Coating processes – With post-treatment of coating or coating material – Heating or drying
Reexamination Certificate
2001-03-08
2002-10-08
Cameron, Erma (Department: 1762)
Coating processes
With post-treatment of coating or coating material
Heating or drying
C427S388100
Reexamination Certificate
active
06461682
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a composition and method for its use in preventing or inhibiting corrosion of metals, particularly aluminum and aluminum alloys. Treatment of the metal surfaces with the composition of this invention inhibits corrosion of that surface and also improves adhesion of subsequent organic coatings to the treated surface.
Many metals are susceptible to corrosion. In this regard, atmospheric corrosion is of particular concern. Such corrosion may affect the performance and/or appearance of the metals affected, and the products produced therefrom. In addition, when polymer coatings such as paints, adhesives or sealants are applied to the metal, corrosion of the underlying metal may cause a loss of adhesion between the polymer coating and the base metal. A loss of adhesion between the polymer coating and the base metal may similarly lead to corrosion of the metal. Aluminum and aluminum alloys frequently require corrosion protection and improvements in adhesion between the base aluminum (or aluminum alloys) and subsequent polymer coatings. Aluminum alloys, in particular, can be susceptible to corrosion since the alloying elements used to improve the metal's mechanical properties may decrease corrosion resistance.
Specifications for testing the effectiveness of the corrosion inhibition and adhesion promotion of various treatments have been established. Examples of the foregoing include ASTM standard D3359-87, Military specification MIL-C-5541D and ASTM standard B117.
Prior art techniques for improving corrosion resistance of metals widely employ the use of chromate conversation coatings to passivate the surface. Such chromate treatments are undesirable, however, because the chromium used is highly toxic, carcinogenic, and environmentally undesirable. Phosphate conversion coatings are also used, but generally provide substantially less corrosion protection unless used in conjunction with a chromate.
Recently, various techniques for eliminating the use of chromates in corrosion inhibition and adhesion promotion treatments have been proposed. U.S. Pat. No. 5,108,793 discusses treating the metal with an inorganic silicate followed by treating the silicate coating with a silane. U.S. Pat. No. 5,292,549 teaches the treatment of metals with a solution of a silane and a crosslinking agent in order to form a siloxane film for temporary corrosion protection.
U.S. Pat. No. 5,200,275 discusses treating a lead or tin coated steel surface with a silicate and a metal salt followed by a further optional treatment with a silane. U.S. Pat. No. 5,759,629 discusses the use of a hydrolyzed vinyl silane in a method for inhibiting corrosion on metal sheet. U.S. Pat. No. 5,322,713 discusses the treatment of metal sheet with an alkaline aluminate coating followed by a rinse with a hydrolyzed organofunctional silane.
U.S. Pat. No. 5,759,629 discusses treatment of a metal sheet with a hydrolyzed vinyl silane for corrosion inhibition. U.S. Pat. No. 5,750,197 discusses treatment of metals with a solution containing a multifunctional silane having at least two trisubstituted silyl groups, wherein the substituents are either alkoxy or acetoxy. An optional second treatment solution containing an organofunctional silane may also be employed, particularly if the metal is to be painted.
Many of the foregoing proposed techniques, however, have been proven to be ineffective, or to require time consuming, energy inefficient, multi-step processes. Thus, there remains a need for a simple, low cost, effective technique for inhibiting corrosion of metals, particularly for aluminum and aluminum alloys.
It is an object of this invention to provide an improved method of inhibiting corrosion of metals, especially aluminum and aluminum alloys, which is simple to apply, cost effective and environmentally friendly. It is another object of this invention to provide a treatment for metals which improves the adhesion of subsequent organic coatings to the metal while at the same time improving the corrosion resistance of the metal.
SUMMARY OF THE INVENTION
The foregoing objectives can be accomplished by treating a metal, particularly aluminum or aluminum alloys, with a treatment composition comprising a mercapto-substituted silane. The treatment composition is applied directly to the metal surface, preferably without any intervening treatment other than cleaning, deoxidizing or etching, by spray, flood or other means of direct contact. The treatment composition is preferably applied at room temperature and preferably has a pH between 3 and 5.
Preferably, the metal surface is cleaned, deoxidized and/or etched prior to treatment with the mercapto-substituted silane. A variety of known cleaners, deoxidizers and etchants may be employed for this purpose, with the appropriate choice being made with the specific metal surface to be prepared in mind.
Once the mercapto-substituted silane is applied to the metal surface the treated metal should be baked in order to dry and cure the coating. After curing, the treatment with the mercapto-substituted silane may be repeated and baked again, as desired.
The compositions and methods of this invention are particularly suitable for treating aluminum and aluminum alloys. The inventors have found that treating aluminum or aluminum alloys with the mercapto-substituted silane containing composition of this invention provides both increased corrosion resistance and enhanced adhesion of subsequent organic coatings to the treated surface.
DETAILED DESCRIPTION OF THE INVENTION
The inventors have found that the corrosion resistance of metal, particularly aluminum and aluminum alloys, can be greatly enhanced by applying a treatment solution comprising a mercapto-substituted silane, preferably followed by a bake dry and cure. The mercapto-substituted silane of this invention is preferably a multifunctional mercapto-substituted organic silane of the following chemical formula:
Wherein R
1
, R
2
and R
3
are independently selected from the group consisting of alkoxy groups (e.g., methoxy, ethoxy, or similar alkoxy groups), alkyl groups and hydrogen groups, and wherein n is an integer from 0 to 10, but is preferably an integer from 1 to 4. A particularly preferred mercapto-substituted silane is gamma-mercaptopropyltrimethoxy silane which has the following structure:
The inventors have found gamma-mercaptopropyltrimethoxy silane to be the most preferred mercapto-substituted silane for use in this invention. Gamma-mercaptopropyltrimethoxy silane is available from the Witco Corporation of Greenwich, Conn. under the tradename SILQUEST® A-189 Silane. Other useful mercapto-substituted silanes include (3-mercaptopropyl) methyl dimethoxy silane available from the Fluka Chemie AG of Buchs, Switzerland. The inventors have unexpectedly discovered that the mercapto group on these mercapto-substituted silanes provides significantly better corrosion protection than is provided by other silanes which do not have the mercapto substituted group. Thus, the inventors believe that the mercapto-substitution on the silane provides significant and unexpected improvements in corrosion protection and adhesion promotion in relation to non-mercapto-substituted silanes.
The mercapto-substituted silane is preferably solubilized, with a suitable solvent, into a treatment solution. The concentration of mercapto-substituted silane in the treatment solution may be from 0.5 to 10% by weight but is preferably from 2 to 4% by weight. Mercapto-substituted silanes, such as gamma-mercaptopropyltrimethoxy silane, may not be soluble in water alone to the extent necessary to practice this invention effectively. The inventors have discovered that a combination of water with an appropriate solvent are required to effectively solubilize these mercapto-substituted silanes in a water matrix. Appropriate solvents for this purpose include N-methyl-2-pyrrolidone, butyrolactone, diethylene glycol butyl ether, hexylene glycol, ethylene glycol monobutyl ether and alcohols. Most preferred as a solubilizati
Crotty David
Girard Justin
Nahlawi Tarek
Cameron Erma
Carmody & Torrance LLP
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