Stock material or miscellaneous articles – Composite – Of epoxy ether
Reexamination Certificate
1998-10-28
2001-05-15
Gorgos, Kathryn (Department: 1741)
Stock material or miscellaneous articles
Composite
Of epoxy ether
C204S488000, C204S478000
Reexamination Certificate
active
06231984
ABSTRACT:
The present invention relates to a multilayer coating film formation process comprising coatings with a cationic electrodeposition coating, an intermediate coating and a top coating, and especially relates to a process to form a multilayer coating film in which a gene-ration of popping caused by baking is prevented and the smoothness of the coating surface is improved.
It is known to form a multilayer coating film by coating with a top coating, after coating and curing by heating of an intermediate coating on the cured coating surface of a cationic electrodeposition coating, and the formed multilayer coating film is excellent in smoothness etc. and has been adopted in many fields. However, in these days, it is proposed, for a reason to shorten the operation steps etc., to omit the heating step of the intermediate coating film, to coat said uncured coating surface with a top coating, and then to cure these coating films by heating at the same time.
However, the cured coating surface of a cationic electrodeposition coating has usually fine unevenness and its smoothness is essentially bad. Therefore, when an intermediate coating is coated on this coating surface and a top coating is further coated before curing, there is a disadvantage that the unevenness of the coating surface of a cationic electrodeposition coating appears also on the top coating surface and the smoothness is not sufficient. In order to improve the smoothness, the intermediate coating film may be made thicker. However, in that case, there arises a problem of generation of popping caused by a bumping of organic solvent in the coating film, when it is heated after the coating with a top coating.
The purpose of the present invention is to propose a process to form a multilayer coating film, which is excellent in smoothness or finishing appearance and capable of preventing the generation of popping caused by baking even thickly coated, without curing the intermediate coating film by heating (operation steps shortening) in a coating process to successively coat with a cationic electrodeposition coating, an intermediate coating, and a top coating.
Means to Solve the Problem
As a result of an intensive study aiming at achieving the above-mentioned purpose, the present inventors have found this time, that the above-mentioned purpose can be achieved by using as an intermediate coating a coating material, which can be cured both by irradiation with an actinic energy ray and heating, coating with a top coating after the intermediate coating film has been at least partly cured by irradiation with an actinic energy ray, and then by curing by heating the coating films, and completed the present invention.
Thus according to the present invention, there is proposed a multilayer coating film formation process characterized by that after a cationic electrodeposition coating has been coated and cured by heating, an intermediate coating, comprising polymerizable unsaturated compound, photopolymerization initiator and thermal polymerization initiator, and further optionally polyester resin and crosslinking agent, is coated, and said coating film is cured by irradiation with an actinic energy ray, and a thermocurable top coating is coated and cured by heating.
The multilayer coating film formation process of the present invention is hereinafter described in more detail.
Substrate
Substrates, to which the process of the present invention is applied, are not particularly limited so long as they are formed articles having an electroconductive surface capable of cationic electrodeposition coating. However, the process of the present invention is particularly useful for coating of outer panels of automobile body, for example, roof, door outer panel, bonnet hood, trunk lid, fender, front apron etc.; and of inner panels, for example, door inner panel, inside of the bonnet hood, trunk room etc.
Cationic Electrodeposition Coating (A)
As a cationic electrodeposition coating (A) to be coated on the above-mentioned substrates, a cationic electrodeposition coating comprising a base resin having hydroxyl group and cationizable group (a-1) and a crosslinking agent such as block polyisocyanate compound (a-2) is preferable.
Here, as a base resin (a-1), a resin, obtained by reacting a cationizing agent with an epoxy resin obtained by reacting a polyphenol compound and epichlorohydrin, namely, a polyglycidyl ether of a polyphenol compound, is preferable.
An epoxy resin, before reacting with a cationizing agent, has two or more than two epoxy groups in the molecule and may have a number-average molecular weight in a range of more than 200, preferably 400-4,000, and more preferably 600-3000, and an epoxy equivalent in a range of 190-2,000, preferably 400-1,000, and more preferably 500-800.
As a polyphenol compound, which can be used to prepare said epoxy resin, there can be mentioned, 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′-dihydroxydiphenylsulphone, phenol novolac, cresol novolac etc.
As a cationizing agent to be used to introduce a cationizable group in said epoxy resin, there can be mentioned, for example, amine compound such as primary amine, secondary amine, tertiary amine, polyamine etc. They are reacted preferably with almost all or all epoxy groups existing in the epoxy resin. They react with epoxy group and form cationizable groups such as secondary amino groups, tertiary amino groups, quaternary ammonium base etc.
Hydroxyl groups of the base resin (a-1) include, for example, a primary hydroxyl group introduced by an alkanolamine to be able to be used as a cationizing agent, ring-opening of caprolactone, which may be reacted with an epoxy resin to modify said resin, or by reaction with a polyol etc.; a secondary hydroxyl group formed by ring-opening of an epoxy group in an epoxy resin, etc. Among them, a primary hydroxyl group introduced by a reaction with an alkanolamine is excellent in crosslinking reactivity with a block polyisocyanate compound (crosslinking agent) and preferable.
The base resin (a-1) has preferably a hydroxyl group equivalent in a range of 20-5,000 mgKOH/g, particularly 100-1,000 mgKOH/g, and more particularly 200-800 mgKOH/g, and especially a primary hydroxyl group equivalent in a range of 200-1,000 mgKOH/g, above all 230-750 mgKOH/g. On the other hand, cationizing groups are sufficient with more than an amount necessary to be able to stably disperse said base resin in water and preferably in a range of generally 3-200, particularly 5-150, and more particularly 10-80, calculated as KOH (mg/g solid content) (amine value).
The base resin (a-1) is desirable not to contain a free epoxy group in principle.
In the cationic electrodeposition coating (A), as a crosslinking agent (a-2) to cure the base resin (a-1) by crosslinking, a block polyisocyanate compound is mainly used.
A block polyisocyanate compound is a polyisocyanate compound whose isocyanate groups are all blocked by a blocking agent to make them inactive at normal temperature. When it is heated to a temperature, which is higher than the prescribed temperature, preferably higher than 120° C., the blocking agent is dissociated and the original isocyanate group is regenerated to take part in the crosslinking reaction.
A polyisocyanate compound is a compound having two or more, preferably 2-3 free isocyanate groups in the molecule and includes, for example, aliphatic diisocyanates such as hexamethylene diisocyanate, trimethylene diisocyanate, tetramethylene diisocyanate, dimer acid diisocyanate, lysine diisocyanate etc.; alicyclic diisocyanates such as isophorone diisocyanate, methylenebis(cyclohexylisocyanate), methylcyclohexane diisocyanate, cyclohexane diisocyanate, cyclopentane diisocyanate etc.; aromatic diisocyanates such as xylylene diisocya
Horibe Kyoichi
Suwama Masami
Gorgos Kathryn
Kansai Paint Co. Ltd.
Tran Thao
Wenderoth , Lind & Ponack, L.L.P.
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