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

[0043] The other coating film-forming resin or resins preferably have an acid value of 10 to 100 mg KOH / g, more preferably 20 to 80 mg KOH / g. Above the upper limit, the water resistance of the coating film will decrease. Below the lower limit, the dispersibility of the resin in water will be decreased. Further, the resins preferably have hydroxyl values within the range of 20 to 180 mg KOH / g, more preferably 30 to 160 mg KOH / g. Above the upper limit, the water resistance of the coating film will be decreased while, below the lower limit, the curability of coating will be decreased.

[0047] The water-based base coat composition mentioned above may contain a polyether polyol. Incorporation of a polyether polyol contributes to the flip-flop properties of the composite coating film and, hence, improves the appearance of the coating film.

[0050] The water-based base coat composition may contain an additional viscosity modifier to prevent imbibing with the top coat coating film and / or secure good workability in application. The viscosity modifier may be any of those generally showing thixotropic properties. As the viscosity modifier, there may be mentioned, for example, crosslinked or noncrosslinked resin particles, swollen dispersions of fatty acid amide, amide type fatty acids, polyamides such as long-chain polyaminoamide phosphates, polyethylene type ones such as colloidal swollen dispersions of oxidized polyethylene, organic acid smectite clays, montmorillonite and like organic bentonite type ones, inorganic pigments such as aluminum silicate and barium sulfate, and flat pigments capable of generating viscosity owing to the shape thereof.

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.