Coating processes – Direct application of electrical – magnetic – wave – or... – Polymerization of coating utilizing direct application of...
Reexamination Certificate
2002-09-11
2004-02-24
Cameron, Erma (Department: 1762)
Coating processes
Direct application of electrical, magnetic, wave, or...
Polymerization of coating utilizing direct application of...
C427S508000, C427S409000, C427S410000
Reexamination Certificate
active
06696106
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to radiation curable coatings for aluminum alloy bodies. More specifically, the invention relates to an improved primer composition for coatings containing radiation curable polymer precursors. The product is preferably polymer coated aluminum alloy sheet suitable for shaping into end panels for food and beverage containers. Another aspect of the invention relates to polymer coated aluminum alloy extrusions for architectural uses.
BACKGROUND OF THE INVENTION
Aluminum alloy sheet coated with a polymeric composition on one or more surfaces is commonly used for shaping into end panels for food and beverage containers. The coating can be applied by processes such as reverse roll coating, gravure coating, electrocoating, spraying, powder coating, and forward roll coating. Coatings are applied to the sheet to better preserve foods and beverages stored in metal containers produced from the coated sheet, and to improve their taste characteristics. Coatings also improve the corrosion resistance, formability and appearance of the metal.
Commonly used commercial coating processes involve the use of solvent based systems that generate volatile organic compounds (VOC's) into the air. To reduce or eliminate the generation of such volatile organics during the coating process, solids coating systems have been proposed for application to the metal surface during the coating process. Examples of solids coating systems are those known as ultraviolet (UV) or electron beam (EB) curable coatings. Such coating systems generally contain monomers and oligomers so that they can be applied to substrates in a fluid state. Some coating systems may also contain small amounts of a solvent to improve fluidity during application. The monomers and oligomers react to form 100% solids coatings.
However, UV and EB curable coatings often separate from the metal substrate during end formation operations.
Accordingly, a need exists for a polymer coated metal sheet which is produced using coatings that do not generate VOC's after being cured and which has sufficient polymer adhesion to adhere to the metal during subsequent fabrication of the metal sheet into ends for beverage or food containers.
Thus, an objective of the present invention is to provide an improved primer composition for making aluminum alloy sheet using coatings that do not generate VOC's during curing and which have sufficient adhesive strength to adhere to the metal surface during subsequent fabrication of the sheet into ends for beverage or food containers.
Although aluminum and aluminum alloys protect themselves against corrosion by forming a natural oxide coating, the protection is not complete. In the presence of moisture and certain electrolytes, aluminum and its alloys may corrode very quickly. Such moisture and electrolytes may originate in acid rain water, water puddles on roadways, salted winter roads, or food materials, among others. Coatings, such as organic polymer and silicone coatings, may protect aluminum substrates to a limited extent by themselves, but are usually poorly adherent without an intermediate treatment or layer. Also, thin coatings are frequently porous, and therefore require an intermediate coating or layer to enhance corrosion resistance of the aluminum substrate.
Accordingly, there is a need to treat aluminum substrates with primers or other chemicals that provide improved corrosion resistance as well as strong bonding affinity for subsequent coatings. Suitable aluminum substrates for practice of the present invention include sheet, plate, castings, and wrought products such as forgings and stampings.
In the prior art, chemical conversion coatings have been formed on aluminum by “converting” a surface of the metal into a tightly adherent coating, part of which consists of an oxidized form of aluminum. Chemical conversion coatings provide high corrosion resistance and improved bonding affinity for polymer coatings. A chromate conversion coating is typically provided by contacting aluminum with an aqueous solution containing hexavalent or trivalent chromium ions, phosphate ions and fluoride ions. In recent years, concerns have arisen regarding the pollution effects of chromates and phosphates discharged into waterways by such processes. Because of the high solubility and strongly oxidizing character of hexavalent chromium ions, expensive waste treatment procedures must be employed to reduce the hexavalent chromium ions to trivalent chromium ions for waste disposal.
Attempts have been made in the prior art to produce acceptable chromate-free conversion coatings for aluminum. For example, some chromate-free conversion coatings contain zirconium, titanium, hafnium and/or silicon, sometimes combined with flourides, surfactants and polymers such as polyacrylic acid. In spite of the extensive efforts that have been made previously, there is still no entirely satisfactory non-chromate conversion coating or primer for improving the adhesion and corrosion resistance of coated aluminum. Two of the major problems are that the chromate free conversion coatings usually provide weaker adhesion of the subsequent coating to the substrate, or the chromate free conversion coatings usually provide less corrosion resistance in aggressive environments, or both.
A principal objective of our invention is to provide aluminum with a chromium free primer layer which provides better corrosion resistance and better adhesion of subsequent coatings than a chromate conversion coating.
A related objective of our invention is to provide coated aluminum having a primer layer comprising a reaction product of a vinylphosphonic acid-acrylic acid copolymer and an aluminum oxide or hydroxide layer on the sheet.
Additional objectives and advantages of our invention will become apparent to persons skilled in the art from the following detailed description of some particularly preferred embodiments.
SUMMARY OF THE INVENTION
In accordance with the present invention there is provided a process for making an aluminum-polymer composite. The composite comprises an aluminum alloy body coated with a radiation cured coating composition. The aluminum alloy body may be a casting, extrusion, plate, wrought product, or sheet and is preferably a sheet having a thickness of about 0.008 to 0.015 inch. Some aluminum alloys suitable for the present invention include aluminum-manganese alloys of the AA 3000 series, aluminum-magnesium alloys of the AA 5000 series, and aluminum-magnesium-silicon alloys of the AA 6000 series. For making polymer coated sheet suitable for shaping into food container bodies or food or beverage container end panels, we prefer aluminum-magnesium alloys of the AA 5000 series and particularly the AA 5042 and AA 5182 alloys. For making polymer coated sheet suitable for use on vehicle bodies we prefer aluminum-magnesium-silicon alloys of the AA 6000 series.
Aluminum alloys suitable for container end panels such as AA5182 are provided as an ingot or billet or slab by casting. Before working, the ingot or billet is subjected to elevated temperature homogenization. The alloy stock is then hot rolled to provide an intermediate gauge sheet. For example, the material may be hot rolled at a metal entry temperature of about 700-950° F. to provide an intermediate product having a thickness of about 0.130 inch to about 0.190 inch. This material is cold rolled to provide a sheet ranging in thickness from about 0.007 to 0.014 inch. A preferred metal sheet is AA5182 aluminum alloy sheet in either the H19 or H39 temper.
Aluminum alloys such as AA5042 are provided as an ingot that is homogenized. This procedure is followed by hot rolling to an intermediate gauge of about 0.125 inch. Typically, the intermediate gauge product is annealed, followed by cold rolling to a final gauge of about 0.007 to 0.014 inch. A preferred metal sheet is AA5042 aluminum alloy sheet in the H2E72 temper.
The aluminum alloy sheet is generally cleaned with an alkaline surface cleaner to move any residual lubricant adhering to th
Ankney Ronald G.
Levendusky Thomas L.
Schultz Paul B.
Alcoa Inc.
Cameron Erma
Klepac Glenn E.
LandOfFree
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