Stock material or miscellaneous articles – Composite – Of metal
Reexamination Certificate
1999-04-30
2002-07-16
Acquah, Samuel A. (Department: 1711)
Stock material or miscellaneous articles
Composite
Of metal
C528S272000, C528S296000, C528S298000, C528S299000, C528S300000, C528S301000, C528S302000, C528S306000, C528S307000, C528S308000, C528S310000, C525S010000, C525S011000, C525S015000, C525S020000, C525S025000, C525S034000, C525S035000, C525S041000, C525S043000, C525S044000, C525S063000, C525S437000, C525S441000, C525S445000, C428S457000, C428S482000, C524S081000
Reexamination Certificate
active
06420040
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to coating compositions for metal substrates, methods of coating a metal substrate, and metal articles having a coating composition applied thereon. The coating composition comprises: (a) an acrylate copolymer having pendant hydroxy groups, (b) a hydroxy-terminated polyester, and (c) a crosslinker in (d) a nonaqueous carrier, and is free of a halide-containing vinyl polymer. The coating composition, after curing, is useful as a topcoat for the interior of metal closures and demonstrates excellent flexibility and adhesion to primer coats and to plastisol gaskets.
BACKGROUND OF THE INVENTION
It is well known that an aqueous solution in contact with an untreated metal substrate can result in corrosion of the untreated metal substrate. Therefore, a metal article, such as a metal closure or container for a water-based product, like a food or beverage, is rendered corrosion resistant in order to retard or eliminate interactions between the water-based product and the metal article. Generally, corrosion resistance is imparted to the metal article, or to a metal substrate in general, by passivating the metal substrate or by coating the metal substrate with a corrosion-inhibiting coating.
Investigators continually have sought improved coating compositions that reduce or eliminate corrosion of a metal article and that do not adversely affect an aqueous product packaged in the metal article. For example, investigators have sought to improve the imperviousness of the coating in order to prevent corrosion-causing ions, oxygen molecules, and water molecules from contacting and interacting with a metal substrate. Imperviousness can be improved by providing a thicker, more flexible, and more adhesive coating, but often improving one particular advantageous property is achieved at the expense of another advantageous property.
In addition, practical considerations limit the thickness, adhesive properties, and flexibility of a coating applied to a metal substrate. For example, thick coatings are expensive, require a longer cure time, can be esthetically unpleasing, and can adversely affect the process of stamping and molding the coated metal substrate into a useful metal article. Similarly, the coating should be sufficiently flexible such that the continuity of the coating is not destroyed during stamping and molding of the metal substrate into the desired shape of the metal article.
Investigators also have sought coatings that possess chemical resistance in addition to corrosion inhibition. A useful coating for the interior of a metal closure or container must be able to withstand the solvating properties of the packaged product. If the coating does not possess sufficient chemical resistance, components of the coating can be extracted into the packaged product and adversely affect the product. Even small amounts of extracted coating components can adversely affect sensitive products, such as beer, by imparting an off-taste to the product.
Organic solvent-based coating compositions provide cured coatings having excellent chemical resistance. Such solvent-based compositions include ingredients that are inherently water insoluble, and, thereby, effectively resist the solvating properties of water-based products packaged in the metal container.
Epoxy-based coatings and polyvinyl chloride-based coatings have been used to coat the interior of metal closures and containers for foods and beverages because these coatings exhibit an acceptable combination of adhesion, flexibility, chemical resistance, and corrosion inhibition. Polyvinyl chloride-based coatings and vinyl acetate/vinyl chloride copolymer-based (i.e., solution vinyl) coatings also have been the topcoat of choice for the interior of metal closures because these coatings provide excellent adhesion to plastisol sealer gaskets applied over the cured topcoat. However, epoxy-based coatings and polyvinyl chloride-based coatings have serious disadvantages that investigators still are attempting to overcome.
For example, polyvinyl chloride-based coating compositions are thermoplastic. Thermoplastic coatings used as the topcoat of the interior coating of metal closures have inherent performance disadvantages, such as potential softening during the closure manufacturing process or under food processing conditions. Therefore, coating compositions having a thermosetting character have been investigated.
In addition, coatings based on polyvinyl chloride or a related halide-containing vinyl polymer, like polyvinylidene chloride, possesses the above-listed advantageous properties of chemical resistance and corrosion inhibition, and are economical. However, curing a polyvinyl chloride or related halide-containing vinyl polymer can generate toxic monomers, such as vinyl chloride, a known carcinogen. In addition, the disposal of a halide-containing vinyl polymer, such as by incineration, also can generate toxic monomers. The generated vinyl chloride thereby poses a potential danger to workers in metal can and closure manufacturing plants, in food process and packaging plants, and at disposal sites. Disposal of polyvinyl chloride and related polymers also can produce carcinogenic dioxins and environmentally harmful hydrochloric acid.
Government regulators are acting to eliminate the use of polyvinyl chloride-based coating compositions that contact food, and thereby eliminate the environmental and health concerns associated with halide-containing vinyl polymers. Presently, however, polyvinyl chloride-based compositions are still used to coat the interior of food and beverage containers and closures.
To overcome the environmental concerns and performance problems associated with polyvinyl chloride-based coating compositions, epoxy-based coating compositions recently have been used to coat the interior of food and beverage containers. However, epoxy-based coatings also possess disadvantages. For example, epoxy-based coating compositions are more expensive than polyvinyl chloride-based coating compositions.
In addition, epoxy-based coatings are prepared from monomers such as bisphenol A and bisphenol A diglycidyl ether (BADGE), for example.
Epoxy resins have a serious disadvantage in that residual amounts of glycidyl ether and bisphenol monomers are present in the resin, typically in an amount of about 0.5% by weight. The presence of such monomers, and especially a glycidyl ether monomer, raises serious environmental and toxicological concerns, especially because a glycidyl ether monomer can be extracted from a cured coating on the interior of a metal container by a product stored in the container. Accordingly, regulatory agencies have promulgated regulations reducing the amount of a glycidyl ether monomer in coating compositions, and especially coating compositions used on the interior of food and beverage containers.
Coating compositions also typically include a phenolic resin. Phenolic resins prepared from bisphenol A or similar bisphenols also can contain residual bisphenol monomers, similar to epoxy-based coatings. Phenolic resins also have disadvantages in that the resins can generate formaldehyde, which can adversely affect a product stored in a coated metal container. Accordingly, it would be an advance in the art to overcome the problems and disadvantages associated with coating compositions for metal substrates that contain an epoxy resin, a halide-containing vinyl polymer, and/or a phenolic resin.
With respect to a metal closure for a food container, the interior of a metal closure conventionally can be coated with three separate coating compositions, i.e., a three-coat system. First, an epoxy/phenolic primer is applied to the metallic substrate and cured, then a vinyl-based middle coat is applied over the cured primer. Finally, after curing the middle coat, a specially formulated topcoat capable of adhering to a plastisol sealer is applied over the cured middle coat. The plastisol sealer is applied over the cured topcoat, and formed into a gasket during manufacture of a metal closure from a metal s
Parekh Girish G.
Seibel Lawrence P.
Acquah Samuel A.
Marshall Gerstein & Borun
The Valspar Corporation
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