Stock material or miscellaneous articles – Composite – Of metal
Utility Patent
1999-05-11
2001-01-02
Gorgos, Kathryn (Department: 1741)
Stock material or miscellaneous articles
Composite
Of metal
48, 48, C148S257000, C204S487000
Utility Patent
active
06168868
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an improved process for applying a lead-free coating via electrodeposition to a metal substrate, including ferrous substrates such as cold rolled steel and electrogalvanized steel, and to the coated substrates produced by this process.
Pretreating metal substrates with a phosphate conversion coating and chrome-containing rinses has long been conventional for promoting corrosion resistance. To maximize corrosion resistance over steel substrates, cationic electrodeposition compositions are conventionally formulated with lead as either a pigment or a soluble lead salt and are applied over pretreated (phosphated and chrome rinsed) substrates. Disadvantages associated with phosphating include the amount of plant space required for processing due to multiple (usually eleven to twenty-five) stages; high capital cost; and generation of waste streams containing heavy metals, requiring expensive treatment and disposal. Additionally, lead and chromium used in the electrodepositable composition can cause environmental concerns. The lead may be present in the effluent from electrodeposition processes and chromium may be present in the effluent from pretreatment processes, and these metals need to be removed and disposed of safely, which again requires expensive waste treatment processes.
Nickel-free phosphate solutions and chrome-free rinsing compositions demonstrating corrosion resistance comparable to the nickel- and chrome-containing forerunners are now being sought. Likewise, lead-free electrodepositable compositions are being developed.
U.S. Pat. No. 3,966,502 discloses treatment of phosphated metals with zirconium-containing rinse solutions. International Patent publication WO 98/07770 discloses lead-free electrodepositable compositions for use over phosphated metals. Neither reference teaches treatment or coating processes for bare metal substrates; i.e., metals that have not been phosphated.
It would be desirable to provide a process for coating metal substrates, particularly bare ferrous metals, using compositions that overcome the environmental drawbacks of the prior art and which demonstrate excellent corrosion resistance.
SUMMARY OF THE INVENTION
In accordance with the present invention, an improved process for applying a lead-free coating by electrodeposition to an untreated metal substrate is provided. By “untreated” is meant a bare metal surface; i.e., one that has not been phosphated. The process comprises the following steps:
a) contacting the substrate surface with a group IIIB or IVB metal compound in a medium, typically an aqueous medium, that is essentially free of accelerators needed to form phosphate conversion coatings; followed by
b) electrocoating the substrate with a substantially lead-free, curable electrodepositable composition; and
c) curing the electrodepositable composition.
The process may further include initial steps of cleaning the substrate with an alkaline cleaner and rinsing with an acidic rinse.
DETAILED DESCRIPTION
Other than in the operating examples, or where otherwise indicated, all numbers expressing quantities of ingredients or reaction conditions used in the specification and claims are to be understood as modified in all instances by the term “about”.
The process of the present invention is typically used to treat cold rolled steel substrates, but can be used to treat other metal substrates such as galvanized steel and aluminum, which are used in the assembly of automobile bodies along with cold rolled steel. Moreover, the bare metal substrate being treated by the process of the present invention 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 by employing conventional cleaning procedures and materials. These would include mild or strong alkaline cleaners such as are commercially available and conventionally used in metal pretreatment processes. Examples of alkaline cleaners include Chemkleen 163 and Chemkleen 177, both of which are available from PPG Industries, Pretreatment and Specialty Products. Such cleaners are generally followed and/or preceded by a water rinse.
Following the optional cleaning step, the metal surface is contacted with a group IIIB or IVB metal compound which is in a medium that is essentially free of accelerators needed to form phosphate conversion coatings. Such accelerators include hydroxylamine, sodium nitrite, and other accelerators known in the art. It is believed that because no phosphate crystal structures are to be formed on the metal substrate surface, no acclerators are necessary. The medium may also be substantially free of phosphates, particularly phosphates of other metals such as zinc, iron, and other metals typically used in phosphating pretreatment processes.
The group IIIB or IVB metal compound is typically in an aqueous medium, usually in the form of an aqueous solution or dispersion depending on the solubility of the metal compound being used. The aqueous solution or dispersion of the group IIIB or IVB metal compound may be applied to the metal substrate by known application techniques, such as dipping or immersion, which is preferred, spraying, intermittent spraying, dipping followed by spraying or spraying followed by dipping. Typically, the aqueous solution or dispersion is applied to the metal substrate at solution or dispersion temperatures ranging from ambient to 150° F. (ambient to 65° C.), and preferably at ambient temperatures. The contact time is generally between 10 seconds and five minutes, preferably 30 seconds to 2 minutes when dipping the metal substrate in the aqueous medium or when the aqueous medium is sprayed onto the metal substrate.
The IIIB or IVB transition metals and rare earth metals referred to herein are those elements 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).
Preferred group IIIB and IVB transition metal compounds and rare earth metal compounds are compounds of zirconium, titanium, hafnium, yttrium and cerium and mixtures thereof. Typical zirconium compounds may be selected from hexafluorozirconic acid, alkali metal and ammonium salts thereof, ammonium zirconium carbonate, 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 preferred. An example of the titanium compound is fluorotitanic acid and its salts. An example of the hafnium compound is hafnium nitrate. An example of the yttrium compound is yttrium nitrate. An example of the cerium compound is cerous nitrate. The group IIIB or IVB metal compound is present in the medium in an amount of 10 to 5000 ppm metal, preferably 100 to 300 ppm metal. The pH of the aqueous medium usually ranges from 2.0 to about 7.0, preferably about 3.5 to 5.5. The pH of the medium may be adjusted using mineral acids such as hydrofluoric acid, fluoroboric acid, phosphoric acid, and the like, including mixtures thereof; organic acids such as lactic acid, acetic acid, citric acid, or mixtures thereof; and water soluble or water dispersible bases such as sodium hydroxide, ammonium hydroxide, ammonia, or amines such as triethylamine, methylethyl amine, diisopropanolamine, or mixtures thereof.
Additionally, the medium may contain a resinous binder. Suitable resins include reaction products of one or more alkanolamines and an epoxy-functional material containing at least two epoxy groups, such as those disclosed in U.S. Pat. No. 5,653,823. Preferably, such resins contain beta hydroxy ester, imide, or sulfide functionality, incorporated by using dimethylolpropionic acid, phthalimide, or mercaptoglycerine as an additional reactant in the preparation of the resin. Alternatively, the
Hauser Brian T.
Karabin Richard F.
Lingenfelter Thor G.
Gorgos Kathryn
Keehan Christopher M.
PPG Industries Ohio Inc.
Uhl William J.
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