Coating processes – Applying superposed diverse coating or coating a coated base – Synthetic resin coating
Reexamination Certificate
2000-01-14
2001-04-10
Cameron, Erma (Department: 1762)
Coating processes
Applying superposed diverse coating or coating a coated base
Synthetic resin coating
C427S410000, C427S412100, C428S447000, C428S450000, C428S457000, C525S100000, C525S101000, C525S107000
Reexamination Certificate
active
06214418
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a novel thermosetting, high solids coating composition and method of forming topcoat by using the same.
BACKGROUND ART
Thermosetting topcoat compositions comprising a hydroxyl-containing acrylic resin and a melamine resin have been heretofore chiefly used for coating automotive exterior panels. However, in recent years, acid rain has posed a worldwide problem of etching and blots or like stains on coating films. With the widespread use of car washers, scratches made on coating films by car washers have presented another problem. In this situation, there is a need for topcoat compositions, particularly clear coat compositions, which are capable of forming coating films satisfactory in both acid resistance and scratch resistance.
Usually, the scratch resistance of a coating film can be improved by increasing the crosslinking density of the film. The acid resistance of a coating film can be improved by incorporating an acid resistant crosslinking system into the film. However, a method has been scarcely proposed for giving both acid resistance and scratch resistance to a coating film.
For example, Japanese Unexamined Patent Publication No. 222,753/1990 discloses that a coating film having high crosslinking density, and thus having high scratch resistance, can be obtained by baking an acrylic resin of high hydroxyl value in the presence of a monomeric melamine resin and an acid catalyst. However, this crosslinking system has a drawback of poor acid resistance since the coating film crosslinked by a melamine resin readily decomposes when contacted with an acid.
Methods have been proposed for imparting both acid resistance and scratch resistance to a coating film by incorporating an acid-resistant crosslinking system to a less acid-resistant melamine resin crosslinking system. The proposed systems include, for example, a composite crosslinking system having a combination of carboxyl group/epoxy group/hydroxyl group/melamine resin (Japanese Unexamined Patent Publication No. 247,264/1990), a composite crossllnking system having a combination of hydroxyl group/alkoxysilyl group/melamine resin (WO91/16,383), etc. However, these systems have not satisfactorily achieved significant improvements in acid resistance because of the presence of a melamine resin.
Melamine resin-free crosslinking systems have also been proposed. The proposals include a crosslinking system having only a combination of carboxyl group/epoxy group or carboxyl group/epoxy group/hydroxyl group (e.g., Japanese Unexamined Patent Publications Nos. 87,288/1987, 45,577/1990 and 287,650/1991). Yet, these systems have a shortcoming that the resulting coating films, although superior in acid resistance, are inferior in scratch resistance owing to the low crosslinking density of the cured films.
A resin composition comprising a carboxyl-containing acrylic polymer and an epoxy- and hydrolyzable silyl-containing compound is known as a crosslinking system having a combination of carboxyl group/epoxy group/hydrolyzable silyl group (Japanese Unexamined Patent Publication No. 187,749/1987), but this composition has a defect that the curing reaction of the polymer with the compound is unsatisfactory because of the steric hindrance caused by the presence of the epoxy group and hydrolyzable silyl group in the same molecule.
A resin composition comprising a hydroxyl- and carboxyl-containing silicone polymer, a carboxyl- and carboxylic acid ester group-containing polymer and a hydroxyl- and epoxy-containing polymer has been proposed as a crosslinking system having a combination of carboxyl group/epoxy group/hydroxyl group which contains the silicone polymer as a base resin (Japanese Unexamined Patent Publication No. 166,741/1994). However, the cured coating of this composition is defective in that it is not always fully satisfactory in crosslinking density and is poor in the recoat adhesion which is one of the important characteristics of coating compositions for automotive exterior panels.
On the other hand, it is of urgent necessity in the field of coating compositions to take measures for the control on the use of organic solvents, from the viewpoints of prevention of air pollution and conservation of resources. As one of such measures, there is a demand for the development of high solids coating compositions which contain a less amount of solvents and have a higher solid concentration.
DISCLOSURE OF INVENTION
An object of the present invention is to provide a novel thermosetting high solids coating composition free of the foregoing prior art drawbacks, and a method of forming a topcoat using said composition.
Another object of the invention is to provide a novel thermosetting high solids coating composition which is capable of forming a coating film excellent in both acid resistance and scratch resistance, and a method for forming a topcoat using said composition.
A further object of the invention is to provide a novel thermosetting high solids coating composition excellent in storage stability and recoat adhesion, and a method for forming a topcoat using said composition.
Other objects and features of the invention will become apparent from the following description.
The present invention provides a thermosetting high solids coating composition comprising:
(A) a carboxyl-containing compound,
(B) a polyepoxide, and
(C) a copolymer prepared by polymerizing monomer components comprising (a) 30 to 50% by weight of vlnyltrimethoxysilane and/or vinyltriethoxysilane, (b) 5 to 15% by weight of N-methylol(meth)acrylamide alkyl ether and (c) 35 to 65% by weight of another polymerizable unsaturated monomer.
Further, the present invention provides a method for forming a topcoat comprising the step of successively forming a colored base coat and a clear coat on a substrate, said colored base coat and/or clear coat being formed from the above coating composition.
The inventors of the present invention conducted extensive research to achieve the above objects and found that the foregoing objects can be achieved by a thermosetting high solids coating composition comprising a carboxyl-containing compound (A), a polyepoxide (B) and the above-specified copolymer (C). The present invention has been completed based on this novel finding.
The high solids coating composition and the method for forming a topcoat according to the invention will be described below in further details.
The high solids coating composition of the invention essentially comprises (A) the carboxyl-containing compound, (B) the polyepoxide and (C) the specific copolymer containing at least one hydrolyzable alkoxysilyl group selected from methoxysilyl and ethoxysilyl, and at least one alkyl-etherified N-methylol group.
The carboxyl-containing compound (A) for use in the invention contains a carboxyl group in its molecule and usually has an acid value of 50 to 500 mg KOH/g, preferably 80 to 300 mg KOH/g.
An acid value of less than 50 mg KOH/g of the compound (A) is likely to lower the curability of the resulting composition and to thereby reduce the acid resistance, scratch resistance and stain resistance of the coating film. On the other hand, an acid value of more than 500 mg KOH/g tends to decrease the compatibility with the polyepoxide (B) and the copolymer (C) and to thereby reduce the storage stability of the composition. Hence an acid value of the compound (A) outside said range is undesirable.
The following compounds (A-1) to (A-4) are usable as the compound (A).
(A-1): a polymer having, in its molecule, a group formed by half esterification of an acid anhydride group
The group formed by half esterification of an acid anhydride group consists of a carboxyl group and a carboxylic acid ester group, and is obtained by subjecting an acid anhydride group and an aliphatic monohydric alcohol to addition reaction for ring opening, namely half esterification. Hereinafter the group may be referred to simply as “half ester group”.
The compound (A-1) can be easily prepared by copolymerizing an unsaturated monomer having a half ester group a
Igarashi Hiroshi
Kitagawa Hiroshi
Nagai Kenichi
Okumura Yasumasa
Saika Masaaki
Armstrong, Westerman, Hattori, McLeland & Naughton, LLD
Cameron Erma
Kansai Paint Co. Ltd.
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