Coating processes – Applying superposed diverse coating or coating a coated base – Synthetic resin coating
Reexamination Certificate
2002-10-24
2004-07-13
Cameron, Erma (Department: 1762)
Coating processes
Applying superposed diverse coating or coating a coated base
Synthetic resin coating
Reexamination Certificate
active
06761933
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to corrosion-resistant coated metal substrates and, more particularly, to ferrous and non-ferrous metal substrates having an environmentally friendly chrome-free and nickel-free pretreatment that inhibits corrosion of the metal substrate.
BACKGROUND OF THE INVENTION
Pretreating metal substrates with a phosphate conversion coating and rinsing with a chrome-containing sealer are well known for promoting corrosion resistance and improving the adhesion of subsequently applied decorative and protective coatings. Cationic electrodeposition compositions are typically applied over phosphated steel substrates to further improve corrosion resistance. While the combination of phosphate conversion coating and electrodeposited coating provides superior corrosion resistance, heavy metals typically used in such coatings can provide environmental disposal concerns. For example, phosphate conversion coating compositions typically contain heavy metals such as nickel, and post-rinses contain chrome. Also, conventional phosphating processes can require several stages that occupy a large amount of physical space in a plant and require significant capital investment. Another drawback of conventional phosphating processes is the difficulty in coating mixed-metal objects including aluminum. In addition, many pretreatment and post-rinse compositions are suitable for use over only a limited number of substrates or over substrates that must be phosphated first, or are not suitable for use without some other treatment.
It would be desirable to provide a simplified pretreatment process free of heavy metals for coating metal substrates, including mixed metal substrates such as those commonly found on today's automobile bodies. Such a pretreatment process, when combined with heavy-metal free coatings, would provide an environmentally friendly alternative for providing corrosion resistance to metal substrates.
SUMMARY OF THE INVENTION
The present invention is directed to a method for coating an untreated metal substrate by contacting the substrate sequentially with two different pretreatment solutions. The first pretreatment solution comprises a Group IIIB and/or IVB metal-containing compound and nitrate. The second pretreatment solution comprises a reaction product of at least one epoxy-functional material or derivative thereof and at least one material selected from the group consisting of phosphorus-containing materials, amine-containing materials, sulfur-containing materials and mixtures thereof. Following the two steps, the substrate is coated with a composition comprising a film-forming resin. The resin can then be cured by any means appropriate for curing the resin.
It is significant that the present methods are directed to untreated metal substrates. As used herein, the term “untreated” means a bare metal surface; that is, the metal surface has not been phosphated or subjected to any other type of conversion coating. Following contact with the two pretreatment solutions, the substrate is then rinsed and/or directly coated with, for example, a pigmented coating comprising a film-forming resin. The coated substrates that result exhibit excellent corrosion resistance. It is significant that this corrosion resistance is achieved with the use of chrome-free and heavy metal-free pretreatment solutions. Thus, an environmentally friendly method is provided, wherein corrosion resistance is not sacrificed.
DETAILED DESCRIPTION OF THE INVENTION
The present invention is directed to a method for coating an untreated metal substrate comprising contacting the substrate with a first pretreatment solution comprising a Group IIIB and/or IVB metal-containing compound and nitrate; subsequently contacting the substrate with a second pretreatment solution comprising a reaction product of at least one epoxy-functional material or derivative thereof and at least one material selected from the group consisting of phosphorus-containing materials, amine-containing materials, sulfur-containing materials and mixtures thereof; and coating the substrate with a composition comprising a film-forming resin.
Both ferrous and non-ferrous metal substrates can be treated according to the present invention. Examples of ferrous metals include cold rolled steel substrates, galvanized steel substrates, including electrogalvanized steel, hot dipped galvanized steel, galvanneal (an Fe/Zn alloy), and stainless steel. Nonferrous metals including, for example, aluminum, magnesium, and copper. It will be appreciated that many substrates that are suitable for treatment according to the present invention will include both ferrous and non-ferrous metals (i.e. “mixed metals”). For example, many automobile assemblies contain both galvanized steel and aluminum. It is an advantage of the present invention that the same composition can be used to treat all of these substrates, with suitable corrosion protection being offered to each. In addition, the untreated metal substrate suitable for use in the present methods may be a cut edge of a substrate that is otherwise treated and/or coated over the rest of its surface.
The substrate to be coated is usually first cleaned to remove grease, dirt, or other extraneous matter. This is done with conventional cleaning procedures and materials, including mild or strong alkaline cleaners that are commercially available and conventionally used in metal pretreatment processes. Examples of alkaline cleaners include CHEMKLEEN 611L, CHEMKLEEN 163 and CHEMKLEEN 177, all of which are available from PPG Industries, Inc. Such cleaners are generally followed and/or preceded by a water rinse.
Following the optional cleaning step, the metal surface is contacted with the first pretreatment solution. As noted above, this solution comprises a Group IIIB and/or IVB metal. The Group IIIB and IVB metals referred to herein are those elements referred to as transition metals and rare earth metals and which are included in such groups in the CAS Periodic Table of the Elements as is shown, for example, in the
Handbook of Chemistry and Physics
, 63rd Edition (1983). It will be appreciated that Group IIIB includes the lanthanides and actinides. Especially suitable Group IIIB and IVB transition metals and rare earth metals are those that contain zirconium, titanium, hafnium, yttrium and cerium and mixtures thereof. The Group IIIB and/or IVB metals can be introduced in various forms, including as nitrates, acetates, sulfamates, lactates, glycolates, formates and dimethylol propionates, where such compounds exist. Typical zirconium compounds may be selected from hexafluorozirconic acid and alkali metal and ammonium salts thereof, ammonium zirconium carbonate, zirconium sulfate, zirconyl nitrate, zirconium carboxylates and zirconium hydroxy carboxylates such as hydrofluorozirconic acid, zirconium acetate, zirconium oxalate, ammonium zirconium glycolate, ammonium zirconium lactate, ammonium zirconium citrate, and mixtures thereof. Hexafluorozirconic acid is especially suitable. An example of a titanium compound is fluorotitanic acid and its salts. An example of a hafnium compound is hafnium nitrate. An example of a yttrium compound is yttrium nitrate. An example of a cerium compound is cerous nitrate.
The first pretreatment solution also comprises nitrate. The nitrate can be introduced in any form; sodium nitrate is especially suitable. It will be appreciated that nitrate can be introduced to the first pretreatment solution when the Group IIIB and/or IVB metals are in their nitrate forms. The amount of nitrate introduced in this manner, however, will typically not be sufficient to achieve the desired molar ratio of nitrate to Group IIIB/IVB metal, discussed further below. For example, the introduction of 500 ppm of zirconyl nitrate to the solution will result in a nitrate to zirconium molar ratio of about 2:1. Increasing the amount of zirconyl nitrate added to the solution would serve to increase the concentration of both the zirconium and the nitrate without changing the molar ratio
Sudour Michel
Tolz Andreas
Warburton Yi J.
Cameron Erma
Meyers Diane R.
PPG Industries Ohio Inc.
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