Coating processes – With post-treatment of coating or coating material – Heating or drying
Patent
1992-11-24
1995-02-28
Lusignan, Michael
Coating processes
With post-treatment of coating or coating material
Heating or drying
4274071, 427409, 524512, B05D 302
Patent
active
053935705
DESCRIPTION:
BRIEF SUMMARY
The invention relates to the production of a multicoat protective and/or decorative coating on a substrate surface, in which process (1) a pigmented basecoat composition is applied to the substrate surface, (2) a polymeric film is formed from the composition applied in stage (1), (3) a transparent aqueous topcoat composition containing a water-thinnable polyacrylate resin as binder and an amino resin as crosslinking agent is applied to the basecoat obtained in this way, and subsequently (4) the basecoat is baked together with the topcoat.
The invention also relates to aqueous coating compositions, to water-thinnable polyacrylate resins and to a process for the production of water-thinnable polyacrylate resins.
The process according to the preamble of claim 1 is known. It is used especially for the production of automotive metallic finishes (cf. for example U.S. Pat. No. 3,639,147, DE-A-3,333,072 and EP-A-38,127).
For economic and ecological reasons it is desirable to employ aqueous topcoat compositions in stage (3) of the so-called basecoat/clearcoat process described above.
The topcoat compositions must be capable of application by spraying using automatic painting equipment. For this purpose their solids content must, at the spray viscosity, be sufficiently high to obtain paint films of adequate thickness with two passes of the gun (cross passes), and the compositions must furnish baked paint films of good appearance (good flow-out, high gloss, good topcoat appearance . . . ).
The aqueous topcoat composition disclosed in Example 2 of EP-A-38,127 does not meet all of the above requirements.
The object of the present invention consists in providing aqueous topcoat compositions which meet the above requirements.
Surprisingly, this object is achieved by topcoat compositions containing a water-thinnable polyacrylate resin as binder, which resin can be obtained by adding, either successively or alternately in part amounts,
(I) one carboxyl group per molecule and is copolymerizable with (b1), (b2), (b3) and (a2), or of a mixture of such monomers, together with from carboxyl groups and is copolymerizable with (b1), (b2), (b3) and (a1), or of a mixture of such monomers, and a mixture (b), consisting of (a1) and (a2) and is essentially free from carboxyl groups, or a mixture of such (meth)acrylic acid esters, and (b1), (b3), (a1) and (a2), contains at least one hydroxyl group per molecule and is essentially free from carboxyl groups, or a mixture of such monomers, and, if appropriate, (b1), (b2), (a1) and (a2), is essentially free from carboxyl groups and is different from (b1) and (b2), or a mixture of such monomers, to an organic solvent or solvent mixture and carrying out a polymerization in the presence of at least one polymerization initiator, and,
(II) at the end of the polymerization, neutralizing, at least partially, the resultant polyacrylate resin and dispersing it in water, the sum of the proportions by weight of (a1) and (a2) always being 100% by weight and the type and amount of (b1), (b2), (b3), (a1) and (a2) being chosen so that the polyacrylate resin has a hydroxyl value of 40 to 200, preferably 60 to 140, an acid value of 20 to 100, preferably 25 to 50, and a glass transition temperature (T.sub.G) of -40.degree. C. to +60.degree. C., preferably -20.degree. C. to +40.degree. C.
Aqueous coating compositions containing water-thinnable amino resins and polyacrylate resins, prepared by grafting vinyl monomers containing hydroxyl-substituted or N-alkoxyalkyl-substituted amide groups to a carboxyl-containing polyacrylate resin, are described in JP-A-1,104,665. These coating compositions are used for the coating of foodstuff cans. JP-A-1,104,665 makes no reference whatsoever to the "basecoat-clearcoat" process and to the problems related to the automatic spray application of aqueous topcoat compositions which occur in this process.
The pigmented basecoat compositions to be applied in stage (1) of the process according to the invention are well known (cf. for example U.S. Pat. No. 3,639,147, EP-A-38,127, D
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Dangschat Manfred
Reimann Manfred
Schwarte Stephan
Westermann Carlos
BASF Lacke+Farben Aktiengesellschaft
Lusignan Michael
Sabourin Anne Gerry
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