Coating processes – With post-treatment of coating or coating material – Chemical agent applied to treat coating
Reexamination Certificate
2000-07-14
2002-05-28
Cameron, Erma (Department: 1762)
Coating processes
With post-treatment of coating or coating material
Chemical agent applied to treat coating
C427S388400
Reexamination Certificate
active
06395336
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a method for coating metal surfaces in order to impart corrosion resistance thereto. More particularly, the invention relates to a method for the anticorrosion coating of metal surfaces in which a resin coating is formed on a metal surface using an autodepositing resin composition and the resulting resin coating is post-treated while still in its uncured condition in order to improve the corrosion resistance and adherence of the resin coating.
BACKGROUND OF THE INVENTION AND RELATED ART
Resin compositions that have the ability to form a resin coating on a metal surface by contact between the metal surface and an acidic aqueous autodeposition composition containing an organic coating-forming resin are already known in the art. Various examples thereof are taught in Japanese Published Patent Application (Kokoku or Examined) Number Sho 47-17630 (17,630/1972), Japanese Published Patent Application (Kokoku or Examined) Number Sho 52-21006 (21,006/1977), Japanese Published Patent Application (Kokoku or Examined) Number Sho 54-13435 (13,435/1979), and Japanese Patent Application Laid Open (Kokai or Unexamined) Number Sho 61-168673 (168,673/1986). A characteristic feature of these known resin coating compositions is that their contact with a suitable clean metal surface results in the formation of a resin coating whose thickness or weight increases with the time of contact. In addition, since coating formation in this technology is achieved by the chemical activity of the resin composition at the metal surface (metal ions eluted from the metal surface by etching are believed to act on the resin particles to induce deposition thereof on the metal surface), this technology can be distinguished from electrodeposition by its ability to efficiently form a resin coating on metal surfaces without the external application of electricity to the metal on which the coating is formed.
However, the coatings afforded by the prior-art autodepositing resin compositions have not always evidenced a satisfactory corrosion resistance or adherence. This has resulted in the development of a variety of tactics for engendering additional improvements in the corrosion resistance and adherence of the autodeposited resin coating residing on the metal surface. For example, a number of different methods are known in which the still uncured autodeposited resin coating is subjected to a chemical treatment (post-treatment).
U.S. Pat. No. 3,795,546 teaches that the post-drying corrosion resistance of the resin coating can be improved by exposing the uncured autodeposited resin coating prior to its thermal drying to an aqueous solution that contains approximately 2.5 to 50 g/L polyacrylic acid and hexavalent chromium. However, environmental pollution considerations make it desirable to avoid the use of chromium-containing treatment baths.
Japanese Patent Application Laid Open (Kokai or Unexamined) Number Sho 52-68240 (68,240/1977) teaches that the post-drying corrosion resistance of the resin coating can be improved by exposing the uncured autodeposited resin coating prior to its thermal drying to an aqueous dispersion or aqueous solution whose base component is 5 to 100 g/L nitrogenous organic compound, for example, at least 1 selection from the group consisting of amines, the amine salts of carboxylic adds, amino acids, melamine, and amides.
The post-treatment of autodeposited resin coatings with a nitrogenous organic compound is an effective means for improving the corrosion resistance and adherence of the ultimately obtained resin coating. However, since autodeposition coating is typically implemented by dipping or immersion, the acid component and/or metal ions component (for example, the iron ions) present in the uncured autodeposited resin coating will elute into the treatment solution used as the post-treatment agent (the post-treatment solution). Since this will frequently cause a loss of stability of the post-treatment solution, this approach may not always be acceptable when viewed from the perspective of producing a resin coating that evidences a continuously stable product quality.
PROBLEM(S) TO BE SOLVED BY THE INVENTION
The present invention provides a method for the anticorrosion coating of metal surfaces that produces an excellent adherence and resistance to corrosion and that maintains its ability to generate these properties even after its post-treatment solution has been used to treat a substantial amount of metal surface.
BRIEF SUMMARY OF THE INVENTION
It has been discovered that a high-quality autodeposited resin coating can be obtained by forming an uncured resin coating on a metal surface by bringing the metal surface into contact with an autodepositing coating composition; by then executing a post-treatment on the uncured resin coating prior to its thermal drying using a post-treatment solution that contains as its essential components a water-soluble amino resin bearing specific reactive functional groups and an amine compound; and by thereafter drying and curing the post-treated resin coating.
DETAILED DESCRIPTION OF THE INVENTION
A process according to the present invention for the anticorrosion coating of metal surfaces characteristically comprises operations of:
(I) forming an adherent wet uncured resin coating on a metal surface by bringing the metal surface into contact with an aqueous autodepositing resin composition that contains oxidizing agent, acid, and water-dispersible or water-soluble organic resin, said resin containing a product of the polymerization of at least one selection from carboxyl-functional condensation polymerizable monomers and hydroxyl-functional condensation polymerizable monomers;
(II) subjecting the resulting uncured resin coating to a post-treatment by bringing the aforesaid resin coating, while it is still in its uncured condition, into contact with an aqueous solution that contains:
from 10 to 100, or more preferably from 20 to 50, grams per liter, hereinafter usually abbreviated as “g/L”, of water-soluble amino resin that bears in each molecule at least two reactive functional groups selected from the alkoxymethylamino, methylolamino, and imino groups; and
from 0.5 to 5.0 g/L, or more preferably from 1.0 to 2.0 g/L, of at least one organic amine compound that does not contain more than 20, or, with increasing preference in the order given, not more than 18, 16, 14, 12, 10, or 8 carbon atoms per molecule; and
(III) subjecting the resin coating afforded by operation (II) as described immediately above to drying and curing.
The solution for the post-treatment of aqueous autodepositing resin compositions according to the present invention preferably contains its water soluble amino resin and its other amine compound as described above in a weight ratio from 2.0:1.0 to 200:1.0, or more preferably from 5.0:1.0 to 50:1.0.
When the post-treatment agent and post-treatment solution contain less than the specified amount of water-soluble amino resin, the ultimately obtained resin coating will usually exhibit an inadequate corrosion resistance and adherence. The effects provided by the water-soluble amino resin do not increase at amounts in excess of the specified upper limit, and such amounts are also problematic because they cause the content of the amine compound to be too low on a relative basis. More specifically, a too low content of amine compound causes the resulting post-treatment solution to have an unsatisfactory stability during continuous use; this results in variations in the quality of the resin coating product. A too high content of the amine compound causes the content of the water-soluble amino resin to be too low on a relative basis, resulting in an unsatisfactory corrosion resistance and adherence.
Each molecule of the water-soluble amino resin used by the present invention preferably contains at least two reactive functional groups represented by at least one OF the following chemical moiety formulas (1), (2), and (3):
The water-soluble amino resin used in the present invention can be selected from
Honda Takumi
Yanagi Mutsumi
Cameron Erma
Harper Stephen D.
Henkel Corporation
Jaeschke Wayne C.
Wisdom, Jr. Norvell E.
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