Coating processes – Applying superposed diverse coating or coating a coated base – Synthetic resin coating
Reexamination Certificate
2000-01-07
2001-07-17
Cameron, Erma (Department: 1762)
Coating processes
Applying superposed diverse coating or coating a coated base
Synthetic resin coating
C427S409000, C428S447000, C428S450000, C428S457000, C525S100000, C525S101000
Reexamination Certificate
active
06261642
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a novel thermosetting high solids coating composition and a method for forming a topcoat 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 world-wide 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. On the other hand, 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 a 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 crosslinking system having a combination of hydroxyl group/alkoxysilyl group/melamine resin (WO91/16,383), etc. However, these systems do not always achieve satisfactory improvements in acid resistance because of the presence of a melamine resin.
On the other hand, melamine resin-free crosslinking systems have 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 film.
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 the 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 the composition is defective in that it is not always fully satisfactory in crosslinking density and Is poor in the recoat adhesion that 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. Such measures include development of high solids coating compositions that contain a less amount of solvents and have a higher solids concentration.
DISCLOSURE OF THE 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 for 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 copolymer prepared by polymerizing monomer components comprising (a) 30 to 50 wt. % of vinyltrimethoxysilane and/or vinyltriethoxysilane, (b) 5 to 15 wt. % of N-methylol(meth)acrylamide alkyl ether and (c) 35 to 65 wt. % of another polymerizable unsaturated monomer,
(B) a hydroxyl-containing resin having a hydroxyl value of 10 to 200 mg KOH/g, and
(C) a curing catalyst.
The present invention also 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 carried out extensive research to achieve the above objects, and found that the foregoing objects can be achieved with a thermosetting high solids coating composition comprising the above-specified copolymer (A), the hydroxyl-containing resin (B) and the curing catalyst (C). The present invention has been completed based on this novel finding.
The thermosetting high solids coating composition and the method for forming a topcoat according to the present invention will be described below in more detail.
The thermosetting high solids coating composition of the invention essentially comprises (A) a specific copolymer containing at least one hydrolyzable alkoxysilyl group selected from methoxysilyl and ethoxysilyl, and at least one alkyl-etherified N-methylol group, (B) a hydroxyl-containing resin having a specific hydroxyl value, and (C) a curing catalyst.
The copolymer (A) in the composition of the invention is a copolymer prepared by polymerizing 100 wt. % of monomer components consisting of (a) 30 to 50 wt. % of vinyltrimethoxysilane and/or vinyltriethoxysilane, (b) 5 to 15 wt. % of N-methylol(meth)acrylamide alkyl ether and (c) 35 to 65 wt. % of another polymerizable unsaturated monomer. A monomer (a) content of less than 30 wt. % in the copolymer is likely to lower the curability of the resulting composition and reduce the scratch resistance of the coating film. On the other hand, more than 50 wt. % of monomer (a) tends to decrease recoat adhesion. A monomer (b) content of less than 5 wt. % in the copolymer is likely to decrease recoat adhesion. On the other hand, more than 15 wt. % of monomer (b) is likely to cause yellowing of the film.
The copolymer (A) usually has a hydrolyzable alkoxysilyl content of 0.5 to 4.0 mmol/g, preferably 1.0 to 3.5 mmol/g, the hydrolyzable alkoxysilyl group being methoxysilyl and/or ethoxysilyl. The copolymer (A) usually has a number average molecular weight of 1,000 to 5,000, preferably 1,200 to 4,000. A hydrolyzable alkoxysilyl content of less than 0.5 mmol/g is likely to lower the curability of the resulting coating composition and
Aida Haruhiko
Igarashi Hiroshi
Nagai Kenichi
Okumura Yasumasa
Saika Masaaki
Armstrong Westerman Hattori McLeland & Naughton LLP
Cameron Erma
Kansai Paint Co. Ltd.
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