Coating processes – With post-treatment of coating or coating material – Heating or drying
Reexamination Certificate
1999-11-01
2001-07-17
Cameron, Erma (Department: 1762)
Coating processes
With post-treatment of coating or coating material
Heating or drying
C427S409000, C427S387000, C427S388100, C427S388400
Reexamination Certificate
active
06261638
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of preventing corrosion of metal. More particularly, the method comprises applying a solution containing one or more multi-functional silanes to a metal substrate. If the metal is to be painted, this treatment may be followed by applying a solution of an organofunctional silane. The method is particularly useful for the treatment of cold-rolled steel, aluminum and aluminum alloys.
2. Description of Related Art
Most metals are susceptible to corrosion, in particular atmospheric corrosion. Such corrosion will significantly affect the quality of such metals, as well as that of the products produced therefrom. Although this corrosion may sometimes be removed from the metal, such steps are costly and may further diminish the utility of the final product. In addition, when polymer coatings such as paints, adhesives, or rubbers are applied to the metal, corrosion of the base metal material 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 can likewise lead to corrosion of the metal. Aluminum alloys are particularly susceptible to corrosion as the alloying elements used to improve the metals mechanical properties (e.g., copper, magnesium and zinc) will decrease corrosion resistance.
Prior art techniques for improving corrosion resistance of metal, particularly metal sheet, include passivating the surface by means of a heavy chromate treatment. Such treatment methods are undesirable, however, because the chromium is highly toxic, carcinogenic and environmentally undesirable. It is also known to employ a phosphate conversion coating in conjunction with a chromate rinse in order to improve paint adherence and provide corrosion protection. It is believed that the chromate rinse covers the pores in the phosphate coating, thereby improving the corrosion resistance and adhesion performance. Once again, however, it is highly desirable to eliminate the use of chromate altogether. Unfortunately, the phosphate conversion coating is generally not effective without the chromate rinse.
Recently various techniques for eliminating the use of chromate have been proposed. These include the steps of providing an aqueous alkaline solution comprising an inorganic silicate and a metal salt in an amount to coat a steel sheet, followed by treating the silicate coating with an organofunctional silane (U.S. Pat. No. 5,108,793). U.S. Pat. No. 5,292,549 teaches the rinsing of metal sheet with an aqueous solution containing low concentrations of an organofunctional silane and a crosslinking agent, such as bistrimethoxy silylethane in order to provide temporary corrosion protection. The crosslinking agent cross links the organofunctional silane to form a denser siloxane film. In one embodiment an organofunctional silane solution is used to treat a metal surface followed by application of a second treatment solution 3 containing a crosslinking agent. One significant drawback to the methods of this patent is that the organofunctional silane will not bond well to the metal surface, and thus the coating may be easily rinsed off.
U.S. Pat. No. 5,393,353 describes a method for preparing a chromium-free black zinc-nickel alloy surface containing at least about 8% nickel in the alloy. The method comprising contacting said surfaces with an aqueous acid solution of an inorganic acid, preferably a phosphorus acid, the solution optionally additionally containing at least one silane, such as bistrimethoxysilylethane.
U.S. Pat. No. 4,689,085 describes coupling agent and primer compositions comprising a conventional silane couplying agent and a disilyl cross linker compound in the same composition solution.
U.S. Pat. No. 5,433,976 teaches the rinsing of a metal sheet with an alkaline solution containing a dissolved silicate or aluminate, an organofunctional silane and a cross-linking agent in order to form an insoluble composite layer containing siloxane.
The water solubility of some silanes may be limited creating solubility problems in the aqueous solution. Additionally, some metal substrates are particularly prone to corrosion upon contact with water, thus, an aqueous solution may be undesirable in some circumstances.
Various other techniques for preventing the corrosion of metal sheets have also been proposed. Many of these proposed techniques, however are ineffective, or require time-consuming, energy-inefficient, multi-step processes. Thus, there is a need for a simple, low-cost technique for preventing corrosion of metals, particularly metal sheet and cast aluminium or aluminium alloys, including those instances wherein a polymer coating such as paint is to be applied to the metal.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an improved method of preventing corrosion of metals.
It is another object of the present invention to provide a method of preventing corrosion of metals, wherein the treatment compositions employed need not be removed prior to painting or coating with another polymer such as adhesives or rubber coatings.
The foregoing objects can be accomplished, in accordance with one aspect of the present invention, by providing a method of pretreating a metal substrate to improve corrosion resistance, comprising the steps of:
(a) providing a metal substrate; and
(b) applying a first treatment solution directly onto the surface of the metal, the first treatment solution containing at least one multi-functional silane having at least two trisubstituted silyl groups wherein the substituents are individually chosen from the group consisting of alkoxy and acetoxy, wherein the multi-functional silane has been at least partially hydrolyzed.
The first treatment solution preferably has a pH of less than about 7 to ensure proper hydrolysis. One significant advantage of the present invention is that the first treatment solution may be applied directly onto the surface of the metal without an underlying layer of a silicate, aluminate or other coating as required by many of the prior art treatment methods.
More than one multi-functional silane may be employed, and each may comprise:
wherein X is either a bond, an aliphatic or aromatic group, each R is an alkyl or acetyl group, and n is 2 or 3. Preferably each R is individually chosen from the group consisting of: ethyl, methyl, propyl, iso-propyl, butyl, iso-butyl, sec-butyl, ter-butyl and acetyl. X may be chosen from the group consisting of: C
1
-C
6
alkyl, C
1
-C
6
alkenyl, C
1
-C
6
alkyl substituted with at least one amino group, C
1
-C
6
alkenyl substituted with at least one amino group, benzyl, and benzyl substituted with C
1
-C
6
alkyl. The preferred multi-functional silane is 1,2-bis-(triethoxysilyl)ethane (BTSE).
The multi-functional silane coating is also preferably cured at a temperature of between about 60° C. and about 250° C. for between about one minute and about 2 hours. After curing, the first treatment solution may be reapplied, and cured if desired.
If the metal substrate is to be painted or coated with another polymer such as an adhesive or rubber, a second treatment solution may be applied after application of the first treatment solution. This second treatment solution contains at least one (and possibly multiple) organofunctional silane which has been at least partially hydrolyzed. The organofunctional silane preferably has a trisubstituted silyl group, wherein the substituents are individually chosen from the group consisting of alkoxy and acetoxy, and at least one organofunctional group. The organofunctional group may be chosen from the group consisting of: amino (with any number of amino moieties), vinyl, ureido, epoxy, mercapto, cyanato, methacrylato, and vinylbenzyl. Particularly preferred organofunctional silanes include: &ggr;-APS, &ggr;-MPS, &ggr;-UPS, &ggr;-GPS, vinyltrimethoxysilane, vinyltriethoxysilane, and vinyltriacetoxysilane. The methods of the present invention are particularly suitable for treating aluminum and a
Subramanian Vijay
van Ooij Wim J.
Zhang Chunbin
Cameron Erma
Dinsmore & Shohl LLP
University of Cincinnati
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