Metal treatment – Process of modifying or maintaining internal physical... – Processes of coating utilizing a reactive composition which...
Reexamination Certificate
2000-07-28
2002-08-20
Sheehan, John (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Processes of coating utilizing a reactive composition which...
C148S273000, C204S489000, C428S457000
Reexamination Certificate
active
06436201
ABSTRACT:
TECHNICAL FIELD
This invention relates to a cationic electrodeposition coating composition capable of forming a coating film which exhibits an excellent rust-proofing effect on untreated steel plates or sheets and has good smoothness, without using harmful substances such as lead compounds and chromium compounds.
BACKGROUND ART
Since cationic electrodeposition coatings exhibit excellent throwing power and can form a coating film having excellent properties such as durability and anticorrosive power, they are being widely used as undercoatings for automobile bodies, electrical appliances and other articles which require such properties.
In order to maintain the anticorrosive power of the coating film, these electrodeposition coatings may contain lead compounds and chromium compounds such as lead chromate, basic lead silicate and strontium chromate. However, these compounds are very harmful substances and their use involves problems from the viewpoint of environmental protection. For this reason, it has been proposed to use, in place of these harmful substances, nontoxic or less toxic compounds such as zinc phosphate, iron phosphate, aluminum phosphate, calcium phosphate, zinc molybdate, calcium molybdate, zinc oxide, iron oxide, aluminum phosphomolybdate and zinc phosphomolybdate (see, for example, Japanese Patent Publication No. 7224/'91). However, as compared with the aforesaid electrodeposition coatings containing a lead or chromium compound, such electrodeposition coatings have poor anticorrosive power and are hence unsatisfactory from a practical point of view.
Moreover, Japanese Patent Laid-Open No. 136303/'94 suggests a cationic electrodeposition coating comprising a resin composition obtained by reacting an epoxy resin with a compound having a P—OH, S—H or 3,4,5-trihydroxyphenyl group and further reacting the resulting prepolymer with a compound having an amino group capable of reacting with the epoxy group, and hydrotalcite (a mineral comprising a hydrated basic carbonate of magnesium and aluminum). However, this cationic electrodeposition coating fails to form a coating film having satisfactory anticorrosive power.
Furthermore, a coating film formed by applying a cationic electrodeposition coating containing no lead compound or chromium compound to untreated cold-rolled steel plates or sheets (which have not been subjected to a chemical conversion treatment with zinc phosphate or the like) has poor anticorrosive power. Moreover, in order to cure this coating film, it must be heated to a temperature of about 170 to about 180° C. Within the heating oven, however, some parts of the coating film may reach temperatures above 200° C. It is pointed out that this causes the problem of reducing the anticorrosive power and adhesion of those parts of the coating film which have been exposed to such high temperatures.
An object of the present invention is to provide a cationic electrodeposition coating capable of forming a coating film which exhibits an excellent rust-proofing effect even on untreated steel plates or sheets, shows no reduction in anticorrosive power or adhesion even if exposed to temperatures above 200° C., and has good smoothness, without using harmful substances such as lead compounds and chromium compounds.
It has now been found that this object can be accomplished by combining a resin for cationic electrodeposition coatings with a compound containing a specific metal and a metal hydroxide having a specific composition. The present invention has been completed on the basis of this finding.
DISCLOSURE OF THE INVENTION
Thus, the present invention provides a cationic electrodeposition coating composition comprising (A) a resin for cationic electrodeposition, (B) a compound containing a metal selected from among bismuth, lanthanum and molybdenum, (c) a metal hydroxide represented by the following formula:
M
(1−x)
Al
x
(OH)
(2+x)
.m
H
2
O
wherein M is Mg or Zn, and x and m are values satisfying the relationships: 0≦x≦1 and 0≦m<2.
EMBODIMENTS OF THE INVENTION
The cationic electrodeposition coating composition of the present invention will be more specifically described hereinbelow.
Resin for Cationic Electrodeposition (A)
As the resin for cationic electrodeposition (A) contained in the coating composition of the present invention, the may be used, for example, a common resin which has a crosslinking functional group (e.g., the hydroxyl group) and a cationic group, and is suitable for use in cationic electrodeposition coatings. The skeleton of the resin may comprises any resin selected from among epoxy resins, acrylic resins, polybutadiene, alkyd resins, polyester resins and the like. However, in order to improve the anticorrosive properties of the coating film, it is generally preferable to use a polyamine resin formed by the addition of an amine to an epoxy resin (i.e., an amine-added epoxy resin).
Examples of the amine-added epoxy resin include (i) the adduct of a polyepoxide compound with a primary mono- or polyamine, a secondary mono- or polyamine, a mixed primary and secondary polyamine, or the like (see, for example, U.S. Pat. No. 3,984,299); (ii) the adduct of a polyepoxide compound with a secondary mono- or polyamine having a ketiminized primary amino group (see, for example, U.S. Pat. No. 4,017,438); and (iii) the reaction product obtained by the etherification of a polyepoxide compound with a hydroxyl compound having a ketiminized primary amino group (see, for example, Japanese Patent Laid-Open No. 43013/'84).
The polyepoxide compound used for the preparation of the aforesaid amine-added epoxy resins is a compound having two or more epoxy groups in the molecule. Suitable polyepoxide compounds generally have a number-average molecular weight of at least 200, preferably 400 to 4,000, and more 800 to 2,000. Among others, polyepoxide compounds formed by the reaction of a polyphenol compound with epichlorohydrin are preferred. The polyphenol compound which can be used for the formation of such polyepoxide compounds include, 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-butylphenyl)-2,2-propane, bis(2-hydroxynaphthyl)methane, 1,5-dihydroxynaphthalene, bis(2,4-dihydroxyphenyl)methane, tetra(4-hydroxyphenyl)-1,1,2,2-ethane, 4,4-dihydroxydiphenyl sulfone, phenolic novolac and cresylic novolac.
The polyepoxide compounds may further be partially reacted with a polyol, polyether polyol, polyester polyol, polyamideamine, polycarboxylic acid, polyisocyanate compound or the like. Moreover, the polyepoxide compounds onto which &egr;-caprolactone, an acrylic monomer or the like has been graft-polymerized may also be used.
The above-described base resins may be either of the external crosslinking type or of the internal (self-)crosslinking type. The base resins of the external crosslinking type are used in combination with a curing agent. As the curing agent, there may be used any of the conventionally known crosslinking agents such as polyisocyanate compounds which may be blocked, and amino resins. Among other, blocked polyisocyanate compounds are preferred. The base resins of the internal (self-)crosslinking type are resins which can be crosslinked and cured by themselves without the aid of a curing agent. Examples thereof include resins which are obtained by reacting a partially blocked polyisocyanate compound with a base resin, and hence have both an active hydrogen-containing functional group and a blocked isocyanate group in the same molecule.
The blocked polyisocyanate compounds which can be used in combination with the base resins of the external crosslinking type are the fully blocked addition products of polyisocyanate compounds. The polyisocyanate compounds are compounds having two or more isocyanate groups in the molecule. Examples thereof include aromatic, alicyclic and aliphatic polyisocyanate compounds such as tolylene diisocyanate, xylylene diisocyanate, phenylene diisocyanate, bis(isocyanatomethyl)
Sugisaki Katsuhisa
Sugita Masaru
Kansai Paint Co. Ltd.
Oltmans Andrew L.
Sheehan John
Wenderoth Lind & Ponack LLP
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