Coating processes – With post-treatment of coating or coating material – Heating or drying
Patent
1999-03-05
2000-11-14
Cameron, Erma
Coating processes
With post-treatment of coating or coating material
Heating or drying
106 1412, 106 1413, 106 1415, 106 1418, B05D 302, B05D 716
Patent
active
061467049
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to the use of aqueous polymer dispersions that comprise as component I) at least one polymer and as component II) at least one colorant and/or a corrosion inhibitor, and also, if desired, a UV stabilizer, component II) being present almost completely dispersed or dissolved in the polymer phase, for protecting metallic surfaces against corrosion.
Corrosion, especially in connection with metallic surfaces, is a serious problem. Corrosion damage, including secondary damage as a result of corrosion, gives rise to costs which amount to several percent of the gross national product of Germany (see Rompps Chemie-Lexikon under "Korrosion"). Because of this, a wide variety of techniques have been developed for protecting metallic materials against corrosion. Examples of these which might be mentioned include coating, plating with gold, with chromium and other metals, galvanizing, and the electrolytic zinc-plating of metallic surfaces. The skilled worker is also aware of the combination of various metallizing techniques with a coating operation. Not all coatings are suitable for protecting the base material against corrosion or as the metallizing layer. Some coating materials, especially those applied from aqueous dispersion, often indeed make a direct contribution to the corrosion of the substrate. Moreover, under corrosive conditions, many coating materials give rise to decorative damage, such as embrittlement, cracking of the coating film, clouding or discoloration of the coating film.
In principle, however, it should be possible to solve the above-mentioned problems by the use of appropriate polymers as coatings. On environmental grounds, preference is given in this context to those polymers which are applied from aqueous dispersion to the surface that is to be protected, thereby making it possible to avoid the use of solvents relative to the earlier solvent-containing coating materials. A disadvantage that is found, however, is that emulsifiers are generally required when aqueous polymer dispersions are prepared. These emulsifiers remain in the dispersions, with the consequence that the coatings produced from the dispersions likewise comprise the emulsifiers. Under prolonged weathering, these emulsifiers may diffuse out of the coatings and thereby reduce the strength of the coating or even attack the metallic surface that is to be protected.
EP-A 201 702 and U.S. Pat. No. 4,693,909 describe emulsifier-free dispersions based on ethylene-acrylic acid copolymers, which are suitable for coating and thus for the protection of metallic surfaces against corrosion. Disadvantageous, however, is the fact that discoloration of the metal surface may occur in the course of curing the coating film at elevated temperature. Since the coating dispersions are generally colorless, the coated articles have a relatively unattractive appearance and are therefore no longer marketable.
This problem can in principle be remedied if white or colored pigments are added to the dispersions. By adding corrosion-inhibiting pigments it is possible, moreover, to enhance the protective effect of the coatings as well. Customarily, however, pigments of this kind are incorporated into the finished coating dispersion by means of an emulsifier, or are added to the dispersion in the form of a ready-made pigment dispersion that has been produced using an emulsifier. In addition to the fundamental disadvantages resulting from the use of emulsifiers, it is found that the handling properties are impaired by the pigments. For example, edge coverage of the workpieces is poor when only a single coat is applied. In addition, the color effect brought about by the pigments are generally pale and not particularly decorative. If the amount of emulsifier is reduced, such dispersions are generally no longer stable; if it is increased, the mechanical stability of the coating drops. Pigment-containing dispersions based on ethylene-acrylic acid copolymers are described, for example, in prior US Application No. U.S. Ser. No. 08/625,151.
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Brodt Gregor
Kingma Arend Jouke
Schroder Ulrich
BASF - Aktiengesellschaft
Cameron Erma
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