Process for the preparation of a two-coat finish, and aqueous co

Details Process for the preparation of a two-coat finish, and aqueous co Process for the preparation of a two-coat finish, and aqueous co

1996-05-16

1999-12-14

Dudash, Diana

Coating processes

Applying superposed diverse coating or coating a coated base

Synthetic resin coating

524458, 524460, 524461, 524591, B05D 136, B05D 716, C08L 7504

Patent

active

060014245

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND AND SUMMARY OF THE INVENTION

The invention relates to a process for the preparation of a two-coat finish on a substrate surface, in which and subsequently
The invention also relates to aqueous coating materials which can be employed in this process as pigmented basecoats.
The process described above is employed in particular for the preparation of two-coat automotive finishes of the basecoat/clearcoat type. In this context the quality of the two-coat finish prepared by this process depends quite critically on the aqueous basecoat employed in step (1) of the process.
EP-A-353 797 describes aqueous coating materials which can be employed as basecoats in step (1) of the above-described process. The aqueous coating materials described in EP-A-353 797 contain as binder a polymer which is obtainable by subjecting acrylate and/or methacrylate monomers to an emulsion polymerization which is initiated by water-soluble initiators, in the presence of an anionic polyurethane resin which may also contain vinyl groups.
If the aqueous coating materials described in EP-A-353 797 are employed as basecoats in the above-described process, two-coat finishes are obtained which have an unsatisfactory stability with respect to condensed moisture. This disadvantage manifests itself in particular in refinishes, which are only cured at temperatures up to 80.degree. C. Moreover, the aqueous coating materials described in EP-A-353 797 have an unsatisfactory storage stability if they contain a melamine resin as additional binder component.
EP-A-297 576 describes a process for the preparation of two-coat finishes of the type described above, in which the aqueous coating materials employed as basecoat contain an aqueous polymer dispersion which is obtainable by polymerizing ethylenically unsaturated monomers in an aqueous dispersion in the presence of a polyurethane resin which contains urea groups but no vinyl groups. If the aqueous coating materials described in EP-A-297 576 are employed as basecoats in the above-described process for the preparation of two-coat finishes, then the two-coat finishes obtained are in need of improvement with respect to their resistance to condensed water. Moreover, the aqueous coating materials described in EP-A-297 576 often display deficiencies in storage stability, and defects which can be traced back to instances of incompatibility, if combinations of different binders are employed.
DE-A-40 10 176 describes a process for the preparation of a two-coat finish, in which an aqueous basecoat is employed which contains as binder a polymer which is obtainable by polymerizing ethylenically unsaturated monomers in an organic solvent in the presence of a polyurethane resin which contains polymerizable double bonds and converting the resulting reaction product to an aqueous dispersion. If basecoats which contain metal pigments are employed in the process described in DE-A-40 10 176, the two-coat metallic finishes obtained are in need of improvement with respect to their metallic effect. Moreover, a disadvantage in the preparation of the aqueous basecoats described in DE-A-40 10 176 is that that a large quantity of organic solvents is required.
The object of the present invention was to provide a new process for the preparation of two-coat finishes of the type described above, with which two-coat finishes are obtained whose properties are improved in comparison with the prior art and in which, in particular, the above-described disadvantages of the prior art are absent or lessened.
This object is surprisingly achieved by a process for the preparation of a two-coat finish on a substrate surface, which process consists in going through the steps (1) to (4) described above and is characterized in that the basecoat contains as binder a polymer which can be obtained by subjecting an ethylenically unsaturated monomer or a mixture of ethylenically unsaturated monomers to free-radical polymerization in an aqueous dispersion of a polyurethane resin which has a number-average molecular weight of from 1000 to 3

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