The invention relates to a method of enhancing the intercoat adhesion in a multicoat paint system.
A process for producing multicoat paint systems, wherein                1) the substrate is coated with a primer, in particular an electrocoat material,        2) the primer film applied in stage 1, in particular an electrocoat film, is optionally predried and cured,        3) the cured primer coat, in particular electrocoat, is coated with a first aqueous coating material,        4) the coating film applied in stage 3 is optionally predried and, without a curing step, is coated with a second aqueous coating material,        5) the coating film applied in stage 4) is coated with a transparent coating material, and        6) the films are cured together,is known, for example, from European patent EP 0 788 523 B 2. This process is referred to below for the sake of brevity as an “integrated wet-on-wet process”. It results in a multicoat paint system made up of        primer, especially electrocoat 1,        aqueous basecoat or functional or surfacer coat 2,        aqueous basecoat 3, and        clearcoat 4.        
With this process it is possible to obtain multicoat paint systems having particularly low coat thicknesses without quality detractions particularly in connection with the stonechip resistance. Critical to the process is that the aqueous coating material or aqueous basecoat material 1 applied in stage 3) comprises a binder composed of a water-dilutable polyurethane resin which has an acid number of from 10 to 60 and a number-average molecular weight of from 4000 to 25000 and is preparable by reacting with one another                a polyester- and/or polyetherpolyol having a number-average molecular weight of from 400 to 5000 or a mixture of such polyester- and polyetherpolyols,        a polyisocyanate or a mixture of polyisocyanates,        a compound containing per molecule at least one isocyanate-reactive group and at least one group capable of forming anions, or a mixture of such compounds, and, if desired,        a hydroxyl- and/or amino-containing organic compound having a molecular weight of from 40 to 400 or a mixture of such compounds and at least partly neutralizing the resultant reaction product.        
German patent applications DE 198 41 842 A 1, DE 100 01 442 A 1, and DE 100 55 464 A 1 disclose pseudoplastic powder clearcoat slurries free from organic solvents and external emulsifiers and preparable by    1) emulsifying an organic solution comprising binder and crosslinker to give an emulsion of the oil-in-water type,    2) removing the organic solvent or solvents, and    3) replacing some or all of the solvent volume removed with water to give the powder clearcoat slurry containing solid spherical particles.
This process is referred to below as the “secondary dispersion process”.
Furthermore, German patent application DE 101 26 651 A 1 and German patent 100 06 673 C 1 disclose powder clearcoat slurries preparable by    1) emulsifying at least one constituent of the powder clearcoat slurry in the liquid state, in particular in the melt state, in an aqueous medium to form an aqueous emulsion of liquid particles and    2) cooling the emulsion so that an aqueous suspension of dimensionally stable particles is formed.
This process is referred to below as the “melt emulsification process”.
Additionally, the powder clearcoat slurries produced by means of the secondary dispersion process or the melt emulsification process are referred to below as “second generation powder clearcoat slurries”.
If these known second generation powder clearcoat slurries prepared by the secondary dispersion process or melt emulsification process are employed instead of the two-component clearcoat material in stage 5) of the integrated wet-on-wet process known from European patent EP 0 788 523 B 2 they result in multicoat paint systems which have very good optical properties, very good leveling, very good intercoat adhesion, high chemical resistance, and weathering stability. Moreover, the multicoat paint systems are free from pops and from cracks (mud cracking). Within the art it has been found, however, that the adhesion between the aqueous basecoat 2 and the clearcoat is lost following condensation exposure and/or stonechip exposure, with the consequence of clearcoat delamination on subsequent exposure to steam jets and/or hot water jets.
This problem does not occur to the same extent with the multicoat paint systems known from German patent applications DE 199 48 004 A 1 and DE 100 39 262 A, produced by means of the integrated wet-on-wet technique. The clearcoats of these multicoat paint systems are produced, however, from powder clearcoat slurries produced by extrusion of the constituents, comminution of the resulting cooled melt, dry grinding of the resulting chips, and wet grinding, as is known, for example, from European patent application EP 0 652 264 A 1, international patent application WO 96/32452 A 1, American patent U.S. Pat. No. 4,268,542 A 1 or German patent applications DE 195 18 392 A 1, DE 196 13 547 A 1, and DE 198 14 471 A 1. The powder clearcoat slurries produced by this process are referred to below as “first generation powder clearcoat slurries”.
This process for producing first generation powder clearcoat slurries is, however, considerably more complicated than the secondary dispersion process or the melt emulsification process. Moreover, the mechanical load on second generation powder clearcoat slurries during their preparation is lower than on the first generation powder clearcoat slurries. In addition, the physical composition of the second generation powder clearcoat slurries can be varied very much more widely than that of the first generation powder clearcoat slurries.
Accordingly, the use of the second generation powder clearcoat slurries in the integrated wet-on-wet process would be preferred over the use of the first generation powder clearcoat slurries if only it were possible to eliminate the problems of intercoat adhesion following exposure to condensation and/or stone chipping in the multicoat paint systems in question.
German patent DE 100 27 290 C 2 and German patent application DE 100 27 292 A 1 disclose multicoat paint systems which can be produced by the integrated wet-on-wet process. It is also possible here to use second generation powder clearcoat slurries, among others.
The aqueous coating materials used in stage 3) can be selected from a large number of aqueous surfacers or aqueous basecoat materials. Thus all of the aqueous surfacers described in documents EP 0 401 565 A 1 or EP 0 817 684 A 1, U.S. Pat. No. 4,537,926 A 1, EP 0 529 335 A 1, EP 0 595 186 A 1, EP 0 639 660 A 1, DE 44 38 504 A 1, DE 43 37 961 A 1, WO 89/10387, U.S. Pat. No. 4,450,200 A 1, U.S. Pat. No. 4,614,683 A 1 or WO 94/26827 and all of the aqueous basecoat materials described in documents EP 0 089 497 A 1, EP 0 256 540 A 1, EP 0 260 447 A 1, EP 0 297 576 A 1, WO 96/12747, EP 0 523 610 A 1, EP 0228 003 A 1, EP 0 397 806 A 1, EP 0 574 417 A 1, EP 0 531 510 A 1, EP 0 581 211 A 1, EP 0 708 788 A 1, EP 0 593 454 A 1, DE 43 28 092 A 1, EP 0 299 148 A 1, EP 0 394 737 A 1, EP 0 590 484 A 1, EP 0 234 362 A 1, EP 0 234 361 A 1, EP 0 543 817 A1, WO 95/14721, EP 0 521 928 A1, EP 0 522 420 A1, EP 0 522 419 A 1, EP 0 649 865 A 1, EP 0 536 712 A 1, EP 0 596 460 A 1, EP 0 596 461 A 1, EP 0 584 818 A 1, EP 0 669 356 A 1, EP 0 634 431 A 1, EP 0 678 536 A 1, EP 0 354 261 A 1, EP 0 424 705 A 1, WO 97/49745, WO 97/49747, and EP 817 684 A 1 are said to be suitable.
Some of these aqueous basecoat materials comprise dispersed graft copolymers whose particles are composed of                a hydrophobic core of at least one copolymerized olefinically unsaturated monomer and        a hydrophilic shell which comprises or consists of at least one polyurethaneand are prepared by free-radical polymerization of the monomers in the presence of a polyurethane having terminal and/or lateral allyl and/or (meth)acrylate groups. No differentiation is made between these graft copolymers and other polyurethanes which are not graft copolymers in terms of the performance properties of the paint systems produced therewith.        
The examples of German patent DE 100 27 290 C 2 and of German patent application DE 100 27 292 A 1, however, merely reveal multicoat paint systems produced by the integrated process of European patent EP 0 788 523 B 2 using first generation powder clearcoat slurries, and accordingly these documents offer no suggestions or indications toward solving the described problems of the intercoat adhesion.