroxyphenyl)-2,2-propane, 4,4′-dihydroxybenzophenone, bis(4-hydroxyphenyl)-1,1-ethane, bis(4-hydroxyphenyl)-1,1-isobutane, bis(4-hydroxy-tert-butyl-phenyl)-2,2-propane, bis(2-hydroxybutyl)methane, 1,5-dihydroxynaphthalene, bis(2,4-dihydroxyphenyl)methane, tetra(4-hydroxyphenyl)-1,1,2,2-ethane, 4,4′-dihydroxydiphenyl ether, 4,4′-dihydroxydiphenylsulfone, phenol novolak and cresol novolak.
These epoxy resins obtained by reacting the polyphenol compounds with epichlorohydrin can further be modified with acryl resins, polybutadiene, alkyd resins, polyester resins and polyamide resins.
The cationizing agent includes, for example, amine compounds such as primary amines secondary amines, tertiary amines and polyamines, and they are preferably reacted with almost all epoxy groups which are present in the epoxy resins. They are reacted with the epoxy groups to form cationizable groups such as a secondary amino group, a tertiary amino group and a quaternary ammonium salt group. Further, basic compounds such as ammonia, hydroxylamine, hydrazine, hydroxyethylhydrazine and N-hydroxyethylimidazoline used as a cationizing agent may be reacted with the epoxy groups to thereby form basic groups, and they may be protonated with acids to form cationizable groups.
A primary hydroxyl group introduced by reaction with alkanolamine which can be used as a cationizing agent is suited to a hydroxyl group in the base resin because of an excellent cross-linking reac

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