Coating processes – With post-treatment of coating or coating material – Chemical agent applied to treat coating
Reexamination Certificate
2001-11-21
2003-09-02
Barr, Michael (Department: 1762)
Coating processes
With post-treatment of coating or coating material
Chemical agent applied to treat coating
C427S341000, C427S386000, C427S388400, C427S410000, C427S435000, C148S243000
Reexamination Certificate
active
06613387
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a method of improving the anticorrosive properties of an autodeposition coating on a metal substrate by a post-bath rinse using an aqueous rinse solution in order to form what is believed to be a modified phosphate at the surface of the substrate. More particularly, the invention relates to a method of enhancing the anticorrosive properties of an autodeposition coating on a metal substrate using an aqueous rinse solution containing Group IIA and/or IIB metal cations and phosphate anions.
BACKGROUND OF THE INVENTION
Over the last few decades, various water-based coatings for metallic surfaces have been developed that are commonly referred to in the field as autodeposition coatings. These coatings utilize an emulsion (latex) or dispersion of a resin that is able to form a protective coating when cured. The coating typically is applied by immersing the metallic surface in a bath containing the resin emulsion or dispersion, acid, and an oxidizing agent to form an adherent coating that is initially wet. The thickness of the coating can be affected, for example, by such factors as total solids, pH and oxidant concentration. The coating thickness also is a function of the immersion time. The initial wet coating is sufficiently adherent to remain attached to the metal surface on which it is formed against the influence of normal gravity and, if desired, can be rinsed before being cured by heating to convert the wet coating to a dry, solid and even more adherent coating. However, a coating produced in this manner does not always provide adequate resistance against corrosion for the metal substrate, as determined, for example, by standard cyclic corrosion testing. These coatings are not always stable and can delaminate when exposed to superheated steam, boiling water, or salt spray.
The corrosion resistance of certain autodeposited coatings is significantly improved by rinsing the adhered coating, prior to curing, in an aqueous solution containing chromium ions. However, appreciable chromium ion concentrations are required to give acceptable coatings. The chromium rinse step is undesirable from an economic and environmental perspective, since chromium compounds are generally both expensive and highly toxic.
Examples of the above-described autodeposition coating compositions and coating and rinsing procedures are more fully described in U.S. Pat. Nos. 3,063,877; 3,585,084; 3,592,699; 3,647,567; 3,791,431; 4,030,945; 4,186,226; 3,795,546; 4,636,265; 4,636,264; and 4,800,006, each of which is incorporated herein by reference in its entirety.
Although these prior processes and compositions have been reasonably effective for the intended purpose, there is a continuing need in the industry for improved coating processes.
SUMMARY OF THE INVENTION
The present invention is directed to a method for enhancing the corrosion resistance of autodeposition coatings. More particularly, the invention is directed to a method of improving the corrosion resistance of an autodeposition coating by using a rinse solution to form what is believed to be a modified metal phosphate at the surface of the metal.
In one embodiment, the present invention is a directed to a method of improving the corrosion resistance of a metallic surface having a cured autodeposited coating adhered thereto. The process comprises contacting an uncured autodeposited coating present on a metallic surface with an aqueous rinse containing effective amounts of at least one Group IIA or Group IIB metal cation source and at least one phosphate source.
Accordingly, one aspect of the invention is to provide a method of improving the corrosion resistance of an autodeposition coating using a rinse containing calcium nitrate and a phosphate source.
Another aspect of the invention is to provide a method of improving the corrosion resistance of an autodeposition coating using a rinse solution containing alkaline earth metal cations and phosphoric acid.
Another aspect of the invention is to provide a method of improving the corrosion resistance of an autodeposition coating using a rinse solution containing zinc cations and a phosphate source.
A further aspect of the invention is to provide a method of improving the corrosion resistance of an autodeposition coating using a rinse solution containing an alkaline earth metal compound, phosphoric acid and an accelerator such as hydroxylamine.
Still another aspect of the invention is to provide the foregoing method where the resin comprises an epoxy resin, an acrylic resin, or a combination of epoxy and acrylic resins.
Another aspect of the invention is to provide the foregoing method wherein the rinse solution step is maintained at a temperature of from about 20° C. to about 100° C. during contact with the uncured autodeposition coating.
A further aspect of the invention is to provide the foregoing method wherein the aqueous solution has a Group IIA and Group IIB metal cation concentration of from about 2 to about 300 mM/L, a phosphate source, and a pH of about 3.5 to about 4.0.
Another aspect of the invention is to provide the foregoing method wherein the rinse solution has a phosphate concentration of from about 10 mM/L to about 1000 mM/L.
In another embodiment, this invention provides a method for improving the anticorrosive properties of a resin (preferably, an epoxy resin, acrylic resin or epoxy-acrylic blended resin) autodeposited on a metal substrate, where the method comprises:
(a) contacting the metal substrate with an autodeposition bath containing the resin in emulsion form and an autodeposition activator until a layer of the resin of desired thickness (typically, about 5 to about 40 micrometers) is autodeposited on the metal substrate;
(b) rinsing the metal substrate having the layer of resin autodeposited thereon with a chromium-free aqueous solution prepared using from about 0.05 to about 5 weight percent (more preferably, about 0.1 to about 1 weight percent) of calcium nitrate; from about 0.1 to about 5 weight percent (more preferably, about 0.3 to about 1 weight percent) phosphoric acid; and from about 0.05 to about 5 weight percent (more preferably, about 0.1 to about 1.0 weight percent) hydroxylamine at a temperature of about 20° C. to about 100° C. at a pH of from about 3.5 to about 4.0 for an effective time to improve the anticorrosive properties of the resin; and
(c) curing the layer of resin autodeposited on said metal substrate following the rinsing step (b).
The process described herein does not require the use of chromium compounds of any type, yet surprisingly furnishes coatings which are very effective in protecting metallic substrates against corrosion, even under very severe environmental conditions. Moreover, high quality coatings may be easily achieved using the present process (i.e., the appearance of the cured autodeposited coating is not adversely affected by the rinse). Another advantage of the present process is that since contacting the substrate with the rinse solution takes place after the coating is deposited on the substrate surface, no aspect of the autodeposition step need be changed from what is conventionally practiced. That is, while it may in theory be possible to treat the surface of the metal substrate with a phosphating solution in order to form a phosphate conversion coating on the substrate surface prior to autodeposition, such a phosphate conversion coating would likely interfere with the desired deposition of the resin on the substrate surface so as to require significant readjustment of the autodeposition conditions. It was unexpected that such a phosphating step could effectively be practiced after the autodeposition coating had been formed on the substrate surface, since it was quite uncertain whether reaction of the metal surface could be effected with the autodeposited coating covering the metal surface and whether such reaction, if achieved, would adversely alter the curing of the autodeposited resin and the appearance and other properties of the cured coating.
The above-noted aspects of the inve
Agarwal Rajat K.
Brown Douglas
Butcher Graham
Fristad William E.
Iqbal Zafar
Barr Michael
Cameron Mary K.
Harper Stephen D.
Henkel Corporation
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