Method of forming composite coating film

Abstract

The present invention provides a method of forming a composite coating film which makes it possible to attain both appearance and workability in a high-humidity environment. The method of forming a composite coating film according to the present invention comprises applying a water-based base coat composition on a substrate, and then a clear coat thereon, wherein said water-based base coat composition comprises a pigment and a resin emulsion obtained by emulsion polymerization of an alpha, beta-ethylenically unsaturated monomer mixture, said alpha, beta-ethylenically unsaturated monomer mixture having an acid value of 3 to 50 and containing at least 65% by weight of a (meth)acrylic ester whose ester moiety contains 1 or 2 carbon atoms.

Description

FIELD OF THE INVENITIO[0001] The present invention relates to a method of forming a composite coating film on car bodies and so on and a composite coating film obtained by the method.PRIOR ART[0002] From environmental considerations, efforts have been made in recent years to develop water-based coating systems for reducing the levels of organic solvents in coatings. In automotive coating where it is common practice to apply a base coating and, then, a clear coating, attempts have been made to apply a water-based system to the base coating.[0003] As a water-based base coat composition suited to automotive use, Japanese Kokai Publication Sho-63-193968 discloses a water-based coating composition comprising a water-dispersible film-forming acrylic emulsion polymer, a hydrophobic melamine resin and a pigment and Japanese Kokai Publication Sho-63-265974 discloses a water-based paint comprising a resin emulsion having a core component comprised of a C.sub.4-12 alkyl-containing monomer and ...

AI Technical Summary

Benefits of technology

The method allows for improved workability and appearance of the composite coating film, maintaining water resistance and physical properties, even in high-humidity environments, by optimizing the acid value, hydroxyl value, and glass transition temperature of the resin emulsion.

Problems solved by technology

However, with any of the water-based base coat compositions heretofore available generally, inclusive of those mentioned above, it has been found difficult to attain both coat appearance and workability, particularly workability under highly humid conditions.

If the proportion is less than 65%, the coating film obtained will be poor in appearance.

Any acid value less than 3 will not contribute to improved workability and an acid value exceeding 50 will lead to decreased water resistance of the coating film.

A hydroxyl value less than 10 will fail to provide sufficient curability while a hydroxyl value exceeding 150 will lead to decreased water resistance of the coating film.

A particle diameter larger than 1.0 .mu.m may possibly impair the appearance of the resulting coating film.

Above the upper limit, the coating film obtained will be poor in appearance.

Outside the above range, insufficient curability will result.

When the molecular weight is less than 3,000, coating workability is poor and curability is insufficient and when the molecular weight exceeds 50,000, the nonvolatile fraction is so low as to decrease the workability.

When the resin emulsion accounts for less than 5% by weight, workability tends to be adversely affected.

When its proportion exceeds 95% by weight, the coating film-forming properties tend to become poor.

If the above requirements are not satisfied, the water resistance tends to be low and / or the expected improvement in appearance may not be obtained.

Above the upper limit, the image sharpness may decrease or troubles such as unevenness or runs may occur in the step of application.

Below the lower limit, the substrate cannot be masked effectively.

Above the upper limit, troubles such as foaming or sagging may occur in the step of coating.

Below the lower limit, the unevenness of the substrate cannot be masked.

Above the upper limit, the coat may become hard and brittle and, below the lower limit, a sufficient level of hardness cannot be obtained.